The 1-naphthol molecule has been the subject of intense research activity for the past 60 years due to its complex behavior as a photoacid upon optical excitation. We have utilized femtosecond mid-infrared spectroscopy and time-resolved fluorescence spectroscopy to investigate the excited-state proton-transfer reaction of 1-naphthol-3,6-disulfonate (1N-3,6diS) and several 5-substituted 1-naphthol derivatives. The proton dissociation rate constant of 1N-3,6-diS was found to be about 3 times faster and the pKa* about 2 pKa units more acidic than the values previously reported in the literature. A Marcus (free-energy) plot of excited-state proton dissociation rate constants vs the excited-state equilibrium constant of the photoacids, Ka*, was constructed using the C-5 series of 1-naphthol derivatives. The newly measured values for the ESPT rate constant and pKa* of 1N-3,6diS was found to fit well with the Marcus correlation. We discuss our findings in the context of the photoacidity phenomenon in general, and the photoacidity of 1-naphthol and its derivatives in particular.

The introduction of naphthol derivatives that exhibit excited‐state acidities comparable to strong mineral acids, has opened the way for direct investigation of ultrafast proton‐transfer in aqueous solutions and alcohols. We report on the direct measurement of the isotope and temperature effects on 5‐cyano‐1‐naphthol excited‐state proton dissociation in the full composition range of water‐methanol mixtures. The results indicate that in pure water the proton‐transfer rate is almost activationless in the proton coordinate and is mainly controlled by the solvent. In pure methanol the reaction shows down considerably and is mainly controlled by activation along the proton coordinate.

The back proton recombination following the photo‐dissociation of 1‐naphthol was studied by a time‐correlated single‐photon counting apparatus. The measured picosecond time‐resolved fluorescence decay profiles were analyzed using the numerical solution of the Debye‐Smoluchowski equation with coupled reversible and irreversible proton back‐recombination reactions at the source. A full kinetic analysis shows that both the reversible geminate recombination reaction, which assumes at long times a t−3/2 dependence, and the irreversible geminate quenching reaction, which assumes at long times a t−1/2 dependence, are significant in determining the excited‐state kinetics of 1‐naphthol.

We have carried out temperature dependence studies of the geminate quenching reaction of 1-naphthol utilizing steady-state and time-resolved fluorescence spectroscopy. The outcome was analyzed using both analytic and numeric procedures. A novel method for estimating the on-contact quenching reaction rate from steadystate quantum-yield measurements is described.