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 the fact that they can combine the properties of high color purity, high luminescence efficiency, and circularly polarized luminescence (CPL).
Among all the chiral moieties, chiral binaphthol and chiral octahydronaphthol units have been widely used to construct CP-TADF structures due to their abundant commercially available compositions and stable axial chiral configurations. However, most of the CP-TADF structures containing binaphthol and octahydronaphthol units were constructed using a chiral perturbation strategy and the distribution of chiral sources in the front molecular orbitals was rare. Therefore, the CPL properties of the materials and corresponding CP-OLEDs are not satisfactory.
To solve this problem, Prof. You-Xuan Zheng’s group proposed a steric-hindrance-assisted dual-core strategy for CP-MR-TADF materials in their previous work (Adv. Mater., 2022, 34(36), 2204253), which allowed the chiral octahydronaphthol unit to participate in the frontier molecular orbital distributions (FMOs) and improved the CPL performance of the materials. However, since the octahydronaphthol moiety does not account for a high enough percentage in the FMOs, the g factor is only 1.0 × 10-3. Compared with the chiral octahydronaphthol unit, the [1,1’-bidibenzo[b,d]furan]-2,2’-diol molecule is more distorted with more intramolecular group overlaps and large spatial site resistance, which is conducive to achieving better CPL properties. Therefore, it is expected that efficient CP-MR-TADF materials with high dissymmetry factors (g) can be obtained by combining this chiral unit with the MR-TADF skeleton.
Figure 1. Design strategy of CP-MR-TADF molecules
By using conjugated extended (1,1’-bidibenzo[b,d]furan)-2,2’-diol and (1,1’-bidibenzo[b,d]thiophene)-2,2’-diol as the chiral sources, two pairs of intrinsically axial chiral MR-TADF materials (R/S-BDBF-BNO and R/S-BDBT-BNO) were designed and synthesized. R/S-BDBF-BNO and R/S-BDBT-BNO realize mirror-symmetric CPL and narrowband emission. The chiral moieties effectively engage in frontier molecular orbital distributions through the establishment of covalent bonds with boron atoms. The proportions of (1,1’-bidibenzo[b,d]furan)-2,2’-diol in the highest occupied and lowest unoccupied natural transition orbitals (HONTO/LUNTO) are 25.3% and 64.7%, respectively. In addition, BDBT-BNO processes a dual-channel transition mode, thus the (1,1’-bidibenzo[b,d]thiophene)-2,2’-diol constitutes 22.8%, 24.3% and 54.0%, 46.9% of the distributions of HONTO, HONTO-1 and LUNTO, LUNTO+1, respectively. The high proportions endow R/S-BDBF-BNO and R/S-BDBT-BNO with good CPL properties with higher |gPL| factors of 1.7/1.8 × 10-3.
Figure 2. (a) CD spectra of R/S-BDBF-BNO and R/S-BDBT-BNO in toluene solutions. Calculated μe (red arrow), μm (blue arrow) and θe,m of S1 state of (b) S-BDBF-BNO and (c) S-BDBT-BNO. TEDM and TMDM densities of (d) S-BDBF-BNO and (e) S-BDBT-BNO. (f) CPL spectra of R/S-BDBF-BNO and R/S-BDBT-BNO in toluene solutions.
Correspondingly, the circularly polarized organic light-emitting diodes based on the enantiomers exhibit mirror symmetrical circularly polarized electroluminescence with |gEL| factors of 1.5/1.6 × 10-3. Furthermore, the introduction of sulphur atoms enables BDBT-BNO to have a high maximum external quantum efficiency of 35.7% in device. The results demonstrate that increasing the conjugation of chiral groups can effectively improve theCPL performance of CP-MR-TADF materials, which provides a certain reference and reference for the subsequent design of CP-MR-TADF materials with high g factor for efficient CP-OLEDs.
Figure 3. OLED performances of D1 and D2: (a) Energy-level diagrams and device structures. (b) Angle-dependent p-polarized PL intensity and simulation curve for the emitting layer with the 10 wt% doping concentration. (c) Normalized EL spectra measured at 7 V, EL emission of the corresponding OLEDs and CIE coordinates. (d) Current efficiency–luminance and external quantum efficiency–luminance curves. (e) Current density-voltage-luminance (J–V–L) characteristics; (f) CPEL spectra of R/S-D1 and R/S-D2.
This work was published inAdv. Funct. Mater. (2024, DOI: 10.1002/adfm.202403803), Dr. Li Yuan and doctoral candidate Yi-Fan Yang are the co-first authors of the paper, and Prof. You-Xuan Zheng is the corresponding author. Dr. Zhi-Ping Yan contributed to the theoretical calculations of the materials. This work is supported by the National Natural Science Foundation of China (92256304, U23A20593, 22105084).