Point-to-point excited state proton transfer between an acidic and a basic functional groups of a photoacid may be either irreversible or reversible processes. The two types of proton transfer reactions are distinguishable by analyzing the geminate recombination of the ejected proton back to reversibly reform the acidic site. We report on the steady-state and time-resolved measurements of a reversible proton transfer between the OH (2-position) and SO3- (8-position) groups in the photoacid 2-naphthol-6,8-disulfonate. In bulk water we find the proton to transfer reversibly to the SO3- group by bulk diffusion while when conducting the reaction in polar aprotic solvents such as acetonitrile and propylene carbonate containing sub-molar concentration of water proton transfer occurs via a proton wires which connect between the two functional groups. The proton transfer results in a zwitterion which red shifts the fluorescence of the anion of the photoacid by about 50 nm. The point-to-point proton transfer reaction is terminated in presence of molar concentrations of water when the proton transfer reaction occurs from photoacid to bulk water.

Reversible point-to-point excited state proton transfer in bifunctional photoacids

2019 Fall National Meeting

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