Reversible Photoswitching and Isomer-Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface

Azobenzene is a prototypical molecular switch that can be reversibly photoisomerized between the nearly planar and apolar trans form, and the distorted, polar cis form. Thus far, most studies related to azobenzene derivatives have focused on planar adsorbed molecules. We present here the study of a three-dimensional shape-persistent molecular architecture consisting of four tetrahedrally arranged azobenzene units and adsorbed on a Ag(111) surface. While the azobenzenes of the tripod in contact with the surface lose their switching ability, different isomers of the upright standing arm of the tetramer are obtained reversibly and efficiently by illumination at different wavelengths, revealing time constants of only a few minutes. Diffusion on the surface turns out to be dependent on the isomeric state - trans or cis - of the upright oriented azobenzene group. Hence, molecular mobility can be modulated via their isomeric state, which suggests that for instance molecular growth processes could be controlled by external stimuli.