Surface influence on the metalation of porphyrins at the solid-liquid interface

Abstract

The insertion of a metal atom in the central cavity of adsorbed porphyrins is an important subject as it allows specific control over the functionality of the molecule. In this work, metalation of protoporphyrin IX (PPIX) molecules bonded directly to a Au(111) surface or to a self-assembled monolayer (SAM) on Au(111) was studied at the solid-liquid interface. X-ray photoelectron spectroscopy (XPS) and scannning tunneling microscopy demonstrate that the molecules bind to the SAM without forming aggregates and with a surface coverage below the monolayer. Nearedge X-ray absorption fine structure spectroscopy and surface-enhanced resonance Raman spectroscopy demonstrate that the molecules are bonded to the SAM with a tilted molecular plane. XPS measurements show that PPIX molecules bonded to the SAM and exposed to Zn2+ containing aqueous solutions are metalated to a small extent at room temperature and fully metalated at 350 K. In contrast, PPIX molecules adsorbed directly to the Au(111) surface are fully metalated at room temperature. These results show that surface-molecule interactions could have an impact on the metalation of porphyrin molecules.