Stabilization versus competing de-metalation, trans-metalation and (cyclo)-dehydrogenation of Pd porphyrins at a copper surface

Metal-porphyrins are studied intensively due their potential applications, deriving from the variety of electronic and chemical properties, tunable by selecting metal centers and functional groups. Metalation, de- and trans-metalation processes are fundamental in this sense to investigate both the synthesis and the stability of these molecular building blocks. More specifically, Pd coordination in tetrapyrroles revealed to be potentially interesting in the fields of cancer therapy, drug delivery and light harvesting. Thus, we focused on the stability of palladium tetraphenyl porphyrins (PdTPPs) on a copper surface by means of combined spectroscopy and microscopy approaches. We find that PdTPPs undergo coverage-dependent trans-metalation accompanied by steric rearrangements already at room temperature, and fully trans-metalate to CuTPPs upon mild annealing. Side reactions such as (cyclo)-dehydrogenation and structural reorganization affect the molecular layer, with Pd-Cu alloying and segregation occurring at higher temperature. Instead, oxygen passivation of the Cu support prevents the metal-involving reactions, thus preserving the layer and increasing the chemical and temperature stability of the Pd porphyrins.

Ab Initio Electronic, Magnetic, and Optical Properties of Fe Phthalocyanine on Cr2O3(0001)

The organic molecules adsorbed on antiferromagnetic surfaces can produce interesting interface states, characterized by charge transfer mechanisms, hybridization of molecular-substrate orbitals, as well as magnetic couplings. Here, we apply an ab initio approach to study the adsorption of Fe phthalocyanine on stoichiometric Cr2O3(0001). The molecule binds via a bidentate configuration forming bonds between two opposite imide N atoms and two protruding Cr ones, making this preferred over the various possible adsorption structures. In addition to the local modifications at these sites, the electronic structure of the molecule is weakly influenced. The magnetic structure of the surface Cr atoms shows a moderate influence of molecule adsorption, not limited to the atoms in the close proximity of the molecule. Upon optical excitation at the onset, electron density moves toward the molecule, enhancing the ground state charge transfer. We investigate this movement of charge as a mechanism at the base of light-induced modifications of the magnetic structure at the interface.

The spatial distribution of cobalt phthalocyanine and copper nanocubes controls the selectivity towards C2 products in tandem electrocatalytic CO2 reduction

The coupling of CO-generating molecular catalysts with copper electrodes in tandem schemes is a promising strategy to boost the formation of multi-carbon products in the electrocatalytic reduction of CO₂. While the spatial distribution of the two components is important, this aspect remains underexplored for molecular-based tandem systems. Herein, we address this knowledge gap by studying tandem catalysts comprising Co-phthalocyanine (CoPc) and Cu nanocubes (Cu_cub). In particular, we identify the importance of the relative spatial distribution of the two components on the performance of the tandem catalyst by preparing CoPc-Cu_cub/C, wherein the CoPc and Cu_cub share an interface, and CoPc-C/Cu_cub, wherein the CoPc is loaded first on carbon black (C) before mixing with the Cu_cub. The electrocatalytic measurements of these two catalysts show that the faradaic efficiency towards C₂ products almost doubles for the CoPc-Cu_cub/C, whereas it decreases by half for the CoPc-C/Cu_cub, compared to the Cu_cub/C. Our results highlight the importance of a direct contact between the CO-generating molecular catalyst and the Cu to promote C-C coupling, which hints at a surface transport mechanism of the CO intermediate between the two components of the tandem catalyst instead of a transfer via CO diffusion in the electrolyte followed by re-adsorption.

On surface chemical reactions of free-base and titanyl porphyrins with r-TiO2(110): a unified picture

In this Perspective we present a comprehensive study of the multiple reaction products of metal-free porphyrins (2H-Ps) in contact with the rutile TiO₂(110) surface. In the absence of peripheral functionalization with specific linkers, the porphyrin adsorption is driven by the coordination of the two pyrrolic nitrogen atoms of the macrocycle to two consecutive oxygen atoms of the protruding O_br rows via hydrogen bonding. This chemical interaction favours the iminic nitrogen uptake of hydrogen from near surface layers at room temperature, thus yielding a stable acidic porphyrin (4H-P). In addition, a mild annealing (∼100 °C) triggers the incorporation of a Ti atom in the porphyrin macrocycle (self-metalation). We recently demonstrated that such a low temperature reaction is driven by a Lewis base iminic attack, which lowers the energy barriers for the outdiffusion of Ti interstitial atoms (Ti_int) [Kremer et al., Appl. Surf. Sci., 2021, 564, 150403]. In the monolayer (ML) range, the porphyrin adsorption site, corresponding to a TiO-TPP configuration, is extremely stable and tetraphenyl-porphyrins (TPPs) may even undergo conformational distortion (flattening) by partial cyclo-dehydrogenation, while remaining anchored to the O rows up to 450 °C [Lovat et al., Nanoscale, 2017, 9, 11694]. Here we show that, upon self-metalation, isolated molecules at low coverage may jump atop the rows of five-fold coordinated Ti atoms (Ti_5f). This configuration is associated with the formation of a new coordination complex, Ti-O-Ti_5f, as determined by comparison with the deposition of pristine titanyl-porphyrin (TiO-TPP) molecules. The newly established Ti-O-Ti_5f anchoring configuration is found to be stable also beyond the TPP flattening reaction. The anchoring of TiO-TPP to the Ti_5f rows is, however, susceptible to the cross-talk between phenyls of adjacent molecules, which ultimately drives the TiO-TPP temperature evolution in the ML range along the same pathway followed by 2H-TPP.

Comparing Cyanophenyl and Pyridyl Ligands in the Formation of Porphyrin-Based Metal-Organic Coordination Networks

In recent studies, porphyrin derivatives have been frequently used as building blocks for the fabrication of metal-organic coordination networks (MOCNs) on metal surfaces under ultrahigh vacuum conditions (UHV). The porphyrin core can host a variety of 3d transition metals, which are usually incorporated in solution. However, the replacement of a pre-existing metal atom in the porphyrin core by a different metallic species has been rarely reported under UHV. Herein, we studied the influence of cyanophenyl and pyridyl functional endgroups in the self-assembly of structurally different porphyrin-based MOCNs by the deposition of Fe atoms on tetracyanophenyl (Co-TCNPP) and tetrapyridyl-functionalized (Zn-TPPyP) porphyrins on Au(111) by means of scanning tunneling microscopy (STM). A comparative analysis of the influence of the cyano and pyridyl endgroups on the formation of different in-plane coordination motifs is performed. Each porphyrin derivative formed two structurally different Fe-coordinated MOCNs stabilized by three- and fourfold in-plane coordination nodes, respectively. Interestingly, the codeposited Fe atoms did not only bind to the functional endgroups but also reacted with the porphyrin core of the Zn-substituted porphyrin (Zn-TPyP), i.e., an atom exchange reaction took place in the porphyrin core where the codeposited Fe atoms replaced the Zn atoms. This was evidenced by the appearance of molecules with an enhanced (centered) STM contrast compared with the appearance of Zn-TPyP, which suggested the formation of a new molecular species, i.e., Fe-TPPyP. Furthermore, the porphyrin core of the Co-substituted porphyrin (Co-TCNPP) displayed an off-centered STM contrast after the deposition of Fe atoms, which was attributed to the binding of the Fe atoms on the top site of the Co-substituted porphyrin core. In summary, the deposition of metal atoms onto organic layers can steer the formation of structurally different MOCNs and may replace pre-existing metal atoms contained in the porphyrin core.

Structural Transformation of Surface-Confined Porphyrin Networks by Addition of Co Atoms

The self-assembly of a nickel-porphyrin derivative (Ni-DPPyP) containing two pyridyl coordinating sites and two pentyl chains at trans meso positions was studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) on Au(111). Deposition of Ni-DPPyP onto Au(111) gave rise to a close-packed network for coverages smaller or equal to one monolayer as revealed by STM and LEED. The molecular arrangement of this two-dimensional network is stabilized via hydrogen bonds formed between the pyridyl's nitrogen and hydrogen atoms from the pyrrole groups of neighboring molecules. Subsequent deposition of cobalt atoms onto the close-packed network and post-deposition annealing at 423 K led to the formation of a Co-coordinated hexagonal porous network. As confirmed by XPS measurements, the porous network is stabilized by metal-ligand interactions between one cobalt atom and three pyridyl ligands, each pyridyl ligand coming from a different Ni-DPPyP molecule.

On-surface porphyrin transmetalation with Pb/Cu redox exchange

Metal complexes at surfaces and interfaces play an important role in many areas of modern technology, including catalysis, sensors, and organic electronics. An important aspect of these interfaces is the possible exchange of the metal center, because this reaction can drastically alter the properties of the metal complex and thus of the interface. Here, we demonstrate that such metal exchange reactions are indeed possible and can proceed already at moderate temperatures even in the absence of solvents. Specifically, we studied the redox transmetalation of a monolayer of lead(ii)-tetraphenylporphyrin (PbTPP) with copper from a Cu(111) surface under ultrahigh-vacuum (UHV) conditions using multiple surface-sensitive techniques. Temperature-dependent X-ray photoelectron spectroscopy (XPS) reveals that the Pb/Cu exchange starts already below 380 K and is complete at 600 K. The identity of the reaction product, CuTPP, is confirmed by mass spectrometric detection in a temperature-programmed reaction (TPR) experiment. Scanning tunneling microscopy (STM) sheds light on the adsorbate structure of PbTPP at 300 K and uncovers the structural changes accompanying the transmetalation and side-reactions of the phenyl substituents. Moreover, individual free Pb atoms are observed as a product of the metal exchange.

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.

Exploring the Adsorption Mechanism of Tetracene on Ag(110) by STM and Dispersion-Corrected DFT

Self-assembled strategy has been proven to be a promising vista in constructing organized low-dimensional nanostructures with molecular precision and versatile functionalities on solid surfaces. Herein, we investigate by a combination of scanning tunneling microscopy (STM) and dispersion-corrected density functional theory (DFT), the adsorption of tetracene molecules on the silver substrate and the mechanism mediating the self-assembly on Ag(110). As expected, ordered domain is formed on Ag(110) after adsorption with adjacent molecules being imaged with alternating bright or dim pattern regularly. While such behavior has been assigned previously to the difference of molecular adsorption height, herein, it is possible to investigate essentially the mechanism leading to the periodic alternation of brightness and dimness for tetracene adsorbed on Ag(110) thanks to the consideration of Van der Waals (vdW) dispersion force. It is demonstrated that the adsorption height in fact is same for both bright and dim molecules, while the adsorption site and the corresponding interfacial charge transfer play an important role in the formation of such pattern. Our report reveals that vdW dispersion interaction is crucial to appropriately describe the adsorption of tetracene on the silver substrate, and the formation of delicate molecular architectures on metal surfaces might also offers a promising approach towards molecular electronics.

Recent Progress with In Situ Characterization of Interfacial Structures under a Solid-Gas Atmosphere by HP-STM and AP-XPS

: Surface science is an interdisciplinary field involving various subjects such as physics, chemistry, materials, biology and so on, and it plays an increasingly momentous role in both fundamental research and industrial applications. Despite the encouraging progress in characterizing surface/interface nanostructures with atomic and orbital precision under ultra-high-vacuum (UHV) conditions, investigating in situ reactions/processes occurring at the surface/interface under operando conditions becomes a crucial challenge in the field of surface catalysis and surface electrochemistry. Promoted by such pressing demands, high-pressure scanning tunneling microscopy (HP-STM) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS), for example, have been designed to conduct measurements under operando conditions on the basis of conventional scanning tunneling microscopy (STM) and photoemission spectroscopy, which are proving to become powerful techniques to study various heterogeneous catalytic reactions on the surface. This report reviews the development of HP-STM and AP-XPS facilities and the application of HP-STM and AP-XPS on fine investigations of heterogeneous catalytic reactions via evolutions of both surface morphology and electronic structures, including dehydrogenation, CO oxidation on metal-based substrates, and so on. In the end, a perspective is also given regarding the combination of in situ X-ray photoelectron spectroscopy (XPS) and STM towards the identification of the structure-performance relationship.

Revealing the Adsorption and Decomposition of EP-PTCDI on a Cerium Oxide Surface

Cerium oxide has constantly attracted intense attention during the past decade both in research and industry as an appealing catalyst or a noninert support for catalysts, for instance, in the water-gas shift reaction and hydrogenation of the ketone group. Herein, the cerium oxide surface has been chosen to investigate the adsorption and decomposition behaviors of the N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxdiimide (EP-PTCDI) molecule by photoelectron spectroscopy. As expected, EP-PTCDI molecules self-assemble on the cerium oxide surface comprising both trivalent and tetravalent cerium at room temperature. Interestingly, the EP-PTCDI molecule exhibits selective adsorption on cerium oxide after the heating treatment. It was found that the ketone group of EP-PTCDI first undergoes hydrogenation after annealing to 400 °C, which is probably related to the fact that high temperature annealing provides sufficient thermal energy to trigger the reaction between the ketone group and trivalent cerium. Furthermore, EP-PTCDI molecules are discovered to start to decompose hierarchically on the ceria substrate from annealing at 400 °C due to the strong molecule-substrate interaction and the effective catalysis by the trivalent cerium, whereas the decomposition sequence of functional groups is revealed to be, first, the ethyl propyl group (-C₅H₉), followed by the hydrogenated ketone (alcohols) group. Finally, our study may provide a new platform for the fundamental understanding of complex organic reactions on the cerium oxide surface.

Stability of metallo-porphyrin networks under oxygen reduction and evolution conditions in alkaline media

Transition metal atoms stabilised by organic ligands or as oxides exhibit promising catalytic activity for the electrocatalytic reduction and evolution of oxygen. Built-up from earth-abundant elements, they offer affordable alternatives to precious-metal based catalysts for application in fuel cells and electrolysers. For the understanding of a catalyst's activity, insight into its structure on the atomic scale is of highest importance, yet commonly challenging to experimentally access. Here, the structural integrity of a bimetallic iron tetrapyridylporphyrin with co-adsorbed cobalt electrocatalyst on Au(111) is investigated using scanning tunneling microscopy and X-ray absorption spectroscopy. Topographic and spectroscopic characterization reveals structural changes of the molecular coordination network after oxygen reduction, and its decomposition and transformation into catalytically active Co/Fe (oxyhydr)oxide during oxygen evolution. The data establishes a structure-property relationship for the catalyst as a function of electrochemical potential and, in addition, highlights how the reaction direction of electrochemical interconversion between molecular oxygen and hydroxyl anions can have very different effects on the catalyst's structure.

On-surface transmetalation of metalloporphyrins

Increasing the complexity of 2D metal-organic networks has led to the fabrication of structures with interesting magnetic and catalytic properties. However, increasing complexity by providing different coordination environments for different metal types imposes limitations on their synthesis if the controlled placement of one metal type into one coordination environment is desired. Whereas metal insertion into free-base porphyrins at the vacuum/solid interface has been thoroughly studied, providing detailed insight into the mechanisms at play, the chemical interaction of a metal atom with a metallated porphyrin is rarely investigated. Herein, the breadth of metalation reactions is augmented towards the metal exchange of a metalloporphyrin through the deliberate addition of atomic metal centers. The cation of Fe(ii)-tetraphenylporphyrins can be replaced by Co in a redox transmetalation-like reaction on a Au(111) surface. Likewise, Cu can be replaced by Co. The reverse reaction does not occur, i.e. Fe does not replace Co in the porphyrin. This non-reversible exchange is investigated in detail by X-ray absorption spectroscopy complemented by scanning tunneling microscopy. Density functional theory illuminates possible reaction pathways and leads to the conclusion that the transmetalation proceeds through the adsorption of initially metallic (neutral) Co onto the porphyrin and the expulsion of Fe towards the surface accompanied by Co insertion. Our findings have important implications for the fabrication of porphyrin layers on surfaces when subject to the additional deposition of metals. Mixed-metal porphyrin layers can be fabricated by design in a solvent-free process, but conversely care must be taken that the transmetalation does not proceed as an undesired side reaction.

van der Waals Interaction Activated Strong Electronic Coupling at the Interface between Chloro Boron-Subphthalocyanine and Cu(111)

In this article, we investigate the interface between shuttlecock-shaped chloro boron-subphthalocyanine molecules and the Cu(111) surface. We highlight how molecular planarization induced by van der Waals forces can fundamentally alter the interface properties and how it can enable a particularly strong hybridization between molecular and metal states. In our simulations, we start from a situation in which we disregard van der Waals forces and then introduce them gradually by rescaling the interaction parameter, thereby "pulling" the molecule toward the surface. This reveals two adsorption regimes with significantly different adsorption distances, molecular conformations, and adsorbate-induced changes of the work function. Notably, the above-mentioned massive hybridization of electronic states, also observed in photoelectron spectroscopy, is obtained solely for one of the regimes. We show that this regime is accessible only as a consequence of the planarization of the molecular backbone resulting from the van der Waals attraction between the molecule and the surface. The results of this study indicate that for certain metal-molecule combinations unusually strong interfacial electronic interactions can be triggered by van der Waals forces creating a situation that differs from the usually described cases of physisorptive and chemisorptive interactions.

Ranking the Stability of Transition-Metal Complexes by On-Surface Atom Exchange

Surface-adsorbed macrocycles exhibit a number of interesting physical and chemical properties; many of them are determined by their transition-metal centers. The hierarchical exchange of the central metal atom in such surface-adsorbed complexes is demonstrated, specifically in the porphyrin-like macrocycle pyrphyrin adsorbed on Cu(111). Using scanning tunneling microscopy and X-ray photoelectron spectroscopy, we show that Cu as central metal atom is easily exchanged with Ni or Fe atoms supplied in trace amounts to the surface. Atom exchange of Ni centers with Fe atoms also occurs, with moderate yield. These results allow ranking the stability of the surface-adsorbed Cu, Ni, and Fe complexes. The fact that the atom exchange occurs at 423 K shows that surface-adsorbed macrocycles can be surprisingly easily transformed.

Unveiling orbital coupling at the CoPc/Bi(111) surface by ab initio calculations and photoemission spectroscopy

Orbital coupling is revealed at the CoPc/Bi(111) interface with the local magnetic moment retained in CoPc.