Photoinduced quantum dynamics of ortho- and para-fulvene: hindered photoisomerization due to mode selective fast radiationless decay via a conical intersection.

In this study, we investigate the photoinduced nonadiabatic dynamics of para- and ortho-fulvene by a combination of quantum chemical ab initio calculations and quantum dynamical simulations. We explore the competition between two different pathways, the photoisomerization and radiationless decay via a conical intersection (CI) at planar configuration. For this purpose, we extend a previous two-dimensional model which included the molecular torsion and the antisymmetric stretch as a coupling mode [Grohmann et al., Chem. Phys. 338, 252 (2007)] to a three-dimensional model, taking into account also the symmetric stretch as additional vibrational mode. Quantum dynamical simulations show that upon excitation with a single short laser pulse, the mode selective motion along the symmetric stretch drives the system to the CI, followed by radiationless decay before photoisomerization of fulvene can take place, thus confirming previous semiclassical calculations [Bearpark et al., J. Am. Chem. Soc. 118, 5253 (1996)]. Moreover, we show that the competition between photoisomerization and radiationless decay at a planar geometry depends on the nonadiabatic coupling strength.