Development of an Organic Emitter Exhibiting Reverse Intersystem Crossing Faster than Intersystem Crossing

In thermally activated delayed fluorescence (TADF)‐based organic light‐emitting diodes (OLEDs), acceleration of reverse intersystem crossing (RISC) and suppression of intersystem crossing (ISC) are demanded to shorten a lifetime of triplet excitons. As a system realizing RISC faster than ISC, inverted singlet‐triplet excited states (iST) with a negative energy difference (ΔEST) between the lowest excited singlet and the lowest triplet states have been gathering much attention recently. Here, we have focused on an asymmetric hexa‐azaphenalene (A6AP) core to obtain a new insight into iST. Based on A6AP, we have newly designed A6AP‐Cz with the calculated ΔEST of −44 meV. The experimental studies of a synthesized A6AP‐Cz revealed that the lifetime of delayed fluorescence (τDF) was only 54 ns, which was the shortest among all organic materials. The rate constant of RISC (kRISC = 1.9×107 s−1) was greater than that of ISC (kISC = 1.0×107 s−1). The negative ΔEST of A6AP‐Cz was experimentally confirmed from 1) the kRISC and kISC (−45 meV) and 2) the temperature‐dependent τDF. 3) The onsets of fluorescence and phosphorescence spectra at 77 K also supported the evidence of negative ΔEST (−73 meV). This study demonstrated the potential of A6AP as an iST core for the first time.