Light-Gated Control of Brownian Rotation by a Molecular Motor

Controlling stochastic molecular motion in a predictable manner remains a major challenge in the development of functional molecular systems. Here we demonstrate that a light-driven molecular motor can reversibly regulate Brownian rotation through remote steric interactions. We integrate an overcrowded alkene motor within a rigid triptycene scaffold, where it acts as a reversible steric gate for an alkynyl-linked anthracene rotor. Photoisomerization modulates the steric environment around the rotor axis, switching the system between fast Brownian rotation and a sterically hindered regime. Variable-temperature NMR spectroscopy reveals that one thermodynamically stable motor state exhibits rapid Brownian rotation of the anthracene unit about the alkynyl C─C≡C─C axis, whereas the other stable state displays hindered rotation, giving rise to two distinct rotameric states at low temperature. Density functional theory (DFT) calculations reproduce this behavior and show that the rotational barrier changes by 31 kJ mol^-1 between the two states, corresponding to an approximately five-order-of-magnitude difference in the calculated rotational rates at 20 °C. These results demonstrate that light-driven molecular motors can reversibly regulate stochastic molecular motion through steric control, providing a general strategy for coupling directional and Brownian dynamics within a single molecular system.