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 C5 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.