International Journal of Molecular Sciences Review Supramolecular Chirality in Azobenzene-Containing Polymer System: Traditional Postpolymerization Self-Assembly Versus In Situ Supramolecular Self-Assembly Strategy Xiaoxiao Cheng, Tengfei Miao, Yilin Qian, Zhengbiao Zhang, Wei Zhang * and Xiulin Zhu State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China; [email protected] (X.C.); [email protected] (T.M.); [email protected] (Y.Q.); [email protected] (Z.Z.); [email protected] (X.Z.) * Correspondence: [email protected]; Tel.: +86-512-6588-4243; Fax: +86-512-6588-2787 Received: 29 June 2020; Accepted: 31 July 2020; Published: 27 August 2020 Abstract: Recently, the design of novel supramolecular chiral materials has received a great deal of attention due to rapid developments in the fields of supramolecular chemistry and molecular self-assembly. Supramolecular chirality has been widely introduced to polymers containing photoresponsive azobenzene groups. On the one hand, supramolecular chiral structures of azobenzene-containing polymers (Azo-polymers) can be produced by nonsymmetric arrangement of Azo units through noncovalent interactions. On the other hand, the reversibility of the photoisomerization also allows for the control of the supramolecular organization of the Azo moieties within polymer structures. The construction of supramolecular chirality in Azo-polymeric self-assembled system is highly important for further developments in this field from both academic and practical points of view. The postpolymerization self-assembly strategy is one of the traditional strategies for mainly constructing supramolecular chirality in Azo-polymers. The in situ supramolecular self-assembly mediated by polymerization-induced self-assembly (PISA) is a facile one-pot approach for the construction of well-defined supramolecular chirality during polymerization process. In this review, we focus on a discussion of supramolecular chirality of Azo-polymer systems constructed by traditional postpolymerization self-assembly and PISA-mediated in situ supramolecular self-assembly. Furthermore, we will also summarize the basic concepts, seminal studies, recent trends, and perspectives in the constructions and applications of supramolecular chirality based on Azo-polymers with the hope to advance the development of supramolecular chirality in chemistry. Keywords: supramolecular chirality; Azo-containing polymers; postpolymerization self-assembly; in situ polymerization-induced self-assembly 1. Introduction Chirality is ubiquitous and can be observed at various hierarchical levels from subatomic and molecular to supramolecular and macroscopic scales [1–3]. As chiral phenomena widely exist in nature (such as the D-sugars as the main components of DNA and the secondary R-helix structure of proteins) and various artificial and biomimetic systems [4,5], they are very important not only for fundamental sciences but also for practical applications in many areas, such as molecular and chiral recognition [6], chiral sensors [2], chiroptical switches [7] and asymmetric catalysis [8]. The studies of chirality in terms Int. J. Mol. Sci. 2020, 21, 6186; doi:10.3390/ijms21176186 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, 6186 2 of 36 of mechanismsInt. J. Mol. Sci. 2020 and, 21 superstructures, x FOR PEER REVIEW have always attracted significant attention from scientists.2 of 36 Indeed, fromscientists. the understanding Indeed, from of chiralitythe understanding phenomena of ch inirality natural phenomena systems toin the natural increasing systems application to the to artificialincreasing intelligence application systems, to artificial subjects intelligence related to systems, chirality subjects will clearly related become to chirality more will developed clearly and providebecome broader more guidance developed for and academic provide research broader andguid industrialance for academic application research [9–11 ].and industrial applicationChirality has [9–11]. been regarded as a basic characteristic of living organisms and nature, which can be manifestedChirality at various has been hierarchical regarded levelsas a basic (Figure characteristic1)[ 12]. Theof living research organisms on the and mechanism nature, which of induction, can transferbe manifested and amplification at various of hierarchical chirality plays levels an (F importantigure 1) [12]. role The in the research evolution on the of nature,mechanism theorigin of of life andinduction, the development transfer and ofamplification chiral materials of chirality [13]. Atplays the an subatomic important level,role in left-handed the evolution helical of nature, neutrinos the origin of life and the development of chiral materials [13]. At the subatomic level, left-handed indicates that chirality is related to parity conservation. Covalently bonded molecules can be chiral helical neutrinos indicates that chirality is related to parity conservation. Covalently bonded by asymmetrically arranging atoms in space around a center (point chirality), axis (axial chirality) or molecules can be chiral by asymmetrically arranging atoms in space around a center (point chirality), planeaxis (planar (axial chirality) or [2 plane]. The (planar deeper chirality) investigations [2]. The deeper of molecular investigations chirality of molecular can provide chirality guidance for thecan design provide of guidance drugs and for functionalthe design of materials. drugs and Furthermore, functional materials. chirality Furt alsohermore, can be chirality expressed also at the supramolecularcan be expressed level at (suchthe supramolecular as proteins andlevel DNA) (such as [14 proteins–16]. Supramolecular and DNA) [14–16]. chirality Supramolecular involves the nonsymmetricchirality involves arrangement the nonsymmetric of building arrangement blocks in a of noncovalent building blocks self-assembly in a noncovalent [17]. Itself-assembly should be noted that although[17]. It should supramolecular be noted that chiralityalthoughis supramolecular strongly related chirality to the is chirality strongly of related its components, to the chirality however, of it is notits necessary components, that however, all building it is not blocks necessary have that chiral all nature.building In blocks most have cases, chiral supramolecular nature. In most chirality cases, can be constructedsupramolecular from chirality chiral orcan achiral be constructed buildingblocks from chiral by weak or noncovalentachiral building interactions, blocks by suchweak as π-π noncovalent interactions, such as π-π stacking, hydrogen bonding, electrostatic interactions, van der stacking, hydrogen bonding, electrostatic interactions, van der Waals force and so on [18–22]. Chirality Waals force and so on [18–22]. Chirality at the macroscopic level, such as the helical structure of at thecucumber macroscopic vines and level, the such spiral as movement the helical of the structure cosmic ofnebula, cucumber provides vines a powerful and the inspiration spiral movement for of themankind cosmic to nebula, manufacture provides controllable a powerful macroscopic inspiration helical for materials. mankind Among to manufacture these various controllable levels macroscopicdescribed helicalabove, materials.supramolecular Among chirality these variousis of vital levels importance described because above, it supramolecularis closely related chiralityto is ofchemistry, vital importance physics, biomedical because it sciences is closely and relatednanoma toterials chemistry, [23]. Chirality physics, at a biomedicalsupramolecular sciences level and nanomaterialshas attracted [23 great]. Chirality attention atdue a supramolecularto rapid developments level hasin supramolecular attracted great chemistry attention and due self- to rapid developmentsassembly. in supramolecular chemistry and self-assembly. Figure 1. Chiral architectures at various scales, from neutrino to enantiomeric molecules, proteins and Figure 1. Chiral architectures at various scales, from neutrino to enantiomeric molecules, proteins and DNADNA biomacromolecules, biomacromolecules, macroscopic macroscopic livingliving systems and and galaxy. galaxy. It isIt well is well known known that that self-assembly self-assembly plays an an important important role role in inchemistry chemistry due dueto the to rapid the rapid developmentsdevelopments of supramolecularof supramolecular chemistry chemistry and an nanotechnology.d nanotechnology. Generally, Generally, the termthe term “self-assembly” “self- can beassembly” defined can as thebe processdefined as of the autonomous process of arrangementautonomous arrangement of the system’s of the components system’s components into predefined architecturesinto predefined under architectures given boundary under conditions given bounda [24],ry which conditions is a “bottom-up”[24], which is approach a “bottom-up” to produce intriguingapproach nanostructures to produce intriguing [25]. In general, nanostructures two approaches [25]. In general, for preparing two approaches nanoassemblies for preparing are usually considerednanoassemblies in the fieldare usually of self-assembly considered in chemistry the field of self-assembly [26]. The first chemistry one is [2 the6]. mostThe first commonly one is the used methodmost called