title: Computational study on the working mechanism of a stilbene light-driven molecular rotary motor: sloped minimal energy path and unidirectional nonadiabatic photoisomerization.
abstract: The working mechanism of a geometrically overcrowded, chiral stilbene light-driven molecular rotary motor [(2R,2R)-2,2',7,7'-tetramethyl-1,1'-bis(indanylidene), 3] has been investigated by a potential energy surface (PES) study. The reaction paths of the two photoinitiated cis-trans (or E/Z) isomerization processes, namely, (P,P)-stable-cis→(M,M)-unstable-trans-3 and (P,P)-stable-trans→(M,M)-unstable-cis-3, have been explored at the CASPT2//CASSCF level of theory. The minimal energy reaction paths (MEPs) of these two processes are nearly parallel on the PESs, separated by a ridge of high inversion barrier. The MEPs have a remarkably steep slope, which drives C═C bond rotation unidirectionally. The asymmetric bias on the excited-state MEPs is caused by the substituents on the "fjord" region as well as by the phenyl moieties. The overall photoisomerization reaction can be described as a three-state (1B→2A→1A) multimode mechanism: The molecule excited to the 1B state first crosses one of the sloped 1B/2A seams, and then follows two cooperative torsional reaction modes to cross preferentially one of the two 2A/1A conical intersections to reach the isomerized ground-state product.
authors.affiliation: Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan.
authors.forename: Fengyi
authors.initials: F
authors.lastname: Liu
authors.forename: Keiji
authors.initials: K
authors.lastname: Morokuma
mesh_terms: D007536:Isomerism; D008027:Light; D010777:Photochemistry; D013267:Stilbenes / Q000737:chemistry*
chemical_list: D013267:Stilbenes