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

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Abstract
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.