Photoinduced strong acid-weak base reactions in a polar aprotic solvent

The excited-state proton transfer (ESPT) of the strong photoacid, N-methyl-7-hydroxyquinolinium, was studied in the presence of different weak bases such as methanol, ethanol, and dimethyl sulfoxide in an aprotic solvent of acetonitrile. Here, we present chemical kinetics analysis of the ESPT mechanism to explain biphasic fluorescence decay of the parent photoacid and the sign reversal of the rise and decay of the resulting conjugate-base fluorescence. The ESPT of the free photoacid showed a molecularity of 2 with reacting alcohol molecules. In the ground state, it was found that a fraction of the photoacid formed 1 : 2 hydrogen-bonded complexes with the residual water present in the aprotic solvent or 1 : 1 complexes with the additive alcohols. In the excited state, these adducts underwent proton transfer when complexed further with diffusing alcohol molecules.