The triplet state in stilbene cistrans photoisomerization

Abstract

Stilbene triplets are further characterized by their interaction with quenchers. A previously proposed model for stilbene triplet decay and quenching, together with rate constants for excitation transfer from indeno[2,l-a]indene triplets to azulene, is used to account for the temperature dependence of the azulene effect on stilbene photoisomerization and is shown to lead to erroneous predictions concerning the lifetime and geometry of stilbene triplets. This conclusion is based on oxygen quenching experiments suggesting a twisted geometry for stilbene triplets in benzene solution with a lifetime of ~120 ns at 30°. Azulene quenching experiments show that bromine substitution at the para position of stilbene markedly increases 1S → T intersystem crossing efficiency while substitution at the meta position does not enhance this process at 30°. These results are consistent with the fact that in contrast to the para position, the meta position is a near node in the highest occupied and lowest unoccupied molecular orbitals of stilbene. © 1975, Walter de Gruyter. All rights reserved.