On-Surface Porphyrin Transmetalation by Pb/Cu Redox Exchange

Metal complexes at surfaces and interfaces play a prominant role in many areas of modern technology, including catalysis, sensors, and organic electronics. An important aspect of these interfaces is the possible exchange of the metal center, because this reaction can drastically alter the properties of the metal complex and thus of the interface. Here, we demonstrate that such metal exchange reactions are indeed possible and can proceed already at moderate temperatures even in the absence of solvents. Specifically, we studied the redox transmetalation of a monolayer of lead(II)-tetraphenylporphyrin (PbTPP) with copper from a Cu(111) surface under ultrahigh-vacuum (UHV) conditions using multiple surface-sensitive techniques. Temperature dependent X-ray photoelectron spectroscopy (XPS) reveals that the Pb/Cu exchange starts already below 380 K and is complete at 600 K. The identity of the reaction product, CuTPP, is confirmed by mass spectrometric detection in a temperature-programmed reaction (TPR) experiment. Scanning tunneling microscopy (STM) sheds light on the adsorbate structure of PbTPP at 300 K and uncovers the structural changes accompanying the transmetalation and side-reactions of the phenyl substituents. Moreover, individual free Pb atoms are observed as a product of the metal exchange. Our study suggests that surfaces functionalized with metal complexes may consist of other species than intended under operation conditions, which often involve elevated temperatures.