Space-confined through-space charge-transfer emitters based on CN-substituted fluorene acceptors: Design, synthesis, and thermally-activated delayed fluorescence

Abstract

Space-confined through-space charge-transfer (TSCT) emitters have been extensively explored for developing thermally-activated delayed fluorescence (TADF), but their donor/acceptor units need to be expanded for tuning the emission properties. Herein we report TSCT-TADF emitters 1–3 by chemically fixing cyano-substituted fluorenes as the acceptors. For 1 and 2, 9H-fluorene-2,5-dicarbonitrile (2,5-CN-FL) and 9H-fluorene-2,7-dicarbonitrile (2,7-CN-FL) are used as the acceptors, respectively, and 1,3,6,8-tetramethyl-9H-carbazole is used as the donor. For emitter 3, 2,7-CN-FL and benzo[5,6][1,4]oxazino[2,3,4-kl]phenoxazine are used as the acceptor and the donor, respectively. Single-crystal structures and theoretical calculations reveal close, cofacial donor/acceptor alignments and thus effective TSCT in 1–3. In doped 2,8-bis(diphenylphosphoryl) dibenzo[b,d]furan films, emitters 1 and 2 show similar blue-green emission centered at around 510 nm, and emitter 3 shows yellow emission centered at 562 nm, with photoluminescent efficiencies at 0.66−0.76. Compared to emitters 1 and 2, emitter 3 shows much faster reverse intersystem crossing due to its much smaller singlet-triplet energy splitting. Organic light-emitting diodes (OLEDs) using 1–3 as the emitters afford blue-green or yellow electroluminescence with high external quantum efficiencies up to 14.9%. The work demonstrates that cyano-substituted fluorenes with a planar and rigid geometry are promising acceptors for developing TSCT-TADF emitters for OLED applications.