Excited state properties of cross-conjugated 1,2- and 1,3-distyrylbenzene and some aza-analogues

On the line of a previous work on the photophysical and photochemical properties of some aza-analogues of EE-1,4-distyrylbenzene, the kinetic competition between the radiative and reactive relaxation channels of the lowest excited states has been investigated for the cross-conjugated 1,3- and 1,2-distyrylbenzene and some aza-analogues where the two side phenyl rings are replaced by pyridyl rings. In the hydrocarbons, the radiative pathway largely prevails on the reactive one even if the yield of the radiative decay processes is drastically reduced in the sterically crowded 1,2 compound where internal conversion plays an important role. The introduction of the heteroatom in the side rings was found to markedly affect the photobehaviour leading to an increase in photoreactivity to detriment of the fluorescence emission. Information on conformational equilibria of the 1,3 compounds, due to internal (restricted) rotation around the quasi single bonds with the ethenic carbons, was obtained by selective photoexcitation.