Open-shell TADF: Quartet-derived luminescence with dark radicals

TL;DR

This paper introduces a new class of molecules combining TADF chromophores with radicals to enable luminescence from high-spin quartet states, expanding possibilities for quantum technologies and OLED applications.

Contribution

It demonstrates quartet-derived luminescence in radical-TADF molecules and establishes design rules for tuning their energetics and emission properties.

Findings

Quartet states can produce detectable luminescence.

Up to 72% of photons originate from quartet state reverse intersystem crossing.

Radical-triplet exchange coupling measured at 0.7 GHz.

Abstract

High-spin states in organic molecules offer promising tuneability for quantum technologies. Photogenerated quartet excitons are an extensively studied platform, but their applications are limited by the absence of optical read-out via luminescence. Here we demonstrate a new class of synthetically accessible molecules with quartet-derived luminescence, formed by appending a non-luminescent TEMPO radical to Thermally Activated Delayed Fluorescence (TADF) chromophores previously used in OLEDs. The low singlet-triplet energy gap of the chromophore opens a luminescence channel from radical-triplet coupled states. We establish a set of design rules by tuning the energetics in a series of compounds based on a naphthalimide (NAI) core. We observe generation of quartet states and measure the strength of radical-triplet exchange (0.7 GHz). In DMAC-TEMPO, up to 72% of detected photons emerge after reverse intersystem crossing from the quartet state repopulates the state with singlet character. This design strategy does not rely on a luminescent radical to provide an emission pathway from the high-spin state. The large library of TADF chromophores promises a greater pallet of achievable emission colours.