image: Figure 1. Synthesis of BN-CP reagents and conditions: (i) Pd2(dba)3, X-Phos, NaO(t-Bu), dry xylene, 140°C, 24 h. 61%; (ii) BBr3, o-dichlorobenzene, 185°C, 48 h; (iii) DIPEA, o-dichlorobenzene, 185°C, 72 h; (iv) MesMgBr, 45°C, 24 h, 14.7%.
Credit: ©Science China Press
In organic light-emitting diode (OLED) display technology, blue-light devices consistently underperform red and green counterparts in critical metrics, notably luminous efficiency and operational lifetime. This represents a primary bottleneck for OLED advancement. Thus, narrowband emissive blue emitters have gained significant attention because their reduced excited-state energy at equivalent color coordinates enables substantial improvements in device efficiency and operational stability compared to conventional broad-spectrum materials.
To address this challenge, researchers modified a [1,4] B/N-embedded heterocyclic core with a broad spectrum through macrocyclization. This strategy increases structural rigidity, which suppresses molecular vibrations and narrows the emission spectrum. This results in narrowband deep-blue emission with a full width at half maximum (FWHM) of merely 24 nm. This approach establishes a new design paradigm for next-generation narrowband emitters.
1. Synthesis
First, researchers constructed the macrocyclic framework via Buchwald coupling. Then, they performed a one-pot triple intramolecular Bora-Friedel-Crafts reaction to embed the [1,4]azaborine heterocyclic core. This strategy produced BN-CP, the first macrocyclic [1,4]azaborine system with narrowband emission.
2. OLED performance
Leveraging the exceptional intrinsic properties of BN-CP—a photoluminescence FWHM of 24 nm and a quantum yield of 97%—the fabricated deep-blue OLED device (CIEy = 0.04) maintains a small FWHM value of 32 nm while achieving a peak external quantum efficiency (EQE) of 23.3%. This EQE value is among the highest reported for deep-blue devices that meet the stringent CIEy < 0.05 standard.
3. Theoretical analysis
The spectral bandwidth is mainly determined by the structural displacement (K) between the excited state S1 and the ground state S0 as well as high-frequency vibrational modes. Theoretical calculations showed that the target molecule BN-CP and the wide-spectrum [1,4] boron-nitrogen binary heteroarene L-BN have similar high-frequency stretching vibrational modes, with their electron density distributions both concentrated within the molecular framework of L-BN. Therefore, the key to the narrowed spectrum of BN-CP lies in the enhanced conformational rigidity brought by its cyclic structure. Specifically, the bending vibration of the plane where the two smallest [1,4] boron-nitrogen heteroarene units (BN1) were located in the L-BN structure has been significantly suppressed in BN-CP.
Spectral bandwidth is primarily governed by the structural displacement (K) between the excited state S1 and the ground state S0 as well as high-frequency vibrational modes.
Theoretical calculations reveal that the target molecule, BN-CP, and the broad-spectrum linear analog, L-BN (1,4-diazaborine heteroarene), have similar high-frequency vibrational modes. These modes are localized within the L-BN molecular framework. Thus, the key to spectral narrowing in BN-CP is its enhanced conformational rigidity, which is conferred by the macrocyclic structure. Specifically, the bending modes of the planes containing the two minimal 1,4-diazaborine units (BN1) — which undergo significant motion in L-BN — are substantially suppressed in BN-CP.
These results were published in the National Science Review (NSR). Tianjiao Fan, a Ph.D. candidate in the Department of Chemistry at Tsinghua University, is the first author. The co-corresponding authors are Professor Lian Duan (Department of Chemistry and Laboratory of Flexible Electronics Technology, Tsinghua University) and Associate Research Fellow Yuewei Zhang (Laboratory of Flexible Electronics Technology, Tsinghua University).