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.