Patterning a surface with functional molecules utilising self-assembly is an important step towards applications such as molecular memory. Here at the Nanoscale Function group we are using AFM to look at the formation of ordered films of rotaxane molecules on a graphite surface. We have observed morphological differences between the films depending on the type of molecule. A simple rotaxane molecule produces monolayer films with morphology influenced only by the solvent evaporation process. On the other hand, the same molecule with external pyridyl groups on the macrocycle formed films with domains preferentially orients relative to the symmetry of the underlying graphite lattice. Further, the measured cross-sectional height of monomolecular films of the exopyridyl rotaxane is dependent upon the hydrogen bond acceptance ability (HBA) of the solvent used in the deposition process. Solvents of higher HBA yield films with a greater height relative to those of low dielectric constant, due to their stronger interaction with the h-bonding system of the rotaxane. The height, and hence the conformation, corresponds closely to appropriate dimensions as measured by X-ray diffraction. Thus by controlling the HBA of the solvent and the molecular structure of the rotaxane, one can exert some control over the assembly and conformation; conditions which may prove useful in accessing conformations that could be conducive to achieving sub-molecular motion at surfaces.

Our rotaxanes are provided via a collaboration with the group of Prof. Dave Leigh, University of Edinburgh.