Acridin-9(10H)-one based thermally activated delayed fluorescence material: simultaneous optimization of RISC and radiation processes to boost luminescence efficiency

Abstract

It is strongly desired for thermally activated delayed fluorescence (TADF) compounds to possess a high rate constant of radiation (K_R) and a high rate constant of reverse intersystem crossing (K_RISC) simultaneously. A novel TADF compound, 3,6-di(10H-phenoxazin-10-yl)-10-phenylacridin-9(10H)-one (3,6-DPXZ-AD), is designed by attaching phenoxazine as the electron donor at the 3,6-sites of acridin-9(10H)-one as the acceptor. The unique sp²-hybridization of the nitrogen atom of the acridone ring leads to the quasi-equatorial conformation and the high molecular rigidity of 3,6-DPXZ-AD, which suppress conformation relaxation and finally generate a high K_R of 1.4 × 10⁷ s⁻¹. The phosphorescence of 3,6-DPXZ-AD with unexpected higher energy than its fluorescence is proved to originate from the intermolecular through-space charge transfer state (TSCT) and the locally excited triplet states (³LE). Due to the multichannel RISC process from the TSCT and ³LE states to the ¹CT state, a high K_RISC of 1.1 × 10⁶ s⁻¹ is realized simultaneously. 3,6-DPXZ-AD shows a short TADF lifetime of 1.6 μs and a high fluorescence quantum yield of 94.9%. The yellow organic light-emitting diode with 3,6-DPXZ-AD as the doped emitter exhibits excellent performance with a low turn-on voltage of 2.2 V, an external quantum efficiency of 30.6% and a power efficiency of 109.9 lm W⁻¹, being among the best values ever reported for acridone based materials.