Iridium(III) - American Chemical Society

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Organometallics 2010, 29, 2882–2891 DOI: 10.1021/om100186k

Iridium(III) Bis-tridentate Complexes with 6-(5-Trifluoromethylpyrazol3-yl)-2,20 -bipyridine Chelating Ligands: Synthesis, Characterization, and Photophysical Properties Jing-Lin Chen,*,†,‡ Yu-Hui Wu,*,‡ Li-Hua He,† He-Rui Wen,† Jinsheng Liao,† and Ruijin Hong† †

School of Materials and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, People’s Republic of China, and ‡State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China Received March 9, 2010

A novel family of iridium(III) bis-tridentate complexes are reported that contain the deprotonated, N∧N∧N-coordinated 6-(5-trifluoromethylpyrazol-3-yl)-2,20 -bipyridine (fpbpyH) ligand. The monocationic homoleptic iridium(III) complex [Ir(fpbpy)2](PF6) (1) was first prepared by treatment of IrCl3 3 3H2O with 2.2 equiv of fpbpyH in degassed ethylene glycol at 196 C and structurally characterized by single-crystal X-ray crystallography. The reaction of Ir(tpy)Cl3 with an equimolar amount of fpbpyH under comparable conditions generated a dicationic heteroleptic iridium(III) complex, [Ir(fpbpy)(tpy)](PF6)2 (2), featuring one 2,20 :60 200 -terpyridine (tpy) in place of one fpbpy. The charge-neutral heteroleptic complex [Ir(fpbpy)(dppy)] (3) (dppyH2 = 2,6-diphenylpyridine) was also afforded successfully via a solvent-free method, giving a complex with one dppy bound to the Ir(III) ion through two carbon atoms for the cyclometalation reaction. Complexes 1-3 are emissive in acetonitrile solution at ambient temperature, perhaps best assigned to the MLCT character, mixed with intraligand charge transfer (ILCT) transition inside fpbpy and ligand-to-ligand charge transfer (LLCT) π(fpbpy) f π*(tpy) or π(dppy) f π*(fpbpy) transition, respectively, which are supported by DFT calculations. It is noteworthy that the photoluminescence of 3 displays a significant red-shifting compared to those of 1 and 2 due to the large reduction of the HOMO-LUMO energy gap as a consequence of the introduction of the more electron-donating and strong ligand-field cyclometalate dppy chelate.

Introduction The second- and third-row transition metal complexes incorporating polypyridyl and related cyclometalating ligands have been intensively investigated during the past few decades due to their useful spectroscopic and photophysical properties, offering potential applications in photovoltaics, organic light-emitting diodes, and luminescent probes and sensors.1 Divalent metal complexes including *To whom correspondence should be addressed. E-mail: [email protected] (J.-L.C.); [email protected] (Y.-H.W.). (1) (a) Schanze, K. S.; Schemehl, R. H., Eds. Applications of Inorganic Photochemistry in the Chemical and Biological SciencesContemporary Developments. J. Chem. Educ. 1997, 74, 633-702. (b) Hung, I. S.; Chen, C. H. Mater. Sci. Eng. Rep. 2002, 39, 143. (c) Lo, K. K. W.; Hui, W. K.; Chung, C. K.; Tsang, K. H. K.; Ng, D. C. M.; Zhu, N. Y.; Cheung, K. K. Coord. Chem. Rev. 2005, 249, 1434. (d) Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. Rev. 2006, 250, 2093. (e) Nazeeruddin, M. K.; Gratzel, M. Struct. Bonding (Berlin, Ger.) 2007, 123, 113. (2) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Vol Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (b) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. Rev. 1996, 96, 759. (c) Chen, Y.; Meyer, T. J. Chem. Rev. 1998, 98, 1439. (d) Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani, V. Top. Curr. Chem. 2007, 280, 117. (e) Chou, P.-T.; Chi, Y. Eur. J. Inorg. Chem. 2006, 3319. pubs.acs.org/Organometallics

Published on Web 06/15/2010

Ru(II),2 Os(II),3 and Pt(II),4 as mono- and multinuclear species, have played a major role in these fields. Recently, a great deal of effort has been invested in trivalent iridium complexes, which can give bright emission across the entire visible spectral region and probably be more used in various devices such as photoactive materials.5 Of particular interest are Ir(III) complexes with the formula [Ir(C∧N)(3-n)(L∧X)n] (n = 0-3),6-10 L∧X = auxiliary monoanionic chelating ligand, which display the anticipated blue, green, and red phosphorescence in both fluid and solid states and have charge-neutral (3) (a) Kalyanasundaram, K. J. Ind. Chem. Soc. 1993, 70, 433. (b) De Cola, L.; Belser, P. Coord. Chem. Rev. 1998, 177, 301. (c) Baxter, S. M.; Jones, W. E., Jr.; Danielson, E.; Worl, L.; Strouse, G.; Younathan, J.; Meyer, T. J. Coord. Chem. Rev. 1991, 111, 47. (d) Kochi, J. K. J. Organomet. Chem. 1990, 383, 339. (e) Kumaresan, D.; Shankar, K.; Vaidya, S.; Schmehl, R. H. Top. Curr. Chem. 2007, 281, 101. (4) (a) Williams, J. A. G. Top. Curr. Chem. 2007, 281, 205. (b) Eryazici, I.; Moorefield, C. N.; Newkome, G. R. Chem. Rev. 2008, 108, 1834. (5) (a) Ayala, N. P.; Flynn, C. M., Jr.; Sacksteder, L.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. Soc. 1990, 112, 3837. (b) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750. (c) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Top. Curr. Chem. 2007, 281, 143. (d) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S. Adv. Mater. 2009, 21, 4418. (e) Chi, Y.; Chou, P.-T. Chem. Soc. Rev. 2010, 39, 638. r 2010 American Chemical Society

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character and hence are highly desirable for the fabrication of phosphorescent OLEDs. Owing to their prominent photophysical properties, such iridium(III) species often serve as a type of favorable molecular building block for the construction of various photoactive multimetallic supramolecular systems.11 Almost all such work on luminescent iridium(III) complexes has centered on tris-bidentate complexes up to now, mainly of the general types discussed above. In sharp contrast, the six-coordinate Ir(III) complexes possessing two tridentate chelating ligands have been much less investigated, despite the fact that bis-tridentate Ir(III) complexes have well-recognized structural advantages over tris-bidentate species.12 Compared to the latter, the former usually show higher geometrical symmetry, which can be more helpful and effective for constructing a variety of linear redox-active and photoactive supramolecular assemblies and minimizing possible stereoisomers arising from the asymmetric nature of the coordinated chelating ligands. In addition, the improved stabilities of such bis-tridentate complexes may be envisaged as an important impetus for switching from bidentate to tridentate ligands, and also the design of such systems with tridentate chelates provides a new approach for tuning excited-state energies and improving luminescence properties. As a result, luminescent and redox-active Ir(III) complexes with C∧N∧C-, N∧C∧N-, N∧N∧C-, or N∧N∧Ncoordinating tridentate ligands have been increasingly explored recently.13 Analogous to the chemistry of the N∧N∧C ligands such as 6-phenyl-2,20 -bipyridine, the modified N∧N∧N-coordinating ligand 6-(5-trifluoromethylpyrazol-3-yl)-2,20 -bipyridine (fpbpyH) can also be employed as an anionic tridentate (6) (a) Dedeian, K.; Shi, J.; Shepherd, N; Forsythe, E.; Morton, D. C. Inorg. Chem. 2005, 44, 4445. (b) Chew, S.; Lee, C. S.; Lee, S.-T.; Wang, P.; He, J.; Li, W.; Pan, J.; Zhang, X.; Kwong, H. Appl. Phys. Lett. 2006, 88, 093510. (c) Nazeeruddin, Md. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.; Zuppiroli, L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790. (7) (a) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904. (b) He, G.; Schneider, O.; Qin, D.; Zhou, X.; Pfeiffer, M.; Leo, K. J. Appl. Phys. 2004, 95, 5773. (c) Yang, D.; Li, W.; Chu, B.; Zhang, D.; Zhu, J.; Su, Z.; Su, W.; Han, L.; Bi, D.; Chen, Y.; Yan, F.; Liu, H.; Wang, D. Appl. Phys. Lett. 2008, 92, 253309. (8) Okada, S.; Okinaka, K.; Iwawaki, H.; Furugori, M.; Hashimoto, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Tsuboyama, A.; Takiguchi, T.; Ueno, K. Dalton Trans. 2005, 1583. (9) (a) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thoma, J. C.; Perter, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 1713. (b) Ionkin, A. S.; Marshall, W. J.; Fish, B. M. Organometallics 2006, 25, 1461. (10) (a) Shih, P.-I.; Chien, C.-H.; Chuang, C.-Y.; Shu, C.-F.; Yang, C.-H.; Chen, J.-H.; Chi, Y. J. Mater. Chem. 2007, 17, 1692. (b) Chang, C.-J.; Yang, C.-H.; Chen, K.; Chi, Y.; Shu, C.-F.; Ho, M.-L.; Yeh, Y.-S.; Chou, P.-T. Dalton Trans. 2007, 1881. (c) Wang, Y.-M.; Teng, F.; Gan, L.-H.; Liu, H.-M.; Zhang, X.-M.; Fu, W.-F.; Wang, Y.-S.; Xu, X.-R. J. Phys. Chem. C 2008, 112, 4743. (d) Lee, T.-C.; Chang, C.-F.; Chiu, Y.-C.; Chi, Y.; Chan, T.-Y.; Cheng, Y.-M.; Lai, C.-H.; Chou, P.-T.; Lee, G.-H.; Chien, C.-H.; Shu, C.-F.; Leonhardt, J. Chem. Asian J. 2009, 4, 742. (11) (a) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni, L.; Encinas, S.; Barigelletti, F. Chem. Soc. Rev. 2000, 29, 385. (b) Cooke, M. W.; Hanan, G. S. Chem. Soc. Rev. 2007, 36, 1466. (12) (a) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. Rev. 1994, 94, 993. (b) Williams, J. A. G. Chem. Soc. Rev. 2009, 38, 1783. (13) (a) Whittle, V. L.; Williams, J. A. G. Inorg. Chem. 2008, 47, 6596. (b) Whittle, V. L.; Williams, J. A. G. Dalton Trans. 2009, 3929. (c) Auffrant, A.; Barbieri, A.; Barigelletti, F.; Collin, J.-P.; Flamigni, L.; Sabatini, C.; Sauvage, J.-P. Inorg. Chem. 2006, 45, 10990. (d) Williams, J. A. G.; Wilkinson, A. J.; Whittle, V. L. Daltons Trans. 2008, 2081. (e) Polson, M.; Ravaglia, M.; Fracasso, S.; Garavelli, M.; Scandola, F. Inorg. Chem. 2005, 44, 1282.

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chelate via cleavage of the pyrazolyl N-H bond, in a way similar to that of the C-linked 2-pyridyl azolate bidentate ligands.14,15 The fpbpyH ligand, as a monoanionic tridentate chelate, has been successfully used to prepare Ru(II), Os(II), and Pt(II) complexes exhibiting excellent chemical stability and intriguing photophysical properties.16 Herein, we describe the syntheses, characterizations, and photophysical properties of a novel family of Ir(III) bis-tridentate complexes utilizing the deprotonated fpbpyH ligand as the monoanionic N∧N∧N-coordinating chelate. It is demonstrated that the monocationic homoleptic iridium(III) complex [Ir(fpbpy)2](PF6) (1) is highly emissive in both solution and solid state, and the replacement of one fpbpy of 1 via other tridentate chelating ligands such as dppyH2 has a very important influence on the photophysical properties of the corresponding iridium(III) complexes.

Experimental Section General Procedures and Materials. All reactions were performed under a nitrogen atmosphere using anhydrous solvents or solvents treated with an appropriate drying reagent. Commercially available reagents were used without further purification unless otherwise stated. IrCl3 3 3H2O, 2,20 :60 200 -terpyridine (tpy), and 2,6-diphenylpyridine (dppyH2) were commercially available (Acros). The precursor compounds Ir(tpy)Cl3 and 6-(5-trifluoromethylpyrazol-3-yl)-2,20 -bipyridine (fpbpyH) were prepared according to the literature method.16,17 Preparation of [Ir(fpbpy)2](PF6) (1). IrCl3 3 3H2O (100 mg, 0.283 mmol) and 6-(5-trifluoromethylpyrazol-3-yl)-2,20 -bipyridine (fpbpyH) (180 mg, 0.620 mmol) were heated in degassed ethylene glycol (8 mL), under a nitrogen atmosphere and in the dark, kept at 100 C for 1 h, and then raised to 196 C for 3 h. After the mixture was cooled to room temperature, the precipitate was obtained by addition of a saturated aqueous solution of KPF6. The crude product was purified by recrystallization in acetone/diethyl ether. Pure yellow crystalline product 1 was obtained by slow diffusion from an acetone/hexane (1:3) mixture; yield: 114 mg, 0.124 mmol, 44%. MS (FAB, 193Ir): m/z 771 (Mþ - PF6). 1H NMR (500 MHz, acetone-d6, 298 K): δ 8.78 30 (2H, d, J = 8.0 Hz, H ), 8.72 (2H, d, J = 7.5 Hz, H3), 8.54 (2H, 0 t, J = 8.0 Hz, H4 ), 8.47 (2H, d, J = 8.0 Hz, H5), 8.25 (2H, t, J = 4 60 7.5 Hz,0 H ), 8.13 (2H, d,0 0 J = 4.5 Hz, H ), 7.55 (2H, t, J = 6.5 Hz, H5 ), 7.29 (2H, s, H4 ). 19F NMR (470 MHz, acetone-d6, 298 K): δ -61.31 (s, 6F, CF3), -72.51 (d, JP-F = 708.8 Hz, 6F, PF6). Anal. Calcd for C28H16F12N8PIr 3 1/2CH3COCH3: C, 37.51; H, 2.03; N, 11.86. Found: C, 37.77; H, 2.39; N, 11.43. Preparation of [Ir(fpbpy)(tpy)](PF6)2 (2). This complex was prepared by a synthetic procedure similar to that of 1 using Ir(tpy)Cl3 (50 mg, 0.094 mmol) and fpbpyH (28 mg, 0.096 mmol) instead of IrCl3 3 3H2O (100 mg, 0.283 mmol) and fpbpyH (180 mg, 0.620 mmol); yield: 58 mg, 0.0577 mmol, 61%. MS (FAB, 193Ir): m/z 860 (Mþ - PF6), 715 (Mþ - 2PF6 þ 1). 1H NMR (500 MHz, acetone-d6, 298 K): δ 9.14 (2H, d, J = 8.0 Hz, H6 tpy), (14) (a) Chi, Y.; Chou, P.-T. Chem. Soc. Rev. 2007, 36, 1421. (b) Chou, P.-T.; Chi, Y. Chem.;Eur. J. 2007, 13, 380. (15) (a) Chang, S.-Y.; Chen, J.-L.; Chi, Y.; Cheng, Y.-M.; Lee, G.-H.; Jiang, C.-M.; Chou, P.-T. Inorg. Chem. 2007, 46, 11202. (b) Tung, Y.-L.; Lee, S.-W.; Chi, Y.; Chen, L.-S.; Shu, C.-F.; Wu, F.-I.; Carty, A. J.; Chou, P.T.; Peng, S.-M.; Lee, G.-H. Adv. Mater. 2005, 17, 1059. (16) (a) Chen, J.-L.; Chang, S.-Y.; Chi, Y.; Chen, K.; Cheng, Y.-M.; Lin, C.-W.; Lee, G.-H.; Chou, P.-T.; Wu, C.-H.; Shih, P.-I; Shu, C.-F. Chem. Asian J. 2008, 3, 2112. (b) Chen, K.-S.; Liu, W.-H.; Wang, Y.-H.; Lai, C.-H.; Chou, P.-T.; Lee, G.-H.; Chen, K.; Chen, H.-Y.; Chi, Y.; Tung, F.-C. Adv. Funct. Mater. 2007, 17, 2964. (c) Chen, K.; Cheng, Y.-M.; Chi, Y.; Ho, M.-L.; Lai, C.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H. Chem. Asian J. 2007, 2, 155. (17) Collin, J.-P.; Dixon, I. M.; Sauvage, J.-P.; Williams, J. A. G.; Barigelletti, F.; Flamigni, L. J. Am. Chem. Soc. 1999, 121, 5009.

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8.90 (2H, d, J = 7.5 Hz, H3 H5 tpy), 8.89-8.850 (3H, m, H4 30 tpy and H H3 fpbpy), 8.71 (1H, t, J = 8.7 Hz, H4 fpbpy), 8.62 td, J = 7.7, 1.3 Hz, (1H, d, J = 8.0 Hz, H5 fpbpy), 8.35 (2H, 0 H4 tpy), 8.32-8.27 (2H, m, H4 H6 fpbpy), 8.07 (2H, d, J = 6.5 Hz, H3 tpy), 7.64 (2H, td, J = 6.6, 1.5 Hz, H5 tpy), 7.56 50 40 0 (1H, t, J = 7.0 Hz, H fpbpy), 7.40 (1H, s, H fpbpy). 19F NMR (470 MHz, acetone-d6, 298 K): δ -61.60 (s, 3F, CF3), -72.42 (d, JP-F = 706.4 Hz, 12F, 2PF6). Anal. Calcd for C29H19F15N7P2Ir 3 H2O: C, 34.06; H, 2.07; N, 9.59. Found: C, 33.99; H, 2.41; N, 9.25. Preparation of [Ir(fpbpy)(dppy)] (3). A solution of IrCl3 3 3H2O (71 mg, 0.201 mmol) and fpbpyH (59 mg, 0.203 mmol) in ethanol was gently refluxed for 2 days under a nitrogen atmosphere. After the mixture was cooled to room temperature and the solvent was removed under vacuum, the residue was dissolved in a small amount of acetone, and a large amount of diethyl ether was added to induce an orange-yellow precipitate. The precipitate was collected, washed with diethyl ether, and dried under vacuum to obtain an orange-yellow solid (107 mg). Without further purification, a mixture of the above Ir(III) intermediate (107 mg), 2,6-diphenylpyridine (dppyH2) (465 mg, 2.01 mmol), and silver(I) trifluoromethanesulfonate (AgOTf) (140 mg, 0.545 mmol) was ground together to afford a fine powder and then heated at 110 C with stirring under a nitrogen atmosphere for 24 h. After cooling, the product was extracted into dichloromethane (30 mL) and the remaining solid removed by filtration. Removal of solvent from the filtration produced an orange residue under reduced pressure. The residue was subjected to column chromatography on silica gel and eluted with dichloromethane to give an orange-red product, 3; yield: 18 mg, 0.0253 mmol, 12%. MS (FAB, 193Ir): m/z 711 (Mþ). 1H NMR0 (500 MHz, acetone-d6, 298 K): δ 8.58 (1H, d, J = 8.0 Hz, H3 fpbpy), 8.53 (1H, d, J = 7.5 Hz, H3 fpbpy),0 8.25 (1H, d, J = 8.0 Hz, H5 fpbpy), 8.13 (1H, t, J = 8.0 Hz, H4 fpbpy), 7.95-7.880 4 40 30 (4H, m, H fpbpy and H H dppy), 7.79 (1H, d, J = 6.0 Hz, H6 fpbpy), 7.75 (2H, td, J = 7.5 Hz, H30 0 dppy), 7.24 (1H, td, J = 6.6, 50 1.3 Hz, H fpbpy), 7.08 (1H, s, H4 fpbpy), 6.82 (2H, t, J = 7.5 Hz, H4 dppy), 6.63 (2H, t, J = 6.9 Hz, H5 dppy), 6.22 (2H, d, J = 7.0 Hz, H6 dppy). 19F NMR (470 MHz, acetone-d6, 298 K): δ -60.17 (s, 3F, CF3). Anal. Calcd for C31H19F3N6Ir: C, 52.39; H, 2.69; N, 9.85. Found: C, 52.16; H, 3.09; N, 9.67. X-ray Crystallography Measurements. Crystals of 1 were grown by slow diffusion of hexane into a 1:3 mixture of acetone and 1,2-dichloroethane. Single-crystal X-ray diffraction data of 1 were measured on a Bruker SMART CCD diffractometer using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). Absorption corrections were applied using SADABS supplied by Bruker. The structure was solved by the direct method using the SHELX-97 program package.18 The positions of the metal atom and its first coordination sphere were located from direct-method E-map; other non-hydrogen atoms were found using alternating Fourier syntheses and least-squares refinement cycles and, during the final cycles, were refined anisotropically. Hydrogen atoms were placed in calculated position and refined as riding atoms with a uniform value of Uiso. The crystallographic data of 1 are summarized in Table 1, and selected bond distances and angles are given in Table 2. Physical Measurements. Mass spectra were obtained on a JEOL SX-102A instrument operating in electron impact (EI) mode or fast atom bombardment (FAB) mode. 1H and 19F NMR spectra were recorded on Varian Mercury-400 or INOVA-500 instruments. Elemental analyses (C, H, N) were conducted at the NSC Regional Instrumentation Center at National Chiao Tung University. UV-vis absorption spectra in acetonitrile solutions were measured on a Perkin-Elmer Lambda 25 UV-vis spectrometer. The photoluminescence properties in acetonitrile solution and solid state were determined on an (18) Sheldrich, G. M. SHELX-97, Program for Solution and Refinement of Crystal Structures; University of G€ ottingen: G€ ottingen, Germany, 1997.

Table 1. Crystallographic Data for Complex 1 empirical formula fw temp, K radiation (λ, A˚) cryst syst space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z Fcalcd, g/cm-3 μ, mm-1 data/restraints/params GOF R1, wR2 (I > 2σ(I)) R1, wR2 (all data) largest diff peak and hole, e A˚-3

C28H16F12IrN8P 915.68 293(2) 0.71073 triclinic P1 9.538(3) 10.937(3) 16.426(4) 77.220(3) 76.753(3) 68.676(3) 1535.7(7) 2 1.980 4.508 4768/0/460 1.043 0.0451, 0.0894 0.0731, 0.1076 0.803 and -0.718

Table 2. Selected Bond Lengths (A˚) and Angles (deg) for Complex 1 Ir1-N1 Ir1-N5

2.079(8) Ir1-N2 2.066(9) Ir1-N6

1.988(8) Ir1-N3 1.989(8) Ir1-N7

2.028(8) 2.001(9)

N1-Ir1-N2 N1-Ir1-N6 N2-Ir1-N5 N3-Ir1-N5 N5-Ir1-N6

79.3(3) 98.5(3) 101.0(4) 91.5(3) 79.7(4)

158.8(3) 95.5(3) 177.7(3) 102.6(3) 158.5(3)

92.0(3) 79.6(3) 100.2(3) 88.7(3) 79.3(4)



Edinburgh analytical instrument (F900 fluorescence spectrometer) with a thermoelectrically cooled Hamamatsu R3809 photomultiplier tube. The emission quantum yields (Φem) of 1 and 2 in acetonitrile solutions at room temperature were calculated by Φs = Φr(Br/Bs)(nr/ns)2(Ds/Dr) using fluorescein in H2O as the standard (Φem = 0.79), while the quantum yield of 3 was determined relative to that of [Ru(bpy)3](PF6)2 in acetonitrile (Φem = 0.062), where the subscripts r and s denote reference standard and the sample solution, respectively, and n, D, and Φ are the refractive index of the solvents, the integrated intensity, and the luminescence quantum yield, respectively. The quantity B is calculated by B = 1 - 10-AL, where A is the absorbance at the excitation wavelength and L is the optical path length.19 An integrating sphere (Lab sphere) was applied to measure the quantum yield of 1 in the solid state. Cyclic voltammetry was carried out in a three-compartment cell using a glassy carbon disk working electrode, a platinum counter electrode, and a Ag/Agþ (CH Instruments, 10 mM AgNO3 in MeCN) reference electrode. All experiments were performed in dry acetonitrile containing 0.1 M tertrabutylammonium hexafluorophosphate as the supporting electrolyte. The potentials are quoted versus the ferrocenium/ferrocene couple in dry acetonitrile (Fcþ/0 = 0.42 V). Theoretical Methodology. All the calculations were carried out by using the Gaussian 03 program package.20 The density functional theory (DFT)21 at the gradient-corrected correlation functional22 PBE1PBE and unrestricted PBE1PBE (UPBE1PBE) approaches was used to optimize the ground-state and excitedstate geometries of complexes 1-3, since the method employed gave the best results in terms of structural parameters and simulation of absorption properties by comparing different functionals (Table S1, Supporting Information). Spin contamination due to the admixture of excitations of higher multiplicity (19) (a) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (b) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung, K.-K.; Zhu, N. Chem.;Eur. J. 2001, 7, 4180. (c) Xu, H.-B.; Chen, X.-M.; Zhang, Q.-S.; Zhang, L.-Y.; Chen, Z.-N. Chem. Commun. 2009, 7318.

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was rather small: the expectation values of the spin operator ÆS2æ are 2.03 for triplet states. We first performed geometry optimization from the X-ray structure in the ground state without symmetry constraint and confirmed that the stable geometry of 1 was C2 symmetry, and those of both 2 and 3 were Cs symmetries in the ground state (Figure S1). Therefore, the stable geometries of 1 with C2 symmetry and 2 and 3 with Cs symmetries were recalculated. Based on the optimized ground-state and lowest-energy triplet excited-state geometries, 60 singlet and six triplet excited states for 1-3 were obtained to determine the vertical excitation energies for all the molecules in acetonitrile media using the time-dependent DFT (TD-DFT) method.23,24 The conductor-like polarizable continuum model (CPCM)25 with acetonitrile as solvent was used to calculate all the electronic structures and excited states in solution. In these calculations, the Hay-Wadt double-ξ with a Los Alamos relativistic effect basis set (LANL2DZ)26 consisting of the effective core potentials (ECP) was employed for the iridium(III) atom and the 6-31G(p,d) basis set was used for the remaining atoms. To precisely describe the molecular properties, one additional f-type polarization function was implemented for the iridium(III) atom (R = 0.938).27

Results and Discussion Synthesis and Characterization. The tridentate chelating ligands and bis-tridentate iridium(III) complexes involved in this work are represented in Scheme 1. The required tridentate ligand fpbpyH was afforded by a pseudo-Claisen condensation reaction of 6-acetyl-2,20 -bipyridine and ethyl trifluoroacetate, followed by cyclization with an excess of hydrazine hydrate in refluxing ethanol according to procedures reported previously.16 It has been demonstrated that the ligand fpbpyH has a strong tendency toward forming a monoanionic tridentate chelate via facile deprotonation during the reaction with various metal salt agents such as Pt(II), Ru(II), and Os(II) in basic media, even in weak acidic media.16 (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02: Gaussian, Inc.: Wallingford, CT, 2004. (21) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (22) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396. (23) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439. (24) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218. (25) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (b) Cossi, M.; Regar, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669. (26) (a) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (27) Ehlers, A. W.; B€ ohme, M.; Dapprich, S.; Gobbi, A.; H€ ollwarth, A.; Jonas, V.; K€ ohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenkig., G. Chem. Phys. Lett. 1993, 208, 111.

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Scheme 1. Chemical Formulas of the Tridentate Ligands and Complexes Synthesized

This project was carried out with a designed goal of synthesizing luminescent bis-tridentate complexes of iridium(III), particularly the charge-neutral species, featuring the deprotonated fpbpy ligand as a tridentate monoanionic ligand. It is also expected that the fpbpyH ligand can complex favorably with the Ir(III) metal center in a monoanionic N∧N∧N-binding mode via the direct cleavage of the pyrazolyl N-H bond using its three nitrogens. As depicted in Scheme 1, the homoleptic 2:1 complex [Ir(fpbpy)2](PF6) (1) was first afforded in a moderate yield of 44% by treatment of iridium metal salt agent IrCl3 3 3H2O with 2.2 equiv of fpbpyH in degassed ethylene glycol at 196 C, in the absence of light. Due to the unsymmetrical nature of the fpbpy ligand, complex 1 is chiral and isolated as a racemic mixture, substantiated by its crystal packing diagram (see Figure 2). Its further characterization was executed employing single-crystal X-ray structural analysis, FAB mass spectrometry, 1H and 19F NMR spectroscopies, and elemental analyses. For the C2 symmetry of 1, only seven aromatic proton signals (δ 8.78-7.55 ppm) appeared in the 1H NMR spectrum in acetone-d6, besides the C-H protons of two pyrazolate rings at δ 7.29 ppm as one singlet peak. However, the unique N-H signal was not observed under this condition, which suggests that the fpbpyH ligand apparently becomes a tridentate monoanionic ligand through the deprotonated process; binding of fpbpy in such a fashion was also found in the previously reported Pt(II), Ru(II), and Os(II) complexes bearing the fpbpy chelates.16 In its 19F NMR spectrum, one singlet peak is at δ -61.31 ppm, corresponding to the CF3 group; the other doublet is at δ -72.51 ppm with JP-F = 708.8 Hz, corresponding to PF6-. Moreover, the integral ratio (1:1) of the two peaks further indicates that 1 is a cationic complex with a PF6- anion. This is also supported by an intense molecular ionic peak fragment (m/z, 771, [M PF6]þ) displayed in the FAB-MS spectrum of 1. A single-crystal X-ray diffraction study of 1 was carried out to reveal its exact molecular structure and the binding of fpbpy around the Ir(III) ion. As indicated in Figure 1, its overall coordination geometry is best described as a distorted octahedron owing to the constraints of the deprotonated ligand fpbpy, while the fpbpy chelate serves as a monoanionic tridentate chelating ligand via cleavage of the pyrazolyl N-H bond, as observed in previously reported fpbpy complexes.16 By comparison with related Ir(III) complexes such as [Ir(terpy)2](PF6)3 (terpy = 2,20 :60 ,200 -terpyridine), [Ir(tterpy)3](PF6)2 (tterpy = 40 -(4-tolyl)-2,20 :60 ,200 -terpyridine),

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Figure 1. ORTEP drawing of the cation of 1 showing 30% probability thermal ellipsoids and atom-labeling scheme. The PF6 anion and hydrogen atoms are omitted for clarity.

Figure 2. Side view depicting the accompanying packing diagram of the cation of 1 in the crystal lattices.

and m-[Ir(fppz)3] (fppzH = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole),17,28 it is noted that the Ir-N bond lengths and the N-Ir-N angles in 1 are very close to those reported data, among which the central Ir-N distances of the fpbpy ligand are shorter than those of the outer nitrogen atoms, mainly because of the internal strain exerted by the chelate bonding, as documented in the literature.29 Ir-N average bond lengths are 1.989 and 2.073 A˚ for the central and peripheral pyridine rings, respectively, and 2.015 A˚ for the flanking pyrazole cycles. It is notable that two bipyridyl fragments of each iridium molecule show a favorable side-to-side stacking with one bipyridyl group of its neighboring enantiomeric molecules (see Figure 2), respectively, thus forming an extended linear chain-like arrangement of complexes via partial π-π stacking of intermolecular fpbpy ligands. The interplanar distances are estimated in the range 3.5-3.6 A˚, suggesting that a weak aromatic π-π stacking interaction between intermolecular bipyridyl moieties is likely operating.30 The achiral heteroleptic complex [Ir(fpbpy)(tpy)](PF6)2 (2) can be accessible in 61% yield via reaction of Ir(tpy)Cl3 (28) (a) Yoshikawa, N.; Yamabe, S.; Kanehisa, N.; Kai, Y.; Takashima, H.; Tsukahara, K. Eur. J. Inorg. Chem. 2007, 1911. (b) Yeh, Y.-S.; Cheng, Y.-M.; Chou, P.-T.; Lee, G.-H.; Yang, C.-H.; Chi, Y.; Shu, C.-F.; Wang, C.-H. ChemPhysChem 2006, 7, 2294. (29) (a) Lo, K. K.-W.; Chung, C.-K.; Ng, D. C.-M.; Zhu, N. New J. Chem. 2002, 26, 81. (b) Polson, M.; Fracasso, S.; Bertolasi, V.; Ravaglia, M.; Scandola, F. Inorg. Chem. 2004, 43, 1950. (c) Yoshikawa, N.; Yamabe, S.; kanehisa, N.; Kai, Y.; Takashima, H.; Tsukahara, K. Inorg. Chim. Acta 2009, 362, 361. (30) Hunter, C. A.; Meah, M. N.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5773.

Chen et al.

with an equivalent amount of fpbpyH in degassed ethylene glycol at 196 C. Complex 2 was also well characterized by FAB-MS spectrometry, 1H and 19F NMR, and elemental analyses. For 2 (Cs symmetry), the terminal pyridyl segments of the tpy ligand are equivalent by symmetry on the NMR time scale, and only 14 aromatic proton resonances appear in the downfield range δ 9.14-7.56 ppm, in addition to the pyrazolyl C-H proton signal at δ 7.40 ppm. Similar to 1, the unique N-H signal of 2 was also not observed, implying that the fpbpyH ligand becomes a negatively charged chelate due to deprotonation of the pyrazolyl-NH. Moreover, the ionic nature of 2 including one fpbpy chelate and two PF6- counterions is characterized by the integral ratio (1:4) of two peaks corresponding to CF3 (δ, -61.31 ppm) and PF6- (δ, -72.51 ppm) in the 19F NMR spectrum. This assignment is also confirmed by a FAB-MS spectrum, in which two intense molecular ionic peak fragments 860 (Mþ - PF6) and 715 (Mþ - 2PF6 þ 1) were observed. In order to obtain charge-neutral bis-tridentate complexes of iridium(III) bearing one monoanionic fpbpy chelate, it is necessary that dianionic tridentate chelates, such as the C∧N∧C-bound ligand 2,6-diphenylpyridine (dppyH2), are introduced as the second tridentate ligand bound to the central metal. The formation of bis-tridentate iridium complexes is notoriously difficult, requiring harsh conditions such as high temperature to overcome the kinetic inertness of the iridium(III) ion.31 Light must also be obviated as much as possible during the reaction. In the present instance, many attempts to give the targeted complex [Ir(fpbpy)(dppy)] (3), including raising the temperature, the alteration of reaction solvents, and the addition of silver salts to facilitate the dechlorination, were consistently in vain. The desired complex 3, with bis-cyclometalating dppy, was ultimately obtained according to the reported procedures with little modification.32 The key intermediate was first synthesized by refluxing the fpbpyH ligand and IrCl3 3 3H2O reagent in ethanol, which is believed to be a complex mixture of Ir(III) complexes without a dominant product, as checked by 1H NMR and FAB-MS. Due to the absence of well-characterized product, the orange-yellow intermediate (i.e., the mixture of iridium(III) complexes with the fpbpy chelate) was employed directly as the following starting material. Thus, the orange complex 3 was isolated in 12% overall yield by heating the above intermediate with both dppyH2 and silver trifluoromethanesulfonate (AgOTf) at 110 C, in the absence of any other solvent, followed by silica gel chromatographic purification. The structure of the charge-neutral bis-tridentate complex 3 (i.e., containing one N∧N∧N--coordinating and one C∧N∧C-coordinating ligand) was characterized and confirmed by satisfactory elemental analyses, 1H and 19F NMR spectroscopies, and FAB-MS spectrometry (see Supporting Information). In fact, such an approach using a lowmelting-point organic reagent as the reaction solvent has been successfully reported in the synthesis of [Ir(ppy)3] and derivatives.33 (31) Mamo, A.; Stefio, I.; Parisi, M. F.; Credi, A.; Venturi, M.; Di Pietro, C.; Campagna, S. Inorg. Chem. 1997, 36, 5947. (32) (a) Wilkinson, A. J.; Goeta, A. E.; Foster, C. E.; Williams., J. A. G. Inorg. Chem. 2004, 43, 6513. (b) Wilkinson, A. J.; Puschmann, H.; Howard, J. A. K.; Foster, C. E.; Williams, J. A. G. Inorg. Chem. 2006, 45, 8685. (33) (a) Colombo, M. G.; Brunold, T. C.; Riedener, T.; G€ udel, H. U.; F€ ortsch, M.; B€ urgi, H.-B. Inorg. Chem. 1994, 33, 545. (b) Grushin, V. V.; Herron, N.; LeClous, D. D.; Marshall, W. J.; Petrov, V. A.; Wang, Y. Chem. Commun. 2001, 1494.

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Table 3. Photophysical and Electrochemical Data of Complexes 1-3 at Room Temperature abs λmax/nm (ε 10-3/M-1 cm-1)a 1 2 3

em λmax/ nma

E(Sm)c/nm

260(36.6), 283sh(27.1), 326(15.8), 342(12.0), 361(6.6) 245(29.4), 258(35.2), 266(37.0), 278(33.8), 310(17.2), 322(17.7), 336(15.2), 353sh(6.4), 382(2.3) 248(46.1), 279sh(29.7), 313(19.3), 324(18.7), 378(7.3), 454(3.4), 534(1.2)

348 (S3) 381 (S2) 440 (S2)

E(T1)c/ nm

τ/nsa, Φema

E1/2ox/V [ΔEp/mV]d

E1/2red/V [ΔEp/mV]d

2.04 (irrev)

-1.05 (66), -1.25 (69) -0.83 (66), -1.08 (78), -1.40 (66)

1.21 (80), 0.82 (67)

533, 507 (565, 537, 498)b 545

532 528

748, 0.138 (248, 0.26)b 770, 0.006

789(sh), 703, 602(sh)

892

25, 0.0005

f

-1.50 (69)

a Data were measured in acetontrile solution. b Data were measured in the solid state. c The low-energy absorptions and emissions calculated at the optimized Sm and T1 geometries of 1-3 by the TD-DFT approach; the calculated results corresponding to the experimental data are underlined. d Potentials were quoted versus the Fcþ/Fc reference (Fcþ/0 = 0.42 V); oxidation and reduction potentials were measured in acetonitrile solution; ΔEp is defined as Eap(anodic peak potential) - Ecp(cathodic peak potential), and these data are quoted in millivolts. f Unseen within the solvent window.

Table 4. Partial Molecular Orbital Compositions (%) in the Ground State for 1 in Acetonitrile Solution under the TD-DFT Calculations MO contribution (%) orbital

energy (eV)

Ir

fpbpy1

fpbpy2

LUMOþ4 LUMOþ2 LUMOþ1 LUMO HOMO HOMO-1 HOMO-2 HOMO-4 HOMO-5 HOMO-7

-1.3734 -1.7309 -2.4390 -2.4455 -6.3942 -6.7923 -6.9648 -7.5529 -7.6255 -8.1349

0.9 2.7 5.6 5.0 36.2 4.0 18.3 53.0 21.7 13.7

49.6 48.6 47.2 47.5 31.9 48.0 40.8 23.5 39.2 43.1

49.5 48.7 47.2 47.5 31.9 48.0 40.9 23.5 39.1 43.2

Electrochemical Properties. The electrochemical behavior of complexes 1-3 has been investigated by cyclic voltammetry in dry acetonitrile using ferrocene as the internal standard, and the respective redox data are presented in Table 3. The plots of cyclic voltammograms for 1-3 are shown in Figure S8. For complex 1, an irreversible one-electron oxidation wave is observed at about 2.04 V. This relatively low oxidation potential value shows that the deprotonated fpbpy chelate possesses a strong σ-donating nature similar to that of the 1,3-dipyridyl-4,6-dimethylbenzene cyclometalating ligand.13c,29b DFT calculations reveal that the highest occupied molecular orbital (HOMO) of 1 is substantially delocalized on the Ir-fpbpy fragments (Table 4 and Figure S2), and thus this process cannot be simply attributable to the IrIV/IrIII redox couple and may be more rationally ascribed to the oxidation of the Ir-fpbpy moiety. At negative potentials, there are two reversible one-electron reduction waves at -1.05 and -1.25 V, respectively. This two processes are assigned to the reductions of the bipyridyl fragments of the fpbpy chelates, as supported by DFT calculations of 1 (Table 4 and Figure S2), showing that its lowest unoccupied molecular orbitals (LUMOs) are mainly localized over the bipyridyl moieties of the fpbpy ligands. Unfortunately, for complex 2, the oxidation wave at positive potentials is not seen within the solvent window ( 104 M-1 cm-1 and the relatively weaker absorptions (ε < 7.5 103 M-1 cm-1) in the range 350-470 nm. With reference to previous work on Ir(III) complexes possessing the C∧N∧C-, N∧N∧C-, N∧C∧N-, and N∧N--coordinating ligands,13,31,34,35 the absorptions at (34) (a) Ashizawa, M.; Yang, L.; Kobayashi, K.; Sato, H.; Yamagishi, A.; Okuda, F.; Harada, T.; Kuroda, R.; Haga, M.-a. Dalton Trans. 2009, 1700. (b) Yang, L.; Okuda, F.; Kobayashi, K.; Nozaki, K.; Tanabe, Y.; Ishii, Y.; Haga, M.-a. Inorg. Chem. 2008, 47, 7154. (c) Obara, S.; Itabashi, M.; Okuda, F.; Tamaki, S.; Tanabe, Y.; Ishii, Y.; Nozaki, K.; Haga, M.-a. Inorg. Chem. 2006, 45, 8907. (d) Yutaka, T.; Obara, S.; Ogawa, S.; Nozaki, K.; Ikeda, N.; Ohno, T.; Ishii, Y.; Sakai, K.; Haga, M.-a. Inorg. Chem. 2005, 44, 4737. (35) (a) Yang, C.-H.; Cheng, Y.-M.; Chi, Y.; Hsu, C.-J.; Fang, F.-C.; Wong, K.-T.; Chou, P.-T.; Chang, C.-H.; Tsai, M.-H.; Wu, C.-C. Angew. Chem., Int. Ed. 2007, 46, 2418. (b) Avilov, I.; Minoofar, P.; Cornil, J.; De Cola, L. J. Am. Chem. Soc. 2007, 129, 8247.

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Figure 3. UV-vis absorption spectra of complexes 1 (blue), 2 (black), and 3 (red) recorded in diluted acetonitrile solution at room temperature. The excitation spectrum of 3 (λem = 700 nm) under the same conditions is also shown (green).

230-350 nm are most likely originated from the ligandcentered 1ππ* transitions. Further support is given by the close matching of several absorption bands in the highenergy region (e350 nm) in complexes 2 and 3 with the same fpbpy chelate. The absorption bands at 350-470 nm can be identified as the metal-to-ligand charge transfer (MLCT) transition from the dπ orbital of the (5d6)Ir-metal center to the unoccupied π* orbital of the fpbpy ligand, mixed with some intraligand charge transfer transition from fpbpy. This assignment is further supported by DFT analysis of 1 (see Figures 6 and 7). Accordingly, for 2, the strong absorption bands (e350 nm) can be thus assigned to LC 1π-π* transitions, and the comparatively weak absorptions beyond 350 nm are attributed to the mixed transitions perhaps comprising a MLCT transition, a certain degree of charge transfer transition inside fpbpy, and some ligand-to-ligand charge transfer (LLCT) fpbpy f tpy transition, as confirmed by DFT calculations of 2 (vide infra). The absorption spectrum of complex 3 in acetonitrile solution at ambient temperature shows very strong absorption bands in the UV region (e350 nm), which are assigned to spin-allowed 1π-π* transitions of the ligands. By comparison with the well-established literature data for iridium(III) complexes containing cyclometalated ligands,13,29,31,32,34 a series of weaker, lower-energy features extending well into the visible region can be more appropriately attributable to the MLCT transition, mixed with an appreciable amount of ligand-to-ligand charge transfer (LLCT) and intraligand charge transfer (ILCT) transitions. In fact, for systems with the cyclometalating ligands such as Ir(ppy)3, mixed descriptions are considered to be very general.29b,32,36 It is noted that complex 3, containing the dppy ligand, shows significantly lower energy absorption bands than those of 1 and 2, bearing one additional fpbpy or tpy ligand, an effect that may be attributed to the increase of the HOMO energy as a result of the electron-donating nature of the two cyclometalating carbons of dppy. (36) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. T.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (b) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem. 2001, 40, 1704. (c) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bu, R.; Thompson, M. E. J. J. Am. Chem. Soc. 2003, 125, 7377.

Chen et al.

Figure 4. Emission spectra of complex 1 in acetonitrile solution (solid line) and in the solid state (dashed line) at room temperature.

Figure 5. Emission spectra of complexes 1-3 in acetonitrile solution at room temperature.

Luminescent Properties. The luminescent properties of complexes 1-3 in acetonitrile and the solid emission of 1 are measured at room temperature (Figures 4 and 5). Their photophysical data are collected in Table 3. The homoleptic iridium(III) complex 1 displays a structured emission profile with a maximum at 533 nm. With reference to previous work on luminescent metal complexes possessing the fpbpy chelate,16 the highest occupied molecular orbital of 1 is believed to most probably spread over the Ir(III) ion and two pyridylpyrazolyl moieties of the fpbpy chelates, whereas its lowest unoccupied molecular orbital is thought to be basically localized on two bipyridyl fragments of the fpbpy ligands. Accordingly, in complex 1, two fpbpy ligands can also be thought to make a significant contribution to the HOMO, as well as the Ir(III) center, and hence the emissive excited state is perhaps best regarded as the MLCT character from d(Ir) f π*(fpbpy), mixed with the ILCT character from intraligand π(fpbpy) f π*(fpbpy). Similar conclusions regarding the contribution of intraligand charge transfer (ILCT) transition and, for heteroleptic complexes, ligandto-ligand charge transfer (LLCT) character to the emitting excited state have been reached in DFT studies of related iridium complexes.29b,34,37 The solid-state emission of 1 shows a red-shift of about 32 nm with respect to its fluid emission. (37) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634.

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Table 5. Excitation and Transition Properties of Absorption and Emission of 1, Calculated by the TD-DFT Method with the Conductorlike Polarizable Continuum Model (CPCM) tr

transition

contrib

E, nm (eV)

O.S.

T1

HOMOfLUMO HOMO-1fLUMOþ1 HOMOfLUMO HOMO-1fLUMOþ1 HOMOfLUMOþ2 HOMOfLUMOþ4 HOMO-4fLUMO HOMO-2fLUMOþ2 HOMO-5fLUMOþ1 HOMO-7fLUMOþ1 HOMO-4fLUMOþ2

78% 9% 100% 96% 96% 69% 20% 80% 9% 62% 25%

532 (2.33)

0.0000

411 (3.01) 348 (3.56) 324 (3.83) 293 (4.23)

0.0074 0.0538 0.1553 0.1182

274 (4.53)

0.3737

250 (4.97)

0.2951

S1 S3 S7 S13 S20 S32

Such a red-shifting can be rationalized via the π-π stacking interactions between intermolecular bipyridyl fragments of 1 in the solid state (see Figure 2), resulting in a lower LUMO energy, less influencing the HOMO energy, and hence leading to the reduction of the HOMO-LUMO energy gap. For the heteroleptic complex 2, a single, broad, structureless emission band at 545 nm is observed in solution, typical of phosphorescence from a 3MLCT state. The emission energy (λmax, 545 nm) of 2 is lower than that (λmax, 533 nm) of 1, with a slight red-shifting of around 12 nm, which may be due to the lower energy of the tpy acceptor orbital compared to that of fpbpy, without significantly affecting the metalbased HOMO of 2 containing tpy in place of fpbpy. Therefore, its emission can be reasonably assigned to a mixed d(Ir)/ π(fpbpy) f π*(tpy) (MLCT/LLCT) excited state. In sharp contrast, the heteroleptic complex 3 exhibits only a relatively weak emission maximum at 703 nm in acetonitrile solution, presenting a large red-shift of ca. 170 nm with respect to that of 1, which can be rationalized in terms of a large increase of the HOMO energy upon substitution of one chelating fpbpy in 1 by the much stronger ligand-field cyclometalated dppy ligand. The LUMO of 3 is still believed to be mainly localized on the bipyridyl unit of the fpbpy chelate, such that the introduction of the ligand dppy would be anticipated to have little effect on the LUMO energy. On the other hand, its HOMO can be largely spread over the Ir(III) ion and two phenyl groups of dppy, probably including the pyrazolyl fragment of fpbpy, as revealed by DFT calculations (see Figure 8). Therefore, the emission of 3 is probably assigned to a heavily mixed d(Ir)/π(dppy)/π(fpbpy) f π*(fpbpy) (MLCT/LLCT/ILCT) excited state. Theoretical Studies. To further understand the absorption and emission character of fpbpy-based Ir(III) bis-tridentate complexes 1-3, we first optimized the singlet ground-state S0 and the lowest triplet excited-state T1 geometries of complexes 1-3. Based on the optimized geometries, the timedependent density functional theory TD-DFT/PBE1PBE was used. The relative compositions of different energy levels in terms of the composing fragments and the emission and absorption transition character of 1 are summarized in Tables 4 and 5, respectively. As indicated in Table 4, the highest occupied molecular orbital of 1 is a π orbital, averagely localized on two fpbpy ligands (31.9%) with additional contribution from Ir(5d) (36.2%), while the lowest unoccupied molecular orbital of 1 is basically localized on the π* obitals of two fpbpy ligands with the average contribution of 47.5%. The calculated lowest-energy absorption arising from HOMO f LUMO transition with the oscillator strength of 0.0074 is assigned as 1ILCT (intraligand charge transfer) transition

assignment 3

ILCT/3MLCT ILCT 1 ILCT/1MLCT 1 ILCT 1 ILCT/1MLCT 1 ILCT/1MLCT 1 ILCT/1MLCT 1 ILCT/1MLCT 1 ILCT/1MLCT 1 ILCT/1MLCT 1 ILCT/1MLCT 3

measd value (nm) 533 342 326 283 260

mixed with 1MLCT (metal-to-ligand charge transfer) character, which is mainly responsible for the lowest absorption band at 411 nm. The LUMOþ2 is localized on fpbpy ligands, while the HOMO-1, HOMO-2, HOMO-5, and HOMO7 also primarily reside on the two fpbpy liands with some Ir(5d) compositions. Therefore, the absorption at 324 nm originates from HOMO f LUMOþ2 transition, suggesting an 1ILCT/1MLCT character, which possesses a larger oscillator strength of 0.1553, mainly contributing the measured absorption peak at 326 nm. The absorption at 348 nm is essentially contributed by the HOMO-1 f LUMOþ1 transition with 96% contribution, implying an 1ILCT character. The high-energy absorptions at λabs < 300 nm from HOMO2 f LUMOþ2, HOMO-5 f LUMOþ1, HOMO-7 f LUMOþ1, and HOMO-4 f LUMOþ2 are assigned as the 1 ILCT transition mixed with some 1MLCT character. A comparison of the measured and calculated absorption spectra of 1 in acetonitrile solution is shown in Figure 7. For complex 2, as indicated in Tables S3 and S4, the HOMO is mainly contributed by π(fpbpy) (73.9%) with some Ir(5d) composition (22.2%), while the LUMO is basically localized on the π*(tpy), which is different from that of 1. As a consequence, the lowest-energy absorption transition of 2 is different from that of 1. Therefore, as shown in Table S4, the S1 transition arising from HOMO f LUMO is assigned as a ligand-to-ligand charge transfer (1LLCT) transition mixed with some 1MLCT character. Analogous to the HOMO, the HOMO-1 is also composed of π(fpbpy) and Ir(5d), whereas the LUMOþ1 and LUMOþ3 are mostly contributed by the π*(fpbpy) with ca. 90% contribution. Consequently, the S2 transition coming from HOMO f LUMOþ1 suggests an 1 ILCT transition mixed with some 1MLCT character, which is responsible for the lowest absorption peak at 381 nm, and the absorption at 328 nm originates from HOMO-1 f LUMOþ1 transition with the oscillator strength of 0.1185, supporting a mixed 1ILCT/1MLCT character. Partial molecular orbital compositions of dppy-containing complex 3 are shown in Table S5. The HOMO is mainly contributed by π(dppy) (45.0%) and Ir(5d) (39.0%) with some π(fpbpy) composition (16.0%), while the LUMO is basically localized on the π*(fpbpy). The difference of the composition of the orbitals induces the change of the absorption excitations of 3. Hence, as indicated in Table S6, the lowest-energy absorption at 478 nm contributed by HOMO f LUMO transition can be assigned as being of mixed 1LLCT/1MLCT/1ILCT character. The high-energy absorptions at 275 nm from HOMO-3 f LUMOþ2 and HOMO-2 f LUMOþ3 can be assigned as 1LLCT/1MLCT/1ILCT transitions in nature. The main singlet vertical excitation energies involved in

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Figure 6. Diagrams of energy levels of orbitals and the HOMO and LUMO involved in the absorptions for 1-3 under TD-DFT calculations.

Figure 7. Measured (black solid line) and calculated (red dashed line) absorption spectra of 1 simulated by a Gaussian curve based on the data calculated under the TD-DFT method in acetonitrile media. The singlet excitations calculated are shown as vertical green bars.

absorption of complexes 1-3 from TD-DFT agree well with the experimental values. The energy diagrams of important frontier orbitals and the single electron density diagrams of HOMOs and LUMOs involved in the absorptions for 1-3 are shown in Figure 6. To shed light on the phosphorescent properties of 1-3, the lowest lying triplet excited states were obtained with 3B, 3A00 , 3 0 A geometric structures calculated by full optimizations on 1-3 at the UPBE1PBE level. The electronic structure calculated at the triplet excited-state geometry shows the same orbital pattern calculated for the ground-state geometry, even though the changes in the orbital energies take place (see Tables 4, S3, S5, and S7). According to the optimized excited-state geometries, the emission properties in acetonitrile media were calculated by the time-dependent DFT (TDDFT) method. As indicated in Table 5, the lowest-lying triplet transition of 1 at 532 nm corresponding to an excitation from HOMO to LUMO is in reasonable agreement with the experimentally measured emission (λem = 533 nm) in acetonitrile solution, which is typical of a mixed 3ILCT

Chen et al.

Figure 8. Transitions responsible for the emissions at 532, 528, and 892 nm for 1-3, respectively, calculated in acetonitrile media.

[π(fpbpy) f π*(fpbpy)] and 3MLCT 3[5d(Ir) f π*(fpbpy)] character. The emissions of 2 at 528 nm and 3 at 892 nm are assigned as 3LLCT/3MLCT and 3LLCT/3MLCT/3ILCT mixed transitions, respectively, which coincide well with the low-energy absorption character mentioned above. To provide a clear comparison between calculated and measured values, the data of the low-energy absorptions and emissions of 1-3 calculated by the TD-DFT approach are also listed in Table 3. The HOMO-LUMO energy gaps of the calculated emissions of 1-3 are 3.54, 3.46, and 2.62 eV, respectively (see Table S7 and Figure 8), which are consistent with the measured emission spectral features. Consequently, the ΔE(LUMO-HOMO) values of both the calculated lowestenergy absorptions (3.95, 3.94, and 3.55 eV for 1-3, respectively, see Tables 4, S3 and S5) and emissions of 1-3 exhibit the sequence 1 > 2 > 3 (Figures 6 and 8). Furthermore, by comparing the quantum yields (Table 3) and the energy gaps of HOMO-LUMO (Table S7 and Figure 8), it is noted that the quantum yield increases with the increase of the HOMO-LUMO energy gap. According to the energy gap law,38 the energy difference between HOMO and LUMO increases, the nonradiative efficiency decreases, and the luminescence efficiency increases. Thus, complex 1 shows a high phosphorescent quantum yield (Φem = 0.138). 3

Conclusions In summary, a new series of three iridium(III) complexes featuring the 6-(5-trifluoromethylpyrazol-3-yl)-2,20 -bipyridine (fpbpyH) tridentate ligand, as a novel family of luminescent bis-tridentate complexes, have been designed and synthesized. It is clearly demonstrated that the fpbpyH ligand can be successfully employed for preparation of photoactive iridium(III) complexes as the monoanionic tridentate chelate, confirmed by X-ray crystallography of 1. It is shown that three Ir(III) complexes bearing fpbpy chelates are emissive in both fluid and solid states, and replacement of one fpbpy of 1 has a significant effect on the photophysical properties of the corresponding complexes, as revealed by TD-DFT calculations. We believe that the results presented herein might provide new insight into (38) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 630.

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the further design of tridentate bipyridyl azolate ligands used in the preparation of Ir(III) complexes with potentia lly high luminescent efficiency in the far-visible and NIR regions.

the Start-up Fund of Jiangxi University of Science and Technology (Grant No. 09162). We thank Prof. Yun Chi at National Tsing Hua University for his help in measuring the electrochemical properties.

Acknowledgment. This work was financially supported by the Natural Science Foundations of Jiangxi Province (Grant No. 2009GQH0038) and Fujian Province (Grant No. 2008F3117), the Research Foundation of Education Bureau of Jiangxi Province (Grant No. GJJ10151), and

Supporting Information Available: X-ray crystallographic data file (CIF) of complex 1, characterization data of complexes 1-3, and tables and figures regarding the TD-DFT calculations. This material is available free of charge via the Internet at http:// pubs.acs.org.

Iridium(III) - American Chemical Society

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