Inverting Singlet and Triplet Excited States using Strong Light-Matter Coupling

TL;DR

This paper demonstrates that strong light-matter coupling in organic microcavities can invert the order of singlet and triplet excited states and measures RISC rates, revealing they remain unchanged despite large energy shifts.

Contribution

It provides the first experimental demonstration of singlet-triplet state inversion via strong coupling and measures RISC rates in this regime, showing they are unaffected by energy level shifts.

Findings

Complete inversion of singlet and triplet states achieved.

RISC rate remains unchanged despite energy shifts.

Inversion is limited by the delocalized nature of polaritons.

Abstract

In organic microcavities, hybrid light-matter states can form with energies that differ from the bare molecular excitation energies by nearly 1 eV. A timely question, given recent advances in the development of thermally activated delayed fluorescence materials, is whether strong light-matter coupling can be used to invert the ordering of singlet and triplet states and, in addition, enhance reverse intersystem crossing (RISC) rates. Here, we demonstrate a complete inversion of the singlet lower polariton and triplet excited states. We also unambiguously measure the RISC rate in strongly-coupled organic microcavities and find that, regardless of the large energy level shifts, it is unchanged compared to films of the bare molecules. This observation is a consequence of slow RISC to the lower polariton due to the delocalized nature of the state across many molecules and an inability to compete with RISC to the dark exciton reservoir, which occurs at a rate comparable to that in bare molecules.