Phosphorescent Cyclometalated Iridium(III) Complexes Based on

Aug 1, 2013 - Synthesis and photophysical properties of cyclometalated heteroleptic iridium(III) complexes containing pyridyl/isoquinolyl-imino-isoind...
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Phosphorescent Cyclometalated Iridium(III) Complexes Based on Amidate Ancillary Ligands: Their Synthesis and Photophysical Properties Wei Yang,†,‡ Dawei Wang,† Qijun Song,† Song Zhang,† Quan Wang,† and Yuqiang Ding*,† †

Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Jiangsu Province, People’s Republic of China ‡ Yiwu Entry-Exit Inspection and Quarantine Bureau, Yiwu 322000, Zhejiang Province, People’s Republic of China S Supporting Information *

ABSTRACT: A series of quinolyl-substituted iridium(III) compounds with an amidate ancillary ligand and a fourmembered-ring structure have synthesized and characterized by X-ray crystallography for the first time. The strategy of varying the amidate ancillary ligand gives photoluminescence in the green and orange-yellow region, while pyridyl-substituted compounds only gave green emission. The HOMO and LUMO energy levels and compositions of quinolyl-substituted iridium(III) compounds were investigated by DFT calculations. This showed that the quinoline and amidate ancillary ligand influence the absorption and emission energies of these complexes by tuning the HOMO energies. In addition, the TG experiment showed quinolyl-substituted iridium(III) compounds have excellent stability. ECL experiments were also explored in this paper.

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as acetylacetonate (acac), functionalized β-diketonate, picolinate, triazolate, and others.5 In 2011, we showed that amides are able to bind to an iridium center via a κ2 mode and thus form a four-membered metallacycle. Consequently, a number of amidate Ir(III) complexes containing phenylpyridyl (ppy) and 2-(4,6difluorophenyl)pyridyl (dfppy) cyclometalating ligands have been synthesized and characterized.6,7 However, these compounds only emitted green photoluminescence. The design and synthesis of better-skeleton Ir complexes which have a photoluminescence color tuning function has always been our research goal. After careful analysis, we think the pyridine ring has a strong electron deficiency, which results in pyridyl Ir complexes having a narrow photoluminescence emission region. As an alternative method, quinoline derivatives may be a better choice (Scheme 1B). Therefore, it is necessary to turn our attention to the synthesis of these phenylquinolyl (pq) Ir(III) complexes with amidate ancillary ligands. Herein, we report a series of quinolyl-substituted iridium(III) compounds that have an amidate ancillary ligand and four-membered-ring structure. Good photoluminescence in the green and orangeyellow region was achieved. In addition, X-ray crystallography and DFT calculations gave reasonable explanations.

INTRODUCTION One aspiration for chemists is to find effective solutions or strategies for challenging problems, especially those for achieving effective catalysts and excellent materials.1 For example, emission color tuning in photoluminescence OLEDs is still needed to improve color purity and to produce various colors. Within this context, the selective synthesis of color tuning materials is still attractive and interesting.2 Our interest in photoluminescence was initiated by studies of cyclometalated Ir(III) complexes, which have attracted considerable attention due to their unique photophysical properties and various applications such as dopants for organic light-emitting diodes (OLEDs), electrogenerated chemiluminescence, luminescent biological labels, and photoelectrochemical solar cells.3,4 Generally, there are two kinds of cyclometalated Ir complexes: neutral complexes such as homoleptic Ir(C∧N)3 and heteroleptic (C∧N)2Ir(LX) complexes (C∧N denotes a cyclometalating ligand, such as 2-phenylpyridine; LX denotes a monoanionic ancillary ligand such as acetylacetonate, picolinate, etc.) and cationic complexes such as [Ir(C∧N)2(N∧N)]+, containing diimine ligands. However, ionic iridium(III) complexes are not suitable for OLED applications due to the difficulty of vapor deposition. Neutral complexes (especially heteroleptic complexes) are of particular interest, because they typically exhibit good optical properties. Recently, a number of contributions in the literature described ancillary ligands, such © 2013 American Chemical Society

Received: April 2, 2013 Published: August 1, 2013 4130

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Scheme 1. Two Totally Different Types of QuinolylSubstituted Iridium(III) Compounds from [(pq)2IrCl]2

Scheme 2. Synthesis of Phenylquinoline Skeleton Amidate Ancillary Ligand Ir(III) Complexes

First, the dichloro-bridged complex [(pq)2IrCl]2 was prepared from the reaction of the pq ligand with IrCl3·3H2O according to a general procedure.8 Subsequently, the complex [(pq)2IrCl]2 was treated with amide in ethoxyethanol in the presence of sodium carbonate; however, no reaction was observed. When we changed the reaction conditions, we found that an unexpected and new dinuclear Ir complex (with a phenylquinolyl bridge structure) was obtained, which was not the desired result (Scheme 1A).9 Finally, we succeeded in synthesizing quinoline skeleton Ir(III) complexes. In this paper, we report the synthesis, characterization, structural studies, photophysical properties, and density functional theory (DFT) studies of quinolyl-substituted iridium(III) compounds with an amidate ancillary ligand and a four-membered-ring structure.

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RESULTS AND DISCUSSION Synthesis of (pq)2Ir(amidate) Complexes 2. The chlorobridged dimer 1,8 amide ligand, and base were placed together in a Schlenk tube containing CH2Cl2 solvent under a nitrogen atmosphere. The mixture was stirred at room temperature for 2 days and gave the (pq)2Ir(amidate) complexes 2 in good yields (Scheme 2). Crystal Structures of 2. To unambiguously elucidate the structures, single-crystal X-ray diffraction analyses were performed for these complexes. Single-crystal analyses reveal that the compound 2b belongs to the P21/c space group. Its crystallographic asymmetry unit contains one Ir complex and one CH2Cl2 solvent molecule. In the molecular structure of compound 2b, the Ir(1) atom is octahedrally coordinated to one N-acetylaniline and two pq ligands all in bidentate chelating fashions (Figure 1). N(3) and O(1) from the Nacetylaniline are coordinated to Ir(1) to form a nearly planar Ir(1)−O(1)−C(38)−N(3) four-membered ring, and the N(3)−Ir(1)−O(1) angle is 57.89(8)°. The Ir(1)−N(3) bond length (2.269(2) Å) is much longer than that of Ir(1)−N(3) (2.146(1) Å) in (ppy)2Ir(LX).6 In comparison to 2b, the complexes 2e,f belong to the space group P21/c and their crystallographic asymmetry units also contain one Ir complex. They have also a slightly distorted octahedral coordination geometry around the iridium center. Overall, there are no large geometric differences among the complexes; the Ir(1)−N(3) bond lengths (2.205(5) Å in 2e and 2.219(4) Å in 2f) are slightly shorter than that in 2b (2.269(2) Å), while the N(3)−Ir(1)−O(2) angles (59.10(16)°

Figure 1. ORTEP diagram of 2b with thermal ellipsoids shown at the 30% probability level. The hydrogen atoms have been omitted for clarity.

in 2e and 58.78(13)° in 2f) are also comparable to that of 2b. It is noteworthy that the substituents of the N-acetylaniline in 2e,f are OCH3 and Cl, respectively, but compound 2e is isostructral with 2f (Figures 2 and 3). Obviously, the variation of the substituents in the N-acetylaniline did not cause a difference in their crystal packing. The lattice solvent molecule dichloromethane in 2b is disordered over two positions with 0.554(10) and 0.446(10) site occupancy factors (SOFs). The CF3 group is orientationally disordered over two positions with 0.913(6) and 0.087(6) SOFs. For 2e, the 4-methoxyphenyl group is also disordered over two positions with 0.854(7) and 0.146(7) SOFs. Thermal Properties. At the same time, TG experiments were investigated, and the thermal curve is shown in Figure 4. For example, the experiment shows that compound 2c is stable up to 233 °C, while compound 2a begins to decompose until 288 °C (5% weight loss), which indicates that most of the 4131

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Figure 2. ORTEP diagram of 2e with thermal ellipsoids shown at the 30% probability level. The hydrogen atoms have been omitted for clarity.

Figure 5. ECL spectra of iridium complex/TPA systems. The luminophores (0.1 mM) and TPA (10 mM) are present in CH3CN solution containing 0.1 M TBAPF6 as the supporting electrolyte.

Ru(bpy)32+, it showed that these amidate iridium complexes have some ECL efficiency and they gave us great inspiration for continuing the ECL efficiency studies. At the same time, the photophysical characterization of (pq)2Ir(amidate) complexes 2 was explored (Figure 6). The maximum absorption wavelengths of the complexes were observed at 269, 268, 269, 270, 272, 270, 268, and 269 nm, respectively.

Figure 3. ORTEP diagram of 2f with thermal ellipsoids shown at the 30% probability level. The hydrogen atoms have been omitted for clarity.

Figure 6. UV−vis absorption and normalized PL emission spectra of complexes 2.

Addtionally, the emissive lifetimes, measured in deaerated solutions, are in the range 0.47−1.36 μs (Table 1), which are in agreement with the values reported for other cationic Ir(III) complexes.12 For phosphorescence quantum yields (ΦPL), these complexes 2 were observed between 0.018 and 0.056 (Table 1), slihgtly lower than the yields for pyridine skeleton Ir(III) complexes.6 However, we were very glad to find that

Figure 4. Thermogravimetric curves of complexes 2.

quinoline skeleton Ir(III) complexes with amidate ancillary ligands have excellent thermal stability (Figure 4). Most of the Ir complexes begin to decompose around 300 °C. It should be noted that compound 2d is only stable under 175 °C, which may be due to the nitro group. Photophysical Properties and ECL Studies. In the past several decades, studies on ECL (electrochemiluminescence) have attracted extensive attention in fundamental aspects and analytical applications.10 In addition to the innovative research on Ru(bpy)32+,11 other metal complexes (e.g., Ir complexes) illustrated high photoluminescence efficiency and stable electrochemical properties. In combination with our studies on amidate Ir(III) complexes, ECL experiments were set up in the presence of TPA. The results showed that the ECL efficiency of a quinolyl-based compound was much better than that of a pyridyl-based compound under the same conditions (Figure 5). Although this efficiency was still lower than that of

Table 1. Photophysical Properties of Complexes 2 at Room Temperature complex 2a 2b 2c 2d 2e 2f 2g 2h 4132

abs wavelength λ (nm) 269, 268, 269, 270, 272, 270, 268, 269,

341, 341, 341, 336, 340, 339, 341, 338,

493, 469, 483, 469, 464, 472, 475, 468,

514 484 509 519 516 508 506 497

soln luminescence λ (nm)

ΦPL

lifetime τ (μs)

576 575 579 575 548 581 581 586

0.032 0.018 0.043 0.020 0.049 0.039 0.025 0.024

1.36 0.93 0.47 1.08 0.62 0.58 1.12 1.27

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line and two pq ligands and the amidate ligand can form a stable four-membered ring with iridium metal. The HOMO energy level, LUMO energy level, and compositions of the four iridium complexes were analyzed, and the results are summarized in Figure 7 and Table 3. The HOMO energy levels vary from −4.73 to −5.17 eV, while the LUMO energy levels are very close, with energies ranging from −1.77 to −2.07 eV.

both green light and orange-yellow light emissions were observed using only different quinolyl substituents with an amidate ancillary ligand. When the substituents are both phenyl groups (R1 = R2 = Ph), the wavelengths of solution photoluminescence (PL) of the complexes are observed around 548 nm, which fall into the green light region. At the same time, it was found that an EWG (electron-withdrawing group) group in the benzene ring (2d) gave wavelengths of photoluminescence emission around 575 nm, which belong to the orange-yellow region. As is known, among organofluorine molecules, trifluoromethylated compounds play a unique and important role in organic chemistry, especially in agricultural and medicinal chemistry. Since the trifluoromethyl group is a strong EWG group and has a unique nature, we decided to explore the effect of a trifluoromethyl functional group on photoluminescence emission in these Ir complexes. Therefore, the (trifluoromethyl)phenyl acrylamide and propanamide were synthesized from the starting materials 4-(trifluoromethyl)benzaldehyde in two or three steps according to the literature (Scheme 3).13 Then, using the same method, the correspond-

Figure 7. HOMO and LUMO of 2c.

The HOMO distributions of these complexes on the metallic d orbital and pq ligand are about 26−49% and 22−47%, respectively. These are quite different from that of the complex with a ppy skeleton, in which the contribution of different substituent ligands is almost the same contribution (nearly 38%).6 Importantly, the contribution of the ancillary ligand in 2e is about 51.4%, while the contribution in 2b is only 3.6%. At the same time, the LUMO distributions are focused on cyclometalated ligands (pq), which is similar to the case for the reported iridium analogues.14

Scheme 3. Chemical Structures of the Two Typical Amide Ligands 4a,b

ing iridium compounds were synthesized. With these trifluoro substituent complexes in hand, their photoluminescence emissions were explored. To our delight, it was found that compound 2h gave the highest emission at around 586 nm. Finally, the photophysical properties of complexes 2 at low temperature was also explored. The results are shown in Table 2. All of the samples showed very intense and highly resolved

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CONCLUSIONS In conclusion, we designed, synthesized, and characterized quinolyl-substituted iridium(III) compounds with an amidate ancillary ligand and four-membered-ring structure. It is found that the photoluminescence color could be obtained in the green and orange-yellow region by varying different quinolyl substituents and ancillary ligands. In addition, these compounds were structurally characterized by X-ray crystallography and confirmed by DFT calculations. Furthermore, TG experiments showed that quinolyl-substituted iridium(III) compounds have excellent stability.

Table 2. Photophysical Properties of Complexes 2 at Low Temperature (−98 °C) complex

λmax (nm)

τ (μs)

2a 2b 2c 2d 2e 2f 2g 2h

594 587 583 581 549 587 582 589

1.96 1.66 0.66 1.78 0.93 0.79 1.65 1.74

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EXPERIMENTAL SECTION Materials. IrCl3·3H2O and other chemicals were obtained from commercial sources and used without further purification. The chloro-bridged dimer (pq)2Ir(μ-Cl)2Ir(pq)2 and amide derivatives were prepared according to the literature. All solvents were dried by standard methods. General Experiments. All of the synthetic procedures involving iridium species, sodium methanolate, and other bases were carried out under a nitrogen atmosphere by using a standard Schlenk tube, the main concern being the oxidative stability of intermediate complexes and moisture-sensitive sodium methanolate and other bases used in the reaction. Typical Procedure for (C∧N)2Ir(amidate) Complexes 2. The chloro-bridged dimer 1g (0.10 mmol), amide ligand 4 (0.33 mmol), and sodium methanolate or potassium carbonate (1.0 mmol) were placed in a Schlenk tube containing 20 mL of CH2Cl2 solvent under a nitrogen atmosphere (Scheme 2). The mixture was stirred at room temperature for 48 h. The precipitate was filtered off, and the solvent was removed in

bands that exhibited small rigidochromism with a bathochromic shift of about 20 nm for most of the samples, while in the case of 2g almost no shift was found. The strong similarity between room-temperature and low-temperature emission bands, as well as the comparable excited-state lifetimes, were slightly different from the values reported for other cationic Ir(III) complexes.12 DFT Calculations. To further understand the electronic structures of these complexes, density functional theory calculations with the B3LYP method were conducted to study the ground state configuration and frontier orbital composition and energy. These calculations revealed that the Ir atom is approximately octahedrally coordinated to N-acetylani4133

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Table 3. Frontier Orbital Compositions (%) of Iridium Complexes 2 from DFT HOMO entry (complex) 1 2 3 4 5 6 7 8

(2a) (2b) (2c) (2d) (2e) (2f) (2g) (2h)

EL (eV)

pq

Ir

ami

pq

Ir

ami

−4.82 −5.06 −4.79 −5.17 −4.67 −4.90 −4.93 −4.90

−1.77 −1.93 −1.77 −2.07 −1.76 −1.85 −1.88 −1.82

37.44 47.49 34.30 43.51 22.17 33.23 44.26 43.44

43.73 48.93 40.07 47.77 26.44 38.54 44.88 43.81

18.83 3.58 25.63 8.72 51.39 28.23 10.86 12.75

94.50 93.27 94.16 88.06 94.11 93.55 94.44 94.38

4.18 4.60 4.14 3.93 1.71 4.48 4.06 1.67

1.32 2.13 1.70 8.01 4.18 1.97 1.50 3.95

vacuo. The residue was washed with deionized water (5 mL × 5), hexane (5 mL × 3), diethyl ether (5 mL × 3), and ethanol (5 mL × 3) successively to afford the crude product, which was further recrystallized in CH2Cl2/hexane and the desired crystal product 2 was obtained. (pq)2Ir(N-phenylacetamide) (2a). Red crystal, yield 76%. IR (cm−1, KBr): 1937 w, 1605 s, 1578 s, 1541 s, 1514 s, 1454 s, 1444 s, 1410 s, 1339 s, 1290 m, 1275 m, 1251 m, 1242 m, 1160 m, 1026 m, 1067 m, 1038 m, 1028 m, 962 m, 826 s, 762 s, 734 s, 695 m, 571 m. 1H NMR (δ, CD2Cl2): 8.90 (d, J = 8.4 Hz, 1H), 8.37 (m, 2H), 8.24 (m, 2H), 8.16 (d, J = 8.8 Hz, 1H), 7.97−7.87 (m, 4H), 7.65−7.58 (m, 3H), 7.33 (m, 1H), 7.06− 6.90 (m, 5H), 6.64−6.48 (m, 3H), 6.31 (d, J = 8.4 Hz, 1H), 6.13 (d, J = 7.2 Hz, 2H), 1.52 (s, 3H); 13C NMR (δ, CD2Cl2): 178.17, 170.20, 169.97, 151.92, 148.88, 148.06, 147.80, 146.62, 145.89, 145.59, 138.46, 138.42, 134.60, 134.07, 130.80, 130.59, 129.23, 129.00, 128.99, 128.38, 128.13, 128.08, 127.90, 127.87, 127.49, 126.37, 126.23, 125.95, 125.91, 123.98, 122.88, 121.34, 120.30, 116.62, 116.34, 21.26. Anal. Found: C, 62.29; H, 3.91; N, 5.59. Calcd: C, 62.11; H, 3.78; N, 5.63.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving detailed experimental procedures and IR, 1H NMR, and 13C NMR data for 2a−h and crystallographic data for 2b,e,f. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.D.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Nos. 20971058, 21175060) for financial support.

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LUMO

EH (eV)

REFERENCES

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Organometallics

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dx.doi.org/10.1021/om400275m | Organometallics 2013, 32, 4130−4135