Photoresponsive Hosts

Nature uses chemical signalling in sensory systems snch as taste and smell. The most important means for information processing in nature is based on information exchange between DNA and RNA our brain use signals relying on the behaviour of ions, etc. This 

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Bohmer, A. Dalla Cort, L. Mandohni, Counteranion effect on complexation of quats by a neutral calix[5]arene receptor, J. Org. Chem., 2001, 66, 1990-1902 b) A. Arduini, E. Brindani, G. Gioigi, A. Pochini, A. Secchi, Ainion effects on the recognition of ion pairs by calix[4]arene-based heteroditopic receptors, J. Org. Chem., 67, 2002, 6188-6194. 

Bartoli, S. Roelens, Electrostatic attraction of counterion dominates the cation-rr interaction of acetylcholine and tetramethylammonium with aromatics in chloroform, J. Am. Chem. Soc., 1999, 121, 11908-11909. 

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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-... 

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. 

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...

Layered materials serve as good hosts for accommodating photoresponsive molecular species. Ogawa and Kuroda [150] reviewed the photoprocesses in layered hosts. 

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. 

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... 

Photoresponsive molecules are required for the fabrication and photomodulation of photoresponsive CLC materials regardless of being employed as chiral mesogens, achiral LC host, or chiral/achiral dopants. The photoisomerization of molecules leads to change in molecular shape (geometry/conformation) and alter the bulk properties of LC material, which constitutes the basis for the photomodulation in chiral LCs [28, 29]. There are many types of photoresponsive molecules and some examples are shown in Fig. 5.4. 

Similar to the phase transition from nematic to isotropic phase induced by azobenzene molecules, the trans-cis isomerization also destabilize the SmC phase composed of calamitic mesogens and lower the Curie point, which is a transition temperature where the SmC will transform from ferroelectric to non-ferroelectric. Some examples have been reported with an early demonstration by Ikeda et al. [130-135]. For example, a photoresponsive SmC was formulated by doping 3 mol % of 4,4 -disubstituted azobenzene 29 into a FLC host 27 and the UV irradiation at 260 nm resulted in the lowering of Curie point and the coercive force Ec required to switch the SSFLC due to the destabilization of bent shape cis isomers [130] (Fig. 5.23). When the electric field was close to Ec before irradiation, the flip of polarization of SmC was achieved. It is noteworthy that its response time 500 ps is much faster than normally observed for photochemical N-I phase transitions [131]. 

Photoresponsive psewdorotaxanes, were also obtained from heteromacrocycle combination of CD, cucurbituril and calixarene hosts, to achieve multiple functions. ... 

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). 

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