Based on density functional theory calculations combined with experimental results, we report and discuss an extremely distorted, "inverted" adsorption geometry of free-base tetraphenylporphyrin on Cu(111). The current findings yield new insights into a well-studied system, shedding light on the peculiar molecule-substrate interaction and the resulting intramolecular conformation.

We have studied the adsorption and self‐metalation of tetraphenylporphyrin (2HTPP) on TiO2(110) ‐ 1 × 1 with X‐ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). Upon adsorption at room temperature the first monolayer of 2HTPP is protonated by protons from the surface, forming porphyrin diacid (4HTPP2+). In STM, single molecules with low mobility and no long‐range order were observed. As the surface is heated to 400 K, any unprotonated 2HTPP molecules in the second layer metalate, forming titanyl tetraphenylporphyrin (TiOTPP), but the protonated molecules in the first layer remain. As the surface is heated further to 550 K even the protonated 4HTPP2+ molecules metalate and only TiOTPP remain on the surface.

The reaction rate of the self-metalation of free-base tetraphenylporphyrins (TPPs) on Cu(111) increases with the number of cyano groups (n=0, 1, 2, 4) attached at the para positions of the phenyl rings. The findings are based on isothermal scanning tunneling microscopy (STM) measurements. At room temperature, all investigated free-base TPP derivatives adsorb as individual molecules and are aligned with respect to densely packed Cu substrate rows. Annealing at 400 K leads to the formation of linear dimers and/or multimers via CN-Cu-CN bonds, accompanied by self-metalation of the free-base porphyrins following a first-order rate equation. When comparing the non-cyano-functionalized and the tetracyano-functionalized molecules, we find a decrease of the reaction rate by a factor of more than 20, corresponding to an increase of the activation energy from 1.48 to 1.59 eV. Density functional theory (DFT) calculations give insights into the influence of the peripheral electron-withdrawing cyano groups and explain the experimentally observed effects.

Die Reaktionsgeschwindigkeit der Selbstmetallierung von Freie‐Base‐Tetraphenylporphyrinen (TPPs) auf Cu(111) nimmt mit zunehmender Zahl der Cyangruppen in para‐Position der Phenylringe (n=0, 1, 2, 4) systematisch ab. Die Ergebnisse basieren auf Messungen mit isothermer Rastertunnelmikroskopie. Bei Raumtemperatur adsorbieren alle untersuchten Freie‐Base‐TPP‐Derivate als individuelle Moleküle mit klar definierter Orientierung entlang den dichtest gepackten Cu‐Substratreihen. Durch Heizen auf 400 K bilden sich durch intermolekulare CN‐Cu‐CN‐Bindungen lineare Dimere und/oder Multimere. Parallel dazu kann die Selbstmetallierung der Freie‐Base‐Porphyrine nach einer Reaktion 1. Ordnung stattfinden. Für Tetracyan‐funktionalisierte Moleküle nimmt die Reaktionsgeschwindigkeit um einen Faktor von >20 gegenüber der von nicht‐funktionalisierten Molekülen ab. Dies entspricht einer Zunahme der Aktivierungsenergie von 1.48 auf 1.59 eV. DFT‐Rechnungen geben einen tieferen Einblick in die Rolle der elektronenziehenden Cyangruppen.

Porphyrins are large organic molecules that are interesting for different applications, such as photovoltaic cells, gas sensors, or in catalysis. For many of these applications, the interactions between adsorbed molecules and surfaces play a crucial role. Studies of porphyrins on surfaces typically fall into one of two groups: (1) evaporation onto well-defined single-crystal surfaces under well-controlled ultrahigh vacuum conditions or (2) more application-oriented wet chemical deposition onto less well-defined high surface area surfaces under ambient conditions. In this study, we will investigate the wet chemical deposition of 5-(monocarboxyphenyl)-10,15,20-triphenylporphyrin (MCTPP) on well-defined rutile TiO2(110) single crystals under ambient conditions. Prior to deposition, the TiO2(110) crystals were also cleaned wet-chemically under ambient conditions, meaning none of the preparation steps were done in ultrahigh vacuum. However, after each preparation step, the surfaces were characterized in ultrahigh vacuum with X-ray photoelectron spectroscopy (XPS) and the result was compared with porphyrin layers prepared in ultrahigh vacuum (UHV) by evaporation. The differences of both preparations when exposed to zinc ion solutions will also be discussed.