Measuring reversible photomechanical switching rates for a molecule at a surface

We have used single-molecule-resolved scanning tunneling microscopy to measure the photomechanical switching rates of azobenzene-derived molecules at a gold surface during exposure to UV and visible light. This enables the direct determination of both the forward and reverse photoswitching cross sections for surface-mounted molecules at different wavelengths. In a dramatic departure from molecular behavior in solution-based environments, visible light does not efficiently reverse the reaction for azobenzene-derived molecules at a gold surface. (c) 2008 American Institute of Physics.