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

Abstract

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.