Band Structure Engineering in 2D Metal-Organic Frameworks.

The design of 2D metal-organic frameworks (2D MOFs) takes advantage of the combination of the diverse electronic properties of simple organic ligands with different transition metal (TM) centers. The strong directional nature of the coordinative bonds is the basis for the structural stability and the periodic arrangement of the TM cores in these architectures. Here, direct and clear evidence that 2D MOFs exhibit intriguing energy-dispersive electronic bands with a hybrid character and distinct magnetic properties in the metal cores, resulting from the interactions between the TM electronic levels and the organic ligand π-molecular orbitals, is reported.

Emergence of Band Structure in a Two-Dimensional Metal-Organic Framework upon Hierarchical Self-Assembly.

Two-dimensional metal-organic frameworks (2D-MOFs) represent a category of atomically thin materials that combine the structural tunability of molecular systems with the crystalline structure characteristic of solids. The strong bonding between the organic linkers and transition metal centers is expected to result in delocalized electronic states. However, it remains largely unknown how the band structure in 2D-MOFs emerges through the coupling of electronic states in the building blocks.

Stabilization 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.

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

Coordination polymers may be synthesized by linear bridging ligands to metal ions with conventional chemistry methods ( 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 synthesis approach, where the anchoring points are provided by a monolayer of metal(II)-tetraphenylporphyrin (M-TPP, M = Cu, Zn, Co) grown in on the rutile-TiO(110) surface.

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

We deposit azafullerene CN radicals in a vacuum on the Au(111) surface for layer thicknesses between 0.35 and 2.1 monolayers (ML).

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.

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.

Double Photoionization of Nitrosyl Chloride by Synchrotron Radiation in the 24-70 eV Photon Energy Range.

The behavior of nitrosyl chloride (ClNO) exposed to ionizing radiation was studied by direct probing valence-shell electrons in temporal coincidence with ions originating from the fragmentation process of the transient ClNO. Such a molecular dication was produced by double photoionization with synchrotron radiation in the 24-70 eV photon energy range. The experiment has been conducted at the Elettra Synchrotron Facility of Basovizza (Trieste, Italy) using a light beam linearly polarized with the direction of the polarization vector parallel to the ClNO molecular beam axis.

Electronic state influence on selective bond breaking of core-excited nitrosyl chloride (ClNO).

The potential for selective bond breaking of a small molecule was investigated with electron spectroscopy and electron-ion coincidence experiments on ClNO. The electron spectra were measured upon direct valence photoionization and resonant core excitation at the N 1s- and O 1s-edges, followed by the emission of resonant-Auger (RA) electrons. The RA spectra were analyzed with particular emphasis on the assignment of the participator and spectator states.

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.

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.

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)-(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).

On surface chemical reactions of free-base and titanyl porphyrins with r-TiO(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 TiO(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 O rows 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).

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.

Tailoring surface-supported water-melamine complexes by cooperative H-bonding interactions.

The water-splitting photo-catalysis by carbon nitride heterocycles has been the subject of recent theoretical investigations, revealing a proton-coupled electron transfer (PCET) reaction from the H-bonded water molecule to the CN-heterocycle. In this context, a detailed characterization of the water-catalyst binding configuration becomes mandatory in order to validate and possibly improve the theoretical modeling. To this aim, we built a well-defined surface-supported water/catalyst interface by adsorbing water under ultra-high vacuum (UHV) conditions on a monolayer of melamine grown on the Cu(111) surface.

Photoionization Dynamics of the Tetraoxo Complexes OsO and RuO.

The photoionization dynamics of OsO and RuO, chosen as model systems of small-size mononuclear heavy-metal complexes, has been theoretically studied by the time-dependent density functional theory (TDDFT). Accurate experimental measurements of photoionization dynamics as a benchmarking test for the theory are reported for the photoelectron asymmetry parameters of outer valence ionizations of OsO, measured in the 17-90 eV photon energy range. The theoretical results are in good agreement with the available experimental data.

Angular Distribution of Ion Products in the Double Photoionization of Propylene Oxide.

A photoelectron-photoion-photoion coincidence technique, using an ion imaging detector and tunable synchrotron radiation in the 18.0-37.0 eV photon energy range, inducing the ejection of molecular valence electrons, has been applied to study the double ionization of the propylene oxide, a simple prototype chiral molecule.

Investigating core-excited states of nitrosyl chloride (ClNO) and their break-up dynamics following Auger decay.

The fragmentation of ClNO upon resonant core-electron excitation to the LUMO and LUMO+1 orbitals at the N and O -edges is investigated. The produced fragment ions were detected in coincidence with a position sensitive ion time-of-flight detector which enables deduction of the angular distribution of the ions. This facilitates a comparison between the two resonances and the two -edges with respect to fragmentation time, transition dipole moment orientation, fragment yield of single-ion and ion-pair channels, and fragmentation mechanisms.

Double photoionization of propylene oxide: A coincidence study of the ejection of a pair of valence-shell electrons.

Propylene oxide, a favorite target of experimental and theoretical studies of circular dichroism, was recently discovered in interstellar space, further amplifying the attention to its role in the current debate on protobiological homochirality. In the present work, a photoelectron-photoion-photoion coincidence technique, using an ion-imaging detector and tunable synchrotron radiation in the 18.0-37.

Single Photon Thermal Ionization of C_{60}.

We report on experiments which show that C_{60} can ionize in an indirect, quasithermal boiloff process after absorption of a single photon. The process involves a large number of incoherently excited valence electrons and yields electron spectra with a Boltzmann distribution with temperatures exceeding 10^{4}  K. It is expected to be present for other molecules and clusters with a comparatively large number of valence electrons.

NEXAFS spectroscopy and site-specific fragmentation of N-methylformamide, N,N-dimethylformamide, and N,N-dimethylacetamide.

Near-edge X-ray absorption fine-structure (NEXAFS) spectra measured at the C, N, and O K-edges for three molecules containing the amide moiety, N-methylformamide (HCONHCH3), N,N-dimethylformamide (HCON(CH3)2), and N,N-dimethylacetamide (CH3CON(CH3)2) are presented. These molecules have similar structures and differ by the number of methyl groups located at the molecular ends. The fragmentation of these molecules after resonant excitation at different K-edge resonances is also investigated, using a 3D-ion imaging time-of-flight spectrometer.

A Study of H2O2 with Threshold Photoelectron Spectroscopy (TPES) and Electronic Structure Calculations: Redetermination of the First Adiabatic Ionization Energy (AIE).

In this work, hydrogen peroxide has been studied with threshold photoelectron (TPE) spectroscopy and photoelectron (PE) spectroscopy. The TPE spectrum has been recorded in the 10.0-21.

Experimental and theoretical XPS and NEXAFS studies of N-methylacetamide and N-methyltrifluoroacetamide.

Experimental Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectra of N-methyltrifluoroacetamide (FNMA), which is a peptide model system, measured at the C, N, O and F K-edges are reported. The features in the spectra have been assigned by Static-Exchange (STEX) calculations. Using the same method, we have also assigned previously measured NEXAFS spectra of another peptide model system, N-methylacetamide (NMA).

Complete dissociation branching fractions and Coulomb explosion dynamics of SO2 induced by excitation of O 1s pre-edge resonances.

Fragmentation processes of SO2 following excitation of the six main O 1s pre-edge resonances, as well as above the ionization threshold and below the resonances, are studied using a position-sensitive time-of-flight ion imaging detector, and the associated dissociation branching ratios and break-up dynamics are determined. In order to distinguish between the O(+) and S(2+) fragments of equal mass-to-charge ratio, the measurements have been performed with the isotopically enriched S(18)O2 sample. By analysis of the complete set of the fragment momentum vectors, the β values for the fragments originating from the SO(+) + O(+) break-up and the kinetic energy release for fragmentation channels of both SO2 (2+) and SO2 (3+) parent ions are determined.