Figure 1. Research background and molecular design strategy
Current research primarily employs two strategies to regulate molecular spectrum of the MR-TADF materials (Figure 1a): extending the resonance system or introducing strong electron-donating/electron-accepting groups. However, these strategies are often inefficient in achieving effective spectral narrowing, as evidenced by the following limitations: conventional resonance extension struggles to achieve both significant red-shifting and fine control of molecular orbitals; purely conjugated extensions, such as fused benzene systems, tend to disrupt the MR effect, resulting in the loss of TADF properties; the incorporation of strong electron-donating/accepting groups often reduces molecular rigidity, enhancing vibrational relaxation, and induces long-range charge transfer (LRCT) characteristics. Even within rigid molecular frameworks, the occurrence of LRCT cannot be completely avoided. Collectively, these factors inevitably lead to spectral broadening, making the molecular design of materials with simultaneous long-wavelength and ultra-narrowband emission extremely challenging.
Recently, You-Xuan Zheng’s research group achieved a significant breakthrough in the field of long-wavelength ultra-narrowband luminescent materials. By precisely embedding two boron atoms into a tetraazacyclophane framework, they innovatively constructed a molecular framework capable of ultra-narrowband long-wavelength emission, which further lead to the development of an ultra-narrowband yellow-emitting material (HBN) (Figure 1b).
Figure 2.Photophysical properties of HBN
Experimental results demonstrate that, compared to the H-tetraazacyclododecane precursor, the incorporation of dual boron atoms achieve an impressive red shift of up to 165 nm in the emission spectrum (Figure 2). Furthermore, HBN exhibits ultra-narrowband yellow emission peaking at 572 nm with a FWHM of 17 nm in dilute toluene solution, while its emission FWHM is further narrowed to 12 nm in n-hexane, setting a new performance benchmark in the medium-to-long wavelength region.
Figure 3. Theoretical calculations of HBN molecule
Theoretical calculations reveal that the HBN molecule possesses a highly symmetrical and planar molecular structure (Figure 3a), with its HOMO and LUMO orbitals also exhibiting symmetry (Figure 3b). The low structural reorganization energy (0.17 eV) indicates its minimal structural relaxation characteristics (Figure 3c). Additionally, the calculated singlet-triplet energy gap (ΔEST = 0.23 eV) of HBN is in excellent agreement with the experimental value (0.22 eV) (Figure 3d). Further electron-hole analysis shows that the increase in the electron-hole center distance (D index) significantly affects the emission FWHM, particularly in the pure red emission region. This phenomenon highlights the critical influence of LRCT properties on the material's spectral characteristics (Figure 3e).
Figure 4. Device performances of OLEDs based on HBN molecule
The non-sensitized device D1 achieves a narrow electroluminescence (EL) emission peaking at 580 nm (FWHM: 24 nm) with a maximum external quantum efficiency (EQEmax) of 26.7%. However, the efficiency roll-off is serious. After introducing a phosphorescent-sensitized fluorescence (PSF) architecture, employing the iridium(III) complex sensitizer Bt₂Ir(acac) and optimizing the doping concentration, the PSF device D4 demonstrates a high EQEmax (36.1%), narrow EL emission (581 nm, FWHM: 25 nm), and reduced efficiency roll-off. Furthermore, it maintains excellent performances under high brightness, showcasing its broad application potential in high-performance optoelectronic devices.
The findings of this study demonstrate that the precise modulation of molecular architecture and material performance offers critical guidance for designing ultra-narrowband pure green-to-red emissive materials, laying a solid foundation for the advancement of wide color gamut display technology.
The above findings were published in Angew. Chem. Int. Ed. (DOI: 10.1002/anie.202421102), with PhD candidate Jia-Jun Hu as the first author, and Prof. You-Xuan Zheng corresponding authors. Dr. Xiao Liang and Dr. Zhi-Ping Yan made significant contributions to material testing and theoretical calculations. Prof. Jing-Lin Zuo also guided this research. This research was funded by the Jiangsu Provincial Natural Science Foundation (BK20243010, BK20242021) and the National Natural Science Foundation of China (92256304, U23A20593).