Direct Observation and Kinetic Mapping of Point-to-Point Proton Transfer of a Hydroxy-Photoacid to Multiple (Competing) Intramolecular Protonation Sites

We study excited-state proton-transfer (ESPT) reactions in bifunctional photoacids 2-naphthol 8-sulfonate (2N8S) and 2-naphthol 6, 8-sulfonate (2N6dS) photoacids. We distinguish between the solvent-coupled diffusion-controlled reversible ESPT reactions of the two side groups and the point-to-point through one-water-bridge ESPT process between the primary photoacidic OH group and the 8-position SO 3 − group of the two photoacids, the SO 3 − side group functioning as a secondary (reversible) proton acceptor. The diffusion-controlled ESPT reactions are carried out in water. Through one-water-bridge ESPT reactions are carried out in acetonitrile (ACN) in presence of sub-0.3 M of water which forms 1: 1 hydrogen-bonding (bridge) complexes with the two bifunctional photoacids. The kinetics of the ensuing ESPT processes following short laser-pulse excitation by time-resolved-fluorescence (TRF) are analyzed. Detailed kinetic analysis allows us to describe the various mechanisms of the observed two coupled ESPT reactions of the bifunctional photoacids in terms of proton antennas.