Reversible intermolecular-coupled-intramolecular (RICI) proton transfer occurring on the reaction-radius a of 2-naphthol-6,8-disulfonate photoacid

Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) from a reversibly dissociating photoacid, 2-naphthol-6,8-disulfonate (2N68DS). The reaction was carried out in water and in acetonitrile–water solutions. We find by carefully analyzing the geminate recombination dynamics of the photobase–proton pair that follows the ESPT reaction that there are two targets for the proton back-recombination reaction: the original O⁻ dissociation site and the SO₃⁻ side group at the 8 position which is closest to the proton OH dissociation site. This observation is corroborated in acetonitrile-water mixtures of χ_water < 0.14, where a slow intramolecular ESPT occurs on a time scale of about 1 ns between the OH group and the SO₃⁻ group via H-bonding water. The proton-transferred R^*O⁻ fluorescence band in mixtures of χ_water < 0.14 where only intramolecular ESPT occurs is red shifted by about 2000 cm⁻¹ from the free R^*O⁻ band in neat water. As the water content in the mixture increases above χ_water = 0.14, the R^*O⁻ fluorescence band shifts noticeably to the blue region. For χ_water > 0.23 the band resembles the free anion band observed in pure water. Concomitantly, the ESPT rate increases when χ_water increases because the intermolecular ESPT to the solvent (bulk water) gradually prevails over the much slower intramolecular via the water-bridges ESPT process.