On-surface nickel porphyrin mimics the reactive center of an enzyme cofactor

Abrupt Change from Ionic to Covalent Bonding in Nickel Halides Accompanied by Ligand Field Inversion

The electronic configuration of transition metal centers and their ligands is crucial for redox reactions in metal catalysis and electrochemistry. We characterize the electronic structure of gas-phase nickel monohalide cations via nickel L2,3-edge X-ray absorption spectroscopy. Comparison with multiplet charge-transfer simulations and experimental spectra of selectively prepared nickel monocations in both ground- and excited-state configurations are used to facilitate our analysis. Only for [NiF]+ with an assigned ground state of 3Π can the bonding be described as predominantly ionic, while the heavier halides with assigned ground states of 3Π or 3Δ exhibit a predominantly covalent contribution. The increase in covalency is accompanied by a transition from a classical ligand field for [NiF]+ to an inverted ligand field for [NiCl]+, [NiBr]+, and [NiI]+, resulting in a leading 3d9 L̲ configuration with a ligand hole (L̲) and a 3d occupation indicative of nickel(I) compounds. Hence, the absence of a ligand hole in [NiF]+ precludes any ligand-based redox reactions. Additionally, we demonstrate that the shift in energy of the L3 resonance is reduced compared to that of isolated atoms upon the formation of covalent compounds.

A local point of view of the Cu(100) → NiTPP charge transfer at the NiTPP/Cu(100) interface

A precise understanding, at the molecular level, of the massive substrate → adsorbate charge transfer at the NiTPP/Cu(100) interface has been gained through the application of elementary symmetry arguments to the structural determination of the NiTPP adsorption site by photoelectron diffraction (PED) measurements and Amsterdam density functional calculations of the free D4h NiTPP electronic structure. In particular, the PED analysis precisely determines that, among the diverse NiTPP chemisorption sites herein considered (fourfold hollow, atop, and bridge), the fourfold hollow one is the most favorable, with the Ni atom located at 1.93 Å from the surface and at an internuclear distance of 2.66 Å from the nearest-neighbors of the substrate. The use of elementary symmetry considerations enabled us to provide a convincing modeling of the NiTPP-Cu(100) anchoring configuration and an atomistic view of the previously revealed interfacial charge transfer through the unambiguous identification of the adsorbate π* and σ* low-lying virtual orbitals, of the substrate surface atoms, and of the linear combinations of the Cu 4s atomic orbitals involved in the substrate → adsorbate charge transfer. In addition, the same considerations revealed that the experimentally reported Ni(II) → Ni(I) reduction at the interface corresponds to the fingerprint of the chemisorption site of the NiTPP on Cu(100).

Sodium Metabisulfite in Food and Biological Samples: A Rapid and Ultra-Sensitive Electrochemical Detection Method

The primary benefit of using sulfites as a food additive is their antimicrobial and antioxidant properties, which stop fungi and bacteria from growing in a variety of foods. The application of analytical methods is necessary to ensure food quality control related to the presence of sulfites in a variety of foods. For the detection of sodium metabisulfite in food and urine samples, two sensors based on reduced graphene oxide doped with Pd paste and modified with 5,10,15,20-tetraphenyl-21H,23H-porphyrin and 5,10,15,20-tetrakis (pentafluorophenyl chloride)-21H,23H-iron (III) porphyrin were proposed. The new sensors were evaluated and characterized using square wave voltammetry. The response characteristics showed that the detection limits for the sensors were 3.0 × 10^-12 mol L^-1 for TPP/rGO@Pd0 based sensors and 3.0 × 10^-11 mol L^-1 for Fe(TPFPP)Cl/rGO@Pd0 based sensors while the quantification limits were 1.0 × 10^-11 mol L^-1 for TPP/rGO@Pd0 based sensors and 1.0 × 10^-10 mol L^-1 for Fe(TPFPP)Cl/rGO@Pd0 based sensors. The sensors can be used to determine sodium metabisulfite in a concentration range between 1.0 × 10^-11 and 1.0 × 10^-7 mol L^-1 for TPP/rGO@Pd0 based sensors and between 1.0 × 10^-10 mol L^-1 and 1.0 × 10^-6 mol L^-1 for Fe(TPFPP)Cl/rGO@Pd0 based sensors. A comparison between the proposed methods' results and other analytical applications is also presented.

Nonplanar porphyrins: synthesis, properties, and unique functionalities

Porphyrins are variously substituted tetrapyrrolic macrocycles, with wide-ranging biological and chemical applications derived from metal chelation in the core and the 18π aromatic surface. Under suitable conditions, the porphyrin framework can deform significantly from regular planar shape, owing to steric overload on the porphyrin periphery or steric repulsion in the core, among other structure modulation strategies. Adopting this nonplanar porphyrin architecture allows guest molecules to interact directly with an exposed core, with guest-responsive and photoactive electronic states of the porphyrin allowing energy, information, atom and electron transfer within and between these species. This functionality can be incorporated and tuned by decoration of functional groups and electronic modifications, with individual deformation profiles adapted to specific key sensing and catalysis applications. Nonplanar porphyrins are assisting breakthroughs in molecular recognition, organo- and photoredox catalysis; simultaneously bio-inspired and distinctly synthetic, these molecules offer a new dimension in shape-responsive host-guest chemistry. In this review, we have summarized the synthetic methods and design aspects of nonplanar porphyrin formation, key properties, structure and functionality of the nonplanar aromatic framework, and the scope and utility of this emerging class towards outstanding scientific, industrial and environmental issues.

Sulfur-Doped Graphene-Based Electrochemical Sensors for Fast and Sensitive Determination of (R)-(+)-Limonene from Beverages

Two sensors based on sulfur-doped graphene, a gold nanoparticle paste modified with 5,10,15,20-tetraphenyl-21H,23H-porphine and 5,10,15,20-tetrakis (pentafluorophenyl chloride)-21H,23H-iron (III) porphyrin, were proposed for the determination of R-limonene in beverages (triple sec liqueur and limoncello). Differential pulse voltammetry was the method used to characterize and validate the proposed sensors. The response characteristics showed that the detection limits for both sensors were 3 × 10^-6 mol L^-1, while the quantification limits were 1 × 10^-5 mol L^-1. Both sensors can be used to determine R-limonene in a concentration range between 1 × 10^-5-6 × 10^-4 mol L^-1 for TPP/AuNPs-S-Gr and 1 × 10^-5-1 × 10^-3 mol L^-1 for Fe(TPFPP)Cl/AuNPs-S-Gr. The highest sensitivity (0.7068 µA/mol L^-1) was recorded when the TPP/AuNPs-S-Gr sensor was used, proving that the electrocatalytic effect of this electrocatalyst is higher compared to that of Fe(TPFPP)Cl/AuNPs-S-Gr. High recoveries (values greater than 99.00%) and low RSD values (%) (below 5.00%) were recorded for both sensors when used to determine R-limonene in triple sec liqueur and limoncello.

Disproportionation of Nitric Oxide at a Surface‐Bound Nickel Porphyrinoid

Uncommon metal oxidation states in porphyrinoid cofactors are responsible for the activity of many enzymes. The F₄₃₀ and P450nor co‐factors, with their reduced Ni^I‐ and Fe^III‐containing tetrapyrrolic cores, are prototypical examples of biological systems involved in methane formation and in the reduction of nitric oxide, respectively. Herein, using a comprehensive range of experimental and theoretical methods, we raise evidence that nickel tetraphenyl porphyrins deposited in vacuo on a copper surface are reactive towards nitric oxide disproportionation at room temperature. The interpretation of the measurements is far from being straightforward due to the high reactivity of the different nitrogen oxides species (eventually present in the residual gas background) and of the possible reaction intermediates. The picture is detailed in order to disentangle the challenging complexity of the system, where even a small fraction of contamination can change the scenario.

Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid

Uncommon metal oxidation states in porphyrinoid cofactors are responsible for the activity of many enzymes. The F430 and P450nor co-factors, with their reduced NiI- and FeIII-containing tetrapyrrolic cores, are prototypical examples of biological systems involved in methane formation and in the reduction of nitric oxide, respectively. Herein, using a comprehensive range of experimental and theoretical methods, we raise evidence that nickel tetraphenyl porphyrins deposited in vacuo on a copper surface are reactive towards nitric oxide disproportionation at room temperature. The interpretation of the measurements is far from being straightforward due to the high reactivity of the different nitrogen oxides species (eventually present in the residual gas background) and of the possible reaction intermediates. The picture is detailed in order to disentangle the challenging complexity of the system, where even a small fraction of contamination can change the scenario.

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.

Room-Temperature On-Spin-Switching and Tuning in a Porphyrin-Based Multifunctional Interface

Molecular interfaces formed between metals and molecular compounds offer a great potential as building blocks for future opto-electronics and spintronics devices. Here, a combined theoretical and experimental spectro-microscopy approach is used to show that the charge transfer occurring at the interface between nickel tetraphenyl porphyrins and copper changes both spin and oxidation states of the Ni ion from [Ni(II), S = 0] to [Ni(I), S = 1/2]. The chemically active Ni(I), even in a buried multilayer system, can be functionalized with nitrogen dioxide, allowing a selective tuning of the electronic properties of the Ni center that is switched to a [Ni(II), S = 1] state. While Ni acts as a reversible spin switch, it is found that the electronic structure of the macrocycle backbone, where the frontier orbitals are mainly localized, remains unaffected. These findings pave the way for using the present porphyrin-based system as a platform for the realization of multifunctional devices where the magnetism and the optical/transport properties can be controlled simultaneously by independent stimuli.

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.

Insight into intramolecular chemical structure modifications by on-surface reaction using photoemission tomography

The sensitivity of photoemission tomography (PT) to directly probe single molecule on-surface intramolecular reactions will be shown here. PT application in the study of molecules possessing peripheral ligands and structural flexibility is tested on the temperature-induced dehydrogenation intramolecular reaction on Ag(100), leading from CoOEP to the final product CoTBP. Along with the ring-closure reaction, the electronic occupancy and energy level alignment of the frontier orbitals, as well as the oxidation state of the metal ion, are elucidated for both the CoOEP and CoTBP systems.

Ferrous to Ferric Transition in Fe-Phthalocyanine Driven by NO2 Exposure

Due to its unique magnetic properties offered by the open‐shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d‐states of FePc and the sp‐band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The Fe^II ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO₂ dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO₂ at room temperature.

On-Surface Synthesis of Nonmetal Porphyrins

We report the on-surface synthesis of a nonmetal porphyrin, namely, silicon tetraphenylporphyrin (Si-TPP), by the deposition of atomic silicon onto a free-base TPP layer on a Ag(100) surface under ultrahigh vacuum (UHV) conditions. Scanning tunneling microscopy provides insights into the self-assembly of the TPP molecules before and after Si insertion. Silicon coordinates with all four nitrogen atoms of the TPP macrocycle and interacts with a silver atom of the substrate as confirmed by scanning tunneling spectroscopy, X-ray photoelectron spectroscopy, and complementary density functional theory calculations. The Si-TPP complex presents a saddle-shaped conformation that is stable under STM manipulation. Our study shows how protocols established for the on-surface metalation of tetrapyrroles can be adopted to achieve nonmetal porphyrins. Complementary experiments yielding Si-TPP and Ge-TPP on Ag(111) highlight the applicability to different main group elements and supports. The success of our nonmetal porphyrin synthesis procedure is further corroborated by a temperature-programmed desorption experiment, revealing the desorption of Ge-TPP. This extension of interfacial complex formation beyond metal elements opens promising prospects for new tetrapyrrole architectures with distinct properties and functionalities.

Exploration of advanced porous organic polymers as a platform for biomimetic catalysis and molecular recognition

This Feature article summarizes our progress in the design of biomimetic POPs for catalysis and molecular recognition with enhanced performance.

Coexisting Charge States in a Unary Organic Monolayer Film on a Metal

The electronic and geometric structures of tetracene films on Ag(110) and Cu(110) have been studied with photoemission tomography and compared to that of pentacene. Despite similar energy level alignment of the two oligoacenes on these surfaces revealed by conventional ultraviolet photoelectron spectroscopy, the momentum-space resolved photoemission tomography reveals a significant difference in both structural and electronic properties of tetracene and pentacene films. Particularly, the saturated monolayer of tetracene on Ag(110) is found to consist of two molecular species that, despite having the same orientation, are electronically very different-while one molecule remains neutral, another is charged because of electron donation from the substrate.