Photoresponsive guest

Photochemical modulation of the helical screw sense and pitch of a cholesteric phase was achieved with the combination of a nematic liquid crystalline host and an optically active photoresponsive guest as illustrated in Scheme 25.[92] Doping of 4 -(pentyloxy)-4-biphenylcarbonitrile 41 with P-trans-17b (1 wt%) converts the nematic phase into a cholesteric phase. 

Photoresponsive host-guest systems based on azobenzene-substituted crown ethers have been shown to be particularly effective in the control of molecular recognition by light, due to their large geometrical changes upon E-Z isomerization. 55 A num-... 

To obtain fast LC photoresponse, a new guest/host system was developed, in which ferroelectric LCs (FLCs) were used as a host LC. FLCs exhibit spontaneous polarization (Ps) and show microsecond responses to change in applied electric field (flip of polarization) in a surface-stabilized state.1261 If a flip of polarization of FLC molecules in the surface-stabilized state can be induced by light in the presence of an applied electric field, photoresponse in the microsecond time region might be achievable. 

Fig. 3 Photoresponsive polymer surface sensitive to pH and light. Adsorption and release of cytochrome c triggered by pH (b, c, and d) release of the polymer layer and cytochrome c by breaking the host-guest interactions between surface-tethered azo dye and cyclodextrin via light irradiation (a and d). The molecular structure on the right represents the host-guest complexa-tion of the azo dye with the cyclodextrin-modified poly(acrylic acid). Reprinted, with permission, from [68]. Copyright (2009) Wiley Interscience...

T. Ikeda, T. Miyamoto, S. Kurihara, M. Tsukada, and S. Tazuke, Effect of structure of photoresponsive molecules on photochemical phase transition of liquid crystals. II. Photochemical phase transition behavior of photochromic guest/host mixtures Mol. Cryst. Liq. Cryst. 182, 373-385 (1990). 

The various examples of photoresponsive supramolecular systems that have been described in this chapter illustrate how these systems can be characterized by steady-state and time-resolved spectroscopic techniques based on either absorption or emission of light. Pertinent use of steady-state methods can provide important information in a simple vay stoichiometry and stability constant(s) of host-guest complexes, evidence for the existence of photoinduced processes such as electron transfer, energy transfer, excimer formation, etc. Investigation of the dynamics of these processes and characterization of reaction intermediates requires in most cases time-resolved techniques. Time-resolved fluorometry and transient absorption spectroscopy are frequently complementary, as illustrated by the study of photoinduced electron transfer processes. Time-resolved fluorometry is restricted to phenomena whose duration is of the same order of magnitude as the lifetime of the excited state of the fluorophores, whereas transient absorption spectroscopy allows one to monitor longer processes such as diffusion-controlled binding. 

Sasaki T, Ikeda T. 1995b. Photochemical control of properties of ferroelectric liquid crystals. 2. Effect of the structure of guest photoresponsive molecules on the photo chemical switching of polarization. J Phys Chem 99 13008 13012. 

Pouliquen G, Amiel C, Tribet C. 2007. Photoresponsive viscosity and host guest associa tion in aqueous mixtures of poly cyclodextrin with azobenzene modified poly(acrylic) acid. J Phys Chem B 111(20) 5587 5595. 

Harada, A., 2012. Expansion-contraction of photoresponsive artificial muscle regulated by host-guest interactions. Nat. Commun. 3,1270. 

As chiral LCs can be formed by chiral mesogens or induced by chiral guest molecules, photoresponsive chiral LCs can be derived from these two systems by photosensitization. One strategy is to photosensitize the existing constituent molecules in chiral LCs, i.e. LC host or chiral dopant. The other strategy is to dope additional photoresponsive agents to the existing chiral LC systems. These two... 

Fig. 8 Structure of amphiphilic CD vesicles and photoresponsive adhesion/dispersal behaviour with azobenzene-containing ditopic guest. Adapted with permission from ref. 59. Copyright 2010 John Wiley and Sons.

Fig. 9 structure of the azobenzene-spermine ditopic guest (azo trans, yellow-green azo cis, red-green), anchoring to DNA and photoresponsive vesicles of amphiphilic CDs of Fig. 15, in solution. Adapted with permissioh from ref. 61. Copyright 2010 Johh Wiley and Sons. 

Fig. 11 Schematic representation of the photoresponse of the p-CD trimer with the bis-azobenzene guest. Reproduced from ref. 65 with permission of The Royal Society of Chemistry.

Supramolecular spherical assemblies of NPs with photoresponsive adhesion/dispersal behaviour were also obtained in a ternary system hierarchically combining the host-guest interaction of different types of CDs toward porphyrin and azobenzene. The inclusion complexation of an azobenzene modified water soluble porphyrin (1) with phthalo-cyanine-grafted permethyl (3-CDs (2) could be reversibly cross-linked to relatively larger nanospheres with naphthyl bridged bis(a-CD)s (3). The large spheres (12 -3) turned reversibly to small-sized particles (1 2) upon photoisomerization of the azoaromatic group in 1 (Fig. 13). 

Hydrogel photoresponsiveness was obtained in a copolymer of acrylic acid and N-dodecylacrylamide in the presence of a-CD and 4,4 -azodibenzoic acid. The gel, formed by hydrophobic association of the copolymer side chains, turned to sol on addition of a-CD because of the inclusion of the hydrophobic chains. In the presence of 4,4 -azodibenzoic acid, as competitive guest for a-CD, the sol-gel transition could be reversibly controlled via the trans-cis photoisomerization of the azocompound. ... 

PPR hydrogels were obtained for the first time by Harada and coworkers with PEG chains threading a-CD units. Photoresponsive hydrogels were also created from PEG-a-CD systems by addition of l- p-(phenylazo) benzyljpyridinium bromide as competive guest for a-CD. Photoreversible control of sol-gel transition was achieved through trans-cis photoisomerization. (Fig. 17). A light controlled fluorescent PPR... 

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