UC Berkeley

The possible reduction of functional devices to molecular length scales provides many exciting possibilities for enhanced speed, device density, and new functionality. Optical actuation of nanomechanical systems through the conversion of light to mechanical motion is particularly desirable because it promises reversible, ultra-fast, remote operation. Past studies in this area have mainly focused on solution-based molecular machine ensembles, but surface-bound photomechanical molecules are expected to be important for future applications in molecular machines, molecular electronics, and functional surfaces. Cryogenic ultra-high-vacuum scanning tunneling microscopy has been employed to study the surface-based photomechanical switching properties of a promising class of photomechanical molecule called azobenzene.

In the case of tetra-tert-butyl-functionalized azobenzene (TTB-AB) molecules adsorbed on Au(111) reversible switching by means of ultraviolet and visible excitation is experimentally observed at the single-molecule level. The presence of the metallic surface leads to a significant change of the molecular photoswitching properties: (i) photoswitching cross section is significantly reduced compared to the molecules in solution environment, (ii) photoswitching directionality is strongly affected. (iii) correlation between molecular ordering, electronic structure, and photoswitching capability is observed. (iv) new photoswitching dynamical mechanisms become operative.

The results presented in this thesis provide insight into our understanding of photoswitching and adsorption properties of surface-bound molecules and elucidate the important role of molecule-surface interactions and molecule-molecule interactions.