Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24339−24348
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Solution-Processed Highly Efficient Bluish-Green Thermally Activated Delayed Fluorescence Emitter Bearing an Asymmetric Oxadiazole−Difluoroboron Double Acceptor Di Zhou,†,§ Denghui Liu,† Xu Gong,‡ Huili Ma,⊥ Gaowei Qian,† Shaolong Gong,‡ Guohua Xie,*,‡ Weiguo Zhu,*,† and Yafei Wang*,†,∥
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National Experimental Demonstration Center for Materials Science and Engineering, Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Engineering Laboratory of Light-Electricity-Heat Energy-Converting Materials and Applications, School of Materials Science & Engineering, Changzhou University, Changzhou 213164, China ‡ Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, China § College of Chemistry, Xiangtan University, Xiangtan 411105, China ∥ Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, 149 Yanchang Road, Shanghai 200072, China ⊥ Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China S Supporting Information *
ABSTRACT: Difluoroboron (BF2)-containing dyes have attracted great interest owing to their exceptionally high luminescence efficiency and good electronwithdrawing properties. However, only a few reports on difluoroboron-based thermally activated delayed fluorescence (TADF) have been addressed. In this contribution, a novel BF2-containing TADF molecule of BFOXD, which contains two acceptor fragments of oxadiazole (OXD) and BF2 and one donor unit of 9,9-dimethylacridine, was synthesized and characterized. For comparison, the precursor of OHOXD bearing one acceptor unit was also investigated. Both molecules clearly show TADF characteristics with sky-blue emission in solution and film state. Additionally, OHOXD undergoes excited-state intramolecular proton transfer-coupled intramolecular charge transfer processes. Using 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) as the host, the organic light-emitting diodes fabricated via a solution process show maximum external quantum efficiency (EQE) of 2.98 and 13.8% for OHOXD- and BFOXD-based devices, respectively. While the bipolar TADF host of 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) is utilized instead of CzSi, the OHOXD- and BFOXD-based devices exhibit better performances with the maximum EQEs of 12.1 and 20.1%, respectively, which render the most efficient and the bluest emission ever reported for the BF2-based TADF molecules. This research demonstrates that introduction of one more acceptor unit into the TADF molecule could have a positive effect on emission efficiency, which opens a new way to design high-efficiency TADF molecules. KEYWORDS: thermally activated delayed fluorescence, difluoroboron complex, oxadiazole unit, acridine unit, organic light-emitting diodes
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INTRODUCTION Since Adachi and co-workers harvested both singlet and triplet excitons in organic light-emitting diodes (OLEDs) with purely organic thermally activated delayed fluorescence (TADF) materials in 2012,1 TADF has become a very popular participant as the organic electroluminescent materials for harnessing triplet excitons.2−5 Intrinsic TADF involves the reverse intersystem crossing (rISC) of a triplet (T1) exciton to the singlet state (S1), followed by emission from the singlet state.6−9 Compared with traditional phosphorescent materials bearing heavy metal atoms such as platinum and iridium, © 2019 American Chemical Society
TADF materials can theoretically achieve 100% internal quantum efficiency without a metal, which improves its cost effectiveness and environmentally friendliness. Over the past several years, substantial progress has been made in shifting the emission of TADF materials from blue to red.10−16 Usually, a very small energy gap (ΔEST) between S1 and T1 can facilitate the rISC and may result in TADF.17−20 With this in mind, Received: April 29, 2019 Accepted: June 12, 2019 Published: June 12, 2019 24339
DOI: 10.1021/acsami.9b07511 ACS Appl. Mater. Interfaces 2019, 11, 24339−24348
Research Article
ACS Applied Materials & Interfaces
theoretical methods. Compounds OHOXD and BFOXD clearly showed TADF with sky-blue emission in solution and film state. In addition, OHOXD displayed excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) characteristics. Encouraged by the TADF-sensitized fluorescence emission,61 OLEDs employing the TADF material CzAcSF as the host, exhibiting the maximum EQE values of 12.1 and 20.1% for the OHOXDbased and BFOXD-based devices, respectively. This research reveals the best result reported for the BF2-based TADF materials (Figure 1), and it paves the way for the design of novel high-efficiency TADF molecules.
research on TADF molecules has largely been focused on the twisted donor (D)−acceptor (A) skeletons, which can efficiently decrease the ΔEST through spatially separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).21−23 Among these TADF molecules, phenylamine,24−26 carbazole,27−29 acridine,30,31 and phenoxazine32,33 are usually regarded as the donor moieties, whereas sulfonyldibenzene,34−36 benzophenone,37−39 1,3,5-triazine,40,41 pyrazine-2,3-dicarbonitrile,42,43 and cyano groups44−46 are the major acceptor moieties. Generally, there is only one acceptor moiety in these reported TADF molecular frameworks, such as D-A, D-A-D, or starshaped D3-A. How about several acceptor units in one TADF molecule? To this end, we would like to explore the effect of the number of acceptor units on the photophysical property of the TADF emitter. Difluoroboron (BF2)-containing chromophores are of particular interest in organic semiconductors due to their exceptional electron-accepting abilities, high extinction coefficients, and high-efficiency luminescence in both the solution and solid state.47,48 However, it is unclear why BF2-containing dyes have not become popular in OLEDs, let alone in TADF OLEDs. To the best of our knowledge, there are only a couple of examples of BF2-based TADF materials.49−54 Adachi and co-workers reported a D-A-D boron difluoride curcuminoid derivative that showed TADF with near-infrared emission (700−800 nm) and a maximum external quantum efficiency (EQE) of 10% in an OLED.55 Although some difluoroboron βdiketonate-based small molecules and polymers also showed TADF properties,56,57 their device performances have not been reported. Notably, these reported BF2-based TADF molecules are β-diketonate complexes and their emission spectra were mainly in the range of long wavelength (>500 nm). Therefore, we seek to apply difluoroboron (BF2)-based chromophores in TADF materials and devices, especially for the shortwavelength emission (
