Circularly polarized multi-resonance thermally activated delayed fluorescence (CP-MR-TADF) materials are crucial for the development of circularly polarized organic electroluminescent devices (CP-OLEDs) and 3D displays due to their high quantum yield, excellent color purity, and circularly polarized luminescence (CPL). However, incorporating chiral sources into the rigid skeleton of MR-TADF often results in the broadening of the emission spectrum. Moreover, the CPL performance of CP-MR-TADF is often unsatisfactory due to the spatial separation between chiral source and luminescence center.
To address these issues, Zheng’s group conducted a series of chiral luminescent materials focusing on axial chiral biphenyls with good performances. In a previous study, axial biphenyl TADF molecules based on cyano and carbazole/phenoxazine exhibited promising chiroptical properties (Adv. Sci., 2020, 7, 2000804). Nevertheless, the axial chiral biphenyl strategy has yet to be applied to CP-MR-TADF molecules.
Figure 1. Design strategy of axial chiral CP-MR-TADF molecules.
In this work, we chose biphenyls with larger steric hindrance substituents (thiobenzene: -SPh/ sulfonylbenzene: -SO₂Ph) as chiral sources, and combined them with classical MR-TADF fragment (DtBuCzB). This approach leads to the synthesis of two pairs of axial chiral CP-MR-TADF enantiomers, (R/S)-S-AX-BN and (R/S)-SO₂-AX-BN (Figure 1), which exhibit both intense CPL and excellent photophysical performance. By introducing the biphenyl skeletons at the para-position of B atom, the frontier molecular orbital of MR fragment extends to the axial chiral source. Additionally, the electron-donating/-accepting effects of -SPh/-SO₂Ph effectively modulate the luminescent properties of the whole molecule. Moreover, the heavy-atom effect of sulfur can enhance the reverse intersystem crossing process in MR-TADF molecules, resulting in high efficiency and reduced efficiency roll-off in CP-OLED.
Figure 2. Normalized UV-Vis absorption and fluorescence spectra of (a) (rac)-S-AX-BNand (b) (rac)-SO2-AX-BN in toluene (1 × 10-5 M) at room temperature; CPPL spectra and gPL-wavelength curves of (c), (d) (R/S)-S-AX-BN and (e), (f) (R/S)-SO2-AX-BN in doped films (5 wt% in 2,6-DCzppy).
(R/S)-S-AX-BN and (R/S)-SO2-AX-BN display green emissions, with peaks at 489 and 495 nm, full-width at half-maximum (FWHM) values of 21 and 20 nm in toluene. Owing to the favorable integration of axial chiral source with the luminescence center, (R/S)-S-AX-BN and (R/S)-SO2-AX-BN exhibit mirror-symmetric CPL spectra in toluene with asymmetry factors (|gPL|) of 2.2 × 10-3 and 1.4 × 10-3, respectively. In doped films, these enantiomers also demonstrate mirror-symmetric CPPL spectra with |gPL| values of 3.5 × 10-3 and 2.3 × 10-3, respectively.
The CP-OLEDs based on (R/S)-S-AX-BN and (R/S)-SO2-AX-BN exhibit extremely narrow electroluminescence at 495 and 500 nm with FWHMs of 22 and 21 nm, respectively, representing the narrowest emissions in CP-OLEDs based on CP-MR-TADF materials known to us. They also demonstrate maximum external quantum efficiencies of 33.5% and 31.5%, respectively, with relatively low efficiency roll-offs. Most importantly, they achieve stable and mirror-symmetric circularly polarized electroluminescence spectra with |gEL| factors of 3.3 × 10-3 and 2.2 × 10-3, respectively, which are the highest values among axial CP-MR-TADF analogs.
Figure 3. (a) The device structure and the energy–level diagrams, (b) EL spectra, (c) CIE coordinates, (d) luminance–voltage–current-density characteristics, and (e)/(f) Current efficiency /external quantum efficiency curves of D-(rac)-S-AX-BN andD-(rac)-SO2-AX-BN devices.
Figure 4.(a), (c) CPEL spectra and (b), (d) gEL values of D-(R/S)-S-AX-BN and D-(R/S)-SO2-AX-BN.
This work introduces a novel approach for designing axial CP-MR-TADF molecules with high efficiency and goodchiroptical properties, demonstrating the success of our axial chiral biphenyl design strategy. Moreover, the results provide a valuable reference for fabricating narrowband CP-OLEDs with high efficiency and intense CPEL.
This work was published in Adv. Funct. Mater. (2024, DOI: 10.1002/adfm.202412044). Master Xiang-Zhi Wang is the first author of the paper, and Prof. You-Xuan Zheng is the corresponding author. Thanks to Prof. Jing-Lin Zuo for his support in this work! This work is supported by the National Natural Science Foundation of China (92256304、U23A20593)and the Natural Science Foundation of Jiangsu Province (BK20243010, BK20242021).