Theoretical Study of Topographical Features around the Conical Intersections of Fluorene-Based Light-Driven Molecular Rotary Motor

Topographical features around the conical intersection (CIX) of a fluorene-based light-driven molecular rotary motor have been examined by means of ab initio molecular orbital calculations. The model molecule is a derivative of 9-(2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene (PCPF) where PCPF is bridged by a pentamethylene chain between the 2 position of the phenyl group and the pseudoaxial position of the C5 atom in the 2-cyclopenten-1-ylidene ring. This model molecule (denoted by M5-PCPF) has been very recently reported as a candidate for a light-driven molecular motor with constant rotation. The conical intersection for the photoprocess of the ethylenic CC torsion reported there (denoted by CIX1), which exclusively leads to a product of P′-M5-PCPF, is found to be classified as a sloped-type CIX. Another CIX reported at the present time (CIX2) exclusively goes back to a reactant of P-M5-PCPF, whereas CIX2 is also classified as a sloped-type CIX. At the stable geometry in S1 around the CIX region (S1-geometry), the 2-cyclopenten-1-ylidene rotor takes a perpendicular twist against the fluorene stator, but the fluorene stator does not wag so much against the CC rotary axis. The wagging motions of the fluorene stator from S1-geometry to the opposite directions lead to CIX1 and CIX2, respectively.