Recent advances in inverted single triplet emitters and solid-state NMR analysis of organic TADF emitters

In TADF-based-OLEDs, reducing the lifetime of excitons is necessary to overcome the efficiency roll-off and device lifetime, which can be achieved by accelerating reverse intersystem crossing (RISC)/radiative decay and suppressing intersystem crossing (ISC). The inverted excited singlet-triplet excited states (iST) emitters are now attracting attention because of their negative energy gap (ΔEST) between the lowest excited singlet and lowest triplet states, which can potentially realize faster RISC than ISC. Here, we show a new iST emitter, named A6AP-Cz, employing an asymmetric hexa-azaphenalene (A6AP) core skeleton, which is predicted to have the negative ΔEST of −44 meV from the quantum chemical calculation. The negative ΔEST was experimentally confirmed for A6AP-Cz, with the kRISC (1.9×10^7 s−1) larger than kISC (1.0×10^7 s−1). We will also present our recent study on conformational analysis of TADF emitters in an amorphous aggregates using solid-state NMR.