Homolytic cleavage of molecular oxygen by manganese porphyrins supported on Ag(111)

Tuning Surface Organic Structures by Small Gas Molecules through Catassembly and Coassembly

The field of molecular assembly has seen remarkable advancements across various domains, such as materials science, nanotechnology, and biomedicine. Small gas molecules serve as pivotal modulators, capable of altering the architecture of assemblies via tuning a spectrum of intermolecular forces including hydrogen bonding, dipole-dipole interactions, and metal coordination. Surface techniques, notably scanning tunneling microscopy and atomic force microscopy, have proven instrumental in dissecting the structural metamorphosis and characteristic features of these assemblies at an unparalleled single-molecule resolution. Recent research has spotlighted two innovative approaches for modulating surface molecular assemblies with the aid of small gas molecules: "catassembly" and "coassembly". This Perspective delves into these methodologies through the lens of varying molecular interaction types. The strategies discussed here for regulating molecular assembly structures using small gas molecules can aid in understanding various complex assembly processes and structures and provide guidance for the further fabrication of complex surface structures.

Self-assembly of C60 on a ZnTPP/Fe(001)-p(1 × 1)O substrate: observation of a quasi-freestanding C60 monolayer

Fullerene (C₆₀) has been deposited in ultrahigh vacuum on top of a zinc tetraphenylporphyrin (ZnTPP) monolayer self-assembled on a Fe(001)-p(1 × 1)O substrate. The nanoscale morphology and the electronic properties of the C₆₀/ZnTPP/Fe(001)-p(1 × 1)O heterostructure have been investigated by scanning tunneling microscopy/spectroscopy and ultraviolet photoemission spectroscopy. C₆₀ nucleates compact and well-ordered hexagonal domains on top of the ZnTPP buffer layer, suggesting a high surface diffusivity of C₆₀ and a weak coupling between the overlayer and the substrate. Accordingly, work function measurements reveal a negligible charge transfer at the C₆₀/ZnTPP interface. Finally, the difference between the energy of the lowest unoccupied molecular orbital (LUMO) and that of the highest occupied molecular orbital (HOMO) measured on C₆₀ is about 3.75 eV, a value remarkably higher than those found in fullerene films stabilized directly on metal surfaces. Our results unveil a model system that could be useful in applications in which a quasi-freestanding monolayer of C₆₀ interfaced with a metallic electrode is required.

Role of the Supporting Surface in the Thermodynamics and Cooperativity of Axial Ligand Binding to Metalloporphyrins at Interfaces

Abstract:

: Metalloporphyrins have been shown to bind axial ligands in a variety of environments including the vacuum/solid and solution/solid interfaces. Understanding the dynamics of such interactions is a desideratum for the design and implementation of next generation molecular devices which draw inspiration from biological systems to accomplish diverse tasks such as molecular sensing, electron transport, and catalysis to name a few. In this article, we review the current literature of axial ligand coordination to surface-supported porphyrin receptors. We will focus on the coordination process as monitored by scanning tunneling microscopy (STM) that can yield qualitative and quantitative information on the dynamics and binding affinity at the single molecule level. In particular, we will address the role of the substrate and intermolecular interactions in influencing cooperative effects (positive or negative) in the binding affinity of adjacent molecules based on experimental evidence and theoretical calculations.

Stabilization and activation of molecular oxygen at biomimetic tetrapyrroles on surfaces: from UHV to near-ambient pressure

Recent advances in the development of surface science methods have allowed bridging, at least partially, the pressure gap between the ultra-high vacuum environment and some applicative conditions. This step has been particularly critical for the characterization of heterogenous catalytic systems (solid-liquid, solid-gas interfaces) and, specifically, of the electronic, structural, and chemical properties of tetrapyrroles at surfaces when arranged in 2D networks. Within a biomimetic picture, in which 2D metalorganic frameworks are expected to model and reproduce in a tailored way the activity of their biochemical proteic counterparts, the fundamental investigation of the adsorption and activation of small ligands at the single-metal atom reaction sites has progressively gained increasing attention. Concerning oxygen, biology offers a variety of tetrapyrrole-based transport and reaction pockets, as e.g. in haemoglobin, myoglobin or cytochrome proteins. Binding and activation of O2 are accomplished thanks to complex charge transfer and spin realignment processes, sometimes requiring cooperative mechanisms. Within the framework of surface science at near-ambient pressure (towards and beyond the mbar regime), recent progress has unveiled novel and interesting properties of 2D metalorganic frameworks and heterostacks based on self-assembled tetrapyrroles, thus opening possible, effective applicative routes in the fields of light harvesting, heterogenous (electro-)catalysts, chemical sensing, and spintronics.

Quantifying reversible nitrogenous ligand binding to Co(ii) porphyrin receptors at the solution/solid interface and in solution

We present a quantitative study comparing the binding of 4-methoxypyridine, MeOPy, ligand to Co(ii)octaethylporphyrin, CoOEP, at the phenyloctane/HOPG interface and in toluene solution. Scanning tunneling microscopy (STM) was used to study the ligand binding to the porphyrin receptors adsorbed on graphite. Electronic spectroscopy was employed for examining this process in fluid solution. The on surface coordination reaction was completely reversible and followed a simple Langmuir adsorption isotherm. Ligand affinities (or ΔG) for the binding processes in the two different chemical environments were determined from the respective equilibrium constants. The free energy value of -13.0 ± 0.3 kJ mol^-1 for the ligation reaction of MeOPy to CoOEP at the solution/HOPG interface is less negative than the ΔG for cobalt porphyrin complexed to the ligand in solution, -16.8 ± 0.2 kJ mol^-1. This result indicates that the MeOPy-CoOEP complex is more stable in solution than on the surface. Additional thermodynamic values for the formation of the surface ligated species (ΔH_c = -50 kJ mol^-1 and ΔS_c = -120 J mol^-1) were extracted from temperature dependent STM measurements. Density functional computational methods were also employed to explore the energetics of both the solution and surface reactions. At high concentrations of MeOPy the monolayer was observed to be stripped from the surface. Computational results indicate that this is not because of a reduction in adsorption energy of the MeOPy-CoOEP complex. Nearest neighbor analysis of the MeOPy-CoOEP in the STM images revealed positive cooperative ligand binding behavior. Our studies bring new insights to the general principles of affinity and cooperativity in the ligand-receptor interactions at the solution/solid interface. Future applications of STM will pave the way for new strategies designing highly functional multisite receptor systems for sensing, catalysis, and pharmacological applications.

Stabilisation of tri-valent ions with a vacant coordination site at a corrole-metal interface

By exploiting an established on-surface metallation strategy, we address the ability of the corrolic macrocycle to stabilise transition metal ions in high-valent (III) oxidation states in metal-supported molecular layers. This approach offers a route to engineer adsorbed metal complexes that cannot be easily fabricated by organic synthesis methods and bear a vacant axial coordination site for catalytic conversions.

Dioxygen at Biomimetic Single Metal-Atom Sites: Stabilization or Activation? The Case of CoTPyP/Au(111)

By means of a combined experimental and computational approach, we show that a 2D metal–organic framework self-assembled at the Au(111) termination is able to mimic the O2 stabilization and activation mechanisms that are typical of the biochemical environment of proteins and enzymes. 5,10,15,20-tetra(4-pyridyl)21H,23H-porphyrin cobalt(III) chloride (CoTPyP) molecules on Au(111) bind dioxygen forming a covalent bond at the Co center, yielding charge injection into the ligand by exploiting the surface trans-effect. A weakening of the O–O bond occurs, together with the development of a dipole moment, and a change in the molecule’s magnetic moment. Also the bonding geometry is similar to the biological counterpart, with the O2 molecule sitting on-top of the Co atom and the molecular axis tilted by 118°. The ligand configuration lays between the oxo- and the superoxo-species, in agreement with the observed O–O stretching frequency measured in situ at near-ambient pressure conditions.

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.

Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation

Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn–Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) [Formula: see text] is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.

Molecular-Scale Mechanistic Investigation of Oxygen Dissociation and Adsorption on Metal Surface-Supported Cobalt Phthalocyanine

Ultrahigh vacuum scanning tunneling microscopy and density functional theory are used to investigate adsorption of oxygen on cobalt phthalocyanine (CoPc), a promising nonprecious metal oxygen reduction catalyst, supported on Ag(111), Cu(111), and Au(111) surfaces at the molecular scale. Four distinct molecular and atomic oxygen adsorption configurations are observed for CoPc supported on Ag(111) surfaces, which are assigned as O₂/CoPc/Ag(111), O/CoPc/Ag(111), CoPc/(O)₂/Ag(111), and (O)₂/CoPc/Ag(111). In contrast, no oxygen adsorption is observed for CoPc supported on Cu(111) and Au(111) surfaces. The results show that for Ag(111), atomic O that is predominantly catalytically produced from the dissociation of molecular O₂ at metal surface step edges is responsible for the observed adsorption configurations. However, Cu(111) binds atomic O too strongly, and Au(111) does not produce atomic O. These results show the active role of the supporting metal surface in facilitating oxygen adsorption on CoPc.

Implementing Functionality in Molecular Self-Assembled Monolayers

The planar heterocyclic molecules 1,6,7,12-tetraazaperylene on a Ag(111) metal substrate show different charging characteristics depending on their local environment: next to vacancies in self-assembled islands, molecules can be charged by local electric fields, whereas their charge state is fixed otherwise. This enables the activation of selected molecules inside islands by vacancy creation from scanning-probe-based manipulation. This concept allows for combining the precise mutual atomic-scale alignment of molecules by self-assembly, on one hand, and the implementation of specific functionality into otherwise homogeneous monolayers, on the other. Activated molecules in the direct neighborhood influence each other in their charging characteristics, suggesting their use as molecular quantum cellular automata. Surprisingly, only very few interacting molecules exhibit a rich spectroscopic signature, which offers the prospect of implementing complex functionality in such structures in the future.

O2, NO2 and NH3 coordination to Co-porphyrin studied with scanning tunneling microscopy on Au(111)

The coordination structure between small molecules and metalloporphyrins plays a crucial role in functional reactions such as bio-oxidation and catalytic activation. Their vertical, tilting, and dynamic structures have been actively studied with diffraction and resonance spectroscopy for the past four decades. Contrastingly, real-space visualization beyond simple protrusion and depression is relatively rare. In this paper, high-resolution scanning tunnelling microscopy (STM) images are presented of di-, tri-, and tetra-atomic small molecules (O2, NO2, and NH3, respectively) coordinated to Co-porphyrin on Au(111). A square ring structure was observed for O2, a rectangular ring structure for NO2, and a bright-center structure for NH3 at 80 K. The symmetries of experimental STM images were reproduced in density functional theory (DFT) calculations, considering the precession motion of the small molecules. Thus, this study shows that the structure of small molecules coordinated to metalloporphyrins can be visualized using high-resolution STM and DFT calculations.

Local adsorption structure and bonding of porphine on Cu(111) before and after self-metalation

We have experimentally determined the lateral registry and geometric structure of free-base porphine (2H-P) and copper-metalated porphine (Cu-P) adsorbed on Cu(111), by means of energy-scanned photoelectron diffraction (PhD), and compared the experimental results to density functional theory (DFT) calculations that included van der Waals corrections within the Tkatchenko-Scheffler approach. Both 2H-P and Cu-P adsorb with their center above a surface bridge site. Consistency is obtained between the experimental and DFT-predicted structural models, with a characteristic change in the corrugation of the four N atoms of the molecule's macrocycle following metalation. Interestingly, comparison with previously published data for cobalt porphine adsorbed on the same surface evidences a distinct increase in the average height of the N atoms above the surface through the series 2H-P, Cu-P, and cobalt porphine. Such an increase strikingly anti-correlates the DFT-predicted adsorption strength, with 2H-P having the smallest adsorption height despite the weakest calculated adsorption energy. In addition, our findings suggest that for these macrocyclic compounds, substrate-to-molecule charge transfer and adsorption strength may not be univocally correlated.

Unraveling the Oxidation and Spin State of Mn-Corrole through X-ray Spectroscopy and Quantum Chemical Analysis

The interplay between Mn ions and corrole ligands gives rise to complex scenarios regarding the metal centers' electronic properties expressing a range of high oxidation states and spin configurations. The resulting potential of Mn-corroles for applications such as catalysts or fuel cells has recently been demonstrated. However, despite being crucial for their functionality, the electronic structure of Mn-corroles is often hardly accessible with traditional techniques and thus is still under debate, especially under interfacial conditions. Here, we unravel the electronic ground state of the prototypical Mn-5,10,15-tris(pentafluorophenyl)corrole complex through X-ray spectroscopic investigations of ultrapure thin films and quantum chemical analysis. The theory-based interpretation of Mn photoemission and absorption fine structure spectra (3s and 2p and L_2,3-edge, respectively) evidence a Mn(III) oxidation state with an S = 2 high-spin configuration. By referencing density functional theory calculations with the experiments, we lay the basis for extending our approach to the characterization of complex interfaces.

Practicably efficient ethylbenzene oxidation catalyzed by manganese tetrakis(4-sulfonatophenyl)porphyrin grafted to powdered chitosan

Manganese tetrakis(4-sulfonatophenyl)porphyrin chloride was grafted onto powdered chitosan via an acid–base reaction and ligation. The grafted catalyst was characterized by transmission electron microscopy, ultraviolet and visible spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectrometry and thermogravimetry. Ethylbenzene oxidation with O[Formula: see text] by the catalyst in the absence of additives and solvents can achieve moderate yields (approximately 30%) of acetophenone and phenethyl alcohol. The grafted catalyst can be reused four times for oxidation reactions. The results indicate that the catalytic activity of manganese tetrakis(4-sulfonatophenyl)porphyrin chloride is promoted by the ligation and grafting function of the amino groups in the powdered chitosan.

Redox-active ligand controlled selectivity of vanadium oxidation on Au(100)

Metal-organic coordination networks at surfaces, formed by on-surface redox assembly, are of interest for designing specific and selective chemical function at surfaces for heterogeneous catalysts and other applications. The chemical reactivity of single-site transition metals in on-surface coordination networks, which is essential to these applications, has not previously been fully characterized. Here, we demonstrate with a surface-supported, single-site V system that not only are these sites active toward dioxygen activation, but the products of that reaction show much higher selectivity than traditional vanadium nanoparticles, leading to only one V-oxo product. We have studied the chemical reactivity of one-dimensional metal-organic vanadium - 3,6-di(2-pyridyl)-1,2,4,5-tetrazine (DPTZ) chains with O2. The electron-rich chains self-assemble through an on-surface redox process on the Au(100) surface and are characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, and density functional theory. Reaction of V-DPTZ chains with O2 causes an increase in V oxidation state from VII to VIV, resulting in a single strongly bonded (DPTZ2-)VIVO product and spillover of O to the Au surface. DFT calculations confirm these products and also suggest new candidate intermediate states, providing mechanistic insight into this on-surface reaction. In contrast, the oxidation of ligand-free V is less complete and results in multiple oxygen-bound products. This demonstrates the high chemical selectivity of single-site metal centers in metal-ligand complexes at surfaces compared to metal nanoislands.

Substrate involvement in dioxygen bond dissociation catalysed by iron phthalocyanine supported on Ag(100)

The first evidence is provided of the role played by the metal support in the oxygen reduction reaction catalysed by Ag(100)-adsorbed iron phthalocyanine molecules.

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.

Direct quantitative identification of the "surface trans-effect"

The strong parallels between coordination chemistry and adsorption on metal surfaces, with molecules and ligands forming local bonds to individual atoms within a metal surface, have been established over many years of study. The recently proposed "surface trans-effect" (STE) appears to be a further manifestation of this analogous behaviour, but so far the true nature of the modified molecule-metal surface bonding has been unclear. The STE could play an important role in determining the reactivities of surface-supported metal-organic complexes, influencing the design of systems for future applications. However, the current understanding of this effect is incomplete and lacks reliable structural parameters with which to benchmark theoretical calculations. Using X-ray standing waves, we demonstrate that ligation of ammonia and water to iron phthalocyanine (FePc) on Ag(111) increases the adsorption height of the central Fe atom; dispersion corrected density functional theory calculations accurately model this structural effect. The calculated charge redistribution in the FePc/H2O electronic structure induced by adsorption shows an accumulation of charge along the σ-bonding direction between the surface, the Fe atom and the water molecule, similar to the redistribution caused by ammonia. This apparent σ-donor nature of the observed STE on Ag(111) is shown to involve bonding to the delocalised metal surface electrons rather than local bonding to one or more surface atoms, thus indicating that this is a true surface trans-effect.

Hydrogen bond-directed encapsulation of metalloporphyrin into the microcages of zeolite imidazolate frameworks for synergistic biomimetic catalysis

In efforts to replicate the 3D model and desirable function of haemoglobin, the zeolite imidazolate framework (ZIF-8) was delineated for an ideal host matrix to accommodate custom-designed porphyrin molecules via hydrogen bonding.

Reversible Tuning of Interfacial and Intramolecular Charge Transfer in Individual MnPc Molecules

The reversible selective hydrogenation and dehydrogenation of individual manganese phthalocyanine (MnPc) molecules has been investigated using photoelectron spectroscopy (PES), low-temperature scanning tunneling microscopy (LT-STM), synchrotron-based near edge X-ray absorption fine structure (NEXAFS) measurements, and supported by density functional theory (DFT) calculations. It is shown conclusively that interfacial and intramolecular charge transfer arises during the hydrogenation process. The electronic energetics upon hydrogenation is identified, enabling a greater understanding of interfacial and intramolecular charge transportation in the field of single-molecule electronics.

Hydrogen capture by porphyrins at the TiO2(110) surface

Metal-free porphyrin molecules adsorb on the rutile TiO2(110) surface with their pyrrolic nitrogen atoms atop the O-bridge rows, whereas the iminic nitrogen atoms capture two additional hydrogen atoms. Hydrogenation occurs spontaneously at room temperature, irrespective of the distance of the polypyrrolic macrocycle from the surface, as varied by changing the porphyrin functionalization.

Immobilised molecular catalysts and the role of the supporting metal substrate

This work demonstrates that immobilising molecular catalysts on metal substrates can attenuate their reactivity. In particular, the reactivity towards molecular oxygen of both ruthenium tetraphenyl porphyrin (Ru-TPP) and its Ti analogue (Ti-TPP) on Ag(111) was studied as benchmark for the interaction strength of such metal-organic complexes with possible reactants. Here, Ru-TPP proves to be completely unreactive and Ti-TPP strongly reactive towards molecular oxygen; along with comparison to work in the literature, this suggests that studies into immobilised catalysts might find fruition in considering species traditionally seen as too strongly interacting.

Mimicking enzymatic active sites on surfaces for energy conversion chemistry

Metal-organic supramolecular chemistry on surfaces has matured to a point where its underlying growth mechanisms are well understood and structures of defined coordination environments of metal atoms can be synthesized in a controlled and reproducible procedure. With surface-confined molecular self-assembly, scientists have a tool box at hand which can be used to prepare structures with desired properties, as for example a defined oxidation number and spin state of the transition metal atoms within the organic matrix. From a structural point of view, these coordination sites in the supramolecular structure resemble the catalytically active sites of metallo-enzymes, both characterized by metal centers coordinated to organic ligands. Several chemical reactions take place at these embedded metal ions in enzymes and the question arises whether these reactions also take place using metal-organic networks as catalysts. Mimicking the active site of metal atoms and organic ligands of enzymes in artificial systems is the key to understanding the selectivity and efficiency of enzymatic reactions. Their catalytic activity depends on various parameters including the charge and spin configuration in the metal ion, but also on the organic environment, which can stabilize intermediate reaction products, inhibits catalytic deactivation, and serves mostly as a transport channel for the reactants and products and therefore ensures the selectivity of the enzyme. Charge and spin on the transition metal in enzymes depend on the one hand on the specific metal element, and on the other hand on its organic coordination environment. These two parameters can carefully be adjusted in surface confined metal-organic networks, which can be synthesized by virtue of combinatorial mixing of building synthons. Different organic ligands with varying functional groups can be combined with several transition metals and spontaneously assemble into ordered networks. The catalytically active metal centers are adequately separated by the linking molecules and constitute promising candiates for heterogeneous catalysts. Recent advances in synthesis, characterization, and catalytic performance of metal-organic networks are highlighted in this Account. Experimental results like structure determination of the networks, charge and spin distribution in the metal centers, and catalytic mechanisms for electrochemical reactions are presented. In particular, we describe the activity of two networks for the oxygen reduction reaction in a combined scanning tunneling microscopy and electrochemical study. The similarities and differences of the networks compared to metallo-enzymes will be discussed, such as the metal surface that operates as a geometric template and concomitantly functions as an electron reservoir, and how this leads to a new class of bioinspired catalysts. The possibility to create functional two-dimensional coordination complexes at surfaces taking inspiration from nature opens up a new route for the design of potent nanocatalyst materials for energy conversion.