Photoresponsive Molecules

Since the term photochromism applies only to photophysical phenomena related to the reversible change in color triggered by light, a new denomination had to be found for molecules which show a reversible response to photoirradiation by a change in a chemical function or in a physical property other than color. They were named photoresponsive molecules. By analogy, photoresponsive polymers are polymers which undergo a change in various properties and functions upon photoirradiation. 

The molecular conformation of a macromolecule is one of the fundamental physical properties of polymers, since it controls macroscopic properties, such as viscosity or solubility. There have been many attempts to stimulate reversible changes in polymer conformation under controlled and reproducible conditions in order to create responsive polymers. One approach is to induce a structural change in photosensitive groups incorporated into the polymer chains, such as a trans-cis isomerization. Another method is to generate ionic charges along the polymer chains. The repulsive interactions thus created force the chain to adopt a different conformation. 

As an example of the first technique, let us consider the case of a polyamide containing photoresponsive azobenzene groups on its main chain (14). The distance between the two para-positions is 9.0 A in trans-azobenzene, and only 5.5 A in the cis-isomer. The cis-trans photoisomerization of these groups is reflected directly by a molecular expansion of the 

As an example of conformational change induced by the introduction of electric charges along a macromolecule, let us consider the polymer (15) which in its side chains carries triarylmethane groups in their leuco-form (15). Upon photoirradiation this neutral group dissociates into triphenylmethyl cation and hydroxyl anion. The polymer chain of the irradiated sample adopts an expanded conformation to alleviate the unfavorable ionic interactions between adjacent cations. These changes are reflected by a marked increase in solution viscosity upon photoirradiation in aqueous or methanolic solutions of a polyacrylamide with side chains of structure (15). The viscosity recovers its initial value when irradiation is discontinued. 

Photoisomerization of azobenzene moieties has been used also to trigger ion transport in photoresponsive membranes.161 A schematic representation of the photoinduced membrane transport of potassium ion is given in Table 2.10 . The bis(crown ether) (16) can form a 1 2 metal/crown ether complex with large metal ions which cannot be accommodated within simple crown ethers. This photoresponsive function of the azobenzene bis(crown ether) accelerates the transport of potassium and calcium cations across liquid membranes under photoirradiation. 

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To date, a number of optically active polymers, particularly helical polymers with an excess one-handedness, have been prepared with much interest. However, helical poly (triaryl methacrylates) are the only examples of successful application to HPLC stationary phases [22,103]. Helical polymers combined with photoresponsive molecules will offer promising functions and applications as realized by natural macromolecules. 

Mauser diagrams are very versatile. Every wavelength-dependent property can be used for their construction, e.g., the refractive index or, for optically active photoresponsive molecules, the ellipticity, a property quite sensitive to asymmetric perturbations of the environment. An example is given in Figure 1.6," which shows that on fixation of the azo steroid in PMMA, an optically inactive matrix, the sites are not equivalent. 

S. Kurihara, T. Ikeda, and S. Tazuke, Photochemically induced isothermal phase transition in liquid crystals. Effect of interaction of photoresponsive molecules with matrix mesogens, Mol. Cryst. Liq. Cryst. 178, 117-132(1990). 

Photocontrol of chemical and physical functions could be used in this system (Fig. 6.21). Stilbazole, a compound related to natural photoresponsive molecules, was used as the photoresponsive switch [136]. Because stilbazole undergoes isomerization from fhe trans form to fhe cis form on UV irradiation, and fhe reverse in visible hght, via complexation of the pyridine group to fhe metal center of metalloporphyrins, fhe (TPP)Al-OMe-stilbazole system on irradiation wifh UV light serves as an on-switch to speed up the reaction whereas in visible hght fhe system worked as an off-switch to reduce the rate of the once accelerated reaction. 

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. 

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. 

A number of systems where photoresponsive molecules are used as ligands to form metal complexes, mono- or multimetallic, have been reported during the last decade. Such complexes exist for a wide range of metal centers (Cr, Mo, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pt, Cu, Ag, Au, Zn) and more recently lanthanides (Eu). The photochromic properties of the organic framework were found, in some cases, to be enhanced by complexation and, as its organic counterpart, also effective in single-crystalline phase. 

Although overcrowded afkenes are attractive due to the efficient switching of LC handedness, the delicate synthesis of overcrowded alkenes remains a limitation for their practical applications. Normally the separation of two enantiomers of crude racemic product to obtain optically pure isomer has to rely on the preparative high performance liquid chromatography (HPLC), which is not suitable for a large-scale preparation. Consequently, researchers are seeking for other types of photoresponsive molecules for the switching of helix handedness. 

When azobenzene groups possess flexible alkyl substituents in their phenyl rings, they often self-assemble into ordered LC phases, as shown in Figure 7.4. In this case, azobenzene moieties may play both the role of photoresponsive molecule and rod-Uke mesogen. Upon irradiation with unpolarized UV light, trans-to-cis photoisomerization occurs, which converts the rod-like mesogen into a more globular molecule... 

Polymer films comprising photoresponsive molecules such as 2-nitrobenzyl [260, 261], spiropyran [262-264], azobenzene [265], cinnamoyl [266], and coumarin [267] represent other attractive candidates for controlling protein and cell adhesion on... 

Polymer or SiUca Film Photoresponsive Molecules Containing an Azobenzene Unit... 

Dendrimers belong to a new class of multi-branched macromolecules " and have attracted much attention in recent years as photoresponsive molecules. This interest stems from the fact that the precise placement of a photoactive group in a well-defined macromolecule is possible, and predictable control of the reversible photoinduced molecular change for altering their function can be expected for these molecules. 

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