Ultrafast excited-state proton transfer from dicyano-naphthol

The rate of proton transfer from electronically excited 5,8-dicyano-2-naphthol (DCN2) to the solvent is studied by time-resolved fluorescence. Unlike most naphthol derivatives, excited DCN2 is a strong acid (pK * ≈ -4.5) and therefore is capable of transferring protons to alcohols and other moderate proton acceptor solvents. The rate constant of proton transfer, κ_d, at low temperatures (< 250 K) is slightly larger than the inverse dielectric relaxation time, 1/τ_D and has the same activation energy of the dielectric relaxation. On the other hand, at temperatures above 250 K the temperature dependence of the proton transfer rate decreases monotonically with increasing temperature, while the dielectric relaxation activation energy maintains the low temperature value.