Singlet Mechanism for Trans —• Cis Photoisomerization of Quaternary Salts of 4-Substituted 4-Azastilbenes

The trans-»eis photoisomerization of a series of substituted stilbazolium salts (A t X , At: trans-l-alkyl-4-[4-R-styryl]-pyridinium and -quinolinium, R = CN. H, CH3 and OCH3, X " = T and CIO4) was studied by laser flash photolysis and steady state irradiation measurements. The quantum yields of eis ^ trans photoisomerization (<£c_t and $ t _ c ) and of fluorescence of the trans isomers (f) were determined in several solvents at room temperature and at low temperatures in mixtures of either 2-methyltetrahydrofuran-dichloromethane or ethanolmethanol (E-M). In polar solvents at room temperature <£t_c is substantial (S 0.3) and f is small (10-10~). Competition of fluorescence and an activated step in the trans-»eis pathway is indicated by the effects of temperature on (f){ and t_c (activation energy: 2 3 kcal/mol). A transient, observed only at low temperatures (lifetime iT > 0.5 ms in E-M below 1 7 0 ° C ) , is assigned to the lowest triplet state with trans configuration. On the basis of the effects of temperature on (/>f, t_>c, rT , and the triplet yield and those of quenchers on f and t_c, involvement of the triplet state in the twisting process at room temperature is excluded. Therefore, a singlet mechanism is suggested for the trans-»•cis photoisomerization of the stilbazolium salts examined. Significant reduction of t_c for iodides in solvents of moderate polarity, where ion pairs are present, is accounted for by photoinduced electron transfer in competition to trans -»• cis photoisomerization.