Photobehavior of the geometrical isomers of two 1,4-distyrylbenzene analogues with side groups of different electron donor/acceptor character

The photobehavior of two 1,4-distyrylbenzene analogues where the central benzene ring is asymmetrically substituted with a 4’-pyridylethenyl group at one side and 2’-thienylethenyl or p-nitrostyryl groups at the other side, has been studied in two solvents at room temperature. The four geometrical isomers (EE, ZE, EZ and ZZ) of each compound were separated by the combined use of HPLC and spectrophotometric techniques. The radiative/reactive competition in their excited state relaxation was particularly examined: the diabatic/adiabatic contributions were estimated and a reasonable interpretation of the photoisomerization mechanism was proposed. The role of the conformational isomers was also investigated by measured and computed spectral data. Since the different electron donor/acceptor character of the side groups of these molecules can induce charge transfer phenomena which can affect the relaxation pathways of their excited states, the photobehavior was compared in inert and polar solvents to clarify the role of the intramolecular charge transfer. The latter was found to affect markedly the relaxation properties and to induce interesting fluorosolvatochromic effects, particularly in the p-nitro-derivative. The participation of the triplet state in the reaction mechanism of the latter was also investigated by flash photolysis and sensitized experiments.