Pseudo-stilbenes mechanism

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-... 

Isomerization needs some extra sweep volume. The volumes for the two mechanisms of azobenzene should be quite different—ca. 0.25 nm for rotation and ca. 0.12 nm for in version. This bears out in restricted spaces. In some zeolites azobenzene can isomerize whereas stilbene does not. Kuriyama and Oishi found that there are two separate AH versus AS lines for azobenzenes isomerizing by inversion (azobenzene type) and rotation (pseudo-stilbene type). ... 

The photoisomerization mechanism in aminoazobenzene- and pseudo-stilbene-type compounds has attracted far less attention than the mechanism for azobenzenes. In pseudo-stilbenes, the (n,7t ) state is buried under the intense it —> Jt band and cannot be populated selectively. No state-specific quantum yields are available because the yields are independent of the exciting wavelength.There is only a very narrow experimental basis for a discussion of these two mechanisms. However, this may change when pseudo-stilbenes are subjected to ultrashort-time experiments. 

As in the azobenzene type systems, the triplet pathway seems to be decoupled from the singlet route in pseudo-stilbenes. Little is known about the mechanism in the triplet state. The only information comes from calculations, and these show that the triplet surfaces are frequently similar in shape to the singlet surfaces. Thus, both mechanisms may be operative in the triplet state, too. 

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