Covalent Adsorption of N-Heterocyclic Carbenes on a Copper Oxide Surface

Adsorption and reaction of 2-methylbenzimidazole molecules on a partially oxidized Cu(110) surface

N-Heterocyclic carbenes (NHCs) have emerged as a promising candidate for functionalizing and modifying metal surfaces. Despite extensive research, the influence of oxides, which frequently occur on metal surfaces, on the adsorption behavior of NHCs has received limited attention at the nanometric scale. In this study, the adsorption configurations and reactions of 2-methylbenzimidazole (MBI) molecules on a CuO/Cu(110) surface were investigated using scanning tunneling microscopy and non-contact atomic force microscopy (nc-AFM). Following the deposition of MBI molecules, Cu islands were observed, and the molecules predominantly adsorbed in a flat-lying configuration. Comparative experiments conducted on a bare Cu(110) surface indicate that the dehydrogenation of MBI molecules due to the cleavage of N-H bonds leads to CuO reduction and the release of Cu adatoms, which subsequently aggregate into islands. Upon annealing at 378 K, molecules adsorbed at the CuO strips assume a tilted flat-lying configuration, suggesting the formation of coordination bonds between nitrogen and copper atoms. On the copper regions, molecules assemble into double-chains, adopting an upright configuration. High-resolution nc-AFM images reveal a similarity between molecules in the double-chains and those at step edges, implying that Cu atoms are extracted from terraces to form slots where molecules preferentially occupy.

Insights into the surface chemistry of N-heterocyclic carbenes

N-heterocyclic carbenes (NHCs) have emerged as a versatile and powerful class of ligands in surface chemistry, offering remarkable stability and tunability when bound to surfaces, including metals, metal oxides, and semiconductors. Understanding their surface and interfacial mechanisms at the atomic-level is essential for precise control of molecule-surface interaction, as well as intermolecular interactions, which directly influence material performance and functionalities. Research in surface chemistry focusing on molecular binding modes, self-assembly, on-surface reactions, and electronic properties is crucial for the rational design of efficient catalysts, customized materials, and high-performance devices. This review highlights these critical aspects of NHCs on surfaces, beginning with their robust and multiple binding modes, which underpin their stability and versatility. The covalent NHC-surface bonds allow NHCs to form stable attachments, often surpassing the strength of traditional thiol-based modifiers, promoting robust anchoring across diverse materials. Another focus is the self-assembly of NHCs into highly ordered monolayers, which facilitates the design of functional nanostructures. Emerging topics also include on-surface reactions, surface electronic properties, and interfacial charge transfer of NHCs, emphasizing their dependence on the substrate and NHC molecular structure. By consolidating recent advancements in the study of NHCs on surfaces, we aim to provide a comprehensive overview of their transformative potential in surface chemistry at the atomic scale, while also identifying key challenges and future directions in the field.

Optical Spectroscopic Probing and Atomic Visualization of the Motion of N-Heterocyclic Carbenes on Ag(111)

The application of N-heterocyclic carbenes (NHCs) as versatile anchors for planar surface modifications has been well documented over the past decade. Despite its fundamental importance to the formation of self-assembled NHC monolayers on surfaces, the microscopic mechanism behind the mobility of NHCs has primarily been explored through theoretical studies; an atomic-level experimental understanding of NHC motion on surfaces remains elusive. Here, we combine tip-enhanced Raman spectroscopy (TERS) and scanning tunneling microscopy (STM) to investigate the mobility of a model NHC on Ag(111). Two distinct molecular behaviors are observed, depending on substrate preparation. Room-temperature deposition leads to diffusing NHC-Ag adatom complexes exhibiting a ballbot-like motion, chemically identified by TERS through their spectroscopic fingerprint. By contrast, NHCs deposited at low temperature are stabilized on Ag(111) as isolated single molecules directly bound to the substrate. Significantly, a desorption/readsorption scenario is suggested for the displacement of NHCs by moving otherwise immobile single NHCs deposited at low temperature via STM manipulation, with their trajectory traced to atomic precision. This study provides chemical and atomic-level insights into the mobility of NHCs, which will advance the understanding of the fundamental properties of NHC-based surface modifications at the spatial limit.

Odd-Even Effects in the Structure and Thermal Stability of Carboxylic Acid Anchored Monolayers on Naturally Oxidized Aluminum Surface

Self-assembled monolayers (SAMs) are broadly used for molecular engineering of surfaces and interfaces, which demands control over their structure and properties. An important tool in this context is the so-called odd-even effects exploiting the dependence of the SAM structure on the parity of the number of building blocks forming the backbone of SAM-building molecules. Even though these effects influence parameters crucial for SAM applications, they have been mainly studied on coinage metals (Au and Ag) until now. Here, using the series of biphenyl-substituted carboxylic acids (BPnCOO, n = 0-4), we show that structural odd-even behavior occurs as well on technologically relevant surface of naturally oxidized aluminum (representative of other oxide surfaces), with the even-numbered monolayers exhibiting higher packing density and lower molecular inclination than the odd-numbered analogs. Despite these structural changes, the SAM desorption energy remains nearly constant at a high value (∼1.5 eV) making BPnCOO/AlOx a promising system for organic electronics applications.

Self-Assembled Monolayer of N-Heterocyclic Carbene as a Primer in a Dual-Layer Coating for Corrosion Protection on Iron

The development of highly stable coatings on iron is essential for mitigating corrosion formation. Herein, it is demonstrated that a self-assembled monolayer of N-Heterocyclic Carbene (NHC) can be electrodeposited on iron foil and function as a binder for a secondary, crosslinked polymer network coating. The dual layer coating, constructed of a monolayer of NHCs and a polymer film, as a primary and a secondary coating, respectively, effectively preventes corrosion formation with a protective efficiency of 99.6 ± 0.2 %, as determined by polarization measurements in 3.5 wt.% NaCl solution. Spectroscopic analysis identified the formation of a chemical interaction between the NHC monolayer and the polymer film. The strong anchoring of NHC to iron along with its chemical interaction with the polymer film induced high stability and durability of the dual-layer coating to effectively protect the coated iron from corrosion formation.

Geometric Effects on C-C Coupling of Aryl-Carbenes under 2D Confinement

It is well established that the confinement of reactants to two dimensions influences their reactivity. However, such confinement is often dominated by charge transfer effects between the reactants and the confining walls, in particular if the walls are conductive. Also, the reactivity of carbenes on metal surfaces is significantly affected by the charge transfer between the carbene and the metal, rendering the carbene more nucleophilic or electrophilic. Here, we investigate the geometrical effects of 2D confinement without an influence of the supporting metal for a photoinduced reaction of an aryl carbene on an ionic decoupling layer. We demonstrate the decoupling concept for the C-C coupling of 3-methoxy-9-fluorenylidene (C14H10O) on a NaBr(100) bilayer on Ag(111). We combine scanning tunneling microscopy with infrared reflection absorption spectroscopy to follow the photoinduced C-C coupling of the carbene from its diazo-protected precursor in two dimensions. Our study demonstrates that the NaBr decoupling bilayer efficiently suppresses the effects of the metal surface, facilitating carbene chemistry under geometrical confinement.

N-Heterocyclic Carbene Deposition on a Copper Powder Surface Using Mechanochemistry

Metal powders are crucial precursors for manufacturing surfaces through thermal spraying, cold spraying, and 3D printing methods. However, surface oxidation of these precursors poses a challenge to the coherence of the metallic materials during manufacturing processes. Herein, we introduce a method for surface modification of copper powder with N-heterocyclic carbenes (NHCs) using mechanochemistry to mitigate surface oxidation. A resonant acoustic mixer was used to deposit five different carbenes on copper powders using benzimidazolium hydrogen carbonate precursors and a trace amount of solvent. Significant oxide reduction was observed by X-ray photoelectron spectroscopy (XPS), and the immobilization of NHCs on the powder was confirmed by mass spectrometry and XPS. The consistent morphology of the modified copper powder minimizes any potential impact on subsequent manufacturing processes. Moreover, a life cycle assessment indicates the potential environmental hotspots, leading to recommendations to develop lower-footprint processes. Overall, the mechanochemical method to produce NHC-modified metal powders with a higher metallic content provides great prospects for powder precursors to produce coatings from thermal spray, cold spray, and additive manufacturing processes.

A Strongly Reducing sp2 Carbon-Conjugated Covalent Organic Framework Formed by N-Heterocyclic Carbene Dimerization

Covalent organic frameworks linked by carbon-carbon double bonds (C=C COFs) are an emerging class of crystalline, porous, and conjugated polymeric materials with potential applications in organic electronics, photocatalysis, and energy storage. Despite the rapidly growing interest in sp2 carbon-conjugated COFs, only a small number of closely related condensation reactions have been successfully employed for their synthesis to date. Herein, we report the first example of a C=C COF, CORN-COF-1 (CORN=Cornell University), prepared by N-heterocyclic carbene (NHC) dimerization. In-depth characterization reveals that CORN-COF-1 possesses a two-dimensional layered structure and hexagonal guest-accessible pores decorated with a high density of strongly reducing tetraazafulvalene linkages. Exposure of CORN-COF-1 to tetracyanoethylene (TCNE, E1/2=0.13 V and -0.87 V vs. SCE) oxidizes the COF and encapsulates the radical anion TCNE⋅- and the dianion TCNE2- as guest molecules, as confirmed by spectroscopic and magnetic analysis. Notably, the reactive TCNE⋅- radical anion, which generally dimerizes in the solid state, is uniquely stabilized within the pores of CORN-COF-1. Overall, our findings broaden the toolbox of reactions available for the synthesis of redox-active C=C COFs, paving the way for the design of novel materials.

Chemically Interrogating N-Heterocyclic Carbenes at the Single-Molecule Level Using Tip-Enhanced Raman Spectroscopy

N-heterocyclic carbenes (NHCs) have been established as powerful modifiers to functionalize metal surfaces for a wide variety of energy and nanoelectronic applications. To fundamentally understand and harness NHC modification, it is essential to identify suitable methods to interrogate NHC surface chemistry at the spatial limit. Here, we demonstrate tip-enhanced Raman spectroscopy (TERS) as a promising tool for chemically probing the surface properties of NHCs at the single-molecule scale. We show that with subnanometer resolution, TERS measurements are capable of not only unambiguously identifying the chemical structure of individual NHCs by their vibrational fingerprints but also definitively determining the binding mode of NHCs on metal surfaces. In particular, by investigating low-temperature NHC adsorption on Ag(111), our TERS studies provide insights into the temperature dependence of the adsorption properties of NHCs. This work suggests the potential of single-molecule vibrational spectroscopy for investigations of NHC surface modification at the most fundamental level.

Over 19% Efficient Inverted Organic Photovoltaics Featuring a Molecularly Doped Metal Oxide Electron-Transporting Layer

Molecular doping is commonly utilized to tune the charge transport properties of organic semiconductors. However, applying this technique to electrically dope inorganic materials like metal oxide semiconductors is challenging due to the limited availability of molecules with suitable energy levels and processing characteristics. Herein, n-type doping of zinc oxide (ZnO) films is demonstrated using 1,3-dimethylimidazolium-2-carboxylate (CO2-DMI), a thermally activated organic n-type dopant. Adding CO2-DMI into the ZnO precursor solution and processing it atop a predeposited indium oxide (InOx) layer yield InOx/n-ZnO heterojunctions with increased electron field-effect mobility of 32.6 cm2 V-1 s-1 compared to 18.5 cm2 V-1 s-1 for the pristine InOx/ZnO bilayer. The improved electron transport originates from the ZnO's enhanced crystallinity, reduced hydroxyl concentrations, and fewer oxygen vacancy groups upon doping. Applying the optimally doped InOx/n-ZnO heterojunctions as the electron-transporting layers (ETLs) in organic photovoltaics (OPVs) yields cells with improved power conversion efficiency of 19.06%, up from 18.3% for devices with pristine ZnO, and 18.2% for devices featuring the undoped InOx/ZnO ETL. It is shown that the all-around improved OPV performance originates from synergistic effects associated with CO2-DMI doping of the thermally grown ZnO, highlighting its potential as an electronic dopant for ZnO and potentially other metal oxides.

N-Heterocyclic Carbene Monolayers on Metal-Oxide Films: Correlations between Adsorption Mode and Surface Functionality

N-Heterocyclic carbene (NHC) ligands have been self-assembled on various metal and semimetal surfaces, creating a covalent bond with surface metal atoms that led to high thermal and chemical stability of the self-assembled monolayer. This study explores the self-assembly of NHCs on metal-oxide films (CuOx, FeOx, and TiOx) and reveals that the properties of these metal-oxide substrates play a pivotal role in dictating the adsorption behavior of NHCs, influencing the decomposition route of the monolayer and its impact on work function values. While the attachment of NHCs onto CuOx is via coordination with surface oxygen atoms, NHCs interact with TiOx through coordination with surface metal atoms and with FeOx via coordination with both metal and oxygen surface atoms. These distinct binding modes arise due to variances in the electronic properties of the metal atoms within the investigated metal-oxide films. Contact angle and ultraviolet photoelectron spectroscopy measurements have shown a significantly higher impact of F-NHC adsorption on CuOx than on TiOx and FeOx , correlated to a preferred, averaged upright orientation of F-NHC on CuOx.

Tailored Monolayers of N-Heterocyclic Carbenes by Kinetic Control

Despite the substantial success of N-heterocyclic carbenes (NHCs) as stable and versatile surface modification ligands, their use in nanoscale applications beyond chemistry is still hampered by the failure to control the carbene binding mode, which complicates the fabrication of monolayers with the desired physicochemical properties. Here, we applied vibrational sum-frequency generation spectroscopy to conduct a pseudokinetic surface analysis of NHC monolayers on Au thin films under ambient conditions. We observe for two frequently used carbene structures that their binding mode is highly dynamic and changes with the adsorption time. In addition, we demonstrate that this transition can be accelerated or decelerated to adjust the binding mode of NHCs, which allows fabrication of tailored monolayers of NHCs simply by kinetic control.

Self-Assembled Monolayers of N-Heterocyclic Olefins on Au(111)

Self-assembled monolayers (SAMs) of N-heterocyclic olefins (NHOs) have been prepared on Au(111) and their thermal stability, adsorption geometry, and molecular order were characterized by X-ray photoelectron spectroscopy, polarized X-ray absorption spectroscopy, scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The strong σ-bond character of NHO anchoring to Au induced high geometrical flexibility that enabled a flat-lying adsorption geometry via coordination to a gold adatom. The flat-lying adsorption geometry was utilized to further increase the surface interaction of the NHO monolayer by backbone functionalization with methyl groups that induced high thermal stability and a large impact on work-function values, which outperformed that of N-heterocyclic carbenes. STM measurements, supported by DFT modeling, identified that the NHOs were self-assembled in dimers, trimers, and tetramers constructed of two, three, and four complexes of NHO-Au-adatom. This self-assembly pattern was correlated to strong NHO-Au interactions and steric hindrance between adsorbates, demonstrating the crucial influence of the carbon-metal σ-bond on monolayer properties.

Surface Modification of ITO with N-Heterocyclic Carbene Precursors Results in Electron Selective Contacts in Organic Photovoltaic Devices

Surface modification of indium tin oxide (ITO) electrodes with organic molecules is known to tune their work function which results in higher charge carrier selectivity in corresponding organic electronic devices and hence influences the performance of organic solar cells. In recent years, N-heterocyclic carbenes (NHCs) have also been proven to be capable to modify the work function of metals and semimetals compared to the unfunctionalized surface via the formation of strong covalent bonds. In this report, we have designed and performed the modification of the ITO surface with NHC by using the zwitterionic bench stable IPr-CO2 as the NHC precursor, applied via spin coating. Upon modification, the work function of ITO electrodes was reduced significantly which resulted in electron selective contacts in corresponding organic photovoltaic devices. In addition, various characterization techniques and analytical methods are used to elucidate the nature of the bound species and the corresponding binding mechanism of the material to the ITO surface.

Geometric and Electronic Effects in the Binding Affinity of Imidazole-Based N-Heterocyclic Carbenes to Cu(100)- and Ag(100)-Based Pd and Pt Single-Atom Alloy Surfaces

We have conducted nonlocal periodic density functional theory (DFT) calculations of N-heterocyclic carbenes (NHCs) adsorbed to Pd/Cu(100), Pt/Cu(100), Pd/Ag(100), and Pt/Ag(100) single atom alloys (SAAs) utilizing the nonlocal optPBE-vdW functional. NHCs with electron donating groups (EDGs) are predicted to bind more strongly to the SAA surface compared to NHCs functionalized with electron withdrawing groups (EWGs). Our calculations show that NHCs typically bind to SAA geometries containing a small space between the heteroatom sites for the SAAs considered. Generally, this pattern is predicted to persist for a single NHCs or for a pair of NHCs bound to the SAA surfaces. Approximate linear relationships between NMR-based parameters and NHC-SAA binding energies are uncovered. We predict that the binding of NHCs to SAA surfaces is composition-dependent and heteroatom geometry dependent.

NHC stabilized copper nanoparticles via reduction of a copper NHC complex

The bottom-up synthesis of plasmonic NHC@CuNPs from common starting reagents, via the formation of the synthetically accessible NHC-Cu(I)-Br complex and its reduction by NH3·BH3 is reported. The resulting NHC@CuNPs have been characterized in detail by XPS, TEM and NMR spectroscopy. The stability of NHC@CuNPs was investigated under both inert and ambient conditions using UV-Vis analysis. While the NHC@CuNPs are stable under inert conditions for an extended period of time, the NPs oxidize under air to form CuxO with concomitant release of the stabilizing NHC ligand.

Stability of N-Heterocyclic Carbene Monolayers under Continuous Voltammetric Interrogation

N-Heterocyclic carbenes (NHCs) are promising monolayer-forming ligands that can overcome limitations of thiol-based monolayers in terms of stability, surface functionality, and reactivity across a variety of transition-metal surfaces. Recent publications have reported the ability of NHCs to support biomolecular receptors on gold substrates for sensing applications and improved tolerance to prolonged biofluid exposure relative to thiols. However, important questions remain regarding the stability of these monolayers when subjected to voltage perturbations, which is needed for applications with electrochemical platforms. Here, we investigate the ability of two NHCs, 1,3-diisopropylbenzimidazole and 5-(ethoxycarbonyl)-1,3-diisopropylbenzimidazole, to form monolayers via self-assembly from methanolic solutions of their trifluoromethanesulfonate salts. We compare the electrochemical behavior of the resulting monolayers relative to that of benchmark mercaptohexanol monolayers in phosphate-buffered saline. Within the -0.15 to 0.25 V vs Ag|AgCl voltage window, NHC monolayers are stable on gold surfaces, wherein they electrochemically perform like thiol-based monolayers and undergo similar reorganization kinetics, displaying long-term stability under incubation in buffered media and under continuous voltammetric interrogation. At negative voltages, NHC monolayers cathodically desorb from the electrode surface at lower bias (-0.1 V) than thiol-based monolayers (-0.5 V). At voltages more positive than 0.25 V, NHC monolayers anodically desorb from electrode surfaces at similar voltages to thiol-based monolayers. These results highlight new limitations to NHC monolayer stability imposed by electrochemical interrogation of the underlying gold electrodes. Our results serve as a framework for future optimization of NHC monolayers on gold for electrochemical applications, as well as structure-functionality studies of NHCs on gold.

N-Heterocyclic Carbenes: Molecular Porters of Surface Mounted Ru-Porphyrins

Ru-porphyrins act as convenient pedestals for the assembly of N-heterocyclic carbenes (NHCs) on solid surfaces. Upon deposition of a simple NHC ligand on a close packed Ru-porphyrin monolayer, an extraordinary phenomenon can be observed: Ru-porphyrin molecules are transferred from the silver surface to the next molecular layer. We have investigated the structural features and dynamics of this portering process and analysed the associated binding strengths and work function changes. A rearrangement of the molecular layer is induced by the NHC uptake: the NHC selective binding to the Ru causes the ejection of whole porphyrin molecules from the molecular layer on silver to the layer on top. This reorganisation can be reversed by thermally induced desorption of the NHC ligand. We anticipate that the understanding of such mass transport processes will have crucial implications for the functionalisation of surfaces with carbenes.