Self-metalation of tetraphenyl porphyrin on Au(111): Structural characterisation via X-ray standing wave analysis

Abstract

The study of surface-confined reactions provides a route towards characterising the mechanistic processes which underpin heterogeneous catalysis. On-surface reactions provide alternative pathways to solution phase synthesis, and can be studied by a range of techniques which can provide chemical and structural information. Importantly, these kinds of studies give us the insight required to potentially control the selectivity and efficiency of these reactions, as well as allowing for the design of molecular systems with specific functionalities. Porphyrin molecules are frequently selected as participants in these systems as they offer significant customisability, with respect to the reactivity of pendant groups and metal atoms which can be incorporated within the porphyrin macrocycle. Utilising the normal incidence X-ray standing wave (NIXSW) technique in conjunction with scanning probe methods and X-ray photoelectron spectroscopy (XPS) enables the generation of detailed models of on-surface systems, including the adsorption geometries of individual molecules. Here the NIXSW technique has been used to perform structural characterisation of three distinct phases of an on-surface reaction of tetraphenyl porphyrin (TPP) on Au(111); close-packed, diffuse and metalated. Using the chemical specificity of XPS and NIXSW we gain insight into chemical and structural changes exhibited by nitrogen atoms within the porphyrin macrocycle at several stages within the reaction process.