Unusual Demetalation and Ordered Adsorption of a Pyridine-Appended Zinc Phthalocyanine at Metal–Electrolyte Interfaces Studied by in Situ Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy

A combination of cyclic voltammetry, in situ electrochemical scanning tunneling microscopy (EC-STM), and ex situ X-ray photoelectron spectroscopy (XPS) studies was employed to investigate the electrochemical behavior and to characterize the structure of adsorbed layers of a redox-active pyridine-appended zinc phthalocyanine (ZnPc) at Cu(100)/electrolyte interfaces. Additional cyclic voltammograms (CVs) of a HOPG electrode in phthalocyanine- and iodide-containing electrolyte showed four redox peak pairs related to the phthalocyanine macrocycle. Focusing on the reduction processes, the four observed cathodic waves were assigned to four single electron transfer steps causing four successive reductions of the phthalocyanine. Due to its narrow potential window, the CV of Cu(100) in the same electrolyte covers only the first three reduction features in the acidic solution superimposed by the CuI formation/dissolution processes. Surprisingly, the XPS data from the iodide and phthalocyanine covered Cu(100) surface, however, revealed a demetalation of the molecules, that is, a replacement of the central zinc atom of adsorbed ZnPc+ by hydrogen atoms resulting in the formation of the corresponding free base phthalocyanine H2Pc(+). These H2Pc(+) molecules form highly ordered adlayers on the iodide-modified Cu(100) surface with the coexistence of different phases, that is, one with a square-shaped unit cell enclosing an angle of (91 +/- 2)degrees and a rhombic one with an angle of (66 +/- 2)degrees, and intermolecular distances of 2.0 +/- 0.1 nm and 2.3 +/- 0.1 nm, respectively. The main symmetry axes of the H2Pc(+) adlayer run parallel neither to the commensurate direction of the iodide layer underneath nor to the close-packed atomic rows of the copper substrate. Detailed molecular models are proposed which are in agreement with the patterns experimentally observed with EC-STM.