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Novel adamantane-bridged phenanthroimidazole molecule for highly efficient full-color organic light-emitting diodes
Dyes and Pigments 177 (2020) 108273
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Novel adamantane-bridged phenanthroimidazole molecule for highly efficient full-color organic light-emitting diodes Hui-Min Guan a, b, Yong-Xu Hu a, Guo-Yong Xiao a, Wen-Ze He a, Hai-Jun Chi a, Yan-Li Lv a, Xiao Li a, *, Dong-Yu Zhang c, **, Zhi-Zhi Hu a, *** a b c
School of Chemical Engineering, University of Science and Technology Liaoning (USTL), Anshan, 114051, People’s Republic of China College of Chemistry, Chemical Engineering and Environment Engineering, Liaoning Shihua University, Fushun, 113001, People’s Republic of China Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou, 215123, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Deep blue Host Phenanthroimidazole Adamantane OLEDs
Highly efficient deep blue phenanthroimidazole fluorescent material by incorporating an adamantane moiety (AD-BPI) is designed and synthesized. AD-BPI exhibits excellent photophysical properties including a deep blue emission, high triplet energy level and photoluminescence efficiency, fine morphological and thermal stability, and ambipolar nature. Non-doped deep blue organic light-emitting diodes (OLEDs) following the construction of ITO (Indium–Tin Oxide)/TAPC (Bis [4-[N, N-di (4-tolyl) amino] phenyl]-cyclohexane, 30 nm)/AD-BPI (100 nm)/ TPBi (1, 3, 5-tris (2-N-phenylbenzimidazolyl) benzene, 50 nm)/Liq (8-hydroxyquinolatolithium, 2 nm)/Al (100 nm) utilizing AD-BPI as emissive layer achieve the peak external quantum efficiency (EQE) of 5.8% with the CIE coordinates of (0.15, 0.07). Using AD-BPI as universal host, the corresponding green, yellow and red phos phorescent devices are also fabricated and exhibit the maximum EQEs of 23.3%, 16.7% and 19.1% accompa nying with negligible efficiency roll-off under ultrahigh luminance. These experimental performances are intensely competitive with the recently reported advanced results of full-color OLEDs. It is the first demon stration of adamantane-based phenanthroimidazole molecule featuring as both host and emitter.
1. Introduction
employing deep blue luminescent materials with high triplet energy level (ET) which serve as hosts for phosphorescent emitters simulta neously [16–18]. However, the design and synthesis of blue-emitting hosts for green, yellow and red phosphors is still a great challenge because these characteristics including mild photoluminescence quan tum yield (PLQY), high ET and good carrier-transporting ability should be integrated into one molecule system. Recently, phenanthroimidazole derivatives have received extensive attention as efficient deep blue/blue emissive materials and/or hosts used in OLEDs due to their high PLQYs, balanced carrier injection and transporting ability, excellent thermal stability, easy synthesis and modification [19–31]. In addition, the near-spherical adamantane pos sesses high ET, good thermal and film formation, and its rigid non-conjugated structure can effectively interrupt the conjugation and electronic transfer between different functional groups, to maintain high ET of the corresponding molecular [32–34]. In view of those, a molecular (1s,3s,5r,7r)-1,3-bis(4-(2-phenyl-1H-
OLEDs (organic light-emitting diodes) have been considered as promising candidate in the field of white solid-state lighting and the next-generation full-color display [1–3]. Meanwhile, diverse red, green and blue luminescent materials which act as important roles in OLEDs have been well exploited one after another for realizing highly efficient OLEDs [4–10]. However, those luminescent materials especially TADF (thermally activated delayed fluorescence) and phosphorescent emitters are usually doped in host materials for achieving further high electro luminescence (EL) performances, which require stringent device process to increase the production cost and extremely restrict the further commercialization of OLEDs [11–15]. To solve this thorny problem, it is very important and meaningful to achieve highly efficient red, green and blue EL via an unsophisticated material system for streamlining the manufacturing process and minimizing the production cost of materials. Recently, it is a wise choice for achieving the full-color OLEDs
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (D.-Y. Zhang), [email protected] (Z.-Z. Hu). https://doi.org/10.1016/j.dyepig.2020.108273 Received 22 October 2019; Received in revised form 7 February 2020; Accepted 7 February 2020 Available online 8 February 2020 0143-7208/© 2020 Elsevier Ltd. All rights reserved.
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phenanthro [9,10-d]imidazol-1-yl)phenyl)adamantane (AD-BPI) with two phenanthroimidazole moieties bridged by an adamantane unit is purposefully designed and synthesized to address above issues. Nondoped blue OLEDs adopting AD-BPI as active layer achieves the peak external quantum efficiency (EQE), power efficiency and current effi ciency of 5.8%, 3.9 lm w 1 and 4.4 cd A 1 with the CIE coordinates of (0.15, 0.07). With the adamantane-bridged phenanthroimidazole molecule as universal host material, highly efficient red (19.1%, 14.5 lm W 1, 20.8 cd A 1), yellow (16.7%, 35.6 lm W 1, 50.3 cd A 1) and green (23.3%, 57.7 lm W 1, 82.5 cd A 1) phosphorescent OLEDs are realized with negligible efficiency roll-off.
H NMR (500 MHz, CDCl3) δ 7.17 (d, J ¼ 7.6 Hz, 4H), 6.65 (d, J ¼ 7.7 Hz, 4H), 2.24 (s, 2H), 1.87 (s, 8H), 1.73 (s, 4H).
2.2.3. Synthesis of AD-BPI 9, 10-phenanthrene quinone (10 mmol, 3.08 g), 4,4’-((1s,3s,5r,7r)adamantane-1,3- diyl)dianiline (5 mmol, 1.62 g), ammonium acetate (40 mmol, 3.08 g), benzaldehyde (10 mmol, 1.0 mL) were added into the flask in turn and then glacial acetic acid (100 mL) was charged. The system then reacted for 10 h under heating reflux with N2 atmosphere. After that, the solution was poured into 500 mL sodium chloride solu tion. The crude solid products were received by extraction and filtration. White powder was obtained through column chromatography (silica gel: 300–400 mesh; petroleum ether: ethyl acetate ¼ 20: 1, V/V). Yield: 53%. HRMS (APCI, MþHþ), Exact mass: 872.3879; Obtained mass: 873.4805. 1 H NMR (500 MHz, CDCl3) δ 8.93 (d, J ¼ 7.9 Hz, 2H), 8.79 (d, J ¼ 8.3 Hz, 2H), 8.73 (d, J ¼ 8.3 Hz, 2H), 7.77 (t, J ¼ 7.4 Hz, 3H), 7.65 (d, J ¼ 17.8, 14.4, 7.3 Hz, 10H), 7.55–7.47 (m, 6H), 7.38–7.25 (m, 10H), 7.22 (d, J ¼ 8.4 Hz, 2H), 2.49 (s, 2H), 2.28 (s, 2H), 2.14 (d, J ¼ 3.0 Hz, 7H). 13 C NMR (126 MHz, CDCl3) δ 151.69, 150.32, 135.63, 128.85, 128.69, 128.44, 128.16, 127.65, 127.54, 126.68, 126.02, 125.59, 125.02, 124.24, 123.48, 122.45, 122.24, 120.26, 76.63, 76.38, 76.13, 48.06, 41.64, 37.05, 35.03, 28.86. Elemental Analysis: Anal. Cacld. for C64H48N4: C, 88.04%; H, 5.54%; N, 6.42%. Found: C, 88.77%; H, 5.26%; N, 6.16%.
2. Experimental section 2.1. General information Raw materials and reagents involved in this experiment including 1adamantanol, 9,10-phenanthroquinone, acetanilide and benzaldehyde are commercially available. All the reactions are operated in standard glass reactor under inert gas (nitrogen or argon). The silica gel (300–400 mesh) is used for column chromatography. HRMS (high-resolution mass spectrometry) is undertaken on an Agilent 6530B Q-TOF LCMS. NMR spectra with TMS (tetramethylsilane) as the internal standard are ob tained via Bruker AC 500 spectrometer. Elemental analyses are acquired by Vario Micro cube analyzer. DSC (differential scanning calorimetry) is recorded in American TA company DSC-Q20 with the heating rate of 10 � C min 1 from 50 to 400 � C under nitrogen. The Tm (melting point) and Tg (glass transition temperature) are obtained from the first and the second heating scan, respectively. TGA (thermogravimetric analysis) is carried out on American TA company TG-DTA Q600 thermal analyzer at a heating rate of 10 � C min 1 from 50 to 800 � C in the atmosphere of nitrogen. The PL (photoluminescence) and UV–Vis absorption spectra are received on LS 55 and Lambda 900 spectrophotometer both from PerkinElmer company, respectively. CV (cyclic voltammetry), which was calibrated by Fc/Fcþ redox couple as the internal reference in dichloromethane containing Bu4NPF6 (tetrabutylammonium hexa fluorophosphate, 0.1 M), is carried out on an AUTOLAB PGSTAT 302 N workstation with a scan rate of 50 mV s 1 under the atmosphere of ni trogen. DFT (density functional theory) calculations are implemented through Gaussian 09 employing the B3LYP functional together with 631G (d, p) basis set.
2.3. Devices fabrication and characterization Devices were made on pre-patterned ITO-coated glass substrates (15 Ω cm 2). Prior to device, the substrates were rinsed with soap and ul trapure H2O, and sonicated for 15 min. Afterwards, two subsequent rinses and 12-min sonication baths in acetone and isopropanol were performed sequentially. All organic layers as well as the Al cathode were deposited in a vacuum thermal evaporator at not less than 6 � 10 4 Pa. The luminance and EL spectra of OLEDs were obtained on PR650 spectrometer. The single-carrier devices and voltage-current properties of the OLEDs were received by Keithley 2400 source meter. All mea surements of devices without encapsulations at ambiet temperature were performed in the dark. 3. Results and discussion 3.1. Synthesis and characterization
2.2. Synthesis
AD-BPI’s chemical structure and its complete synthesis routes are shown in Scheme 1. The key intermediate 4,4’-((1s,3s,5r,7r)-ada mantane-1,3-diyl)dianiline was obtained via two-step reaction including Friedel-Crafts alkyl reaction and hydrolysis reaction, respectively, with the total yield of 35%. The classic Debus-Radziszewski reaction was then adopted to smoothly prepare the target compound AD-BPI in one pot with high isolated yield of 53%. All the intermediates and target com pound were characterized by elemental analysis, HRMS and NMR spectrometry.
2.2.1. Synthesis of N,N’-(((1s,3s,5r,7r)-adamantane-1,3-diyl)bis(4,1phenylene)) diacetamide A two-necked flask was charged with 1-adamantanol (2.5 mmol, 0.375 g), acetanilide (5 mmol, 0.78 g) and 98% sulfuric acid (7.5 mL) at room temperature (25 � C), and then reacted for 5 h under magnetic stirring. Subsequently, white crude solid was obtained once H2O was poured into the mixture. The pure white solid was got via column chromatography (silica gel, 300–400 mesh). Yield: 56%. HRMS (APCI, MþHþ): Exact mass: 402.2307; Obtained mass: 403.2380. 1H NMR (500 MHz, DMSO‑d6) δ 9.85 (d, J ¼ 4.5 Hz, 2H), 7.49 (d, J ¼ 6.7 Hz, 4H), 7.31 (d, J ¼ 6.7 Hz, 4H), 2.23 (s, 1H), 2.01 (s, 7H), 1.87 (d, J ¼ 18.8 Hz, 10H), 1.72(s,2H).
3.2. Thermal property The morphological stability and thermal property of AD-BPI were determined by DSC and TGA at a heating rate of 10 � C min 1 under streaming N2, respectively. TGA and DSC plots of AD-BPI were presented in Fig. 1. As showed in Fig. 1, AD-BPI exhibited the high decomposition temperature (Td, 5% decomposition temperature) of 509 � C, and simi larly a relatively high melting point Tm of 358 � C (Fig. S1) and Tg of 188 � C were observed via DSC analysis (Inset of Fig. 1), which could guar antee the subsequent vacuum evaporation smoothly. In addition, the Tg and Tm of AD-BPI roughly followed the Boyer-Kauzmann rule (Tg/Tm � 0.7) on the Kelvins temperature [35]. The slightly high value of Tg/Tm (0.73) based AD-BPI indicated strong intermolecular interaction in the
2.2.2. Synthesis of 4,4’-((1s,3s,5r,7r)-adamantane-1,3-diyl)dianiline N,N’-(((1s,3s,5r,7r)-adamantane-1,3-diyl)bis(4,1-phenylene)) diac etamide (0.75 mmol, 0.30 g) in ethanol (4 mL) and H2O (1 mL) including sodium hydroxide (12 mmol, 0.48 g) was put into the flask in turn. The mixture was refluxed with constant stirring for 6 h. Following that, the solution was cooled to ambient temperature, and then saturated brine was added to obtain the yellowish-colored raw solid. Further recrys tallization with ethanol enabled to the white solid product. Yield: 63%. HRMS (APCI, MþHþ), Exact mass: 318.2096; Obtained mass: 319.2222. 2
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Scheme 1. Synthesis routes and molecular structure of AD-BPI.
AD-BPI presented the blue-violet emission peaked at 398 nm in degassed dichloromethane accompanying with mild fluorescent PLQY of 0.76. Furthermore, AD-BPI exhibited red-shift of ca. 20 nm in neat film compared with that in CH2Cl2 solution, implying that π-π interaction in solid state was effectively restrained by the rigid and large twisted molecular configuration of AD-BPI. The PL spectra of AD-BPI in different solvents were also investigated and displayed small changes upon increasing solvent polarity. As indi cated in Fig. S2, the maximum emission of AD-BPI shifted from 398 nm in low polar CH2Cl2 to 405 nm in high polar acetonitrile solution with only 7 nm variation, confirming the localized excited property of the lowest singlet excited state of AD-BPI [38] and negligible intramolecular charge transfer. The ET of AD-BPI was obtained to be 2.64 eV via the low-temperature phosphorescent spectrum, which was even slightly higher than that of single 1,2-diphenyl-1H-phenanthro [9,10-d] imidazole molecular (2.61 eV) [39], suggesting that introduction of the insulated and rigid adamantane unit in this molecular system could provide the steric hindrance while maintaining a high ET. Furthermore, the high ET of AD-BPI was fully satisfied with energy transfer to red/ yellow/green phosphors. Therefore, AD-BPI could not only be utilized as a deep-blue fluorescent emitter but also a promising host in the green/yellow/red phosphorescent OLEDs.
Fig. 1. The TGA curve of AD-BPI. Inset: DSC curve of AD-BPI.
glass state of AD-BPI [36]. Taken together, the introduction of rigid adamantane unit in the currently investigated molecular definitely helps to improve thermal stability of material. Those excellent thermal prop erties of AD-BPI should facilitate device stability during operation of OLEDs. 3.3. Photophysical property
3.4. Electrochemical property
In Fig. 2, UV–vis absorption and PL (photoluminescent) curves of AD-BPI in dilute solution (10 5 M) are shown. In Table 1, all photo physical parameters are outlined. The absorption peaks of AD-BPI in CH2Cl2 were mainly concentrated on 200–370 nm, of which strong ab sorption peaks positioned at 262 and 310 nm were classified into π-π* transition of the corresponding benzene tied to N1 position in imidazole unit and phenanthroimidazole section, respectively [37,38].
CV plot of the compound AD-BPI is shown in Fig. 3. The compound AD-BPI exhibited a reversible oxidation curve and its threshold oxida tion potential (Eox onset) was 1.17 eV. And then the HOMO (highest occu pied molecular orbital) of AD-BPI could be deduced to be 5.57 eV via EHOMO ¼ -(4.4þEox onset). Interestingly, HOMO of AD-BPI matched well with that of common hole-transporting material TAPC (4,4’-(cyclo hexane-1,1-diyl)bis(N,N-di-p-tolylaniline)) (5.5 eV), suggesting that very small hole injection barrier (0.07 eV) existed between AD-BPI and TAPC, which was beneficial to lower the turn-on voltage. The LUMO (lowest unoccupied molecular orbital) of AD-BPI was acquired to be 2.28 eV according to the HOMO and energy gap deduced from UV ab sorption spectrum. 3.5. Theoretical calculations To learn more about the effects of the introduction of adamantane unit on the electronic property and geometrical structure of AD-BPI, DFT calculation of AD-BPI (Fig. 4) was handled at B3LYP/6-31G (d, p) level. All dihedral angles such as adamantane/its linked benzenes and phenanthroimidazole/N1-benzenes were more than 70� . The SP3 hy bridizations of N1 in imidazole and C in adamantane unit led to the nonflatness of AD-BPI, which thus increased steric hindrance and effectively
Fig. 2. The UV–Vis absorption and PL spectra in dilute CH2Cl2 solution (10-5 M), PL spectrum in neat film of AD-BPI at room temperature, and the phos phorescence spectrum in 2-methyltetrahydrofuran at 77 K. 3
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Table 1 Photophysical, thermal and Electrochemical properties of AD-BPI. Compound
λaabs (nm)
λbem (nm)
Φc
ETd (eV)
Td/Tm/Tge (� C)
HOMOf (eV)
LUMOf (eV)
Egf (eV)
AD-BPI
262, 310, 360
398/420
0.76/0.62
2.64
509/358/188
5.57
2.28
3.29
a b c d e f
Measured in CH2Cl2 solution with 1 � 10 5 mol L 1. Fluorescence emission measured in CH2Cl2 solution and neat film, respectively. The quantum yields measured in CH2Cl2 using 9,10-diphenylanthrecene (Φ ¼ 0.90) as the standard and in neat film. Triplet energy level measured in 2-MeTHF at 77 K, ET ¼ 1240.8/λPh. Decomposition temperature of 5% weight loss, melting point and glass transition temperature, respectively. EHOMO (eV) ¼ -e(Eoxonset þ 4.4), Eg ¼ 1240.8/λ, ELUMO (eV) ¼ Eg þ EHOMO.
unit possessed both electron- and hole-transporting properties. Slightly different from CV data, the calculated HOMO and LUMO levels of AD-BPI based DFT were 5.22 and 1.38 eV, respectively. 3.6. Electroluminescent performance Firstly, non-doped OLEDs with the traditional structure of ITO/TAPC (30 nm)/AD-BPI (100 nm)/TPBi (1,3,5-Tri(1-phenyl-1H-benzo[d]imi dazol-2-yl)phenyl, 50 nm)/Liq (2 nm)/Al (100 nm) (marked as: B-D1) was fabricated to assess the deep blue device performance of AD-BPI. Al and ITO served as the cathode and anode. TAPC and TPBi functioned as the hole-transporting layer and the hole-blocking/electron-transporting layer. The device configuration and involved materials’ structures are described in Fig. 5. EL spectrum, voltage-current density-luminance (JV-L) plots, current/power efficiency-luminance (CE/PE-L) characteris tics and EQE-luminance curve of non-doped deep blue OLEDs are depicted in Fig. 6, and the corresponding EL performances are summa rized in Table 2. B-D1 exhibited deep blue emission peaked at 422 nm as shown in Fig. 6(a), which was almost identical to that of AD-BPI neat film, demonstrating that all resulting carriers (hole and electron) of device could be limited solely in the luminescent layer and the emissive peak derived from AD-BPI itself. The CIE coordinates (0.15, 0.07) of B-D1 at 8 V conformed with the deep blue standard as specified via NTSC (Na tional Television Systems Committee) of the United States (CIEy<0.08)
Fig. 3. The CV curve of AD-BPI in anhydrous CH2Cl2 solution with 0.1 M TBAPF6 as supporting electrolyte.
reduced π-π stacking between molecules. Both HOMO and LUMO of AD-BPI almost localized at phenan throimidazole unit and the distribution of its LUMO extended on N1linked benzene ring. These overlaps between the orbitals allowed ADBPI functional to the flow of intramolecular electronic, which might act as an efficient emitter in OLEDs [40]. Besides, those features of electron distribution could be an evidence that phenanthroimidazole
Fig. 4. The optimized structure and the frontier molecular orbitals of AD-BPI.
Fig. 5. (a) The device configuration; (b) The device energy-level diagram; (c) Molecular structures of the materials used in OLEDs. 4
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Fig. 6. Characterizations of non-doped AD-BPI-based OLED (B-D1 and B-D2): (a) EL spectra at 8 V; (b) J-V-L characteristic curves; (c) CE/PE-L characteristic curves; (d) EQE-L curves.
[41]. It was worth mentioning that EL spectra of B-D1 remained virtu ally unchanged from 5 to 9 V (shown in Fig. S3), indicating that the AD-BPI-based non-doped deep blue OLED had good light-color stability. B-D1 showed the peak brightness of 12703 cd m 2 and low turn-on voltage of 3.1 V (1.0 cd m 2), which maybe mainly benefited from small energy barrier of HOMO between TAPC and AD-BPI. B-D1 exhibited excellent device performance with the maximum efficiencies of 5.8%, 4.4 cd A 1 and 3.9 lm w 1, respectively. Furthermore, at the luminance of 100 and 1000 cd m 2, B-D1 manifested nearly negligible efficiency roll-off, and its EQE remained 5.2%, 5.1%, respectively. Such high ef ficiency at high luminescence rendered AD-BPI a promising candidate for practical application. The performances of non-doped AD-BPI-based OLED were extraordinarily competitive with those of the reported phenanthroimidazole-based non-doped deep blue OLEDs (CIEy � 0.08, Table S1). Then we further investigated the theoretical EQE of AD-BPI-based OLED via the formula EQE ¼ γ � ηout � ϕPL � ηfr [42], where γ (the charge balance factor of holes and electrons) is usually assumed to be 1 if the resulting hole and electron are balanced and recombined in emitting zone, ηout stands for light-out coupling efficiency (typically ca. 20%), ϕPL represents the PLQY of AD-BPI film (0.62), and ηfr refers to the efficiency of radiative exciton production (usually 25% for fluorescent OLEDs). Based on this, the maximum EQE of AD-BPI-based OLED was theoreti cally estimated to be only 3.1%. Evidently, the actual maximum EQE of AD-BPI-based OLED (5.8%) was much larger than the theoretical value of 3.1% and surmounted the EQE limit (5%) of the traditional fluores cent OLEDs. We couldn’t explain it at the moment and further
Table 2 Summary of EL properties of non-doped deep blue OLED and 10% Ir-doped devices with AD-BPI as host. Device
Vturn-on (V) a
Lmax (cd m 2) b
EQE (%)c
CE (cd A 1) c
PE (lm W 1) c
CIE (x, y)d
B-D1
3.1
12703
3.1
4037
G-D
2.7
42718
Y-D
2.9
92806
R-D
3.2
36846
4.4/4.2/ 3.7 5.9/ 3.9/82.5/ 83.8/ 79.7 50.3/ 48.7/ 43.2 20.8/ 17.9/ 17.2
3.9/2.9/ 1.8 3.6/ 1.4/57.7/ 40.5/ 30.7 35.6/ 23.9/ 18.3 14.5/ 6.5/5.8
(0.15,0.07)
B-D2
5.8/5.2/ 5.1 6.9/ 4.9/23.3/ 21.9/ 21.8 16.7/ 16.4/ 14.2 19.1/ 16.6/ 13.7
(0.15,0.08) (0.28, 0.64) (0.50, 0.50) (0.66, 0.34)
a
Turn-on voltage at 1.0 cd m 2. Maximum luminance. c Order of measured efficiency values: maximum, values at 100, 1000 cd m 2 for B-D1 and B-D2; the others values at 5000, 10000 cd m 2 for G-D, Y-D and RD. d Commission International de I’Eclairage (CIE) coordinates measured at 10 V. b
5
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experiments will be executed to uncover the possible mechanism. AD-BPI’s carrier-transporting properties were then studied via single-carrier devices following: ITO/TPBi (10 nm)/AD-BPI (60 nm)/ TPBi (10 nm)/LiF (1 nm)/Al (100 nm) (electron-only device) and ITO/ TAPC (10 nm)/AD-BPI (60 nm)/TAPC (10 nm)/Al (100 nm) (hole-only device). TPBi and TAPC acted as hole-blocking and electron-blocking layer, respectively, to interrupt the injection of electron/hole from the electrode. As shown in the voltage-current density graphs depicted in Fig. S4, the hole-transporting ability of AD-BPI was much higher than its electron-transporting ability. Overall, AD-BPI could transport hole and electron, and therefore presented ambipolar feature, which was one of vital factors to achieve high device efficiency with little efficiency rolloff. Considering AD-BPI’s much higher hole-transporting ability, we tentatively optimized device structure by merely replacing TPBi with TmPyPB, and fabricated OLED of ITO (40 nm)/TAPC (30 nm)/AD-BPI (100 nm)/TmPyPB (50 nm)/Liq (2 nm)/Al (100 nm) (B-D2). The cor responding electro-optical performance plots are all outlined in Fig. 6. The EL efficiency of B-D2 was remarkably improved and a maximum EQE of 6.9% was obtained accompanying with CIE of (0.15, 0.08) at 8 V, which would be mainly due to deep HOMO of TmPyPB (6.75 eV of TmPyPB vs. 6.2 eV of TPBi) [43] and the better electron-transporting property (1 � 10 3 cm2/V vs. 3.3 � 10 5 cm2/Vs) [44,45]. Those findings demonstrate that adamantane-based phenanthroimidazole is a class of potential deep blue candidate and undoubtedly advance in EL efficiency through further optimizing device structure is within reach. Further efforts on optimizing the device structure with AD-BPI as emitter for acquiring better EL performances are ongoing. Due to the higher ET of AD-BPI than those of the classical red, yellow and green emissive materials, AD-BPI was also studied as host for red/ yellow/green phosphorescent luminescent materials in the uniform device configuration: ITO/TAPC (40 nm)/AD-BPI: dopant (30 nm)/TPBi
(30 nm)/Liq (2 nm)/Al (100 nm) (simplified as R-D, Y-D and G-D). Here, the optimized 10%-doped phosphor ((piq)2Ir (acac)/(bt)2Ir(acac)/Ir (ppy)3) in AD-BPI was adopted as emissive layer. The device configu ration and chemical structure of materials used are described in Fig. 5. Those device’s performances are generalized in Table 2. By examining the energy level diagram (Fig. 5(b)), the HOMO/ LUMO energy levels of G/Y/R phosphorescent emitters were all embraced in those of AD-BPI, which meant that both hole and electron could be directly trapped in phosphorescent dopant. Therefore, the dominate charge-trapping mechanism probably appeared in current OLEDs. In addition, high ET of AD-BPI as host ensured to efficient sensitize the phosphors ((piq)2Ir(acac)/(bt)2Ir(acac)/Ir(ppy)3) via exothermic energy transfer mechanism. As shown in Fig. 7, all OLEDs demonstrated a major EL emission peak originating from the corresponding phosphorescent emitter and no un foreseen peaks from the transporting layer material and/or host appeared, suggesting that energy transfer from AD-BPI to dopants was complete. Furthermore, those phosphorescent OLEDs exhibited low voltage of 2.7–3.2 V at 1.0 cd m 2, which mainly resulted from mild stepwise energy barrier among the adjacent functional-layer materials. For green phosphorescent devices, G-D showed the maximum CE, PE and EQE of 82.5 cd A 1, 57.5 lm W 1 and 23.2%, which was one of best reported green phosphorescent OLEDs [18,46,47]. For yellow devices, Y-D possessed the maximum CE, PE and EQE of 50.3 cd A 1, 35.6 lm W 1 and 16.7%. Particularly, Y-D achieved the ultrahigh luminescence of 92806 cd m 2. As expected, (piq)2Ir(acac)-doped red device (R-D) also exhibited the excellent EL performances with the maximum CE, PE and EQE of 20.8 cd A 1, 14.5 lm W 1 and 19.1% with CIE coordinates of (0.66, 0.34), which was even better than that of Y-D. All phosphorescent OLEDs showed almost no efficiency roll-off in pace with the increasing brightness, which was rare among the reported phosphorescent OLEDs. For example, at ultrahigh brightness of 5000 cd
Fig. 7. Characterizations of phosphorescent OLEDs with AD-BPI as host: (a) EL spectra at 10 V; (b) J-V-L characteristic curves; (c) CE/PE-L characteristic curves; (d) EQE-L curves. 6
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m 2, G-D, Y-D and R-D still demonstrated the high EQEs of 21.9%, 16.4% and 16.6% with small efficiency roll-off of 6%, 2% and 13%, respectively. Those prominent EL performances among the present phosphorescent OLEDs were mainly attributed to two factors: the balanced carrier (hole and electron) existing in active layer to broaden the recombination zone of excitons [48,49]; good thermal stability of AD-BPI restraining the strong intermolecular interactions of the phos phors to diminish the triplet-triplet annihilation [50]. Those experimental results confirm that AD-BPI is both a promising deep blue fluorescent emitter and a versatile host for green/yellow/red phosphorescent emitters in EL device.
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4. Conclusions A novel adamantane-bridged phenanthroimidazole molecule ADBPI, which showed largely twisted conformation, good thermal prop erty, high PLQY and ET as well as ambipolar nature was successfully designed and synthesized. Influence of adamantane moiety on the op toelectronic nature of the corresponding phenanthroimidazole molecule was thoroughly examined. AD-BPI was not only employed as deep blue emitter to establish non-doped fluorescent device, but also as host to fabricate highly efficient red/yellow/green phosphorescent OLEDs. Especially, the AD-BPI-based non-doped deep blue OLEDs manifested the ceiling EQE of 5.8% as well as the CIEs of (0.15, 0.07). What’s more, green, yellow and red phosphorescent OLEDs with AD-BPI as the host achieved outstanding device performances with the peak EQEs of 23.3%, 16.7% and 19.1% paralleling with little efficiency roll-off as changing current. It is the first demonstration of adamantane-based phenanthroimidazole molecule featuring as both emitter and host. Our results suggest that the utilization of the adamantane unit into phe nanthroimidazole is an effective strategy to realize impressive opto electronic materials for low-cost and full-color OLEDs. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. CRediT authorship contribution statement Hui-Min Guan: Writing - original draft. Yong-Xu Hu: Conceptual ization, Data curation. Guo-Yong Xiao: Resources. Wen-Ze He: Formal analysis, Investigation. Hai-Jun Chi: Methodology, Validation. Yan-Li Lv: Resources, Software. Xiao Li: Supervision, Writing - review & editing, Funding acquisition. Dong-Yu Zhang: Supervision. Zhi-Zhi Hu: Supervision. Acknowledgements This research was funded through University of Science and Tech nology Liaoning Talent Project Grants (201603) and National Key Research and Development Program of China (Project No. 2017YFB0404501). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2020.108273. References [1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987; 51:913. [2] Sasabe H, Kido J. Development of high performance OLED for general lighting. J Mater Chem C 2013;1:1699–707.
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Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
Novel adamantane-bridged phenanthroimidazole molecule for highly efficient full-color organic light-emitting diodes Hui-Min Guan a, b, Yong-Xu Hu a, Guo-Yong Xiao a, Wen-Ze He a, Hai-Jun Chi a, Yan-Li Lv a, Xiao Li a, *, Dong-Yu Zhang c, **, Zhi-Zhi Hu a, *** a b c
School of Chemical Engineering, University of Science and Technology Liaoning (USTL), Anshan, 114051, People’s Republic of China College of Chemistry, Chemical Engineering and Environment Engineering, Liaoning Shihua University, Fushun, 113001, People’s Republic of China Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou, 215123, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Deep blue Host Phenanthroimidazole Adamantane OLEDs
Highly efficient deep blue phenanthroimidazole fluorescent material by incorporating an adamantane moiety (AD-BPI) is designed and synthesized. AD-BPI exhibits excellent photophysical properties including a deep blue emission, high triplet energy level and photoluminescence efficiency, fine morphological and thermal stability, and ambipolar nature. Non-doped deep blue organic light-emitting diodes (OLEDs) following the construction of ITO (Indium–Tin Oxide)/TAPC (Bis [4-[N, N-di (4-tolyl) amino] phenyl]-cyclohexane, 30 nm)/AD-BPI (100 nm)/ TPBi (1, 3, 5-tris (2-N-phenylbenzimidazolyl) benzene, 50 nm)/Liq (8-hydroxyquinolatolithium, 2 nm)/Al (100 nm) utilizing AD-BPI as emissive layer achieve the peak external quantum efficiency (EQE) of 5.8% with the CIE coordinates of (0.15, 0.07). Using AD-BPI as universal host, the corresponding green, yellow and red phos phorescent devices are also fabricated and exhibit the maximum EQEs of 23.3%, 16.7% and 19.1% accompa nying with negligible efficiency roll-off under ultrahigh luminance. These experimental performances are intensely competitive with the recently reported advanced results of full-color OLEDs. It is the first demon stration of adamantane-based phenanthroimidazole molecule featuring as both host and emitter.
1. Introduction
employing deep blue luminescent materials with high triplet energy level (ET) which serve as hosts for phosphorescent emitters simulta neously [16–18]. However, the design and synthesis of blue-emitting hosts for green, yellow and red phosphors is still a great challenge because these characteristics including mild photoluminescence quan tum yield (PLQY), high ET and good carrier-transporting ability should be integrated into one molecule system. Recently, phenanthroimidazole derivatives have received extensive attention as efficient deep blue/blue emissive materials and/or hosts used in OLEDs due to their high PLQYs, balanced carrier injection and transporting ability, excellent thermal stability, easy synthesis and modification [19–31]. In addition, the near-spherical adamantane pos sesses high ET, good thermal and film formation, and its rigid non-conjugated structure can effectively interrupt the conjugation and electronic transfer between different functional groups, to maintain high ET of the corresponding molecular [32–34]. In view of those, a molecular (1s,3s,5r,7r)-1,3-bis(4-(2-phenyl-1H-
OLEDs (organic light-emitting diodes) have been considered as promising candidate in the field of white solid-state lighting and the next-generation full-color display [1–3]. Meanwhile, diverse red, green and blue luminescent materials which act as important roles in OLEDs have been well exploited one after another for realizing highly efficient OLEDs [4–10]. However, those luminescent materials especially TADF (thermally activated delayed fluorescence) and phosphorescent emitters are usually doped in host materials for achieving further high electro luminescence (EL) performances, which require stringent device process to increase the production cost and extremely restrict the further commercialization of OLEDs [11–15]. To solve this thorny problem, it is very important and meaningful to achieve highly efficient red, green and blue EL via an unsophisticated material system for streamlining the manufacturing process and minimizing the production cost of materials. Recently, it is a wise choice for achieving the full-color OLEDs
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (D.-Y. Zhang), [email protected] (Z.-Z. Hu). https://doi.org/10.1016/j.dyepig.2020.108273 Received 22 October 2019; Received in revised form 7 February 2020; Accepted 7 February 2020 Available online 8 February 2020 0143-7208/© 2020 Elsevier Ltd. All rights reserved.
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phenanthro [9,10-d]imidazol-1-yl)phenyl)adamantane (AD-BPI) with two phenanthroimidazole moieties bridged by an adamantane unit is purposefully designed and synthesized to address above issues. Nondoped blue OLEDs adopting AD-BPI as active layer achieves the peak external quantum efficiency (EQE), power efficiency and current effi ciency of 5.8%, 3.9 lm w 1 and 4.4 cd A 1 with the CIE coordinates of (0.15, 0.07). With the adamantane-bridged phenanthroimidazole molecule as universal host material, highly efficient red (19.1%, 14.5 lm W 1, 20.8 cd A 1), yellow (16.7%, 35.6 lm W 1, 50.3 cd A 1) and green (23.3%, 57.7 lm W 1, 82.5 cd A 1) phosphorescent OLEDs are realized with negligible efficiency roll-off.
H NMR (500 MHz, CDCl3) δ 7.17 (d, J ¼ 7.6 Hz, 4H), 6.65 (d, J ¼ 7.7 Hz, 4H), 2.24 (s, 2H), 1.87 (s, 8H), 1.73 (s, 4H).
2.2.3. Synthesis of AD-BPI 9, 10-phenanthrene quinone (10 mmol, 3.08 g), 4,4’-((1s,3s,5r,7r)adamantane-1,3- diyl)dianiline (5 mmol, 1.62 g), ammonium acetate (40 mmol, 3.08 g), benzaldehyde (10 mmol, 1.0 mL) were added into the flask in turn and then glacial acetic acid (100 mL) was charged. The system then reacted for 10 h under heating reflux with N2 atmosphere. After that, the solution was poured into 500 mL sodium chloride solu tion. The crude solid products were received by extraction and filtration. White powder was obtained through column chromatography (silica gel: 300–400 mesh; petroleum ether: ethyl acetate ¼ 20: 1, V/V). Yield: 53%. HRMS (APCI, MþHþ), Exact mass: 872.3879; Obtained mass: 873.4805. 1 H NMR (500 MHz, CDCl3) δ 8.93 (d, J ¼ 7.9 Hz, 2H), 8.79 (d, J ¼ 8.3 Hz, 2H), 8.73 (d, J ¼ 8.3 Hz, 2H), 7.77 (t, J ¼ 7.4 Hz, 3H), 7.65 (d, J ¼ 17.8, 14.4, 7.3 Hz, 10H), 7.55–7.47 (m, 6H), 7.38–7.25 (m, 10H), 7.22 (d, J ¼ 8.4 Hz, 2H), 2.49 (s, 2H), 2.28 (s, 2H), 2.14 (d, J ¼ 3.0 Hz, 7H). 13 C NMR (126 MHz, CDCl3) δ 151.69, 150.32, 135.63, 128.85, 128.69, 128.44, 128.16, 127.65, 127.54, 126.68, 126.02, 125.59, 125.02, 124.24, 123.48, 122.45, 122.24, 120.26, 76.63, 76.38, 76.13, 48.06, 41.64, 37.05, 35.03, 28.86. Elemental Analysis: Anal. Cacld. for C64H48N4: C, 88.04%; H, 5.54%; N, 6.42%. Found: C, 88.77%; H, 5.26%; N, 6.16%.
2. Experimental section 2.1. General information Raw materials and reagents involved in this experiment including 1adamantanol, 9,10-phenanthroquinone, acetanilide and benzaldehyde are commercially available. All the reactions are operated in standard glass reactor under inert gas (nitrogen or argon). The silica gel (300–400 mesh) is used for column chromatography. HRMS (high-resolution mass spectrometry) is undertaken on an Agilent 6530B Q-TOF LCMS. NMR spectra with TMS (tetramethylsilane) as the internal standard are ob tained via Bruker AC 500 spectrometer. Elemental analyses are acquired by Vario Micro cube analyzer. DSC (differential scanning calorimetry) is recorded in American TA company DSC-Q20 with the heating rate of 10 � C min 1 from 50 to 400 � C under nitrogen. The Tm (melting point) and Tg (glass transition temperature) are obtained from the first and the second heating scan, respectively. TGA (thermogravimetric analysis) is carried out on American TA company TG-DTA Q600 thermal analyzer at a heating rate of 10 � C min 1 from 50 to 800 � C in the atmosphere of nitrogen. The PL (photoluminescence) and UV–Vis absorption spectra are received on LS 55 and Lambda 900 spectrophotometer both from PerkinElmer company, respectively. CV (cyclic voltammetry), which was calibrated by Fc/Fcþ redox couple as the internal reference in dichloromethane containing Bu4NPF6 (tetrabutylammonium hexa fluorophosphate, 0.1 M), is carried out on an AUTOLAB PGSTAT 302 N workstation with a scan rate of 50 mV s 1 under the atmosphere of ni trogen. DFT (density functional theory) calculations are implemented through Gaussian 09 employing the B3LYP functional together with 631G (d, p) basis set.
2.3. Devices fabrication and characterization Devices were made on pre-patterned ITO-coated glass substrates (15 Ω cm 2). Prior to device, the substrates were rinsed with soap and ul trapure H2O, and sonicated for 15 min. Afterwards, two subsequent rinses and 12-min sonication baths in acetone and isopropanol were performed sequentially. All organic layers as well as the Al cathode were deposited in a vacuum thermal evaporator at not less than 6 � 10 4 Pa. The luminance and EL spectra of OLEDs were obtained on PR650 spectrometer. The single-carrier devices and voltage-current properties of the OLEDs were received by Keithley 2400 source meter. All mea surements of devices without encapsulations at ambiet temperature were performed in the dark. 3. Results and discussion 3.1. Synthesis and characterization
2.2. Synthesis
AD-BPI’s chemical structure and its complete synthesis routes are shown in Scheme 1. The key intermediate 4,4’-((1s,3s,5r,7r)-ada mantane-1,3-diyl)dianiline was obtained via two-step reaction including Friedel-Crafts alkyl reaction and hydrolysis reaction, respectively, with the total yield of 35%. The classic Debus-Radziszewski reaction was then adopted to smoothly prepare the target compound AD-BPI in one pot with high isolated yield of 53%. All the intermediates and target com pound were characterized by elemental analysis, HRMS and NMR spectrometry.
2.2.1. Synthesis of N,N’-(((1s,3s,5r,7r)-adamantane-1,3-diyl)bis(4,1phenylene)) diacetamide A two-necked flask was charged with 1-adamantanol (2.5 mmol, 0.375 g), acetanilide (5 mmol, 0.78 g) and 98% sulfuric acid (7.5 mL) at room temperature (25 � C), and then reacted for 5 h under magnetic stirring. Subsequently, white crude solid was obtained once H2O was poured into the mixture. The pure white solid was got via column chromatography (silica gel, 300–400 mesh). Yield: 56%. HRMS (APCI, MþHþ): Exact mass: 402.2307; Obtained mass: 403.2380. 1H NMR (500 MHz, DMSO‑d6) δ 9.85 (d, J ¼ 4.5 Hz, 2H), 7.49 (d, J ¼ 6.7 Hz, 4H), 7.31 (d, J ¼ 6.7 Hz, 4H), 2.23 (s, 1H), 2.01 (s, 7H), 1.87 (d, J ¼ 18.8 Hz, 10H), 1.72(s,2H).
3.2. Thermal property The morphological stability and thermal property of AD-BPI were determined by DSC and TGA at a heating rate of 10 � C min 1 under streaming N2, respectively. TGA and DSC plots of AD-BPI were presented in Fig. 1. As showed in Fig. 1, AD-BPI exhibited the high decomposition temperature (Td, 5% decomposition temperature) of 509 � C, and simi larly a relatively high melting point Tm of 358 � C (Fig. S1) and Tg of 188 � C were observed via DSC analysis (Inset of Fig. 1), which could guar antee the subsequent vacuum evaporation smoothly. In addition, the Tg and Tm of AD-BPI roughly followed the Boyer-Kauzmann rule (Tg/Tm � 0.7) on the Kelvins temperature [35]. The slightly high value of Tg/Tm (0.73) based AD-BPI indicated strong intermolecular interaction in the
2.2.2. Synthesis of 4,4’-((1s,3s,5r,7r)-adamantane-1,3-diyl)dianiline N,N’-(((1s,3s,5r,7r)-adamantane-1,3-diyl)bis(4,1-phenylene)) diac etamide (0.75 mmol, 0.30 g) in ethanol (4 mL) and H2O (1 mL) including sodium hydroxide (12 mmol, 0.48 g) was put into the flask in turn. The mixture was refluxed with constant stirring for 6 h. Following that, the solution was cooled to ambient temperature, and then saturated brine was added to obtain the yellowish-colored raw solid. Further recrys tallization with ethanol enabled to the white solid product. Yield: 63%. HRMS (APCI, MþHþ), Exact mass: 318.2096; Obtained mass: 319.2222. 2
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Scheme 1. Synthesis routes and molecular structure of AD-BPI.
AD-BPI presented the blue-violet emission peaked at 398 nm in degassed dichloromethane accompanying with mild fluorescent PLQY of 0.76. Furthermore, AD-BPI exhibited red-shift of ca. 20 nm in neat film compared with that in CH2Cl2 solution, implying that π-π interaction in solid state was effectively restrained by the rigid and large twisted molecular configuration of AD-BPI. The PL spectra of AD-BPI in different solvents were also investigated and displayed small changes upon increasing solvent polarity. As indi cated in Fig. S2, the maximum emission of AD-BPI shifted from 398 nm in low polar CH2Cl2 to 405 nm in high polar acetonitrile solution with only 7 nm variation, confirming the localized excited property of the lowest singlet excited state of AD-BPI [38] and negligible intramolecular charge transfer. The ET of AD-BPI was obtained to be 2.64 eV via the low-temperature phosphorescent spectrum, which was even slightly higher than that of single 1,2-diphenyl-1H-phenanthro [9,10-d] imidazole molecular (2.61 eV) [39], suggesting that introduction of the insulated and rigid adamantane unit in this molecular system could provide the steric hindrance while maintaining a high ET. Furthermore, the high ET of AD-BPI was fully satisfied with energy transfer to red/ yellow/green phosphors. Therefore, AD-BPI could not only be utilized as a deep-blue fluorescent emitter but also a promising host in the green/yellow/red phosphorescent OLEDs.
Fig. 1. The TGA curve of AD-BPI. Inset: DSC curve of AD-BPI.
glass state of AD-BPI [36]. Taken together, the introduction of rigid adamantane unit in the currently investigated molecular definitely helps to improve thermal stability of material. Those excellent thermal prop erties of AD-BPI should facilitate device stability during operation of OLEDs. 3.3. Photophysical property
3.4. Electrochemical property
In Fig. 2, UV–vis absorption and PL (photoluminescent) curves of AD-BPI in dilute solution (10 5 M) are shown. In Table 1, all photo physical parameters are outlined. The absorption peaks of AD-BPI in CH2Cl2 were mainly concentrated on 200–370 nm, of which strong ab sorption peaks positioned at 262 and 310 nm were classified into π-π* transition of the corresponding benzene tied to N1 position in imidazole unit and phenanthroimidazole section, respectively [37,38].
CV plot of the compound AD-BPI is shown in Fig. 3. The compound AD-BPI exhibited a reversible oxidation curve and its threshold oxida tion potential (Eox onset) was 1.17 eV. And then the HOMO (highest occu pied molecular orbital) of AD-BPI could be deduced to be 5.57 eV via EHOMO ¼ -(4.4þEox onset). Interestingly, HOMO of AD-BPI matched well with that of common hole-transporting material TAPC (4,4’-(cyclo hexane-1,1-diyl)bis(N,N-di-p-tolylaniline)) (5.5 eV), suggesting that very small hole injection barrier (0.07 eV) existed between AD-BPI and TAPC, which was beneficial to lower the turn-on voltage. The LUMO (lowest unoccupied molecular orbital) of AD-BPI was acquired to be 2.28 eV according to the HOMO and energy gap deduced from UV ab sorption spectrum. 3.5. Theoretical calculations To learn more about the effects of the introduction of adamantane unit on the electronic property and geometrical structure of AD-BPI, DFT calculation of AD-BPI (Fig. 4) was handled at B3LYP/6-31G (d, p) level. All dihedral angles such as adamantane/its linked benzenes and phenanthroimidazole/N1-benzenes were more than 70� . The SP3 hy bridizations of N1 in imidazole and C in adamantane unit led to the nonflatness of AD-BPI, which thus increased steric hindrance and effectively
Fig. 2. The UV–Vis absorption and PL spectra in dilute CH2Cl2 solution (10-5 M), PL spectrum in neat film of AD-BPI at room temperature, and the phos phorescence spectrum in 2-methyltetrahydrofuran at 77 K. 3
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Table 1 Photophysical, thermal and Electrochemical properties of AD-BPI. Compound
λaabs (nm)
λbem (nm)
Φc
ETd (eV)
Td/Tm/Tge (� C)
HOMOf (eV)
LUMOf (eV)
Egf (eV)
AD-BPI
262, 310, 360
398/420
0.76/0.62
2.64
509/358/188
5.57
2.28
3.29
a b c d e f
Measured in CH2Cl2 solution with 1 � 10 5 mol L 1. Fluorescence emission measured in CH2Cl2 solution and neat film, respectively. The quantum yields measured in CH2Cl2 using 9,10-diphenylanthrecene (Φ ¼ 0.90) as the standard and in neat film. Triplet energy level measured in 2-MeTHF at 77 K, ET ¼ 1240.8/λPh. Decomposition temperature of 5% weight loss, melting point and glass transition temperature, respectively. EHOMO (eV) ¼ -e(Eoxonset þ 4.4), Eg ¼ 1240.8/λ, ELUMO (eV) ¼ Eg þ EHOMO.
unit possessed both electron- and hole-transporting properties. Slightly different from CV data, the calculated HOMO and LUMO levels of AD-BPI based DFT were 5.22 and 1.38 eV, respectively. 3.6. Electroluminescent performance Firstly, non-doped OLEDs with the traditional structure of ITO/TAPC (30 nm)/AD-BPI (100 nm)/TPBi (1,3,5-Tri(1-phenyl-1H-benzo[d]imi dazol-2-yl)phenyl, 50 nm)/Liq (2 nm)/Al (100 nm) (marked as: B-D1) was fabricated to assess the deep blue device performance of AD-BPI. Al and ITO served as the cathode and anode. TAPC and TPBi functioned as the hole-transporting layer and the hole-blocking/electron-transporting layer. The device configuration and involved materials’ structures are described in Fig. 5. EL spectrum, voltage-current density-luminance (JV-L) plots, current/power efficiency-luminance (CE/PE-L) characteris tics and EQE-luminance curve of non-doped deep blue OLEDs are depicted in Fig. 6, and the corresponding EL performances are summa rized in Table 2. B-D1 exhibited deep blue emission peaked at 422 nm as shown in Fig. 6(a), which was almost identical to that of AD-BPI neat film, demonstrating that all resulting carriers (hole and electron) of device could be limited solely in the luminescent layer and the emissive peak derived from AD-BPI itself. The CIE coordinates (0.15, 0.07) of B-D1 at 8 V conformed with the deep blue standard as specified via NTSC (Na tional Television Systems Committee) of the United States (CIEy<0.08)
Fig. 3. The CV curve of AD-BPI in anhydrous CH2Cl2 solution with 0.1 M TBAPF6 as supporting electrolyte.
reduced π-π stacking between molecules. Both HOMO and LUMO of AD-BPI almost localized at phenan throimidazole unit and the distribution of its LUMO extended on N1linked benzene ring. These overlaps between the orbitals allowed ADBPI functional to the flow of intramolecular electronic, which might act as an efficient emitter in OLEDs [40]. Besides, those features of electron distribution could be an evidence that phenanthroimidazole
Fig. 4. The optimized structure and the frontier molecular orbitals of AD-BPI.
Fig. 5. (a) The device configuration; (b) The device energy-level diagram; (c) Molecular structures of the materials used in OLEDs. 4
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Fig. 6. Characterizations of non-doped AD-BPI-based OLED (B-D1 and B-D2): (a) EL spectra at 8 V; (b) J-V-L characteristic curves; (c) CE/PE-L characteristic curves; (d) EQE-L curves.
[41]. It was worth mentioning that EL spectra of B-D1 remained virtu ally unchanged from 5 to 9 V (shown in Fig. S3), indicating that the AD-BPI-based non-doped deep blue OLED had good light-color stability. B-D1 showed the peak brightness of 12703 cd m 2 and low turn-on voltage of 3.1 V (1.0 cd m 2), which maybe mainly benefited from small energy barrier of HOMO between TAPC and AD-BPI. B-D1 exhibited excellent device performance with the maximum efficiencies of 5.8%, 4.4 cd A 1 and 3.9 lm w 1, respectively. Furthermore, at the luminance of 100 and 1000 cd m 2, B-D1 manifested nearly negligible efficiency roll-off, and its EQE remained 5.2%, 5.1%, respectively. Such high ef ficiency at high luminescence rendered AD-BPI a promising candidate for practical application. The performances of non-doped AD-BPI-based OLED were extraordinarily competitive with those of the reported phenanthroimidazole-based non-doped deep blue OLEDs (CIEy � 0.08, Table S1). Then we further investigated the theoretical EQE of AD-BPI-based OLED via the formula EQE ¼ γ � ηout � ϕPL � ηfr [42], where γ (the charge balance factor of holes and electrons) is usually assumed to be 1 if the resulting hole and electron are balanced and recombined in emitting zone, ηout stands for light-out coupling efficiency (typically ca. 20%), ϕPL represents the PLQY of AD-BPI film (0.62), and ηfr refers to the efficiency of radiative exciton production (usually 25% for fluorescent OLEDs). Based on this, the maximum EQE of AD-BPI-based OLED was theoreti cally estimated to be only 3.1%. Evidently, the actual maximum EQE of AD-BPI-based OLED (5.8%) was much larger than the theoretical value of 3.1% and surmounted the EQE limit (5%) of the traditional fluores cent OLEDs. We couldn’t explain it at the moment and further
Table 2 Summary of EL properties of non-doped deep blue OLED and 10% Ir-doped devices with AD-BPI as host. Device
Vturn-on (V) a
Lmax (cd m 2) b
EQE (%)c
CE (cd A 1) c
PE (lm W 1) c
CIE (x, y)d
B-D1
3.1
12703
3.1
4037
G-D
2.7
42718
Y-D
2.9
92806
R-D
3.2
36846
4.4/4.2/ 3.7 5.9/ 3.9/82.5/ 83.8/ 79.7 50.3/ 48.7/ 43.2 20.8/ 17.9/ 17.2
3.9/2.9/ 1.8 3.6/ 1.4/57.7/ 40.5/ 30.7 35.6/ 23.9/ 18.3 14.5/ 6.5/5.8
(0.15,0.07)
B-D2
5.8/5.2/ 5.1 6.9/ 4.9/23.3/ 21.9/ 21.8 16.7/ 16.4/ 14.2 19.1/ 16.6/ 13.7
(0.15,0.08) (0.28, 0.64) (0.50, 0.50) (0.66, 0.34)
a
Turn-on voltage at 1.0 cd m 2. Maximum luminance. c Order of measured efficiency values: maximum, values at 100, 1000 cd m 2 for B-D1 and B-D2; the others values at 5000, 10000 cd m 2 for G-D, Y-D and RD. d Commission International de I’Eclairage (CIE) coordinates measured at 10 V. b
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experiments will be executed to uncover the possible mechanism. AD-BPI’s carrier-transporting properties were then studied via single-carrier devices following: ITO/TPBi (10 nm)/AD-BPI (60 nm)/ TPBi (10 nm)/LiF (1 nm)/Al (100 nm) (electron-only device) and ITO/ TAPC (10 nm)/AD-BPI (60 nm)/TAPC (10 nm)/Al (100 nm) (hole-only device). TPBi and TAPC acted as hole-blocking and electron-blocking layer, respectively, to interrupt the injection of electron/hole from the electrode. As shown in the voltage-current density graphs depicted in Fig. S4, the hole-transporting ability of AD-BPI was much higher than its electron-transporting ability. Overall, AD-BPI could transport hole and electron, and therefore presented ambipolar feature, which was one of vital factors to achieve high device efficiency with little efficiency rolloff. Considering AD-BPI’s much higher hole-transporting ability, we tentatively optimized device structure by merely replacing TPBi with TmPyPB, and fabricated OLED of ITO (40 nm)/TAPC (30 nm)/AD-BPI (100 nm)/TmPyPB (50 nm)/Liq (2 nm)/Al (100 nm) (B-D2). The cor responding electro-optical performance plots are all outlined in Fig. 6. The EL efficiency of B-D2 was remarkably improved and a maximum EQE of 6.9% was obtained accompanying with CIE of (0.15, 0.08) at 8 V, which would be mainly due to deep HOMO of TmPyPB (6.75 eV of TmPyPB vs. 6.2 eV of TPBi) [43] and the better electron-transporting property (1 � 10 3 cm2/V vs. 3.3 � 10 5 cm2/Vs) [44,45]. Those findings demonstrate that adamantane-based phenanthroimidazole is a class of potential deep blue candidate and undoubtedly advance in EL efficiency through further optimizing device structure is within reach. Further efforts on optimizing the device structure with AD-BPI as emitter for acquiring better EL performances are ongoing. Due to the higher ET of AD-BPI than those of the classical red, yellow and green emissive materials, AD-BPI was also studied as host for red/ yellow/green phosphorescent luminescent materials in the uniform device configuration: ITO/TAPC (40 nm)/AD-BPI: dopant (30 nm)/TPBi
(30 nm)/Liq (2 nm)/Al (100 nm) (simplified as R-D, Y-D and G-D). Here, the optimized 10%-doped phosphor ((piq)2Ir (acac)/(bt)2Ir(acac)/Ir (ppy)3) in AD-BPI was adopted as emissive layer. The device configu ration and chemical structure of materials used are described in Fig. 5. Those device’s performances are generalized in Table 2. By examining the energy level diagram (Fig. 5(b)), the HOMO/ LUMO energy levels of G/Y/R phosphorescent emitters were all embraced in those of AD-BPI, which meant that both hole and electron could be directly trapped in phosphorescent dopant. Therefore, the dominate charge-trapping mechanism probably appeared in current OLEDs. In addition, high ET of AD-BPI as host ensured to efficient sensitize the phosphors ((piq)2Ir(acac)/(bt)2Ir(acac)/Ir(ppy)3) via exothermic energy transfer mechanism. As shown in Fig. 7, all OLEDs demonstrated a major EL emission peak originating from the corresponding phosphorescent emitter and no un foreseen peaks from the transporting layer material and/or host appeared, suggesting that energy transfer from AD-BPI to dopants was complete. Furthermore, those phosphorescent OLEDs exhibited low voltage of 2.7–3.2 V at 1.0 cd m 2, which mainly resulted from mild stepwise energy barrier among the adjacent functional-layer materials. For green phosphorescent devices, G-D showed the maximum CE, PE and EQE of 82.5 cd A 1, 57.5 lm W 1 and 23.2%, which was one of best reported green phosphorescent OLEDs [18,46,47]. For yellow devices, Y-D possessed the maximum CE, PE and EQE of 50.3 cd A 1, 35.6 lm W 1 and 16.7%. Particularly, Y-D achieved the ultrahigh luminescence of 92806 cd m 2. As expected, (piq)2Ir(acac)-doped red device (R-D) also exhibited the excellent EL performances with the maximum CE, PE and EQE of 20.8 cd A 1, 14.5 lm W 1 and 19.1% with CIE coordinates of (0.66, 0.34), which was even better than that of Y-D. All phosphorescent OLEDs showed almost no efficiency roll-off in pace with the increasing brightness, which was rare among the reported phosphorescent OLEDs. For example, at ultrahigh brightness of 5000 cd
Fig. 7. Characterizations of phosphorescent OLEDs with AD-BPI as host: (a) EL spectra at 10 V; (b) J-V-L characteristic curves; (c) CE/PE-L characteristic curves; (d) EQE-L curves. 6
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m 2, G-D, Y-D and R-D still demonstrated the high EQEs of 21.9%, 16.4% and 16.6% with small efficiency roll-off of 6%, 2% and 13%, respectively. Those prominent EL performances among the present phosphorescent OLEDs were mainly attributed to two factors: the balanced carrier (hole and electron) existing in active layer to broaden the recombination zone of excitons [48,49]; good thermal stability of AD-BPI restraining the strong intermolecular interactions of the phos phors to diminish the triplet-triplet annihilation [50]. Those experimental results confirm that AD-BPI is both a promising deep blue fluorescent emitter and a versatile host for green/yellow/red phosphorescent emitters in EL device.
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4. Conclusions A novel adamantane-bridged phenanthroimidazole molecule ADBPI, which showed largely twisted conformation, good thermal prop erty, high PLQY and ET as well as ambipolar nature was successfully designed and synthesized. Influence of adamantane moiety on the op toelectronic nature of the corresponding phenanthroimidazole molecule was thoroughly examined. AD-BPI was not only employed as deep blue emitter to establish non-doped fluorescent device, but also as host to fabricate highly efficient red/yellow/green phosphorescent OLEDs. Especially, the AD-BPI-based non-doped deep blue OLEDs manifested the ceiling EQE of 5.8% as well as the CIEs of (0.15, 0.07). What’s more, green, yellow and red phosphorescent OLEDs with AD-BPI as the host achieved outstanding device performances with the peak EQEs of 23.3%, 16.7% and 19.1% paralleling with little efficiency roll-off as changing current. It is the first demonstration of adamantane-based phenanthroimidazole molecule featuring as both emitter and host. Our results suggest that the utilization of the adamantane unit into phe nanthroimidazole is an effective strategy to realize impressive opto electronic materials for low-cost and full-color OLEDs. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. CRediT authorship contribution statement Hui-Min Guan: Writing - original draft. Yong-Xu Hu: Conceptual ization, Data curation. Guo-Yong Xiao: Resources. Wen-Ze He: Formal analysis, Investigation. Hai-Jun Chi: Methodology, Validation. Yan-Li Lv: Resources, Software. Xiao Li: Supervision, Writing - review & editing, Funding acquisition. Dong-Yu Zhang: Supervision. Zhi-Zhi Hu: Supervision. Acknowledgements This research was funded through University of Science and Tech nology Liaoning Talent Project Grants (201603) and National Key Research and Development Program of China (Project No. 2017YFB0404501). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2020.108273. References [1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987; 51:913. [2] Sasabe H, Kido J. Development of high performance OLED for general lighting. J Mater Chem C 2013;1:1699–707.
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