Using Displacement-Assisted Desorption to Identify Reactive Intermediates: Ring-Fused Porphyrins on Cu(111)

Because of its excellent chemical sensitivity, temperature-programmed desorption (TPD) is a widely-used method to study on-surface reactions, but it is limited to study adsorbed species that desorb before they decompose. In this study, we present a novel displacement-assisted TPD approach that displaces the adsorbed species from the monolayer into the multilayer prior to desorption, and thus allows for the detection and unambiguous identification of species that would otherwise decompose on the surface before desorption. The method employs a three-step process consisting of (i) controlled pre-annealing to initiate an on-surface reaction, (ii) deposition of a displacement agent, enabling displacement of the species in the monolayer into the multilayer and (iii) subsequent annealing to desorb the multilayer. This approach is demonstrated by investigating the temperature-dependent self-metalation and subsequent ring fusions (cyclodehydrogenation) of tetraphenyl-porphyrin (2HTPP) on Cu(111), using phenylphosphonic acid (PPA) as a displacement agent. The results reveal the quantitative conversion of CuTPP to 1-, 2-, 3-, and finally 4-fold-fused derivatives, species that otherwise would require high-resolution STM or AFM to distinguish between.