Charge-transfer Spectra Research Articles

Optical excitations and coupling constants in FeO42− and CrO44− complexes in oxides: Density functional study

The experimental crystal field (CF) and charge transfer (CT) spectra due to Fe6+-doped K2XO4 (X=S, Se, Cr) and Cr4+ in oxides have been investigated by density functional (DF) calculations on FeO42− and CrO44− complexes at different values of the metal–ligand distance, R. To this aim the coupling constants, Aα and Aε, with the A1 and the Jahn–Teller E modes have also been calculated. The present results explain reasonably the transition energies and coupling constants observed for Fe6+ and Cr4+ in oxides and account for the following experimental features: (i) The different nature of the first CF and CT excitations observed on passing from Fe6+ to Cr4+ in oxides. (ii) Why for Fe6+ in oxides the dominant coupling is with the E mode for the CF states while with the A1 mode for CT states. (iii) Why on going from Cr4+ to Fe6+ in oxides the coupling constant Aα of CT states decreases while the opposite is found for CF ones. The role played by the covalency on these questions is explained on simple grounds, stressing that it drastically decreases the Aα value for CT states. This fact together with the value να=820 cm−1 accounts for the low value of the Huang–Rhys factor Sα=2.5 in the assigned 1t1↓→4t2↓ CT excitation of K2SeO4:Fe6+. As a salient feature the R dependence of 10Dq (which determines Aα for the T23 CF state) is found to be greatly sensitive to the small admixture (≈0.5%) of 2s(O) orbitals in the antibonding 2e level. The electronic relaxation decreases the energy of the first CT transition of FeO42− by 19 000 cm−1, the charge distribution being however the same as that of the ground state. The important role played by the trigonal distortion for reducing the Jahn–Teller effect in the first excited state of Cr4+ in oxides is analyzed through a phenomenological model. Though no further neighbors of the host lattice are included, the calculated equilibrium distances for FeO42− and CrO44− are only 3% and 8% higher than experimental values found for Cr4+ to Fe6+ in oxides. Moreover the calculated value of the να frequency at the right equilibrium distance essentially coincides with experimental findings indicating that force constants between oxygen and further neighbors likely play a minor role. In comparison with recent ab initio plus configuration interaction results on FeO42− the DF scheme offers a simpler description of ground and CT states.

Study of the local structure of Jahn-Teller distorted CuX2-M 4 (X = Cl, Br) systems by polarized charge transfer spectroscopy: Influence of chemical pressure along A2MX4 (M = Mn, Zn, Cd, Hg)

The aim of this work is to investigate the Charge Transfer (CT) spectra of CuCl2- 4 and CuBr2- 4 complexes formed in Cu2+-doped A2MX4 crystals and their dependence with chemical pressure when Cu2+ replaces the divlent cation at the MCl2- 4 tetrahedral site. A salient feature of this study is the weak sensitivity exhibited by the CT bands to structural changes of the complex. A similar phenomenon has also been observed for pure chlorocuprates. The analysis carried out on the available family of chlorocuprates indicates that the CT band-shift induced by structural distortions in CuCl2- 4 is smaller than that of the corresponding Crystal Field (CF) bands. We present a perturbative model for explaining the weak sensitivity of the CT bands. We present a perturbative model for explaining the weak sensitivity of the CT bands. in comparison to the CF ones. The estimates of the model accounts for the spectroscopic results observed along the A2MCl4: Cu2+ series.

Optical transition and ionicity of C60/amine charge-transfer complexes studied by optical spectroscopy

Abstract We have investigated absorption spectra of charge-transfer (CT) complexes of C60 with various aromatic amines with different ionization potentials. An absorption band due to CT transition is observed in the visible region; with increasing ionization potential of amines, the peak energy shifts to the higher energy side and the oscillator strength of CT transition increases. We extract degrees of charge transfer by analyzing the transition energy of the CT band as a function of ionization potential and Raman spectra. The experimental results show that the CT complexes studied here have a neutral character in their ground state.

MOLECULAR COMPLEXES OF CROWN ETHERS, PART 5 : COMPLEXES OF DB18C6 WITH SOME ACCEPTORS

The UV-Visible spectra of DB18C6 as a donor with TCNE (Tetracyanoethylene), and DDQ (2,3-dichloro-5,6-dicyano-1,4- benzoquinone) as acceptors were studied. Charge transfer spectra were obtained for these systems from which it was possible to calculate the formation constant, Kc. The effects of potassium halides were studied. This study shows that in the presence of the anion there is an electron transfer from the anion to the acceptor. This process is enhanced by the presence of the crown ether, CE. The formation of the anion salt with the acceptor, in the presence of the CE, follows the trend F = I > Br > Cl. It is also indicated in this study that the interaction between DB18C6 and the acceptor follow the trend DDQ > TCNE.

Molecular Complexes of Crown Ethers, Part 6: Complexes of Cryptand (2,2,2) and Kryptofix 5 with Some Acceptors

The UV-Visible spectra of Cryptand (2,2,2) and Kryptofix 5 as donors, and TCNE (tetracyanoethylene), DDQ (2,3-dichloro-5,6- dicyano-1,4 benzoquinone) and PA (picric acid) as acceptors were studied. Charge transfer (CT) spectra were obtained for these systems. It was found that potassium halides have little effect on the spectra. The major process was due to an electron transfer from the donor to the acceptor. This revealed itself in very high conductivity values for the CT solutions in comparison to that of the donor or the acceptor solutions. The infrared and proton NMR spectra of the complexes indicated a strong interaction between the donor and the acceptor.

Thermochromic properties of the ferroelectric -doped : study of the temperature-induced dichroism

A new thermochromic material formed by doping into the ferroelectric crystal is presented. The change of colour from green (room temperature) to red (low temperature) under polarized light observation is accompanied by a strong temperature-induced dichroism at low temperature. The origin of these phenomena is investigated through the polarized charge transfer spectra associated with complexes formed in the title compound, and their dependence on temperature in the 10-300 K range. Attention is paid to correlating the optical spectra with the local structure around . The results are compared with those obtained in the tribromide and , and the tetrabromide and crystals, where forms complexes of nearly symmetry, and tetrahedral distorted complexes of symmetry, respectively. An interesting feature is the presence of a strongly polarized absorption band at 15 800 in , which is responsible for the observed dichroism and thermochromism. The existence of this band is associated with the formation of highly distorted complexes in the monoclinic host crystal. The enhancement of dichroism exhibited by this crystal with decreasing temperature is noteworthy. The analysis of the spectra reveals that this unusual thermal behaviour can be explained in terms of thermally activated reorientations rather than structural changes of the complex.

Effects of chemical pressure on the charge-transfer spectra of complexes formed in -doped (M = Zn, Mn, Cd, Hg; X = Cl, Br)

This work investigates the charge-transfer spectra of and complexes formed in anisotropic (X = Cl, Br) crystals. Attention is paid to the variations of the charge-transfer transition energies induced by chemical pressure effects when substituting along the crystal series. A salient feature of this study is the weak sensitivity of these bands to structural changes of the complex in comparison to the sensitivity of the corresponding crystal field (d-d) bands. The knowledge of these structural-induced shifts is important since they are responsible to a great extent for the thermo- and piezochromic properties exhibited by some compounds containing units as chromophores. We present a tentative model based on a perturbed tetrahedral complex for explaining the weak sensitivity exhibited by the charge-transfer transitions in the title compounds. Furthermore the estimates of this model can also explain the big difference between the energy shift of d-d and charge-transfer transitions due to structural changes of the complex in pure chlorocuprates.

Molecular Complexation and Kinetics for Diels−Alder Condensation of Naphthylalkenes with Tetracyanoethylene

The charge-transfer spectra of 1:1 molecular complexes of seven 1-alkyl-1-(2-naphthyl)ethenes (where R = alkyl group: H, Me, Et, n-Pr, i-Pr, t-Bu, Neopent) and 1-vinylnaphthalene with TCNE were determined in ClCH2CH2Cl as solvent at 27.1 °C. Equilibrium constants, K, for these complexes vary with R, as based on two opposing factors, viz. (a) the polar substituent factor σ* and (b) the angle-of-twist θ between the planes of the naphthyl and vinyl groups. For R = Neopent (σ* dominant) K is 50 times as large as for R = t-Bu (θ dominant). Except for R = t-Bu, kinetics of reaction conform with the equation D + TCNE ⇄ (K) D·TCNE ⇄ (k1/k-1) P, where D is the donor alkene, P is the Diels−Alder 1,4-cycloaddition product, and k1 and k-1 are first-order reaction rate constants. Values of k1 vary from 0.71 min-1 (R = H) to 55.5 min-1 (R = Neopent) and the corresponding relative second-order rate constants k2 (or k1K) from 1 to 4000. The rate constant k-1 was measured only for 1b (R = Me, 0.0017 min-1) in the solvent mixture p-xylene/ClCH2CH2Cl. Formation of 1b·TCNE complex gives ΔH = 10.0 kcal/mol and ΔS = 38.4 eu, and conversion to P shows an Arrhenius activation energy of Ea = 7.24 kcal/mol. It is proposed that the preferred conformation of a naphthylalkene for complexation has the R and vinyl groups projecting outward from opposite sides of the plane of the naphthalene ring. The TCNE molecule then aligns parallel to the naphthalene ring on the vinyl side where (except for R = t-Bu) it can slide into the geometry of the transition state to form P.

Charge-Transfer Spectra and Bonding in Tetrahedral Mn(VI), Cr(V), and V(IV) and Mn(VII), Cr(VI), and V(V) Oxo Anions.

Density functional theory (DFT) calculations on the tetrahedral Mn(VI), Cr(V), and V(IV)(d(1)) oxo anions in their ground and lowest excited d-d and O --> M charge transfer (CT) states are reported and used to assign the electronic absorption spectra by reference to the spectra of the isoelectronic Mn(VII), Cr(VI), and V(V) (d(0)) and the Mn(V) and Cr(IV) (d(2)) anions. Calculated geometrical shifts along the totally symmetric metal-ligand vibration (alpha(1)) for electronic excitations are in agreement with data deduced from experimental vibronic fine structures, supporting the proposed assignments. Using a CT model including (as different from DFT) configuration interaction (CICT), it is shown that the CT excited states of MnO(4)(2)(-) at 17 000, 23 300, and 28 200 cm(-)(1) are due to d(2) (3)A(2)(2e(2)), (1)E(2e(2)), and (3)A(2)(2e(2)) final states combining with a single hole (L) on the ligand 1t(1) and 4t(2) orbitals, respectively. The higher 10Dq and smaller B values for the d(2)L(d(1)) states compared to those of the d(2) systems correlate with the shortening of the metal-ligand bond accompanying the removal of electrons from the antibonding d orbitals, leading to an increase in covalency and a change in the ordering of CT states for Cr(V) with (3)T(2)(2e(1)5t(2)(1))L (10Dq) at a higher energy than (1)E(2e(2))L (8B + 2C) as compared to Cr(IV) with nearly degenerate (3)T(2)(2e(1)5t(2)(1)) and (1)E(2e(2)) terms. This allows one to estimate the energy of the (3)A(2)(2e(2))L --> (1)E(2e(2))L transition from the CT (d(2)L) spectrum of Cr(V)(d(1)), which could not be observed for Cr(IV). From a comparison of calculated and experimental oscillator strengths and Huang-Rhys factors (S) for the lowest CT band in the V(V), Cr(VI), and Mn(VII) (d(0)) and the V(IV), Cr(V), and Mn(VI) (d(1)) oxo anions, it is shown that the increase in covalency from left to right in this series is accompanied by a reduction in band intensity and S for the progression in the alpha(1) vibration. An explanation of this result in terms of ionic contributions to the metal-ligand bond increasing from Mn(VI) to Cr(V) and V(IV) is proposed. Intensities of "d-d" transitions display the opposite trend; increasing covalency leads to stronger mixing between d --> d and CT excited states and thus an increase in intensity.

Quantum dynamics of a time-dependent multidimensional non-linear oscillator and modelling of ultrafast intensity oscillations in charge transfer spectra

Quantum dynamics of a coupled two-dimensional non-linear oscillator which has its harmonic frequencies varying with time is worked out in a time-dependent variational as well as perturbative framework. The time-dependent variational calculations are done by invoking the time-dependent Fourier grid Hamiltonian method. A more general case in which the harmonic frequencies, along with the equilibrium positions, are allowed to vary with time is also investigated. The model is used to interpret ultrafast intensity oscillations that may accompany an intramolecular charge transfer transition in which the time scales of charge separation and vibration are not widely different and therefore temporally non-separable.

Ion size effect on the charge transfer and Raman spectrum of the (Pb 0.65Sr 0.35)Sr 2(R 0.5Ca 0.5)Cu 2O y compound

The effect of rare earth ion size on the charge transfer and Raman spectra of the rare-earth-substituted Pb-based 1212 compounds (Pb 0.65Sr 0.35)Sr 2(R 0.5Ca 0.5)Cu 2O y (R = Y, La, Ce, Pr, Nd, Yb, Lu) has been studied systematically. X-ray diffraction showed that all the rare-earth-substituted samples were single phase of the Pb-1212 except R = Ce. The compound with R = Ce did not form under the same synthesis conditions. Both the a and c parameters increased monotonically with the substitution of larger size rare earth. AC susceptibility measurement revealed that all the samples showed superconducting transitions between 43 and 68 K except the largest ion La-substituted sample. T c showed a tendency to decrease with rare earth radius. Raman spectra of these samples illustrated that only one peak at about 540 cm −1 appeared for all the rare-earth-substituted samples in the range of 100 to 800 cm −1. This peak was attributed to the vibration of the apical-O(2) in this system. Its frequency showed a tendency to increase with rare earth ion. The relationship between lattice parameter, superconductivity, apical-O(2) vibrational characteristic and ionic radius were discussed in this paper. Based on several experimental facts, our present Pb-1212 samples are considered to lie in the underdoped region of the T c- n diagram.

Dependence of the charge transfer spectra of (C 2H 5NH 3) 2CdCl 4 : Cu 2+ with Hydrostatic pressure: Structural changes around Cu 2+

This paper investigates the variation of the charge transfer (CT) spectra of the Cu 2+ doped (EtNH 3) 2CdCl 4 ( Et = C 2 H 5) layer perovskite under hydrostatic pressure. The aim is to explain the strong redshift (−4000 cm −1) undergone by the first CT band of the formed CuCl 4− 6 complex on passing from (EtNH 3) 2CdCl 4: Cu 2+ to the isomorphous (EtNH 3) 2MnCl 4: Cu 2+ crystal, which is accompanied by a change of color from light yellow to deep red. The pressure experiments show that the first Cl − → Cu 2+ CT band experience an abrupt shift of − 1200 cm −1 between 23 and 26 kbar. The comparison of these results with those previously obtained in the pure (EtNH 3) 2CuCl 4 suggests that this shift is associated with a local change around the Cu 2+ from an elongated octahedron geometry ( P = 1 atm) to a more compressed situation similar to that found for the Mn crystal.

Electron-molecular vibration coupling in organic charge-transfer salts: application of spectroscopic and Hubbard models

Spectroscopic and Hubbard models were utilized for analysis of vibrational spectra of organic monovalent charge-transfer (CT) salts with dimerized structures. Within the framework of a dimer approach we showed origin of intense infrared active electron-vibrational (vibronic) bands and obtained an expression for their frequency shifts relatively to matching Raman active totally symmetric vibrations. The theory predicts an important relationship between intensities and frequencies of the vibronic bands and is in good agreement with experimental data for BEDT-TTF-based and TTF-based CT complexes. Existence of the universal relationship between the two easy measurable quantities allows to make a simple evaluation of coupling constants for the substances directly from the experimental spectra.

Infrared photoexcitation spectra of conducting polymer/methanofullerene films

We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)-methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index, ΔN = Δn(ω)+iΔϰ(ω), are obtained. The results indicate that the PIA spectra are dominated by Δϰ(ω), while the PIR spectra are dominated mainly by Δn(ω). The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear optical applications of these materials.

Electronic Structure and Dynamics of the Excited State in CT Microcrystals As Revealed by Femtosecond Diffuse Reflectance Spectroscopy

Transient absorption spectra of charge-transfer (CT) crystals between methyl- or methoxy-substituted benzene derivatives and pyromellitic dianhydride (PMDA) were measured by femtosecond diffuse reflectance spectroscopy, and the electronic structure and dynamics of the excited state depending on the nature of the electron donor (D) are discussed. For such weak CT complexes, it was confirmed that excitation energy is localized in one donor−acceptor pair and the mixing of CT and locally excited (LE) configurations is important in the excited CT singlet state. The CT degree of the excited state depends on the oxidation potential of D and on the mutual configuration of D and electron acceptor (A) molecules. The second-order decay constant of the excited state was observed under usual photolysis conditions, from which a motion-limited diffusion was considered. A first-order decay of the transient absorption was obtained, when the excitation intensity was weak, and ascribed to charge recombination to the ground state. The energy gap (−ΔG) dependence of the charge recombination rate constant (kCR) was confirmed to give a linear relationship between ln(kCR) and |−ΔG|.

Thermally-Induced Dichroism in Cu2+ Doped [(CH3)4N]HgBr3: Study of the Thermochromic Properties*

This work investigates the polarized Charge-transfer spectra associated with Cu2+ impurities in [(CH,)4N]HgBr,:Cu2+ in the 10—300 temperature range. Attention is paid for correlating the optical spectra with the local structure around Cu2+. The spectra are compared with those obtained in the Cu24 doped [(CH,)4N]CdBr, and [(CH,)4N]MnBr,, and [(CH,)4N]2HgBr4 and [(CH,)4N]2CdBr4 crystals, where the Cu2+ ions form CuBr^ complexes of nearly D4/, symmetry and tetrahedral distorted CuBr2 complexes of D2J symmetry, respectively. The presence of a strongly-polarized absorption band at 15900 cm' in [(CH.OjNlHgBr^Cu2* is interpreted in terms of a highly distorted CuBrj complex. The enhancement of dichroism exhibited by this crystal with decreasing temperature is noteworthy. The analysis of the spectra reveals that this unusual thermal behaviour is mainly associated with orientational motions of the Cu2f complex rather than with structural changes of the complex.

The charge transfer state of excited bianthryl and a derivative: solvatochromism, emission CT spectra broadening in homogeneous solvents

Abstract The photophysical properties of a recently synthesized molecule, the bis-9,9′-(2-ethyl-10-hexyl)anthryl (BOA), have been studied with the parent molecule, the 9,9′-bianthryl (BA), in solvents of various polarity. The comparative analysis of the photophysical behaviour of these two probes has been achieved in order to emphasise environmental effects of the alkyl substituent on the aromatic chromophore. The results obtained in perfluoroalkane, reported for the first time in this paper, suggest that the charge transfer state (CT) of BA or BOA contributes to the fluorescence emission even in non polar solvents like cyclohexane. The fluorescence spectra of these two probes in perfluoroalkane have been introduced as standard spectra of the locally excited state form (LE) and used to obtain a new decomposition of the other overall fluorescence spectra into two emission components: one coming from the CT state and the second from the initial excited singlet state (LE). We have analysed the solute-solvent interactions not only in term of Stokes shift but also using the inhomogeneous band broadening parameter. The vibrational band decomposition found its validation in the agreement observed between the thermodynamic and spectroscopic properties of both excited states. A correlation between the inhomogeneous broadening and the Stokes shift of the charge transfer spectrum is exhibited. The existence of up to 50% of the CT state even in non polar solvents like cyclohexane has been shown. The dipole moment of the CT state of BA and of BOA can be estimated in the range of 6.2 to 10.3 D depending on the solvatochromic model used. It has been shown that the alkyl substituent of BOA destabilises the CT state and only four methylene groups of each lateral chain of BOA contribute to the local environment of the aromatic chromophore.

Photoinduced absorption and photoinduced reflectance in conducting polymer/methanofullerene films: Nonlinear-optical changes in the complex index of refraction.

We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)/methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index \ensuremath{\Delta}N=\ensuremath{\Delta}n(\ensuremath{\omega})+i\ensuremath{\Delta}\ensuremath{\kappa}(\ensuremath{\omega}) are obtained. The results indicate that the PIA spectra are dominated by \ensuremath{\Delta}\ensuremath{\kappa}(\ensuremath{\omega}); the subgap electronic absorptions peak at approximately 0.1 and 1.6 eV. The PIR spectra are dominated mainly by \ensuremath{\Delta}n(\ensuremath{\omega}), which exhibits a derivative spectral shape with zero crossing at 0.1 eV. The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear-optical applications of these materials. \textcopyright{} 1996 The American Physical Society.

Development of a two-site Hubbard model for analysis of the electron-molecular vibration coupling in organic charge-transfer salts

A two-site Hubbard model has been utilized for the analysis of the intense electron-vibrational bands in the i.r. spectra of organic charge-transfer (CT) salts with dimerized structures. It is shown that intermolecular charge oscillations in totally symmetric anti-phase vibrations are accompanied by a decrease of the energy of the highest occupied two-electron state of the dimer. This effect is responsible for frequency shifts of the electron-vibrational bands compared with relevant Raman active modes and can be described by dimensionless coupling constants. The absorption coefficient for the bands was found to be proportional to the frequency shift. Calculated values are in agreement with the experimental data on BEDT-TTF based monovalent CT complexes.

Polarized electronic spectra of the (CH 3NH 3) 2Cd 1 − xMn xCl 4 ( x = 0–1) perovskite layer doped with Cu 2+: Study of the Cl− → Cu2+ charge transfer intensity enhancement along the series

The polarized optical absorption spectra of the 2-D A 2Cd 1 − x Mn x Cl 4 ( x = 0–1; A = CH 3 NH 3) crystals doped with Cu 2+ are investigated. The analysis of both the charge transfer and crystal field spectra indicates that the copper impurities form CuCl 6 4− complexes with an elongated D 2h (nearly D 4h) symmetry. A salient feature is the enhancement of charge transfer band intensity as well as the presence of new intense bands at 21000 and 25000cm −1 observed on passing from x = 0 to x = 1 along the series. These bands are associated with Mn-Cu aggregates whose superexchange pathways involve the short (equatorial) CuCl bond (25000 cm −1) and the long (axial) CuCl bond (21000 cm −1) of the CuCl 6 4− complex. A noteworthy fact of these exchange coupled MnCu systems is that the transition energy of the first Mn 2+ excitations are resonant with the Cl − → Cu 2+ charge transfer transitions. This effect seems to play a fundamental role in the enhancement of intensity. The temperature dependence of the oscillator strength allowed us to estimate a ground state exchange constant J ≈ 70cm −1 for the MnCu pair. The results are compared with those obtained in MnCu aggregates in fluorides.