Optical Probes of the Quantum-Entangled Triplet-Triplet State in a Heteroacene Dimer

TL;DR

This paper develops a comprehensive many-electron theoretical framework to analyze and predict optical absorption spectra of triplet-triplet states in heteroacene dimers, aiding the understanding of quantum entanglement in correlated-electron systems.

Contribution

It introduces a detailed many-electron theory for triplet-triplet states and their excited state absorptions, enabling direct comparison with experiments and physical interpretation.

Findings

The theory matches existing experimental spectra.

Predictions for spectral features of triplet-triplet states.

Provides physical insights into transient absorption processes.

Abstract

The nature and extent of the spin-entanglement in the triplet-triplet biexciton with total spin zero in correlated-electron $\pi$-conjugated systems continues to be an enigma. Differences in the ultrafast transient absorption spectra of free triplets versus the triplet-triplet can give a measure of the entanglement. This, however, requires theoretical understandings of transient absorptions from the optical spin-singlet, the lowest spin-triplet exciton as well as from the triplet-triplet state, whose spectra are often overlapping and hence difficult to distinguish. We present a many-electron theory of the electronic structure of the triplet-triplet, and of complete wavelength-dependent excited state absorptions (ESAs) from all three states in a heteroacene dimer of interest in the field of intramolecular singlet fission. The theory allows direct comparisons of ESAs with existing experiments as well as experimental predictions, and gives physical understandings of transient absorptions within a pictorial exciton basis that can be carried over to other experimental systems.