Photoinduced coupled charge and proton transfers in gradually twisted phenol-pyridinium biaryl series

A detailed photophysical analysis of a phenol-pyridinium biphenyl series with gradual twisted geometry is presented in this paper. The low-energy CT absorption band of the compounds undergoes a decrease of intensity with a progressive blue shift by increasing the twist angle of the central bond (Theta(AD)). These effects are well described and quantified within the framework of the Mulliken-Murrel approach, which allows us to extend such a model to the charge-shift process. The biaryl compounds exhibit broadened fluorescence bands assigned to the radiative deactivation of a charge shift (CSh) species generated by an intramolecular twisting relaxation of the locally excited (LE) state. Parallel to the rotamerism of the central single bond, excited-state proton-transfer (ESPT) processes are occurring from both excited states and lead to the nonemissive phenolate forms. Solvatochromic shifts of the emission bands are correlated by the Kamlet-Taft parameters (pi*, alpha, and beta). The correlation first confirms the pi* dependence of the CSh band shift but also demonstrates a clear beta dependence. The contribution of the latter parameter to the band hypsochromy is markedly increasing with Theta(AD). Such an unusual effect was ascribed to a much higher ESPT rate relative to the highly twisted conformation with respect to that of more planar geometry. Despite the suppression of the geometrical relaxation in ethanol glass at 77 K, the fluorescence of the phenolate species produced by ESPT from LE state is detected. The relative increase of its fluorescence band intensity with Theta(AD) confirms the gradual enhancement of the excited states acidity.