How Does the Flexibility of Molecules Affect the Performance of Molecular Rotors?

In research on molecular machines, the flexibility of the molecules has been shown to significantly affect the performance of such "soft" machines and thus lead to unexpected phenomena that differ from rigid machines in the macroscopic world. Taking several typical rotational molecules as examples, we examine how the deformation of the molecule (commonly caused by curving parts of a molecule due to its interaction with other molecules) affects the effectiveness of a molecular machine system, such as a chain of molecular gears. From the viewpoint of quantum chemistry and classical mechanics, we introduce a torque analysis strategy to quantitatively analyze the strength of the repulsion/attraction force induced by the deformation of such molecules. By comparing different types of chemical bonds, we show that a bond connecting to an aromatic ring exhibits a larger stiffness than bonds that do not directly connect to an aromatic ring. We thereby highlight that the inclusion of aromatic rings in a molecular machine can considerably increase the stiffness of the machine, which is an important factor in designing effective molecular machines.