In this work, the adsorption and on-surface reactions of different porphyrins on rutile TiO2(110) and on Cu(111) were investigated with X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The first part of my research was focused on the adsorption of different porphyrin molecules on rutile TiO2 while the second part dealt with the on-surface reactions of benzoporphyrins on the Cu(111) surface. On the rutile TiO2(110) surface, the adsorption energies of different metallo-tetraphenyl-porphyrins were compared. Because of the irreversible adsorption of the monolayer, temperature programmed desorption cannot be used as an investigation method here. Instead, mixed layers of two different metallo-tetraphenylporphyrins were adsorbed on the substrate. By heating the sample, the molecules become mobile and can diffuse between mono- and multilayer until an equilibrium state is reached where the porphyrin with the higher adsorption energy enriches on the surface by replacing the porphyrin with the lower adsorption energy. The latter was then depleted at the surface and enriched in the multilayer. From the observed ratios of the two porphyrins in the remaining monolayer after multilayer desorption, we were able to calculate the equilibrium constant of the layer exchange process. The equilibrium constant is dependent on the adsorption energies of the porphyrins and the change in entropy of the layer-exchange process. We thereby expect no changes in entropy during the exchange process of two very similar molecules, which only differ in their metal center. This way, we were able to calculate the differences in adsorption energy between pairs of different metallo-tetraphenylporphyrins. Additionally, we investigated the coadsorption of carboxyl-functionalized (monocarboxy-phenyl triphenylporphyrin) and non-functionalized tetraphenylporphyrin (Zn-tetraphenyl-porphyrin) on rutile TiO2(110). Our initial expectations were that, due to the strongly-binding carboxyl group, a layer-exchange process would occur, where the equilibrium would lie completely on the side of the carboxyporphyrin populating the monolayer and completely replacing the unfunctionalized porphyrin from the surface. Instead, we found that only a small proportion of the Zn-tetraphenylporphyrin molecules were replaced by the carboxyporphyrin. Using the N 1s and Zn 2p3/2 signal of grazing angle and normal emission XPS, we were able to identify that the majority of the carboxyporphyrin molecules are adsorbed above the Zn-tetraphenylporphyrin molecules, most-likely anchored upright standing in between left open gaps between the underneath flat-lying Zn-tetraphenylporphyrin molecules. The second main part of my research was focused on the on-surface reactions of benzo-functionalized tetraphenylporphyrins on the Cu(111) surface. Here, STM measurements together with DFT calculations suggested that a metalation reaction of free-base tetraphenyl transdibenzoporphyrin with surface copper atoms takes place already at room temperature. This metalation of the molecules results in island formation, stabilized by T-type interactions between the isoindole and the phenyl groups of adjacent molecules. The metalation reaction was confirmed by XPS where the N 1s region showed a transition from unmetalated porphyrins with two characteristic nitrogen signals to one single signal of metalated porphyrins. Furthermore, STM, XPS and H2 TPD showed that the rate of the metalation reaction is strongly coverage- and temperature-dependent. Using H2 TPD and STM, we furthermore found that, at higher temperatures, after the metalation reaction has taken place, the benzoporphyrins undergo an intramolecular ring fusion reaction, where every phenyl group forms a new bond with neighboring carbon atoms within the same molecule. At even higher temperatures, a polymerization reaction occurs, during which the molecules lose all their hydrogens. We furthermore compared these result to H2 TPD measurements of Cu-tetraphenyltransdibenzoporphyrin, tetraphenyltetrabenzoporphyrin and tetraphenyl porphyrin. While all molecules showed similar coverage-dependent behavior for the ring fusion and polymerization reactions, all free-base molecules showed a completely different coverage dependence in their metalation reactions. The abrupt coverage-dependent changes of the metalation rate of the investigated free-base porphyrins are most likely caused by coverage-dependent transitions in their adsorption structure.
Maximilian Muth (Thu,) studied this question.