Prediction of infrared light emission from pi-conjugated polymers: a diagrammatic exciton basis valence bond theory

TL;DR

This paper develops a diagrammatic exciton basis valence bond theory to predict infrared light emission in modified pi-conjugated polymers, addressing the challenge of designing polymers that emit beyond visible wavelengths.

Contribution

It introduces a new theoretical framework to understand electron correlation effects in modified pi-conjugated polymers for infrared emission.

Findings

Predicted infrared optical gap in substituted polymers

Showed reduced effective on-site correlation in modified systems

Indicated structural modifications can enable infrared light emission

Abstract

There is currently a great need for solid state lasers that emit in the infrared, as this is the operating wavelength regime for applications in telecommunications. Existing $\pi$--conjugated polymers all emit in the visible or ultraviolet, and whether or not $\pi$--conjugated polymers that emit in the infrared can be designed is an interesting challenge. On the one hand, the excited state ordering in trans-polyacetylene, the $\pi$--conjugated polymer with relatively small optical gap, is not conducive to light emission because of electron-electron interaction effects. On the other hand, excited state ordering opposite to that in trans-polyacetylene is usually obtained by chemical modification that increases the effective bond-alternation, which in turn increases the optical gap. We develop a theory of electron correlation effects in a model $\pi$-conjugated polymer that is obtained by replacing the hydrogen atoms of trans-polyacetylene with transverse conjugated groups, and show that the effective on-site correlation in this system is smaller than the bare correlation in the unsubstituted system. An optical gap in the infrared as well as excited state ordering conducive to light emission is thereby predicted upon similar structural modifications.