Mechanism of unidirectional motions of chiral molecular motors driven by linearly polarized pulses

Abstract

The mechanism of the unidirectional rotational motion of a chiral molecular motor driven by linearly polarized laser pulses was theoretically studied. A simple aldehyde molecule was adopted as a chiral molecular motor, in which a formyl group (–CHO) was the rotating part of the motor. Temporal evolutions of the instantaneous angular momentum averaged over an ensemble of randomly oriented motors were taken as a measure of the unidirectional motion. The contour plots of the averaged instantaneous angular momentum were obtained by using a quantum master equation approach that took into account relaxation effects and a classical trajectory approach. Two regimes are found in the contour plots. One is an intense laser field regime in which the laser–motor interaction energy exceeds the asymmetric potential barrier. In this regime, the motors are unidirectionally driven in the intuitive direction, i.e., the gentle slope of the potential. The other regime is a subthreshold laser intensity regime in which unintuitive rotational motions also occur. This unintuitive rotation is found to be a quantum effect, as indicated by contour plots calculated by taking into account temperature effects.