[Paper Review] Inverting Singlet and Triplet Excited States using Strong Light-Matter Coupling

Elad Eizner, Luis Á. Martínez-Martínez+2 more|arXiv (Cornell University)|Mar 21, 2019

Strong Light-Matter Interactions4 citations

TL;DR

This study demonstrates complete inversion of singlet and triplet excited state ordering in organic microcavities via strong light-matter coupling, achieving a lower polariton state that is energetically below the triplet state. Despite large energy shifts, reverse intersystem crossing (RISC) rates to the lower polariton remain unchanged compared to bare molecules due to delocalization and competition with dark exciton reservoirs.

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.

Motivation & Objective

• To investigate whether strong light-matter coupling can invert the energy ordering between singlet and triplet excited states in organic microcavities.
• To determine the impact of strong coupling on reverse intersystem crossing (RISC) rates in organic semiconductors.
• To measure and compare RISC rates in strongly-coupled systems versus bare molecular films.
• To understand the role of delocalized polariton states and dark exciton reservoirs in RISC dynamics under strong coupling.
• To explore the potential of polariton engineering for enhancing thermally activated delayed fluorescence (TADF) materials.

Proposed method

• Fabrication of organic microcavities incorporating TADF molecules to enable strong light-matter coupling.
• Excitation of the system using pulsed laser sources to probe ultrafast dynamics of singlet and triplet states.
• Use of time-resolved photoluminescence spectroscopy to measure RISC rates in both bare films and strongly-coupled microcavities.
• Analysis of energy level shifts and polariton splitting to confirm the inversion of singlet and triplet state ordering.
• Comparison of RISC rates between the lower polariton state and the dark exciton reservoir to isolate dominant relaxation pathways.
• Employment of quantum mechanical modeling to interpret the observed energy level inversions and RISC dynamics.

Experimental results

Research questions

• RQ1 Can strong light-matter coupling induce a complete inversion of the singlet and triplet excited state energy levels in organic microcavities?
• RQ2 How does strong coupling affect the rate of reverse intersystem crossing (RISC) to the lower polariton state?
• RQ3 What is the role of delocalized polariton states in modifying RISC dynamics compared to isolated molecules?
• RQ4 How does the presence of a dark exciton reservoir influence RISC rates in strongly-coupled systems?
• RQ5 To what extent do large energy shifts in polariton states alter the fundamental RISC mechanism in TADF materials?

Key findings

• Complete inversion of the singlet lower polariton and triplet excited states is experimentally demonstrated, with the lower polariton state lying below the triplet state.
• The RISC rate to the lower polariton state remains unchanged compared to bare molecular films, despite energy level shifts of nearly 1 eV.
• The unchanged RISC rate is attributed to the delocalized nature of the lower polariton, which hinders efficient RISC due to symmetry and coherence constraints.
• RISC to the dark exciton reservoir occurs at a rate comparable to that in bare molecules, indicating that this pathway remains dominant.
• The study confirms that strong coupling does not enhance RISC rates in the lower polariton, challenging assumptions about polariton-mediated rate acceleration.
• The results highlight the importance of competing relaxation pathways and the limitations imposed by delocalization in strongly-coupled organic systems.