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. .

Self-Metalation of Porphyrins at the Solid-Gas Interface.

Self-metalation is a promising route to include a single metal atom in a tetrapyrrolic macrocycle in organic frameworks supported by metal surfaces. The molecule-surface interaction may provide the charge transfer and the geometric distortion of the molecular plane necessary for metal inclusion. However, at a metal surface the presence of an activation barrier can represent an obstacle that cannot be compensated by a higher substrate temperature without affecting the layer integrity. The formation of the intermediate state can be facilitated in some cases by oxygen pre-adsorption at the supporting metal surface, like in the case of 2H-TPP/Pd(100). In such cases, the activation barrier can be overcome by mild annealing, yielding the formation of desorbing products and of the metalated tetrapyrrole. We show here that the self-metalation of 2H-TPP at the Pd(100) surface can be promoted already at room temperature by the presence of an oxygen gas phase at close-to-ambient conditions via an Eley-Rideal mechanism.

Cyclopropenylidenes as Strong Carbene Anchoring Groups on Au Surfaces.

The creation of stable molecular monolayers on metallic surfaces is a fundamental challenge of surface chemistry. N-Heterocyclic carbenes (NHCs) were recently shown to form self-assembled monolayers that are significantly more stable than the traditional thiols on Au system. Here we theoretically and experimentally demonstrate that the smallest cyclic carbene, cyclopropenylidene, binds even more strongly than NHCs to Au surfaces without altering the surface structure. We deposit bis(diisopropylamino)cyclopropenylidene (BAC) on Au(111) using the molecular adduct BAC-CO2 as a precursor and determine the structure, geometry, and behavior of the surface-bound molecules through high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Our experiments are supported by density functional theory calculations of the molecular binding energy of BAC on Au(111) and its electronic structure. Our work is the first demonstration of surface modification with a stable carbene other than NHC; more broadly, it drives further exploration of various carbenes on metal surfaces.

The Environment-Dependent Behavior of the Blatter Radical at the Metal-Molecule Interface.

Stable organic radicals have potential applications for building organic spintronic devices. To fulfill this potential, the interface between organic radicals and metal electrodes must be well characterized. Here, through a combined effort that includes synthesis, scanning tunneling microscopy, X-ray spectroscopy, and single-molecule conductance measurements, we comprehensively probe the electronic interaction between gold metal electrodes and a benchtop stable radical-the Blatter radical. We find that despite its open-shell character and having a half-filled orbital close to the Fermi level, the radical is stable on a gold substrate under ultrahigh vacuum. We observe a Kondo resonance arising from the radical and spectroscopic signatures of its half-filled orbitals. By contrast, in solution-based single-molecule conductance measurements, the radical character is lost through oxidation with charge transfer occurring from the molecule to metal. Our experiments show that the stability of radical states can be very sensitive to the environment around the molecule.

A Ru-Ru pair housed in ruthenium phthalocyanine: the role of a "cage" architecture in the molecule coupling with the Ag(111) surface.

A number of studies have investigated the properties of monomeric and double-decker phthalocyanines (Pcs) adsorbed on metal surfaces, in view of applications in spintronics devices. In a combined experimental and theoretical study, we consider here a different member of the Pcs family, the (RuPc)2 dimer, whose structure is characterized by two paired up magnetic centers embedded in a double-decker architecture. For (RuPc)2 on Ag(111), we show that this architecture works as a preserving cage by shielding the Ru-Ru pair from a direct interaction with the surface atoms. In fact, while noticeable surface-to-molecule charge transfer occurs with the ensuing quenching of the molecular magnetic moment, such phenomena occur here in the absence of a direct Ru-Ag coupling or structural rearrangement, at variance with other Pcs and thanks to the above shielding effect. These unique properties of the (RuPc)2 architecture are expected to permit an easy control of the surface-to-molecule charge-transfer process as well as of the molecular magnetic properties, thus making the (RuPc)2 dimer a significant paradigm for innovative "cage" structures as well as a promising candidate for applications in spintronics nano or single-molecule devices.

Ultrafast Charge Transfer Pathways Through A Prototype Amino-Carboxylic Molecular Junction.

Charge transport properties of a vertically stacked organic heterojunction based on the amino-carboxylic (A-C) hydrogen bond coupling scheme are investigated by means of X-ray resonant photoemission and the core-hole clock method. We demonstrate that hydrogen bonding in molecular bilayers of benzoic acid/cysteamine (BA/CA) with an A-C coupling scheme opens a site selective pathway for ultrafast charge transport through the junction. Whereas charge transport from single BA layer directly coupled to the Au(111) is very fast and it is mediated by the phenyl group, the interposition of an anchoring layer of CA selectively hinders the delocalization of electrons from the BA phenyl group but opens a fast charge delocalization route through the BA orbitals close to the A-C bond. This evidences that hydrogen bonding established upon A-C recognition can be exploited to spatially/orbitally manipulate the charge transport properties of heteromolecular junctions.

Ultrafast electron injection into photo-excited organic molecules.

Charge transfer rates at metal/organic interfaces affect the efficiencies of devices for organic based electronics and photovoltaics. A quantitative study of electron transfer rates, which take place on the femtosecond timescale, is often difficult, especially since in most systems the molecular adsorption geometry is unknown. Here, we use X-ray resonant photoemission spectroscopy to measure ultrafast charge transfer rates across pyridine/Au(111) interfaces while also controlling the molecular orientation on the metal. We demonstrate that a bi-directional charge transfer across the molecule/metal interface is enabled upon creation of a core-exciton on the molecule with a rate that has a strong dependence on the molecular adsorption angle. Through density functional theory calculations, we show that the alignment of molecular levels relative to the metal Fermi level is dramatically altered when a core-hole is created on the molecule, allowing the lowest unoccupied molecular orbital to fall partially below the metal Fermi level. We also calculate charge transfer rates as a function of molecular adsorption geometry and find a trend that agrees with the experiment. These findings thus give insight into the charge transfer dynamics of a photo-excited molecule on a metal surface.

Morphological modulation of graphene-mediated hybridization in plasmonic systems.

We investigated the plasmonic response of a 2-dimensional ordered array of closely spaced (few-nm apart) Au nanoparticles covered by a large-area single-layer graphene sheet. The array consisted of coherently aligned nanoparticle chains, endowed with a characteristic uniaxial anisotropy. The joint effect of such a morphology and of the very small particle size and spacing led to a corresponding uniaxial wrinkling of graphene in the absence of detectable strain. The deposition of graphene redshifted the Au plasmon-resonance, strongly increased the optical absorption of the array and, most importantly, induced a marked optical anisotropy in the plasmonic response, absent in the pristine nanoparticle array. The experimental observations are accounted for by invoking a graphene-mediated resistive coupling between the Au nanoparticles, where the optical anisotropy arises from the wrinkling-induced anisotropic electron mobility in graphene at optical frequencies.

Ultrafast Bidirectional Charge Transport and Electron Decoherence at Molecule/Surface Interfaces: A Comparison of Gold, Graphene, and Graphene Nanoribbon Surfaces.

We investigate bidirectional femtosecond charge transfer dynamics using the core-hole clock implementation of resonant photoemission spectroscopy from 4,4'-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core-hole creation in all systems, opening an additional decay channel for the core-hole, involving electron donation from substrate to the molecule. Furthermore, using the core-hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the core-excited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface.

Length-Independent Charge Transport in Chimeric Molecular Wires.

Advanced molecular electronic components remain vital for the next generation of miniaturized integrated circuits. Thus, much research effort has been devoted to the discovery of lossless molecular wires, for which the charge transport rate or conductivity is not attenuated with length in the tunneling regime. Herein, we report the synthesis and electrochemical interrogation of DNA-like molecular wires. We determine that the rate of electron transfer through these constructs is independent of their length and propose a plausible mechanism to explain our findings. The reported approach holds relevance for the development of high-performance molecular electronic components and the fundamental study of charge transport phenomena in organic semiconductors.

Association of USF1 and APOA5 polymorphisms with familial combined hyperlipidemia in an Italian population.

BACKGROUND: Familial combined hyperlipidemia (FCH) is a polygenic and multifactorial disease characterized by a variable phenotype showing increased levels of triglycerides and/or cholesterol. The aim of this study was to identify single nucleotides (SNPs) in lipid-related genes associated with FCH. METHODS AND RESULTS: Twenty SNPs in lipid-related genes were studied in 142 control subjects and 165 FCH patients after excluding patients with mutations in the LDLR gene and patients with the E2/E2 genotype of APOE. In particular, we studied the 9996G > A (rs2073658) and 11235C > T (rs3737787) variants in the Upstream Stimulatory Factor 1 gene (USF1), and the -1131T > C (rs662799) and S19W (rs3135506) variants in the Apolipoprotein A-V gene (APOA5). We found that the frequencies of these variants differed between patients and controls and that are associated with different lipid profiles. At multivariate logistic regression SNP S19W in APOA5 remained significantly associated with FCH independently of age, sex, BMI, cholesterol and triglycerides. CONCLUSIONS: Our results show that the USF1 and APOA5 polymorphisms are associated with FCH and that the S19W SNP in the APOA5 gene is associated to the disease independently of total cholesterol, triglycerides and BMI. However, more extensive studies including other SNPs such as rs2516839 in USF1, are required.

Trimethyltin-mediated covalent gold-carbon bond formation.

We study the formation of covalent gold-carbon bonds in benzyltrimethylstannane (C10H16Sn) deposited on Au in ultra-high-vacuum conditions. Through X-ray photoemission spectroscopy and X-ray absorption measurements, we find that the molecule fragments at the Sn-benzyl bond when exposed to Au surfaces at temperatures as low as -110 °C. The resulting benzyl species is stabilized by the presence of Au(111) but only forms covalent Au-C bonds on more reactive Au surfaces like Au(110). We also present spectroscopic proof for the existence of an electronic "gateway" state localized on the Au-C bond that is responsible for its unique electronic properties. Finally, we use DFT-based nudged elastic band calculations to elucidate the crucial role played by the under-coordinated Au surface in the formation of Au-C bonds.

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.

Changes of the molecule-substrate interaction upon metal inclusion into a porphyrin.

Metal-dependent conformations: a change in the adaptation of tetraphenylporphyrins (TPPs) on Ag(111) was observed in the presence of a metal ion in the macrocycle. Upon annealing at T>575 K, 2H-TPP molecules increase the overlap of the phenyl π orbitals with the substrate, thus reducing the distance. The presence of Co creates a strong bond between Co dz(2) and the Ag sp states, leaving the porphyrin macrocycle at a larger distance to the surface.

C-reactive protein levels are associated with paraoxonase polymorphism L55M in patients undergoing cardiac SPECT imaging.

BACKGROUND: Oxidation and inflammation are linked processes playing an important role in the development and progression of coronary artery disease (CAD). The relation between oxidation and inflammation markers with myocardial ischemia is still controversial. We assessed the association between paraoxonase (PON) polymorphisms (rs854560, rs662, rs7493) and high sensitivity C-reactive protein levels with stress-induced ischemia in patients with suspected CAD. MATERIALS AND METHODS: Patients (n = 203; 78 men; mean age 59 ± 10 years), with suspected CAD underwent on the same day stress/rest Tc-99m sestamibi cardiac single-photon emission computed tomography and venous blood samples collection, to assess PON polymorphisms, lipid profile and high sensitivity C-reactive protein levels. RESULTS: At cardiac tomography, 43 (21%) patients had stress-induced myocardial ischemia and 160 (79%) did not. At logistic regression analysis, diabetes (p < 0.005), sex (p = 0.020) and high-density lipoprotein (HDL)-cholesterol levels (p < 0.050) were independently associated with stress-induced ischemia. No differences of PON1 and PON2 genotype frequencies were observed between patients with and without stress-induced ischemia. Multiple linear regression analysis showed that LL genotype for PON1 (p < 0.03), high body mass index (BMI) values (p < 0.001) and low HDL-cholesterol levels (p < 0.05) are associated with high C-reactive protein levels independently from presence of stress-induced ischemia, age, sex, diabetes, hypertension, statin therapy, smoking and total cholesterol levels. CONCLUSIONS: The results of this study identified a weak association between the M55L polymorphism in PON1 gene and C-reactive protein levels. BMI showed a major role in the determination of C-reactive protein levels. No association between PON polymorphisms and stress-induced ischemia was found.

Substrate influence for the Zn-tetraphenyl-porphyrin adsorption geometry and the interface-induced electron transfer.

In molecular devices, the importance of interfaces cannot be neglected as they determine charge injection and charge flow and, therefore, the device performance. Herein we report on the interaction of one single layer of Zn-tetraphenyl-porphyrin with Ag(110) and Si(111). Photoemission, near-edge X-ray absorption, and resonant photoemission are used to study the bonding nature, the adsorption geometry as well as the dynamics of electron transfer between the molecules and the metal or semiconductor surfaces. Molecule-substrate charge transfer is driven by the overlap with the molecular pi orbitals. In particular, the coupling of the phenyl legs with the substrate and the relative excited charge injection are dramatically different for the two surfaces considered.

Mesoscopic donor-acceptor multilayer by ultrahigh-vacuum codeposition of Zn-tetraphenyl-porphyrin and C70.

The peculiar electrochemical and photophysical properties of porphyrin and fullerene molecules make them promising candidates for the construction of two- and three-dimensional organic-based materials. An important question is how pristine fullerene and porphyrin will organize when deposited on surfaces via in vacuum molecular beam evaporation. Here we show that codeposition of C(70) and Zn-tetraphenyl-porphyrin (ZnTPP) induces the self-assembly of electron-rich flat aromatic molecules at the curved surface of C(70), thus enhancing the chromophore interaction and forming a supramolecular multilayer donor-acceptor structure. While the ground-state electronic spectra almost reflect a simple summation of ZnTPP and C(70) components, the excited-state electrons at the porphyrin macrocycle can rapidly delocalize to the fullerene. The excited charge transfer time scale is faster than 1-2 fs, as shown by resonant photoemission for the core-excited charges.

Decreased paraoxonase-2 expression in human carotids during the progression of atherosclerosis.

OBJECTIVE: Many gene products involved in oxidation and inflammation are implicated in the pathogenesis of atherosclerosis. We investigated paraoxonase 2 (PON2), 5-lipoxygenase (5-LO), and 5-LO activating protein (FLAP) expression and malondialdehyde (MDA) levels in carotid lesions to assess their involvement in plaque formation. METHODS AND RESULTS: We measured gene expression and MDA levels in atherosclerotic plaques from 59 patients undergoing carotid endarterectomy, and in plaque-adjacent tissue from 41/59 patients. Twenty-three fetal carotids and 6 mammary arteries were also investigated. Real-time polymerase chain reaction and immunohistochemistry revealed decreased PON2 expression in plaques versus adjacent regions (P<0.005, P<0.001, respectively), mammary arteries (P<0.031, P<0.001, respectively), and fetal carotids (both P<0.001). mRNA levels of 5-LO and FLAP were higher (P<0.038, P<0.005, respectively) in lesions versus fetal carotids. MDA was higher in plaques versus plaque-adjacent tissue and fetal carotids. PON2 mRNA was downregulated by oxidative stress in 5 ex vivo experiments, thereby indicating its possible atheroprotection role. CONCLUSIONS: We demonstrate that PON2 mRNA and protein are decreased in plaques versus plaque-adjacent tissue, mammary arteries, and fetal carotids. Our data indicate that the protective effect of PON2 could fail during atherosclerosis exacerbation; this was confirmed by the increase of MDA levels. The increase of 5-LO and FLAP mRNA expression confirms their role as inflammatory markers associated to atherosclerosis.

On-surface trapping of alkali atoms by crown ethers in ultra high vacuum.

Crown ethers, assembled into a regular 2D array via a chemical guest-host recognition process, have been successfully employed to trap sodium atoms on a surface, under ultra-high vacuum conditions.

Determination of the structure and geometry of N-heterocyclic carbenes on Au(111) using high-resolution spectroscopy.

N-heterocyclic carbenes (NHCs) bind very strongly to transition metals due to their unique electronic structure featuring a divalent carbon atom with a lone pair in a highly directional sp2-hybridized orbital. As such, they can be assembled into monolayers on metal surfaces that have enhanced stability compared to their thiol-based counterparts. The utility of NHCs to form such robust self-assembled monolayers (SAMs) was only recently recognized and many fundamental questions remain. Here we investigate the structure and geometry of a series of NHCs on Au(111) using high-resolution X-ray photoelectron spectroscopy and density functional theory calculations. We find that the N-substituents on the NHC ring strongly affect the molecule-metal interaction and steer the orientation of molecules in the surface layer. In contrast to previous reports, our experimental and theoretical results provide unequivocal evidence that NHCs with N-methyl substituents bind to undercoordinated adatoms to form flat-lying complexes. In these SAMs, the donor-acceptor interaction between the NHC lone pair and the undercoordinated Au adatom is primarily responsible for the strong bonding of the molecules to the surface. NHCs with bulkier N-substituents prevent the formation of such complexes by forcing the molecules into an upright orientation. Our work provides unique insights into the bonding and geometry of NHC monolayers; more generally, it charts a clear path to manipulating the interaction between NHCs and metal surfaces using traditional coordination chemistry synthetic strategies.