Supramolecular Chirality in Self-Assembled Systems

Review

pubs.acs.org/CR

Supramolecular Chirality in Self-Assembled Systems Minghua Liu,* Li Zhang, and Tianyu Wang Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China 4.3.6. Sonication 7339 4.3.7. pH Value 7340 4.4. Chiral Amplification in Supramolecular Sys- tems 7340 4.4.1. Analogue-Induced Chiral Amplification 7340 4.4.2. Chiral Amplification in Binary Systems 7343 4.4.3. Chiral Amplification to Nanoscale 7343 4.4.4. Unexpected Amplification in Racemate Assemblies 7344 4.5. Chiral Memory in Supramolecular Systems 7344 4.5.1. Helicity Memory in Noncovalently-In- duced Helical Polymers 7344 4.5.2. Chiral Memory in Aggregates Such as J and H Aggregates 7346 CONTENTS 4.5.3. Helicity Memory in Chiral Cages from Coordination Compounds 7347 1. Introduction 7305 5. Spontaneous Symmetry Breaking and Emergence 2. Basic Concepts Related to Molecular and Supra- of Supramolecular Chirality in Self-Assembled molecular Chirality 7305 Systems from Exclusively Achiral Molecules 7347 2.1. Configuration and Conformation Chirality 7306 5.1. Liquid-Crystal and Banana-Shaped Mole- 2.2. Induced Chirality 7306 cules 7348 2.3. Helicity or Helical Chirality 7307 5.2. Solution Systems, Micelles 7349 3. Characterization of Supramolecular Chirality 7308 5.3. Gel Systems 7353 3.1. Morphology Observation 7308 5.4. Air/Water Interface and LB Films 7354 3.2. Spectroscopic Methods for Characterization 5.5. Controlling Handedness of Supramolecular of Chirality 7309 Chirality 7358 3.2.1. CD Spectra of Supramolecular Systems 7309 5.5.1. Vortices and Spin Coating 7358 3.2.2. Measurement Aspects 7309 5.5.2. Circularly-Polarized Light 7361 3.2.3. CD Spectra and Interpretation 7309 5.5.3. Surface Pressure 7362 4. Supramolecular Chirality in Self-Assembled Sys- 5.6. Self-Assembly of Racemic Systems 7362 tems Containing Chiral Molecular Components 7310 6. Applications of Supramolecular Chirality 7365 4.1. Supramolecular Chirality in Assemblies of 6.1. Supramolecular Chiral Recognition and Sens- Chiral Components 7310 ing 7366 4.1.1. Amphiphiles 7310 6.2. Supramolecular Chiroptical Switches 7372 4.1.2. C -Symmetric Molecules 7313 3 6.3. Supramolecular Chiral Catalysis 7374 4.1.3. π-Conjugated Molecules 7316 6.4. Optics and Electronics Based on Supra- 4.1.4. Molecules with Multiple Chiral Centers 7324 molecular Chiral Assembly 7381 4.2. Chirality Transfer in Systems Containing 6.5. Circularly Polarized Luminescence (CPL) Chiral and Achiral Molecules 7325 Based on Chiral Supramolecular Assemblies 7381 4.2.1. Chirality Transfer through Noncovalent 6.6. Biological Applications of Supramolecular Bonds 7325 Chirality 7383 4.2.2. Chirality Transfer from Solvent to Assem- 7. Conclusions 7384 blies 7329 Author Information 7384 4.2.3. Chirality Transfer from Low Molecular Corresponding Author 7384 Weight Molecules to Macromolecules 7330 Notes 7384 4.3. Dynamic Features and Regulation of Supra- Biographies 7384 molecular Chirality 7331 4.3.1. Solvents 7332 4.3.2. Temperature 7335 4.3.3. Redox Effect Chirality 7336 Special Issue: 2015 Supramolecular Chemistry 4.3.4. Photoirradiation 7336 Received: December 8, 2014 4.3.5. Chemical Additives 7336 Published: July 20, 2015

Acknowledgments 7385 biological systems and self-assembly, and many assist in References 7385 developing new drugs and materials. In this review, we present an overview of the progress in supramolecular chirality in self-assembly systems, which mainly include self-assembly in solution or in dispersion systems, 1. INTRODUCTION supramolecular gels, organized molecular films such as Langmuir − fi Chirality is a basic characteristic of living matter and nature. and Langmuir Blodgett lms, and others. Although there are During the evolution of life on our planet, nature has favored one several reviews detailing supramolecular chirality, self-assembly, 7−13 fi kind of chirality, thereby selecting the L-amino acids (with the as well as chiral nanomaterials and nanostructures, the eld exception of glycine) as the main component of proteins and has grown rapidly, and many new exciting results and enzymes and D-sugars as the main components of DNA and phenomena have emerged. Furthermore, a general view of the RNA. In addition, chirality is universal and can be observed at chirality issue through the prism of supramolecular chirality will various hierarchical levels from subatomic and molecular to be helpful in better understanding many emergent chiral supramolecular, nanoscopic, macroscopic, and galactic scales.1 phenomena. In this review, we try to provide a comprehensive Figure 1 illustrates some typical chiral substances and objects at understanding of various organized chiral systems from the these various scales. perspective of supramolecular chirality, with reference mainly to At a subatomic level, chirality is connected to parity work published after 2010. First, we will provide a general fi conservation. Therefore, only left-handed helical neutrinos are overview of the supramolecular chirality, its de nition, and found. At a molecular level, there are a huge number of chiral special features through a comparison with the molecular molecules in natural system such as amino acids, sugars, and chirality. Second, we will simply introduce the various modern terpenes, and many synthetic compounds are also chiral. techniques of characterization used in supramolecular chirality. Furthermore, there are many biological macromolecules or Third, we will show in relative detail how molecular chirality supramolecular systems with chirality, microorganisms with could be transferred or related to supramolecular chirality in self- helix-shaped viruses, and bacteria such as tobacco mosaic virus assembled systems containing chiral molecular components. and Helicobacter pylori, respectively, and macroscopic living Here, we will further show some special features of supra- systems such as snails. On a larger scale, one finds that many molecular chirality such as dynamic chirality, the principles fi plants express chiral sense, such as mountain climbing vines. On a governing the chiral ampli cation, and chiral memory. In the light-year scale, our galaxy system is also chiral. fourth part, we will discuss how achiral molecules can self- Among these various levels, chirality at a molecular and assemble into a chiral system, i.e., symmetry breaking and the supramolecular level is of vital importance since it is strongly emergence of supramolecular chirality in systems containing related to chemistry, physics, biology, materials, and nano- exclusively achiral molecules. A great challenge in the supra- science, which treat the matter in scales from atomic to molecular molecular chiral systems constructed from achiral molecules is and supramolecular.2 The concept of molecular chirality has long the control of the chirality of system. Thus, we will discuss the been recognized and provided guidance in the design of drugs manner of controlling the supramolecular chirality in systems and functional molecules, while chirality at a supramolecular level composed of achiral molecules. Finally, we will show some is currently attracting great attention due to rapid developments typical applications of supramolecular chiral systems in electro- in supramolecular chemistry and molecular self-assembly. optics, sensing, asymmetric catalysis, biological applications, Supramolecular chemistry is the chemistry beyond molecules among others. In this portion, we will focus on the uniqueness of ff or the chemistry of entities generated by intermolecular chiral supramolecular systems, how they di er from molecular noncovalent interactions.3,4 Supramolecular chemistry is chiral systems, and the new properties that emerge from strongly related to self-assembly, which has been defined as the supramolecular chirality. autonomous organization of components into patterns or Currently, with the rapid development of supramolecular structures without human intervention.5 Both molecular self- chemistry, self-assembly, and nanoscience, chirality has become an important issues, and many new chirality-related topics have assembly and supramolecular chemistry are connected by − appeared, such as chirality at a surface,14 16 chirality in a noncovalent bonds and/or certain nano/microsized architec- 17−22 23 tures. Molecular self-assembly plays an important role in coordination system, and plasmonic chirality. These biological systems, the transfer and storage of genetic topics have been discussed and reviewed but are beyond the information in nucleic acids, and the folding of proteins into scope of this review. efficient molecular machines.6 During such biological processes, supramolecular chirality, which can be simply regarded as 2. BASIC CONCEPTS RELATED TO MOLECULAR AND chirality at a supramolecular level, is the result of biological SUPRAMOLECULAR CHIRALITY molecular self-assembly. A typical example is the secondary Chiralityisusedtodescribeanobjectthatcannotbe structures of proteins, which can exhibit various conformations superimposed on its mirror image. When a molecule is not such as α-helix, β-sheet, and random coil structures with different superimposable on its mirror image, then the molecule can be supramolecular chirality. During the molecular self-assembly, termed a chiral molecule. However, in practice, when judging supramolecular chirality is also the result of the special spatial whether a molecule is chiral, it is preferable to see if there is an arrangements of the molecules. Although supramolecular asymmetric carbon atom in the molecule. An asymmetric carbon chirality is strongly related to the chirality of the component atom or chiral carbon is a sp3 carbon atom that is attached to four chiral molecules, it is not necessary that all components be chiral. different types of atoms or four different groups of atoms. In To this end, achiral molecules can also possibly produce addition, if a molecule possesses two noncoplanar rings that are supramolecular chirality in a self-assembled system. Therefore, dissymmetrically connected and cannot easily rotate about the a deeper exploration of chirality at the molecular and chemical bond connecting them or the molecule possesses an supramolecular level will provide a better understanding of axis about which a set of substituents is held in a spatial

7305 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 1. Chiral architectures at various scales, from neutrinos to enantiomeric molecules, nanosized biomacromolecules with chiral structures (DNA and proteins), self-assembled micrometer-sized helical ribbons, microorganisms (helix-shaped bacteria), macroscopic living systems (seashells and plants), and galaxies. SEM image showing a helix is reprinted with permission from ref 285. Copyright 2014 John Wiley & Sons. The picture of a protein structure was obtained from Wikipedia (http://upload.wikimedia.org/wikipedia/commons/f/f3/T7RNA_polymerase_at_work.png) and reprinted under the “fair use” under Wikipedia’s license. Pictures are obtained from the following Web sites and apply to “fair use”: bacteria (http://tech.sina.com. cn/d/2010-01-29/10113816919.shtml), seashell (http://news.hainan.net/hainan/yaowen/tupian/2014/08/14/2016639.shtml), and flower (http:// www.chla.com.cn/htm/2011/0403/80069.html). The picture of a galaxy is a free stock graphic obtained from http://www.rgbstock.com/bigphoto/ mVErmjU%2FSpiral+Galaxy. arrangement that is not superimposable on its mirror image, the polymerize into polymers to form chiral primary structures and molecule can also be chiral even if it lacks an asymmetric carbon then self-assemble into secondary and tertiary structures through atom. Such chirality is termed planar chirality and axial chirality, noncovalent bonds, where both molecular and supramolecular respectively. Thus, molecular chirality can be essentially classified chirality are involved. The key to their difference originates from as point, plane, and axis chirality. the differences in the covalent and noncovalent bonds. There are Since supramolecular chemistry is based on the chemistry of a some unique features of supramolecular chirality, as shown in noncovalent bond, supramolecular chirality is produced by Table 1. For example, supramolecular chirality is generally nonsymmetric arrangement of molecules through a noncovalent dynamic and changes in response to external stimuli and the bond. Therefore, supramolecular chirality can be produced from environment. Chiral memory effects can also be seen in many chiral component molecules, the combination of chiral and supramolecular systems. Molecular chirality can originate from achiral molecules, or exclusively achiral molecules. Supra- the tetrahedral geometry of certain atoms or the asymmetric axes molecular chirality is largely dependent on the manner of and planes, while supramolecular chirality can be due to self- assembly of the molecular components, but the chirality of the assembled helical, spiral structures and chiral sheets or chiral component molecule plays an important role in determining this domain structures on surfaces. It should be noted that herein we manner in supramolecular systems. Generally, chiral molecules mean molecular chirality generally refers to that of small tend to form specific chiral structures with determined molecules. If we consider the chirality of polymer systems, the supramolecular chirality. In the combination of chiral and achiral distinctions between this and supramolecular chirality are less molecules, achiral molecules can be induced into chiral obvious. For example, the sergeant−soldier rule and the majority assemblies if there is a strong interaction between the chiral rule of chirality were originally proposed based on polymers and and the achiral molecules. In most cases, the supramolecular are also applicable to supramolecular systems. chirality of the system is also determined and may follow the Below are some general terms related to molecular and chirality of the chiral molecules. In the case of exclusively achiral supramolecular chirality. components, supramolecular chirality can result from the 2.1. Configuration and Conformation Chirality formation of supramolecular systems but in general will be racemic in the resulting macroscopic system. Configuration refers to the permanent geometry resulting from Table 1 lists a simple comparison between molecular and the spatial arrangement of a system’s bonds. Conformation refers supramolecular chirality. Both share some common features and to the spatial arrangement of substituent groups that are free to are strongly related. When we speak of supramolecular chirality, assume different positions in space without breaking any bonds, molecular chirality should be frequently considered. For because of the freedom of bond rotation. While configuration example, in the case of peptides, the chiral monomers covalently chirality is generally used in the case of molecular chirality, such as the absolute configuration of a chiral molecule, conformation Table 1. Comparison between Molecular and Supramolecular chirality usually refers to supramolecular chirality in systems such Chirality as the secondary and tertiary structures of proteins. However, this term has not always been rigorously used based on this molecular definition. chirality supramolecular chirality 2.2. Induced Chirality composition atom molecule, building block, tacton bond covalent bond noncovalent bond Induced chirality generally refers to those chiral supramolecular chiral geometry tetrahedron, axis, helical, spiral, chiral sheet, chiral domain systems where chirality is induced in an achiral guest molecule as plane a result of asymmetric information transfer from a chiral host or manifestation of point, axis, and conformation, secondary and tertiary vice versa. This host could be a chiral molecule, chiral pocket, chirality plane structures, helicity, induced chirality, etc. cavity, or chiral nanostructure. In order to produce the induced naming R/S, L/D, M/PM/P chirality, it is necessary for the achiral molecule to have a strong convention interaction with the chiral host through a noncovalent bond. A special feature fixed chirality, dynamic, sergeant−soldier rule, majority typical example of induced chirality is the encapsulation of a recognition rule, chiral memory, recognition chromophore into the cavity of cyclodextrin.24

7306 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 2. Some typical chiral molecules and their corresponding naming conventions.

Figure 3. (Top) Comparison of various microscopies used to characterize the chiral architectures. (Bottom) (A) AFM images of xerogels self-assembled from L- and D-HDGA (N,N-hexadecanedioyl-diglutamic acid). Reprinted with permission from ref 27. Copyright 2010 Royal Society of Chemistry. (B) STM images of the chiral twin chains from PVBA. Reprinted with permission from ref 28. Copyright 2001 The American Physical Society. (C) Mirror- imaged nanorods self-assembled from TPPS and (1R,2R)- or (1S,2S)-1,2-diaminocyclohexane. Reprinted with permission from ref 29. Copyright 2013 Royal Society of Chemistry. (D) TEM image of a chiral twist self-assembled from pyridine-containing L-glutamide. Reprinted with permission from ref 30. Copyright 2011 Royal Society of Chemistry.

2.3. Helicity or Helical Chirality molecule with axial chirality. If the substituents are molecules Helicity is a special form of axial chirality, which is deﬁned as an held together along the axis by noncovalent bonds then the entity that has an axis about which a set of substituents is held in a assemblies can be regarded as helical and have helical chirality. spatial arrangement that is not superimposable on its mirror image. If these substituents are atoms of molecular groups Helicity is very common in the supramolecular systems, and in covalently attached to the axis then it can be classiﬁed as a chiral particular, such chirality can often be visualized through AFM,

7307 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

SEM, and TEM observations. Helicity can be classified as M or P Atomic force microscopy (AFM) is based on measurement of helicity, which will be discussed later. the force between a sharp tip and a sample’s surface. The sample Besides these chirality concepts, the naming conventions of is mounted on a piezoelectric scanner that moves the sample chirality are complicated, and the detailed descriptions have been beneath a tip mounted on a soft cantilever. As the sample passes − published.25 27 For the reader’s convenience, Figure 2 illustrates beneath the tip, the force between the tip and the surface can be some typical chiral compounds or assemblies with certain types measured, which forms an AFM image. AFM has been used of chiral conventions. successfully to probe the surfaces at scales down to the atomic The R/S system is the most important and general level in vacuum, air, or other environments. The sample is nomenclature system for denoting enantiomers. Hereby, each generally fabricated on a very flat surface such as those of silica or chiral center is labeled R or S according to a system where its mica. For example, AFM was used to observe self-assembled substituents are each assigned a priority, according to the Cahn− chiral nanotubes obtained through the gelation of bolaamphi- 27 Ingold−Prelog priority rules (CIP). The D/L system (coined philes terminated with L-orD-glutamic acids. On the basis of from the Latin dexter and laevus, right and left) is related to the AFM observation, we can directly judge the supramolecular glyceraldehyde, whose two chiral isomers are labeled D and L.In chirality of the nanotube. L-HDGA formed a right-handed helical this system, compounds are named by analogy to glyceraldehyde. nanotube, while D-HDGA formed a left-handed one. Many biological molecules are labeled using this method. The Scanning tunneling microscopy (STM) technology is a M/P chirality generally refers to a supramolecular system or the technology based on quantum tunneling. When a conducting axial or planar molecular chirality. Viewing from either end of a tip is brought very close to a conductive surface and a bias voltage molecule or supermolecule downward along the helical axis, the is applied, a tunneling current flows between the tip and the system has P helicity if the rotation is clockwise and M helicity if surface. The resulting tunneling current is a function of the gap 31 the rotation is anticlockwise. The Λ and Δ chirality terms are between the tip and the surface. If the tunneling current is used for defining coordination compounds. The enantiomers can monitored and kept constant by adjusting the gap, the elevation be designated as Λ for a left-handed twist of the propeller of the surface can be traced and thus displayed an STM image. described by the ligands and Δ for a right-handed twist, as This technique provides an excellent means for controlling the illustrated in Figure 2. distance between the probe and the surface and a very high resolution image of the samples mounted on an atomically flat 3. CHARACTERIZATION OF SUPRAMOLECULAR conductive substrate such as HOPG. The STM technique CHIRALITY provides a molecular level resolution and is used to directly discriminate the absolute configuration of chiral molecules.32 An important step in the research of supramolecular chirality is Further, the technique is also applicable to observation of the the characterization of the chirality. Although there are many supramolecular chirality at surfaces.14 For example, Figure 3B ways to characterize the chiral features of a supramolecular shows mirror-imaged chiral twin chains that were self-assembled system, two classes of characterization of supramolecular from PVBA (4-[trans-2-(pyrid-4-vinyl)]benzoic acid) adsorbed chirality are usually applied. One is the morphological on a palladium substrate. The twin chains display supramolecular observation by various microscopes, with which one can directly chirality.28 observe the chiral molecules and chiral structures. With the rapid The scanning electron microscope (SEM) is the most widely development of STM, AFM, SEM, and TEM technologies, direct used electron microscope for investigating the surface features of observation of chiral structures has been made possible, and materials. When electrons interact with atoms in the sample they these techniques have significantly assisted in the development of produce various signals that can be detected including scattered research on chirality, especially in the self-assembled supra- electrons and X-rays. SEM uses electron illumination to form molecular systems. The other class of characterization is images from the reflected electrons. SEM is critical in all fields spectroscopy techniques such as circular dichroism (CD), that require characterization of solid materials. The SEM image is vibrational CD (VCD), and Raman optical activity (ROA) seen in three dimensions, but the result is a two-dimensional spectroscopy. With these techniques, the dynamic features of the photograph; thus, it is especially useful for detecting chiral supramolecular chirality can be followed and the self-assembly structures such as helices or twists. Figure 3C shows our results process can be unveiled. Although X-ray structural analysis is with self-assembled twisted nanorods by treating water-soluble useful in determining the absolute configuration of the chiral TPPS with (1R,2R)- or (1S,2S)-1,2-diaminocyclohexane in molecules, it requires that samples form into crystals. Self- which the mirror-imaged left-handed and right-handed helices 29 assembly systems are generally not crystallized and thus not were formed, respectively. applicable for X-ray crystallography. However, the molecular Transmission electron microscopy (TEM) is a microscopy configuration and packing information from the X-ray studies technique in which a beam of electrons is transmitted through an can help in understanding the self-assembly process. Herein, we ultrathin specimen, interacting with the specimen as it passes do not attempt to explain all of the possible characterization through. A TEM image is formed from the interaction of the methods in detail but provide a general introduction as to how electrons transmitted through a very thin specimen (<200 nm). these techniques are applied to the study of supramolecular In addition, using TEM, a diffraction pattern can also be ∼ chirality. obtained. TEM provides very high resolution ( 0.1 nm) and is useful in determining the structures of nanomaterials. It is also 3.1. Morphology Observation useful for observing chiral structures. For example, using TEM, a Seeing is believing. The rapid development of research in self-assembled helical twist structure formed by a pyridine- 30 supramolecular chiral systems is largely dependent on those containing amphiphilic L-glutamide was clearly observed. techniques that lead to direct visualization of the chiral Although all of the above techniques can be used to observe nanostructures. Figure 3 shows typical chiral images obtained chiral nanostructures, not all of the techniques are suitable for by these techniques and a comparison between these techniques. these observations. It is important to select the characterization

7308 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review method based on the samples and the self-assembled systems. be very useful in monitoring dynamic processes if the system Taking self-assembled gel systems as an example, AFM is contains chiral elements. generally applicable for investigating transparent gels, while SEM As a result, CD spectroscopy has developed rapidly because of is preferable for observation of white gels, since the sizes of the its application in supramolecular systems and been extended to nanostructures are different. For use of STM, a conductive investigating nanosystems such as plasmonic chirality research. sample is generally needed, while for TEM observation, materials This has become the most important technique for character- with heavy metals or metal ions are relatively easily observed. izing molecular, supramolecular, and nanoassembly systems. 3.2. Spectroscopic Methods for Characterization of Chirality With respect to CD measurements of supramolecular systems, there exist excellent reviews on this technique.34,37 Thus, herein, Spectroscopy provides a powerful method for detecting the only a few important measurement techniques will be chiral characteristics of supramolecular systems. Generally, mentioned, together with an explanation of the use of CD spectroscopy methods used for characterization of molecular spectroscopy in the measurement of supramolecular chirality. chirality in solution can also be used for characterizing 3.2.2. Measurement Aspects. The CD technique is supramolecular systems. Depending on the light source, typically suitable for viewing samples in isotropic solutions. measured physical variables, and principles, the measurement methods can be divided into linear and nonlinear optical Appropriate selection of the solvent, solute concentration, and methods. Linear optical methods include the CD, VCD, and measurement cell can facilitate measurement of the CD VOA spectroscopies apparatus, which are commercially spectrum of a system. However, when measuring a supramolecular system such as solid or colloid dispersions, membranes available. Nonlinear optical methods include SHG and SFG fi methods, which are not available as commercial instruments. or lms, gels, or liquid crystals, many factors such as the turbidity, Many of these techniques have been introduced in detail in books birefringence, and anisotropic chiral nanostructures will seriously − ff or reviews,33 36 so they will not be described here in detail. a ect the data collection and subsequent data interpretation. A Among these techniques, CD spectroscopy is the most widely major interference can result from linear dichroism (LD). A fi used for chirality characterization. We will take CD measurement useful method for the CD measurement of lms and to exclude ff as an example to show how supramolecular chirality is most often the e ect of LD has been proposed by Spitz et al. to distinguish characterized. intrinsic chirality from possible parasitic artifacts, and this 38,39 3.2.1. CD Spectra of Supramolecular Systems. Circular method was applied to Langmuir−Blodgett (LB) films. In dichroism (CD) is the differential absorption of left versus right some self-assembly systems, the LD could be enlarged if there is circularly polarized light. A CD spectrum records the circular an ordered arrangement of the molecules. Thus, simultaneous dichroism as a function of wavelength. Thus, a CD spectrum is measurement of the LD spectra may give clear warnings about strongly related to the absorption spectrum of certain the accuracy of the CD spectra. In addition, the contribution compounds. If the molecules do not contain any chromophore from linear and circular birefringence (CB) cannot be neglected or the absorption is outside the wavelength region then no CD in some anisotropic liquid systems. A more powerful trans- signal can be detected. For this reason, the two types of spectra, mission two-modulator generalized ellipsometry is proposed to UV−vis and CD spectroscopies, should be considered together completely determine the linear and circular birefringence (CB), in most cases. the linear dichroism and circular dichroism, and the depolariza- Circular dichroism spectroscopy was developed to study tion of the sample.40 This measurement is particularly important molecular chirality. However, through a series of detailed in symmetry-breaking systems where achiral molecules or investigations using CD spectroscopy in supramolecular systems, building blocks produce macroscopic chirality. Although it has been found that this technique is particularly useful for relatively little attention has been paid to these measurements monitoring self-assembled systems for reasons described below. in the past, more recent literature provides a deeper under- First, self-assembly is usually a dynamic process where standing of these effects. assembly and disassembly occurs simultaneously. These 3.2.3. CD Spectra and Interpretation. In CD spectral dynamics generally occur in the time scale of CD measurements. measurement, two types of spectra are generally obtainable, as Thus, CD spectroscopy is very useful in obtaining information, illustrated in Figure 4A. such as the formation dynamics of the chiral nanostructures, as well as the interaction between the chiral species and the chiral substrates. Second, CD signals originate from the electronic transitions of the chromophore and are generally strong and sensitive to the chromophores as well as the chromophore packing. During self- assembly, molecular packing plays an important role in determining the chiral nanostructures, and the CD spectrum can give detailed information on packing. In many cases, we can detect exciton coupling in the self-assembled systems. The exciton CD can experimentally assign molecular conformations, absolute configurations, and molecular interactions, even for rather complicated supramolecular systems. In many cases, it is more sensitive than UV−vis spectra and gives more detailed information on chiral interactions. Third, CD spectroscopy is also useful for detecting the chiral Figure 4. (A) Typically classified CD spectra in supramolecular systems. perturbations of the assemblies. Many self-assembled systems are (B) Qualitative Kramers−Kronig-consistent transformation of a CD dynamic and responsive to external stimuli; thus, CD spectra will bisignate band.

7309 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Observation of a peak or valley in the CD spectrum is referred 4.1. Supramolecular Chirality in Assemblies of Chiral to as the Cotton effect, which is the characteristic change in Components circular dichroism in the vicinity of an absorption band of a The transfer of chirality from a chiral center to aggregates or ff substance. The Cotton e ect is deemed positive if the circular assemblies is widely found in molecular self-assemblies. It is fi fi dichroism rst increases as the wavelength decreases (as rst generally accepted that chiral molecules easily form chiral fi observed by Cotton) and negative if the CD decreases rst. supramolecular systems. However, such a transfer, in fact, ff When measuring the CD spectrum, the Cotton e ect generally depends on a number of factors including the distance of the − corresponds to the absorption maximum in the UV vis chiral center to the assembly site, the strength of the noncovalent spectrum, while the sign of the CD is determined by the bonds, and the competition of the chiral and achiral interactions handedness of the supramolecular assemblies. If the enantiomers to name just a few. The supramolecular chirality of the assemblies are measured, mirror images should result. can be determined by their CD spectral measurement and/or Another technique is the exciton-type CD spectrum, as shown morphological observation using AFM, SEM, and TEM. in Figure 4A with dashed lines. This CD spectrum is commonly Generally, in their monomeric or free state, the CD signal of referred to as a bisignate band, which is Kramers−Kronig the compounds is silent in the chromophore portion if the 40 consistent with the measured CB, as illustrated in Figure 4B. In chromophore is located far from the chiral center. However, general, in the case of solutions, there is a direct correlation through self-assembly, the entire assembly becomes chiral and between the regions of absorption and the CD. In the case of a supramolecular chirality can be produced and thereby detected noncoupled chromophore, the shapes of the two spectra by CD spectroscopy. During the self-assembly of the chiral observed for the enantiomers are similar, although the vibrational components, the chromophores are held together by various fine structure could be different. If two or more strongly noncovalent bonds and tend to adopt a spatial nonsymmetric or absorbing chromophores are oriented chirally with respect to chiral arrangement that lowers the energy of the system. In many each other, an exciton spectrum is observed and characterized by cases, such assemblies appear as chiral nanostructures, which can the presence of two bands with opposite signs. The zero CD be easily visualized using AFM or SEM observation. However, it between the valley and the peak is the crossover, which is usually is not necessary that a chiral system that exhibits a CD signal in the position of the absorption maximum in the UV−vis should always be composed of chiral nanostructures. The spectra. In supramolecular systems, the exciton CD is frequently chirality transfer will depend on the structure of the chiral observed. While the exciton CD spectrum is generally sym- molecules. Here, we summarize the correlation between the metrical in solutions, it is quite common that the two bands of an supramolecular chirality of typical chiral supramolecular systems exciton spectrum do not have the same intensity in the based on their component molecules. supramolecular system. This is because there are many 4.1.1. Amphiphiles. Molecules that contain both hydro- aggregated or cooperatively interacting chromophores, which philic and hydrophobic groups are called amphiphiles. A typical can lead to nondegenerate couplings of the same chromophore, amphiphile consists of a polar hydrophilic group, usually called which alters the relative intensity of the two bands. In addition, in the head, which is joined to a nonpolar hydrophobic moiety, some cases, the scattering effect may affect the shape of the CD referred to as the tail. Amphiphiles are some of the most spectra.41 extensively investigated building blocks in self-assembly systems. According to the number of polar head(s) and hydrophobic 4. SUPRAMOLECULAR CHIRALITY IN tail(s) and the connection between them, amphiphiles are usually fi SELF-ASSEMBLED SYSTEMS CONTAINING CHIRAL classi ed as (1) single head/single or double tail amphiphiles, (2) MOLECULAR COMPONENTS bolaamphiphiles, in which two hydrophilic heads are connected by a hydrophobic skeleton group, (3) Gemini or “dimeric” During the chiral self-assembly, an important issue is how the amphiphiles made up of two hydrocarbon tails and two ionic supramolecular chirality or the chiral nanostructures are groups linked by a spacer, or (4) dendritic amphiphiles, where produced. Propagation of chiral information through specific the headgroup or the hydrophobic chain is dendritic. interactions and organization in materials or supramolecular Amphiphilic molecules self-assemble in water or in an organic assemblies is generally called chirality transfer. The chirality phase to form various kinds of ordered structures including transfer from the molecules to the supramolecular system micelles, vesicles, microemulsions, and liquid-crystalline meso- represents an important origin of supramolecular chirality. This morphic phases. In addition, chiral self-assembly can hierarchi- chirality transfer in self-assemblies containing chiral molecular cally lead to a rich variety of more organized nanostructures such components can be divided into two cases: (1) the chiral as fibers, ribbons, helices, “superhelices”, and tubes when the information on a chiral center (generally an asymmetric carbon amphiphiles are endowed with chiral elements. During this atom or axial chirality) is imposed to the whole aggregate or process, supramolecular chirality is often, but not always, assemblies containing a chromophore, which usually can be expressed in the morphology of these aggregates at a length detected by CD spectroscopy, and (2) the chiral sense of one scale of nanometers or micrometers. component is transferred to achiral components to form a 4.1.1.1. Conventional Amphiphiles. Over the past three complex system exhibiting supramolecular chirality. The bridges decades, a number of synthetic chiral amphiphiles with diverse for this chiral transfer are various noncovalent interactions, such molecular structures have been designed to form supramolecular as hydrogen bonding, electrostatic interactions, metal−ion chiral nanostructures. In the design of amphiphiles, both the coordination, donor−acceptor interactions, host−guest inter- position of the chiral center and the noncovalent bonding sites actions, and van der Waals interactions. In this section, we will should be considered. When chiral amphiphiles with long alkyl discuss the chirality transfer in systems composed of chiral chains self-assemble, a basic bilayer structure is generally formed molecules and in those containing mixed chiral/achiral in the initial stage, which can further stack into multibilayer molecules. Chiral systems composed exclusively from achiral structures. With the help of chiral sense in the amphiphiles, these molecules will be the topic of the next section. sheet-like bilayer membranes (single or multiple) often distort to

7310 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review form chiral nanostructures with a large curvature and/or a high to the superassemblies resulted from their differences in the aspect ratio, leading to twists, helices, superhelices, tubes, and so hydrogen bonding between the gelator molecules. Both the 3b on. In 1984, Kunitake et al. reported the first evidence of the and the 3c gelators self-assembled through strong intermolecular formation of nanoassemblies with a helical sense from a bilayer of H bonds and π−π stacking, while 3a tended to form an totally synthetic dialkyl amphiphiles.42,43 intramolecular H bond. Such differences in the H bonding could To date, a large number of amphiphiles have been reported to further lead to varying morphologies, in which the gels from 3b form chiral fibrous aggregates based on distorted bilayers.44 and 3c formed nanotwists and nanotubes, respectively, exhibiting These amphiphiles include diacetylenic phospholipids, glyco- a clear chiral feature in their morphologies, while 3a formed lipids with open or cyclic sugars and various quantities and types conventional nanofibers without a clear chiral feature. of unsaturation in the tail (double bonds, diacetylenic moieties),45,46 amino-acid-based amphiphiles,47 as well as hexamethylenediamine-based amphiphiles. More detailed re- − views can be found elsewhere.13,48 50 Herein, we show recent examples of self-assemblies of the L-glutamic-acid-based amphiphiles. Liu’s group and the Ihara group designed a series of chiral amphiphiles based on L-orD-glutamide, and Liu et al. investigated the chirality transfer from the molecular to supramolecular level or nanostructures, as shown in Figure − 5.30,51 66 Self-assembled chiral nanostructures are generally

Figure 6. (A) Illustration of the self-assembly manner of different isomeric molecules. Bilayer units were first formed for 3a and 3c and then stacked into multibilayers to form nanofibers and nanotubes, respectively. Only one bilayer unit is shown for clarity. 3b stacked into square columns and then into a nanotwist. TEM images obtained from various DMSO gels: (B) 3a, (C) 3b, and (D) 3c. Reprinted with permission from ref 30. Copyright 2011 Royal Society of Chemistry.

4.1.1.2. Gemini Amphiphiles and Bolaamphiphiles. In contrast to conventional amphiphiles with only one headgroup, both Gemini amphiphiles and bolaamphiphiles have two headgroups covalently connected by an alkyl spacer and, as such, have attracted great interest in their manner of self- assembly and structure. During self-assembly, the cooperation of these two headgroups together with the covalently linked spacer plays an important role. Oda et al. pioneered work on the helical self-assemblies through gelation of cationic Gemini surfactants with chiral − counterions.67 69 They found that the coassembly of cationic Gemini amphiphiles with chiral tartrate counterions in chloroform leads to the formation of stable twist ribbons. L-Tartrate produced exclusively right-handed helices, while the D- enantiomer produced left-handed helices. Bolaamphiphiles are molecules that contain two hydrophilic groups covalent linked by a hydrophobic skeleton (e.g., one, two, 70 Figure 5. Structure of glutamide amphiphiles (gelators) that formed or three alkyl chains, a steroid, or a porphyrin). Bolaamphi- supramolecular chiral systems through gelation. philic molecules have attracted much research interest due to the fact that they can be found in archaebacteria, which are able to obtained by a supramolecular gelation procedure in which the survive in a volcanic environment, i.e., in hot sulfuric acid. It is amphiphiles are dispersed into a solvent (water or an organic believed that the monolayer structure of a membrane together solvent) by heating and/or sonication, and then the transparent with its helix formation can stiﬀen the cell membrane, enabling solution is allowed to cool to room or a designated temperature, the archaebacteria to survive in the hostile volcanic conditions. yielding supramolecular gels with an expression of supra- The most important feature of assemblies formed by molecular chirality and various chiral nanostructures. bolaamphiphiles is their monolayer lipid membrane (MLM) For instance, three isomeric pyridine-containing (ortho, meta, intermediate, which is very similar to a bilayer structure but the and para isomers) L-glutamic amphiphiles can easily form two polar layers are covalently connected. The Shimizu research organogels with DMSO (3a−c).30 Although none of the gelator group conducted a series of studies on the self-assembly of the molecules exhibited CD signals in solution, when they formed bolaamphiphiles that revealed their manner of assembly the organogels, the assemblies based on the m-pyridine- systematically.71 During the self-assembly process, many of substituted (3b)andp-pyridine-substituted (3c)gelators these molecules formed into single- or multi-MLMs and then exhibited CD signals. However, the o-pyridine gels (3a) did self-assembled into nanotubes containing a variety of wall not. This diﬀerence in the chiral transfer from the molecular scale thicknesses, lengths, and exterior/interior diameters.71 If the

7311 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review headgroup is modified and the MLM is allowed to stabilize, and could be spun unto yarns using an automatic spinning single-walled chiral nanotubes could possibly be fabricated from machine.75 The self-assembled supramolecular nanotube yarns the bolaamphiphiles. For example, Liu and co-workers designed had a nominal tensile strength of 45−60 MPa with Young’s a simple bolaamphiphile containing two L-glutamic acids as the moduli as high as 6.8−9.9 GPa, which is comparable to many hydrophilic head (15a, 15b) and found that this molecule can covalently linked polymers. This result illustrates that a complex self-assemble into a helical nanotube.27,72 The wall of the supramolecular polymer with relative strong mechanical proper- nanotube consisted of a single molecular layer, and the ties can be obtained through hierarchical self-assembly of a small nanotubes were sufficiently long to form entanglements between molecule. Here, use of the chirally terminated amino acids may adjacent tubes. However, introduction of a rigid benzene play an important role. segment into the hydrophobic chain, in molecule 17, changed While the design of symmetric bolaamphiphiles is relative the morphology of the resulting assemblies to nanofibers or simple, the self-assembly of unsymmetrical bolaamphiphiles nanoribbons.73 Another bolaamphiphile containing a methyl provides more diversity in the control of the inner and outer ester of L-histidine (18) as the terminal group was also found to surfaces of the resulting nanotube. Shimizu et al. designed 74 form single-wall nanotubes at a low pH. unsymmetrical L-glucosamide bolaamphiphiles that possessed headgroups varying in size or properties, and these were used to construct a lipid nanotube and an unsymmetrical lipid membrane. Unsymmetrical MLM-based nanotubes possess both an outer surface containing sugar hydroxyl groups and an inner surface containing carboxylic acid groups. They were able to control the inner diameters of the unsymmetrical MLM nanotubes by varying the length of the spacer chain.76 Such nanotubes offer two advantages when compared with those prepared from the monopolar amphiphiles: (i) they possess distinctive inner and outer surfaces, which can be used for efficient encapsulation of materials, in particular, for selective surface functionalization, and (ii) the diameter of these nanotubes can be controlled by changing the molecular shape. It is common practice to attach the chiral centers as the headgroup when designing a chiral amphiphile. Thereby, bolaamphiphiles with two headgroups of the same chirality are generally designed. However, it would be interesting to observe the chiral assembly process when the bolaamphiphiles contain two headgroups with opposite chirality. 4.1.1.3. Peptide Amphiphiles. Among the many available headgroups in an amphiphile, peptide amphiphiles (abbreviated as PA) possessing peptides as the headgroup have attracted increasing attention recently owing to their close relationship to biological systems. Many types of peptides can be introduced into the amphiphilic molecules ranging from simple amino acid units to complicated peptide sequences with bioactive functions. Figure 7. Structures of bolaamphiphiles that were found to form chiral The group of Stupp77,78 developed a class of peptide nanotubes or twists. amphiphiles capable of self-assembly into cylindrical nanofibers with high aspect ratios. The chiral sense of the peptide offers an opportunity to fabricate cylindrical intertwining nanofibers that form multiple helices or superhelices for biomedical applications.77,78 In the systematic study of peptide amphiphiles containing valine−glutamic acid dimers, Stupp et al. found that the dimeric repeat unit promoted self-assembly into belt-like flat assemblies.79 The lateral growth of these assemblies can be controlled in the range from 100 nm to as little as 10 nm as the number of Figure 8. AFM images of hydrogels of (A) 15c, (B) 18, and (C) 19. dimeric repeat units increased from two to six. With the growth Reprinted with permission from refs 72, 74, and 75. Copyright 2005 and of the peptide sequence, these flat β-sheet assemblies appeared to 2013 Royal Society of Chemistry and Copyright 2013 John Wiley & twist (Figure 9). Experimental results demonstrated that the Sons. peptide amphiphile sequences can profoundly affect the subsequent supramolecular morphologies in water. Interestingly, when compound 18 was hydrolyzed to its Of greater interest was the finding that the sequence of the carboxylic acid, 19,75 it exhibited a hierarchical self-assembly into amino acids in the peptide amphiphiles could significantly multiwalled ultralong supramolecular nanotubes in slightly influence the one-dimensional (1D) chiral nanostructures. It was alkaline aqueous solution (pH 8−9) as a result of hydrogen found that four peptide amphiphile isomers, with identical bonding and electrostatic interaction. Of greater interest, these composition but varying sequences of four amino acids, had a nanotubes then formed bundles consisting of thousands of drastic effect on the resulting 1D nanostructures under identical ultralong supramolecular nanotubes packed in a parallel manner environmental conditions.80 The molecules with a peptide

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show strong CD signals in their absorption regions via self- assembly, which means the chirality can be transferred from the periphery to the core and the whole assembly. A helical tube fabricated from a dendritic glutamide lipid87 (21) exhibited a wall with a thickness of 4 nm, which corresponds to a bilayer thickness (Figure 11). While many other compounds generally form multiwalled nanotubes, these dendron molecules appeared to form a double-walled nanotube, because of the multiple H bonds between the dendron heads that stabilize the bilayer structures. More interestingly, the nanotube structure is very stable in a wide pH range. C 4.1.2. 3-Symmetric Molecules. Among the many supramolecular building motifs, C3-symmetric molecules have gained special interest in forming organized nanoassemblies either 88,89 chirally or nonchirally. C3-symmetric building blocks, usually consisting of a central aromatic or cyclohexane ring functionalized at the 1, 3, and 5 positions, have been frequently exploited. In the design of the C3-symmetric molecules, other functional groups can be attached via amide or urea bonds with tricarboxylic acids or triamines. If chiral elements are introduced into these Figure 9. (A) Structures of 20 (VE)2, (VE)4, and (VE)6. (B−D) molecules, chiral supramolecular assemblies can be easily Cryogenic transmission electron micrographs of (VE)2, (VE)4, and obtained. The C3-symmetric molecules often adopt a propeller- (VE)6 nanostructures formed in 5 mM solutions of PA. Scale bars: 200 like conformation, because of the steric hindrance surrounding nm. Reprinted with permission from ref 79. Copyright 2013 American the central aromatic core and the wedged substituent group Chemical Society. which causes rigidity through intramolecular interactions (mainly hydrogen bonding). During stacking, the chiral elements sequence of alternating hydrophobic and hydrophilic amino play an important role in determining the chiral sense of the acids, such as VEVE and EVEV, are able to self-assemble into a assemblies. The benzene-1,3,5-tricarboxamide (BTA) motif ﬂat nanostructure. EVEV was shown to form 3D twisted ribbons comprising either three N-centered or three CO-centered or helical ribbons. In addition, occasional nanotubes can coexist amides attached to a benzene core (Figure 12) has been widely with the twisted and helical ribbons. By contrast, nonalternating isomers such as VVEE and EEVV result only in the formation of employed in the assembly of chiral supermolecules. The three cylindrical nanoﬁbers that lack a chiral sense. This result is an amide bonds in these molecules form H bonds, which contribute excellent demonstration that the peptide sequences can to the one-dimensional growth of the monomers into a determine the supramolecular chirality as well as the possible columnar-type supramolecular polymer. The chiral information chiral architectures (Figure 10).80 at the substituents of the molecule can be transferred to the 4.1.1.4. Dendritic Amphiphiles. Although dendritic amphi- central aromatic rings and induce preferential formation of a philes have not been frequently reported in the past decade, they single helicity in their supramolecular architectures. During this exhibit some interesting assembly features with regard to the process, chirality transfer is largely dependent on the size of the 81−86 expression of chirality. For example, aromatic rings attached C3 core and the connection between the wedged substituents to the core of a dendron with chiral glutamic acid substituents can and the core.

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Figure 11. Structure of dendron amphiphiles (left) and height AFM images of hydrogels of 21 (right): (A) pH 3, (B) pH 7, (C) pH 10, (D) pH 11. (E) AFM image analysis of the helical pitch. (F) TEM image of a hydrogel obtained at pH 7. AFM and TEM images reprinted with permission from ref 87. Copyright 2011 John Wiley & Sons.

NH bond compared to the Ph−CO bond in the monomer. Therefore, N-centered BTAs may exhibit less cooperation in self- assembly relative to that of the CO-centered BTAs, which results in a lower degree of amplification of chirality.101 To rationalize the chirality transfer mechanism in the self- assembly of C3 molecules, variations of the molecular structure, including the distance of the chiral center to the central cores, the number of chiral side chains, as well as the central core, have been conducted. When replacement of a hydrogen atom by deuterium was used as the source of chiral information, only a small energy difference between the diastereomerically related right- and left- handed helical aggregates was observed (Figure 14).92 The authors concluded that the value of the molar CD effect was Figure 12. (Top) Propeller-like shape of C3-symmetric molecules and approximately three times lower than that of 34a, although the their assembly into a helical superstructure upon stacking in columns. sign and shape of the CD spectrum of the deuterated isomer 33a (Bottom) Structure of CO-centered and N-centered BTA. Reprinted was similar to that of 34a. with permission from refs 101 and 222. Copyright 2010 and 2007 John The structure in the central core has been found to have a Wiley & Sons. profound effect on the chirality transfer. Luis Sancheź et al.102,103 utilized oligo(phenyleneethynylene)-based tricarboxamides Meijer et al. presented pioneering work on the chiral self- (OPETAs) as the central core of the C3-symmetric discotics. 90−93 assembly of BTA molecules. The self-assembly of the parent They compared the self-assembly of two series of C3-symmetric compound, CO-centered BTA equipped with chiral aliphatic discotics based on benzene-1,3,5-tricarboxamides (35) and side chains, has been found to self-assemble into helical, one- oligo(phenyleneethynylene)-based tricarboxamides (OPETAs) dimensional aggregates by means of strong, 3-fold H bonding, (36), in which the peripheral groups were decorated with chiral which was confirmed by a strong Cotton effect centered around N-(2-aminoethyl)-3,4,5-trialkoxybenzamide units (35b and 220 nm. Later, several groups extended these studies by 36b) (Figure 15). Unexpectedly, the chiral BTAs here were synthesizing BTA derivatives with increased π−π surfaces. practically CD silent, which differed from previously reported Examples include bipyridine,94,95 tetrathiafulvalene (TTF),96 BTA derivatives. The authors speculated that the outer amide NDI,97and porphyrins98,99 substituted with chiral side chains, functionalities of one molecule were too far apart to form which were connected to the BTA core. Due to the large intermolecular, helical supramolecular polymers. However, the conjugated surface, intermolecular self-assembly in solution takes CD spectrum of chiral OPE−TA 36b at a concentration of 1 × − place via strong π−π interactions and solvophobic effects.100 10 5 M in MCH exhibited an intense bisignated Cotton effect, The self-assembly of N-BTAs shows self-assembly behavior which suggested that this compound had self-assembled into a that is similar to that of their CO-centered counterparts, but left-handed helical structure. These results confirmed the the amplification of chirality is less pronounced as the result of combinatory influence of the π surface of the central aromatic the relatively weaker aggregation. A theoretical simulation reveals core and the branched nature of the peripheral side chains on the that there is a higher energy penalty for rotation around the Ph− chirality transfer.

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Figure 13. Structures of CO-centered BTAs having intramolecular hydrogen bonds with increased π−π surface.

Two series of oligo(phenyleneethynylene) (OPE)-based C3 Meijer et al. designed a C3-symmetric molecule that combined molecules have been synthesized to study the influence of the conjugated structures of OPEs with amide groups to structural features of the OPE discotics on chiroptical properties rebalance the π−π stacking and H-bonding interactions.105 For − (37, 38). Initially, the author compared the effect of the groups the chiral compound 40 at a concentration of 8 × 10 6 Min linked between the aromatic core and the peripheral side chains chloroform, no CD effect was detected, suggesting that 40 was on the supramolecular chirality. The OPE-based trisamides with molecularly dissolved at this concentration. However, in ff a variable number of chiral side chains (compounds 37b−d) self- methylcyclohexane (MCH), a bisignate Cotton e ect was assembled into helical aggregates. By contrast, the triangle- observed, which indicated a preferred helicity in the columnar shaped OPEs with ether and amide functional groups did not aggregates formed by 40, induced by the optical activity. ff Interestingly, when the authors changed the N-centered OPE- exhibit e ective chirality transfer (compounds 38), as demon-  strated by the corresponding CD studies, even though there was based discotics to C O-centered OPE-based discotics, the CD spectra of the supramolecular polymers were opposite in sign an absolute configuration of the stereogenic centers at all of the when keeping with the configuration of the identical stereogenic peripheral chains. Therefore, the cooperation of the three highly center. This suggests that the helical preference of OPE-based directional H bonds between the amide functional groups plays fi C3-symmetric molecules is governed by both the con guration of an important role in the hierarchical self-assembly and the stereogenic center and the manner of the amide connectivity, corresponding chirality transfer. Second, the CD spectra of the which affects the conformation of the amides with respect to the OPE-based trisamides with a variable number of chiral side π-conjugated core. Furthermore, the authors evaluated the chains demonstrated that only one stereogenic center was impact of symmetrization of the discotic structure on the chirality needed to achieve a helical organization with a preferred of the supramolecular polymer when C2-symmetric discotic tris- handedness. In addition, the ability to amplify the chirality amide 41 was synthesized. Despite 40 and 41 having an identical increased with an increase in the number of stereocenters at the configuration of the stereogenic centers, opposite chiral peripheral side chains. The study presented herein improved the information for the helical aggregates of 40 and 41 was observed. understanding of the structural rules that regulate the chiral Temperature-dependent CD measurements directly reflected 104 supramolecular organization of discrete molecules. weaker noncovalent interactions for the C2-symmetric 41 than

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Figure 14. Structures of N,N′,N″-trialkylbenzene-1,3,5-tricarboxamides (BTAs) (33, 34). Reprinted with permission from ref 92. Copyright 2010 Nature Publishing Group. for 40. While the number of amide bonds was the same in both systems, the strength of the hydrogen bonding varied. Combined π−π with the weaker stacking interactions in the C2-symmetric discotics relative to the C -symmetric discotics, the chirality Figure 15. (A) Structures of C3-symmetric BTAs (35) and triangle- 3 − transfer may have occurred in a different manner. shaped OPETAs (36). (B) CD spectra of compound 36b (2.5 × 10 6 M in MCH) at room temperature and 90 °C. Insets depict the cooling The chirality of the chiral center can further be transferred to −1 determine the supramolecular chirality of the nanostructures. curves of compound 36b from 363 to 288 K at intervals of 0.5 K min . For example, enantiomeric C compounds containing π- Reprinted with permission from ref 102. Copyright 2013 John Wiley & 3 Sons. functional tetrathiafulvalene units (compound 31)self- assembled into helical aggregates showing a preferential helicity twist over several length scales (Figure 17).96 The formation of ester gelator was found to form hexagonal tubes ranging from primary helices as twisted stacks was investigated by CD nano- to micrometer scale depending on the solvents used.106 In measurements and further confirmed by the theoretical addition, through antisolvent gelation in a wide range of mixed calculations. molecular mechanics (MM) and molecular solvents, hexagonal nanotubes are formed instantly upon mixing dynamics (MD) simulations were used to evaluate the relative at room temperature. stability of the P and M conformations of the stacks, and the 4.1.3. π-Conjugated Molecules. π-Conjugated molecules results indicated that the (S)-31 enantiomer provided the M occupy a very important position in the research on supra- helix, which showed greater stability (2 kcal mol−1) per molecule molecular chirality of self-assembled systems. For this there are than the P helix, which was in agreement with the optical activity several reasons. First, π-conjugated molecules possess inherent observed in the CD spectra. However, it was a surprise to obtain electronic properties. They are unique because of their potential mesoscopic-size chiral fibers of this compound in higher use in organic electronic devices, such as organic solar cells, field- concentrations, which exhibited inverted helicity, i.e., P helices effect transistors (FETs), light-emitting diodes (LEDs), for the S enantiomer and M helices for the R one. Although the etc.107,108 If endowed with chirality, these molecules may inversion of helicity between the primary twisted stack in represent new structures with novel properties. Second, π−π solution and the secondary helical aggregates for the solid fibers, stacking is one of the most important forms of noncovalent which can be seen as superhelices from hierarchical assembly, bonding, which frequently determines if a system can perform seems to be incomprehensible, it is a common phenomenon that self-assembly and also determines the self-assembly path- has been reported in many supramolecular systems. A more way.109,110 Third, π-conjugated molecules have strong absorp- detailed and exhaustive study needs to be conducted to explore tion in the UV−vis region, which allows their chiral assembly the link between molecular chirality, supramolecular chirality, processes to be easily characterized with CD spectra and other and the higher order of chiral expression or chiral nanostructures. morphological observations. π C3-symmetric benzene- or cyclohexane-centered chiral mole- In order to realize chiral self-assemblies based on -conjugated cules often self-assemble into nanotube structures by column molecules, it is important to introduce chiral elements into the π- stacking of the assembly unit. When chiral units are introduced, conjugated molecules. This can generally be done through the self-assembly process and formation of the nanotubes seems functional group substitutions at different positions on the to be easier. For example, a C3-symmetric L-glutamic acid ethyl aromatic rings or in the substituted alkyl chains. In addition, the

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Figure 16. Structure of OPEs derivatives. Reprinted with permission from refs 103 and 104. Copyright 2011 and 2012 American Chemical Society. chiral feature of the building unit itself, the substituent group, the nature of the π-conjugated backbone, solvent properties, Figure 17. Molecular structure of C3-symmetric compounds containing temperature, and even the stimuli factors such as light, heat, tetrathiafulvalene units (31), and an SEM image of helical aggregates. sonication, magnetic fields, etc., can have a significant influence Reprinted with permission from ref 96. Copyright 2011 American on the chiral assemblies. Herein we demonstrate the typical chiral Chemical Society. assembly features for important π-conjugated molecules such as polycyclic aromatic hydrocarbons, thiophene and its derivatives, oligo(p-phenylenevinylene) (OPV) and its derivatives, and perylene bisimide (PBI) and its related compounds. Since the self-assembly of a π-conjugated molecule in a gel has been reviewed,111,112 herein we focus on those materials with supramolecular chirality. 4.1.3.1. Pyrene (a Polycyclic Aromatic Hydrocarbon). The pyrene moiety is well known for its ability to form excimers at specified concentrations in solution. In addition, the strong π−π stacking of the chromophore makes it easy to perform self- assembly, during which supramolecular chirality can also be observed in the CD spectra if the chiral unit is introduced in the vicinity of the chromophore. For example, compounds 42 and 43 contain a pyrene moiety covalently connected to the chiral unit R R through the urethane moiety, forming organogels in isooctane Figure 18. (a) CD spectra of isooctane gels of 42 (solid line) and 43 (dotted line) at 293 K (1 mm path length). (b) Variable-temperature and n-dodecane, respectively. Although the solution of the CD spectra on an isooctane gel of 43R (31 mM). Reprinted with compounds did not show a CD signal, they exhibited CD signals permission from ref 113. Copyright 2010 American Chemical Society. at gel states in the region of pyrene chromophores, indicating that the chirality was transferred from the asymmetric carbon atom to the whole assembly. A temperature-dependent CD assemblies, the chirality in the L-glutamide moiety can be spectrum indicated that as the gel was progressively heated, the transferred to the self-assembled nanostructures, but the CD signal decreased and completely disappeared as the gel expression of the chirality at a supramolecular level appeared melted (Figure 18b), suggesting that chiral supramolecular to be dependent on the spacer and solvents. The 4b gels showed assembly was responsible for the CD signal.113 the same P chirality both in polar and in nonpolar solvents; Two pyrene-conjugated glutamide derivatives were designed however, the gel of 4a displayed an interesting chiral inversion to investigate the effect of the link spacer on the chirality transfer. induced by solvent polarity, i.e., M chirality in nonpolar solvents The pyrene moiety was linked to amphiphilic L-glutamide and P chirality in polar solvents (Figure 19). It was concluded directly (4a) or with three methylene spacers (4b). In both that the spacer between the amide groups and the pyrene ring

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Figure 19. Possible packing mode in the organogels of 4a (top) and 4b (bottom). For the 4b organogel, the steric hindrance between the pyrene moieties was reduced by the spacer. The units packed in the same direction both in polar and in nonpolar solvents. For the 4a gel, similar packing as in the 4b gel is observed in polar solvents. In nonpolar solvents, due to the large steric hindrance and the stronger H bond between the conjugated amide, opposite packing was adopted and the chirality was inverted. Reprinted with permission from ref 60. Copyright 2013 Royal Society of Chemistry. effectively regulated the hydrogen bonding and π−π interactions and then influenced the assembly mode as well as the corresponding supramolecular chirality. 4.1.3.2. Hexa-peri-hexabenzocoronene (HBC). Amphiphilic hexa-peri-hexabenzocoronenes (HBC) developed by Aida et al.114 are a unique class of amphiphilic aromatic molecules that exhibit excellent self-assembly behavior. Although the achiral HBC was found to self-assemble into chiral nanotubes, it simultaneously formed two mirrored chiral assemblies of equal quantity. Therefore, they canceled each other, and no macroscopic chirality could be detected. However, if some chiral elements were introduced into the HBC molecules, a particular supramolecular chirality can be obtained. For example, Aida et al. Figure 20. Structures of HBC derivatives 44 and 45 and formation of found that a hexa-peri-hexabenzocoronene having two chiral self-assembled graphitic nanotubes. (a) Schematic illustrations of the oxyalkylene side chains along with two lipophilic side chains structure of self-assembled graphitic nanotubes consisting of HBC amphiphiles. (b) Formation of chiral graphitic nanotubes with one- (denoted HBC 44) yielded graphitic nanotubes in MeTHF (2- π methyltetrahydrofuran).114 The CD spectra measurements handed helical arrays of -stacked HBC units through translation of revealed that the solution of (S)-44 at 50 °C was CD silent. molecular chirality into supramolecular helical chirality. Reprinted with permission from ref 114. Copyright 2005 National Academy of Sciences, Cooling of the solution resulted in the appearance of positive CD U.S.A. bands at 389, 400, and 423 nm, which gradually intensified with time. This result suggested that the tubular assembled 44 most likely contained a helical molecular arrangement of the π-stacked electronic and photonic devices through the formation of self- HBC units, whose helical sense was determined by the absolute assembled structures. There are many reports on the emergence configuration of the chiral centers in the hydrophilic side chains. of supramolecular chirality based on PBIs as building units.115,116 Thus, the molecular chirality was successfully transferred into the For example, the aggregates of dipeptides and perylene resulting supramolecular helical nanotubular assembly (Figure bisimide conjugates (glycine-tyrosine, GY, or glycine-aspartic 20). By contrast, the HBC amphiphile 45 bearing branched acid, GD) have been reported to show different chiral self- asymmetric centers in the paraffinic side chains produced few assembly depending on the nature of the peptide used. There is a nanotubular assemblies, which may be due to its branched competition between H bonding among the peptides and the paraffinic side chains that prevent the formation of a bilayer tape aromatic π−π stacking of PBI. Most interestingly, the peptide and further the chiral nanostructures. This indicated that not all sequence has a profound effect on the chirality transfer. In an ff fi of the molecular chirality can be transferred to the supra- aqueous bu er, PBI-[GY]2 formed chiral nano bers. In the molecular system. corresponding CD spectra of PBI-[GY]2 aggregates, a negative 4.1.3.3. Perylenebisimide (PBI). Perylenebisimide (PBI) is a Cotton effect at 447 nm and two positive Cotton effects were well-known dye which displays photostability and outstanding observed at approximately 515 and 552 nm, indicating that the optical and electronic properties as well as strong hydrophobic chiral sense of the peptide was transferred to the PBI moiety. By π−π interactions and stacking, which are of potential use in contrast, the PBI-[GD]2, which formed spherical aggregates

7318 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review rather than ﬁbers, was CD silent. This work established an association of the chiral transfer from a molecularly chiral component to supramolecular chirality as well as the morphology of the assemblies. It further provides a strategy for using short peptides and speciﬁcally their sequence structure to manipulate the PBI chiral nanostructure through chirality transfer.117

Figure 22. Chemical Structures of the S and R isomers of the compound 46 [R = CH(C6H13)2] and illustration of the self-assemblies structures on the value of luminescence dissymmetry factor (glum). Reprinted with Figure 21. Circular dichroism spectra of PBI-[GY]2 and PBI-[GD]2 aggregates in buffer solution (pH 10.8), concentration 1 × 10−3 M. permission from ref 119. Copyright 2014 American Chemical Society. Reprinted with permission from ref 117. Copyright 2014 American Chemical Society. an even number of L-alanine groups in the side chains were considerably less well defined and exhibited much smaller molar Many PBI helical assemblies are formed with the help of ellipticities. Moreover, they observed a 2-fold odd−even effect, intermolecular π−π interactions because of their large planar i.e., on one hand, expressed by an alternating reversal of the 118 structure. However, Zhu et al. found that chiral carboxylic- Cotton effect upon increasing the number of L-alanine units acid-functionalized PBI systems spontaneously self-assembled within a series of molecules with the same spacer length. On the into supramolecular helices via intermolecular hydrogen bonding other hand, the sign of the Cotton effect also alternated in reverse rather than π−π stacking based on fluorescence spectra with the increase in the spacer length for molecules with an measurements. The fluorescence of the PBI unit herein did not identical number of L-alanine units. change much upon formation of supramolecular assemblies, 4.1.3.4. Phenylenes. Phenylenes are a typical class of rigid-rod which was in contrast to the π−π arrangement, which led to molecules that has been studied in the area of self-assembly as significant fluorescence quenching. well as optical properties. By introducing a chiral group in the coil In addition to the chiral transfer from the chiral centers, the moiety of phenylenes, the chiral information can be transferred axial chirality of binap was also found to be efficiently transferred to the phenylene moieties through the self-assembly process.121 to perylenebisimides (PBIs) with the help of molecular stacking. For example, the bent rod-shaped rod molecule 47 was found to The supramolecular chirality of the PBI assemblies was optically self-assemble into the hollow tubules in dilute aqueous solutions. probed by circular dichroism (CD), vibrational circular Circular dichroism (CD) spectra of the aqueous solutions dichroism (VCD), and circularly polarized luminescence showed a significant Cotton effect above certain concentrations (CPL). The biPBI derivatives 46 formed one-dimensional (0.002 wt %) in the region of the aromatic chromophore, aggregates in methylcyclohexane (MCH) and spherical indicating that the tubules adopted a one-handed helical aggregates in chloroform at a higher concentration.119 The structure. Combining vapor pressure osmometry (VPO) one-dimensional aggregates exhibited twice the value of the measurements and CPK models, Lee et al.122 proposed that luminescence dissymmetry factor (glum) when compared with the compound 47 self-assembles via a fully overlapped packing spherical aggregates. The sum of excitonic couplings between the arrangement into the hexamericmacrocycles, which, in turn, individual chromophore units contributed to the high CPL stack on top of each other with mutual rotation in a single dissymmetry of the nanostructures. direction to form helical tubules (Figure 23). When a pyridine Frauenrath and co-workers found an unprecedented 2-fold unit was introduced into the concave side of the apex of the bent- odd−even effect during the investigation of oligopeptide− shaped aromatic compound, 48, the pulsating motions of the polymer-substituted perylene bisimides comprising a varying tubules were found to show a chiral inversion by virtue of the 120 number of L-alanines. Depending upon the number of L- pyridine forming water clusters through hydrogen bonding and alanine units, the observed CD activity was alternatively strong resulting in adjacent molecules that slide into a looser packing 122 and weak. The molecules bearing an odd number of L-alanines in arrangement. the side chains exhibited well-defined spectra with molar Another interesting example was taken from the helical self- ellipticities that increased directly with the number of L-alanine assembly of oligo-p-phenylene-based organogelators 49−52 residues. By contrast, the CD spectra of the compounds bearing (Figure 24), which has been found to be dependent on the

7319 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 23. (A) Structures of 47 and 48. (B) CD spectra of S-48 in aqueous solution at various concentrations. (C) Temperature-dependent CD spectra of S-48 (0.01 wt %) in aqueous solution. (D) Schematic representation of reversible switching of the tubules between expanded and contracted states with chirality inversion. Reprinted with permission from ref 122. Copyright 2012 American Association for the Advancement of Science.

Figure 24. Structures of 49−52, and schematic illustration of the aggregation pathways. At low concentrations, low temperatures, and short times, the helical organization of 50 is dominated by the atropisomerism of the central biphenyl unit and metastable P-type helices are formed. 51 and 52, and also 50 at higher concentrations, higher temperatures, and longer times, self-assemble into supramolecular structures of the opposite helicity (M-type). Reprinted with permission from ref 123. Copyright 2013 John Wiley & Sons. phenyl ring in the core.123 The OPPS with more than two phenyl stereocenters determined the left-handed supramolecular helix rings in the core self-assembled into left-handed helices, but through thermodynamic control. By contrast, compounds 50 when it contained a biphenyl core the resulting molecule showed and 51, with three and four phenyl rings, respectively, self- exchangeable helicity depending on the reaction time, temper- assembled exclusively into left-handed aggregates, with no ature, and concentration, exhibiting a competitive modulation of inversion of the helicity. This indicated that the inﬂuence of supramolecular helicity controlled by a kinetic versus thermody- the oligophenyl atropisomerism is much weaker than that of the namic process. At low temperature, low concentration, and short external stereocenters. This study provides not only a good assembling times, compound 50 formed right-handed metastable example of kinetically controlled modulation of supramolecular supramolecular helices under kinetic control. In this case, the chirality but also a better understanding of the chemical and supramolecular chirality was determined by the axial chirality of topological control in the generation of helical supramolecular the biphenyl core. On the contrary, at higher temperature, higher structures and the impact of the synergy between diﬀerent chiral concentration, and longer reaction times, the external S elements.

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4.1.3.5. Porphyrin. Porphyrin is one of the most extensively synthesized by Jiang et al.128 The self-assembly of this novel investigated π-conjugated compounds and exhibits excellent porphyrin−pentapeptide conjugate in THF/hexane and THF/ assembly capability and biocomptability. Like other π-conjugated water was comparatively investigated to illustrate the effect of the molecules, the introduction of chiral elements into the peptide’s second conformation on the helical arrangement of macrocyclic ring induces chiral self-assembly and produces porphyrin in the assemblies. The positive chirality of porphyrin supramolecular chirality. Porphyrin derivatives based on a pentapeptide aggregates was observed from conjugates in THF/ symmetrical amide-substituted discotic with chiral hydrocarbon hexane, suggesting the helical arrangement of the porphyrin side chains were designed by Meijer et al.124 (Figure 25). At chromophore with a P helicity. The negative chirality was found for the aggregates fabricated in THF/water, which was opposite that observed in THF/hexane. This work confirmed that the secondary conformation of the peripheral peptide tuned by solvent polarity further influenced the porphyrin chromophore packing mode and supramolecular chirality in aggregates. This result not only represented an example of organic nanostructures self-assembled from a covalently linked porphyrin−pentapeptide conjugate but provided a strategy for controlling and tuning the morphology and, in particular, the supramolecular chirality of porphyrin nanostructures. 4.1.3.6. Oligo(p-phenylenevinylenes). Oligo(p-phenylenevinylenes) (OPVs) are another class of linear π-conjugated molecules which are widely used in the fabrication of chiral supermolecules, due to the electronic properties of the π system, which is sensitive to intermolecular interactions, particularly the way in which the chromophores are organized. Figure 25. (Top) Structure and temperature-dependent CD spectrum Ajayaghosh and co-workers reported many helical nanostruc- ° ° of 53 in methylcyclohexane between 20 and 90 C with 10 C intervals. tures composed of OPV assemblies that are formed by attaching (Bottom) A model in which aggregates, monomers, and monomeric and hydrocarbon chains with asymmetric carbons to the OPV dimeric porphyrin pyridine adducts are connected by equilibrium backbone, which have been reviewed recently.111 They mainly constants. Reprinted with permission from ref 124. Copyright 2010 John Wiley & Sons. focused on the emergence of supramolecular chirality during the OPV organogelation, and a detailed description of the process room temperature, 53 was molecularly dissolved in chlorofor- can be found in his review. λ George and co-workers129 studied the self-assembly of OPVs mand. It had a sharp Soret band at max = 422 nm but lacked a CD ff signal. In MCH, porphyrin 53 exhibited a large blue shift to a bearing a chiral side chain and obtained two di erent kinds of λ assemblies which depended on the solvents employed and the broadened band at max = 390 nm, which is a typical band observed for cofacially arranged porphyrins or H aggregates.125 system temperature. Two aggregates were found, corresponding The CD measurements revealed an intense bisignate Cotton to State A (2.5% THF) with sheet morphology and State B (10% effect with a crossover at 390 nm, indicating a helical THF) with rolled nanotube structure. Remarkably, circular dichroism (CD) studies performed on State B showed that these arrangement of the chromophores in the aggregate. The assemblies were CD silent, while State A was marked by the porphyrin assemblies were disrupted by heating, accompanied appearance of a bisignated CD signal with a positive Cotton by a disappearance of the CD response. Upon cooling at a − effect at 415 nm and a negative Cotton effect at 375 nm which is concentration of 5.0 × 10 5 M, the CD effect reappeared at 69 °C characteristic of exciton-coupled OPV chromophores. The (Figure 25). chiroptical properties could be due to the different packing of A unique property of porphyrin compounds is their axial the chromophores. Notably, in State B, an annealing process was coordination ability. Changes of the axial ligand will cause the found to improve the molecular ordering in the assemblies and supramolecular chirality of the system to be regulated. For convert tubes to the nanosheets (similar to State A), which was example, the addition of the axial ligand pyridine to aggregates of confirmed by a sudden appearance of the bisignated CD signals 53 was found to alter the supramolecular chirality of the system. during cooling, characteristic of excitonically coupled chromo- Without pyridine, the porphyrin Soret band appears at 390 nm. phores. Thus, the supramolecular chirality in the self-assembled When 40 equiv of pyridine was added, a new red-shifted and split systems of the π-conjugated molecules is strongly related to their band at 418 and 427 nm appeared. The exciton splitting energy π−π stacking even when they had chiral substituents. −1 of 500 cm is indicative of a dimeric porphyrin pyridine Meijer, Schenning, and co-workers investigated the chiral 126 adduct. With the dissociation of porphyrin aggregates, the CD assembly of two OPVs through chiral peptide segments spectrum in the exciton split band region became weak. composed of either a glycinyl-alanyl-glycinyl-alanyl-glycine Ultimately, at a pyridine molar excess of 80 000, this split Soret (GAGAG), silk-inspired β-sheet, or a glycinyl-alanyl-asparagyl- band gradually converted into a single, narrow, CD-silent band at prolyl-asparagyl-alanyl-alanyl-glycine (GANPNAAG), β-turn- 430 nm. This band was identical in shape and position to a forming oligopeptide sequence.130 Due to the different nature monomeric porphyrin pyridine adduct,127 which suggested that of the two peptides, OPV-GAGAG dissolved molecularly in THF at this rather high pyridine concentration, the porphyrin and could only form a left-handed helix in water and MCH, while aggregates have dissociated. OPV-GANPNAAG formed a left-handed helix in THF and Another feature of the porphryin derivatives is their chloroform but a right-handed helix in water. In addition, the modification in the central core by metal ions. A porphyrinato stability of the formed chiral structures was remarkable. The zinc complex covalently linked with a peptide was designed and temperature-dependent CD spectra in water showed that the

7321 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review helical aggregates of OPV-GAGAG were completely destroyed at suprastructures with weak CD signals: this was positive for the above 20 °C, while CD could still be observed at 90 °C for OPV- high molecular weight polymer and negative for the low GANPNAAG. The chiral assembly of OPV-peptide conjugates molecular weight polymer. On increasing the poor solvent in depended greatly on solvent polarity, temperature, and, in the solution, the chirality of the high molecular weight polymer particular, the peptide nature. This was a good example of showed no distinct changes in its CD spectrum, while for the low regulation of supramolecular chirality of π-conjugated systems molecular weight polymer, the single Cotton effect became a using peptide secondary structures. bisignate Cotton effect with increased intensity. It was suggested Oxadiazole-containing OPVs have been chemically attached to that the chiral centers can only offer small chiral perturbation to an α-helical peptide, and the effect of the relative spacing and the high molecular weight polymer systems but can impart helical orientation of the chromophores in the peptide on the chiral 131 suprastructures to the low molecular weight polymer systems. assembly of the OPVs was explored (Figure 26). Oxa-6D OPV has been incorporated into the main chain of chiral poly(L-lactic acid)(PLLA), and the chirality transfer from PLLA to OPV was investigated in the solid state.133 Under these circumstances, the chirality information on PLLA was found to be transferred to the self-assembled OPV chromophores. This suggests that the molecular packing and supramolecular chirality of OPV in the aggregates can be tuned by the PLLA. It was observed that the mole percent incorporation of the OPV chromophore can greatly affect the chirality transfer from PLLA, and 3 mol % proved to be the incorporation ratio where the strongest CD intensity was observed. 4.1.3.7. Oligo- and Poly(thiophenes). Thiophene and its derivatives are another class of π-conjugated molecules whose chiral packing has been extensively studied. Chiral sexithio- Figure 26. (Left) Structures of Oxa-OPV, Oxa-6D, Oxa-7O, and Oxa- phenes can form helical aggregates in water, butanol, and the 11D. (Right) CD spectra of Oxa-6D, Oxa-7O, Oxa-11D, and Oxa-6D solid state. In water and butanol, the chiral assemblies show mono. A representative absorption spectrum for Oxa-11D is also shown. different “melting transition” temperatures. Furthermore, chiral Reprinted with permission from ref 131. Copyright 2008 American sexithiophenes have been found to induce the chiral packing of Chemical Society. achiral sexithiophenes. Interestingly, both thermodynamically stable and kinetically favored mixed aggregates with opposite mono (equipped with a single Oxa-OPV) showed no exciton- supramolecular chirality were obtained. Later, Schenning and co- − ff coupled CD signal. Oxa-6D and Oxa-7O, both of which have two workers investigated the odd even e ect of the chiral center Oxa-OPVs with different orientations, generated very similar position on the oligo(ethylene oxide) chains away from the ff ff thiophene backbone and the number of thiophene rings on the negligible positive split Cotton e ects. The weak Cotton e ect 134 might be due to the high degree of overlap of the side supramolecular chirality of the assemblies (Figure 27). It was chromophores for Oxa-6D and the isolated Oxa-OPVs for Oxa- found that bisignate Cotton effects were observed for all these 7O. Oxa-11D equipped with two chromophores on the same side molecules. As expected, the sign of the Cotton effect was reversed with a lager spacing than Oxa-6D showed negative Cotton effects for the aggregates of T6βS and T6βR due to the opposite with the greatest intensity among the three molecules. This configuration of the stereocenter. Notably, the Cotton effect strong CD intensity suggested that the side chains of Oxa-11D showed positive signs for T5βS and T7βS but a negative sign for were in sufficient proximity to one another for exciton coupling. T6βS. In addition, when the chiral center was moved from the α Chiral assemblies obtained from OPV polymers have also been to the ε position, there was a positive CD signal for T6αS and investigated.132 It was found that the chiral polymers (Rac and R) T6εS and a negative CD signal for T6βS and T6σS. Therefore, with both high and low molecular weight could form helical both the number of thiophene moieties and the chiral center on

Figure 27. Structures of chiral oligothiophenes, and (a) CD spectra of T5βS,T6βS, and T7βS in butanol (2.6 × 10−5 M) at 283 K. (b) CD spectra for all chiral T6 derivatives (8 × 10−5 M) at 283 K. Reprinted with permission from ref 134. Copyright 2006 American Chemical Society.

7322 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 28. Influence of the addition of methanol to a chloroform solution of the block copolymers 54−57.(A−D) CD spectra of compounds 54−57 in mixtures of methanol and chloroform. Reprinted with permission from ref 138. Copyright 2010 American Chemical Society. the oligo(ethylene oxide) chains effected the chiral assembly in have different effects on the self-assembly of chiral polythio- an obvious odd−even manner. phenes. Bauerlë et al.135 explored how the chirality of the biomolecules Thiophene block copolymers equipped with a chiral side chain affected the chiral self-assembly of oligothiophenes. Initially, he on one or both of the blocks were synthesized and investigated to decorated the tetrathiophenes with carbohydrates, which determine which chiral side chain played a key role in the chirality resulted in the chirality of the carbohydrate directing the chirality transfer. Koeckelberghs et al.138 synthesized P3AT(S*)-b- of the assemblies of the carbohydrate−thiophene. The self- P3AOT, P3AT(R*)-b-P3AOT(S*), and P3AT(S*)-b-P3AOT- assembly of thiophenes into chiral superstructures can be tuned (S*)(54−57) composed of an alkyl- and an alkoxy-substituted by the choice of saccharidic building blocks with suitable polythiophene block and investigated their aggregation behavior stereochemistry. The authors further synthesized thiophenes using UV−vis, CD, and emission spectroscopies. Through the containing a single chiral amino acid (proline) to study the chiral introduction of poor solvent, the chiral aggregation of the assembly of the conjugates.135 It was demonstrated that proline P3AOT block occurred first owing to its lower solubility in the with two chiral centers could induce a defined helical packing of chosen solvent than that of P3AT. Upon further addition of the the conjugates, whose helicity was controlled by the config- poor solvent, the P3AT block also aggregated, thereby adopting uration of the amino acid moiety. Since proline has two chiral the same helical supramolecular organization as the P3AOT centers, the authors also studied how the proline with opposite block. The results obtained from CD spectroscopy suggested chirality affects the self-assembly of proline−thiophene.136 The that the P3AOT block transferred its helical supramolecular thiophene functionalized with a proline unit of opposite chirality structure to the P3AT block, because the chiroptical behavior of showed a reversed Cotton effect in the region of the π−π* P3AT(S*)-b-P3AOT significantly contrasted with that of the transition of thiophene, which indicated that the chirality was other three polymers. The achiral P3AOT block aggregated in an transferred to the thiophene stacks and that the supramolecular achiral way regardless of the chirality of the P3AT block. chirality was related to the stereochemistry of the proline residue. However, the substituent on the P3AT unit can complicate the The thiophene containing diastereomeric proline showed a silent stacking, as expressed by the intensity of the CD spectra. These CD spectrum due to the lack of chirality in the formed studies revealed that for all block copolymers the initial block aggregates. The mixture of the two enantiomers showed a CD aggregation addition of a nonsolvent has a major influence on the spectrum in which the CD signal nearly vanished. stacking and the chiroptical behavior of the other block. Besides thiophene oligomers, polythiophenes have also been 4.1.3.8. Alkynylmetal. Yam et al.139 reported an example of attracting much attention. Inganas̈ et al.137 investigated the chiral the control of the chiral supramolecular structures exerted by assembly of polythiophenes with synthetic peptides. In this work variation of the counteranions in a single gelator molecule of a it was observed that positively and negatively charged peptides luminescent chiral alkynylplatinum(II)−terpyridyl. Through

7323 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review metal−metal and π−π interactions, molecule 58 can form centers (heterochiral) can affect the supramolecular chirality of metallogels in DMSO and CD signals were obtained in the gels. the assemblies is a very interesting consideration. The important This result together with the lack of activity in the CD spectrum question here is which chiral center will control the supra- of a solution of 58−OTf in dichloromethane at the same molecular chirality? Homochiral, heterochiral, and achiral concentration indicated that the CD signals observed in the peptide auxiliaries appended with naphthalenediimides (NDIs) metallogels originated from the helical chirality of the self- were designed and synthesized, as shown in Figure 30. It was assembled chromophores, transferred from the chiral group, rather than from the intrinsic chirality of the gelator molecules. The chiral supramolecular structures of the metallogels are influenced by varying the counteranions. The metallogels with various counteranions in DMSO showed different CD spectral patterns with a negative Cotton effect in the region of the MMLCT transition. The CD signal of 58−OTf was much stronger than that of other counteranions. The authors inferred that 58−OTf formed highly ordered helical supramolecular structures. The different CD spectral patterns of 58 associated with the different counteranions suggested that the variation of the counteranions would give rise to different chiral supramolecular structures in the gel phase by varying the degree of aggregation through Pt···Pt and π−π interactions. Yam et al.140 also incorporated alkynylplatinum(II) terpyridine units into the single-turn backbone of a binaphthol derivative, 59 (Figure 29). The complex experienced a transition

Figure 29. Structure of alkynylplatinum(II) derivatives 58 and 59. Reprinted with permission from refs 139 and140. Copyright 2009 John Wiley & Sons and Copyright 2013 National Academy of Sciences, Figure 30. Structures of compound 60, homochiral (LL and DD), U.S.A. heterochiral (LD and DL), and achiral (AA) peptide conjugates of NDI. Proposed models: (a) Schematic representation of left-handed (LL and LD) and right-handed (DD and DL) chiral supramolecular assemblies; (b) from random coils to single-turn helical strands in which the schematic illustration of sergeants-and-soldiers effect in which the conformational transition was controlled by the Pt···Pt and π−π achiral soldier AA follows the chiral sergeant, LL or DD. Reprinted with interactions of alkynylplatinum(II) terpyridine moieties based permission from ref 141. Copyright 2012 John Wiley & Sons. on the solvents used and temperature. The bisignate Cotton effect in the circular dichroism spectra was indicative of the found that in the case of the heterochiral peptide conjugates (LD cooperative transformation from a random coil state to a and DL) the chirality of the first stereocenter (irrespective of the compact single-turn M or P helix. The metal···metal and π−π stereochemistry of the second stereocenter) adjacent to the NDI interactions of the alkynylplatinum(II) terpyridine moieties were core determined the supramolecular helicity. Remarkably, supposed to stabilize the metallofoldamers, as defined by density homochiral LL and DD peptide-modified NDIs self-assembled functional theory calculations.140 into 1D hierarchical supramolecular polymers with opposite 4.1.4. Molecules with Multiple Chiral Centers. Thus far, helicity, while the heterochiral peptide conjugates LD and DL supramolecular chirality based on molecules with one or multiple formed microspheres.141 homochiral centers has been the central topic of this review. Yang et al.142 reported an interesting control of handedness of However, how a molecule with two or more opposite chiral the alanine dipeptide self-assemblies by the chirality of the

7324 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review alanine moiety adjacent to the hydrophobic headgroups.142 The often used for translation of chiral information from one 143−149 peptide amphiphiles termed (L,L)-61 and (L,D)-61 showed component to others in multicomponent assemblies. negative CD signals, and (D,D)-61 and (D,L)-61 exhibited For example, the pyridine-ended OPV 62 is a pregelator, opposite signals. Meanwhile, the handedness of the nanoribbons which can form supramolecular organogels via molecular 147 of (L,L)-61 and (D,L)-61 were left-handed. The nanoribbons of recognition of the chiral forms of tartaric acid (TA). The (L,D)-61 and (D,D)-61 were right-handed. The morphologies of molecule 62 with M-TA (meso-tartaric acid) did not form a gel, the assemblies were consistent with the CD spectral data, which suggesting that the spatial orientation of the −COOH moiety in also indicated that the handedness of these organic self- the chiral TA played a key role in determining the gel formation assemblies was controlled by the chirality of the alanines at the mode. The chiral information on the TA enantiomer led to the terminals. formation of P- and M-helical fibers in complexes containing D- TA and L-TA, respectively, while both helices existed in the complexes upon induction by M-TA. This result was consistent with CD spectra measurements, in which the combination of 62 and enantiomeric TA revealed a bisignated Cotton effect. A virtually mirror-image spectrum was observed for the complexes with two enantiomers (L- and D-TA), indicating the transfer of the chiral information on TA to the self-assembled chromophores in a helical sense. The complexes containing M-TA showed no helical bias. Chiral diamines were successfully employed as triggers to transfer their chiral information to achiral tetracarboxymetallophthalocyanine 63 in DMSO/CHCl3 through hydrogen bonding between carboxylic acid and amine.149 The sign and amplitude of the supramolecular chirality was effected by the structures of the amine molecules, volume ratio of the poor/good cosolvents, type of poor solvents, molar ratio of chiral molecular diamine to tcPcM, cavity metal of phthalocyanine, and addition order of the amines. Chirality transfer through hydrogen bonding between pyridine and carboxylic acid was reported by the Liu group.150 As shown in Figure 34, a simple supramolecular approach has been proposed to achieve chirality transcription and resulted in twisted nanostructures in a two-component system consisting of L-glutamic-acid-based amphiphiles 64 and bipyridines 4Py. Compound 64 can self-assemble into nanofibers in water. Figure 31. Molecular structures of compound 61; CD and UV spectra of Upon coassembly with bipyridine, the nanostructures underwent −1 the hydrogels at a concentration of 30.0 g L . Reprinted with exciting changes to chiral twists due to strong hydrogen bonding permission from ref 142. Copyright 2013 American Chemical Society. between the carboxylic acid and the pyridyl nitrogen atoms. The molecular chirality of gelator molecules can be transferred to the 4.2. Chirality Transfer in Systems Containing Chiral and bipyridine aggregates by strong hydrogen bonding. Supra- Achiral Molecules molecular chirality is expressed not only by the CD signals in Another important system of chirality transfer is that of a chiral the corresponding absorption band of bipyridine but also by the component to an achiral one and the extension over the whole chiral twist structures. system. Here the induction of the chirality in the achiral De Feyter, Schenning, and Lazzaroni et al. reported the chiral assembly of OPVs assisted by nucleobases and nucleo- components is of utmost importance. In order to induce the − chirality of the achiral components, the interaction between the sides.151 153 Both achiral and chiral OPVs can form chiral chiral molecules and the achiral molecules plays a very important rosette structures. After addition of thymidine molecules, the role. Therefore, the design of molecules with matched bonding morphology of the OPVs transformed from rosettes to lamella sites and their cooperations are of utmost importance. All of the structures composed of dimers. The chirality of the lamella noncovalent bondshydrogen bonds, electrostatic interactions, depended on the chirality of the thymidine even for the chiral host−guest interactions, as well as hydrophobic interactions OPVs. In addition, the OPVs can coassemble with thymine into could be utilized to perform the chirality transfer between the chiral patterns. It was found that the achiral guest molecule chiral and the achiral molecules. In some cases, chiral spaces or thymine can induce the formation of diastereomers from an environments can also endow achiral components with chirality. enantiomeric OPV. The chirality of OPVs can then be tuned by An obvious merit of the chiral transfer in these system is that coadsorption with nucleobases or nucleosides. instead of the tedious organic synthesis required to introduce 4.2.1.2. Electrostatic Interaction. Electrostatic interactions chiral units, a simple mixing of the functional achiral units with play an essential role in specific molecular recognition and the commercially available chiral molecules can produce molecular assembly.154 An anionic chiral compound can induce a functional supramolecular chiral assemblies. cationic π-conjugated polymer to form an interchain helically π- 4.2.1. Chirality Transfer through Noncovalent Bonds. stacked assembly that is stabilized by both electrostatic and π−π 4.2.1.1. H-Bond-Directed Chirality Transfer. Hydrogen bond- interactions, which hierarchically self-organize into super- ing, which is directional and relatively strong, is the most molecules with circularly polarized blue luminescence. A water- important interaction in self-assembly. Hydrogen bonding is soluble poly(p-phenylene) derivative (PPP, 66) was synthesized

7325 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 32. Molecular structure of pyridine-end OPV 62, AFM topographic images showing (i) M helix for 62 + L-TA, (ii) P helix for 62 + D-TA, and (iii) a mixture of M and P helices for 62 + M-TA. Reprinted with permission from ref 147. Copyright 2012 Royal Society of Chemistry.

Figure 33. Structures of tetracarboxymetallophthalocyanine 63 and chiral amines. Reprinted with permission from ref 149. Copyright 2011 John Wiley & Sons. by introducing tetraalkylammonium cations at the terminal sites of |10−2|−|10−1| in luminescence. Further, the polymer of the side chains, forming a complex with a water-soluble assemblies gathered to form spherulites, which can be regarded diaxially chiral binaphthyl derivative (BNP) bearing two as semicrystalline nanospheres, and the spherulites exhibited sulfonate anions at terminal sites of the substituents, as shown circularly polarized blue luminescence. This work provides a in Figure 36.155 A complex of 66 (PPP) and BNP exhibited a CD simple way to fabricate chiral spherulites, which may find band in the π−π* transition region of the π-conjugated backbone application in novel chiral nanomaterials for the next generation (Figure 36a). The strong bisignate Cotton effects observed at of plastic optoelectronics. 376 and 341 nm implied the presence of an exciton-coupling Liu and co-workers reported that the enantiomer of phenomenon between the main chains. These results confirmed diaminocyclohexane induced a water-soluble porphyrin that the assembly showed induced chirality of the polymer (TPPS) to form helical nanorods in organic solvents.29 Mirror- moiety, which was caused by chirality transfer from the axially imaged helical nanorods were observed in these systems, chiral compound (BNP) to the achiral polymer (66). The indicating that the transferred chirality or induced chirality of electrostatic interactions here are essential to the chirality achiral π-conjugated molecules can manifest not only via CD imposed on the polymers, resulting in large dissymmetry factors spectra but also via direct helical nanostructures. Such chirality

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(CD), circularly polarized luminescence (CPL), optical rotation), chiroptical switching processes, and nonlinear optical (NLO) activity. A supramolecular assembly of compound 68 (perylenebisimide (PBI) functionalized with dipicolylethylenediamine−zinc (DPA−Zn) binding sites), which can specifically bind to phosphates, showed an induced chirality when binding with ATP (Figure 38).156 When DPA−Zn was mixed with 1 equiv of ATP, a positive bisignate CD signal was observed, i.e., positive at 518 nm and then negative at 480 nm, in the PBI absorption region with a zero crossing at 507 nm. This is characteristic of an excitonically coupled right-handed helical organization of the PBI chromophores, indicating the efficient chirality induction to an achiral chromophoric assembly through the specific binding of the phosphate guest molecules to the DPA−Zn sites. Kleij described trinuclear Schiff base host complexes in which the conformation was rigidified by a central Zn ion.157 The coordination of a series of suitable monotopic ligands to this central Zn ion caused the effective chirality transfer to the host as characterized by circular dichroism (CD) spectroscopy. The chirality transfer provides the possibility for the development of substrate-specific host systems that are useful for determination Figure 34. Structures of compounds 64 and bipyridines (xPy). of the absolute configuration of various types of organic Morphologies of coassembled 64/4Py (a, b, c), 64/4ePy (d), and 64/ molecules. 2Py (e) at molar ratios of 1:2. Insets are photographs of the samples. Metal−ligand interactions were also attributed to express Reprinted with permission from ref 150. Copyright 2011 John Wiley & chirality at the nanoscale via extending the π-conjugated system Sons. and enhancing the molecular interactions. Liu et al. reported that the Cu2+ ions triggered the amphiphilic Schiff base assemblies to 158 control over a large length scale from molecules to nanostruc- form twist nanofibers. The square-planar coordination 2+ tures could have implications in the design of asymmetric between Cu and the Schiff base was attributed to the extension nanocatalysts. of π-conjugated system and further enhancement of the 4.2.1.3. Metal−Ligand Coordination. The interactions intramolecular interactions, leading to the chirality being between metals and ligands are at the heart of a wide variety of expressed on the nanoscale, because the chiral interaction chemical, physical, and biological phenomena. Metal−ligand accumulated in a confined space. Another example is a interactions allow the design of materials with controlled terephthalic-acid-substituted amphiphilic L-glutamide (com- topology and with specific physical properties such as redox, pound 6) gel in DMSO.57 A left-handed uniform helical twist magnetic, or photochemical properties. Combined with a chiral was obtained in the presence of a wide range of metal ions, sense, metal-based materials can be designed that have unique including Na+,Li+,Fe3+,Co2+,Ni2+,Cu2+,Zn2+,Cd2+,Mn2+, properties including chiroptical properties (circular dichroism Mg2+,Ca2+,Ag+,Eu3+, and Tb3+ (Figure 39). First, the ligand

Figure 35. Structures of OPVs 65, thymidine, and thymine. Reprinted with permission from refs 151 and152. Copyright 2011 and 2013 American Chemical Society. Reprinted with permission from ref 153. Copyright 2014 Royal Society of Chemistry.

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Figure 36. Poly(p-phenylene) derivative (PPP) 66 and binaphthyl derivatives (BNPs) with (R)- and (S)-configurations. A plausible model of the π−π − electrostatic and interactions between two 66 repeating units and one (R)-BNP molecule. (a) UV vis absorption, CD, and gabs spectra, and (b) PL, − − CPL, and glum spectra of PPP, BNP, and a mixture (PPP BNP) (1.0:2.0 mol/mol) in methanol water (50:50 v/v). Reprinted with permission from ref 155. Copyright 2012 John Wiley & Sons. molecules formed a flat multibilayer structure through π−π encapsulated, two diastereomeric isomers were formed, stacking between the benzene rings and the H bonds between the suggesting that the P and M helicities of the capsule can be amide groups as well and the carboxylic acids. The interaction of biased by the chiral guest encapsulation. the metal ions with the carboxylic acid may change the flat A class of oligothiophene-based organogelator bearing two multibilayer structure to a left-handed chiral twist due to the crown ethers at both ends was found to gelatinize several organic chiral nature of the gelator molecules, providing an easy way to solvents in the presence of ammonium, forming one-dimensional tune the chiral twist by simply changing the metal ions. fibrous aggregates (Figure 40).162 The helical one-dimensional 4.2.1.4. Host−Guest Interaction. The binding or encapsula- assemblies were induced by the chirality of 1,2-bisammonium tion of a chiral guest in an achiral cavity has been proven to be an guests through host−guest interactions. It was interesting to note effective method for chirality transfer. Rebek and co-workers that the chirality of an oligothiophene-based organogel can be developed a hydrogen-bonded dimeric capsule composed of two created by thermal gelation, whereas it was silent in thixotropic achiral monomers that produced a dissymmetric space in which a gelation. small chiral guest can be encapsulated in a diastereoselective 4.2.1.5. Hydrophobic Interactions. Chirality transfer based fashion.159,160 This capsule was generally expressed as racemic, on hydrophobic interactions is rarely reported, which may be due because the enantiomers of thecapsulearedynamically to the weakness of this interaction relative to other noncovalent interconvertible through dissociation and recombination. The interactions. However, this is possible in gel systems where alkyl energy between the diastereomeric complexes would be different chains of a chiral gelator and achiral guest molecules can entangle as a result of the chiral interior recognizing the shape of the chiral each other to form chiral assemblies. We have presented chirality guest when a chiral guest is captured. The group of Haino transfer by taking advantage of this concept.51 The chirality can developed a calixarene-based capsule, which can encapsulate a be transferred to porphyrin chromophores through interchain variety of guest molecules and heterodimeric hydrogen-bonded interaction between the alkyl chains of both the porphyrin and pairs of carboxylic acids.161 When the chiral guest was the gelator during the coassembly. By contrast, when porphyrins

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Figure 38. Structure of molecule 68 (PDPA), and schematic of the guest-induced regulation of supramolecular chirality in PDPA assemblies. Reprinted with permission from ref 156. Copyright 2014 Royal Society of Chemistry.

Figure 37. (Top) Structures of TPPS (67) and chiral amines. (Bottom) (A and B) Typical SEM images of the 67 (TPPS) nanostructures assembled with the assistance of R-DAC (A) and S-DAC (B). (C) UV− vis (bottom) and CD (top) spectra of the dispersion of TPPS obtained in the presence of R-DAC (black) and S-DAC (red). (D−G) Typical TEM and HRTEM images of the TPPS nanostructures assembled with the assistance of R-DAC (D and E) and S-DAC (F and G). (Inset in Figure 39. Illustration of the self-assembly of 6 in DMSO. In the absence panels E and G) FFT of the corresponding HRTEM image. TPPS/DAC of the metal ions, the bilayer structure was initially formed, many of ratio is 1/4. At the bottom is a schematic illustration of the formation of which further assembled into nanofiber structures. When metal ions mirror-imaged 67 nanorods with the assistance of DAC molecules. were present, they reacted with the headgroup and caused a twist of the Reprinted with permission from ref 29. Copyright 2013 Royal Society of multibilayer structure. When Eu3+ and Tb3+ were added, red and green Chemistry. emissive chiral twists were formed. For the sake of simplicity, only one bilayer is shown. Reprinted with permission from ref 57. Copyright 2012 Royal Society of Chemistry. without long alkyl chains were used in the gels, no CD signals were observed in either the mixed solutions or the organogels. An interesting example of the chirality transfer through This fact indicated that the entanglements or hydrophobic hydrophobic interactions was reported by Ghosh and co- interactions of long alkyl chains played an important role in workers. They investigated an H-bonding-mediated assembly chirality transfer. In this case, the situation is more akin to achiral in bis-amide-functionalized chiral acceptor (NDI) and achiral molecules doped in the chiral liquid crystals. The chiral three- donor (DAN) molecules.164 Two types of homoaggregated dimensional microenvironment provided by the chiral gels is fibers were obtained due to the mismatch in the distance between believed to be responsible for the induction of the chirality of the two amide groups. CD experiments revealed a helical TPPOC12H25 assemblies. Besides this approach, when an achiral assembly for both the donor and the acceptor stacks, although a Schiff base bearing long alkyl chains (70) was mixed with chiral chiral center was present only in the acceptor building block. The gels formed by 2L and 2D, the chiral information in the gelator authors suggested that the induction of helical bias was from the molecules was transferred to the Schiff base chromophore and acceptor stack to the donor stack via hydrophobic interaction supramolecular chirality was obtained.163 On the basis of the among the peripheral alkyl chains. dynamic covalent chemistry of the imine, the pH-responsive 4.2.2. Chirality Transfer from Solvent to Assemblies. property of the supramolecular chirality was explored and a pH- The solvent, as the second supramolecular partner of each soft driven chiroptical switch was obtained upon treatment with acid self-assembly system, is crucial in determining the thermody- and base alternatively. Thus, a supramolecular chiroptical switch namic process of a self-assembly system. There are some was established based on supramolecular chirality transfer and interesting cases that illustrate the chirality transfer from solvents dynamic covalent chemistry. to supramolecular assemblies. The first chiral solvent effect was

7329 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 40. Structure of oligothiophene derivative 69; CD spectra of the gel phase of 69 (straight line), 69·(R,R)-diammonium (dashed line; triangle), and 69·(S,S)-diammonium (dotted line; circle). (a) ICD spectra of the 1:1 mixture. (b) CD titration study at 370 nm. Reprinted with permission from ref 162. Copyright 2012 John Wiley & Sons. reported for the emergence of Cotton CD signals due to the OPV4UT in S-citronellol at room temperature showed a strong twisted form of CD-silent benzyl molecules dissolved in (2S,3S)- bisignated Cotton effect, which is characteristic of the exciton- butanediol.165 The second example was a helical preference coupled helically ordered chromophore, similar to that reported revealed by a study of the CD characteristics of poly- for homochiral OPV4UT analogues and the induced chirality 166 (hexylisocyanate) in nonracemic chlorinated chiral solvents. from citronellic acid guest molecules. This indicated the transfer A further study showed that helix formation can be induced in a of chirality from the chiral solvent molecules to the racemic 167,168 cosolvent containing chiral and achiral solvents. stacks of achiral OPV molecules. The mirror image CD spectra Recently, it was reported that the self-assembly of an achiral obtained for A-OPV4UT in the other enantiomeric chiral perylenebisimide (PBI) organogelator (73) with two 3,4,5- solvent, R-citronellol, provided proof of chirality transfer. Similar tridodecyloxybenzoylaminoethyl substituents at the imide mirror-image, bisignated CD spectra and morphologies were positions was chiroptically silent in achiral solvents. However, observed for A-OPV3UT in enantiomerically pure chiral in reality, it was found that this system formed both left- and alcohols, showing that chiral induction in supramolecular stacks right-handed helices in equal amounts, which canceled any 144 169 through chiral solvents is possible. chiroptical signal. When (R)- or (S)-limonene was used as a Zhang et al.172 synthesized an azo-containing π-conjugated chiral solvent, it was shown that a preferential population of a polymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-4,40- certain handedness of the helical assemblies can be selected by azobenzene](F8AZO), 74, and found that the solvent chirality choosing a chiral solvent. Interestingly, the enantiomeric of (S)- and (R)-limonenes was successfully transferred to main- selectivity depended on the assembly process. With dilute chain polymers, which generated optically active 74 aggregates. solutions and sufficient equilibration time (thermodynamic The intense circular dichroism (CD) signals corresponding to 74 conditions), the enantiomeric excess was close to 100%, whereas fi for the assemblies fabricated by a controlled kinetic self-assembly in the visible region con rmed the chirality transfer from solvents process (higher concentration), the enantiomeric excess was to polymer. More interestingly, the reversible chiroptical switch only 20%. It was inferred that the fast gelation process at high was achieved upon alternating photoirradiation at 405 (trans concentration was controlled by nonequilibrated nuclei in a form) and 546 nm (cis form). kinetic rather than a thermodynamic self-assembly process. In general, the helical bias induced by a chiral solvent is not as Under these conditions the chiral induction from the homochiral strong as the helical bias induced by a chiral monomer. For solvent may not be adequate to effectively impose a single example, chiral solvents (limonene) triggered assembly of a handedness on helices. racemic bisurea into a helical nanotube, which was characterized The solvents not only induced the chirality transfer to by its circular dichroism signature. However, the helical bias was supermolecules assembled from low-mass molecules but can only 33%, much lower than that induced by a chiral monomer. impose the chiral information on solvents into the polymer or However, this method offers a simple way to impose chirality on oligopolymer assemblies.170,171 Achiral oligo(p-phenyleneviny- supramolecular assemblies without introduction of a chiral lene) (OPV) derivatives equipped with either ureidotriazine (A- matrix or auxiliary.173 OPVUTs) or diaminotriazine (A-OPVTs) H-bonding arrays 4.2.3. Chirality Transfer from Low Molecular Weight were found to self-assemble into columnar stacks in apolar Molecules to Macromolecules. While the synthesis of the solvents. When using enantiomerically pure R- and S-citronellol main-chain chiral polymers is an important topic, the regulation as solvents, circular dichroism spectroscopy (CD) of A- or control of chirality of the polymer main chain through the

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chiral small molecules. Various acyclic and cyclic, primary, secondary, and tertiary amines were introduced in the side chains of the poly(acetylene)s. The same acid−base chemistry as above holds for the interaction of these polymers with chiral carboxylic acids. In addition, the resulting optical activity is strongly related to the structure of the amine-functionalized polymer. Other noncovalent interactions, such as metal−ligand interactions, host−guest interactions, and electrostatic interactions, also contribute a great deal to inducing the formation of one-handed helices from achiral polymers. For example, when crown ethers were present in the pendants,184 a predominantly one-handed helical conformation was formed in aqueous solutions (HClO4) upon complexation with various chiral compounds, such as amino acids, peptides, amino, sugars, amines, and amino alcohols. Addition of NaCl or KCl decreased the magnitude of the CD spectra, which suggested the importance of the crown ether−ammonium complexation in acidic water. Interactions between chiral molecules and the functional group in the polymer side chains were also observed for other polymer backbones. Other achiral polymers, including polyisocyanates, poly(phenylisocyanide), poly(thiophene), and poly- (guanidine), can also be endowed with dynamic chirality by various noncovalent interactions. Vandeleene et al. reported poly(phenyleneethynylene-alt- bithiophene) copolymers with chiral pendants and pendants bearing carboxylic acid groups in solution and in films.185 Here, it was noticed that the addition of chiral primary amines resulted in ff chiral aggregation of the polymers. When the chiral centers of the Figure 41. Structures of (A) dopant Schi base compound 70 and (B) pendants and amines were the same, cooperation between them 2L and 2D. (C) Schematic illustration of their coassembly. (a) A Schiff base based on a dynamic covalent bond. (b) In the coassembly of 70 and in helical stacks was observed by CD spectroscopy. The opposite 2, 70 can be inserted into the alkyl chains of 2 molecules. On the basis of situation holds when they are opposed. hydrophobic interactions, a supramolecular assembly with twist Inoue and co-workers synthesized a poly(m-ethynylpyridine) structure can be formed, and the supramolecular chirality can be polymer that comprised at least 72 pyridine moieties with a transferred from 2 to the Schiff base moiety. The supramolecular molecular weight of ca. 4500.186 When a chiral saccharide was chirality showed “on” and “off” states through the alternate treatment of enclosed in the inner sphere of the polymers, helical structures of acid and base. Reprinted with permission from ref 163. Copyright 2013 polymers were guided by uncharged hydrogen-bonding Royal Society of Chemistry. interactions with saccharides. Circular dichroism studies revealed the nature of the chirality induction and how the achiral host interaction with a chiral unit in their side chain provides a new senses differently the chiral structure of a range of saccharides. arena for study (Figure 46). fi 4.3. Dynamic Features and Regulation of Supramolecular The pioneering studies in this eld have been reviewed Chirality thoroughly by Yashima et al.174 The first helical polymer induced by chiral amines through acid−base interactions was reported at Dynamic exchanges and rearrangements of building blocks in 1995.175 Upon interacting with chiral amines in DMSO, a assemblies present challenges in supramolecular chemistry. This preferred helical handedness of a cis-transoidal, stereoregular is also true for supramolecular chirality. In contrast to a system poly((4-carboxyphenyl) acetylene) (75-f) is instantaneously under thermodynamic control, which often exhibits a single, induced in the polymer, showing a characteristic ICD in the π- simple assembly route, supramolecular chirality based on conjugated polymer backbone region, indicating that the chirality supramolecular chemistry also shows the complexity and of the amines was imposed into the main-chain polymers.175 diversity of kinetic direction. Various noncovalent interactions Later, this helical sense induction concept through noncovalent may result in nonequilibrium self-assembly, in which structural chirality transfer was applied to the synthesis of a variety of diversity is achieved by forming several kinetic products based on chirality-responsive PPAs by introducing a specific functional a single covalent building block. The multiple available − group as the pendant group.176 183 Yashima and co-workers interaction sites and the flexibility of the interaction modes systematically investigated the formation of one-handed helices make the supramolecular chirality dependent on the kinetics of from achiral polymers by acid−base complexation of chiral self-assembly. amines, amino alcohols, and amino acids with organic acid Stupp and co-workers187 demonstrated that the preparation − functions in their side chains.175 180 During this process, the protocols of the peptide amphiphiles self-assembling in water can rather irregular twist of the adjacent double bonds around a result in the formation of different supramolecular morphologies, single bond is transformed into a helical conformation with a either long filaments containing β-sheets or smaller aggregates predominant handedness. This was confirmed by bisignate containing peptide segments in random coil conformations. The Cotton effects in the polymer backbone absorption, which peptide amphiphiles (PA) were found to exist as monomers in showed a mirror-image relationship between two enantiomers of the good solvent HFIP and formed assemblies upon addition of

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Figure 42. Schematic illustration of chiral induction by helical neighbors. Reprinted with permission from ref 164. Copyright 2011 Royal Society of Chemistry. nonsolvent−water. Two peptide amphiphile assemblies of the molecular chirality that occurred in process I can be transferred same composition were prepared in two different ways, as shown to a positive one, which is similar to that of process II. in Figure 47.187 Although the two systems have the same HFIP In comparison with molecular chirality, supramolecular and PA content, clear differences can be observed between the chirality can be easily altered by external factors such as solvent, CD spectra of solutions 1 and 2 (Figure 47b). Solution 1 temperature, sonication, photoirradiation, redox potential, and exhibited a random coil CD spectrum, whereas the spectrum of chemical additives. This provides an opportunity to regulate the solution 2 had β-sheet character. The presence of β-sheets in supramolecular chirality in self-assembled systems. solution 2 can be rationalized by the fact that for this solution the 4.3.1. Solvents. Solvent is the medium for self-assembly fl fi stock solution of PA1 in HFIP is initially added to water. In processes and can strongly in uence self-assembly via the speci c solution 2, although the amount of HEIP reached 20%, these β- interactions between solvent and solute. The basic features of solvents such as polarity, viscosity, and solubility for the solute sheets do not disassemble completely, because there was no ff transition back to the random coil conformation. These results and other compounds could also a ect the supramolecular chirality of a supramolecular system. The majority of reports in demonstrate that β-sheet assemblies have high kinetic stability the field of supramolecular chirality focus on the influence of the and, once formed, do not readily disassemble. It is evident that molecular structure on assembly and largely ignore the role of the insights into the characteristic dynamics of a supramolecular ffi solvent. However, understanding how solvent properties system can provide an e cient way to select the optimum influence chiral structures can help provide a deep understanding assembly pathway necessary for function. of how supramolecular chirality is produced. Zhang and Liu investigated the aggregation of an anionic For example, an L-glutamate-based amphiphilic gelator bearing porphyrin (TPPS) (compound 67) on the a cationic polypeptide 188 an azobenzene segment 5 formed organogels that showed an (poly(lysine)) controlled by dynamic assembly. Through excellent photoregulated gel−sol transition.56 It was found that simple adjustment of the mixing sequences of TPPS with totally opposite CD signals were observed in DMSO and toluene poly(lysine), opposite CD signals from TPPS J aggregation were gels. The DMSO gel exhibited a positive Cotton effect, while a obtained. When PLL was dropped into the TPPS solution negative Cotton effect was observed in the toluene gel, as shown (process I), a negative CD signal was observed. By contrast, a in Figure 49. The opposite Cotton effect obtained from different positive Cotton effect was found when TPPS was added to the solvents implied that the supramolecular chirality was reversed as PLL solution (process II). The time scan of the CD spectra a result of different molecular orientations at the molecular level. revealed that process II was controlled by thermodynamics, while According to the results of XRD and temperature-dependent process I was controlled by kinetics. The negative supra- UV−vis spectroscopy, two kinds of molecular stacking models

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Figure 43. Molecular structure of PBI derivative 73. AFM images of a ﬁlm spin coated (2000 rpm) from a solution of PBI 1 in (S)-limonene (c =1× 10−4 M) onto HOPG. (a and b) Height images. (c) Phase image. Scale bars in a and c correspond to 450 nm; the z scale in a and b is 9 nm. The statistical graph of M and P helices is derived from image a. (a−b and c−d) Cross-section analyses of the ﬁbers. Reprinted with permission from ref 169. Copyright 2013 John Wiley & Sons.

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Figure 45. Schematic illustration of unpolarized-light-driven chiroptical switching in CHCl3/(1R or 1S)/IPA (0.3/1.5/1.2, v/v/v). In 74-trans, chiral aggregation manifested as a bright yellow and turbid solution, while nonaggregated 74-cis formed a yellow transparent solution. Reprinted with permission from ref 172. Copyright 2011 American Chemical Society.

Figure 46. Schematic illustration of helical polymers obtained from achiral polymers induced by achiral guests and structures of poly(phenylacetylene)s, poly(thiophene)s, and poly(phosphazene)s. Reprinted with permission from ref 174. Copyright 2009 American Chemical Society. have been proposed: the azobenzene groups packed face to face been suggested that the interaction between the pyridylpyrazole (π−π stacking), and the amino acids packed by forming H bonds, headgroup and the solvents may subtly change the stacking of the as shown in Figure 49. molecules and thus their self-assembled nanostructures. Thus, by In the case of a pyridylpyrazole-linked L-glutamide organo- choosing appropriate solvents, a transition in morphology from gelator,58 diverse nanostructures over a wide scale range from nanofibers to chiral twists to nanotubes and to microtubes can be nanofiber to nanotube and microtubes were obtained based on achieved. the polarity of the solvent. The nanofiber, nanotwist, nanotube, Solvent-driven morphological transitions may dominate and microtube structures of 9 were obtained in toluene, supramolecular self-assembly in many cosolvent mixtures.189 chloroform, DMF, and DMSO, respectively. Such morphological For example, Liu et al. found that the addition of a small amount changes can also occur with xerogels in the solvent vapors. It has of water to organic solvents, either water miscible or immiscible,

7334 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 49. (A) CD spectra of a gel of 5 in various states: DMSO gel (▲) with a concentration of 4.3 mM, and toluene gel (▽) with a concentration of 5.0 mM. (B) Schematic illustration of molecular packing in DMSO and toluene. Reprinted with permission from ref 56. Copyright 2011 American Chemical Society. Figure 47. Assembly of PA1 is dependent on the preparation protocol. (a) Two PA1 solutions (50 μgmL−1) in 20% HFIP were prepared via handed helix in nonpolar solvents to a right-handed helix in polar two methods that diﬀer by the order in which pure HFIP and the PA/ solvents. HFIP stock solution were added to water. Even though both solutions 1 and 2 contain the same PA concentration and HFIP content, clear diﬀerences can be observed in CD (b) and DLS (c). Time-dependent CD (200 nm) acquired on solutions 1 and 2, shown in the inset of panel b, demonstrates the large hysteresis involved. Reprinted with permission from ref 187. Copyright 2014 American Chemical Society.

Figure 48. (A) CD spectra of a PLL/TPPS (67) mixture in different mixing sequences: (a) PLL was added to TPPS (process I) and (b) TPPS was added to PLL solution (process II). (B) Time-dependent CD spectra of the PLL/TPPS mixture. Illustration of the formation of TPPS aggregate on polymer. The green block represents TPPS, and its charges Figure 50. Chemical structure of the PTCDI−HAG amphiphile were omitted. (a) Pending-type aggregate in which one site of TPPS molecule 81. (a, c) CD spectra and (b, d) TEM images of the fi − ff binds on the PLL, while the other unit is stacked on the rst in a head-to- PTCDI HAG molecules 81 in di erent volume ratios of CHCl3/n- tail manner as a J aggregate. When less PLL presented in the solution or C8H18 or THF/H2O. Reprinted with permission from ref 190. PLL was added into TPPS, such aggregates were predominantly formed Copyright 2011 Royal Society of Chemistry. and the process is a dynamic one. (b) Wrapping-type aggregation in which every TPPS unit is wrapping around the polymer chain, while these units formed head-to-tail stacks as J aggregates. Reprinted with 4.3.2. Temperature. As mentioned above, hydrogen permission from ref 188. Copyright 2009 American Chemical Society. bonding is the most important interaction in self-assembly: the strength of this interaction decreases with increasing temperature. Therefore, chiral assemblies based on hydrogen bonding are especially sensitive to the adjustment of temperature. The can trigger the formation of chiral nanostructures of a cationic most popular instances of this are reported in the supramolecular amphiphile (7).61 In ethanol, nanofibrous structures without any gels based on H bonds, in which CD signals are silent in solution chiral sense evolved into the helical nanostructures, and or in the monomer state as the system temperature is increased. furthermore, the helical pitch could be tuned by the amount of Supramolecular chirality is induced by the gel formation process. water present. In nonpolar solvents, helical tube structures were In other words, gelation-induced supramolecular chirality is quite produced upon the addition of water. sensitive to the temperature-regulated sol−gel transforma- The supramolecular interactions of the PBI derivatives were tion.192 successfully modulated by solvents,190,191 which not only Meijer and co-worker investigated a series of supramolecular 90−95 induced a CD signal inversion but also the macroscopic polymers based on C3-symmetric molecules. In these properties could be modulated by the solvent, from a left- systems, temperature-dependent supramolecular chirality is

7335 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review widely found. In general, at higher temperatures, the molecules exist in the monomeric state, in which the CD is silent. Upon cooling, the chirality appears gradually during the self-assembly. Thus, temperature-dependent CD spectroscopy is often performed to illustrate the mechanism of self-assembly. With the exception of the thermally dependent change of chirality in the supramolecular assemblies based on hydrogen bonding, a thermally reversible method for the inversion of chirality was developed by changing of the lattice symmetry. A thermally reversible inversion of chirality was discovered in helical supramolecular columns formed by C3-symmetric self- assembling dendrimers based on dendrons connected at their apex via trisesters and trisamides of 1,3,5-benzenetricarboxylic acid.86 The authors demonstrated a change in the lattice symmetry as follows: negative chirality for 2D and 3D phases with triangular symmetry (columnar hexagonal) and positive chirality for 2D and 3D phases with rectangular symmetry Figure 51. (Top) Cu(10)2 molecules self-assembled into gels through (columnar rectangular and orthorhombic) which was attributed coordination, hydrogen bonding, and hydrophobic interactions, as well to a thermally-induced inversion. This is the first example of the as through π−π stacking. On reduction, this π−π stacking was impeded, elucidation of the mechanism of reversible inversion of helical and accordingly, the gels changed into a sol. (Bottom) CD spectra of 10 (a) and Cu(10)2 gels (b), the sol after reduction (c), and the revived gel chirality in supramolecular dendrimers. The structural changes − reported can be used to design complex functions based on after redox (d). (Inset) Enlargement of the spectra in the range 400 ff 500 nm. Reprinted with permission from ref 54. Copyright 2013 John helical supramolecular dendrimers with di erent degrees of Wiley & Sons. packing on their periphery.86 4.3.3. Redox Effect Chirality. When metal ions or moieties with variable valence states are inserted into a molecular building from 50 nm to 1 mm with lengths of several micrometers (Figure block, the redox chemistry of this moiety will cause the variation 52a). After irradiation at 323 K followed by cooling, the helicity 193,194 of supramolecular chirality. Liu et al. reported redox- of the fibers was found to be left handed (M), as shown in Figure responsive chiral organogels based on a Cu(II)−quinolinol 52b. AFM analysis of (S)-83 revealed reversal of their native 54 derivative (10). With the gels of the Cu(II)−quinolinol helicity to the induced opposite screw sense after irradiation. derivative, a positive Cotton effect at 280 nm and a negative band In the absence of heating, the helicity did not change even after at 240 nm with a crossover at 260 nm and a single positive band at irradiation for several hours. Even if there could be slow 351 nm and a negative band at 475 nm were present, which isomerization of the molecules within the helical fibers, the suggested that the chirality was transferred onto the metallogel helical twist did not change significantly. This work demon- assemblies. Upon reduction of Cu(II) to Cu(I) by ascorbic acid, strated that the handedness of a photoresponsive supramolecular the signals at around 475 and 351 nm disappeared, whereas the object can be tuned with the cooperation of light and heat, exciton band at around 260 nm was retained. This result showed without changing the inherent molecular chirality of the that the reduction was mainly localized on the central Cu(II) individual building blocks. ions, whereas the π−π stacking of the aromatic rings, which were Cone-shaped alkoxyazobenzenes dimers functionalized with brought proximal to the amide groups, was not destroyed. Owing amide groups were synthesized, and the presence of amide to the formation of this different structure, the chirality could not hydrogen-bonding sites in one side of the folded molecules propagate the L−L band; thus, the bands at 351 and 475 nm prevented the antiparallel stacking favored by asymmetric disappeared. Subsequently, as the system was oxidized by O2, the structures, facilitating the formation of toroidal aggregates, in negative Cotton effect at around 475 nm and the positive band at which the chirality was transferred to supramolecular chirality. 351 nm reappeared. These results revealed that the supra- The resulting toroidal structures have large π surfaces on their molecular chirality could be tuned by redox chemistry and a top and bottom and can hierarchically organize into tubular redox-driven chiroptical switch could be realized.54 nanostructures. Irradiation of 84 with UV light quantitatively 4.3.4. Photoirradiation. Photoirradiation is a noninvasive, converted aggregative trans-azobenzene moieties to nonaggre- easy, and fast external stimulus that is often utilized to adjust the 204 − gative cis isomers (Figure 53a), collapsing nearly all aggregates structures of supramolecular assemblies.56,195 199 Photoinduced to monomers (DLS study). However, a different situation was isomerization of azobenzenes,200,201 dithienylethenes,202 and achieved when 84 was irradiated with visible light at 470 nm: the spiropyrans is the most often-used strategy for developing reversible trans−cis isomerization of the azobenzene moieties phototriggered chiroptical switches. An azobenzene-linked could be promoted because both isomers have absorptions at 470 phenyleneethynylene bearing chiral groups (82) showed an nm, while the fraction of the aggregative trans isomer is kept in intense positive signal at 464 nm with two negative signals at 407 large excess because the cis isomer has a greater molar extinction and 322 nm with a zero crossing at 421 nm corresponding to the coefficient at this wavelength (Figure 53b). In this situation, a π−π* transition of the PE moiety. Surprisingly, the CD spectrum rapid generation and evanescence of polar cis-azobenzene after UV irradiation at 323 K, followed by cooling the solution to moieties occurred within the toroidal aggregates. This is capable a lower temperature, showed a reversal of the CD spectrum; of promoting hierarchical growth of the aggregates by increasing however, there was a lower intensity in the CD bands. In this nanostructure surface polarity in nonpolar environments. case, photoirradiation resulted in a reversal of the CD signal.203 4.3.5. Chemical Additives. The addition of guest molecules SEM analysis of (S)-83 before photoirradiation showed and metal ions was found to be effective in adjusting the chiral entangled right-handed (P) helical ropes of diameters ranging superstructures of assemblies through guest−host interaction

7336 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 52. Photoisomerization of the azobenzene-linked phenyleneethynylene derivatives 82 and 83. A mixture of E,E, E,Z, and Z,Z isomers is possible. SEM images of (S)-83 (a) before and (b) after photoisomerization; AFM images for (S)-83 (c) before and (d) after photoisomerization. Reprinted with permission from ref 203. Copyright 2012 John Wiley & Sons.

− and metal−ligand cooperation.57,158,205 208 For example, an Yamagishi et al.209 reported a highly stable 1:1 host−guest achiral-guest-triggered chiral inversion in a novel supramolecular complex formed by pseudo[1]catenane 1,1,4-dicyanobutane assembly fabricated by pillar[5]arenes has been reported.209 The (G1) as a guest and pillar[5]arenes as a host. Inclusion of G1 in planar chirality of pillar[5]arenes is caused by the substitution the cavity of pillar[5]arenes causes dethreading of the alkyl chain position of the alkoxy groups, which have two equivalent stable moiety from the cavity of pillar[5]arenes, which caused the achiral guest G1 to induce the planar chiral inversion from in-pS- conformations (pS and pR), as shown in Figure 54B(a). In order 1 to out-pR-1. The chiral inversion was characterized by the to isolate the two enantiomers, the rotation of these units should observed CD spectra, which changed dramatically from positive be inhibited because the interconversion between pS and pR to negative with increasing G1 concentration. An ammonium occurs by rotation of these units in solution. General approaches cation (G2) was another guest that could be added to the second to hinder this rotation are to modify both rims with bulky fraction (in-pR-1), and a decrease in the CD intensities was substituents or form a rotaxane consisting of a pillar[5]arene observed. The authors concluded that the absence of chiral wheel and a guest axle. inversion was due to the weaker association constants between

7337 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 53. (A) Schematic illustration of the self-assembly of 84. (B) Photoinduced (a and b) UV−vis and (c and d) CD spectral change of 84 (c = 3.0 × 10−4 M) in MCH at 20 °C. (a and c) Changes upon irradiation of a trans-rich solution with 365 nm UV light. (b and d) Changes upon irradiation of the cis-rich solution with 470 nm visible light. (e) Plots of the fraction of cis-azobenzene moieties (red marks, left axis), and maximum Δε values (blue marks, right axis) versus irradiation time of UV (left side) and visible (right side) light. Aggregation states are shown with graduated background colors representing monomeric (water blue) and aggregated (orange) states. Reprinted with permission from ref 204. Copyright 2012 American Chemical Society.

7338 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 54. (A) Molecular structure of pillar[5]arene, competitive guests (G1 and G2), and a competitive host ([24,8]). (B) Representations of (a) the planar chiral inversions triggered by achiral guest G1 and (b) alternating addition of achiral guest G2 and host [24,8]. Reprinted with permission from ref 209. Copyright 2013 John Wiley & Sons.

G2 and pillar[5]arenes than those between G1 and pillar[5]- arenes. More interestingly, G2 could be removed from the cavity of pillar[5]arenes with the assistance of the crown ether [24]crown-8 ([24,8]), resulting in CD intensities that nearly recovered to their initial state. This result displayed the host− guest complexation as a valid driving force for the chiral inversion, and this guest-triggered chiral inversion system will be useful for chiral switching or sensing systems. Naphthalene diimide amphiphiles functionalized with the − dipicolylethylenediamine Zn motif were synthesized in order to Figure 55. (a) CD spectra and (b) schematic of the dynamic helical promote a guest-induced self-assembly and chiral induction reversal of NDPA-Amph/ADP assemblies upon competitive guest fi 210 − through speci c binding interactions. Titration of NDPA binding experiments with ATP (c =7× 10 5 M, 70% aq HEPES buffer in amphiphiles with increasing molar ratios of ADP resulted in the THF). Reprinted with permission from ref 210. Copyright 2012 Royal gradual evolution of strong Cotton effects, indicating that ADP Society of Chemistry. binding induced a preferred helical handedness to the resulting assemblies of achiral NDIs. The binding of ATP induced opposite handedness to NDI assemblies, as evident from the preparation.217 The aggregates prepared by the heating−cooling positive bisignated CD signal, with positive and negative maxima method possessed ordered molecular packing and enhanced at 390 and 359 nm, respectively. The mirror-image Cotton optical chirality. In contrast, ultrasonication resulted in molecular ff e ects of NDPA-Bola assemblies obtained with ADP and ATP aggregates with less ordered packing and opposite supra- indicated the induction of chirality with opposite handedness. molecular chirality to the sample prepared via a heating−cooling Interestingly, addition of 0.5 equiv of ATP to NDPA-Amph/ method. This heating−cooling method caused the nanofibers to ADP assemblies resulted in positive bisignated CD signals which have extended length and a prominent helical twist. The S isomer exactly match with those of NDPA-Amph/ATP stacks alone. gave left-handed M helices, and R isomers provided right-handed This clearly suggested the competitive replacement of ADP by chiral sense. In contrast, the assemblies prepared by the ATP from the assemblies as expected and an instantaneous ultrasonication method exhibited thinner fibers (10−15 nm) reversal of its helical handedness. The authors also revealed a with the opposite twist in helices for the corresponding isomers, dynamic helix reversal procedure through an intrastack left- (M) and right-handed (P) twists for the R and S isomers, mechanism. respectively. The procedure which uses a heating−cooling cycle 4.3.6. Sonication. Ultrasound is often used as a source of is thermodynamically driven and results in the formation of more energy to cleave and homogenize H-bonding, π−π stacking, and stable nanostructures. In contrast, ultrasonication is a fast process hydrophobic interactions of molecular building blocks and to leading to a kinetically stable product in a shorter time frame. The − reshape the packing mode and the morphology.211 216 tunable chiroptical properties in these supramolecular systems The self-assembly of bichromophoric perylene bisimide into make them potential candidates for applications in the field of chiral nanostructures, and the supramolecular helicity of the optical and electronic device fabrication based on organic nanostructures could be controlled by varying the method of nanostructures.

7339 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 56. Schematic illustration of the diﬀerence in molecular packing leading to reversal of supramolecular chirality of the aggregates formed by the two methods. Reprinted with permission from ref 217. Copyright 2011 American Chemical Society.

4.3.7. pH Value. The helicity inversion can often be found to Figure 58. Illustration of chiral amplification and the model of two be triggered by the change of pH value through the change of principles: sergeants-and-soldiers rule and majority rule. conformation of peptide and the interaction between building − blocks that are based on amino acids.218 220 Lednev et al. reported that a small pH change initiated controlling the supramolecular chirality of the system, and in the spontaneous transformation of insulin fibrils from one case of majority rule, how small can a chiral bias be while still polymorph to another.220 These authors found that the sign of determining the chirality of an entire system. the VCD band pattern from filament chirality can be controlled In the first case, a large amount of achiral units (the soldiers) by adjusting the pH of the incubating solution, above pH 2 for obey the rule of a small number of chiral molecules (the “normal” left-hand helical filaments and below pH 2 for sergeants). Majority rules refers to a slight initial excess of a single “reversed” right-hand helical filaments. Later, they extended enantiomer leading to a strong bias toward the same helical sense this to other proteins and peptide fragments and again found that in the whole aggregate. pH variation triggered filament chirality change. 4.4.1. Analogue-Induced Chiral Amplification. Meijer et al.94 first attempted to apply these two rules to explain the chiral amplification in supramolecular systems in solutions. When a small amount of a chiral disc-shaped molecule was added to a solution of an achiral analogue in hexane, the CD effect of the bipyridine transition showed a nonlinear response to the amount of chiral sergeant.94,96,127,222 Fitting the data to a theoretical model showed that on introducing on average one molecule of chiral 32a per 80 molecules of achiral 32b, the chiral component (the sergeant) dictated the helical sense of the total stack (of soldiers). The mixtures of the enantiomers 32a and 32c showed a Figure 57. AFM images of prion fibrils grown in pH 2.0 (a) and 3.9 (b). nonlinear response of the CD effect on the enantiomeric excess, Reprinted with permission from ref 220. Copyright 2014 American fi Chemical Society. indicating that chiral ampli cation in these systems corresponds to the majority-rules effect. Many of the supramolecular systems were found to obey these fi 4.4. Chiral Ampli cation in Supramolecular Systems rules. Coronenebisimides (CBIs), as potential candidates for Amplification of chirality is a well-known phenomenon in novel liquid-crystalline materials and active n-type semi- classical covalent polymers, the pioneering studies of which were conductor molecules in organic electronics, were assembled in performed by Green and co-workers using the poly- nonpolar methylcyclohexane.223 Derivatives of CBIs bearing (alkylisocyanates) system.221 They defined two effects that chiral and achiral 3,4,5-trialkoxyphenyl groups at the imide influence the amplification of chirality and named them the position (85 and 86) self-assembled mainly through π-stacking “sergeants-and-soldiers” principle and the “majority-rules” and van der Waals interactions in methylcyclohexane, resulting in effects. In recent decades, interest in the amplification of chirality long 1D fibrillar stacks. Different amounts of 85 were has broadened to supramolecular polymer systems based on coassembled with 86 (c = 2.5 × 10−5 M), and their chiroptical noncovalent interactions. In the supramolecular assemblies, properties were probed. Even with a small amount (3%) of the chiral amplification has been described as a phenomenon where chiral derivative (85) as the sergeant the CD spectrum of the local chirality of a small fraction of chiral bias decides the chiral coassembly showed a bisignated Cotton effect (Figure 60a). The sense of the entire assembly and is in general followed by anisotropy factor or g value Δε/ε monitored at λ = 320 nm manifestation in the CD signals (Figure 58). The two principles, showed nonlinear behavior (Figure 60b), which reached the sergeants-and-soldiers and majority rules, often describe the corresponding value of the pure chiral assembly at around 50% of strong amplification of a small chiral imbalance at the molecular the sergeant. This result suggested a chiral amplification based on level to a supramolecular chirality. The basic concepts of sergeants-and-soldiers rule in this system. amplification of chirality in the self-assembled systems are The effect of chemical structure on the amplification of illustrated in Figure 58. chirality was studied by systematic variation of the chemical The key challenge in the case of the sergeant-and-soldiers rule structure of benzene-1,3,5-tricarboxamide derivatives 87−92 is how small can the amount of sergeant molecules be while still (Figure 61).224 Since each BTA comprises three side groups,

7340 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 59. (a) Hydrophobic disc-shaped compounds with bipyridine units (32a−c). (b) Ampliﬁcation of chirality observed upon mixing solutions of 32a and 32b in hexane results in a nonlinear relationship between the CD eﬀect and the amount of chiral 32a added to achiral 32b. (c) Net helicity as a function of enantiomeric excess measured by CD spectroscopy of mixtures of 32a and 32c in n-octane. The line indicates the theoretical result that gives the closest agreement with the experiment. Reprinted with permission from ref 96. Copyright 2011 American Chemical Society.

Figure 60. Coronene bisimide molecules 85 and 86 and coassembly of 85 and 86 and resultant chiral amplification. All experiments were done in MCH (c = 2.5 × 10−5 M). (a) CD spectra of the coassembly at different percentages of 85 in a 1 cm cuvette at 20 °C. The arrow indicates the spectral change with an increase in the percentage of the sergeant. (b) Anisotropy value or g value monitored at λ = 320 nm as a function of the percentage of 85. The dashed line that connects the fraction of 85 indicates the linear variation of the g value in the absence of any chiral amplification. Reprinted with permission from ref 223. Copyright 2013 John Wiley & Sons. asymmetrically substituted monomers have been synthesized to soldiers experiments, it was found that 90 showed a stronger study the effect of the number of stereocenters and the position amplification of chirality in majority-rules experiments, i.e., a net of the stereogenic center on the degree of chiral amplification. helicity of 1 was reached at lower ee for 90. First, when the position of the stereogenic center in asymmetri- In the study of the influence of the number of stereogenic cally substituted BTAs is varied, an odd−even effect can be centers on the chiral amplification, the results suggested that discerned, which was characterized by the appearance of a lowering of the number of stereocenters caused an enhancement positive Cotton effect as for (R)-89 and (R)-91 and a negative of the degree of chiral amplification.224 Cotton effect for (R)-90. For the (R)-89:88 and (R)-91:88 To quantify the majority-rules and sergeants-and-soldiers data, mixtures, a net helicity of 1 is obtained at a sergeant fraction of Meijer et al. proposed two free energy penalties, i.e., HRP and 0.15 in both of these systems, while more than 30% sergeant (R)- MMP, in the chiral amplification in supramolecular assemblies. 90 is needed to obtain a net helicity of 1 in the (R)-90:88 Herein, the HRP (helix reversal penalty) describes the energy mixture. penalty of a helix reversal in the aggregate.225,226 This energy Along with sergeants-and-soldiers experiments, the authors penalty is paid when in a helical stack of these building blocks the further performed majority-rules experiments. In contrast with handedness of the stack is reversed, i.e., going from a left-handed the weaker amplification of chirality of (R)-90 in sergeants-and- to a right-handed helical segment or vice versa. The HRP value is

7341 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 61. CD spectra of mixtures of (R)-89:88 (A), (R)-90:88 (B), and (R)-91:88 (C). Net helicity versus fraction of sergeant for mixtures of (R)- 89:88,(R)-90:88, and (R)-91:88 (D). Reprinted with permission from ref 224. Copyright 2010 American Chemical Society. related to intermolecular interaction, as once a handedness is trimer 94 would be able to act as a sergeant and control the chosen strong intermolecular interactions are favorable to overall helicity of a columnar stack consisting of soldiers of 93. maintain this handedness throughout the stack. The MMP However, it was surprising to find that a total absence of (mismatch penalty) is related to the incorporation of a chiral amplification of chirality was found in this system. The author monomer in a helical aggregate of its unpreferred helicity. In proposed that the interactions between 93 and 94 were not in the majority-rules and sergeants-and-soldiers experiments, the HRP correct balance to express chirality from the sergeants to the would be very similar, since it is related to the intermolecular soldiers. For porphyrin trimers 95 and 96, which, in comparison ff interactions. The MMP, on the other hand, has di erent physical to trimers 93 and 94, lack three of the four meso-phenyl rings at meaning in the two types of experiments. For the sergeants-and- the porphyrin moieties, intermolecular interactions between soldiers experiment, a MMP arises when the chiral sergeant is these molecules in the columnar assemblies would be stronger incorporated in a stack of achiral molecules of its unpreferred than between the molecules of 93 and 94 in their respective helicity. For the majority-rules experiment, the MMP arises when stacks. This is because 95 and 96 can approach each other as the one chiral enantiomer is incorporated in a stack formed from fi result of the reduced steric hindrance and thus enhance the chiral monomers of opposite stereocon guration with corre- π−π fi sponding opposite helicity. For the BTA derivatives discussed intermolecular stacking interactions. The ampli cation of above, the HRP value is similar in all systems, but the MMP is chirality in toluene was thus very successful, while in n-heptane it directly related to the number of stereocenters present in the was completely absent. This might be due to the columnar stacks molecules. Increasing this number from one to three resulted in of porphyrintrimers in n-heptane which are kinetically inert an increase in this energy penalty while leaving the HRP assemblies, and as a result, the chiral and achiral stacks cannot unaffected. These findings can help gain a better understanding dynamically exchange their building blocks. The sergeant and of the ultimate limits of chiral amplification. soldiers experiments have proven to be an excellent method for Elemans et al. further illustrated the effect of molecular revealing this behavior. The work presented here shows the structure on chiral amplification through the synthesis of an influence of variations in the molecular structure and the choice analog of porphyrin trimers based on benzenetricarboxyamide of solvent amplification of chirality on system chiral (BTA), 93, 94, 95, and 96.99 In this case, the chiral porphyrin amplification.

7342 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 62. (A) Molecular structures of the porphyrin trimers 93−96. (B) Schematic representation of the self-assembly of porphyrin trimers in helical columnar stacks. Reprinted with permission from ref 99. Copyright 2012 Royal Society of Chemistry.

4.4.2. Chiral Amplification in Binary Systems. Chiral indeed diverged from linearity, indicating that the amplification amplification was also studied in binary complex systems.227 In of chirality obeyed the majority rules and occurred at the level of this case, melamines equipped with two PBI chromophores and the hydrogen-bonded complexes. two 3,7-dimethyloctyl chiral handles were mixed with cyanuric 4.4.3. Chiral Amplification to Nanoscale. Chiral acid to form a discotic supramolecular complex (CA).228 It was amplification can be expressed in the form of nano/micro- found that the sergeants-and-soldiers effect where a few chiral structures. Compound 98 is an achiral molecule and the analogue building blocks can control the helical sense of the large number of compound 99, which has four chiral centers in its alkyl chains. of structurally related achiral ones was not applicable for this The self-assembly of 98 could only give a flat nanostructure. system. When mixed chiral 97S was used as sergeant with the However, when 98 was coassembled with 99, the flat lamellae optically inactive achiral 97A as the soldiers, plots of Δε versus were transformed into twisted ribbons.229 The presence of only 5 the amount of 97S for these ternary mixtures (97S/97A/CA) mol % of the sergeant 99 was able to transfer the chirality showed a decrease in their optical activities compared to the embedded in the peripheral chains to the remaining 95% of the chiral complex, indicating the absence of any sergeant-and- soldiers (98), as revealed by the corresponding SEM images, in soldiers effect. In contrast to the sergeants-and-soldiers effect, the which the micrometer-long twisted ribbons of high aspect ratio other chiral amplification effect, the majority-rules effect, appeared. The presence of stereogenic centers in the coassembly occurred at the level of the hydrogen-bonded complexes. of achiral 98 with chiral 99 provoked the chiral propagation of Enantiomeric mixtures of 97S and 97R showed almost linear the H bonding of the amide functionalities reinforcing the dependence of Δε on ee in the absence of CA (recipe i), formation of twisted ribbons. Increasing the percentage of chiral suggesting that the self-aggregation of the enantiomers (self- sergeant 99 in the coassembly caused the formation of twisted sorting) or coaggregation occurs; they obey their own preferred ribbons to increase. The results represent an excellent example of helicities. In the presence of CA (recipe ii), the Δε/ee plots the study of homochirality on surfaces and, at the same time,

7343 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 63. (a) Chemical structures of 97 and CA. (b) Proposed structure of 3:1 hydrogen-bonded complex 97S·CA. (c) Schematic illustration of helical columns. Reprinted with permission from ref 228. Copyright 2011 John Wiley & Sons. contribute to the knowledge about the set of rules governing the 4.5. Chiral Memory in Supramolecular Systems generation of chiral objects that hold great potential for the The phenomenon of chirality memory describes a supra- development of supramolecular devices. molecular system in which chirality is first induced and then 4.4.4. Unexpected Amplification in Racemate Assem- maintained after the chiral source is erased or replaced by an blies. Liu et al. found an interesting chiral amplification in self- achiral component. Thus, these complexes have a memory for assembled systems based on L-andD-alanine derivatives the chirality of the species that induced the system’s asymmetry containing an N-fluorenyl-9-methoxycarbonyl (Fmoc) moiety after the removal of these inductor molecules. and a long alkyl chain.230 It has been found that both the Generally, chiral memory is difficult to induce in noncovalent enantiomeric and the racemic assemblies showed CD signals. supramolecular assemblies, partly because additives often interfere with the noncovalent interactions that hold the The enantiomer 100 showed mirror-imaged CD spectra, and the ff sign of the CD spectra for the assemblies followed the molecular assembly together. However, in recent decades, more e orts to explore supramolecular chirality memory systems have proven to chirality. However, the supramolecular chirality of the racemate be successful through the efforts to design the chiral and achiral assembly was not certain. It was revealed that the slight excess of units and control the dynamics. In order to realize the chiral one enantiomer in the racemic mixtures may result in an active memory, there are several important elements. First, the induced CD signal since the exact 1:1 mixture at a molecular level cannot chiral nanostructure should be generally stable. Thus, even when be reached. When mixing the two enantiomers (100L/100D) you remove the chiral species, the chirality is maintained. Second, with different molar ratios it was found that an excess of 100L a small amount of the chiral substance should be able to induce resulted in a negative Cotton effect, whereas mirror-imaged CD chirality in the system. A successful chiral memory system may spectra were obtained for mixtures with an excess of 100D. contain (1) noncovalently induced helical polymers, (2) strong Furthermore, the CD signals observed for a nonequimolar aggregates from achiral building blocks such as J or H aggregates, mixture of the enantiomers were more intense than those for the or (3) chiral cages from coordination compounds. pure enantiomers. For the system obeying the majority-rules 4.5.1. Helicity Memory in Noncovalently-Induced Helical Polymers. principle, the CD intensity generally decreased when their Helicity memory in the noncovalently induced helical polymers has been thoroughly investigated by mixing ratio deviated from the pure one. However, in the case of 174 L D fi Yashima and co-workers. On the basis of a chiral memory the mixed 100 /100 system, the CD signals intensi ed when system of noncovalent helicity induction in optically active the mixing ratio approached 1:1, as shown in Figure 65. This polymers, Yashima et al. developed an excellent macromolecular indicated that the self-assembly of the racemic mixture is very memory system of a helical polyacetylene in the solid state.231 sensitive to a slight enantiomeric excess and the system could be They synthesized a polyacetylene derivative, 101, and induced its used for the detection of a broad range of chiral amino acid preferred-handed helicity in the presence of (S)-phenylethanol in derivatives. n-hexane through weak hydrogen-bonding interactions. 101 was

7344 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 65. (A) G values centered at 309 and 255 nm as a function of the Figure 64. (Top) Structure of bisamides 98 and 99, schematic ee value of nonequimolar mixtures of 100L and 100D. The G value of illustration of the self-assembly of 98 into sheets, and the coassembly of the racemate was set to zero. (B) SEM images of the nanostructures 98 and 99 into twisted ribbons. (Bottom) SEM images of twisted formed from mixtures of 100L and 100D with various ee values. ribbons formed by the coassembly of the achiral soldier 98 and chiral Reprinted with permission from ref 230. Copyright 2013 John Wiley & sergeant 99 at different percentages of chiral component: 95/5 (a and Sons. b), 90/10 (c), and 85/15 (d). Reprinted with permission from ref 229. Copyright 2010 Royal Society of Chemistry. then recovered by filtration followed by washing with methanol to completely remove (S)-phenylethanol. Upon further dissolving 101 in n-hexane at 20 °C, an apparent ICD band exhibited an intensity that increased with time. This intensity finally became nearly equal to that induced in an n-hexane solution in the presence of (S)-phenylethanol after 1 h. The macromolecular helicity memory in the solid state was more stable than in solution, because the ICD intensity in the solid state persisted for at least 11 months at 25 °C. More interestingly, by simply immersing the polymer with macromolecular helicity memory by (S)-phenylethanol in a solution containing (R)- phenylethanol followed by washing with methanol, the helicity of 101 was completely inverted and memorized. Thus, there was no doubt that a preferred-handed helix of the optically inactive polymer 101 was induced via weak noncovalent bonding interactions in the solid state simply by immersing 101 in a nonracemic liquid, and this helicity could be memorized and switched automatically. This chiral memory and switchable fi Figure 66. Reversible switching and memory of macromolecular helicity system in the solid state will be bene cial to the development of a of 101 in the solid state. Preferred-handed macromolecular helicity of chiral stationary phase for the separation of enantiomers. 101 is induced and subsequently memorized in the optically inactive Inoue and Takashima et al. designed a m-ethynylpyridine 101 via noncovalent interactions with a nonracemic alcohol (S-orR- polymer that has a metal coordination site at the 4 position of phenylethanol) followed by complete removal of phenylethanol in the each pyridine unit, which showed a chiral memory effect on a m- solid state. The polymer’s helical handedness and axial twist sense are ethynylpyridine oligomer.232 It is found that the polymer can switched reversibly in the solid state in the presence of the opposite form CD-active helical complexes with various kinds of guest enantiomeric alcohol. Reprinted with permission from ref 231. saccharides by the interaction of hydrogen bonds between the Copyright 2014 Nature Publishing Group.

7345 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review nitrogen atoms of the pyridine rings and the hydroxy groups of the saccharides. Moreover, the ICD band was remarkably enhanced by the addition of Cu(OTf)2 (0.5 equiv to pyridine units) and o-phenanthroline (phen) as a result of stabilization of the helical structure of the polymer. Even when an equimolar amount of another enantiomer of glucopyranoside was added to make the whole system apparently racemic, the ICD of the ethynylpyridine polymer was memorized and remained for several weeks.

Figure 67. Assumed mechanism of helix stabilization of m- ethynylpyridine host oligomer 102 by coordination of copper and phen inside the helix. Reprinted with permission from ref 232. Copyright 2012 Royal Society of Chemistry.

4.5.2. Chiral Memory in Aggregates Such as J and H Aggregates. Purrello et al. reported that a number of self- assembly systems could memorize the chirality of complexes when the chiral auxiliary had been removed. They first induced the formation of aggregates between the cationic porphyrin CuTMP and anionic porphyrin H4TPPS, 63, with the chiral 233 matrix of either L-orD-polyglutamic acid. Upon the formation of the ternary complex, the induced CD of binary CuTMP and TPPS was obtained in the α-helix structure of polyglutamic acid. Interestingly, the induced CD signals of the porphyrin complexes Δ Λ Figure 68. Schematic structures of H2TPPS and -or -Co(III) remained even when the conformation of the polyglutamic acid complexes and Schematic illustration of the induction, memory, and fi Λ Δ was switched to a random coil by increasing the pH to 12. This ampli cation of chirality in H2TPPS with -or -Co(III) complexes. suggested that during the formation of the ternary complex the Reprinted with permission from ref 242. Copyright 2010 Royal Society chirality of the polymer was transferred to and memorized by the of Chemistry. porphyrin complexes. Interestingly, the chiral complex was stable and maintained even when a 5-fold excess of a competing and antipodal chirality source; namely, poly-D-glutamate, was added and achiral Cu porphyrins through a dynamic control of 243,244 to the CuTMP/TPPS/poly-L-glutamate system. A similar self- assemblies. The chiral Zn porphyrins were used as a assembly memory system based on the aggregation of the sergeant to transfer their chirality to the achiral Cu−porphyrin oppositely charged CuTMP and TPPS in the presence of soldiers. After the sergeant was removed from the coaggregates − enantiopure aromatic amino acids was also found.234 238 by axial ligation with a Lewis base (quinuclidine), the chiral Further, Shi et al. and He et al. found simple TPPS J aggregates information in the remaining aggregate was preserved as a result − also showed chiral memory effects.239 242 For instance, He et of slow conformational dynamics, which revealed a chiral 242 al. found that with the existence of L-orD-enantiomers of cis- memory effect. [CoBr(NH3)(en)2]Br2 as chiral triggers for the J aggregates of Such chiral memory based on the aggregation has been achiral 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin expanded to other systems. Jiang et al. established a J aggregate of (TPPS), 67 could be fabricated into chiral assemblies, during an achiral perylene dianhydride (PDA) in CTAB micelle solution which the metal-centered chirality can be transferred to the J and employed small molecule D- and/or L-tartaric acid as the aggregates. In addition, the chirality was memorized in the chiral auxiliary to induce transfer of the chirality to PDA porphyrin J aggregates. These authors initially synthesized the aggregates.245 The ICD signal of the J aggregates was also found porphyrin aggregates with the L-Co(III) complex, and then the D- to remain unchanged upon addition of a large excess of the Co(III) complex was added to the system. During the addition of alternative enantiomer of tartaric acid, implying the imprinting of this opposite D-Co(III) chiral species, the UV absorption at the the chirality in the J aggregates. TPPS Soret band (434 nm) gradually decreased; however, the Aida et al. synthesized an elaborate “nanotubular” helical induced CD signal of the porphyrin aggregates increased (rather architecture with 60% de (80:20 diastereomeric ratio) by the self- Py than inverting) after the addition of excess D-Co(III) complex. assembly of a hexabenzocoronene derivative, HBC , carrying a This suggested that the chirality of TPPS J aggregates induced by chiral (BINAP)Pt(II) moiety as a detachable chiral auxiliary. The one enantiomer of Co(III) coordination was maintained even optically active nanotubes did not racemize after removing the with the addition of an excess of the opposite enantiomer of the chiral auxiliary through the addition of ethylenediamine. Once Co(III) complex. the helical tubular structure of HBC formed, the addition of Meijer et al. developed a class of highly tunable porphyrin- (BINAP)Pt(II) with an absolute configuration opposite to the based chiral memory system based on the chiral Zn porphyrins original one did not cause the helical inversion. These results

7346 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

stereocenters of the structure enabled retention of conﬁguration upon replacement of the chiral subcomponents. This memory eﬀect allows for the stereoselective preparation of a metal− organic capsule that ultimately contains only achiral subcomponents, which can extend the application of metal−organic capsules in stereoselective guest recognition and sensing and as asymmetric reaction vessels.

5. SPONTANEOUS SYMMETRY BREAKING AND EMERGENCE OF SUPRAMOLECULAR CHIRALITY IN SELF-ASSEMBLED SYSTEMS FROM EXCLUSIVELY ACHIRAL MOLECULES Through self-assembly, not only chiral molecules but also Figure 69. (A) Chiral/achiral amide-functionalized zinc/copper completely achiral molecules can form chiral supramolecular tetraphenylporphyrins. (B) Schematic depiction of selective depolyme- assemblies. This situation results from spontaneous symmetry rization with chirality retention and temperature-induced switching of breaking,10 which is one of the most important issues in the chiral memory. Reprinted with permission from ref 244. Copyright obtaining assemblies with macroscopic chirality248 or optical 2010 American Chemical Society. activity249 instead of producing the same amount of the enantiomeric nanostructures. On the other hand, there is a real ﬀ possibility that the origin of life could have depended on demonstrated a good example of a stereochemical memory e ect 250 in HBC nanotubular helical architecture.246 molecular chirality and supramolecular chirality. Although we 4.5.3. Helicity Memory in Chiral Cages from Coordina- still do not clearly know the origin of natural homochirality, II supramolecular chirality can now be created from achiral tion Compounds. A class of Fe 4L4 capsules with chirotopic cavities has been prepared by the in situ metal-templated imine molecular building blocks. In general, asymmetric environments condensation between tris(formylpyridyl) benzene and a chiral are necessary for symmetry breaking, which provide the amine.247 This capsule maintained the stereochemistry of the supramolecular chirality to the assemblies of achiral mole- 236,251−253 cage framework (99% ee) even when the chiral amine was cules. However, the most intriguing possibility would replaced by an achiral one. The cage retained its stereochemistry be symmetry breaking without discernible chiral conditions. In after 4 days at 90 °C. The author inferred that the strong this section, we will show examples where supramolecular cooperative stereochemical coupling between the iron(II) chirality emerged from achiral building blocks.

Figure 70. Schematic illustration of a series of experiments for investigating the dynamic nature of nanotubularly assembled HBCPy. Reprinted with permission from ref 246. Copyright 2013 American Chemical Society.

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Figure 71. Route i: formation of racemic cage 2 through subcomponent self-assembly. Route ii then route iii: enantioselective formation of cage 2 through subcomponent substitution. Reprinted with permission from ref 247. Copyright 2013 American Chemical Society.

5.1. Liquid-Crystal and Banana-Shaped Molecules Even though the origin of chirality and natural homochirality has attracted much attention over the past decades, supramolecular chirality resulting from the self-assembly of achiral molecular Figure 72. Molecular structures of silicon-containing bent-core achiral building blocks has been of major interest as a result of the early molecules, and the temperature-induced inversion of supramolecular work conducted on liquid crystals. The first report of this can be chirality. Reprinted with permission from ref 256. Copyright 2006 found in the work of Young and co-workers.254 These authors American Chemical Society. designed and synthesized a series of stilbene derivatives and studied the possibility of forming nematic liquid crystals from the workers (Figure 74).258 In this instance, the self-assembly of assembly of these molecules. Interestingly, with the racemic banana-shaped (bent-core) achiral molecules 106 can result in stilbene mixtures they observed small cholesteric liquid- strong supramolecular chirality. Most interestingly, these crystalline regions in the nematic field under polarized light assemblies do not exhibit anisotropy at the macroscopic scale. microscopy. These assemblies are macroscopically isotropic fluids that In the use of achiral molecules to form chiral liquid crystals, the possess only short-range orientation and positional order, just most famous building blocks are the banana-shaped or bent-core like a true liquid. Therefore, the self-assembly of achiral achiral molecules.255 In the field of liquid crystals, the self- molecules was found to form isotropic fluids with supramolecular assembly of banana-shaped achiral molecules has been widely chirality. In particular, the assembly of bent-core achiral investigated. For example, Tschierske and colleagues studied the molecules can form a phase that exhibits smectic layering with liquid-crystalline phases formed by silicon-containing polyphilic fluid order within the smectic layers. Thus, the coherent length of bent-core achiral molecules 104. These authors found that the smectic layering is very short, smaller than ca. 100 nm. The temperature-induced inversion of chirality in a supramolecular mechanism of formation of this property has been attributed to system formed by achiral molecules can be established (Figure the formation of saddle−splay deformations involving the elastic 72).256 constant K24. Saddle−splay director fields are not space filling, The research group of Cheng constructed chiral propellers and smectic layers are unstable in saddle−splay deformations and from the self-assembly of achiral molecules (BPCA-Cn-PmOH), thus incompatible with long-range order. which were composed of 4-biphenylcarboxylic acids connected With respect to the banana-shaped (bent-core) achiral with phenol via alkoxyl chains 105. The achiral BPCA-Cn- molecules forming supramolecular chirality, Hough, Clark, and PmOH molecules can form individual head-to-head dimers, and co-workers (Figure 75) also studied helical nanofilament phases, the twisting of the dimers can lead to chiral N phases. These in which a local chiral structure is expressed as twisted layers.259 results demonstrate that neither molecular chirality nor a Although composed of achiral molecules 107, the layers in these molecular bend is necessary to form a chiral phase (Figure 73).257 filaments are twisted and rigorously homochiral, demonstrating For chiral assemblies formed by banana-shaped (bent-core) broken symmetry. This work was published in the same issue of achiral molecules, a notable work is that of Hough, Clark, and co- Science as that of isotropic chiral fluids. In contrast to isotropic

7348 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 73. (A) Structure of the dimeric building block of BPCA-C7- PmOH 105. (B) Chiral propellers from the self-assembly of this achiral molecule 105 (BPCA-Cn-PmOH). Reprinted with permission from ref 257. Copyright 2006 John Wiley & Sons. Figure 75. (A) Structure and phase sequence upon cooling of the four B4 phase-forming compounds studied. (B) Mechanism and TEM images of hierarchical self-assembly of the nanoﬁlament (NF) phase starting with bent-core mesogenic molecules. Reprinted with permission from ref 259. Copyright 2009 The American Association for the Advancement of Science.

both bent-core molecules and rod-like molecules.260 This process can be considered a general method for producing homochiral helical nanofilaments. Banana-shaped molecules (108, 109) are in the B4 phase, while rod-like molecules (110, 111) can form a nematic phase. The mixture of different molecules has the phase sequence N−Bx(B4/N), and homochiral helical nanofilaments can be obtained upon cooling (Figure 76).261 To understand the helical nanofilament phases assembled by banana-shaped (bent-core) achiral molecules, the corresponding hierarchical nanostructures have attracted much attention. It was found that the arrangement of different helical nanofilaments can be very important to the assembly process. Jaklí et al. found that some properties, especially structural color, of the B4-phase liquid crystals formed by bent-core achiral molecules 112 cannot be explained by the nanostructures of nanofilaments alone. In Figure 74. (A) Structures, phase sequences, layer spacing, and their study, they found that different helical nanofilaments do not correlation lengths (as measured by XRD and freeze−fracture TEM) form parallel packing. Instead, these helical nanofilaments were of achiral 106. Freeze−fracture TEM images of the dc phase of 106. (B) arranged at an angle of 35−40° with respect to each other, Frustration between molecular fragments in a tilted bent-core molecule forming a doubly twisted nanostructure, which caused the can be relieved by saddle-splay curvature of the layers. Reprinted with unusual structural color of these liquid crystals (Figure 77).262 permission from ref 258. Copyright 2009 The American Association for 5.2. Solution Systems, Micelles the Advancement of Science. Asymmetric environments or conditions are usually necessary for symmetry breaking within supramolecular systems, in which the chiral fluids assembled by achiral building blocks, these helical aggregation of achiral molecular building blocks can lead to nanofilament phases exhibit birefringence, indicating long-scale supramolecular chiral assemblies. Thus, chiral dopants202 or a ordering. chiral matrix263 can assist achiral molecules to assemble into In the production of helical nanofilament phases from the chiral nanostructures. Solid surfaces264,265 also can provide two- assembly of banana-shaped (bent-core) achiral molecules, dimensional confined environments that lead to the symmetry Takezoe and co-workers developed a mixture system containing breaking.

7349 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

obvious approaches to make the achiral molecules form chiral assemblies in organic or aqueous solution. Indeed, there are some reports showing that achiral molecules self-assemble into helical nanostructures. However, within these systems, the mixture of both left- and right-handed helices in equal amounts does not − fulfill the requirements for true supramolecular chirality.266 268 In this case, the assemblies are overall racemic mixtures and show no macroscopic optical activity with a silent CD. Researchers in the field of supramolecular chirality are very interested in the formation of assemblies with unequal amounts of left- and right-handed helices using purely achiral molecular building blocks in solution. This type of symmetry breaking should be able to be detected by CD spectral measurements. There are only a few published reports of the aggregation of achiral molecules in solution to form supramolecular chirality. Amazingly, the first paper on this issue was published in 1996.269 Dahnë and co-workers found that one achiral charged dye molecule containing long alkyl chains (113) can form supramolecular chiral assemblies in solution. In this system, two types of noncovalent interactions play very important roles. Thus, both Figure 76. (A) Chemical structures of bent-core and rod-like the self-association of organic dyes and hydrophobic interactions components of the mixtures: (a) 12OAzo5AzoO12 (108), (b) P8-O- between long alkyl chains cause the achiral molecules to undergo PIMB (109), (c) 5CB (110), and (d) 5PCB (111). (B) AFM image of J aggregation with supramolecular chirality, which can be Bx surface. (C) (a) UV−vis absorption spectra of 12OAzo5AzoO12 confirmed by CD spectra. It is suggested that the dye molecule (108) (bent-core) and ZLI2293 (rod-like). (b) CD spectra of the Bx can form twisted herringbone-type assemblies, which may phase. (c) CD spectra showing a longer wavelength peak made from TN possibly lead to supramolecular chirality (Figure 78). cells of various cell thicknesses: 0.4, 0.6, 0.7, and 0.9 μm. (Inset) CD intensity as a function of cell thickness. Reprinted with permission from The Liu group studied the aggregation of achiral molecular ref 261. Copyright 2013 John Wiley & Sons. building blocks for forming chiral assemblies in oil/water mixed micelle dispersions.270 The work is based on a surfactant-assisted self-assembly (SAS). In this work, an aqueous solution of In a homogeneous solution, the birth of chiral information cetyltrimethylammonium bromide (CTAB) and a chloroform from totally achiral systems can be very subtle, so there are no solution of another achiral molecular building block was added

Figure 77. (A) Molecular structure and phase sequence of PnOPIMB (112)(n = 7, 8, 9 and 12) during cooling at 1 °C min−1. (B) TEM image and models showing the double-twist structure. Reprinted with permission from ref 262. Copyright 2014 Nature Publishing Group.

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assembly of ZnTPyP. Interestingly, no supramolecular chirality was detected in these nanotubes or nanofibers (Figure 79). With regard to symmetry breaking upon self-assembly, different achiral porphyrins have been used as model molecular building blocks. In aqueous solution, the self-assembly of some water-soluble porphyrins (such as tetrakis(4-sulfonatophenyl)- porphine, TPPS, 67) has been widely investigated and is further discussed below. On the other hand, the uncharged achiral water- soluble porphyrin was also found to form chiral assemblies in aqueous solution. Mineo et al. synthesized a TPP-based porphyrin containing polyethyleneoxy substituents (PPeg4, 115).271 This uncharged porphyrin can spontaneously form self-assembled structures in water. Although PPeg4 is achiral, this molecule forms a chiral assembly in water. Most interestingly, the supramolecular chirality from the assembly of PPeg4 was found to be induced by a weak thermal force. In this case, the intensity of the CD Figure 78. Spontaneous formation of supramolecular chirality in J signal of the PPeg4 assemblies could be enhanced by increasing aggregates. the temperature (Figure 80). Meijer and co-workers recently investigated the symmetry breaking of an assembly of achiral molecular building blocks in dropwise into the aqueous solution with stirring. This method is organic solvents.272 They found that the self-assembly of achiral related to the formation of a microemulsion, where the fl chloroform evaporates during the self-assembly process. Using partially uorinated benzene-1,3,5-tricarboxamide molecules the SAS method via an oil/aqueous medium, supramolecular 116 could produce supramolecular chirality (Figure 81A). For assemblies with different nanostructures and properties could be these systems, they carefully studied the kinetics of the self- assembly, and the unique two-step self-assembly behavior was obtained in a controllable manner from very simple achiral fi molecular building blocks. For example, when a chloroform con rmed. Moreover, they found that true symmetry breaking solution of zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine could happen during a kinetically controlled secondary (ZnTPyP) (114) was added dropwise to a cetyltrimethylammo- nucleation (Figure 81B). Thus, within the initial self-assembly fl nium bromide (CTAB) aqueous solution, different nanostruc- process, achiral partially uorinated benzene-1,3,5-tricarboxa- tures with varying supramolecular chirality can be obtained mide molecules could assemble into equal amounts of one- depending on the concentration of CTAB and the aging time. dimensional left- (M) and right-handed (P) helical aggregates, Using the 0.9 mM CTAB system for assembly with subsequent whereas the systems as a whole did not show any optical activity. aging for 3 days, chiral nanorods can be produced by the CTAB- During kinetically controlled secondary nucleation, these one- assisted self-assembly of ZnTPyP. The supramolecular chirality dimensional helical aggregates could bundle into nanofibers, was proven by CD spectral measurements. However, when the which could be optically active.272 Here, the partially fluorinated concentration of CTAB and the time for aging were changed, molecular structures are important, which produced the chiral different supramolecular nanostructures, such as nanotubes and bias that was then amplified by different noncovalent long nanofibers, were produced by the CTAB-assisted self- interactions. Hydrogen bonding was considered to dominate

Figure 79. Schematic illustration showing the controlled synthesis of various porphyrin nanostructures with varied supramolecular chirality by means of an SAS, where an oil/aqueous medium was employed. CD spectra of ZnTPyP (114) nanorods fabricated in sample B that was aged for 3 days. Black and red curves are the results detected from the samples prepared in diﬀerent batches. Reprinted with permission from ref 270. Copyright 2010 American Chemical Society.

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Figure 80. (A) Structure of PPeg4 (115). (B) Circular dichroism spectra at T =26°C (full line), 30 °C (dotted line), and 36 °C (dashed line) of the PPeg4 aqueous solution. Reprinted with permission from ref 271. Copyright 2014 The Royal Society of Chemistry. Figure 81. (A) Chemical structures of the partially fluorinated BTAs (116). (B) (a) AFM image of drop-casted solutions of BTA-F8H on the formation of the helical aggregates, while dipole−dipole mica (scale bar = 0.5 mm). (b and c) Results of stopped-flow interactions dominate the secondary nucleation. experiments for solutions of BTA-F8H. (d) Self-assembly mechanism of Hao and Sun et al. found that achiral bolaamphiphilic 116. Reprinted with permission from ref 272. Copyright 2012 John Wiley & Sons. azobenzene 117 was able to form chiral assemblies in aqueous solution.199 It was shown that the precipitation process was important. The achiral bolaamphiphilic azobenzene containing many carboxyl groups (Figure 82A) was found to dissolve in ments show that only the slow aggregation process can lead to water at pH 6.56, but when the pH value was adjusted to 2.77 by supramolecular chiral assembly (Figure 83). adding HCl solution, the bolaamphiphilic azobenzene formed For symmetry breaking upon the self-assembly of achiral precipitates. TEM and SEM measurements of the material building blocks, a recently published THF/water system is worth showed that these precipitates consisted of nanotubes with mentioning. Since THF and water are not completely miscible, a supramolecular chirality (Figure 82B), which was detected by liquid/liquid interface is present in THF/water mixtures with CD spectral measurements (Figure 82C). Furthermore, since the volume ratios of 1/1. When a THF solution of achiral carboxyl azobenzene derivatives are photosensitive, the morphology and azobenzene derivatives (Figure 84A) was added dropwise to an chirality of the self-assembled nanostructures derived from the aqueous solution of melamine, supramolecular chiral assemblies achiral bolaamphiphilic azobenzene could change in response to can be obtained at the THF/water interface along with the external stimuli such as light and heat. permeation and volatilization of THF (Figure 84C).274 For the An acidification process was also implemented for the self- formation of coassemblies, hydrogen bonding between carboxyl assembly of TPPS (67) in aqueous solution. TPPS is a well- azobenzene derivatives and melamine plays a very important known building block for the occurrence of symmetry breaking role. TEM and AFM measurements show that these assemblies during the self-assembly. The acidification of TPPS could help in are long and helical fibers with intrinsic conformational chirality the formation of J aggregates with supramolecular chirality. (Figure 84B). Furthermore, the morphology and chirality of the Interestingly, the emergence of supramolecular chirality on supramolecular assemblies are photoresponsive, which is TPPS assemblies in aqueous solution can be controlled induced by the photoisomerization of the azobenzene kinetically by modulating the speed of the acidification of the components within the self-assembled nanostructures. porphyrin. In a recently published paper by Scolaro et al., detailed For supramolecular chirality from an assembly of achiral kinetic investigations of the self-assembly of acidified TPPS molecules in solution, it seems that π-conjugated molecules are demonstrate that the rate of the aggregation process strongly always necessary. Within these self-assemblies, the aromatic rings affects the chiral induction.273 In this study, the aggregation tend to overlap through π−π stacking. Thus, the displacement of process in aqueous solution could be changed by adding the aromatic rings within the molecular packing shows a slight angle porphyrin as the first (PF) or last reagent (PL). PF represents the between neighboring rings, which will produce a chiral bias. If slow aggregation process, while PL represents the fast acid- this bias is repeated by forming a helix in a certain direction, ification and aggregation process. The CD spectral measure- supramolecular chirality would emerge and be amplified.

7352 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 82. (A) Molecular structure of achiral bolaamphiphilic azobenzene 117 and its packing mode within the self-assembly. (B) (a) TEM and (b) SEM observation of the self-assembled coiled and tubular nanostructures of 117 from water at pH 2.77. (c) TEM image of the twisted ribbon. (d) HR- TEM image of the nanotube. (C) CD spectra of the self-assembled suspension 117 at pH 2.77 (solid line) and 117 itself (dashed line). Reprinted with permission from ref 199. Copyright 2012 The Royal Society of Chemistry.

Figure 83. (A) Molecular structure of TPPS (67). (B) Schematic illustration showing kinetic control of the supramolecular chirality of the assembly of acidiﬁed TPPS with J aggregation. Reprinted with permission from ref 273. Copyright 2014 American Chemical Society.

Figure 84. (A) Chemical structures of carboxyl azobenzene derivatives and melamine, and the schematic representation of their complex. (B) (a, b) TEM and (c and d) AFM observation of the self-assembled helixes and supercoils with labeled handedness. (C) CD measurements of the assemblies. Reprinted with permission from ref 274. Copyright 2014 The Royal Society of Chemistry.

5.3. Gel Systems helps the formation of hydrogen bonding. Thus, spontaneous chiral symmetry breaking through the steric effect of imidazole As a very important form of soft matter with extensive potential 282 applications, supramolecular gels fabricated by noncovalent upon gelation was achieved. − bonds have attracted great interest recently.275 280 Chiral Another chiral supramolecular gel assembled by achiral supramolecular gels are usually fabricated by chiral gelators and molecules is also related to the complexation of the imidazole show macroscopic optical activity and/or helical nanostructures. unit with metal ions. You and co-workers demonstrated that A few achiral gelators have been found to self-assemble into simple achiral molecules containing an imidazole unit could form + helical nanostructures with an equivalent amount of left- and supramolecular polymers by complexation with Ag , which can right-handedness,266,268,281 but macroscopic optical activity was further gel in a variety of solvents. The supramolecular metal gels 283 barely detected from these supramolecular gels. However, there formed by simple achiral molecules can be optically active. are really some examples of optically active gels obtained from Recently, Liu and Wang et al. synthesized an achiral C3- achiral gelators. symmetric benzene-1,3,5-tricarboxamide substituted with ethyl Kimura et al. synthesized an achiral disk-shaped molecule cinnamate (BTAC, 119) and studied its supramolecular gelation having one imidazole unit (Figure 85A). This achiral molecule and macroscopic chirality from the self-assembly of BTAC in the 118 was found to form a supramolecular gel in 2-methoxyethanol DMF/H2O mixture. They found that upon gelation this achiral and assemble into long and twisted nanoscopic fibers (Figure compound can simultaneously self-assemble into unequal 85C). Although the molecular building block is achiral, it forms amounts of left- and right-handed twists, thus resulting in supramolecular gels with optical activity, which can be macroscopic chirality without any chiral additives (Figure 86). demonstrated by CD spectral measurements (Figure 85B). The symmetry breaking and formation of macroscopic chirality During the emergence of the optical activity, it was found that the from the assembly of this type of molecules is quite rare. The imidazole substituent is very important, because it increases the hierarchical self-assembly of an uneven number of different chiral asymmetrical characteristics of the molecular building block and assemblies produces the unbalanced left- and right-handed

7353 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Langmuir−Blodgett (LB) films through the air/water interfacial self-assembly. The Liu group first reported air/water interfacial chiral assemblies from achiral molecular building blocks in 2003.39 In this work, it was found that when an achiral amphiphilic molecule containing a naphtha[2,3]imidazole headgroup and long alkyl chain (NpImC17, 120) was assembled with Ag(I) ions at the air/ water interface, the compound could coordinate with AgNO3 to form a stable monolayer, which can be then further transferred onto a solid substrate to produce a Langmuir−Blodgett (LB) film. The CD spectra of these LB films showed a strong Cotton effect after removing the possible LD effects, which suggested that the supramolecular chirality resulted from the assembly of achiral molecular building blocks. It was suggested that the overcrowded stacking of the achiral chromophores in a helical sense would produce such macroscopic supramolecular chirality.40 Interestingly, in this system containing only achiral Figure 85. (A) Molecular structures of an achiral disk-shaped molecule molecules, the sign of the supramolecular chirality cannot be having one imidazole unit (118). (B) CD spectra of the supramolecular determined in the air/water interfacial assembly. One can obtain gels obtained in different batches at 20 °C. (C) (a) TEM and (b) AFM ff fi M-orP-chiral assemblies in di erent batches. It is worth images of twisted nano bers formed from the assembly of 118. mentioning that air/water interfacial chiral assemblies obtained Reprinted with permission from ref 282. Copyright 2010 American ff Chemical Society. from achiral molecules are totally di erent from those obtained from racemic mixtures, in which no optical activity can be detected. In contrast, these LB films are optically active, even fi twists, as con rmed by a series of CD spectral measurements and though the handedness of the supramolecular chirality can SEM studies. Importantly, the overcrowded stacking of the change from batch to batch. cinnamate rings within the assembly plays a very important role With air/water interfacial assembly, it is not rare to obtain the in the spontaneous symmetry breaking and production of an supramolecular chirality from achiral molecular building blocks. uneven number of helical nanostructures with different handed- 284 For many achiral molecules, this process of making supra- ness. Furthermore, this phenomenon may generally occur in molecular chiral assemblies is feasible. For example, Liu et al. the gel systems of BTA derivatives. It is expected that many of the further designed an achiral amphiphilic barbituric acid 121 and other BTA derivatives will show similar properties. obtained supramolecular chiral assemblies with optical activity at 5.4. Air/Water Interface and LB Films the air/water interface (Figure 88A). These molecules further The aggregation of adjacent molecules plays an important role self-assembled into spiral nanoarchitectures (Figure 88B). The during the self-assembly of achiral molecules with the emergence hydrogen bonding between the headgroups and the over- of the supramolecular chirality. The initial stacking at a certain crowded arrangement of the aromatic ring within the assembly angle from the neighboring molecules can produce a chiral bias, played very important roles.285 and this dislocation can be left-handed or right-handed. If this The relationship between molecular structures of achiral chiral bias can be further grown or amplified, chiral structures molecules and the supramolecular chirality of their assemblies at with left- and right-handedness are obtained. Moreover, most the air/water interface has been thoroughly investigated. It was importantly, if the growth rate of the two biases is different during found that many achiral molecules with larger steric hindrance the self-assembly, optical activity of the system can be expected, during self-assembly can form chiral assemblies on the surface of with an unequal amount of left-handed or right-handed chiral water. For example, some achiral arylbenzimidazoles with 2- nanostructures. This type of symmetry breaking is sometimes substituted anthryl groups were found to interact with AgNO3 in observed in the aforementioned processes of self-assembly in the subphase and form chiral assemblies.286 In the case of two solution and gel systems. However, when the assembly of achiral achiral coumarin derivatives, 7-octadecyloxylcoumarin (7- molecules was studied at the air/water interface, it was found that CUMC18, 122) and 4-octadecyloxylcoumarin (4-CUMC18, this kind of symmetry breaking is quite general in nature. Thus, 123) substituted at different positions, the Langmuir−Schaefer achiral molecules could be fabricated into optically active (LS) film of 4-CUMC18, which has larger steric hindrance,

Figure 86. Formation of optically active supramolecular gels with chiral nanostructures and optical activity by the hierarchical self-assembly of achiral 119. Reprinted with permission from ref 284. Copyright 2014 John Wiley & Sons.

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Figure 87. (A) Chemical structures of NpImC17 (120), and the model for the formation of the chiral NpImC17−Ag(I) coordination assemblies. (B) Circular dichroism (CD) (A) and UV (B) spectra of a 10-layer NpImC17 LB ﬁlm transferred from the pure water surface (a) and a 20-layer Ag(I)− NpImC17 LB ﬁlm (b). Reprinted with permission from ref 39. Copyright 2003 American Chemical Society.

Figure 88. (A) Structure of achiral amphiphilic barbituric acid 121. (B) AFM images of one-layer LB films deposited at various surface pressures at 20 °C (a) and (b) 7, (c) 20, and (d) 30 mN/m after the inflection point. Reprinted with permission from ref 285. Copyright 2004 American Chemical Society. showed supramolecular chirality, while 7-CUMC18 did not (Figure 89).287 The azobenzene derivatives can easily undergo trans−cis isomerization; thus, the aggregation behavior of two azobenzene isomers is often different. Liu et al. found that the trans form of the achiral azobenzene derivative 4-octyl-4′-(5-carboxypentamethyleneoxy) azobenzene (trans-C8AzoC5, 124) showed a strong Cotton effect in its LB films, while the assemblies of cis-C8AzoC5 did not (Figure 90).288 The aggregation of porphyrin derivatives can be one of the most attractive models for understanding supramolecular assemblies. Different molecular packing of porphyrins can lead to H or J aggregation, which have different properties. Achiral porphyrins can also assemble into helical nanostructures with supramolecular chirality. For example, through the air/water interfacial assembly of TPPS (67) with achiral positively charged amphiphiles, Zhang and Liu et al. obtained chiral assemblies, as shown in Figure 91. The emergence of the supramolecular chirality could be due to the helical stacking of the TPPS units in Figure 89. (A) Molecular structures of achiral coumarin derivatives. (B) 289 a confined two-dimensional interface. CD spectra of 4-CUMC18 (a, c) and 7-CUMC18 (b) LS films The packing mode of the supramolecular assembly of achiral transferred from the water surface at 20 mN/m. Reprinted with porphyrins can be modulated upon protonation of their central permission from ref 287. Copyright 2007 American Chemical Society. nitrogen atoms. If such protonation was performed in situ at the air/water interface, many achiral porphyrins can assemble into

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Figure 90. (A) Structure of 4-octyl-4′-(5-carboxypentamethyleneoxy)- azobenzene (trans-C8AzoC5, 124). (B) Formation of optically active and inactive supramolecular assemblies from trans-C8AzoC5 (top, 124) and cis-C8AzoC5 (bottom), respectively. Reprinted with permission from ref 288. Copyright 2006 American Chemical Society.

Figure 91. (A) CD spectra of 20-layer LS films of (a) ODA, (b) CTAB, and (c) DOAB with TPPS transferred at 30 mN/m. (B) Schematic illustration of TPPS/amphiphile coassemblies and the J aggregation of TPPS. Reprinted with permission from ref 289. Copyright 2003 American Chemical Society. Figure 93. (A) Structure of amphiphilic diacetylene derivative (tricosa- − fi 290 10,12-diynoic acid, TDA, 125). (B) UV vis and CD spectra of LB lms optically active LB films, as illustrated in Figure 92. Similarly to deposited from in situ photopolymerized PDA films (a) from pure water fi these protonated porphyrins, introduction of the metal ions into and (b) from Cu(NO3)2. (C) TEM images of PDA lms in situ the centers of the porphyrin can also cause some of the achiral polymerized on pure water. Reprinted with permission from ref 292. porphyrins to form chiral assemblies at the air/water interface.291 Copyright 2002 The Royal Society of Chemistry. Generally, the chiral assemblies obtained from achiral molecules at the air/water interface are not so stable. Thus, if the air/water interface is used as the platform to form the chiral assemblies first and then introduce more covalent bonds to the systems, the stability can be increased. One of the best ways is the transform through the topochemical polymerization. For example, Liu at al. found that achiral amphiphilic diacetylene derivatives 125 can form chiral assemblies at the air/water interface, and these diacetylene derivatives can be polymerized upon photoirradiation. Interestingly, the photopolymerized organized molecular films of polydiacetylene showed strong optical activity as well as helical nanostructures (Figure 93).292 Zou et al. synthesized different achiral azobenzene-substituted diacetylene monomers (Figure 94) and studied the air/water interfacial assembly of these molecules.293 The results show that only the achiral diacetylene monomer containing one hydro- Figure 94. Molecular structures of the three azobenzene-substituted phobic chain can form chiral assemblies at the air/water interface. diacetylene monomers. When these LB films of compounds 126 and 127 were irradiated

Figure 92. Protonation of achiral water-insoluble free-base porphyrins can lead to optically active supramolecular assemblies at the air/water interface. Reprinted with permission from ref 290. Copyright 2007 John Wiley & Sons.

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Figure 95. (A) Molecular structures of different porphyrin derivatives. (B) Possible stacking of the porphyrin TPPA3 (133) and TPPA0 (129) assembly on the air/water interface. Reprinted with permission from ref 296. Copyright 2008 The Royal Society of Chemistry. by left- or right-handed circularly polarized UV light (CPUL), and Guo et al. designed a series of TPP-based achiral porphyrins supramolecular chirality with polymerization of diacetylene with different alkyl chains and hydrophilic substituents and groups can be detected. Interestingly, for the LB films formed investigated the supramolecular chirality within the LB films as by compound 128 containing two hydrophobic diacetylene well as in situ in the monolayers. Depending on the number of chains, polymerization cannot be achieved, even though alkyl chains, different supramolecular chirality can be detected supramolecular chirality can be detected from the CD signals from the LB films of these porphyrin derivatives (Figure 95).296 of azobenzene chromophores. Furthermore, the production of the supramolecular chirality in In addition to amphiphilic diacetylenes, the chiral polymer the monolayers of these porphyrins at the air/water interface was from achiral phthalocyanine derivatives was also achieved. The studied by means of second-harmonic generation linear achiral phthalocyanine building block silicon dichroism (SHG-LD). Interestingly, achiral TPPA2a (131) was 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocya- demonstrated to form chiral assemblies by in situ SHG-LD nine dihydroxide (Pc 1) was spreading at the air/water interface measurements even though the LS film did not have clear CD to form a monolayer and subsequently transferred onto solid signals. This is because the SHG-LD can provide more sensitive substrates to produce LS films with optical activity. Interestingly, signals than conventional CD spectra. In addition, a subtle upon heating at 180 °C in a high vacuum for 10 h, the chiral interaction at the air/water interface can affect the supra- assemblies within the LS films could be converted into chiral molecular chirality in the monolayers, which can be monitored 294 covalent polymers with the CD signals increased significantly. by the SHG-LD. For instance, using the SHG-LD technique, it This result indicated that the transform from the noncovalent to was found that Cu2+ ions in the subphase enhance the the covalent bond not only increased the stability of the chiral supramolecular chirality of the TPPA2a monolayer, while Zn2+ fi 297 assemblies but also ampli ed the supramolecular chirality of the ions inhibit the formation of chiral assemblies (Figure 96). systems in these interfacial films. In these above examples, for the supramolecular chirality formed by air/water interfacial assembly of achiral molecules, the CD spectra have always been measured in the corresponding LB films. Thus, one may doubt whether the supramolecular chirality originated from the deposition process for making LB films or merely originates from the assembly of achiral molecules on the water surface. Therefore, when achiral molecules were placed on the water surface, the in situ measurement of the supramolecular chirality could be very important. Recently, the SHG-LD technique has been developed for detecting supramolecular chiral assemblies in situ on water surfaces.295 It was confirmed that the supramolecular chirality is produced from the assembly Figure 96. In situ supramolecular chirality from the assembly of achiral of achiral molecules at the air/water interface. In addition, some TPPA2a (131), which can be modulated by adding different metal ions subtle effects of the chirality change of the systems can also be to the subphase. Reprinted with permission from ref 297. Copyright detected from in situ measurements. For example, Liu, Wang, 2014 American Chemical Society.

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5.5. Controlling Handedness of Supramolecular Chirality sulfophenyl)porphyrin (135). Within the assemblies, the achiral Constructing supramolecular chiral assemblies from achiral porphyrins were found to undergo J aggregation. In addition, molecular building blocks is a fascinating possibility. A further AFM measurements on the stirred solutions showed helical nanoribbons, which indicated supramolecular chirality at the step would be to control the handedness of supramolecular 299 chirality containing only achiral molecular building blocks. nanoscale (Figure 98). Unfortunately, there are still very few methods available for With regard to vortex motion or stirring introduced into the supramolecular chiral assemblies, achiral porphyrins are controlling handedness. However, we can gain hints from several π−π important cases. Herein, we focus on the effects of mechanical important model molecules. In this context, the interactions force and circularly polarized light. between the aromatic rings play very important roles for forming 5.5.1. Vortices and Spin Coating. For the formation of chiral assemblies. Aida and co-workers studied dendritic zinc porphyrins with two carboxylic acid groups (136). These supramolecular chiral assemblies from achiral molecular building π−π blocks, the macroscopic force field may play a very important porphyrins could form J aggregates in CHCl3 through role. It is worth mentioning that the handedness of chiral interactions from the porphyrin rings and also from the aromatic assemblies formed by achiral molecules can be controlled by the rings within their dendritic substituents. Moreover, hydrogen direction of vortex stirring. This is the advantage of symmetry bonding between the carboxylic acid groups can also be a very breaking with vortex stirring or spin coating. Considering that the important driving force for forming J aggregates. The spin coating of the J-aggregate solutions could produce very stable chiral information is exhibited below the nanoscale, while fi mechanical perturbations occur on macroscopic scales, the optically active lms. Interestingly, the handedness of the 298 supramolecular chirality of the films can be controlled by modulation of supramolecular chirality is remarkable. 248 With regard to this issue, the first significant work was changing the direction of the spin coating (Figure 99). published by Ribóand co-workers.249 The molecular building Aside from spin coating, the stirring of benzene solutions of blocks for these studies were various achiral diprotonated meso- achiral dendritic zinc porphyrins with two carboxylic acid groups sulfonatophenyl-substituted porphyrins. A vortex motion with (136) can also produce supramolecular chirality. Strong CD either clockwise or anticlockwise direction was generated using a signals can be detected from the porphyrin solution upon rotary evaporator with different directions of rotation. Upon stirring. Interestingly, when the direction of stirring was changed from clockwise (CW) to counterclockwise (CCW), the opposite rotary evaporation, the achiral diprotonated meso-sulfonato- fi phenyl-substituted porphyrins could form assemblies with CD signals were detected. For this system, the nano bers formed optical activity. Interestingly, the chirality sign of these from the assembly of porphyrins via J aggregation play very assemblies can be selected by vortex motion during the important roles. Some of the observed chiroptical activity could aggregation process, as confirmed by circular dichroism (CD) also originate from the macroscopic helical alignment of fi 300 spectra. In this context, the aggregation of the porphyrins could nano bers (Figure 100). be modulated to show different supramolecular chirality by For achiral porphyrins in aqueous solution under stirring, Purrello et al. carefully studied the changes of CD spectra of changing the rotational direction of the rotary evaporator (Figure 301 97). TPPS (67) aqueous solutions in cuvettes upon stirring. These ff The supramolecular chirality from the assembly of achiral authors found that by stirring in di erent directions the ff meso-sulfonatophenyl-substituted porphyrins under vortex mo- protonated TPPS could form two di erent J aggregates. The tion or stirring can be detected from the CD spectra and results showed that the CD signal of the solution inverted with an observed directly from TEM and AFM measurements. Ribóet al. increasing in stirring intensity when the direction of stirring was studied stirred solutions of 5-phenyl-10,15,20-tris(4- altered (Figure 101B). Remarkably, with stirring, assemblies of the porphyrin were deposited on the cuvette wall, and the chirality of the assemblies on the solid surface were different than those in solution. However, the directions of the CD signals of the porphyrin aggregations on the cuvette wall were also found to be dependent on the stirring direction.301 In some cases, strong CD signals can be detected from assemblies of achiral molecules, but the supramolecular chirality of these assemblies is still in doubt. Linear polarization properties, such as linear dichroism (LD) and linear birefringence (LB), can hamper the actual circular dichroism (CD). Images of the true CD signal can be obtained using Mueller matrix spectroscopy (MMS). For some assemblies, formed by using achiral molecular building blocks, using MMS may overcome the problems related to linear polarization properties. Okano and Yamashita et al. developed a two- component system to investigate the supramolecular chirality that originates from the assembly of achiral molecules.303 In this study, the achiral ionic oligomer (poly[pyridinium-1,4-diylimi- no-carbonyl-1,4-phenylene-methylene chloride], 138) formed hydrogels at high concentration. At lower concentrations in fi Figure 97. Achiral diprotonated meso-sulfonatophenyl-substituted water, the achiral ionic oligomer could form brous nanostruc- porphyrins form optically active assemblies upon rotary evaporation. tures with supramolecular chirality with vortex stirring. To Reprinted with permission from ref 249. Copyright 2001 The American remove the influence of the linear polarization properties for Association for the Advancement of Science. identifying the supramolecular chirality, another achiral dye

7358 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 98. (A) Structure of 5-phenyl-10,15,20-tris(4-sulfophenyl)porphyrin (135). (B) AFM images of assemblies of 135 showing the onset of folding in stagnant and stirred solutions. Reprinted with permission from ref 299. Copyright 2006 John Wiley & Sons.

Figure 99. (A) J aggregation of achiral dendritic zinc porphyrin (136) and macroscopic chirality from spinning. (B) Mechanism for the formation of chiral assemblies. Reprinted with permission from ref 248. Copyright 2004 John Wiley & Sons.

molecule, Congo Red (CR, 137), was added to the system. As a result, the real supramolecular chirality of the assemblies of achiral ionic oligomers, i.e., the chiral information, was transferred to the Congo Red. In this context, a strong CD signal from the Congo Red can be detected by Mueller matrix spectroscopy. Amazingly, due to the monosignate shape of the UV−vis spectra of Congo Red, which suggested the absence of chromophore coupling, the chirality of Congo Red was attributed to its single chromophore, instead of its aggregates.302 These results suggest that the supramolecular chirality can in fact be generated from an assembly of achiral molecules (Figure 102). Similar chiral assemblies from achiral two-component molecular building blocks were studied using circularly polarized luminescence. For this study, the concentration of the achiral ionic oligomer (138) was increased to form hydrogels, and Rhodamine B dye (139) was embedded into the hydrogel. Vortex stirring was performed during the slow cooling from the sol to the gel. The supramolecular chirality was studied by circularly polarized luminescence (CPL) from Rhodamine B dye, and supramolecular chirality was detected in these hydrogels. When the samples were heated to change the gel to a solution, the resulting supramolecular chirality disappeared. Interestingly, these results also show that the sense of the CPL could be Figure 100. Molecular structure of achiral dendritic zinc porphyrin controlled by switching the stirring direction from clockwise 303 (136) and the corresponding supramolecular chiral assembly generated (CW) to counterclockwise (CCW) (Figure 103). from vortex ﬂows. Reprinted with permission from ref 300. Copyright The mechanism of the formation of chiral assemblies formed 2007 John Wiley & Sons. by achiral molecular building blocks with vortex stirring has been

7359 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 103. (A) Structure of achiral ionic oligomer (138) and Rhodamine B dye (139). (B) (a) Photoluminescence (PL) spectra λ × −5 ( ex = 520 nm) of Rhodamine B (139) (1.6 10 M) in an aqueous solution of 138 (0.6 wt %). (B) Circularly polarized luminescence spectra (CPL) of the solution with clockwise (CW, red) and counterclockwise (CCW, blue) stirring at 1000 rpm and unstirred Figure 101. (A) Schematic structure of the protonation of TPPS. (B) (black). Reprinted with permission from ref 303. Copyright 2011 John Schematic representation of the possible eﬀects of the stirring on a Wiley & Sons. racemate (A). CW (B) and CCW (C) stirring favor Δ and Λ J aggregates, respectively. Reprinted with permission from ref 301. Copyright 2010 John Wiley & Sons. porphyrins (Figure 104A) can be a hierarchical noncovalent polymerization process preceded by a critical nucleation stage. In this case, the primary nucleation is a very slow process, while the secondary nucleation stage is much faster. During the primary nucleation process, signiﬁcant enantiomeric excesses can be obtained from the formation of a few primary nuclei, and the vortex stirring controlled the secondary nucleation by assisting

Figure 102. Chiral induction to an achiral molecule (Congo Red dye, 137) from the chiral assembly induced by an achiral oligomer (138) was demonstrated by Mueller matrix spectroscopy analysis of the optical polarization properties. The true CD image of the clockwise (top) and counterclockwise (bottom) stirred solution of achiral ionic oligomer 138 are shown. Reprinted with permission from ref 302. Copyright 2011 Figure 104. (A) Molecular structures of diﬀerent achiral diprotonated John Wiley & Sons. meso-sulfonatophenyl-substituted porphyrins. (B) CD spectra of μ H4TPPF5S3 (140) (10 m, HCl 0.1 M) J aggregates obtained under vigorous-shaking conditions (6 h, full line) and in magnetically stirred studied by Ribóet al. Their results show that the assembly of conditions (24 h, dashed line). Reprinted with permission from ref 304. diﬀerent achiral diprotonated meso-sulfonatophenyl-substituted Copyright 2012 John Wiley & Sons.

7360 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review the growth of the primary chiral nuclei.304 This mechanism can be demonstrated from the difference between the CD spectra measured under vigorous shaking and magnetic stirring (Figure 104B). Thereby, chiral assemblies can be obtained from achiral building blocks. Although vortex stirring can help achiral molecules to form chiral assemblies, both rotational and magnetic forces provided more efficient modulation of the supramolecular chirality during the assembly of achiral molecules. This issue is fully demonstrated by Scolaro et al. using the aggregation of an achiral porphyrin, tris(4-sulfonatophenyl) phenylporphyrin (TPPS3, 135), to form chiral assemblies under both rotational and magnetic forces.305 Interestingly, the influence of the magnetic forces can be tuned to an effective gravity which plays a very important role, while the magnetic orientation of the aggregates is also essential (Figure 105). Moreover, this study also suggested that application of rotational and magnetic forces during the primary nucleation process is sufficient to form chiral assemblies. Figure 106. (a−d) Self-assembly leading to the formation of a two- dimensional emulsion featuring chirally resolved domains. trans- Azobenzene surfactant molecules (b, 141), resulting from the spontaneous isomerization of the cis isomer (a), self-assemble at the air/water interface into segregated chirally resolved circular domains surrounded by the cis-rich matrix (d); (e) molecular structures of achiral trans-azobenzene surfactant 141 and its chiral analogues 142 and 143. (f) Coupling between the chiral modifier and the vortical flow. Reprinted with permission from ref 306. Copyright 2012 Nature Publishing Group.

molecular assemblies containing only achiral molecular building

Figure 105. Chiral self-assembly of TPPS3 under rotational and blocks were irradiated by circularly polarized light, chirality can magnetic forces, showing the relationship between the observed chirality be introduced into the systems. and the applied physical forces. Reprinted with permission from ref 305. Oriol et al. studied liquid crystals with chiral organization Copyright 2012 Nature Publishing Group. formed by achiral molecules upon irradiation with circularly polarized light (Figure 107). The achiral building block used was Nevertheless, supramolecular chirality originating from a polymer with methoxyazobenzene groups in the side chain. assemblies of achiral molecular building blocks upon vortex When thin films of the nematic glassy phase of this polymer were stirring has been generally acknowledged. On the other hand, it is irradiated with 488 nm circularly polarized light (CPL), a chiral well known that some chiral dopants can coassemble with achiral arrangement of the azobenzene groups with helical nanostruc- molecular building blocks to form chiral supramolecular tures resulted from the selective reflection of visible light. aggregations. When the chiral species and the vortex stirring Furthermore, the CPL-induced supramolecular chirality in the meet together, which effect will be dominate? Sagueś and copolymer was confirmed by CD spectra and vibrational circular workers studied the chiral competition between vortex rotating dichroism (VCD) spectra.201 and chiral dopants.306 In their study, achiral trans-azobenzene Zou et al. synthesized an achiral amphiphilic azobenzene surfactant 141 was used as the primary building block, while its derivative (144), which can assemble into a monolayer on the chiral analogues 142 and 143 were the chiral dopants. Vortex surface of water (Figure 108A). Supramolecular chirality with rotating was performed to form monolayer assemblies, and the helical packing of azobenzene derivatives can be introduced into supramolecular chirality was studied using Brewster angle these monolayers by irradiation with CPL. The resulting microscope (BAM) measurements. The results showed that handedness of the supramolecular chirality of these monolayers the influence of the vortex rotation can be comparable to that of was controlled by the handedness of the CPL. This is an excellent the chemical induction processes. However, the influence of the example that supramolecular chirality from an achiral component vortex rotation can be more easily controlled by changing the be controlled by CPL. Furthermore, these chiral azobenzene direction or speed of stirring (Figure 106). monolayers can be used as a chiral template, which supports the 5.5.2. Circularly-Polarized Light. Circularly polarized light contention that the diacetylene derivatives can form chiral (CPL) can be regarded as a type of energy that contains chiral polydiacetylene under normal UV irradiation (Figure 108B).309 information. When achiral supramolecular assemblies are Moreover, Zou et al. also prepared discotic hydrogen-bonding irradiated by circularly polarized light, chiral information from complexes of diacetylene (DA) units. When these complexes the illumination can be transferred to the systems to form were irradiated by circularly polarized ultraviolet light within the supramolecular chiral assemblies. liquid-crystal phase, helical polydiacetylene can be obtained For more than a century, scientists have considered CPL as a (Figure 109A).310 Most interestingly, by using linearly polarized possible cause of natural homochirality. Because CPL has been light irradiation together with a magnetic field, the enantiose- observed in star formation, CPL could be the source of chiral lective polymerization of these complexes can also be achieved information during the origins of life.307,308 When supra- within the liquid-crystal phase (Figure 109B).311 In this case, the

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control on the supramolecular chirality in self-assembled systems. 5.5.3. Surface Pressure. Indeed, many supramolecular chiral assemblies can be obtained from different achiral molecular building blocks via air/water interfacial assembly. However, for this method, a great remaining challenge is the control of the macroscopic chirality of these systems. Normally, the handedness of the chirality of the assemblies obtained from the air/water interfacial assembly from achiral molecular building blocks is random. In order to control the macroscopic chirality of air/ water interfacial assembly from achiral molecules, a new method using unilateral compression geometry has been developed. In this approach, only one of the two movable Langmuir barriers was used for the compression. Using this kind of unidirectional compression, Chen and Liu et al. found that the assemblies deposited from the mirror regions of the LB trough can display mirror-image macroscopic chirality.312 During the compression, vortex-like flows are suggested to be generated, and the direction of this compression-generated vortex-like flow can determine the macroscopic chirality of the formed assemblies (Figure 110). 5.6. Self-Assembly of Racemic Systems Molecular or supramolecular chirality plays a very important role in many self-assembly processes. In contrast to constructing chiral assemblies from pure achiral building blocks, the self- assembly manner of racemic systems is unique and largely dependent on the mixing of chiral enantiomers with different handedness. Figure 107. Model for circularly polarized-light introduced chirality in First, the mixing of an enantiomer and its mirror image may azomaterials. Reprinted with permission from ref 201. Copyright 2007 meet with problems such as phase separation, so that the John Wiley & Sons. situation of these systems can be very complicated. In general, for self-assembly of racemic systems, regardless of the chiral molecules or the chiral nanostructures, some enantiomers can preferentially aggregate with themselves, while other enantiomers might prefer to form complexes with their mirror images. Therefore, self-recognition can be established when some enantiomers recognize themselves and form homochiral assemblies, while in the case of an R enantiomer forming a complex with its S enantiomer, this kind of self-sorting has been regarded as self-discrimination to form heterochiral coassem- − blies.313 317 For the self-assembly of racemic systems, in most cases, there are much more possibilities for forming homochiral assemblies than that of forming heterochiral assemblies. However, because the energy difference between the enantiomers with different handedness is quite small, phase separation in racemic systems is not very significant.72 Würthner et al. studied the chiral self-sorting of perylene bisimide (PBIs) assemblies.318 In this case, different chiral PBI molecules containing oligoethylene glycol bridges have been synthesized, and the coassembly of these enantiomers with Figure 108. (A) Molecular structures of achiral amphiphilic azobenzene different handedness was investigated. The results showed that derivative 144 and achiral diacetylene derivative 125. (B) Schematic chiral self-recognition always prevails over self-discrimination for illustrations of the generation of chirality from PTDA/DBA hybrid films these PBIs assemblies. The coassembly of this system tends to with the azobenzene-containing monolayer irradiated by left- and right- form homochiral aggregates (Figure 111). handed CPL. Reprinted with permission from ref 309. Copyright 2011 Percec et al. synthesized chiral hat-shaped dendronized The Royal Society of Chemistry. cyclotriveratrylene (CTV) and studied the racemic assembly of these molecules. The results showed that these molecules can handedness of the helical polydiacetylene chains can be form heterochiral assemblies at low temperature. When these controlled by changing the orientation of the linearly polarized systems were heated below 60 °C for 2 h, homochiral assemblies light and the magnetic field. The enantioselective recognition of formed upon self-sorting (Figure 112).319 D-orL-lysine using these helical polydiacetylene assemblies was Although chiral self-sorting can be very significant for a achieved. Combination of these physical vectors showed effective racemic assembly, systems containing enantiomers with different

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Figure 109. (A) Structure of discotic hydrogen-bonding complex of diacetylene (DA) units. (B) Asymmetric polymerization of the corresponding complex by using linearly polarized light irradiation and a magnetic ﬁeld. Reprinted with permission from ref 310. Copyright 2014 The Royal Society of Chemistry. Reprinted with permission from ref 311. Copyright 2014 Nature Publishing Group.

Figure 110. Schematic illustrations of the chirality selection phenomenon induced by compression-generated vortex-like flow. Reprinted with permission from ref 312. Copyright 2011 John Wiley Figure 111. (A) Structures of chiral perylene bisimides (PBIs) & Sons. containing oligoethylene glycol bridges (146). (B) Energy diagram of the different supramolecular diastereomers of PBIs. Reprinted with permission from ref 318. Copyright 2011 American Chemical Society. handedness can still show many new features, of which the nanostructures and properties are different from corresponding Changing the ratio of the pseudoenantiomers produced various pure chiral assemblies.67 For example, for the formation of gel systems (Figure 113).321 supramolecular gels, the racemates are usually believed to be With the exception of the formation of supramolecular gels, poor gelators. However, in some special cases, only racemates the self-assembly of racemic molecular building blocks can also were found to form supramolecular gels while the corresponding improve the macroscopic properties of soft matter. For example, enantiomer cannot.320 the mechanical properties of peptide hydrogels can be improved For example, Yamaguchi et al. found that homochiral upon racemic self-assembly. Schneider et al. found that the pseudoenantiomeric ethynylhelicene oligomers (147) cannot hydrogels prepared from racemic β-hairpins showed non- form organogels, while the mixture of P oligomer and M additive, synergistic enhancement in material rigidity compared oligomer can form two-component organogels in toluene. to gels prepared from either pure enantiomer. Therefore, racemic

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Figure 114. (A) (a) Assembly mechanisms for enantiomeric peptides leading to the formation of a fibrillar network that defines hydrogelation. (b) Sequences of enantiomers MAX1, DMAX1, and the nonisomeric control peptide. D-Amino acid residues are italicized. (B) Dynamic time sweep rheological data measuring the storage moduli of 1 wt % Figure 113. (A) Structures of pseudoenantiomeric ethynylhelicene hydrogels containing pure MAX1 (□), 3.1 MAX1.DMAX1 (▲), 1.1 oligomers. (B) Minimal thermoreversible gelation concentration in MAX1.DMAX1 (●), 1.3 MAX1.DMAX1 (▼), and pure DMAX1 (⧫). millimolar for 1.1 mixtures of (M)-n and (P)-n (n =1−6) in toluene. S = Reprinted with permission from ref 322. Copyright 2011 American soluble at room temperature (solubility > 5 mm), C = crystallization. Chemical Society. Reprinted with permission from ref 321. Copyright 2010 John Wiley & Sons. The gelation properties, supramolecular chirality, and nano- self-assembly can be a powerful tool for developing novel structures of the racemic hydrogels can be regulated by changing functional soft matters (Figure 114).322 molar ratios of different molecular building blocks (Figure Wang and Liu et al. studied the coassembly of the glutamic- 115).323 acid-based bolaamphiphile racemates with melamine. In this In addition to forming supramolecular gels, the mixing of system, the coassembly of melamine with pure enantiomeric enantiomers has also been used to tune the properties of different glutamic-acid-based bolaamphiphile (HDGA, 15b) cannot form supramolecular assemblies. Oda et al. studied the twists and gels. The assembly of the glutamic-acid-based bolaamphiphile nanotubes formed from the coassembly of nonchiral dicationic n- racemate produced only precipitates. Mixing the glutamic-acid- 2-n Gemini amphiphiles with chiral tartrate anions. They found based bolaamphiphile racemate with melamine produced good that the morphologies of the assemblies, such as the twist pitch of supramolecular gels. Remarkably, the racemic hydrogels showed the ribbons, can be continuously modulated by varying the a lower CGC value, enhanced mechanical rigidity, and dual pH- enantiomeric excess of tartrate anions. For instance, adding 10 responsive ability compared to the pure enantiomer hydrogels. mol % of the opposite enantiomer of tartrate anions led to a 15%

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Figure 115. (A) Molecular structure of the glutamic-acid-based bolaamphiphile (HDGA, 15b). (B) Schematic presentation of the racemic hydrogels formed by the coassembly of HDGA racemate and melamine. (C) Gelation properties of the (L + D)-HDGA mixtures and the (L + D)-HDGA/melamine mixtures with different ee values. Reprinted with permission from ref 323. Copyright 2014 American Chemical Society. increase in the diameter of assembled supramolecular nanotubes (Figure 116).69 Liu et al. synthesized enantiomeric L-orD-glutamic-acid-based lipids (1) and investigated the self-assembly of these chiral enantiomers. Although both L- and D-enantiomeric molecules self-assembled into ultralong nanotubes, mixing D- and L- enantiomers with different molar ratios further changed the nanostructures consecutively from helical nanotubes to nano- fl 53 Figure 116. (A) Coassembly of nonchiral dicationic n-2-n Gemini twists to at nanoplates (Figure 117). amphiphiles with chiral tartrate anions. (B) Transition from twisted In most cases, the self-assembly of chiral molecules can lead to ribbons to helical ribbons and then to tubules observed by TEM helical nanostructures, while racemates usually assemble into flat measurement. Reprinted with permission from ref 69. Copyright 2007 nanostructures. However, this situation has exceptions. Liu et al. American Chemical Society. synthesized the enantiomeric L-orD-alanine derivatives (AlaC17, 100), and the self-assembly and gelation properties of the corresponding individual enantiomers and the racemates were investigated. The self-assembly of individual enantiomers of AlaC17 can form only flat nanostructures, even though both L- AlaC17 and D-AlaC17 can form gels in different organic solvents. Interestingly, racemic AlaC17 was found to self-assemble into beautiful twisted ribbons. Moreover, these twists are very sensitive to a slight enantiomeric excess of many other amino acids, showing remarkable macroscopic chirality. Therefore, these racemic assemblies can be used for the discrimination of − various amino acid derivatives (Figure 118).230 Figure 117. (A) Correlative plot of the vibration bands of N H, amide I, and amide II and the d spacing of the nanostructures in the mixed gels 6. APPLICATIONS OF SUPRAMOLECULAR CHIRALITY against the enantiomeric excess value (ED/L). (B) Proposed mechanism for the formation of various nanostructures upon mixing enantiomeric L- Recently, self-assembled chiral supramolecular systems have or D-glutamic-acid-based lipids. Reprinted with permission from ref 53. attracted greater attention due to the many potential applications Copyright 2010 John Wiley & Sons. of forms of soft matter. Such applications can further enhance our understanding of supramolecular chirality. In particular, new properties that single chiral molecules do not have emerge from − supramolecular chiral systems. Certainly, for many applications pressed on different materials within diverse scales.11,324 326 of functional soft matters, supramolecular chirality plays the Herein, we address the most prominent fields for the application critical role, which is also dependent on the properties and of supramolecular chirality, such as chiral recognition, sensing, characteristics of the supramolecular chiral information ex- and catalysis. Some optical devices based on supramolecular

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Figure 118. (A) Molecular structure of the enantiomeric alanine derivatives AlaC17 (100). (B) Schematic illustration of the molecular packing for a single enantiomer and the racemate. The backgrounds are the SEM images of the transparent hexane gel formed by L-AlaC17 and the twisted ribbons formed by the racemic mixture. (C) Intensity of the CD signals (centered at 309 nm) of the racemate upon addition of 2 mol % of various amino acid derivatives. LBG is a glutamic acid derivative with two long alkyl chains. Reprinted with permission from ref 230. Copyright 2013 John Wiley & Sons. chirality and biological applications of chiral soft matters will also be demonstrated by NMR, MS, light-scattering, and calorimetric be discussed. measurements. 6.1. Supramolecular Chiral Recognition and Sensing Chiral recognition is a very important issue in supramolecular chemistry. In general, interactions between different enantiomers and another chiral molecule can produce totally different results. These different interactions can be detected by spectral measurements or determination of their crystal struc- − tures,318,327 329 representing the chiral recognition. Thus far, studies on host−guest chemistry related to chirality have focused − on chiral recognition,330 337 and many excellent works have been published. For example, work on chiral recognition using cyclodextrinsisveryfamousandhasbeenextensively − reviewed.338 346 On the other hand, chiral recognition not only describes the interactions between different chiral molecules but also can represent the interactions between chiral supramolecular assemblies and chiral molecules. Herein, we focus on the chiral Figure 119. Chiral recognition of (S)-2-methylbutylamine from achiral − recognition of supramolecular assemblies.347 349 Thus, when cucurbiturils (CBs) incorporated with chiral 2-methylpiperazine. molecules with contrary chirality interact with chiral supra- Reprinted with permission from ref 351. Copyright 2006 American molecular assemblies, the chiral recognition shows some new Chemical Society. features, which are different from the case of chiral recognition between different chiral molecules. For example, when molecules Although chiral recognitions based on cyclodextrins have been with different chirality are mixed with chiral supramolecular gels, widely investigated, supramolecular polymers constructed from chiral recognition can be achieved from the change in appearance cyclodextrins also showed interesting recognition properties. For or rheological properties of the related supramolecular gels. example, Yashima et al. synthesized polymers containing β- More importantly, these chiral recognitions, detected as sol−gel cyclodextrin substituents and studied enantioselective gelation of transformation or color changes of the supramolecular gels, are these polymers in response to the chirality of a chiral amine. It has visible to the naked eye.350,192 been found that (S)-1-phenylethylamine can help the polymers Although chiral recognition related to the host−guest to form organogels but that (R)-1-phenylethylamine cannot.352 chemistry of cyclodextrin has been widely investigated, chiral Shinkai et al. incorporated 4,4′-biphenyldicarboxylic acid into recognition based on highly symmetrical cucurbituril molecules cyclodextrin (CD) as a bridging ligand, which further interacted is still intriguing. Thus, when achiral cucurbiturils form a very with TbIII to form polyrotaxane-type metallosupramolecular stable complex with chiral molecules, the corresponding polymers. Due to the chirality of cyclodextrin as well as the strong assemblies are able to completely discriminate other enan- fluorescence of rare earth complexes, the recognition of small tiomers. This work has been reported by Inoue and co- chiral molecules by this supramolecular polymer was achieved workers.351 In this study, achiral cucurbiturils (CBs) were based on both fluorescence and circular dichroism spectral incorporated into (R)- or (S)-2-methylpiperazine, and the changes.353 resulting complex showed significant enantiomeric discrim- One of the most important advantages of the chiral ination of various chiral organic amines, such as (S)-2- recognitions from different chiral soft matters can be the methylbutylamine (Figure 119). The chiral recognition could alteration of their macroscopic properties, which also can be

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Figure 120. (A) Schematic illustration of the fabrication of a TbIII-based supramolecular polymer. (B) Fluorescence (ex. 270 nm) and CD Figure 122. Visual chiral recognition of binap through enantioselective spectra before and after adding D-orL-tartaric acid (final concentration; III metallogel collapsing. Reprinted with permission from ref 355. L, Tb , and D/L-tartaric acid, 10 μm, α-CD, 300 μm). (dotted line) L + III Copyright 2011 John Wiley & Sons. α-CD + Tb , (dashed line) D-tartaric acid, and (solid line) L-tartaric acid. Reprinted with permission from ref 353. Copyright 2013 John Wiley & Sons. Liu et al. synthesized novel L-glutamide-based amphiphilic gelators containing Schiff base moieties and long alkyl chains (o- fi SLG, 8a and p-SLG, 8b). The self-assembly and gelation identi ed by the naked eye. Therefore, visual chiral recognitions ff can be realized. For example, in the case of supramolecular gels, properties of these amphiphiles with di erent metal ions have been investigated in many organic solvents. In particular, adding the recognition of chiral gelators can be achieved from simply fi identifying whether the systems are a “gel”. metal ions to the organogels can signi cantly change the self- assembled nanostructures and the spectral characteristics of For supramolecular gel systems, chiral recognition by the 2+ fi naked eye may be simply achieved from the sol−gel trans- these systems. Thus, Cu can transform the nano ber gel into a fi chiral twist, while adding Mg2+ ions enhanced the fluorescence of formation. The work of Pu et al. is the rst report of this 2+ phenomenon. They synthesized a Cu(II) terpyridine complex the gels. Remarkably, organogels containing Mg ions have very ′ good chiral recognition ability. For example, when D-orL-tartaric containing 1,1 -bi-2-naphthol (BINOL) substituents (148), fl which can form stable supramolecular gels in CHCl upon acid was introduced into the system, the uorescence quenching 3 processes of these organogels can be totally different (Figure sonication. Interestingly, some chiral amino alcohols were found 158 to change these gels into sols, while their enantiomers failed. 123). Therefore, the chiral recognition of this organogel to some chiral Liu et al. designed and synthesized a series of amphiphilic − molecules containing both glutamide moieties and long alkyl amino alcohols can be achieved from the sol gel transformation, ff which can be recognized by the naked eye. In this context, (S)- chains with di erent hydrophobic headgroups (Figure 5). The supramolecular assemblies of some of these amphiphiles can be phenylglycinol (0.10 equiv) can break the CHCl3 gel network, while (R)-phenylglycinol cannot. With chiral 1-amino-2- used for the recognition of chiral molecules. Except for the propanol, the same enantioselective gel-collapsing process can organogels prepared from o-SLG and p-SLG, the assemblies also be observed (Figure 121).354 based on quinolinol-functionalized L-glutamides (HQLG, 10) and metal ions were also found to have good chiral recognition capability.55 HQLG (10) can form complexes with different metal ions, such as Li+,Zn2+, and Al3+. Although these chiral complexes do not show a CD signal or chiral recognition properties in solution, they can form fluorescent metallogels with optical activity upon gelation in several organic solvents. The recognition of the small chiral organic molecules by these fluorescent metallogels can be detected by the changes in their CD spectra or fluorescence spectra. For example, metallogels formed by Zn2+/HQLG (10) ff fl Figure 121. Chiral recognition from enantioselective gel collapsing, complex showed a totally di erent uorescent color when which formed from a Cu(II) terpyridine complex. Reprinted with treated with (R,R)- or (S,S)-1,2-diaminocyclohexane. Therefore, permission from ref 354. Copyright 2010 American Chemical Society. using metallogels, chiral recognition can be achieved by the naked eye. It is believed that the self-assembled nanostructures Tu et al. synthesized a gelator containing steroidal substituents play very important roles for chiral recognition (Figure 124). and a platinum complex (149) and prepared metallogels from Porphyrins are very important molecular building blocks for the assembly of these gelators. A visual chiral recognition can be the study of chiral supramolecular assembly. Ihara et al. realized from the sol−gel transformation of the metallogels. synthesized an L-glutamide-functionalized zinc porphyrin (g- When (R)-binap was introduced into the system, the metallogels TPP/Zn, 150) and investigated the enantioselective recognition were destroyed. In contrast, (S)-binap cannot change the of different amino acids by assemblies thereof (Figure 125A). g- situation of metallogels (Figure 122).355 TPP/Zn (150) can form organogels in different organic solvents.

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Figure 125. (A) Schematic image of enantioselective recognition Figure 123. (A) Molecular structures and self-assembly of the through chirally ordered porphyrin assembly. (B) CD spectra of g-TPP/ amphiphilic Schiﬀ bases with metal ions. (B) (a) Assembly mechanism μ μ 2+ Zn (150) (50 M) with and without L- and D-His-OMe (50 M) in of o-SLG (8a) and the chiral twist in the presence of Cu ions. (b) o- cyclohexane at 20 °C. (C) Fluorescence spectra of the g-TPP/Zn (150) SLG (8a) formed a complex with Mg2+ ions and transferred the chirality μ μ 2+ (50 M) assembly with and without L- and D-His-OMe (50 M) in to the whole assembly. When D-tartrate approached the Mg ion, the D- ° 2+ cyclohexane at 20 C. Reprinted with permission from ref 356. enantiomer was favored. (c) Cu ions reacted with p-SLG (8b) and Copyright 2012 The Royal Society of Chemistry. caused gelation. Reprinted with permission from ref 158. Copyright 2012 John Wiley & Sons.

results show that the coassembly of L-lysine dendrons with chiral The CD spectra of the self-assembly of g-TPP/Zn in cyclohexane amines can form supramolecular gel fibers, and the chirality of showed strong optical activity. Interestingly, L-andD- the amine could control the corresponding diastereomeric enantiomers of many different α-amino acid derivatives can be complexes. When both R and S amines were incorporated into differentiated by organogels of g-TPP/Zn. For example, when L- the systems, the L-lysine dendrons could selectively coassemble histidine methyl ester (L-His-OMe) or D-histidine methyl ester with the R enantiomer, because the L-lysine dendron/R amine (D-His-OMe) was mixed with g-TPP/Zn cyclohexane gels, very complexes formed the most stable gel. Moreover, when R amine different CD and fluorescence spectra were obtained (Figure enantiomers were added to the supramolecular gels formed by 356 125B and 125C). the L-lysine dendron and pure S amines, the diffusion of R amines Smith and co-workers thoroughly investigated two-compo- and displacement of the original S amines from the “solid-like” 357 nent organogels based on the coassembly of an L-lysine dendron fibers can be detected (Figure 126B). These results suggest (151) with different amines (Figure 126A). For these systems, that the two-component organogels are very sensitive to the when the chiral amines were used for the coassembly, very molecular chirality of gelators and have great potential for chiral interesting chiral recognition phenomena can be detected. The recognitions.

Figure 124. (A) Molecular structures of the HQLG ligand molecule (10) and its metal complexes. (B) Chiral recognition brought about by the metallogels. Reprinted with permission from ref 55. Copyright 2013 American Chemical Society.

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substrates was determined using this porphyrin tweezer (Figure 127).368

Figure 127. Fluorinated porphyrin tweezer working as chiral sensor for the discrimination of the absolute conﬁgurations of amino alcohols. Reprinted with permission from ref 368. Copyright 2008 American Chemical Society.

Suzuki et al. synthesized secondary terephthalamide deriva- Figure 126. (A) Chiral gelation system of an L-lysine dendron (G2-Lys, tives containing four aryl blades (153a-H), which can be used as 151) and chiral amines (C6R/S). (B) Schematic of thermodynamically the host molecule for chiral sensing. The conformation of this controlled gel evolution upon addition of C6R to a gel made from G2- host can be changed from a nonpropeller anti form to a propeller- Lys (151) and C6S. Reprinted with permission from ref 357. Copyright shaped syn form with the formation of a complex with some 2014 American Chemical Society. chiral molecules, such as p-xylylenediammonium derivatives. Moreover, depending on the chirality of the guest enantiomers coassembled with secondary terephthalamide derivatives, the In principle, if the chiral recognition can be triggered by a very complex can be biased to prefer a particular handedness, with the small amount of chiral organic molecules, these assemblies with enhancement of CD signal. This secondary terephthalamide chiral recognition ability can be used as a chiral sensor. derivative can be used as chiral sensor for the discrimination of Compared with the recognition of chiral molecules, chiral the very important neurotransmitter, (−)-phenylephrine (Figure sensors based on supramolecular assemblies can be more difficult 128).369 to prepare. On the other hand, some wide-sense chiral sensors Some biomacromolecules can also be used for chiral are available based on organic molecules, polymers, and recognition and sensing systems. As one of the most important supramolecular assemblies.192,358,359 Herein, we discuss some biomacromolecules, DNA contains chiral information from the typical examples of chiral sensors that have been recently molecular to supramolecular level. Moreover, the stability of reported. DNA can be a huge advantage for building different devices for Very simple but efficient chiral sensors can be prepared from chiral sensing. the biaryls, which have been developed mainly by Wolf et Qu et al. constructed electrochemical DNA sensors for the al.360,361 For example, naphthalene derivatives containing chiral sensing. In this system, DNA molecules modified with thiol salicylaldehyde units and pyridyl N-oxide fluorophores have groups were covalently bonded to a gold electrode. When small been synthesized. This is the result of the salicylaldehyde unit chiral molecules interacted with DNA, changes in the electro- which can form a complex with amino alcohols with different chemical characteristics of the gold electrode could be detected, chirality and subsequently change the conformation of the thus demonstrating chiral sensing ability. It is worth mentioning molecules. The chiral sensing can be determined from the that this system offers great advantages for distinguishing chiral changes in the CD and fluorescence spectra. By using this system, metallosupramolecular complexes. For example, the authors the absolute configuration of many amino alcohols can be reported a three-way junction based on an E-DNA sensor. In this analyzed.42 study, the same palindromic DNA labeled with a redox-active Covalently connected porphyrin dimers, which were named methylene blue (MB) tag at the 5′-terminus was used as the porphyrin tweezer systems, have been thoroughly studied by support. Discrimination of chiral metallosupramolecular com- Nakanishi, Berova, and co-workers. These porphyrin tweezer plexes with an enantioselective recognition ratio of about 3.5 was systems can be used to determine the stereochemistry of many realized (Figure 129A and 129B).370 In another case, the authors − small chiral organic molecules.362 367 reported a similar electrochemical DNA (E-DNA) chiral sensor Indeed, some of the porphyrin tweezers can be very good based on the human telomeric G-quadruplex formation. An chiral sensors with determination of the absolute chirality of the enantioselective recognition on zinc-finger-like chiral metal- guest molecules. For example, Borhan et al. designed and losupramolecular assemblies reaches ratios higher than 5 (Figure synthesized an electron-deficient fluorinated porphyrin tweezer 129C).371 (152) and demonstrated that it is a good chiral sensor for the As previously described, supramolecular chiral assemblies have discrimination of many different small organic molecules good capabilities for the recognition of low molecular weight containing two chiral centers. Depending on the chirality of chiral organic molecules. Worthy of note are the chiral the small organic substrates, the changes in the CD spectrum of supramolecular assemblies that can recognize very small amounts the supramolecular assemblies formed by the porphyrin tweezers of chiral molecules. In this context, we will show some and chiral guests can be detected. In this case, the absolute representative examples of chiral sensing based on supra- stereochemical configuration of a variety of erythro and threo molecular assemblies.

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Figure 128. (A) Molecular structures of the secondary terephthalamide derivatives containing four aryl blades (153) and their diﬀerent conformations. (B) Schematic image showing the conformational changes from a nonpropeller anti form to a propeller-shaped syn form upon forming complexes with chiral guests. Reprinted with permission from ref 369. Copyright 2009 American Chemical Society.

Figure 129. (A) Schematic illustration of a three-way junction based on E-DNA for distinguishing chiral metallo-supramolecular complexes. (B) Changes of current from the E-DNA sensor showing discrimination of chiral metallosupramolecular complexes. (C) Schematic representation of the human telomeric DNA-based electrochemical DNA (E-DNA) sensor. Reprinted with permission from refs 370 and 371. Copyright 2012 The Royal Society of Chemistry.

Figure 130. Self-assembly of zinc porphyrin dimers can lead to box-shaped tetramers, which shows chiroptical sensing for limonene. Reprinted with permission from ref 372. Copyright 2007 John Wiley & Sons.

Porphyrins are very important building blocks for the study of substituents (154). This zinc porphyrin dimer can self-assemble supramolecular chirality. Tsuda and Aida synthesized a zinc into box-shaped tetramers. In a solution of asymmetric porphyrin dimer containing a rigid linker and pyridine hydrocarbons, such as limonene, the self-assembly of the zinc

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Figure 131. (A) Four-component reversible covalent assembly for secondary alcohol binding. OTf is trifluoromethanesulfonate (triflate). (B) Exploration of four-component assembly for chirality sensing and ee determination. CD spectra of 1-phenylethanol-induced assembly with different ee’s of the alcohol (from top to bottom −100%, −80%, −60%, −40%, −20%, 0%, 20%, 40%, 60%, 80%, 100%). Reprinted with permission from ref 373. Copyright 2011 Nature Publishing Group. porphyrin dimer can lead to a homochiral box-shaped tetrameric assembly. This self-assembled porphyrin box is enantiomerically enriched and optically active, showing that chiroptical sensing can be realized (Figure 130). From the CD spectra of the porphyrin box, the absolute configuration of limonene can be determined. Interestingly, in the case of very small enantiomeric enrichment of limonene, extremely large molecular ellipticities of the porphyrin boxes were detected.372 Chiral sensing based on multicomponent assemblies containing reversible covalent bonding are also worth mentioning. Anslyn et al. constructed a four-component reversible assembly containing carbonyl activation and hemiaminal ether stabilization (155) (Figure 131A). In this system, reversible binding of the monoalcohol has been achieved. Moreover, the tetradentate ligand of the assembly renders close incorporation of secondary alcohols. By binding and exchange of chiral alcohols, chiral sensing can be demonstrated by CD spectral measurements (Figure 131B). This chiral sensor can be used for determination of the enantiomeric excess of mixed chiral alcohols with different ee values (Figure 131B).373 A chiral sensor with the ability to differentiate many different chiral molecules, such as amino acids, peptides, proteins, and even some aromatic drugs, has been developed by Biedermann and Nau. These systems are based on ternary complexes formed between the macrocyclic host cucurbit[8]uril, dicationic dyes, and chiral aromatic analytes (Figure 132A). Although both cucurbit[8]uril and dicationic dyes are achiral, the chirality of the aromatic analytes with low micromolar concentrations in water Figure 132. (A) Schematic illustration of the chiral sensing based on the can be detected by the changes in the CD spectra. Remarkably, ternary complexes between the macrocyclic host cucurbit[8]uril, by using these chiral sensors, peptide sequences can also be dicationic dyes, and chiral aromatic analytes. (B) Examples of reaction recognized. Most interestingly, since the chiral sensors are monitoring. Reprinted with permission from ref 374. Copyright 2014 constructed via noncovalent interactions with good reversibility, John Wiley & Sons. real-time monitoring of the chirality of analytes can be achieved. Therefore, for certain enzyme-catalyzed chemical reactions, the rate of reactions, the yield of products, and the ee values of the For example, Kadodwala et al. built an ultrasensitive chiral sensor products can be detected in real time by measuring the CD based on chiral metamaterials, which had the potential to spectra (Figure 132B).374 discriminate between large biomolecules with similar levels of Although we would like to focus on supramolecular chirality sensitivity and subtle structural differences at pictogram within self-assembled systems in this review, we still need to quantities. In this study, the optical excitation of plasmonic cover some of the chiral metal nanomaterials in this portion to planar chiral metamaterials can generate superchiral electro- fully address chiral sensing. Because of the free electrons on the magnetic fields, which are highly sensitive for the detection of surface, the plasmonic absorption or CD spectra from the chiral chiral peptide nanostructures. The sensitivity of this chiral sensor metal nanomaterials can be very sensitive for detecting the was found to be up to 106 times greater than that of the optical − interactions with other chiral molecules,375 380 and chiral metal polarimetry measurements. The largest differences were nanomaterials can be constructed as ultrasensitive chiral sensors. observed for proteins with high β-sheet content. The system

7371 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 133. Changes induced in the chiral plasmonic resonances of the planar chiral metamaterials (PCM) are readily detected using CD spectroscopy. (A) CD spectra collected from PCMs immersed in distilled water. (B) Influence of the adsorbed proteins hemoglobin, β-lactoglobulin, and thermally denatured β-lactoglobulin on the CD spectra of the PCMs. (C) Hemoglobin (top) and β-lactoglobulin (bottom) (α-helix, cyan cylinder; β-sheet, ribbons), shown adopting a well-defined arbitrary structure with respect to a surface. The figure illustrates the more anisotropic nature of adsorbed β- lactoglobulin. Reprinted with permission from ref 381. Copyright 2010 Nature Publishing Group. has great potential for detecting amyloid diseases and certain types of viruses (Figure 133).381 6.2. Supramolecular Chiroptical Switches In the field of supramolecular chemistry and nanotechnology, constructing assemblies that act like a “switch” is one of the most interesting topics. In general, any assembly in which specific properties can be reversibly changed with an external stimulus can be regarded as a supramolecular switch. A supramolecular chiroptical switch is based on reversible changes of supramolecular chirality, which is seen as externally stimulated optical − activity.382 384 The changes of supramolecular chirality can be achiral to chiral, either reversible or reversible from left-handed chirality to right-handed chirality. Various supramolecular assemblies, such as liquid crystals,385 − host−guest complexes,386 LB films,387 389 and supramolecular gels,51,56 have been developed as supramolecular chiral switches. In addition, some small organic molecules or polymers can also be used as chiroptical switches based on photoirradiation or − interaction with other small molecules.391 396 A supramolecular chiroptical switch based on an amorphous azobenzene polymer (156) has been constructed by Kim et al. When the thin films of an achiral epoxy-based polymer containing photoresponsive azobenzene groups were irradiated by elliptically polarized light (EPL), supramolecular chirality was introduced into the system, which was confirmed by the CD spectral measurements. The helical arrangement of the azobenzenes plays a very important role in the photoinduced supramolecular chirality. When the irradiation at 488 nm was changed from right-handed elliptical polarization to left-handed elliptical polarization, the handedness of the chiral supra- Figure 134. (A) Chemical structure of an achiral epoxy-based polymer molecular assembly also changed reversibly. This supramolecular containing photoresponsive azobenzene groups (156) and the helical ff chiroptical switch was able to operate several times before fatigue arrangement of the azobenzenes with di erent handedness. (B) (a) resistance occurred (Figure 134).391 Chiroptical switching of the CD spectra by alternating irradiation with r- and l-EPL. (b) Intensity of the CD signal at 410, 510, and 700 nm. Polythiophene is a very important conductive polymer. fi Reprinted with permission from ref 391. Copyright 2006 John Wiley & Yashima et al. constructed the rst reversible supramolecular Sons. chirality switch based on chiral polythiophene aggregates. When fl copper(II) tri uoromethanesulfonate [Cu(OTf)2] was added to chiral aggregates of chiral regioregular polythiophene in a addition or removal of an electron from the corresponding chloroform−acetonitrile mixture, the CD signal of the assemblies polymer main chain.397 disappeared due to the oxidative doping of the polymer main Chiral LS films constructed from the air/water interfacial chain. However, when amines, such as triethylenetetramine assembly of achiral molecular building blocks can be used as (TETA), were added to the system to undope the polymer, the supramolecular chiral switches. In this context, the handedness of CD signals reappeared. Thus, a supramolecular chiral switch can the supramolecular chirality of assemblies constructed by achiral be prepared from the assembly of chiral polythiophenes with the molecules can be changed by an external stimulus. This flexibility

7372 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review is a typical characteristic of assemblies based on noncovalent interactions, and supramolecular chiral switches have been achieved from the assembly of amphiphilic molecules on a water surface. For example, achiral 5-(octadecyloxy)-2-(2- thiazolylazo)phenol (TARC18, 157) can form chiral LS films via an air/water interfacial assembly. Supramolecular chiral switches based on the LS films of achiral TARC18 were fabricated by alternately exposing the film to HCl gas and to air (Figure 135).387

Figure 136. (A) Molecular structure of azobenzene-substituted diacetylene (NADA, 158). (B) Schematic illustrations of (a) enantioselective polymerization with CPUL irradiation and (b) chirality modulation for NADA LB ﬁlms with CPL treatment. Reprinted with permission from ref 388. Copyright 2009 The Royal Society of Chemistry.

Figure 135. (A) Molecular structure of TARC18 (157). (B) Schematic 399−401 illustration of the possible helical stacking of the TARC18 (157) their well-known work on light-driven molecular motors. molecules in the LS films. M and P chiralities of the films were formed by In this context, the modified light-driven molecular motor was chance. Reprinted with permission from ref 387. Copyright 2006 John covalently connected to the terminus of poly(n-hexyl isocyanate) Wiley & Sons. (PHIC), and the chiral information from the headgroups of the polymer was found to be expressed at both the supramolecular fi and the macromolecular levels. Upon irradiation with two Chiral thin lms that were constructed from achiral ff phthalocyanine derivatives via air/water interfacial assembly di erent wavelengths of light, a chiroptical switch has been can also be used as supramolecular chiral switches.382 In demonstrated, as evidenced by the CD spectral measurements and images obtained from an optical microscope equipped with particular, supramolecular chiroptical switches based on the 385 assembly of achiral phthalocyanine derivatives were found to be crossed polarizers (Figure 137). very stable. Certainly, the π−π interactions between phthalocyanine rings can increase the stability of the assembly. Most importantly, the polymerization of achiral phthalocyanine derivatives can produce chiral assemblies based on covalent bonds. When LS films containing polymerized chiral assemblies of achiral phthalocyanine derivatives were alternately exposed to HCl and NH3, a reversible change in the CD spectra was detected. This process can be repeated many times without any decrease in CD signal intensity. Zou et al. constructed LB films of azobenzene-substituted diacetylene (NADA, 158). Although NADA is achiral, the Figure 137. Schematic representation of the full photocontrol of the NADA LB films show supramolecular chirality. Moreover, the magnitude and sign of the supramolecular helical pitch of a cholesteric fi LC phase generated by a polyisocyanate with a single chiroptical assemblies within the LB lms can be polymerized with molecular switch covalently linked to the polymer’s terminus. Reprinted photoirradiation. When left- and right-handed circularly with permission from ref 385. Copyright 2008 American Chemical polarized ultraviolet light (CPUL) was applied, polymerized Society. NADA (PNADA) LB films with different chirality were obtained, as confirmed by the corresponding CD spectra from both azobenzene chromophores and polydiacetylene (PDA) chains. Chiral polymers containing azobenzene groups were also Interestingly, these polymerized LB films NADA (PDA LB films) found to form liquid crystals. Chiral switches based on these can be used as chiroptical switches after irradiation using left- and chiral liquid-crystalline polymers were investigated by Angiolini right-handed circularly polarized lasers (CPL, 442 nm), due to and co-workers. It was found that the chiral polymers containing alternation of the stereoregular packing of azobenzene azobenzene groups and L-lactic acid formed a smectic A1/2 (fully chromophores.388 interdigitated) liquid-crystalline phase. The resulting chiroptical The self-assembly of banana-shaped achiral molecules was switching of the system was achieved by irradiation with found to lead to chiral liquid crystals, and chiroptical switches circularly polarized light (CPL) with different handedness.402 based on liquid crystals containing only achiral molecular Chiroptical switches based on some soft matters, such as building blocks were also achieved. Tschierske et al. synthesized supramolecular gels or supramolecular assemblies in solution, bent-core mesogens carrying branched oligosiloxane units. A have attracted increased attention recently. Cucurbituril is a very chiroptical switch based on the phase transition was obtained by important building block for preparing chiral supramolecular applying an electric field or changing the temperature.398 assemblies. Supramolecular chiroptical switches based on the An elegant light-driven supramolecular chiroptical switch was coassembly of chiral binaphthalene−bipyridinium guests togeth- developed by Feringa and co-workers. This work was based on er with cucurbituril hosts have been developed by Venturi and

7373 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Tian et al.403 In this study, three molecular tweezers containing 4,4′-bipyridinium (BPY2+) and (R)-2,2′-dioxy-1,1′-binaphthyl (BIN) units with diﬀerent length alkyl chains were designed and synthesized (159). In aqueous solution, these chiral binaphthalene−bipyridinium guests formed complexes with cucurbituril hosts. In this system, chiroptical switching was achieved from the reversible changes of the helicity of the BIN units, which was triggered by reduction of the BPY2+ units. In addition, the alkyl linkers also play an important role in association to cucurbiturils. Thus, changing the length of the alkyl chains modulated the properties of the chiral switches, such as the molar ratio of the complex and the dihedral angle of BIN (Figure 138).

Figure 139. Schematic illustration of the self-assembled azobenzene- containing lipid showing multiresponsibility for chiral switching. Reprinted with permission from ref 56. Copyright 2011 American Chemical Society.

nanotubes, this hierarchical assembly cannot be used as a chiroptical switch. Only when Azo was coassembled with the bolaamphiphile at the molecular level could the corresponding assemblies show reversible changes both in the UV−vis and in the CD spectra upon alternative UV−vis irradiation of Azo (Figure 140).404 Chiral supramolecular nanotubes assembled by L-orD- glutamic-acid-based bolaamphiphiles (HDGA, 15b) in water can also be used as templates to produce silica nanotubes. Most interestingly, only the inner walls of the formed silica nanotubes were found to have supramolecular chirality. When the photoactive azobenzene moieties were loaded onto the inner chiral silica nanotubes, chiroptical switches based on inorganic nanomaterials resulted (Figure 141).27 6.3. Supramolecular Chiral Catalysis In synthetic chemistry, construction of chiral molecules has been found to be extremely important. Therefore, developing different − chiral homogeneous catalysts and heterogeneous catalysts has Figure 138. (A) Structure of chiral binaphthalene bipyridinium guests − (159). (B) Pictorial representation of the conformational rearrange- become a significant research goal.405 411 For example, ments of 159 in response to two-electron reduction and/or complex- coordination complexes containing metal ions and chiral ligands ation with either CB[8] or CB[7]. Reprinted with permission from ref have been widely investigated as highly efficient chiral catalysts. 403. Copyright 2012 John Wiley & Sons. Many outstanding reviews have been published examining this − issue.410 416 In particular, as a class of very important The very good stimulus-responsive properties of self- coordination complexes with crystalline structures, metal− assembled systems based on noncovalent interactions can help organic frameworks (MOFs), which have infinite network these systems form chiral switches with unique performance. For structures built with multitopic organic ligands and metal ions, example, Liu et al. synthesized a glutamic-acid-based lipid have been thoroughly studied as potential asymmetric catalysts. containing an azobenzene headgroup (azo-LG2C18, 5), which This topic has also been extensively reviewed recently.417 can form organogels with supramolecular chirality in different Similarly, the catalytic properties of cyclodextrin derivatives organic solvents. Remarkably, the resulting organogels can be have also attracted increased attention recently.418,419 As the used as chiroptical switches with multiresponsibility. Thus, the chiral host, cyclodextrin derivatives can provide a suitable chiral supramolecular chirality can be changed reversibly by photo- environment, and reactions at the guest molecules can provide irradiation, temperature variation, or solvent polarity (Figure reaction products with chirality. Moreover, the molecular 139).56 structures of cyclodextrin derivatives can be further modified A supramolecular chiroptical switch based on multicompo- to obtain more efficient chiral catalysts.420 There are also many nent self-assembled soft matters has also been developed by Liu good review articles concerning chiral catalysis based on group. The self-assembly of chiral glutamic-acid-based bolaam- cyclodextrin derivatives. For example, Inoue and co-workers phiphiles (HDGA, 15b) can lead to hydrogels and chiral summarized supramolecular photochirogenesis based on cyclo- supramolecular nanotubes. For the construction of multi- dextrin derivatives.421,422 component soft matters, the azobenzene derivative 4- In a general sense, “supramolecular chiral catalysis” is presently (phenylazo)benzoic acid sodium salt (Azo) can be coassembled a major topic of research interest, but we cannot address every with either HDGA molecules (15b) or chiral supramolecular aspect of this field of research. In this review, we concentrate on nanotubes. Although very strong supramolecular chirality can be the catalytic properties of some chiral supramolecular assemblies. detected from the coassembly of Azo with the preformed chiral Although the catalytic properties of metal complexes have been

7374 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 140. (A) Structures of the hydrogelators HDGA (15b) and Azo. (B) Gels formed by L-HDGA in water in the absence and presence of Azo. (C) Illustration of the coassembly of Azo with HDGA. Reprinted with permission from ref 404. Copyright 2011 The Royal Society of Chemistry.

induction and reusability in the catalysis of asymmetric hydrogenation of dehydro-α-amino acid and enamide derivatives (Figure 142).423 Furthermore, Ding et al. synthesized a heteroditopic ligand containing a 2,2′:6′,2″-terpyridine (tpy) unit and Feringa’s MonoPhos (160). The selective coordination of this ligand with FeII and RhI ions can produce chiral supramolecular polymers, which can be used as chiral bimetallic self-supported catalysts. In the hydrogenation of α-dehydroamino acid, enamide, and itaconic acid derivatives, these reusable heterogeneous asymmetric catalysts can lead to very high reaction rates with excellent enantioselectivity (90−97% ee) (Figure 143).424 Figure 141. Creating chirality in the inner walls of silica nanotubes Among various supramolecular chiral catalysis from self- through a hydrogel template, and the chiroptical switching of these assembly, ion pair catalysts, which have been developed in recent nanotubes. Reprinted with permission from ref 27. Copyright 2010 The years, are very important systems with many distinctive Royal Society of Chemistry. characteristics. Ooi et al. developed supramolecular assemblies containing ion pairs through intermolecular hydrogen bonding. well investigated, supramolecular polymers based on coordina- This system was formed by the coassembly of a chiral tion interactions have very special characteristics for supra- tetraaminophosphonium cation, two phenols, and a phenoxide molecular chiral catalysis. anion and was found to have chiral catalytic activity (161). In For example, Ding et al. constructed polymeric supra- solution, this ion pair complex promotes a highly stereoselective molecular chiral catalysts based on self-assembly. The authors conjugate addition of acyl anion equivalents to α,β-unsaturated synthesized an organic ligand containing ureido-4[1H]- ester surrogates with a broad substrate scope (Figure 144).425 ureidopyrimidone (UP) and Feringa’s MonoPhos motifs. By Ishihara et al. also produced self-assembled chiral catalysts mixing the organic ligand with [Rh(cod)2]BF4, supramolecular without metal ions. These supramolecular catalysts are based on polymers can be produced by orthogonal self-assembly via in situ coassembly from chiral diols, arylboronic acids, and hydrogen-bonding and ligand-to-metal coordination interac- tris(pentaﬂuorophenyl)borane (162). In the Diels−Alder tions. This supramolecular polymer shows excellent asymmetric reactions of cyclopentadiene with diﬀerent acroleins, these

Figure 142. (A) Polymeric supramolecular chiral catalyst based on self-assembly. (B) Asymmetric hydrogenation of dehydro-α-amino acid and enamide derivatives. Reprinted with permission from ref 423. Copyright 2006 John Wiley & Sons.

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Figure 143. (A) Molecular structure of a ligand containing the 2,2′:6′,2″-terpyridine (tpy) unit and Feringa’s MonoPhos (160), and schematic illustration of supramolecular catalysts through orthogonal coordination of two diﬀerent metal ions with a single ditopic ligand. (B) Asymmetric hydrogenation of dehydroamino acid, enamide, and itaconic acid derivatives using a catalyst with high reaction rate and excellent enantioselectivity. Reprinted with permission from ref 424. Copyright 2010 John Wiley & Sons.

Supramolecular catalysts based on the coassembly of chiral amines and poly(alkene glycol)s have been reported by Xu et al. These systems were found to be highly eﬃcient in the asymmetric catalysis of the unusual Diels−Alder reaction between cyclohexenones and nitrodienes, nitroenynes, or nitrooleﬁns, providing excellent chemo-, regio-, and enantioselectivities (Figure 146).427

Figure 146. Coassembly of chiral amines and poly(alkene glycol)s showing highly eﬃcient asymmetric catalysis of Diels−Alder reactions. Reprinted with permission from ref 427. Copyright 2011 John Wiley & Sons.

Although many chiral supramolecular catalysts have been developed via self-assembly, the chiral catalysis characteristics of Figure 144. (A) Structures of chiral tetraaminophosphonium cations. · systems are still largely dependent on chirality at the molecular (B) Oak Ridge thermal ellipsoid plot diagram of 161a (OPh)3H2. (C) Scope of α,β-unsaturated acylbenzotriazole. Reprinted with permission level. By contrast, even if many chiral nanostructures have been from ref 425. Copyright 2009 The American Association for the constructed via self-assembly, the relationship between chirality Advancement of Science. at the nanoscale and chiral catalysis at the molecular level is still rarely discussed. As mentioned previously, the self-assembly of chiral glutamic- acid-based bolaamphiphiles (HDGA, 15b) led to hydrogels and catalysts have very good endo/exo selectivities and high chiral supramolecular nanotubes. When Cu2+ ions were added to enantioselectivities (Figure 145).426 the system, a monolayer nanotube was transformed into a

Figure 145. (A) Structures of chiral supramolecular catalyst (162). (B) Enantioselective Diels−Alder reactions with anomalous endo/exo selectivities using chiral supramolecular catalysts. Reprinted with permission from ref 426. Copyright 2011 John Wiley & Sons.

7376 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review multilayer nanotube with a tubular wall thickness of about 10 nm. Interestingly, the resulting Cu2+-containing supramolecular nanotubes were useful as an asymmetric catalyst for the Diels− Alder reaction between cyclopentadiene and aza-chalcone, which accelerates the reaction rate and enhances enantiomeric selectivity. Thus, asymmetric catalysis of the molecular reaction can be achieved by chiral nanostructures. It was suggested that through the Cu2+-mediated nanotube formation the substrate molecules could be anchored on the nanotube surfaces to produce a stereochemically favored alignment (Figure 147). Therefore, when adducts reacted with the substrate, both the enantiomeric selectivity and the reaction rate were found to increase.428

Figure 148. Self-assembly of vesicles regulated by compressed CO2 and the proposed transition-state model for the direct asymmetric aldol reaction. Reprinted with permission from ref 429. Copyright 2013 John Wiley & Sons.

Yashima et al. synthesized a chiral polymer containing riboflavin units as the main chain (165). Within the polymers, the 5-ethylriboflavinium cations can be reversibly transformed into 4a-hydroxyriboflavins upon hydroxylation/dehydroxylation, which renders significant changes in the absorption and circular dichroism (CD) spectra of the polymers. It is believed that the 2+ Figure 147. Illustration of the assembly mechanism of the Cu −L- face-to-face stacking of the intermolecular riboflavinium units HDGA nanotubular structure and its asymmetric catalysis of the Diels− within the polymer produced twisted helical nanostructures with Alder reaction of aza-chalcone with cyclopentadiene. Reprinted with supramolecular chirality. This optically active polymer contain- permission from ref 428. Copyright 2011 American Chemical Society. ing 5-ethylriboflavinium cations was found to efficiently catalyze the asymmetric organocatalytic oxidation of sulfides with hydrogen peroxide, yielding optically active sulfoxides with up Besides catalytic nanotubes, chiral catalysis based on supra- to 60% ee (Figure 150).431 molecular nanostructures has also been observed using vesicles. DNA is a very important genetic material of living organisms. Liu et al. synthesized amphiphilic molecules containing a proline However, it can also be regarded as a very useful chiral functional ff headgroup (PTC12, 163). The self-assembly of PTC12 in water polymer for di erent applications. As a polymer, DNA has many ff under compressed CO2 can produce vesicles. These assemblies di erent molecular chiral centers and charged substituents, were found to catalyze the asymmetric aldol reaction with high which can increase its solubility in water. Moreover, the folding of enantiomeric selectivity without any additives. Importantly, the DNA can produce very ordered nanostructures via hydrogen size of the PTC12 assemblies and subsequently catalyst activity bonding, which can also be modulated by changing the and stereoselectivity can be dynamically modulated by changing sequences in the DNA. It should be noted that DNA is relatively the status of the compressed CO2. Moreover, because CO2 can stable in comparison to other biomacromolecules, such as RNA. be easily removed from the system, it is very convenient for the Therefore, developing DNA-based supramolecular chiral cata- separation and purification of products, as well as the reuse of the lysts has recently attracted interest. chiral supramolecular catalysts (Figure 148).429 The first DNA-based asymmetric catalyst containing copper In the development of supramolecular chiral catalysis, chiral ions was reported by Roelfes and Feringa. In this study, the covalent polymers, including some biomacromolecules, such as authors synthesized an achiral ligand containing a DNA- DNA and polypeptides, can be used as building blocks. The intercalating moiety (9-aminoacridine), alkyl chain spacer, and catalytic capability of these polymers may originate from the metal-binding group (166). A DNA-based asymmetric catalyst molecular chiral centers within these polymers but may also was fabricated from the coassembly of a copper(II)-enclosing result from their folding characteristics and hierarchical ligand and DNA. In the Diels−Alder reaction, this chiral catalyst nanostructures. could transfer the chirality of the DNA into the products, with Meijer and co-workers synthesized water-soluble segmented the an ee value up to 90% (Figure 151A).432 terpolymers containing PEG and chiral benzene-1,3,5-tricarbox- For many other organic chemical reactions, a DNA-based amide side chains as well as a ruthenium complex (164). Due to chiral catalyst containing copper(II) was also found to be very the chiral self-assembly of the benzene-1,3,5-tricarboxamide side useful. For example, Roelfes et al. developed a DNA-based chains, the folding of these polymers can produce a helical asymmetric catalyst containing copper(II) and an achiral ligand structure in the apolar core around a ruthenium-based catalyst. for catalyzing the Michael reaction in water to achieve high This catalyst, resulting from the folding of polymers, was found enantioselectivity. These reactions can be performed on a to catalyze the transfer hydrogenation of ketones (Figure relatively large scale, allowing recycling of the supramolecular 149).430 chiral catalyst. For this system, many simple achiral ligands, such

7377 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 149. (A) Water-soluble segmented terpolymer containing PEG and chiral benzene-1,3,5-tricarboxamide side chains as well as a ruthenium complex (164). (B) Supramolecular single-chain folding of polymers in water aﬀording a compartmentalized catalyst for the transfer hydrogenation of ketones. Reprinted with permission from ref 430. Copyright 2011 American Chemical Society.

Figure 150. Optically active polymers consisting of riboflavin units catalyze the asymmetric organocatalytic oxidation of sulfides. Reprinted with permission from ref 431. Copyright 2012 American Chemical Society. as dipyridine, can be used for coassembly with DNA and copper sulfones in water, high enantioselectivities with ee values of up to ions (Figure 151B). The reactants in this study included α,β- 84% were achieved.435 unsaturated 2-acylimidazoles working as the Michael acceptors Most interestingly, use of DNA-based chiral catalysts and nitromethane and dimethyl malonate as the nucleophiles. containing copper(II) for chemical reactions in biological Upon chiral catalysis by the DNA-based assemblies containing systems, such as enantioselective addition of water to olefins in ffi copper(II) and achiral ligand, the enantioselectivities of the an aqueous environment, resulted in good e ciency under the Michael reaction were found to be up to 99% ee.433 laboratory conditions. For the enantioselective hydration of β Furthermore, Feringa and Roelfes also studied asymmetric enones, the chiral -hydroxy ketone product can be obtained Friedel−Crafts alkylation with olefins in water catalyzed by a with an ee up to 82% upon catalysis with DNA-based assemblies DNA-based chiral catalyst containing copper(II). In this system, (Figure 151D). Moreover, the reaction was also found to be ′ ′ diastereospecific, with the formation of only the syn hydration 4,4 -dimethyl-2,2 -bipyridine (dmbpy) was used as the achiral 436 ligand for the complex with copper(II) and coassembly with product. The folding of DNA can produce different nanostructures, DNA. For the asymmetric Friedel−Crafts reaction of α,β- which can also be modulated by changing the DNA sequences or unsaturated 2-acylimidazoles with heteroaromatic π nucleo- other conditions for assembly. Besides double-helix DNA, philes, good yields and high enantioselectivities were obtained telomeric G-quadruplex DNA was also studied to construct using a very small of amount of DNA-based chiral catalysts DNA-based chiral catalysts. Moses et al. constructed supra- (Figure 151C). In this study, the catalytic efficiency of both ff molecular chiral catalysts based on the assembly of telomeric G- double-stranded DNA and single-stranded DNA with di erent quadruplex DNA, achiral ligands, and copper(II). These catalytic sequences was investigated. The results showed that only the systems were found to catalyze Diels−Alder reactions success- chiral catalysts assembled by the double-stranded DNA can fully with modest enantioselectivities.437 introduce high enantioselectivities by catalyzing the asymmetric Another class of G-quadruplex-DNA-based chiral catalysts was Friedel−Crafts alkylation. In addition, the highest enantiose- developed by Li and co-workers. These supramolecular chiral lectivities (up to 93%) were obtained by the supramolecular catalysts were constructed by self-assembly of human telomeric 434 catalysts assembled using d(TCAGGGCCCTGA)2 DNA. G4DNA and different metal ions. In this case, additional achiral A DNA-based chiral catalyst containing copper(II) and achiral ligands were not needed for building the catalysts. In an ligand was also studied in the reaction of asymmetric asymmetric Diels−Alder reaction, the complex of human intramolecular cyclopropanation (Figures 151−155). For the telomeric G4DNA and Cu2+ ions provided a significant asymmetric intramolecular cyclopropanation of α-diazo-β-keto enhancement in the reaction rate with good enantioselectivity

7378 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Figure 151. (A) Asymmetric Diels−Alder reaction of cyclopentadiene with aza-chalcone, catalyzed by copper complexes of the ligand in the presence of DNA. (B) Asymmetric Michael addition reaction catalyzed by complexes formed between copper(II) ions and achiral ligands in the presence of DNA. (C) Cu−dmbpy/st-DNA-catalyzed Friedel−Crafts alkylation. Reprinted with permission from refs 432, 433, and434. Copyright 2005, 2007, and 2009 John Wiley & Sons. (D) DNA-based catalyst and general reaction scheme of the catalytic enantioselective hydration of a variety of α,β-unsaturated 2- acyl-(1-alkyl)imidazole substrates, and overview of ligands used in this study. Reprinted with permission from ref 436. Copyright 2010 Nature Publishing Group. (E) Intramolecular cyclopropanation of α-diazo-β-keto sulfones in water using a DNA-based catalyst. Reprinted with permission from ref 435. Copyright 2013 The Royal Society of Chemistry.

Figure 152. Enantioselective Diels−Alder reactions with G-quadruplex-DNA-based catalysts; the absolute conﬁguration of the products can be reversed when the conformation of the G4DNA is switched from antiparallel to parallel. Reprinted with permission from ref 438. Copyright 2012 John Wiley & Sons.

(74% ee). In addition, the rate and enantioselectivity of the For the DNA-based chiral catalysis, the relationship between reaction can be modulated by changing the DNA sequence and the handedness of the DNA helix and the molecular chirality of metal ions used to form the complex. Interestingly, the absolute products was investigated by Smietana and Arseniyadis et al. ff configuration of the products can be controlled by the assembly They constructed di erent DNA-based supramolecular chiral catalysts from the assembly of both L-DNA and D-DNA. The L- of chiral catalysts (Figure 152). Thus, when the conformation of DNA, which contains deoxyribose with an L-conformation, can the G4DNA was switched from antiparallel to parallel, the self-assemble into left-helical nanostructures, while the folding of absolute configuration of the products obtained from Diels− normal DNA only produces right-helical nanostructures. There- 438 Alder reactions could be reversed. fore, the L-DNA-based and D-DNA-based supramolecular chiral

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Figure 153. Tuning the absolute configuration in DNA-based asymmetric catalysis. Reprinted with permission from ref 439. Copyright 2013 John Wiley & Sons. catalysts have totally different supramolecular chirality (Figure covalent interactions between amino acids during the folding of 153). In the case of Friedel−Crafts reactions and Michael polypeptides, which produce more flexible nanostructures. For additions using many different substrates, enantiomers of the supramolecular chiral catalysis under laboratory conditions, such products can be obtained by the catalysis of L-DNA- or D-DNA- complicated and often unstable nanostructures are not easy to based supramolecular chiral catalysts.439 handle. Therefore, even though nature has used enzymes for In the construction of DNA-based chiral catalysts, even though catalyzing many very subtle chemical reactions for billions of metal ions are very important, they are not always necessary. years in catalyzing chemical reactions under artificial conditions, Andreassoń et al. studied the asymmetric closing reaction of DNA is still a better candidate. dithienylethene derivatives by complexion with DNA upon Nevertheless, polypeptide-based chiral catalysts have also been photoirradiation. In this study, fluorinated dithienylethene constructed recently. For example, Roelfes et al. modulated derivatives containing methylpyridinium and methylquinolium natural bovine pancreatic polypeptides with nonproteinogenic substituents were bound to DNA in both the open and the closed amino acids for binding Cu2+ ions. The resulting metalloenzymes forms. Cyclization of dithienylethene derivatives upon photo- catalyze Diels−Alder and Michael addition reactions in water 441 irradiation could produce the closed form of these molecules with high enantioselectivities (Figure 155). with significant enantioselectivity (Figure 154). In this case, Herrmann et al. developed chiral catalysts based on natural chirality was transferred from the DNA to the products.440 polypeptides without many chemical modifications. These polypeptides are cyclic peptides formed by intramolecular disulfide linking of cysteine residues at both ends of the peptide. The chiral catalysts were constructed by binding the cyclic peptide with Cu2+ ions. The advantage of this system is small- sequence constriction and flexibility in the amino acids of the polypeptides. In catalyzing Diels−Alder and Friedel−Crafts reactions, these cyclic-peptide-based chiral catalysts achieved high enantioselectivities of up to 99% ee and 86% ee, respectively (Figure 156). Furthermore, in this work, Herrmann et al. also

Although the most popular natural chiral catalysts (enzymes) are polypeptides, constructing artiﬁcial enzymes via the Figure 156. (a) Cyclic peptide ligand with constrained conformation coassembly of polypeptides with other molecules has been through an intramolecular disulﬁde bridge. (b) D−A reaction catalyzed only partially successful. Polypeptides are generally much more by cyclic peptide ligand and Cu2+. Reprinted with permission from ref complicated biomacromolecules than DNA, due to the greater 442. Copyright 2014 John Wiley & Sons. array of molecular building blocks available for the formation of polypeptides and the relatively weak but complicated non-

Figure 155. Enantioselective artiﬁcial metalloenzymes based on a bovine pancreatic polypeptide scaﬀold, showing catalytic Diels−Alder and Michael addition reactions in water with high enantioselectivities. Reprinted with permission from ref 441. Copyright 2009 John Wiley & Sons.

7380 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review systematically studied the relationship between amino acid crystals, which formed from the assembly of coronene, are at the sequences and the corresponding enantioselectivities of the core of the matter. Thus, stacking at certain twisting angles within catalytic reactions. The results show that the position of alanine the discotic liquid crystals increased charge carrier mobility of within the sequences plays a very important role.442 these devices.454 6.4. Optics and Electronics Based on Supramolecular Chiral Assembly Development of functional devices is one of the main objectives of research on supramolecular assembly. In the application of supramolecular chirality from self-assembled systems, chiral electronic or optical devices are also worth discussing. Recently, − these issues have attracted increasing interest.443 448 Chiral optics and electronics are directly dependent on many different functions of supramolecular chiral assemblies. For example, chiral sensors can be used to construct chiral electronic devices, as shown by Wei and co-workers. They prepared ultraordered superhelical microfibers with clear screws and favorable monodispersity from chiral polyaniline (PANI). When these superhelical microfibers were treated with chiral aminohexane vapor with different handedness, very different electrical conductivity in these microfibers was detected (Figure 156).449,450

Figure 157. Superhelical conducting microfibers with homochirality for enantioselective sensing. Reprinted with permission from ref 449. Figure 158. (A) Molecular structure of coronene (168). (B) Charge Copyright 2013 American Chemical Society. mobilities as a function of temperature as measured by the pulse radiolysis time-resolved microwave conductivity (PR-TRMC) technique. Reprinted with permission from ref 454. Copyright 2009 Nature In the development of chiral electronic or optic devices, the Publishing Group. most important aspect is the relationship between the optical/ electrical properties of the materials and the supramolecular chirality. The first important work concerning chiral optics was The photocurrent properties of the supramolecular chiral published by Verbiest et al. in 1998. The authors prepared LB assemblies formed at the air/water interface were investigated by films of helicene with different supramolecular chirality and the Liu group. They found that an anthracene derivative (AN) could be controllably assembled to nanocoils and straight studied the second-order nonlinear optical (NLO) properties of ff these LB films. The results show that the second-order NLO nanoribbons on water surfaces depending on the di erent susceptibility of the chiral assemblies can be 30 times larger than surface pressures. Most interestingly, the nanoribbons exhibited 451 a switchable photocurrent, while the nanocoils did not show a that of the racemic material with the same chemical structure. 455 Except for optical properties, the electrical properties of some photocurrent response (Figure 159). supramolecular assemblies were also found to be dependent on Not only can a photocurrent be generated from the chiral the material’s chirality. For example, Fourmiguéet al. studied the supramolecular assemblies, devices based on chiral supra- electronic conductivity of chiral salts of tetrathiafulvalene molecular assemblies can also be used as sensors for detecting − circularly polarized light. These results were reported by Fuchter methyl oxazoline derivatives. The results showed that the fi conductivity of the pure enantiomeric salts can be an order of and Campbell et al. In this study, they constructed organic eld 452 effect transistors from the assembly of helicene (Figure 160A), magnitude higher than the conductivity of the racemic salts. fi Wei et al. fabricated hierarchical chiral assemblies of the and a highly speci c photoresponse to circularly polarized light conducting polyaniline (PANI) with different nanostructures was detected. Importantly, the photoresponse to circularly polarized light was found to be directly related to the handedness and superstructures by controlling the interactions between 456 molecules. The anisotropic electrical transport properties based of the helicene molecule (Figure 160B). on the arrangement of molecules and nanostructures were 6.5. Circularly Polarized Luminescence (CPL) Based on Chiral probed.453 Supramolecular Assemblies The semiconductor properties of supramolecular assemblies Circularly polarized light is inherently chiral and has been has always been very interesting. Remarkably, supramolecular regarded as one possible origin of natural homochirality457 and chirality was also found to play a very important role in this issue. the source of chiral information during the emergence of Müllen et al. studied field-effect transistor devices based on the life.308,309 As we described previously, supramolecular chirality assembly of coronene (168). In this system, discotic liquid with controlled handedness can be introduced into the

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Figure 159. Controllable fabrication of supramolecular nanocoils and nanoribbons and their morphology-dependent photoswitching. Reprinted with permission from ref 455. Copyright 2009 American Chemical Society.

Akagi et al. synthesized chiral polythiophenes and chiral thiophene−phenylene copolymers and found that these polymers, with different molecular structures and aggregation states, could exhibit red, green, and blue fluorescent. Remarkably, mixing these different fluorescent polymers generated a unique, circularly polarized white luminescence.465

Figure 160. (A) Molecular structure and device architecture of the circularly polarized light-detecting helicene OFETs. (B) Response of helicene OFETs to circularly polarized light. Reprinted with permission from ref 456. Copyright 2013 Nature Publishing Group. supramolecular assemblies containing only achiral molecular building blocks via irradiation with circularly polarized light (CPL). Among the different circularly polarized light, the circularly polarized luminescence from chiral assemblies, abbreviated CPL, can be extremely important and is attracting increasing research interest. In the generation of circularly polarized luminescence (CPL) from chiral supramolecular assemblies, the chiral arrangement of fl the luminescent chromophores is an essential prerequisite. When Figure 161. Mixture of red, green, and blue uorescent polymers luminophores exist in a dissymmetric environment within the generated a unique, circularly polarized white luminescence. Reprinted with permission from ref 465. Copyright 2012 American Chemical photoexcited state, circularly polarized luminescence (CPL) is Society. generated. The study of the circularly polarized luminescence (CPL) from chiral assemblies has been widely dominated by lanthanide complexes owing to their ability to exhibit high CPL Another type of important chiral supramolecular system for dissymmetry.458,459 generating circularly polarized luminescence is the assembly of On the other hand, many different chiral supramolecular helicenes. The aggregation of chiral helicenes was found to show assemblies resulting from organic molecular building blocks have large CPL dissymmetry owing to the strong helical distortion of π − also been recently found to be very important sources for systems.466 469 generating circularly polarized luminescence (CPL). This Maeda et al. introduced the BINOL−boron moiety to situation can be another important application of chiral dipyrrolyldiketones and fabricated the chiral conformation of supramolecular assemblies. The most prominent efforts are π-conjugated system. The anions-triggered strong circularly based on the π-conjugated polymers with chiral side chains or polarized luminescence (CPL) was observed from these helical aggregated nanostructures that have been reported to assemblies.468 − show intense CPL signals.460 465 In addition, Nakashima and Kawai et al. reported chiral For example, Swager et al. synthesized chiral poly(p- bichromophoric perylene bisimides as active materials for phenylenevinylene) derivatives and studied the circularly circularly polarized emission. They found that the compounds polarized luminescence (CPL) spectroscopy of the assemblies formed chiral aggregates with solvent variations. A large from this polymer. Interestingly, using the same polymer with enhancement in the dissymmetry of circularly polarized same molecular chirality, different supramolecular assemblies luminescence was achieved by the aggregated structures. It was were found to produce opposite CPL spectra.463 further found that the spacer between the chiral center and the

7382 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review chromophoric units played a crucial role in the effective 1,4-benzenedicarboxamide phenylalanine derivative (170)as enhancement of chiroptical properties in these self-assembled supramolecular gelators to construct different supramolecular structures.470 hydrogels, and the cell adhesion within these supramolecular hydrogels was studied. It was found that cell adhesion and proliferation can be influenced by the chirality of the nanofibers. Thus, the left-handed helical nanofibers increased cell adhesion and proliferation, while the right-handed nanofibers decrease cell adhesion and proliferation. The stereospecific interaction between chiral nanofibers and fibronectin plays a critical role in these effects (Figure 163).476

Figure 163. Schematic representation of the culture of cells in Figure 162. Enhancement in the dissymmetry of circularly polarized supramolecular hydrogels and the different cell-adhesion and cell- luminescence from the assembly of chiral bichromophoric perylene proliferation behavior in the enantiomeric nanofibrous hydrogels (d: bisimides. Reprinted with permission from ref 470. Copyright 2013 John right-handed helical nanofibers; l: left-handed helical nanofibers). The Wiley & Sons. molecular structure of the gelator enantiomers (170) is shown. Reprinted with permission from ref 476. Copyright 2014 John Wiley & Sons. 6.6. Biological Applications of Supramolecular Chirality The existence of life and biological evolution directly depend on both molecular chirality and supramolecular chirality. This situation can be demonstrated from the homochirality of amino As mentioned above, the handedness of self-assembled acids, nucleic acids, and many other biomolecules as well as the nanostructures can influence cell proliferation. However, Zouani helical nanostructures formed by the folding of DNA and et al. demonstrated that helical nanostructures with the same proteins. Many biomedical applications are also closely related to handedness, but different shapes and periodicities show totally molecular chirality and supramolecular chirality. For example, different capabilities for inducing human mesenchymal stem cell most drugs used for clinical application are chiral molecules. (hMSCs) adhesion and commitment into osteoblast lineage. In In general, every aspect of biological applications is dependent this study, mineralization of helical organic nanoribbons, which on chirality. We cannot address all of these issues in this review. formed from the self-assembly of Gemini-type amphiphiles, For a further understanding of supramolecular chirality from self- ff assembled systems, we will discuss some aspects of surpramo- could produce chiral silica nanoribbons with two di erent shapes lecular chirality effects on cell adhesion. and periodicities. Interestingly, helical silica nanoribbons with a fi ± fi Supramolecular hydrogels formed from the self-assembly of speci c periodicity of 63 nm ( 5 nm) helped the speci c cell peptide derivatives or nucleic acid derivatives have been studied adhesion and stem cell differentiation, while silica twists with a − for different biological applications.471 473 Certainly, the specific periodicity of 100 nm (±15 nm) did not (Figure 164). chirality always plays a very important role. For example, for These results indicate that stem cells could interpret helical the hydrogels formed by short peptides, L-peptides have been nanostructures with supramolecular chirality.477 474 found to be labile to proteases. Marchesan et al. recently Recently, Liu et al. synthesized gelators bearing amphiphilic L- ff studied the e ects of amino acid chirality on tripeptide self- glutamide and D-orL-pantolactone (abbreviated as DPLG and assembly and hydrogelation at physiological pH and cytocom- LPLG, 171). The self-assembly of DPLG and LPLG produced fi ff patibility in broblast cell culture. In this study, di erent nanostructures with opposite supramolecular chirality. The uncapped hydrophobic tripeptides with all combinations of D- ability of proteins to adhere to these nanostructures was found and L-amino acids were prepared. The self-assembly and to be dependent on their supramolecular chirality, as hydrogelation was found to be dependent on the chirality of demonstrated from quartz crystal microbalance measurements. the amino acids, and combinations of D, L-amino acids are very useful for maintaining the viability and proliferation of fibroblasts Thus, the supramolecular nanostructures formed by DPLG have in vitro.475 stronger adhesive ability to human serum albumin. Interestingly, Interestingly, the cell adhesion in the supramolecular hydro- the distinction of protein adhesion ability was only found at the gels was found to be dependent on the handedness of the self- supramolecular level. At the molecular level, however, no clear assembled nanofibers. Feng et al. used the two enantiomers of a difference could be detected (Figure 163).478

7383 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397 Chemical Reviews Review

Supramolecular chirality can be produced in systems containing chiral and/or achiral molecules. In contrast to the molecular chirality, it is still difficult to quantitatively evaluate the purity of supramolecular chirality. In a system containing chiral molecules, the main questions are do they form only one kind of supramolecular chirality or does there exist a percentage of assemblies with the opposite chirality? Occasionally, supramolecular chirality has emerged from assemblies based on achiral molecules. Even though one can observe microscopic chirality, two enantiomers coexisted. Understanding the emergence of chirality and how to evaluate the enantiomeric excess of a chiral assembly remains difficult. Supramolecular chirality is generally dynamic and strongly related to the self-assembly process. While we achieved many controls over supramolecular chirality, the characterization techniques of the dynamic processes of the Figure 164. SEM images of helical silica nanoribbons and silica twists; adhesion and differentiation of stem cells on helical silica nanoribbon chiral assemblies in particular, the development of time- substrates. Reprinted with permission from ref 477. Copyright 2013 dependent spectroscopy and imaging technology is urgently American Chemical Society. necessary. Although we acquiredmuchknowledgeabout supramolecular chirality in self-assembly systems related to intermolecular interactions, structural control, and function development, many of these are limited to one to two components. There is a lack of understanding of how to tune many different molecules into a complex chiral system in a cooperative or syndetic way as is accomplished in a living cell. Nanostructured chiral materials offer many opportunities to develop entirely new functional materials, which justifies research into supramolecular chirality. In this regard, can new catalytic, optical, opto-electrical, and magnetic materials result from work on chiral self-assembly systems? Chirality effects are fundamental to biological systems, such as different enantiomers that can be a Figure 165. Self-assembled nanostructures with opposite supra- ff useful drug and or poisonous depending on their chirality. How molecular chirality showing di erent adhesive ability to human serum to construct the chiral/biointerface? Therefore, further efforts on albumin. Reprinted with permission from ref 478. Copyright 2014 American Chemical Society. supramolecular chirality research should integrate new ideas from supramolecular chemistry, biology, medical science, pharmacology, and material and nanosciences.

7. CONCLUSIONS AUTHOR INFORMATION Chiral self-assembly from the molecular to the supramolecular Corresponding Author level represents one of the most attractive and promising areas in *Phone: +86 10 82615803. E-mail: [email protected]. supramolecular chemistry and self-assembly. The supramolecu- Notes lar chirality in these self-assembled systems is the expression of ﬁ the noncovalent interactions between the component molecules, The authors declare no competing nancial interest. where chiral transfer from a chiral component to the whole Biographies assembly plays an important role. In addition, supramolecular chirality can also emerge through symmetry breaking even when only achiral molecules are involved. Due to the dynamic features of the self-assembly system, the supramolecular chirality can be regulated through the design of the chiral molecules themselves, external conditions such as pH, metal ions, photoirradiation, solvents, temperature, sonication, and so on. Diﬀerent from molecular chirality, supramolecular chirality can exhibit unique properties such as the sergeant-and-soldier principle, the majority-rule principle, and chiral memories in several systems. Supramolecular chirality in self-assembled systems has been found to be useful in chiral sensing, chiral molecular recognition, and asymmetric catalysis. Some new functions such as chiroptical switching, chiroptics, and CPL have also been observed. Furthermore, chiral nanostructures showed some interesting properties when interacting with the biological systems. This Minghua Liu, born in 1965 in China, is a Professor at the Institute of review has described many examples of the emergence, Chemistry of the Chinese Academy of Sciences (CAS). He graduated regulation, and unique features or functions of the supra- from Nanjing University in 1986 and received his Ph.D. degree in 1994 molecular chirality; however, there are still many unknowns in Materials Science from Saitama University, Japan, under the related to supramolecular chirality. supervision of Prof. Kiyoshige Fukuda. He then joined the Institute of

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7397 DOI: 10.1021/cr500671p Chem. Rev. 2015, 115, 7304−7397