Speaker
Description
On-surface reactions including self- and trans-metalation of organic molecules on surfaces play a major role in the fields of organic electronics and molecular self-assembly[1]. They can on one hand provide additional pathways, or even the only method, to obtain specific structures, thereby providing a tool to tailor the properties of molecular layers; but on the other hand, unwanted reactions can negatively affect their stability and limit their applications.
While such types of reactions have been extensively studied on metal surfaces, it was long believed that they would be hindered on oxides, due top the stronger bonds within such solids. However, in previous experiments, we have shown that porphyrins, which represent a versatile class of organic molecules, can self-metalate when adsorbed on ultrathin magnesium oxide film[2].
On such surface, self-metallation is not the only chemical process occurring: charge transfer can also be observed, depending on the difference between the electron affinity of the adsorbate and the surface work function[3], with either process having been observed regardless of the other[4].
Here we show that the interplay between these two processes is much more subtle, with the charge state of the molecule affecting primarily the adsorption geometry and thereby also the energy barriers for the self-metallation of porphyrins of different size. Temperature-resolved XPS measurements, combined with DFT calculations, reveal that charge transfer actually increases the barrier for self-metallation reactions, thereby stabilising the structure at least up to room temperature. These results provide fundamental understanding to the parameters which must be tuned to fully control on-surface reactions on oxide surfaces and thin films.
[1] Carnes et al., Chem. Soc. Rev., 2014, 43, 1825-1834
[2] Egger et al., Angew. Chem. Intl. Ed., 2021, 60, 5078-5082
[3] Hurdax et al., Adv. Mater. Interf., 2020, 7, 2000592
[4] Presel et al., Phys. Chem. Chem. Phys., 2022, 24, 28540-28547