Mechanically Interlocked Molecular Rotors on Pb(100)

The mechanical coupling between molecules represents a promising route for the development of molecular machines. Constructing molecular gears requires easily rotatable and mutually interlocked pinions. Using scanning tunneling microscopy (STM), it is demonstrated that aluminum phthalocyanine (AlPc) molecules on Pb(100) exhibit these properties. Unlike other phthalocyanines on this substrate, isolated AlPc molecules fluctuate between two azimuthal orientations. Density functional theory (DFT) calculations confirm two stable orientations of single molecules and indicate a relatively low rotation barrier. In STM-constructed dimers and trimers, fluctuations diminish, and various molecular orientations are stabilized. Induced collective rotation of all molecules in the trimers is observed, demonstrating their mechanical interlocking. Potential functions describing angle and distance dependencies of intermolecular and molecule-substrate interactions are derived from DFT calculations of dimers; 52 experimentally determined trimer geometries are reproduced using these potentials. This intuitive approach may prove to be useful in modeling larger structures beyond the scope of quantum mechanical descriptions.