The process of self assembly at multiple lengthscales is studied by low temperature scanning tunnelling microscopy for bis-urea substituted toluene evaporated on Au(111) surfaces in high vacuum. The design of the molecules at the supramolecular chemistry level (hydrogen bonds can be formed at specific sites) allows for the
formation of long polymeric chains as observed in solution. However,
bringing these molecules to surfaces in vacuum, weaker interactions become also highly relevant and impose an unexpectedly important influence on the
two-dimensional organisation of these molecules, leading to the formation of hierarchically structured patterns covering lengthscales from the molecular building
units up to perfectly organized supramolecular assemblies of areas larger
than some 104nm2 (F. Vonau et al., Phys. Rev. Lett. 94, 066103 (2005).
Due to the electronic properties and the particular conformation of these molecules, a tunnel transparency allows us to detect simultaneously both the supramolecular layer and the gold surface underneath. As the Au(111) surface reconstruction defines a specific crystallographic direction, it is possible to follow the interlocking of the supramolecular layer with the substrate as a function of surface coverage and annealing time (from several months up to one year under UHV conditions at room temperature). Our results demonstrate the interplay between molecule-molecule and comparatively weaker molecule-substrate interactions. The latter are becoming less influential as the size of the perfectly organized crystalline domains (i.e. the number of coherently interacting molecules) increases with annealing time. This quasi-decoupling from the substrate is nicely demonstrated by the observation that a large 2D-layer of perfectly ordered supramolecular polymers (obtained after a long maturation time) is able to cross over monoatomic steps in the substrate without
perturbation of its 2D structure. We present a possible mechanism for explaining the change in interlocking with the substrate by a cooperative process which involves a large number of supramolecular polymers. We also show a mechanism characteristic for the self-assembly of supramolecular polymers and impossible for covalently formed polymers. |