Strain Energy Induced Rotary Speed Acceleration in a Light-Driven Molecular Motor.

Light-driven molecular motors based on overcrowded alkenes represent a major type of molecular machines that are able to rotate unidirectionally. The regulation of the rotary speed without altering the core structure of a motor is crucial and remains a major challenge. In the present study, we reported that the rotary speed of molecular motors can be significantly enhanced by harnessing the strain energy of cycloparaphenylene (CPP). A series of molecular motors incorporated in CPP with varying sizes were synthesized, and their photochemical and thermal isomerization behaviors were meticulously examined using UV-vis and H NMR spectroscopy. The remarkable increase of the acceleration effect of the rotary speed with decreasing macrocycle sizes, up to 389-fold, can be attributed to the strain energy induced bending in the stator part, which reduces steric hindrance in the "fjord region" of the molecule, supported by a detailed computational study employing density functional theory. This work provides systematic insight into the behavior of molecular motors under strain energy, thereby paving the way for the application of motor-incorporating CPPs as a general strategy to accelerate the rotary speed of molecular motors.