Deep-Blue-Emitting Heteroleptic Iridium(III) Complexes Suited for

Article pubs.acs.org/cm

Deep-Blue-Emitting Heteroleptic Iridium(III) Complexes Suited for Highly Efficient Phosphorescent OLEDs Cheng-Han Yang,† Matteo Mauro,† Federico Polo,† Soichi Watanabe,‡ Ingo Muenster,‡ Roland Fröhlich,§ and Luisa De Cola*,† †

Physikalisches Institut and Center for Nanotechnology (CeNTech), Westfälische Wilhelms-Universität Münster, Heisenbergstrasse 11, 48149 Münster, Germany ‡ BASF SE, 67056 Ludwigshafen, Germany § Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster, Germany S Supporting Information *

ABSTRACT: We report on the design, synthesis, and characterization of four new heteroleptic iridium(III) complexes bearing 2′,6′-difluoro-2,3′-bipyridine and pyridyl-azole ligands. The photophysical properties and cyclic voltammetry of the complexes were also investigated. All compounds display highly efficient genuine blue phosphorescence (λmax ca. 440 nm), at room temperature in solution and in thin film, with quantum yield in the range 0.77− 0.87 and 0.62−0.93, respectively. We found that introduction of the bulky tert-butyl substituents on the cyclometalated or azolated chelates can effectively reduce detrimental aggregation, which results in a loss of color purity. Comprehensive density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches have been performed on the ground and excited states of the here reported complexes, in order to gain deeper insights into their structural and electronic features as well as to ascertain the nature of the excited states involved into the electronic absorption processes. Moreover, electron spin density analysis and total electron density difference at the lowest-lying triplet state (T1) were performed for shedding light onto the nature of the emitting excited state. Finally, the fabrication of the organic light-emitting diodes (OLEDs), employing the bulkiest derivative among the here reported phosphorescent dopants, was successfully made. The devices exhibit remarkable maximum external quantum efficiency (EQE) as high as 7.0%, in nonoptimized devices, and power efficiency (PE) of 4.14 lm W−1, together with a true-blue chromaticity CIEx,y = 0.159, 0.185 recorded at 300 cd m−2. KEYWORDS: iridium complexes, blue-emission, phosphorescence, organic-light emitting diode, pyridil-azolate ligand

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INTRODUCTION Phosphorescent organic light-emitting diodes (OLEDs) are under intensive investigation because of their great potential in both display and lighting applications.1 In such devices, the electrophosphorescence is easily generated from both singlet and triplet excited states and can reach a theoretical level of 100% as internal quantum efficiency.2 Thus, great effort has been devoted to the second- and third-row transition metal complexes, such as Ir(III), Pt(II), Os(II) or Ru(II), for developing highly efficient phosphors that can emit all three primary colors.3 Among third-row transition metals, iridium(III)-based compounds are particularly promising candidates because of their high emission efficiency, short triplet radiative lifetimes (μs range), good thermal stability and wide range of emission color when compared to the other metal complexes. In comparison with the high efficient and stable red and green phosphorescent iridium(III) complexes, blue and deep-blue phosphorescent emitters for OLED are still limited and challenging. The most well-known example, i.e., bis(4′,6′difluorophenylpyridinato) iridium(III) picolinate (FIrpic) has © 2012 American Chemical Society

proved to be an excellent dopant for greenish-blue phosphorescent OLEDs.4 Plenty of improvements were made by substituting picolinate with other ancillary ligands such as tetrakis(1-pyrazolyl)borate,5 pyridyl azolates,6 picolinate Noxide,7 or N-phenyl pyrazole8 in order to push the emission into the blue region. Recently, Chi et al. reported on blue and true-blue heteroleptic iridium(III) complexes comprising ancillary phosphine chelate,9 and other ingenious molecular designs.10 Furthermore, the use of high field-strength ligands such as N-heterocyclic carbenes11 also resulted in a shift toward higher energy of the emission and an increase of the blue phosphorescent efficiency. For example, Cheng et al. prepared several blue-emitting iridium compounds using N-heterocyclic carbenes and pyridyl azolates ligands. The devices employing [Ir(fpmi)2(pypz)] and [Ir(fpmi)2(tfpypz)] {fpmi=1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C,C2′; pypz=2-(1H-pyraReceived: April 3, 2012 Revised: August 24, 2012 Published: August 25, 2012 3684

dx.doi.org/10.1021/cm3010453 | Chem. Mater. 2012, 24, 3684−3695

Chemistry of Materials

Article

Synthesis of 4-(tert-Butyl)-2′,6′-difluoro-2,3′-bipyridine (dfpybpy). 2-Chloro-4-tert-butylpyridine (2.2 g, 12.96 mmol), 2,6-difluoropyridyl-3-boronic acid (2.1 g, 13.2 mmol), triphenylphosphine (0.34 g, 1.29 mmol), K2CO3 (3.58 g, 25.9 mmol), and palladium acetate (0.07 g, 0.42 mmol) were dissolved in 30 mL of 1,2-dimethoxyethane. The reaction mixture was heated at 90 °C for 18 h. The crude mixture was cooled down and poured into water. The organic components were extracted with CH2Cl2 (3 × 200 mL) and dried over MgSO4. Finally, silica gel column chromatography gave the pure product as a transparent oil (2.51 g, 10.1 mmol, 78%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.60 (m, 2H), 7.82−7.77 (m, 1H), 7.29 (td, J = 5.5, 1.9 Hz, 1H), 6.98−6.89 (m, 1H), 1.33 (d, J = 6.4 Hz, 9H). 19F{1H} NMR (282 MHz, CDCl3) δ: −68.41 (1F), −69.67 (1F). Synthesis of [(dfpypy)2Ir(μ-Cl)]2. IrCl3·3H2O (1.63 g, 4.65 mmol) and dfpypy (1.8 g, 9.36 mmol) were dissolved in 30 mL 2ethoxyethanol and refluxed at 140 °C for 18 h. After the solution was cooled, the addition of 50 mL of H2O gave a light yellow precipitate that was filtered and washed with diethyl ether. The crude product was used for the next reaction without further purification (2.44 g, 2.99 mmol, 86%). Synthesis of [(dfpybpy)2Ir(μ-Cl)]2. IrCl3·3H2O (1.50 g, 4.28 mmol) and dfpybpy (2.19 g, 8.80 mmol) were dissolved in 30 mL 2ethoxyethanol and refluxed at 140 °C for 18 h. The addition of 50 mL of H2O gave a light yellow precipitate that was filtered and washed with diethyl ether. The crude product was used for the next reaction without further purification (2.22 g, 1.55 mmol, 72%). Synthesis of Complexes 1, 2, 3, and 4. A mixture of 3trifluoromethyl-5-(2-pyridyl) pyrazole, namely (fppz)H, or 3-(trifluoromethyl)-5-(4-t-butylpyridyl) pyrazole, namely (fpbpz)H, and [(dfpypy)2Ir(μ-Cl)]2 or [(dfpybpy)2Ir(μ-Cl)]2 (0.16 mmol) was stirred overnight in a 3:1 mixture of dichloromethane and ethanol (40 mL) at 60 °C. The solvent was removed by evaporation under reduced pressure. The white solid was purified by using silica gel column chromatography with CH2Cl2:methanol in 9:1 ratio as eluent, giving the desired complex as light yellow powder with the following yields: 1 (82.5%), 2 (84.2%), 3 (81.3%) and 4 (76.8%). All crystalline materials were obtained from a layered solution of CH2Cl2 and methanol at room temperature. Spectra Data of Complex Ir(Dfpypy)2(fppz) (1). 1H NMR (300 MHz, CD2Cl2) δ: 8.27 (dd, J = 14.3, 8.3 Hz, 2H), 7.91−7.77 (m, 4H), 7.75−7.67 (m, 1H), 7.59 (ddd, J = 11.7, 5.8, 0.8 Hz, 2H), 7.09 (m, 3H), 7.00 (s, 1H), 5.83 (t, J = 2.2 Hz, 1H), 5.72 (t, J = 2.0 Hz, 1H).19F{1H} NMR (282 MHz, CD2Cl2) δ: −60.3 (3F), −69.09 (1F), −69.49 (1F), −71.06 (1F), −71.59 (1F). HRMS calcd for C29H15F7IrN7 787.0906 ([M+H]+); found 788.0974. Elem. anal. Calcd (%) for C29H15F7IrN7: C, 44.28; H, 1.92; N, 12.46. Found: C, 44.38; H, 1.98; N, 12.56. Spectra Data of Complex Ir(Dfpypy)2(fpbpz) (2). 1H NMR (300 MHz, CD2Cl2) δ: 8.27 (ddd, J = 16.7, 8.4, 0.7 Hz, 2H), 7.85 (m, 2H), 7.77 (dd, J = 2.1, 0.6 Hz, 1H), 7.64−7.53 (m, 3H), 7.09 (tdd, J = 8.2, 6.0, 1.4 Hz, 3H), 7.02 (s, 1H), 5.82 (t, J = 2.2 Hz, 1H), 5.72 (t, J = 2.0 Hz, 1H), 1.34 (s, 9H). 19F{1H} NMR (282 MHz, CD2Cl2) δ: −60.45 (3F), −69.27 (1F), −69.57 (1F), −71.23 (1F), −71.70 (1F). HRMS calcd for C33H23F7IrN7 843.1532 ([M+H]+); found 844.1612. Elem. anal. calcd (%) for C33H23F7IrN7: C, 47.03; H, 2.75; N, 11.63. Found: C, 47.46; H, 2.97; N, 11.82. Spectra Data of Complex Ir(dfpybpy)2(fppz) (3). 1H NMR (300 MHz, CD2Cl2) δ: 8.31 (t, J = 2.1 Hz, 1H), 8.23 (t, J = 2.0 Hz, 1H), 7.89−7.77 (m, 2H), 7.75−7.69 (m, 1H), 7.45 (dt, J = 8.0, 4.0 Hz, 1H), 7.35 (dt, J = 12.3, 6.1 Hz, 1H), 7.14−7.02 (m, 3H), 6.99 (s, 1H), 5.84 (t, J = 2.2 Hz, 1H), 5.73 (t, J = 2.0 Hz, 1H), 1.37 (s, 8H), 1.35 (s, 9H). 19 1 F{ H} NMR (282 MHz, CD2Cl2) δ: −60.43 (3F), −69.62 (1F), −69.91 (1F), −71.79 (1F), −72.27 (1F). HRMS calcd for C37H31F7IrN7 899.2158 ([M+H]+); found 898.2197. Elem. anal. Calcd (%) for C37H31F7IrN7: C, 49.44; H, 3.48; N, 10.91. Found: C, 50.01; H, 3.52; N, 11.01. Spectra Data of Complex Ir(dfpybpy)2(fpbpz) (4). 1H NMR (300 MHz, CD2Cl2) δ: 8.31 (t, J = 2.1 Hz, 1H), 8.23 (t, J = 2.0 Hz, 1H), 7.77 (dd, J = 2.1, 0.6 Hz, 1H), 7.61 (dd, J = 6.0, 0.6 Hz, 1H), 7.51− 7.41 (m, 1H), 7.38 (dd, J = 6.2, 0.4 Hz, 1H), 7.15−7.04 (m, 3H), 7.01

zol-5-yl)pyridinato and tfpypz=2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridinato)} as dopant emitters showed good performances with EQEs of 14.1%, and 7.6% and with CIE(x,y) values of (0.14, 0.18) and (0.14, 0.10), respectively.12 Blue phosphorescent Ir(III) complexes can also be obtained from the modified difluorophenylpyridine (dfppy) ligands, which is able to shift to even higher energy region relative to that of a homoleptic blue-green complexes, fac-Ir(dfppy)3.13 Morover, tris-cyclometaled blue or deep-blue iridium complexes using nonpyridine-based ligands that have higher triplet energies than dfppy ligand, such as phenylpyrazole,13 phenyltriazole,14 pyridylazolate,15 or imidazolphenathridine,16 have been prepared. More recently, Lee et al. reported two emissive Ir(III) complexes with fluorine-substituted 2,3′-bipyridine (dfpypy), namely fac-[Ir(dfpypy)3,17 and (3,5-difluoro-4-cyanophenyl)pyridine cyclometalates, FCNIr.18 These complexes were synthesized and found to exhibit deep-blue phosphorescence with high emission quantum yields in fluid solution at room temperature. Furthermore, remarkable EQE (above 20%) was achieved in deep-blue phosphorescent OLEDs using FCNIr as dopant.19a To attain deep-blue phosphorescent emitters, using the above-described concept, fluorine-substituted 2,3′-bipyridine (dfpypy) as cyclometalated ligand represents a good candidate for replacing the dfppy ligand.19b Herein, we report on synthesis, single-crystal X-ray diffractometric analysis, photophysical, electrochemical, and theoretical characterization of a novel class of heteroleptic Ir(III) complexes bearing two cyclometalating 2′,6′-difluoro2,3′-bipyridyl (dfpypy) chelates and one pyridyl pyrazolate ligand. Efficient deep-blue phosphorescence with excellent color chromaticity was observed. To prevent detrimental aggregation phenomena, we have introduced bulky tert-butyl groups on the pyridyl moieties. Indeed, the strategy resulted very successful and great color purity was achieved even in OLED devices made by vapor deposition of the complexes. Finally, all the experimental data were supported by DFT and TD-DFT calculations.

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EXPERIMENTAL SECTION

General Information and Materials. The solvents were dried using standard procedures. All other reagents were used as received from commercial sources, unless otherwise stated. NMR spectra were recorded on an ARX 300 or an AMX 400 from Bruker Analytische Messtechnik (Karlsruhe, Germany). The 1H NMR chemical shifts (δ) of the signals are given in ppm and referenced to residual protons in the deuterated solvents: chloroform-d1 (7.26 ppm), dimethyl sulfoxide-d6 (2.50 ppm), or acetone-d6 (2.09 ppm). The 19F NMR chemical shifts are referenced to CFCl3 (0.00 ppm) as an internal standard. The signal splitting are abbreviated as follows: s = singlet; d = doublet; t = triplet; q = quartet; m = multiplet. All coupling constants (J) are given in Hertz (Hz). Mass spectrometry was performed in the Department of Chemistry, University of Münster. Electrospray ionization (ESI) mass spectra were recorded on a Bruker Daltonics (Bremen, Germany) MicroTof with loop injection. Elemental analysis was performed at the University of Milan, Italy. The pyridyl azole and phenyl pyridine chelates, namely 3trifluoromethyl-5-(2-pyridyl) pyrazole(fppz)H,20 3-(trifluoromethyl)5-(4-t-butylpyridyl) pyrazole (fpbpz)H, 2′,6′-difluoro-2,3′-bipyridine (dfpypy),17 2-Chloro-4-tert-butylpyridine were prepared according to the literature procedures.21 4-(tert-butyl)-2′,6′-difluoro-2,3′-bipyridine (dfpybpy) was newly synthesized and fully characterized in this work. The iridium dimer complexes [(C∧N)2Ir(μ Cl)]2 (C∧N = dfpypy or dfpybpy) were synthesized using IrCl3·nH2O and dfpypy or dfbpypy in 2-ethoxyethanol according to literature method.22 3685



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Scheme 1. Schematic Synthetic Diagram and Structural Drawing of Complexes 1−4

Photophysical Characterization. Absorption spectra were measured on a Varian Cary 5000 double-beam UV−vis-NIR spectrometer and baseline corrected. Steady-state emission spectra were recorded on a HORIBA Jobin-Yvon IBH FL-322 Fluorolog 3 spectrometer equipped with a 450 W xenon-arc lamp, double-grating excitation and emission monochromators (2.1 nm mm−1 dispersion; 1200 grooves mm−1), and a Hamamatsu R928 photomultiplier tube or a TBX-4-X single photon-counting detector. Emission and excitation spectra were corrected for source intensity (lamp and grating) by standard correction curves. Time-resolved measurements were performed using the time-correlated single-photon counting (TCSPC) option on the Fluorolog 3. NanoLED (402 nm; FHWM < 750 ps) with repetition rates between 10 kHz and 1 MHz was used to excite the sample. The excitation sources were mounted directly on

(d, J = 0.6 Hz, 1H), 5.83 (t, J = 2.2 Hz, 1H), 5.71 (t, J = 2.0 Hz, 1H), 1.37 (s, 8H), 1.36 (s, 8H), 1.35 (d, J = 1.9 Hz, 9H). 19F{1H}NMR (282 MHz, CD2Cl2) δ: −60.41 (3F), −69.80 (1F), −69.97 (1F), −71.93 (1F), −72.37 (1F). HRMS calcd for C41H39F7IrN7 955.2783 ([M+H]+); found 956.2846. Elem. anal. Calcd (C)for C41H39F7IrN7: C, 51.56; H, 4.12; N, 10.27. Found: C, 52.02; H, 4.32; N, 10.35. X-ray Crystallographic Analysis. Data sets were collected with a Nonius KappaCCD diffractometer, equipped with a rotating anode generator. Programs used were as follows: data collection COLLECT23 data reduction Denzo-SMN,24 absorption correction SORTAV,25,26 Denzo,27 structure solution SHELXS-97,28 structure refinement SHELXL-97,29and graphics SCHAKAL.30 The crystallographic refinement parameters of complexes 1 and 4 are summarized in Table S1 in the Supporting Information. 3686



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of 2-MeTHF as solvent described by the IEFPCM.34 In order to simulate the 2-MeTHF solvent, that is not included as default in Gaussian09, we used ε = 6.97, εf = 1.978, Rsolv = 2.71, as static dielectric constant, dynamic dielectric constant and solvent radius (in Å) of the solvent as parameters, respectively.35 All the calculations were performed with Gaussian09W program package.36 Device Fabrication and Characterization. The current density−voltage−luminance (J−V−L) characteristics of the OLEDs were measured by a Keithley sourcemeter 2400 and a calibrated Advantest Q8221 Optical Multi Power Meter, respectively. EL spectra were taken by a Tec5 Spectrometer. EQEs were calculated from the luminance, current density, and EL spectrum, assuming a Lambertian distribution.37

the sample chamber at 90° to a double-grating emission monochromator (2.1 nm mm−1 dispersion; 1200 grooves mm−1) and collected by a TBX-4-X single-photon-counting detector. The photons collected at the detector are correlated by a time-to-amplitude converter (TAC) to the excitation pulse. Signals were collected using an IBH Data Station Hub photon counting module, and data analysis was performed using the commercially available DAS6 software (HORIBA Jobin Yvon IBH). The quality of the fit was assessed by minimizing the reduced chi squared function (χ2) and visual inspection of the weighted residuals. Luminescence quantum yields were measured with a Hamamatsu Photonics absolute PL quantum yield measurement system (C9920− 02) equipped with a L9799−01 CW Xenon light source (150 W), monochromator, C7473 photonic multichannel analyzer, integrating sphere and employing U6039−05 PLQY measurement software (Hamamatsu Photonics, Ltd., Shizuoka, Japan). All solvents were spectrometric grade. Deaerated samples were prepared by purging argon for 30 min. Cyclic Voltammetry. The electrochemical characterization for the metal complexes herein reported has been performed in acetonitrile/ 0.1 M tetra-butylammonium hexafluorophosphate (TBAH). The concentration of the samples was 1 mM. Acetonitrile (Acros, extra dry, 99.9%) was used as arrived without any further purification. TBAH (electrochemical grade, ≥ 99%, Fluka) was used as supporting electrolyte, which was recrystallized from a 1:1 ethanol−water solution and dried at 60 °C under vacuum. For the electrochemical measurements, a CHI750C Electrochemical Workstation (CH Instruments, Inc., Astin, TX, USA) was used. A homemade glassy carbon (Tokai Inc.) electrode has been employed as the working electrode, and a silver wire as a quasireference (QRE) electrode, which was separated from the catolyte solution by a glass frit (vycor). A platinum wire served as the counterelectrode. The working electrode was stored in ethanol, and before each experiments it was mechanically polished with a 0.05 μm diamond suspension (Metadi Supreme Diamond Suspension, Buehler) and ultrasonically rinsed with ethanol for 5 min. The electrode was electrochemically activated in the background solution by means of several voltammetric cycles at 0.5 V s−1 between the anodic and cathodic solvent/electrolyte discharges, until the same quality features were obtained. The reference electrode was calibrated at the end of each experiment against the ferrocene/ferricenium couple, whose formal potential is 0.460 V against the KCl saturated calomel electrode (SCE); in the following, all potential values will be reported against SCE. The measurements were performed in a glass cell under an Ar atmosphere. To minimize the ohmic drop between the working and the reference electrodes, we employed the feedback correction. Computational Method. Geometries were optimized by means of density functional theory (DFT) employing the exchange correlation hybrid functional B3LYP.31 The standard valence doubleζ polarized basis set 6-31G(d,p)32and the triple-ζ polarized basis set including diffuse functions 6-311++G(d,p) were used for C, H, F, and N, for ground and excited state optimizations, respectively. For Ir, the Stuttgart-Dresden (SDD) effective core potential was employed along with the corresponding valence triple-ζ basis set. Single-point DFT calculation of the S0 state at the T1-optimized geometry were performed by using the same level of theory employed for excited state optimizations. All the calculations were performed without any symmetry constrains. The nature of all the stationary points was checked by computing vibrational frequencies, and all the species were found to be true potential energy minima, as no imaginary frequency were obtained (NImag = 0). To simulate the absorption electronic spectrum down to about 250 nm, for each complex the lowest 30 singlet (S0 → Sn) as well as the 3 lowest triplet excitation energies (S0 → Tn) were computed on the optimized geometry at the S0 state by means of time-dependent density functional calculations (TD-DFT), at the same level of accuracy of the ground state.33 Oscillator strengths were deduced from the dipole transition-matrix elements (for single states only). All these calculations were performed in vacuum. For all the compounds, TD-DFT calculations were also done in the presence

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RESULTS AND DISCUSSION Synthesis and Characterization. All the iridium complexes were synthesized by reaction of [(dfpypy)2Ir(μ-Cl)]2 or

Figure 1. ORTEP diagram of 1 with thermal ellipsoids shown at 30% probability level. The CH2Cl2 solvent molecules hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Ir−C (52) = 2.009(5), Ir−C(32) = 2.013(5), Ir−N (41) = 2.044(4), Ir−N (21) = 2.047(5), Ir−N(1) = 2.092(5), Ir−N (11) = 2.152(4).

Figure 2. Absorption and normalized emission spectra of complexes 1 (black trace), 2 (orange trace), 3 (red trace), and 4 (blue trace) in 2MeTHF solution at room temperature. The samples were excited at 345 nm.

[(dfpybpy)2Ir(μ-Cl)]2 with the fppzH or fpbpzH according to literature method reported for similar systems 38 (see 3687



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Table 1. Most Meaningful Photophysical and Electrochemical Data for the Complexes 1−4 entry

λPLa (nm) 440, 440, 441, 440,

1 2 3 4

469 470 470 470

Φa

τobs(μs)

kr (× 105s−1)

knr (× 105s−1)

Eoxb (V)

Eredb (V)

0.79 0.87 0.81 0.77

3.23 2.24 2.49 1.65

2.44 3.88 3.24 4.67

0.65 0.58 0.76 1.39

1.29 1.27 1.25 1.23

−2.25 −2.26 −2.33 −2.32

In 2-MeTHF. (λex = 345 nm; Φ = photoluminescence quantum yield; τ = excited state lifetime. bElectrochemical data versus Fc+|Fc (Fc is ferrocene) were collected in CH3CN/0.1 M TBAH (tetra-butylammonium hexafluorophosphate). a

slightly elongated Ir−N distances of 2.092(3) and 2.152(3) Å vs those of the trans-orientated Ir−N distances of the dfpypy ligands. Moreover, the overall structural arrangement is similar when compared to several previously reported examples.17,39 For complex 4, similar structural data were obtained but, as expected, the introduction of the tert-butyl groups leads to a larger distance among the iridium complex. In complex 1, the closest distance between two metal centers (Ir···Ir* (*: symmetry code −X + 2, −Y, −Z + 2)) is 7.3564(3) Å, in 4 the closest one is between the two independent Ir-atoms in the asymmetric unit (Ir1A···Ir1B 9.5222(6) Å). Photophysical Characterization. The absorption spectra of all complexes are depicted in Figure 2 and were recorded at room temperature in 2-MeTHF solution. In general, the lowest-lying absorption band (shoulder) with maximum centered at 345−355 nm can reasonably be assigned to an admixture of spin-allowed π → π* (ligand centered, 1LC) and metal-to-ligand charge-transfer (1MLCT) transitions. The spin forbidden 3MLCT transitions are not visible due to their low intensity. The highest energy bands with maxima at 265−270 nm for all the complexes are 1LC transitions mostly localized on the dfpypy ligands but the energies of the ancillary ligand are also very close. These assignments were supported by theoretical calculations (vide infra). Figure 2 shows normalized emission spectra of the investigated complexes in 2-MeTHF solution. The vibronic structure of the emission bands indicates a certain degree of mixing between the ligand centered (3LC) and 3MLCT state. In particular, Figure 2 shows that the investigated complexes exhibited strong phosphorescence emissions consisting of two bands centered at 440 and 470 nm, similar to the values reported for fac-Ir(dfpypy)3 (438 and 463 nm). The photoluminescence emission quantum yields (PLQY) of the complexes 1−4 measured in 2-MeTHF solution with the absolute method, by employing an integrating sphere setup, are 0.79, 0.87, 0.81, and 0.71 for 1, 2, 3, and 4,

Table 2. Photophysical Data of Thin-Film for the Compounds 1−4 entry

λmaxa [nm]

Φa

τa (μs)

λmaxb (nm)

Φb

τb (μs)

1

440, 469,497(sh) 441, 469,495(sh) 441, 470, 494(sh) 440, 469, 493(sh)

0.82

4.40

444,474,506(sh)

0.05

0.08

0.62

3.99

444,473,508(sh)

0.09

0.11

0.93

3.16

441, 470

0.04

0.14

0.86

3.78

441, 470

0.06

0.09

2 3 4

a In 10 wt.% PMMA film. bNeat Film (λex = 345 nm; Φ = photoluminescence quantum yield; τ = excited state lifetime.).

experimental section for details). A pictorial synthetic pathway leading to the desired Ir(III) complexes is depicted in Scheme 1. After purification and recrystallization of the compounds detailed characterizations were carried out using high-resolution mass spectrometry, 1H and 19F NMR spectroscopy, and elemental analysis. To prevent aggregation and to improve the solubility, the introduction of tert-butyl groups on the pyridyl moieties was indeed a good strategy. The derivatives 3 and 4 showed better solubility and narrower spectra, color purity, in comparison with 1 and 2. Single crystals of 1 and 4 suitable for X-ray diffractometric analysis were grown from dichloromethane and methanol. As depicted in Figure 1, complex 1 showed the slightly distorted octahedral geometry with two cyclometalated dfpypy ligand and one fppz chelate surrounding the iridium metal center. The dfpypy ligands adopt a mutually eclipsed configuration with the nitrogen atoms N(21) and N(41) residing at the trans locations and the Ir−N distances lie between 2.047(3) and 2.044(3) Å. The cyclometalated carbon atoms C(32) and C(52) are mutually cis on the iridium and showing similar distances 2.013(5) and 2.009(5) Å. The third fppz ligand displays a

Figure 3. Emission spectra of iridium complexes 1 (black trace), 2 (orange trace), 3 (red trace), and 4 (blue trace) in thin film (a) neat (b) 10 wt % doping in PMMA matrix. The samples were excited at 345 nm. 3688



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Figure 4. Left: Partial molecular orbital diagram for complexes 1−4. The arrows are intended to highlight the HOMO−LUMO energy gaps. Right: Partial molecular orbital diagram for complex 1 with some selected isodensity frontier molecular orbital mainly involved in the electronic transitions. All the DFT energy values are given in eV.

Table 3. Computed Excitation Energies and Oscillator Strengths for the S0 → Sn Transitions of the Complexes 1−4a 1 λ (nm, eV) ( f) expansion coefficient

369, 3.362 (0.004) (S1) 0.667 HOMO → LUMO 316, 3.928 (0.086) 0.437 HOMO − 2 → LUMO + 2 0.421 HOMO − 2 → LUMO + 1 290, 4.282 (0.062) 0.519 HOMO → LUMO + 5

271, 4.568 (0.106) 0.394 HOMO − 2 → LUMO + 5 −0.289 HOMO − 4 → LUMO + 3

2

3

4

370, 3.348 (0.004) (S1) 364, 3.406 (0.007) (S1) 0.664 HOMO → LUMO 0.666 HOMO → LUMO 313, 3.968 (0.092) 327, 3.791 (0.071) 0.446 HOMO − 4 → LUMO 0.594 HOMO − 1 → LUMO + 2 0.324 HOMO − 2 → LUMO +2 292, 4.252 (0.110) 298, 4.157 (0.087) 0.365 HOMO − 4 → 0.498 HOMO − 3 → LUMO + 2 LUMO + 2 −0.342 HOMO → 0.350 HOMO → LUMO + 5 LUMO + 4 285, 4.355 (0.060) 293, 4.227 (0.052) 0.509 HOMO − 2 → −0.463 HOMO − 5 → LUMO + 3 LUMO

366, 3.389 (0.005) (S1) 0.669 HOMO → LUMO 313, 3.965 (0.110) 0.427 HOMO − 2 → LUMO + 1 0.371 HOMO − 4 → LUMO

270, 4.593 (0.097) 0.363 HOMO − 2 → LUMO + 5 −0.338 HOMO − 3 → LUMO + 3 0.322 HOMO − 6 → LUMO + 1

279, 4.450 (0.071) 0.385 HOMO − 2 → LUMO + 3 −0.315 HOMO − 5 → LUMO + 2

284, 4.363 (0.126) 0.550 HOMO → LUMO + 5 −0.348 HOMO − 3 → LUMO + 2

279, 4.447 (0.072) −0.337 HOMO − 2 → LUMO + 3 −0.314 HOMO − 6 → LUMO + 1

298, 4.158 (0.066) 0.374 HOMO − 3 → LUMO + 2 0.301 HOMO → LUMO + 4 285, 4.346 (0.088) 0.514 HOMO → LUMO + 5

269, 4.601 (0.137) 0.435 HOMO − 2 → LUMO + 5 0.331 HOMO − 4 → LUMO + 3 269, 4.617 (0.069) 0.485 HOMO − 4 → LUMO + 3

Except for S1 transitions, only calculated excitation with f ≥ 0.06 are listed. Also, only single excitation configurations with the highest contributions are reported, together with the corresponding transition symmetry and nature of the involved orbitals. a

(PL) spectra in thin film are depicted (Figure 3a, as neat film and Figure 3b, as 10 wt % in PMMA). Spin-coated thin-film at 10 wt % doped PMMA for all complexes exhibited PL spectra similar to those observed in the corresponding fluid solution (see Figure 3b). At such doping concentration, thin films in PMMA matrix showed PLQY higher for complexes with bulky tert-butyl groups, i.e. 3 and 4, than for complexes 1 and 2, being 93, 86, 82, and 62%, respectively. On the other hand, all these four complexes are weakly emissive in neat film, with a low quantum efficiency and short observed phosphorescence

respectively (see Table 1). These values are slightly higher than the fac-Ir(dfpypy)3, being 0.71 the value reported in CH2Cl2 solution.17 The excited-state lifetimes of all of the investigated complexes are monoexponential and in the range of 1.65−3.23 μs, highlighting the triplet nature of the emitting excited state. The main photophysical data are summarized in Table 1. The emission properties of the complexes 1−4 as thin film, in a polymeric matrix, at concentration in the range 10−100 wt %, were investigated and the corresponding photophysical data are summarized in Table 2. In Figure 3, photoluminescence 3689



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Table 4. Electroluminescent Device Performance of 4 Devices (ITO/Plexcore® OC/50 wt % MoO3:DPBIC (40 nm)/DPBIC (10 nm)/10, 15, and 20 wt % 4:SimCP (20 nm)/SimCP (5 nm)/P3PyPB (40 nm)/CsF (2 nm)/Al (100 nm)) dopant concentration (w %) 10 15 20

Figure 5. Comparison between experimental absorption (solid line) spectrum in 2-MeTHF of complex 1 and computed vertical transitions (vertical bars) with the corresponding oscillator strengths calculated in gas phase.

CIE (x,y)

voltage @ 300 cd m−2 (V)

luminance @ max. (lm W1−)

EQE @ max. (%)

0.159, 0.185 0.160, 0.190 0.162, 0.192

5.68

3.54

4.90

5.42

3.59

5.30

5.78

4.14

7.00

bear bulkier tert-butyl groups. Indeed the bulky groups play an important role in order to prevent stacking of the complexes, thus reducing the quenching effect in the film and consequently increasing the emission quantum yield.10,41 Computational Investigation. Molecular structure of the complexes 1−4 was optimized at their electronic ground state (S0) by means of DFT at B3LYP/(6-31G(d,p) + SDD) level. The main computed geometrical parameters are listed in Table S2 in the Supporting Information. The calculated S0 structures nicely correspond to the geometrical parameters obtained experimentally by single-crystal X-ray diffractometric analysis, within the known limitation of the density functional used.31c,42 The modeled geometries possess a distorted octahedral

lifetime (see Table 2), most probably due to triplet−triplet annihilation (TTA).40 Also, the spectra of 1 and 2 as neat film are slightly broadened and shifted with respect to the spectra in solution. We conclude that aggregation for complexes 1 and 2 in the solid is responsible for such reduced emission quantum yield and bathochromic shift. This observation was corroborated by the fact that no significant change in the emission properties has been observed for the complexes 3 and 4, which

Figure 6. Isodensity surface plots of the highest and lowest singly occupied molecular orbitals, HSOMO and LSOMO, respectively, along with the corresponding electron spin density and total density difference, for the compounds 1−4 at their T1 equilibrium geometry. Blue and green colors show regions of positive and negative difference between the alpha and the beta electron densities, respectively. Purple and turquoise colors show regions of decrease and increase in electron density, respectively. 3690



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Figure 7. Shematic device structure and the chemical formulas of materials used for the device preparation.

arrangement around the iridium center, with C1 point group symmetry, and reproduce the Ir−C(dfpypy) distances, being for (1) 2.015 and 2.031 Å for Ir−C(52) and Ir−C(32), versus corresponding experimental values of 2.009 and 2.013 Å, respectively. The Ir−C(32) bond distance appears to be slightly elongated with respect to Ir−C(52), due to the stronger trans effect of the formally anionic N atom of the pyrazole ligand coordinated to the metal center, when compared with the pyridyl N atom (see above). As already reported, Ir− N(pyridine) bond distances are slightly overestimated by the B3LYP functional, of a degree within the range 2−4%.31c,42 This nice agreement confirms the suitability of the employed theoretical model for describing the geometrical parameters of the here investigated complexes. Also, the Becke’s three parameter B3LYP exchange-correlation functional has been already widely proved to properly describe the electronic and optical properties of phosphorescent cyclometalated iridium complexes.43 In Figure 4 is reported the schematic representation of the energy levels of the orbitals closer to the frontier region for the complexes 1−4, and in Figure S4 in the Supporting Information are depicted their isodensity surface plots for relevant molecular orbitals. The complete list of the energies of the orbitals from HOMO − 10 to LUMO + 6, along with the HOMO−LUMO energy gap, is reported in Table S3 in the Supporting Information, for all the complexes. As far as the complex 1 is concerned, the HOMO is an Ir(t2g) orbital in antibonding combination with the π orbitals of the pyrazole ring lying at −6.122 eV, while the LUMO (−2.025 eV) has a π* character delocalized on one of the two dfpypy moieties, mainly localized on the pyridyl fragment and with minor contribution of the d metal orbitals. For 3, the introduction of moderate electron-donating groups, namely tert-butyl, on the pyridyl moieties destabilizes both HOMO and LUMO due to inductive effect. Nonetheless, the destabilization of the LUMO is found to be at higher degree than for the HOMO, being the energetic difference 0.176 and 0.137 eV, respectively. Indeed, the tertbutyl is bound on a fragment of the molecule (pyridine in dfpypy) strongly involved in the LUMO, but much less in the HOMO. Similar considerations can be adducted for the complex 4. In the complex 2, the presence of the tert-butyl on a pyridine (fppz), which is now not directly involved in the LUMO, destabilizes the lowest-lying virtual orbital but also rises up the energy of the HOMO. The relative destabilization

of the HOMO (0.076 eV) is higher than the corresponding destabilization of the LUMO (0.055 eV) with and without substituent. The effect of the substituents on the frontier orbitals is also qualitatively confirmed by electrochemical data (vide infra). A good linear correlation between theoretical and electrochemical energy gaps is shown in Figure S5 in the Supporting Information. Starting from the HOMO and going to more negative energies, HOMO − 1 and HOMO − 2 are encountered and involve other Ir(t2g) orbitals with, in the case of HOMO − 1, the cyclometalating difluoro-pyridyl moiety, while in the case of HOMO − 2, the pyrazole ring, both in antibonding fashion. The two nonequivalent cyclometalating difluoro-pyridines strongly participate to the occupied molecular orbitals, in turn, in the HOMO − 3 and HOMO − 4. Starting from LUMO and increasing the energy, the orbitals LUMO +1 and LUMO + 2 show, respectively, a strong contribution of the pyridyl fragment of the fppz and pyridyl moiety of dfpypy, which was not involved in the LUMO. Such description is valid for the complexes 1, 3 and 4. In 2, the presence of a tert-butyl on the pyridyl moiety of the fppz destabilizes the virtual orbital located on this fragment, being not compensated by the presence of further tert-butyl on the pyridine of the dfpypy moieties (as for 3), thus yielding a switch of the nature of LUMO + 1 and LUMO + 2 (see Figure S4 in the Supporting Information for the MO plots). To gain deeper insight into the electronic properties of the transitions involved in the optical absorption processes, we investigated the here reported complexes by means of timedependent density functional theory (TD-DFT) in both vacuum and 2-MeTHF solvent by means of IEFPCM model (see Computational Method for details). The computed vertical transitions were calculated at the equilibrium geometry of the S0 state and described in terms of one-electron excitations of molecular orbitals of the corresponding S0 geometry (see Experimental Section for further details). The most relevant transitions involved along with their energy, character and oscillator strengths computed in vacuum are listed in Table 3 (for a more complete list see Table S4 of the Supporting Information), and the comparison between the experimental spectrum and computed vertical transitions is depicted in Figure 5 for complex 1 (for the other complexes, see Figures S6−S8 of Supporting Information). For all the complexes, the lowest-energy singlet vertical excitation (S0 → 3691



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In general, because of the fact that the occupied molecular orbitals closer to the frontier region show strongly mixed Ir(t2g)−π(dfpypy or fppz) character, all the computed transitions can be better described as metal−ligand-to-ligand charge-transfer (MLLCT), with different amount of π−π* character, for all the complexes. As far as the complex 1 is concerned, several transitions with moderate intensities ( f = 0.02−0.05) can be envisaged going from the lower to the higher energy region of the spectrum. Nonetheless, the first transition with sizable intensity that can be encountered moving to higher energy is computed at 316 nm (3.928 eV, f = 0.086) and essentially consists of the combination of HOMO − 2 → LUMO + 2 and HOMO − 2 → LUMO + 1. The next intense process is calculated at 290 nm (4.282 eV, f = 0.062) with HOMO → LUMO + 5 character. The more intense and higher energy feature present in the experimental absorption spectrum (λabs = 264 nm) is computed, in excellent agreement, at 271 nm (4.568 eV, f = 0.106) and can be described in terms of linear combination of the HOMO − 2 → LUMO + 5 and HOMO − 4 → LUMO + 3. Similar considerations can be brought for the complexes 2, 3 and 4. For the complex 2, even though a different ordering of the three lowest virtual orbitals is present, a consistence of the electronic nature of the transitions can be found. Furthermore, the three lowest-lying formally spin-forbidden singlet-to-triplet excitations (S0 → Tn, with n = 1−3) were computed in gas phase for all the complexes at their corresponding S0 equilibrium geometry, and the values are listed in Table S4 in the Supporting Information. The S0 → T1 transitions were calculated at 420−424 nm and are mainly described as excitations involving linear combination of different d(Ir)π(pyrazole) → π*(pyridineppz) orbitals, for 1, 3, and 4, whereas as combination of different d(Ir)π(pyrazole) → π*(dfpypy), for 2. In all these cases, the S0 → T1 excitation can be described as 3MLLCT transition. The computed resonance frequencies are in very good agreement with the corresponding onset values of the experimental emission spectra in 2-MeTHF (vide infra). To evaluate the solvent effect, we also performed TD-DFT computations in 2-MeTHF solution by means of IEFPCM. The most relevant transitions involved along with their energy and oscillator strength are listed in Table S5 of the Supporting Information. As expected, when transition with similar character are compared to those computed in vacuum, for the lower energy transitions with MLLCT nature the IEFPCM results show only slight hypsochromic shift (

Deep-Blue-Emitting Heteroleptic Iridium(III) Complexes Suited for

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