Azobenzene molecules

Photorefractivity is a property exhibited by some materials in which the redistribution in space of photogenerated charges will induce a nonuniform electric space-charge field which can, in turn, affect the refractive index of the material. In a new material the active species is a highly efficient cyclopalladated molecule97,98 shown in Figure 5. The palladium-bonded azobenzene molecule is conformationally locked, and gratings derived from cis—trans isomerizations can be safely excluded. 

Zhang C, Du MH, Cheng HP, Zhang XG, Roitberg AE, Krause JL (2004) Coherent electron transport through an azobenzene molecule a light-driven molecular switch. Phys Rev Lett 92(15) 158301... 

In this paper we will present recent experiments on different types of azobenzene molecules with transient visible and IR-spectroscopy which show that ultrafast structural changes of peptide molecules occur on the time-scale of 10 ps. 

Coordination chemistry reveals how two ArN species can be coupled into one azobenzene molecule. In the case of the reaction of Fe3(CO)12 with aromatic nitro compounds in benzene1161, formation of derivatives such as in structure 111 has been proven by X-ray diffraction. Azoxybenzene can be formed by reaction of nitrene with nitrosobenzene, formed by reduction of nitrobenzene. 

In addition, more recently, an interesting approach to fast response of photorespon-sive LCs has been reported. Crosslinked PLC networks containing azobenzene molecules were prepared by polymerization of ternary mixtures of monofunctional and difunctional LC monomers together with a LMW azobenzene LC, as shown in Figure... 

A closely related phenomenon induced by linearly polarized light was found independently by Gibbons et al., who employed a polyimide (PI) film doped with azobenzene molecules as a dichroic dye and showed that the direction of homogeneous... 

The relevant vibrations for this review are the N=N and C-N (Ph-N) stretching vibrations and, perhaps, torsional vibrations around the C-N bond. The E-azobenzene molecule has a center of inversion, and therefore the N=N vibration is infrared-inactive, but Raman-active, and has been found to be at 1442 cm". By IR spectroscopy, Kiibler et al. located the symmetric C-N stretching vibration at 1223 cm" in E- and at 866 cm in Z-azobenzene. The N=N vibration in Z-azobenzene is at 1511 cm" (in KBr pellets). These numbers are confirmed by newer work Biswas and Umapathy report 1439 and 1142 cm for the N=N and C-N vibrations (in CCE), and Fujino and Tahara found nearly identical results (1440 cm" and 1142 cm ). A thorough vibrational analysis of the E-isomer is given by Amstrong et al. The vibrations in the (n,7t ) excited state are very similar 1428 cm" and 1130 cm"h... 

The use of Walsh diagrams, based on one-electron molecular orbitals, shows that on n —> 7t excitation the azobenzene molecule is stretched, which is the beginning of inversion. All calculations and suggestions for an inversion mechanism agree that the potential energy curve for inversion has a relatively steep slope at the E- and the Z- geometries. This is corroborated by the experimental evidence of a continuous n tc absorption band in both isomers. In fact, a structured n band in an azo compound that can isomerize has never been observed. 

The process of reorientation during cis—>trans thermal isomerization can be seen at the value of in Equation 3.11, which shows that the cis anisotropy does not contribute to the trans anisotropy if the trans isomer loses total memory of the orientation in the cis isomer Q2 = 0). It is informative to note that in the realistic physical case—i.e., the case of the azobenzene molecule chemically attached to a polymer, where the cis and trans diffusion rates are negligible in comparison to the cis— trans isomerization rate—the relaxation of the cis and trans anisotropy, AA and can be written respectively in the form ... 

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. 

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... 

Kurihara, S., Sakamoto, A., Yoneyama, D., and Nonaka, T. Photochemical switching behavior of liquid crystalline polymer networks containing azobenzene molecules. Macromotecutes 32, 6493 (1999). 

Kurihara, S., Matsumoto, K., and Nonaka, T. Optical shutter driven photochemieally from anisotropic polymer network containing liquid crystalline and azobenzene molecules. J. App/. P s. Lea. 73,160 (1998). 

With azobenzenes 661, vinylidene carbene, generated from silylvinyl triflate, gives tetrazoles 662 as a result of [3+2]-cycloaddition of the intermediate azomethine imines 663 to the N = N bond of a second azobenzene molecule (84JA6015). 

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