Showing papers on "Molecule published in 2017"

The atom, the molecule, and the covalent organic framework

Abstract: Just over a century ago, Lewis published his seminal work on what became known as the covalent bond, which has since occupied a central role in the theory of making organic molecules. With the advent of covalent organic frameworks (COFs), the chemistry of the covalent bond was extended to two- and three-dimensional frameworks. Here, organic molecules are linked by covalent bonds to yield crystalline, porous COFs from light elements (boron, carbon, nitrogen, oxygen, and silicon) that are characterized by high architectural and chemical robustness. This discovery paved the way for carrying out chemistry on frameworks without losing their porosity or crystallinity, and in turn achieving designed properties in materials. The recent union of the covalent and the mechanical bond in the COF provides the opportunity for making woven structures that incorporate flexibility and dynamics into frameworks.

Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution

Abstract: Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton-electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ 18O isotope labelling mass spectrometry, we provide direct experimental evidence that the O2 generated during the OER on some highly active oxides can come from lattice oxygen. The oxides capable of lattice-oxygen oxidation also exhibit pH-dependent OER activity on the reversible hydrogen electrode scale, indicating non-concerted proton-electron transfers in the OER mechanism. Based on our experimental data and density functional theory calculations, we discuss mechanisms that are fundamentally different from the conventional scheme and show that increasing the covalency of metal-oxygen bonds is critical to trigger lattice-oxygen oxidation and enable non-concerted proton-electron transfers during OER.

Photophysics of thermally activated delayed fluorescence molecules

Abstract: Thermally activated delayed fluorescence (TADF) has recently emerged as one of the most attractive methods for harvesting triplet states in metal-free organic materials for application in organic light emitting diodes (OLEDs). A large number of TADF molecules have been reported in the literature with the purpose of enhancing the efficiency of OLEDs by converting non-emissive triplet states into emissive singlet states. TADF emitters are able to harvest both singlets and triplet states through fluorescence (prompt and delayed), the latter due to the thermally activated reverse intersystem crossing mechanism that allows up-conversion of low energy triplet states to the emissive singlet level. This allows otherwise pure fluorescent OLEDs to overcome their intrinsic limit of 25% internal quantum efficiency (IQE), which is imposed by the 1:3 singlet-triplet ratio arising from the recombination of charges (electrons and holes). TADF based OLEDS with IQEs close to 100% are now routinely fabricated in the green spectral region. There is also significant progress for blue emitters. However, red emitters still show relatively low efficiencies. Despite the significant progress that has been made in recent years, still significant challenges persist to achieve full understanding of the TADF mechanism and improve the stability of these materials. These questions need to be solved in order to fully implement TADF in OLEDs and expand their application to other areas. To date, TADF has been exploited mainly in the field of OLEDs, but applications in other areas, such as sensing and fluorescence microscopies, are envisaged. In this review, the photophysics of TADF molecules is discussed, summarising current methods to characterise these materials and the current understanding of the TADF mechanism in various molecular systems.

Structural and hydrodynamic properties of an intrinsically disordered region of a germ cell-specific protein on phase separation

Abstract: Membrane encapsulation is frequently used by the cell to sequester biomolecules and compartmentalize their function. Cells also concentrate molecules into phase-separated protein or protein/nucleic acid "membraneless organelles" that regulate a host of biochemical processes. Here, we use solution NMR spectroscopy to study phase-separated droplets formed from the intrinsically disordered N-terminal 236 residues of the germ-granule protein Ddx4. We show that the protein within the concentrated phase of phase-separated Ddx4, [Formula: see text], diffuses as a particle of 600-nm hydrodynamic radius dissolved in water. However, NMR spectra reveal sharp resonances with chemical shifts showing [Formula: see text] to be intrinsically disordered. Spin relaxation measurements indicate that the backbone amides of [Formula: see text] have significant mobility, explaining why high-resolution spectra are observed, but motion is reduced compared with an equivalently concentrated nonphase-separating control. Observation of a network of interchain interactions, as established by NOE spectroscopy, shows the importance of Phe and Arg interactions in driving the phase separation of Ddx4, while the salt dependence of both low- and high-concentration regions of phase diagrams establishes an important role for electrostatic interactions. The diffusion of a series of small probes and the compact but disordered 4E binding protein 2 (4E-BP2) protein in [Formula: see text] are explained by an excluded volume effect, similar to that found for globular protein solvents. No changes in structural propensities of 4E-BP2 dissolved in [Formula: see text] are observed, while changes to DNA and RNA molecules have been reported, highlighting the diverse roles that proteinaceous solvents play in dictating the properties of dissolved solutes.

A series of energetic metal pentazolate hydrates

Abstract: Metal complexes of the pentazole anion exhibit multiple coordination modes, through ionic, covalent and hydrogen-bonding interactions, and good thermal stability with onset decomposition temperatures greater than 100 °C. Polynitrogen compounds can decompose to N2 with an extraordinarily large energy release, which makes them promising candidate materials for explosives but difficult to produce in a stable form. Compounds containing five-membered all-nitrogen rings have attracted particular interest in the search for a stable polynitrogen molecule. Yuangang Xu et al. report five metal complexes containing the pentazole anion, cyclo--N5−, four of which exhibit good thermal stability and a range of different bonding interactions for stabilization. Given their energetic properties and stability, and the adaptability of the cyclo-N5− species in terms of its bonding interactions, these complexes might lead to the development of a new class of high-energy-density materials and of other unusual polynitrogen complexes. Singly or doubly bonded polynitrogen compounds can decompose to dinitrogen (N2) with an extremely large energy release. This makes them attractive as potential explosives or propellants1,2,3, but also challenging to produce in a stable form. Polynitrogen materials containing nitrogen as the only element exist in the form of high-pressure polymeric phases4,5,6, but under ambient conditions even metastability is realized only in the presence of other elements that provide stabilization. An early example is the molecule phenylpentazole, with a five-membered all-nitrogen ring, which was first reported in the 1900s7 and characterized in the 1950s8,9. Salts containing the azide anion (N3−)10,11,12 or pentazenium cation (N5+)13 are also known, with compounds containing the pentazole anion, cyclo-N5−, a more recent addition14,15,16. Very recently, a bulk material containing this species was reported17 and then used to prepare the first example of a solid-state metal–N5 complex18. Here we report the synthesis and characterization of five metal pentazolate hydrate complexes [Na(H2O)(N5)]·2H2O, [M(H2O)4(N5)2]·4H2O (M = Mn, Fe and Co) and [Mg(H2O)6(N5)2]·4H2O that, with the exception of the Co complex, exhibit good thermal stability with onset decomposition temperatures greater than 100 °C. For this series we find that the N5− ion can coordinate to the metal cation through either ionic or covalent interactions, and is stabilized through hydrogen-bonding interactions with water. Given their energetic properties and stability, pentazole–metal complexes might potentially serve as a new class of high-energy density materials19 or enable the development of such materials containing only nitrogen20,21,22,23. We also anticipate that the adaptability of the N5− ion in terms of its bonding interactions will enable the exploration of inorganic nitrogen analogues of metallocenes24 and other unusual polynitrogen complexes.

Introducing DDEC6 atomic population analysis: part 3. Comprehensive method to compute bond orders

Abstract: Developing a comprehensive method to compute bond orders is a problem that has eluded chemists since Lewis's pioneering work on chemical bonding a century ago. Here, a computationally efficient method solving this problem is introduced and demonstrated for diverse materials including elements from each chemical group and period. The method is applied to non-magnetic, collinear magnetic, and non-collinear magnetic materials with localized or delocalized bonding electrons. Examples studied include the stretched O2 molecule, 26 diatomic molecules, 3d and 5d transition metal solids, periodic materials with 1 to 8748 atoms per unit cell, a biomolecule, a hypercoordinate molecule, an electron deficient molecule, hydrogen bound systems, transition states, Lewis acid–base complexes, aromatic compounds, magnetic systems, ionic materials, dispersion bound systems, nanostructures, and other materials. From near-zero to high-order bonds were studied. Both the bond orders and the sum of bond orders for each atom are accurate across various bonding types: metallic, covalent, polar-covalent, ionic, aromatic, dative, hypercoordinate, electron deficient multi-centered, agostic, and hydrogen bonding. The method yields similar results for correlated wavefunction and density functional theory inputs and for different SZ values of a spin multiplet. The method requires only the electron and spin magnetization density distributions as input and has a computational cost scaling linearly with increasing number of atoms in the unit cell. No prior approach is as general. The method does not apply to electrides, highly time-dependent states, some extremely high-energy excited states, and nuclear reactions.

Origin of the Substitution Mechanism for the Binding of Organic Ligands on the Surface of CsPbBr3 Perovskite Nanocubes.

Abstract: Optoelectronic properties of CsPbBr3 perovskite nanocubes (NCs) depend strongly on the interaction of the organic passivating molecules with the inorganic crystal. To understand this interaction, we employed a combination of synchrotron-based X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR) spectroscopy, and first-principles density functional theory (DFT)-based calculations. Variable energy XPS elucidated the internal structure of the inorganic part in a layer-by-layer fashion, whereas NMR characterized the organic ligands. Our experimental results confirm that oleylammonium ions act as capping ligands by substituting Cs+ ions from the surface of CsPbBr3 NCs. DFT calculations shows that the substitution mechanism does not require much energy for surface reconstruction and, in contrast, stabilizes the nanocrystal by the formation of three hydrogen bonds between the −NH3+ moiety of oleylammonium and surrounding Br– on the surface of NCs. This substitution mechanism and its origin ar...

Probing the electronic and catalytic properties of a bimetallic surface with 3 nm resolution

Abstract: Tip-enhanced Raman spectroscopy in conjunction with scanning tunnelling microscopy can be used to correlate chemical properties and surface topography of bimetallic catalysts with high spatial resolution. An atomic- and molecular-level understanding of heterogeneous catalysis is required to characterize the nature of active sites and improve the rational design of catalysts1,2,3. Achieving this level of characterization requires techniques that can correlate catalytic performances to specific surface structures, so as to avoid averaging effects1. Tip-enhanced Raman spectroscopy4,5,6,7 combines scanning probe microscopy with plasmon-enhanced Raman scattering and provides simultaneous topographical and chemical information at the nano/atomic scale from ambient8,9,10 to ultrahigh-vacuum11,12 and electrochemical environments13,14. Therefore, it has been used to monitor catalytic reactions15,16,17,18 and is proposed to correlate the local structure and function of heterogeneous catalysts19. Bimetallic catalysts, such as Pd–Au, show superior performance in various catalytic reactions20,21, but it has remained challenging to correlate structure and reactivity because of their structural complexity. Here, we show that TERS can chemically and spatially probe the site-specific chemical (electronic and catalytic) and physical (plasmonic) properties of an atomically well-defined Pd(sub-monolayer)/Au(111) bimetallic model catalyst at 3 nm resolution in real space using phenyl isocyanide as a probe molecule ( Fig. 1a ). We observe a weakened N≡C bond and enhanced reactivity of phenyl isocyanide adsorbed at the Pd step edge compared with that at the Pd terrace. Density functional theory corroborates these observations by revealing a higher d-band electronic profile for the low-coordinated Pd step edge atoms. The 3 nm spatial resolution we demonstrate here is the result of an enhanced electric field and distinct electronic properties at the step edges.

Straightforward Loading of Imidazole Molecules into Metal–Organic Framework for High Proton Conduction

Abstract: A one-step straightforward strategy has been developed to incorporate free imidazole molecules into a highly stable metal-organic framework (NENU-3, ([Cu12(BTC)8(H2O)12][HPW12O40])·Guest). The resulting material Im@(NENU-3) exhibits a very high proton conductivity of 1.82 × 10-2 S cm-1 at 90% RH and 70 °C, which is significantly higher than 3.16 × 10-4 S cm-1 for Im-Cu@(NENU-3a) synthesized through a two-step approach with mainly terminal bound imidazole molecules inside pores. Single crystal structure reveals that imidazole molecules in Im-Cu@(NENU-3a) isolate lattice water molecules and then block proton transport pathway, whereas high concentration of free imidazole molecules within Im@(NENU-3) significantly facilitate successive proton-hopping pathways through formation of hydrogen bonded networks.

Pursuit of Record Breaking Energy Barriers: A Study of Magnetic Axiality in Diamide Ligated DyIII Single-Molecule Magnets

Abstract: DyIII single-ion magnets (SIMs) with strong axial donors and weak equatorial ligands are attractive model systems with which to harness the maximum magnetic anisotropy of DyIII ions. Utilizing a rigid ferrocene diamide ligand (NNTBS), a DyIII SIM, (NNTBS)DyI(THF)2, 1-Dy (NNTBS = fc(NHSitBuMe2)2, fc = 1,1′-ferrocenediyl), composed of a near linear arrangement of donor atoms, exhibits a large energy barrier to spin reversal (770.8 K) and magnetic blocking (14 K). The effects of the transverse ligands on the magnetic and electronic structure of 1-Dy were investigated through ab initio methods, eliciting significant magnetic axiality, even in the fourth Kramers doublet, thus demonstrating the potential of rigid diamide ligands in the design of new SIMs with defined magnetic axiality.

Tetrel, pnictogen and chalcogen bonds identified in the gas phase before they had names: a systematic look at non-covalent interactions

Abstract: The terms tetrel bond, pnictogen bond and chalcogen bond were coined recently to describe non-covalent interactions involving group 14, 15 and 16 atoms, respectively, acting as the electrophilic site that seeks a nucleophilic region of another molecule, for example a non-bonding electron pair or π-electron pair of a Lewis base Many complexes containing these non-covalent bonds were identified and characterised in isolation in the gas phase by rotational and vibrational spectroscopy long before they were given these names In this article, the geometries so determined for selected examples of complexes of each type are rationalised in terms of the molecular electrostatic surface potentials of the component molecules Examples of chalcogen-bonded complexes considered are based mainly on sulfur dioxide, with the region near the sulfur atom as the electrophilic site that interacts with n-electron and π-electron pairs for a range of simple Lewis base molecules For tetrel bonds, the examples discussed involve the carbon atom of carbon dioxide as the electrophilic centre, while for pnictogen bonds the central nitrogen of the closely related molecule nitrous oxide is chosen Geometrical similarities within each series allow simple definitions of each type of non-covalent bond that are conformal with that recently advanced for the halogen bond, a related non-covalent interaction

Resolving Transition Metal Chemical Space: Feature Selection for Machine Learning and Structure–Property Relationships

Abstract: Machine learning (ML) of quantum mechanical properties shows promise for accelerating chemical discovery. For transition metal chemistry where accurate calculations are computationally costly and available training data sets are small, the molecular representation becomes a critical ingredient in ML model predictive accuracy. We introduce a series of revised autocorrelation functions (RACs) that encode relationships of the heuristic atomic properties (e.g., size, connectivity, and electronegativity) on a molecular graph. We alter the starting point, scope, and nature of the quantities evaluated in standard ACs to make these RACs amenable to inorganic chemistry. On an organic molecule set, we first demonstrate superior standard AC performance to other presently available topological descriptors for ML model training, with mean unsigned errors (MUEs) for atomization energies on set-aside test molecules as low as 6 kcal/mol. For inorganic chemistry, our RACs yield 1 kcal/mol ML MUEs on set-aside test molecul...

Origin of hydrophobicity and enhanced water hydrogen bond strength near purely hydrophobic solutes

Abstract: Hydrophobicity plays an important role in numerous physicochemical processes from the process of dissolution in water to protein folding, but its origin at the fundamental level is still unclear. The classical view of hydrophobic hydration is that, in the presence of a hydrophobic solute, water forms transient microscopic “icebergs” arising from strengthened water hydrogen bonding, but there is no experimental evidence for enhanced hydrogen bonding and/or icebergs in such solutions. Here, we have used the redshifts and line shapes of the isotopically decoupled IR oxygen–deuterium (O-D) stretching mode of HDO water near small purely hydrophobic solutes (methane, ethane, krypton, and xenon) to study hydrophobicity at the most fundamental level. We present unequivocal and model-free experimental proof for the presence of strengthened water hydrogen bonds near four hydrophobic solutes, matching those in ice and clathrates. The water molecules involved in the enhanced hydrogen bonds display extensive structural ordering resembling that in clathrates. The number of ice-like hydrogen bonds is 10–15 per methane molecule. Ab initio molecular dynamics simulations have confirmed that water molecules in the vicinity of methane form stronger, more numerous, and more tetrahedrally oriented hydrogen bonds than those in bulk water and that their mobility is restricted. We show the absence of intercalating water molecules that cause the electrostatic screening (shielding) of hydrogen bonds in bulk water as the critical element for the enhanced hydrogen bonding around a hydrophobic solute. Our results confirm the classical view of hydrophobic hydration.

Porous MOF with Highly Efficient Selectivity and Chemical Conversion for CO2.

Abstract: A new Co(II)-based MOF, {[Co2(tzpa)(OH)(H2O)2]·DMF}n (1) (H3tzpa = 5-(4-(tetrazol-5-yl)phenyl)isophthalic acid), was constructed by employing a tetrazolyl-carboxyl ligand H3tzpa 1 possesses 1D tubular channels that are decorated by μ3–OH groups, uncoordinated carboxylate O atoms, and open metal centers generated by the removal of coordinated water molecules, leading to high CO2 adsorption capacity and significantly selective capture for CO2 over CH4 and CO in the temperature range of 298–333 K Moreover, 1 shows the chemical stability in acidic and basic aqueous solutions Grand canonical Monte Carlo simulations identified multiple CO2-philic sites in 1 In addition, the activated 1 as the heterogeneous Lewis and Bronsted acid bifunctional catalyst facilitates the chemical fixation of CO2 coupling with epoxides into cyclic carbonates under ambient conditions

Interface confined hydrogen evolution reaction in zero valent metal nanoparticles-intercalated molybdenum disulfide

Abstract: Interface confined reactions, which can modulate the bonding of reactants with catalytic centres and influence the rate of the mass transport from bulk solution, have emerged as a viable strategy for achieving highly stable and selective catalysis. Here we demonstrate that 1T′-enriched lithiated molybdenum disulfide is a highly powerful reducing agent, which can be exploited for the in-situ reduction of metal ions within the inner planes of lithiated molybdenum disulfide to form a zero valent metal-intercalated molybdenum disulfide. The confinement of platinum nanoparticles within the molybdenum disulfide layered structure leads to enhanced hydrogen evolution reaction activity and stability compared to catalysts dispersed on carbon support. In particular, the inner platinum surface is accessible to charged species like proton and metal ions, while blocking poisoning by larger sized pollutants or neutral molecules. This points a way forward for using bulk intercalated compounds for energy related applications. Interface confined reactions are a viable strategy for achieving stable and selective catalysts. Here, the authors demonstrate that 1T'-enriched lithiated MoS2 can reduce metal ions in situ, forming zero valent platinum nanoparticle-intercalated MoS2, with enhanced hydrogen evolution activity.

Synthesis of Layered Carbonitrides from Biotic Molecules for Photoredox Transformations

Abstract: The construction of layered covalent carbon nitride polymers based on tri-s-triazine units has been achieved by using nucleobases (adenine, guanine, cytosine, thymine and uracil) and urea to establish a two-dimensional semiconducting structure that allows band-gap engineering applications. This biomolecule-derived binary carbon nitride polymer enables the generation of energized charge carrier with light-irradiation to induce photoredox reactions for stable hydrogen production and heterogeneous organosynthesis of C-O, C-C, C-N and N-N bonds, which may enrich discussion on chemical reactions in prebiotic conditions by taking account of the photoredox function of conjugated carbonitride semiconductors that have long been considered to be stable HCN-derived organic macromolecules in space.

High-spatial-resolution mapping of catalytic reactions on single particles

Abstract: The critical role in surface reactions and heterogeneous catalysis of metal atoms with low coordination numbers, such as found at atomic steps and surface defects, is firmly established. But despite the growing availability of tools that enable detailed in situ characterization, so far it has not been possible to document this role directly. Surface properties can be mapped with high spatial resolution, and catalytic conversion can be tracked with a clear chemical signature; however, the combination of the two, which would enable high-spatial-resolution detection of reactions on catalytic surfaces, has rarely been achieved. Single-molecule fluorescence spectroscopy has been used to image and characterize single turnover sites at catalytic surfaces, but is restricted to reactions that generate highly fluorescing product molecules. Herein the chemical conversion of N-heterocyclic carbene molecules attached to catalytic particles is mapped using synchrotron-radiation-based infrared nanospectroscopy with a spatial resolution of 25 nanometres, which enabled particle regions that differ in reactivity to be distinguished. These observations demonstrate that, compared to the flat regions on top of the particles, the peripheries of the particles-which contain metal atoms with low coordination numbers-are more active in catalysing oxidation and reduction of chemically active groups in surface-anchored N-heterocyclic carbene molecules.

Single molecule magnet with an unpaired electron trapped between two lanthanide ions inside a fullerene.

Abstract: Increasing the temperature at which molecules behave as single-molecule magnets is a serious challenge in molecular magnetism. One of the ways to address this problem is to create the molecules with strongly coupled lanthanide ions. In this work, endohedral metallofullerenes Y2@C80 and Dy2@C80 are obtained in the form of air-stable benzyl monoadducts. Both feature an unpaired electron trapped between metal ions, thus forming a single-electron metal-metal bond. Giant exchange interactions between lanthanide ions and the unpaired electron result in single-molecule magnetism of Dy2@C80(CH2Ph) with a record-high 100 s blocking temperature of 18 K. All magnetic moments in Dy2@C80(CH2Ph) are parallel and couple ferromagnetically to form a single spin unit of 21 μB with a dysprosium-electron exchange constant of 32 cm−1. The barrier of the magnetization reversal of 613 K is assigned to the state in which the spin of one Dy centre is flipped. Single molecule magnets have demonstrated promise for information storage, molecular spintronics and quantum computing, but are limited by their low operational temperatures. Here, Popov and coworkers prepare a SMM with a high blocking temperature of 18 K by trapping two lanthanide ions with a single-electron bond inside a fullerene.

Interfering RNA molecules

Abstract: The present invention is related to a ribonucleic acid comprising a double stranded structure whereby the double-stranded structure comprises a first strand and a second strand, whereby the first strand comprises a first stretch of contiguous nucleotides and whereby said first stretch is at least partially complementary to a target nucleic acid, and the second strand comprises a second stretch of contiguous nucleotides whereby said second stretch is at least partially identical to a target nucleic acid, and whereby the double stranded structure is blunt ended.

Microstructure of carbon nitride affecting synergetic photocatalytic activity: Hydrogen bonds vs. structural defects

Abstract: Carbon nitride has emerged as one of the most attractive materials for developing photocatalysts with low cost, high efficiency and structural stability. However, fast charge recombination caused intrinsically by the π–π conjugated electronic system severely limits its photocatalytic performance. Constructing carbon nitride photocatalysts with modulated electronic structures is thus a promising but challenging task. In this paper, carbon nitride with different microstructural features, such as degree of polymerization, hydrogen bonds, bandgap, structural defects and ratio of C/N, were synthesized by polymerization of different types of nitrogen-rich precursors. Synergetic reactions were rationally designed for hydrogen production and the efficient and simultaneous removal of multiple contaminants, using carbon nitrides as metal-free photocatalysts. The significant impact of hydrogen bonds on synergetic photocatalysis was comprehensively demonstrated. With the smallest amount of hydrogen bonds, carbon nitride derived from urea exhibited fast charge transfer between interlayers, which is a prerequisite for superior photoactivity. By contrast, the polymerization of melamine and cyanamide was favorable for the formation of abundant hydrogen bonds and intrinsic vacancy defects. It was found that the coexistence of nitrogen deficiency and oxygen-doped microstructures could facilitate the activation of oxygen molecules, and thereby contributed to their moderate photoactivity. This research provides fundamental insights into the microstructural engineering of carbon nitride for high-performance synergetic applications.

Femtosecond response of polyatomic molecules to ultra-intense hard X-rays

Abstract: Upon exposure to ultra-intense, hard X-ray pulses, polyatomic molecules containing one heavy atom reach a much higher degree of ionization than do individual heavy atoms, contrary to previous assumptions X-ray free-electron lasers offer many new applications such as the ability to structurally probe fast biological processes This requires the use of hard and intense X-ray pulses, but the behaviour of matter under such conditions has not been fully explored Artem Rudenko et al show that when exposing small polyatomic molecules that contain one heavy atom to hard X-ray pulses with ultra-high intensities, the response is qualitatively different from what is seen in experiments carried out under less extreme conditions The observed ionization of the molecule is considerably enhanced compared to that of an individual heavy atom under the same conditions, owing to ultrafast charge transfer within the molecule that replenishes the electrons removed from the heavy atom, enabling further ionization Being able to account for this effect will aid further use of X-ray free-electron lasers for studying biological systems X-ray free-electron lasers enable the investigation of the structure and dynamics of diverse systems, including atoms, molecules, nanocrystals and single bioparticles, under extreme conditions1,2,3,4,5,6,7 Many imaging applications that target biological systems and complex materials use hard X-ray pulses with extremely high peak intensities (exceeding 1020 watts per square centimetre)3,5 However, fundamental investigations have focused mainly on the individual response of atoms and small molecules using soft X-rays with much lower intensities8,9,10,11,12,13,14,15,16,17 Studies with intense X-ray pulses have shown that irradiated atoms reach a very high degree of ionization, owing to multiphoton absorption8,12,13,18, which in a heteronuclear molecular system occurs predominantly locally on a heavy atom (provided that the absorption cross-section of the heavy atom is considerably larger than those of its neighbours) and is followed by efficient redistribution of the induced charge14,15,16,17,19,20 In serial femtosecond crystallography of biological objects—an application of X-ray free-electron lasers that greatly enhances our ability to determine protein structure2,3—the ionization of heavy atoms increases the local radiation damage that is seen in the diffraction patterns of these objects21,22 and has been suggested as a way of phasing the diffraction data23,24 On the basis of experiments using either soft or less-intense hard X-rays14,15,16,17,18,19,25, it is thought that the induced charge and associated radiation damage of atoms in polyatomic molecules can be inferred from the charge that is induced in an isolated atom under otherwise comparable irradiation conditions Here we show that the femtosecond response of small polyatomic molecules that contain one heavy atom to ultra-intense (with intensities approaching 1020 watts per square centimetre), hard (with photon energies of 83 kiloelectronvolts) X-ray pulses is qualitatively different: our experimental and modelling results establish that, under these conditions, the ionization of a molecule is considerably enhanced compared to that of an individual heavy atom with the same absorption cross-section This enhancement is driven by ultrafast charge transfer within the molecule, which refills the core holes that are created in the heavy atom, providing further targets for inner-shell ionization and resulting in the emission of more than 50 electrons during the X-ray pulse Our results demonstrate that efficient modelling of X-ray-driven processes in complex systems at ultrahigh intensities is feasible

Imaging the halogen bond in self-assembled halogenbenzenes on silver

Abstract: Halogens are among the most electronegative elements, and the variations in size and polarizability of halogens require different descriptions of the intermolecular bonds they form. Here we use the inelastic tunneling probe (itProbe) to acquire real-space imaging of intermolecular-bonding structures in the two-dimensional self-assembly of halogenbenzene molecules on a metal surface. Direct visualization is obtained for the intermolecular attraction and the “windmill” pattern of bonding among the fully halogenated molecules. Our results provide a hitherto missing understanding of the nature of the halogen bond.

Donor-Acceptor Interaction Determines the Mechanism of Photoinduced Electron Injection from Graphene Quantum Dots into TiO2: π-Stacking Supersedes Covalent Bonding.

Abstract: Interfacial electron transfer (ET) constitutes the key step in conversion of solar energy into electricity and fuels. Required for fast and efficient charge separation, strong donor–acceptor interaction is typically achieved through covalent chemical bonding and leads to fast, adiabatic ET. Focusing on interfaces of pyrene, coronene, and a graphene quantum dot (GQD) with TiO2, we demonstrate the opposite situation: covalent bonding leads to weak coupling and nonadiabatic (NA) ET, while through-space π-electron interaction produces adiabatic ET. Using real-time time-dependent density functional theory combined with NA molecular dynamics, we simulate photoinduced ET into TiO2 from flat and vertically placed molecules and GQD containing commonly used carboxylic acid linkers. Both arrangements can be achieved experimentally with GQDs and other two-dimensional materials, such as MoS2. The weak through-bond donor–acceptor coupling is attributed to the π-electron withdrawing properties of the carboxylic acid gro...

Water-Soluble and Ultrastable Ti4L6 Tetrahedron with Coordination Assembly Function

Abstract: We have successfully constructed a tetrahedral Ti4L6 cage with calixarene-like coordination-active vertices. It further features high solubility and stability in H2O and DMF/H2O solution, affording an interesting stepwise assembly function with other metal ions. Through trapping of different amounts of Co or Ln ions, the Ti4L6 tetrahedra can be organized into various dimensional architectures, including a Ti4L6-Co3 cage, a Ti4L6-Ln2 cage, a Ti4L6-Ln2 chain, and a three-dimensional Ti4L6-Ln framework. An unusual mixed-valence phenomenon was observed in the Ti4L6 cage, whose Ti3+/4+ compositions were adequately identified by electron spin resonance and X-ray photoelectron spectroscopy analyses. More remarkably, the calixarene-like oxygen vertices of the Ti4L6 cage can also be used for the recognition of C3-symmetric dye molecules through N–H···O hydrogen bonding. Accordingly, driven by visible light, selective and efficient homogeneous photodecomposition of acid blue 93 and alkali blue 4B were successfully ...

Enantiomer-Specific State Transfer of Chiral Molecules.

Abstract: State-selective enantiomeric excess is realized using microwave-driven coherent population transfer. The method selectively promotes either $R$ or $S$ molecules to a higher rotational state by phase-controlled microwave pulses that drive electric-dipole allowed rotational transitions. We demonstrate the enantiomer-specific state transfer method using enantiopure samples of 1,2-propanediol. This method of state-specific enantiomeric enrichment can be applied to a large class of asymmetric, chiral molecules that can be vaporized and cooled to the point where rotationally resolved spectroscopy is possible, including molecules that rapidly racemize. The rapid chiral switching demonstrated here allows for new approaches in high-precision spectroscopic searches for parity violation in chiral molecules.

g-C3N4 Hydrogen-Bonding Viologen for Significantly Enhanced Visible-Light Photocatalytic H2 Evolution

Abstract: Graphitic carbon nitride (g-C3N4) has recently emerged as a promising metal-free photocatalytic material for the conversion of solar energy into chemical energy under visible-light irradiation. Unfortunately, the photocatalytic activity of g-C3N4 is still unsatisfactory due to the serious recombination of photogenerated electron–hole pairs. Here, we develop a strategy to construct a type of g-C3N4-based composite photocatalyst (C3N4/CBV2+), a g-C3N4 surface coupled with a viologen redox mediator (1,1′-bis(4-carboxylatobenzyl)-4,4′-bipyridinium dichloride, denoted as CBV2+) through hydrogen bonds, for enhanced H2 production from water under visible-light irradiation. The CBV2+ molecules not only provide sites for metal particle formation but also act as an efficient electron transfer mediator to transfer the photoinduced electrons from g-C3N4 to platinum nanoparticles (Pt NPs). The vectorial charge transfer results in an efficient spatial separation of electrons and holes in the C3N4/CBV2+ composite photoc...