Coherent Proton Transfer in the Excited State of Salophen Driven by a Specific Low-Frequency Skeletal Vibration

Identifying key vibrational modes directly coupled to a chemical reaction is essential for understanding molecular reaction dynamics in condensed phases, yet remains challenging because of mode mixing, rapid dephasing, and strong multimode effects. Here, we combine ultrahigh-time-resolution fluorescence measurements with Born-Oppenheimer molecular dynamics simulations to study excited-state intramolecular proton transfer (ESIPT) in salophen. Notably, nuclear motions following photoexcitation are described by projecting simulated trajectories onto the normal modes of the relevant chemical species. Contrary to the common expectation that multimode effects dominate for such a large molecule in the condensed phase, we find that a single skeletal vibrational mode almost exclusively shortens the distance between the proton donor (oxygen) and acceptor (nitrogen) significantly, rendering the reaction potential energy surface effectively barrierless. This approach provides a practical route to identifying mode-specific driving motions in condensed-phase coherent reaction dynamics.