Types of paramagnetic centers in Cu2+ complexes with model neuromelanins

Electron paramagnetic resonance (EPR) spectroscopy was used to examine free radical properties of model neuromelanins obtained by oxidative polymerization of noradrenaline, adrenaline and dopamine. We compared the lineshape of the experimental spectra, type and concentration of free radicals in the analyzed samples. The effect of different concentrations of Cu²⁺ on free radicals in melanins was studied. The total concentration of free radicals (about 10¹⁸ to 10¹⁹ spin/g) in the studied melanins increases as follows: adrenaline-melanin < dopamine-melanin < noradrenaline-melanin. EPR spectra of dopamine-melanin and adrenaline-melanin were a single EPR line (ΔB _pp, 0.50 and 0.55 mT, respectively). o-Semiquinone free radicals with the characteristicg-value of 2.0040 exist in these melanins. EPR spectra of noradrenaline-melanin were a superposition of two lines (ΔB _pp, 0.45 and 0.81 mT). o-Semiquinone free radicals were responsible for the narrower component. Nitrogen free radicals with ag-factor of 2.0030 were probably responsible for the broader component. Paramagnetic copper ions quenched the EPR signals of melanin free radicals in the studied samples. For melanin-Cu²⁺ complexes, broad EPR lines (ΔB _pp, 10–32 mT) of copper ions with ag-value of about 2.1 appeared. The influence of the microwave power on the EPR spectra of these complexes demonstrated the fast spin-lattice relaxation in the copper system in melanins.     

X-ray absorption spectral investigations on mixed-valence copper complexes of penicillamine and cysteamine

The X-ray K-absorption spectra of copper in some of its complexes involving ligands like penicillamine (known detoxicating agent for heavy metal poisoning) and cysteamine (a known radiation protector) have been investigated using a 40 cm curved crystal spectograph. These complexes are described in the literature as being mixed-valence [1, 2]. The copper-penicillamine complex shows two distinct edges, K₁ and K₂, indicating mixed-valency in the complex. The cysteamine complexes do not show any splitting of the edges. In one of the cysteamine complexes, which is said to involve molecular oxygen, the magnitudes of shifts of the main edge and principal absorption maximum fall close to copper (II) compounds whereas in the other complex, these shifts are in the range of copper (I) compounds. The probable reasons for the observed deviations have been discussed.     

An ESCA investigation of some copper complexes

A series of copper complexes have been investigated by ESCA. All complexes were salts of the tetraphenylphosphonium ion. The binding energies of all the atoms in the complexes were determined. From the binding energies of the ligand atoms we estimated the effective charges on these atoms. For this purpose we used linear relations of the formE_b = kq + E_bO which had been established previously within our scheme of C 1s (phenyl) as internal standard. From the data thus obtained, the effective charge on the copper atom was estimated. A linear relation between binding energy and the effective charge on the copper atom was found, i.e.,E_b(Cu) = 1.52q_Cu + 932.2ESCA spectra were recorded for the complexes bis(1,3-diphenyl-1,3-propanediono) copper (II) and bis(3-phenyl-2,4-pentanediono) copper (II). By a combination of the XPS binding energies and IR intensities of the ν_CH vibrations of the phenyl groups in the complexes with empirical relations between these entities and the effective charges of the atoms and groups, a fairly complete mapping of the charge distributions of these complexes has been achieved.     

Structure of New Bis-Dicarbollyl Derivatives of Transition Metals

The dipyridyl complexes of the copper salts of bis-o-dicarbollyliron(III), -cobalt(III), and -nickel(III) are investigated by means of EPR spectroscopy. Theoretical models of the EPR spectra of copper(II) ions in these complexes are constructed and their spectral parameters are determined. It is shown that in all cases the tetragonally distorted octahedron is the most probable coordination sphere of the Cu²⁺ ions.     

Electron spin resonance with g eff ≈ 4.2 in YBa2Cu3O6.35. Model of chain copper-oxygen fragments

Systematic measurements are made of ESR spectra with g ≈ 4.2 in YBa₂Cu₃O_6+y compounds with various doping indices y. Temperature dependences of the spectrum intensity show that the ground states of the centers are singlet and the energies of the excited states M _S = ±1 correspond to 8?11 K. In general, the intensity of the ESR spectra varies with time and depends on the sample preparation technology. The most probable models for the paramagnetic centers studied are considered to be chain fragments of copper ions of variable valence. General laws governing the energy structure of these centers are described.     

Ab initio investigation of potential indium and gallium free chalcopyrite compounds for photovoltaic application

The amazing opto-electronic properties of pseudo-ternaries Cu(In,Ga)(S,Se)₂ compounds are, for a great deal, related to their defect chemistry, for which ab initio calculation have already provided new insights. As Indium and Gallium are rare and expensive elements, the search of an alternative may become an important issue. Ab initio calculation of energies and electronic structures of various chalcopyrite type structures replacing In by other elements has been performed exploring over 16 compounds with some 10 different elements of the periodic table. Among the different possibilities, the substitution of In by the isoelectronic couples such as (Zn, Sn) and other cationic couples having an average valency of III have been evaluated in details, as well as the influence of the anion (S or Se). The most important point defects and defect complexes have been explored. Low copper vacancy (V_Cu) and defect complexes (such as V_Cu+Zn_Cu) formation energies have been found in these new compounds: for instance the formation energy of V_Cu in Cu₂SnZnSe₄ is 0.49 eV as compared to 0.48 eV in CuInSe₂ (CIS). In addition to point defect characteristics, different electronic properties (such as band structure, bonding character) are compared to the one of CIS, which is considered as our reference structure. The method used is based on the density functional theory within the framework of pseudo-potentials and plane waves basis. The results are discussed in view of the existing data, models and calculations. Based on these properties as well as the ionic/covalent bonding character some trends are exposed.     

Liquid-crystalline dendrimer Cu(II) complexes and Cu(0) nanoclusters based on the Cu(II) complexes: An electron paramagnetic resonance investigation

New nanostructured materials, namely, the liquid-crystalline copper(II) complexes that contain poly(propylene imine) dendrimer ligands of the first (ligand 1) and second (ligand 2) generations and which have a columnar mesophase and different copper contents (x = Cu/L), are investigated by EPR spectroscopy. The influence of water molecules and nitrate counterions on the magnetic properties of complex 2 (x = 7.3) is studied. It is demonstrated that water molecules can extract some of the copper ions from dendrimer complexes and form hexaaqua copper complexes with free ions. The dimer spectra of fully hydrated complex 2 (x = 7.3) are observed at temperatures T < 10 K. For this complex, the structure is identified and the distance between the copper ions is determined. It is shown that the nitrate counterion plays the role of a bridge between the hexaaqua copper(II) complex and the dendrimer copper(II) complex. The temperature-induced valence tautomerism attended by electron transport is revealed for the first time in blue dendrimer complexes 1 (x = 1.9) with a dimer structure. The activation energy for electron transport is estimated to be 0.35 meV. The coordination of the copper ion site (NO₄) and the structural arrangement of green complexes 1 (x = 1.9) in the columnar mesophase are determined. Complexes of this type form linear chains in which nitrate counterions serve as bridges between copper centers. It is revealed that green complexes 1 (x = 1.9) dissolved in isotropic inert solvents can be oriented in the magnetic field (B ₀ = 8000 G). The degree of orientation of these complexes is rather high (S _z = 0.76) and close to that of systems with a complete ordering (S _z = 1) in the magnetic field. Copper(0) nanoclusters prepared by reduction of complex 2 (x = 7.3) in two reducing agents (NaBH₄, N₂H₄ · H₂O) are examined. A model is proposed for a possible location of Cu(0) nanoclusters in a dendrimer matrix.     

The CuPL defect and the Cus1Cui3 complex

The Cu_PL defect is characterized by intense photoluminescence (PL) emission with a 1014 meV zero-phonon line and has since long been called the ‘copper-pair’ defect. However, recent PL studies in isotopically pure ²⁸Si samples show clear evidence for four Cu atoms in this defect. This defect is one of a family of complexes that contain four (sometimes five) metal impurities. No complex containing two or three metal impurities (at least one of which is a transition metal) has been reported. We have performed systematic calculations of a priori possible Cu₄ complexes, including the Cu_s1Cu_i3 complex proposed by Shirai et al. [18]. This complex has the lowest formation energy of all the Cu₄ defects. We have studied its properties using first-principle theory. In addition to the structures, binding energies, and vibrational spectra, we studied the formation pathway in an attempt to figure out why smaller (or larger) complexes do not form. We find that Cu_s1Cu_in with n=0, 1 or 2 continue to trap Cu_i⁺ if the Fermi level is midgap, and that the process stops when n=3. © 2011 Elsevier Science. All rights reserved     

An X-ray photoelectron spectroscopic study of some nickel (II) amine complexes

The X-ray photoelectron (XPS) spectra of several Ni(II) diamine complexes, a Ni(II) triamine complex and several Ni(II) diimine complexes are reported in terms of the Ni 2p, Cl 2p and N 1s core level binding energies. Theoretical models are presented to account for the quadratic relationship observed between XPS Ni 2p_{$\text{3}\text{2}$}binding energy shifts and the N-H infrared rocking frequency as related to these complexes. On the basis of this correlation, it is possible to discriminate between free counter-ions coordinated to the metal or involved in cluster-ion formation. Individual XPS data for these complexes are interpreted in terms of the intrinsic nature of the metal environment and an attempt is made to calculate the metal and donor nitrogen atomic charges for selected complexes.     

Electron Spin Polarization of Photo-Excited Copper Coproporphyrin I: From Monomers to Dimers

The results of the electron paramagnetic resonance (EPR) and transient EPR (TREPR) of copper complexes of coproporphyrin I in different solvents before and after the laser pulse photo-excitation have been presented. Continuous-wave EPR spectra of the CuCPP-1 complex in o-terphenyl indicate the presence of only monomer fragments, while in the solution of the chloroform and isopropanol mixture, the complexes dimerize and the amount of dimers is five times larger than that of monomer complexes. Parameters describing EPR spectra of monomer and dimer CuCPP-1 complexes have been determined. It was established that the fine structure tensor of the dimer complex is rotated with respect to the g-tensor, which coincides with the tensor of monomer complexes. TREPR spectra of CuCPP-1 complexes in o-terphenyl and in the chloroform and isopropanol mixture after the laser photo-excitation are mainly due to spin-polarized ground states of monomer and dimer complexes, respectively. The TREPR spectra of the monomer CuCPP-1 show the emissive spin-polarized signal of the ground state. For dimer fragments, the net polarization is observed in the form of absorption and there is a small contribution from the multiplet polarization, which decays fast in time. The time dependence of TREPR of CuCPP-1 complexes in the chloroform and isopropanol mixture is described with allowance for these contributions from the ground state of the dimer and the contribution from the ground state of the monomer, which is manifested at larger times. Differences in the spin polarization of ground states and their possible origin are discussed.     

Luminescent properties of the oxygen impurity centres in A1N

Emission luminescence spectra of A1N with oxygen impurity are composed of several overlapping bands ranging from 2.37 eV to 3.51 eV. The intensity of these bands depends on the excitation conditions. The Stokes shift of corresponding excitation bands is ～0.93 eV; this shift is compatible with the experimental half-width of individual bandsΘ ～ 0.46 eV. The observed complex spectra are ascribed to the excitation and recombination processes in D-A complexes formed of substitutional oxygen donors O_N and aluminum vacancies V _Al . Tentative models of these centers are proposed.     

Optical spectra of Cu porphin and VO porphin in single crystal triphenylene

Polarized optical absorption spectra of copper porphin and vanadyl porphin in single crystals of triphenylene have been obtained at room temperature, 77°K and 4.2°K. Luminescence spectra of these samples are also reported. The gross features of the spectra of these two metalloporphins are similar but the spectral lines in vanadyl porphin are much sharper than those in copper porphin, revealing much more satellite structure. It is shown that the central metal can substantially affect metalloporphyrin absorption spectra, contrary to the case of broad band solution spectra. The data is consistent with porphin molecules in substitutional sites of triphenylene and a crystal field reduction of the usual D_4h metalloporphyrin symmetry to D_2h.     

K-absorption spectral investigations on some dimeric copper(II) carboxylate complexes

The X-ray K-absorption edge position and the extended fine structure of copper in some copper(II) carboxylate complexes involving metal-metal exchange interaction have been investigated using a 40 cm bentcrystal spectrograph. It has been found that the edge position in these complexes shifts towards the higher energy side as the pK_a value of the corresponding acids increases. Sharp and narrow main peaks have been observed in all these complexes except the monomeric complex, copper(II) trichloroacetate. Estimates of metal-ligand average bond distances have also been made.     

Spectral and luminescent properties of coumarins in nanoparticles from metal ion complexes

We have studied the absorption spectra of complexes of trivalent ions Y, La, Lu, and Sc with p-phenylbenzoyltrifluoroacetone that were introduced into a solution of 90% H₂O + 10% iso-C₃H₇OH in the absence and presence of either 1,10-phenanthroline or coumarins 6, 7, or 30. We have shown that these coumarins, as well as phenanthroline, are synergistic bidentate ligands that are incorporated into complexes up to concentrations comparable with concentrations of complexes and that stabilize them in the solution. We have studied the dependences of the fluorescence (cofluorescence) intensity (I _cofl) of the coumarins on their concentration in nanoparticles from the complexes mentioned above. We have shown that, in nanoparticles from complexes of Y(III), Ln(III), and Sc(III), I _cofl of coumarin 30 at high concentrations is higher than I _cofl of coumarins 6 and 7. In addition, up to concentrations of coumarin 30 comparable with the concentration of complexes in nanoparticles, there is no concentration quenching of its fluorescence. For coumarins 6 and 7, which are prone to association in the solution under study, the process of incorporating coumarins into complexes competes with their association, which leads to concentration quenching and changes in the shape of their cofluorescence spectra.     

Studies of EPR and ENDOR spectra of14N and15N label bis(2-hydroxyacetophenyl ketoxime)-63Cu(II) and-65Cu(II) complexes in disordered system

The EPR spectra of isotopic label bis(2-hydroxyacetophenyl ketoxime)-Cu(II) [N?Cu-HAP] complexes, such as¹⁴N?⁶³Cu-HAP,¹⁵N?⁶³Cu-HAP and¹⁴N?⁶⁵Cu-HAP in frozen THF solution below 77 K, and their ENDOR spectra in frozen DMSO/EtOH (5∶1) solution below 20 K were studied. The exact values of the components ofg-tensor, of hyperfine tensors of copper isotopes, and of superhyperfine interaction tensors of copper with nuclei of nitrogen isotopes and¹H nuclei, and of the¹⁴N nuclear quadrupolar moment coupling tensor were obtained. The bond parameters α, β, δ, γ and the corresponding energy levels of N?Cu-HAP complexes were calculated by using EPR and ENDOR data. It was shown that the unpaired, electron is delocalized not only to the nearest N atom but also to the H atom, of ligands, which is more far from the Cu ion.     

EPR of Spin Transitions in Complexes of Cu(hfac)2 with tert-Butylpyrazolylnitroxides

Polymer chain complexes [Cu(hfac)₂L^R] _n exhibit thermally and light-induced magnetic anomalies in many aspects similar to a spin crossover. These compounds attracted significant attention in the field of molecular magnetism and have been extensively studied by electron paramagnetic resonance (EPR) during the last several years. All compounds studied so far were based on copper(II) ions bridged by pyrazolyl-substituted nitronylnitroxides. The present work reports the first EPR study of complexes of Cu(hfac)₂ with tert-butylpyrazolylnitroxides—a new type of nitroxide ligand expected to modify exchange interaction pathways and physical properties of the crystals. The Q-band EPR spectra of three representative novel compounds are principally different from those studied previously, supporting the assumption that the magnetic motif of the compound has changed. Dominant intercluster exchange interactions are now found along the structural polymer chains. This complicates the EPR detection of phase transitions to some extent; however, theoretical modeling of the observed spectral changes allows for unambiguous assignment of different spin states and transitions between them. The magnitudes of intercluster exchange interaction were estimated to be ca. 0.1–1.5 cm^?1 for the studied compounds.     

Deactivation of an excited charge-transfer complex II. Influence of rigidity of medium on radiative and radiationless transitions

Fluorescence spectra, quantum yields and decay times for charge-transfer complexes of tetracyanoethylene with benzene, toluene, ortho-xylene and para-xylene in low-temperature glassy solutions of different viscosities have been investigated. It was established that the influence of viscosity or rigidity of medium on charge-transfer fluorescence consists in the decrease of efficiency of radiationless processes when viscosity or rigidity increases. Fluorescence spectra of complexes of tetracyanoethylene with toluene, o-xylene and p-xylene have a composite double-band character at high viscosities of medium. The intensity of both subbands changes in a different way with change of viscosity. The excitation spectra of these complexes are dependent on the wavelength of observation. These facts are discussed in terms of an existence of different orientational isomers of a charge-transfer complex of low symmetry.     

ESR and Chemical Bonding of Some Copper Complexes

ESR spectra of copper (II) was measured in 33 copper complexes which were synthesized in this laboratory and the bonding parameter α²of copper was calculated according to the method proposed by Kivelson. ⁽¹⁾ The results of these calculations are classified, together with the data quoted from the literature,^1-5into four groups according to the kinds of ligand atoms, i.e., α²is 0.72 to 0.87 for 0₄ - type complexes, 0.69 to 0.86 in N₂O₂, 0.74 to 0.82 in N₄, and 0.54 to 0.62 in S₄ and S₂₃N₂ type complexes, where 0, H and S refer the oxygen, nitrogen and sulfur atoms of ligands, respectively, that directly coordinate with copper.     

Enhanced luminescence of rare-earthTb (III) by Tm (III) in bis(benzoylmethyl) sulfoxide complexes and intra-molecular energy transfer

Rare-earth complexes [(Tb_xTm_y)L₅(ClO₄)₂](ClO₄)·3H₂O(x:y=1.000:0.000, 0.999:0.001, 0.995:0.005, 0.990:0.010, 0.950:0.050, 0.900:0.100, 0.800:0.200, 0.700:0.300; L=C₆H₅COCH₂SOCH₂COC₆H₅) were synthesized and characterized with elemental analysis, infrared spectra (IR) and ¹H NMR. The photophysical properties of these complexes were studied in detail with ultraviolet absorption spectra, fluorescent spectra and lifetimes. The fluorescence spectra and decay curves of complexes indicated that the fluorescence emission intensity was enhanced and the fluorescence lifetime was prolonged by Tm (III), which may be due to the intra-molecular energy transfer between inert rare-earth ions and active rare-earth ions. The complexes showed the best properties when the mole ratio of Tb (III) to Tm (III) is 0.995:0.005. The intensity of fluorescence can be increased to 208%. Additionally, the energy-transfer mechanisms between the ligand and the central Tb (III) ions were discussed.     

Orientation of the imidazole type ligands in copper complexes studied by ESEEM

Effects of nitrogen quadrupole coupling tensors on ESEEM spectra are discussed. It is shown from theoretical considerations that the spectra, especially the double quantum transition line, are strongly affected by changes of tensor orientation in the magnetic field. Model calculations for angle-selected nitrogen ESEEM spectra of copper complexes in the disordered state show that the double quantum transition line changes its intensity with the resonance magnetic field set position in the EPR spectrum. These model calculations also show that estimation of the orientation of the imidazole type ligands with respect to theg principal axes may be easily made without any tedious processes of computer simulation of the whole ESEEM spectral pattern. The usefulness of the method is experimentally shown by applying it to the structural studies of the copper complexes of 2-(2′-pyridyl)imidazole and 2(2′-pyridyl) benzimidazole and their six coordinated complexes with nitrogeneous bases.