Concentration and Coverage Dependent Adlayer Structures: From Two-Dimensional Networks to Rotation in a Bearing

A bisterpyridine based molecule, 3,3′-BTP, shows a variety of adlayer structures at the interface between highly oriented pyrolytic graphite (HOPG) and the liquid depending on the concentration in solution. Three closely related linear and one hexagonal 2D patterns are found. Comparison with the self-assembly at the HOPG|gas interface shows that in the absence of the solvent one of the linear and the hexagonal structures can be found. The concentration dependent order of appearance of the different surface structures is rationalized by a thermodynamic model. In the adlayer unit cell, the hexagonal phase offers a central void which is mostly filled with a seventh 3,3′-BTP molecule. In the presence of the solvent, those molecules are presumably rotating, whereas at the HOPG|gas interface no clear rotation can be observed.