Thermally Activated Delayed Fluorescent Emitters Based on Cyanobenzene Exhibiting Fast Reverse Intersystem Crossing to Suppress the Efficiency Roll-Off

Abstract

Thermally activated delayed fluorescent (TADF) materials exhibiting the fast reverse intersystem crossing (RISC) process are essential for improving the stability of organic lightemitting diodes (OLEDs). However, for most TADF emitters, the upconversion processes from the lowest triplet state (T 1 ) to the lowest singlet state (S 1 ) are still inefficient due to the low RISC (k RISC ) rate below 10 5 s −1 . Herein, we report two TADF molecules, 2DACz-mCN and 2DACz-oCN, that possess multiple donor units to minimize singlet−triplet energy splitting (ΔE ST ) and enhance the spin−orbit coupling matrix elements. Our work shows that the rate constants of RISC of 2DACz-mCN and 2DACz-oCN are as fast as ∼2.7 × 10 6 and ∼8.6 × 10 6 s −1 , which are 1 order of magnitude higher than the other benzonitrilebased TADF molecules. The short delayed fluorescent lifetimes of ∼1.58 and ∼1.16 μs in doped films are achieved. The OLEDs by utilizing 2DACz-oCN as emitter exhibit green electroluminescence (EL) with CIE chromaticity coordinates of (0.28, 0.49) and high maximum quantum efficiency of ∼25.1% with the suppressed efficiency roll-off, which still remains ∼21% at the luminance of 1000 cd/m 2 .