Novel iridium(III) complexes based on 2-(2,2’-bithien-5-yl)-quinoline. Synthesis, photophysical, photochemical and DFT studies

Materials Chemistry and Physics xxx (2015) 1e11

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Novel iridium(III) complexes based on 2-(2,2’-bithien-5-yl)-quinoline. Synthesis, photophysical, photochemical and DFT studies _ Grazyna Szafraniec-Gorol a, *, Aneta Słodek a, Michał Filapek a, Bartosz Boharewicz b, _ a, Marta Sołtys a, Joanna Pisarska a, Agnieszka Iwan b, Maria Jaworska a, Lidia Zur a c Iwona Grudzka-Flak , Sylwia Czajkowska , Maciej Sojka d, Witold Danikiewicz d, Stanisław Krompiec a a

Institute of Chemistry, Faculty of Mathematics, Physics and Chemistry, University of Silesia, Szkolna 9, 40-007 Katowice, Poland Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 55/61, 50-369 Wroclaw, Poland c Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland d Institute of Organic Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw, Poland b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Iridium(III) complexes bearing 2bithienylquinolines as main ligands were examined.
Optical and electrochemical measurements were compared with DFT calculations.
Organic BHJ solar cells were constructed using iridium(III) complexes.
The one of solar cell has shown a power conversion efficiency (PCE) of 0.25%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 January 2015 Received in revised form 14 May 2015 Accepted 8 June 2015 Available online xxx

Four novel cyclometalated iridium(III) complexes: [Ir(q-bt-Ph)2(phen)]PF6, [Ir(q-bt-Ph)2(acac)], [Ir(q-btMe)2(bpy)]PF6 and [Ir(q-bt-Me)2(acac)] (where q-bt-Ph, q-bt-Me correspond to 2-(2,2’-bithien-5-yl)-4phenylquinoline and 2-(2,2’-bithien-5-yl)-4-methylquinoline), are reported. The complexes were characterized by NMR, FTIR and HRMS. The optical, electrochemical properties and thermal stability of novel iridium(III) complexes were thoroughly investigated. The complexes emit a light in the narrow range of 693 e707 nm. The optical study showed that replacement of fragment in the main quinoline ligand did not affect wavelength of the emitted light. On the other hand, the modification of the ancillary ligand and substituent in the quinoline ring caused the increase of the photoluminescence quantum yields. Electrochemical experiments demonstrate that the oxidation process for complexes [Ir(q-bt-Ph)2(phen)]PF6 and [Ir(q-bt-Ph)2(acac)] was reversible (or quasi-reversible) and well detectable whereas for complexes with quinoline substituted by methyl group was irreversible, even at low temperature (70  C). The electrochemical and photophysical studies have been well confirmed by density functional theory (DFT) calculations. In addition, bulk heterojunction polymer solar cells based on complexes [Ir(q-bt-Ph)2(phen)]PF6 and [Ir(q-bt-Ph)2(acac)] were fabricated. Only the solar cell incorporating [Ir(q-bt-Ph)2(acac)] exhibited a photovoltaic effect. The architecture of the cell was ITO/PEDOT:PSS/P3HT:PCBM:[Ir(q-bt-Ph)2(acac)]/Al. A power conversion efficiency of 0.25% was measured under 1 sun illumination using an AM 1.5G filter to simulate the solar spectrum. © 2015 Elsevier B.V. All rights reserved.

Keywords: Organometallic compounds Photoluminescence spectroscopy Thermogravimetric analysis (TGA) Electrochemical properties Optical properties

* Corresponding author. E-mail address: [email protected] (G. Szafraniec-Gorol). http://dx.doi.org/10.1016/j.matchemphys.2015.06.020 0254-0584/© 2015 Elsevier B.V. All rights reserved.

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1. Introduction Iridium(III) complexes have been investigated in detail for recent two decades especially with regard to their properties as organic light emitting diodes (OLEDs) [1e4], light-emitting electrochemical cells (LECs) [5e7], dye sensitized solar cells (DSSCs) [8e11], or as a multifunctional devices - organic light-emitting transistors (OLETs) [12,13]. They are widely used as singlet oxygen sensitizers [14], sensitizers for visible-light-driven hydrogen generation [15] and chemosensors for wide range of ions [16]. In addition, they play an essential role as component in the synthesis of anticancer drugs and as bioimaging reagents [17,18]. The phosphorescent iridium(III) complexes are mainly applied as emitter for the OLEDs. Nowadays, organic-light emitting devices exhibiting the highest external quantum efficiency (EQE) are based on bis[2(pyridinyl)phenyl-C2,N](2,4-pentanedionato-O2,O4)iridium(III) [Ir(ppy)2(acac)] as emitter [19]. It was shown that the [Ir(ppy)2acac] exhibited an EQE of 30%, whereas the photoluminescence quantum yield (F) of the aforementioned complex in the dichloromethane solution corresponded to 0.34 [20]. Recently, iridium(III) complexes have also attracted attention as material possessing unconventional physical properties interesting for applications in material science as absorbers for organic photovoltaics (OPVs). Among the large numbers of inorganic solar cells [21], the organic photovoltaic devices have emerged as promising materials due to their low-cost fabrication, high flexibility, and easy processing. In addition, OPVs based on small molecules such as transition metal complexes, especially iridium(III), seem to be the desired photosensitizers exhibiting uniform and well-designed structure, higher open circuit voltage (VOC), enhanced hole mobility as compared to the corresponding polymer-based photovoltaic devices [21,22]. The power conversion efficiency (PCE) in the range of 7e12% has been recently achieved for small-molecule organic photovoltaics devices (SM-OPVs) [21,23,24]. This value is still three times lower than that obtained for inorganic solar cells (for example thin-film GaAs device, where 28% efficiency has been measured) [22]. Therefore, extensive efforts to design and synthesize new iridium(III) complexes that will be used as efficient emitters and photosensitizers have to be made. The chemical modification of iridium(III) complexes can affect the electronic structure and thus their optoelectronic properties. The possibility of straightforward exchanging of the ancillary ligand and the changes in the structure of cyclometalating ligand can considerably alter the electronic properties of iridium(III) complexes, allowing to modify emission color from blue to red [25,26]. Through modification of the chemical structure of complexes one can control the HOMO and LUMO levels [23] and the emission quantum efficiency of iridium(III) complexes [3]. A large number of iridium(III) complexes containing quinoline (q) derivatives have been recently synthesized and investigated mainly in OLEDs [27e29]. Among numerous iridium(III) complexes, only a few complexes with quinoline substituted by thienyl (th) group are known and described. For instance, the 2-(thien-2-yl)quinolinebased iridium(III) complex - [Ir(thq)2(acac)] was presented and applied for fabrication of polymer-based light-emitting diodes (PLEDs) [27]. On the other hand, the aforementioned complex was tested as multisignaling chemosensor for Hg2þ ions [28]. The replacement of acetoacetonate (acac) with 1,3-diphenyl-4phenylacetyl-5-pyrazolone (dpap) ligand (ancillary ligands) in 2(thien-2-yl)quinoline-based iridium(III) complex has led to changes in the photoluminescence spectra and improved the photoluminescence efficiency [29]. In addition to intensive study of iridium(III) complexes as dopant in OLEDs, they have lately attracted a great deal of interest as photovoltaics cells mainly owing to their high quantum efficiency. For example, iridium(III)

complexes bearing 2,4-difluorophenyl-pyridine (dfppy) as donortype materials [30] have exhibited a broad absorption spectrum in the range of 380e562 nm and a power conversion efficiency equals 2.8%. Moreover, they turned out to possess highly thermal stability and tunable HOMO and LUMO energy levels [30]. In our previous study we presented the quinoline derivatives containing the 2,2'-bithien-5-yl motif as potential candidates for cyclometalating ligands [31]. We showed the influence of substituents in the quinoline rings on their electrochemical and photophysical properties. Taking into account the attractive electrochemical and optical properties of quinoline derivatives we synthesized the novel bis-cyclometalated iridium(III) complexes with 2-(2,2’-bithien-5-yl)-4-phenylquinoline and 2-(2,2’-bithien5-yl)-4-methylquinoline as virtual candidates for optoelectronics application. To the best of our knowledge, only a few iridium(III) complexes with 2,2’-bithien-5-yl moiety as p-donating part of C^N cyclometalating ligands have been reported (where N-donating fragment was pyrdinie) [32,33]. In this paper we report synthesis and characterization of bis-cyclometalated iridium(III) complexes with 2-(2,2’-bithien-5-yl)-4-phenylquinoline and 2-(2,2’-bithien5-yl)-4-methylquinoline as main ligands. We choose these type of ligands because the nature of substituents in the quinoline ring strongly influences the electronic features and, hence, complexes containing quinoline derivatives can be good emitters of the red light. The high donor character of bithienyl group can modify the conducting properties, shift the emission band to longer wavelength and, in addition, broaden the absorption spectrum of compounds [33]. Herein, we discuss the electrochemical, physical and optical properties of the novel quinoline-based iridium(III) complexes. The electronic spectra of the studied complexes were computed with the use of time-dependent density functional theory (TDDFT) method. The bulk heterojunction polymer solar cells with iridium(III) complexes as an active layer have been fabricated and their properties were investigated. The main goal of this study was to investigate and obtain new iridium(III) complexes as luminescence and photovoltaic materials. 2. Experimental 2.1. Materials and methods All reagents and solvents were purchased from SigmaeAldrich, while IrCl3 hydrate was purchased from ABCR and ITO was purchased from Ossila company. All reactions were carried out under argon. NMR spectra were obtained with a Bruker Avance 500 MHz instrument. IR spectra were obtained with FT-IR Magna 560 Nicolet instrument with a resolution of 2 cm1. Electrospray ionization (ESI) high resolution mass spectra were recorded with Synapt G2S HDMS (Waters) mass spectrometer using leucine enkephaline as the internal standard. The resolving power of mass spectrometer was 40 000 (FWHM) and the accuracy of measurements was better than 2 ppm. Samples were introduced as the methanol solutions. Surface resistance of ITO was about 20 U/square. UVeVis spectra were recorded with Hewlett Packard models 8453 UVeVis spectrophotometer. Fluorescence measurements were performed with a PTI QuantaMaster QM40 coupled with tunable pulsed optical parametric oscillator (OPO), pumped by a third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD). The luminescence spectra and luminescence decay curves were registered for compounds in the dichloromethane solution. A cell path length of 1 cm was employed. The luminescence was dispersed by double 200 mm monochromators. The luminescence spectra were recorded using a multimode UVVIS PMT (R928) detector controlled by a computer. The excitation correction for real time correction was applied for excitation spectra. Luminescence decay curves were recorded and

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stored by a PTI ASOC-10 [USB-2500] oscilloscope with an accuracy of ±1 ms. All measurements were carried out at room temperature under argon atmosphere. The concentration of examined solution was equal 2*106 mol/dm3. Fluorescence quantum yield of complex using tetraphenyl porphyrin in acetonitrile solution (F ¼ 0.15) as a standard was calculated [34]. Electrochemical measurements were carried out using Eco Chemie Autolab PGSTAT128n potentiostat, platinum coil and silver wire as auxiliary and reference electrode, respectively. Glassy carbon electrode (diam. 2 mm) or platinum electrode (diam. 1 mm) were used as working electrode. Potentials are referenced with respect to ferrocene (Fc), which was used as the internal standard. Cyclic and differential pulse voltammetry experiments were conducted in a standard one-compartment cell, in CH2Cl2 (Carlo Erba, HPLC grade), under argon. Bu4NPF6 (Aldrich; 0.2 M, 99%) was used as the supporting electrolyte. Thermogravimetric studies of iridium(III) complexes were carried out with a thermogravimetric analyser, TGA type Q-1500, MOM Budapest, in pure nitrogen, with a flow rate of 7 L/h. Samples (30 mg) were heated from temperature 20e650  C at a heating rate of 10  C/min. The calculations were performed with the use of Gaussian09 package [35]. The B3LYP functional together with 6-31G(d) basis set for the N, C, O and H atoms and MWB60 basis for the Ir atom was used. Full geometry optimization was performed for cations of complexes 1 and 3 and for complexes 2 and 4. The electronic spectra of the studied complexes were computed with the TDDFT method. The PCM solvent model was used throughout the calculations with CH2Cl2 as the solvent [36,37]. 2.2. Construction and characterization of organic solar cells Solar cells were constructed on an indium tin oxide (ITO)-coated glass substrate, with the structure ITO/PEDOT:PSS/P3HT:PCBM:(X)/ Al. The ITO-coated glass substrate was first cleaned with deionised water and then ultrasonicated in isopropanol for about 20 min. PEDOT:PSS was spin cast (5000 rounds per minute, 25 s) from aqueous solution, to form a film on the ITO substrate. A mixture of P3HT:PCBM:X in chloroform solution with 5% additives of iridium complex was then spin cast on top of the PEDOT:PSS layer. An aluminum electrode was deposited by thermal evaporation in vacuum of about 5  105 Torr and heated at 130  C for about 15 min. Current densityevoltage (JeV) characteristics of the solar cells were measured using Solar Simulator Model SS100AAA with AM 1.5G. For simulation of solar irradiation a xenon lamp with irradiation intensity of 100 mW/cm2 was used. The area of one photovoltaic pixel was 4.5 mm2. 2.3. Synthesis of iridium(III) complexes The synthesis of C^N cyclometalating ligands 2-(2,2’-bithien-5yl)-4-phenylquinoline (q-bt-Ph) and 2-(2,2’-bithien-5-yl)-4methylquinoline (q-bt-Me) was previously described [31]. Twostep synthesis of the iridium(III) complexes 1e4 is shown in Fig. 1. Cyclometalated chloride-bridged iridium dimer [Ir2(C^N)4Cl2] was prepared by modified literature procedure [27]. Iridium chloride (IrCl3*3H2O) and 2.2 eq of precursor of cyclometalating ligands were refluxed for 24 h in a mixture of 2-etoxyethanol:water (3:1). After cooling to room temperature, a dimer was precipitated to water, filtered and dried. The product was then recrystallized from CH2Cl2/hexane mixture by slow evaporation of a more volatile CH2Cl2 on a rotary evaporator at 40  C under slightly reduced pressure. The product was collected by filtration, washed with hexane, and dried in vacuum at 60  C to give a red solid of dimeric iridium(III) complex with 50e60% yield. The dimer was used subsequently without further purification. Synthesis of 1 e [(1,10-phenantroline-N,N)-bis[2-(2,2’-

3

bithien-5-yl)-4-phenylquinolinato-C4,N]iridium(III)] hexafluorophosphate - [Ir(q-bt-Ph)2(phen)]PF6. To a solution of dimer iridium(III) complex - [Ir2(q-bt-Ph)4Cl2] (395 mg, 0.205 mmol) in 20 ml of MeOH/CH2Cl2 (3:1) 1,10-fenantroline (92 mg, 0.511 mmol) was added and the resulting mixture was heated at 70  C under argon atmosphere for 17 h. Then, NH4PF6 (334 mg, 2.05 mmol) in 10 ml of methanol was added and the mixture was refluxed for additional 5 h. After cooling to room temperature, the solvent was removed under reduced pressure. A crude product was dissolved in CH2Cl2 and inorganic solid was filtered off. The filtrate was concentrated under reduced pressure to a small amount of solvent, which then was triturated with Et2O, filtered to give a dark orange solid of [Ir(q-bt-Ph)2(phen)]PF6 (179 mg; 0.143 mmol). Yield 35%. 1 H NMR (400 MHz, DMSO) d 8.83 (d, J ¼ 8.1 Hz, 2H, H8quin), 8.68 (d, J ¼ 4.6 Hz, 2H, H2phen), 8.19 (s, 2H), 8.16 (m, 2H), 7.90 (s, 2H, H3quin), 7.59 (m, 12H, H5quin, Ph), 7.49 (d, J ¼ 4.8 Hz, 2H, H4phen), 7.30 (d, J ¼ 1.2 Hz, 2H, H5phen), 7.23e7.15 (m, 2H, H3'bt), 7.06 (m, 4H, H5'bt, H6quin), 6.86 (m, 2H, H4'bt), 6.60 (s, 2H, H3bt). 13C NMR (126 MHz, CDCl3) d 164.69, 155.79, 152.36, 149.07, 148.76, 147.19, 144.34, 140.10, 136.57, 136.23, 131.66, 131.35, 130.09, 129.71, 129.65, 129.61, 129.52, 129.02, 128.98, 128.10, 127.64, 127.62, 127.10, 126.02, 125.91, 125.41, 124.79, 122.71, 117.71. HRMS: C58H36F6IrN4PS4 [Mþ] calc. 1109.1452, found:1109.1447. Synthesis of 2 bis[2-(2,2’-bithien-5-yl)-4phenylquinolinato-C4,N]iridium(III) (2,4-pentanedionatoO2,O4) - [Ir(q-bt-Ph)2(acac)]. To a solution of [Ir2(q-bt-Ph)4Cl2] (520 mg, 0.27 mmol) in 25 ml of CH2Cl2/MeOH (1:1) a pentane-2,4dione (Hacac) (100 ml, 0.946 mmol) and tetramethylammonium hydroxide (3.4 ml, 25% in MeOH) were added and the resulting mixture was heated at 70  C under argon atmosphere for 20 h. The solvent was removed under reduced pressure. The crude product was dissolved in CH2Cl2, washed with water (3  15 ml) and organic layer was dried over MgSO4. The product was then recrystallized from CH2Cl2/MeOH mixture by slow evaporation of the more volatile CH2Cl2 on a rotary evaporator at 40  C under slightly reduced pressure. The product was collected by filtration, washed with methanol and dried in vacuum at 60  C to give a dark orange solid of [Ir(q-bt-Ph)2(acac)] (106 mg, 0.103 mmol). Yield 19%. 1H NMR (400 MHz, DMSO) d 8.34 (d, J ¼ 8.8 Hz, 2H, H8quin), 7.81 (d, J ¼ 7.3 Hz, 2H, H7quin), 7.75 (m, 2H), 7.74 (s, 2H, H3quin), 7.69e7.64 (m, 4H), 7.61 (dd, J ¼ 9.7, 7.2 Hz, 6H), 7.51e7.45 (m, 2H, H6quin), 7.40 (d, J ¼ 5.0 Hz, 2H, H3'bt), 7.13 (d, J ¼ 2.9 Hz, 2H, H5'bt), 6.97 (dd, J ¼ 4.9, 3.8 Hz, 2H, H4'bt), 6.34 (s, 2H, H3bt), 4.95 (s, 1H, -CH-), 1.64 (s, 6H, -CH3). 13C NMR (126 MHz, DMSO) d 185.45 (-C]O), 164.88, 154.39, 150.43, 149.82, 140.75, 139.08, 136.51, 136.31, 136.05, 130.97, 130.47, 129.82, 129.73, 129.18, 128.88, 128.83, 128.62, 128.42, 126.23, 124.84, 124.44, 123.81, 116.71, 101.05 (-CH- acac), 27.94 (-CH3). HRMS: C51H35IrN2O2S4 [Mþ] calc. 1028.1211, found 1028.1199. Synthesis of 3 e [(2,2’-bipyridine)-bis[2-(2,2’-bithien-5-yl)-4methylquinolinato-C4,N]iridium(III)] hexafluorophosphate [Ir(q-bt-Me)2(bpy)]PF6. To a vigorously stirred solution of dimeric iridium(III) complex - [Ir2(q-bt-Me)4Cl2] (52 mg, 0.033 mmol) in 12 ml of CH2Cl2/MeOH (2:1), 2,2’-bipyridine (11 mg, 0.73 mmol) was added and the resulting mixture was heated at 70  C under argon atmosphere for 20 h. Then, NH4PF6 (50 mg, 0.3 mmol) in 6 ml of methanol was added, and the reaction mixture was kept at 70  C under argon atmosphere for additional 5 h. The solvent was removed under reduced pressure. The crude product was dissolved in CH2Cl2, filtered, and the filtrate was concentrated. The product was then recrystallized from CH2Cl2/EtOH mixture by slow evaporation of the more volatile CH2Cl2 under slightly reduced pressure. The product was filtered, washed with EtOH and dried in vacuum at 60  C to give orange solid of [Ir(q-bt-Me)2(bpy)]PF6 (45 mg, 0.041 mmol). Yield 62%. 1H NMR (400 MHz, Acetone) d 8.56 (d, J ¼ 8.4 Hz, 2H, H8quin), 8.48 (d, J ¼ 5.6 Hz, 2H, H2bpy), 8.19 (m, 2H,

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Fig. 1. Synthesis of Iridium(III) complexes 1e4.

H7quin), 7.99 (dd, J ¼ 8.0, 1.2 Hz, 2H, H5quin), 7.90 (s, 2H, H3quin), 7.83e7.75 (m, 2H, H3bpy), 7.42 (dd, J ¼ 7.2, 1.2 Hz, 2H, H4bpy), 7.40 (dd, J ¼ 5.2, 1.2 Hz, 2H, H3'bt), 7.25 (dd, J ¼ 8.8, 0.8 Hz, 2H, H5bpy), 7.20 (dd, J ¼ 4.0, 1.2 Hz, 2H, H5'bt), 7.10 (m, 2H, H6quin), 7.02 (dd, J ¼ 5.2, 4.0 Hz, 2H, H4'bt), 6.54 (s, 2H, H3bt), 2.96 (s, 6H, -CH3). 13C NMR (126 MHz, DMSO) d 164.52, 156.56, 155.72, 149.99, 148.73, 147.56, 143.54, 140.56, 139.76, 136.05, 131.39, 129.28, 129.24, 129.06, 127.34, 126.31, 126.29, 126.06, 126.03, 124.82, 123.16, 118.70, 18.81 (-CH3 quin). HRMS: C46H32IrN4S4 [Mþ] calc. 961.1139, found 961.1156. Synthesis of 4 bis[2-(2,2’-bithien-5-yl)-4methylquinolinato-C4,N]iridium(III) (2,4-pentanedionatoO2,O4) - [Ir(q-bt-Me)2(acac)]. To a solution of dimeric iridium(III) complex - [Ir2(q-bt-Me)4Cl2] (160 mg, 0.098 mmol) in 15 ml of CH2Cl2/MeOH (2:1), pentane-2,4-dione (Hacac) (25 ml, 0.247 mmol) and solution tetramethylammonium hydroxide (0.9 ml, 25% in MeOH) were added and the resulting mixture was heated at 70  C under argon atmosphere for 20 h. The solvent was removed under reduced pressure. The crude product was dissolved in CH2Cl2, washed with water (3  15 ml) and organic layer was dried over MgSO4. The product was then recrystallized from CH2Cl2/MeOH mixture by slow evaporation of the more volatile CH2Cl2 in a rotary evaporator at 40  C under slightly reduced pressure. The product was collected by filtration, washed with hexane and dried in vacuum at 60  C to give dark orange solid of [Ir(q-bt-Me)2(acac)] (64 mg, 0.071 mmol). Yield 35%. 1H NMR (400 MHz, CDCl3) d 8.35 (m, 2H, H8quin), 7.88 (m, 2H, H7quin), 7.51 (s, 2H, H3quin), 7.44 (m, 4H, H5,6quin), 7.06 (dd, J ¼ 4.8, 1.2 Hz, 2H, H3'bt), 7.02 (dd, J ¼ 3.6, 1.2 Hz, 2H, H5'bt), 6.87 (dd, J ¼ 5.2, 4.0 Hz, 2H, H4'bt), 6.37 (s, 2H, H3quin), 4.82 (s, 1H, -CH-), 2.87 (s, 6H, -CH3 quin), 1.62 (s, 6H, -CH3 acac). 13C NMR (101 MHz, CDCl3) d 185.47(-C]O), 165.63, 154.43, 149.78,

146.28, 140.83, 139.64, 137.88, 131.41, 130.36, 127.53, 125.79, 125.38, 124.42, 123.98, 123.87, 117.50, 100.70 (-CH- acac), 28.24(-CH3 acac), 19.02(-CH3 quin). HRMS: C41H31IrN2O2S4 [Mþ] calc. 904.0898, found 904.0892. 3. Results and discussion 3.1. Optical properties The maxima of absorption and emission, lifetimes and Flum of iridium(III) complexes 1e4 were recorded at room temperature and are collected in Table 1. The absorption spectra of complexes 1e4 are shown in Fig. 2. The electronic transitions for the iridium(III) complexes 1e4 calculated with the TD DFT method are described in Table 2. The orbital contour plots are presented in Table S1. The absorption peaks of novel complexes 1e4 were compared with the calculated transitions which allowed for ascription of their electronic character. The intense bands (ε > 104 M1cm1) in a wide range of 200e380 nm are attributed to both pp* ligand-centered (LC) and metal-to-ligand charge transfer (MLCT) transitions colligated with the C^N ligands. The next bands with a smaller intensity (ε ~ 104 M1cm1) are observed at 427, 434, 409 and 431 nm for complex 1, 2, 3, and 4, respectively. These bands could be assigned mainly as transitions between ligands, both ligandeligand (LL) and intraligand (IL) charge transfer. The lowest energy bands are observed at 498 (complex 1), 537 (complex 2), 488 (complex 3) and 526 nm (complex 4). The molar absorption coefficients of the lowest energy bands are 8221, 5214, 10731, and 4777 M-1cm1 for complex 1, 2, 3, and 4, respectively. The value of the molar absorption coefficient is considerably higher for cationic iridium

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Table 1 Photophysical data of quinoline-based iridium(III) complexes 1e4. Compound

Absorption lmaxa [nm] (ε[M1cm1])

Emissionb lmax [nm]

tb [ms]

Flumb,c

Stokes shift [nm] (cm1)

1 2 3 4

273 337 341 337

696 707 693 703

4.7 4.6 5.0 5.1

0.0009 0.0034 0.0006 0.0009

197 170 206 182

a b c

(21216), (21207), (20662), (15512),

347 (23045), 427 (11751), 466 (9347), 498 (8221) 389 (16119), 434 (9409), 502 (5220), 537 (5214) 409 (13969), 456 (10331), 488 (10731) 379(15715), 431 (8447), 495 (5011), 526 (4777)

(5672) (4478) (6104) (4969)

measured at 293 K in CH2Cl2 (2*106 M). measured at 293 K in degassed CH2Cl2 of (2*106 M), at lmax on the lowest energy transition. Flum is shown relative to tetraphenyl porphyrin in acetonitrile solution (F ¼ 0.15) [33].

complexes than for neutral ones. The obtained values of molar absorption coefficients for 1e4 are close to values of iridium(III) complexes bearing azaperylene (560 nm, ε ¼ 6000 cm1M1) [30], and izoquinoline motif (545 nm, ε ¼ 7000 cm1M1), which were tested in photovoltaic devices [38]. The simulated absorption spectra of complexes 1e4 with the DFT method are presented in Fig. 3. The MO diagrams together with HOMO and LUMO contour plots are depicted in Fig. 4. The shape and position of absorption maxima obtained after calculations are very close to experimental bands of complexes 1e4. The change of ancillary ligand from neutral to anionic caused a shift of the lowest energy band by 256 cm1 to the red region. These bands are mainly attributed to HOMO/LUMOþ1 transitions for complexes 1, 2 and 3 whereas the transition HOMO/LUMO is responsible for lower energy absorption band in compound 4 (Table 2). For the complex 1, the HOMO is mainly located on the central iridium atom and bithiophene fragment while the LUMOþ1 is distributed on the iridium and ancillary ligand (phenantroline fragment). Therefore, the lowest energy band can be described as MLCT and LL transitions. In the case of complex 2 with acetoacetonate (acac) as ancillary ligand the HOMO is localized on the metal and bithiophene moiety while the LUMOþ1 is entirely delocalized on the cyclometalating ligands (both quinoline and bithiophene fragments). The lowest energy band of complex 2 is generated by MLCT/LL/IL transitions. For the complex 3, the electron density of the HOMO is mainly located on the central iridium atom and bithiophene moiety, while the LUMOþ1 is located on the both main ligands and central metal atom. Thus, the lowest energy band is due to MLCT/LL/IL transitions. In the case of complex 4 HOMO is located on the central iridium metal and bithiophene fragment, while the LUMO is delocalized on one part of the main ligand i.e. bithienylquinoline. It points out that the absorption band at 526 nm is due to MLCT/IL transition. It is worth noting that only in the case of

4 the oscillator strength for HOMO-LUMO transition is higher than 0.1. This transition has no significant contribution in UVeVis spectrum for other iridium complexes, though, the tail at around 550 nm for complexes 1 and 3 was observed. The emission spectra of complexes 1e4 are shown in Fig. 5. The spectra were registered under excitation at 499 nm (complex 1), 537 nm (complex 2), 487 nm (complex 3), and 521 nm (complex 4). Our luminescence study indicates that all examined compounds are red emitting light systems in CH2Cl2 solution. The luminescence band for complex 1 is located at 696 nm but the total emission quantum yield is low (F (1) ¼ 0.0009). The compound 2 has red emission band at 707 nm and possesses higher quantum yield than the cationic complex 1, determined to be 0.0034. The emission band for complex 3 is observed at 693 nm, whereas the neutral complex 4 has emission band at 703 nm. Moreover, the luminescence bands are slightly shifted to longer wavelengths for compounds 2 and 4. The Stokes shifts were also determined and their values are given in Table 1. For all studied complexes 1e4, the Stokes shifts are relatively large, where values for cationic complexes 1 and 3 are higher than that for neutral ones 2 and 4. It is worth noting that the photoluminescence quantum yield for complex 2 is the highest among the examined complexes determined to be F (2) ¼ 0.0034, however, this value remains still considerably lower than that of the described bis-cyclometalated iridium(III) complexes (F ¼ 0.1e0.8) [2]. Luminescence decay curves for complexes 1e4 have been analyzed in details. The experimental decays are well fitted to a nearly single exponential function (Fig. S13 in SI). The observed lifetimes are consistent with emission from a triplet state. The values of luminescence lifetimes slightly vary from 4.7 (1), 4.6 (2) through 5.0 (3) to 5.1 ms (4). It is worth mentioning, that obtained values seem to be independent from the type of complex. Regardless of whether the studied complexes are cationic (1, 3) or neutral (2, 4), the luminescence lifetimes is around 5 ms. The presented results are in good agreement with previously published results for iridium(III) complexes, where [Ir(2,4difluorophenylpyridine)2(4,40 -dimethylamino-2,20 -bipyridine)](PF6) has exhibited the luminescence lifetime equal to 4.1 ms [39]. On the other hand, the investigated complexes 1e4 possess longer luminescence lifetimes than [Ir(bt-py)2(acac)] (1.3 ms) [32]. It suggests, that the spin-orbit coupling causes more efficient triplet radiative degradation in the [Ir(bt-py)2(acac)] than in the presented complexes 1e4. The occurrence of additional fused benzene ring in quinoline (compared to pyridine) and phenyl substituent in main ligand induces an elongation of the luminescence lifetime [32]. 3.2. Cyclic voltammetry

Fig. 2. The absorption spectra of iridium(III) complexes 1e4 in CH2Cl2 solution.

Electrochemical properties of the studied compounds 1e4 were investigated in CH2Cl2 solution. The electrochemical oxidation and reduction onset potentials were used for estimation of the HOMO and LUMO energies (or rather, ionization potentials (IP) and electron affinities (EA)) of the materials (assuming that IP of ferrocene

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Table 2 The electronic transitions for the complexes 1e4 calculated with B3LYP functional by TDDFT method.

l [nm]

f

(1) [Ir(q-bt-Ph)2(phen)]þ 550 0.0096 500 0.0792 464 0.2225 463 0.3547 443 0.394 371 0.2794 365 0.1311 361 0.1208 354 339 (2) [Ir(q-bt-Ph)2(acac)] 529 464 458 430 393 387 346

0.1827 0.2297 0.2114 0.2633 0.1429 0.197 0.3785 0.2278 0.5911

(3) [Ir(q-bt-Me)2(bpy)]þ 538 0.0042 495 0.2211 455 0.2694 452 0.5454 381 0.2781 376 0.1999 335 0.4128 (4) [Ir(q-bt-Me)2(acac)] 518 0.1095 484 0.1771 458 0.2679 443 0.3421 413 0.1247 376 0.3763 369

0.1791

Transitions

Character

HOMO / LUMO (94%) HOMO / LUMOþ1 (90%) HOMO / LUMOþ3 (77%) HOMO-1 / LUMOþ2 (80%) HOMO-1 / LUMOþ3 (89%) HOMO / LUMOþ4 (51%) HOMO-3 / LUMOþ2 (53%) HOMO-1 / LUMOþ4 (27%) HOMO / LUMOþ5 (34%) HOMO-2 / LUMOþ3 (43%) HOMO-4 / LUMOþ3 (64%)

dIr/pbt(2)/pbt(1) / p*phenantr dIr/pbt(2)/pbt(1) / p*phenantr dIr/pbt(2)/pbt(1) / dIr/p*quin-bt(2) dIr/pbt(1)/pquin-bt(2) / p*quin-bt(1)/p*phenantr dIr/pbt(1)/pquin-bt(2) / dIr/p*quin-bt(2) dIr/pbt(2)/pbt(1) / p*quin-bt(1)/p*quin-bt(2) dIr/pbt(1) / p*quin-bt(1)/p*phenantr dIr/pbt(1)/pquin-bt(2) / p*quin-bt(1)/p*quin-bt(2) dIr/pbt(2)/pbt(1) / p*quin-bt(1)/p*quin-bt(2) dIr/pquin(1)/pbt(2) / dIr/p*quin-bt(2) dIr/pquin(2) / dIr/p*quin-bt(2)

HOMO / LUMOþ1 (82%) HOMO-2 / LUMO (87%) HOMO-1 / LUMOþ1 (92%) HOMO-2 / LUMOþ1 (85%) HOMO-3 / LUMOþ1 (92%) HOMO-1 / LUMO (89%) HOMO-5 / LUMO (63%) HOMO-1 / LUMOþ3 (31%)

dIr/pbt(2)/pbt(1) / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/pacac / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/p bt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/pacac / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/pbt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/p bt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/pquin(1)/pquin(2) / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/p bt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2)

HOMO / LUMO (97%) HOMO / LUMOþ1 (96%) HOMO-1 / LUMOþ1 (92%) HOMO-1 / LUMOþ2 (95%) HOMO-2 / LUMOþ1 (93%) HOMO-2 / LUMOþ2 (85%) HOMO-1 / LUMOþ6 (74%)

dIr/pbt(1)/pbt(2) / dIr/p*bpy dIr/pbt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2) pbt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2) pbt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/pbt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2) dIr/pbt(1)/pbt(2) / dIr/p*quin-bt(1)/p*quin-bt(2) pbt(1)/pbt(2) / p*quin-bt(1)/p*quin-bt(2)

HOMO / LUMO (93%) HOMO / LUMOþ1 (92%) HOMO-1 / LUMO (94%) HOMO-1 / LUMOþ1 (86%) HOMO-2 / LUMO (86%) HOMO-4 / LUMO (49%); HOMO / LUMOþ2 (21%) HOMO-3 / LUMOþ1(66%)

dIr/pquin-bt(1)/pbt(1) / dIr/p*quin-bt(2) dIr/pquin-bt(1)/pbt(1) / dIr/p*quin-bt(1) dIr/pquin-bt(1)/pquin-bt(2) / dIr/p*quin-bt(2) dIr/pquin-bt(1)/pquin-bt(2) / dIr/p*quin-bt(1) dIr/pacac/pbt(2) / dIr/p*quin-bt(2) dIr/pbt(2)/pquin(1) / dIr/p*quin-bt(2); dIr/pquin-bt(1)/pbt(1) / p*quin-bt(1)/p*quin-bt(2) dIr/pbt(1)/pquin(2) / dIr/p*quin-bt(1)

Exp l [nm]

498 466 427 347

537 502 434 389 337

488 456 409 341 526 495 431

379

Fig. 3. Simulated absorption spectra of iridium(III) complexes 1e4 in CH2Cl2 media with the TDDFT B3LYP method.

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Fig. 4. Contour plots of the HOMOs and LUMOs of Ir(III) complexes 1e4 with energy levels from DFT calculations.

equals 5.1 eV [40]). The calculated HOMO and LUMO levels together with electrochemical energy band gaps (Eg) are presented in Table 3.

Fig. 5. The emission spectra of iridium(III) complexes 1e4 in CH2Cl2 solution.

Firstly, the electrochemical properties during oxidation process were investigated. For 1 and 2 the first [Ir]/[Ir]þ and the second [Ir]þ/[Ir]2þ oxidations were reversible (or quasi-reversible) and well detectable. For 1 E1ox was estimated at 0.32 V (peak onset) while E2ox was at 0.69 V, however in both cases (from thermodynamical point of view) the process is quasi-reversible in nature (i.e. peak-topeak separation DE > 60 mV). Next, we conducted the experiment in dry ice/acetone bath (obtaining 70  C). Under these conditions [Ir]/[Ir]þ and [Ir]þ/[Ir]2þ become fully reversible with DE ¼ 59 mV (Figs. 6 and 7a). Due to the fact that in lower temperature geometrical rearrangements are prevented (or slowed enough) electron transfer is maintained at the values required by the Nernst equation. Similar behavior was observed for complex 2 where E1ox (0.27 V) and E2ox (0.61 V) were quasi-reversible at r.t. while at 70  C were converted to fully reversible (see Fig. 7a). Quite different results were obtained for complexes 3 and 4 with methyl substituted quinoline ligands. In this case, oxidation occurred at much higher potential i.e. 0.92 and 0.89 V for 3 and 4, respectively. In addition, this process was irreversible, even at lower temperature and higher scan rates. It indicates that formal charge at complex core does not affect oxidation while structure of cyclometalating moieties is crucial. The nature of electronic features are shifted e for 1 and 2 the oxidation undergoes with dominant metal character while for 3 and 4 is located mainly at bithienyl substituents. This observation is also confirmed by DFT calculations where in complexes with q-bt-Ph main ligand, HOMO is located mainly on iridium atom, while for complexes 3 and 4 on bithiophene moiety.

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Table 3 Electrochemical properties of iridium(III) complexes 1e4. CODE

Ox1

1 1f 2 2f 3 4

0.32 0.33 0.27 0.33 0.92 0.86

a b c d e f g

Ox2 (100)e (59)e (80)e (59)e

0.69 0.57 0.61 0.81 e e

(80)e (59)e (80)e (60)e

Red1

Red2

Eg(CV)a

Eg(opt)b

HOMOc

LUMOd

HOMOg

LUMOg

1.73 1.89(60)e 2.04 1.99(60)e 1.42 1.31

e 2.12(60)e e 2.2 (60)e e e

2.05 2.22 2.31 2.32 2.34 2.17

2.17 e 2.10 e 2.30 2.14

5.42 5.43 5.37 5.43 6.02 5.96

3.37 3.21 3.06 3.11 3.68 3.79

5.53 e 5.01 e 5.49 5.03

2.73 e 2.15 e 2.62 2.07

Eg ¼ Eox (onset)  Ered (onset). Eg optical ¼ 1240/lonset. HOMO ¼ 5.1 e Eox [40]. LUMO ¼ 5.1 e Ered [40]. E1/2red e formal redox potential calculated as (Eox þ Ered)/2 from cyclic voltammetry. Measured in dry ice/acetone bath (70  C). Calculated by TDDFT method.

In the next step, the behavior of complexes 1e4 during reduction was investigated. Compound 1 underwent two-step (reversible) reduction at 1.73 (E1red) and 2.03 V (E2red) (Fig. 7b), whereas the corresponding processes in the case of 2 were observed at 1.89 and 2.12 V (Fig. 7c). In contrast, 3 and 4 have exhibited irreversible reduction at 1.40 and 1.31 V, respectively (Fig. 7d). As indicated by DFT calculations, for complexes 3 and 4 LUMO is located mainly on bithiophene moiety, while for 1 on phenantroline, and for 2 on iridium atom. Probably, reduction (occurring on the bithiophene part) is followed by cleavage at LeIr bond. It is worth noting that discussed molecules 1e4 possess optical band gap very similar to estimated by cyclic voltammetry experiments (Table 3). This, in turn, points out that HOMO and LUMO overlapping is s 0 and photo-excited electron can be transferred between these two orbitals. In addition, due to the fact that in case of 1 and 2 electrochemical processes were found to be reversible or quasi-reversible (what is in the great importance in scope of photovoltaic's research), we selected only this two complexes (1 and 2) for further investigations. 3.3. Thermogravimetric analysis The thermogravimetric plots are shown in Fig. 8. The TGA data show that iridium complexes 1 and 2 possess an excellent thermal stability. The onset temperature of weight loss was at 317  C (for

compound 1) and at 299  C (for compound 2). For compound 1 the temperatures of 5 and 10% weight loss were determined to be 355 and 467  C, respectively. Moreover, the temperatures of 5% (323  C) and 10% (348  C) weight loss of complex 2 were slightly lower than for complex 1. Complexes 1 and 2 have shown better thermal stability than iridium(III) complexes with 2-phenylquinoxaline ligands, the latter were stable only in the range of 220e290  C [41]. The complex 2 has exhibited a lower decomposition temperature than complex 1. Obviously, it is caused by the presence of more volatile acac moiety in the complex 2. In addition, the ionic character of the complex 1 significantly affects the rise of decomposition temperature. The high decomposition temperature of complexes 1 and 2 is required for their use as the active layer in photovoltaic cells. 3.4. Study of photovoltaics devices Compounds 1 and 2 were preliminary investigated as an additive in bulk heterojunction (BHJ) polymer solar cells based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was used as hole transporting layer (HTL). Photovoltaic properties were measured for the BHJ devices with the architecture ITO/PEDOT:PSS/ P3HT:PCBM:(2)/Al. The device with the active layer P3HT:PCBM:(2) showed a power-conversion efficiency PCE of 0.25%. On the other hand, BHJ device with complex 1 in active layer did not exhibit PV properties measured under 100 mW/cm2 AM 1.5G solar illumination. Our study showed that chemical structure of an additive in the active layer influences the performance of polymer solar cells. The presented photovoltaic experiments suggested that neutral iridium complex 2 with acac as ancillary ligand is better for the photovoltaic applications than complex 1. J-V characteristic of the BHJ polymer solar cell with 2 is presented in Fig. 9 along with architecture of device and PV parameters. Taking into consideration the present trends in the organic solar cells it is very important and prosperous to apply new small organic compounds in PV towards to control morphology of active layer, investigation the intermolecular interactions and miscibility towards increase the performance of OPV [42e44]. 4. Conclusion

Fig. 6. Cyclic voltammogramm of 1 during oxidation process; GC as working electrode; sweep rate n ¼ 100 mV/s, 0.2 M Bu4NPF6 in CH2Cl2 at 70  C.

Four cyclometalated iridium(III) complexes bearing 2-bithienylquinoline as the main ligand have been synthesized, characterized and studied with regard to their photophysical and

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Fig. 7. Cyclic voltammogramms of 2 during oxidation process (a), reduction of: 1 (b); 2 (c) and 4 (d); GC as working electrode; sweep rate n ¼ 100 mV/s (for CV); potential step ¼ 2.5 mV/s (for DPV), 0.2 M Bu4NPF6 in CH2Cl2.

Fig. 8. Thermogravimetric analysis curves of complexes 1 and 2.

electrochemical properties. The complexes 1e4 show a wide absorption spectrum in the range of 280e540 nm, caused by mixed transitions MLCT/LLCT/ILCT. Interestingly, the transition HOMO/LUMO, for cationic complexes 1 and 3 was characterized with extremely low oscillator strengths, determined to be 0.009-0.004. Notwithstanding, these transitions are seen in the absorption spectra as the tail at around 550 nm. The iridium(III) complexes 1e4 are a red emitters with maxima bands located approximately at 700 nm. Moreover, the emission quantum yield of complex 2 (3.4%) was much better compared to the emission quantum yield of the other ones. It was demonstrated that the values of band gaps obtained electrochemically, optically, and by DFT are very close to each other. The electrochemical measurements showed that in case of complex 1 and 2 the oxidation and reduction processes were reversible (or quasi-reversible), thus they were examined as active layers in the photovoltaics. The bulk heterojunction solar cells based on complexes 1 and 2 were fabricated. It was found that complex 2 e [Ir(q-bt-Ph)2(acac)] displayed better parameters. The device based on complex 2 exhibits a power conversion efficiency (PCE) of 0.25%. The value of PCE for 2 (0.25%) is not satisfactorily high but it could be a good starting point to look for an efficient photovoltaics based on transition metal complexes, especially iridium(III).

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Fig. 9. J-V curves of BHJ organic solar cell under AM 1.5 G-simulated solar illumination (100 mW/cm2) along with architecture of device and PV parameters.

Acknowledgments This work was supported by National Science Centre of Poland, Projects No. 2011/01/B/ST5/06309 and the National Centre for Research and Development PBS2/A5/40/2014. G. Szafraniec-Gorol, M. Sołtys acknowledge a scholarship from the DoktoRIS project co-financed by the European Social. The Gaussian09 calculations were carried out in the Wroclaw Centre for Networking and Supercomputing, WCSS, Wroclaw, Poland. http://www.wcss.wroc. pl, under calculational Grant No. 18. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2015.06.020. References [1] C. Ulbricht, B. Beyer, C. Friebe, A. Winter, U.S. Schubert, Adv. Mater. 21 (2009) 4418e4441. [2] H.-W. Hong, T.-M. Chen, Mat. Chem. Phys. 101 (2007) 170e176. [3] E. Baranoff, J.-H. Yum, M. Graetzel, MdK. Nazeeruddin, J. Organomet. Chem. 694 (2009) 2661e2670. [4] M.S. Lowry, S. Bernhard, Chem. Eur. J. 12 (2006) 7970e7977. [5] J. Zhang, L. Zhou, H.A. Al-Attar, K. Shao, L. Wang, D. Zhu, Z. Su, M.R. Bryce, A.P. Monkman, Adv. Funct. Mater. 23 (2013) 4667e4677. [6] A.M. Bünzli, H.J. Bolink, E.C. Constable, C.E. Housecroft, J.M. Junquerandez, M. Neuburger, E. Ortí, A. Perteg rez, D. Tordera, Herna as, J.J. Serrano-Pe J.A. Zampese, Dalton Trans. 43 (2014) 738e750. [7] C.D. Sunesh, O. Sunseong, M. Chandran, D. Moon, Y. Choe, Mat. Chem. Phys. 136 (2012) 173e178. [8] Y. Shinpuku, F. Inui, M. Nakai, Y. Nakabayashi, J. Photoch. Photob. A 222 (2011) 203e209. [9] Z. Ning, Q. Zhang, W. Wu, H. Tian, J. Organom. Chem. 694 (2009) 2705e2711. [10] E.I. Mayo, K. Kilsa, T. Tirrell, P.I. Djurovich, A. Tamayo, M.E. Thompson, N.S. Lewis, H.B. Gray, Photochem. Photobiol. Sci. 5 (2006) 871e873. [11] C. Dragonetti, A. Valore, A. Colombo, S. Righetto, V. Trifiletti, Inorg. Chim. Acta 388 (2012) 163e167. [12] J. Li, G. Dong, L. Duan, D. Ma, T. Hu, Y. Zhang, L. Wang, Y. Qiu, RSC Adv. 4 (2014) 51294e51297. [13] Y. Zhou, S. Han, G. Zhou, W.-Y. Wong, V.A.L. Roy, App. Phys. Lett. 102 (2013) 083301.

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Please cite this article in press as: G. Szafraniec-Gorol, et al., Novel iridium(III) complexes based on 2-(2,2’-bithien-5-yl)-quinoline. Synthesis, photophysical, photochemical and DFT studies, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.020