EFFICIENT ADIABATIC AND DIABATIC SINGLET-STATE CIS-TRANS PHOTOISOMERIZATIONS OF A STYRYLSTILBENEDICARBALDEHYDE

The mechanism for cis-trans photoisomerization of the three isomers of 4'-styrylstilbene-4,4''-dicarbaldehyde has been investigated. Single and twofold isomerizations of the Z,Z-isomer have been observed. In methylcyclohexane the isomerization is mainly an adiabatic process in the singlet excited state although with contributions of a diabatic as well as of a triplet-state isomerization. In methylene chloride a diabatic process in the singlet excited state is dominating. Quantum yields of the steady-state isomerizations as well as of the adiabatic isomerizations are presented. From temperature studies on the adiabatic isomerization quantum yields, excited-state energy barriers and preexponential factors of the isomerizations are determined, and in the last part of the paper these preexponential factors are used to discuss the shape of the singlet excited-state energy surface.