Jeong Min Choi

Ideal Molecular Design of Blue Thermally Activated Delayed Fluorescent Emitter for High Efficiency, Small Singlet–Triplet Energy Splitting, Low Efficiency Roll-Off, and Long Lifetime

Highly efficient thermally activated delayed fluorescent (TADF) emitters, 5-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (oBFCzTrz), 5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (mBFCzTrz), and 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (pBFCzTrz), were synthesized to study the effects of ortho-, meta-, and para- linkages between donor and acceptor moieties. oBFCzTrz having ortho- linked donor and acceptor moieties showed smaller singlet-triplet energy gap, shorter excited state lifetime, and higher photoluminescence quantum yield than mBFCzTrz and pBFCzTrz which are interconnected by meta- and para- positions. The TADF device using oBFCzTrz as a blue emitter exhibited high external quantum efficiency over 20%, little efficiency roll-off, and long device lifetime.

Mechanochromism and electroluminescence in positional isomers of tetraphenylethylene substituted phenanthroimidazoles

The study of aggregation-induced emission (AIE) luminogens has gained momentum due to their remarkable luminogenic properties and applications in mechano-sensors and organic light-emitting diodes (OLEDs). In this article we have studied three positional isomers (ortho, meta, and para) of phenanthroimidazoles 3a–3c and explored their AIE, mechanochromic and electroluminescence behavior. The phenanthroimidazoles 3a–3c were synthesized by the Suzuki cross-coupling reaction of (2-bromo/3-bromo/4-bromo)phenathroimidazoles 2a–2c with 4-(1,2,2-triphenylvinyl)phenylboronic acid pinacol ester in good yields. The phenanthroimidazoles 3a–3c exhibit strong AIE. The mechanochromic study reveals reversible mechanochromism with good color contrast between blue and green colors. The ortho (3a) and meta (3b) isomers exhibit a grinding induced spectral shift of 98 nm while the para-isomer (3c) exhibits a spectral shift of 43 nm. Moreover, 3a–3c were explored as non-doped blue emitters in efficient organic light-emitting diodes. Among the three emitters, 3c provided a high quantum efficiency of 4.0% in a non-doped blue device.

Blue thermally activated delayed fluorescent emitters having a bicarbazole donor moiety

Bis(4-(9′-phenyl-9H,9′H-[3,3′-bicarbazol]-9-yl)phenyl)methanone (BPBCz) and 9′,9′′′-((6-phenyl-1,3,5-triazine-2,4-diyl)bis(4,1-phenylene))bis(9-phenyl-9H,9′H-3,3′-bicarbazole) (TrzBCz) were investigated as thermally activated delayed fluorescent (TADF) emitters to exhibit blue emission color. A 3,3′-bicarbazole was a donor in the two TADF emitters, and benzophenone and triazine were acceptors of BPBCz and TrzBCz, respectively. Both BPBCz and TrzBCz behaved as high efficiency TADF emitters through a small singlet–triplet energy gap for the activation of delayed fluorescence. The BPBCz and TrzBCz TADF emitters offered high quantum efficiencies of 23.3% and 23.6% in the blue TADF devices.

Bis(phenylsulfone) as a strong electron acceptor of thermally activated delayed fluorescent emitters

Bis(phenylsulfone) was developed as a strong electron acceptor of thermally activated delayed fluorescent emitters. The connection of two electron withdrawing phenylsulfone moieties through meta-position of phenyl produced the bis(phenylsulfone) acceptor and the strong electron acceptor strength of bis(phenylsulfone) enabled preparation of sky-blue and green thermally activated delayed fluorescent emitters in combination with weak carbazole donors. The bis(phenylsulfone) acceptor and carbazole donor combined organic materials performed as thermally activated delayed fluorescent emitters with a high quantum efficiency of 18.3%.