Enhanced Excited-State Proton Transfer via a Mixed Water–Methanol Molecular Bridge of 1-Naphthol-5-Sulfonate in Methanol–Water Mixtures

We used steady-state and time-resolved fluorescence techniques to study the excited-state proton transfer (ESPT) and the nonradiative properties of two irreversible photoacids, 1-naphthol-4-sulfonate (1N4S) and 1-naphthol-5-sulfonate (1N5S). We found that the ESPT rate constant of 1N4S in water is 2.2 × 1010 s–1, whereas in methanol, it is smaller by about 3 orders of magnitude and is not observed. The ESPT process of 1N5S competes with a major nonradiative process of equal rate and kPT of 2.2 × 1010 s–1. In methanol–water mixtures of χH2O = 0.2, the fluorescence lifetime of the ROH form of 1N5S is lower by a factor of 10 than that in pure methanol. In the steady-state fluorescence spectra of 1N5S in methanol–water mixtures, there are two iso-emissive points, one for χH2O 0.3. This large reduction in fluorescence intensity and the two iso-emissive points are explained by the existence of a mixed water–methanol bridge of about three molecules that connects the proton donor 1-OH wit...