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-mCN and 2DACz-oCN, that possess multiple donor units to minimize singlet-triplet energy splitting (Delta E-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 similar to 2.7 x 10(6) and similar to 8.6 x 10(6) s(-1), which are 1 order of magnitude higher than the other benzonitrile-based TADF molecules. The short delayed fluorescent lifetimes of similar to 1.58 and similar to 1.16 mu s 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 similar to 25.1% with the suppressed efficiency roll-off, which still remains similar to 21% at the luminance of 1000 cd/m(2)