The adsorption of N-heterocyclic carbenes (NHCs) on Cu(111) and Au(111) surfaces is studied with density-functional theory. The role of the molecular side groups as well as the surface morphology in determining the adsorption geometry are explored in detail. Flat-laying NHCs, as observed experimentally for NHC with relatively small side groups, result from the adsorption at adatoms and give rise to the so-called ballbot configurations, which are more stable than adsorption on flat surfaces and provide an efficient precursor for the formation of bis(NHC) dimers. On Au(111), the resulting (NHC)2 Au complexes are purely physisorbed and thus mobile. On the more reactive Cu(111), in contrast, the central Cu atom in the (NHC)2 Cu dimer is still covalently bound to the surface, resulting in a mobility, which has to be thermally activated.

N-Heterocyclic carbenes (NHCs) are promising modifiers and anchors for surface functionalization and offer some advantages over thiol-based systems. Because of their strong binding affinity and high electron donation, NHCs can dramatically change the properties of the surfaces to which they are bonded. Highly ordered NHC monolayers have so far been limited to metal surfaces. Silicon, however, remains the element of choice in semiconductor devices and its modification is therefore of utmost importance for electronic industries. Here, a comprehensive study on the adsorption of NHCs on silicon is presented. We find covalently bound NHC molecules in an upright adsorption geometry and demonstrate the formation of highly ordered monolayers exhibiting good thermal stability and strong work function reductions. The structure and ordering of the monolayers is controlled by the substrate geometry and reactivity and in particular by the NHC side groups. These findings pave the way towards a tailor-made organic functionalization of silicon surfaces and, thanks to the high modularity of NHCs, new electronic and optoelectronic applications.

The $\mathrm{Si}(111)\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}\text{\ensuremath{-}}\mathrm{B}\phantom{\rule{0.28em}{0ex}}[\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}]$ surface has evolved into a particularly interesting surface in the context of on-surface molecular self-assembly. Photoemission spectroscopy is a powerful tool to understand the interaction between the surface and the adsorbates. Previous studies of $\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}$ contain many inconsistencies with regard to the Si $2p$ core level and valence-band dispersion. Here we employ synchrotron-based core-level and angle-resolved photoemission spectroscopy measurements in combination with density functional theory (DFT) calculations to address these issues. DFT calculations of the Si $2p$ core-level spectra accurately identify contributions from one bulk and five surface components, which allows us to obtain a comprehensive understanding of the spectra recorded at different photon energies. As an archetypal example, this refined decomposition is employed to understand the changes in Si $2p$ spectra upon the adsorption of cobalt phthalocyanine molecules. Regarding the valence-band dispersion of the clean $\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}$ surface, our comprehensive DFT and photoemission investigations are able to reconcile the inconsistencies appearing in previous studies and reveal several yet unreported surface states. Furthermore, we are able to theoretically and experimentally resolve the distribution of these surface states in constant energy plots.

Inorganic-organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), an SF molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-ray absorption fine structure (NEXAFS) and Xray photoelectron spectroscopy (XPS) experiments with density functional theory (DFT) calculations. For samples grown at or below substrate temperatures of 265 K, the resulting ultrathin Tc films are dominated by almost upright-standing molecules. The molecular arrangement is very similar to the Tc bulk phase, with only a slightly higher average angle between the conjugated molecular plane normal and the surface normal (alpha) around 77 degrees. Judging from carbon K-edge X-ray absorption spectra, the orientation of the Tc molecules are almost identical when grown on H-Si(111) and a-Si:H substrates as well as for (sub)mono- to several-monolayer coverages. Annealing to room temperature, however, changes the film structure toward a smaller alpha of about 63 degrees. A detailed DFT-assisted analysis suggests that this structural transition is correlated with a lower packing density and requires a well-chosen amount of thermal energy. Therefore, we attribute the resulting structure to a distinct monolayer configuration that features less inclined, but still well-ordered molecules. The larger overlap with the substrate wave functions makes this arrangement attractive for an optimized interfacial electron transfer in SF-assisted silicon solar cells.

The rapid and reliable detection of lethal agents such as sarin is of increasing importance. Here, density-functional theory (DFT) is used to compare the interaction of sarin with single-metal-centered phthalocyanine (MPc) and MPc layer structures to a benign model system, i.e., the adsorption of dimethyl methylphosphonate (DMMP). The calculations show that sarin and DMMP behave nearly identical to the various MPcs studied. Among NiPc, CuPc, CoPc, and zinc phthalocyanine (ZnPc), we find the interaction of both sarin and DMMP to be the strongest with ZnPc, both in terms of interaction energy and adsorption-induced work function changes. ZnPc is thus proposed as a promising sensor for sarin detection. Using X-ray photoelectron spectroscopy, the theoretically predicted charge transfer from DMMP to ZnPc is confirmed and identified as a key component in the sensing mechanism.

Inorganic-organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), a SF molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) experiments with density functional theory (DFT) calculations. For samples grown at or below substrate temperatures of 265 K, the resulting ultrathin Tc films are dominated by almost uprightstanding molecules. The molecular arrangement is very similar to the Tc bulk phase, with only slightly higher average angle between the conjugated molecular plane normal and the surface normal () around 77°. Judging from carbon K-edge X-ray absorption spectra, the orientation of the Tc molecules are almost identical when grown on H-Si(111) and a-Si:H substrates as well as for (sub)monoto severalmonolayer coverages. Annealing to room temperature, however, changes the film structure towards a smaller  of about 63°. A detailed DFT-assisted analysis suggests that this structural transition is correlated with a lower packing density and requires a well-chosen amount of thermal energy. Therefore, we attribute the resulting structure to a distinct monolayer configuration that features less inclined, but still well-ordered molecules. The larger overlap with the substrate wavefunctions makes this arrangement attractive for an optimized interfacial electron transfer in SF-assisted silicon solar cells.

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 L2,3-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.

We demonstrate here that theory-assisted near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy enables the site-sensitive monitoring of on-surface chemical reactions, thus, providing information not accessible by other techniques. As a prototype example, we have used free-base 5,10,15-tris(pentafluorophenyl)corroles (3H-TpFPC) adsorbed on Ag(111) and present a detailed investigation of the angle-dependent NEXAFS of this molecular species as well as of their thermally induced derivatives. For this, we have recorded experimental C and N K-edge NEXAFS spectra and interpret them based on XAS cross-section calculations by using a continuous fraction approach and core-hole including multiprojector PAW pseudopotentials within DFT. We have characterized the as-deposited low temperature (200 K) phase and unraveled the subsequent changes induced by dehydrogenation (at 330 K) and ring-closure reactions (at 430 K). By exemplarily obtaining profound insight into the on-surface chemistry of free-base corrolic species adsorbed on a noble metal this work highlights how angle-dependent XAS combined with accurate theoretical modeling can serve for the investigation of on-surface reactions, whereby even highly similar molecular structures, such as tautomers and isomers, can be distinguished.

Radical cyclization is among the most powerful and versatile reactions for constructing mono- and polycyclic systems, but has, to date, remained unexplored in the context of on-surface synthesis. We report the controlled on-surface synthesis of stable corrole radicals on Ag(111) via site-specific dehydrogenation of a pyrrole N-H bond in the 5,10,15-tris(pentafluoro-phenyl)-corrole triggered by annealing at 330 K under ultrahigh-vacuum conditions. We reveal a thermally induced regioselective cyclization reaction mediated by a radical cascade and resolve the reaction mechanism of the pertaining cyclodefluorination reaction at the single-molecule level. Via intramolecularly resolved probing of the radical-related Kondo signature, we achieve real space visualization of the distribution of the unpaired electron density over specific sites within the corrole radical. Annealing to 550 K initiates intermolecular coupling reactions, producing an extended π-conjugated corrole system.

Corrole compounds attract increasing interest due to their potential to stabilize high-valent metal states. Xray spectroscopy is a powerful tool for the investigation and development of functional interfaces. For corrolic species, however, the required reference data are missing. Here, we employ a multitechnique X-ray investigation of thin films of the prototypical free-base 5,10,15-tris(pentafluoropheny1)corrole (3H-TpFPC) grown on the Ag(111) surface under ultrahigh vacuum conditions. Ultrapure corrole multilayer samples are prepared and characterized by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine Structure (NEXAFS) spectroscopy. In parallel, the X-ray fingerprints are simulated using the continued-fraction approach within density functional theory (DFT) for extended, (quasi-)periodic molecular structures. An excellent agreement between experimental and theoretical spectra enables a thorough interpretation of the detailed spectral features and proves an accurate description of the free-base corrole electronic structure within the present DFT approach. The present study provides X-ray spectroscopic references for all relevant core-level regions and absorption edges of intact molecular species and, thus, represents an ideal starting point for the comprehensive understanding of the complex chemistry of corroles in the adsorbed state toward the development of related functional interfaces.

Non-trivial arrangement of molecules within a molecular network complicates structure determination due to interdigitation, partial overlap, or stacking. We demonstrate that combined imaging and lateral manipulation with a scanning tunneling microscope resolves the intricate structure of a molecular network in two-dimensions in a straightforward manner. The network, formed by a monolayer of 5,10,15-tris(pentafluorophenyl)-corrole molecules on Ag(111), is manipulated for the first time with single-molecule precision. Our results reveal a shingle-like packing of partially overlapping corrole molecules. Density functional theory calculations support our findings.

Abstract Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H+/4 e− process, while oxygen can be fully reduced to water by a 4 e−/4 H+ process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2 −. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

We report a combined experiment-theory study on low energy vibrational modes in fluorescence spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules. Using very low coverages, isolated molecules were adsorbed on terrace sites or at sites located at residual steps on (100) oriented alkali halide films (KCl and NaCl). The low energy modes couple to the optical transition only because the PTCDA molecule is geometrically distorted (C2v) upon adsorption on the surface; they would be absent for the parent planar (D2h) PTCDA molecule. The modes differ in number and energy for molecules adsorbed on regular terrace sites and molecules adsorbed at step edge sites. Modes appearing for step edge sites have the character of frustrated rotations. Their coupling to the optical transition is a consequence of the further reduced symmetry of the step edge sites. We find a larger number of vibrational modes on NaCl than on KCl. We explain this by the stronger electrostatic bonding of the PTCDA on NaCl compared to KCl. It causes the optical transition to induce stronger changes in the molecular coordinates, thus leading to larger Franck-Condon factors and thus stronger coupling. Our results demonstrate how optical spectroscopy can be used to gain information on adsorption sites of molecules at low surface concentrations.

We report a detailed analysis of the line shapes of the 0-0 transitions in the fluorescence (FL) and fluorescence excitation (FLE) spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules adsorbed at terrace and step edge sites of (100)-oriented alkali halide films (KC1 and NaCl). At low temperatures (6-20 K), we find very narrow FLE lines with a fwhm of 4.5 cm(-1) (3.0 cm(-1)) on KC1 (NaCl). These line shapes are dominated by inhomogeneous broadening related to the structural variation of the environment of the PTCDA molecules. We explain this site broadening on the basis of structural models for the adsorption sites at the step edges from earlier scanning microscopy data and density functional theory calculations. With increasing temperatures, the 0-0 lines in the FL and FLE spectra broaden; e.g., in the FL, the fwhm increases to 26 cm(-1) (18 cm(-1)) at 100 K on the KC1 (NaCl) surface. This temperature induced broadening is of Lorentzian shape and can be described by the theory of Hsu and Skinner, based on dephasing by coupling to acoustic phonons of the substrate. Discrepancies remain for experimentally observed small line shifts. We discuss how surfaces can be used and optimized as sample systems for a highly resolved optical spectroscopy of molecules.

First principles calculations have been employed to investigate the adsorption of single PTCDA molecules on KCl(100) and NaCl(100) surfaces. The lateral and rotational diffusion barriers as well as the electronic and the geometric aspects of single PTCDA molecules adsorbed on planar terraces as well as at defective steps have been studied in detail.

First-principles calculations were combined with scanning tunneling microscopy (STM) measurements to analyze the adsorption of diindenoperylene (DIP) molecules on Cu(111) surfaces. The influence of the substrate on the geometry of single adsorbed molecules, their diffusion barriers, as well as the role of step-edges and intermolecular interactions for molecular self-assembly and structure growth are studied. Long-range ordered arrangements of DIP molecules are found to be most favorable irrespective of the terrace width. Energetically less favored short-range order structures, however, are observed as well.

First-principles calculations on the adsorption of diindenoperylene molecules on Cu(111) are presented. Single molecules as well as molecular monolayers on planar surface have been studied in detail, both with respect to the adsorption structure and the modification of the molecular charge density. Among the geometries studied here, long-range ordered arrangements of DIP molecules are found to be most favorable.

Scanning tunneling microscopy at low temperature and low molecular coverage (similar to 1%) was combined with density-functional theory calculations to investigate the adsorption of isolated perylene-3,4,9,10-tetracarboxyl acid dianhydride (PTCDA) molecules on KCl(100) surfaces. Experimentally, we used epitaxial thin KCl(100) films on Ag(100) of about 3 layers in thickness. After deposition at 100 K, the PTCDA molecules are statistically distributed on the terraces with an azimuthal orientation of the long axis along the polar < 110 > orientation. After annealing at about 150 to 200 K the molecules are exclusively found at step-edge sites. Thereby, several configurations are observed, the most typical being a site where the PTCDA molecules protrude into the step edge, forming vacancies at the step edge. The total-energy calculations predict this step-edge adsorption site to be energetically favorable compared to the adsorption on terraces. The corrugation of the calculated potential energy surface is below 1 eV, with diffusion barriers of about 0.6 eV, which explains the mobility of the PTCDA molecules at elevated temperatures.