Seeding the vertical growth of laterally coherent coordination polymers on the rutile-TiO2(110) surface.

Coordination polymers may be synthesized by linear bridging ligands to metal ions with conventional chemistry methods (e.g. in solution). Such complexes can be hardly brought onto a substrate with the chemical, spatial and geometrical homogeneity required for device integration. Instead, we follow an in situ synthesis approach, where the anchoring points are provided by a monolayer of metal(II)-tetraphenylporphyrin (M-TPP, M = Cu, Zn, Co) grown in vacuum on the rutile-TiO2(110) surface. We probed the metal affinity to axial coordination by further deposition of symmetric dipyridyl-naphthalenediimide (DPNDI). By NEXAFS linear polarization dichroism, we show that DPNDI stands up on Zn- and Co-TPP thanks to axial coordination, whereas it lies down on the substrate for Cu-TPP. Calculations for a model pyridine ligand predict strong binding to Zn and Co cations, whose interaction with the O anions underneath is disrupted by surface trans effect. The weaker interactions between pyridine and Cu-TPP are then overcome by the strong attraction between TiO2 and DPNDI. The binding sites exposed by the homeotropic alignment of the ditopic DPNDI ligand on Zn- and Co-TPP are the foundations to grow coordination polymers preserving the lateral coherence of the basal layer.

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.

Oxygen Promoted On-Surface Synthesis of Polyboroxine Molecules.

We present our findings on the on-surface synthesis of polyboroxine molecules derived from boroxine molecules precursors. This process is promoted by oxygen species present on the Au(111) surface: oxygen atoms facilitate the detachment of naphthalene units of trinaphthyl-boroxine molecules and bridge two unsaturated boroxine centers to form a boroxine-O-boroxine chemical motif. X-ray spectroscopic characterization shows that, as the synthesis process proceeds, it progressively tunes the electronic properties of the interface, thus providing a promising route to control the electron level alignment. .

Mechanistic insights into on-surface reactions from isothermal temperature-programmed X-ray photoelectron spectroscopy.

On-surface synthesis often proceeds under kinetic control due to the irreversibility of key reaction steps, rendering kinetic studies pivotal. The accurate quantification of reaction rates also bears potential for unveiling reaction mechanisms. Temperature-Programmed X-ray Photoelectron Spectroscopy (TP-XPS) has emerged as an analytical tool for kinetic studies with splendid chemical and sufficient temporal resolution. Here, we demonstrate that the common linear temperature ramps lead to fitting ambiguities. Moreover, pinpointing the reaction order remains intricate, although this key parameter entails information on atomistic mechanisms. Yet, TP-XPS experiments with a stepped temperature profile comprised of isothermal segments facilitate the direct quantification of rate constants from fitting time courses. Thereby, rate constants are obtained for a series of temperatures, which allows independent extraction of both activation energies and pre-exponentials from Arrhenius plots. By using two analogous doubly versus triply brominated aromatic model compounds, we found that their debromination on Ag(111) is best modeled by second-order kinetics and thus proceeds via the involvement of a second, non-obvious reactant. Accordingly, we propose that debromination is activated by surface supplied Ag adatoms. This hypothesis is supported by Density Functional Theory (DFT) calculations. We foresee auspicious prospects for this TP-XPS variant for further exploring the kinetics and mechanisms of on-surface reactions.

Enhancing Haloarene Coupling Reaction Efficiency on an Oxide Surface by Metal Atom Addition.

The bottom-up synthesis of carbon-based nanomaterials directly on semiconductor surfaces allows for the decoupling of their electronic and magnetic properties from the substrates. However, the typically reduced reactivity of such nonmetallic surfaces adversely affects the course of these reactions. Here, we achieve a high polymerization yield of halogenated polyphenyl molecular building blocks on the semiconducting TiO2(110) surface via concomitant surface decoration with cobalt atoms, which catalyze the Ullmann coupling reaction. Specifically, cobalt atoms trigger the debromination of 4,4″-dibromo-p-terphenyl molecules on TiO2(110) and mediate the formation of an intermediate organometallic phase already at room temperature (RT). As the debromination temperature is drastically reduced, homocoupling and polymerization readily proceed, preventing presursor desorption from the substrate and entailing a drastic increase of the poly-para-phenylene polymerization yield. The general efficacy of this mechanism is shown with an iodinated terphenyl derivative, which exhibits similar dehalogenation and reaction yield.

Noncontact Layer Stabilization of Azafullerene Radicals: Route toward High-Spin-Density Surfaces.

We deposit azafullerene C59N• radicals in a vacuum on the Au(111) surface for layer thicknesses between 0.35 and 2.1 monolayers (ML). The layers are characterized using X-ray photoemission (XPS) and X-ray absorption fine structure (NEXAFS) spectroscopy, low-temperature scanning tunneling microscopy (STM), and by density functional calculations (DFT). The singly unoccupied C59N orbital (SUMO) has been identified in the N 1s NEXAFS/XPS spectra of C59N layers as a spectroscopic fingerprint of the molecular radical state. At low molecular coverages (up to 1 ML), films of monomeric C59N are stabilized with the nonbonded carbon orbital neighboring the nitrogen oriented toward the Au substrate, whereas in-plane intermolecular coupling into diamagnetic (C59N)2 dimers takes over toward the completion of the second layer. By following the C59N• SUMO peak intensity with increasing molecular coverage, we identify an intermediate high-spin-density phase between 1 and 2 ML, where uncoupled C59N• monomers in the second layer with pronounced radical character are formed. We argue that the C59N• radical stabilization of this supramonolayer phase of monomers is achieved by suppressed coupling to the substrate. This results from molecular isolation on top of the passivating azafullerene contact layer, which can be explored for molecular radical state stabilization and positioning on solid substrates.

Charge Transfer and Orbital Reconstruction at an Organic-Oxide Interface.

The two-dimensional electron system (2DES) located at the surface of strontium titanate (STO) and at several other STO-based interfaces has been an established platform for the study of novel physical phenomena since its discovery. Here we report how the interfacing of STO and tetracyanoquinodimethane (TCNQ) results in a charge transfer that depletes the number of free carriers at the STO surface, with a strong impact on its electronic structure. Our study paves the way for efficient tuning of the electronic properties, which promises novel applications in the framework of oxide/organic-based electronics.

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).

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.

Two-Dimensional Self-Assembly Driven by Intermolecular Hydrogen Bonding in Benzodi-7-azaindole Molecules on Au(111).

The control of molecular structures at the nanoscale plays a critical role in the development of materials and applications. The adsorption of a polyheteroaromatic molecule with hydrogen bond donor and acceptor sites integrated in the conjugated structure itself, namely, benzodi-7-azaindole (BDAI), has been studied on Au(111). Intermolecular hydrogen bonding determines the formation of highly organized linear structures where surface chirality, resulting from the 2D confinement of the centrosymmetric molecules, is observed. Moreover, the structural features of the BDAI molecule lead to the formation of two differentiated arrangements with extended brick-wall and herringbone packing. A comprehensive experimental study that combines scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory theoretical calculations has been performed to fully characterize the 2D hydrogen-bonded domains and the on-surface thermal stability of the physisorbed material.

Surface-Mediated Spin Locking and Thermal Unlocking in a 2D Molecular Array.

Molecule-based functional devices may take advantage of surface-mediated spin state bistability. Whereas different spin states in conventional spin crossover complexes are only accessible at temperatures well below room temperature, and the lifetimes of the high-spin state are relatively short, a different behavior exhibited by prototypical nickel phthalocyanine is shown here. Direct interaction of the organometallic complex with a copper metal electrode mediates the coexistence of a high spin and a low spin state within the 2D molecular array. The spin state bistability is extremely non-volatile, since no external stimuli are required to preserve it. It originates from the surface-induced axial displacement of the functional nickel cores, which generates two stable local minima. Spin state unlocking and the full conversion to the low spin state are only possible by a high temperature stimulus. This spin state transition is accompanied by distinct changes in the molecular electronic structure that might facilitate the state readout at room temperature, as evidenced by valence spectroscopy. The non-volatility of the high spin state up to elevated temperatures and the controllable spin bistability render the system extremely intriguing for applications in molecule-based information storage devices.

On-surface synthesis of metal-organic frameworks: the critical role of the reaction conditions.

Two different metal-organic frameworks with either a honeycomb or Kagome structure were grown on Cu(111) using para-aminophenol molecules and native surface adatoms. Although both frameworks are made up from the same chemical species, they are structurally different emphasizing the critical role being played by the reaction conditions during their growth. This work highlights the importance of the balance between thermodynamics and kinetics in the final structure of surface-supported metal-organic networks.

Conservation of Nickel Ion Single-Active Site Character in a Bottom-Up Constructed π-Conjugated Molecular Network.

On-surface chemistry holds the potential for ultimate miniaturization of functional devices. Porphyrins are promising building-blocks in exploring advanced nanoarchitecture concepts. More stable molecular materials of practical interest with improved charge transfer properties can be achieved by covalently interconnecting molecular units. On-surface synthesis allows to construct extended covalent nanostructures at interfaces not conventionally available. Here, we address the synthesis and properties of covalent molecular network composed of interconnected constituents derived from halogenated nickel tetraphenylporphyrin on Au(111). We report that the π-extended two-dimensional material exhibits dispersive electronic features. Concomitantly, the functional Ni cores retain the same single-active site character of their single-molecule counterparts. This opens new pathways when exploiting the high robustness of transition metal cores provided by bottom-up constructed covalent nanomeshes.

The influence of adsorption geometry on the reduction affinity of nitroaromatics on Au(111).

Chemoselective reduction of nitro groups in multifunctional nitroaromatics is a challenging catalytic process with high interest due to the importance of the resulting anilines for the chemical industry. Molecular-level understanding of the ways by which adsorption geometry of nitroaromatics influence their affinity toward nitro reduction will enable the development of highly selective reactions. Herein, taking advantage of the well-ordered self-assembly of para- and ortho-nitrothiophenol (p-NTP and o-NTP, respectively) monolayers on Au(111), we examined the correlation between adsorption geometry and nitro reduction affinity. The anchoring geometry of NTPs and their nitro reduction affinity were determined by conducting polarized X-ray absorption spectroscopy while the influence of NTPs' adsorption geometry on the interaction with the Au surface was analyzed by density functional theory (DFT) calculations. Exposure of surface anchored p-NTPs to reducing conditions led to their reorientation from a tilt angle of 52° to 25°, which enabled strong interactions between the π system of the molecules and the Au surface. Direct correlation was identified between the surface proximity of the nitro group, its parallel position to the surface and the resulting reduction yield. The asymmetric structure of o-NTP led to a tilted adsorption geometry in which the nitro group was rotated away from the plane of the aromatic ring and therefore was positioned parallel and in high proximity to the Au surface. This positioning led to surface-bonding that involved the oxygen atoms of o-NTP. The higher surface proximity and stronger surface interactions of the nitro group in o-NTP enabled nitro reduction already at 180 °C, while in p-NTP nitro reduction was achieved only at 230 °C, due to the longer distance between the NO2 group and the Au surface that led to weaker adsorbate-surface interactions. Thus, parallel positioning of the nitro group and high surface proximity were found as essential descriptors for nitro reduction affinity in both p-NTP and o-NTP on the Au surface. These findings provide explicit guidelines for tuning the reactant and surface properties in order to control the reactant's adsorption geometry for selective nitro reduction in multifunctional nitroaromatics.

π-Orbital mediated charge transfer channels in a monolayer Gr-NiPc heterointerface unveiled by soft X-ray electron spectroscopies and DFT calculations.

With the aim to identify charge transfer channels underlying device development and operation, X-Ray Photoelectron Spectroscopy (XPS), Near-Edge X-Ray Absorption Fine Structure (NEXAFS), and Resonant Photoelectron Spectroscopy (ResPES) have been employed to characterize a novel heterointerface obtained by the controlled evaporation of a Nickel Phthalocyanine (NiPc) monolayer on a single layer of Graphene (Gr) on SiC substrate. Indeed, the Gr-NiPc interface could be a promising candidate for different applications in the field of photonics, optoelectronics, and sensing, provided that clear information on the charge transfer mechanisms at the Gr-NiPc interface can be obtained. The analysis of the spectroscopic data has shown the effective functionalization and the horizontally-flat disposition of the NiPc complexes over the Gr layer. With this geometry, the main intermolecular interaction experienced by the NiPc species is the coupling with the Gr substrate, through π-symmetry orbitals, as revealed by the different behaviour of the valence band photoemission at resonance with the N K-edge and Ni L3-edge. These results have been supported by the analysis of density functional theory (DFT) calculations, that allowed for a rationalization of the experimental data, showing that charge transfer at the interface occurs from the doubly degenerate eg LUMO orbital, involving mainly N and C (pyrrole ring) pz states, to the holes in the p-doped graphene layer.

Ordered assembly of non-planar vanadyl-tetraphenylporphyrins on ultra-thin iron oxide.

Stabilizing ordered assemblies of molecules represents the first step towards the construction of molecular devices featuring hybrid (organic-inorganic) interfaces where molecules can be easily functionalized in view of specific applications. Molecular layers of planar metal-tetraphenylporphyrins (MTPP) grown on an ultrathin iron oxide [namely Fe(001)-p(1 × 1)O] show indeed a high degree of structural order. The generality of such a picture is tested by exploiting non-planar porphyrins, such as vanadyl-TPP (VOTPP). These molecules feature a VO2+ ion in their center, with the O atom protruding out of the plane of the porphyrin ring. In this work, by employing diffraction, photoemission and X-ray absorption, we prove that non-planar VOTPP can nevertheless form a square and ordered superstructure, where porphyrin molecules lie flat with respect to the underlying substrate. Ab initio density functional theory simulations are used to elucidate the VO bond orientation with respect to the iron substrate.

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 Obr 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 (Tiint) [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 (Ti5f). This configuration is associated with the formation of a new coordination complex, Ti-O-Ti5f, as determined by comparison with the deposition of pristine titanyl-porphyrin (TiO-TPP) molecules. The newly established Ti-O-Ti5f anchoring configuration is found to be stable also beyond the TPP flattening reaction. The anchoring of TiO-TPP to the Ti5f 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.

Adsorption of glutamic acid on clean and hydroxylated rutile TiO2(110): an XPS and NEXAFS investigation.

Due to its biocompatibility, TiO2is a relevant material for the study of bio-interfaces. Its electronic and chemical properties are influenced by defects, which mainly consist of oxygen vacancies or adsorbed OH groups and which affect, consequently, also the interaction with biological molecules. Here we report on an x-ray photoemission spectroscopy and near edge adsorption fine structure study of glutamic acid (Glu) adsorption on the rutile TiO2(110) surface, either clean or partially hydroxylated. We show that Glu anchors to the surface through a carboxylate group and that the final adsorption state is influenced by the presence of hydroxyl groups on the surface prior to Glu deposition. Indeed, molecules adsorb both in the anionic and in the zwitterionic form, the former species being favored on the hydroxylated substrate.

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.

Clarifying the Adsorption of Triphenylamine on Au(111): Filling the HOMO-LUMO Gap.

In this article, we analyze the electronic structure modifications of triphenylamine (TPA), a well-known electron donor molecule widely used in photovoltaics and optoelectronics, upon deposition on Au(111) at a monolayer coverage. A detailed study was carried out by synchrotron radiation-based photoelectron spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, scanning tunneling microscopy (STM), and ab initio calculations. We detect a new feature in the pre-edge energy region of the N K-edge NEXAFS spectrum that extends over 3 eV, which we assign to transitions involving new electronic states. According to our calculations, upon adsorption, a number of new unoccupied electronic states fill the energy region between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the free TPA molecule and give rise to the new feature in the pre-edge region of the NEXAFS spectrum. This finding highlights the occurrence of a considerable modification of the electronic structure of TPA. The appearance of new states in the HOMO-LUMO gap of TPA when adsorbed on Au(111) has crucial implications for the design of molecular nanoelectronic devices based on similar donor systems.