Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations

TL;DR

This study uses advanced DFT calculations to understand and guide the design of vinylene-linked heterocyclic conducting polymers with low band gaps for photovoltaic applications, emphasizing the role of aromaticity and chemical stability.

Contribution

It introduces a rational, first-principles approach to designing low band gap polymers by destabilizing the ground state towards aromatic-quinoidal level-crossing, challenging previous assumptions about aromaticity.

Findings

Hybrid DFT functionals accurately predict band gaps.

Destabilizing the ground state lowers the band gap.

Aromaticity alone does not guarantee low band gaps.

Abstract

The band structure and electronic properties in a series of vinylene-linked heterocyclic conducting polymers are investigated using density functional theory (DFT). In order to accurately calculate electronic band gaps, we utilize hybrid functionals with fully periodic boundary conditions to understand the effect of chemical functionalization on the electronic structure of these materials. The use of predictive first-principles calculations coupled with simple chemical arguments highlights the critical role that aromaticity plays in obtaining a low band gap polymer. Contrary to some approaches which erroneously attempt to lower the band gap by increasing the aromaticity of the polymer backbone, we show that being aromatic (or quinoidal) in itself does not insure a low band gap. Rather, an iterative approach which destabilizes the ground state of the parent polymer towards the aromatic \leftrightarrow quinoidal level-crossing on the potential energy surface is a more effective way of lowering the band gap in these conjugated systems. Our results highlight the use of predictive calculations guided by rational chemical intuition for designing low band gap polymers in photovoltaic materials.