Theoretical design of a fluorene-based light-driven molecular rotary motor with constant rotation

A fluorene-based light-driven molecular rotary motor with constant rotation has been designed by means of ab initio molecular orbital calculations. A model molecule is obtained by a chemical modification of 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene (MPCPF) which we reported recently. Despite that MPCPF has a great advantage that the thermal helical inversion proceeds with a much lower energy barrier than those in previous model molecules, a small energy difference between the M- and the P-helical isomers is possible to cause a fast equilibration between them after electronic relaxation around the conical intersection (CIX), and therefore, a backward rotation from the M-helical isomer is not always suppressed effectively. In order to overcome this defect of MPCPF, we modified MPCPF by a bridge of a pentamethylene chain between the 2 position of the phenyl group and the psesudoaxial position of the C(5) atom in the 2-cyclopenten-1-ylidene ring. The modified molecule with a pentamethylene bridge (denoted by M5-PCPF) energetically destabilizes a conformation in the M-helical region and so passes through the M-helical region without any trap in the full rotary process, which leads to direct conversion from a stable P-helical isomer to another stable P-helical isomer via CIX. Therefore, M5-PCPF is expected to be a light-driven molecular rotary motor with constant rotation speed as well as unidirectionality.