In this Article we review the field of photoacids. The rate of excited-state proton transfer (ESPT) to solvent spans a wide range of time scales, from tens of nanoseconds for the weakest photoacids, to short time-scales of about 100 fs for the strongest photoacids synthesized so far. We divide the photoacid strength into four regimes. Regime I includes the weak photoacids 0< pKa*<3. These photoacids can transfer a proton only to water or directly to a mild base molecule in solution. The ESPT rate to other protic solvents, like methanol or ethanol, is too small in comparison with the radiative rate. The second regime includes stronger photoacids whose pKa* range from -4 to 0. They are capable of transferring a proton to other protic solvents rather than only to water. The third regime includes even stronger photoacids. Their pKa*~-6 and the ESPT rate constant kPT is limited by the solvent orientational time characterized by the average solvation correlation function S(t). The fourth regime set a new time limit for the ESPT rate of the strongest photoacids synthesized so far. The kPT value of such photoacids is 1013s-1 and tPT =100 fs. We attribute this new time limit (beyond the solvent control) to intermolecular vibration between the two heavy atoms of the proton donor and the proton acceptor, which assist the ESPT by lowering the potential barrier height and width, thus enhancing the ESPT rate.

How fast a proton transfer reaction can be? Beyond the solvent control limit

248th National Meeting (2014)

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