Search

Capturing dinitrogen at a high-valent metal centre using a cavitand-based ligand.

Defects play a critical role for the functionality and performance of materials, but the understanding of the related effects is often lacking, because the typically low concentrations of defects make them difficult to study. A prominent case is the topological defects in two-dimensional materials such as graphene. The performance of graphene-based (opto-)electronic devices depends critically on the properties of the graphene/metal interfaces at the contacting electrodes. The question of how these interface properties depend on the ubiquitous topological defects in graphene is of high practical relevance, but could not be answered so far. Here, we focus on the prototypical Stone–Wales (S–W) topological defect and combine theoretical analysis with experimental investigations of molecular model systems. We show that the embedded defects undergo enhanced bonding and electron transfer with a copper surface, compared to regular graphene. These findings are experimentally corroborated using molecular models, where azupyrene mimics the S–W defect, while its isomer pyrene represents the ideal graphene structure. Experimental interaction energies, electronic-structure analysis, and adsorption distance differences confirm the defect-controlled bonding quantitatively. Our study reveals the important role of defects for the electronic coupling at graphene/metal interfaces and suggests that topological defect engineering can be used for performance control.\ud \ud

The synthesis and characterization of a new silver iodido bismuthate with a low optical band gap highlights the decisive role of building unit connectivity for the optical properties in this class of materials.

Electrophilic anions of type [B12 X11 ]- posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B12 X11 ]- with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N2 as σ-donor-π-acceptors. Temperature-dependent formation of [B12 X11 NG]- indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B12 X11 ]- and steric effects. The binding of CO and N2 to [B12 X11 ]- is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B12 X11 CO]- and [B12 X11 N2 ]- were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N2 stretching frequencies νCO and νN2 , respectively. Observed shifts of νCO and νN2 are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π-backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B12 X11 ]- reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo-borate anions.

The additive‐free tetrazine/enol ether click reaction was performed in ultra‐high vacuum (UHV) with an enol ether group covalently linked to a silicon surface: Dimethyl 1,2,4,5‐tetrazine‐3,6‐dicarboxylate molecules were coupled to the enol ether group of a functionalized cyclooctyne which was adsorbed on the silicon (001) surface via the strained triple bond of cyclooctyne. The reaction was observed at a substrate temperature of 380 K by means of X‐ray photoelectron spectroscopy (XPS). A moderate energy barrier was deduced for this click reaction in vacuum by means of density functional theory based calculations, in good agreement with the experimental results. This UHV‐compatible click reaction thus opens a new, flexible route for synthesizing covalently bound organic architectures., Catalyst‐free coupling of a tetrazine derivative and an enol ether group, the latter being covalently attached on a Si(001) surface via cyclooctyne, has been observed in ultra‐high vacuum by X‐ray photoelectron spectroscopy. DFT calculations identified the associated reaction meachanism. This click chemistry scheme allows the synthesis of covalently bound molecular architectures on highly reactive surfaces in a UHV environment.

Iodido metalates of heavy main group elements have seen much research interest in the last years due to their possible application as absorbers in photovoltaics. However, for materials based on the non-toxic element bismuth one challenge lies in narrowing the optical band gap for sufficient solar absorption. Here, we present a new iodido silver bismuthate, [SMe

In this density functional theory study, the influence of the bonded dimer vacancy (DV) on the reactivity of the Si(001) surface is investigated. To this end, electronic and structural properties o...

The reaction of methyl enol ether functionalized cyclooctyne on the silicon (001) surface was investigated by means of X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Three different groups of final states were identified; all of them bind on Si(001) via the strained triple bond of cyclooctyne but they differ in the configuration of the methyl enol ether group. The majority of molecules adsorbs without additional reaction of the enol ether group; the relative contribution of this configuration to the total coverage depends on substrate temperature and coverage. Further configurations include enol ether groups which reacted on the silicon surface either via ether cleavage or enol ether groups which transformed on the surface into a carbonyl group., Binding via strained bond: Methyl enol ether functionalized cylooctyne is shown to adsorb on Si(001) preferentially via the strained triple bond of cyclooctyne. It can thus serve as an interface between the silicon surface and organic architectures which are coupled via the intact enol ether group. Side reactions include ether cleavage and the formation of an aldehyde group as observed by XPS and DFT.

An approach for the possible production of novel bimetallic self-doped porphyrin-based compounds of potential interest in material science is reported. Heating Cu(II)tetraphenylporphyrin (TPPCu) with chromocene at 120°C in benzonitrile affords the crystalline multimetal porphyrin system TPPCu/TPPCr in good yield. The X-ray single crystal structural analysis reveals a random distribution of TPPCu and TPPCr, with a Cu:Cr ratio of 71(2):29(2)%. Exploratory DFT calculations of TPPCu/TPPCr indicate little if any electron transfer. In contrast, calculations of a hypothetical cationic TPPCu/TPPRu system indicates the possibility of self-doping.

Halogenido metalates of heavy main group elements are versatile semiconductor materials with broad applications. Especially the iodido metalates generally show small optical band gaps, making them suitable for photovoltaics. However, the most promising results have been generated using toxic lead-based materials, raising environmental concerns, while the related and far less toxic bismuth compounds show band gaps too large for direct use in photovoltaics. The introduction of heterometals such as copper can significantly lower the band gap of iodido bismuthates and antimonates, but no clear trend could yet be established in this regard. In this work a short overview over all known copper iodido bismuthates and antimonates is given and this small family of compounds is expanded with nine charged as well as neutral complexes [EkMlIm(P(R)3)n]q− (E = Sb, Bi; M = Cu, Ag; R = Ph, o-tol). The compounds’ crystal structures, stability and optical properties are investigated and compared to the findings of quantum chemical investigations. The main excitation is shown to be a copper to antimony or copper to bismuth charge transfer while the relative energetic position of the organic ligand orbitals influences the magnitude of the band gap. This reveals that the nature of the ligands and the coordination environment at the copper atom is crucial for designing new copper iodido antimonates and bismuthates with specific band gaps.

The Sulfur (VI) Fluoride Exchange (SuFEx) reaction is an emerging scheme for connecting molecular building blocks. Due to its broad functional group tolerance and rather stable resulting linkage it is seeing rapid adoption in various fields of chemistry. Still, to date the reaction mechanism is poorly understood which hampers further development. Here, we show that the mechanism of the SuFEx reaction for the prototypical example of methanesulfonyl fluoride reacting with methylamine can be understood as an SN2-type reaction. By analyzing the reaction path with the help of density functional theory in vacuo and under consideration of solvent and co-reactant influence we identify the often used complementary base as crucial ingredient to lower the reaction barrier significantly by increasing the nucleophilicity of the primary amine. With the help of energy decomposition analysis (EDA) at the transition state structures we quantify the underlying stereo-electronic effects and propose new avenues for experimental exploration of the potential of SuFEx chemistry.

The reaction of pyrazine with the Ge(100)-2×1 surface has been investigated by X-ray photoelectron spectroscopy (XPS) and density functional theory calculations. Results show that pyrazine reacts w...

Corroles on metal surfaces show substantial reactivity and aromaticity-driven interfacial electron transfer of their transient σ/π-radicals. These effects are much more pronounced than for the clos...

The reaction of a methyl-substituted benzylazide on the silicon (001) surface was investigated by means of X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density f...

On-surface synthesis has become a thriving topic in surface science. The Ullmann coupling reaction is the most applied synthetic route today, but the nature of the organometallic intermediate is still under discussion. We investigate the bonding nature of prototypical intermediate species (phenyl, naphthyl, anthracenyl, phenanthryl, and triphenylenyl) on the Cu(111) surface with a combination of plane wave and atomic orbital basis set methods using density functional theory calculations with periodic boundary conditions. The surface bonding is shown to be of covalent nature with a polarized shared-electron bond supported by π-back donation effects using energy decomposition analysis for extended systems (pEDA). The bond angle of the intermediates is determined by balancing dispersion attraction and Pauli repulsion between adsorbate and surface. The latter can be significantly reduced by adatoms on the surface. We furthermore investigate how to choose computational parameters for pEDA of organic adsorbates on metal surfaces efficiently and show that bonding interpretation requires consistent choice of the density functional.

The modification of the nature and size of bandgaps for III-V semiconductors is of strong interest for optoelectronic applications. Strain can be used to systematically tune the bandgap over a wide range of values and induce indirect-to-direct transition (IDT), direct-to-indirect transition (DIT), and other changes in bandgap nature. Here, we establish a predictive first-principles approach, based on density functional theory, to analyze the effect of uniaxial, biaxial, and isotropic strain on the bandgap. We show that systematic variation is possible. For GaAs, DITs are observed at 1.56% isotropic compressive strain and 3.52% biaxial tensile strain, while for GaP an IDT is found at 2.63% isotropic tensile strain. We additionally propose a strategy for the realization of direct-to-indirect transition by combining biaxial strain with uniaxial strain. Further transition points are identified for strained GaSb, InP, InAs, and InSb and compared to the elemental semiconductor silicon. Our analyses thus provide a systematic and predictive approach to strain-induced bandgap tuning in binary III-V semiconductors.

With the aim to mimic the yet unknown covalent deposition of metal chalcogenide clusters on transition metal dichalcogenide (TMDC) MoS2 or WS2 layers, and thereby explore the interaction between the two systems and potential consequences on physical properties of the TMDC material, we synthesized heterobimetallic model systems. The heterocubane-type cluster [(SnCl3)(WCp)3S4] (1), the organotin-sulfidomolybdate cluster [{(PhSn)3SnS6}{(MoCp)3S4}] (2), and the corresponding tungstate [(PhSn)3SnS6{(WCp)3S4}] (3) were obtained in ligand exchange reactions from [(PhSn)4S6] and [M(CO)3CpCl]. Indeed, the {M3S4} cages in 1-3 resemble a section of the respective TMDC monolayers, altogether representing minimal molecular model systems for the adsorption of organotin sulfide clusters on MoS2 or WS2. The interaction between the {(MCp)3S4} and {(PhSn)3SnS6} subunits is characterized by multicenter bonding, rendering the respective Sn atom as Sn(II), hence driving the clusters into a mixed-valence Sn(IV)/Sn(II) situation, and the M atoms as M(IV) upon an in situ redox process. The attachment is thus weaker than via regular covalent M-S bonds, but definitely stronger than via van der Waals interactions that have been characteristic for all known interactions of clusters on TMDC surfaces so far. Computational analyses of an extended model mimicking the electronic situation in the cluster prove the analogy to a covalent attachment on TMDCs.

A crucial factor determining charge transport in organic semiconductors is the electronic coupling between the molecular constituents, which is heavily influenced by the relative arrangement of the molecules. This renders quinacridone, with its multiple, structurally fundamentally different polymorphs and their diverse intermolecular interactions an ideal test case for analyzing the correlation between the electronic coupling in a specific configuration and the configuration's energetic stability. To provide an in-depth analysis of this correlation, starting from the $\alpha$-polymorph of quinacridone, we also construct a coplanar model crystal. This allows us to systematically compare the displacement-dependence of the electronic coupling with that of the total energy. In this way, we identify the combination of Pauli repulsion and orbital rehybridization as the driving force steering the system towards a structure in which the electronic coupling is minimal (especially for the valence band and at small displacements). The general nature of these observations is supported by equivalent trends for an analogous pentacene model system. This underlines that the design of high-performance materials cannot rely on the "natural" assembly of the $\pi$-conjugated backbones of organic semiconductors into their most stable configurations. Rather, it must include the incorporation of functional groups that steer crystal packing towards more favorable structures, where aiming for short-axis displacements or realizing comparably large long-axis displacements appear as strategies worthwhile exploring., Comment: This article has been removed by arXiv administrators because the submitter did not have the rights to agree to the license at the time of submission

The reactivity and bonding of an ethinyl-functionalized cyclooctyne on Si(001) is studied by means of density functional theory. This system is promising for the organic functionalization of semiconductors. Singly bonded adsorption structures are obtained by [2 + 2] cycloaddition reactions of the cyclooctyne or ethinyl group with the Si(001) surface. A thermodynamic preference for adsorption with the cyclooctyne group in the on-top position is found and traced back to minimal structural deformation of the adsorbate and surface with the help of energy decomposition analysis for extended systems (pEDA). Starting from singly bonded structures, a plethora of reaction paths describing conformer changes and consecutive reactions with the surface are discussed. Strongly exothermic and exergonic reactions to doubly bonded structures are presented, while small reaction barriers highlight the high reactivity of the studied organic molecule on the Si(001) surface. Dynamic aspects of the competitive bonding of the functional groups are addressed by ab initio molecular dynamics calculations. Several trajectories for the doubly bonded structures are obtained in agreement with calculations using the nudged elastic band approach. However, our findings disagree with the experimental observations of selective adsorption by the cyclooctyne moiety, which is critically discussed.

Electrophilic anions of type [B12X11]− posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B12X11]− with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ‐donors and (ii) CO/N2 as σ‐donor‐π‐acceptors. Temperature‐dependent formation of [B12X11NG]− indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B12X11]− and steric effects. The binding of CO and N2 to [B12X11]− is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B12X11CO]− and [B12X11N2]− were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N2 stretching frequencies νCO and νN2 , respectively. Observed shifts of νCO and νN2 are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π‐backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B12X11]− reactivety dependent on the substituent X and provides first systematic data on π‐backdonation from delocalized σ‐electron systems of closo‐borate anions., A new type of π‐backbonding is discussed, which was observed between the π‐acceptors CO/N2 and the closo‐dodecaborate fragments [B12X11]− (X=F, Cl, Br, I, CN). The highly reactive [B12X11]− anions transfer electron density from their delocalized σ‐electron systems into antibonding orbitals of the respective acceptor molecules. Moreover, the comprehensive set of data provided with this study gives insights into the reactivity of [B12X11]− anions depending on the substituent X.

In this density functional theory study, the influence of the dimer vacancy on the reactivity of the Si(001) surface is investigated. To this end, electronic and structural properties of the defect are analyzed. Band structure calculations reveal a higher-lying valence band which would suggest increased reactivity. However, the opposite is found when organic molecules for interface formation (acetylene, ethylene and cyclooctyne) are adsorbed at the defect. Significant reaction barriers have to be overcome in order to form bonds with defect atoms, while adsorption on the pristine surface is mostly direct. This suggests the presence of a, rather weak, Si-Si bond across the defect which must be dissociated before organic adsorbates can react. A rich adsorption and reaction network is found in addition to the structures known from the pristine surface. All three investigated adsorbates show different bonding characteristics. For acetylene and ethylene, the preferred thermodynamic sink is the insertion into the defect, with the latter molecule even dissociating. Bulky cyclooctyne on the other hand avoids reaction with the defect due to steric demands imposed by the small defect cavity. The DV has no effect on reactivity of neighboring dimers. A combination of defect creation and hydrogen-precoverage could be a promising approach for selective surface functionalization. We thus show the influence of a non-ideal surface on organic functionalization and interface build-up reactions for a prototypical interface.

Previous reports in the literature describe that the crystallization of hexaphenyl carbodiphosphorane (CDPPh) from a variety of solvents gives a “bent” geometry for the P–C–P moiety as the solid-st...

Heating Cu(II)tetraphenylporphyrin (TPPCu) with chromocene at 120 °C affords the crystalline self-doped multimetal porphyrin system TPPCu/TPPCr in good yield. The X-ray structural analysis reveals a random configuration of TPPCu and TPPCr with a Cu:Cr ratio of about 71:29 %. Exploratory DFT calculations indicate significant electron transfer in a hypothetical cationic TPPCu/TPPRu system, in contrast to TPPCu/TPPCr.

Disclosed herein is a visible light mediated cyclization of methyl (α-naphthyl) acrylates and heteroaromatic analogues yielding substituted acenaphthenes and azaacenaphthenes. This highly functional-group-tolerant transformation was put to the test in an enantioselective formal synthesis of delavatine A. Mechanistic details were elucidated by DFT-calculations revealing an unusual intramolecular H-transfer mediated by a primary amine. The generality of this transformation enables a novel synthetic strategy of five membered ring annulation at an advanced stage, allowing reliance upon naphthalene chemistry up to the point of acenaphthene construction.

Herein, we present a series of isomerically pure, peripherally alkyl substituted, soluble and low aggregating azaphthalocyanines as well as their new, smaller hybrid homologues, azasubphthalocyanines. The focus lies on the effect of the systematically increasing number of aza building blocks [-N[double bond, length as m-dash]] replacing the non-peripheral [-CH[double bond, length as m-dash]] units and their influence on the physical and photophysical properties of these chromophores. The absolute and relative HOMO-LUMO energies of azaphthalocyanines were analyzed using UV-Vis and CV and compared to the density functional theory calculations (B3LYP, TD-DFT). The lowering of the HOMO level is revealed as the determining factor for the trend in the adsorption energies by electronic structure analysis. Crystals of substituted subphthalocyanines, N2-Pc*H2 and N4-[Pc*Zn·H2O], were obtained out of DCM. For the synthesis of the valuable tetramethyltetralin phthalocyanine building block a new highly efficient synthesis involving a nearly quantitative CoII catalyzed aerobic autoxidation step is introduced replacing inefficient KMnO4/pyridine as the oxidant.

Surprising adsorption dynamics in terms of an either direct or pseudodirect pathway in contrast to common precursor-mediated surface reactions is shown for the title system by theoretical investigations. We present a computational protocol for the description of the adsorption dynamics in this complex system that is transferable to other large molecules. The system size prohibits the use of established accurate methods, and approximations have to be made and validated. Our approach combines potential energy surface scans, reaction path determination methods, statistical thermodynamics, and ab initio molecular dynamics simulations based on density functional theory (DFT). This leads to a conclusive picture of adsorption dynamics in the limits of DFT accuracy and shows how a thoughtful selection of methods can comprehensively describe the adsorption dynamics of an experimentally relevant system that might seem too complex for ab initio approaches at first glance.

Wir zeigen mit quantenchemischen Methoden, dass sich die Adsorption von Ether-Molekulen auf Si(001) im Ultrahochvakuum durch klassische Konzepte der organischen Molekulchemie verstehen lasst. Wie unsere detaillierte Analyse zeigt, spiegelt der zweistufige Mechanismus – 1) Ausbildung einer dativen Bindung zwischen dem Sauerstoffatom am Ether und einem Lewis-sauren Oberflachenatom, 2) nukleophiler Angriff eines nahegelegenen Lewis-basischen Oberflachenatoms – die saurekatalysierte Etherspaltung in Losung wider. Zudem ist die dative O-Si-Bindung die starkste ihrer Art und die Reaktivitat in Schritt 2 verlauft entgegen dem Bell-Evans-Polanyi-Prinzip. Mithilfe einer neuartigen Methode der Bindungsanalyse wird auserdem die Umverteilung von Elektronendichte wahrend des C-O-Bindungsbruchs visualisiert. Hierbei wird deutlich, dass der Mechanismus nukleophiler Substitutionen auf Halbleiteroberflachen identisch zu molekularen SN2-Reaktionen ist. Unsere Untersuchungen verdeutlichen, wie die Forschungsfelder der Oberflachenwissenschaften und Molekulchemie voneinander profitieren konnen und so unerwartete Einblicke ermoglichen.

Using computational chemistry, we show that the adsorption of ether molecules on Si(001) under ultra-high vacuum conditions can be understood with textbook organic chemistry. The two-step reaction mechanism of (1) dative bond formation between the ether oxygen and a Lewis acidic surface atom and (2) a nucleophilic attack of a nearby Lewis basic surface atom is analysed in detail and found to mirror the acid-catalysed ether cleavage in solution. The O-Si dative bond is found to be the strongest of its kind and reactivity from this state defies the Bell-Evans-Polanyi principle. Electron rearrangement during the C-O bond cleavage is visualized using a newly developed bonding analysis method, which shows that the mechanism of nucleophilic substitutions on semiconductor surfaces is identical to molecular chemistry SN2 reactions. Our findings thus illustrate how the fields of surface science and molecular chemistry can mutually benefit and unexpected insight can be gained.