Non-covalent chemistry on surface-confined, isolated dendrimers

A bottom-up fabrication of nanometer-scale dendritic structures via the growth of isolated, nanosized supramolecular structures on gold surfaces is presented. A novel dendritic wedge containing peripheral pyridines and a focal dialkylsulfide chain (9) was synthesized and its coordination chemistry in solution was tested with a second-generation Fréchet dendron functionalized with a focal SCS PdII pincer moiety (13). Replacement of the labile acetonitrile in 13 by the pyridine groups of 9 converts dendritic wedge 9 into fourth-generation metallodendron 9 · 134 in one step. Based on these results the spatially confined growth on a gold surface was studied. The dendritic molecules with reactive pyridine groups at the periphery (9) were first spatially isolated by inserting them into decanethiol self-assembled monolayers (SAMs) on a gold surface. Subsequently, the peripheral pyridines were reacted via metal-ligand coordinative interactions by exposing the monolayer to a solution of dendritic wedges containing focal SCS PdII pincer moieties (13). The isolated molecules inserted into the original SAM in the first step were resolved by ex-situ tapping-mode atomic force microscopy (TM-AFM). The subsequent PdII-pyridine coordination resulted in a significant increase in size of the individually resolved molecules. Reference experiments demonstrated that the increased size of the isolated dendrimers on the gold substrate was a result of specific metal-ligand interactions. The methodology presented herein allows the creation of surface-confined architectures and potentially provides a viable complement to currently employed top-down methods in nanofabrication.