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1 Violet-Blue Aggregation-Induced Emission Emitters for Non-Doped Oleds with Ciey Smaller Than 0.046 Pengbo Han, Chengwei Lin, D

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1 Violet-Blue Aggregation-Induced Emission Emitters for Non-Doped Oleds with Ciey Smaller Than 0.046 Pengbo Han, Chengwei Lin, D Violet-blue Aggregation-induced Emission Emitters for Non-doped OLEDs with CIEy Smaller than 0.046 Pengbo Han, Chengwei Lin, Dongge Ma,* Anjun Qin* and Ben Zhong Tang P. Han, C. Lin, Prof. D. Ma, Prof. A. Qin, Prof. B. Z. Tang State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou, 510640, China. E-mail: [email protected]; [email protected] Prof. B. Z. Tang Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. Keywords: Aggregation-induced emission, violet-blue emitter, non-doped device, organic light-emitting diode, tetraphenylbenzene High emission efficiency and finite molecular conjugation in aggregate state are two desirable features in violet-blue emitters. Aggregation-induced emission luminogens (AIEgens) have surfaced as promising luminescent materials that possess both features. Herein, we report the design and synthesis of a group of violet-blue emissive AIEgens with photoluminescence quantum yield higher than 98% in their film states. When utilizing these AIEgens as non- doped emitting layers, the fabricated organic light-emitting diode exhibit a maximum external quantum efficiency of 4.34% with Commission Internationale de L’Eclairage (CIE) coordinates of (0.159, 0.035), which are amenable to next generation Ultra-high Definition Television (UHDTV) display standard. 1. Introduction Organic light-emitting diodes (OLEDs) have advanced substantially owing to their outstanding contrast ratio, low driving voltage, flexible display, fast response, etc. in the past decade.[1-5] To date, green, red and blue emitters are being used in the commercial products.[6-8] However, there is a great challenge that developing efficient and stable violet-blue emitters with Commission Internationale de L’Eclairage (CIE) coordinates (x, y) = (0.131, 0.046), defined by Ultra-high Definition Television (UHDTV) ITU-R BT.2020.[9] 1 In principle, rigid planar structures with finite molecular conjugated skeleton and intrinsic wide bandgap are used for the construction of violet-blue emitters. However, these luminescent materials usually suffer from the aggregation-caused quenching (ACQ) effect due to inevitable intermolecular π-π stacking.[10-13] In addition, their charge transport and charge injection are also unbalanced in OLEDs, which in turn increase driving voltages and decrease device efficiency.[14-17] In consequence, many violet-blue emitters have to be doped into suitable host materials with wide band gap and higher triplet energy. Although the doping device can solve these problems, the device configurations are complicated, making the practical cost increased. To avoid these adverse factors, a strategy of molecularly melding aggregation-induced emission (AIE) core with donor (D) and acceptor (A) is proposed, which has been used for the fabrication of non-doped OLEDs.[18-26] Furthermore, these AIE luminogens (AIEgens) can improve the charge injection and carrier transport in OLEDs, leading to an enhance electroluminescence efficiency of devices when using them as emitting layers.[27-31] However, the reports on the efficient and stable AIEgen-based violet-blue OLEDs with CIEy smaller than 0.046 are rare. To design violet-blue AIEgens, an AIE core with ultraviolet emission decorated by a weak D and an A is essential because the core could maintain the violet-blue emission and the D and A groups could slightly red-shift its emission. Following this design principle, tetraphenylbenzene (TPB), a new AIEgen with a solid emission peak at 363 nm, is much suitable for construction violet-blue emitter by attaching D and A groups.[32] When cyano group was used as A and triphenylamine (TPA) or diphenylamine (DPA) moieties as D, the generated AIEgens show high photoluminescence quantum yields. Using these TPB-based AIEgens as non-doped EMLs, the fabricated OLEDs exhibit excellent device performance. However, their CIEy values are still larger than 0.08.[33-36] Above results suggest that there might be a strong intramolecular charge transfer (ICT) between the A and D groups, which induce the emission red-shifted. Thus, to further blue- 2 shift the emission of TPB-based AIEgens, one of the feasible strategies is to decrease the ICT process by weakening the electron accepting or donating ability. In consideration that the cyano group on the TPB core could decrease the lowest unoccupied molecular orbital (LUMO) energy level and reduce electron injection barrier in OLEDs,[37] we thus tried to alter the D group from TPA or DPA to carbazole derivatives with weaker electron donating ability.[38,39] Furthermore, the carbazole moieties can enhance carrier transport to enhance the device performance. Following this strategy, we designed and synthesized three TPB derivatives of TPBCzC1- TPBCzC3 (Figure 1), in which TPB serves as a π-conjugate bridge and is used to reduce the intramolecular interference between D and A groups as well as intermolecular interaction to enhance the emission efficiency in the solid state. The photophysical property investigation in THF/water mixtures indicates that TPBCzC1-TPBCzC3 show the aggregation-enhanced emission (AEE) with peaks at 409-422 nm. Their emission decreased when the water fractions (fw) in THF/water mixtures are lower than 60% accompanying with red-shifted peaks due to the twisted intramolecular charge transfer (TICT). Afterward, the emission intensified with blue-shifed peaks upon adding water because of the formation of aggregates and activation of restriction of intramolecular motion (RIM) process.[40] Excitingly, these AIEgens show high photoluminescence (PL) quantum yields (F) over 95% in their film states. As a result, the OLEDs using them as non-doped EMLs exhibit stable device efficiency and violet-blue emission. The TPBCzC1-based OLED achieves a maximum forward-viewing external quantum efficiency (EQE) of 4.34% with CIE coordinates (0.160, 0.035). Meanwhile, the TPBCzC2-based OLED gives a maximum EQE of 4.78% and CIE coordinates of (0.159, 0.060). Thus, this work provides a new guideline for developing violet-blue emitters for next generation UHDTV display standard. 2. Results and Discussion 2.1. Synthesis 3 The synthetic routes to TPBCzC1-TPBCzC3 are shown in Scheme S1. The reagent of 4-(4- bromo-2,5-diphenyl)phenyl-cyanobenzene 1 could be facilely synthesized according to the reported procedures.[35] Then, the Suzuki couplings of 1 and carbazole-subsitituted phenylboronic acid 2 readily furnish TPBCzC1-TPBCzC3 in the yields over 80%. The structures of TPBCzC1-TPBCzC3 were fully characterized by 1H and 13C NMR and high resolution mass (HRMS) spectroscopies, and satisfactory results have also been obtained. TPBCzC1-TPBCzC3 are soluble in commonly used organic solvents, such as tetrahydrofuran (THF) and dichloromethane (DCM), but insoluble in water. 2.2. Thermal stability Thermal and morphological stabilities of the emitters are crucial for the fabrication and operation of OLEDs. Therefore, the thermal properties of TPBCzC1-TPBCzC3 were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) methods under N2. As shown in Figure S1, TPBCzC1-TPBCzC3 show excellent thermal stabilities o with the Td (the temperatures for 5% weight loss) as high as 407, 433 and 426 C, respectively. The glass transition temperatures (Tg) of TPBCzC2 and TPBCzC3 were recorded to be 145 o and 142 C, respectively, while no apparent Tg peak could be detected for TPBCzC1. o Moreover, TPBCzC3 exhibits evident Tc (crystallization temperature) at 238 C. These data reveal that TPBCzC1-TPBCzC3 are suitable for OLED fabrication by vacuum thermal evaporation technique. 2.3. Photophysical Properties After confirming the structures and studying the thermal stabilities of TPBCzC1-TPBCzC3, we investigated their photophysical properties. Figure 2A presents the UV-vis spectra of TPBCzC1-TPBCzC3 in THF solutions with a concentration of 10 μM. They show similar profiles with absorption bands in the range of 200 to 320 nm. The absorption bands located at 200-260 nm are considered as the π–π∗ transition of TPB,[9] whereas, the long-wavelength absorption at 320 nm might be attributed to effective ICT transition.[35] From the onset of 4 absorptions of TPBCzC1-TPBCzC3 in THF solutions, the optical bandgap (Eg) levels were deduced to be 3.52, 3.43, 3.42 eV, respectively (Table 1). The PL spectra of TPBCzC1-TPBCzC3 in THF solutions with a concentration of 10 μM and their evaporated films are shown in Figures S2 and 2B. In THF solutions, TPBCzC1- TPBCzC3 emit at violet-blue and deep-blue regions with peaks at 429, 420 and 438 nm, suggesting that the substitution greatly affects the PL. In the vacuum-deposited neat films, TPBCzC1-TPBCzC3 show violet-blue emission peak at 411, 409 and 426 nm, respectively. In contrast with a single molecule in THF solutions, the emission peaks of TPBCzC1-TPBCzC3 films were considerably blue-shifted, implying that a shorter conjugation length in the solid [41] state than that in solution. Notably, the ΦF values of TPBCzC1-TPBCzC3 in THF solutions were recorded to be 81.3, 86.9 and 94.5%, whereas, those in vacuum-deposited neat films are enhanced to be 99.9, 98.9 and 98.6%, respectively, indicating that they possess typical AEE feature. To further confirm the AEE feature of TPBCzC1-TPBCzC3, their PL spectra were conducted in THF/water mixtures with different water fractions. As shown in Figures 3 and S3, the PL intensity of TPBCzC1-TPBCzC3 keeps decreasing and slowly red-shifting upon [42] adding water, which is ascribed to the TICT effect. With further increasing fw, their PL gradually blue-shifted along with the increase in intensity owing to the formation of [40] aggregates and in turn activating the process of RIM. Moreover, the PL lifetime (τd) of TPBCzC1-TPBCzC3 films fabricated by vacuum evaporation were measured to be to 2.05, 2.12 and 1.92 ns, respectively, suggesting that they emit fluorescence (Figure S4).
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