Photoelectron spectroscopy investigation of the temperature-induced deprotonation and substrate-mediated hydrogen transfer in a hydroxyphenyl-substituted porphyrin

TL;DR

This study uses photoelectron spectroscopy to explore how heating affects the chemical structure and bonding of a hydroxyphenyl-substituted porphyrin on different metal surfaces, revealing temperature-dependent deprotonation and hydrogen transfer processes.

Contribution

It provides detailed insights into the temperature-induced chemical transformations of the porphyrin on Au(111) and Ag(110) surfaces, highlighting substrate effects on molecular stability and reactions.

Findings

Deprotonation occurs stepwise with increasing temperature.

Hydrogen transfer and dissociation are substrate-dependent.

High-temperature annealing induces covalent bonding and structural changes.

Abstract

The temperature dependent stepwise deprotonation of 5,10,15,20-tetra(p-hydroxyphenyl)porphyrin is investigated using photoelectron spectroscopy. An abundance of pyrrolic relative to iminic nitrogen and a decrease in the ratio of the amount of -NH- to -N= with increasing annealing temperature is found. In contrast to the molecules adsorbed on Au(111), on the more reactive Ag(110) surface, partial dissociation of the hydroxyl groups and subsequent diffusion and rebonding of hydrogen to the central nitrogen atoms resulting in a zwitterionic molecule was clearly observed. Moreover, partial C-H bond cleavage and the formation of new covalent bonds with adjacent molecules or the surface starts at a relatively high annealing temperature of 300{\deg}C. This reaction is identified to occur at the carbon atoms of the pyrrole rings, which leads also to a shift in the N 1s signal and changes in the valence band of the molecules. Our results show that annealing can significantly alter the molecules which were deposited depending on the maximum temperature and the catalytic properties of the specific substrate. The thermal stability should be considered if a molecular monolayer is prepared from a multilayer by desorption, or if annealing is applied to enhance the self-assembly of molecular structures.