Effect of stereoisomerism on the radiative and reactive relaxation channels of two thio-analogues of distyrylbenzene

The photophysical and photochemical properties of the ZE and ZZ stereoisomers of two thio-analogues of distyrylbenzene, where thiophene rings replaced the central or the side benzene rings, have been investigated. The kinetic competition between the radiative and reactive relaxation channels of the lowest excited singlet state has been compared with that previously found for the corresponding EE stereoisomers and the parent hydrocarbon. The photobehaviour markedly depends on the position of the heteroatom (central or side rings). Its main effect is an increase in the S1 → T1 intersystem crossing, which may open the way to isomerization in the triplet manifold. The triplet reactivity, sensitized by biacetyl, was also investigated. Adiabatic formation of the excited singlet and triplet EE∗ isomer by irradiation of the other isomers (in the case of ZZ, a “one photon - two bonds” mechanism is operative) was evidenced by fluorescence and flash photolysis measurements.