Search

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]

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]

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]

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]

3,6-di- tert -butyl-9H-carbazole and 3,6-diphenyl-9H-carbazole have been widely used to construct blue thermally activated delayed fluorescence (TADF) emitters. However, it is difficult for related emitters to achieve maximum external quantum efficiency (EQE max) above 20 % with the Commission Internationale de l'Eclairage (CIE) y ≤ 0.12. A new type of donor was developed to construct blue TADF molecules, namely CF 3 P-BuCz-TRZ and CF 3 P-PhCz-TRZ. The weak electron-withdrawing trifluoromethylphenyl (CF 3 P) with moderate steric hindrance was introduced into the 1-site of carbazole donor for improving the color purity for blue TADF emitters. More importantly, the introduction of CF 3 P tightly compacts the energy levels for the significant excited states in RISC process. The energy gap between the lowest singlet excited (S 1) state and the lowest triplet (T 1) state (Δ E (S 1 − T 1) ), as well as between T 1 and the higher triplet (T n , n ≥ 2) states (Δ E (T n − T 1) ) were reduced, so that T n (n ≥ 2) could be effectively harvested to further promote reverse intersystem crossing (RISC). As a result, the OLEDs based on CF 3 P-BuCz-TRZ and CF 3 P-PhCz-TRZ displayed blue electroluminescence with CIE coordinates of (0.156, 0.107) and (0.155, 0.115). In particular, the maximum EQEs of the CF 3 P-PhCz-TRZ device reached 20.43 % without employing any light out-coupling enhancement or TADF-sensitized structure. This strategy should be valuable for designing more pure-blue and deep-blue TADF emitters. The weak electron-withdrawing trifluoromethylphenyl (CF 3 P) with moderate steric hindrance was introduced into the 1-site of carbazole donor. The k RISC s were remarkably facilitated with incorporation of CF 3 P group in CF 3 P-PhCz-TRZ. CF 3 P-PhCz-TRZ exhibited both blue-shifted EL in OLED with CIE (0.155, 0.115) and improved maximum EQE of 20.43 %, which are superior to the parent emitter PhCz-TRZ with CIE (0.154, 0.146) and EQE of 15.18 %. [Display omitted] • Trifluoromethylphenyl (CF 3 P) is introduced to 1-site of carbazole as new donors for blue TADF emitters. • The electron withdrawing CF 3 P group weakens donor strength, making hyprochromic shift by 10 nm. • CF 3 P group compacts the excited states energy levels and reduce energy differences Δ E ST and Δ E TT and facilitates RISC. • CF 3 P based TADF emitters exhibit higher EQE = 20 % (vs 15 %) and better CIE y = 0.115 (vs 0.146) in OLEDs. • Modifying carbazole donors with CF 3 P is a universal design strategy for better blue color and higher efficiencies. [ABSTRACT FROM AUTHOR]

It is crucial for thermally activated delayed fluorescence (TADF) emitters to have both high photoluminescence quantum yield (PLQY) and high reverse intersystem crossing rate (k RISC). A novel TADF emitter, namely 3,6-bis(3,6-bis(diphenylamino)-9 H -carbazol-9-yl)-10-phenylacridin-9(10 H)-one (3,6-DPCz-AD), was designed by introducing N 3 , N 3 , N 6 , N 6 -tetraphenyl-9 H -carbazole-3,6-diamine (DPCz) at 3,6-sites of the acceptor acridin-9(10 H)-one (AD) by nucleophilic substitution. On the acridin-9(10 H)-one ring, the sp2-hybridization of the 10-site N atom leads to high rigidity, which inhibited conformation relaxation of excited state to reduce non-radiative decay. The introduction of large conjugated diphenylamine groups at the 3,6-sites of carbazole not only facilitated the RISC by increasing intramolecular charge transfer and generating a low S 1 -T 1 energy gap (Δ E ST), but also enhanced the radiative transition by enlarging the highest occupied molecular orbital (HOMO) delocalization. As a result, even without the traditional highly distorted structure, 3,6-DPCz-AD exhibited strong TADF characteristics with high k RISC of 5.8 × 105 s−1, high k r of 3.4 × 107 s−1 and a high PLQY of 99% simultaneously. The green organic light-emitting diode (OLED) of 3,6-DPCz-AD showed both a maximum external quantum efficiency (EQE max) of 30.1% and a power efficiency of 83.3 lm W−1. An EQE of 20.1% was still remained at a high brightness of 1000 cd m−2. This is one of the best performances for acridin-9(10 H)-one based TADF emitters ever reported so far. A novel thermally activated delayed fluorescence emitter 3,6-DPCz-AD based on acridone acceptor and 3,6-bis(diphenylamino)-carbazole donors realizes effective trade-off between reverse intersystem crossing and radiative transition through appropriate design strategy, which results in high k RISC > 5 × 105 s−1, k r > 107 s−1 and PLQY of 99% simultaneously and finally leads to high external quantum efficiency over 30% with good efficiency stability in simple-structured green OLED. [Display omitted] • A novel thermally activated delayed fluorescence (TADF) emitter 3,6-DPCz-AD was developed for OLED application. • Incorporating diphenylamine on carbazole donors enhanced TADF feature despite small donor-acceptor angles. • Combination of DPCz as donors and acridone as acceptor facilitated both RISC and radiative transition processes. • High k RISC of 5.8 × 105 s−1, k r of 3.4 × 107 s−1 and PLQY of 99.2% were realized simultaneously. • 3,6-DPCz-AD exhibited a very high EQE max of 30.1% in green OLED with slow efficiency roll-off. [ABSTRACT FROM AUTHOR]

The non-doped organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) materials are attracting increasing attentions due to the excellent performance and simple fabrication. We report an effective design strategy of TADF emitters to improve the non-doped performance by adjusting the valence state of the S atom in TADF molecules and utilizing the steric hindrance effect. Two novel TADF materials PT-BZ-DMAC and PS-BZ-DMAC were designed and synthesized with benzophenone acceptor and acridine donor. In particular, the thioether or sulfone group was attached to benzophenone. Remarkably, the bluish-green TADF-OLED with non-doped PT-BZ-DMAC turned on at 2.5 V and realized a maximum external quantum efficiency (EQE) of 17.34%, which was comparable with its doped OLED (17.71%) but showed much slower roll-off. At 100 and 1000 cd m−2, the non-doped PT-BZ-DMAC device still maintained acceptable efficiencies at 14.65% and 12.59%. In contrast, sulfone-modified PS-BZ-DMAC exhibited good efficiency of 20.55% only in doped OLED. The single crystal analysis and DFT calculation revealed that the phenyl thioether unit in PT-BZ-DMAC hardly contributes to the LUMO or the electronic transitions, instead determines the highly twisted geometry and acts as the barrier group, finally improving the performance of PT-BZ-DMAC in neat film device. Two TADF emitters are developed by attaching thioether or sulfone groups to the benzophenone acceptor. Unlike the strong electron withdrawing sulfone group that contributes to the electronic transition and TADF, the phenyl thioether blocked the conjugation of molecular backbone. The steric hindrance effect of phenyl thioether moiety along with the intermolecular hydrogen bondings can effectively prevents exciton quenching, thus further raising the EQE of non-doped TADF-OLED to 17.34% with low efficiency roll-off. [Display omitted] • Thioether and sulfone were incorporated as substituents to design TADF molecules PT-BZ-DMAC and PS-BZ-DMAC. • Thioether was found to strongly suppress intermolecular interactions and prevent exciton quenching via steric hindrance effect. • Non-doped OLED of thioether based PT-BZ-DMAC exhibited comparably high efficiency (17.34%) to its doped device (17.71%), but with much slower roll-off. • Sulfone based PS-BZ-DMAC exhibited high efficiency of 20.55% only in doped OLED, while non-doped device did not work. [ABSTRACT FROM AUTHOR]

A group of D-π-A type emitters were designed and synthesized by using triazine (Trz) as electron acceptor (A), carbazole (Cz), 10-dimethylacridine (DMAC) or 10H-phenoxazine (PXZ) as electron donors (D), and phenylene as π-bridge between D and A. The electron withdrawing ability of A unit and thus the intramolecular charge transfer (ICT) strength play the most essential role to determine the luminescence mechanism and the emission color. By gradually enhancing the ICT extent through selecting strong electron donating D and/or replacing methyl with electron withdrawing cyano group on the π-bridge, the luminescence is tuned from normal fluorescence to thermally activated delayed fluorescence (TADF) and the emission color is successfully tuned from pure-blue (434 nm) to orange-red (616 nm). Organic light-emitting diodes (OLEDs) based on these emitters exhibit bright electroluminescence covering the major visible-light range. An external quantum efficiency (η ext) of 4.8% with excellent color coordinate of (0.16, 0.08) is obtained for pure blue fluorescent OLED of TrzCz-Me. TrzCz-CN, TrzDMAC-Me, and TrzPXZ-Me exhibited higher η ext values of 14.40% (19.73 cd A−1), 19.01% (41.12 cd A−1), 16.50% (51.17 cd A−1) in their blue, sky-blue, and green TADF OLEDs. This study provides a practical strategy related to ICT strength to facilitate generation of TADF emitters and tune the emission colors over the visible light range. A group of D-π-A emitters were developed by combining triazine acceptor with different donors and incorporating either cyano or methyl on phenylene bridge. The triazine/carbaozle based blue emitter was tuned from normal fluorescence to thermally activated delayed fluorescence (TADF) by incorporating cyano group, and the emission color was tuned from deep blue to orange-red. It was proved the intramolecular charge-transfer strength played the most important role to facilitate both the emission mechanism evolution and color tuning. The TADF emitters exhibited high quantum efficiencies of 14–19% in organic light-emitting diodes. Image 1 • A series of D-p-A type emitters were developed with various donors. • Cyano or methyl was grafted on phenylene bridge to tune the acceptor strength and molecular steric conformation. • The triazine/carbazole compound was tuned from normal fluorescence to TADF by incorporating cyano group. • The emission color was continuously tuned from deep blue to orange-red by tuning the intramolecular charge-transfer strength. [ABSTRACT FROM AUTHOR]