Directionality of Double-Bond Photoisomerization Dynamics Induced by a Single Stereogenic Center.

In light-driven single-molecule rotary motors, the photoisomerization of a double bond converts light energy into the rotation of a moiety (the rotor) with respect to another (the stator). However, at the level of a molecular population, an effective rotary motion can only be achieved if a large majority of the rotors rotate in the same, specific direction. Here we present a quantitative investigation of the directionality (clockwise vs counterclockwise) induced by a single stereogenic center placed in allylic position with respect to the reactive double bond of a model of the biomimetic indanylidene-pyrrolinium framework. By computing ensembles of nonadiabatic trajectories at 300 K, we predict that the photoisomerization is >70% unidirectional for the Z → E and E → Z conversions. Most importantly, we show that such directionality, resulting from the asymmetry of the excited state force field, can still be observed in the presence of a small (ca. 2°) pretwist or helicity of the reactive double bond. This questions the validity of the conjecture that a significant double-bond pretwist (e.g., >10°) in the ground state equilibrium structure of synthetic or natural rotary motors would be required for unidirectional motion.