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9,9‐Dimethyl‐9,10‐dihydroacridine (DMAC) is one of the most widely used electron donor for constructing high‐performance thermally activated delayed fluorescence (TADF) emitters. However, DMAC‐based emitters often suffer from the imperfect color purity, particularly in blue emitters, due to its strong electron‐donating capability. To modulate donor strength, 2,7‐F‐Ph‐DMAC and 2,7‐CF3‐Ph‐DMAC were designed by introducing the electron‐withdrawing 2‐fluorophenyl and 2‐(trifluoromethyl)phenyl at the 2,7‐positions of DMAC. These donors were used, in combination with 2,4,6‐triphenyl‐1,3,5‐triazine (TRZ) acceptor, to develop novel TADF emitters 2,7‐F‐Ph‐DMAC‐TRZ and 2,7‐CF3‐Ph‐DMAC‐TRZ. Compared to the F‐ or CF3‐free reference emitter, both two emitters showed hypsochromic effect in fluorescence and comparable photoluminescence quantum yields without sacrificing the reverse intersystem crossing rate constants. In particular, 2,7‐CF3‐Ph‐DMAC‐TRZ based OLED exhibited a blue shift by up to 39 nm and significantly improved Commission International de l′Éclairage (CIE) coordinates from (0.36, 0.55) to (0.22, 0.41), while the external quantum efficiency kept stable at about 22.5 %. This donor engineering strategy should be valid for improving the color purity of large amount of acridine based TADF emitters. It can be predicted that pure blue TADF emitters should be feasible if these F‐ or CF3‐modifed acridine donors are combined with other weaker electron acceptors. [ABSTRACT FROM AUTHOR]

Coexistence of through-bond charge transfer (TBCT) and through-space charge transfer (TSCT) is observed to strongly enhance the performance of thermally activated delayed fluorescence (TADF) materials that contain multiple donors and acceptors. A group of TADF emitters were developed with carbazole as the donor, benzophenone as the acceptor, and phenylene as the linking bridge. TBCT was responsible for the TADF feature of the para-linked analogue p-tCz-BP , while both TBCT and TSCT were observed in the ortho-linked isomer o-tCz-BP. The multiple-donor–acceptor analogue D-tCz-D-BP was proved to exhibit multi-channel TBCT and TSCT processes with more near-degenerate excited states and more TSCT (81.0%) contribution than o-tCz-BP (66.3%). Thus D-tCz-D-BP combines the merits of both the high oscillator strength of p-tCz-BP and the tiny energy splitting (ΔEST) between the lowest singlet and triplet excited states caused by the twisted conformation of o-tCz-BP , leading to a high rate constant of radiation (kr) of 1.01 × 107 s−1, of reverse intersystem crossing (krisc) of 0.56 × 106 s−1 and a high photoluminescence quantum yield of 96.8%. The sky-blue organic light-emitting diode (OLED) of D-tCz-D-BP exhibited an external quantum efficiency (EQE) of 24.9%, much higher than those of p-tCz-BP (6.3%) and o-tCz-BP (11.1%). D-tCz-D-BP was also capable of hosting an orange-red iridium phosphor to form a two-emitting-component white OLED that realized an EQE of 18.8% and a high CRI of 80. [ABSTRACT FROM AUTHOR]

High-performance blue phosphorescent iridium(III) complexes having good efficiency and color purity simultaneously are still a huge challenge. Two blue phosphorescent iridium(III) complexes, (dfdmappy)2Ir(phim) (Ir1) and (dfdmapypy)2Ir(phim) (Ir2), were developed by employing 2-(2,4-difluorophenyl)-N,N-dimethylpyridin-4-amine (dfdmappy) and 2′,6′-difluoro-N,N-dimethyl-[2,3′-bipyridin]-4-amine (dfdmapypy) as cyclometalating ligands and N-heterocyclic carbene (phim) as an ancillary ligand. The introduction of a strong electron-donating dimethylamino (dma) group onto the N-coordinating pyridine ring of the main ligands and the adoption of high field-strength phim effectively raise the cyclometalating ligand-centered emitting triplet states (i.e.3LC). Meanwhile, the phim ligand with strong σ-donating electron character destabilizes the non-radiative d–d*state as well. These strategies make both complexes achieve charge transfer (CT) state dominated emission, i.e., metal-to-ligand/ligand-to-ligand charge transfer (3MLCT/3LLCT) dominated emission from the T1 state and resulted in Ir1 and Ir2 single-peak blue phosphorescence with high photoluminescence quantum yields (PLQYs) of 91% and 60%, respectively, and shorter excited-state lifetimes of 1.10 and 3.33 μs, respectively. Quantum chemical calculations verified the CT-dominated feature and lower root-mean-square displacement/deviation (RMSD) value of Ir1versus more metal centered d–d*transition and larger RMSD value of Ir2, well accounting for the higher PLQY and superior color purity of Ir1. Pure blue organic light-emitting diodes (OLEDs) of Ir1 exhibit a maximum external quantum efficiency (EQE) of 28% with Commission Internationale de I'Eclairage (CIE) coordinates of (0.16, 0.21), which is one of the best performances for blue phosphorescent OLEDs reported so far. [ABSTRACT FROM AUTHOR]

By linking the carbazole unit to the nitrogen atom of acridone through phenyl or pyridyl, two compounds, named 10‐(4‐(9H‐carbazol‐9‐yl)phenyl)acridin‐9(10H)‐one (AC‐Ph‐Cz) and 10‐(5‐(9H‐carbazol‐9‐yl)pyridin‐2‐yl)acridin‐9(10H)‐one (AC‐Py‐Cz) were designed and synthesized. These two materials, characterized with highly twisted and rigid structure, good thermal stability, and balanced carrier‐transporting properties, were employed as host materials for green phosphorescent and thermally activated delayed fluorescent organic light‐emitting diodes (OLEDs). The carbazole group, despite its small contribution to the highest occupied molecular orbitals (HOMOs) of these two materials, plays an essential role as an intramolecular host in energy delivering and improving the hole transporting ability of these two hosts. The incorporation of the electron‐deficient pyridyl group as a linking group slightly improves the electron transporting capability of AC‐Py‐Cz. The green phosphorescent OLED (PhOLED) based on AC‐Py‐Cz exhibited excellent device performance with a turn‐on voltage of 2.5 V, a maximum power efficiency and an external quantum efficiency (ηext) of 89.8 lm W−1 and 25.2 %, respectively, benefitting from the better charge‐balancing ability of AC‐Py‐Cz host due to the presence of the pyridyl bridge. More importantly, all the devices based on these two hosts showed low efficiency roll‐off at high brightness due to the suppressed non‐radiative transition in the emitting layer. In particular, the AC‐Py‐Cz‐hosted green PhOLED exhibited an efficiency roll‐off of 1.6 % from the maximum next at a high brightness of 1000 cd m−2 and a roll‐off of 15.9 % at an extremely high brightness of 10000 cd m−2. This study manifests that acridone‐based host materials have great potential in fabricating OLEDs with low efficiency roll‐off. [ABSTRACT FROM AUTHOR]

Blue thermally activated delayed fluorescence (TADF) emitters usually suffer from poor color purity and low efficiencies, especially deep-blue emitters. Here, acridin-9(10H)-one (acridone, AD), featuring an orthogonal and highly rigid conformation, was used as an acceptor to construct a series of deep-blue TADF emitters (3,6-DCz-AD, 3,6-DPhCz-AD, 3,6-DtBuPhCz-AD, 3,6-DDPhCz-AD and 3,6-DDtBuPhCz-AD), which effectively restricted intramolecular relaxation and produced narrow full widths at half maximum of ∼55 nm. By extending the π-skeleton of the carbazole donor by tuning the peripheral groups on the carbazole ring to slightly increase the donor strength, both the energy splittings between the S1 (1CT) and T1 (3LE) states and the T1 and T2 (3CT) states were gradually reduced, which facilitated the multichannel reverse intersystem crossing (RISC) and realized high kRISC values of 105 s−1 for 3,6-DDPhCz-AD and 3,6-DDtBuPhCz-AD. At the same time, the extended transition dipole moment along with high molecular rigidity led to an extremely high radiative transition rate constant kR of 108 s−1. 3,6-DDPhCz-AD and 3,6-DDtBuPhCz-AD exhibited external quantum efficiencies of 17.4% and 17.3% in doped organic light-emitting diodes (OLEDs) with CIE coordinates of (0.15, 0.11) and (0.15, 0.13), respectively. Tuning the peripheral groups on carbazole, even without changing donor distortion, proved to be a practical strategy for enhancing TADF efficiencies while maintaining color purity. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials that can exhibit high efficiencies in non‐doped organic light‐emitting diodes (OLEDs) are highly desired. On the base of a reference TADF molecule (4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)(phenyl)methanone (BP‐DMAC) containing benzoyl/acridine as acceptor/donor, 9‐phenyl‐9H‐3,9"‐bicarbazole (PBCz) is incorporated to the benzoyl side to design and develop a novel emitter, (4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)(9‐phenyl‐9H‐[3,9″‐bicarbazol]‐6‐yl)methanone (PBCz‐BP‐DMAC). The introduction of PBCz unit is proved to endow PBCz‐BP‐DMAC the additional self‐host feature, in addition to its original aggregation‐induced emission and TADF characters. As a result, the non‐doped OLED of PBCz‐BP‐DMAC exhibits sky‐blue emission with remarkable external quantum efficiency (EQE) of 23.0% and rather low efficiency roll‐off (6%) at a brightness of 1000 cd m−2, while its doped device reveals a maximum EQE of 27.5%; the spectra and color coordinates of the doped and non‐doped devices are almost identical. It also acts as a host to sensitize green iridium phosphor (fac‐tris(2‐phenylpyridine) iridium(III), Ir(ppy)3) with high EQE of 23.6%. More fascinatingly, the simplified single‐emitting‐layer all‐TADF white OLED utilizing PBCz‐BP‐DMAC as host emitter in combination with an orange TADF emitter (4CzTPN‐Ph) realizes a state‐of‐the‐art EQE of 20.6%. Evidently PBCz‐BP‐DMAC is one of the best sky‐blue TADF materials and exhibits multi‐functional applications in OLEDs with excellent performance. [ABSTRACT FROM AUTHOR]

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]

It is strongly desired for thermally activated delayed fluorescence (TADF) compounds to possess a high rate constant of radiation (KR) and a high rate constant of reverse intersystem crossing (KRISC) simultaneously. A novel TADF compound, 3,6-di(10H-phenoxazin-10-yl)-10-phenylacridin-9(10H)-one (3,6-DPXZ-AD), is designed by attaching phenoxazine as the electron donor at the 3,6-sites of acridin-9(10H)-one as the acceptor. The unique sp2-hybridization of the nitrogen atom of the acridone ring leads to the quasi-equatorial conformation and the high molecular rigidity of 3,6-DPXZ-AD, which suppress conformation relaxation and finally generate a high KR of 1.4 × 107 s−1. The phosphorescence of 3,6-DPXZ-AD with unexpected higher energy than its fluorescence is proved to originate from the intermolecular through-space charge transfer state (TSCT) and the locally excited triplet states (3LE). Due to the multichannel RISC process from the TSCT and 3LE states to the 1CT state, a high KRISC of 1.1 × 106 s−1 is realized simultaneously. 3,6-DPXZ-AD shows a short TADF lifetime of 1.6 μs and a high fluorescence quantum yield of 94.9%. The yellow organic light-emitting diode with 3,6-DPXZ-AD as the doped emitter exhibits excellent performance with a low turn-on voltage of 2.2 V, an external quantum efficiency of 30.6% and a power efficiency of 109.9 lm W−1, being among the best values ever reported for acridone based materials. [ABSTRACT FROM AUTHOR]

Blue iridium phosphors are always precious and desirable for both fundamental research and industrial applications in organic light-emitting diodes (OLEDs). Three heteroleptic cyclometalated iridium(III) complexes, namely, Ir1, Ir2 and Ir3, incorporating 2-(3,5-bis(trifluoromethyl)phenyl)-pyrimidine (tfmphpm) or 2-(2,4-difluorophenyl)-pyrimidine (dfppm) as the main ligand, and 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine (fppz) or 2-(5-(4-(trifluoromethyl)phenyl)-2H-1,2,4-triazol-3-yl)pyridine (Htaz) as the ancillary ligand, were developed for application in OLEDs. Owing to the introduction of the strong-field CF3-containing ancillary ligand and the incorporation of electron-withdrawing F or CF3 groups on the HOMO-lying C-related aromatic rings of the C^N cyclometalating ligands, the phosphorescence of these Ir(III) complexes peaked at 457–459 nm. In comparison with the well-known commercial sky-blue iridium complex FIrpic (bis(4′,6′-difluorophenylpyridinato) iridium(III) picolinate, 475 nm), the phosphorescence of these complexes is hypsochromically shifted by 16–18 nm, more closely approaching standard blue light. They have high phosphorescence quantum yields in solution. The phosphorescent OLEDs employing these complexes as doped emitters display impressive performance. In particular, the PhOLED based on Ir3 achieves an outstanding maximum external quantum efficiency of 21.23% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.26), which is about 27% higher than that of the control device using FIrpic as a dopant (16.63%). [ABSTRACT FROM AUTHOR]

It is essential to manage the alignment of the locally excited (3LE) and charge-transfer triplet (3CT) and singlet (1CT) states of thermally activated delayed fluorescence (TADF) materials to accelerate reverse intersystem crossing (RISC) process. A series of donor-acceptor (D-A) type TADF emitters were developed by attaching acridine (DMAC) or phenoxazine (PXZ) donor at 3-site of acridin-9(10 H)-one (i.e. acridone, AD) acceptor. Regardless of the donor, the lowest triplet excited states (T 1) are always dominated by the 3LE A state with certain contribution from the 3CT state. By introducing methyl (Me) or trifluoromethyl (CF 3) at 6-site of AD acceptor, the energy levels of 1CT, 3CT and 3LE states were tuned in different ways. When the donor is DMAC, incorporating Me on AD ring (3-DMAC-6-Me-AD) destructed the emission performance, while CF 3 (3-DMAC-6-CF 3 -AD) strongly stimulated the TADF by pulling down the 1CT state level and reducing the energy splitting Δ E ST to almost zero. In the case of strong donor PXZ, decorating acceptor with Me (3-PXZ-6-Me-AD) did not bring extinct effect on the TADF property. In particular, 3-DMAC-6-CF 3 -AD and 3-PXZ-6-Me-AD exhibited high rate constants of RISC (k RISC) of 10−6 and of radiation (k R) of 107 S−1, respectively. They achieved the maximum external quantum efficiency (EQE max) of 21.6% and 23.3% in the doped OLEDs, and realized EQE max of 14.7% and 17.6% with low efficiency roll-offs in non-doped devices. Highly rigid acridone is used as acceptor to construct donor-acceptor type thermally activated delayed fluorescence (TADF) emitters. Both incorporating trifluoromethyl on acceptor and adopting stronger donor can realize the energetically favorable 1CT ≈ 3CT ≈ 3LE status, increasing the k RISC from 105 s−1 to 106 s−1. High EQEs of 21.6%/14.7% and 23.3%/17.6% are achieved in doped and non-doped OLEDs. [Display omitted] • A series of thermally activated delayed fluorescence (TADF) emitters were developed using highly rigid acridone as acceptor. • Design strategies were focused on tuning the substituents on acceptor or varying donor strength, not changing twisting angles. • Energy level differences among 1CT, 3CT and 3LE states were reduced to facilitate the RISC to 106 s−1. • High EQEs of 21.6%/14.7% and 23.3%/17.6% were obtained in the doped and non-doped OLEDs. [ABSTRACT FROM AUTHOR]

Two novel tricyclometalated heteroleptic iridium complexes, namely Ir(dfbpy) 2 (pt) and Ir(dfbpy) 2 (dfppy), were designed and synthesized for application as blue phosphors in optoelectronic devices. 2′,6′-difluoro-2,3′-bipyridine (dfbpy) was used as the cyclometalating ligand. In comparison with the parent homoleptic Ir(dfbpy) 3 , the newly developed 3-(1-benzyl-1 H -1,2,3-triazol-4-yl)pyridine (pt) with higher triplet energy than dfbpy or 2-(2,4-difluorophenyl)pyridine (dfppy) was used as the ancillary ligands to tune the electronic properties of these heteroleptic complexes. Both complexes emit strong blue phosphorescence with high quantum yields. The incorporation of high-energy ancillary ligand pt in Ir(dfbpy) 2 (pt) makes the highest-energy sub-band intensity in the phosphorescence spectrum increase relative to those of the parent Ir(dfbpy) 3 , and leads to narrower phosphorescence spectrum than the analogue Ir(dfbpy) 2 (dfppy) as well, which may provide a practical method to adjust the phosphorescence spectra and color purity of the complexes. [ABSTRACT FROM AUTHOR]

A series of thermally activated delayed fluorescence (TADF) materials, namely o -Ac2BBP, o -Px2BBP, D-BP-DMAC and D-BP-PXZ, were designed with double acridine (DMAC) or phenoxazine (PXZ) donors and double carbonyl acceptors. Phenylene and biphenylene were selected as the central bridges for these emitters, and each benzene ring on the central bridge was substituted at ortho-sites. The biphenylene bridge based D-BP-DMAC and D-BP-PXZ show 3D dimensional non-planar conformations and multi-fold layout, which result in multiple intramolecular interactions including C–H⋯O hydrogen bonds and C–H···π interactions and finally strongly stabilized and rigidified the whole molecular structures by locking the molecular geometry. As a result, the nonradiative transition rates (k nr) were effectively suppressed by 5 times in comparison with the corresponding phenylene bridged analogues, and the photoluminescence quantum yields were increased to over 95%. The green and yellow OLEDs with D-BP-DMAC and D-BP-PXZ as doped emitters exhibited the maximum external quantum efficiencies (EQE) of 24.3% and 17.3%, which are almost double of the corresponding values (13.1% and 9.3%) of the phenylene bridged analogues o -Ac2BBP and o -Px2BBP. These results indicate that to increase molecular rigidity by multi-fold-layout is an effective design strategy to enhance the performance of TADF materials and OLEDs. By incorporating a biphenylene as central linking bridge in thermally activated delayed fluorescence emitters, a multi-fold layout is generated, which enables multiple intramolecular interactions, strongly increases molecular rigidity, and thus suppresses non-radiative decays and increases PLQY to 95%. The OLEDs exhibit high EQEs of 24.3%, being twice of those for the less-folded reference analogues. [Display omitted] • A group of TADF emitters are designed with phenylene or biphenylene as central linking bridge. • The biphenylene bridge endows D-BP-DMAC and D-BP-PXZ with 3D non-planar conformations and unique multi-fold layout. • The multi-fold layout enables multiple interactions, increases molecular rigidity, and suppresses non-radiative decays. • The k nr of D-BP-DMAC and D-BP-PXZ are suppressed by 5 times, and the PLQYs are increased to over 95%. • OLEDs of D-BP-DMAC and D-BP-PXZ exhibit maximum EQEs of 24.3% and 17.3%, being twice of the phenylene bridged analogues. [ABSTRACT FROM AUTHOR]

A group of heteroleptic cyclometalated iridium complexes Ir1–Ir4 were designed with 2-phenylpyridine (ppy) derivatives as cyclometalating ligands and 2-(3-(trifluoromethyl)-1 H -pyrazol-5-yl)pyridine (fppz) or 2-(5-(4-(trifluoromethyl)phenyl)-2 H -1,2,4-triazol-3-yl)pyridine (Htaz) as ancillary ligands. The incorporation of electro-donating group into ppy ligands to elevate ligands-centered 3π-π* energy level and utilization of fppz or Htaz ancillary ligands combined to tune the intrinsic electronic properties. These complexes showed greenish-blue or green phosphorescence and are characterized by high photoluminescence quantum yields of up to 70% and short lifetimes below 1 μs. The organic light-emitting diodes (OLEDs) with Ir1–Ir4 as doped emitters all exhibited high efficiencies with low efficiency roll-off. Particularly, the Ir3 device exhibited distinguished performances with the maximum current efficiency of 86.9 cd A−1 and an external quantum efficiency (EQE) of 33.5%. Even at high brightness of 1000 cd m−2, the EQE still remained at 24.6%. Evidently the EQE of 33.5% of Ir3 device is one of the highest efficiencies ever reported for greenish-blue phosphorescence OLEDs. By selecting dimethylamino or ethoxyl decorated 2-phenylpyridine as cyclometalating ligands and fppz and Htaz as ancillary ligands, a group of heteroleptic iridium complexes were developed with characters of high photoluminescence quantum yields up to 70% and short lifetimes below 1 μs. All complexes exhibited good efficiencies over 20% in OLEDs, and the Ir3 device gave the highest EQE max of 33.5% and low efficiency roll-off, being one of the highest EQEs for greenish-blue phosphorescence OLEDs so far. [Display omitted] • Highly efficient iridium complexes are developed with decorated phenylpyridine as cyclometalating ligands. • Rich-nitrogen heterocyclic ligands were utilized as the ancillary ligands. • These complexes are characterized by high PLQY, short excited state lifetimes below 1 μs, and high EQEs over 20% in OLEDs. • The Ir3 based OLED gave a maximum EQE of 33.5%, being among the highest efficiencies for greenish-blue OLEDs so far. • Fluorine-free feature of the cyclometalating ligands was attributed to be one reason for extra efficiency stability. [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]

9,9-Dimethyl-9,10-dihydroacridine (DMAC) is one of the most widely used electron donor for constructing high-performance thermally activated delayed fluorescence (TADF) emitters. However, DMAC-based emitters often suffer from the imperfect color purity, particularly in blue emitters, due to its strong electron-donating capability. To modulate donor strength, 2,7-F-Ph-DMAC and 2,7-CF 3 -Ph-DMAC were designed by introducing the electron-withdrawing 2-fluorophenyl and 2-(trifluoromethyl)phenyl at the 2,7-positions of DMAC. These donors were used, in combination with 2,4,6-triphenyl-1,3,5-triazine (TRZ) acceptor, to develop novel TADF emitters 2,7-F-Ph-DMAC-TRZ and 2,7-CF 3 -Ph-DMAC-TRZ. Compared to the F- or CF 3 -free reference emitter, both two emitters showed hypsochromic effect in fluorescence and comparable photoluminescence quantum yields without sacrificing the reverse intersystem crossing rate constants. In particular, 2,7-CF 3 -Ph-DMAC-TRZ based OLED exhibited a blue shift by up to 39 nm and significantly improved Commission International de l'Éclairage (CIE) coordinates from (0.36, 0.55) to (0.22, 0.41), while the external quantum efficiency kept stable at about 22.5 %. This donor engineering strategy should be valid for improving the color purity of large amount of acridine based TADF emitters. It can be predicted that pure blue TADF emitters should be feasible if these F- or CF 3 -modifed acridine donors are combined with other weaker electron acceptors., (© 2024 Wiley-VCH GmbH.)

By linking the carbazole unit to the nitrogen atom of acridone through phenyl or pyridyl, two compounds, named 10-(4-(9H-carbazol-9-yl)phenyl)acridin-9(10H)-one (AC-Ph-Cz) and 10-(5-(9H-carbazol-9-yl)pyridin-2-yl)acridin-9(10H)-one (AC-Py-Cz) were designed and synthesized. These two materials, characterized with highly twisted and rigid structure, good thermal stability, and balanced carrier-transporting properties, were employed as host materials for green phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes (OLEDs). The carbazole group, despite its small contribution to the highest occupied molecular orbitals (HOMOs) of these two materials, plays an essential role as an intramolecular host in energy delivering and improving the hole transporting ability of these two hosts. The incorporation of the electron-deficient pyridyl group as a linking group slightly improves the electron transporting capability of AC-Py-Cz. The green phosphorescent OLED (PhOLED) based on AC-Py-Cz exhibited excellent device performance with a turn-on voltage of 2.5 V, a maximum power efficiency and an external quantum efficiency (η ext ) of 89.8 lm W -1 and 25.2 %, respectively, benefitting from the better charge-balancing ability of AC-Py-Cz host due to the presence of the pyridyl bridge. More importantly, all the devices based on these two hosts showed low efficiency roll-off at high brightness due to the suppressed non-radiative transition in the emitting layer. In particular, the AC-Py-Cz-hosted green PhOLED exhibited an efficiency roll-off of 1.6 % from the maximum next at a high brightness of 1000 cd m -2 and a roll-off of 15.9 % at an extremely high brightness of 10000 cd m -2 . This study manifests that acridone-based host materials have great potential in fabricating OLEDs with low efficiency roll-off., (© 2022 Wiley-VCH GmbH.)