Theoretical Study on Conformational-Dependent Photochemical Behaviors of a Fluorene-Based Light-Driven Molecular Rotary Motor

Abstract

Abstract Photochemical behaviors which depend on the pentamethylene conformations of fluorene-based light-driven molecular rotary motor (denoted by M5-PCPF) have been examined by ab initio complete active space self-consistent-field calculations. In the conical intersection region where the ethylenic rotary axis is perpendicularly twisted, the stable geometries in S1 which are distinguished from each other by the pentamethylene conformations are separated by a low energy barrier. In consequence, conformational interchanges are allowed on the S1 surface in the conical intersection region. Two conical intersections (CIXs) are located in the region where the fluorene stator wags to the negative and positive directions against the ethylenic rotary axis, respectively. After electronic relaxation at CIX for a forward rotation, M5-PCPF with some conformations directly goes to P′-helical isomer, whereas M5-PCPF with other conformations is trapped in the M′-helical region and no longer reaches the P′-helical region thermally. The former types of conformers exhibit unidirectional rotation through the direct photochemical P–P′ and P′–P photochemical conversions, whereas the latter exhibit oscillatory photochemical behavior between P- and M′-helical (and P′- and M-helical) isomers. However, the conformation with oscillatory behavior can interchange into another conformation with unidirectional rotation in the conical intersection region. Therefore, M5-PCPF exhibits a net unidirectional rotation.