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Recently, numerous efforts have been devoted to sky blue or greenish blue organic light-emitting diodes (OLEDs), nevertheless, the deep blue and pure blue OLEDs still remain a formidable challenge. Herein, a series of deep blue and pure blue donor–acceptor–donor (D–A–D) type emitters, namely Cz-DMTD, PhCz-DMTD and TMCz-DMTD, were constructed. For PhCz-DMTD which exhibited no delayed fluorescence component, the extended delocalization of donors and the enhancement of oscillator strength were both observed through theoretical calculation. For methyl-engineered TMCz-DMTD, the degenerate charge transfer singlet excited (1CT) and triplet excited (3CT) states and localized triplet excited (3LE) state endowed a thermally activated delayed fluorescence (TADF) character. Moreover, PhCz-DMTD and TMCz-DMTD based OLEDs displayed emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.164, 0.077) and (0.157, 0.107) and with maximum external quantum efficiencies (EQEmax) of 4.6% and 8.7%, respectively. It is the first time that the electroluminescence wavelength of TMCz based OLEDs reached below 450 nm with a CIEy below 0.11. Molecular engineering of the carbazole moiety is expected to provide a valuable strategy for molecular design of more deep blue emitters. [ABSTRACT FROM AUTHOR]

Incorporating pyridyl uniquely from its 2-position onto 10-site of acridone ring generated intramolecular hydrogen bond in a group of acridone-carbazole hybrid thermally activated delayed fluorescence emitters, which universally pulled down non-radiative transition rates by three orders and elevated reverse intersystem crossing rates by two orders, and thus boosted PLQYs to almost unity and EQEs of OLEDs to 30+%, representing the state-of-the-art efficiencies of deep-blue TICT-TADF materials and devices with CIEy≤0.10. [Display omitted] • Intramolecular-hydrogen-bond boosts performance of deep blue TADF emitters. • H-bond reduces k nr by 3 orders and increases k RISC by 2 orders. • PLQYs≈100 %, k RISC s ≥ 105 s−1 and horizontal orientation ratios ≥90 % are obtained. • Deep blue OLEDs exhibit record-high EQEs of 31–33 % with CIE y of 0.09–0.12. A group of D-A-D type thermally activated delayed fluorescence (TADF) emitters were developed with 10-(pyridin-2-yl)acridin-9(10 H)-one as acceptor and substituted carbazoles as donors. The design was focused on incorporating pyridyl to the 10-site nitrogen atom of acridin-9(10 H)-one ring, which improved the reverse intersystem crossing rate constant k RISC from 103 s−1 to 105 s−1. More importantly, the linkage of pyridyl uniquely from its 2-position to acridin-9(10 H)-one ring induced intramolecular hydrogen bond, which suppressed nonradiative transition rates by three to four orders of magnitude and universally increased the photoluminescence quantum yields from 54 % to nearly 100 %. In combination with the intrinsic high horizontal orientation ratios over 90 % of acridin-9(10 H)-one based molecules, such pyridyl decoration boosted the external quantum efficiencies of simple-structure deep blue organic light-emitting diodes to 31–33 % with CIE x of 0.15 and CIE y of 0.09 ∼ 0.12, being double of that for hydrogen-bond-free parent emitter and representing the state-of-the-art efficiencies for the deep blue twisted intramolecular charge transfer type TADF materials with CIEy ≤ 0.10 so far. These TADF emitters are comparable with the prevailing multiresonance blue emitters in terms of distinctive efficiencies, but advantageous in much facile syntheses. [ABSTRACT FROM AUTHOR]

Developing deep blue thermally activated delayed fluorescence (TADF) materials with high efficiencies and good color purities is still a formidable challenge. Here, two novel deep blue TADF emitters, namely MePhCz-AD and MePhCz-AD-Py, were synthesized by incorporating two 3,6-diphenyl-9 H -carbazole donors onto the methyl-modified acridin-9(10 H)-one (AD) acceptor, respectively. The high rigidity of AD acceptor effectively suppressed non-radiative transition and vibration relaxation of excited states, which is favorable for a good emitting quantum yield and narrow spectral width. The presence of methyl groups at 2,7-sites of AD ring slightly reduced the acceptor strength and contributed to the deep blue emission color. Incorporation of phenyls at 3,6-sites of carbazole ring delocalized the distribution of the highest occupied molecular orbital (HOMO), promoting the radiative decay process. As a result, these two emitters showed deep blue TADF with fast radiative decay rates k R > 107 s−1. In addition, with the introduction of pyridyl (Py) group at 10-site of AD ring instead of phenyl, both the spin-orbital coupling (SOC) constant between high-lying triplet states and the lowest singlet excited state (S 1) and thus the reverse intersystem crossing rate constant (k RISC) were dramatically promoted for MePhCz-AD-Py. The organic light-emitting diode with MePhCz-AD-Py as doped emitter exhibited a maximum external quantum efficiency (EQE max) of 15.4% with a relatively narrow FWHM of 60 nm and the deep-blue color coordinates of (0.15, 0.15). A deep blue TADF emitter MePhCz-AD-Py based on diphenylcarbazole as donors and acridin-9(10 H)-one as acceptor with methyl at 2,7-sites and pyridyl at 10-site exhibited both fast k RISC of 3.7 × 104 s−1 and k R of 4.1 × 107 s−1 due to a large SOC and an enlarged oscillator strength. The deep blue OLED with MePhCz-AD-Py as doped emitter realizes an EQE max of 15.4% with CIE coordinates of (0.15, 0.15). [Display omitted] • Deep blue TADF emitters are developed with methyl-modified acridin-9(10 H)-one as acceptor. • Pyridyl is introduced to 10-site of acceptor to increase the SOC and RISC rate constant k RISC. • High EQE of 15.4% and CIE (0.15, 0.15) were obtained for deep blue OLED. [ABSTRACT FROM AUTHOR]

Carbazole is a weak electron donor suitable to construct blue thermally activated delayed fluorescence (TADF) emitters, but the less repulsion effect of its five-member-ring structure can't guarantee the frontier molecular orbital separation and TADF feature. A group of carbazole-π-triazine based blue fluorescence emitters is designed by introducing the steric hindrance groups with different electronic properties at the donor or π-bridge. When electron-withdrawing pyridyl with moderate steric hindrance is introduced to the 1-site of 3,6-Di-tert-butyl-9H-carbazole, PyBuCz-TRZ and PyBuCz-MeTRZ gain the blue-shifted TADF. The observed charge transfer triplet excited state within the donor (3CT D) between carbazole and pyridyl units has different spatial orbital angular momentum and allowed spin-orbit coupling (SOC) with the charge transfer singlet excited state (1CT) of the whole molecule from carbazole donor to triazine acceptor, thus, acting as the media state to facilitate efficient multi-channel reverse intersystem crossing (RISC) process. This is the first time to introduce an electron-withdrawing group into the 1-position of carbazole donors to construct TADF molecules, not only achieving the blue-shifted TADF at 455 nm but also improving the external quantum efficiency to 15.3% in organic light-emitting diodes. This donor strategy should be valid to design more blue TADF materials. A donor design strategy for blue TADF is developed by incorporating electron-withdrawing and bulky pyridyl at 1-site of carbazole, not only leading to blue-shift of TADF but also facilitating multi-channel reverse intersystem crossing via the participation of the charge transfer triplet state of the donor (3CT D). An EQE of 15.3% is achieved in pure blue OLED of these TADF emitters. [Display omitted] • Electron-withdrawing pyridyl was introduced at 1-site of carbazole to construct thermally activated delayed fluorescence (TADF) emitters. • Blue shifted emission and improved EQE were simultaneously achieved in the pure blue OLEDs. • A new triplet state 3CT D was detected within the donor and facilitated multi-channel RISC process. • Too strong electron-withdrawing substituent on carbazole like cyano was proved to make TADF feature disappear. • This is the first report to incorporate electron-withdrawing substituent at 1-site of carbazole for blue TADF materials. [ABSTRACT FROM AUTHOR]