Trends in geometric and electronic properties of thiophene- and cyclopentadiene-based polymers

Abstract

ABSTRACT:In this work, we present theoretical evidence illustrating that cyano derivatives of conducting polymers such as polythiophene, polycyclopentadiene, and polyfulvene have smaller intrinsic band gaps than those of their parent polymers. The geometric and electronic properties of the parent and the derivative polymers were Ž . studied with the use of two methodologies: 1 the pseudo-one-dimensional bandstructure calculations performed using the semi-empirical molecular orbital theory Ž . Ž. MNDO, AM1 and 2 oligomer calculations performed using the ab initio molecular orbital theory both at the Hartree᎐Fock and configuration interaction levels. In particular, Ž . Ž . we found that an organic polymer, poly dicyanomethylene cyclopentadifulvene PCNFv , Ž . Ž has a comparable possibly lower band gap to the one observed in poly dicyanomethylene . Ž . Ž . cyclopentadithiophene PCNTH which has a band gap of 0.8 eV . The precursor of Ž . Ž . PCNFv is poly dicyanomethylene cyclopentadicyclopentadiene PCNCY in which two cyclopentadiene rings are connected by a dicyanomethylene group. The additional bond Ž . conjugation in contrast to PCNCY perpendicular to the chain axis makes PCNFv very rigid and fully planar. Trends in structural properties indicate that the lower band gaps in the cyano-substituted polymers, in comparison to their parent polymers, are accompanied by a decrease in bond alternations in the aromatic or trans᎐cisoid forms and by an increase in bond alternations in the quinoid or cis᎐transoid forms.

Evolution of physical properties of conjugated systems

“…Experimental and theoretical investigations of the energy gaps of conjugated polymers via the analysis of the HOMO–LUMO gaps of a series of finite oligomers received considerable attention 33, 35, 40, 42, 44, 48, 77. The computed HOMO–LUMO gaps for the five families of studied oligomers are presented in Fig.…”

“…Here, to investigate the evolution, we explicitly consider oligomers built of the following trans ‐connected monomers: cis ‐1,3‐butadiene, cyclopentadiene, pyrrole, furan, and thiophene. In principle, two mesomeric forms are possible for the studied systems, aromatic and quinoid (or trans ‐cisoid and cis ‐transoid, respectively) 20, 44, which differ by the relative position of double bonds in the carbon backbone. Since it was demonstrated 40 that the former structure is substantially lower in energy, we consider only the aromatic‐like oligomers and polymers.…”

“…A complete review of these studies is out of scope of the present work. However, the concise compilation of references given here for the polymers and oligomers built of the acetylene 33, 46–49, cyclopentadiene 18, 19, 32, 33, 44, 48, pyrrole 12, 19, 23, 32–35, 42, 43, 48, 50–53, furan 19, 32–35, 42, 48, 50, 52, and thiophene 19, 32–35, 42–44, 48, 52, 54, 55 units is supposed to give a representative sample.…”

Conductivity and Activation Energy in Polymers Synthesized by Plasmas of Thiophene

“…In chemically synthesized PTh, H. Subramanian theoretically studied some PTh derivatives which could have lower activation energy than the PTh itself [1]. M. Granstrom experimentally studied light emitting diodes using polythiophenes, where the electronic band gap influences the color of the emitted light [2].…”