Benchmarking the molecular mechanics valence bond method: Photophysics of styrene and indene

We have recently studied the excited states of the simplest aryl olefin styrene (Bearpark, M. J.; Olivucci, M.; Wilsey, S.; Bernardi, F.; Robb, M. A. J. Am. Chem: Sec. 1995, 117, 6944-6953) at the CASSCF/4-31G level of theory. Full geometry optimization was shown to be essential in characterizing decay funnels for internal conversion (activated) and intersystem crossing (activationless) processes in this molecule. Here, we demonstrate that the CASSCF potential energy surfaces for styrene excited states can be simulated to an acceptable level of accuracy using a hybrid molecular mechanics-valence bond method (MMVB), which is many orders of magnitude less expensive computationally than CASSCF. The nonradiative deactivation of styrene and indene from Si is compared. Because ethylene torsions are restricted, the mechanism proposed for styrene S-1 decay (involving S-1/T-2, T-2/T-1, and T-1/S-0 surface crossings) is much less likely to occur in indene. The existence of both S-2/S-1 and S-1/S-0 conical intersections is consistent with the lack of fluorescence observed after exciting indene to S-2 in the gas phase and suggests that rearrangement reactions may be due to vibrationally excited S-0*.