Direct QM/MM excited-state dynamics of retinal protonated Schiff base in isolation and methanol solution

We use the full multiple spawning (FMS) dynamics approach with a hybrid quantum mechanics/molecular mechanics (QM/MM) reparameterized semiempirical method to investigate the excited-state dynamics of retinal protonated Schiff base (RPSB) in isolation, in neat methanol solution, and in methanol solution with a Cl(-) counterion. The excited-state lifetime is dramatically affected by MeOH solvent, which slows down the photoisomerization by an order of magnitude. We show that this is due to charge migration in the RPSB chromophore and the concomitant solvent friction in polar media. Simulation results are compared to experiments where available, with good agreement for excited-state lifetimes, bond selectivity of isomerization, and the time/energy-resolved fluorescence spectrum. We find that the inclusion of a Cl(-) counterion in the simulations has little effect on lifetimes, mechanism, or bond selectivity. In contrast to previous studies limited to RPSB and a surrounding counterion, we find that the placement of the counterion has little effect on bond selectivity. This suggests that dielectric screening can spoil the effect of a counterion in directing excited-state reactivity.