Competitive radiative and reactive relaxation channels in the excited state decay of some thio-analogues of EE-distyrylbenzene

The photophysical and photochemical properties of some thio-analogues of symmetrically substituted EE-1,4-distyrylbenzene(linear conjugation) and EE-1,3-distyrylbenzene(crossed conjugation), where thiophene rings replace the side benzene rings or the central benzene ring, have been investigated. The kinetic competition between the radiative and reactive relaxation channels of the lowest excited singlet state has been compared with that found for the corresponding hydrocarbons. The photobehaviour markedly depends on the type of conjugation and on the position (2' or 3')of the ethenic bridge with respect to the sulfur atom. The main effect of the heteroatom is an increase in the photoisomerization yield due to a decrease of the torsional barrier in S-1 and to an increase in the S-1 -> T-1 intersystem crossing, which opens the way to isomerization in the triplet manifold. Conformational equilibria, due to restricted rotation around the quasi-single bonds with the ethenic carbons, have also been investigated by selective photoexcitation for the compounds with side 3'-thienyl groups.