The influence of viscosity on the functioning of molecular motors.

Light driven molecular motors based on sterically overcrowded alkenes achieve repetitive unidirectional rotation through a sequential series of photochemical and thermal steps. The influence of highly viscous environments on the functioning of unidirectional light driven molecular motors is established in the present report using three distinct media. Liquefied propane, a reference medium due to its low viscosity even at 85 K, and two binary solvent systems that undergo a well-defined glass-transition are used to evaluate the influence of glass-like viscosities on both photochemical equilibria and on thermal helix inversion in the molecular motors. It is found that the greater molecular volume of the stable conformations relative to the unstable conformations is responsible for a shift of the photochemical equilibrium from 20% to 100% in favour of the unstable form when irradiated in high viscosity media. These results demonstrate the critical role excluded solvent volume can play in immobilized photo responsive molecular systems. The volume expansion associated with the thermal reversal to the most stable isomer can be used to determine the maximum possible power output of the rotary cycle.