Chiral materials have potential application in circularly polarized organic light-emitting diode (CP-OLED) and 3D display, due to its left and right circularly polarized luminescence (CPL). Generally, the intensity of CP light is characterized by the dissymmetry factor (g), but the inferior g values of reported CPL materials frustrate their application in 3D display.
You-Xuan Zheng group takes the lead research in the property of chiral emitter and CP-OLED. The findings include chiral iridium complexes (Sci. Rep., 2015, 5, 14912；Adv. Optical Mater., 2016, 4, 1726; Chem. Commun., 2019, 55, 8215), chiral platinum complexes (Chem. Eur. J., 2019, 25, 5672), chiral thermally activated delayed fluorescence (TADF) materials (Adv. Mater., 2019, 31, 201900524; J. Mater. Chem. C, 2019, 7, 7045), chiral organic room-temperature phosphorescence (Angew. Chem. Int. Ed., 2019, 58, 17220), helical polypeptides (J. Am. Chem. Soc., 2019, 141, 12697; Adv. Optical Mater., 2020, 1902122) and chiral perovskite ferroelectric enantiomers (J. Am. Chem. Soc., 2020, 142, 4756).
Recently, Zheng group efficiently synthesized four chiral 1,2-diaminocyclohexane based fluorescence enantiomers by simple nucleophilic reaction of corresponding absolute configuration N,N'-bismethyl-1,2-cyclohexanediamine with high yields (up to 92%) and high enantiomeric excesses (ee >99%). By vary of the electron donating groups of N-methyl, carbazole and diphenylamine, these materials achieved multi-color from blue (420 nm) to red (610 nm) with g values in 10-4-10-3 in toluene. CP-OLEDs using these enantiomers showed maximum external quantum efficiency (EQEmax) of 5.5% with low efficiency roll-off and obvious circularly polarized electroluminescence (CPEL) signals with gEL of -1.4×10−3/+1.3×10−3. The results open the door for future studies on multi-color 3D-displays (ACS Appl. Mater. Interfaces 2020, DOI: 10.1021/acsami.0c02400).
CPL emitters based on chiral carbon and corresponding devices perform moderate g values because of little distributions of HOMO/LUMO. To boost intrinsic CPL properties of chiral TADF materials and corresponding devices, Zheng groupdeveloped a pair of axially chiral biphenyl compound-based dual-core TADF emitters, named (R/S)-BPPOACZ, containing the cyano as electron-withdrawing moieties, carbazole and phenoxazine as electron-donating units. The CP-TADF enantiomers exhibited both high PLQY of 86.10% and excellent CPL activities with |gPL| values of 9.7´10-3in toluene and 1.8 × 10-2 in doped film, which are among the best values of previously reported small chiral organic materials. It can be observed from computational simulation that dissymmetric biphenyl actively participate in the frontier molecular orbitals (FMOs) distributions as a chiral source, which could induce strong CPL. The resulting CP-OLEDs also exhibited a brightness of 35711 cd m-2 and an EQEmax up to 16.6% with extremely low efficiency roll-offand obvious CPEL signals with gEL of +4.5 × 10-3 and -2.8 × 10-3 (Adv. Sci., 2020, 2000804).
These work were supported by the National Natural Science Foundation of China (51773088 and 21975119). Thanks for help from Prof.Jing-Lin Zuo, Dr. Yi Wang and Prof. Yi. Pan.