Nanophotonics and Supramolecular Chemistry
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© 2013 Science Wise Publishing & DOI 10.1515/nanoph-2013-0025 Nanophotonics 2013; 2(4): 265–277 Review article Katsuhiko Ariga*, Hirokazu Komatsu and Jonathan P. Hill Nanophotonics and supramolecular chemistry Abstract: Supramolecular chemistry has become a key top down (engineering down) nanofabrications. These area in emerging bottom-up nanoscience and nanotech- top-down approaches are based on suitable lithographic nology. In particular, supramolecular systems that can or ion implantation techniques and have resulted in produce a photonic output are increasingly important various types of densely integrated device structures that research targets and present various possibilities for make an immense contribution to high technology fields practical applications. Accordingly, photonic proper- such as silicon-integrated chip technology. However, ties of various supramolecular systems at the nanoscale according to the so-called Moore’s law [6, 7], the current are important in current nanotechnology. In this short rate of miniaturization in silicon device technology will review, nanophotonics in supramolecular chemistry will very soon be adversely affected by the physical limits of be briefly summarized by introducing recent examples device dimensions imposed by ultra-violet, electron/ion of control of photonic responses of supramolecular sys- beam and soft X-ray lithographic techniques. tems. Topics are categorized according to the fundamental In a breakthrough made possible by nanoscience and actions of their supramolecular systems: (i) self-assembly; nanotechnology, an alternate approach based on bottom- (ii) recognition; (iii) manipulation. up (engineering-up) concepts has been introduced. These approaches rely on spontaneous processes of self-assem- Keywords: supramolecular chemistry; photonic response; bly [8–11] and subsequent formation of nanostructure self-assembly; molecular recognition; manipulation. formation [12–15]. Self-assembly processes are often con- ducted under mild conditions unlike top-down nanofab- rication procedures that often require ultrahigh vacuum *Corresponding author: Katsuhiko Ariga, World Premier and laser irradiation. Thus, bottom-up approaches are International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials applicable for the formation of nanostructures of various Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan, soft materials including organic molecules, biomolecules Phone: +81-29-860-4597, Fax: +81-29-852-4832, and their hybrids. Therefore, supramolecular chemistry, e-mail: [email protected]; and Japan Science and which deals with molecule-molecule interactions, has Technology Agency, CREST, 1-1 Namiki, Tsukuba 305-0044, Japan become a key science in emerging bottom-up nanoscience Hirokazu Komatsu: World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute and nanotechnology. for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan In order to convert scientific knowledge to a technical Jonathan P. Hill: World Premier International (WPI) Research Center application, basic phenomena should be integrated into for Materials Nanoarchitectonics (MANA), National Institute for functional systems such as stimuli-responsive devices. Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan; Nanotechnology involving bottom-up approaches often and Japan Science and Technology Agency, CREST, 1-1 Namiki, requires supramolecular systems to produce an output Tsukuba 305-0044, Japan implying some useful functionality including electronic, Edited by Volker Sorger photonic, chemical and biological outputs [16–18]. Of these, systems based on photonic outputs are of increas- ing current interest with various possibilities for practical 1 Introduction application. Therefore, photonic properties of nanosized forms of various supramolecular systems will be of crucial Over the past 20 years, major discoveries in science and importance in nanotechnology in the near future. innovation in technology have led to the development of In this short review, nanophotonic aspects of supra- nanoscience and nanotechnology where the fabrication of molecular chemistry are briefly summarized by intro- ultrasmall objects and/or control of highly precise struc- ducing typical recent examples of control of photonic tures are critical [1–5]. Successful techniques for nano- responses from supramolecular systems. The topics are structure control currently rely heavily on sophisticated roughly categorized according to the fundamental actions 266 K. Ariga et al.: Nanophotonics and supramolecular chemistry © 2013 Science Wise Publishing & of the supramolecular systems: (i) self-assembly; (ii) rec- ognition; (iii) manipulation. The examples introduced illustrate clearly the critical relationship between nano- photonics and supramolecular chemistry. 2 Self-assembly Organic dye molecules when contained in supramo- lecular assemblies exhibit very different spectroscopic characteristics compared to those observed for their mono-dispersed states in solution [19–21]. As is known for different assembly modes such as J- and H-aggregates, assembly motifs sensitively influence the spectroscopic properties of both their absorption and emission profiles. Therefore, photonic properties of self-assembled struc- tures of chromophore molecules are attractive research targets in supramolecular chemistry. Ajayaghosh and coworkers have investigated the photonic characteristics of gel-type assemblies of rather small chromophore molecules [22–24]. For example, they researched thienylenevinylene-based gels (Figure 1A) that exhibit epitaxial self-assembly to form aligned supra- molecular wires [25]. The hydrogen-bonded networks obtained possessed enhanced electrical properties with high charge carrier mobility. In addition, thienylenevi- Figure 1 Gel-forming chromophores: (A) thienylenevinylene-based nylene molecules with amide end groups exhibited a gelator; (B) perfluoroarene-based oligo(p-phenylenevinylene) gelator. considerable difference in their absorption spectra when contained in supramolecular assemblies. A large blue shift for the assembled structures was attributed to a highly beneficial if used in a simple and low-cost pro- large oscillator strength and strong exciton coupling tocol for the on-site detection of TNT on contaminated based on formation of a one-dimensional arrangement specimens. of the molecules in H-type aggregates. Reversible forma- Ajayaghosh et al. have also demonstrated photo- tion of the aggregates from individual molecules was induced reversible dot to rod variation of assembled also confirmed in variable temperature experiments. The structures of azobenzene derivatives 1, 2 [27]. Irradia- same research group reported attogram sensing of the tion of nanodots with UV light induced partial conver- explosive 2,4,6-trinitrotoluene (TNT) by observing the sion of trans-azobenzene 2 to its cis-isomer 1. Even photonic behavior of a self-assembled gel of perfluoro- though the conversion yield is not very large a locally arene-based oligo(p-phenylenevinylene) (Figure 1B) due high concentration of the cis-isomer 1 on the surface of to their fluorescence responses [26]. When predeposited the dots enhanced the surface dipole moment making on disposable paper strips the gel could be used to detect the nanodot structures unstable due to an increase TNT at the attogram (10-18 g) level with a detection limit in ζ-potential. This situation facilitates interparti- of 0.23 ppq. TNT molecules fit vertically into the free cle association, resulting in formation of rod-shaped spaces of the gel through electrostatic interactions with structures. They also synthesized azobenzene-linked its fibers. Edge-to-face π-stacking between the electron- phenyleneethynylenes that could form helical supra- deficient aromatic core of trinitrotoluene and the elec- molecular assemblies [28]. Helicity of the assemblies tron-rich perfluoroarene moieties of the gel resulted in formed were of a specific chirality that could be revers- formation of a strongly bound complex. The entrapped ibly switched to the opposite helical sense through trinitrotoluene molecules play the role of a fluorescence a chiral-center-controlled photoisomerization of the trap. The reported attogram detection level would be attached azobenzene moieties. © 2013 Science Wise Publishing & K. Ariga et al.: Nanophotonics and supramolecular chemistry 267 HO OH RO OR RO OR RO N HO N OR N N RO OR OH 1. cis-azobenzen derivative 2. trans-azobenzen derivative George and coworkers used an unsymmetrical coro- HN nene monoimide 3 for formation of hydrogen-bonding- N O O H H O N mediated supramolecular gels. Gels are formed through N O H H the concept of repulsion between dipoles to maintain a O O N head-head staggered organization [29]. Hydrogen-bond- 4 NH ing enforcement minimizes dipolar repulsion between chromophores. The structures of supramolecular assemblies alter RO depending on the relative positions of carboxylic acids at O the benzene unit, leading to diverse photonic properties. RO O HN Some of the assemblies exhibited a strong luminescence RO N enhancement by doping with europium (III) chloride. O This feature is difficult to achieve using normal synthetic 3 polymers. Kim and coworkers investigated fluorescence prop- The formation of head-head assembly is highly erties of pyrene-containing poly(arylene ether sulfone) advantageous for nonlinear optical activity. Sierra, Bar-