Polaron Structure Research Articles

An ab initio investigation of the charge-transfer complexes of alkali atoms with oligo (α,α′) thiophenes and oligoparaphenylenes: A model calculation on polaronic and bipolaronic defect structures

Ab initio self-consistent field (SCF), second- and third-order Mo/ller–Plesset calculations on the charge-transfer complexes of one and two alkali atoms with oligothiophenes and oligophenylenes are reported. Complexes up to quaterthiophene and quaterphenyl with Li, Na, and Cs have been investigated. The mono-alkali complexes are related to polaron and the di-alkali complexes to bipolaron defects. Extensive basis set investigations have been performed. The quinoid structures which result from the interaction of the alkali atoms with the oligomers are well reproduced by the SCF method, whereas for accurate interaction energies electron correlation effects are very important. Our calculations show that the spatial extension of the polaronic and bipolaronic defects on the chain depends strongly on the fact whether counterions are explicitly taken into account or not. From previous quantum chemical calculations one finds that, e.g., in the case of oligothiophene dications the bipolaron defects extend over 9–11 thiophene units whereas our calculations under inclusion of the electron donating alkali atoms show that in this case the defects are much more localized. A very crude estimate of the energy balance between two polaron defects and one bipolaron gives about 15 kcal/mol in favor of the bipolaron structure. We also find a rather similar behavior along the alkali series as concerns interaction energies, defect structures, and charge distribution patterns.

Robustness of the single dressing fraction characterization of polaron structure in multi-mode partial dressing theory

In approaches to polaron theory which attempt to unify small and large polaron behavior, the notion of “partial dressing” plays a central role in determining the allocation of interaction energy between correlations of the small and large polaron types. This paper examines some of the consequences of generalizing from a single dressing fraction to a distribution of dressing fractions. Employing a distribution of dressing fractions allows distinct phonon modes to allocate interaction energy in distinct ratios, thereby supporting a more flexible description of polaron structure than that based on a single dressing fraction applied uniformly to all modes. The distribution of optimal dressing fractions can be characterized by a mean dressing fraction whose properties are in close agreement with the single dressing fraction used previously.

Polaron excitations in doped C60: Effects of disorder.

Effects on C$_{60}$ by thermal fluctuations of phonons, misalignment of C$_{60}$ molecules in a crystal, and other intercalated impurities (remaining C$_{70}$, oxygens, and so on) are simulated by disorder potentials. The Su-Schrieffer-Heeger--type electron-phonon model for doped C$_{60}$ is solved with gaussian bond disorders and also with site disorders. Sample average is performed over sufficient number of disorder configurations. The distributions of bond lengths and electron densities are shown as functions of the disorder strength and the additional electron number. Stability of polaron excitations as well as dimerization patterns is studied. Polarons and dimerizations in lightly doped cases (C$_{60}^{-1,-2}$) are relatively stable against disorders, indicated by peak structures in distribution functions. In more heavily doped cases, the several peaks merge into a single peak, showing the breakdown of polaron structures as well as the decrease of the dimerization strength. Possibility of the observation of polaronic lattice distortions and electron structures in doped C$_{60}$ is discussed.

Optical properties of polyaniline

Optical absorption has been used to study polyaniline at various levels of oxidation and protonation. The conversion of leucoemeraldine base to its salt accounts for the blue shift of the π-π∗ absorption from 3.94 to 4.17 eV. The emeraldine base has an absorption peak at 2.1 eV due to the localized benzenoid HOMO (highest occupied molecular orbital) to quinoid LUMO (lowest unoccupied molecular orbital) excitation. Protonation of the emeraldine base causes a lattice distortion of polyaniline to form a polaronic lattice structure, which accounts for the shift of the 2.1 eV absorption to 1.5 eV. Further protonation of emeraldine salt causes the breakdown of the polaronic lattice to a confined bipolaron lattice. When the acid strength of the solution is very high (96% H 2SO 4), most of the amine nitrogens are protonated. Characteristic absorptions of isolated quinoid imine units (∼2.5 eV) and benzene units (∼4.8 eV) have been found. The evolution of band structure corresponding to the optical-absorption change during the protonation of polyaniline at each specific oxidation state is studied by gradually changing the acid strength of the solution to which the polymer is subjected. We have also studied the optical-absorption change at one fixed acid strength by slowly oxidizing leucoemeraldine to emeraldine then to pernigraniline by electrochemical and chemical means. Detailed information has been obtained on the band structure changes in relation to lattice structure interconversions during the transitions between various oxidation and protonation states.

Random-phase-approximation approach to phonon spectra in doped and undoped halogen-bridged transition-metal compounds.

We have developed a method based on a real-space random-phase approximation (RPA) to study the phonon spectra of doped and undoped halogen-bridged transition-metal-chain complexes. The method is very convenient in cases of spatially inhomogeneous mean-field structures. Thus, we are able to find the usual extended phonon modes as well as local modes associated with polaron, bipolaron, kink, exciton, or impurity structures in intrinsically or extrinsically doped systems. The corresponding infrared absorption spectra and Raman spectra (as a function of the excitation energy) are calculated. We demonstrate the method for a homogeneous charge-density-wave system, and for electron and hole polarons. We find that each type of defect has specific signatures in both infrared and Raman spectra, which can help to identify different types of defects experimentally.

The polaron structure in the conducting layers of the high- temperature superconductors

The theory of the polaron-dimensional lattices arising in the conducting layers is constructed. The discussion is carried out using the model of two groups of charge carriers, namely: light holes and the heavy holes from the narrow band inducing the large radius polarons. It is shown that the light holes destroy the polaron state. The critical shielding length at which the polaron is destroyed is calculated. The dependence of the hole energy in the polaron well, the polaron size and the polaron mass on the concentration of the light carries, has been determined. The polaron lattice structures have been investigated. The energy of the polaron lattices and the spectrum of the smll oscillations were determined. It is shown that the results strongly depend on the shielding electron spectrum dimension. In particular, for a two-dimensional dispersion law the characteristic polaron properties do not depend on the shielding electron concentration.

Characterization of a bipolaronic form in poly(2-methylaniline)

The electropolymerization of 2-methylaniline gave uniform electroactive polymer films which were analyzed by cyclic voltammetry and spectroelectrochemistry in different acidic conditions. It has been found that a protonated (bipolaronic) form can be stabilized in the fully oxidized polymer at pH < −0.5 and this new electronic structure is characterized by an absorption band at 630 nm. However, in-situ conductivity measurements have shown an insulating behaviour for this protonated fully oxidized form whereas the partially oxidized (polaronic) structure is conducting.

Will bipolarons be formed in heavily oxidized polyaniline?

The energetic stability and electronic structure of a polaron lattice generated in heavily oxidized polyaniline (PAn) were theoretically examined using the one-dimensional tight-binding self consistent field-crystal orbital (SCF-CO) method. The spin system in the polaron lattice was first explicitly treated by the calculation for PAn. Geometry for the heavily oxidized PAn was essentially taken from the experimental data for the oligomer salt. The calculation results show that the formation of a polaron structure is dominant over that of bipolarons. The reason for the dominance of the polaron lattice is discussed.

Bipolar absorption of light in polyarylenephthalides

The idea of self-localized states in polymers, which are created in the polymers by introducing nor p-type impurities [i, 2], turned out to be very fruitful for explaining the properties of polyacetylene. The case of trans-polyacetylene with a degenerate ground state is, however, still unique, since most of the currently known conducting macromolecules do not have a degenerate ground state, for example, cis-polyacetylene, polyphenylene, polypyrrole, etc. Self-localized states of the polar type are realized in such polymers [3]; these states are accompanied by interesting physical phenomena: generation of local levels in the gap, photoluminescence with anomalous Stokes shift, absence of spin on the charge carriers, photostimulated absorption, etc. In polyaromatic systems the polarons are electrons or holes, surrounded by the deformed part of the polymer chain (quinoid structure). According to existing calculations the number of deformed rings ranges from four for a bipolaron to six for a polaron [4]. Such a polaron structure does not have anything in common with the structure of self-localized states in existing inorganic crystals. The most efficient methods for studying polaron states in polymers experimentally are optical and electrophysical methods.

Slow electrons in condensed matter: The large polaron.

We discuss the interactions and energy dissipation of subexcitation electrons in homogeneous and isotropic condensed materials in the framework of a perturbative, generalized polaron theory. The polaron structure includes only Fourier components of the Coulomb interaction with momentum transfers \ensuremath{\Elzxh}q less than the polaron momentum p, thus limiting the treatment to long-range coupling. The coupling between the electron and rovibrational modes of the material is represented by off-diagonal form factors. The energy contributed to the polaron by virtual excitations is represented by the real part of a perturbation term, whose imaginary part represents energy dissipated (irreversibly) into other excitations. Quantitative knowledge of the relevant parameters remains undeveloped.

Fine structure of triplet exciton polarons in polydiacetylene molecules

Triplet states on conjugated polydiacetylene chains which are created by UV excitation are examined experimentally using ODMR spectroscopy. The observed fine structure shows that the triplet state can be ascribed to the conjugated chain rather than to the side groups and that it is localized. This leads to the suggestion of an exciton polaron. In the theoretical part the wave function of the exciton polaron is calculated using the configuration model in analogy to the description of pz radical electrons on carbene chain ends of reactive short-chain intermediates. The total fine structure tensor is obtained by summing up the fine structure tensors of the individual configurations weighted by their probability densities. The transfer integral t of the configuration model is fitted with respect to good agreement between experimentally observed and calculated fine structure parameters. Finally, comparison between the experimentally observed ESR linewidth and the calculated hyperfine structure splitting shows that the linewidth is motionally narrowed leading to the conclusion that the exciton polaron is mobile.

On the Charge Structure of Polarons in Trans‐Polyacetylene

AbstractThe charge structure of polarons (centered at long bonds) and antipolarons (centered at short bonds) is investigated on rings with N = 4 M sites in the SSH‐model and in the continuum model. In contrast with previous statements the charge distribution has to be described by two smooth penetrating functions. They can be reasonable well obtained within the continuum theory.

Ab initio SCF MO calculation on model compounds of doped polyacetylene

The electronic structure of alkaline metal doped polyacetylene (PA) is studied by the ab initio SCF MO calculation on the model compounds. It is found that two Li atoms coordinate to the polyene molecule and donate electrons to the LUMO of polyene. The results on model compounds, C 12H 14Li 2, showed that the triplet charge transfer state is more stable than the singlet state. The charge density in the HOMO of the polyene is strongly polarized, showing a model of the CDW state. Its polyene chain has a characteristic structure, half has a charged soliton structure and the other half shows little bond alternation. The model provides an atomistic view of the sliding and pinning of the CDW and the origin of paramagnetism in the polaron structure of the heavily doped PA chain.

Inverse Spectral Theory and Incommensurate Peierls Phases

AbstractA simple version of the inverse spectral theory for finite‐gap potentials of the Hermitean Zhakarov‐Shabat operator is presented, and the kink and polaron structures in quasi‐one‐dimensional systems with a nearly half‐filled band are discussed in terms of this theory. The gap equation for one kink band in systems with a nearly 1/3‐filled band is solved for special parameters, but no general finite‐gap solution is found in this case.