Photoisomerization 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. 

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. 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

The photoisomerization of the push-pull azobenzene derivative disperse red 1 (DRl) has also been shown recently to occur efficiently in poly-(methyl metacrylate) (PMMA) thin films at room temperature [23,24]. DRl is a pseudo-stilbene type azobenzene molecule, as classified by Rau. This means that the high-energy n-n transition is overlapping the low energy n-n transition, which leads to a large structureless band in the trans isomer and... 

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