Structural variations in self-assembled monolayers of 1-pyrenehexadecanoic acid and 4,4′-bipyridyl on graphite at the liquid-solid interface

Static and dynamic structures in self-assembled monolayers containing 1-pyrenehexadecanoic acid (PHDA) at a liquid-solid interface were investigated with scanning tunneling microscopy. Uni-component adsorption layers made a specific structure having ring figures corresponding to pyrene groups, whereas the co-adsorption of PHDA and 4,4'-bipyridyl resulted in two different structures depending on their concentrations and the ambient temperature. When the concentration of 4,4'-bipyridyl was high and the temperature at around 290 K, a dynamic bistable structure was observed within a stacked row of molecules without causing any disorder to the assembly as a whole. The bistability is considered to be due to molecular desorption-adsorption process occurring at the anti-parallel configuration of two neighbouring PHDA molecules.