Phosphorescent organic light-emitting devices (PhOLEDs) based on heteroleptic bis-cyclometalated complexes using acetylacetonate as the ancillary ligand

Synthetic Metals 198 (2014) 131–136

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Phosphorescent organic light-emitting devices (PhOLEDs) based on heteroleptic bis-cyclometalated complexes using acetylacetonate as the ancillary ligand Frédéric Dumur a,∗ , Marc Lepeltier b , Hossein Zamani Siboni c,∗∗ , Didier Gigmes a , Hany Aziz c a

Aix-Marseille Université, CNRS, ICR, UMR 7273, F-13397 Marseille, France Institut Lavoisier de Versailles, UMR 8180 CNRS, Université de Versailles Saint-Quentin en Yvelines, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France c Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue West Waterloo, Ontario, Canada N2L 3G1 b

a r t i c l e

i n f o

Article history: Received 26 April 2014 Received in revised form 9 September 2014 Accepted 22 September 2014 Available online 18 October 2014 Keywords: Iridium complex Acetylacetonate OLED Phosphorescence Electroluminescence

a b s t r a c t A series of four heteroleptic iridium (III) complexes comprising acetylacetonate as the ancillary ligand is synthesized and tested as phosphorescent dopants in OLEDs. It was found that the substitution pattern of the cyclometalated ligand strongly influences the device performance. Notably, reduced device performances were observed for the fluorinated complexes as a result of poor exciton confinement in the guest molecules. © 2014 Elsevier B.V. All rights reserved.

1. Introduction During the past decade, intense research efforts have been devoted to develop highly emissive dopants for organic lightemitting devices (OLEDs). In this aim, phosphorescent emitters have attracted the attention of researchers as these emitters grant superior advantages over fluorescent materials. Notably and as the most appealing feature of these emitters, both triplet and singlet excitons can be both advantageously harvested for light emission. Phosphorescent materials are generally based on heavy metal complexes and the most efficient complexes are undoubtedly those based on iridium (III) [1]. Homoleptic and heteroleptic complexes have been both successfully utilized in OLEDs [2–10]. Over the years, a wide range of cyclometalated ligands have also been used to design these different iridium-based emitters. However, although photochemical properties of numerous complexes have been examined and reported, some of them have never been

used or tested in devices. For example, the heteroleptic complex Ir(tpy)2 (acac) C1 where tpy stands for 2-(p-tolyl)pyridine (L1) is one of those. Although Li et al. [11] reported this complex and its photophysical properties in 2004, to the best of our knowledge, this complex has never been tested in OLEDs. Ir(dfppy)2 (acac) C2 (with dfppy = 2-(2,4-difluorophenyl)pyridine) L2 also named Firacac is a second example of complex studied from a theoretical standpoint in at least four publications, yet has never been tested in devices [12–15]. Similarly, Ir(fppy)2 (acac) C3 (with fppy = 2-(4difluorophenyl)pyridine) L3 with only one fluorine atom per ligand is yet another example [16]. Therefore, these three complexes have been studied in this report for the first time as emitters in OLEDs. To evaluate the electroluminescence properties of the three emitters, a well-known dopant, namely Ir(ppy)2 (acac) C4 [17,18], has been studied for comparison. The chemical structures of the four emitters (i.e. C1–C4) are presented in Fig. 1.

2. Experimental ∗ Corresponding author at: Aix-Marseille Université, CNRS, ICR, UMR 7273, F-13397 Marseille, France. Tel.: +33 04 91 28 27 48; fax: +33 04 91 28 87 58. ∗∗ Corresponding author. Tel.: +1 519 888 4567x32872. E-mail addresses: [email protected] (F. Dumur), [email protected] (H.Z. Siboni). http://dx.doi.org/10.1016/j.synthmet.2014.09.026 0379-6779/© 2014 Elsevier B.V. All rights reserved.

2.1. General informations 1 H and 13 C NMR spectra were determined at room temperature in 5 mm o.d. tubes on a Bruker Avance 300 spectrometer

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Fig. 1. Structures of the four heteroleptic bis-cyclometalated complexes C1–C4 investigated in this study.

equipped with a QNP probe head: 1 H (300 MHz) and 13 C (75 MHz). The 1 H chemical shifts were referenced to the solvent peak: CDCl3 (7.26 ppm), and the 13 C chemical shifts were referenced to the solvent peak: CDCl3 (77.0 ppm). All starting materials and solvents were purchased from Aldrich or Lumtec and used as supplied commercially. 2-(4-Fluorophenyl)pyridine L3 [19] and 2-(4-methylphenyl) pyridine L4 [20] were prepared according to procedures previously reported in the literature. Cyclometalated iridium dimers were synthesized under inert atmosphere according to the Nonoyama route by refluxing IrCl3 ·3H2 O with 2–2.5 equiv. of cyclometalating ligand in a 3:1 mixture of 2-ethoxyethanol and water [21]. Ir(tpy)2 (acac) C1 [22] and Ir(fppy)2 (acac) C3 [23] were synthesized according to procedures previously reported in the literature, without modification and in similar yields. 2.1.1. Synthesis of the dimer precursors Ir2 (L)4 Cl2 D1–D4 To a suspension of LH (3.5 mmol) in 2-ethoxyethanol/water (75:25, 40 mL) was added IrCl3 ·3H2 O (0.344 g, 1.0 mmol). The reaction mixture was stirred at reflux for 24 h. Then, water (50 mL) was added and the product was filtered, washed, with ethanol and diethyl ether. The product was then isolated as a powder. [(tpy)2 Ir(-Cl)]2 D1. Yellow powder, 89%. NMR characterizations were consistent with those previously reported [24]. 1 H NMR (300 MHz, CDCl3 , ppm): 9.18 (d, 3 J = 5.1 Hz, 4H), 7.82 (d, 3 J = 8.1 Hz, 4H), 7.71 (t, 3 J = 8.1 Hz, 4H), 7.38 (d, 3 J = 8.1 Hz, 4H), 6.73 (t, 3 J = 6.3 Hz, 4H), 6.57 (d, 3 J = 7.5 Hz, 4H), 5.76 (s, 4H), 1.94 (s, 12H). [(dfppy)2 Ir(-Cl)]2 D2. Yellow powder, 93%. NMR characterizations were consistent with those previously reported [25]. 1 H NMR (300 MHz, CDCl3 , ppm): 9.15 (d, 3 J = 6.0 Hz, 4 J = 1.5 Hz, 4H), 8.32 (d, 3 J = 8.4 Hz, 4H), 7.85 (dt, 3 J = 7.8 Hz, 4 J = 0.9 Hz, 4H), 6.84 (m, 4H), 6.35 (m, 4H), 5.30 (d, 3 J = 9.0 Hz, 4 J = 2.1 Hz, 4H). [(fppy)2 Ir(-Cl)]2 D3. Yellow powder, 97%. NMR characterizations were consistent with those previously reported [26]. 1 H NMR (300 MHz, CDCl3 , ppm): 9.14 (d, 3 J = 5.7 Hz, 4H), 7.80 (m, 8H), 7.52 (dd, 3 J = 8.4 Hz, 3 J = 5.7 Hz, 4H), 6.81 (m, 4H), 6.53 (dt, 3 J = 8.4 Hz, 4 J = 2.4 Hz, 4H), 5.54 (dd, 3 J = 9.9 Hz, 4 J = 2.4 Hz, 4H). [(ppy)2 Ir(-Cl)]2 D4. Yellow powder, 99%. NMR characterizations were consistent with those previously reported [27,28]. 1 H NMR (300 MHz, CDCl , ppm): 9.25 (d, 3 J = 5.4 Hz, 4H), 7.88 3 (d, 3 J = 8.1 Hz, 4H), 7.75 (dt, 3 J = 7.2 Hz, 4 J = 1.5 Hz, 4H), 7.50 (dd, 3 J = 7.8 Hz, 4 J = 1.2 Hz, 4H), 6.77 (m, 8H), 6.57 (dt, 3 J = 7.8 Hz, 4 J = 0.9 Hz, 4H), 5.94 (d, 3 J = 7.5 Hz, 4H). 2.1.2. Synthesis of Ir(L)2 (acac) C1–C4 To a suspension of the dimer [(L)2 Ir(-Cl)]2 (0.2 mmol) in 2ethoxyethanol (30 mL) was added acetylacetone (60 mg, 0.6 mmol) and sodium carbonate (0.212 g, 2.0 mmol). The reaction mixture was stirred at reflux for 15 h. Then, water (50 mL) was added and the product was filtered and purified by chromatography on silica gel to be isolated as a powder. Ir(tpy)2 (acac) C1. Yellow powder (153 mg, 61%). NMR characterizations were consistent with those previously reported [29]. 1 H NMR (300 MHz, CDCl , ppm): 8.49 (d, 3 J = 5.4 Hz, 2H), 7.81 (d, 3

3 J = 7.8 Hz,

2H), 7.71 (t, 3 J = 7.5 Hz, 2H), 7.45 (d, 3 J = 8.1 Hz, 2H), 7.10 (m, 2H), 6.64 (d, 3 J = 7.8 Hz, 2H), 6.08 (s, 2H), 5.21 (s, 1H), 2.06 (s, 6H), 1.79 (s, 6H). HRMS (ESI) m/z: [M]+ . calcd for C29 H27 IrN2 O2 628.1702; found 628.1704. Ir(dfppy)2 (acac) C2. Yellow powder (135 mg, 51%). NMR characterizations were consistent with those previously reported [30]. 1 H NMR (300 MHz, CDCl , ppm): 8.45 (d, 3 J = 5.4 Hz, 2H), 8.26 (d, 3 3 J = 8.4 Hz, 2H), 7.81 (dt, 3 J = 7.8 Hz, 4 J = 1.2 Hz, 2H), 7.20 (m, 2H), 6.34 (m, 2H), 5.67 (dd, 3 J = 9.0 Hz, 4 J = 2.4 Hz, 2H), 5.27 (s, 1H), 1.83 (s, 6H). HRMS (ESI) m/z: [M]+ . calcd for C27 H19 F4 IrN2 O2 672.10; found 672.1046. Ir(fppy)2 (acac) C3. Yellow powder (173 mg, 72%). Yellow powder (135 mg, 51%). NMR characterizations were consistent with those previously reported [16]. 1 H NMR (300 MHz, CDCl3 , ppm): 8.45 (d, 3 J = 6.0 Hz, 2H), 7.78 (m, 4H), 7.55 (dd, 3 J = 6.0 Hz, 3 J = 8.7 Hz, 2H), 7.17 (m, 2H), 6.55 (dt, 3 J = 9.0 Hz, 4 J = 2.4 Hz, 2H), 5.87 (dd, 3 J = 9.6 Hz, 4 J = 2.4 Hz, 2H), 5.24 (s, 1H), 1.81 (s, 6H). HRMS (ESI) m/z: [M]+ . calcd for C27 H21 F4 IrN2 O2 636.1200; found 636.1207. Ir(ppy)2 (acac) C4. Yellow powder (154 mg, 64%). NMR characterizations were consistent with those previously reported [31,32]. 1 H NMR (300 MHz, CDCl , ppm): 8.52 (d, 3 J = 5.8 Hz, 2H), 7.86 3 (d, 3 J = 8.0 Hz, 2H), 7.74 (dt, 3 J = 7.4 Hz, 4 J = 1.4 Hz, 2H), 7.56 (dd, 3 J = 7.6 Hz, 4 J = 1.0 Hz, 2H), 7.15 (dt, 3 J = 5.8 Hz, 4 J = 1.4 Hz, 2H), 6.82 (dt, 3 J = 7.4 Hz, 4 J = 1.0 Hz, 2H), 6.70 (dt, 3 J = 7.4 Hz, 4 J = 1.8 Hz, 2H), 6.28 (dd, 3 J = 7.4 Hz, 4 J = 1.2 Hz, 4H), 5.23 (s, 1H), 1.80 (s, 6H). HRMS (ESI) m/z: [M]+ . calcd for C27 H23 IrN2 O2 600.14; found 600.1412.

2.2. OLEDs fabrication and measurements OLEDs were fabricated onto cleaned indium tin oxide (ITO) glass substrates with sheet resistance of 10–12 /sq. Prior to organic layer deposition, the ITO substrates were successively washed with acetone and isopropanol in an ultrasonic bath for 10 min. Organic layers were then sequentially deposited onto the ITO substrate at a ˚ and a base pressure below 5 × 10−6 Torr. Aluminum rate of 2–4 A/s cathode was formed with a shadow mask by thermal evaporation. Devices have the following general structure (see Fig. 3): ITO was used as a transparent anode electrode, MoO3 (5 nm) as a hole injecting material/CBP (4,4 -bis(N-carbazolyl)-1,1 -biphenyl) (35 nm) as a hole transporting layer/CBP doped with the iridium complexes as the emission layer (EML) (10 nm), TPBI (1,3,5tris(N-phenylbenzimidizol-2-yl)benzene) (40 nm) as an electron transporting material, LiF (0.6 nm) as an electron injecting material, and Al (80 nm) as a cathode electrode material. Doped layers were made by co-evaporation from carefully temperature controlled organic sources. All materials except the emitters used in the devices were purchased from Lumtec with the best purity available and used as received. Current density–Voltage–Luminance (J–V–L) measurements were carried out using Agilent 4156 C and Minolta Chroma Meter CS-100. Devices were stored and characterized under nitrogen without encapsulation. Fig. 1 shows the schematic structure of the devices.

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Scheme 1. Synthetic route to Ir(L)2 (acac) C1–C4.

3. Results and discussion 3.1. Synthesis of the heteroleptic complexes Iridium trichloride reacts with the 2-phenylpyridine derivatives L1–L4 to give the precursor dimeric complexes D1–D4 in a mixture 2-ethoxyethanol/water (75:25) in almost quantitative yields (from 89% to 99%). The heterocyclic rings of the ligands are in a trans-configuration and those complexes are isolated as a racemic mixture of / isomers, confirmed by NMR 1 H [33]. The complexes Ir(L)2 (acac) C1–C4 are obtained by reacting the corresponding dimers D1–D4 in 2-ethoxyethanol at 80 ◦ C with acetylacetone (2.5 equivalents) with the presence of Na2 CO3 with relative good yields (from 51% to 72%) after purification by chromatography on silica gel (see Scheme 1). 3.2. Photophysical properties Fig. 2 shows the absorption and the emission spectra of Ir(tpy)2 (acac) C1, Ir(dfppy)2 (acac) C2, Ir(fppy)2 (acac) C3 and Ir(ppy)2 (acac) C4 respectively measured in CH2 Cl2 under ambient conditions. These spectra are characteristic of (CˆN)2 Ir(LX) complexes. Although all of them have the same “Ir(acac)” fragment, a clear blue shift of the lowest energy absorption band is observed

Fig. 2. Absorption and emission spectra of the four heteroleptic complexes C1–C4 in CH2 Cl2 under ambient conditions.

along with a blue shift of the maximum emission wavelengths as the number of fluorine atoms on the cyclometalated ligand is increased. In accordance with the literature, intense bands in the ultraviolet part of the spectra between 250 and 350 nm are

Fig. 3. Chemical structure of the different materials, the layer structure and the orbital diagram of the PhOLEDs investigated in this study.

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Table 1 Photophysical properties of heteroleptic (CˆN)2 Ir(acac) complexes C1–C4. Iridium complex

abs (nm)

em a (nm)

b



T1 (eV)

HOMO (eV)

LUMO (eV)

Ir(tpy)2 (acac) C1 Ir(dfppy)2 (acac) C2 Ir(fppy)2 (acac) C3 Ir(ppy)2 (acac) C4

266, 338, 362, 404, 458 250, 328, 387, 436 257, 328, 393, 442, 475 259, 340, 406, 461, 490

514 481 492 517

3.1 ␮s [20] 1.0 ␮s [23] 1.5 ␮s [16] 1.6 ␮s [25]

0.31 [20] 0.64 [23] 0.40 [16] 0.53 [26]

2.60 2.77 2.71 2.60

−5.62 [20] −5.44 [24] −5.26 [16]c −5.18 [26]

−2.52 [22] −2.86 [24] – −2.77 [26]

a b c

Upon excitation at 350 nm. Measured in degassed CH2 Cl2 (∼10−5 M) using quinine sulfate (0.5 M H2 SO4 ) as a standard ( = 0.55). Extrapolated from the electrochemical characteristics reported in the literature.

because of the spin-allowed 1 (␲–␲*) transitions of the CˆN ligands and the weaker bands at the lower energy (i.e. 350–500 nm) are attributed to singlet and triplet MLCT transitions [34–36,15,37–39]. The photoluminescence (PL) spectra of all iridium (III) complexes in degassed CH2 Cl2 solutions at room temperature showed intense emissions at 450–550 nm under UV-light irradiation at 350 nm. Introduction of fluorine atoms on the cyclometalated ligands had a marked effect on the phosphorescence spectrum resulting a blue shift in the emission. The photophysical properties of the emitters are summarized in Table 1. 3.3. Electroluminescence performance The electroluminescence (EL) characteristics of the devices are summarized in Table 2. It should be noted that the performance of the control device (i.e. C4) in this current work is comparable to the previously reported values. Notably, a maximum power efficiency of 18 lm/W was previously achieved at 1 mA/cm2 [34] for this complex while in this work the device with the C4 emitter showed the power efficiency of 28.3 lm/W. Similarly, the maximum current efficiency of the device at 8 wt% guest concentration is 28.3 cd/A which is slightly higher than the previously reported value [31]. Interestingly, all devices fabricated with C1–C4 showed blue and green emission similar to their PL emission spectra in solution. EL spectra of the four complexes at 8 wt% dopant concentrations are shown in Fig. 4. Interestingly, fluorinated complexes Ir(dfppy)2 (acac) C2 and Ir(fppy)2 (acac) C3 exhibited lower device efficiencies in comparison to the none-fluorinated complexes Ir(tpy)2 (acac) C1 and Ir(ppy)2 (acac) C4 (see Fig. 5). The lower performance in the devices with Ir(dfppy)2 (acac) C2 and Ir(fppy)2 (acac) C3 is possibly due to the triplet energy level mismatch between the host and the guests. Notably, the triplet energy levels of CBP, C2 and C3 are 2.60, 2.77 and 2.71 eV, respectively.

Fig. 4. EL spectra of devices with dopant concentration at 8 wt% for the four emitters C1–C4.

Therefore, the energy transfer from CBP into the guests is inefficient due to the low triplet energy level of CBP [40]. The CBP with lower triplet energy gap also induces back energy transfer from the guest to the host, leading to the confinement of excitons in the host rather than guests [41–43]. Fig. 5 shows EL spectra of the devices with various dopant concentrations. Additional peak in the EL spectra of these emitters (i.e. C2 and C3) at 420 nm wavelength further verifies the incomplete energy transfer from the host to the guests suggesting that CBP is not a suitable host for the fluorinated dopants [44]. On the contrary, a perfect matching between the triplet energy levels of C1 and C4) and the CBP host can be observed (see Table 1), resulting very efficient devices (see Fig. 5). Interestingly, no residual EL emission from the host or adjacent layers can be observed, indicating an efficient energy transfer from

Fig. 5. (a) Luminous efficiency vs. current density at 8 wt%. (b) Power efficiency vs. current density at 8 wt%.

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Table 2 Summary of performances of the fabricated PhOLEDs. Doping conc. of Ir(CˆN)2 (acac) (%)

ligand CˆN

Vturn-on a

CIE (x,y)b

EL (nm)

EM (nm)

L (cd/m2 )

Current eff. (cd/A)g

Power eff. (lm/W)g

Doping conc. of Ir(CˆN)2 (acac) (%)

2 wt% (C1) 4 wt% (C1) 8 wt% (C1) 10 wt% (C1) 2 wt% (C2) 4 wt% (C2) 8 wt% (C2) 10 wt% (C2) 2 wt% (C3) 4 wt% (C3) 8 wt% (C3) 10 wt% (C3) 2 wt% (C4) 4 wt% (C4) 8 wt% (C4) 10 wt% (C4)

tpy tpy tpy tpy dfppy dfppy dfppy dfppy fppy fppy fppy fppy ppy ppy ppy ppy

3.0 2.7 2.7 3.0 4.5 4.2 3.7 3.7 2.7 2.5 2.5 2.4 3.0 2.7 2.5 2.5

0.323, 0.629 0.324, 0.628 0.323, 0.629 0.316, 0.630 0.186, 0.345 0.186, 0.363 0.188, 0.386 0.187, 0.384 0.219, 0.481 0.221, 0.501 0.224, 0.516 0.222, 0.515 0.315, 0.618 0.322, 0.623 0.329, 0.624 0.331, 0.625

520 520 520 520 483 483 483 483 493 493 493 493 520 520 520 520

514 514 514 514 481 481 481 481 492 492 492 492 517 517 517 517

41,650c 48,370c 58,750c 53,330c 7300d 10,050d 16,100d 14,250d 14,870e 25,240e 31,270e 30,590e 39,890f 51,370f 57,610f 54,930f

28.6 33.0 40.2 39.2 5.8 8.6 15.1 13.0 10.8 18.4 24.9 25.0 41.5 43.7 47.2 41.0

21.0 24.2 29.3 28.5 2.5 3.6 6.3 5.3 8.0 13.6 18.4 18.6 24.6 26.2 28.3 24.6

2 wt% (C1) 4 wt% (C1) 8 wt% (C1) 10 wt% (C1) 2 wt% (C2) 4 wt% (C2) 8 wt% (C2) 10 wt% (C2) 2 wt% (C3) 4 wt% (C3) 8 wt% (C3) 10 wt% (C3) 2 wt% (C4) 4 wt% (C4) 8 wt% (C4) 10 wt% (C4)

a b c d e f g

Turn-on voltage at a brightness of 1 cd/m2 . Values measured at 4 mA/cm2 . Values obtained at 12 V. Values obtained at 17.5 V. Values obtained at 13 V. Values obtained at 14 V. Values obtained at 1 mA/cm2 .

Fig. 6. EL spectra of devices with dopant concentration ranging from 2 to 10 wt% with Ir(dfppy)2 (acac) C2 (a) and Ir(fppy)2 (acac) C3 (b) as the dopant.

Fig. 7. EL spectra of devices with dopant concentration ranging from 2 to 10 wt% with Ir(tpy)2 (acac) C1(a) and Ir(ppy)2 (acac) C4 (b) as the dopant.

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CBP to the guests and subsequent exciton confinement in the guests (see Fig. 7). Their EL spectra clearly superimpose their PL spectra, proving that the emission is produced exclusively from the dopant at various dopant concentrations. Current efficiencies as high as 47.2 and 40.2 cd/A and power efficiencies of 28.3 and 29.3 lm/W are obtained at 1 mA/cm2 for Ir(ppy)2 (acac) C4 and Ir(tpy)2 (acac) C1, respectively. All devices show optimum performance at 8 wt% dopant concentration (Figs. 6 and 7). 4. Conclusion

[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

A series of four heteroleptic iridium (III) complexes comprising acetylacetonate as the ancillary ligand is synthesized and tested as phosphorescent dopants in OLEDs. It was found that the substitution pattern of the cyclometalated ligand strongly influences the device performance. Notably, reduced device performances were observed for the fluorinated complexes as a result of poor exciton confinement in the guest molecules.

[25] [26]

[27] [28] [29] [30]

Acknowledgements The authors thank the CNRS, Aix-Marseille University, the Institut Carnot STAR and the Natural Science and Engineering Research Council of Canada (NSERC) for financial supports.

[31] [32] [33] [34]

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