In artificial small-molecule machines,molecular motors can be used to perform work on coupled systems by applying a mechanical load-such as strain-that allows for energy transduction.Here,we report how ring strain influences the rotation of a rotary molecular motor.Bridging the two halves of the motor with alkyl tethers of varying sizes yields macrocycles that constrain the motor's movement.Increasing the ring size by two methylene increments increases the mobility of the motor stepwise and allows for fine-tuning of strain in the system.Small macrocycles(8-14 methylene units)only undergo a photochemical EIZ isomer-ization.Larger macrocycles(16-22 methylene units)can perform a full rotational cycle,but thermal helix inversion is strongly dependent on the ring size.This study provides systematic and quantitative insight into the behavior of molecular motors under a mechanical load,paving the way for the development of complex coupled nanomachinery.
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