Molecular Dynamics of the Ultrafast Excited-State Proton Transfer of the Super-Photoacid NM7HQ+ to the Aprotic Solvent DMSO

Photoacids exhibit an increase in acidity upon electronic excitation, providing powerful tools for studying excited-state proton transfer (ESPT) reactions. While these photoacids have been used extensively in experiments, theoretical studies, particularly on the dynamics, are largely lacking. Here, we report on the ultrafast ESPT of the super-photoacid N-methyl-7-hydroxyquinolium (NM7HQ+) to the aprotic solvent DMSO, studied using excited-state ab initio molecular dynamics (AIMD). We combine TDDFT with the implicit solvent model PCM to simulate 400 trajectories for 3 ps, starting in the S1 state. Our simulations reveal a short-lived equilibrium between the initially excited photoacid-DMSO complex (CPX) and the so-called Eigen complex (EC), the ESPT product, established within 1 ps. Subsequently, the EC undergoes a slow separation via a rotational motion on the ps timescale, further driving the ESPT. These results imply that the ESPT within the CPX may be significantly faster (< 1 ps) than previously expected (2–3 ps).