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 M5-PCPF). CPF is a parent molecule of M5-PCPF but exhibits neither helicity nor unidirectionality. A stable geometry in S1 (S1-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 S1-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 M5-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 M5-PCPF, the intrinsic reaction coordinate was followed from the ethylenic ππ* state at the stable geometry in S0 into S1-geometry. Thereby, it was confirmed that a spectroscopically dark state due to the π­(fluorene)­π*­(ethylene) excitation contributes less to the photochemical process of the ethylenic bond torsion, as is the case of M5-PCPF.本研究开展了从头算分子轨道(ab initio molecular orbital)计算，以考察9-(2-环戊烯-1-亚基)-9H-芴(CPF)光异构化过程中锥形交叉点(CIXs)周围的拓扑特征。本研究的动机源于此前对一类具有平滑旋转特性的芴基光驱动分子马达（记为M5-PCPF）的锥形交叉区域拓扑特征的计算发现。CPF是M5-PCPF的母体分子，但既不具备手性也不表现单向旋转特性。 第一激发单重态(S1)下的稳定几何构型（S1构型）位于烯键旋转轴发生垂直扭转的区域，此时芴基团相对于旋转轴的摇摆幅度极小，这一点与M5-PCPF的情况一致。然而，CPF的锥形交叉点周围的拓扑特征与M5-PCPF存在差异：从S1构型出发，芴定子向负方向的摇摆运动可直接抵达一个同时对应正向与反向旋转的锥形交叉点；而仅向正方向的摇摆运动则无法抵达锥形交叉点，还需额外的几何形变。根据额外几何形变的方向不同，正摇摆区域内存在两个分别作为正向与反向旋转出口通道的锥形交叉点。 CPF与M5-PCPF在锥形交叉区域的拓扑特征差异，可归因于五亚甲基链的影响。由于CPF的计算工作量远低于M5-PCPF，且其电子结构与M5-PCPF相似，本研究追踪了基态(S0)稳定构型下烯键ππ*态到S1构型的内禀反应坐标(intrinsic reaction coordinate)。结果证实，由π(芴)π*(乙烯基)激发产生的光谱暗态，对烯键扭转的光化学过程贡献极小，这一点同样与M5-PCPF的情况一致。