How Fast Can a Proton-Transfer Reaction Be beyond the Solvent-Control Limit?

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 < pK_a* < 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 pK_a*’s range from −4 to 0. They are capable of transferring a proton to other protic solvents and not only to water. The third regime includes even stronger photoacids. Their pK_a* is ∼ –6, and the ESPT rate constant, k_PT, is limited by the orientational time of the solvent which is characterized by the average solvation correlation function ⟨S(t)⟩. The fourth regime sets a new time limit for the ESPT rate of the strongest photoacids synthesized so far. The k_PT value of such photoacids is 10¹³ s^–1, and τ_PT = 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 height and width of the potential barrier, thus enhancing the ESPT rate.