J.T. López Navarrete

A simple interpretation of the vibrational spectra of undoped, doped and photoexcited polyacetylene: Amplitude mode theory in the GF formalism

Abstract It is shown that the Amplitude Mode Theory worked out for obtaining a unified interpretation of the vibrational spectra of Polyacetylene can be reformulated in a simpler and more complete way in terms of straightforward concepts of moleculare dynamics.

Lattice dynamics and vibrational spectra of polythiophene. I. Oligomers and polymer

The vibrational dynamics of oligomers of thiophene is treated theoretically and the experimental infrared and Raman spectra are studied with the main purpose to derive information for the interpretation of the spectra of polythiophene. The optimized geometries and the vibrational force field are calculated for the oligomers Thn (n=1, 2, 3, 4, and 6) and the parameters are used for PTh. For PTh and two isotopic derivatives dispersion curves and k=0 phonons frequencies are calculated and a vibrational assignment is proposed.

Efficient π electrons delocalization in α,α′‐dimethyl end‐capped oligothiophenes: A vibrational spectroscopic study

α,α′‐dimethyl substituted oligothiophenes with increasing chain length (from the dimer up to the hexamer) were recently synthesized by chemical methods. In this paper we have investigated the vibrational Fourier transform‐IR and Fourier transform‐Raman spectra of solid α,α′‐dimethyl substituted oligothiophenes in the neutral state. The data are consistent with the existence of a chain‐length dependent π electron delocalization: a large frequency dispersion with conjugation length is observed for some Raman and infrared active vibrational modes, particularly at the high‐energy side of the aromatic C=C stretching region. Vibrational assignments are proposed for the main vibrational features in the whole spectral range. This vibrational spectroscopic analysis of the solid samples thus becomes a tool for deriving information on the structure of these neutral materials in solution and in the doped state.

Vibrational spectra of charged defects in a series of α,α′-dimethyl end-capped oligothiophenes induced by chemical doping with iodine

We present the results of a detailed study carried out on a few α,α′-dimethyl end-capped oligothiophenes which have been in situ doped with dry iodine vapors, at different concentrations. The doping process of these well sized oligomers (from dimer to hexamer) has been monitored by using Fourier transform infrared absorption and Fourier transform raman scattering spectroscopies. These data, when combined with the vibrational full assignment of the oligomers in the neutral state and the electronic absorption bands recorded for both neutral and doped compounds, provide a complete spectroscopic characterization of a full series of positive polaron-type model defects of doped polythiophene. We have also performed density functional theory quantum-chemical calculations in order to analyze the effects of ionization on the geometries and vibrational spectra of these systems.

Lattice dynamics and vibrational spectra of polythiophene. II. Effective coordinate theory, doping induced, and photoexcited spectra

It is shown that the infrared and Raman spectra of polythiophene, pristine, doped or photoexcited as well as the spectra of the oligomers pristine and doped are accounted for by the effective conjugation coordinate theory. An effective conjugation force constant associated with the delocalization of the π electrons is measured. It is proven that delocalization extends at least over five thiophene rings and that the samples of the polymer consist of a distribution of conjugation lengths with a confinement length of at least five thiophene rings. The infrared spectrum of the doped and photoexcited polymer is consistent with the existence of a bipolaronic conjugational defect confined within ∼5 thiophene rings.

Ab initio study of torsional potentials in 2,2'-bithiophene and 3,4'- and 3,3'-dimethyl-2,2'-bithiophene as models of the backbone flexibility in polythiophene and poly(3-methylthiophene)

Ab initio molecular orbital theory is employed to calculate the gas‐phase barrier to internal rotation in 2,2’‐bithiophene. Ground state geometries are fully optimized at the restricted Hartree–Fock level of theory using the 3‐21G* and 6‐31G** basis sets. Methylation in β‐positions modulates the geometry, the inter‐ring twist angle and the conformational properties of thiophene dimers. These methyl substitution effects have been assessed by calculations on the 3,4’ and 3,3’‐dimethyl derivatives in a number of selected conformations. A meaningful picture of the molecular relaxation on rotation is attained by allowing for full geometry optimization at both levels of calculation.

Chain flexibility in polyheteroaromatic polymers part I. Electronic properties, structure and vibrational spectra of oligomers as models of polypyrrole and polythiophene

Abstract The conformational properties of dimers and timers of thiophene and pyrrole as obtained from experiments and quantum chemical calculations are discussed with the aim of analysing the validity and limitations of theoretical calculations. The roles played by intramolecular and intermolecular interactions are pointed out. New quantum chemical calculations are reported with full geometry optimization which includes torsional angles; their physical and spectroscopic relevance are discussed. A vibrational assignment is proposed for bithiophene.

A molecular viewpoint of lattice dynamics and spectra of conducting polymers

Abstract The well known features of vibrational spectra of pristine and doped polyacetylene are reinterpreted extending standard concepts of molecular dynamics. In particular, dipersion of Raman frequencies with the exciting wavelength, activation of new bands by doping or photoinduction and the appearance of the so-called S-modes may receive an explanation.

An interpretation of the vibrational spectra of insulating and electrically conducting poly(3-methylthiophene) aided by a theoretical dynamical model

It is shown that the relevant spectral features which arise in the infrared spectrum of poly(3‐methylthiophene) upon chemical doping or photoexcitation (i.e., in the electrically conducting form) can be properly explained by means of the effective conjugation coordinate (ECC) formalism. This theoretical dynamical model accounts for the intramolecular hopping of π electrons in the class of polyconjugated aromatic systems. A complete assignment for the infrared and Raman spectra of the polymer in the pristine state (i.e., in the insulating form) is proposed as the result of a theoretical vibrational potential function derived from semiempirical calculations on short oligomers. The dependence of the bandgap energy on the internal rotation about the inter‐ring single bond is analyzed theoretically in dimers as model molecules and the results are compared with experiments.

Lattice dynamics and vibrational spectra of pristine and doped polyconjugated polyfuran

The interpretation of the infrared and Raman spectra of polyconjugated polyfuran prepared by electroreduction of 2,5‐dibromofuran is presented as the result of lattice dynamical calculations. Geometries and vibrational force fields are derived from semiempirical calculations carried out on oligomers and the parameters are used for the polymer. Finally, it is shown that the infrared and Raman spectra of polyfuran pristine and doped are accounted for by effective conjugation coordinate theory, thus showing that this material also constitutes a polyconjugated material.