Excited-State Proton Transfer from the Photoacid 2-Naphthol-8-sulfonate to Acetonitrile/Water Mixtures

Steady-state and time-resolved fluorescence techniques were used to study excited-state proton transfer (ESPT) to water of the reversible photoacid 2-naphthol-8-sulfonate (2N8S) in acetonitrile/water mixtures. In acetonitrile-rich mixtures, up to χ_water ≤ 0.12, we found a slow ESPT process on the order of nanoseconds. At χ_water ≈ 0.15, the RO^- fluorescence band intensity is at the minimum, whereas at χ_water ≈ 0.030, it is at the maximum. The steady-state fluorescence spectra of these mixtures show that the intensity of the RO^- fluorescence band at χ_water ≈ 0.030 is about 0.24 of that of the ROH band. We explain this unusual phenomenon by the presence of water clusters that exist in the acetonitrile-rich CH₃CN/H₂O mixtures. We propose that a water bridge forms between the 2-OH and 8-sulfonate by preferential solvation of 2N8S, and this enables the ESPT process between the two sites of the molecular structure of 2N8S. In mixtures of χ_water ≥ 0.25, the ESPT process takes place to water clusters in the bulk mixture. The higher the χ_water in the mixture, the greater the ESPT rate constant. In neat water, the rate constant is rather small, 4.5 × 10⁹ s^-1. TD-DFT calculations show that a single water molecule can bridge between 2-OH and 8-sulfonate in the excited state. The activation energy for the ESPT reaction is about 9 kcal/mol, and the RO^-(S₁) species is energetically above the ROH(S₁) species by about 1.6 kcal/mol.