Youxuan Zheng and Wenhua Zheng's Group Reported the Planar Chiral MR-TADF Materials and CP-OLEDs

Time:2022-12-21Viewed:10

Organic light emitting diodes (OLEDs) have been widely used in modern life. The next generation of circularly polarized OLEDs (CP-OLED) and 3D displays can be realized through circularly polarized luminescence (CPL). High performance CP-OLED has the advantages of generating tunable emission CPL directly, simple device structure and high efficiency, making it one of the most feasible strategies for 3D display. In the past years, chiral fluorescence materials including polymers and small molecules, chiral phosphorescence metal complexes and chiral thermally activated delayed fluorescence (TADF) materials were applied in CP-OLEDs. Because both singlet (S1) and triplet (T1) excitons can be utilized through the reverse intersystem crossing (RISC) process,the device has high performance without the assistance of noble metal, which make chiral TADF materials attracted much attention. However, the wide emission spectrum of traditional chiral TADF materials (full-width at half-maximum FWHM > 60 nm) makes it difficult to achieve ultrapure color emission.

Multiple resonance TADF (MR-TADF) materials through the opposite resonance effect of electron-deficient boron/carbonyl and electron-rich oxygen/nitrogen, minimize the bonding/antibonding characteristics of FMOs (frontier molecular orbitals)to reduce the vibrational relaxation of the fused-ring structures, leading to the narrow FWHM and high luminescence efficiency. Most of them display excellent OLED performances. Therefore, circularly polarized MR-TADF (CP-MR-TADF) materials have the potential to prepare high performance and color purity CP-OLEDs. Up to now there are limited studies on the development of CP-MR-TADF emitters with axial, helical and carbon central chirality, respectively. However, as one of the important chiral elements, planar chirality has not been reported in the field of CP-MR-TADF materials.

Recently, Professor You-Xuan Zheng and Wen-Hua Zheng's group introduced [2.2]paracyclo (1,4)carbazolophane (Czp) into the B/N multiple resonance system based on 3,6-di(tert-butyl)-carbazole / phenoxazine, two pairs of planar chiral CP-MR-TADF enantiomers (R/S)-Czp-tBuCzB / (R/S)-Czp-POABwere obtained. The highly rigid structure is conducive to obtain narrow emission and small bandgap between S1 and T1 (DEST).(R)-Czp-tBuCzB and (R)-Czp-POAB showed the emissions peaking at 478 and 497 nm with FWHMs of 23 and 36 nm in toluene with the photoluminescence quantum yields in the doped films of 98% and 93%, respectively. The electroluminescence (EL) spectrum of CP-OLED based on (R)-Czp-tBuCzBdisplayed a FWHM of only 24 nm, and the maximum external quantum efficiency reached 32.1%. When the luminance reaches 1000 cd/m2, the efficiency roll-off is only 3.7%, which is rarely reported in blue CP-OLEDs. The device based on (R)-Czp-POAB showed the first near pure green CPEL based on CP-MR-TADF materials, with EL peak of 513 nm and CIE coordinates of (0.23, 0.65). Both CP-OLEDs displayed symmetrical CPEL spectra, with |gEL| of 1.54 × 10-3 and 1.30 × 10-3.

In this work, the rigid planar chiral structure Czp was introduced into the B/N based CP-MR-TADF materials for the first time. The corresponding CP-OLEDs with high luminescence efficiency, narrow FWHM and high asymmetry factor were obtained. Among them, the efficiency roll-off of blue CP-OLED device was the smallest in similar studies, and the CPEL closed to pure green light was shown for the first time. Therefore, introducing planar chirality into multiple resonance system and preparing CP-OLEDs is an effective strategy, which is of reference significance for future 3D display research.

The work was published in Angelw Chem. Int. Ed. (2022, DOI: 10.1002/anie.202217045). Doctoral students Liao Xiangji and Pu Dongdong are both first-authors of the paper, and Professor You-Xuan Zheng and Wen-Hua Zheng are the corresponding authors of the paper. Thanks to Professor Jing-Lin Zuo of Nanjing University for the support and help. This work was supported by the National Natural Science Foundation's major integration project 92256304.