Moderate Charge Transfer Research Articles

Hybrid Local and Charge-Transfer Material with Ultralong Room Temperature Phosphorescence for Efficient Organic Afterglow Light-Emitting Diodes.

Organic ultralong room temperature phosphorescence (OURTP) materials capable of combining various emission behaviors for diversified optoelectronic properties and applications have recently gained a vigorous development, but it remains a forbidden challenge in designing OURTP molecules with hybrid local and charge-transfer (HLCT) feature, possibly due to the elevated difficulties in simultaneously meeting the stringent requirements of both HLCT and OURTP emitters. Here, through introducing multiple heteroatoms into one-dimensional fused ring of coumarin with moderate charge transfer perturbation in donor-π-acceptor architecture, we demonstrate a HLCT-featured OURTP molecule showing both promoted fluorescence with a quantum yield of 77 % in solution and long-lived OURTP with a lifetime of 251 ms in conventional host material used in electroluminescent device. Thus, efficient OURTP organic light-emitting diodes (OLEDs) were fabricated, exhibiting bright electroluminescence with an exciton utilization efficiency of 85 % and yellow OURTP lasting over 2 s for afterglow. Impressively, the HLCT OURTP-OLEDs can be further optimized to reach an unprecedented total external quantum efficiency (EQE) of ~12 % and OURTP EQE up to 3.11 %, representing the highest performance among the reported OURTP-OLEDs. These impressive results highlight the significance to fuse HLCT and OURTP together in enriching OURTP materials and improving the afterglow OLED performances.

Effect of Lattice Misfit on the Stability of the Misfit Layer Compound (SnS)1+xNbS2

The prototype misfit layer compound (SnS)1.17NbS2 consists alternatingly of a metallic triatomic NbS2 layer, in which Nb atoms are sandwiched by S atoms, and an insulating SnS double layer featuring a NaCl-type structure. Here we investigate the effect of lattice misfit on the stability and chemical bonding in the misfit layer compound using a first-principles density functional theory approach. The calculations show that for the (SnS)1+xNbS2 approximants, the most stable one has x = 0.167, close to the experimental observations. Charge analysis finds a moderate charge transfer from SnS to NbS2. Sn or S vacancies in the SnS part affect the electronic properties and interlayer interactions. The obtained information here helps in understanding the mechanism of formation and stability of misfit layer compounds and ferecrystals and further contributes to the design of novel multilayer compounds and emerging van der Waals heterostructures.

Effective pseudocapacitive performance of binder free transparent α-V2O5 thin film electrode: Electrochemical and some surface probing

Vanadium oxide nanomaterial was successfully prepared on conductive substrate via a simple and facile chemical route with no binder or additive. The film was characterized to investigate its microstructural, optical, electrical and electrochemical properties. The SEM micrographs of the annealed film presented enabling environment that is required for effective photocatalysis. It also demonstrated enhanced interaction between the film and the substrate and adequate pore size distribution that can allow free electrolytic ion intercalation/deintercalation for efficient supercapacitive response. EDX confirmed the elemental composition of the film. XRD and Raman spectroscopy showed substantial characteristic peaks that are attributes of orthorhombic crystal structure of V2O5 powder. Optical studies showed that the film exhibited high visible light transmittance and its energy band gap was found to be 2.77 eV. Pseudocapacitive performance of the V2O5 electrode was investigated via a three-electrode cell configuration. The film electrode exhibited highest areal capacity of 1.18 μAh cm−2, energy density of 0.485 μWhcm−2 at a power density of 107.71 μWcm−2 and current density 0.25 mAcm−2, moderate charge transfer resistance and superior cycling stability. The study demonstrated the viability of V2O5 film as electrode material for the development of high-performance supercapacitors.

Adsorption of cyanogen halides (X-CN; X = F, Cl and Br) on pristine and Fe, Mn doped C60: A highly potential gas sensor

Reliable monitoring of toxic and pollutant gases is a prime concern of the technological and scientific communities, simulated by the terrible environmental pollution crisis. Herein, we systematically investigated the gas (X-CN; X = F, Cl, Br) sensing mechanism of pristine, transition metal (Fe and Mn) doped fullerene C60 using first-principles calculations. The pure C60 surface show quite weak adsorption behavior towards X-CN gas molecules that gives rise to physisorption in these complexes, while chemisorption is observed for the Fe, Mn doped C60/X-CN configurations. Moreover, the prominent adsorption strength, moderate charge transfer and appreciable electronic conductivity demonstrates high sensitivity of metal doped C60 cage towards cyanogen halide gas molecules. The simulated electronic band structure and optical absorption spectra using HSE06 hybrid functional show strong variations after the adsorption of X-CN gases, revealing the potential capability of metal doped C60 as reliable gas sensor. Especially, the band gaps of Fe, Mn doped C60/X-CN complexes were reduced from that of pure counterpart, representing a strong signal for the detection of cyanogen halide gases. Further, the sensitivity of X-CN gases were correlated with π electron occupancy of the host materials. As we extol the remarkable sensitivity of fullerene C60, we believe that these findings may capture widespread attention towards wearable chemical sensors for future applications.

Activating the Highly Reversible Mo4+/Mo5+ Redox Couple in Amorphous Molybdenum Oxide for High-Performance Supercapacitors.

Molybdenum oxide provides high theoretical capacity, but the moderate charge transfer kinetics and cycle life have limited the applications for capacitive energy storage. Herein, the electrochemical performance of molybdenum oxide is modified by tuning the active redox couple through an electrochemical activation process. The activated electrode K-MoOx with a 10 mg cm-2 loading shows excellent pseudocapacitive behavior within the large potential range of -1.2-0 V and delivers 313 F g-1 capacitance at 5 mA cm-2. It also presents an ultrastable cycle life, with a 98% capacitance retention after 10,000 cycles. The activation process involves the insertion of K+ into MoOx, which modifies the Mo electronic structure and introduces Mo4+ sites according to X-ray photoelectron spectroscopy. As a result, the charge storage redox couple shifts from Mo5+/Mo6+ to Mo4+/Mo5+, with the latter delivering higher electrochemical activity due to improved conductivity. Electrochemical impedance spectroscopy also suggests faster ion diffusions and thus higher power capability in K-MoOx, resulting in the enhanced performance. A 2.4 V asymmetric supercapacitor is assembled using K-MoOx as the anode with a MnOx cathode. The work demonstrates a feasible and facile strategy to promote the pseudocapacitive behavior of metal oxide materials.

Structural, Magnetic and Thermoelectric Properties of hp‐Mn3Ge5

Mn3Ge5 was obtained by high‐pressure, high temperature synthesis. The compound adopts a Nowotny‐Chimney‐Ladder type crystal structure [tetragonal, space group P4n2, a = 5.7449(1) Å, c = 13.9096(4) Å, Z = 4]. Magnetic measurements reveal a ferromagnetic transition around 40 K and metallic conductivity in the temperature range from 3 K to 350 K. Despite a low thermal conductivity, the metallic character of the sample and the low Seebeck coefficient result in low values for the thermoelectric Figure of merit, ZT. Band‐structure calculations show that the Fermi level is located slightly below a pseudo‐gap in the electronic density of states. Chemical bonding analysis in position space discloses moderate charge transfer and two‐ as well as three‐atomic directed, heteroatomic bonding involving both the manganese and the germanium atoms.

Oriented External Electric Fields and Ionic Additives Elicit Catalysis and Mechanistic Crossover in Oxidative Addition Reactions.

Judiciously applied oriented external electric fields (OEEFs) exert catalytic effects on the kinetics and improve the thermodynamics of chemical reactions. Herein, we examine the ability of OEEFs to assist catalysts and show that the rate of oxidative addition between palladium catalysts and alkyl/aryl electrophiles can be controlled by an OEEF applied along the direction of electron reorganization (the "reaction axis"). The concerted mechanism of oxidative addition proceeds through a transition state with moderate charge transfer character. We demonstrate that OEEFs along the reaction axis can control this charge transfer and impart electrostatic catalysis. When the applied field exceeds a certain critical value (∼0.15 V/Å), we observed a mechanistic crossover from the concerted to a dissociative CSNAr type of reactivity for aryl electrophiles. To our surprise, alkyl electrophiles follow a hitherto unexplored SN2 pathway for the reaction with large transition state stabilization at relatively low OEEFs. A valence-bond state correlation diagram (VBSCD) is employed to comprehend the results. Finally, although the catalytic effect of salt additives in oxidative addition is known, its mechanism is still under debate. Our findings further show evidence that salt additives exert electric-field effects on the rate of cross-coupling reactions, and their cocatalytic effects can be judiciously reproduced by applied external electric fields. As such, we propose that the use of additives (anionic or cationic) is an experimentally viable strategy to implement external electric-field effects in routinely used oxidative addition catalysis.

Theoretical treatment of semiconductor heterojunctions for photocatalysis: the WO3/BiVO4 interface

The valence and conduction band (CB) alignments and the nature of the WO3/BiVO4 heterojunction have been analysed in detail based on hybrid functionals density functional theory calculations. The WO3/BiVO4 junction is widely studied in photocatalysis for its capability to reduce electron–hole recombination and to improve efficiency. This is assumed to be due to a favourable band alignment of the junction’s components, which generates a flow of negative charge carriers towards WO3, and positive ones towards BiVO4. This conclusion is often based on the properties of the two isolated, non-interacting units. Here, we propose an explicit interface model where the (0 0 1) surface of WO3 is put in contact with the (0 1 0) surface of BiVO4 rotated by about 45°, which leads to a small strain and a favourable cation-anion matching. The interface displays a moderate charge transfer and a small interface dipole. This leads to only moderate effects on the band alignment, which remains qualitatively similar to that obtained from the two independent oxides. We also considered in detail the role of the amount of exact exchange used in the description of the heterojunction, and in particular of the BiVO4 component, for which rather different hybrid functional approaches have been proposed in the literature.

Electron-phonon coupling in a honeycomb borophene grown on Al(111) surface

Recently, a honeycomb borophene was reported to grow successfully on Al(111) surface. Since the metallic $\sigma$-bonding bands of honeycomb boron sheet play a crucial role in the 39 K superconductivity of MgB$_2$, it is physically interesting to examine whether similar property exists in this material. We have calculated the electronic structures and the electron-phonon coupling for honeycomb borophene by explicitly considering the substrate effect using first-principles density functional theory in conjunction with the Wannier interpolation technique. We find that the $sp^2$-hybridized $\sigma$-bonding bands of honeycomb borophene are metallized due to moderate charge transfer from the Al substrate, similar as in MgB$_2$. However, the electron-phonon coupling in honeycomb borophene is much weaker than in MgB$_2$ due to the hardening of the bond-stretching boron phonon modes and the reduction of phonon density of states. Nevertheless, the interlayer coupling between Al-associated phonons and electrons in borophene is strong. Based on this observation, we predict that a 6.5 K superconducting transition can be observed in a free-standing borophene decorated by a single Al layer, namely monolayer AlB$_2$. Accordingly, similar superconducting transition temperature could be expected in honeycomb borophene on Al(111).

Effect of external strain on the charge transfer: Adsorption of gas molecules on monolayer GaSe

First-principles calculations were performed to investigate the interaction of small gas molecules, including H2, O2, H2O, NH3, NO, NO2, and CO, with monolayer GaSe. The energetics, charge transfer and band structures were evaluated by considering the Grimme-D2 correction. Due to the low adsorption energies and the moderate charge transfer, monolayer GaSe could be a promising candidate as a sensor for O2 and NO2. The theoretical results for adsorption of O2 on monolayer GaSe are consistent well with the most recent experimental observation. Diverse projected band structures of these gas molecule-adsorbed systems demonstrate that there exist two kinds of charge transfer mechanisms: traditional and orbital mixing theories. Based on the proposed charge transfer mechanisms, external strain exerted different influences on the charge transfer for gas molecules adsorbed on the GaSe monolayer. The present study provides theoretical insight leading to a better understanding of the novel two-dimensional materials, such as graphene, phosphorene, and transition metal dichalcogenides.

Charge transfer across transition-metal oxide interfaces: Emergent conductance and electronic structure

We perform density functional theory plus dynamical mean field theory calculations to investi- gate internal charge transfer in an artificial superlattice composed of alternating layers of vanadate and manganite perovskite and Ruddlesden-Popper structure materials. We show that the elec- tronegativity difference between vanadium and manganese causes moderate charge transfer from VO2 to MnO2 layers in both perovskite and Ruddlesden-Popper based superlattices, leading to hole doping of the VO2 layer and electron doping of the MnO2 layer. Comparison of the perovskite and Ruddlesden-Popper based heterostructures provides insights into the role of the apical oxy- gen. Our first principles simulations demonstrate that the combination of internal charge transfer and quantum confinement provided by heterostructuring is a powerful approach to engineering electronic structure and tailoring correlation effects in transition metal oxides.

Conformational control of benzophenone-sensitized charge transfer in dinucleotides

Charge transfer in DNA cannot be understood without addressing the complex conformational flexibility, which occurs on a wide range of timescales. In order to reduce this complexity four dinucleotide models 1X consisting of benzophenone linked by a phosphodiester to one of the natural nucleosides X = A, G, T, C were studied in water and methanol. The theoretical work focuses on the dynamics and electronic structure of 1G. Predominant conformations in the two solvents were obtained by molecular dynamics simulations. 1G in MeOH adopts mainly an open geometry with a distance of 12–16 Å between the two aromatic parts. In H2O the two parts of 1G form primarily a stacked conformation yielding a distance of 5–6 Å. The low-lying excited states were investigated by electronic structure theory in a QM/MM environment for representative snapshots of the trajectories. Photo-induced intramolecular charge transfer in the S1 state occurs exclusively in the stacked conformation. Ultrafast transient absorption spectroscopy with 1X reveals fast charge transfer from S1 in both solvents with varying yields. Significant charge transfer from the T1 state is only found for the nucleobases with the lowest oxidation potential: in H2O, charge transfer occurs with 3.2 × 10(9) s(-1) for 1A and 6.0 × 10(9) s(-1) for 1G. The reorganization energy remains nearly unchanged going from MeOH to the more polar H2O. The electronic coupling is rather low even for the stacked conformation with H(AB) = 3 meV and explains the moderate charge transfer rates. The solvent controls the conformational distribution and therefore gates the charge transfer due to differences in distance and stacking.

Control of the CT interaction between electron-donor and -acceptor moieties of a 1,4-dicyanonaphthalene–arene dyad for intermolecular exciplex or excimer formation in crystals

Colorless crystals of 1,4-dicyano-2-(4′-methoxybenzyloxy)methylnaphthalene ( 2), which is a 1,4-dicyano-2-methylnaphthalene (DCMN)–4-methylanisole (MA) dyad linked by an ether unit, selectively form a fluorescent intermolecular DCMN–MA exciplex (greenish blue, λ f exciplex = 456 nm ). In contrast, 1,4-dicyano-2-(4′-methylbenzyloxy)methylnaphthalene ( 3), which is a DCMN– p-xylene dyad, forms a fluorescent intermolecular DCMN–DCMN excimer in the crystalline state (blue, λ f excimer = 404 nm ). These findings demonstrate that a moderate charge transfer interaction takes place between the DCMN moiety of 2 and MA moieties of the adjacent molecules of 2, which successfully facilitates the preparation of light-emissive organic crystals.

A DFT study of methanol dissociation on isolated vanadate groups

Molecular and dissociative adsorption processes of methanol on isolated vanadate groups have been studied by means of density functional theory calculation at the B3LYP computing level. The catalyst is represented by an isolated vanadia unit in two environments: hydrated and supported. First, a OV(OH) 3 model considers hydrated conditions and second, a OV(OTiO 2H) 3 cluster accounts for the titania-supported site. The stationary points on the potential energy surface have been characterized and their geometries, relative energies and also the vibrational spectra have been obtained. In addition, the nature of chemical bonding has been highlighted by means of the analysis of the electron localization function. Our results show that methanol dissociation is a favourable process leading to methoxide groups. The molecular mechanism for dissociation on V OH or supported V O Ti sites is preferred to that involving vanadyl V O groups. Hydrogen bonding plays a key role stabilizing molecular methanol intermediates in hydrated conditions, while electrostatic acid/base interactions prevail in supported systems. Our calculated vibrational spectra confirm the experimental band assignment. An ELF bonding analysis identifies the ionic character of catalyst–methanol interactions. A moderate charge transfer of −0.34 electrons to the methoxy group would result in a blue-shift of the vanadyl band.

Experimental and theoretical characterization of the Zn—Zn bond in [Zn2(η5-C5Me5)2

The existence and characterization of a bond between the Zn atoms in the recently synthesized complex [Zn(2)(eta(5)-C(5)Me(5))(2)], as well as between Zn and ligand C atoms is firmly based on neutron diffraction and low-temperature X-ray synchrotron diffraction experiments. The multipolar analysis of the experimental electron density and its topological analysis by means of the 'Atoms in Molecules' (AIM) approach reveals details of the Zn-Zn bond, such as its open-shell intermediate character (the results are consistent with a typical metal-metal single bond), as well as many other topological properties of the compound. Experimental results are also compared with theoretical ab initio calculations of the DFT (density functional theory) and MP2 (Møller-Plesset perturbation theory) electron densities, giving a coherent view of the bonding in the complex. For instance, charges calculated from the AIM approach applied to the atomic basin of each Zn atom are, on average, +0.72 e from both the experimental and the theoretical electron density, showing a moderate charge transfer from the metal, confirmed by the calculated topological indexes.

Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights

The temperature dependence and anisotropy of optical spectral weights associated with different multiplet transitions is determined by the spin and orbital correlations. To provide a systematic basis to exploit this close relationship between magnetism and optical spectra, we present and analyze the spin-orbital superexchange models for a series of representative orbital-degenerate transition metal oxides with different multiplet structure. For each case we derive the magnetic exchange constants, which determine the spin wave dispersions, as well as the partial optical sum rules. The magnetic and optical properties of early transition metal oxides with degenerate ${t}_{2g}$ orbitals (titanates and vanadates with perovskite structure) are shown to depend only on two parameters, viz. the superexchange energy $J$ and the ratio $\ensuremath{\eta}$ of Hund's exchange to the intraorbital Coulomb interaction, and on the actual orbital state. In ${e}_{g}$ systems important corrections follow from charge transfer excitations, and we show that ${\mathrm{KCuF}}_{3}$ can be classified as a charge transfer insulator, while ${\mathrm{LaMnO}}_{3}$ is a Mott insulator with moderate charge transfer contributions. In some cases orbital fluctuations are quenched and decoupling of spin and orbital degrees of freedom with static orbital order gives satisfactory results for the optical weights. On the example of cubic vanadates we describe a case where the full quantum spin-orbital physics must be considered. Thus information on optical excitations, their energies, temperature dependence, and anisotropy, combined with the results of magnetic neutron scattering experiments, provides an important consistency test of the spin-orbital models, and indicates whether orbital and/or spin fluctuations are important in a given compound.

Coordination of Li+, Ca+, V+, and Cu+ to the Molecules S8 and S4 — A Computational Study

AbstractFor Abstract see ChemInform Abstract in Full Text.

Coordination of Li+, Ca+, V+, and Cu+ to the Molecules S8 and S4 –A Computational Study

AbstractThe complex formation between the Li+ cation and the sulfur homocycle S8 has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Starting with various isomers of S8, the formation of LiS8 heterocycles and clusters is preferred over complexes with a monodentate ligand. The binding energies of the cation in the 23 complexes investigated range from –95 to –217 kJ·mol–1. The global minimum structure of [LiS8]+ is of C4v symmetry with the S8 homocycle in the well‐known crown conformation and four Li–S bonds of length 254.2 pm (binding energy: –156.5 kJ·mol–1). The S–S bonds of the various ligands are slightly weakened by the complex formation and a more or less strong bond length alternation is induced. Relatively unstable isomers of S8 (chair, tub, exo–endo ring, branched rings, triplet chain) are partly stabilized and partly destabilized by complex formation with Li+. The interaction between the cation and the S8 ligands is mainly due to ion–dipole attraction with little to moderate charge transfer (0.04–0.27 electrostatic units). In the four most stable isomers of [LiS8]+, the number of sulfur–sulfur bonds is at a maximum and the coordination number of Li+ is either 4 or 3. Complexes of the type [Li(S4)2]+ are much less stable than isomers with an eight‐atomic ligand. The Li–S bond lengths in all of these complex cations (230–273 pm) depend on the coordination number of Li and on the atomic charge of the donating sulfur atom(s). In contrast to [LiS8]+, the complexes of composition [MS8]+ with M = Ca, V, and Cu are more stable as [M(S4)2]+ than with an eight‐atomic crown‐shaped ligand. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Investigation of Superfilling and Electrical Characteristics in Low-Impurity-Incorporated Cu Metallization

Complete filling of 0.13-µm vias and deposition of Cu with a low resistivity of about 2.3 µΩ·cm were obtained using an electrolyte with 2-aminobenzothiazole (2ABT) as the filling promoter. Due to the moderate charge transfer polarization characteristic realized by the addition of 2ABT, the film resistivity and the activation energy for thermal grain growth were similar to those for electrolytes without it. After rapid thermal annealing (RTA) at 400°C for 30 s, the resistivity was further reduced to ∼1.9 µΩ·cm.

Charge transfer in rare earth-trichloride graphite intercalation compounds

We report the results of 151Eu Mössbauer and optical reflectivity measurements on intercalation compounds formed either by the reaction of graphite with EuCl 3, or with EuCl 3: AlCl 3 mixtures, in the presence of Cl 2. Results on graphite flakes and highly-oriented-pyrolytic graphite (HOPG) indicate the presence of a single narrow Mössbauer line characteristic of Eu 3+, which we identify with intercalated EuCl 3. No evidence for Eu 2+ was found in the Mössbauer spectra. Since Eu 3+ is the most easily reduced of the rare earth ions, we conclude that no reduction of any rare earth trichlorides takes place under similar conditions of intercalation. The optical reflectance spectrum of the HOPGEuCl 3Cl 2 exhibits a sharp Drude edge at ∼ 1.2 eV consistent with metallic conduction and a moderate charge transfer. These results taken collectively suggest that the enhanced metallic conductivity in rare earth chloride intercalation compounds is associated with charge transfer from the graphite to the co-intercalated Cl 2 forming Cl - and is consistent with measured stoichiometries C n EuCl 3+δ, where δ≅0.2.