Thienylene Vinylene Research Articles

Nonlinear optical processes in conjugated polymers: configuration interaction description of linear polyenes and VEH/SOS evaluation of polyarylene vinylenes

We have used a Multi-Reference Determinant (MRD) Configuration Interaction (CI) approach, including double excitations within a Pariser-Parr-Pople Hamiltonian, to investigate linear polyenes containing from four to sixteen carbons. We have calculated the low-lying excited states, the two-photon absorption spectrum, and the Third Harmonic Generation response. We have analyzed the length dependence of the static third-order susceptibility, χ (3): an indication of saturation behavior has been found. Focusing our attention on the two-photon resonance peak present in the free-electron laser THG measurements on polyacetylene, we conclude that the experimental data can be explained within a strongly correlated electron model. The Valence Effective Hamiltonian (VEH) method has been extended to calculate nonlinear optical coefficients and has been applied to investigate the static and dynamic third-order polarizabilities in poly( p-phenylene vinylene) and poly(thienylene vinylene). The relation between aromaticity and hyperpolarizability is discussed.

Electro-Absorption Spectroscopy on Poly(Arylene Vinylene)s

Abstract We report spectrally resolved electroabsorption (EA) measurements on poly(arylene vinylene)s. The materials poly(thienylene vinylene), PTV, and poly(p-phenylene vinylene), PPV, are particularly attactive as they can be prepared from soluble precursors which allow ready fabrication of films. The subsequent thermal conversion of the precursor yields samples with good optical and mechanical properties and environmental stability. Both materials give a strong electro-optic response, an induced change in the optical absorption leading to a refractive index change of order 10or applied electric fields between 25 and 100 kV/cm. The EA signal scales quadratically with applied field and shows an anisotropy of 2. 3 with respect to the polarisation of the incident light at the main peak.

The effect of side-group substitution on the energy gaps of phenylene and thienylene oligomers and polymers

Polymer properties were studied by band structure calculations on polymers and using large oligomers. MNDO semi-empirical energy band theory was used for determination of the fully optimized geometry of poly(thienylene vinylene), poly( p-phenylene), poly(phenylene vinylene) and their substituted derivatives. The optimized structures were used in extended Hückel energy band structure calculations to determine the variation of the bandgap caused by substitutions. Polymer properties were also modeled by CNDO/S calculations of the electronic spectra of large oligomers. Both these approaches explain the observed reduction of the energy gap induced by the methoxy side group.

Theoretical investigation of the electronic properties of poly(thienylene vinylenes) and poly(furylene vinylenes)

We present nonempirical valence effective Hamiltonian (VEH) calculations on the electronic structure ofpoly(thienylene vinylene) (PTV) and poly(furylene vinylene) (PFV). We discuss the effects of substitution with electron donating groups such as methyl and methoxy groups on the calculated electronic properties. The VEH results obtained for the ionization potentials and band gaps of PTV and PFV are found to be in excellent agreement with experimental data. Substitution of the heteroatom produces a decrease in the ionization potential and an increase in the band gap in going from PTV to PFV. Substitution with electron donating groups, especially with methoxy groups, implies an additional decrease in the ionization potential and a reduction of the band gap. These effects are more clearly seen for PFV.

The vibrational properties and defect structures in vinylene-linked low-band-gap conducting polymers

We have performed a normal mode analysis and used a newly developed vibrational analysis method for polymers, based on ab initio calculations for large oligomers, to determine the valence force field of poly(p-phenylene vinylene) (PPV), poly(thienylene vinylene) (PTV) and related polymers, and to calculate theoretical IR and Raman spectra of these oligomers and polymers. The calculated C-C stretch constants are used to describe the structure of neutral and doped PPV in terms of bond lengths r and Δr, the bond length alternation parameter. The results are compared to bond length alternation patterns from MNDO for cations (polarons) and dications (bipolarons) of large model oligomers. The results favor the formation of bipolarons in PPV.

Experimental and theoretical investigation of the vibrational properties of poly(thienylene vinylene) (PTV) and poly(furylene vinylene) (PFV)

In this work, a complete assignment of Raman and IR modes is reported for both poly(thienylene vinylene) and poly(furylene vinylene) whose chains are composed of an alternation of aromatic rings and vinyl groups. In order to achieve these attributions, we have developped a valence force field model. The vibrations of an aromatic ring show some similarities in a series of compounds with the same cycle. So the same set of parameters is used to calculate vibrational modes for both polymers and model compounds such as trans-bis(2-thienyl)ethylene, a molecule composed of two thiophene rings separated by a vinyl group. Therefore, we present an experimental comparison between Raman spectra of poly(thienylene vinylene), thiophene and trans-bis(2-thienyl)ethylene, and the assignment of the vibrational modes in the latter molecule is also discussed.

Theoretical analysis of the Raman spectra of PTV and PFV

In this work, we report experimental Raman and IR results obtained on two different polymers composed of heteroaromatic rings separated by a vinyl group, namely poly(thienylene vinylene) (PTV) and poly(furylene vinylene) (PFV). PTV and PFV exhibit a rather similar behaviour and, in particular, in both cases the observed Raman bands undergo a slight shift when the excitation wavelength is changed from 676.7 to 457.9 nm. By using a dynamical model based on valence force field calculations, we have assigned all IR and Raman modes. In order to obtain a good set of force constants, we started calculations by considering those of polythiophene and, also, we used a model compound such as trans-bis(2-thienyl)ethylene. It was found that the numerical values given for these force constants are consistent with the electronic structure usually assigned for these polymers.

Asymmetrical Diphenylpolyene Pendant Polymers for Nonlinear Optical Activity

ABSTRACTRecent activity in the development of nonlinear optical (NLO) materials with high second order susceptibilities has prompted the development of a wide variety of pendant polymers which possess desirable properties for device development, including high Tg minimal relaxation of the poled matrix, and high laser damage threshold. Ideal candidates for such systems are asymmetrically substituted oligomers of polyacetylenes such as dimethylaminonitrostilbene (DANS) and corresponding higher order oligomers. We report here a preliminary study of the synthesis of asymmetrically substituted donor/acceptor diphenyl-capped polyenes and thienylene vinylene oligomers attached as pendant moieties on polyvinylphenol (PVP) for second harmonic generation.

An electrochemical investigation of the doping processes in poly(thienylene vinylene)

Cyclic voltametry (CV) and electrochemical voltage spectroscopy (EVS) have been used to investigate the doping processes in poly(thienylene vinylene). CV measurements show that the kinetics of the doping process are diffusion limited with respect to the dopant species. EVS experiments indicate the presence of considerable band-tailing around the band edges. Hysteresis between the doping and undoping curves are considered to be due to diffusion control. The electrochemical band gap was estimated at around 1.7 eV, a value consistent with optical data.

The electronic and electrochemical properties of poly(phenylene vinylenes) and poly(thienylene vinylenes): An experimental and theoretical study

The electronic and electrochemical properties of poly(p-phenylene vinylene), poly(thienylene vinylene), and their derivatives with electron donating moieties such as methyl, methoxy, and ethoxy are studied using the newly developed electrochemical potential spectroscopy (ECPS) and optical spectroscopy. It is shown that electrochemically derived band gaps agree well with band gap values obtained from optical measurements. Substitution with electron donating groups substantially lowers the ionization potentials and band gaps. A similar effect can be attributed to the incorporation of a vinylene linkage between rings of the polymer backbone. Our results imply that through a proper choice of substituents and backbone structure one can adjust the electrochemical potentials over a wide range as well as red shift the absorption edge of these polymers. In the case of the alkoxythienylene vinylenes the absorption edge is shifted through the visible range of the spectrum into the near infrared (NIR) yielding polymers which become transparent and substantially colorless upon doping with electron donors or acceptors. The structure and the substitution effects of these polymers were modeled using the semiempirical quantum chemical modified neglect of differential overlap (MNDO) method. The MNDO-determined structure served as basis for the valence effective Hamiltonian (VEH) technique which was employed to calculate band structures, ionization potentials, and band gaps, and to study theoretically the effect of substituents on the band structure. Good agreement between experimental and theoretical values of ionization potentials, band gaps and the change of these parameters with substitution is found with the exception of methoxy (or ethoxy) groups. This fact is attributed to a failure of VEH to correctly account for the role of the oxygen atoms in these groups.