Dynamics of thioether molecular rotors: Effects of surface interactions and chain flexibility

Recent single-molecule experiments indicated that thioethers (dialkyl sulfides) on gold surfaces act as thermally-or mechanically activated molecular rotors, although the mechanisms for these phenomena are not yet clearly understood. Here we present theoretical and experimental investigations of the rotational dynamics of these thioether molecules. Single-molecule studies utilizing low-temperature scanning tunneling microscopy allowed us to determine rotational rates and activation energies for the rotation of symmetric dialkyl sulfides. It was found that the rotational energy barriers increased as a function of alkyl chain length but then quickly saturated. Molecular dynamics simulations have also been performed in order to understand the molecular rotations of thioethers, and our theoretical calculations agree well with experimental observations. It is argued that the observed rotational dynamics of dialkyl sulfides are determined by the effective interactions with the surface and the flexibility of the alkyl chains. These results suggest possible ways to control and utilize thioether rotors at the single-molecule level.