Chiral multiple resonance thermally activated delayed fluorescence (CP-MR-TADF) materials show great promise in the preparation of circularly polarized organic electroluminescent devices (CP-OLEDs) for 3D and wide color gamut displays due to their properties of high color purity, high luminescence efficiency, and circularly polarized luminescence (CPL). However, the skeletons of CP-MR-TADF molecules reported so far are limited to mono- or diboron structures, whereas triboron or polyboron CP-MR-TADF materials are still unexplored, whose syntheses and investigations will promote the further development of chiral materials and lead to new possibilities of applications.
Figure 1. Design strategy of polyboron CP-MR-TADF molecules
Recently, the Prof. You-Xuan Zheng and Prof. Cheng-Hui Li team made progress in the study of polyboronated CP-MR-TADF materials and CP-OLEDs. Two pairs of tetraborated intrinsically axial chiral CP-MR-TADF enantiomers (R/S)-BDBF-BOH and (R/S)-BDBT-BOH containing two B-OH groups were obtained by using diphenylamine with small steric resistance as the electron donor unit.In toluene solution, both materials showed ultra-pure blue emission with maximum emission peaks of 458 and 459 nm with full width at half maximum (FWHM) of 27 nm, with CIE coordinates of (0.14, 0.08) and (0.14, 0.09), respectively. Due to the effective participation of chiral units in the frontier molecular orbital distributions, the (R/S)-BDBF-BOH and (R/S)-BDBT-BOH enantiomers showed mirror-symmetric CPL spectra in toluene with asymmetry factors (|gPL|) of 6.8 × 10-4 and 8.5 × 10-4, respectively.
Figure 2. (a) CD spectra and (b) gabs versus wavelength curves of R/S-BDBF-BOH and R/S-BDBT-BOH in toluene. (c) and (f) Calculated μe (red arrow), μm (blue arrow), and θe,m of S1 state of S-BDBF-BOH and S-BDBT-BOH. (d) CPL spectra of R/S-BDBF-BOH and R/S-BDBT-BOH, and (e) gPL versus wavelength curves of R/S-BDBF-BOH and R/S-BDBT-BOH in toluene.
Unlike other polyboron MR-TADF structures containing B-OH groups, the (R/S)-BDBF-BOH and (R/S)-BDBT-BOH structures have a special position of B-OH, which makes it easy to form intramolecular hydrogen bonding instead of intermolecular hydrogen bonding. This greatly reduces the intermolecular forces and facilitates the preparation of vapor-deposited devices from the materials. In addition, sulfur atoms were introduced to utilize their heavy atom effect to accelerate thereverse intersystem crossing process of the molecules in order to improve the luminescence efficiency of (R/S)-BDBT-BOH. As a result, the vapor-deposited OLED based on BDBF-BOH presents a pure blue emission peaking at 467 nm with a FWHM of 30 nm, and a maximum external quantum efficiency (EQEmax) of 29.5%. And the BDBT-BOH-based OLED showed better performances with an EQEmax of 30.1%. Moreover, the enantiomer-based CP-OLEDs exhibited obvious CPEL spectra with |gEL| factors of 0.59 × 10-3 and 1.2 × 10-3, respectively.
The results demonstrate the importance of chiral polyboron MR-TADF materials and their promising applications in high-efficiency CP-OLEDs. This work increases the structural diversity of CP-MR-TADF materials and provides some guidance and inspiration for the design of CP-MR-TADF materials.
Figure 3. OLED performances of D1 and D2: (a) Normalized EL spectra measured at 7 V and corresponding CIE coordinates; (b) current efficiency–luminance and external quantum efficiency–luminance curves; (c) Current density-voltage-luminance (J–V–L) characteristics; (d) and (e) CPEL spectra of R/S-D1 and R/S-D2; (f) gEL versus wavelength curves of R/S-D1 and R/S-D2.
This work was published in Angew. Chem. Int. Ed. (Li Yuan, Jun-Wei Xu, Zhi-Ping Yan, Yi-Fan Yang, dan Mao, Jia-Jun Hu, Hua-Xiu Ni, Cheng-Hui Li,* Jing-Lin Zuo, You-Xuan Zheng,* Tetraborated Intrinsically Axial Chiral Multi-resonance Thermally Activated Delayed Fluorescence Materials, Angew. Chem. Int. Ed., 2024, DOI: 10.1002/anie.202407277). This work is supported by the National Natural Science Foundation of China (92256304, U23A20593, 22105084), and Fundamental Research Funds for the Central Universities (020514380272).