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The affects of alkali-metal atoms on the electronic structure of disordered and highly ordered thin films of the medium-size aromatic hydrocarbon hexa-peri-hexabenzocoronene (HBC) have been investigated by valence and core level photoelectron spectroscopies-ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS)-and accompanying quantum-chemical calculations. Deposition of Li or Na atoms in situ leads to new spectral features in the UPS spectra, which are related to formerly unoccupied molecular states. The binding energies and intensities of these features depend on the nature of the counterion. The smaller Li ion exhibits a stronger influence on the electronic structure than its sodium counterpart. In the intercalation of sodium into ordered films, a high degree of molecular order is preserved, and, at high deposition levels, a surface dipole is formed that is associated with the layered structure of the compound. Remarkably, high levels of alkali-metal storage of at least one alkali-metal atom for each four carbon atoms have been observed, indicating clearly the potential use of these graphene materials in lithium-ion batteries with a high charge-storage capacity.

The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1 s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure. © 2001 American Institute of Physics.

The electronic structure of pristine and sodium-doped poly(p-pyridine) has been studied using both ultraviolet and x-ray photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum-chemical calculations. Electronic band-structure calculations are performed for isolated chains with different connectivity patterns (head-to-tail and head-to-head), using the valence effective Hamiltonian (VEH) method, with geometries derived from optimizations using the Austin Model 1 Hamiltonian. The density-of-valence-states are derived directly from the VEH band structure. Excellent agreement is obtained between the theoretical simulations and the experimental data, which allows for a detailed assignment of the different peaks in the spectra. The C(1s) and N(1s) shake-up spectra of poly(p-pyridine) are analyzed on the basis of corresponding data for pyridine in the gas phase. Upon sodium doping of poly(p-pyridine), new states are observed within the otherwise forbidden energy gap. These ...

Correlated semi-empirical quantum-chemical calculations are performed to rationalize the origin of the large first- (α), second- (β), and third- (γ) order molecular polarizabilities in carotenoidlike molecules capped at a single end by an acceptor group. The degree of ground-state polarization of the chromophores is made to vary via the application of an external homogeneous electric field in the range 107–108 V/cm. The most interesting feature is a stepwise evolution of the longitudinal component of the dipole moment with field, caused by charge transfer toward the acceptor end of the molecule. Since the steps are abrupt, this results in very large values of the molecular polarizabilities, in accordance with the derivative relationships among the dipole moment and α, β, and γ. The longitudinal components of the molecular polarizabilities are analyzed as a function of both the external field and the bond-order alternation (BOA) parameter. The ability of simple models to describe the evolution of the molec...

The behavior of lithium atoms deposited on the surfaces of ultra-thin spin-coated films of poly(dioctylfluorene), and of condensed molecular solid films of tris(8-hydroxyquinoline) aluminum, have b ...

We extend the symmetrized density-matrix renormalization-group method to compute the dynamic nonlinear optical coefficients for long chains. By computing correction vectors in the appropriate symmetry subspace, we obtain the dynamic polarizabilities, alpha(ij)(omega), and third-order polarizabilities gamma(ijkl)(omega,omega,omega) of the Hubbard and "U-V'' chains in an all transpolyacetylene geometry, with and without dimerization. We rationalize the behavior of and on the basis of the low-lying excitation gaps in the system.

The effect of nonadiabaticity on optical linear polarizability ({alpha}) and third-order polarizability ({gamma}) is investigated numerically for a Holstein-Hubbard finite chain system. The variations of {alpha} and {gamma} are studied as a function of the nonadiabaticity parameter, electron-phonon coupling strength and electron-electron interactions. For a specific range of values of each of these parameters, {alpha} and {gamma} are significantly enhanced. Disorder in either electronic or phonon degrees of freedom is also found to enhance optical nonlinearity. {copyright} {ital 1999} {ital The American Physical Society}

The polarized absorption spectra in the bc face of an α-sexithienyl (T6) single crystal have been measured at 4.2 K. The origin of the lowest electronic transition is at 18 360 cm−1 and has been assigned to the lowest b-polarized au Davydov component of the 1 1Bu molecular level. The second optically allowed Davydov component (bu) is polarized in the ac crystal plane and is located at 20 945 cm−1. Therefore, the interchain interaction leads to a Davydov splitting of about 2600 cm−1. A quantum chemical model, which considers the total molecular wavefunctions for each transition, shows good agreement with the experimental findings for the energy and polarization of the optically allowed crystal levels. The vibronic manifold in the absorption spectra has been interpreted in terms of the Herzberg–Teller and Franck–Condon vibronic coupling. In particular, two false origins due to the coupling of the molecular electronic levels 1 1Bu and 2 1Bu have been identified at 18 486 and 18 657 cm−1. Consistently, the em...

Using a symmetry adapted density matrix renormalization group (DMRG) formulation, we have calculated various optical spectra, including linear absorption, electroabsorption, third-harmonic generation, and two-photon absorption, for a series of linear polyene molecules (up to 40 carbon sites) modeled by an extended Hubbard–Peierls Hamiltonian within the intermediate correlation regime. The theoretical two-photon absorption spectrum for trans-octatetraene is in good agreement with experiment. By comparing the experimental linear absorption spectra and theoretical higher Ag state energies, it is suggested that the so-called cis-band actually corresponds to the threshold of the mAg band.

As a first step towards the theoretical investigation of polymer alloy composites formed by the dispersion of carbon black particles throughout a polyethylene/polystyrene blend, we discuss here the results of ab initio Hartree—Fock quantum-chemical calculations, including correlation effects via second-order Moller—Plesset perturbation theory, on small model systems. These are chosen to simulate the various interfaces that appear in the composites: the interfaces between the polymers and the graphitic surface of carbon black are modeled by the complexes formed by benzene (taken as the substrate) with methane, ethane, propane, and ethylbenzene while the polymer/polymer interface is modeled by the propane/ethylbenzene system. The functionalization of the benzene substrate with hydroxyl, amine, carboxylic, or quinoid groups is also investigated in order to determine the influence of such moieties that can be present on the carbon surface. Our main goal by studying these model systems is to characterize the primary binding interaction sites and to evaluate the energies involved in the bonding; our results can also serve as a reference for molecular mechanics calculations on more extended systems.

The hyperpolarizabilities for electric dipole second-harmonic generation (SHG) due to the bilinear combination of an electric field and either the electric-field gradient (μeeq) or the associated magnetic field (τeem), and for electric quadrupole SHG quadratic in the electric field (μqee) have been calculated for the C60 molecule at pump photon energies from 0.5 to 2.2 eV using the valence effective Hamiltonian method with a minimal STO-3G basis; magnetic dipole SHG quadratic in the electric field (τmee) is zero by symmetry in C60. The coefficients exhibit resonances at photon energies near 1.9 for μeeq, 1.4 for μqee, and 2.0 eV for τeem. They are combined with empirical local-field factors and a theoretical local field-gradient tensor to calculate the composite susceptibility χ(2) for SHG from crystalline C60 in the same energy range. The susceptibility shows several resonances that arise from those in μeeq, μqee, and τeem. The results are able to reproduce the magnitudes and signs reported experimentall...

The lowest singlet and triplet excited states in cyano-substituted phenylene vinylene oligomers are characterized by means of configuration interaction calculations. First, the vertical singlet-singlet, S 0 → S 1 , singlet-triplet, S 0 → T 1 , and triplet-triplet T 1 → T n , excitation energies are evaluated in oligomers ranging in size from two to five phenylene rings; the spatial extent of the S 1 , T 1 , and T n excited states is estimated on the basis of a simple analysis of their wavefunctions. We then pay attention to the lattice distortions taking place in the lowest singlet and triplet excited states of these model oligomers. In each case, the results are compared to those obtained for the corresponding unsubstituted oligo(phenylene vinylene)s. Besides the bathochronic shift associated with the electroactive character of the substituents, an overall localization of the excited state wavefunction is found when going from unsubstituted to cyano-substituted oligomers.

The one- and two-photon singlet excited states, relevant for the description of the nonlinear optical response of the octatetraene molecule, have been characterized by the ab initio complete active space self-consistent-field/multiconfiguration second-order perturbation theory (CASSCF/CASPT2) technique. We compare the nature of the main electronic couplings among excited states, as obtained at the ab initio level, to that provided by the semiempirical Pariser-Parr-Pople (PPP) method. Both approaches indicate the presence of a dominant Ag excited state at about 1.6 times the optical gap. However, in contrast to the PPP results, other two-photon states are found in the ab initio calculations to contribute significantly to the cubic polarizability.

Using a symmetrized density-matrix renormalization-group formulation, we have investigated the electronic binding energy of the lowest optically allowed exciton (1B{sub u}) within an extended Hubbard-Peierls model (with parameters U, V, and {delta}) for conjugated chains with N=80 sites. Three symmetries, the C{sub 2}, spin parity, and electron-hole symmetries, have been applied to construct the projector operator. Analysis of our results on the ratios between exciton binding energies and exciton energies, sheds light on the current experimental and theoretical controversy on exciton binding for polyacetylene and polyparaphenylene vinylene. We show that in the absence of dimerization, the exciton binding energy is vanishingly small for U/t up to 5 and V/t up to 2.3; for a finite dimerization, it is only when V is large enough that the exciton gets to be significantly bound. This result questions the applicability to conjugated polymers of the strong correlation picture, in which the exciton binding energy is equal to V. {copyright} {ital 1997} {ital The American Physical Society}

Evidence for exceptionally high dipole moments in a series of nonlinear optical chromophores doped into poly(methyl methacrylate) films has been found. The strength of the poling field/dipole interaction is measured directly from incidence angle dependence of the second harmonic (SH) intensity generated from corona poled films. Values of the dipolar orientation parameter, , greater than 0.5 can be achieved for externally applied fields of only MV m −1 . Quantum chemical calculations of the evolution of the dipole moments of the compounds versus local reaction field support the conclusion that extremely high dipole moments characterise these molecules.

We investigate theoretically the chemical structure and vibrational properties of the interface between aluminum and polythiophene, taken as a prototype conjugated polymer, considering both the deposition of the metal on a polymer substrate and the adsorption of the polymer on the metal surface. We perform quantum-mechanical calculations to determine the geometric and electronic structure and vibrational frequencies in a series of molecular model systems for the Al-polythiophene interfaces.

Using a symmetrized density matrix renormalization group formulation, we have investigated the electronic binding energy (E b ) of the lowest optically allowed exciton (1B) and the crossover behavior of the 1B/2A states within an extended Hubbard-Peierls model (with parameters U, V, and 5) for conjugated polymer chains with N=80. The E b value is found to be around 0.1 eV for polyacetylene and 0.22–0.45 eV for PPV when considering a realistic range of parameters. The 1B/2A crossover behavior is investigated for a wide range of parameters.

This contribution aims at providing a broad overview of the quantum-chemistry and density-functional theory techniques that are currently used to describe the electronic structure of conjugated molecules and polymers. Rather than dwelling on detailed mathematical descriptions, we emphasize the limitations and approximations involved in each type of approach and illustrate in which ways different methods are related to one another. A specific example is then discussed in order to illustrate the usefulness of quantum-mechanical calculations in the field of conjugated compounds.

We extend the density-matrix renormalization-group (DMRG) method to exploit parity, ${\mathit{C}}_{2}$ (rotation by \ensuremath{\pi}), and electron-hole symmetries of model Hamiltonians. We demonstrate the power of this method by obtaining the lowest-energy states in all eight symmetry subspaces of Hubbard chains with up to 50 sites. The ground-state energy, optical gap, and spin gap of regular U=4t and U=6t Hubbard chains agree very well with exact results. This development extends the scope of the DMRG method and allows future applications to study of optical properties of low-dimensional conjugated polymeric systems. \textcopyright{} 1996 The American Physical Society.

We present a joint experimental and theoretical investigation of the electronic excitations in transition metal‐containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that π conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain‐length dependence of the singlet–singlet, S0→S1, singlet–triplet, S0→T1, and triplet–triplet, T1→Tn, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T1, is strongly localized on a single phenylene ring while the S1 and Tn states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang–Rhys analysis of the triplet emission spectrum in model systems.

We investigate the physisorption of a polyene molecule onto the basal plane of graphite. We use an ab initio Hartree—Fock quantum-chemical approach, including correlation effects via second-order Moller—Plesset perturbation theory, to study the geometric and electronic structure of the butadiene/benzene complex chosen as a model system. The aim of this work is to define general trends concerning the evolution in complex stability and nature of the interactions taking place between the substrate and the adsorbate, according to the configuration adopted by the complex.

A physical model is presented that aims at rationalizing the selectivity of bond breakage observed when polymethylmethacrylate is irradiated by ions in the 10–500 Kev energy range. This model, previously proposed by Brandt and Ritchie, is based on electronic collective effects. The coupling between the pure plasma oscillation at ωp and the oscillation of free electrons at (ω2 k0)½ makes the whole electronic population resonant at the frequency ωrp = (ω2 p+ (ω2 k0))½. By computing the valence-band density of states, we are able to calculate (ω2 k0) and then to deduce the theoretical value of ωrp. On the other hand, we provide an experimental measurement of ωrp and study its dependence on ion fluence by electron-energy-loss spectroscopy. The validity of the model of Brandt and Ritchie is then discussed in the light of both theoretical and experimental data.

The electronic structure of conjugated polymers of current interest in the development of novel organic light-emitting diodes, poly( p -phenylenevinylene) and its derivatives, has been studied experimentally by ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy as well as theoretically using quantum-chemical calculations. The π-electronic structure near the Fermi level, which plays a major role in such phenomena as charge carrier injection, interface formation and light emission, has been the focus of our study on the pristine polymers. Also, the early stages of metal/polymer interface formation have been studied, namely aluminum and sodium on poly (2,5,2′,5′-tetrahexyloxy-8,7′-dicyano-di- para -phenylenevinylene).

We report the results of a theoretical study on torsion and its effect on the structural and electronic properties of cyano-substituted poly ( p -phenylene vinylene) and its dimethoxy and dimethyl ring-substituted derivatives. Geometries, inter-ring torsion potentials, and potentials associated with torsion of the ring substituents are calculated using the semiempirical quantum-chemical Austin model 1 method. The evolution of the bandgap, ionization potential, and electron affinity is evaluated using the pseudopotential valence effective Hamiltonian method.

We have applied the correction vector (CV) method coupled with the intermediate neglect of differential overlap-single and double configuration interaction semiempirical approach to calculate the nonlinear optical coefficients of donor-acceptor conjugated compounds consisting in ortho-, meta-, and para-substituted benzene derivatives as well as the corresponding octupolar molecules. The CV method has been shown to be both exact and computationally effective. We find that for weak donors such as a methyl group, the NLO response efficiency orders as: octu > para > meta > ortho; however, if the donor becomes stronger and consists in an amino group, then: para > ortho > meta > octu. The results are in agreement with experimental findings and can be rationalized in terms of a competition between the effects related to structural resonance and charge separation length.

By means of a high-level configuration interaction technique, we describe the nature of the lowest singlet and triplet excited states in oligo(phenylene vinylene)s and oligothiophenes. We first calculate the evolution with chain length of the singlet-singlet and triplet-triplet excitation energies, as well as that of the singlet-triplet energy difference. The theoretical results compare favorably with the available experimental data. We then investigate the geometry relaxation phenomena occurring in the lowest excited states of model oligomers. We found that the lowest triplet state is characterized by more pronounced and local lattice distortions than the singlet state, especially in the case of the phenylene vinylene oligomers.

We investigate the electric-field-induced change in the absorbance (i.e. electroabsorption response) of samples of β-carotene and α,ω-bis(1,1-dimethylheptyl) -1,3,5,7,9,11,13,15-hexadecaoctaene dispersed in a polystyrene matrix. From the change in absorbance, the refractive index and the thickness of the polymer film, the third-order nonlinear d.c. Kerr susceptibility, χ (3) (-ω; 0,0,ω), can be calculated via the Kramers-Kronig relationship. Results are also presented for the simulated electroabsorption of β-carotene, derived from calculation of the absorption both in the presence and absence of an applied electric field.

The electronic structure of cyano‐substituted poly(2,5‐dihexyloxy‐p‐phenylene‐vinylene), or CN‐PPV, has been studied in both pristine and doped states. Ultraviolet photoelectron spectroscopy (UPS) and x‐ray photoelectron spectroscopy (XPS), as well as optical absorption spectroscopy have been carried out under ultrahigh vacuum (UHV) conditions, and the results have been interpreted with the help of quantum‐chemical calculations. For the pristine polymer, the addition of cyano groups to the vinylene units does not affect the width of the π‐bands closest to the Fermi level; however, the positions of the flat parts of the upper π‐bands are shifted by approximately 0.4 eV towards higher binding energies relative to the Fermi energy, as compared with the corresponding bands of other alkoxy‐substituted poly(p‐phenylenevinylene)s. On the other hand, there are only marginal differences in the optical absorption spectra; the interband absorption onset is comparable to the values for alkoxy‐substituted poly(p‐pheny...

The chemical nature and the electronic structure of metal/conjugated polymer interfaces are investigated in the context of polymer-based light-emitting diodes. We consider the interaction of low-workfunction metals (Al, Ca) with the surface of conjugated polymers or model oligomer molecules with a combined experimental and theoretical approach. The early stages of the interface formation are followed with X-ray and ultraviolet photoelectron spectroscopies and the experimental data are compared to the results of quantum chemical calculations. The reactions of Al and Ca with the organic surface are found to be fundamentally different: while the former forms new covalent bonds onto the polymer backbone, the latter tends to dope the conjugated system. Both types of reaction are expected to modify drastically the electronic properties of the polymer semiconductor.

The aluminum/polyethylene terephthalate interface is investigated with a combined theoretical and experimental approach, in order to understand the interactions occurring at the molecular level when the metal is deposited onto the polymer surface. The theoretical approach consists in performing quantum‐chemical calculations on short molecular model systems interacting with a few Al atoms. The geometric structure of the organometallic complex is optimized and its stability as well as the changes in charge density due to Al bonding are evaluated. The theoretical results are compared to experimental x‐ray photoelectron spectroscopy data collected during the early stages of interface formation. In particular, the evolution of the polymer core level peaks is paralleled to the Al‐induced modification of the charge density on the model molecules. Al is found to react preferentially with the ester group by forming covalent bonds with the oxygen and/or the carbon atom of that group.

The electronic structure of a conjugated polymer of current interest in organic LED's, poly(p-phenylenevinylene), or PPV, has been studied by ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. The focus of this work is on the nature of the pi-electronic band structure nearest the Fermi level and the physical influence of finite torsion angles, the geometry of the polymer backbone, on the electronic properties of the system. Details of the ct-electronic bands, as reflected in the associated density-of-states, are observed clearly in the spectra, from which some underlying geometrical details of the polymer system can be deduced. The experimental spectra have been analyzed theoretically using band structure calculations based upon the valence effective Hamiltonian (VEH) model. In addition, in order to control the band structure, three ring-substituted derivatives of PPV, each of which induces a different bonding geometry in the backbone, have been studied. The changes in the experimental results can be explained on the basis of both physical and chemical interactions of the substituents with the backbone, which lead to geometrical changes along the backbone, which influence the ct-bandwidths and contribute to differences in both the optical absorption threshold and the binding energy of the valence band edge.

Summary form only given. We present a detailed theoretical analysis of the geometric and electronic structure of polydicyanomethylene-cyclopenta-dicyclopentadiene, a polymer made by the succession of bicyclopentadiene units in which the two rings are linked by a dicyanomethylene moiety. The geometry optimization is carried out by means of the semiempirical AMl technique. The results indicate that the introduction of the withdrawing group between the two cyclopentadiene rings of the unit-cell does not significantly affect the geometric structure of the polymer. Based on the analysis of the bonding-antibonding electronic patterns appearing on the HOMO and LUMO levels of the polymer, as obtained from VEH band-structure calculations, it is shown that the small calculated bandgap, 0.17 eV, is mainly due to a strong stabilizing interaction occuring between the LUMO of the conjugated skeleton and the LUMO of the withdrawing group.

We present a theoretical investigation by the sum‐over‐states formalism of the static first‐ and third‐order polarizabilities of thienylenemethylidene oligomers; following the incorporation of a single conjugated carbon between the rings, these compounds present a partially quinoid geometry. We use a multireference determinant single–double configuration interaction approach to calculate excited‐state energies, state dipole moments, and transition dipole moments which are introduced in the sum‐over‐states expressions. All the results are discussed in comparison to those obtained in thiophene oligomers of similar sizes; they indicate that the third‐order polarizabilities are smaller for the methine‐bridged compounds than for the corresponding thiophene oligomers. This feature is rationalized by analyzing the main optical (virtual excitation) channels that contribute to the nonlinear response.

The semi-empirical Austin Model 1 and the non-empirical pseudo-potential valence effective Hamiltonian (VEH) methods as well as the local spin density (LSD) approximation technique have been applied to the investigation of the doping-induced electronic and geometrical changes in some conjugated molecules related to poly(p-phenylene) and poly(p-phenylenevinylene) (PPV): biphenyl, stilbene and a phenyl-capped dimer of PPV. The theoretical results are compared with experimental valence band spectra, as recorded by ultraviolet photoelectron spectroscopy (UPS). The experimental UPS studies show that two ingap states are detected upon doping with alkali metals. The energy splitting between the two in-gap states increases as the molecule size decreases. The results of the LSD calculations agree very well with the experimental results, while the VEH method overestimates the energy splitting for the small molecules. The LSD modelling also indicates a destabilization of several high binding energy valence levels, due to the presence of counter-ions, in agreement with experiment.

A brief overview of measurement methods and quantum chemical calculation techniques for studies of the chemical and electronic structure of conjugated polymer surfaces and interfaces with metals is presented. Some new results are mentioned, which are published as separate contributions in these proceedings.

We develop the Valence Effective Hamiltonian method to calculate the magnetic dipole (MD) and electric quadrupole (EQ) contributions to the hyperpolarizability β of molecular systems. For both the C60 and C70 molecules, we find that the MD contribution leads to a β value which is several times larger than that of the EQ contribution. Taking into account the local field correction factor for C60 in solid state, we calculate x (2) susceptibility values that are in excellent agreement with recent experimental data.

The early stages of metal/polymer interface formation between calcium and poly(2,5-diheptyl-p-phenylenevinylene) (PDHPV) have been studied using both X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Charge transfer is observed from the metal atoms to the polymer; as a result the calcium atoms at the interface are ionic, and negative bipolarons appear as the charge-carrying species on the polymer chains. This n-type doping of PDHPV by calcium leads to the appearance of new electronic states in the polymer bandgap. The calcium atoms appear to diffuse into the near surface region of the polymer, rather than forming a well-defined overlayer on the organic films.

Preliminary results are presented for a study of the electronic band structure of thin single crystals of 4-BCMU polydiacetylene using angle-dependent ultraviolet photoelectron spectroscopy with polarized light from synchrotron radiation. In particular, the crystal momentum dependence of the lowest binding energy π-band, the π-band dispersion along the b - axis , has been mapped. The experimental results are compared with the results of quantum chemical calculations.

The interactions between aluminum atoms and model molecules representing trans‐polyacetylene are studied quantum chemically by a local density functional method. We focus on the chemical and electronic structure of the organoaluminum complexes. Special emphasis is put on a comparison between results at the local spin density approximation and ab initio Hartree–Fock levels. In unmetallized polyenes, the density functional method provides a very good description of the carbon–carbon bond lengths of conjugated systems; in the case of hexatriene, it reproduces the bond dimerization in very good agreement with experimental measurements. Upon metallization, a strong covalent interaction between aluminum and carbon is found. The Al–C bond formation induces an interruption of the bond alternation pattern and reduces the π‐conjugation in the oligomer, in qualitative agreement with photoelectron spectroscopy data and previous theoretical results at the Hartree–Fock level. Notably, the π‐electron levels in the organoaluminum complexes maintain delocalization. In contrast to Hartree–Fock results where an aluminum atom binds to a single carbon, the interactions calculated with the local spin density approximation lead to (i) formation of multicenter aluminum–carbon bonding; (ii) near planarity of the polyene molecule; and (iii) a lower degree of charge transfer from the metal atom to the polymer.

The interactions between different low work function metals aluminium,calcium and sodium, and α,ω‐diphenyltetradecaheptaene, a model molecule for certain conjugated polymers, have been investigated using both x‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum chemical calculations performed within the local spin density (LSD) approximation methodology. The metals are found to interact with the conjugated system in very different ways. Aluminium forms a covalent bond, which strongly modifies the π‐electronic structure of the conjugated molecule, while both the sodium and the calcium atoms act as doping agents, inducing new states in the otherwise forbidden bandgap. These new gap states can be viewed as a soliton–antisoliton pair for the Na/DP7 and a bipolaronic‐like defect for Ca/DP7.

The evolution of doping-induced electronic states within the otherwise forbidden energy gap has been studied as a function of the polyene length in a series of diphenylpolyenes. The chemical and electronic structures ha ve been studied using both X-ray and ultraviolet photoelectron spectroscopies. The results are interpreted with the help of quantum chemical calculations, performed using the semiempirical Austin Model 1 and valence effective Hamiltonian methods. The molecules studied area series of diphenylpolyenes, DPx, with x = 1-7 C=C double bonds in the pol yene part of the molecule. Since the frontier or bitals of the diphenylpolyenes are localized on the polyene chain portion of the molecule, there is a high degree of separation of the phenyl and polyene parts of the 11"-systems. Hence, many chemical and electronic properties of diphenylpolyenes are expected to be similar to those of short-chain trans-polyacetylene. For the longer molecules, n = 6 or 7, the present results indicate the presence of doubly charged, interacting soliton-antisoliton pairs, which appear as two new energy levels in the otherwise forbidden energy gap. In diphenyldecaheptaene to stilbene, i.e. 1 ≤ x ≤ 5, however, a singly charged state is formed at intermediate doping levels, after which the soliton-antisoliton pairs appear for the fully doubly charged systems. These results show that, remarkably, even for very short polyene segments, charges transferred are stored in the form of ( confined) solitons.

We investigate theoretically the electronic structure of regular and statistical copolymer chains containing segments of poly(p‐phenylene vinylene) (PPV) and its dimethoxy‐substituted derivative, poly(2,5‐dimethoxy‐1,4‐phenylene vinylene) (DMeOPPV). Such copolymers have recently been shown to present remarkable electroluminescence properties. The calculations on the copolymer chains are performed on the basis of the negative factor counting technique coupled to the valence effective Hamiltonian method. The eigenfunctions of the upper occupied and lower unoccupied levels are calculated explicitly using the inverse iteration technique, and the degree of localization is determined via the inverse participation number approach. It is shown that the electronic structure of PPV/DMeOPPV copolymers is strongly affected by both the concentrations and relative locations of the parent polymer moieties along the chains. The appearance of clusters of dimethoxy‐substituted units leads to energy gaps that are smaller th...

The resonance Raman excitation profiles of the vibrational modes of the emeraldine salt form of polyaniline protonated with camphor sulfonic acid are remarkably and selectively enhanced, indicating improved structural and electronic order. We demonstrate that the electronic states near the Fermi energy (EF) of metallic polyaniline interact with a specific optical phonon mode. The 1598 cm−1 Raman-active vibrational mode (Ag symmetry) exhibits a distinct resonance enhancement associated with the mid-infrared (mid-IR) absorption in metallic PANI. Since the mid-IR oscillator strength results from the intraband free-carrier Drude absorption, the symmetry of the electronic wavefunctions near EF matches the vibrational pattern of the 1598 cm−1 normal mode. In contrast, the same mode shows no resonance enhancement with the interband absorption at 2.6 eV, implying that the symmetry of the bond relaxation of the excited state is orthogonal to the vibrational pattern of the 1598 cm−1 mode. By contrast, the resonance Raman excitation profile of the 1626 cm−1 mode is in strong resonance with the 2.6 eV absorption, but this mode is not as strongly in resonance with the free-carrier Drude absorption.