Surface-Mediatedin SituMetalation of Porphyrins at the Solid–Vacuum Interface

Abstract

The investigation of porphyrin derivatives at the solid-vacuum interface has become a vivid research field with the prospect to tailor functional molecular architectures and as prototype examples to study the fundamental properties of porphyrin derivatives in regard to their vital role in many natural processes. The functional properties of the porphyrin derivatives are mainly determined by the central metal atom. Thus, the recent exploration of the surface-confined in situ metalation of porphyrins is an important step toward the realization of molecule-based functional devices. The corresponding metalation reaction of free base porphyrin derivatives can be conveniently realized in situ in ultrahigh vacuum by post- or predeposition of metal atoms or directly with substrate atoms in the so-called self-metalation. Moderate heating above room temperature (RT) might be necessary either to realize the transport of the metal to the porphyrin via diffusion or to overcome an activation barrier determined by the redox reaction itself. Surface science techniques like scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) are powerful tools to scrutinize the reaction and give valuable insights into the metalation process. For example, the completed metalation can be reflected in an enhanced apparent height of the corresponding porphyrin in STM or can be evidenced by characteristic changes in the N 1s region in XPS. These signatures allow monitoring of the progress of the metalation, and it was found that the reaction generally proceeds with very high yield. Surface diffusion of the coadsorbed metal atoms mediates the reaction and is crucial for the high yields of the corresponding reactions with pre- and postadsorbed metals. It was also demonstrated that the completed metalation can indeed significantly alter the adsorption behavior and the electronic properties and thus the functionality of the porphyrin molecules. These alterations can be used to monitor the kinetics of a particular porphyrin self-metalation reaction by STM and to estimate the activation barrier for that reaction based on isothermal measurements at different temperatures. Also TPD measurements of the H2 and D2 signals allow for the determination of corresponding activation energies for the metalation of free base porphyrins and their deuterized analogues. Gas phase DFT calculations of the metalation of the "bare" free base porphyrin macrocycle identify intermediate reaction steps with the transfer of the first hydrogen atom to the metal center being the main barrier to overcome. The values from these calculations are in fair agreement with experimentally determined ones. However, TPD based results indicate that exchanges of deuterium and hydrogen between the central nitrogen and the surface occur, which indicate an active role of the surface and challenge the findings from gas phase DFT. The in situ metalation of porphyrins at the solid-vacuum interface is established as a novel and conveni...

On‐Surface Metalation and 2D Self‐Assembly of Pyrphyrin Molecules Into Metal‐Coordinated Networks on Cu(111)

“…However, the TD spectra resolve both 2HPyr and CuPyr desorbing at around 550 K from the multilayer as intact molecules. Our results are consistent with the literature, where metalation in the multilayer has also been observed for tetrapyrroles …”

“…Surface functionalization with macrocycles, such as porphyrin or phthalocyanine, presents a fascinating platform for studying physical and chemical phenomena at interfaces and provides the basis for many applications. These predominantly planar macrocycles can coordinate a metal ion, rendering them interesting model systems to study coordination chemistry at surfaces . In addition, the central metal ions can coordinate an axial ligand at the site opposite to the surface .…”

“…metalation) in‐situ directly at the surface under ultrahigh vacuum (UHV) conditions. The metal atoms can be provided either by the surface (sometimes termed as ‘self‐metalation’) or supplied by co‐deposition through evaporation …”

Porphyrin Metalation at MgO Surfaces: A Spectroscopic and Quantum Mechanical Study on Complementary Model Systems

“…[17] The inability of silver to metalate porphyrins should also prevents ilver atoms, which might be diffusing through the MgO thin film, from reactingw ith the adsorbed porphyrins. [17] The inability of silver to metalate porphyrins should also prevents ilver atoms, which might be diffusing through the MgO thin film, from reactingw ith the adsorbed porphyrins.…”

“…[1], [15]- [17] and references therein),a nd by temperature-programmed desorption. In case of the free-base porphyrin, the two protons transfer between the two pairs of opposing nitrogen atoms at the molecule'sc enter.O nm etals, metalation reactions have been studied extensively, down to al evel of atomic resolution, by using scanning probe microscopiesa nd X-ray photoelectrons pectroscopy( see refs.…”