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

Thermally activated delayed fluorescent (TADF) materials exhibiting the fast reverse intersystem crossing (RISC) process are essential for improving the stability of organic light-emitting 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- m CN and 2DACz- o CN, that possess multiple donor units to minimize singlet–triplet energy splitting (Δ E ST ) and enhance the spin–orbit coupling matrix elements. Our work shows that the rate constants of RISC of 2DACz- m CN and 2DACz- o CN 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 benzonitrile-based TADF molecules. The short delayed fluorescent lifetimes of ∼1.58 and ∼1.16 μs in doped films are achieved. The OLEDs by utilizing 2DACz- o CN 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 .