Photoisomerization of azobenzene molecule

UV (360 nm) and blue (450 nm) light irradiations of the ultrathin azo-silane SAMs clearly induce the forth, i.e., trans—>cis, and back, i.e., cis—>trans, photoisomerization of azobenzene molecules (see Figure 4.2A). The real-time dependence of the absorbance of the sample during the thermal cis—>trans back reaction is not a monoexponential decay (see Figure 4.2B). This decay shows a complex multiexponential relaxation behavior that could be fit neither by a monoexponential decay nor by a biexponential relaxation. Nevertheless, a monoexponential decay could be fit to the data acquired over... 

The decrease in order parameter in LCs caused by the photoisomerization of azobenzene molecules are also known to induce phase transition from chiral smectic phase to cholesteric phase. This type of transition is mainly observed in chiral mesogenic compounds. In 1999, My et al. conducted a systematic research on the phase transition behavior of a series of chiral azobenzene LCs [44]. A photo... 

In reality, the liquid crystalline-to-isotropic phase transition temperature Tc) of the copolymer or LCP mixture with the cw-form Tec) is lower than that with the trans-isomer (Tct). The presence of the cw-azobenzene moieties depresses the transition temperature of the host liquid crystalline phase based on the concentration. If the temperature of the mixture T) is set between Tct and Tec and the sample is irradiated to cause trans-to-cis photoisomerization of azobenzene molecules, Tc decreases with an accumulation of the number of cw-isomer. When Tc becomes lower than the irradiation temperature, the liquid crystalline-to-isotropic phase transition is induced. The photoinduced phase transitions are interpreted in terms of a change in the phase transition temperature of PLCP systems on accumulation of one isomer of the photochromic molecule. As expected from Figure 7.5, AT (= Tct - Tec) is one of the most important parameters in the phase transition [1], When the temperature of systems is set below Tec, no phase transition is induced. When the temperature is set close to Tct, the amount of cw-azobenzene needed to lower Tc is relatively small, enabling one to effectively induce phase transition upon UV irradiation [10]. 

Switching devices that are reversible and work on the molecular level are essential features of nanomachinery. Control of the access to capsules, the transport of molecules in and out of the cavities, is desirable and we examined a well-established system that uses light as a switching device the cis-trans photoisomerization of azobenzenes [58, 59]. The azobenzenes have been applied in the supramolecular chemistry of crown ethers [60-62], cyclodextrins [63,64], and even proteins [65, 66]. The photoisomerization changes the shape in a predictable way and we used azobenzene photoisomerization in an indirect sense to control reversible encapsulation. 

Shen, Y.-Q., and Rau, H. (1991). The environmentally controlled photoisomerization of probe molecules containing azobenzene moieties in solid poly(methylmethacrylate). Makromol. Chem. 192, 945-957. 

This chapter is organized as follows. Section 4.2 addresses the study of photoisomerization and photoinduced orientation of azobenzene molecules at the molecular level in SAMs of azo-silane molecules. Section 4.3 discusses photoinduced effects in supramolecular assemblies, i.e., LBK multilayer structures containing azobenzene molecules, and compares the photoinduced movement of azobenzenes in these structures to that observed in spin-cast films. Section 4.4 focuses on the isomerization and sub-Tg photoinduced orientation in a series of very high Tg (up to 350°C) nonlinear optical polyimide and thermoplastic donor-embedded polyurethane polymers containing azo dye, especially focusing on polymer structure-Tg-photoinduced molecular movement relationships. Section 4.5 describes pressure effects on photoisomerization and photo-orientation in films of a PMMA polymer containing azo dye. Finally, we make some concluding remarks in Section 4.6. 

We designed an amphiphilic molecule, in which three functional parts are involved azobenzene as the switching unit, an alkyl chain as the transmission unit, and a TCNQ charge transfer salt as the working unit (APT(8-12), Fig. 3). The molecule is assembled in an ordered array by a horizontal lifting method on a solid substrate. In this arrangement, the cis-trans photoisomerization of azobenzene induces the reversible change in the lateral conductivity of the LB film. 

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