Protonation effect on the excited state behaviour of some aza‐analogues of EE‐distyrylbenzene

The photobehaviour of four aza‐analogues of EE ‐distyrylbenzene bearing the heteroatom in the two side rings [1,4‐di‐(2‐pyridylethenyl)benzene] or in the central ring [2,5‐di‐(phenylethenyl)pyridine, 2,6‐di(phenylethenyl)pyridine and the corresponding diene, 2,6‐di(phenylbutadienyl)pyridine] has been investigated in aqueous solutions at pH 2 and 9, where the molecules are in mono‐protonated and neutral forms, respectively. The type of conjugation, linear for the first two compounds and crossed for the others, is particularly important in determining the spectral and photochemical properties. The main effects of the protic solvent are a reduction of the radiative deactivation, prevalent in the corresponding hydrocarbons, and an increase of the reactive deactivation, very modest in the hydrocarbons. The increase in photoreactivity to detriment of fluorescence, observed in protic medium with respect to non‐polar solvents, is drastically emphasized in acid medium where the fluorescence quantum yield decreases by an order of magnitude leading to high values of the EE → ZE photoisomerization quantum yield. The presence of conformational isomerism (two main conformers were detected) has been observed in both basic and acid media for compounds with crossed conjugation and for the linearly conjugated 2,5 substituted pyridine. The results obtained give a picture of the different photobehaviour of the neutral and protonated species and indicate the experimental conditions to favour a specific relaxation pathway.