Synthesis, characterization and photophysical properties of iridium complexes with amidinate ligands

Journal of Organometallic Chemistry 772-773 (2014) 68e78

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Synthesis, characterization and photophysical properties of iridium complexes with amidinate ligands Cigdem Sahin a, *, Aysen Goren a, Canan Varlikli b a b

Department of Chemistry, Art&Science Faculty, Pamukkale University, Denizli, Turkey Solar Energy Institute, Ege University, 35100 Bornova, Izmir, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 July 2014 Received in revised form 29 August 2014 Accepted 30 August 2014 Available online 6 September 2014

Iridium(III) complexes containing 2-(3-fluorophenyl)-4-methylpyridine and amidinate ligands with alkyl chains were synthesized and characterized by UV/Visible, fluorescence, FTIR, NMR spectroscopies and cyclic voltammetry. Thermal properties of the iridium complexes have been investigated using thermogravimetric analysis (TGA). The influence of alkyl side groups on the photoluminescence spectra of iridium complexes has been investigated. The spectra of all complexes show similar emission which results in the blue region. However, the increase in the solution concentrations of the complexes leads to excimer emission in the amidinate complexes. The effects of different polarities of solvents on photophysical properties have been investigated. The HOMO and LUMO energy levels of the complexes are in the range of 5.32e5.35 eV and 2.48e2.51 eV, respectively. The energy states, thermal and electrochemical stability of the complexes are appropriate for their utilization in OLED applications. © 2014 Elsevier B.V. All rights reserved.

Keywords: Amidinate 2-Phenylpyridine Iridium(III) complexes Excimer emission

Introduction Cyclometalated iridium(III) complexes have attracted attention in organic light emitting diodes (OLEDs) [1], chemosensors [2], biological labeling reagents [3] and catalysts [4] due to their stability, photophysical, photochemical, and electrochemical properties. These properties of iridium(III) complexes can be controlled by cyclometalated ligands variations which cause large d-orbital splitting in iridium atom [5]. OLEDs represent one of the most popular application areas of cyclometalated iridium(III) complexes. OLEDs have gained great interest in full-color display applications because of their high efficiency, high brightness and, resolution and lower cost [6]. White light emitting OLEDs are becoming increasingly successful in lighting applications. However, there is need for improvement of the efficiency, color purity and stability [1]. Developing new molecules is one of the ways to improve performance and stability of an OLED. Iridium complexes show the best efficiency in this area of technology due to their high quantum efficiency and color tunability [7]. These complexes exhibit high triplet quantum yields due to mixing the singlet and the triplet excited states via spin-orbit coupling, leading to high phosphorescence efficiencies [8]. Iridium complexes usually contain cyclometalated ligands such as phenylpyridyl, phenyl-imidazole for OLED applications [9].

* Corresponding author. Tel.: þ90 258 2963526; fax: þ90 258 2963535. E-mail address: [email protected] (C. Sahin). http://dx.doi.org/10.1016/j.jorganchem.2014.08.031 0022-328X/© 2014 Elsevier B.V. All rights reserved.

The aim of this study is the synthesis of new iridium complexes containing 2-(3-fluorophenyl)-4-methylpyridine and amidinate ligands with alkyl chains to obtain a wide range of emission. In 2phenylpyridine ligand, electron withdrawing fluoro unit on the phenyl ring and electron donating methyl group on the pyridyl ring increases the HOMO-LUMO gap and emission energy which leads to blue shifted emission band [10]. Furthermore, polar F atoms and 2-phenylpyridine ligands can induce intermolecular interaction and p-p interaction between molecules [11,12]. The donation from p-electrons on the amidinate ligand to iridium metal can provide chelating bonding mode which increases ligand and metal interaction [13,14]. The influences of different solvent polarities and concentrations of the iridium complexes on photophysical properties are discussed. It is found that molecules show excimer emission with increasing solution concentrations and their energy states, electrochemical and thermal stabilities are appropriate for their utilization in OLED applications. Experimental section Materials 3-fluorophenylboronic acid, tetrakis (triphenylphosphine) 0 0 palladium (0), N,N -di-tert-butylcarbodiimide, N,N -diisopropylcarbodiimide, sodium carbonate, butyllithium solution (1.6 M in

C. Sahin et al. / Journal of Organometallic Chemistry 772-773 (2014) 68e78

hexane) were provided from Aldrich. Iridium (III) trichloride hydrate (IrCl3$nH2O), 4-bromotoluene, 1-bromo-4-n-propylbenzene, 1-bromo-4-n-butylbenzene, 4-bromophenetole 2-ethoxyethanol was obtained from Alfa Aesar. 1-Bromo-3, 5-dimethoxybenzene, 2-bromo-4-methylpyridine were purchased from across. All reactions and manipulations of air-sensitive materials were carried out under argon atmosphere using standard Schlenk techniques. Solvents were dried and freshly distilled prior to use. All other chemicals were used as received. Measurements UVeVis and photoluminescence (PL) spectra were recorded in a 1 cm path length quartz cell using a Shimadzu UV-1601 UVeVis spectrophotometer and Perkin Elmer LS55 fluorescence spectrometers, respectively. Lifetime measurements were obtained using an Edinburg FLS920P fluorescence spectrophotometer with an EPL-470 diode laser. Infrared spectra were recorded on a Perkin Elmer Spectrum Two FT-IR Spectrometer with a diamond ATR. NMR spectra were recorded at probe temperature on a Varian Mercury AS 400 NMR instrument. The reported chemical shifts are referenced to tetramethylsilane (TMS). Elemental analyses were performed using a Carlo Erba 1106 elemental analyzer. Mass spectra were recorded on Bruker Microflex LT MALDI-TOF MS. The thermal properties of the complexes were obtained by a Shimadzu DTG-60 equipped with TGA unit. Electrochemical data were obtained using a CH Instrument 660 B Model Electrochemical Workstation. Synthesis and characterization Synthesis of 2-(3-fluorophenyl)-4-methylpyridine (FMeppy) 2-(3-fluorophenyl)-4-methylpyridine was prepared by a Suzuki coupling reaction using a modified literature procedure [15]. 3-

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Fluorophenylboronic acid (250 mg, 1.79 mmol) 2-bromo-4methylpyridine (307.34 mg, 1.79 mmol) and tetrakis (triphenylphosphine) palladium (0) (70.20 mg, 0.061 mmol) were dissolved in a mixture of toluene (12 mL) and ethanol (6 mL). Aqueous sodium carbonate solution (2 N, 12 mL) was added slowly and the reaction mixture was stirred at 80  C for 24 h. The mixture was cooled to room temperature and extracted with ethyl acetate (2  15 mL). The combined organic phases were dried over anhydrous magnesium sulfate, and were evaporated under vacuum to remove ethyl acetate. The residue was purified by silica gel chromatography using hexane/ethyl acetate (3:1). The collected product was dried under vacuum. The pure product was obtained as yellow liquid with 78% yield. 1H NMR (CDCl3) d ppm: 8.55 (d, J ¼ 4.8 Hz, 1H), 7.74 (d, J ¼ 6.8 Hz, 1H), 7.70 (dt, J ¼ 7.6, 0.8 Hz, 1H), 7.53 (s, 1H), 7.42 (q, 1H), 7.10 (m, 2H), 2.43 (s, 3H). Synthesis of [(FMeppy)2Ir(m-Cl)]2 The dimer [(FMeppy)2Ir(m-Cl)]2 was synthesized according to literature [16]. IrCl3$XH2O (19.1 mg, 0.064 mmol) and 2-(3fluorophenyl)-4-methylpyridine (30 mg, 0.16 mmol) in a mixture of ethoxyethanol and water (3:1, v/v) were heated at 110  C for 12 h and then cooled to room temperature. The resulting precipitate was filtered off and washed with diethyl ether and dried to give the yellow solid with 70% yield. Synthesis of [Ir(FMeppy)2(N,N0 -diisopropyl-4-methyl-benzamidine) (CS127) CS127 was prepared using a modified literature procedure [16]. A solution of butyllithium (1.6 M in hexane, 0.15 mL) was added to a solution of 4-bromotoluene (8.52 mg, 0.050 mmol) in hexane (15 mL). After stirring for 1 h, N,N0 -diisopropylcarbodiimide (6.29 mg, 0.050 mmol) was added dropwise. The solution was stirred for 1 h at room temperature and then the dimer

Scheme 1. The summary of synthetic procedure followed.

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[(FMeppy)2Ir(m-Cl)]2 (30 mg, 0.025 mmol) was added. The mixture was heated overnight at 70  C. After being cooled to room temperature, the resulting precipitate was collected and washed with diethyl ether. The dried product was obtained as a yellow powder. Yield 65%. FTIR (ATR, cm1): 3071, 2963, 2936, 2877, 1620, 1505, 1417, 1264, 1087, 1034, 862. 1H NMR (DMSO-d6) d ppm: 9.61 (d, J ¼ 6 Hz, 1H), 9.31 (d, J ¼ 6.4 Hz, 1H), 8.20 (d, J ¼ 5.6 Hz, 1H), 8.10 (s, 1H), 8.01 (s, 1H), 7.71 (t, J ¼ 8.4 Hz, 1H), 7.62 (d, J ¼ 7.6 Hz, 1H), 7.45 (d, J ¼ 5.6 Hz, 1H), 7.33 (d, J ¼ 7.6 Hz, 1H), 7.21 (d, J ¼ 5.2 Hz, 1H), 6.72 (m, 2H), 6.60 (m, 2H), 6.18 (d, J ¼ 8.4 Hz, 1H), 5.55 (d, J ¼ 7.6 Hz, 1H), 4.31 (m, 1H), 4.10 (m, 1H), 3.31 (s, 3H), 2.63 (s, 3H), 2.58 (s, 3H), 1.01 (d, J ¼ 6.8 Hz, 6H), 0.87 (d, J ¼ 6.4 Hz, 6H). MALDI-TOF MS m/z 781.9 ([M]þ). Anal. Calcd. for C38H39F2IrN4(%): C, 58.37; H, 5.03; N, 7.16. Found: C, 58.41; H, 5.06; N, 7.19. Synthesis of [Ir(FMeppy)2(N,N0 -diisopropyl-4-propyl-benzamidine) (CS128) [Ir(FMeppy)2(N,N0 -diisopropyl-4-propyl-benzamidine) was synthesized using the same reaction condition and purification step of CS127 except that 1-bromo-4-n-propylbenzene was used instead of 4-bromotoluene. Yield 55%. FTIR (ATR, cm1): 3068, 2963, 2935, 2871, 1619, 1504, 1422, 1262, 1088, 1033, 862. 1H NMR (DMSO-d6) d ppm: 9.59 (d, J ¼ 6.4 Hz, 1H), 9.30 (d, J ¼ 6 Hz, 1H), 8.17 (d, J ¼ 6.8 Hz, 1H), 8.08 (s, 1H), 7.98 (s, 1H), 7.67 (d, J ¼ 8.4 Hz, 1H), 7.60 (d, J ¼ 8.4 Hz, 1H), 7.44 (q, 1H), 7.31 (d, J ¼ 6.4 Hz, 1H), 7.18 (d, J ¼ 6.4 Hz, 1H), 6.93 (m, 2H) 6.63 (m, 2H), 6.16 (t, J ¼ 7.6 Hz, 1H), 5.60 (d, J ¼ 6.8 Hz, 1H), 4.09 (m, 2H), 3.15 (t, J ¼ 7.2 Hz, 2H), 2.62 (s, 3H), 2.57 (s, 3H), 1.50 (m, 2H), 1.06 (t, J ¼ 6.8 Hz, 3H), 1.01 (d, J ¼ 6.4 Hz, 6H), 0.87 (d, J ¼ 6.4 Hz, 6H). MALDI-TOF MS m/z 810.0 ([M]þ). Anal. Calcd. for C40H43F2IrN4(%): C, 59.31; H, 5.35; N, 6.92. Found: C, 59.33; H, 5.37; N, 6.95.

Table 1 Mass loss (%) of the iridium complexes in the different temperature ranges. Complex Stage

TGA temp. Mass loss (%) Decomposition product loss  range ( C) Found Calc.

CS127

I II III Residue I II III IV Residue I II

30e117 117e500 500e900 >900 30e117 117e450 450e705 705e900 >900 30e110 110e515

2.28 31.51 43.01 23.21 2.26 19.13 18.65 37.03 22.91 2.21 20.03

2.25 31.92 43.05 22.78 2.17 19.24 18.77 37.48 22.34 2.13 20.58

III Residue I II III IV Residue I II

515e900 >900 30e115 115e455 455e784 784e900 >900 30e115 115e486

55.71 22.05 2.09 19.94 18.29 38.01 21.67 2.20 33.37

55.05 22.24 2.16 19.44 18.69 37.40 22.31 2.12 34.68

III Residue I II III Residue I II III IV Residue I II

486e900 >900 30e110 110e495 495e900 >900 30e110 110e492 492e758 758e900 >900 30e114 114e475

38.68 25.75 2.28 33.27 42.59 21.86 2.16 20.90 18.27 36.87 21.80 2.07 22.20

40.70 22.5 2.17 33.47 43.50 20.86 2.10 21.89 18.12 36.26 21.63 2.06 23.15

III Residue I II III IV Residue I II

475e900 >900 30e110 110e449 449e790 790e900 >900 30e116 116e422

53.64 22.09 2.25 21.94 18.21 36.34 21.26 2.15 36.99

53.27 21.52 2.10 22.07 18.08 36.17 21.58 2.06 36.78

CS128

CS129

CS130

CS131

CS132

CS133

CS134

Synthesis of [Ir(FMeppy)2(N,N0 -diisopropyl-4-butyl-benzamidine) (CS129) [Ir(FMeppy)2(N,N0 -diisopropyl-4-butyl-benzamidine) was synthesized using the same reaction condition and purification step of CS127 except that 1-bromo-4-n-butylbenzene was used instead of 4-bromotoluene. Yield 63%. FTIR (ATR, cm1): 3066, 2961, 2934, 2876, 1620, 1499, 1417, 1263, 1088, 1033, 861. 1H NMR (DMSO-d6) d ppm: 9.58 (d, J ¼ 6.4 Hz, 1H), 9.29 (d, J ¼ 6 Hz, 1H), 8.16 (d, J ¼ 6.8 Hz, 1H), 8.09 (s, 1H), 7.96 (s, 1H), 7.65 (d, J ¼ 8 Hz, 1H), 7.60 (d, J ¼ 8 Hz, 1H), 7.43 (q, 1H), 7.32 (d, J ¼ 6.4 Hz, 1H), 7.17 (d, J ¼ 6.4 Hz, 1H), 6.92 (m, 2H), 6.61 (m, 2H), 6.16 (t, J ¼ 7.4 Hz, 1H), 5.60 (d, J ¼ 6.8 Hz, 1H), 4.07 (m, 2H), 3.15 (t, J ¼ 7.2 Hz, 2H), 2.63 (s, 3H), 2.58 (s, 3H), 1.48 (m, 4H), 1.05 (t, J ¼ 6.8 Hz, 3H), 1.01 (d, J ¼ 6.4 Hz, 6H), 0.87 (d, J ¼ 6.4 Hz, 6H). MALDI-TOF MS m/z 824.0 ([M]þ). Anal. Calcd. for C41H45F2IrN4(%): C, 59.76; H, 5.50; N, 6.80. Found: C, 59.74; H, 5.54; N, 6.77. Synthesis of [Ir(FMeppy)2(N,N0 -diisopropyl-4-ethoxy-benzamidine) (CS130) [Ir(FMeppy)2(N,N0 -diisopropyl-4-ethoxy-benzamidine) was synthesized using the same reaction condition and purification step of CS127 except that 4-bromophenetole was used instead of 4bromotoluene. Yield 58%. FTIR (ATR, cm1): 3058, 2967, 2936, 2878, 1619, 1499, 1421, 1262, 1172, 1088, 1033, 862. 1H NMR (DMSO-d6) d ppm: 9.62 (d, J ¼ 6 Hz, 1H), 9.33 (d, J ¼ 6.4 Hz, 1H), 8.20 (d, J ¼ 6.8 Hz, 1H), 8.11 (s, 1H), 8.02 (s, 1H), 7.71 (d, J ¼ 8 Hz, 1H), 7.63 (dd, J ¼ 7.6 Hz, 1H), 7.45 (q, 1H), 7.34 (d, J ¼ 6 Hz, 1H), 7.21 (d, J ¼ 6 Hz, 1H), 6.77e6.55 (m, 4H), 6.19 (q, 1H), 5.48 (d, J ¼ 6.8 Hz, 1H), 4.20 (m, 2H), 3.91 (q, 2H), 2.60 (s, 3H), 2.56 (s, 3H), 1.28 (t, J ¼ 7.2 Hz, 3H), 1.08 (d, J ¼ 6.8 Hz, 6H), 1.01 (d, J ¼ 6.4 Hz, 6H). MALDI-TOF MS m/z 812.7 ([M]þ). Anal. Calcd. for C39H41F2IrN4O(%): C, 57.69; H, 5.09; N, 6.90. Found: C, 57.75; H, 5.14; N, 6.92.

CS135

CS136

III 422e900 Residue >900

39.01 39.98 21.85 21.18

H2O 2CH3 þ C7H14N2 þ C6H4eCH3 2F þ 2C11H10N Ir H2O 2CH3 þ CH2CH2CH3 þ 2CH(CH3)2 NCN þ C6H4 þ 2F 2C11H10N Ir H2O 2CH3 þ CH2CH2CH2CH3 þ 2CH(CH3)2 NCN þ C6H4 þ 2F þ 2C11H10N Ir H2O 2CH3 þ OCH2CH3 þ 2CH(CH3)2 NCN þ C6H4 þ 2F 2C11H10N Ir H2O 2CH3 þ C7H14N2 þ C6H3 þ 2OCH3 2F þ 2C11H10N Ir H2O 2CH3 þ C9H18N2 þ C6H4eCH3 2F þ 2C11H10N Ir H2O 2CH3 þ CH2CH2CH3 þ 2C(CH3)3 NCN þ C6H4 þ 2F 2C11H10N Ir H2O 2CH3 þ CH2CH2CH2CH3 þ 2C(CH3)3 NCN þ C6H4þ2F þ 2C11H10N Ir H2O 2CH3 þ OCH2CH3 þ 2C(CH3)3 NCN þ C6H4 þ 2F 2C11H10N Ir H2O 2CH3 þ C9H18N2 þ C6H3 þ 2OCH3 2F þ 2C11H10N Ir

Fig. 1. TGA curves of the iridium complexes.

C. Sahin et al. / Journal of Organometallic Chemistry 772-773 (2014) 68e78

Synthesis of [Ir(FMeppy)2(N,N0 -diisopropyl-3,5-dimethoxybenzamidine) (CS131) [Ir(FMeppy)2(N,N0 -diisopropyl-3,5-dimethoxy-benzamidine) was synthesized using the same reaction condition and purification step of CS127 except that 1-bromo-3,5-dimethoxybenzene was used instead of 4-bromotoluene. Yield 50%. FTIR (ATR, cm1): 3057, 2964, 2937, 2878, 1621, 1504, 1417, 1265, 1218, 1176, 1087, 1034, 861. 1 H NMR (DMSO-d6) d ppm: 9.62 (d, J ¼ 6 Hz, 1H), 9.33 (d, J ¼ 5.6 Hz, 1H), 8.20 (d, J ¼ 6.8 Hz, 1H), 8.11 (s, 1H), 8.02 (s, 1H), 7.71 (t, J ¼ 8 Hz, 1H), 7.63 (dd, J ¼ 7.6 Hz, 1H), 7.46 (q, 1H), 7.34 (d, J ¼ 6 Hz, 1H), 7.21 (d, J ¼ 5.6 Hz, 1H), 6.72e6.41 (m, 3H), 6.19 (t, J ¼ 6.4 Hz, 1H), 5.48 (d, J ¼ 7.2 Hz, 1H), 4.19 (m, 2H), 3.92 (s, 3H), 3.65 (s, 3H), 2.61 (s, 3H), 2.56 (s, 3H), 1.08 (d, J ¼ 6.8 Hz, 6H), 1.00 (d, J ¼ 6.4 Hz, 6H). MALDITOF MS m/z 828.0 ([M]þ). Anal. Calcd. for C39H41F2IrN4O2(%): C, 56.57; H, 4.99; N, 6.77. Found: C, 56.53; H, 5.02; N, 6.73. Synthesis of [Ir(FMeppy)2(N,N0 -di-tert-butyl-4-methylbenzamidine) (CS132) [Ir(FMeppy)2(N,N0 -di-tert-butyl-4-methyl-benzamidine) was synthesized according to the procedure described for CS127 except that N,N0 -di-tert-butylcarbodiimide was used instead of N,N0 -diisopropylcarbodiimide. Yield 68%. FTIR (ATR, cm1): 3070, 2962, 2936, 2876, 1620, 1505, 1417, 1262, 1087, 1033, 863. 1H NMR (DMSOd6) d ppm: 9.60 (d, J ¼ 6.4 Hz, 1H), 9.32 (d, J ¼ 6 Hz, 1H), 8.19 (s, 1H),

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8.10 (s, 1H), 8.00 (s, 1H), 7.72 (d, J ¼ 7.2 Hz, 1H), 7.62 (d, J ¼ 7.6 Hz, 1H), 7.45 (t, J ¼ 6.8 Hz, 1H), 7.32 (d, J ¼ 6 Hz, 1H), 7.21 (d, J ¼ 5.2 Hz, 1H), 6.69 (t, J ¼ 6.8 Hz, 2H), 6.62 (t, J ¼ 5.6 Hz, 2H), 6.19 (t, J ¼ 5.2 Hz, 1H), 5.51 (t, J ¼ 6.4 Hz, 1H), 3.20 (s, 3H), 2.63 (s, 3H), 2.58 (s, 3H), 0.97 (s, 9H), 0.88 (s, 9H). MALDI-TOF MS m/z 810.0 ([M]þ). Anal. Calcd. for C40H43F2IrN4(%): C, 59.31; H, 5.35; N, 6.92. Found: C, 59.29; H, 5.36; N, 6.97. Synthesis of [Ir(FMeppy)2(N,N0 -di-tert-butyl-4-propyl-benzamidine) (CS133) [Ir(FMeppy)2(N,N0 -di-tert-butyl-4-propyl-benzamidine) was synthesized using the same reaction condition and purification step of CS132 except that 1-bromo-4-n-propylbenzene was used instead of 4-bromotoluene. Yield 53%. FTIR (ATR, cm1). 3074, 2962, 2936, 2877, 1620, 1505, 1423, 1262, 1087, 1034, 863. 1H NMR (DMSO-d6) d ppm: 9.61 (d, J ¼ 6.4 Hz, 1H), 9.32 (d, J ¼ 5.6 Hz, 1H), 8.19 (d, J ¼ 5.4 Hz, 1H), 8.10 (s, 1H), 8.00 (s, 1H), 7.69 (d, J ¼ 8 Hz, 1H), 7.62 (d, J ¼ 8.8 Hz, 1H), 7.46 (t, J ¼ 6.8 Hz, 1H), 7.33 (d, J ¼ 7.6 Hz, 1H), 7.21 (d, J ¼ 6.8 Hz, 1H), 6.94 (t, J ¼ 5.6 Hz, 2H) 6.59 (t, J ¼ 8 Hz, 2H), 6.17 (t, J ¼ 5.6 Hz, 1H), 5.62 (t, J ¼ 6.4 Hz, 1H), 3.15 (t, J ¼ 7.2 Hz, 2H), 2.62 (s, 3H), 2.58 (s, 3H), 1.50 (m, 2H), 1.06 (t, J ¼ 6.8 Hz, 3H), 0.97 (s, 9H), 0.88 (s, 9H). MALDI-TOF MS m/z 838.1 ([M]þ). Anal. Calcd. for C42H47F2IrN4(%): C, 60.19; H, 5.65; N, 6.69. Found: C, 60.13; H, 5.69; N, 6.63.

Fig. 2. UVeVis absorption spectra of 5  105 M solution of the iridium complexes in different solvents: dichloromethane, chlorobenzene, toluene.

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Synthesis of [Ir(FMeppy)2(N,N0 -di-tert-butyl-4-butyl-benzamidine) (CS134) [Ir(FMeppy)2(N,N0 -di-tert-butyl-4-butyl-benzamidine) was synthesized using the same reaction condition and purification step of CS132 except that 1-bromo-4-n-butylbenzene was used instead of 4-bromotoluene. Yield 65%. FTIR (ATR, cm1): 3074, 2963, 2936, 2877, 1620, 1511, 1423, 1262, 1087, 1034, 863. 1H NMR (DMSO-d6) d ppm: 9.68 (d, J ¼ 5.6 Hz, 1H), 9.38 (d, J ¼ 6 Hz, 1H), 8.26 (d, J ¼ 6.4 Hz, 1H), 8.16 (s, 1H), 8.07 (s, 1H), 7.76 (d, J ¼ 7.2 Hz, 1H), 7.68 (d, J ¼ 8 Hz, 1H), 7.50 (q, 1H), 7.39 (d, J ¼ 6.4 Hz, 1H), 7.26 (d, J ¼ 6.8 Hz, 1H), 6.98 (m, 2H), 6.69 (m, 2H), 6.25 (t, J ¼ 5.6 Hz, 1H), 5.68 (d, J ¼ 6 Hz, 1H), 3.15 (t, J ¼ 7.2 Hz, 2H), 2.69 (s, 3H), 2.64 (s, 3H), 1.49 (m, 4H), 1.04 (t, J ¼ 6.8 Hz, 3H), 0.97 (s, 9H), 0.88 (s, 9H). MALDI-TOF MS m/z 852.1 ([M]þ). Anal. Calcd. for C43H49F2IrN4(%): C, 60.61; H, 5.80; N, 6.58. Found: C, 60.58; H, 5.75; N, 6.62. Synthesis of [Ir(FMeppy)2(N,N0 -di-tert-butyl-4-ethoxybenzamidine) (CS135) [Ir(FMeppy)2(N,N0 -di-tert-butyl-4-ethoxy-benzamidine) was synthesized using the same reaction condition and purification step of CS132 except that 4-bromophenetole was used instead of 4-bromotoluene. Yield 59%. FTIR (ATR, cm1): 3057, 2967, 2937, 2879, 1620, 1495, 1417, 1263, 1213, 1106, 1087, 1039, 860. 1H NMR (DMSO-d6) d ppm: 9.59 (d, J ¼ 5.6 Hz, 1H), 9.30 (d, J ¼ 5.6 Hz, 1H), 8.17 (d, J ¼ 5.6 Hz, 1H), 8.08 (s, 1H), 7.79 (s, 1H), 7.67 (d, J ¼ 6.4 Hz, 1H), 7.60 (d, J ¼ 8 Hz, 1H), 7.44 (t, J ¼ 6 Hz, 1H), 7.31 (d, J ¼ 6 Hz, 1H), 7.18 (d, J ¼ 6.4 Hz, 1H), 6.67 (t, J ¼ 8 Hz, 2H), 6.58 (t, J ¼ 8.8 Hz, 2H), 6.17 (t, J ¼ 7.2 Hz, 1H), 5.58 (t, J ¼ 8.8 Hz, 1H), 3.90 (q, 2H), 2.60 (s, 3H), 2.56 (s, 3H), 1.26 (t, J ¼ 6.4 Hz, 3H) 1.01 (s, 9H), 0.92 (s, 9H). MALDI-TOF MS m/z 840.0 ([M]þ). Anal. Calcd. for C41H45F2IrN4O(%): C, 58.62; H, 5.40; N, 6.67. Found: C, 58.70; H, 5.66; N, 6.72.

Synthesis of [Ir(FMeppy)2(N,N0 -di-tert-butyl-3,5-dimethoxybenzamidine) (CS136) [Ir(FMeppy)2(N,N0 -di-tert-butyl-3,5-dimethoxy -benzamidine) was synthesized using the same reaction condition and purification step of CS132 except that 1-bromo-3,5-dimethoxybenzene was used instead of 4-bromotoluene. Yield 53%. FTIR (ATR, cm1): 3061, 2964, 2940, 2913, 1620, 1477, 1423, 1264, 1211, 1153, 1088, 1034, 861. 1 H NMR (DMSO-d6) d ppm: 9.59 (d, J ¼ 5.6 Hz, 1H), 9.32 (d, J ¼ 6 Hz, 1H), 8.19 (d, J ¼ 6 Hz, 1H), 8.09 (s, 1H), 8.02 (s, 1H), 7.70 (t, J ¼ 6.4 Hz, 1H), 7.63 (d, J ¼ 8 Hz, 1H), 7.46 (t, J ¼ 6.4 Hz, 1H), 7.32 (d, J ¼ 6 Hz, 1H), 7.21 (d, J ¼ 6.4 Hz, 1H), 6.63e6.39 (m, 3H), 6.17 (t, J ¼ 7.2 Hz, 1H), 5.45 (d, J ¼ 8 Hz, 1H), 3.92 (s, 3H), 3.65 (s, 3H), 2.60 (s, 3H), 2.55 (s, 3H), 1.00 (s, 9H), 0.91 (s, 9H). MALDI-TOF MS m/z 856.0 ([M]þ). Anal. Calcd. for C41H45F2IrN4O2(%): C, 57.53; H, 5.30; N, 6.54. Found: C, 57.51; H, 5.32; N, 6.57. The summary of synthetic procedure of the molecules is given in Scheme 1.

Results and discussion Structural characterization 1

H NMR spectrum of the 2-(3-fluorophenyl)-4-methylpyridine shows proton signals for aromatic rings and proton signal at 2.43 ppm indicates the presence of the methyl substituent on the pyridine ring. The ratios of aromatic resonance peaks in the 1H NMR spectra of all iridium complexes prove the presence of two 2phenylpyridine ligands. The two singlet peaks between 2.70 and 2.56 ppm are attributable to the methyl substituent of the pyridine ring of iridium complexes. The complexes (CS127, CS128, CS129, CS130, CS131) show multiplet signals in the range of 4.07e4.20 and two doublet signals at 1.00 and 0.90 ppm due to isopropyl moieties on amidinate ligands, while two singlet peaks of tert-butyl

Table 2 Absorption and emission data of the iridium complexes (5  105 M). Complex

Solvent

4 1 labs cm1) max , (nm) (ε/10 M

CS127

CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene

258 287 283 259 288 284 257 289 283 258 289 284 259 288 284 258 289 284 258 287 288 254 289 284 258 289 285 258 289 284

CS128

CS129

CS130

CS131

CS132

CS133

CS134

CS135

CS136

(2.33) (2.08) (1.11) (1.55) (1.18) (1.15) (0.66) (0.62) (0.78) (1.23) (1.42) (1.01) (1.78) (0.70) (0.80) (2.36) (0.61) (0.86) (1.21) (1.39) (1.27) (0.42) (1.61) (0.51) (0.96) (1.44) (1.39) (1.97) (0.94) (0.91)

300 406 410 300 408 411 299 408 411 300 409 411 300 409 412 300 408 411 302 408 411 301 409 413 301 409 412 302 409 411

(1.16) (0.44) (0.17) (0.70) (0.21) (0.18) (0.34) (0.10) (0.09) (0.64) (0.28) (0.16) (1.19) (0.12) (0.11) (1.05) (0.12) (0.16) (0.58) (0.31) (0.19) (0.21) (0.15) (0.09) (0.48) (0.24) (0.13) (0.90) (0.20) (0.14)

336 452 455 337 453 456 335 453 456 336 453 456 337 452 456 340 453 460 337 448 459 337 450 460 336 451 461 337 451 461

(0.55) (0.30) (0.13) (0.36) (0.14) (0.15) (0.16) (0.06) (0.07) (0.30) (0.19) (0.13) (0.37) (0.09) (0.09) (0.38) (0.09) (0.13) (0.27) (0.24) (0.14) (0.09) (0.11) (0.07) (0.25) (0.17) (0.09) (0.40) (0.15) (0.10)

lem max ðnmÞ 402 (0.32) 448 (0.19)

403 (0.18) 448 (0.13)

402 (0.06) 448 (0.06)

402 (0.14) 452 (0.08)

403 (0.20) 448 (0.12)

402 (0.20) 448 (0.11)

403 (0.14) 451 (0.09)

401 (0.04) 450 (0.02)

403 (0.03) 450 (0.02)

402 (0.21) 443 (0.14)

464, 510 529 465, 514 530 465, 511 529 465, 513 530 464, 512 530 465, 510 530 464, 514 529 464, 513 530 464, 514 529 465, 513 528

495

496

496

496

494

495

496

495

497

496

C. Sahin et al. / Journal of Organometallic Chemistry 772-773 (2014) 68e78

substituents of amidinate ligands in the complexes (CS132, CS133, CS134, CS135, CS136) are observed at 1.00 and 0.90 ppm. Further the structural confirmations of iridium complexes were carried out by FTIR spectroscopy. The spectra of the complexes show peaks in the range of 1620 to 1417 cm1 and the peaks around 3070 cm1 which are characteristic of aromatic rings of 2-phenylpyridine. The peak at 852 cm1 is due to the out of plane bending vibrations of aromatic CeH groups. The characteristic vibration bands of the alkyl chains are observed between 2967 and 2871 cm1. The peak at 1260 cm1 is assigned to the CeF stretching [17]. The thermal properties of the compounds have been investigated using thermal gravimetric analysis (TGA) under nitrogen atmosphere with a heating rate of 20  C/min. The thermoanalytical data of the iridium complexes are summarized in Table 1. The TGA curves of the complexes with isopropyl groups on amidinate ligands (CS127, CS128, CS129, CS130, CS131) show high decomposition temperatures above 360  C (Fig. 1). The first weight loss of the complexes (2.2%) between 30 and 120  C, is attributable to the removal of adsorbed water molecules. The mass loss of the complexes (CS127, CS131) (31.51e33.37%) was observed between 120 and 500  C due to the elimination of the

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one amidinate ligand with isopropyl groups and two methyl units of 2-(3-fluorophenyl)-4-methylpyridine in the second step. Finally, mass loss (38.68e43.01) in the temperature range 500e900  C shows the removal of the two fluoro units and two 2-phenylpyridine ligands. After this decomposition step, the black colored residue with the mass 23% may be assigned to the iridium. Thermal behavior of the complexes (CS128, CS129, CS130) show similar decomposition steps which follow lower mass loss than CS127 and CS131. Furthermore, the effect of N-alkyl substituents of amidinate ligands on thermal stability of iridium complexes has been investigated and no significant influence on the decomposition steps was observed. However, the iridium complexes with isopropyl groups on amidinate ligands (CS127, CS128, CS129, CS130, CS131) give higher thermal stability than the iridium complexes with isobutyl groups on amidinate ligands (CS132, CS133, CS134, CS135, CS136). This suggests that a decrease in the degree of branching may enhance intermolecular interaction and results the higher thermal stability of complexes [18]. The obtained results indicate that the synthesized complexes have high thermal stability required for OLED applications [19].

Fig. 3. PL spectra of 5  107 M, 5  106 M, 5  105 M solutions of the CS127 and CS132 in different solvents (dichloromethane, chlorobenzene, toluene) by excitation at their lowest energy MLCT absorption maxima band at room temperature.

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C. Sahin et al. / Journal of Organometallic Chemistry 772-773 (2014) 68e78

Absorption and emission studies The absorption spectra of the iridium complexes were obtained in different solvents (dichloromethane, chlorobenzene, toluene) at room temperature (Fig. 2). The maximum absorption wavelengths and corresponding absorption coefficients are given in Table 2. The bands in the UV region (257e300 nm) generate from the p / p* ligand centered charge transfer (LC) transitions of 2-(3-fluorophenyl)-4-methylpyridine and absorption bands between 350 and 410 nm are assigned to spin-allowed singlet-tosinglet metal-to-ligand charge-transfer (1MLCT) from Ir(III) to 2(3-fluorophenyl)-4-methylpyridine ligand [20]. The weaker absorption bands around 455 nm are associated with spinforbidden singlet-to-triplet metal-to-ligand charge-transfer (3MLCT) transitions (Ir(III) to 2-(3-fluorophenyl)-4methylpyridine ligand, 3MLCTC^N transition) [21]. The 3MLCT transitions are quite intense despite the spin-forbidden character compared to the spin-allowed 1MLCT transitions. This is attributed to the heavy metal atom effect of iridium(III) which causes a strong spin-orbit coupling [22]. The p / p* and MLCT bands of the iridium complexes are red shifted with the decreasing polarity of the solvents (toluene < chlorobenzene < dichl oromethane).

Fig. 3 shows the normalized photoluminescence (PL) spectra of the synthesized iridium complexes in dichloromethane solutions of different concentrations (5  107, 5  106 and 5  105 M). All iridium complexes exhibit similar emission spectra which give blue emission bands at 465 nm and 495 nm (Table 2). The intensity ratio between the emission bands of 495 nm and 465 nm, increases from 1.35 to 2.39 with the increase in solution concentrations from 5  107 to 5  105 M. The increase in emission band at 495 nm is attributed to excimer emission of the molecules [11] which is further confirmed by the excitation spectra. As it was expected, the excitation bands are shifted to longer wavelengths and broadened out with the increasing solution concentrations (Fig. 4) [23]. Excimer is simply the dimer formation between the excited and ground states of the same molecule and expected to have longer excited state lifetimes. The lifetime (t) values of the complexes were measured by excitation at 472 nm with a pulsed diode laser in the deoxygenated dichloromethane solution at room temperature. The lifetime of the emission peak at 495 nm is longer than that of the peak at 465 nm [11] and for the solution concentrations of 5  105 M the t values are at around 75 ns and 40 ns, respectively. t values of the complexes are increasing with the increase in solution concentrations from 5  106 to 5  104 M (Table 3). This concentration-dependent t increase can be attributed to the

Fig. 4. The excitation spectra of 5  107 M, 5  106 M, 5  105 M solutions of the CS127 and CS132 in different solvents: dichloromethane, chlorobenzene, toluene.

C. Sahin et al. / Journal of Organometallic Chemistry 772-773 (2014) 68e78

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Table 3 Quantum yields and emission lifetimes (t) at the emission maxima of the iridium complexes. Complex

Solvent

5  106 M

FPL CS127

CS128

CS129

CS130

CS131

CS132

CS133

CS134

CS135

CS136

a b

CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene CH2Cl2 Chlorobenzene Toluene

0.06 0.03 0.16 0.05 0.05 0.18 0.05 0.06 0.15 0.08 0.06 0.28 0.05 0.06 0.17 0.05 0.06 0.19 0.04 0.02 0.14 0.03 0.05 0.15 0.04 0.04 0.19 0.04 0.03 0.13

5  105 M

t1(ns)

t2(ns)

a

b

4.2 30.8 22.2 4.3a 32.4 22.4 4.3a 30.3 22.0 5.1a 30.2 23.1 5.4a 34.4 24.2 4.1a 30.2 19.6 3.8a 32.4 19.2 3.5a 31.4 20.6 4.1a 36.8 21.8 4.4a 33.5 20.7

18.8

19.0b

19.6b

20.5b

20.2b

18.2b

12.4b

11.9b

18.6b

18.8b

FPL 0.13 0.04 0.21 0.10 0.06 0.23 0.11 0.09 0.23 0.15 0.07 0.33 0.10 0.09 0.27 0.11 0.08 0.24 0.09 0.04 0.24 0.10 0.06 0.24 0.13 0.07 0.35 0.11 0.06 0.26

5  104 M

t1(ns)

t2(ns)

t1(ns)

t2(ns)

a

b

a

129b

41.2 50.7 42.3 42.8a 54.7 42.9 44.5a 51.6 43.0 46.4a 51.9 44.3 47.4a 55.5 45.7 41.2a 50.3 42.8 40.1a 55.1 42.2 40.5a 53.7 43.7 40.5a 59.4 47.4 40.2a 52.6 45.6

77.7

76.9b

77.3b

80.9b

90.9b

73.7b

70.0b

72.6b

72.5b

74.3b

69.1 60.3 55.5 65.3a 63.4 55.3 66.4a 61.3 57.6 71.2a 65.8 58.0 73.4a 67.2 57.4 59.4a 60.3 56.4 60.8a 64.1 56.4 58.5a 63.5 54.6 60.6a 66.6 58.0 63.9a 63.4 57.4

122b

125b

140b

150b

119b

120b

123b

120b

128b

Life time for lem ¼ ~465. Life time for lem ¼ ~495.

formation of excimer [24,25]. In the synthesized molecules, FMeppy ligands containing polar F atoms can induce intermolecular interactions [11,12] and may cause excimer formation and emission. The iridium complexes containing tert-butyl substituents on amidinate ligands (CS132, CS133, CS134, CS135, CS136) present a decrease in the intensity ratios of excimer bands (495 nm) to monomer bands (465 nm) compared to the complexes with isopropyl groups on amidinate ligands (CS127, CS128, CS129, CS130, CS131). It can be attributed to the steric effect of tert-butyl substituents which decrease the interaction between molecules [26]. However, no significant effect of the side groups on phenyl rings on emission spectrum is dedected. The normalized PL spectra of the iridium complexes in chlorobenzene and toluene (Fig. 3) show similar behavior depending on concentration with the dichloromethane solution of the complexes. But for the concentration of 5  105 M excimer emission becomes dominant. For all of the synthesized complexes, red shift in the emission band with the decreasing polarity of the solvents is obtained. Independent from the excimer formation, this is attributed to the reduction of the interactions between the complex and solvent molecules and aggregate formation [27]. The PL spectra of 5  105 M of the synthesized complexes in different solvents are given in (Fig. 5). The relative emission quantum yields of the iridium complexes were calculated according to the Eq. (1) and by using fac-Ir(bpy)3 as the reference material (FPL ¼ 0.4 in toluene) [28], where FPL is the photoluminescence quantum yield, A is the absorption intensity, S is the integrated emission band area and n is the solvent reflective index, u and s refer the unknown and standard, respectively [29].

fPL ¼ fPLs

Su As n2u Ss Au n2s

(1)

The obtained FPL values are around 0.25 in toluene and 0.1 in chlorobenzene and dichloromethane for the concentration of 5  105 M. An increase in quantum yield is observed at increasing concentrations (Table 3). This increase in quantum yield supports the formation of excimer [30]. Furthermore, it was found that the iridium complexes show a decrease in emission intensities and as a consequence in FPL values with the increasing polarity of solvents. This may be attributed non-radiative transitions from exited states to the ground state of the iridium complexes [31]. The obtained results indicate that the polarity of solvent influences on the photophysical properties of the iridium complexes and the photophysical properties of the iridium complexes may be appropriate to be used as emitter material in OLED applications. But it worths mentioning here, because that the solubilities of these complexes are limited in regular organic solvents and they tend to aggregate in the common film preparation solvents, i.e. toluene and chlorobenzene, their OLED applications will be sentenced to vacuum evaporation. Electrochemical data Electrochemical properties of the synthesized iridium complexes have been investigated by cyclic voltammetry using a cell containing glassy carbon working electrode, Pt wire counter electrode and Ag wire reference electrode in acetonitrile solution containing 0.1 M TBAPF6. The electrochemical data are summarized in Table 4. The differential cyclic voltammograms of all iridium complexes (Fig. 6) show one oxidation and two reduction peaks. The extent of the reversible oxidation peak can be attributed to the contribution of the N atoms of the amidinate ligand and the iridium 5d orbitals to the highest occupied molecular orbital (HOMO) and

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C. Sahin et al. / Journal of Organometallic Chemistry 772-773 (2014) 68e78

Fig. 5. PL spectra of 5  105 M solutions of the iridium complexes in different solvents: dichloromethane, chlorobenzene, toluene by excitation at 350 nm at room temperature.

C. Sahin et al. / Journal of Organometallic Chemistry 772-773 (2014) 68e78 Table 4 The electrochemical data of the iridium complexes in acetonitrile. Complex

Eox1 (V)

Ered1 (V)

Ered2 (V)

HOMO (eV)

LUMO (eV)

Band gap (DEg)(V)

CS127 CS128 CS129 CS130 CS131 CS132 CS133 CS134 CS135 CS136

1.17 1.24 1.22 1.22 1.19 1.07 1.12 1.10 1.11 1.12

1.84 1.76 1.78 1.86 1.87 1.96 1.91 1.91 1.96 1.97

2.03 1.98 2.00 2.02 2.06 2.16 2.10 2.11 2.11 2.12

5.51 5.58 5.56 5.56 5.53 5.41 5.46 5.44 5.45 5.46

2.50 2.58 2.56 2.48 2.47 2.38 2.43 2.43 2.38 2.38

3.01 3.00 3.00 3.08 3.06 3.03 3.03 3.01 3.07 3.07

reductions peaks are related to 2-(3-fluorophenyl)-4methylpyridine ligands [16,32]. The redox potentials of CS128 and CS129 containing propyl and butyl groups are shifted to anodic area compared to CS127. The chain length may cause steric effects which change the electrochemical properties of the compounds [33,34]. The effects of propyl and butyl groups on steric conformations can result in the anodic shift of the redox potentials. The oxidation and reduction peaks of CS130 and CS131 containing eOCH3 and eOCH2CH3 groups are slightly shifted cathodically which are related to the stronger electron donor property of alkoxy groups when compared to iridium complexes with alkyl chains (CS127, CS128, CS129) [35]. The tert-butyl groups on the N atoms of the amidinate ligands of the complexes (CS132, CS133, CS134, CS135, CS136) increase the energy levels of HOMO and LUMO due to their electron-donating abilities compared to the complexes with isopropyl groups on amidinate ligands (CS127, CS128, CS129, CS130, CS131) (Fig. 6). The differential cyclic voltammogram of complexes show an additional reduction peak at 0.70 eV due to the moisture sorption properties of molecules. The HOMO and LUMO energy level of the complexes were calculated using the maximum of first oxidation and reduction potentials [36]. Ferrocene was used as an internal standard (0.46 V vs. Ag/Agþ). The calculated HOMO and LUMO energy levels of the complexes are in the range of 5.58 to 5.41 eV and 2.58 to 2.38 eV, respectively. The HOMO energy levels of the synthesized complexes are lower than the related classes of iridium(III) complexes, (ppy)2Ir(dipba) (4.78 eV) and Ir(ppy)3 (5.2 eV) [16]. This lower HOMO energy levels can allow hole injection from commonly used hole transport layers, (e.g. NPB: 4,4-bis(N-1-

Fig. 6. a) Differential cyclic voltammograms of complexes (CS127, CS128, CS129, CS130, CS131) b) differential cyclic voltammograms of CS127 and CS132 c) 4 consecutive cyclic voltammograms of CS127 measured in acetonitrile solutions containing 0.1 M TBAPF6 with scan speed of 100 mV s1.

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naphthyl)-N-phenylamino)-biphenyl) [16]. The substitution of fluoro as electron withdrawing group to the phenyl ring of 2phenylpyridine and the methyl as electron donating group to the pyridyl ring increase the energy band gap of the complexes to achieve the blue shift in emission color [10]. Consecutive cyclic behaviors of the complexes have been investigated in order to determine their electrochemical stability. No significant change in peak currents and potentials of anodic and cathodic areas are observed. Furthermore, the reversibility of the redox peaks exhibits the electrochemical stability of molecules. Conclusion In the present study, we reported the synthesis, characterization, thermal, photophysical and electrochemical properties of new iridium complexes containing 2-(3-fluorophenyl)-4-methylpyridine and amidinate ligands with different substituents. PL spectra of the complexes were obtained in different solvents. The results exhibited that the polarity of solvent has important influence on PL spectra of the complexes and the formation of excimer emission with increasing solution concentrations from 5  107 to 5  105 M. The thermal properties of the complexes show high decomposition temperatures above 360  C. The electrochemical studies of the iridium complexes exhibit that electron donating methyl group on the pyridyl ring and electron withdrawing fluoro unit on the phenyl ring increase HOMO-LUMO gap. The spectroscopic, electrochemical and thermal properties of synthesized iridium complexes suggest that these molecules can be used utilized in vacuum evaporated OLED applications. Acknowledgments We acknowledge the project support fund of the Scientific Research Council of Turkey (TUBITAK) (Project Number: 112T357). References [1] Y.H. Lan, C.H. Hsiao, P.Y. Lee, Y.C. Bai, C.C. Lee, C.C. Yang, M.K. Leung, M.K. Wei, T.L. Chiu, J.H. Lee, Org. Electron 12 (2011) 756e765. [2] K.K.W. Lo, J.S.Y. Lau, D.K.K. Lo, L.T.L. Lo, Eur. J. Inorg. Chem. (2006) 4054e4062. [3] Y. You, S. Cho, W. Nam, Inorg. Chem. 53 (2014) 1804e1815. [4] N.D. McDaniel, F.J. Coughlin, L.L. Tinker, S. Bernhard, J. Am. Chem. Soc. 130 (2008) 210e217. [5] F.D. Angelis, L. Belpassi, S. Fantacci, J. Mol. Struct. (Theochem) 914 (2009) 74e86. [6] Q. Mei, L. Wang, B. Tian, B. Tong, J. Weng, B. Zhang, Y. Jiang, W. Huang, Dyes Pigm 97 (2013) 43e51. [7] S.S. Yoon, J.Y. Song, E.J. Na, K.H. Lee, S.K. Kim, D.W. Lim, S.J. Lee, Y.K. Kim, Bull. Korean Chem. Soc. 34 (2013) 1366e1370. [8] S. Zhang, F. Wu, W. Yang, Y. Ding, Inorg. Chem. Commun. 14 (2011) 1414e1417. [9] E. Baranoff, S. Fantacci, F.D. Angelis, X. Zhang, R. Scopelliti, M. Greatzel, M. Khaja Nazeeruddin, Inorg. Chem. 50 (2011) 451e462. [10] H.W. Ham, Y.A. Yang, Y.S. Kim, J. Korean Phys. Soc. 57 (2010) 1695e1698. [11] J. Wang, F. Zhang, B. Liu, Z. Xu, J. Zhang, Y. Wang, J. Phys. D Appl. Phys. 46 (2013) 015104, 9pp. [12] C.H. Shin, J.O. Huh, M.H. Lee, Y. Do, Dalton Trans. (2009) 6476e6479. [13] T. Hayashida, H. Nagashima, Organometallics 21 (2002) 3884e3888. [14] C. Sahin, Th Dittrich, C. Varlikli, S. Icli, M. Ch Lux-Steiner, Sol. Energy Mat. Sol. C. 94 (2010) 686e690. [15] H.S. Lee, Y. Ha, Mol. Cryst. Liq. Cryst. 504 (2009) 67e75. [16] Y. Liu, K. Ye, Y. Fan, W. Song, Y. Wang, Z. Hou, Chem. Commun. (2009) 3699e3701. [17] T. Oh, K.M. Lee, K.S. Kim, C.K. Choi, J. Korean Phys. Soc. 45 (2004) 705e708. [18] H. Tang, Y. Li, Q. Chen, B. Chen, Q. Qiao, Wei Yang, H. Wu, Y. Cao, Dyes Pigm 100 (2014) 79e86. [19] A.F. Ma, H.J. Seo, S.H. Jin, U.C. Yoon, M.H. Hyun, S.K. Kang, Y.I. Kim, Bull. Korean Chem. Soc. 30 (2009) 2754e2758. [20] X. Zhang, J. Gao, C. Yang, L. Zhu, Z. Li, K. Zhang, J. Qin, H. You, D. Ma, J. Organomet. Chem. 691 (2006) 4312e4319. [21] S. Ikawa, S. Yagi, T. Maeda, H. Nakazumi, H. Fujiwara, Y. Sakurai, Dyes Pigm 95 (2012) 695e705.

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