Divergent growth of coordination dendrimers on surfaces.

Divergent growth of surface-initiated dendritic nanostructures on gold surfaces in a highly controlled, stepwise manner is demonstrated, using metal-organic coordination as the binding motif. The repeat unit for dendrimer growth was a branched, C3-symmetrical ligand building block bearing three bis-hydroxamate groups. The surface initiation sites for dendrimer growth were obtained by the formation of a mixed monolayer comprising isolated bis-hydroxamate disulfide anchor ligands and octanethiol (OT) at very low anchor/OT ratios. Following functionalization of the surface with spaced anchors, alternate immersion in solutions of Zr4+ ions and the branched ligand afforded surface-confined dendrimers of increasing generation, where the number of generations is conveniently controlled by the number of coordination binding sequences. The heights of different generation dendrimers are in excellent agreement with values predicted by molecular models, as well as with thicknesses of branched multilayers prepared by the same procedure on full anchor monolayers. At higher generation numbers, gradual dendrimer overlap and coalescence are observed, eventually resulting in a continuous overlayer and a transition from 3D to 1D growth. A mechanism for the development of dendritic coordination nanostructures on surfaces is discussed.