Effect of pressure on the proton-transfer rate from a photoacid to a solvent. 2. DCN2 in propanol

The reversible proton dissociation and geminate recombination of photoacids is studied as a function of pressure in liquid propanol. For this purpose we used a strong photoacid, 5,8-dicyano-2-naphthol (DCN2) (pK(a)* similar to -4.5 in water), capable of transferring a proton to alcohols. The time-resolved emission data are explained by the reversible diffusion-influenced chemical reaction model. At low pressure, the proton-transfer rate slightly increases with pressure whereas, at high pressure, the rate constant decreases significantly as the pressure increases. The pressure dependence is explained using an approximate stepwise two-coordinate proton-transfer model. The model is compared with the Landau-Zener curve-crossing proton tunneling formulation. Decrease of the proton-transfer rate at high-pressures reflects the solvent-controlled limit, and the increase in rate at low-pressures reflects the proton tunneling nonadiabatic limit. The results are compared with our recent studies of the pressure dependence of proton transfer from 2-naphthol-6-sulfonate (2N6S) to water and DCN2 to ethanol. Though in 2N6S-water, the proton transfer is controlled by proton tunneling, in our current work we find that, at high pressure, the solvent controls the rate of the process.