Polymer light-emitting diodes based on cationic iridium(III) complexes with a 1,10-phenanthroline derivative containing a bipolar carbazole–oxadiazole unit as the auxiliary ligand

Optical Materials xxx (2014) xxx–xxx

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Polymer light-emitting diodes based on cationic iridium(III) complexes with a 1,10-phenanthroline derivative containing a bipolar carbazole–oxadiazole unit as the auxiliary ligand Huaijun Tang a,⇑, Liying Wei a, Guoyun Meng a, Yanhu Li b, Guanze Wang a, Furui Yang a, Hongbin Wu b, Wei Yang b, Yong Cao b a b

Engineering Laboratory of Polylactic Acid-Based Functional Materials of Yunnan, School of Chemistry and Biotechnology, Yunnan Minzu University, Kunming 650500, China State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 13 January 2014 Received in revised form 15 June 2014 Accepted 20 August 2014 Available online xxxx Keywords: Cationic iridium complex Polymer light-emitting diode 1,10-Phenanthroline derivative Carbazole Oxadiazole

a b s t r a c t A 1,10-phenanthroline derivative (co-phen) containing a bipolar carbazole–oxadiazole unit was synthesized and used as the auxiliary ligand in cationic iridium(III) complexes [(ppy)2Ir(co-phen)]PF6 (ppy: 2phenylpyridine) and [(npy)2Ir(co-phen)]PF6 (npy: 2-(naphthalen-1-yl)pyridine). Two complexes have high thermal stability with the glass-transition temperatures (Tg) of 207 °C and 241 °C, and the same 5% weight-reduction temperatures (DT5%) of 402 °C. Both of them were used as phosphorescent dopants in solution-processed polymer light-emitting diodes (PLEDs): ITO/PEDOT: PSS/PVK: PBD: complex (mass ratios 100: 40: x, x = 1.0, 2.0, and 4.0)/CsF/Al. The maximum luminances of the PLEDs using [(ppy)2Ir(cophen)]PF6 and [(npy)2Ir(co-phen)]PF6 were 12567 cd m 2 and 11032 cd m 2, the maximum luminance efficiencies were 17.3 cd A 1 and 20.4 cd A 1, the maximum power efficiencies were 9.8 lm W 1 and 10.3 lm W 1, and the maximum external quantum efficiencies were 9.3% and 11.4% respectively. The CIE color coordinates were around (0.37, 0.57) and (0.44, 0.54) respectively, corresponding to the yellow green region. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Since the significant breakthrough made by Tang and Van Slyke [1], organic light-emitting diodes (OLEDs, including small-molecule and polymer light-emitting diodes) have been widely researched and rapidly developed for over twenty years, due to their potential applications in solid-state lighting sources, fullcolor flat-panel displays and liquid crystal display backlights [2–5]. In contrast with other already reported organic electroluminescent (EL) materials, phosphorescent organic iridium(III) complexes have exhibited some prominent advantages, such as high efficiency of 100% theoretical internal quantum efficiency (QE), ease of spectral tuning via various ligands, short triplet state lifetimes, high stability and so on, and have been considered as the most promising candidates for practical application [2–5]. Although it is now more than ten years since organic iridium(III) complexes were developed for OLEDs [6,7] and large quantities of organic iridium(III) complexes have been synthesized and applied [2–5], however, most of them are neutral, and very few ⇑ Corresponding author. Tel.: +86 871 65913043; fax: +86 871 65910017. E-mail address: [email protected] (H. Tang).

ionic iridium(III) complexes have been tried. Ionic organic iridium(III) complexes, especially, the cationic iridium(III) complexes have similar photochemical and photophysical properties to the neutral iridium(III) complexes, can be more easily synthesized and purified with much higher yield, and have been widely applied in light-emitting electrochemical cells (LECs) [8–12]. However, most of cationic iridium(III) complexes are difficult to be applied in OLEDs, because their EL devices to be fabricated by solution-processing technology or vacuum-deposition technology have been hindered by their low solubility, easy crystallization, bad compatibility with host materials or poor sublimability [13,14]. Nevertheless, in recent years, some cationic iridium(III) complexes without aforementioned issues had been successfully applied in OLEDs, mainly, in polymer light-emitting diodes (PLEDs) fabricated by solution-processing technology [13–20]. The results suggest that cationic iridium(III) complexes can be applied in OLEDs like neutral iridium(III) complexes with similar EL performance when they were designed reasonably. In this paper, a 1,10-phenanthroline derivative containing a bipolar carbazole–oxadiazole unit was designed, synthesized and used as the neutral auxiliary ligand in two cationic iridium(III) complexes. Similar to in neutral iridium(III) complexes, carbazole

http://dx.doi.org/10.1016/j.optmat.2014.08.013 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

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H. Tang et al. / Optical Materials xxx (2014) xxx–xxx

and oxadiazole in such cationic iridium(III) complexes also are used as hole- and electron-transporting functional groups respectively, and introduced in the complexes for good EL performances [4,14], especially, the bipolar unit containing both hole- and electron-transporting functional groups is expected to better promote their EL performance due to the balance of hole and electron fluxes being improved [14]. Additionally, a 2-ethylhexyl on carbazole was designed to improve solubility, anti-crystallization and compatibility with host materials [4,19]. After the thermal stability, electrochemical properties, ultraviolet–visible (UV–Vis) absorption spectra and photoluminescence (PL) spectra of the cationic iridium(III) complexes being carefully investigated, both of them were used as phosphorescent dopants in PLEDs. 2. Experimental 2.1. General information Chemicals and reagents were purchased from commercial sources and used without further purification unless otherwise stated. 1H NMR spectra were recorded on a Bruker AV400 spectrometer operating at 400 MHz, tetramethylsilane (TMS) was used as internal standard. Mass spectra (MS) were obtained on a Bruker amaZon SL liquid chromatography mass spectrometer (LC-MS) with an electrospray ionization (ESI) interface using acetonitrile as matrix solvent. Elemental analyses (EA) were performed on a Vario EL III Elemental Analysis Instrument. Ultraviolet–visible (UV–Vis) absorption spectra were recorded on a HP8453E spectrophotometer. Photoluminescence (PL) spectra were recorded on a Jobin Yvon FL3-21 spectrofluorometer at room temperature and a 450 W xenon lamp was used as excitation source (the spectrofluorometer being used as factory defaults without extra calibration). Differential scanning calorimetry (DSC) curves were obtained on a Netzsch DSC200 analyzer at a heating rate 10 °C min 1 under N2 after the first heating cycles of the iridium(III) complexes from room temperature to 250 °C under the same conditions. Thermogravimetry (TG) curves were obtained on a Netzsch STA 449F3 thermal analyzer at a heating rate 10 °C min 1 under N2. Cyclic voltammetry (CV) was performed on a computer-controlled CHI660D electrochemical analyzer with a conventional three-electrode configuration consisting of a glassy carbon working electrode, a platinum wire counter electrode, and a saturated calomel electrode (SCE) as reference electrode, in anhydrous and Ar-saturated acetonitrile solutions of the cationic iridium(III) complexes at 1  10 3 mol L 1 at a scanning rate of 100 mV s 1 using 0.1 mol L 1 tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6) as supporting electrolyte. Ferrocene (Fc, 4.8 eV under vacuum) was used as the internal standard. 2.2. Synthesis and characterization of the cationic iridium(III) complexes The synthetic routes of the cationic iridium complexes are shown in Scheme 1, experimental details and characterization data are given in the following. 2.2.1. Synthesis of 9-(2-ethylhexyl)-9H-carbazole-3-carbonitrile (3) 9-(2-Ethylhexyl)-9H-carbazole (1) and 3-Formyl-9-(2-ethylhexyl)-9H-carbazole (2) were synthesized according to the published procedures [19]. Compound 2 (6.15 g, 20.0 mmol) was dissolved in tetrahydrofuran (THF, 25 mL), then ammonia (25– 28%, 50 mL) and I2 (5.58 g, 22.0 mmol) were added in and the mixture was stirred at room temperature for 1.5 h. Na2S2O3 was added in until the blackish–brown vanished. The reaction mixture was extracted with CH2Cl2 (3  50 mL), washed with water and then

dried with anhydrous Na2SO4. The solvent was distilled off and the residue was chromatographed over silica gel, eluting with petroleum ether (60–90 °C). Yield 90.0% (5.48 g), yellow oil. 1H NMR (400 MHz, CDCl3, 25 °C, ppm): d = 8.39 (s, 1H, ArH), d = 8.10 (d, 1H, 3J = 8.0 Hz, ArH), d = 7.67–7.71 (m, 1H, ArH), d = 7.55 (t, 1H, 3J = 1.2 Hz, ArH), d = 7.41–7.46 (m, 2H, ArH), d = 7.32 (t, 1H, 3 J = 7.6 Hz, ArH), d = 4.31, 4.18 (dq, 2H, 3J = 7.6 Hz, NACH2A), d = 2.00–2.10 (m, 1H, NACACH—), d = 1.24–1.38 (m, 8H, A(CH2)A and A(CH2)3A), d = 0.92 (t, 3H, 3J = 7.6 Hz, ACH3), d = 0.85 (t, 3H, 3 J = 7.2 Hz, ACH3). Anal. Calc. for C21H24N2 (%): C, 82.85; H, 7.95; N, 9.20. Found (%): C, 82.45; H, 8.00; N, 9.42. 2.2.2. Synthesis of 9-(2-ethylhexyl)-3-(1H-tetrazol-5-yl)-9H-carbazole (4) Compound 3 (6.09 g, 20.0 mmol), NH4Cl (16.05 g, 300.0 mmol) and NaN3 (19.50 g, 300.0 mmol) were added in N,N-dimethylformamide (DMF, 120 mL), then the mixture was heated to 120 °C and kept stirring for 72 h. The reaction mixture were cooled to room temperature and poured into water (about 1 L), then concentrated hydrochloric acid was dropped into with stirring until pH  1.0. The resultant precipitates were filtered and washed with plenty of water, dried in vacuum. Yield 85.0% (5.91 g), white solid. 1H NMR (400 MHz, DMSO-D6, 25 °C, ppm): d = 8.85 (s, 1H, ArH), d = 8.25 (d, 1H, 3J = 7.6 Hz, ArH), d = 8.12 (d, 1H, 3J = 8.4 Hz, ArH), d = 7.78–7.85 (m, 1H, ArH), d = 7.63–7.69 (m, 1H, ArH), d = 7.52 (t, 1H, 3J = 7.6 Hz, ArH), d = 7.29 (t, 1H, 3J = 7.2 Hz, ArH), d = 4.45, 4.33 (dq, 2H, 3J = 7.6 Hz, NACH2A), d = 1.98–2.10 (m, 1H, NACACH—), d = 1.14–1.36 (m, 8H, A(CH2)A and A(CH2)3A), d = 0.87 (t, 3H, 3J = 7.6 Hz, ACH3), d = 0.78 (t, 3H, 3J = 7.2 Hz, ACH3). Element Anal. Calc. For C21H25N5 (%): C, 72.59; H, 7.25; N, 20.16. Found (%): C, 72.42; H, 7.20; N, 20.38. 2.2.3. Synthesis of 4-(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4oxadiazol-2-yl)benzaldehyde (5) 4-Formylbenzoic acid (1.50 g, 10.0 mmol) was added in SOCl2 (20 mL) and refluxed for 12 h. After the larger part of excess SOCl2 was removed by vacuum distillation, residual SOCl2 was further removed with additional anhydrous benzene (10 mL) by vacuum distillation. After the resultant 4-formylbenzoyl chloride was cooled to room temperature, a solution of compound 4 (3.47 g, 10.0 mmol) in 50 mL anhydrous pyridine (distilled after being dried by potassium hydroxide) was dropped in and stirred under reflux in the presence of argon for 6 h. After being cooled to room temperature, the reaction mixture was poured into water, and then the resultant precipitates were filtered and washed with plenty of water. The precipitates were dried and then chromatographed over silica gel, eluting with CH2Cl2. Yield 45.0% (2.03 g), white solid. 1H NMR (400 MHz, CDCl3, 25 °C, ppm): d = 10.12 (s, 1H, CHO), d = 8.86 (s, 1H, ArH), d = 8.36 (d, 2H, 3J = 8.0 Hz, ArH), d = 8.24 (d, 1H, 3 J = 8.4 Hz, ArH), d = 8.20 (d, 1H, 3J = 7.6 Hz, ArH), d = 8.06 (d, 2H, 3 J = 8.0 Hz, ArH), d = 7.49–7.55 (m, 2H, ArH), d = 7.44 (d, 1H, 3 J = 8.4 Hz, ArH), d = 7.30–7.34 (m, 1H, ArH), d = 4.21 (q, 2H, 3 J = 7.6 Hz, NACH2A), d = 2.04–2.10 (m, 1H, NACACH—), d = 1.24– 1.42 (m, 8H, A(CH2)A and A(CH2)3A), d = 0.94 (t, 3H, 3J = 7.6 Hz, ACH3), d = 0.86 (t, 3H, 3J = 7.2 Hz, ACH3). Element Anal. Calc. For C29H29N3O2 (%): C, 77.13; H, 6.47; N, 9.31. Found (%): C, 76.94; H, 6.44; N, 8.97. 2.2.4. Synthesis of 2-(4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2yl)phenyl)-5-(9-(2-ethylhexyl)- 9H-carbazol-3-yl)-1,3,4-oxadiazole (6) 1,10-Phenanthroline-5,6-dione was synthesized according to the reported procedures [21]. A mixture of 1,10-phenanthroline5,6-dione (1.05 g, 5.0 mmol), compound 5 (2.26 g, 5.0 mmol), CH3COONH4 (7.70 g, 100.0 mmol) and glacial acetic acid (30.0 mL) was refluxed for 2 h. After being cooled to room

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Scheme 1. Synthetic route and chemical structure of two cationic iridium(III) complexes.

temperature, the reaction mixture was poured into water (about 100 mL), and then concentrated ammonia was dropped in with stirring until pH  7.0. The resultant yellow precipitates were filtered, washed with water and ethanol, and then dried in a vacuum oven. Yield 86.0% (2.76 g), yellow solid. The product was used directly in the next reaction without further purification and characterization. 2.2.5. Synthesis of 2-(4-(1-ethyl-1H-imidazo[4,5f][1,10]phenanthrolin-2-yl)phenyl)-5-(9-(2- ethylhexyl)-9H-carbazol3-yl)-1,3,4-oxadiazole (co-phen) Fully dried compound 6 (3.21 g, 5.0 mmol) was added in anhydrous DMF (50.0 mL, dried with MgSO4 and distilled under vacuum at 70 °C), and then NaH (0.6 g, 25.0 mmol) was added in with stirring. After the mixture being stirred at room temperature for 1 h, C2H5Br (2.18 g, 20.0 mmol) was added in, and then heated to

110 °C and kept for 15 h in the presence of Ar. The cooled reaction mixture was poured in water and extracted with CHCl3 (3  80 mL). The organic phase was washed with water for several times and dried with anhydrous Na2SO4. After Na2SO4 being filtrated, the solvent was distilled off to afford the crude product. The crude product was further purified by silica gel column chromatography, eluting with CH2Cl2 and CH3OH (volume ratio, 100:1). Yield 80.0% (2.68 g), pale yellow powder. 1H NMR (400 MHz, CDCl3, 25 °C, ppm): d = 9.19–9.21 (m, 2H, ArH), d = 9.07 (d, 1H, 3J = 1.6 Hz, ArH), d = 8.89 (d, 1H, 3J = 1.6 Hz, ArH), d = 8.63 (d, 1H, 3J = 1.6 Hz, ArH), d = 8.42 (d, 2H, 3J = 6.8 Hz, ArH), d = 8.26 (d, 1H, 3J = 1.6 Hz, ArH), d = 8.20 (d, 1H, 3J = 1.6 Hz, ArH), d = 7.99 (d, 2H, 3J = 8.4 Hz, ArH), d = 7.72–7.75 (m, 2H, ArH), d = 7.46–7.54 (m, 3H, ArH), d = 7.33 (t, 1H, 3J = 2.4 Hz, ArH), d = 4.73 (q, 2H, 3J = 7.2 Hz, NACH2A of 2-ethylhexyl), d = 4.22 (q,

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H. Tang et al. / Optical Materials xxx (2014) xxx–xxx

Fig. 1. TG and DSC curves of two cationic iridium complexes. Inset: DSC curves.

2H, 3J = 7.2 Hz, ANACH2A of ethyl), d = 2.04–2.12 (m, 1H, NACACH—), d = 1.68 (t, 3H, 3J = 7.2 Hz, NACACH3), d = 1.25–1.45 (m, 8H, A(CH2)A and A(CH2)3A of 2-ethylhexyl), d = 0.94 (t, 3H, 3 J = 7.6 Hz, ACH3 of 2-ethylhexyl), d = 0.87 (t, 3H, 3J = 7.2 Hz, ACH3 of 2-ethylhexyl). ESI–MS (m/z): 670.3 [M+H]+, 692.3 [M+Na]+; Element Anal. Calc. For C43H39N7O (%): C, 77.10; H, 5.87; N, 14.64. Found (%): C, 76.68; H, 6.00; N, 14.70. 2.2.6. Synthesis of [(ppy)2Ir(co-phen)]PF6 The chloro-bridged dimer (ppy)2Ir(l-Cl)2Ir(ppy)2 (ppy: 2-phenylpyridine) was prepared according to the reported procedures [14]. A mixture of (ppy)2Ir(l-Cl)2Ir(ppy)2 (0.51 g, 0.40 mmol) and co-phen (0.56 g, 0.84 mmol) in glycol (30 mL) was heated at 150 °C in argon with stirring for 16 h. After being cooled to room temperature, 10 mL aqueous solution of NH4PF6 (0.3 mol L 1)

was added to obtain a yellow suspension of floccules. The resultant floccules were filtered, washed with water and then dried in vacuum. The pure product was obtained by silica gel column chromatography, eluting with a mixture of CH2Cl2 and acetonitrile (volume ratio, 10:1). Yield 88.2% (0.93 g), yellow solid. 1H NMR (400 MHz, CDCl3, 25 °C, ppm): d = 9.30 (d, 1H, 3J = 8.0 Hz, ArH), d = 9.12 (d, 1H, 3J = 8.0 Hz, ArH), d = 8.88 (d, 1H, 3J = 1.2 Hz, ArH), d = 8.40 (d, 1H, 3J = 8.4 Hz, ArH), d = 8.25–8.28 (m, 3H, ArH), d = 8.21 (d, 1H, 3J = 7.6 Hz, ArH), d = 7.94–8.03 (m, 5H, ArH), d = 7.70–7.85 (m, 5H, ArH), d = 7.45–7.53 (m, 4H, ArH), d = 7.28– 7.38 (m, 2H, ArH), d = 7.09 (t, 3H, 3J = 8.0 Hz, ArH), d = 6.98 (t, 3H, 3 J = 4.0 Hz, ArH), d = 6.86 (t, 1H, 3J = 8.0 Hz, ArH), d = 6.40–6.43 (m, 2H, ArH), d = 4.78–4.97 (m, 2H, ANACH2A of 2-ethylhexyl), d = 4.22 (q, 2H, 3J = 7.6 Hz, NACH2A of ethyl), d = 2.05–2.15 (m, 1H, NACACH—), d = 1.69 (t, 3H, 3J = 7.2 Hz, NACACH3), d = 1.25– 1.45 (m, 8H, A(CH2)A and A(CH2)3A of 2-ethylhexyl), d = 0.94 (t, 3H, 3J = 7.6 Hz, ACH3 of 2-ethylhexyl), d = 0.86 (t, 3H, 3J = 7.2 Hz, ACH3 of 2-ethylhexyl). ESI–MS (m/Z): 1170.4 [M–PF6]+; Anal. Calc. for C65H55F6IrN9OP (%): C, 59.35; H, 4.21; N, 9.58. Found (%): C, 59.24; H, 4.27; N, 9.80.

2.2.7. Synthesis of [(npy)2Ir(co-phen)]PF6 The chloro-bridged dimer (npy)2Ir(l-Cl)2Ir(npy)2 (npy: 2(naphthalen-1-yl)pyridine) also was prepared according to the reported procedures [19,22]. Then [(npy)2Ir(co-phen)]PF6 was synthesized via the same method as [(ppy)2Ir(co-phen)]PF6. Yield 89.3%, orange solid. 1H NMR (400 MHz, CDCl3, 25 °C, ppm): d = 9.30 (d, 1H, 3J = 8.0 Hz, ArH), d = 9.09 (d, 1H, 3J = 8.4 Hz, ArH), d = 8.87 (d, 1H, 3J = 1.6 Hz, ArH), d = 8.32–8.38 (m, 4H, ArH), d = 8.25 (d, 3H, 3 J = 7.2 Hz, ArH), d = 8.21 (d, 2H, 3J = 7.6 Hz, ArH), d = 8.16 (d, 1H, 3 J = 8.4 Hz, ArH), d = 7.68–7.97 (m, 8H, ArH), d = 7.25–7.60 (m, 12H, ArH), d = 7.10 (t, 1H, 3J = 8.4 Hz, ArH), d = 6.98 (t, 1H,

Fig. 2. UV–Vis absorption spectra. (a) ligand co-phen in CH2Cl2 at 1.0  10 5 mol L 1; (b) two cationic iridium(III) complexes in CH2Cl2 at 1.0  10 5 mol L 1 (inset: a partial enlarged view of the spectra from 425 nm to 600 nm); (c) neat films of two cationic iridium(III) complexes; (d) two cationic iridium(III) complexes doped in PVK films (4.0 wt%, on quartz plates).

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layer in a glove box containing less than 10 ppm oxygen and moisture, then baked at 120 °C in inert atmosphere for 20 min to remove solvent residue (chlorobenzene). Then an electron injection layer of CsF and subsequent aluminum cathode layer were thermally evaporated with a base pressure of 3  10 4 Pa. The thickness of the spin-coated films was measured by an Alfa Step 500 surface profilometer, and the thickness of the thermal deposition layers was monitored by a quartz crystal thickness/ratio monitor (Model: STM-100/MF, Sycon). The current density– luminance–voltage characteristics were measured by a Keithley 236 sourcemeasurement unit and a calibrated silicon photodiode. EL spectra and CIE coordinate were recorded by a spectrophotometer (SpectraScan PR-705, Photo Research). 3. Results and discussion 3.1. Thermal properties

Fig. 3. PL spectra of two cationic iridium(III) complexes: (a) in CH2Cl2 at 1  10 5 mol L 1; (b) in films of pure complexes (on quartz plates); (c) doped in PVK films (4.0 wt%, on quartz plates).

3

J = 8.8 Hz, ArH), d = 6.75 (s, 2H, ArH), d = 4.78–4.97 (m, 2H, ANACH2A of 2-ethylhexyl), d = 4.21 (q, 2H, 3J = 7.6 Hz, NACH2A of ethyl), d = 2.05–2.15 (m, 1H, NACACH—), d = 1.67 (t, 3H, 3 J = 7.2 Hz, NACACH3), d = 1.25–1.45 (m, 8H, A(CH2)A and A(CH2)3A of 2-ethylhexyl), d = 0.94 (t, 3H, 3J = 7.6 Hz, ACH3 of 2ethylhexyl), d = 0.86 (t, 3H, 3J = 7.2 Hz, ACH3 of 2-ethylhexyl). ESI–MS (m/z): 1270.4 [M PF6]+; Anal. Calc. for C73H59F6IrN9OP (%): C, 61.94; H, 4.20; N, 8.91. Found (%): C, 61.68; H, 4.24; N, 8.98. 2.3. Fabrication and measurements of EL devices Patterned indium tin oxide (ITO) coated glass with a sheet resistance of 15–20 X/h was used as substrates and anodes. Before being used, the glass was sufficiently cleaned with acetone, detergent, distilled water and 2-propanol in ultrasonic baths sequentially, and then treated with oxygen plasma for 4 min. Then an anode buffer layer of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT: PSS) was spin-coated on the glass and dried by baking in a vacuum oven at 80 °C for 8 h. The light-emitting layer composed of different weight ratios of poly(N-vinylcarbazole) (PVK, used as host materials), 2-(4-biphenylyl)-5- (4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, used as electron-transporting materials) and cationic iridium(III) complexes (used as phosphorescent dopants) was spin-coated on the buffer

The TG and DSC (inset) curves of two cationic iridium complexes are shown in Fig. 1. The TG results show that both [(ppy)2 Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6 have high thermal stability with a same 5% weight-reduction temperature (DT5%) of 402 °C. Because the coordination bond is the weakest bond in such heteroleptic complexes, neutral auxiliary ligands usually lost firstly in thermal decomposition [19,23], and co-phen have been used equally as the neutral auxiliary ligand in [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6, so both of them have a same thermal decomposition temperature. Such high thermal decomposition temperatures should be caused by the big bulk, irregularity of the auxiliary ligand co-phen, and especially, the intermolecular cross-linking of 2-ethylhexyl on co-phen, which can hinder the loss of the auxiliary ligand. On DSC curves of two cationic iridium complexes, both of them show no melting peaks or crystallization peaks but only high glass-transition temperatures (Tg) at 207 °C and 241 °C respectively. The results suggest that both of them have good amorphous nature, mainly due to the dendritic and long chain-like 2-ethylhexyl [4,19]. High thermal decomposition temperatures and good amorphous nature with high Tg of the complexes suggest their thermal decomposition, crystallization and phase disengagement all are difficult to occur, which are desirable for high stability and good performance of their EL devices [19,24]. 3.2. UV–Vis absorption and PL spectra The UV–Vis absorption spectra of two cationic iridium(III) complexes in three different states and the ligand co-phen (inset) in CH2Cl2 at 1.0  10 5 mol L 1 are shown in Fig. 2. The absorption spectra of [(ppy)2Ir(co-phen)]PF6 in CH2Cl2 (as shown in Fig. 2(b), the maximum absorption wavelength (kabs,max) is 276 nm and the corresponding molar extinction coefficient (e276nm) is 7.90  104 L mol 1 cm 1) should mainly be caused by the absorption of the ancillary ligand co-phen (kabs,max = 218 nm and e218nm = 1.08  105 L mol 1 cm 1), because the peaks on their absorption spectra are similar to each other in amount, location and width, the difference between them probably is caused by the coordination and the effect of 2-phenylpyridine (ppy). Relatively, the difference between [(npy)2Ir(co-phen)]PF6 (kabs,max = 2 85 nm and e285nm = 1.14  105 L mol 1 cm 1) and co-phen is more larger, maybe due to the absorption of [(npy)2Ir(co-phen)]PF6 being more effected by the bigger conjugated system of 2-(naphthalen-1-yl)pyridine (npy). Besides the strong absorption bands of two complexes at 190–400 nm which can be assigned to the spin-allowed 1p–p⁄ transition of the ligands, the extra weak absorption bands of the complexes after 400 nm (400–534 nm for [(ppy)2Ir(co-phen)]PF6 and 400–548 nm for [(npy)2Ir

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H. Tang et al. / Optical Materials xxx (2014) xxx–xxx

Fig. 4. Cyclic voltammograms of two cationic iridium complexes in CH3CN solution at 1  10 100 mV s 1. Potentials were recorded versus Fc+/Fc.

3

mol L

1

, 0.1 mol L

1

n-Bu4NPF6 as supporting electrolyte, at scan rate of

Table 1 Thermal, optical, electrochemical data and energy levels of the cationic iridium complexes.

a b c

Complex

Tg (°C)

DT5% (°C)

kmax,em (nm)a

E1/2,ox (V)b

[(ppy)2Ir(co-phen)]PF6 [(npy)2Ir(co-phen)]PF6

207 241

402 402

567 595

0.60 0.61

E1/2,red (V)b 1.81 1.72

HOMO (eV)c

LUMO (eV)c

5.40 5.41

Measured in CH2Cl2 at 1.0  10 5 mol L 1. Measured in Ar–saturated CH3CN solution at 1  10 3 mol L 1, 0.1 mol L 1 n-Bu4NPF6 as supporting electrolyte, scan rate 100 mV s HOMO = e (E1/2,ox) + ( 4.8) eV, LUMO = e (E1/2,red) + ( 4.8) eV, Eg = LUMO HOMO.

2.99 3.08

Eg (eV)c 2.41 2.33

1

, versus Fc+/Fc couple.

Fig. 5. EL spectra of the devices ITO/PEDOT: PSS/PVK: PBD: cationic iridium complex (mass ratios 100: 40: x, x = 1.0, 2.0, and 4.0)/CsF/Al.

(co-phen)]PF6) are overlapping singlet and triplet metal–ligand charge transfer (1MLCT and 3MLCT) and 3p–p⁄ transition absorption bands, due to the strong spin–orbit coupling induced by the iridium heavy atom [24,25]. As shown in Fig. 2(c), By comparison with the UV–Vis absorption spectra of two cationic iridium(III) complexes in solution, the absorption spectra of pure complex solid films show similar lineshapes but several 5–12 nm redshift, such redshift may be caused by the larger conjugated systems, due to the coplanarity being improved when the swirling of the single bonds between aromatic rings being restrained in the solid state. As shown in Fig. 2(d), the absorption spectra of two com-

plexes doped in PVK films are completely different from (b) and (c), such absorption actually belong to PVK instead of cationic iridium complexes because 4.0 wt% is a very low doping concentration and the absorption of complexes have been covered by that of PVK. PL spectra of two cationic iridium(III) complexes were measured in three states: (a) in CH2Cl2 at 1  10 5 mol L 1, (b) in films of pure complexes, (c) doped in PVK at 4.0 wt%, and the results are shown in Fig. 3. The PL spectrum of [(ppy)2Ir(co-phen)]PF6 in CH2Cl2 solution is mainly located at yellow light band from 500 nm to 725 nm with the maximum emission wavelength (kmax,em) of 567 nm, that of [(npy)2Ir(co-phen)]PF6 is mainly

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7

H. Tang et al. / Optical Materials xxx (2014) xxx–xxx Table 2 Performance of the EL devices. 2

Doping concentration (mass ratio)

Von (V)

Lmax (cd m

[(ppy)2Ir(cophen)]PF6

1.0 2.0 4.0

3.8 5.4 7.0

7598 12567 9356

17.3 15.1 9.6

9.8 5.5 2.4

[(npy)2Ir(cophen)]PF6

1.0 2.0 4.0

4.8 6.0 6.8

8529 11032 10145

20.4 15.6 11.3

10.3 5.5 3.5

located at orange-red light band about from 525 nm to 750 nm with the kmax,em of 595 nm. The PL spectra of two complexes in pure solid complex films are very similar to those in solution, the kmax,em of [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6 are 568 nm and 593 nm respectively, and also are nearly identical to those in solution, which suggest that there is no strong intermolecular interactions and no aggregates of the iridium complexes in their solid films, and suggests the PL emission came from the same energy gaps as the single iridium complex molecules. However, compared with the PL spectra of the complexes in the two aforesaid cases, those of [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(cophen)]PF6 doped in PVK both exhibit obvious blue shifts, though the lineshapes and bandwidths of their PL spectra show no obvious change. The PL spectrum of [(ppy)2Ir(co-phen)]PF6 doped in PVK is mainly located at yellowish green light band from 465 nm to 700 nm with the kmax,em of 534 nm, that of [(npy)2Ir(co-phen)]PF6 is mainly located at yellow light band from 480 nm to 700 nm with the kmax,em of 551 nm. Compared with the PL spectra of the complexes in solution, those of [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6 doped in PVK have blue shifts of 33 nm and 44 nm respectively. Such blue shifts of cationic iridium complexes also have been found in previous research [14], and are assigned to weaker interactions of the complexes in solid PVK film at a low concentration (i.e. diluted effect) [26–28], due to the decreasing polarity and polarisability between themselves caused by dilution and solidification of PVK.

)

LEmax (cd A

1

Complex

)

PEmax (lm W

1

)

QEmax (%)

kmax (nm)

CIE (at 12 mA m

9.3 8.2 5.2

534 538 540

(0.36, 0.57) (0.37, 0.57) (0.38, 0.57)

11.4 8.7 6.3

552 550 554

(0.43, 0.53) (0.44, 0.54) (0.45, 0.52)

2

)

3.4. EL properties In order to investigate the EL properties of two cationic iridium complexes, both of them were used as phosphorescent dopants in multilayer PLEDs. The configuration of these EL devices is ITO/ PEDOT: PSS (40 nm)/PVK: PBD: cationic iridium complex (mass ratios 100: 40: x, x = 1.0, 2.0, and 4.0) (80 nm)/CsF (1.5 nm)/Al (120 nm), the key emitting layer is composed of PVK, PBD and cationic iridium complex in different mass ratios. The EL spectra of the devices are shown in Fig. 5. For [(ppy)2Ir(co-phen)]PF6, three devices with it all exhibit strong emission peaks at 460–700 nm with the maximum emission wavelengths (kmax,em) around 536 nm (specific data being listed in Table 2), which are primarily consistent with the PL spectrum of it doped in PVK at 4.0 wt%. For three devices using [(npy)2Ir(co-phen)]PF6, the EL spectra also are primarily consistent with the PL spectrum of [(npy)2Ir(cophen)]PF6 doped in PVK at 4.0 wt% and exhibit strong emission peaks at 480–700 nm with kmax,em around 552 nm. Besides these strong emission peaks originated from the iridium complexes, very weak emission at 380–480 nm from the PVK–PBD exciplexes or/ and excimers of PVK still can be observed at low doping concentration (at x = 1.0) due to the excitons of PVK not being consumed

3.3. Electrochemical properties and HOMO–LUMO energy levels The electrochemical properties, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of two cationic iridium(III) complexes were investigated by cyclic voltammetry (CV), and the resultant CV curves are shown in Fig. 4. Every complex has a reversible one-electron oxidation couple at relatively low positive potential, which can be assigned to metal-centered IrIII/IrIV oxidation couple [13,29], the corresponding E1/2,ox of [(ppy)2Ir(co-phen)]PF6 and [(npy)2 Ir(co-phen)]PF6 are 0.60 V and 0.61 V respectively. In addition, every complex has an irreversible oxidation couple at relatively high positive potential, which maybe originate from the electrondonating carbazole unit [29]. At negative potentials, each iridium complex has a reversible reduction couple, the E1/2,red are 1.81 V and 1.75 V respectively. Such reversible reduction couple can be assigned to orbitals centered mainly on the Ir-phen fragment and presumably caused by the reduction of the phen unit [30]. The values of HOMOs, LUMOs and energy gaps (Eg) can be calculated from electrochemical data using HOMO = e (E1/2,ox) + ( 4.8) eV, LUMO = e (E1/2,red) + ( 4.8) eV, Eg = LUMO HOMO and are list in Table 1. The HOMO levels of [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6 are 5.40 eV and 5.41 eV, the LUMO levels of them are 2.99 eV and 3.08 eV respectively, these data lie between the HOMO ( 5.80 eV) and LUMO ( 2.2 eV) levels of PVK which suggest PVK is suitable host material for them being used as phosphorescent dopants in EL devices [16].

Fig. 6. Luminance (above) and current density (below) versus voltage characteristics of the devices ITO/PEDOT: PSS/PVK: PBD: cationic iridium complex (mass ratios 100: 40: x, x = 1.0, 2.0, and 4.0)/CsF/Al.

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[(ppy)2Ir(co-phen)]PF6

(a)

x = 1.0 x = 2.0 x = 4.0

15 10 5 0 0

50

100

150

200

-1

-1

20

Current efficiency (cd⋅A )

H. Tang et al. / Optical Materials xxx (2014) xxx–xxx

Current efficiency (cd⋅A )

8

25

(b)

x = 1.0 x = 2.0 x = 4.0

20 15 10 5 0

250

0

-2

100

150

200

250

Current density (mA⋅cm ) 15

x = 1.0 x = 2.0 x = 4.0

9 6 3 0 0

50

100

150

200

250

-2

Current density (mA⋅cm )

-1

[(ppy)2Ir(co-phen)]PF6

(c)

Power efficiency (lm⋅W )

-1

Power efficiency (lm⋅W )

50

-2

Current density (mA⋅cm ) 12

[(npy)2Ir(co-phen)]PF6

12

[(npy)2Ir(co-phen)]PF6

(d)

x = 1.0 x = 2.0 x = 4.0

9 6 3 0 -3

0

50

100

150

200

250

-2

Current density (mA⋅cm )

Fig. 7. Current efficiency and power efficiency versus current density characteristics of the devices ITO/PEDOT: PSS/PVK: PBD: cationic iridium complex (mass ratios 100: 40: x, x = 1.0, 2.0, and 4.0)/CsF/Al.

completely [19,29], and such weak emission peaks disappear at higher doping concentration (at x = 2.0 and 4.0). The luminance and current density versus voltage characteristics of the EL devices using two cationic iridium complexes at different doping concentrations are shown in Fig. 6, current efficiency versus current density characteristics are shown in Fig. 7, and the performance data are summarized in Table 2. The turn-on voltages (Von) of the EL devices increase with the doping concentrations, the large Von values (especially at high doping concentrations) could be due to the fact that the complexes could act as deep traps for electrons or holes (less likely for holes) and reduce carrier mobility. The luminance and current density both increase with the voltage, however, the luminance changing with the doping concentrations is not completely consistent with that of current density, the relationship between the maximum current densities and doping concentrations is not definite, moreover, no inflection points of maximum current density versus applied voltage emerge, but the maximum values of the luminance were obtained at x = 2.0, and concentration quenching happens at higher doping concentrations. For [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6, the maximum luminances (Lmax) are 12567 cd m 2 and 11032 cd m 2 respectively. For [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(cophen)]PF6, the maximum luminance efficiencies (LEmax) were obtained both at x = 1.0, are 17.3 cd A 1 and 20.4 cd A 1 respectively, the maximum power efficiencies (PEmax) were 9.8 lm W 1 and 10.3 lm W 1, and the corresponding maximum external quantum efficiencies (QEmax) are 9.3% and 11.4% respectively, however, the efficiencies at x = 1.0 decline quickly with the increasing current densities because of triplet–triplet annihilation [31] and field-induced quenching effects [32], and the efficiencies at x = 2.0 are higher than those at x = 1.0 under high current densities. Together with the weak emission of PVK–PBD exciplexes or/and excimers of PVK still emerging at x = 1.0 doping concentration, based on an overall evaluation of luminance, efficiency and current density, x = 2.0 is an optimal doping concentration. The

Commission Internationale de L’Eclairage (CIE) color coordinates of the devices using [(ppy)2Ir(co-phen)]PF6 are around (0.37, 0.57), and those of [(ppy)2Ir(co-phen)]PF6 are around (0.44, 0.54), both corresponding to the yellow green region. Although the EL performances of the aforementioned PLEDs using [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6 as phosphorescent dopants are still behind those of many already reported iridium complexes, especially some up-to-date neutral iridium complexes [2,3,33], however, the EL performances of them are better than or near to those of many reported cationic iridium complexes [13–20], also are better than those of some neutral iridium complexes reported recently [34–36]. Better EL performances should be obtained if their EL devices were further optimized. 4. Conclusions Two novel cationic iridium complexes [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6 with a 1,10-phenanthroline derivative containing a bipolar carbazole–oxadiazole unit as the auxiliary ligand were synthesized, both of them have good solubility, thermal stability, amorphous nature and miscibility with host materials. The PLEDs with them exhibited good EL performance, the maximum luminances of them are 12567 cd m 2 and 11032 cd m 2, the maximum luminance efficiencies are 17.3 cd A 1 and 20.4 cd A 1, the maximum power efficiencies were 9.8 lm W 1 and 10.3 lm W 1, and the maximum external quantum efficiencies are 9.3% and 11.4% respectively. Acknowledgements This work was supported by National Nature Science Foundation of China (No. 21262046) and Key Scientific Research Fund of Yunnan Province Education Department (No. 2011Z003).

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H. Tang et al. / Optical Materials xxx (2014) xxx–xxx

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