Effect of Temperature on Photoisomerization Dynamics of a Newly Designed Two-Stroke Light-Driven Molecular Rotary Motor.

The working mechanism of conventional light-driven molecular rotary motors, especially Feringa-type motors, contains two photoisomerization steps and two thermal helix inversion steps. Due to the existence of a thermal helix inversion step, both the ability to work at lower temperatures and the rotation speed are limited. In this work, a two-stroke light-driven molecular rotary motor, 2-(1,5-dimethyl-4,5-dihydrocyclopenta[b]pyrrol-6(1H)-ylidene)-1,2-dihydro-3H-pyrrol-3-one (DDPY), is proposed, which is capable of performing unidirectional and repetitive rotation by only two photoisomerization ( and ) steps. With trajectory surface-hopping simulation at the semi-empirical OM2/MRCI level, the → and nonadiabatic dynamics of DDPY were systematically studied at different temperatures. Both and photoisomerizations are on an ultrafast timescale (ca. 200-300 fs). The decay mode of → photoisomerization is approximately bi-exponential, while that of photoisomerization is found to be periodic. For and isomers of DDPY, after the S0→S1 excitation, the dynamical processes of nonadiabatic decay are both followed by twisting about the central C=C double bond and the pyramidalization of the C atom at the stator-axle linkage. The effect of temperature on the nonadiabatic dynamics of → and photoisomerizations of DDPY has been systematically investigated. The average lifetimes of the S1 excited state and quantum yields for both → and photoisomerization are almost temperature-independent, while the corresponding unidirectionality of rotation is significantly increased (e.g., 74% for → and 72% for at 300 K 100% for → and 94% for at 50 K) with lowering the temperature.