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The coadsorption of porphyrin molecules (TMPyP: tetra(N-methyl-4-pyridyl)-porphyrin), sulfate anions and copper on a Au(111) electrode was investigated by the use of cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy. With decreasing electrode potential the following sequence of surface phases was found: (I) an ordered � √ 3 × √ 7 � R19.1 • − SO 4 2− structure on the unreconstructed Au(111)-(1 × 1) surface; (II) a disordered SO 4 2−-layer on the still unreconstructed Au(111)-(1 × 1); (III) a � √ 3 × √ 3 � R30 • coadsorption structure of 2/3 ML Cu and 1/3 ML SO 4 2− ; (IV) a completed 1 ML Cu covered by a layer of mobile, i.e. not imaged, SO 4 2− anions, moreover, a coadsorption layer of disordered porphyrin molecules and still mobile SO 4 2− anions; (V) overpotentially deposited Cu-multilayers terminated by the well known Moiré-type modulated � √ 3 × √ 7 � R19.1 • − SO 4 2− structure (similar to bulk Cu(111)) and covered by a dense layer of flat lying TMPyP molecules showing a growing square as well as hexagonally ordered arrangement, and at even more negative potential values and low Cu concentrations in the solution (VI) a pseudomorphic underpotentially deposited Cu-monolayer covered by a � √ 3 × √ 7 � R19.1 • − SO 4 2− layer and a dense, ordered porphyrin layer ontop. The formation of the various phases is driven by the potential dependent surface charge density and the resultant electrostatic interaction with the respective ions. A severe imbalance between the copper deposition and desorption current in the CV spectra suggests also the formation of CuTMPyP-metalloporphyrin on the surface which diffuses into the bulk solution.

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