Comparison of the rate of excited-state proton transfer from photoacids to alcohols and water

Steady-state and time-resolved techniques were employed to study the excited-state proton-transfer (ESPT) rate of two newly synthesized photoacids – quinone cyanine diindolium (QCy7) and its dipicolinium analog (QCy9) – in three protic solvents, water, methanol and ethanol. The ESPT rate constant kPT of QCy7, whose p K a * ∼ − 6.5 in water, methanol and ethanol is 1.2 × 1013 s−1, 8 × 1011 s−1, 7 × 1011 s−1 respectively. kPT for QCy9 ( p K a * ∼ − 8.5 ), is 1013 s−1 for all three protic solvents used. We qualitatively explain these results and also the previously found solvent dependence of kPT of weaker photoacids by adiabatic and non-adiabatic proton-transfer formalism.