Thermal isomerization pseudo-stilbenes

One concludes from these facts that pseudo-stilbenes are not suitable for persistent switching of the molecular form. Any information based on E-Z isomerization is quickly lost. If, however, fast interconversion of E- and Z-forms is the aim, as it is in the alignment of the higher-order polarizability tensor of donor/acceptor azobenzenes, then thermal isomerization supports the photoisomerization process. 

For pseudo-stilbene-type molecules, the question of the mechanism of thermal isomerization was taken up again in the early eighties by Whitten et al. and later by Kobayashi et al., " who, on the basis of their isomerization experiments with donor/acceptor-substituted azobenzenes in polar solvents, postulate rotation. Asano and coworkers infer from the isomeriza-... 

E,Z-isomerization can be induced by a direct (singlet) or sensitized (triplet) route from the n,Jt or 7t,7t state. The thermal back-reaction from the Z-isomer is slow in azobenzene and fast in amino- or aminonitro-substituted azobenzenes (pseudo-stilbenes). The color change is more in intensity than in wavelength. In geometrically locked azobenzene (i.e., in 3,4, and 5), the Z-form might be stable and cannot isomerize (Scheme 2). ... 

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