Unusual Temperature Dependence of Excited State Proton Transfer Rates in Alcohols

The reversible proton dissociation and geminate recombination of photoacids is studied as a function of temperature in alcohols. For this purpose we used a strong photoacid 5,8-dicyano-2-naphthol (DCN2) (pK*_a ∼ -4.5 in water) that is capable to transfer a proton to alcohols. The experimental data are analyzed by the Debye-Smoluchowski equation solved numerically with boundary conditions to account for the reversibility of the reaction. We propose a qualitative model to describe the unusual temperature dependence of the proton-transfer rate constant in alcohols in the liquid phase.