Correlated-electron description of the photophysics of thin films of $\pi$-conjugated polymers

TL;DR

This paper develops a theoretical model for excited state charge transfer in $ ext{pi}$-conjugated polymers, predicting optical absorption features that match experimental ultrafast photoinduced absorption spectra.

Contribution

The authors extend Mulliken's theory to excited states in $ ext{pi}$-conjugated oligomers and perform high-order configuration-interaction calculations to predict charge-transfer exciton spectra.

Findings

The model predicts five excited state absorptions consistent with experiments.

Calculated spectra show qualitative agreement with ultrafast photoinduced absorption in PPV films.

Charge-transfer excitons significantly contribute to observed photoinduced absorption.

Abstract

We extend Mulliken's theory of ground state charge transfer in a donor-acceptor complex to excited state charge transfer between pairs of identical $\pi$-conjugated oligomers, one of which is in the optically excited state and the other in the ground state, leading to the formation of a charge-transfer exciton. Within our theory, optical absorptions from the charge-transfer exciton should include a low energy intermolecular charge-transfer excitation, as well as distinct intramolecular excitations from both the neutral delocalized exciton component and the Coulombically bound polaron-pair component of the charge-transfer exciton. We report high order configuration-interaction calculations for pairs of oligomers of poly-paraphenylenevinylene (PPV) that go beyond our previous single configuration-interaction calculation and find all five excited state absorptions predicted using heuristic arguments based on the Mulliken concept. Our calculated excited state absorption spectrum exhibits strong qualitative agreement with the complete wavelength-dependent ultrafast photoinduced absorption in films of PPV derivatives, suggesting that a significant fraction of the photoinduced absorption here is from the charge-transfer exciton. We make detailed comparisons to experiments, and a testable experimental prediction.