Theoretical Study of Topographical Features around the Conical Intersections of 9-(2-Cyclopenten-1-ylidene)-9H-fluorene

Ab initio molecular orbital calculations have been performed to examine the topographical features around the conical intersections (CIXs) for the photoisomerization of 9-(2-cyclopenten-1-ylidene)-9H-fluorene (CPF). The present study is motivated by the computational findings of the topographical features in the CIX region of a fluorene-based light-driven molecular motor with smooth rotation (denoted by MS-PCPF). CPF is a parent molecule of M5-PCPF but exhibits neither helicity nor unidirectionality. A stable geometry in S-1 (S-1-geometry) is located in the region where the ethylenic rotary axis is perpendicularly twisted, but the fluorene part wags little against the rotary axis, as is the case of M5-PCPF. However, the topographical features around the CIXs of CPF are different from those of M5-PCPF. The wagging motion of the fluorene stator to the negative direction from S-1-geometry leads straight to a CIX which implements forward and backward rotations. On the other hand, only the wagging motion to the positive direction does not lead to a CIX, and additional geometrical deformations are needed. Depending on the directions of additional geometrical deformations, two CIXs, which play the roles of respective exit channels for forward and backward rotations, are located in the positive wagging region. The difference in the topographical features in the CIX region between CPF and MS-PCPF is ascribed to the effect of the pentamethylene chain. By virtue of much less computational labor of CPF as well as the electronic structures being similar to those of MS-PCPF, the intrinsic reaction coordinate was followed from the ethylenic pi pi* state at the stable geometry in S-0 into S-1-geometry. Thereby, it was confirmed that a spectroscopically dark state due to the pi-(fluorene)pi*(ethylene) excitation contributes less to the photochemical process of the ethylenic bond torsion, as is the case of M5-PCPF.