In situ generation of acyloxyphosphoniums for mild and efficient synthesis of thioesters.

We present a novel approach for in situ generation of acyloxyphosphoniums by premixing iodobenzene dicarboxylates and triphenylphosphine, resulting in efficient thioester synthesis (up to 100% yield). Stable solid iodobenzene dicarboxylates, achieved via carboxylate exchange, serve as hypervalent iodine precursors. The resulting acyloxyphosphoniums allow convenient one-pot thioester synthesis under mild conditions. Our method demonstrates facile acyloxyphosphonium production from iodobenzene dicarboxylates and Ph3P, enabling diverse thioester preparation. ESI-MS analysis confirms acyloxyphosphonium ion formation, pivotal in acylation. This strategy holds potential for combinatorial thioester synthesis and broader nucleophile modification applications.

Synthesis of Single-Layer Two-Dimensional Metal-Organic Frameworks M3 (HAT)2 (M=Ni, Fe, Co, HAT=1,4,5,8,9,12-hexaazatriphenylene) Using an On-Surface Reaction.

1,4,5,8,9,12-Hexaazatriphenylene (HAT) is one of the smallest polyheterocyclic aromatic building blocks for forming conjugated metal-organic frameworks (cMOFs). However, the strong inter-molecular steric hindrance impedes the growth of HAT-based cMOFs. Here we employ on-surface synthesis to grow single-layer two-dimensional cMOFs of M3 (HAT)2 (M=Ni, Fe, Co). Using scanning tunnelling microscopy and density-functional theory (DFT) analysis, we resolve that the frameworks comprise a hexagonal lattice of HAT molecules and a Kagome lattice of metal atoms. The DFT analysis indicates that Ni, Co and Fe carry a magnetic moment of 1.1, 2.5, and 3.7 µB, respectively. We anticipate that the small π-conjugated core of HAT and strong bidentate chelating coordination give rise to appealing electronic and magnetic properties.

Lung ventilation distribution in patients after traditional full sternotomy and minimally invasive thoracotomy: An observational study.

BACKGROUND: The aim of the study was to examine the post-operative ventilation distribution changes in cardiac surgical patients after traditional full sternotomy (FS) or minimally invasive thoracotomy (MIT). METHODS: A total of 40 patients scheduled for FS with two-lung ventilation or MIT with one-lung ventilation were included. Ventilation distribution was measured with electrical impedance tomography (EIT) at T1, before surgery; T2, after surgery in ICU before weaning; T3, 24 hours after extubation. EIT-based parameters were calculated to assess the ventilation distribution, including the left-to-right lung ratio, ventral-to-dorsal ratio, and the global inhomogeneity index. RESULTS: The global inhomogeneity index increased at T2 and T3 compared to T1 in all patients but only statistically significant in patients with MIT (FS, P = .06; MIT, P < .01). Notable decrease in the dorsal regions (FS) or in the non-ventilated side (MIT) was observed at T2. Ventilation distribution was partially improved at T3 but huge variations of recovery progresses were found in all patients regardless of the surgery types. Subgroup analysis indicated that operation duration was significantly lower in the MIT group (240 ± 40 in FS vs 205 ± 90 minutes in MIT, median ± interquartile range, P < .05) but the incidence of atrial fibrillation/flutter was significantly higher (5% in FS vs 50% in MIT, P < .01). Other exploratory outcomes showed no statistical differences. CONCLUSIONS: Ventilation distribution was impaired after cardiac surgery. The recovery process of ventilation homogeneity was strongly depending on individuals so that MIT was not always superior in this aspect. EIT may help to identify the patients requiring further care after surgery.

On-surface Synthesis of a Semiconducting 2D Metal-Organic Framework Cu3 (C6 O6 ) Exhibiting Dispersive Electronic Bands.

A 2D metal-organic framework (2D-MOF) was formed on a Cu(111) substrate using benzenehexol molecules. By means of a combination of scanning tunneling microscopy and spectroscopy, X-ray photoelectron spectroscopy and density-functional theory, the structure of the 2D-MOF is determined to be Cu3 (C6 O6 ), which is stabilized by O-Cu-O bonding motifs. We find that upon adsorption on Cu(111), the 2D-MOF features a semiconductor band structure with a direct band gap of 1.5 eV. The O-Cu-O bonds offer efficient charge delocalization, which gives rise to a highly dispersive conduction band with an effective mass of 0.45 me at the band bottom, implying a high electron mobility in this material.

Electron Transmission through Coordinating Atoms Embedded in Metal-Organic Nanoporous Networks.

On-surface metal-organic nanoporous networks generally refer to adatom coordinated molecular arrays, which are characterized by the presence of well-defined and regular nanopores. These periodic structures constructed using two types of components confine the surface electrons of the substrate within their nanocavities. However, the confining (or scattering) strength that individual building units exhibit is a priori unknown. Here, we study the modification of the substrate's surface electrons by the interaction with a Cu-coordinated TPyB metal-organic network formed on Cu(111) and disentangle the scattering potentials and confinement properties. By means of STM and angle-resolved photoemission spectroscopy we find almost unperturbed free-electron-like states stemming from the rather weak electron confinement that yields significant coupling between adjacent pores. Electron plane wave expansion simulations match the superlattice induced experimental electronic structure, which features replicating bands and energy renormalization effects. Notably, the electrostatic potential landscape obtained from our ab initio calculations suggests that the molecules are the dominant scattering entities while the coordination metal atoms sandwiched between them act as leaky channels. These metal atom transmission conduits facilitate and enhance the coupling among quantum dots, which are prone to be exploited to engineer the electronic structure of surface electron gases.

Kinetically Controlled Synthesis of Four- and Six-Member Cyclic Products via Sequential Aryl-Aryl Coupling on a Au(111) Surface.

We synthesize four- and six-member cyclic products via sequential multi-step aryl-aryl coupling reactions of 2,3,6,7,10,11-hexabromotriphenylene molecules on a Au(111) surface. The final products as well as the organo-gold intermediate structures are identified using scanning tunneling microscopy and density-functional theory simulation. By adjusting reaction temperature and post-annealing temperature, we enhance/suppress the yields of the four-member and six-member cyclic products. We propose an underlying mechanism which is associated with different reaction kinetics of the first-order and second-order reactions. This work exemplifies intricate kinetically-controlled on-surface synthesis when multiple reactions of different reaction order are involved.

Identifying Multinuclear Organometallic Intermediates in On-Surface [2+2] Cycloaddition Reactions.

We investigate the on-surface [2+2] cycloaddition reaction of 2,3,6,7,10,11-hexabromotriphenylene (HBTP) on Ag(111), Cu(111), Au(111), and Cu-dosed Au(111) surfaces using STM and DFT simulation focusing on the organometallic intermediates. The fully debrominated HBTP molecules form an organo-silver framework on Ag(111) and an organo-copper framework on Cu(111), both incorporating multinuclear metal adatom clusters. The organo-silver framework is converted into porous covalent networks via [2+2] cycloaddition above 240 °C. In contrast, the organo-copper framework is very stable and does not undergo [2+2] cycloaddition even at 300 °C. On Au(111), no organo-gold intermediate of [2+2] cycloaddition is observed. After loading Cu onto Au(111), the partially debrominated HBTP molecules bind to Cu adatom dimers to form multinuclear organo-copper complexes at 100 °C which undergo [2+2] cycloaddition at 140 °C. This study shows that the choice of surface can direct the reaction pathway.

Negative Differential Conductance in Polyporphyrin Oligomers with Nonlinear Backbones.

We study negative differential conductance (NDC) effects in polyporphyrin oligomers with nonlinear backbones. Using a low-temperature scanning tunneling microscope, we selectively controlled the charge transport path in single oligomer wires. We observed robust NDC when charge passed through a T-shape junction, bistable NDC when charge passed through a 90° kink and no NDC when charge passed through a 120° kink. Aided by density functional theory with nonequilibrium Green's functions simulations, we attributed this backbone-dependent NDC to bias-modulated hybridization of the electrode states with the resonant transport molecular orbital. We argue this mechanism is generic in molecular systems, which opens a new route of designing molecular NDC devices.

Stabilizing and Organizing Bi3 Cu4 and Bi7 Cu12 Nanoclusters in Two-Dimensional Metal-Organic Networks.

Multinuclear heterometallic nanoclusters with controllable stoichiometry and structure are anticipated to possess promising catalytic, magnetic, and optical properties. Heterometallic nanoclusters with precise stoichiometry of Bi3 Cu4 and Bi7 Cu12 can be stabilized in the scaffold of two-dimensional metal-organic networks on a Cu(111) surface through on-surface metallosupramolecular self-assembly processes. The atomic structures of the nanoclusters were resolved using scanning tunneling microscopy and density functional theory calculations. The nanoclusters feature highly symmetric planar hexagonal shapes and core-shell charge modulation. The clusters are arranged as triangular lattices with a periodicity that can be tuned by choosing molecules of different size. This work shows that on-surface metallosupramolecular self-assembly creates unique possibilities for the design and synthesis of multinuclear heterometallic nanoclusters.

Donor/Acceptor Properties of Aromatic Molecules in Complex Metal-Molecule Interfaces.

We present a comparative study, combining density functional theory with scanning tunneling microscopy/spectroscopy, of two aromatic molecules bonded with a variable number of Cu adatom(s) on a Cu(111) surface. The two molecules, 1,3,5-tris(pyridyl)benzene (TPyB) and 1,3,5-tris(4-radical-phenyl)benzene (TPB), possess the same aromatic backbone but bond weakly versus strongly to Cu with different terminal groups, respectively. We find that TPyB and TPB exhibit, respectively, small versus large charge transfers between the surface and the molecule; this contrast results in opposite shifts in the calculated density of states distributions and thus explains the opposite STS peak shifts observed in our experiments. The two molecules exhibit weak donor versus strong acceptor characters. This work provides a fundamental understanding, on a single-molecule level, of the principle that selecting specific functional groups can effectively and intentionally modify the molecular electronic properties in a wider class of molecule-metal interfaces.

Self-assembly of a binodal metal-organic framework exhibiting a demi-regular lattice.

Designing metal-organic frameworks with new topologies is a long-standing quest because new topologies often accompany new properties and functions. Here we report that 1,3,5-tris[4-(pyridin-4-yl)phenyl]benzene molecules coordinate with Cu atoms to form a two-dimensional framework in which Cu adatoms form a nanometer-scale demi-regular lattice. The lattice is articulated by perfectly arranged twofold and threefold pyridyl-Cu coordination motifs in a ratio of 1 : 6 and features local dodecagonal symmetry. This structure is thermodynamically robust and emerges solely when the molecular density is at a critical value. In comparison, we present three framework structures that consist of semi-regular and regular lattices of Cu atoms self-assembled out of 1,3,5-tris[4-(pyridin-4-yl)phenyl]benzene and trispyridylbenzene molecules. Thus a family of regular, semi-regular and demi-regular lattices can be achieved by Cu-pyridyl coordination.

Resonant Charge Transport in Conjugated Molecular Wires beyond 10 nm Range.

Using a scanning tunneling microscope, we measured high-bias conductance of single polyporphyrin molecular wires with lengths from 1.3 to 13 nm. We observed several remarkable transport characteristics, including multiple sharp conductance peaks, conductances as high as 20 nS in wires with lengths of >10 nm, and nearly length-independent conductance (attenuation <0.001 Å(-1)). We carried out first-principles simulations on myriad metal-molecule-metal junctions. The simulations revealed that the measured conductance is coherent resonant transport via a delocalized molecular orbital.

Surface-Activated Coupling Reactions Confined on a Surface.

Chemical reactions may take place in a pure phase of gas or liquid or at the interface of two phases (gas-solid or liquid-solid). Recently, the emerging field of "surface-confined coupling reactions" has attracted intensive attention. In this process, reactants, intermediates, and products of a coupling reaction are adsorbed on a solid-vacuum or a solid-liquid interface. The solid surface restricts all reaction steps on the interface, in other words, the reaction takes place within a lower-dimensional, for example, two-dimensional, space. Surface atoms that are fixed in the surface and adatoms that move on the surface often activate the surface-confined coupling reactions. The synergy of surface morphology and activity allow some reactions that are inefficient or prohibited in the gas or liquid phase to proceed efficiently when the reactions are confined on a surface. Over the past decade, dozens of well-known "textbook" coupling reactions have been shown to proceed as surface-confined coupling reactions. In most cases, the surface-confined coupling reactions were discovered by trial and error, and the reaction pathways are largely unknown. It is thus highly desirable to unravel the mechanisms, mechanisms of surface activation in particular, of the surface-confined coupling reactions. Because the reactions take place on surfaces, advanced surface science techniques can be applied to study the surface-confined coupling reactions. Among them, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are the two most extensively used experimental tools. The former resolves submolecular structures of individual reactants, intermediates, and products in real space, while the latter monitors the chemical states during the reactions in real time. Combination of the two methods provides unprecedented spatial and temporal information on the reaction pathways. The experimental findings are complemented by theoretical modeling. In particular, density-functional theory (DFT) transition-state calculations have been used to shed light on reaction mechanisms and to unravel the trends of different surface materials. In this Account, we discuss recent progress made in two widely studied surface-confined coupling reactions, aryl-aryl (Ullmann-type) coupling and alkyne-alkyne (Glaser-type) coupling, and focus on surface activation effects. Combined experimental and theoretical studies on the same reactions taking place on different metal surfaces have clearly demonstrated that different surfaces not only reduce the reaction barrier differently and render different reaction pathways but also control the morphology of the reaction products and, to some degree, select the reaction products. We end the Account with a list of questions to be addressed in the future. Satisfactorily answering these questions may lead to using the surface-confined coupling reactions to synthesize predefined products with high yield.

Cross-coupling of aryl-bromide and porphyrin-bromide on an Au(111) surface.

Cross-coupling is of great importance in organic synthesis. Here it is demonstrated that cross-coupling of aryl-bromide and porphyrin-bromide takes place on a Au(111) surface in vacuo. The products are oligomers consisting of porphyrin moieties linked by p-phenylene at porphyrin's meso-positions. The ratio of the cross-coupled versus homocoupled bonds can be regulated by the reactant concentrations. Kinetic Monte Carlo simulations were applied to determine the activation barrier. It is expected that this reaction can be employed in other aryl-bromide precursors for designing alternating co-polymers incorporating porphyrin and other functional moieties.

Polyphenylsilole multilayers--an insight from X-ray electron spectroscopy and density functional theory.

We present a combined investigation by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy of condensed multilayers of two polyphenylsiloles, namely hexaphenylsilole (HPS) and tetraphenylsilole (TPS). Both compounds exhibit very similar spectroscopic signatures, whose interpretation is aided by density functional theory (DFT) calculations. High-resolution XPS spectra of the Si 2p and C 1s core levels of these multilayers indicate a positively charged silicon ion flanked by two negatively charged adjacent carbon atoms in the silole core of both molecules. This result is corroborated quantitatively by DFT calculations on isolated HPS (TPS) molecules, which show a natural bond orbital partial charge of +1.67 e (+1.58 e) on the silicon and -0.34 e (-0.58 e) on the two neighbouring carbon atoms in the silole ring. These charges are conserved in direct contact with a Cu(111) substrate for films of submonolayer coverage, as evidenced by the Si 2p XPS data. The C K-edge NEXAFS spectra of HPS and TPS multilayers exhibit distinct and differing features. Their main characteristics reappear in the simulated spectra and are assigned to the different inequivalent carbon species in the molecule. The angle-dependent measurements hardly reveal any dichroism, i.e., the molecular π-systems are not uniformly oriented parallel or perpendicular with respect to the surface. Changes in the growth conditions of TPS, i.e., a reduction of the substrate temperature from 240 K to 80 K during deposition, lead to a broadening of both XPS and NEXAFS signatures, as well as an upward shift of the Si 2p and C 1s binding energies, indicative of a less ordered growth mode at low temperature.

Thermodynamic versus kinetic control in self-assembly of zero-, one-, quasi-two-, and two-dimensional metal-organic coordination structures.

Four types of metal-organic structures exhibiting specific dimensionality were studied using scanning tunneling microscopy and Monte Carlo simulations. The four structures were self-assembled out of specifically designed molecular building blocks via the same coordination motif on an Au(111) surface. We found that the four structures behaved differently in response to thermal annealing treatments: The two-dimensional structure was under thermodynamic control while the structures of lower dimension were under kinetic control. Monte Carlo simulations revealed that the self-assembly pathways of the four structures are associated with the characteristic features of their specific heat. These findings provide insights into how the dimensionality of supramolecular coordination structures affects their thermodynamic properties.

Manipulating photon emission efficiency with local electronic states in a tunneling gap.

We demonstrate manipulation of photon emission efficiency in a tunneling gap by tuning the rates of elastic and inelastic electron tunneling processes with local electronic states. The artificial local electronic states are created by a scanning tunneling microscope tip on a CuN nanoisland grown on a Cu(100) surface at cryogenic temperature. These local electronic states can either enhance or suppress the excitation of tip-induced surface plasmon modes at specific bias voltages, and thus the induced photon emission rates. A theoretical model quantitatively analyzing inelastic and elastic tunneling processes associated with characteristic electronic states shows good agreement with experiments. We also show that tip-induced photon emission measurement can be used for probing the electronic states in the tunneling gap.

Manipulation and characterization of aperiodical graphene structures created in a two-dimensional electron gas.

We demonstrate that Dirac fermions can be created and manipulated in a two-dimensional electron gas (2DEG). Using a cryogenic scanning tunneling microscope, we arranged coronene molecules one by one on a Cu(111) surface to construct artificial graphene nanoribbons with perfect zigzag (ZGNRs) or arm-chairedges and confirmed that new states localized along the edges emerge only in the ZGNRs. We further made and studied several typical defects, such as single vacancies, Stone-Wales defects, and dislocation lines, and found that all these defects introduce localized states at or near the Dirac point in the quasiparticle spectra. Our results confirm that artificial systems built on a 2DEG provide rigorous experimental verifications for several long-sought theoretical predications of aperiodic graphene structures.

A single-molecule-level mechanistic study of Pd-catalyzed and Cu-catalyzed homocoupling of aryl bromide on an Au(111) surface.

On-surface Pd- and Cu-catalyzed C-C coupling reactions between phenyl bromide functionalized porphyrin derivatives on an Au(111) surface have been investigated under ultra-high vacuum conditions by using scanning tunneling microscopy and kinetic Monte Carlo simulations. We monitored the isothermal reaction kinetics by allowing the reaction to proceed at different temperatures. We discovered that the reactions catalyzed by Pd or Cu can be described as a two-phase process that involves an initial activation followed by C-C bond formation. However, the distinctive reaction kinetics and the C-C bond-formation yield associated with the two catalysts account for the different reaction mechanisms: the initial activation phase is the rate-limiting step for the Cu-catalyzed reaction at all temperatures tested, whereas the later phase of C-C formation is the rate-limiting step for the Pd-catalyzed reaction at high temperature. Analysis of rate constants of the Pd-catalyzed reactions allowed us to determine its activation energy as (0.41±0.03) eV.

A Cu2(C6O6) metal-organic framework monolayer assembled on silicon carbide grown graphene exhibiting a metallic band structure.

We report the self-assembly of a monolayer metal-organic framework of Cu-benzenehexol (BHO) on a graphene/SiC substrate assisted by in situ Cu-catalyzed deprotonation reactions. The density functional theory calculations reveal that the free-standing framework is a semiconductor with a band gap of 0.485 eV. Interestingly, upon adsorption on the substrate, the Fermi level is up-shifted to the conduction band of the free-standing framework due to the n-doped graphene on SiC, while the other band structure features are largely preserved. The metallic nature corroborates the scanning tunneling microscopy images acquired near the Fermi level. This work demonstrates that the graphene substrate, which interacts weakly with the framework, can be used to tune the Fermi level of the metal-organic framework.