Efficient yellow–green light-emitting cationic iridium complexes based on 1,10-phenanthroline derivatives containing oxadiazole-triphenylamine unit

Dyes and Pigments 100 (2014) 79e86

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Efficient yellowegreen light-emitting cationic iridium complexes based on 1,10-phenanthroline derivatives containing oxadiazoletriphenylamine unit Huaijun Tang a, b, Yanhu Li a, Qiliang Chen c, Bing Chen a, Qiquan Qiao c, Wei Yang a, *, Hongbin Wu a, Yong Cao a a

State Key laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China Engineering Laboratory of Polylactic Acid-Based Functional Materials of Yunnan, School of Chemistry and Biotechnology, Yunnan University of Nationalities, Kunming 650500, China c Center for Advanced Photovoltaics, Electrical Engineering Department, South Dakota State University, Brookings, SD 57007, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 May 2013 Received in revised form 16 July 2013 Accepted 20 July 2013 Available online 29 July 2013

Cationic iridium(III) complexes with 1,10-phenanthroline (phen) derivatives containing oxadiazoletriphenylamine, oxadiazole or triphenylamine unit used as neutral auxiliary ligands, with the formula of [Ir(ppy)2(op-phen)][PF6] (C1), [Ir(ppy)2(o-phen)][PF6] (C2), and [Ir(ppy)2(p-phen)][PF6] (C3) (ppy ¼ 2phenylpyridine) were synthesized, respectively. The complexes exhibit high thermal stability and were used as phosphorescent dopants in polymer light-emitting diodes (PLEDs) with the configuration of ITO/ PEDOT: PSS/PVK (70): PBD (30): complex (x, wt.%)/TPBI/CsF/Al. The complexes C1, C2 and C3 exhibit high electroluminescent performances with the maximum luminance efficiencies of 30.6 cd/A, 28.4 cd/A and 26.3 cd/A, respectively. The Commission Internationale de L’Eclairage (CIE) color coordinates of all complexes are (0.41, 0.55) corresponding to the yellow-green region. The complex C1 containing bipolar unit of oxadiazole-triphenylamine exhibits higher efficiency due to its balance of hole and electron fluxes. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Cationic iridium complex Phenanthroline Triphenylamine Oxadiazole Yellowegreen emission Polymer light-emitting diode

1. Introduction Cationic iridium complexes have attracted considerable interest in recent years due to their unique photophysical properties and applications in light-emitting electrochemical cells (LECs) [1e 9] and organic light-emitting diodes (OLEDs, including smallmolecule and polymer light-emitting diodes) [10e16]. LECs usually require only a single emissive layer without extra layers and use ionic charges to facilitate electronic charge injection from the electrodes into the active layer [1e9]. So LECs can offer several advantages such as simpler device architecture, independence on active layer thickness and being used with air-stable electrodes [1e 9]. Nevertheless, some grievous drawbacks such as slow response, severe excited-state quenching due to being composed of neat film of emissive materials, have hindered their practical applications [10e16]. Comparatively, such drawbacks can be significantly

* Corresponding author. Tel.: þ86 20 87114346x17; fax: þ86 20 87110606. E-mail address: [email protected] (W. Yang). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.07.029

suppressed in OLEDs [10e16]. For example, Plummer et al. used a cationic iridium complex as yellow-emitting dopant in polymer light-emitting diodes (PLEDs) which achieved a maximum current efficiency of 22.5 cd/A [10]. Wong et al. reported a sublimable cationic iridium complex being used as dopant in small-molecule OLEDs with a maximum current efficiency of 19.7 cd/A [11]. Up to now, the cationic iridium complexes emitting from blue to red [10e 16], white [13] and even near-infrared [15] in OLEDs all were reported. As being mentioned above, the performances of cationic iridium complexes in OLEDs are noticeable and comparable with those of some neutral iridium complexes. Moreover, cationic iridium complexes can be more easily synthesized and purified with much higher yield (usually above 80%) than neutral iridium complexes [1e16]. However, there are still some disadvantages of cationic iridium complexes used as phosphorescent dopants in OLEDs: 1) cationic iridium complexes, composed of organic complex cation and inorganic acid anion such as PF6, ClO4and BF4, usually are incompatible with organic host materials and result in the phase segregation between the host and dopants [11]; 2) cationic iridium

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H. Tang et al. / Dyes and Pigments 100 (2014) 79e86

complexes usually have low solubility in ordinary organic solvents such as toluene, chlorobenzene, chloroform, although they are probably soluble in strong polar solvents such as acetonitrile and N,N-dimethylformamide (DMF), however, strong polar solvents can erode the bottom organic layer in solution-processing and result in decreasing the device performances. Moreover, it is also difficult to fabricate OLEDs via vacuum-deposition due to poor sublimability of cationic iridium complexes [11]. To function as effective phosphorescent dopants in OLEDs like neutral iridium complexes, above mentioned issues of cationic iridium complexes need to be addressed. Although the inorganic acid anions such as PF6, ClO4 and BF4 were found to facilitate electronic charge injection from the electrodes into the active layer in LECs [1e9], however, this effect can be neglected in OLEDs because the doping concentration usually is very low and carrier transport layers are used. So the function and mass ratio of inorganic acid anions can be disregarded in designing cationic iridium complexes for OLEDs. Theoretically, some properties of cationic iridium complexes such as solubility in organic solvents, miscibility with organic host materials and amorphous nature would be improved when using irregular and solubilizing organic ligands, which are advantages to their being used in OLEDs [17]. Similar to neutral iridium complexes, the device performances of cationic iridium complexes would be improved when carrier-transporting units such as triphenylamine, carbazole and oxadiazole were contained in their ligands [17e19]. 1,10-Phenanthroline (phen) derivatives are conventional auxiliary ligands for luminescent complexes due to its large conjugated area and good electron fluidity [3,20]. However, some properties such as carrier-transporting property, solubility and thermal stability still need to be further improved [17]. Recently, orange-red lightemitting cationic iridium complexes with 1,10-phenanthroline derivatives containing oxadiazole or carbazole unit were synthesized according to aforementioned strategies [21]. In this work, 1,10phenanthroline was further modified by bipolar carrier-transporting unit of oxadiazole-triphenylamine as auxiliary ligand in cationic iridium complex which was used as phosphorescent dopant in PLEDs. 2. Experimental 2.1. General information Chemicals and reagents were obtained from commercial sources and used without further purification unless otherwise stated. Column chromatography was carried out on silica gel (200e 300 mesh). Yields refer to the isolated pure compound. 1H NMR and 13 C NMR spectra were recorded on a Bruker AV300 spectrometer operating at 300 MHz or a Bruker AV400 spectrometer operating at 400 MHz, tetramethylsilane (TMS) was used as internal standard. Mass spectra (MS) were obtained from a Bruker Esquire HCT PLUS liquid chromatography mass spectrometer (LC-MS) with an electrospray ionization (ESI) interface using MeCN as the matrix solvent. The IR spectra were recorded in KBr on a Shimadzu FTIR8201PC spectrometer. Elemental analyses (EA) were performed on Vario EL Elemental Analysis Instrument. UVeVis absorption spectra were recorded on a HP8453E spectrophotometer. Photoluminescence (PL) spectra were recorded on a Fluorologe3 spectrophotometer. Differential scanning calorimetry (DSC) curves were obtained on a Netzsch DSC200 analyzer via two heating cycles at a heating rate of 10  C min1 in N2 atmosphere. Thermogravimetry (TG) curves were obtained on a Netzsch TG209 thermal analyzer at a heating rate of 20  C min1 in N2. Cyclic voltammetry (CV) was performed on a computer-controlled CHI800C electrochemical analyzer with the cationic iridium complexes dissolved in

anhydrous and Ar-saturated MeCN solutions containing 0.1 mol L1 tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte at a scanning rate of 50 mV s1. A glass carbon electrode was used as working electrode, Ag/AgCl electrode as reference electrode, and platinum wire as counter electrode. Ferrocene (4.8 eV under vacuum) was used as the internal standard. 2.2. Synthesis and characterization 2.2.1. Synthesis of dichloro-bridged iridium dimer ([(ppy)2Ir(mCl)2Ir(ppy)2]) [2,4] IrCl3$3H2O with 2.2 equiv of 2-phenylpyridine (ppy) in a mixed solvent of 2-ethoxyethanol and deionized water (3:1) was heated at 110  C under Ar for 24 h. The yellowegreen solid product was directly used for the next step after being dried in vacuum without further purification and characterization. 2.2.2. Synthesis of 1,10-phenanthroline derivatives 4-(5-(4-(1-Ethyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl) phenyl)-1,3,4-oxadiazol-2-yl)-N,N-diphenylaniline (op-phen): synthesized according to the previously reported procedures [22]. 1 H NMR (400 MHz, CDCl3, 25  C, ppm): 9.16 (d, 2H, 3J ¼ 2.4 Hz, ArH), 9.06 (d, 1H, 3J ¼ 7.2 Hz, ArH), 8.62 (d, 1H, 3J ¼ 8.4 Hz, ArH), 8.33 (d, 2H, 3J ¼ 8.0 Hz, ArH), 7.95 (t, 4H, 3J ¼ 8.8 Hz, ArH), 7.70e7.75 (m, 2H, ArH), 7.33 (t, 4H, 3J ¼ 7.6 Hz, ArH), 7.10e7.20 (m, 8H, ArH), 4.69 (q, 2H, 3J ¼ 7.2 Hz, eCH2e), 1.64 (t, 3H, 3J ¼ 7.2 Hz, eCH3); 13C NMR (400 MHz, CDCl3, 25  C, ppm): 165.2, 163.4, 152.3, 151.4, 149.0, 146.7, 144.8, 144.1, 136.8, 133.0, 130.8, 130.6, 129.8, 128.4, 128.3, 127.3, 125.9, 125.5, 125.1, 124.7, 123.8, 123.0, 121.0, 120.0, 115.6, 41.96, 16.15; FTIR (KBr, cm1): 3443, 2360, 2333, 2027, 1657, 1593, 1487, 1418, 1384, 1334, 1269, 1118, 1070, 1042, 1008, 989, 864, 818, 740, 697, 620, 546; Element anal. calcd. for C41H29N7O (%): C, 77.46; H, 4.60; N, 15.42; found (%): C, 77.52; H, 4.55; N, 15.47. 2-(4-Ttert-butyl)phenyl)-5-(4-(1-ethyl-1H-imidazo[4,5-f][1,10] phenanthrolin-2-yl)phenyl)-1,3,4-oxadiazole (o-phen): synthesized according to the previously reported procedures [20]. 1H NMR (400 MHz, CDCl3, 25  C, ppm): 9.17e9.18 (m, 2H, ArH), 9.05 (dd, 1H, 3 J ¼ 4.0 Hz, 4J ¼ 1.8 Hz, ArH), 8.62 (dd, 1H, 3J ¼ 8.4 Hz, 4J ¼ 1.6 Hz, ArH), 8.36 (dd, 2H, 3J ¼ 6.8 Hz, 4J ¼ 1.8 Hz, ArH), 8.08e8.11 (m, 2H, ArH), 7.94e7.97 (m, 2H, ArH), 7.70e7.73 (m, 2H, ArH), 7.56e7.59 (m, 2H, ArH), 4.72 (q, 2H, 3J ¼ 7.2 Hz, CH2), 1.65 (t, 3H, 3 J ¼ 7.2 Hz, CH3), 1.38 (s, 9H, eC(CH3)3); 13C NMR (400 MHz, CDCl3, 25  C, ppm): 165.2, 163.9, 155.8, 152.2, 149.3, 148.1, 145.1, 144.4, 137.1, 133.4, 130.6, 130.5, 128.2, 127.4, 127.0, 126.3, 125.4, 125.2, 124.2, 123.8, 122.8, 121.0, 120.0, 41.96, 35.27, 31.24, 16.18; FTIR (KBr, cm1): 3445, 2365, 2335, 2026, 1642, 1612, 1572, 1444, 1384, 1353, 1269, 1115, 1052, 1011, 861, 743, 619, 569; Element anal. calcd. for C33H28N6O (%): C, 75.55; H, 5.38; N, 16.02; found (%):C, 75.18; H, 5.47; N, 15.77. 4-(1-Ethyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-N,Ndiphenylaniline (p-phen): synthesized according to the previously reported procedures [23]. 1H NMR (400 MHz, CDCl3, 25  C, ppm): 9.11e9.20 (m, 2H, ArH), 9.04 (dd, 1H, 3J ¼ 8.0 Hz, 4J ¼ 1.6 Hz, ArH), 8.55 (dd, 1H, 3J ¼ 8.4 Hz, 4J ¼ 1.4 Hz, ArH), 7.62e7.69 (m, 2H, ArH), 7.58 (d, 2H, 3J ¼ 8.6 Hz, ArH), 7.26e7.36 (m, 4H, ArH), 7.16e7.21 (m, 6H, ArH), 7.08 (t, 2H, 3J ¼ 7.2 Hz, ArH), 4.64 (q, 2H, 3 J ¼ 7.2 Hz, CH2), 1.59 (t, 3H, 3J ¼ 7.2 Hz, CH3); 13C NMR (400 MHz, CDCl3, 25  C, ppm): 153.6, 149.5, 148.9, 147.6, 147.2, 144.7, 144.2, 136.7, 130.6, 130.4, 129.6, 128.0, 125.3, 124.7, 124.2, 123.9, 123.5, 123.0, 122.6, 122.3, 120.0, 41.96, 16.12; FTIR (KBr, cm1): 3380, 2357, 2330, 2051, 2027, 1662, 1594, 1490, 1467, 1443, 1384, 1333, 1270, 1118, 1042, 1010, 865, 741, 700, 622, 548; Element anal. calcd. for C33H25N5 (%): C, 80.63; H, 5.13; N, 14.25; found (%):C, 80.54; H, 5.17; N, 14.29.

H. Tang et al. / Dyes and Pigments 100 (2014) 79e86

2.2.3. Synthesis of cationic iridium complexes Dichloro-bridged iridium dimer [(ppy)2Ir(m-Cl)2Ir(ppy)2] (322 mg, 0.30 mmol) and 1,10-phenanthroline derivatives (0.63 mmol) were added in 1,2-ethanediol (30 mL). Then the mixture was kept at 150  C in Ar atmosphere with stirring for 16 h. After being cooled to room temperature, the mixture became orange color, then 10 mL aqueous solution of NH4PF6 (0.3 mol L1) was added to obtain a yellow suspension. The solid was filtered, then washed with water and dried in vacuum. The crude product was purified by column chromatography on silica gel, eluting with CH2Cl2/MeCN (volume ratio, 10:1). Complex [Ir(ppy)2(op-phen)][PF6], C1: yellow solid, yield: 90% (690 mg). 1H NMR (300 MHz, DMSO-D6, 25  C, ppm): 9.23 (d, 2H, 3 J ¼ 8.3 Hz, ArH), 8.36 (d, 2H, 3J ¼ 8.3 Hz, ArH), 8.21e8.30 (m, 4H, ArH), 8.01e8.16 (m, 4H, ArH), 7.90e7.98 (m, 6H, ArH), 7.50e7.55 (m, 2H, ArH), 7.42 (t, 4H, 3J ¼ 7.8 Hz, ArH), 7.17e7.23 (m, 6H, ArH), 6.96e 7.09 (m, 8H, ArH), 6.31 (dd, 2H, 3J ¼ 7.2 Hz, 4J ¼ 3.4 Hz, ArH); 4.83 (q, 2H, 3J ¼ 7.0 Hz, eCH2e), 1.58 (t, 3H, 3J ¼ 7.0 Hz, eCH3); FTIR (KBr, cm1): 3443, 2361, 2334, 2048, 2026, 1649, 1604, 1447, 1383, 1353, 1269, 1117, 1069, 1005, 857, 737, 621, 549; ESIeMS (m/Z): 1136.5 [M  PF6]þ; Element anal. calcd. for C63H45F6IrN9OP (%): C, 59.06; H, 3.54; N, 9.84; found (%): C, 58.82; H, 3.41; N, 9.56. Complex [Ir(ppy)2(o-phen)][PF6], C2: orange solid, yield: 85% (600 mg). 1H NMR (300 MHz, DMSO-D6, 25  C, ppm): 9.24 (dd, 2H, 3 J ¼ 8.3 Hz, 4J ¼ 1.2 Hz, ArH), 8.41 (d, 2H, 3J ¼ 8.3 Hz, ArH), 8.21e8.30 (m, 4H, ArH), 8.08e8.13 (m, 6H, ArH), 7.90e7.99 (m, 4H, ArH), 7.68 (d, 2H, 3J ¼ 8.3 Hz, ArH), 7.50e7.53 (m, 2H, ArH), 6.96e7.08 (m, 6H, ArH), 6.29e6.33 (m, 2H, ArH), 4.83 (q, 2H, 3J ¼ 7.0 Hz, eCH2e), 1.59 (t, 3H, 3J ¼ 7.0 Hz, eCH3), 1.35 (s, 9H, eC(CH3)3); FTIR (KBr, cm1): 3442, 2361, 2335, 2026, 1648, 1608, 1580, 1443, 1356, 1268, 1154, 1117, 1069, 1008, 856, 736, 650, 550; ESIeMS (m/Z): 1025.5 [M  PF6]þ; Element anal. calcd. for C55H44F6IrN8OP (%): C, 56.45; H, 3.79; N, 9.58; found (%): C, 56.33; H, 3.88; N, 9.64. Complex [Ir(ppy)2(p-phen)][PF6], C3: orange solid, yield: 88% (600 mg). 1H NMR (300 MHz, CDCl3, 25  C, ppm): 9.29 (s, 1H, ArH), 9.13 (d, 1H, 3J ¼ 8.7 Hz, ArH), 8.23 (dd, 2H, 3J ¼ 5.0 Hz, 4J ¼ 1.6 Hz, ArH), 7.98e8.04 (m, 1H, ArH), 7.92 (t, 2H, 3J ¼ 9.4 Hz, ArH), 7.69e 7.79 (m, 5H, ArH), 7.58 (d, 2H, 3J ¼ 8.7 Hz, ArH), 7.42 (d, 1H, 3 J ¼ 5.8 Hz, ArH), 7.30e7.36 (m, 5H, ArH), 7.19e7.22 (m, 6H, ArH), 7.05e7.14 (m, 4H, ArH), 6.90e6.99 (m, 3H, ArH), 6.82 (t, 1H, 3 J ¼ 6.6 Hz, ArH), 6.39e6.42 (m, 2H, ArH), 4.83 (q, 2H, 3J ¼ 7.7 Hz, e CH2e), 1.68 (t, 3H, 3J ¼ 7.0 Hz, eCH3); FTIR (KBr, cm1): 3449, 2440, 2333, 2054, 2027, 1636, 1607, 1442, 1268, 1120, 1064, 848, 759, 619, 573, 515; ESIeMS (m/Z): 992.4 [M  PF6]þ; Element anal. calcd. for C55H41F6IrN7P (%): C, 58.09; H, 3.63; N, 8.62; found (%): C, 57.88; H, 3.36; N, 8.54.

81

2.3. Quantum chemical calculations Density functional theory (DFT) calculations were carried out with the Gaussian 09 program package at the B3LYP level together with the 6-31G** basis set for C, H, N, and O atoms and the ‘doublex’ quality LANL2DZ basis set for the Ir element without C2 symmetry constraints. 2.4. Fabrication and measurements of devices Indium tin oxide (ITO) coated glass with a sheet resistance of 15e20 U/, was used as substrate and anode. After being sufficiently cleaned and treated with oxygen plasma, an anode buffer layer of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT: PSS) was spin-coated on the ITO substrate and then dried by baking in a vacuum oven at 80  C for 8 h. A light-emitting layer which consists of poly(N-vinylcarbazole) (PVK, host materials), 2-(4-biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (PBD, electron-transporting materials), and cationic iridium(III) complexes (phosphorescent dopants) at different concentrations was spin-coated on the PEDOT: PSS layer in a glove box with less than 10 ppm oxygen and moisture, then was baked at 100  C in inert atmosphere for 20 min to remove solvent (chlorobenzene) residue. A hole-blocking/electron-transporting layer of 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene (TPBI), a CsF electron injection layer and a subsequent aluminum cathode layer were thermally evaporated with a base pressure of 3  104 Pa. The spin-coated film thickness was measured by an Alfa Step 500 surface profilometer, and the thickness during thermal deposition was monitored by a quartz crystal thickness/ratio monitor (Model: STM-100/MF, Sycon). The current densityeluminanceevoltage (IeLeV) characteristics were measured by a Keithley 236 source measurement 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. Synthesis The synthetic routes of the cationic iridium complexes are shown in Scheme 1. 2-Phenylpyridine (ppy) and 1,10phenanthroline derivatives were used as anionic ligand and neutral auxiliary ligands, respectively. Among them, the 1,10phenanthroline derivatives containing oxadiazole-triphenylamine, oxadiazole and triphenylamine unit were used in the complex C1,

Scheme 1. Synthetic routes of the cationic iridium complexes C1, C2 and C3.

82

H. Tang et al. / Dyes and Pigments 100 (2014) 79e86

C2, C3, respectively. The soluble intermediates of cationic iridium complexes using Cl as inorganic acid anion were produced, followed by ion exchange reaction with ammonium hexafluorophosphate, and then the precipitates of the desired products were obtained after Cl being replaced by PF6. All cationic iridium complexes were prepared as yellow or orange solid in high yields above 85%. All of them are soluble in most of ordinary organic solvents, which is beneficial to their EL devices being fabricated by solution-processing technology.

Table 1 Thermal and electrochemical properties of the complexes. Complex

Td ( C)

Tg ( C)

E1/2, (V)

C1 C2 C3

387 399 380

240 249 225

0.63, 0.91 0.89 0.63, 0.92

a HOMO ¼ ee (E1/2, Eg ¼ LUMO-HOMO.

ox)

E1/2, (V)

ox

red

1.74 1.73 1.76

HOMOa (eV)

LUMOa (eV)

Ega (eV)

5.43, 5.71 5.69 5.43, 5.72

3.06 3.07 3.04

2.37, 2.65 2.62 2.39, 2.68

þ (4.8) eV, LUMO ¼ ee (E1/2,

red)

þ (4.8) eV and

3.2. Thermal properties The thermal properties of the complexes measured by differential scanning calorimetry (DSC) and thermogravimetry (TG) are shown in Fig. 1 and summarized in Table 1. No melting or crystal peak but only glass-transition area occurs in the range of 220e 250  C, which suggests that complexes C1, C2 and C3 all are in amorphous. To contrast with the crystal of archetype cationic iridium complex [Ir(ppy)2(phen)][PF6] (ppy ¼ 2-phenylpyridine, phen ¼ 1,10-phenanthroline) [24], the amorphous property of them should be caused by the additional bulky dendritic groups of 2-(triphenylamine)-5-phenyl-1,3,4-oxadiazole, 2,5-diphenyl-1,3,4oxadiazole or triphenylamine in 1,10-phenanthroline derivatives. The high Tg and amorphous nature of the complexes are resistant to crystallization and phase disengagement, which are desirable for OLEDs with high stability and efficiency [25,26]. The complexes C1, C2 and C3 show high thermal stability with the 5% weight-loss temperatures (DT5%, given as the decomposition temperatures, Td) of 387  C, 399  C and 380  C, respectively. The high thermal stability is probably due to high intermolecular interaction, which would hinder the loss of auxiliary ligands. 3.3. Electrochemical and photophysical properties The cyclic voltammograms are shown in Fig. 2 and the results are summarized in Table 1. A reversible one-electron oxidation couple at relatively high positive potentials with E1/2,ox of 0.89e 0.92 V can be attributed to metal-centered IrIII/IrIV oxidation couple [11,27]. Another reversible oxidation couple at relatively low positive potentials with E1/2,ox of 0.63 V, which originates from the electron-donating triphenylamine unit [11,27], was observed in cyclic voltammograms of complexes C1 and C3.

60

Current /a.u.

0.2 Exothermal Heat Flow /a.u.

Mass Loss /%

90

70

0.0

-0.2

C2

C3

-0.4 50

50

C1

C1 C2 C3

100

80

These can be further demonstrated by density functional theory (DFT) calculations shown in Table 2. As shown in the contour plots, the electron density of the highest occupied molecular orbital (HOMO) of complexes C1 and C3 are both located on the electrondonating triphenylamine unit corresponding to the oxidation couple at low positive potentials with E1/2, ox ¼ 0.63 V. For complex C2, the electron density of HOMO lies on electron-withdrawing 2,5diphenyl-1,3,4-oxadiazole unit, which is difficult to be oxidized, therefore only an oxidation couple at relatively high positive potentials corresponds to HOMO-1 (or HOMO-2, an equivalent orbital to HOMO-1) with E1/2, ox ¼ 0.89 V. The electron density of HOMO-1 of complex C1 also mainly lies on the electron-withdrawing 2,5diphenyl-1,3,4-oxadiazole unit, so the oxidation couple at relatively high positive potentials does not correspond to it, but to HOMO-2 with E1/2, ox ¼ 0.91 V. For complex C3, the oxidation couple at high positive potentials corresponds to HOMO-1 with E1/ 2, ox ¼ 0.92 V. These molecular orbitals (i.e. HOMO-2 of complex C1, HOMO-1 or HOMO-2 of complex C2 and HOMO-1 of complex C3) corresponding to oxidation couples at relatively high positive potentials are all composed of a mixture of Ir(III) dp orbitals (t2g) and phenyl p-orbitals of the ppy ligands and therefore exhibits similar or subequal energy levels [6,24]. This point is simultaneously verified by DFT calculations (EHOMO-2,C1 ¼ 7.76 eV, EHOMO1,C2 ¼ 7.96 eV, EHOMO-2,C2 ¼ 7.98 eV and EHOMO-1,C3 ¼ 7.50 eV) and electrochemical measurements (5.71 eV, 5.69 eV and 5.72 eV for complexes C1, C2 and C3, respectively). The differences between the values from cyclic voltammetry measurement and that from DFT calculation is a systematic error due to the parameters used in DFT calculation not completely coinciding with practicality [2,6].

100

100

200

150

200

250

300

300

400

500

o

Temperature / C Fig. 1. TG and DSC (inset) curves of the cationic iridium complexes C1, C2 and C3.

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

+

0.5

1.0

1.5

Potential /V vs. Fc /Fc Fig. 2. Cyclic voltammograms of the complexes C1, C2 and C3 in CH3CN solution of 1  103 mol∙L1 at a scan rate of 50 mV∙s1. Potentials were recorded versus Fcþ/Fc.

H. Tang et al. / Dyes and Pigments 100 (2014) 79e86

83

Table 2 Electronic density contour plots and energy levels of LUMO, HOMO, HOMO-1 and HOMO-2 of the cationic iridium complexes determined by the DFT calculations. Complex C1

Complex C2

Complex C3

LUMO

e4.6495 eV

e4.6454 eV

e4.5371 eV

HOMO

e6.2318 eV

e7.3972 eV

e6.7504 eV

HOMO-1

e7.5776 eV

e7.9638 eV

e7.5004 eV

HOMO-2

e7.7570 eV

e7.9831 eV

e8.1104 eV

1.0

(a)

C1 C2 C3

0.8

1.0 0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 300

400

500

600

700 1.0

(b)

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 300

400

PL Intensity /a.u.

1.0

500

600

PL Intensity /a.u.

Absorption Intensity /a.u.

a rotatable single bond lead to a poor coplanarity. Therefore, the communication of p-electron clouds between 1,10-phenanthroline and functional units is hampered due to the conjugation in these 1,10-phenanthroline derivatives is interrupted [29]. So the p-electrons on HOMO and HOMO-1 of complex C1, HOMOs of complexes C2 and C3, residing on the aforesaid carrier-transporting groups are difficult to communicate with their LUMOs which mainly reside on the 1,10-phenanthroline unit. On the contrary, the HOMO-2 of

Absorption Intensity /a.u.

At negative potentials, complexes C1, C2 and C3 all exhibit a reversible reduction couple caused by the reduction of diimine on 1,10-phenanthroline unit [6,11]. Correspondingly, their electron density of the lowest unoccupied molecular orbitals (LUMOs) also mainly reside on 1,10-phenanthroline unit (as shown in Table 2). The LUMO levels obtained by DFT calculations also are similar to each other with the values of 4.65 eV, 4.65 eV, and 4.54 eV for complexes C1, C2 and C3, respectively. Without regard to systematic error, the LUMO levels obtained by DFT calculation agree with that obtained from cyclic voltammetry measurement, i.e. 3.06 eV, 3.07 eV, and 3.04 eV for complexes C1, C2 and C3, respectively. The results indicate that complexes C1, C2 and C3 all have good oxido-reduction property and stability. The UVeVis absorption and PL spectra of the complexes are shown in Fig. 3. Complex C1 shows a visible-light-harvesting effect, which have a D-A unit of oxadiazole-triphenylamine, whereas complexes C2 or C3, which only has an oxadiazole or a triphenylamine unit, has a much lower absorption in the visible range. The strong absorption band in 250e350 nm can be assigned to the spinallowed 1pep* transition in the ligands [11,28]. The weak absorption band in the visible-light region can be assigned to an admixture of 1MLCT (metaleligand charge transfer), 3MLCT and 3pep* states [11,28]. The admixture of 3MLCT and 3pep* with higher-lying 1 MLCT is caused by the strong spineorbit coupling induced by the iridium heavy atom [11,28]. The PL emission of complexes C1, C2 and C3 in solution and in film are nearly identical peaked around 570 nm. The nearly identical emission of complexes both in film and in solution indicates that no aggregate formed in film, which suggests the emission came from the similar energy gaps. This also can be demonstrated by DFT calculations and electrochemical measurements. As shown in Table 2, the electron densities of LUMOs of all complexes are very similar, which resides on 1,10-phenanthroline unit, and their LUMO levels are also close to each other (i.e. 4.65 eV, 4.65 eV, and 4.54 eV for complexes C1, C2 and C3, respectively). However, in these complexes, the steric hindrance caused by the carriertransporting groups (i.e. triphenylamine-oxadiazole, oxadiazole and triphenylamine units) attached to 1,10-phenanthroline unit via

700

Wavelength /nm Fig. 3. UVeVis absorption and PL spectra of the complexes C1, C2 and C3 in CH2Cl2 solution of 1  105 mol∙L1 (a) and in film (b).

H. Tang et al. / Dyes and Pigments 100 (2014) 79e86

complex C1, HOMO-1 of complexes C2 and C3 are very similar to each other and all composed of a mixture of Ir dp- orbitals (t2g) and phenyl p- orbitals distributed equally among the two ppy ligands [6,17e19], all of them are communicable with the LUMOs, because the electrons can easily transfer from the Ireppy environment to the 1,10-phenanthroline unit (i.e. diimine ligand) via Ir dp-orbitals [17e19]. Therefore, the PL emission of the complexes mainly originated from the electron migration from LUMO to HOMO-2 in complex C1, and from LUMO to HOMO-1 in complexes C2 and C3. The pertinent energy gap (Eg) values obtained from DFT calculations are 2.93 eV, 3.32 eV and 2.96 eV for complexes C1, C2 and C3, respectively. In addition, the Eg values obtained from electrochemical measurement are 2.65 eV, 2.62 eV and 2.68 eV, respectively. The Eg values obtained from DFT calculation are close to each other, so do the Eg values obtained from electrochemical measurement, which results in nearly identical PL emission of the complexes under the same condition. At the same time, it suggests that the modification on 1,10-phenanthroline by aforesaid different carrier-transporting groups had not affected the line shape and bandwidth of the PL emission of the complexes, which is similar to the previous report by R. D. Costa [6]. The PL spectra of the complexes C1, C2 and C3 doped in host matrix are shown in Fig. 4. The emission peaked at 420 nm should originate from the PVK-PBD exciplexes [30] and 530 nm from the iridium complexes, respectively. Compared with the PL emission of complexes C1, C2 and C3 in solution, a blue shift of 40 nm possibly is due to weaker interactions at a low concentration of the complexes in PVK film (i.e. diluted effect) [31e34]. 3.4. Electroluminescent properties The cationic iridium complexes C1, C2 and C3 were used as phosphorescent dopants in polymer light-emitting diodes (PLEDs) with the configuration of ITO/PEDOT:PSS (40 nm)/PVK (70 wt.%): PBD (30 wt.%): iridium complex (x wt.%) (80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al (120 nm), x ¼ 0.5, 1.0 and 2.0, respectively. The EL spectra of the devices are shown in Fig. 5. The EL devices at the different complex concentrations all exhibit a strong emission peaked around 540 nm. The weak emission peaked at 420 nm originating from the PVK-PBD exciplexes is only observed at the doping concentration of 0.5 wt.%, and disappears at 1.0 and 2.0 wt.%. However, the PL spectra still show obvious emission from

Normalized Intensity /a.u.

1.0

C1 C2 C3

0.8 0.6

C1 C2 C3

Intensity /a.u.

84

(b) (a) 400

500

600

700

800

Wavelength /nm Fig. 5. EL spectra of the devices: Ito/PEDOT: PSS/PVK (70%): PBD (30%): complex (x wt.%)/TPBI/CsF/Al, x ¼ 0.5(a), x ¼ 1.0(b), x ¼ 2.0(c).

the PVK-PBD exciplexes at 2.0 wt.% (in Fig. 4). This disagreement indicates that the excitons can be consumed more effectively by iridium complexes in the EL process probably due to more efficient carrier transport under electric potentials [30]. The EL emission of the complexes shows a red shift of 18 nm relative to the PL emission when the complexes being doped at 2.0 wt.% in host matrix (in Fig. 4), at the same time, the full widths at half maximum (FWHMs) of the EL spectra exhibit a broadening of 18 nm possibly due to enhanced interactions between iridium complex cations and inorganic acid anions under electric potentials, which is similar to polarization effect [35,36]. Like PL spectra, the EL spectra of all devices using different complexes are nearly identical under the same conditions which suggests the modification on 1,10-phenanthroline by the aforesaid different carriertransporting groups had not affected the EL spectra of the complexes. The performances of the devices are summarized in Table 3. It can be seen that 1.0 wt.% is an optimal doping concentration to achieve the highest luminance and efficiency for all cationic iridium complexes. The maximum luminance efficiencies (LEmax) and the maximum external quantum efficiencies (QEmax) of the devices using complexes C1, C2 and C3 are 30.6 cd/A, 28.4 cd/A and 26.3 cd/ A, and 10.2%, 9.4% and 8.7%, respectively, which are higher than the EL efficiencies of previously reported similar cationic iridium complexes used in OLEDs [10e16,21]. Complex C1 exhibits the

Table 3 Device performances based on the cationic iridium complexes.a

0.4

Complex

0.2

C1

0.0

C2

350

(c)

400

450

500

550

600

650

700

750

C3

Wavelength /nm Fig. 4. PL spectra of the cationic iridium complexes doped in host matrix, PVK (70 wt.%): PBD (30 wt.%): complex C1, C2 or C3 (2.0 wt.%).

a b

Content (wt.%)

Von (V)

Lmax (cd/m2)

LEmax (cd/A)

QEmax (%)

lmax

0.5 1 2 0.5 1 2 0.5 1 2

5.0 4.3 5.0 5.3 6.3 7.0 5.6 6.3 6.5

6054 8865 8600 5887 9185 9275 6348 7967 7578

16.7 30.6 26.9 17.1 28.4 24.5 17.3 26.3 23.1

5.6 10.2 9.0 5.7 9.4 8.2 5.8 8.7 7.7

544 548 552 542 548 548 546 550 548

CIEb (x, y)

(nm) (0.37, (0.41, (0.41, (0.37, (0.41, (0.42, (0.37, (0.41, (0.41,

Device configuration: ITO/PEDOT: PSS/PVK: PBD: complex/TPBI/CsF/Al. At 12 mA/m2.

0.55) 0.55) 0.55) 0.55) 0.55) 0.55) 0.55) 0.55) 0.55)

H. Tang et al. / Dyes and Pigments 100 (2014) 79e86

85

4. Conclusions 1,10-Phenanthroline derivatives containing oxadiazoletriphenylamine, oxadiazole and triphenylamine units were synthesized and used as neutral auxiliary ligands in cationic iridium complexes. The cationic iridium complexes all own high thermal stability, good solubility and amorphous nature, which are adequate for being used as phosphorescent dopants in PLEDs. The PL and EL spectra of them are identical although these complexes were modified by aforesaid different carrier-transporting groups. By comparison, complex C1 containing bipolar unit of oxadiazoletriphenylamine exhibits higher device performance due to its balanced hole and electron fluxes. Acknowledgments

Fig. 6. Luminance and current density (inset) vs. voltage characteristics of the devices: ITO/PEDOT: PSS/PVK (70%): PBD (30%): complexes C1, C2 or C3 (1.0 wt.%)/TPBI/CsF/Al.

highest efficiency most probably due to the triphenylamineoxadiazole bipolar unit providing a balanced hole and electron fluxes [37e39]. The Commission Internationale de L’Eclairage (CIE) color coordinate of all devices at 1.0 wt.% concentration is (0.41, 0.55) corresponding to the yellow-green region. Fig. 6 shows the luminance and current density versus voltage characteristics of the devices at 1.0 wt.% doping concentration. Compared with complexes C2 and C3, the device of complex C1 shows a lower turn-on voltage and luminance. Fig. 7 represents the current efficiency versus current density at the doping concentration of 1.0 wt.%. Although the efficiencies decline sharply at low current density (<5 mA/cm2), the efficiency decreases slowly and linearly with increasing current density after 5 mA/cm2. The current efficiency of complex C1 is 13 cd/A at 50 mA/cm2, and declines to 8.8 cd/A at high current density of about 100 mA/cm2, indicating a good device stability. The EL efficiencies decrease with increasing the current density because of tripletetriplet annihilation [40] and field-induced quenching effects [41] which are common with most phosphorescent OLEDs.

C1 C2 C3

Current efficiency /cd A

-1

30 20 10 0 -10 -20 0

20

40

60

80

100

-2

Current density /mA cm

Fig. 7. Current efficiency vs. current density characteristics of the devices ITO/PEDOT: PSS/PVK (70%): PBD (30%): complexes C1, C2 or C3 (1.0 wt.%)/TPBI/CsF/Al.

This work was supported by China Postdoctoral Science Foundation (No. 20090450858), National Nature Science Foundation of China (No. 21262046), Key Scientific Research Fund Supported by Education Department of Yunnan Province (No. 2011Z003), Youth Fund of Yunnan Universities of Nationalities (No. 11QN05), and State Key Basic Research Project of China (No. 2009CB623602). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2013.07.029. References [1] Slinker JD, Gorodetsky AA, Lowry MS, Wang J, Parker S, Rohl R, et al. Efficient yellow electroluminescence from a single layer of a cyclometalated iridium complex. J Am Chem Soc 2004;126:2763e7. [2] Tamayo AB, Garon S, Sajoto T, Djurovich PI, Tsyba IM, Bau R, et al. Cationic biscyclometalated iridium(III) diimine complexes and their use in efficient blue, green, and red electroluminescent devices. Inorg Chem 2005;44:8723e32. [3] Slinker JD, Rivnay J, Moskowitz JS, Parker JB, Bernhard S, Abruña HD, et al. Electroluminescent devices from ionic transition metal complexes. J Mater Chem 2007;17:2976e88. [4] Su H-C, Chen H-F, Fang F-C, Liu C-C, Wu C-C, Wong K-T, et al. Solid-state white light-emitting electrochemical cells using iridium-based cationic transition metal complexes. J Am Chem Soc 2008;130:3413e9. [5] He L, Qiao J, Duan L, Dong G, Zhang D, Wang L, et al. Toward highly efficient solid-state white light-emitting electrochemical cells: blue-green to red emitting cationic iridium complexes with imidazole-type ancillary ligands. Adv Funct Mater 2009;19:2950e60. [6] Costa RD, Ortí E, Bolink HJ, Graber S, Housecroft CE, Constable EC. Efficient and long-living light-emitting electrochemical cells. Adv Funct Mater 2010;20: 1511e20. [7] Mydlak M, Bizzarri C, Hartmann D, Sarfert W, Schmid G, Cola LD. Positively charged iridium(III) triazole derivatives as blue emitters for light-emitting electrochemical cells. Adv Funct Mater 2010;20:1812e20. [8] Sun L, Galan A, Ladouceur S, Slinker JD, Zysman-Colman E. High stability lightemitting electrochemical cells from cationic iridium complexes with bulky 5,50 substituents. J Mater Chem 2011;21:18083e8. [9] Hu T, He L, Duan L, Qiu Y. Solid-state light-emitting electrochemical cells based on ionic iridium(III) complexes. J Mater Chem 2012;22:4206e15. [10] Plummer EA, Dijken VA, Hofstraat HW, Cola LD, Brunner K. Electrophosphorescent devices based on cationic complexes: control of switch-on voltage and efficiency through modification of charge injection and charge transport. Adv Funct Mater 2005;15:281e9. [11] Wong W-Y, Zhou G-J, Yu X-M, Kwok H-S, Lin Z. Efficient organic lightemitting diodes based on sublimable charged iridium phosphorescent emitters. Adv Funct Mater 2007;17:315e23. [12] He L, Duan L, Qiao J, Zhang D, Dong G, Wang L, et al. Efficient solutionprocessed electrophosphorescent devices using ionic iridium complexes as the dopants. Org Electron 2009;10:152e7. [13] He L, Duan L, Qiao J, Zhang D, Wang L, Qiu Y. Highly efficient solutionprocessed blue-green to red and white light-emitting diodes using cationic iridium complexes as dopants. Org Electron 2010;11:1185e91. [14] Fernández-Hernández JM, Yang C-H, Beltrán JI, Lemaur V, Polo F, Fröhlich R, et al. Control of the mutual arrangement of cyclometalated ligands in cationic iridium(III) complexes. Synthesis, spectroscopy, and electroluminescence of the different isomers. J Am Chem Soc 2011;133:10543e58.

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