Interaction of free-base tetraphenylporphyrin with magnesium oxide: Influence of MgO morphology on metalation

Ladungsunterstützte Selbstmetallierung von Porphyrinen auf Oxidoberflächen

Das Interesse an Metallierungs‐ und Selbstmetallierungsreaktionen auf Oxidoberflächen ist in jüngster Zeit ständig gewachsen. Der Mechanismus der Selbstmetallierungsreaktion ist jedoch nicht vollständig geklärt. Hier zeigen wir mithilfe von Rastertunnelmikroskopie, Photoemissions‐Spektroskopie und Dichtefunktionaltheorie‐Rechnungen, dass die Selbstmetallierung von 2H‐Tetraphenylporphyrin auf der Oberfläche von ultradünnen MgO(001)‐Filmen durch Ladungstransfer ermöglicht wird. Es wird gezeigt, dass der Ladungszustand und dadurch der Metallierungszustand der Porphyrin‐Moleküle durch die Austrittsarbeit des MgO(001)/Ag(001)‐Substrats gezielt eingestellt werden können.

Charge-Promoted Self-Metalation of Porphyrins on an Oxide Surface

Metalation and self-metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self-metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self-metalation of 2H-tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.

Morphology dependent interaction between Co(II)-tetraphenylporphyrin and the MgO(100) surface

Porphyrins are key elements in organic–inorganic hybrid systems for a wide range of applications. Understanding their interaction with the substrate gives a handle on structural and electronic device properties. Here we investigate a single transition-metal porphyrin, namely Co(ii)-tetraphenylporphyrin (CoTPP), on the MgO(100) surface and the effect of multilayer film formation within hybrid density-functional theory and many-body perturbation theory. We focus on the relevant adsorption sites, simulate their photoemission spectra as a key fingerprint and compare with experiments on MgO(100) films on Ag(100). While we find only weak interaction between the cobalt centre and terrace sites on the MgO(100) surface, a strong interaction manifests itself with the low-coordinated sites. This leads to distinct features in both the valence and core-level regions of the electronic structure, as observed in the ultraviolet and X-ray photoemission spectra, corroborated by simulated spectra and calculated cobalt core-level shifts. Our work thus demonstrates the relevance of morphology-related low-coordinated sites and their properties in the adsorption of CoTPP on the MgO(100) surface.

The adsorption of Co-tetraphenylporphyrin at relevant low-coordinated sites on MgO(100) shows distinct features from terrace-site and multilayer films in the near-valence and corelevel regions of the electronic structure.

Negative electron affinity of adamantane on Cu(111)

Photoelectron spectroscopy is used to show that thick adamantane films on Cu(111) have a negative electron affinity of -0.3 ± 0.1 eV. The ionization potential is obtained as 8.55 ± 0.15 eV resulting in a band gap of 8.9 ± 0.1 eV. For films of about 1.4 monolayer thickness the electron affinity is close to zero and the valence bands are shifted toward the Fermi energy due to charge transfer from Cu 3d bands.

Self-Metalation of Anchored Porphyrins on Atomically Defined Cobalt Oxide Surfaces: In situ Studies by Surface Vibrational Spectroscopy

Metalation of anchored porphyrins is essential for their functionality at hybrid interfaces. In this work, we have studied the anchoring and metalation of a functionalized porphyrin derivative, 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin (MCTPP), on an atomically-defined CoO(100) film under ultrahigh vacuum (UHV) conditions. We follow both the anchoring to the oxide surface and the self-metalation by surface Co2+ ions via infrared reflection absorption spectroscopy (IRAS). At 150 K, MCTPP multilayer films adsorb molecularly on CoO(100) without anchoring to the surface. Upon heating to 195 K, the first layer of porphyrin molecules anchors via formation of a bridging surface carboxylate. Above 460 K, the MCTPP multilayer desorbs and only the anchored monolayer resides on the surface up to temperatures of 600 K approximately. The orientation of anchored MCTPP depends on the surface coverage. At low coverage, the MCTPP adopts a nearly flat-lying geometry, whereas an upright standing film is formed near the multilayer coverage. Self-metalation of MCTPP depends critically on the surface temperature, the coverage and on the molecular orientation. At 150 K, metalation is largely suppressed, while the degree of metalation increases with increasing temperature and reaches a value of around 60 % in the first monolayer at 450 K. At lower coverage higher metalation fractions (85 % and above) are observed, similar as for increasing temperature.

Effect of crystallization on the electronic and optical properties of archetypical porphyrins

Thin porphyrin films as employed in modern optical devices or photovoltaic applications show deviating electronic and optical properties from the gasphase species. Any understanding of the physical origin may pave way to a specific engineering of these properties via ligand or substituent control. Here we investigate the impact of crystallization of prototypical porphyrins on the electronic levels and optical properties in the framework of density functional theory and many-body perturbation theory. Crystallization substantially shrinks the HOMO-LUMO gap based on polarization effects. We find a shift of the HOMO to higher energy is consistent with recent experiment of MgTPP multilayer film on Ag (100) [A. Classen et al., Phys. Rev. B, 2017, 95, 115414]. Calculated excitation spectra demonstrate a significant redshift of excitation bands except for the Q bands. These lowest excitation bands, in stark contrast to the strong HOMO-LUMO gap renormalization, remain essentially the same as in the gas phase. Our work underlines the possibility of band-gap engineering via ligand-controlled modification of the polarizability.

Interfacial Reactions of Tetraphenylporphyrin with Cobalt-Oxide Thin Films

We have studied the adsorption and interfacial reactions of 2H-tetraphenylporphyrin (2HTPP) with cobalt-terminated Co₃ O₄ (111) and oxygen-terminated CoO(111) thin films using synchrotron-radiation X-ray photoelectron spectroscopy. Already at 275 K, we find evidence for the formation of a metalated species, most likely CoTPP, on both surfaces. The degree of self-metalation increases with temperature on both surfaces until 475 K, where the metalation is almost complete. At 575 K the porphyrin coverage decreases drastically on the reducible cobalt-terminated Co₃ O₄ (111) surface, while higher temperatures are needed on the non-reducible oxygen-terminated CoO(111). The low temperature self-metalation is similar to that observed on MgO(100) surfaces, but drastically different from that observed on TiO₂ (110), where no self-metalation is observed at room temperature.