A Quantitative Single-Molecule Study of Thioether Molecular Rotors

This paper describes a fundamental, single-molecule study of the motion of a set of thioethers supported on Au surfaces. Thioethers constitute a simple, robust system with which molecular rotation can be actuated both thermally and mechanically. Low-temperature scanning tunneling microscopy allowed the measurement of the rotation of individual molecules as a function of temperature and the quantification of both the energetic barrier and pre-exponential factor of the motion. The results suggest that movement of the second CH, group from the S atom over the surface is responsible for the barrier. Through a series of single-molecule manipulation experiments, we have switched the rotation on and off reversibly by moving the molecules toward or away from one another. Arrhenius plots for individual dibutyl sulfide molecules reveal that the torsional barrier to rotation is similar to 1.2 kJ/mol, in good agreement with the temperature at which the molecule's appearance changes from a linear to a hexagonal shape in the STM images. The thioether backbone constitutes an excellent test bed for studying the details of molecular rotation at the single-molecule level.