Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

TL;DR

This paper develops a comprehensive theoretical framework for understanding primary photoexcitations in donor-acceptor copolymers, highlighting the mixing of excitonic states and implications for singlet fission.

Contribution

It introduces a generic theory accounting for electron correlation and symmetry breaking effects, revealing the optically allowed triplet-triplet state in donor-acceptor copolymers.

Findings

Triplet-triplet state can have stronger oscillator strength than charge-transfer exciton.

Strong mixing occurs between charge-transfer exciton and triplet-triplet state.

Intramolecular singlet fission from the triplet-triplet state is possible.

Abstract

We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.