New iridium complex as high-efficiency red phosphorescent emitter in polymer light-emitting devices{ Bo Liang, Changyun Jiang, Zhao Chen, Xiuju Zhang, Huahong Shi* and Yong Cao* Received 11th November 2005, Accepted 5th January 2006 First published as an Advance Article on the web 19th January 2006 DOI: 10.1039/b515549e A new heteroleptic iridium complex Ir(1-piq)2pt with 1-phenylisoquinoline and 3-(pyridin-29-yl)- 1H-1,2,4-triazole was synthesized and characterized. The complex was incorporated into phosphorescent polymer light-emitting devices using polyhedral oligomeric silsesquioxane- terminated polyfluorene (PFO-poss) as a host polymer doped with 30% of electron transport materials 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). Red electrophosphorescence was observed with a peak emission at 605 nm. The highest-efficiency polymer light-emitting diode was achieved with PFO-poss doped with 2% Ir(1-piq)2pt. An external quantum efficiency of 10.4% and a luminous efficiency of 9.4 cd A21 were obtained at 10.8 mA cm22. These values were found to be 7.67% and 5.99 cd A21 at 100 mA cm22. Introduction Organic/polymer light emitting diodes (O/PLEDs) have attracted considerable interest among academic and industrial communities because of their potential applications in full- color flat panel displays.1,2 Great progress has been made mainly through the synthesis of efficient lumophores and the development of improved device structures.3���6 The strong spin���orbit coupling caused by heavy metal ions (Os, Ir, Pt) incorporated in the complexes results in efficient intersystem crossing from the singlet to the triplet excited state. These organometallic complexes are highly suitable for phosphorescent light emitting devices due to their relatively short excited state lifetime, high photoluminescence efficiencies and excellent color tunability. Mixing of the singlet and triplet excited states not only removes the spin-forbidden nature of the radiative relaxation of the triplet state but also significantly shortens the triplet state lifetime. Triplet���triplet annihilation is more effectively suppressed because of the shorter lifetime of the triplet-excited state. These devices implement efficient phosphorescent dopants as the light-emitting medium capable of harvesting light from both singlet and triplet excitons to reach an internal quantum efficiency of 100%.7,8 By optimizing the molecular structure of the dopants and the energy transfer process, high external and power efficiencies have been obtained from the green to the red spectral region.9���12 Due to their extremely high efficiency and wavelength tunability over the entire visible spectrum, great efforts have been devoted to the design, synthesis and charac- terization of cyclometalated iridium complexes. It was found that the synthesis of homoleptic cyclometalated iridium complexes was difficult and tedious.13,14 Heteroleptic cyclo- metalated iridium complexes possess comparable properties and device performance.4,15 These complexes have two cyclometalated ligands and a single bidentate, monoanionic ancillary ligand. In general, the ancillary ligands are b-dike- tones. The emission color from the complex is dependent on the choice of cyclometalating ligand, ranging from blue to red. The power efficiency and external quantum efficiency of a device fabricated with Ir(piq)3 as a dopant in a multilayer structure12 are 8.0 lm W21 and 10.3% at 100 cd m22 and 6.3 lm W21 and 9.6% at 300 cd m22. However, the condition for the synthesis of heteroleptic cyclometalated iridium complexes is much milder. Hughes and Bryce16 and Jenekhe et al.17 reviewed and pointed out that the high efficiency is related to the electron-transporting nature of the pt ancillary ligand. A pyrazolyl-borate ligand18 and a triazolyl pyridine ligand19 were reported to be ancillary ligands, which can be used to prepare ������real blue������ heteroleptic cyclometalated iridium complexes. PLEDs have the potential to be used for large area displays, which can be made from the solution. Ir(III) cyclometalated complexes are receiving great attention as efficient phosphor dopants in polymer matrices for applications in the area of organic light-emitting diode devices (OLEDs).20���23 Gong et al. reported high-efficiency red emission of QEext = 5% ph/el and LE = 7.2 cd A21 with the maximum of 600 nm by doping tris(2,5-bis-29-(99,99-dihexyl- fluorene)pyridine)iridium(III) [Ir(HFP)3] into PVK : PBD (40%).24 The external quantum efficiencies of polymer light emitting diodes (PLEDs) based on the phosphorescent dye doped into the polymer host are still much lower than those of small molecule-based OLEDs. Our previous work on red polymer devices has involved the use of polymers PVK and PFO as the host materials doped with PBD and obtained saturated red phosphorescent PLEDs with external quantum efficiency up to 12%.25 In this paper, 1-phenylisoquinoline (1-piq) is chosen as the cyclometalating ligand, and an ancillary ligand 3-(pyridin- 29-yl)-1H-1,2,4-triazole (pt) is synthesized. A corresponding Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Specially Functional Materials and Advanced Manufacturing Technology, South China University of Technology, Guangzhou, 510640, P. R. China. E-mail: poycao@scut.edu.cn Fax: +(86) 20 87110606 Tel: +(86) 20 87114346 { Electronic supplementary information (ESI) available: 1H NMR and 13C NMR spectra for the new complex. See DOI: 10.1039/b515549e PAPER www.rsc.org/materials | Journal of Materials Chemistry This journal is �� The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 1281���1286 | 1281

heteroleptic iridium complex from this ligand was synthesized and characterized. By using the new heteroleptic iridium complex as a guest and polyfluorene as a host, high-efficiency red phosphorescent polymer light-emitting devices were obtained. In comparison, we also synthesized a homoleptic iridium complex Ir(1-piq)3. The devices doped with the homoleptic iridium complex are also demonstrated. Experimental Measurement and characterization The 1H NMR and 13C NMR spectra were collected on a Bruker DRX 400 spectrometer in deuterated chloroform solution operating respectively at 400 or 100 MHz, with tetramethylsilane as reference. Elemental analyses were per- formed on a Vario EL elemental analysis instrument (Elementar Co.). EI-MS were recorded on a LCQ DECA XP Liquid Chromatograph���Mass Spectrometer (Thremo Group) and UV-visible absorption spectra were recorded on a HP 8453 UV-Vis spectrophotometer. Cyclic voltammetry was carried out on a CHI660A electrochemical workstation in a solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) (0.1 M) in dichloromethane at a scan rate of 10 mV s21 at room temperature under argon protection. A platinum electrode was used as the working electrode. A Pt wire was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. Syntheses Reagents. All reagents and solvents were obtained from Aldrich, Acros, and TCI Chemical Co. and used as received. Tetrakis(triphenylphosphine)palladium was stored in an atmosphere of dry argon. The ligands 1-phenylisoquinoline (1-piq)26 and 3-(pyridin-29-yl)-1H-1,2,4-triazole (pt)27 were synthesized according to the literature. N-Phenethyl benzamide (1). Benzoyl chloride(28.10 g, 0.20 mol) was dropped into a mixture of phenethylamine (24.20 g, 0.20 mol) and triethylamine (20.20 g, 0.2 mol) in 300 mL dichloromethane. The mixture was stirred at room temperature for 4 hours. After being washed with water, 5% hydrochloric acid and water, the organic layer was dried with anhydrous magnesium sulfate. The product obtained was purified by recrystallization from ethyl acetate to form white crystals of 3 (37.3 g, 83.3%). GC-MS (225, M+). l-Phenyl-3,4-dihydroisoquinoline (2). 22.51 g (0.1 mol) of 1 were dissolved in 40 mL of xylene. P2O5 (28.60 g, 0.1 mol) and POCl3 (80 mL) were added via stirring, and the mixture was heated to reflux and held at that temperature for 3 hours. Then the solution was allowed to cool to room temperature. The solvent was decanted, and the residual solid was carefully triturated to neutrality with 10% sodium hydroxide solution. The resultant aqueous mixture was extracted with dichloro- methane. The organic layer was dried with anhydrous magnesium sulfate. Evaporating the solvent formed a brown���yellow oily crude product. Further purification by silica gel column using ethyl acetate���dichloromethane (1 : 10) as an elute formed a pale yellow oil (17.11g, 82.4%). 1H NMR (CDCl3, 400 MHz): dH(ppm) 7.60���7.58 (dd, 2 H), 7.41���7.40 (m, 4 H), 7.26���7.21 (m, 3 H), 3.86���3.82 (t, 2 H), 2.80���2.77 (t, 2 H). GC-MS: m/z 207 (M+). l-Phenylisoquinoline (3). 17.01 g (0.082 mol) of 2 were dissolved in 50 mL 1,2,4-trimethylbenzene, and 1.75 g of 10% Pd/C (2% mol) were added, and the mixture refluxed for 3 h at 190 uC in N2. The black power was filtered and washed with dichloromethane several times. Volatiles were removed to leave a yellow liquid, which was stored at 4 uC for 1 h, with a yellow solid being separated out. The product was purified by recrystallization from petroleum ether to form white crystals (12.30 g, 73%). 1H NMR (CDCl3, 400 MHz): dH(ppm) 8.60��� 8.59 (d, 1 H), 8.10���8.08 (d, 1 H), 7.87���7.85 (d, 1 H), 7.70���7.67 (m, 3 H), 7.65���7.62 (t, 1 H), 7.54���7.46 (m, 4 H). GC-MS: m/z 205 (M+). 3-(Pyridin-2-yl)-1H-1,2,4-triazole (4). 5.00 g of 2-cyanopyr- idine were added to 20 ml of methanol and then added to 10 ml of a methanol solution of sodium methoxide (25 wt%), and the resulting solution was stirred at a room temperature for 1 hour. 5 ml of acetic acid were added to the solution dropwise, and 10 g of formylhydrazide were added and stirred at room temperature for 1 hour to precipitate a white crystal. The crystal was separated via filtration, 50 ml of toluene was added into it, and the resulting mixture was stirred under reflux for 3 hours. The mixture was then cooled to room temperature and the solvent was removed to obtain a white solid (4.01 g, 57.0%). 1H NMR (CDCl3, 400 MHz): dH(ppm) 15���14 (br s, 1 H), 8.70���8.68 (d, 1 H), 8.27 (s, 1 H), 8.10���8.08 (d, 1 H), 7.98��� 7.94 (m, 1 H), 7.50���7.47 (m, 1 H). [Ir(1-piq)2Cl]2, tetrakis(l-phenylisoquinoline-C2,N9)(m-chloro- bridged)diiridium(III). Iridium trichloride hydrate (1.318 g, 3.8 mmol) was combined with 1.915 g (9.4 mmol) of 3, dissolved in a mixture of 20 mL 2-ethoxyethanol and water (3 : 1), and refluxed for 24 h in N2. The solution was cooled to room temperature, and the deep-red precipitate was collected on a glass filter frit. The precipitate was washed with 95% ethanol and ethyl ether to form a dark-red power (2.134 g, 90%), which was used directly for the next step without purification.20 Ir(1-piq)3, tris(l-phenylisoquinoline-C2,N9)iridium(III). [Ir(1- piq)2Cl]2 (0.508 g, 0.4 mmol), 3 (0.328 g, 1.6 mmol), acetyl acetone (0.081 g, 0.8 mmol) and triethylamine ((0.080 g, 0.8 mmol) were dissolved in 50 mL of glycol. The solution was refluxed under nitrogen for 48 h. After cooling the reaction mixture to room temperature, 10 mL of 1 M aqueous hydro- chloric acid was added to the solution, resulting in precipita- tion of the product. Then the product was filtered, washed with water and ethyl ether, and dried at 100 uC in vacuum. The purification of the product was carried out by silica gel column chromatography with CH2Cl2 as an eluent to obtain a red powder (0.193 g, 30%). 1H NMR (CDCl3, 400 MHz): dH(ppm) 8.94 (m, 3 H), 8.18 (d, 3 H), 7.71 (m, 3 H), 7.62 (m, 6 H), 7.33 (d, 3 H), 7.09 (d, 3 H), 6.94���6.99 (m, 6 H), 6.84 (t, 3 H). Anal. Calcd for C45H30IrN3: C, 67.14, H, 3.76, N, 5.22. Found: C, 67.24, H, 3.59, N, 5.18%. EIMS: m/z 805 (M + 1)+. 1282 | J. Mater. Chem., 2006, 16, 1281���1286 This journal is �� The Royal Society of Chemistry 2006