Research into organic light emitters employing multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials is presently attracting a great deal of attention due to the potential for efficient deep-blue emission. However, the origins and mechanisms of successful TADF are unclear, as many MR-TADF materials do not show TADF behaviour in solution, but only as particular pure solids. Here, an investigation into a well-known MR-TADF material, DABNA-1, together with other new MR materials (9H-quinolino[3,2,1-kl]phenothiazin-9-one (QPO) and 9H-quinolino-[3,2,1-kl]-phenothiazin-9-one 5,5-dioxide (QP3O)), yields new insights regarding the origin of TADF. Although a material system may support the concept of MR, inefficiency in both forward and reverse intersystem crossings forbids TADF unless a suitable host material allows an exciplex-like host–emitter interaction that boosts TADF. This boosted-TADF mechanism can be generalized to any fluorescence dye that lacks TADF in the photoluminescence measurement but has a thermally accessible S1–T1 energy gap, opening the way to high-performance organic light-emitting diodes. This study reveals the importance of host–guest interactions for effective multiple-resonance thermally activated delayed fluorescence in organic light emitters.

The role of host–guest interactions in organic emitters employing MR-TADF

Molecular design strategies are crucial to develop highly efficient and long-wavelength thermally activated delayed fluorescent (TADF) emitters because the inherent limitation of the energy gap law degrades the efficiency of the red or orange TADF emitters. To resolve the low efficiency issue, we designed and synthesized two TADF emitters, 4,4'-(6-(9,9-dimethylacridin-10(9H)-yl)-7-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCNAc) and 11-(9,9-dimethylacridin-10(9H)-yl)-12-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCNAc), by utilizing fluorine and peripheral cyano-substituted quinoxaline and phenazine acceptors of 4,4'-(6-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCN) and 11-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCN), respectively. A fluorine atom at the ortho position of the acridine donor acts as an auxiliary acceptor to minimize the singlet-triplet energy gap (ΔEST) below 0.1 eV and promotes the reverse intersystem crossing (RISC) process. Organic light-emitting diodes (OLEDs) fabricated with FDQCNAc and FBPCNAc emitters demonstrated high external quantum efficiencies (EQEs) of 27.6 and 23.8% in the yellow-red TADF OLEDs, respectively. In particular, the combination of the F auxiliary acceptor unit and the rigidified FBPCN acceptor unit enabled red-shifted emission by about 58 nm without much sacrifice of the EQE in the red region.

Rational Molecular Design of Highly Efficient Yellow-Red Thermally Activated Delayed Fluorescent Emitters: A Combined Effect of Auxiliary Fluorine and Rigidified Acceptor Unit

A series of blue thermally activated delayed fluorescence (TADF) emitters were designed and synthesized using 2,4,6-triphenyl-1,3,5-triazine as the acceptor unit and indenocarbazole derivatives as the electron-donating moiety. In contrast with other six-membered heterocycles, like phenothiazine, phenoxazine, and dihydroacridine, where the TADF efficiency is affected by the presence of different conformers, indenocarbazole derivatives do not show this effect. Therefore, InCz23FlTz, InCz23DPhTz, InCz23DMeTz, and InCz34DPhTz allow the investigation of the effect of different substituents and substitution positions on TADF properties, without the influence of different conformations. We have demonstrated that the substituted position on the carbazole and different substituents in the same position have clear influence on the donor character of indenocarbazole derivatives. Also, the color purity of blue emission and excited states could be adjusted by substituents and substituted position, and thus excellent b...

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Blue TADF Emitters Based on Indenocarbazole Derivatives with High Photoluminescence and Electroluminescence Efficiencies.

The exploitation of universal spiro-type hosts suitable for full-color phosphors is still a bottleneck for fabricating highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) with a similar device structure. In this work, a novel spiro-configured host 3,6-di(9H-carbazol-9-yl)spiro[indolo[3,2,1-de]acridine-8,9′-thioxanthene]10′,10′-dioxide (TCZSO2) incorporating 9′H-9,3′:6′,9′′-tercarbazole and 9H-thioxanthene 10,10-dioxide moieties is designed and synthesized. Utilizing 9′H-9,3′:6′,9′′-tercarbazole as the building-block in designing new spiro-type host materials in organic light-emitting diodes (OLEDs) is seldom reported. In particular, the three color-based devices based on TCZSO2/bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate (FIrpic), TCZSO2/iridium(III) bis(2-phenylpyridinato-N,C2′)acetylacetonate (Ir(ppy)2(acac)) and TCZSO2/iridium(III) bis(2-methyldibenzo[f,h]quinoxaline)acetylacetonate (Ir(MDQ)2(acac)) exhibit respectable external quantum efficiencies (EQEs) of 22.8%, 24.0% and 21.0%, respectively. More encouragingly, blue and green PHOLED devices have quite low efficiency roll-off (EQE: 22.7%/24.0%, FIrpic/Ir(ppy)2(acac)) at a high brightness of 1000 cd m−2. At a higher brightness of 5000 cd m−2, there is only 5.3% and 4.6% decrease in EQEs from their maximum values, indicating a very low efficiency roll-off. These excellent results confirm a novel spiro-type host design strategy for incorporating reported electron donors with high triplet energy (ET) to achieve high-performance RGB PHOLEDs.

Incorporating a tercarbazole donor in a spiro-type host material for efficient RGB phosphorescent organic light-emitting diodes

Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency. This limit can be overcome in materials that have intramolecular charge-transfer excitonic states and associated small singlet-triplet energy separations; triplets can then be converted to emissive singlet excitons resulting in efficient delayed fluorescence. However, the mechanistic details of the spin interconversion have not yet been fully resolved. We report transient electron spin resonance studies that allow direct probing of the spin conversion in a series of delayed fluorescence fluorophores with varying energy gaps between local excitation and charge-transfer triplet states. The observation of distinct triplet signals, unusual in transient electron spin resonance, suggests that multiple triplet states mediate the photophysics for efficient light emission in delayed fluorescence emitters. We reveal that as the energy separation between local excitation and charge-transfer triplet states decreases, spin interconversion changes from a direct, singlet-triplet mechanism to an indirect mechanism involving intermediate states. Despite advances in the design of thermally activated delayed fluorescence (TADF) emitters for devices, the effect of spin interactions is not well understood. Here, the authors report the role of spin-vibronic coupling in TADF organic emitters using transient electron spin resonance spectroscopy.

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Electron spin resonance resolves intermediate triplet states in delayed fluorescence.

Two thermally activated delayed fluorescence (TADF) emitters 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-fluorophenyl)-9′H-9,3′:6′,9′′-tercarbazole (TCTZ-F) and 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2,6-difluorophenyl)-9′H-9,3′:6′,9′′-tercarbazole (TCTZ-2F) were synthesized and compared to the previously reported molecule 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′H-9,3′:6′,9′′-tercarbazole (TCTZ) to explore the possible effects of the fluorine atom(s) on the molecular conformation, electronic coupling between the donor and acceptor groups, and consequently the photophysical behavior of the emitters. Specifically, the TCTZ-F molecule was calculated to have a smaller dihedral angle than TCTZ-2F and a smaller S1–T1 gap than TCTZ, which led it to have the highest photoluminescence quantum yield (PLQY) among this series of compounds. An OLED fabricated with TCTZ-F as the emitter achieved the highest external quantum efficiencies (22.5%) of this series, indicating that fluorination can play a useful role in TADF emitters.

Jiao Jiao Liang

Papers

The role of fluorine-substitution on the π-bridge in constructing effective thermally activated delayed fluorescence molecules