Self-assembly of porphyrin molecules on a Cu(111) electrode: Influence of different anions and electrode potential

Abstract In this contribution, we provide a view on the role of halide interlayers in the self-assembly of layers of organic molecules at metal-electrolyte interfaces with molecular resolution. In particular, the ordering of tetra(4-trimethylammoniophenyl) porphyrin molecules in cationic form, abbreviated as [H2TTMAPP]4+, has been studied on a Cu(111) electrode surface in aqueous solution containing chloride-, bromide- or iodide-anions using a combination of cyclic voltammetry (CV) and in-situ electrochemical scanning tunneling microscopy (EC-STM). On the one hand the obtained results unveil that the degree of ordering of the self-assembled layer of [H2TTMAPP]4+ cations on the Cu(111) surface changes significantly when changing the halide anion in the working electrolyte, and, thereby, the buffer layer between the Cu(111) surface and the adsorbed [H2TTMAPP]4+ molecular cations. Under the same experimental conditions a fully ordered layer is observed on the chloride precovered Cu(111) surface, but less ordering and even incomplete monolayer formation are found on the bromide and iodide terminated Cu(111) surface, respectively; the degree of order decreases with decreasing electronegativity of the underlying halide. On the other hand potential driven reduction of the organic species lowers the charge density of the organic cations themselves causing a desorption of the reduced molecules at even lower potentials from any of the three differently halide modified electrode surfaces. Both observations underline the importance of electrostatic interactions in the adsorption and structure formation of the porphyrin molecules on the halide modified surfaces.