  We have observed reversible light-induced mechanical switching for a single
organic molecule bound to a metal surface. Scanning tunneling microscopy (STM)
was used to image the features of an individual azobenzene molecule on Au(111)
before and after reversibly cycling its mechanical structure between trans and
cis states using light. Azobenzene molecules were engineered to increase their
surface photomechanical activity by attaching varying numbers of tert-butyl
(TB) ligands ("legs") to the azobenzene phenyl rings. STM images show that
increasing the number of TB legs "lifts" the azobenzene molecules from the
substrate, thereby increasing molecular photomechanical activity by decreasing
molecule-surface coupling.
