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 (ΔE_ST) 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 ΔE_ST of -44 meV. The experimental studies of a synthesized A6AP-Cz revealed that the lifetime of delayed fluorescence (τ_DF) was only 54 ns, which was the shortest among all organic materials. The rate constant of RISC (k_RISC=1.9×10⁷ s^-1) was greater than that of ISC (k_ISC=1.0×10⁷ s^-1). The negative ΔE_ST of A6AP-Cz was experimentally confirmed from 1) the k_RISC and k_ISC (-45 meV) and 2) the temperature-dependent τ_DF. 3) The onsets of fluorescence and phosphorescence spectra at 77 K also supported the evidence of negative ΔE_ST (-73 meV). This study demonstrated the potential of A6AP as an iST core for the first time.