Tuning the magnetic and electronic properties of polynuclear Prussian-blue-like complexes: the role of the organic ligand

Mixed valence cyanide bridged polynuclear complexes of formula [Fe^II(CNFe^IIIL)₆]^x+, where L is a pentadentate ligand, may be considered as structural models for Prussian Blue. A judicious choice of the peripheral ligand allows us to tune their electronic and magnetic properties. A correlation between the nature of the peripheral ligand and the energy of the intervalence band is found. The acceptor power of the ligand shifts the intervalence band towards low energies and leads to an increase of the magnitude of the ferromagnetic interaction. Thus, the charge-transfer excited state is responsible for the ferromagnetic nature of the exchange coupling interaction in Prussian Blue. To cite this article: G. Rogez et al., C. R. Chimie 6 (2003).     

The band structure of one-dimensional (tetrazaporphyrinato)cobalt(II). A semi-empirical self-consistent field crystal orbital analysis

The band structure of (tetraporphyrinato)cobalt(II) (2) has been investigated by means of a semi-empirical INDO (intermediate neglect of differential overlap) crystal orbital approach. The simplest stoichiometric unit of the title compound contains an uneven number of electrons. We have considered metallic and insulating (Mott) electronic distributions in the “half-filled” band. The Hartree-Fock energies for two spatial symmetries have been determined: (i) singly occupied Co 3d_z² band (a_1g) and (ii) singly occupied ligand-centered (a_1u) linear combination. The mean-field energies of insulating “Mott” states have been calculated by means of a grand canonical (GC) averaging procedure. The electronic ground state is of spatial A_1g symmetry (“half-filled” Co 3d_z² dispersion) and corresponds to an insulating band occupation. The calculated width of the a_1g dispersion amounts to 0.13 eV. It is argued that the band structure results of 2 are in qualitative agreement with available experimental results derived for (phthalocyaninato(cobalt(II).     

Optical constants of electrochemically synthesized polyindole and poly(5-carboxilic acid indole)

The optical properties and optical constants of the polyindole and poly(5-carboxilic acid indole) conductive polymers synthesized and doped electrochemically with ClO ₄ ^? in acetonitrile solution were investigated by means of transmittance and reflectance spectra, in the wavelength range of 300–800 nm. Absorption band centered at 425 nm assigned to the direct allowed electron transition (π → π*) from valence band to the conduction band. The optical band gap, E _g , was determined out of the optical absorption spectra. The E _g increases from 2.17 eV for polyindole film to 2.40 eV for poly(5-carboxilic acid indole) polymer thin film, which is attributed to the effect of electron withdrawing carboxylic acid functional group on the growth of chain length of the polymer during the electropolymerization. The oscillator energy E ₀, dispersion energy E _d and other parameters were determined by the Wemple-DiDomenico method.     

Energetics and dynamics of the interface of RuS2 and implications for photoelectrolysis of water

RuS₂ (E_G=1.3 eV), grown from liquid Bi, has proved to be a stable and fairly efficient photoanode for a potential assisted photoelectrolysis of water using visible and near infrared light. Its valence band has a quite pure d-character. Holes generated on d-states in the valence band lead to the formation of Ru-based surface states which induce interfacial coordination bonding. The average positive charge accumulated in these complexes determines the energetic position of the energy bands with the consequence that the band edges are shifted with application of an electrode potential until their position becomes stabilized by electron injection from the electrolyte. This conclusion is derived from capacity measurements performed in the region of light intensity limited and diffusion controlled anodic photocurrents (rotating disk experiments). The energetic limitation of RuS₂ photoelectrodes has mainly to be seen in the position of the surface states which are formed too high above the edge of the valence band. For the present this compound appears to be a very attractive model system for research on low photon energy photooxidation of water.     

Isotope effects—I: Hydrogen isotope effects in acetylacetone and its enol

Aspects of the effects of deuteriation on the properties of acetylacetone (pentan-2,4-dione) and its enol have been reinvestigated and new isotope effects have been sought in order to understand better the nature of the hydrogen bridge in the enol (1E). There is no substantial hydrogen isotope effect on the enthalpy of enolisation of 1 (contrary to Ref. 5) but there are unusually large isotope effects on v_max (contrary to Ref. 5) and on the band shape of the long wavelength ( ~ 37,000 cm^?1) UV absorption and on the ¹³C NMR chemical shift of the Cl(3) atoms in 1E. It is concluded that these and other properties of the enol 1E can be qualitatively explained if the hydrogen bridge in 1E is not quite symmetrical but has two symmetrically placed potential energy minima close together for the hydrogen atom leading to a lengthening of the O?O distance on deuteriation.     

Redox system for perfluoroalkylation of arenes and α-methylstyrene derivatives using titanium oxide as photocatalyst

Photoirradiation of titanium oxide (TiO₂) excites the electrons from the valence band to the conduction band, leaving holes in the valence band. Using these holes and electrons, it is possible to perform one-electron oxidations and reductions. We developed a method for the photocatalytic perfluoroalkylation of aromatic rings such as benzene and its derivatives, naphthalene and benzofuran with perfluoroalkyl iodide by the combination of reduction and oxidation reactions with TiO₂. Perfluoroalkyl iodide was reduced to a perfluoroalkyl radical by the excited electrons in the conduction band of TiO₂, and the resulting radical reacted with an aromatic ring to form an arenium radical that was successively oxidized to a cation by the holes in the valence band of TiO₂. Similarly, the photocatalytic reaction of α-methylstyrene with perfluoroalkyl iodide afforded perfluoroalkylated α-methylstyrene, in which the perfluoroalkyl group is on a methyl carbon.     

The band structure of Ni(H5C3B2). An example for energetic stabilization due to dimerization

The band structure of Ni(H₅C₃B₂) the first experimentally verified one-dimensional (1D) polydecker system, has been investigated by means of a semi-empirical crystal orbital (CO) formalism based on an improved INDO (intermediate neglect of differential overlap) hamiltonian. In order to allow for an analysis of the electronic structure of a 1D arrangement composed by unit cells with an ungerade number of electrons a grand canonical (GC) averaging scheme has been used for the definition of the crystal hamiltonian. The synthesized 1D material with 13 valence electrons per simplest stoichiometric unit is a semiconductor and shows nuclear distortions into the direction of the stacking axis (i.e. formation of non-equivalent metal-ligand contacts). The tight-binding calculations lead to a transparent theoretical explanation of this formal dimerization. The 1D arrangement built up by one Ni(H₅C₃B₂) half-sandwich per unit cell reproducing the full translational symmetry is unstable towards a dimerization that leads to a unit which is formed by two half-sandwiches. The energy of the 1D column with the doubled cell is stabilized due to a symmetry-violation of the spatial wavefunction (i.e. symmetry-breaking for a symmetry atomic arrangement). A nuclear distortion (formation of non-equivalent metal-ligand distances) causes an additional energy lowering of the 1D system. The band structure properties in the outer valence region are analyzed. The calculated band gap is in line with he experimentally observed semiconducting properties of the 1D chain. The microstates of the valence band contain both admixtures from the cyclic organic π ligand (leading contribution) and from the transition metal centers (3d_xz or 3d_yz orbitals).     

Supramolecular host–guest coordination systems: [(G+)(Me3E)3MII(CN)6]∞ as ion exchangers,where (G+=Me3E,Et4N or stp), (E=Sn or Pb) and (M=Fe or Ru)

A number of 3D-coordination polymers, constructed via [^dM(CN)₆] building blocks and (Me₃E) connecting units, have been prepared and characterized by X-ray powder diffraction and different spectroscopic methods. 1-Methyl-4-(4′-R-styryl) or (2′-R-styryl) pyridinium cations (stp) have been successfully encapsulated within the expandable wide channels of the 3D-coordination polymers by tribochemical or ion exchange reactions producing novel molecular composites. Apart from 6, [(4′-OCH₃-stp)(Me₃Sn)₃Fe^II(CN)₆–MeOH]_∞ which exhibits thermochromic behaviour, the molecular composites [(stp)_x(Me₃E)₃Fe^III_1–xFe^II_x(CN)₆]_∞, 1–12 are mixed valence materials exhibiting localized interaction between the mixed valence iron. The results indicated an ion charge transfer interaction between the guest stp-cations and the host matrix. The molecular composites [(stp)(Me₃E)₃ M^II(CN)₆]_∞, 13–18 are due to the facile readiness of the coordination polymers [(Me₃E)₄M(CN)₆]_∞ and [(Et₄N)(Me₃Sn)₃Fe(CN₆)]_∞ to ion exchange.     

The calculation of London coefficients by the variation-perturbation method

The London coefficients for the dispersion interaction between LiLi, BeBe and LiBe are calculated by the variation-perturbation method using the whole atomic hamiltonian as H₀ and the Hartree-Fock approximation for the upper turbed wavefunction ?₀. A single excited valence configuration with optimized orbital exponents gives accurate results for Li, whereas comparable accuracy for Be is obtained when part of the valence correlation energy in the ground as well as in the excited state is accounted for through a limited configuration interaction.     

First-principles study of the pressure effects on the structural and electronic properties of crystalline organic azide C<Subscript>10</Subscript>H<Subscript>8</Subscript>N<Subscript>6</Subscript>O<Subscript>4</Subscript>

Within the density functional theory with regard to the dispersion interaction the crystal structure parameters of organic C₁₀H₈N₆O₄ azide are determined. The pressure effect in the range 0-20 GPa on its structural and electronic properties is studied. Parameters of the equation of state in the Vinet and Birch–Murnaghan models are determined. Within the quasi-particle method (G ₀ W ₀) the energy band structure is calculated. It is shown that the hydrostatic pressure of 20 GPa results in the approach of planes of C₁₀H₈N₆O₄ molecules and their shift relative to each other. This is accompanied by a broadening of the upper valence bands and a decrease in the band gap from 5.07 eV to 3.97 eV.     

XPS study on valence band structures of transition-metal trisulfides,TiS3, NbS3, and TaS3

Transition-metal trisulfides, TiS₃, NbS₃, and TaS₃, with a quasi-one-dimensional structure are investigated by X-ray photoelectron spectroscopic (XPS) measurements to obtain information on the valence band structures. The band structures at the Fermi level of these compounds correspond well to their transport properties. A shoulder is observed at the top of the valence band in NbS₃ and TaS₃, suggesting that this band is made up of the metal d_z² electrons. The d_z² band is occupied in NbS₃ and TaS₂ and empty in TiS₃. The characteristic features at the top of the valence band in NbS₃ imply the occurrence of d_z² band separation, which leads to a semiconducting nature.     

Electron energy structure of GaN when titanium or zinc atoms substitute for gallium

A calculation scheme based on density functional theory with full geometry optimization, modified for structures with translational symmetry is applied to study the electron energy spectrum and magnetic characteristics of hexagonal gallium nitride and structures such as Y _x Ga_1?x N (Y: donor (Ti) or acceptor (Zn) impurity). The dependence of relaxation shifts of interstitial atoms, the position of the chemical potential level, energy band boundaries, valence band widths, and energies corresponding to the intraband maxima of the density of states on the dopant concentration is discussed.     

Electronic spectra parameters as a tool for studying the structure of the catalytic center and the mechanism of activation of d 0-transition metal complexes in olefin polymerization

The quantum-chemical model formerly designed is described, which explains the high reactivity of the d ⁰-transition metal-carbon σ-bond in concerted reactions including polymerization of olefins. In this model, the energy of transition from the ground singlet state to the excited singlet or triplet state corresponding to the transfer of electron density from the metal-carbon bond to vacant d-atomic orbitals correlates with its relative elongation after which a change in the valence state of a metal begins. This change is caused by the difference in the geometries of valence s-, p-, and d-atomic orbitals having close energies; as a result, at a certain bond elongation, partial uncoupling of electrons involved in bonding takes place so that one of them becomes localized on d-atomic orbital. This process facilitates formation of the reactive state of the bond of the biradical type and leads to a reduction in the energy barrier of the insertion of an olefin molecule into this bond. Lower energies of this barrier correspond to lower values of ΔE(S ₀ → S _j ) and ΔE(S ₀ → T _j ). As shown at the example of zirconocenes Cp₂ZrMe₂ and Me₂CCp₂ZrMe₂, alongside with a reduction in the energy barrier of olefin insertion into the Zr-Me bond of the cationic complex being formed, the characteristic absorption band in the spectrum of the corresponding neutral derivative shifts to the long-wave region. This band is attributed to the transfer of the electron density from the Zr-Me bond to the Zr atom. Analysis is performed of the causes for bathochromic shifts of the long-wave absorption band for adduct formation in the systems including metallocene and Al-containing cocatalyst.     

Ring currents that survive bond alternation in constrained 8π and 6π monocycles

Current density maps are computed at the ipsocentric CTOCD-DZ/6-31G^**//RHF/6-31G^** level for angle-constrained planar 1,3,5,7-cyclooctatetraenes (COT) and benzene. Constraint of α(CCH) angles to 90° in D_4h COT (3b) leads to endo-4 and exo-4 valence isomers. The exo structure, with CH bonds perpendicular to long sides of the octagon, is lower in energy by 202 kJ/mol and has stronger bond alternation (ΔR 0.265 Å). However, endo-4, exo-4 and COT 3b at its best planar geometry all sustain intense paratropic ring currents, attributed to the rotational character of the HOMO–LUMO transition, and consistent, mutatis mutandis, with the diatropic current in D_3h benzene 5.     

Electronic structure of Ti<Subscript>4</Subscript>Fe<Subscript>2</Subscript>O as determined from <Emphasis Type="Italic">ab initio</Emphasis> calculations and X-ray spectroscopy studies

The TiL _α, FeL _α and OK _α ultrasoft X-ray emission bands obtained in experiment reflect, respectively, the energy distribution of mainly the Ti3d, Fe3d and O2p electronic states in Ti₄Fe₂O compound, which is an efficient hydrogen absorber for energy cells. Full and partial densities of electronic states for all atoms constituting the indicated oxide were calculated by a modified method of associated plane waves (APW) using the WIEN2k software package. The APW calculation data for Ti₄Fe₂O compound as well as superposition of TiL _α, FeL _α and OK _α ultrasoft X-ray emission bands on a single energy scale indicate that O2p states in the oxide are localized mainly near the bottom of the valence band, the major contribution near the ceiling of the valence band belonging to Fe3d and Ti3d states. According to the APW calculation, the major contribution to the bottom of Ti₄Fe₂O conduction band is made by Fe3d* and Ti3d* states. The APW data for Ti₄Fe₂O are supported by the cluster calculation performed for this compound using a FEFF82 software package.     

Structural and optical characterization of type II In0.14Ga0.86As0.13Sb0.87/GaSb heterostructure doped with zinc grown by liquid phase epitaxy

In_0.14Ga_0.86As_0.13Sb_0.87 quaternary solid solutions lattice-matched to GaSb (0 0 1) substrates were grown by liquid phase epitaxy, which were intentionally doped with Zn in a wide range. Two main vibrational bands are observed in their Raman spectra over the doping range investigated. The assignment of the observed modes to GaAs-like and (GaSb + InAs)-like mixture modes is discussed. The comparison of the experimental results with obtained ones by the modified random-element isodisplacement (MREI) model allows to confirm that the bands correspond to the vibrational modes associated with longitudinal- and transverse-optical (LO and TO) modes of the binary compounds GaAs and (GaSb + InAs). The low-temperature photoluminescence (LT-PL) of p-type In_xGa_1−xAs_ySb_1−y was studied as a function of zinc concentration added to the melt solution. Low temperature photoluminescence spectra show the presence of an emission band that has been related to radiative emission involving Zn-acceptors. For low carrier concentration, the photoluminescence line shape could be explained in terms of a direct transition following a simple k-selection rule. For degenerate concentrations, however, it is properly interpreted in terms of non-k-conserving transitions which arise from indirect recombination of holes in a highly filled valence band.     

Analysis of the electronic structure of Hf(Si0.5As0.5)As by X-ray photoelectron and photoemission spectroscopy

The ternary hafnium silicon arsenide, Hf(Si_xAs_1−x)As, has been synthesized with a phase width of 0.5?x?0.7. Single-crystal X-ray diffraction studies on Hf(Si_0.5As_0.5)As showed that it adopts the ZrSiS-type structure (Pearson symbol tP6, space group P4/nmm, Z=2, a=3.6410(5) Å, c=8.155(1) Å). Physical property measurements indicated that it is metallic and Pauli paramagnetic. The electronic structure of Hf(Si_0.5As_0.5)As was investigated by examining plate-shaped crystals with laboratory-based X-ray photoelectron spectroscopy (XPS) and synchrotron radiation photoemission spectroscopy (PES). The Si 2p and As 3d XPS binding energies were consistent with assignments of anionic Si¹⁻ and As^1-. However, the Hf charge could not be determined by analysis of the Hf 4f binding energy because of electron delocalization in the 5d band. To examine these charge assignments further, the valence band spectrum obtained by XPS and PES was interpreted with the aid of TB-LMTO band structure calculations. By collecting the PES spectra at different excitation energies to vary the photoionization cross-sections, the contributions from different elements to the valence band spectrum could be isolated. Fitting the XPS valence band spectrum to these elemental components resulted in charges that confirm that the formulation of the product is Hf²⁺[(Si_0.5As_0.5)As]²⁻.     

Semiconducting thin films of zinc selenide quantum dots

A novel chemical route for deposition of zinc selenide quantum dots in thin film form is developed. The deposited films are characterized with very high purity in crystallographic sense, and behave as typical intrinsic semiconductors. Evolution of the average crystal size, lattice constant, lattice strain and the optical properties of the films upon thermal treatment is followed and discussed. The band gap energy of as-deposited ZnSe films is blue-shifted by ≈0.50 eV with respect to the bulk value, while upon annealing treatment it converges to 2.58 eV. Two discrete electronic states which originate from the bulk valence band are observed in the UV-VIS spectra of ZnSe 3D quantum dots deposited in thin film form via allowed electronic transitions to the 1S electronic state arising from the bulk conduction band—appearing at 3.10 and 3.50 eV. The splitting between these two states is approximately equal to the spin-orbit splitting in the case of bulk ZnSe. The electronic transitions in the case of non-quantized annealed films are discussed in terms of the direct allowed band-to-band transitions with the spin-orbit splitting of the valence band of 0.40 eV. The effective mass approximation model (i.e., the Brus model) with the static relative dielectric constant of bulk ZnSe fails to predict correctly the size dependence of the band gap energy, while only a slight improvement is obtained when the hyperbolic band model is applied. However, when substantially smaller value for ε_r (2.0 instead of 8.1) is used in the Brus model, an excellent agreement with the experimental data is obtained, which supports some earlier indications that the quantum dots ε_r value could be significantly smaller than the bulk material value. The ionization energy of a deep donor impurity level calculated on the basis of the temperature dependence of the film resistivity is 0.82 eV at 0 K.     

A DFT study on geometric preference of non-bridging form to bridging form in molybdenum complexes with phosphenium ligand

Density functional theory (DFT) calculations were conducted to elucidate why complexes bearing both phosphenium and phosphite ligands in a cis position do not take an OR-bridging form and why complexes bearing both silylene and alkoxysilyl ligands in a cis position prefer an OR-bridging form. Energy profiles, geometries, and electronic structures along the transformation pathways from the non-bridging to the bridging forms were analyzed for four phosphenium phosphite complexes, cis-[Mo(CO)₄{P(NMeCH₂)₂}{P(NMeCH₂)₂(OMe)}]⁺ (1), CpMo(CO){P(NMeCH₂)₂}{P(NMeCH₂)₂(OMe)} (2), CpMo(CO){PMe₂}{PMe₂(OMe)} (3), cis-[Mo(CO)₄(PR₂){PR₂(OMe)}]⁺ (R = Me, Et, n-Pr) (4), and a silylene alkoxysilyl complex CpMo(CO)₂{(SiMe₂)₂(OMe)} (Si1). The DFT B3LYP/SBKJC(d) calculations for phosphenium phosphite complexes 1 and 2 revealed that there are two local minima (LM), both of which are non-bridging forms with similar energy levels, and one bridging form as a transition state (TS), which connects one non-bridging form and its mirror-image complex. These are consistent with the experimental results. In contrast, for silylene alkoxysilyl complex Si1, both bridging and non-bridging forms are LM. The former is more stable than the latter by 21.07 kcal/mol. The TS directly connects them in association with the rotation of the silyl ligand. The quite small activation energy less than 2 kcal/mol and the large energy difference between the two LM are consistent with the experimental results that only the bridging form has been reported to date. Phosphenium phosphite complexes 3 and 4 with alkyl substituents in place of amino substituents on the phosphenium P were also subjected to DFT calculations. The energy profile of 3 was found to be similar to those of 1 and 2. However, non-bridging and bridging forms were both LM for 4. The bridging form was estimated to be easily transformed to the non-bridging form, because the non-bridging form is more stable in energy and the activation energy from the bridging form is less than 1 kcal/mol for 4a. Charge accumulation in the bonding region, nuclear repulsion among the ligands, and the stability of E–O–E bond formation (E = P, Si) were considered to be decisive factors for the geometric feature.