Computational Design of a Fluorene-Based Ethylenoid Bridged by Trimethylene Chain

Abstract

Abstract A fluorene-based ethylenoid bridged by trimethylene chain (denoted by M3-PCPF) has been computationally designed as a candidate for a light-driven molecular rotary motor by ab initio complete active space self-consistent-field and its second-order perturbation calculations. In the conical intersection region where the ethylenic rotary axis is highly twisted, three stable confirmers on S1 are connected with each other through individual transition states. M3-PCPF around each stable conformation on S1 is directly accessible to the four conical intersections (CIXs) which are distinguished by the wagging and rocking angles of the fluorene part against the ethylenic rotary axis. The S0 surface around each CIX is separated into two regions with respect to the ethylenic bond torsion. One leads to the reactant of P-helical isomer without conformational interchange of the trimethylene chain, whereas the other leads to the product of another P′-helical isomer. Electronically excited P-helical isomers with ethylenic ππ* character are correlated with the stable S1 geometry in the CIX region without conformational interchange of the trimethylene chain. In consequence, all the three conformers of M3-PCPF exhibit unidirectional rotation through the direct P–P′ and P′–P photochemical conversions.