Azobenzene molecular motion

In order to obtain stable amorphous states of organic materials, the glass transition temperature (Tg) of the materials should be higher than room temperature. Molecular motion is frozen at temperatures below Tg, and crystal growth is suppressed. The Tg of la was found to be lower than room temperature. Although several photochromic amorphous materials containing azobenzene unit have been synthesized, the practical use of amorphous azobenzenes is limited because of thermal instability of the cis-isomer and low reactivity in the solid state. Figure 17.3 shows some molecular structures... 

The photochromism of azobenzene probe molecules tells us furthermore that bulk materials differ distinctively from silica films.59 The pores in the sol-gel bulk materials were larger than those in the corresponding sol-gel films. The first-order kinetics of the thermal isomerization of the photogenerated Z-isomer revealed that the steric effects restricting the molecular motion increased in the order sol-gel... 

The light-induced isomerization of the azobenzene moiety is a classical example of controlled molecular motion and has provided the basis for the construction of some of the first archetypes of molecular machines [17]. In system 5, the pendant-arm/ring interaction concurs to improve the efficiency of the azobenzene-based engine, which converts photonic energy into a mechanical work, at the molecular level. 

The kinetics of spiropyran and azobenzene photoisomerization deviate from first order when these dyes are entrapped in a solid matrix below Tg.24-34 This behavior has been attributed to the presence of a distribution of free volume within the matrix, as shown in Table 3.11 .35 When the probe is located in sites of free volume Vf greater than the critical volume for isomerization Vfc, the reaction proceeds at the same rate as in solution. For sites of Vf < Vfc, the reaction is retarded, since it becomes controlled by the matrix molecular motions. At low temperature, the local molecular motions are frozen and fluctuations of local free volume become increasingly small as the temperature decreases. Consequently the fraction of sites where Vf < Vk increases. 

Nagamani SA, Norikane Y, Tamaoki N. 2005. Photoinduced hinge like molecular motion studies on xanthene based cyclic azobenzene dimers. J Org Chem 70(23) 9304 9313. 

The temperature depaidence of fdiotoisomerization in polymer solids yields detailed information about the effects of free volume distribution and molecular motion on the reaction process. The present authors measured the rate of trans -+ cis photoisomerization of azobenzene in a polycarbonate film over a very wide range of temperatures (4 K-423 K), and proposed a kinetic model, which consists of chemically... 

Photochemical and thermal isomerization of azobenzene derivatives were studied in silica-gel matrices (Ueda, 1993, 1994). Triethoxysily groups linked with the azobenzene moiety had no effect on isomerization reaction. The first-order kinetics of the thermal isomerization of the photoinduced cis isomer reveals that the steric restrictions to suppress the molecular motion decreased in the following order sol-gel films > poly(methyl... 

Another system constructed based on the unique coordination property of Cu is a molecular photo-electro transducer , which works in a cyclic manner totally powered by light irradiation [60]. The UV/blue light controlled repetitive motion of azobenzene moieties in 6,6/-bis(4"-tolylazo)-4,4/-bis(4-ferf-bulylphcnyl)-2,2 -bipyridine, 74, causes reciprocal Cu1 translocation between two coordination environments in complex 75, resulting in pumping of the redox potential of Cu1 (Scheme 3). Therefore, UV/blue light information can be successfully transformed into an electrode potential change and... 

Natansohn, A., Rochon, P., Meng, X., Barrett, C., Buffeteau, X, Bonenfant, S., and Pezolet, M. Molecular addressing Selective photoinduced cooperative motion of polar ester groups in copolymers containing azobenzene groups. Macromolecules 31, 1155 (1998). 

To summarize, there are three types of photoinduced motions at the molecular level, at the nanometer, or domain, level, and at the micrometer (macroscopic) level. All are the result of photoinduced isomerization of the azobenzene groups. One interesting direction of our research was to try to exploit these phenomena for their photonic applications. We have demonstrated, at least as proof of principle, a few possible photonic functions for the new materials we synthesized. Some of these are summarized in publications they will also be reviewed here. 

DYNAMIC PHOTOCONTROLS OF MOLECULAR ORGANIZATION AND MOTION OF MATERIALS BY TWO-DIMENSIONALLY ARRANGED AZOBENZENE ASSEMBLIES... 

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