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Phosphoryl/Sulfonyl-Substituted Iridium Complexes as Blue Phosphorescent Emitters for Single-Layer Blue and White Organic Light-Emitting Diodes by Solution Process Cong Fan,† Yanhu Li,‡ Chuluo Yang,†,* Hongbin Wu,‡,* Jingui Qin,† and Yong Cao‡ †

Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan, 430072, People’s Republic of China ‡ Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, People’s Republic of China S Supporting Information *

ABSTRACT: Two new phosphoryl/sulfonyl-substituted iridium complexes, POFIrpic and SOFIrpic, have been designed and synthesized on the basis of the structural frame of sky-blue FIrpic. The introduction of phosphoryl/sulfonyl moieties into the 5′-position of phenyl ring makes the emission peak blue-shift to the 460 nm, simultaneously the compounds maintain high photoluminescence quantum yields (PLQYs) of about 50% in solution. Single-layer blue and white polymer organic light-emitting diodes by full solution-process were fabricated with the following configuration: ITO/PEDOT:PSS/PVK:OXD-7:dopants/CsF/Al. The blue device based on POFIrpic shows a maximum current efficiency of 11.1 cd A−1, a maximum external quantum efficiency of 7.1%, which are the highest ever reported for blue PhOLEDs by full solution process. The white device with POFIrpic as blue component reveals a maximum current efficiency of 25 cd A−1, a maximum external quantum efficiency of 15%, and a good CRI value of 82. KEYWORDS: phosphoryl, sulfonyl, iridium complex, phosphorescence, organic light-emitting diodes

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INTRODUCTION Because of the electron spin−orbit coupling (SOC) and fast intersystem crossing (ISC), phosphorescent heavy metal complexes can harvest both electrogenerated singlet and triplet excitons in the emitting layer of organic light-emitting diodes (OLEDs) and achieve 100% internal quantum efficiency.1 Therefore, phosphorescent OLEDs (PhOLEDs) show a bright future as the next-generation flat-panel displays and solid-state lighting sources. Among the trichromatic emissions, blue light plays an important role in saving the energy and achieving high color rendering index (CRI) in white OLEDs for illumination. However, the efficiencies and stabilities of blue PhOLEDs still need to be greatly improved. Tremendous efforts have been made to develop new blue phosphorescent dyes and corresponding host materials.2 To the present, cyclometalated iridium(III) complexes are still the most promising phosphorescent dyes because of their lifetime on the microsecond time-scale, high quantum yields, flexibility in color tuning and thermal stability.3 The emissions of cyclometalated iridium(III) complexes are generally assigned to the mixed metal to ligand charge transfer (MLCT) and the π−π* transition of ligands. The HOMO primarily localizes on the phenyl part of the cyclometalated ligands and the iridium(III) ion, and the LUMO mainly distributes on the pyridine part of the cyclometalated ligands. On the basis of the green phosphorescent dye, bis(2-phenylpyridinatoN,C2′)2iridium(III) (acetylacetonate) [(ppy)2Ir(acac)], a sky© 2012 American Chemical Society

blue iridium complex, bis(2-(4′,6′-difluoro)phenylpyridinatoN,C2′)2iridium(III) picolinate (FIrpic), was designed by introducing electron-withdrawing fluorine atom into the phenyl ring and changing the ancillary ligand from acetylacetonate to picolinate, which can increase the HOMO−LUMO gap4 (See Figure 1). The modifications in the structural frame of FIrpic include: (i) adding electron-withdrawing groups such as −F,5 −CF3,6 −CN,7 −COOMe (−COCF3),8 or alkyl group9 to the 5′-position of phenyl ring; (ii) adding electron-donating groups such as −OMe,10 −NMe211 to the 4-position of pyridine ring; (iii) replacement of fluorine by alkoxy group12 or chlorine (bromine);13 (iv) changing ancillary ligands such as poly(pyrazolyl)borate (FIr6, Figure 1)14 or picolinic acid N-oxide.15 Another strategy to acquire blue iridium complex is to utilize the cyclometalated ligands with inherently large HOMO− LUMO gaps, such as N-phenylpyrazole,16 2′,4′-difluoro-2,3′bipyridine,17 5-(2′-pyridyl)-3-trifluoromethylpyrazole,18 and carbene.19 However, this strategy may bring the LUMO energy level of iridium complex close to the antibonding d orbitals of iridium(III) ion, which may result in nonradiative deactivation of vibronic relaxation involving d−d excited states. On the other hand, this strategy may lower the HOMO energy level, which Received: September 4, 2012 Revised: November 12, 2012 Published: November 12, 2012 4581

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sized according to the reference.6 All reagents commercially available were used as received unless otherwise stated. The solvents (THF, diethyl ether, dichloromethane) were purified by conventional procedure and distilled under argon before using. 1H NMR spectra were measured on Varian Unity 300 MHz spectrometer using CDCl3 as solvent and tetramethylsilane as an internal reference. 13C NMR spectra were measured on Bruker BioSpin 400 MHz spectrometer using CDCl3 as solvent. The chemical shifts reported for the 13C NMR spectra reflected the positions of the observed peaks at the frequency given, some of which were expected to arise due to coupling to 19F and/or 31P, and these couplings had not been assigned. Elemental analysis of carbon, hydrogen and nitrogen was performed on Vario ELIII microanalyzer. EI-MS spectra were recorded on VJ-ZAB-3F-Mass spectrometer. MALDI-TOF (matrix-assisted laser-desorption/ionization time-of-flight) mass spectra (MS) were performed on Bruker BIFLEX III TOF mass spectrometer. Thermogravimetric analysis (TGA) was undertaken with a NETZSCH STA 449C instrument and the thermal stability of the samples under nitrogen atmosphere was determined by measuring their weight loss, heated at a rate of 10 °C min−1 from 25 to 600 °C. Photophysical Characterization. UV−vis absorption spectra were recorded on Shimadzu UV-2550 spectrophotometer with baseline corrected. Steady-state emission spectra were recorded on Hitachi F-4500 fluorescence spectrophotometer. Absolute photoluminescence quantum yields measured in CH2Cl2 were recorded on FLS920 spectrometer with Xenon light source (450 W) through the Edinburgh Instruments integrating sphere. The integrating sphere is 150 mm in diameter and has its inner surface coated with Barium Sulfate (BaSO4). Time-resolved measurements in CH2Cl2 solution were performed on FLS920 spectrometer using the time-correlated single-photon counting (TCSPC) option and the Edinburgh Instruments picoseconds pulsed diode laser (model EPL-375) as the light source. The excited state lifetime data were analyzed using F900 software by minimizing the reduced chi squared function (χ2) and visual inspection of the weighted residuals. All the samples are fresh and carefully prepared. Deaerated samples were prepared by purging argon for 30 min. Quantum yields (measured in 80 nm film with a composition of PVK: OXD-7: dopants = 100:40:10) were recorded on FluoroSENS in Gilden Photonics Ltd. through collecting the light emission with an integrating sphere. Single Crystal. X-ray diffraction data were obtained from a Bruker AXS Smart CCD diffractometer using a graphite-monochromated MoKα (λ = 0.71073 Å) radiation at 77 K. The data were collected using the ω/2θ scan mode and corrected for Lorentz and polarization effects as well as the absorption during data reduction using Shelxtl 97 software. CCDC 878373 contains the supplementary crystallographic data of SOFIrpic for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Cyclic Voltammetry. Cyclic voltammetric measurements were carried out using a CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte. The conventional three-electrode cell with a Pt work electrode of 2 mm diameter, a platinum-wire counter electrode and a Ag/AgCl reference electrode was employed. The scan rate was 0.1 V s−1. At the end of each experiment, the ferrocene/ferricenium (Fc/Fc+) couple was used as the internal standard. The HOMO and LUMO energy levels (eV) of the two compounds are calculated according to the formula: −[4.8 eV + Eox/red (vs EFc/Fc+)]. Device Fabrication and Measurement. Patterned ITO coated glasses with a sheet resistance of 15−20 Ω square−1 were cleaned by a surfactant scrub, then underwent a wet-cleaning process inside an ultrasonic bath, beginning with deionized water, followed by acetone and isopropanol. After oxygen plasma cleaning for 4 min, 40−50 nm PEDOT:PSS (Bayer Baytron P 4083 or Bayer Baytron P 8000) used as a hole-injection layer at the anode interface was spin-coated on the ITO substrate and then dried in a vacuum oven at 120 °C for 20 min. The emissive layer was coated on the top of PEDOT:PSS layer by spin-coating from chlorobenzene/CHCl3 solution, and then annealed at 120 °C for 20 min to remove the solvent residue. The thickness of

Figure 1. Chemical structures and maximum emissions of common green Ir complex to blue Ir complex.

may reduce the contributions from d orbitals of iridium(III) ion and cut the radiative rate (kr) to the ground state.20 Nowadays, using the vacuum-deposited technology, the PhOLEDs based on sky-blue dye of FIrpic or blue dye of iridium(III) bis(4′,6′-difluorophenylpyridinato-N,C2′) tetrakis(1-pyrazolyl) borate14 (FIr6) have achieved the maximum current efficiency of over 40 cd A−1 and the maximum external quantum efficiency (EQE) of over 20%.21 In contrast, solutionprocessed blue PhOLEDs usually show much inferior efficiencies. With a vacuum-deposited or solution-processed electron-transporting layer, the FIrpic-based solution-processed sky-blue PhOLEDs have exhibited gradually evolvement, and a maximum current efficiency of 28 cd A−1 and a maximum EQE of 15% have been achieved.22 However, the efficiency improvement of solution-processed blue PhOLEDs based on FIr6 is in tardy progress. Qiu et al. reported the device with a maximum current efficiency of 11.5 cd A−1 and a maximum EQE of 6.8% by employing a vacuum-deposited electrontransporting layer. Using an electron-injecting material, Jen et al. reported the device with a maximum current efficiency of 14 cd A−1 and a maximum EQE of 6%.23 In this article, we report two new blue iridium complexes, POFIrpic and SOFIrpic, with phosphoryl/sulfonyl substituted 2-(2′,4′-difluorophenyl)pyridine (dfppy) as the cyclometalated ligands. The phosphoryl (PO) and sulfonyl (SO) moieties are strong electron-withdrawing groups capable of polarizing the molecule without extending the π-conjugation of pristine chromophore. We expect the introduction of PO/SO moieties to the 3′-position of the dfppy could lower the HOMO energy level and tune the emission of iridium complex to the blue region; simultaneously, the alteration may leave LUMO level little affected, and facilitate to remain high quantum efficiencies. By using POFIrpic as the blue dopant, we fabricated a fully solution-processed single-active-layer blue device, which acquired a maximum current efficiency of 11.1 cd A−1 and a maximum EQE of 7.1%. Moreover, at practical brightness of 100 cd m−2, the device still retained a high current efficiency of 10.3 cd A−1 and EQE of 6.6%. To the best of our knowledge, these are the highest efficiencies ever reported for blue PhOLEDs by full solution process.

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EXPERIMENTAL SECTION

General Information. All reactions were carried out using Schlenk tube in an argon atmosphere. 2-(2′,4′-Difluorophenyl) pyridine (dfppy) and 2-(2′,4′-difluoro-3′-iodophenyl) pyridine were synthe4582

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the EML was about 80 nm. Finally, a cathode composed of CsF (1.5 nm) and Al (100 nm) layer was evaporated with a shadow mask at a base pressure of 3 × 10−4 Pa. The thickness of the evaporated cathode was monitored by a quartz crystal thickness/ratio monitor (Model: STM-100/MF, Sycon). The overlapping area between the cathode and anode defined a pixel size of 19 mm2. Except for the deposition of the PEDOT:PSS layer, all the fabrication processes were carried out inside a controlled atmosphere of nitrogen drybox (Vacuum Atmosphere Co.) containing less than 10 ppm oxygen and moisture. The current density−luminance−voltage characteristic was measured using a Keithley 236 source measurement unit and a calibrated silicon photodiode. The forward-viewing luminance was calibrated by a spectrophotometer (SpectraScan PR-705, Photo Research) and the forward-viewing luminance efficiency (LE) was calculated accordingly. Throughout the whole manuscript, the reported values of luminance and LE are for forward-viewing direction only. The external quantum efficiency of EL was collected by measuring the total light output in all directions in an integrating sphere (IS-080, Labsphere). The EL spectra were collected by a PR-705 photometer. Synthesis of podfppy. A solution of diisopropylamine (1.1 g, 10.7 mmol) in diethyl ether (60 mL) was treated with 6 mL of n-BuLi (2.38 M in hexane, 14 mmol) under argon at −78 °C. The mixture was stirred for 60 min to form lithium diisopropylamide (LDA). Then, 2-(2′,4′-difluorophenyl) pyridine (dfppy) (1.8 g, 9.4 mmol, liquid) was added dropwise and the mixture was stirred for additional 1.5 h at −78 °C. Finally, 2 mL of chlorodiphenylphosphine (2.4 g, 11 mmol) was added to the mixture and the solution was stirred overnight. The reaction was quenched with water, extracted with ethyl acetate and dried over anhydrous Na2SO4. After removing the solvent, the residue was treated with 40 mL of CH2Cl2 and 40 mL of hydrogen peroxide (30%) (warning: explosive), and then stirred at room temperature for 3 h. The crude product was extracted with CH2Cl2 and purified by column chromatography on silica gel using pure ethyl acetate as eluent to afford 2-(2′,4′-difluoro-3′-(diphenylphosphoryl) phenyl) pyridine (podfppy) as white solid. Podfppy was recrystallized using ethyl acetate to get fine crystal. The overall yield is 46%. 1H NMR (300 MHz, CDCl3) δ (ppm): 8.69 (d, J = 4.5 Hz, 1H), 8.22 (q, J = 6.6 Hz, 1H), 7.83−7.76 (m, 4H), 7.73−7.64 (m, 2H), 7.59−7.46 (m, 6H), 7.25−7.23 (m, 1H), 7.08−7.01 (m, 1H). 13C NMR (100 MHz, CDCl3) δ [ppm]: 164.8, 164.7, 162.5, 162.4, 162.2, 162.1, 159.9, 159.8, 151.3, 149.7, 136.8, 136.7, 136.6, 136.4, 133.4, 132.3, 132.0, 131.0, 130.9, 128.5, 128.4, 124.9, 124.8, 124.7, 124.4, 124.3, 122.7, 113.1, 113.0, 112.8, 111.0, 110.8, 110.6, 110.1, 109.9, 109.7. MS (EI, m/z): [M]+ calcd for C23H16F2NOP, 391.09; found, 391.01. Anal. Calcd for C23H16F2NOP (%): C, 70.59; H, 4.12; N 3.58. Found: C, 70.91; H, 3.95; N, 3.50. Synthesis of sodfppy. A mixture of 2-(2′,4′-difluoro-3′iodophenyl) pyridine (0.95 g, 3 mmol), 4-methylbenzenethiol (0.74 g, 6 mmol) and cuprous oxide (Cu2O) (0.86 g, 6 mmol) in 20 mL of dimethylacetamide (DMAc) was refluxed at 170 °C for 24 h under argon atmosphere. After cooled to room temperature, the reaction was filtrated, extracted with CHCl3, washed by water, and dried over anhydrous Na2SO4. The intermediate, 2-(2′,4′-difluoro-3′-(p-tolylthio) phenyl) pyridine was purified by column chromatography on silica gel using 1: 5 (v:v) ethyl acetate/petroleum as eluent to give a light yellow oil. Without further characterization, the 2-(2′,4′-difluoro-3′-(ptolylthio) phenyl) pyridine was treated with 30 mL of CH2Cl2 and 1.3 g of 3-chloroperbenzoic acid (mCPBA) (7.5 mmol, 2.5 equiv) (warning: explosive), and the reaction was stirred overnight at room temperature. Quenched by sodium bisulfite, the reaction was extracted with CH2Cl2, washed by water, and dried over anhydrous Na2SO4. The product, 2-(2′,4′-difluoro-3′-tosylphenyl) pyridine (sodfppy) was obtained by column chromatography on silica gel using 1: 10 (v:v) ethyl acetate/CH2Cl2 as eluent to give a light yellow solid. The overall yield of the two steps is 65%. 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.69 (d, J = 4.5 Hz, 1H), 8.18 (q, J = 6.6 Hz, 1H), 7.98 (d, J = 8.1 Hz, 2H), 7.80−7.76 (m, 2H), 7.36−7.27 (m, 3H), 7.11 (t, J = 9.6 Hz, 1H), 2.43 (s, 3H). 13C NMR (100 MHz, CDCl3) δ [ppm]: 167.4, 160.7, 158.2, 158.1, 155.6, 155.5, 150.6, 149.5, 145.1, 138.5, 136.7, 136.6, 134.0, 132.5, 132.3, 129.6, 129.4, 129.0, 127.7, 127.5, 125.1, 125.0,

124.9, 124.4, 124.3, 123.0, 119.6, 119.4, 119.3, 113.5, 113.4, 113.2, 21.4. MS (EI, m/z): [M]+ calcd for C18H13F2NO2S, 345.06; found, 345.06. Anal. Calcd for C18H13F2NO2S (%): C, 62.60; H, 3.79; N 4.06. Found: C, 62.35; H, 3.81; N, 3.95. Synthesis of POFIrpic. A mixture of 2.5 equiv of podfppy (0.98 g, 2.5 mmol) and 1 equiv of IrCl3·3H2O (0.35 g, 1 mmol), 15 mL of 2ethoxyethanol, and 5 mL of H2O was refluxed at 120 °C for 24 h under an argon atmosphere. After back to room temperature, the mixture was poured into water and the formed precipitate was filtered, washed by water, ethanol, and diethyl ether to obtain the intermediate, assumed to be a chloro-bridged dimer. Without further purification, a mixture of the intermediate (0.61g, 0.3 mmol), Na2CO3 (0.32 g, 3 mmol), and picolinic acid (0.19 g, 1.5 mmol) was added into a solution of 40 mL of CH2Cl2 and 10 mL of ethanol. The reaction was stirred overnight at room temperature, then extracted with CH2Cl2 and dried over anhydrous Na2SO4. The product, POFIrpic, was obtained by column chromatography on silica gel using 1: 10 (v:v) ethanol/CH2Cl2 as eluent to give a yellow solid. Overall yields: 76%. 1 H NMR (300 MHz, CDCl3) δ [ppm]: 8.70 (d, J = 6.0 Hz, 1H), 8.33 (d, J = 8.1 Hz, 1H), 8.13 (d, J = 9.6 Hz, 1H), 8.07 (d, J = 8.7 Hz, 1H), 7.99 (t, J = 8.1 Hz, 1H), 7.87−7.69 (m, 11H), 7.54−7.38 (m, 14H), 7.18 (t, J = 6.6 Hz, 1H), 6.98 (t, J = 6.6 Hz, 1H), 5.79−5.75 (m, 1H), 5.64−5.59 (m, 1H). 13C NMR (100 MHz, CDCl3) δ [ppm]: 172.4, 160.3, 150.9, 148.5, 148.3, 148.0, 138.8, 138.7, 134.1, 133.7, 133.0, 131.9, 131.7, 131.1, 131.0, 130.9, 130.8, 128.8, 128.7, 128.6, 128.5, 128.4, 128.3, 128.2, 124.1, 123.9, 123.6, 123.4, 123.2, 123.1, 116.0, 115.8. MS (MALDI-TOF, m/z): [M]+ calcd for C52H34F4IrN3O4P2, 1095.16; found, 1096.07. Anal. Calcd for C52H34F4IrN3O4P2 (%): C, 57.04; H, 3.13; N, 3.84. Found: C, 56.78; H, 3.34; N, 3.90. Synthesis of SOFIrpic. A mixture of 2.5 equiv of sodfppy (0.86 g, 2.5 mmol) and 1 equiv of IrCl3·3H2O (0.35 g, 1 mmol), 15 mL of 2ethoxyethanol, and 5 mL of H2O was refluxed at 120 °C for 24 h under argon atmosphere. After being brought back to room temperature, the mixture was poured into water and the formed precipitate was filtered and washed by water, ethanol, and diethyl ether to obtain the intermediate, assumed to be a chloro-bridged dimer. Without further purification, a mixture of the intermediate (0.57 g, 0.3 mmol), Na2CO3 (0.32 g, 3 mmol), picolinic acid (0.19 g, 1.5 mmol), and 15 mL of 1, 2-dichloroethane was refluxed at 80 °C for 24 h under an argon atmosphere. The reaction was quenched with water, extracted with CH2Cl2 and dried over anhydrous Na2SO4. The product, SOFIrpic, was obtained by column chromatography on silica gel using 1: 1 (v:v) ethyl acetate/CH2Cl2 as eluent to give a yellow solid. Overall yields: 58%. 1H NMR (300 MHz, CDCl3), δ [ppm]: 8.66 (d, J = 6.0 Hz, 1H), 8.33−8.27 (m, 3H), 8.04−7.93 (m, 3H), 7.88−7.83 (m, 4H), 7.70 (d, J = 5.1 Hz, 1H), 7.52 (t, J = 6.3 Hz, 1H), 7.39 (d, J = 5.7 Hz, 1H), 7.34−7.24 (m, 5H), 7.08 (t, J = 6.0 Hz, 1H), 5.87 (d, J = 10.2 Hz, 1H), 5.61 (d, J = 10.8 Hz, 1H), 2.41 (s, 6H; CH3). 13C NMR (100 MHz, CDCl3) δ [ppm]: 172.1, 164.0, 162.8, 160.7, 160.6, 160.0, 159.9, 157.9, 150.8, 148.5, 148.1, 148.0, 144.8, 144.5, 139.3, 139.1, 139.0, 129.8, 129.6, 128.9, 128.7, 127.4, 127.2, 124.3, 124.1, 123.8, 123.7, 116.5, 116.4, 116.1, 112.9, 112.7, 112.6, 21.6, 21.5. MS (MALDI-TOF, m/z): [M] + calcd for C42H28F4IrN3O6S2, 1003.10; found, 1004.09. Anal. Calcd for C42H28F4IrN3O6S2 (%): C, 50.29; H, 2.81; N, 4.19. Found: C, 49.95; H, 2.44; N, 3.91.

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RESULTS AND DISSCUSION Synthesis and Characterization. Scheme 1 outlines the synthesis of the two new ligands, 2-(2′,4′-difluoro-3′(diphenylphosphoryl) phenyl) pyridine (podfppy) and 2(2′,4′-difluoro-3′-tosylphenyl) pyridine (sodfppy), as well as the final iridium complexes. Ligand podfppy was obtained in a three-step procedure: regioselective lithiation of dfppy, coupling with chlorodiphenylphosphine and oxidation with hydrogen peroxide. Ligand sodfppy was obtained in a two-step procedure from the start of 2-(2′,4′-difluoro-3′-iodophenyl) pyridine: Ullmann-coupling with 4-methylbenzenethiol and 4583

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wavelength absorptions (374 nm for POFIrpic and 372 nm for SOFIrpic) are due primarily to the π−π* transitions of the ligands, whereas the weak long wavelength absorptions (447 nm for POFIrpic and 445 nm for SOFIrpic) can be assigned to the MLCT transitions. POFIrpic and SOFIrpic show intense blue photoluminescence in CH2Cl2 solution at room temperature, with the emission peaks at 460 nm, 489 nm for POFIrpic, and 459 and 486 nm for SOFIrpic, respectively. Their photoluminescence quantum yields (PLQYs) measured in CH2Cl2 solution were 49% for POFIrpic and 55% for SOFIrpic. Meanwhile, the values measured in 80 nm film with a composition of poly(N-vinylcarbazole (PVK): 1,3-bis[2-(4tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7): dopants (100:40:10) were 54% for POFIrpic and 46% for SOFIrpic, respectively, and under the same conditions, the value was 50% for FIr6. The excited-state lifetimes of the two complexes were monoexponential and in the range of microsecond, indicating the triplet nature of the excited state.16 All the thermal and photophysical data of POFIrpic and SOFIrpic are summarized in Table 1. Electrochemical Properties. The electrochemical properties of POFIrpic and SOFIrpic were probed by the cyclic voltammetry (CV). The CV of FIrpic was also probed under the identical condition for comparison (see Figure S3 in the Supporting Information). All the three iridium complexes exhibited reversible oxidation process in CH3CN solution. The oxidation potentials (Eox vs Fc/Fc+) were 0.91 V for FIrpic, 1.12 V for POFIrpic and 1.21 V for SOFIrpic. Their HOMO energy levels estimated from the oxidation potentials were −5.7 eV for FIrpic, −5.9 eV for POFIrpic, and −6.0 eV for SOFIrpic, respectively. For the reduction process in THF solution, both FIrpic and SOFIrpic exhibited reversible behavior, whereas POFIrpic exhibited quasi-reversible behavior. The reduction potentials (Ered vs Fc/Fc+) were −2.42 V for FIrpic, −2.32 V for POFIrpic and −2.21 V for SOFIrpic. Accordingly, their LUMO energy levels were estimated to be −2.4 eV for FIrpic, −2.5 eV for POFIrpic and −2.6 eV for SOFIrpic, respectively. Both HOMO and LUMO energy levels of POFIrpic and SOFIrpic are lowered because of the electron-withdrawing phosphoryl (PO) and sulfonyl (SO) moieties. The data are summarized in Table 1. Phosphorescent OLEDs. To evaluate the two new iridium complexes as the blue phosphorescent dyes in solutionprocessed devices, we designed the following single-activelayer device structure: ITO/PEDOT:PSS(4083) (40 nm)/ PVK:OXD-7:POFIrpic (device A) or SOFIrpic (device B) (100:40:10, 80 nm)/CsF (1.5 nm)/Al (100 nm), where poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS 4083) was used as hole-injecting layer, and PVK combined with OXD-7 was employed as host matrix. The new blue emitters showed almost the same maximum emissions with commonly used blue phosphor of FIr6 as discussed above. For comparison, the control device C using FIr6 as emitter was

Scheme 1. Synthesis of the ligands, podfppy and sodfppy, and the iridium complexes, POFIrpic and SOFIrpic

then oxidation with 3-chloroperbenzoic acid (mCPBA). The final iridium complexes, POFIrpic and SOFIrpic, were prepared in a two-step procedure: assumed cyclometalated Ir(III) μchloro-bridged dimers were first obtained, and then reacted with 2−2.5 equiv of picolinic acid.24 All the compounds were fully characterized by 1H and 13C NMR spectroscopy, mass spectrum and elemental analysis (see details in the Experimental Section). The structure of SOFIrpic was further confirmed by single-crystal X-ray diffraction analysis, and the molecular structure is displayed in Figure S1 in the Supporting Information. Their thermal stabilities were manifested by their high thermal decomposition temperatures (Td, corresponding to 5% weight loss in the thermogravimetric analysis) of 379 °C for POFIrpic and 395 °C for SOFIrpic (Figure S2), respectively. Photophysical Properties. The UV−vis absorption and photoluminescence spectra of POFIrpic and SOFIrpic in CH2Cl2 solution are shown in Figure 2. The intense short

Figure 2. UV−vis absorption and PL spectra of POFIrpic and SOFIrpic in CH2Cl2 solution at room temperature.

Table 1. Thermal and Photophysical Properties of POFIrpic and SOFIrpic compds

Td (°C)

λabs, maxa (nm) (ε × 103)

λPL, maxa (nm)

Φb (%)

τobsb (μs)

PLQYc (%)

HOMOd (eV)

LUMOd (eV)

POFIrpic SOFIrpic

379 395

374 (9.4), 447 (0.7) 372 (11.9), 445 (0.7)

460, 489 459, 486

49 55

2.3 1.9

54 46

−5.9 −6.0

−2.5 −2.6

a Measured in CH2Cl2 solution (extinction coefficient in parentheses). bΦ = absolute photoluminescence quantum yields in deaerated CH2Cl2; τobs = excited state lifetime. cAbsolute quantum yield measured in PVK/OXD-7 film. dHOMO/LUMO energy levels were estimated from the oxidation/ reduction potentials in the cyclic voltammetry.

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Figure 3. (a) J−V−L characteristics of devices A−C. (b) Current efficiency versus luminance of devices A−C.

Figure 4. (a) J−V−L characteristics of device D−F. (b) Current efficiency versus luminance of device D−F.

Table 2. Performance Summary of Blue Device A-F dopant

device

PEDOT:PSS

Von(V)a

Lmax (cd m−2)b

LE (cd A−1)c

EQE (%)d

CIE (x, y)d

POFIrpic SOFIrpic FIr6 POFIrpic SOFIrpic FIr6

A B C D E F

P4083 P4083 P4083 P8000 P8000 P8000

5.0 4.8 4.6 5.0 5.5 4.5

1212 1227 1285 815 590 823

9.8/9.2/5.4 7.3/7.3/3.8 9.7/7.6/5.5 11.1/10.3/7.6 6.8/6.6/3.7 10.5/9.1/6.5

6.3/5.9/3.5 4.7/4.7/2.5 6.2/4.8/3.5 7.1/6.6/4.9 4.4/4.2/2.4 6.5/5.8/4.1

0.168, 0.176, 0.175, 0.166, 0.174, 0.178,

0.294 0.294 0.315 0.279 0.281 0.305

Turn-on voltages at 1 cd m−2. bMaximum luminance. cOrder of measured efficiency values: maximum, then values at 100, 1000 cd m−2 for devices A−C and 500 cd m−2 for devices D−F. dCommission International de I’Eclairage coordinates (CIE) measured at 2 mA cm−2. a

replaced P4083 in device A−C as the buffer layer. Devices D−F exhibit blue emission in their EL spectra with CIE coordinates of (0.166, 0.279) for device D, (0.174, 0.281) for device E and (0.178, 0.305) for device F. The EL spectra of the devices were almost the same with PL spectra and no PVK emission was observed, suggesting that the emissions completely originated from the triplet emitters (see Figure S5 in the Supporting Information). The color purity of blue light is significantly improved compared with their analogous device A-C. This should be contributed to the use of P8000 that could better balance hole and electron flux, and consequently make the recombination region away from the interface between the emissive layer and cathode.26 The J−V−L characteristics and current efficiency versus luminance of these devices are shown in Figure 4, and the data of device A−F are summarized in Table 2. With the use of P8000, both device D and F show enhanced efficiency compared with their analogous device A and C, respectively. Moreover, device D with POFIrpic as triplet emitter exhibits superiority over device F with FIr6 as emitter in both efficiency and color purity. Device D acquires a maximum current efficiency of 11.1 cd A−1 and a maximum EQE of 7.1% (at 0.73 mA cm−2, 7.8 V). Even at brightness of 100 cd m−2 (at 0.99 mA cm−2, 8 V), device D still reveals high current efficiency of 10.3 cd A−1 and EQE of 6.6%. To the best

also fabricated. The current density−voltage−luminance (J− V−L) characteristics and current efficiency versus luminance of these devices are shown in Figure 3. Device B based on SOFIrpic displays good performance, with a maximum current efficiency of 7.3 cd A−1 and maximum EQE of 4.7% (at 1.3 mA cm−2, 7.2 V). Device A based on POFIrpic shows better performance, with a maximum current efficiency of 9.8 cd A−1 and a maximum EQE of 6.3% (at 1.3 mA cm−2, 7.6 V), which are comparable with the control device C based on FIr6. At practical brightness of 100 cd m−2, device A still exhibits high current efficiency of 9.2 cd A−1 and EQE of 5.9% (at 1.1 mA cm−2, 7.4 V). To further pursue high efficiencies in simple device configuration, the charge balance should be well-manipulated in the single-layer OLEDs. PVK is a hole-transporting material, and a modified buffer layer can reduce the injection efficiency of the hole flux. We recently reported that the use of PEDOT:PSS (P8000) with a low conductivity of 1 × 10−6 S cm−1 can spur a significant increase in device performance for phosphorescent polymer light-emitting devices.25 Accordingly, we fabricated the device D-F with the following configuration: ITO/PEDOT:PSS(8000) (50 nm)/PVK:OXD-7:POFIrpic (device D) or SOFIrpic (device E) or FIr6 (device F) (100:40:10, 80 nm)/CsF (1.5 nm)/Al (100 nm), where P8000 4585

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PO/SO moieties into the 3′-position of the 2-(2′,4′difluorophenyl) pyridine (dfppy) to enlarge the energy gap. The two phosphorescent dyes show blue emission with the maximum emission around 460 nm and high quantum yields of ca. 50% in solution. The single-layer solution-processed blue PhOLEDs based on POFIrpic showed a maximum current efficiency of 11.1 cd A−1 and a maximum EQE of 7.1%. The single-layer trichromatic white PhOLEDs employing POFIrpic as the blue component exhibited a maximum current efficiency of 25 cd A−1 and a maximum EQE of 15%, respectively. These efficiencies are among the highest ever reported for solutionprocessed blue and R-G-B three components white PhOLEDs. Importantly, the white PhOLEDs exhibited a good CRI value of 82 with the new blue phosphor component. We believe that the device performance can be further improved by the device optimization, such as the use of interface materials. This work is in progress.

of our knowledge, these are the highest efficiencies ever reported for blue PhOLEDs by full solution process. These efficiencies are even comparable with the multilayer blue FIr6based devices by using the vacuum-deposited hole/excitonblocking layer of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene) and electron-transporting layer of 1,3,5-tris(N-phenylbenzimidazol2-yl)benzene, with a maximum current efficiency of 12.5 cd A−1 and a maximum EQE of 6.3%.27 The lifetimes of the devices were about half an hour, which should be mainly due to the unstability of the blue iridium complexes. We previously reported a fully solution-processed white OLED using phosphorescent R-G-B three components, achieving a maximum current efficiency of 24 cd A−1 and EQE of 14%.28 However, owing to using the sky-blue FIrpic, the color rendering index (CRI) value of 77 is undesirable for illumination. Considering the excellent performance of the blue phosphorescent dye, POFIrpic, we further built a threecomponent white device G with the configuration: ITO/ PEDOT:PSS(8000) (50 nm)/PVK:OXD-7:POFIrpic:Ir(ppy)3: (piq)2Ir(acac) (80 nm)/CsF (1.5 nm)/Al (100 nm), where the ratio of PVK:OXD-7:dopants is 100:40:10. POFIrpic, iridium tris(2-pyridinato-N,C2′) (Ir(ppy)3) and iridium bis(1-phenylisoquinoline) (acetylacetonate) ((piq)2Ir(acac)) were employed as blue, green and red phosphorescent dyes, respectively, and the dye ratio of blue:green:red is 60:1:2. Device G showed a maximum current efficiency of 25 cd A−1 and a maximum EQE of 15% (at 0.14 mA cm−2, 6.4 V). At practical brightness of 100 cd m−2, device G remained very high current efficiency of 24 cd A−1 and EQE of 14% (at 0.40 mA cm−2, 7.2 V). Even at brightness of 1000 cd m−2, device G still revealed high current efficiency of 16 cd A−1 and EQE of 9.8% (at 6.2 mA cm−2, 10.6 V) (Figure 5). These device efficiencies

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ASSOCIATED CONTENT

* Supporting Information S

ORTEP diagram and selected crystal data of SOFIrpic, TGA thermograms, spectra of cyclic voltammetry, external quantum efficiency versus luminance of devices A−F, EL spectra of devices D−F with P8000 at 2 mA cm−2, EL spectra of white device G at different currents, performance summary of white device G, 1H, 13C NMR, and MS spectra of all the compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.Y.); [email protected] (H.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.Y. thanks the National Science Fund for Distinguished Young Scholars of China (No. 51125013), the National Basic Research Program of China (973 Program 2009CB623602), the National Natural Science Foundation of China (90922020), and the Fundamental Research Funds for the Central Universities of China; H. Wu thanks the National Natural Science Foundation of China (61177022).

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Figure 5. Current efficiency, external quantum efficiency versus luminance of white device G. Device configuration: ITO/PEDOT:PSS (P8000)/host:dopants (B:G:R)/CsF/Al, where the composition of EML is PVK:OXD-7:dopants (B:G:R = 60:1:2) = 100:40:10.

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are among the highest results ever reported for solutionprocessed white PhOLEDs based on the R-G-B three components, and even better than the double-emitting-layer device based on FIr6 with a maximum EQE of 12%.29 Noticeably, as we expected, the CRI value of the device was enhanced to 82 due to the employment of the new blue phosphor.

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CONCLUSION In summary, two new iridium complexes, POFIrpic and SOFIrpic, have been developed by introducing the polarized 4586

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