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 (Delta EST) 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 Delta EST of -44 meV. The experimental studies of a synthesized A6AP-Cz revealed that the lifetime of delayed fluorescence (tau DF) was only 54 ns, which was the shortest among all organic materials. The rate constant of RISC (kRISC=1.9x107 s-1) was greater than that of ISC (kISC=1.0x107 s-1). The negative Delta EST of A6AP-Cz was experimentally confirmed from 1) the kRISC and kISC (-45 meV) and 2) the temperature-dependent tau DF. 3) The onsets of fluorescence and phosphorescence spectra at 77 K also supported the evidence of negative Delta EST (-73 meV). This study demonstrated the potential of A6AP as an iST core for the first time. The inverted singlet-triplet excited states (iST) molecule, A6AP-Cz, based on asymmetric hexa-azaphenalene was designed. A6AP-Cz showed the extremely short lifetime of delayed fluorescence (tau DF) of 54 ns. Moreover, the reverse intersystem crossing (rate constant of RISC: kRISC=1.9x107 s-1) was faster than ISC (rate constant of ISC: kISC=1.0x107 s-1) due to the negative energy gap between lowest excited singlet and triplet states (Delta EST). image