Competing Photoisomerization and Excited-State Intramolecular Proton Transfer in a Visible-Light-Driven Salicylidene Schiff Base Functionalized Molecular Motor: Insights from Ultrafast Nonadiabatic Dynamics Simulations.

We investigate the ultrafast dynamics of a visible-light-driven salicylaldehyde Schiff base-functionalized molecular motor using non-adiabatic molecular dynamics (NAMD) at the orthogonalization-corrected method 2 and the multireference configuration interaction (OM2/MRCI) level. Results reveal a dynamic competitive interplay between E → Z photoisomerization and excited-state intramolecular proton transfer (ESIPT). Importantly, we reveal a direct coupling mechanism between double-bond rotation and proton transfer: within a critical 500-900 fs window, ESIPT transiently stalls rotor rotation while modulating the central double-bond order. The S1 state lifetime is ~1677 fs. Time-resolved fluorescence analysis further predicts rapid fluorescence quenching within ~37 fs accompanied by a pronounced spectral red-shift, indicating that the system quickly enters a dark state with negligible oscillator strength. This study provides atomistic insights into the relaxation pathways of visible-light-driven motors, offering a theoretical basis for designing efficient long-wavelength responsive molecular machines.