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Synthesis, structure and DFT calculation of a facial tris-cyclometalated phosphorescent iridium(III) complex containing substituted phthalazine ligands
Inorganica Chimica Acta 466 (2017) 343–348
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Research paper
Synthesis, structure and DFT calculation of a facial tris-cyclometalated phosphorescent iridium(III) complex containing substituted phthalazine ligands Xue-Mei Wang, Jia-Yan Qiang, Xian-Hua Ni, Ai-Quan Jia, Xiu-Fang Ma, Bihai Tong, Qian-Feng Zhang ⇑ Institute of Molecular Engineering and Applied Chemistry, and Anhui Province Key Laboratory of Metallurgy Engineering & Resources Recycling, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China
a r t i c l e
i n f o
Article history: Received 7 November 2016 Received in revised form 19 May 2017 Accepted 8 June 2017 Available online 9 June 2017 Keywords: Iridium(III) complex Facial tris-cyclometalated complex Crystal structure Photoluminescence DFT calculation
a b s t r a c t The facial tris-cyclometalated iridium(III) complex fac-Ir(dmpopopz)3 (dmpopopzH = 1-(2,6dimethylphenyloxy)-4-(4-phenyloxyphenyl)-phthalazine) was synthesized from the reaction of iridium (III) chloride hydrate and ligand dmpopopzH in refluxing ethoxyethanol. The conformation of complex fac-Ir(dmpopopz)3 is described on the basis of the 1H NMR, 13C NMR, FT-IR and mass spectroscopies, the crystal structure of fac-Ir(dmpopopz)3 was determined by X-ray crystallography, and the theoretical calculation of complex fac-Ir(dmpopopz)3 was also investigated by the DFT method. The absorption and emission spectral results exhibit that complex fac-[Ir(dmpopopz)3] may be used as a red-emitting phosphorescent material. Efficient OLEDs have been achieved using the complex fac-[Ir(dmpopopz)3] in the red-emitting region. Ó 2017 Published by Elsevier B.V.
1. Introduction Mixing of singlet and triplet excited states through spin-orbit coupling which was aroused by the heavy-metal effect, to a large extent, would partially remove the spin-forbidden nature of the T1 ? S0 radioactive relaxation, leading to a fast radio- active decay; therefore, quenching and/or annihilation processes can be comparably small, resulting in a highly intense phosphorescent emission [1,2]. Accordingly, a large amount of effort has been made in investigating the second- and third-row transition-metal complexes, aimed at development of the highly efficient phosphors that can emit all three primary colors [3]. Among the complexes studied to date, cyclometalated tris-chelated iridium(III) complexes have emerged as particularly promising candidate materials since the pioneering works were carried out by the group of Thompson and Forrest [4]. Generally, the tris-cyclometalated iridium(III) complexes were isolated by way of three synthetic routes: (i) direct reaction of iridium(III) chloride hydrate with the three-fold amount (or excess) of the ligand in the refluxing condition [5], (ii) treatment of Ir(acac)3 with 3.0–3.5 equiv of the appropriate ligand in the refluxing glycerol [6], and (iii) a novel one-step method in the presence of AgO2CCF3 [7].
⇑ Corresponding author. E-mail address: [email protected] (Q.-F. Zhang). http://dx.doi.org/10.1016/j.ica.2017.06.021 0020-1693/Ó 2017 Published by Elsevier B.V.
As well-known, iridium(III) complexes are not sufficiently compatible to form homogeneous molecular dispersions in polymer films. This poor solubility, in turn, restricts the device efficiency and lifetime [5]. A widely used approach to enhance the solubility of the iridium(III) complexes has been to attach solubilizing alkyl moieties or to introduce dendritic architectures [8,9]. However, the introduction of alkyl groups perturbs charge carrier transport and lowers the glass transition temperature (tg). Hence, sterically hindered aromatic substituents, which give rise to fewer side effects, are considered to be better solubilizing groups [10–12]. Meanwhile, 1,2-diazenes (AN@N-) are a field of increasing importance and there has been much research on this area given probable intermediates of diazene complexes in biological fixation and the nonezymatic conversion of coordinated dinitrogen into diazene derivatives [13,14]. Their high electron affinities of such complexes would make them promising candidates for the electron injection and transportation. With this in mind, we are interested to introduce sterically hindered phenol groups into diazene derivative to get a novel ligand 1-(2,6-dimethylphenyloxy)-4- (4-phenyloxy-phenyl)phthalazine (dmpopopzH). The corresponding facial tris-cyclometalated iridium(III) complex fac-Ir(dmpopopz)3 was synthesized by the reaction of IrCl3nH2O and dmpopopzH in the refluxing ethoxyethanol overnight. The structural characterization and photoluminescence properties along with theoretical calculation of the complex fac-Ir
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(dmpopopz)3 were reported, and the performance of OLED fabricated by solution process was also investigated in this paper. 2. Experimental section 2.1. General considerations All experiments were performed under a dry dinitrogen atmosphere using standard Schlenk techniques. All solvents were freshly distilled over appropriate drying reagents prior to use. All starting materials were purchased from either Aldrich or Alfa and used without further purification. Ligand 1-(2,6-dimethylphenyloxy)-4- (4-phenyloxy-phenyl)-phthalazine (dmpopopzH) was prepared according to the procedure described in the literature [5]. The silica gel column chromatography was performed with use of silica gel (200–300 mesh). 2.2. Measurements 1
H and 13C NMR spectra were recorded on a Bruker ALX 300 spectrometer operating at 400 MHz (for 1H) and 100 MHz (for 13 C) with tetramethylsilane (TMS) as the internal standard. Infrared spectrum was recorded on a Perkin-Elmer 16 PC FT-IR spectrophotometer with use of pressed KBr pellets. Electronic absorption spectrum was measured in dichloromethane solution on a Shimadzu UV-3000 spectrophotometer. Luminescence properties were measured and recorded using an FLS-920 fluorescence spectrometer. Positive-ion ESI mass spectrum was recorded on a Perkin-Elmer Sciex API 365 mass spectrometer. Photoluminescence (PL) spectra were measured with a Shimadzu RF-5301PC fluorescence spectrophotometer. Luminescence lifetime was determined on an Edinburgh FL920 time-correlated pulsed single-photon-counting instrument. Melting points were measured using capillary melting point apparatus. Elemental analyses were carried out using a Perkin-Elmer 2400 CHN analyzer. 2.3. Synthesis of ligand dmpopopzH (1) 1-Chloro-4-(4-phenoxy)-phthalazine (6 g, 18 mmol), 2,6dimethylphenol (2.2 g, 18 mmol) and K2CO3 (5 g, 36 mmol) were dissolved in DMF (50 mL), and the mixture was heated at 120 °C for 4 h under nitrogen atmosphere. After cooling, the mixture was poured into distilled water and the white solid was precipitated. The crude product was purified by chromatography on a neutral alumina column. Yield: 7.2 g (96%). M.p.: 245–246 °C. 1H NMR (CDCl3, 400 MHz) d: 2.21 (s, 6H, ACH3), 7.10–7.16 (m, 8H, Ar), 7.39 (d, 3JHH = 8.0 Hz, 2H, ACAC6H4AO), 7.73 (d, 3JHH = 8.4 Hz, 2H, ACAC6H4AO), 7.94 (d, 3JHH = 7.2 Hz, 1H, -phthalazine), 7.97 (d, 3JHH = 8.0 Hz, 1H, -phthalazine), 8.14 (d, 3JHH = 7.2 Hz, 1H, -phthalazine), 8.57 (d, 3JHH = 7.6 Hz, 1H, -phthalazine). 2.4. Synthesis of complex [Ir(dmpopopz)3]¼C6H14 (1) IrCl3xH2O (200 mg, 0.56 mmol) and ligand mpopoppzH (0.75 g, 1.8 mmol) were dissolved in ethoxyethanol (15 mL), and the mixture was refluxed at 110 °C overnight under nitrogen atmosphere with exclusion of light. After cooling, the precipitate was filtered off and washed with anhydrous ethanol. The residue was purified by chromatography on a silica gel column using hexane/dichloromethane (v:v = 6:1) as an eluent to yield the facial tris-cyclometalated iridium(III) complex fac-Ir(mpopoppz)3¼C6H14 (1) as a bright red powder. Single crystals of complex 1 were grown from the mixed dichloromethane/n-hexane (1:3) solution by slow evaporation at room temperature. Yield: 250 mg (31%). MS ((+)-ESI): m/ z = 1444.65 (calcd. 1467.64 for C84H63N6O6IrNa, [M + Na+]). 1H
NMR (CDCl3, 400 MHz): d 2.18 (s, 18H, ACH3), 5.90–5.95 (m, 3H, -Ar), 6.40 (d, 3JHH = 12 Hz, 6H, AAr), 6.52 (s, 3H, AIrAC6H3), 6.65 (d, 3JHH = 10 Hz, 6H, -Ar), 6.81–7.00 (m, 12H, -Ar), 7.82–8.03 (m, 9H, -phthalazine), 8.34 (d, 3JHH = 6.4 Hz, 3H, AAr), 8.85 (d, 3 JHH = 11.3 Hz, 3H, -phthalazine). 13C NMR (100 MHz, CDCl3) d 159.39, 158.89, 157.06, 156.98, 150.92, 132.92, 132.45, 132.02, 131.49, 131.08, 130.36, 129.21, 128.60, 126.82, 126.02, 124.27, 123.80, 120.20, 119.96, 118.78, 17.07. FT-IR (KBr disc, cm 1): m (PhCAH) 3044(w), m(ACH3) 2924(m), 2849(w), m(PhC@C) 1633 (w), 1572(s), 1538(m), 1489(m), m(CAO) 1385(s). Anal. Calc. for C84H63N6O6Ir- ¼(C6H14): C, 70.04; H, 4.57; N, 5.73%. Found: C, 69.89; H, 4.54; N, 5.71%. 2.5. OLED fabrication and measurement The indium tin oxide (ITO) glass substrate was washed in turn with a substrate-cleaning detergent, deionized water, acetone, and ethanol for 15 min, under ultrasonic condition, and finally treated with ozone for about 20 min. Poly(3,4-ethlenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT) (Baytron P4083, Bayer AG) in water was spin-coated at a rate of 3000 rpm on the ITO substrate and dried by baking in air at 120 °C for 10 h. Then the solution of poly(vinylcarbazole) (PVK, Mw = 81,800) (68 wt%) blended with 2-tert-butylphenyl- 5-biphenyl-1,3,4-oxadiazole (PBD) (30 wt%) and fac-Ir(dmpopopz)3 (2 wt%) in chloroform was spin-coated at a rate of 1300 rpm on top of the PEDOT layer. Finally, barium and aluminum as the cathode layers were evaporated. Current density- voltage-luminance data were collected using a Keithley 236 source measurement unit and a calibrated silicon photodiode. External quantum efficiency was obtained by measuring the total light output in all directions in an integrating sphere (IS080, Labsphere). Luminance and luminous efficiency were measured by a silicon photodiode and calibrated using a PR-705 photometer (Photo Research). 2.6. Theoretical calculation The ground state molecular geometry full optimization of complex fac-Ir(mpopoppz)3 was carried out with the density functional theory (DFT) at the Becke’s 3LYP (B3LYP) level [15]. Frequency calculations at the same level of theory have also been performed to identify the stationary point as minima (zero imaginary frequency). On the basis of the optimized geometries, the natural bond orbital (NBO) is employed to analyze the molecular orbital compositions (%). The lowest 10 singlet-singlet and singlet-triplet excitations have been computed by means of TDDFT. The effective core potentials (ECPs) of Hay and Wadt with a double f-valence basis set (LANL2DZ) were used for the iridium atom [16], the 6311g⁄ basis set for the atoms bonded to metal iridium atom directly, the 6-31g for the other atoms [17]. The basis set for r iridium atom is a modified LANL2DZ plus an f-type polarization function and the two 4p functions of the standard LANL2DZ are replaced by the optimized 4p functions from Couty and Hall. Starting geometry was taken from X-ray crystal structure. All the calculations were performed with the Gaussian-03 software package [18]. 2.7. X-ray crystallography analysis Intensity data of complex fac-Ir(mpopoppz)3¼C6H14 (1) were collected on a Bruker SMART APEX 2000 CCD diffractometer using graphite-monochromated Mo-Ka radiation (k = 0.71073 Å) at 296 (2) K. The collected frames were processed with the software SAINT. The data was corrected for absorption using the program SADABS. Structures were solved by the direct methods and refined by full-matrix least-squares on F2 using the SHELXTL software
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package [19,20]. All non-hydrogen atoms were refined anisotropically. The positions of all hydrogen atoms were generated geometrically (Csp3AH = 0.96 and Csp2AH = 0.93 Å), assigned isotropic thermal parameters, and allowed to ride on their respective parent carbon or nitrogen atoms before the final cycle of least-squares refinement. The hexane molecules in fac-Ir(mpopoppz)3¼C6H14 were isotropically refined without hydrogen atoms due to obvious disorders of carbon atoms in solvents. Crystallographic data and experimental details for complex fac-Ir(mpopoppz)3¼C6H14 are summarized in Table 1. The refined atomic coordinates and anisotropic thermal parameters are deposited in the Supporting Information. Crystallographic data for the structure reported here have been deposited with the Cambridge Crystallographic Data Center (Deposition No. CCDC-1515243). The data can be obtained free of charge via http://www.ccdc.cam.ac.uk./perl/catreq/catreq. cgi (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax,+44 1223 336033; e-mail, [email protected]).
3. Results and discussion 3.1. Synthesis and characterization The synthetic route of the facial tris-cyclometalated iridium(III) complex fac-Ir(dmpopopz)3 is shown in Scheme 1. The ligand dmpopopzH was easily prepared via a substitution reaction of 1chloro-4-(4-phenoxy)phthalazine and 2,6- dimethylphenol in an equivalent ratio in the presence of potassium carbonate as an inorganic base in N, N’-dimethylformamide solution [5]. The complex fac-Ir- (dmpopopz)3 was obtained by a relatively simple procedure. Direct reaction of IrCl3nH2O and dmpopopzH in ethoxyethanol at reflux overnight resulted in isolation of the final tris-cyclometalated iridium(III) product in a yield of 31%. Complex fac-Ir (dmpopopz)3 is quite stable under air and was well characterized by proton and carbon NMR spectroscopies, positive-ion ESI mass spectroscopy, infrared spectroscopy, and microanalyses. In the 1H NMR spectrum, the complex exhibits eight well-resolved peaks
Table 1 Crystallographic data and experimental details for fac-Ir(dmpopopz)3¼C6H14 1. Ir(dmpopopz)3¼C6H14 C85.5H66.5N6O6Ir 1466.15 Monoclinic 45.2739(5) 13.6920(1) 29.7472(4) 130.256(1) 14072.7(3) C2/c 8 1.384 296(2) 5972 1.958 65950 16072 886 0.0400 0.0355, 0.0825 0.0607, 0.0922 1.028
Complex Empirical formula Formula weight Crystal system a (Å) b (Å) c (Å) b (°) V (Å3) Space group Z Dcalc (g cm 3) Temperature (K) F(000) l(Mo-Ka) (mm 1) Total refln Independent refln No. of parameters Rint R1a, wR2b (I > 2(I)) R1, wR2 (all data) GoFc a b c
P P R1 = Fo Fc/ Fo. P P 1/2 wR2 = [ w(F2o F2c )2/ wF22 . o ] P 2 GoF = [ w(Fo Fc) /(Nobs Nparam)]1/2.
OH
O
Fig. 1. Perspective view of complex fac-Ir(mpopoppz)3. Thermal ellipsoids are shown at the 40% probability level. Selected bond lengths (Å) and angles (deg): Ir (1)-C(71) 2.008(3), Ir(1)-C(111) 2.013(3), Ir(1)-C(31) 2.017(3), Ir(1)-N(3) 2.087(3), Ir (1)-N(5) 2.095(3), Ir(1)-N(1) 2.106(3); C(71)-Ir(1)-C(111) 93.29(13), C(71)-Ir(1)-C (31) 99.82(13), C(111)-Ir(1)-C(31) 97.65(13), C(71)-Ir(1)-N(3) 78.10(12), C(111)-Ir (1)-N(3) 87.14(12), C(31)-Ir(1)-N(3) 174.91(11), C(71)-Ir(1)-N(5) 170.37(12), C (111)-Ir(1)-N(5) 77.58(12), C(31)-Ir(1)-N(5) 84.58(11), C(71)-Ir(1)-N(1) 92.71(11), C (111)-Ir(1)-N(1) 173.28(11), C(31)-Ir(1)-N(1) 78.32(12), N(3)-Ir(1)-N(5) 98.19(10), N(3)-Ir(1)-N(1) 97.07(11), N(5)-Ir(1)-N(1) 96.58(11).
O
O IrCl3 xH2O
Ir N N Cl
K2CO3, DMF
N N O
N N 3
ethoxyethanol/H2O O
Scheme 1. The synthetic route of the facial tris-cyclometalated iridium(III) complex fac-Ir(mpopoppz)3.
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in the region of 5.9–8.9 ppm due to the phenyl rings and multiple peaks in region of 1.3–2.1 ppm due to the methyl groups. The methyl groups of dmpopopz ligand in complex fac-Ir(dmpopopz)3 showed expected signal in the high field of the 13C NMR spectrum (17.07 ppm). The IR spectrum of complex fac-Ir(dmpopopz)3 clearly shows the strong band at 1385 cm 1, which may be attributed to the m(CAO) absorption. The weak band at 3044 cm 1 in the IR spectrum of complex 1 may be tentatively assigned to the m (PhCAH) absorption. The structure of complex fac-Ir(dmpopopz)3 was unambiguously established by single-crystal X-ray diffraction analysis. Relevant crystallographic data and details of the data collection are listed in Table 1, and the molecular structure of complex fac-Ir(dmpopopz)3 along with selected bond lengths and bond angles is shown in Fig. 1. The facial geometry around the iridium(III) center in complex fac-Ir(dmpopopz)3 is distorted octahedral with three nitrogen atoms and three carbon atoms, which is indicated by the average CAIrAC and NAIrAN bond angles being 96.92(12)° and 97.28(11)°, respectively, along with the CAIrAN bond angles ranging from 77.58(12)° to 174.91(11)°. The average Ir C (av. 2.013(3) Å) and IrAN (av. 2.096(3) Å) distances are normal by the comparison with other related tris-cyclometalated iridium(III) complexes [5,21].
3.2. Ground-state DFT calculations Computational investigations have been proved to be very helpful in understanding the HOMO and LUMO properties of the cyclometalated iridium(III) complexes [22,23]. Because the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the host are one of the most important factors for OLEDs, herein we investigated the HOMO and LUMO properties of complex fac-Ir(dmpopopz)3 by density functional theory (DFT) calculations [24]. Fig. 2b displays the LUMO and HOMO orbitals for complex fac-Ir(dmpopopz)3,
and the experimental data of the HOMO-LUMO energy gaps are given in Table 2, which are quite similar to the according data of Ir(mpcppz)3 (mpcppzH = 1-(2,6-dimethyl-phenoxy)-4-(4-chlorophenyl)phthalazine) [5]. From Fig. 2, we can obviously observe that the HOMO and LUMO localized on the iridium(III) atom (mainly on d orbitals) and the phenyl- phthalazine rings, respectively. This is consistent with the property of the excited state of complex facIr(dmpopopz)3 because the luminescence mechanism is the metal to ligand charge transfer (d of the iridium to p⁄ of the phenylphthalazine).
3.3. Absorption and photophysical properties The UV–Vis spectrum of complex fac-Ir(dmpopopz)3 in dichloromethane solution, as illustrated in Fig. 3, exhibits a strong absorption band from 200 to 300 nm, indicating that the electronic transition is mostly ligand-centered (LC) p-p⁄. Intraligand (IL) pp⁄ transition and metal-to-ligand charge transfer (1MLCT, dp(Ir) p(ligand)) transition are observed in the region of 350–460 nm. In addition, a weaker absorption shoulder extending into the visible region, which can be assigned to the formally spin-forbidden transition from the triplet metal-to-ligand charge transfer (3MLCT), are also observed in the UV–Vis spectrum of complex fac-Ir (dmpopopz)3. The photoluminescence (PL) spectroscopy is displayed in Fig. 4. Obviously, complex fac-Ir(dmpopopz)3 in dichloromethane solution emits an intense orange luminescence with emission wavelength at 602 nm. In comparison with emission wavelength at 590 nm for complex Ir(mpcppz)3, it is probably due to the substitution of chloride by a phenoxy group, leading to a little red shift of the emission wavelength. The PL decay curve of complex fac-Ir (dmpopopz)3 is shown in Fig. 5 and the lifetime of complex fac-Ir (dmpopopz)3 is ca. 268 ns, which is obviously shorter than that of complex Ir(mpcppz)3 (2 ls). Because long lifetime will give rise Table 2 Frontier orbital energy and HOMO-LUMO energy gap (eV). Orbital
fac-Ir(dmpopopz)3
ELUMO+1 ELUMO EHOMO EHOMO-1 DEL-H
1.36 1.47 4.76 4.95 3.29
Absorbance
0.6
0.4
0.2
0.0 300
400
500
600
Wavelength(nm) Fig. 2. (a) Simplified structure of complex fac-Ir(dmpopopz)3, hydrogen atoms linked to oxygen atoms represent phenyl rings; (b) Schematic representation of the frontier orbitals of complex fac-Ir(dmpopopz)3.
Fig. 3. UV–Vis absorption dichloromethane.
spectrum
of
complex
fac-Ir(dmpopopz)3
in
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3.4. Electroluminescent properties
Normalized Intensity
200
To further investigate electroluminescent (EL) properties of the complex fac-Ir(dmpopopz)3, the OLEDs using the facial triscyclometalated iridium(III) complex fac-Ir(dmpopopz)3 as a dopant was fabricated by solution process with the following structure: ITO/PEDOT(40 nm)/2 wt% Ir(dmpopopz)3 + 70 wt% PVK + 28 wt% PBD (80 nm)/Ba(4 nm)/Al(120 nm). PEDOT was used as hole-injection material. PVK and PBD were selected as the host material and the electron transport material, respectively. The general structure of the device and the molecular structures of the compounds used in the device are illustrated in Fig. 6. The device has a maximum emission at 616 nm (see Fig. 7), and the fine vibronic structure of the EL spectra leads us to conclude that EL emission originates from a mixed 3MLCT/3p-p⁄ state [25]. The Commission International de L’Eclairage (CIE) coordinates of 0.643 and 0.353 are close to the standard red color coordinates (0.67 and 0.33) of the national television system committee, at a driving voltage of 3.9 V, which is obviously better than that of the previously reported complex Ir(mpcppz)3 [5]. No emission from the PVK-PBD blend is detected, which implies that the energy transfer from the blend to the facial tris-cyclometalated iridium(III) complex fac-[Ir(dmpopopz)3] is efficient under electrical excitation. As shown in Fig. 8, the device gives a maximum luminance of 1330 cd/m2 at a current density of 83.9 mA/cm2. A maximum external quantum efficiency of 1.48% corresponding to a luminous efficiency of 0.81 cd/A may be obtained. Even if the external quantum efficiency is 1.48% only, the maximum brightness is still up to 1030 cd/m2, an indicative of the material itself being an efficient
150
100
50
0 550
600
650
700
750
Wave length (nm) Fig. 4. Photoluminescence dichloromethane.
spectrum
of
complex
fac-Ir(dmpopopz)3
in
0.8
0.6
0.4
1.0
Normalisec PL Intensity
PL Intensity (normalised)
1.0
0.2
0.0 1000
2000
3000
4000
5000
Time (ns) Fig. 5. PL decay curve of complex fac-Ir(dmpopopz)3 in dichloromethane.
to the emission saturation and then decrease the efficiency, suitable lifetimes of the cyclometalated iridium(III) complexes (0.1– 14 ls) make them the ideal candidates for the OLEDs application. Compared with the similar complexes previously reported in the literature [5], complex fac-Ir(dmpopopz)3 has a relatively shorter lifetime.
0.8
0.6
0.4
0.2
0.0 500
600
700
Wavelenth (nm) Fig. 7. EL spectrum of complex fac-Ir(dmpopopz)3 in dichloromethane.
S Al Ba(4 nm)
O
n
ITO Glass substrate
N
O SO 3-
PVK:PBD:Ir complex(80 nm) PEDOT:PSS(40 nm)
m
PEDOT:PSS
PVK N N O PBD
Fig. 6. General structures for the devices and molecular structures of the relevant compounds.
n
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Appendix A. Supplementary data
2
10
0
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2017.06.021.
2
Current Density(mA/cm )
1500 10
1000
References
L(cd/m )
-2
10
2
10-4
500
-6
10
-8
10
0 0
2
4
6
8
10
Voltage(V) Fig. 8. Current density–voltage–luminance curves of the device with complex fac-Ir (dmpopopz)3.
phosphor. Therefore, it may be expected to improve the performance of the device by further optimizing the device structure. 4. Conclusions In summary, a tris-cyclometalated iridium(III) complex fac-Ir (dmpopopz)3 with phenylphthalazine ligands was synthesized and characterized by single-crystal X-ray diffraction along with a series of spectroscopic properties. DFT calculation results show the HOMO and LUMO localized on the iridium(III) atom (mainly on d orbital) and the phenylphthalazine rings, respectively. Complex fac-Ir(dmpopopz)3 may emit intense saturated red light in solid state. A red OLED with a solution processed emissive layer containing complex fac-Ir(dmpopopz)3 is presented in this paper. The device exhibits a maximum luminance of 1330 cd/m2. This red emitter may be potentially used in white OLEDs via a cost effective solution-process. Acknowledgements This project was supported by the National Natural Science Foundation of China (21372007).
[1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151–154. [2] C.-H. Yang, Y.-M. Cheng, Y. Chi, C.-J. Hsu, F.-C. Fang, K.-T. Wong, P.-T. Chou, C.H. Chang, M.-H. Tsai, C.-C. Wu, Angew. Chem. Int. Ed. 46 (2007) 2418–2421. [3] C.-H. Yang, Y.-M. Cheng, Y. Chi, C.-J. Hsu, F.-C. Fang, K.-T. Wong, P.-T. Chou, C.H. Chang, M.-H. Tsai, C.-C. Wu, Inorg. Chem. 48 (2009) 6501–6508. [4] A.B. Tamayo, B.D. Alleyne, P.I. Djurovich, S. Lamansky, I. Tsyba, N.N. Ho, R. Bau, M.E. Thompson, J. Am. Chem. Soc. 125 (2003) 7377–7387. [5] B. Tong, Q. Mei, S. Wang, Y. Fang, Y. Meng, B. Wang, J. Mater. Chem. 18 (2008) 1636–1639. [6] K. Dedeian, P.I. Djurovich, F.O. Garces, C. Carlson, R.J. Watts, Inorg. Chem. 30 (1991) 1685–1687. [7] V.V. Grushin, N. Herron, D.D. LeCloux, W.J. Marshall, V.A. Petrov, Y. Wang, Chem. Commun. (2001) 1494–1495. [8] Y. You, C.-G. An, J.-J. Kim, S.Y. Park, J. Org. Chem. 72 (2007) 6241–6246. [9] S. Jung, Y. Kang, H.-S. Kim, Y.-H. Kim, C.-L. Lee, J.-J. Kim, S.-K. Lee, S.-K. Kwon, Eur. J. Inorg. Chem. (2004) 3415–3423. [10] C.-L. Ho, W.-Y. Wong, Z.-Q. Gao, C.-H. Chen, K.-W. Cheah, B. Yao, Z. Xie, Q. Wang, D. Ma, L. Wang, X.-M. Yu, H.-S. Kwok, Z.-Y. Lin, Adv. Funct. Mater. 18 (2008) 319–331. [11] Y. Zheng, A.S. Batsanov, R.M. Edkins, A. Beeby, M.R. Bryce, Inorg. Chem. 51 (2012) 290–297. [12] Y. Fang, B. Tong, S. Hu, S. Wang, Y. Meng, J. Peng, B. Wang, Org. Electron. 10 (2009) 618–622. [13] N. Xiao, Q. Xu, J. Sun, J.B. Chen, Dalton Trans. (2006) 603–608. [14] Q. Mei, L. Wang, B. Tian, B. Tong, J. Weng, B. Zhang, Y. Jiang, W. Huang, Dyes Pigments 97 (2013) 43–51. [15] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789. [16] J. Oxgaard, R.A. Periana, W.A. Goddard III, J. Am. Chem. Soc. 126 (2004) 11658– 11665. [17] J. Oxgaard, R.P. Müller, W.A. Goddard III, R.A. Periana, J. Am. Chem. Soc. 126 (2004) 352–363. [18] F.A. Cotton, G. Wilkinson, C.A. Murill, Advanced Inorganic Chemistry, sixth ed., A Wiley-Interscience Publication, John-Wiley & Sons, Inc., 1999. [19] G.M. Sheldrick, SHELXTL Software Reference Manual (Version 5.1), Bruker AXS Inc., Madison, WI, 1997. [20] G.M. Sheldrick, Acta Crystallogr. Sect. A 64 (2008) 112–122. [21] A.S. Ionkin, W.J. Marshall, D. Christopher Roe, Y. Wang, Dalton Trans. 64 (2006) 2468–2478. [22] D. Wang, H. Wang, Q. Wang, W. Yang, Y. Ding, J. Organomet. Chem. 749 (2014) 41–46. [23] J.-Y. Qiang, Y.-Q. Xu, B. Tong, X.-F. Ma, Q. Chen, W.-H. Leung, Q.-F. Zhang, Inorg. Chim. Acta 394 (2013) 184–189. [24] C. Shang, J. Xu, Y. Du, J. Zhao, J. Lumin. 175 (2016) 217–224. [25] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971–12979.
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Research paper
Synthesis, structure and DFT calculation of a facial tris-cyclometalated phosphorescent iridium(III) complex containing substituted phthalazine ligands Xue-Mei Wang, Jia-Yan Qiang, Xian-Hua Ni, Ai-Quan Jia, Xiu-Fang Ma, Bihai Tong, Qian-Feng Zhang ⇑ Institute of Molecular Engineering and Applied Chemistry, and Anhui Province Key Laboratory of Metallurgy Engineering & Resources Recycling, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China
a r t i c l e
i n f o
Article history: Received 7 November 2016 Received in revised form 19 May 2017 Accepted 8 June 2017 Available online 9 June 2017 Keywords: Iridium(III) complex Facial tris-cyclometalated complex Crystal structure Photoluminescence DFT calculation
a b s t r a c t The facial tris-cyclometalated iridium(III) complex fac-Ir(dmpopopz)3 (dmpopopzH = 1-(2,6dimethylphenyloxy)-4-(4-phenyloxyphenyl)-phthalazine) was synthesized from the reaction of iridium (III) chloride hydrate and ligand dmpopopzH in refluxing ethoxyethanol. The conformation of complex fac-Ir(dmpopopz)3 is described on the basis of the 1H NMR, 13C NMR, FT-IR and mass spectroscopies, the crystal structure of fac-Ir(dmpopopz)3 was determined by X-ray crystallography, and the theoretical calculation of complex fac-Ir(dmpopopz)3 was also investigated by the DFT method. The absorption and emission spectral results exhibit that complex fac-[Ir(dmpopopz)3] may be used as a red-emitting phosphorescent material. Efficient OLEDs have been achieved using the complex fac-[Ir(dmpopopz)3] in the red-emitting region. Ó 2017 Published by Elsevier B.V.
1. Introduction Mixing of singlet and triplet excited states through spin-orbit coupling which was aroused by the heavy-metal effect, to a large extent, would partially remove the spin-forbidden nature of the T1 ? S0 radioactive relaxation, leading to a fast radio- active decay; therefore, quenching and/or annihilation processes can be comparably small, resulting in a highly intense phosphorescent emission [1,2]. Accordingly, a large amount of effort has been made in investigating the second- and third-row transition-metal complexes, aimed at development of the highly efficient phosphors that can emit all three primary colors [3]. Among the complexes studied to date, cyclometalated tris-chelated iridium(III) complexes have emerged as particularly promising candidate materials since the pioneering works were carried out by the group of Thompson and Forrest [4]. Generally, the tris-cyclometalated iridium(III) complexes were isolated by way of three synthetic routes: (i) direct reaction of iridium(III) chloride hydrate with the three-fold amount (or excess) of the ligand in the refluxing condition [5], (ii) treatment of Ir(acac)3 with 3.0–3.5 equiv of the appropriate ligand in the refluxing glycerol [6], and (iii) a novel one-step method in the presence of AgO2CCF3 [7].
⇑ Corresponding author. E-mail address: [email protected] (Q.-F. Zhang). http://dx.doi.org/10.1016/j.ica.2017.06.021 0020-1693/Ó 2017 Published by Elsevier B.V.
As well-known, iridium(III) complexes are not sufficiently compatible to form homogeneous molecular dispersions in polymer films. This poor solubility, in turn, restricts the device efficiency and lifetime [5]. A widely used approach to enhance the solubility of the iridium(III) complexes has been to attach solubilizing alkyl moieties or to introduce dendritic architectures [8,9]. However, the introduction of alkyl groups perturbs charge carrier transport and lowers the glass transition temperature (tg). Hence, sterically hindered aromatic substituents, which give rise to fewer side effects, are considered to be better solubilizing groups [10–12]. Meanwhile, 1,2-diazenes (AN@N-) are a field of increasing importance and there has been much research on this area given probable intermediates of diazene complexes in biological fixation and the nonezymatic conversion of coordinated dinitrogen into diazene derivatives [13,14]. Their high electron affinities of such complexes would make them promising candidates for the electron injection and transportation. With this in mind, we are interested to introduce sterically hindered phenol groups into diazene derivative to get a novel ligand 1-(2,6-dimethylphenyloxy)-4- (4-phenyloxy-phenyl)phthalazine (dmpopopzH). The corresponding facial tris-cyclometalated iridium(III) complex fac-Ir(dmpopopz)3 was synthesized by the reaction of IrCl3nH2O and dmpopopzH in the refluxing ethoxyethanol overnight. The structural characterization and photoluminescence properties along with theoretical calculation of the complex fac-Ir
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(dmpopopz)3 were reported, and the performance of OLED fabricated by solution process was also investigated in this paper. 2. Experimental section 2.1. General considerations All experiments were performed under a dry dinitrogen atmosphere using standard Schlenk techniques. All solvents were freshly distilled over appropriate drying reagents prior to use. All starting materials were purchased from either Aldrich or Alfa and used without further purification. Ligand 1-(2,6-dimethylphenyloxy)-4- (4-phenyloxy-phenyl)-phthalazine (dmpopopzH) was prepared according to the procedure described in the literature [5]. The silica gel column chromatography was performed with use of silica gel (200–300 mesh). 2.2. Measurements 1
H and 13C NMR spectra were recorded on a Bruker ALX 300 spectrometer operating at 400 MHz (for 1H) and 100 MHz (for 13 C) with tetramethylsilane (TMS) as the internal standard. Infrared spectrum was recorded on a Perkin-Elmer 16 PC FT-IR spectrophotometer with use of pressed KBr pellets. Electronic absorption spectrum was measured in dichloromethane solution on a Shimadzu UV-3000 spectrophotometer. Luminescence properties were measured and recorded using an FLS-920 fluorescence spectrometer. Positive-ion ESI mass spectrum was recorded on a Perkin-Elmer Sciex API 365 mass spectrometer. Photoluminescence (PL) spectra were measured with a Shimadzu RF-5301PC fluorescence spectrophotometer. Luminescence lifetime was determined on an Edinburgh FL920 time-correlated pulsed single-photon-counting instrument. Melting points were measured using capillary melting point apparatus. Elemental analyses were carried out using a Perkin-Elmer 2400 CHN analyzer. 2.3. Synthesis of ligand dmpopopzH (1) 1-Chloro-4-(4-phenoxy)-phthalazine (6 g, 18 mmol), 2,6dimethylphenol (2.2 g, 18 mmol) and K2CO3 (5 g, 36 mmol) were dissolved in DMF (50 mL), and the mixture was heated at 120 °C for 4 h under nitrogen atmosphere. After cooling, the mixture was poured into distilled water and the white solid was precipitated. The crude product was purified by chromatography on a neutral alumina column. Yield: 7.2 g (96%). M.p.: 245–246 °C. 1H NMR (CDCl3, 400 MHz) d: 2.21 (s, 6H, ACH3), 7.10–7.16 (m, 8H, Ar), 7.39 (d, 3JHH = 8.0 Hz, 2H, ACAC6H4AO), 7.73 (d, 3JHH = 8.4 Hz, 2H, ACAC6H4AO), 7.94 (d, 3JHH = 7.2 Hz, 1H, -phthalazine), 7.97 (d, 3JHH = 8.0 Hz, 1H, -phthalazine), 8.14 (d, 3JHH = 7.2 Hz, 1H, -phthalazine), 8.57 (d, 3JHH = 7.6 Hz, 1H, -phthalazine). 2.4. Synthesis of complex [Ir(dmpopopz)3]¼C6H14 (1) IrCl3xH2O (200 mg, 0.56 mmol) and ligand mpopoppzH (0.75 g, 1.8 mmol) were dissolved in ethoxyethanol (15 mL), and the mixture was refluxed at 110 °C overnight under nitrogen atmosphere with exclusion of light. After cooling, the precipitate was filtered off and washed with anhydrous ethanol. The residue was purified by chromatography on a silica gel column using hexane/dichloromethane (v:v = 6:1) as an eluent to yield the facial tris-cyclometalated iridium(III) complex fac-Ir(mpopoppz)3¼C6H14 (1) as a bright red powder. Single crystals of complex 1 were grown from the mixed dichloromethane/n-hexane (1:3) solution by slow evaporation at room temperature. Yield: 250 mg (31%). MS ((+)-ESI): m/ z = 1444.65 (calcd. 1467.64 for C84H63N6O6IrNa, [M + Na+]). 1H
NMR (CDCl3, 400 MHz): d 2.18 (s, 18H, ACH3), 5.90–5.95 (m, 3H, -Ar), 6.40 (d, 3JHH = 12 Hz, 6H, AAr), 6.52 (s, 3H, AIrAC6H3), 6.65 (d, 3JHH = 10 Hz, 6H, -Ar), 6.81–7.00 (m, 12H, -Ar), 7.82–8.03 (m, 9H, -phthalazine), 8.34 (d, 3JHH = 6.4 Hz, 3H, AAr), 8.85 (d, 3 JHH = 11.3 Hz, 3H, -phthalazine). 13C NMR (100 MHz, CDCl3) d 159.39, 158.89, 157.06, 156.98, 150.92, 132.92, 132.45, 132.02, 131.49, 131.08, 130.36, 129.21, 128.60, 126.82, 126.02, 124.27, 123.80, 120.20, 119.96, 118.78, 17.07. FT-IR (KBr disc, cm 1): m (PhCAH) 3044(w), m(ACH3) 2924(m), 2849(w), m(PhC@C) 1633 (w), 1572(s), 1538(m), 1489(m), m(CAO) 1385(s). Anal. Calc. for C84H63N6O6Ir- ¼(C6H14): C, 70.04; H, 4.57; N, 5.73%. Found: C, 69.89; H, 4.54; N, 5.71%. 2.5. OLED fabrication and measurement The indium tin oxide (ITO) glass substrate was washed in turn with a substrate-cleaning detergent, deionized water, acetone, and ethanol for 15 min, under ultrasonic condition, and finally treated with ozone for about 20 min. Poly(3,4-ethlenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT) (Baytron P4083, Bayer AG) in water was spin-coated at a rate of 3000 rpm on the ITO substrate and dried by baking in air at 120 °C for 10 h. Then the solution of poly(vinylcarbazole) (PVK, Mw = 81,800) (68 wt%) blended with 2-tert-butylphenyl- 5-biphenyl-1,3,4-oxadiazole (PBD) (30 wt%) and fac-Ir(dmpopopz)3 (2 wt%) in chloroform was spin-coated at a rate of 1300 rpm on top of the PEDOT layer. Finally, barium and aluminum as the cathode layers were evaporated. Current density- voltage-luminance data were collected using a Keithley 236 source measurement unit and a calibrated silicon photodiode. External quantum efficiency was obtained by measuring the total light output in all directions in an integrating sphere (IS080, Labsphere). Luminance and luminous efficiency were measured by a silicon photodiode and calibrated using a PR-705 photometer (Photo Research). 2.6. Theoretical calculation The ground state molecular geometry full optimization of complex fac-Ir(mpopoppz)3 was carried out with the density functional theory (DFT) at the Becke’s 3LYP (B3LYP) level [15]. Frequency calculations at the same level of theory have also been performed to identify the stationary point as minima (zero imaginary frequency). On the basis of the optimized geometries, the natural bond orbital (NBO) is employed to analyze the molecular orbital compositions (%). The lowest 10 singlet-singlet and singlet-triplet excitations have been computed by means of TDDFT. The effective core potentials (ECPs) of Hay and Wadt with a double f-valence basis set (LANL2DZ) were used for the iridium atom [16], the 6311g⁄ basis set for the atoms bonded to metal iridium atom directly, the 6-31g for the other atoms [17]. The basis set for r iridium atom is a modified LANL2DZ plus an f-type polarization function and the two 4p functions of the standard LANL2DZ are replaced by the optimized 4p functions from Couty and Hall. Starting geometry was taken from X-ray crystal structure. All the calculations were performed with the Gaussian-03 software package [18]. 2.7. X-ray crystallography analysis Intensity data of complex fac-Ir(mpopoppz)3¼C6H14 (1) were collected on a Bruker SMART APEX 2000 CCD diffractometer using graphite-monochromated Mo-Ka radiation (k = 0.71073 Å) at 296 (2) K. The collected frames were processed with the software SAINT. The data was corrected for absorption using the program SADABS. Structures were solved by the direct methods and refined by full-matrix least-squares on F2 using the SHELXTL software
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package [19,20]. All non-hydrogen atoms were refined anisotropically. The positions of all hydrogen atoms were generated geometrically (Csp3AH = 0.96 and Csp2AH = 0.93 Å), assigned isotropic thermal parameters, and allowed to ride on their respective parent carbon or nitrogen atoms before the final cycle of least-squares refinement. The hexane molecules in fac-Ir(mpopoppz)3¼C6H14 were isotropically refined without hydrogen atoms due to obvious disorders of carbon atoms in solvents. Crystallographic data and experimental details for complex fac-Ir(mpopoppz)3¼C6H14 are summarized in Table 1. The refined atomic coordinates and anisotropic thermal parameters are deposited in the Supporting Information. Crystallographic data for the structure reported here have been deposited with the Cambridge Crystallographic Data Center (Deposition No. CCDC-1515243). The data can be obtained free of charge via http://www.ccdc.cam.ac.uk./perl/catreq/catreq. cgi (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax,+44 1223 336033; e-mail, [email protected]).
3. Results and discussion 3.1. Synthesis and characterization The synthetic route of the facial tris-cyclometalated iridium(III) complex fac-Ir(dmpopopz)3 is shown in Scheme 1. The ligand dmpopopzH was easily prepared via a substitution reaction of 1chloro-4-(4-phenoxy)phthalazine and 2,6- dimethylphenol in an equivalent ratio in the presence of potassium carbonate as an inorganic base in N, N’-dimethylformamide solution [5]. The complex fac-Ir- (dmpopopz)3 was obtained by a relatively simple procedure. Direct reaction of IrCl3nH2O and dmpopopzH in ethoxyethanol at reflux overnight resulted in isolation of the final tris-cyclometalated iridium(III) product in a yield of 31%. Complex fac-Ir (dmpopopz)3 is quite stable under air and was well characterized by proton and carbon NMR spectroscopies, positive-ion ESI mass spectroscopy, infrared spectroscopy, and microanalyses. In the 1H NMR spectrum, the complex exhibits eight well-resolved peaks
Table 1 Crystallographic data and experimental details for fac-Ir(dmpopopz)3¼C6H14 1. Ir(dmpopopz)3¼C6H14 C85.5H66.5N6O6Ir 1466.15 Monoclinic 45.2739(5) 13.6920(1) 29.7472(4) 130.256(1) 14072.7(3) C2/c 8 1.384 296(2) 5972 1.958 65950 16072 886 0.0400 0.0355, 0.0825 0.0607, 0.0922 1.028
Complex Empirical formula Formula weight Crystal system a (Å) b (Å) c (Å) b (°) V (Å3) Space group Z Dcalc (g cm 3) Temperature (K) F(000) l(Mo-Ka) (mm 1) Total refln Independent refln No. of parameters Rint R1a, wR2b (I > 2(I)) R1, wR2 (all data) GoFc a b c
P P R1 = Fo Fc/ Fo. P P 1/2 wR2 = [ w(F2o F2c )2/ wF22 . o ] P 2 GoF = [ w(Fo Fc) /(Nobs Nparam)]1/2.
OH
O
Fig. 1. Perspective view of complex fac-Ir(mpopoppz)3. Thermal ellipsoids are shown at the 40% probability level. Selected bond lengths (Å) and angles (deg): Ir (1)-C(71) 2.008(3), Ir(1)-C(111) 2.013(3), Ir(1)-C(31) 2.017(3), Ir(1)-N(3) 2.087(3), Ir (1)-N(5) 2.095(3), Ir(1)-N(1) 2.106(3); C(71)-Ir(1)-C(111) 93.29(13), C(71)-Ir(1)-C (31) 99.82(13), C(111)-Ir(1)-C(31) 97.65(13), C(71)-Ir(1)-N(3) 78.10(12), C(111)-Ir (1)-N(3) 87.14(12), C(31)-Ir(1)-N(3) 174.91(11), C(71)-Ir(1)-N(5) 170.37(12), C (111)-Ir(1)-N(5) 77.58(12), C(31)-Ir(1)-N(5) 84.58(11), C(71)-Ir(1)-N(1) 92.71(11), C (111)-Ir(1)-N(1) 173.28(11), C(31)-Ir(1)-N(1) 78.32(12), N(3)-Ir(1)-N(5) 98.19(10), N(3)-Ir(1)-N(1) 97.07(11), N(5)-Ir(1)-N(1) 96.58(11).
O
O IrCl3 xH2O
Ir N N Cl
K2CO3, DMF
N N O
N N 3
ethoxyethanol/H2O O
Scheme 1. The synthetic route of the facial tris-cyclometalated iridium(III) complex fac-Ir(mpopoppz)3.
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in the region of 5.9–8.9 ppm due to the phenyl rings and multiple peaks in region of 1.3–2.1 ppm due to the methyl groups. The methyl groups of dmpopopz ligand in complex fac-Ir(dmpopopz)3 showed expected signal in the high field of the 13C NMR spectrum (17.07 ppm). The IR spectrum of complex fac-Ir(dmpopopz)3 clearly shows the strong band at 1385 cm 1, which may be attributed to the m(CAO) absorption. The weak band at 3044 cm 1 in the IR spectrum of complex 1 may be tentatively assigned to the m (PhCAH) absorption. The structure of complex fac-Ir(dmpopopz)3 was unambiguously established by single-crystal X-ray diffraction analysis. Relevant crystallographic data and details of the data collection are listed in Table 1, and the molecular structure of complex fac-Ir(dmpopopz)3 along with selected bond lengths and bond angles is shown in Fig. 1. The facial geometry around the iridium(III) center in complex fac-Ir(dmpopopz)3 is distorted octahedral with three nitrogen atoms and three carbon atoms, which is indicated by the average CAIrAC and NAIrAN bond angles being 96.92(12)° and 97.28(11)°, respectively, along with the CAIrAN bond angles ranging from 77.58(12)° to 174.91(11)°. The average Ir C (av. 2.013(3) Å) and IrAN (av. 2.096(3) Å) distances are normal by the comparison with other related tris-cyclometalated iridium(III) complexes [5,21].
3.2. Ground-state DFT calculations Computational investigations have been proved to be very helpful in understanding the HOMO and LUMO properties of the cyclometalated iridium(III) complexes [22,23]. Because the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the host are one of the most important factors for OLEDs, herein we investigated the HOMO and LUMO properties of complex fac-Ir(dmpopopz)3 by density functional theory (DFT) calculations [24]. Fig. 2b displays the LUMO and HOMO orbitals for complex fac-Ir(dmpopopz)3,
and the experimental data of the HOMO-LUMO energy gaps are given in Table 2, which are quite similar to the according data of Ir(mpcppz)3 (mpcppzH = 1-(2,6-dimethyl-phenoxy)-4-(4-chlorophenyl)phthalazine) [5]. From Fig. 2, we can obviously observe that the HOMO and LUMO localized on the iridium(III) atom (mainly on d orbitals) and the phenyl- phthalazine rings, respectively. This is consistent with the property of the excited state of complex facIr(dmpopopz)3 because the luminescence mechanism is the metal to ligand charge transfer (d of the iridium to p⁄ of the phenylphthalazine).
3.3. Absorption and photophysical properties The UV–Vis spectrum of complex fac-Ir(dmpopopz)3 in dichloromethane solution, as illustrated in Fig. 3, exhibits a strong absorption band from 200 to 300 nm, indicating that the electronic transition is mostly ligand-centered (LC) p-p⁄. Intraligand (IL) pp⁄ transition and metal-to-ligand charge transfer (1MLCT, dp(Ir) p(ligand)) transition are observed in the region of 350–460 nm. In addition, a weaker absorption shoulder extending into the visible region, which can be assigned to the formally spin-forbidden transition from the triplet metal-to-ligand charge transfer (3MLCT), are also observed in the UV–Vis spectrum of complex fac-Ir (dmpopopz)3. The photoluminescence (PL) spectroscopy is displayed in Fig. 4. Obviously, complex fac-Ir(dmpopopz)3 in dichloromethane solution emits an intense orange luminescence with emission wavelength at 602 nm. In comparison with emission wavelength at 590 nm for complex Ir(mpcppz)3, it is probably due to the substitution of chloride by a phenoxy group, leading to a little red shift of the emission wavelength. The PL decay curve of complex fac-Ir (dmpopopz)3 is shown in Fig. 5 and the lifetime of complex fac-Ir (dmpopopz)3 is ca. 268 ns, which is obviously shorter than that of complex Ir(mpcppz)3 (2 ls). Because long lifetime will give rise Table 2 Frontier orbital energy and HOMO-LUMO energy gap (eV). Orbital
fac-Ir(dmpopopz)3
ELUMO+1 ELUMO EHOMO EHOMO-1 DEL-H
1.36 1.47 4.76 4.95 3.29
Absorbance
0.6
0.4
0.2
0.0 300
400
500
600
Wavelength(nm) Fig. 2. (a) Simplified structure of complex fac-Ir(dmpopopz)3, hydrogen atoms linked to oxygen atoms represent phenyl rings; (b) Schematic representation of the frontier orbitals of complex fac-Ir(dmpopopz)3.
Fig. 3. UV–Vis absorption dichloromethane.
spectrum
of
complex
fac-Ir(dmpopopz)3
in
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3.4. Electroluminescent properties
Normalized Intensity
200
To further investigate electroluminescent (EL) properties of the complex fac-Ir(dmpopopz)3, the OLEDs using the facial triscyclometalated iridium(III) complex fac-Ir(dmpopopz)3 as a dopant was fabricated by solution process with the following structure: ITO/PEDOT(40 nm)/2 wt% Ir(dmpopopz)3 + 70 wt% PVK + 28 wt% PBD (80 nm)/Ba(4 nm)/Al(120 nm). PEDOT was used as hole-injection material. PVK and PBD were selected as the host material and the electron transport material, respectively. The general structure of the device and the molecular structures of the compounds used in the device are illustrated in Fig. 6. The device has a maximum emission at 616 nm (see Fig. 7), and the fine vibronic structure of the EL spectra leads us to conclude that EL emission originates from a mixed 3MLCT/3p-p⁄ state [25]. The Commission International de L’Eclairage (CIE) coordinates of 0.643 and 0.353 are close to the standard red color coordinates (0.67 and 0.33) of the national television system committee, at a driving voltage of 3.9 V, which is obviously better than that of the previously reported complex Ir(mpcppz)3 [5]. No emission from the PVK-PBD blend is detected, which implies that the energy transfer from the blend to the facial tris-cyclometalated iridium(III) complex fac-[Ir(dmpopopz)3] is efficient under electrical excitation. As shown in Fig. 8, the device gives a maximum luminance of 1330 cd/m2 at a current density of 83.9 mA/cm2. A maximum external quantum efficiency of 1.48% corresponding to a luminous efficiency of 0.81 cd/A may be obtained. Even if the external quantum efficiency is 1.48% only, the maximum brightness is still up to 1030 cd/m2, an indicative of the material itself being an efficient
150
100
50
0 550
600
650
700
750
Wave length (nm) Fig. 4. Photoluminescence dichloromethane.
spectrum
of
complex
fac-Ir(dmpopopz)3
in
0.8
0.6
0.4
1.0
Normalisec PL Intensity
PL Intensity (normalised)
1.0
0.2
0.0 1000
2000
3000
4000
5000
Time (ns) Fig. 5. PL decay curve of complex fac-Ir(dmpopopz)3 in dichloromethane.
to the emission saturation and then decrease the efficiency, suitable lifetimes of the cyclometalated iridium(III) complexes (0.1– 14 ls) make them the ideal candidates for the OLEDs application. Compared with the similar complexes previously reported in the literature [5], complex fac-Ir(dmpopopz)3 has a relatively shorter lifetime.
0.8
0.6
0.4
0.2
0.0 500
600
700
Wavelenth (nm) Fig. 7. EL spectrum of complex fac-Ir(dmpopopz)3 in dichloromethane.
S Al Ba(4 nm)
O
n
ITO Glass substrate
N
O SO 3-
PVK:PBD:Ir complex(80 nm) PEDOT:PSS(40 nm)
m
PEDOT:PSS
PVK N N O PBD
Fig. 6. General structures for the devices and molecular structures of the relevant compounds.
n
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X.-M. Wang et al. / Inorganica Chimica Acta 466 (2017) 343–348
Appendix A. Supplementary data
2
10
0
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2017.06.021.
2
Current Density(mA/cm )
1500 10
1000
References
L(cd/m )
-2
10
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500
-6
10
-8
10
0 0
2
4
6
8
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Voltage(V) Fig. 8. Current density–voltage–luminance curves of the device with complex fac-Ir (dmpopopz)3.
phosphor. Therefore, it may be expected to improve the performance of the device by further optimizing the device structure. 4. Conclusions In summary, a tris-cyclometalated iridium(III) complex fac-Ir (dmpopopz)3 with phenylphthalazine ligands was synthesized and characterized by single-crystal X-ray diffraction along with a series of spectroscopic properties. DFT calculation results show the HOMO and LUMO localized on the iridium(III) atom (mainly on d orbital) and the phenylphthalazine rings, respectively. Complex fac-Ir(dmpopopz)3 may emit intense saturated red light in solid state. A red OLED with a solution processed emissive layer containing complex fac-Ir(dmpopopz)3 is presented in this paper. The device exhibits a maximum luminance of 1330 cd/m2. This red emitter may be potentially used in white OLEDs via a cost effective solution-process. Acknowledgements This project was supported by the National Natural Science Foundation of China (21372007).
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