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

• Two very strong photoacids QCy7, pKa*∼−6.5, and QCy9, pKa*∼−8.5, are studied in three protic solvents, water, MeOH and EtOH.
• Weak photoacids do not transfer a proton to alcohols within the excited state lifetime, whereas strong photoacids do.
• In QCy7 the ESPT rate constant, kPT in water, methanol and ethanol is 1.2 × 1012, 8 × 1011 and 7 × 1011 respectively.
• The ESPT rate of QCy9 in these solvents is the same, kPT ∼ 1013 s−1.
• Non-adiabatic and adiabatic PT theories explain the kPT solvent dependence of weak and very strong photoacids respectively.

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 pKa*∼−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 (pKa*∼−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.

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