Simultaneous enhancement of thermally activated delayed fluorescence and photoluminescence quantum yield via homoconjugation

Abstract

A fundamental problem facing thermally activated delayed fluorescence (TADF) is to overcome the paradox of efficient electronic transitions and a narrow singlet-triplet energy gap (ΔEST) in a single luminophore. We present a quinoxaline-based TADF iptycene as the first clear example that homoconjugation can be harnessed as a viable design strategy toward this objective. Homoconjugation was introduced in an established TADF luminophore by trimerization through an iptycene core. This homoconjugation was confirmed by electrochemistry. As a direct consequence of homoconjugation we observed synergistic improvements to photoluminescence quantum yield (ΦPL), radiative rate of singlet decay (krS), delayed fluorescence lifetime (τTADF), and rate of reverse intersystem crossing (krISC), while narrowing the ΔEST. The cooperative enhancement is rationalised with TD-DFT calculations including spin-orbit coupling (SOC). A facile synthesis of this system, and the ubiquity of the pyrazine motif in state-of-the-art TADF materials across the electromagnetic spectrum, leads to a great potential for generality.