The electron inductive effect of dual non-conjugated trifluoromethyl acceptors for highly efficient thermally activated delayed fluorescence OLEDs

Abstract

Recently, pure organic twisted donor-acceptor (D-A) structured thermally activated delayed fluorescence (TADF) materials have been extensively demonstrated as a new generation emitter in organic light-emitting diodes (OLEDs) due to their low-cost and high-efficiency. Herein, we designed and synthesized a highly twisted TADF emitter 2,3,5,6-tetra(3,6-di-tert-butyl-9H-carbazol-9-yl)bis(trifluoromethyl)benzene (4tCzDCF3Ph) by a facile catalyst-free C-N coupling reaction. The two electron-withdrawing non-conjugated trifluoromethyl (CF3) moieties bearing strong electron inductive effects instead of other widely reported conjugation effects contributes for the necessary D-A interactions for TADF emission. 4tCzDCF3Ph exhibited an extremely small singlet-triplet energy gap (ΔEST) of 0.02 eV, fast kRISC rate of 7.02 × 105 s−1 and modest photoluminescence quantum yield (PLQY) of ~70%. A low turn-on voltage of 2.6 V, along with maximum current efficiency of 50.6 cd A−1, power efficiency of 40.7 lm W−1 and external quantum efficiency (EQE) of 16.2% could be facilely achieved for 4tCzDCF3Ph based green TADF OLEDs, which is significantly higher than <1% of 4CzCF3Ph and 9.5% of 5CzCF3Ph TADF emitters based on single trifluoromethyl acceptor.