Steric and electrostatic effects on photoisomerization dynamics using QM/MM ab initio multiple spawning

Photoisomerization of conjugated systems is a common pathway for photomechanical energy conversion in biological chromophores. There are many examples where the local environment of the chromophore plays an important role in determining the outcome of photoisomerization. We have investigated the effect of simple steric and electrostatic environments on the excited-state photodynamics of ethylene, a simple model for larger conjugated systems. Ab initio electronic structure methods were combined with molecular mechanical force fields to describe the ground and excitedstate potential energy surfaces of ethylene embedded in electrostatic and steric environments. The time evolution of the system following photoabsorption was modeled using the ab initio multiple spawning (AIMS) method for quantum dynamics. We introduce a new method for integration of the equations of motion in AIMS, which detects conical intersections automatically and then decreases the timestep adaptively around them. Neither steric hindrance nor electrostatics have a large effect on the excited-state lifetime, even at effective pressures as large as 2 GPa. However, a nearby point charge creates an electric field that stabilizes one of two symmetry-related conical intersections, biasing the reaction toward a particular photoisomerization pathway. For the larger tetramethylethylene, where steric hindrance is expected to be more pronounced, we also see no effect on the excited-state lifetime. Our results suggest that electrostatic interactions are more effective than steric hindrance in modifying the course of excited-state reactions.