Competing adiabatic and nonadiabatic pathways in the cis-trans photoisomerization of cis-1,2-di(1-methyl-2-naphthyl)ethene. A zwitterionic twisted intermediate.

Identical fluorescence lifetimes and spectra of cis- and trans-1,2-di(1-methyl-2-naphthyl)ethene reveal an adiabatic cis → trans photoisomerization pathway that accounts for a significant fraction of observed cis → trans photoisomerization quantum yields. The fluorescence quantum yields of both isomers decrease as the solvent is changed from methylcyclohexane to the more polarizable toluene or to the more polar acetonitrile and there is a corresponding increase in the photoisomerization quantum yield in the trans → cis direction. The pronounced solvent dependence of the contribution of the adiabatic pathway to cis → trans photoisomerization--methylcyclohexane (70%), toluene (45%), acetonitrile (31%)--is consistent with the participation of a zwitterionic twisted intermediate, (1)p*, on the singlet excited state surface which is stabilized as the polarizability and/or polarity of the solvent is increased. Solvent stabilization of (1)p* favours the nonadiabatic photoisomerization pathways of both isomers and diminishes the cis → trans adiabatic pathway.