Solvents Hinder the Interlocking Rotation between Molecular Gears, as Revealed by Torque Calculations

Molecular motors offer promising applications in the fields of nanodevices and nanofluidics. It is thus highly relevant to study their practical operation processes in fluids. In this work, we adopted the torque approach based on quantum mechanical-calculated results to explicitly demonstrate that liquids hinder the rotation of a cogwheel-gearing system consisting of two nonpolar hexaethynyl-benzene molecules stacked on graphene with π–π bonding. For nine common organic solvents (some of which can be viewed as small models of lubricants)—acetic acid, propylene carbonate, benzene, ethyl acetate, ethanol, tetrahydrofuran, acetone, acetonitrile, and n-hexane–torque profiles reveal a counterintuitive increasing hindrance effect with decreasing solvent viscosity. Through a further analysis by the reduced density gradient method, we find that noncovalent interactions, that is, dispersion forces between the solvents and gears, dominate in obstructing nonpolar gear rotation transfer in the solvents of lower viscosity; our torque approach thus predicts a significant solvent effect on molecular motors. This study shows that the torque approach can help better understand the mechanisms of molecular rotors working in a realistic liquid medium and guide the design of effective molecular motors for viscosity probes or pumping fluids, for example.