A Series of Fused Carbazole/Carbonyl Based Blue to Yellow‐Green Thermally Activated Delayed Fluorescence Materials for Efficient Organic Light‐Emitting Diodes

Abstract

The exploitation of novel fused carbazole/carbonyl emitters is essential to broaden the application of thermally activated delayed fluorescence (TADF) materials for organic light‐emitting diodes (OLEDs). Here, with the aid of the addition and cyclization of the cyano group with the ortho‐carbazole, fused carbazole/carbonyl based TADF emitters are effectively synthesized via this new synthetic strategy. The attachment of ancillary donors including carbazole, diphenylamine, and phenolazine to the fused carbazole/carbonyl skeletons further tunes their emissions from blue to yellow‐green. Particularly, pure‐blue OLEDs incorporating the peripheral carbazole attached emitter exhibit a maximum external quantum efficiency (EQEmax) of 22.3%, with a small full width at half maximum (FWHM) of 48 nm. In addition, the peripheral phenolazine attached yellow‐green emitter shows extremely small singlet−triplet state energy gap (ΔEST) of 0.01 eV, high photoluminescence quantum efficiency of 82.5%, short delayed fluorescence lifetime of 6.2 µs and good OLED performances with an EQEmax of 21.7%, an FWHM of 68 nm and low efficiency roll‐off. These results demonstrate that the new synthetic strategy for fused carbazole/carbonyl molecules provide a valuable reference for the design of high‐efficient TADF emitters.

Narrowband Emissive Thermally Activated Delayed Fluorescence Materials

“…[117] Another type of CO/N-based MR-TADF materials featuring a fused carbonyl/carbazole core was reported by Zheng and co-workers. [118] Similar to other CO/N systems, the attachment of ancillary donor units can tune photophysical properties. Among the set of materials, 2,3-CZ and 2,5-CZ (Figure 9), exhibited narrowband blue emissions (λ PL = 449/459 nm and FWHM PL = 36/41 nm, respectively, in toluene).…”

Efficient narrowband electroluminescence based on a hetero-bichromophore thermally activated delayed fluorescence dyad

“…24 The introduction of general electron donors (arylamines) or acceptors has proven an effective strategy, which creates a long-range intramolecular charge transfer (ICT) state for improving the device efficiency and tuning the bandgap. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] This strategy usually improved the color purity due to the vanished shoulder peak. However, such modification for blue MR-TADF emitters usually enhanced the structural relaxation, broadened the emission spectra, and redshifted the emission band.…”

Simple Phenazine‐Based Compounds Realizing Superior Multicolored Emission

“…[20] Based on this strategy, many researchers tended to modify the subject (acceptor) by integrating more electron donors (Ds), [26][27][28] aiming to utilize the steric hindrance of donors to separate the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). Some previous works are preferred to use special donors, such as 1,8-dimethylcarbazole, [29][30][31] to make a large dihedral angle between donor and acceptor (A). Regardless of which method was used, to achieve high efficiency performance, most compounds either experienced arduously picking of specific subjects (acceptors) that were tedious to synthesize, [21] or were coupled with more and even different donors.…”

“…Regardless of which method was used, to achieve high efficiency performance, most compounds either experienced arduously picking of specific subjects (acceptors) that were tedious to synthesize, [21] or were coupled with more and even different donors. [32,33] Meanwhile, even though the interest in TADF materials is shifting to the operational lifetime, [34] color purity, [23] reverse intersystem crossing rate, and so on, [31,[34][35][36] however, according to recent works reported by Ma [37] and Adachi and co-workers, [24,38] new mechanisms about hybrid local and charge-transfer (HLCT)-type TADF have been proposed. We hope that depending on this work, these new mechanisms could be applied to the conventional TADF configuration, further paving the way for the follow-up studies of the mechanisms of TADF.A new family of phenazine-based compounds is synthesized to achieve multicolored emission from deep blue (418 nm) to red (625 nm).…”