Photochemical and thermal cis/trans isomerization is reported for a stilbazolium betaine (M). Obtained from synthesis in the trans configuration, M cannot be isomerized directly but only via the O-protonated form MH+trans. Photoisomerization of MH+trans yields the cis isomer MH+cis (structure established by 1H NMR) which can be deprotonated to the cis betaine. Mcis represents the first example of a cis isomer in this class of compounds. Though fairly stable in aqueous solution, Mcis can be fully reverted to the trans isomer either thermally or photochemically. The sequence Mtrans {} MH+trans {} MH+cis {} Mcis → Mtrans constitutes a complete molecular reaction cycle which may serve as a chemical model for the storage of information and subsequent regeneration of the information carrier, for instance, in biological systems.
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Steiner et al. (1978) studied this question.