Photoisomerization mechanism of the cis isomers of 1,2-distyrylbenzene and two hetero-analogues

The radiative and reactive excited state relaxations of the ZE and ZZ geometrical isomers of 1,2-distyrylbenzene [1,2-(PhE)2B] and two hetero-analogues, where the side phenyl groups were replaced by 4′-pyridyl or 2′-thienyl groups, were studied and compared with the photobehaviour of the EE isomers previously investigated.The ZE and ZZ isomers of the hydrocarbon photoisomerize by a predominant adiabatic mechanism (a “one photon – two bonds” process is also operative in the case of ZZ) whereas the contribution of the adiabatic pathway is reduced for the thienyl-analogues, which isomerize by a mixed (diabatic/adiabatic) mechanism, whereas a prevalent diabatic mechanism characterizes the behaviour of the pyridyl analogue.