Asymmetric Blue Multiresonance TADF Emitters with a Narrow Emission Band

Abstract

In this study, we synthesized and characterized multiple resonance (MR) type blue thermally activated delayed fluorescence (TADF) emitters. Unlike many boron-based MR-TADF materials, the blue TADF emitters of this work had an asymmetric molecular structure with one boron, one oxygen, and one nitrogen. The aromatic units linked to the nitrogen were changed into diphenylamine, carbazole, dimethylacridine, and diphenylacridine to manage the light emission properties of the emitters. The TADF emitters exhibited a blue emission due to the weak electron-donating oxygen atom and the emission color was controlled by the aromatic unit connected to the nitrogen. The simple diphenylamine unit was effective in achieving real deepblue emission for the BT2020 standard with a high external quantum efficiency (EQE), while the electron-rich nitrogen-based dimethylacridine and diphenylacridine accelerated the reverse intersystem crossing for high EQE and small EQE roll-off. Among the emitters, a diphenylamine-substituted emitter, 7-(tert-butyl)-9-phenyl-9H-5-oxa-9-aza-13b-boranaphtho-[3,2,1-de]anthracene (B-O-dpa), showed a maximum external quantum efficiency of 16.3%, a small full width at half-maximum of 32 nm, and a real deep-blue color coordinate of (0.15, 0.05).

Highly Efficient Asymmetric Multiple Resonance Thermally Activated Delayed Fluorescence Emitter with EQE of 32.8 % and Extremely Low Efficiency Roll‐Off

“…The significantly alleviated efficiency roll‐off of 2Cz‐PTZ‐BN was ascribed to the very fast RISC process, high fluorescent radiative efficiency and effective suppression of aggregation‐induced quenching by the shielding carbazole moieties. Notably, the operational device lifetime of 2Cz‐PTZ‐BN was a slightly longer than that of Cz‐PTZ‐BN (LT 50 =14.1 and 10.3 h, respectively), under an initial luminance at 500 cd m −2 (Figure S23, Supporting Information), which was comparable to the sensitizer‐free MR‐TADF devices, [4f, 5c,f,h, 6c,f] demonstrating the good stability of the asymmetric MR emitter. To the best of our knowledge, these are among the best performance values reported for pure green MR‐TADF OLEDs.…”

“…With higher PLQY and significant steric effect, the 2Cz‐PTZ‐BN‐based device realized much better EL performance with a CE max of 108.5 cd A −1 , a PE max of 92.1 lm W −1 and an EQE max of 32.8 % without any coupling optimization, which is one of the best performance values for green MR‐TADF materials among the reported data (Figure 5a and Table S10, Supporting Information). Noticeably, at a brightness of 100 cd m −2 and 1000 cd m −2 , the EQE values of the 2Cz‐PTZ‐BN‐based device still maintained a high level of 30.8 % and 23.5 %, respectively, which are the highest values for green MR‐TADF emitters at the same brightness (Figure 5b, c and Table S10, Supporting Information) [4a–k, 5b–j, 6a–f, 10b–c] . The significantly alleviated efficiency roll‐off of 2Cz‐PTZ‐BN was ascribed to the very fast RISC process, high fluorescent radiative efficiency and effective suppression of aggregation‐induced quenching by the shielding carbazole moieties.…”

Ultrafast Triplet–Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens

“…Typical MR‐TADF molecules consist of fused polycyclic aromatic frameworks with mutually ortho ‐positioned electron‐deficient boron (B) and electron‐rich N atoms, which can induce complementary resonance effects for the localization and separation of frontier molecular orbitals on different atoms, resulting in Δ E ST values of 0.1–0.2 eV. These features endow the MR‐TADF molecules with a high photoluminescence (PL) quantum yield (Φ PL ) approaching 100 % and an ideal narrow spectral full width at half maximum (FWHM) of <30 nm [23–50] . However, a critical drawback of the current MR‐TADF systems is their rather slow RISC ( k RISC ≤10 5 s −1 ), which remains a major challenge for the practical OLED application.…”

Narrowing the Electroluminescence Spectra of Multiresonance Emitters for High-Performance Blue OLEDs by a Peripheral Decoration Strategy

“…This design enables the emitter to realize a small Δ E ST and suppressed structural relaxation in the excited states concurrently, resulting in TADF with a narrowband emission as well as high photoluminescence quantum yields (PLQYs). A series of MR-TADF materials based on the B–N or carbonyl–nitrogen framework have been developed for high-performance TADF OLEDs with a small fwhm of as low as 18 nm and an external quantum efficiency (EQE) of up to 34%. − Nevertheless, compared to the conventional D–A-type TADF materials, the development of MR-TADF compounds is still in its infancy. The relationship between the chemical structures and their photophysical properties, including the efficiency and fwhm, still lags behind.…”