The pyramidal structures of five-coordinate gallium porphyrins appear to destabilize their binding to highly oriented pyrolytic graphite (HOPG) as compared to planar metalloporphyrins, for porphyrins where side-chain contributions to the binding energy are relatively small.  Overcoming this destabilization is important for forming robust self-assembled monolayers in which covalent axial ligands project functional moieties orthogonal to the surface, where they may possess unusual collective properties. We have investigated the surface chemistry of long-side-chain Ga(III) meso-tetraalkyl-substituted porphyrins [Ga(TRP)X; e.g. R = C10H21,X = Cl, CCR'] using scanning tunneling microscopy (STM ) at the 1-phenyloctane/HOPG interface. It is observed that this class of compounds forms highly ordered self-assembled arrays, with the axial ligands oriented perpendicular to the surface. Compared to derivatives of more compact porphyrins (e.g. octaethylporphyrin), arrays of Ga(TRP)X have greater domain sizes, and the removal of adsorbed molecules during STM imaging has been greatly reduced. Interestingly, the axial ligand exerts a marked structure-directing effect for porphyrins of a given side-chain length, as indicated by comparisons among the packing structures of Ga(TRP)X compounds and to those of non-gallium-containing derivatives. The implications of these results for development of surface-supported functional monolayers will be described.

Scanning tunneling microscopy studies of gallium meso-tetraalkylporphyrins on HOPG

246th National Meeting (2013)

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