Beyond Wigner sampling: Effect of initial conditions and substitution on simulated ESIPT in salicylidene aniline

Upon excitation, salicylidene aniline (SA)-based photoswitches can undergo a sub 100 fs excited-state intramolecular proton transfer (ESIPT), a necessary first step in the overall photoswitching mechanism. Using non-adiabatic ab initio molecular dynamics simulations (NAMD), we have studied ESIPT in SA and selected derivatives with either sterically hindered backbones or electronically active substituents. By comparing different methods used to sample the initial ground-state distribution of the molecule which vary in their capacity to describe low-frequency anharmonic modes and/or quantum effects, we have shown that the ESIPT yield of this molecule is sensitive to the initial conditions, varying by as much as 24%. The ESIPT yield has also been shown to be sensitive to the inclusion of solvents. Finally, we have investigated how molecular substitutions can be used to modulate the ESIPT yield, showing how rational design can be used to increase it by 19%, while also demonstrating the unpredictability of photoinduced reactivity and thus the interest of screening using NAMD.