Resolving Photoinduced Femtosecond Three-Dimensional Solute–Solvent Dynamics through Surface Hopping Simulations

Photoinduced dynamics
in solution is governed by mutual
solute–solvent
interactions, which give rise to phenomena like solvatochromism, the
Stokes shift, dual fluorescence, or charge transfer. Understanding
these phenomena requires simulating the solute’s photoinduced
dynamics and simultaneously resolving the three-dimensional solvent
distribution dynamics. If using trajectory surface hopping (TSH) to
this aim, thousands of trajectories are required to adequately sample
the time-dependent three-dimensional solvent distribution functions,
and thus resolve the solvent dynamics with sub-Ångstrom and femtosecond
accuracy and sufficiently low noise levels. Unfortunately, simulating
thousands of trajectories with TSH in the framework of hybrid quantum
mechanical/molecular mechanical (QM/MM) can be prohibitively expensive
when employing ab initio electronic structure methods. To tackle this
challenge, we recently introduced a computationally efficient approach
that combines efficient linear vibronic coupling models with molecular
mechanics (LVC/MM) via electrostatic embedding [Polonius et al., JCTC 2023, 19, 7171–7186]. This method
provides solvent-embedded, nonadiabatically coupled potential energy
surfaces while scaling similarly to MM force fields. Here, we employ
TSH with LVC/MM to unravel the photoinduced dynamics of two small
thiocarbonyl compounds solvated in water. We describe how to estimate
the number of trajectories required to produce nearly noise-free three-dimensional
solvent distribution functions and present an analysis based on approximately
10,000 trajectories propagated for 3 ps. In the electronic ground
state, both molecules exhibit in-plane hydrogen bonds to the sulfur
atom. Shortly after excitation, these bonds are broken and reform
perpendicular to the molecular plane on timescales that differ by
an order of magnitude due to steric effects. We also show that the
solvent relaxation dynamics is coupled to the electronic dynamics,
including intersystem crossing. These findings are relevant to advance
the understanding of the coupled solute–solvent dynamics of
solvated photoexcited molecules, e.g., biologically relevant thio-nucleobases.