中央研究院 原子與分子科學研究所

The advent of scanning tunneling microscopy (STM) has permitted a detailed atomic view of organic molecules adsorbed on solid surfaces. In this work, we make use of the STM to provide an unprecedented direct single-molecule perspective on the cis−trans photoisomerization of stilbene molecules within ordered monolayers physisorbed on the Ag/Ge(111)−(√3×√3)R30° surface. The STM view of the molecular structure transformation upon irradiation provides direct evidence for the generally accepted one-bond-flip mechanism proposed for the photoisomerization process. We also find that the surface environment produces a profound effect on the reaction mechanism. The reaction is observed to proceed mainly through pairs of co-isomerizing molecules situated at domain boundaries. To explain these observations, we propose a mechanism whereby excitation migrates to the domain boundary and the reaction occurs through a biexciton reaction pathway.

The adsorption and self-organization of trans-azobenzene (TAB) on Ag/Ge(111)-(√3×√3)R30°(Ag/Ge(111)-√3) were studied by low temperature scanning tunneling microscopy (LT-STM) in ultrahigh vacuum (UHV). High-resolution STM images allow the observation of individual TAB molecules and the commensurate TAB chain domains formed via the hydrogen bond enhanced intermolecular interaction and molecule–substrate interaction on Ag/Ge(111)-√3. From in situ observation of the substrate lattice, the TAB monolayers were found to form a (2x1) structure. Some coexisting cis-azobenzene (CAB) molecules were observed on the domain boundary of TAB overlayer. The structural model and the molecule registry corresponding to STM images for the monolayer of TAB on Ag/Ge(111)-√3 are proposed and discussed