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 S 1 and the ground state S 0 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 S 1 and the ground state S 0 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).
National Science Review
Experimental study