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Abstract

There are few general methods for the nanoscale organization of functional modules in three dimensions, an issue that needs to be addressed as device fabrication reaches its lower length-scale limits. The bottom-up self-assembly approach aims to organize such functional subunits into repeating patterns at nanometer length scales in ways that their collective assembly at an interface makes them addressable. The central hypothesis investigated here is that five-coordinate gallium-porphyrin monolayers with covalently bound axial ligands are good candidates for supports in such a bottom-up approach. The two-dimensional assembly of porphyrins on planar surfaces is a richly developed field, and the incorporation of a metal that can axially coordinate to a ligand of interest offers a modular method of extending the assembly into the third dimension. This work describes a dual approach to the functionalization of self-assembled five-coordinate gallium(III)octaethylporphyrin (Ga(OEP)X) monolayers. The first is a covalent functionalization of the axial ligand to incorporate functional modules. To initially investigate the effects of covalently adding a fifth ligand to self-assembled arrays of metalloporphyrins, a series of Ga(OEP)X complexes (X = Cl, Br, I, O3SCF3, CCPh) was studied via density functional theory (DFT) and scanning tunneling microscopy (STM) at the solid-liquid interface. Additional functionality was then incorporated using ethynylferrocene and zinc ethynyltetraphenylporphyrin ligands. The assembly of select complexes was also investigated on a variety of substrates including highly oriented pyrolytic graphite (HOPG), single layer graphene on copper foil, and monolayer MoS2. The second approach for surface functionalization described in this work is the supramolecular incorporation of fullerenes into the inter-ligand cavities formed by the unit cell of the self-assembled monolayer. This approach both takes advantage of the favorable size match between the unit cell in Ga(OEP)X monolayers with that of fullerene guest molecules and further axially incorporates a ligand predicted to have affinity for fullerenes. Various templating ligands were investigated (1-ethynylpyrene, 2-ethynylpyrene, 3-ethynylthiophene, 9-ethynylanthracene, 9-ethynyltriptycene, and 1-pyrenecarboxylate) by molecular modeling and STM. The various factors that affect fullerene templating, such as ligand height, deposition method, and concentration are discussed.

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