1,721,226 research outputs found
Periodic-dimer-cluster Models For the Study of the Optical-properties of Dimerized Half-filled Organic Charge-transfer Crystals
No full textVibrational and Electronic-spectra of One-dimensional Organic Charge-transfer Crystals - Model For A Tetramerized Cluster Structure
No full textRecently synthesized organic charge-transfer crystals have been shown to display an interesting stack structure of the type ...DAADDAAD... (D denotes the donor molecule, A denotes the acceptor molecule), in which a self-charge-transfer interaction AA or DD alternates with a DA one. A theoretical model is developed for a tetrameric cluster DAAD or ADDA which can account for the coexistence of both types of interaction. The model is based on an electronic extended Hubbard Hamiltonian and also takes into account the interaction between electrons and molecular vibrations. A description of the optical and Raman spectra of the model system is achieved and used to study some complicated features observed in the spectra of the charge-transfer complex TTF-TCNE (TTF denotes tetrathiafulvalene, TCNE denotes tetracyanoethylene)
Partial ionic ground state in the self-dimer mixed stack charge transfer crystal of TTF and TCNE
No full textThe vibrational spectra of the charge transfer complex of TTF with TCNE are explored. It is found that a degree of charge transfer of 0.5, namely that one electron has migrated from a dimer of TTF to that of TCNE molecules, characterizes the ground state of the complex. A Holstein-Hubbard model for a symmetric tetramer with two donor and two acceptor molecules allows the understanding of the vibronic bands observed in the infrared and Raman spectra and of the electronic charge transfer excitations
Periodic Cluster-models As A Tool To Understand the Optical-properties of Half-filled Charge-transfer Crystals
No full textMolecular cluster models with periodic boundary conditions (periodic cluster models) are shown to be able to explain the electronic and vibronic excitations of the optical spectra of half-filled charge transfer crystals also as a function of their dimerization
Characterization of Tcnqf4 Charge-transfer Complexes Through Vibrational and Vibronic Studies
No full textThe vibrational and vibronic data which enable one to use the strong electron acceptor TCNQF4 as a probe for the characterization of CT complexes containing this molecular unit are reported and applied to investigate systems of known and as yet unknown structure
Charge-transfer Organic-crystals - Molecular Vibrations and Spectroscopic Effects of Electron-molecular Vibration Coupling of the Strong Electron-acceptor Tcnqf4
No full textA fundamental aspect in the study of the charge–transfer (CT) organic crystals with ionic or partially ionic ground states is the investigation of the spectroscopic effect of the electron–molecular vibration coupling. 7,7,8,8‐tetracyano‐2,3,5,6‐tetrafluoroquinodimethane (TCNQF4), an electron acceptor much stronger than TCNQ, is an outstanding component of many interesting CT systems. A thorough vibrational analysis of the title compound and of its monomeric radical anion is reported. The analysis is based on the Raman depolarization ratio measurements and on infrared data of solutions of both neutral and ionic species as well as on polarized infrared spectra of oriented crystals of the neutral molecule. The vibrational assignment, completed by a normal coordinate analysis (NCA), brings to the identification of the ionization frequency shifts and to the choice of the fundamentals (b1uν19 and b2uν33) whose frequencies are diagnostic of the degree of charge transfer for a TCNQF4 moiety partner of a CT system. Through a combined use of a RHF‐CNDO/S(CT) electronic calculation and of the eigenvectors of the radical anion given by the NCA, the numerical evaluation of the linear e–mv coupling constants is carried out. The relative values of these constants are extracted, by applying the dimer model, from the vibronic features of the infrared spectrum of the low temperature phase of Rb–TCNQF4. This compound is recognized, by comparison with the alkali salts of TCNQ, to display a phase transition at T≂130 K from a regular to a dimerized stack structure. From the analysis of all these data it is possible to characterize the spectroscopic vibronic effects related to e–mv coupling in CT systems of TCNQF4. This is successfully verified by the segregated dimerized stack system of DBTTF–TCNQF4 and applied to the not yet characterized system TSF–TCNQF4 which is recognized here as a fully ionic complex with segregated dimerized stack motif. As a consequence, through the vibrational spectra, TCNQF4 can be fruitfully used as a sensitive probe for structural and electronic characterization of its CT complexes
TTF-TCNE a charge transfer p-molecular crystal with partial ionic ground state: Optical properties and electron-molecular vibration interaction
No full textThe electronic ground state structure of the charge transfer molecular complex TTF-TCNE is investigated on the basis of its electronic and vibrational spectra. Highly oriented polycrystalline films and the spectra of the fully deuterated complex, TTF-d(4)-TCNE, allow one to obtain a full exploitation of the spectra. Using the vibrational frequencies as local probes of the electronic structure one finds a value of 0.5+/-0.1 for the degree of charge transfer of this molecular solid. This partial degree of charge transfer and the alternation of self-dimers of TTF and TCNE along the one-dimensional electronic pi-structure reveal themselves in the vibrational spectra and particularly in the charge transfer vibronic resonances present in the infrared and Raman spectra. These resonances and the electronic spectrum related to the charge transfer excitations are understood on the basis of a Holstein-Hubbard model which allows the determination of the electron-intramolecular vibration coupling constants of TTF and TCNE
Infrared Properties of A 2-d Organic Conductor - Alpha-(bedt-ttf)2i3 In Its High and Low-temperature Phases
No full textPolarized reflectance data (300–10000 cm−1), their Kramers-Kronig transformation and absorbance spectra (400–4000 cm−1) at room temperature and below the metal-insulator transition at T=135K are reported for α-(BEDT-TTF)2I3. A preliminary analysis of the vibrational behavior of the BEDT-TTF molecule is reported and guides the assignment of the vibronic features to specific intramolecular modes. A microscopic mechanism of vibronically induced electron charge oscillations in the presence of a 2-D network of electron transfer interactions is discussed for the first time and the crystal modes active in this mechanism are sorted out on the basis on symmetry arguments. Some characteristics of the low temperature phase are deduced from the analysis of the low temperature spectra
Optical-properties of Molecular Conductors - One-dimensional Systems With Twofold-commensurate Charge-density Waves
No full textA model is presented for the optical properties in the near to far infrared of a twofold-commensurate one-dimensional system of noninteracting molecular-ion chains. Each chain may be subject to an arbitrary type of lattice distortion that doubles the periodicity of the regular chain and is composed of a lattice dimerization (LD) and/or an alternating molecular deformation (AMD). The effect of an external potential induced by the presence of nearby chains of closed-shell counterions is also accounted for. The linear coupling of the electrons to an arbitrary number of intramolecular modes and to one longitudinal acoustic phonon branch is treated in the adiabatic linear-response approximation. No direct electron-electron interaction is explicitly included but the limit case of noninteracting spinless fermions in a large-U system can be dealt with in the present scheme. The results of the model can be directly applied to the analysis of the experimental infrared data of many conducting organic radical salts of 2:1 stoichiometry where LD and/or AMD of small amplitude occur and the on-site electron-electron correlation is thought to play a major role. The model fitting of the data can be used to obtain information on the one-electron bandwidth, the individual contributions to the total gap of charge density waves components centered on the bonds and on the sites, the relevance of the counterion potential, and the strength of the electron-phonon and electron-molecular vibration interactions. Some of these potentialities as well as the ‘‘selection rules’’ governing the infrared activity of the intramolecular and intermolecular modes as phase phonons are illustrated by numerical model calculations. Self-consistent relations and practical criteria that allow one to use the minimal number of adjustable parameters in the calculations are also presented
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