Density functional theory study of the structure and energetics of negatively charged pyrrole oligomers

TL;DR

This study uses first-principles calculations to explore the structure, charge states, and electronic properties of pyrrole oligomers and doped polypyrrole, revealing length-dependent electron affinity and structural changes upon doping.

Contribution

It provides new insights into the structural and electronic behavior of negatively charged pyrrole oligomers and the effects of lithium doping on polypyrrole's band gap.

Findings

Negatively charged oligomers become almost flat due to charge accumulation.

Short oligomers can have negative electron affinity despite stability.

Doping with lithium reduces the band gap and introduces dopant states.

Abstract

First-principles calculations are used to investigate the electronic properties of neutral and negatively charged n-pyrrole oligomers with n= 2-18. Chains of neutral oligomers are bent while the negatively charged oligomers become almost flat due to accumulation of negative charge at the end monomers. Several isomers of the short oligomers (n < 5) display negative electron affinity, although they are energetically stable. For longer oligomers with n > 5, the electron affinity turns positive, increasing with oligomer length. The doping of 12-pyrrole with lithium atoms is studied, showing that negative oxidation states are possible due to charge transfer from dopant to oligomer at locations close to dopant. These molecular regions support extra negative charge and exhibit a local structural change from benzenoid to quinoid in the C-C backbone conjugation. Additional calculations of neutral and doped polypyrrole are conducted showing that the doped infinite polymer chain displays a substantial reduction of the energy band gap and the appearance of dopant-based bands in the gap.