Blue Phosphorescence from Iridium(III) Complexes at Room

Sep 26, 2006 - 1H NMR (400.2 MHz, CDCl3): δ 1.02 (3 H, t, J = 7, CH3), 1.82 (2 H, m, CH2), 2.73 (2 H, t, J = 7.5, CH2), 3.93 (3 H, s, NCH3), 7.75 (3 ...
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Chem. Mater. 2006, 18, 5119-5129

5119

Blue Phosphorescence from Iridium(III) Complexes at Room Temperature Shih-Chun Lo,† Christopher P. Shipley,† Raghu N. Bera,‡ Ruth E. Harding,‡ Andrew R. Cowley,† Paul L. Burn,*,† and Ifor D. W. Samuel*,‡ Chemistry Research Laboratory, Department of Chemistry, UniVersity of Oxford, Mansfield Road, Oxford, OX1 3TA, U.K., and Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, UniVersity of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, U.K. ReceiVed May 18, 2006. ReVised Manuscript ReceiVed July 17, 2006

We report a new family of homoleptic iridium(III) complexes that emit blue phosphorescence at room temperature. The iridium(III) complexes are comprised of phenyltriazole ligands and were easily prepared via short synthetic routes. The parent fac-tris(1-methyl-5-phenyl-3-propyl-[1,2,4]triazolyl)iridium(III) complex exhibits blue photoluminescence (PL) with emission peaks at 449 and 479 nm and has a solution PL quantum yield of 66%. The emission was sequentially blue-shifted by the attachment of one and two fluorine atoms to the ligand phenyl ring with the fac-tris{1-methyl-5-(4,6-difluorophenyl)-3-propyl-[1,2,4]triazolyl}iridium(III) complex having the 1931 Commission Internationale de l’Eclairage coordinates of (0.16, 0.12) at room temperature. In contrast, when the phenyl ring of the ligands was substituted by trifluoromethyl, the PL spectrum was red-shifted when compared to the parent compound whereas if the trifluoromethyl group was attached to the triazole ring, the emission was blue-shifted. The radiative rates of these new blue iridium(III) complexes were found to be in the range of 2-6 × 105 s-1, indicating that the emission had varying amounts of metal-to-ligand charge-transfer character. Molecular orbital calculations showed that for the fluorinated complexes the contribution of the ligand triplet character to the emissive energy state increased with the hypsochromic shift in emission. This was confirmed by timeresolved PL measurements, which showed that the complex with the deepest blue emission had the slowest radiative decay rate.

Introduction Phosphorescent materials are very likely to play a major role in bringing organic light-emitting diode (OLED) displays and lighting to a wide market.1,2 Phosphorescent materials are generally based on heavy metal complexes with the most effective being those based on iridium(III).3-8 Iridium(III) complexes emit from states with a large degree of metalto-ligand charge transfer (MLCT) character and have the advantage over fluorescent materials in that both singlet and triplet excitons can emit light. While highly efficient OLEDs based on red4,9 and green2,10 phosphorescent small molecules * Authors to whom correspondence should be addressed. E-mail: paul.burn@ chem.ox.ac.uk (P. L. Burn); [email protected] (I. D. W. Samuel). † University of Oxford. ‡ University of St Andrews.

(1) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. AdV. Mater. 2005, 17, 1109. (2) D’Andrade, B. W.; Forrest, S. R. AdV. Mater. 2004, 16, 1585. (3) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (4) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.; Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622. (5) Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082. (6) Tokito, S.; Iijima, T.; Suzuri, Y.; Kita, H.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett. 2003, 83, 569. (7) Ikai, M.; Tokito, S.; Sakamoto, Y.; Suzuki, T.; Taga, Y. Appl. Phys. Lett. 2001, 79, 156. (8) Yang, X.; Neher, D.; Hertel, D.; Da¨ubler, T. K. AdV. Mater. 2004, 16, 161. (9) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971.

and dendrimers11-15 have been reported, the same progress for saturated blue phosphorescence has not been achieved. The first reported “blue” phosphorescent devices utilized the heteroleptic complexes of bis[2-(2,4-difluorophenyl)pyridyl]iridium(III) with either an acetylacetonate, [FIr(acac)], or a picolinate, (FIrpic), co-ligand.5 The devices had Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.29), which is a deeper blue than that observed for the photoluminescence (PL) spectra of the same emissive material. The blue shift in the observed electroluminescence arose from the fact that the devices contained a copper phthalocyanine hole transport layer that absorbed some of the longer wavelength emission. Nevertheless, the blue-shifted emission with respect to the parent green fac-tris(2-phenylpyridyl)iridium(III) (Irppy3) complex was attributed to the attachment of the two fluorine atoms to the ligand phenyl ring. While this work has led to some very elegant research on emission (10) Gong, X.; Robinson, M. R.; Ostrowski, J. C.; Bazan, G. C.; Heeger, A. J. AdV. Mater. 2002, 14, 581. (11) Markham, J. P. J.; Lo, S.-C.; Magennis, S. W.; Burn, P. L.; Samuel, I. D. W. Appl. Phys. Lett. 2002, 80, 2645. (12) Lo, S.-C.; Male, N. A. H.; Markham, J. P. J.; Magennis, S. W.; Burn, P. L.; Salata, O. V.; Samuel, I. D. W. AdV. Mater. 2002, 14, 975. (13) Anthopolous, T. D.; Markham, J. P. J.; Namdas, E. B.; Lawrence, J. R.; Samuel, I. D. W.; Lo, S.-C.; Burn, P. L. Org. Electron. 2003, 4, 71. (14) Anthopoulos, T. D.; Frampton, M. J.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W. AdV. Mater. 2004, 16, 557. (15) Lo, S.-C.; Richards, G. J.; Markham, J. P. J.; Namdas, E. B.; Sharma, S.; Burn, P. L.; Samuel, I. D. W. AdV. Funct. Mater. 2005, 15, 1451.

10.1021/cm061173b CCC: $33.50 © 2006 American Chemical Society Published on Web 09/26/2006

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color tuning, there is a limitation on how deep a blue is possible using homoleptic and heteroleptic iridium(III) complexes based on the 2-(2,4-difluorophenyl)pyridyl ligand.16-19 Therefore, to achieve deep blue phosphorescence from iridium(III) complexes, new ligand types need to be developed. In this context arylpyrazolyl ligands have been investigated and emission from fac-tris(arylpyrazolyl)iridium(III) complexes has been observed at 77 K with PL peaks at 414 and 390 nm for fac-tris(phenylpyrazolyl)iridium(III) (Irppz3) and the difluorinated derivative, respectively.20 More recently, iridium(III) complexes with carbene-containing ligands have been reported to give short wavelength emission.21 We have been exploring the effect of replacing the pyridyl moiety of the 2-phenylpyridyl ligand with a triazole ring to give homoleptic fac-tris(5-aryltriazolyl)iridium(III) complexes for blue emission. While the pyridyltriazole ligands22 have been used as a co-ligand for heteroleptic iridium(III) complexes,23-25 homoleptic iridium(III) complexes comprising 5-aryltriazole ligands have not yet been investigated as phosphorescent emitters for OLEDs. The logic for replacing pyridyl with triazolyl was based on molecular orbital calculations of iridium(III) complexes26 and nitrogen-containing cyclic compounds.27 For simple iridium(III) complexes it has been calculated that the lowest unoccupied molecular orbital (LUMO) is almost exclusively ligand-based while the highest occupied molecular orbital (HOMO) has substantial d-orbital character.26 It has also been shown that triazole has a considerably higher LUMO energy than that of pyridine.27 Therefore, replacing the pyridyl moiety with a triazolyl ring in a homoleptic iridium(III) complex with a 5-phenyltriazolyl ligand should give complexes with bluer emission than the corresponding Irppy3-based materials. We also anticipated that the attachment of electron-withdrawing groups such as fluorine to the phenyl ring of the fac-tris(phenyltriazolyl)iridium(III) complexes would result in a further blue-shift in emission in a manner similar to that observed for Irppy3. (16) (a) Holmes, R. J.; D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.; Thompson, M. E. Appl. Phys. Lett. 2003, 83, 3818. (b) Ren, X.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 2004, 16, 4743. (17) Nazeeruddin, Md. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.; Zuppiroli, L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790. (18) Lee, C.-L.; Das, R. R.; Kim, J.-J. Chem. Mater. 2004, 16, 4642. (19) Mak, C. S. K.; Hayer, A.; Pascu, S. I.; Watkins, S. E.; Holmes, A. B.; Ko¨hler, A.; Friend, R. H. Chem. Commun. 2005, 4708. (20) Tamayo, A.; Alleyne, B.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377. (21) Holmes, R. J.; Forrest, S. R., Sajoto, T.; Tamayo, A.; Djurovich, P. I.; Thompson, M. E.; Brooks, J.; Tung, Y.-J.; D’Andrade, B. W.; Weaver, M. S.; Kwong, R. C.; Brown, J. J. Appl. Phys. Lett. 2005, 87, 243507. (22) (a) Yu, J.-K.; Hu, Y.-H.; Cheng, Y.-M.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Carty, A. J.; Tung, Y.-L.; Lee, S.-W.; Chi, Y.; Liu, C.-S. Chem. Eur. J. 2004, 10, 6255. (b) Giuffrida, G.; Ricevuto, V.; Guglielmo, G.; Campagna, S.; Ciano, M. Inorg. Chim. Acta 1992, 194, 23. (c) Dixon, I. M.; Collin, J. P.; Sauvage, J. P.; Flamigni, L.; Encinas, S.; Barigelletti, F. Chem. Soc. ReV. 2000, 29, 385. (d) Fanni, S.; Keyes, T. E.; O’Connor, C. M.; Hughes, H.; Wang, R. Y.; Vos, J. G. Coord. Chem. ReV. 2000, 208, 77. (23) Igarashi, T. U.S. Patent 0134984, 2002. (24) Coppo, P.; Plummer, E. A.; De Cola, L. Chem. Commun. 2004, 1774. (25) Yeh, S. J.; Wu, M.-F.; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho, M.-H.; Hsu, S.-F.; Chen, C. H. AdV. Mater. 2005, 17, 285. (26) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634. (27) Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi, S. J. Am. Chem. Soc. 1996, 118, 11974.

Lo et al. Scheme 1a

a A: Ethyl butyrimidate hydrochloride, NEt , CHCl , r.t., Ar 3 3 (g) or N2(g) and then methylhydrazine hydrate, CHCl3, r.t., Ar(g) or N2(g). B: Ethyltrifluoroacetate, THF, heat, Ar(g), and then benzamidine, NaOH, heat, Ar(g). C: Iridium(III) chloride trihydrate, water, 2-ethoxyethanol, heat, Ar(g) and then silver trifluoromethanesulfonate, ligand (5-8), heat, Ar(g). D: Iridium(III) acetylacetonate, glycerol, heat, N2(g).

In this paper we report for the first time a new class of homoleptic cyclometalated complexes based on fac-tris(5aryl-1,3-disubstituted-[1,2,4]triazolyl)iridium(III) that give blue phosphorescence at room temperature. The synthesis and characterization of the complexes as well as their photophysical and electrochemical properties are described. We discuss effects of ligand substituents on the color and efficiency of the emission and correlate these properties with the calculated electronic properties of the new complexes and compare them with those of Irppy3. We also consider possible nonradiative decay mechanisms including the role of vibrational quenching. Results and Discussion The first step in the study was the synthesis of the parent fac-tris(1-methyl-5-phenyl-3-propyl-1H-[1,2,4]triazolyl)iridium(III) to determine the effect of exchanging the pyridyl moiety with the triazole ring on the emission properties of a homoleptic iridium(III) complex. The second aspect of the work focused on elucidating the effect of electronwithdrawing substituents, namely, fluoro- and trifluoromethyl groups on the emission color and efficiency. The iridium(III) complexes developed in this study are shown in Scheme 1 and include complexes that contain an unsubstituted phenyl ring, 10, or a phenyl ring bearing one (11) or two fluorine atoms (12), or a trifluoromethyl group (13). In addition, the iridium(III) complex (14) with a trifluoromethyl group attached to the triazole ring was prepared to investigate the effect of substituent position on the properties of the complexes. Synthesis and Physical Properties. The syntheses of the five iridium(III) complexes are summarized in Scheme 1. The methods for forming 1,3,5-trisubstituted-[1,2,4]triazoles are well-known,28 and it was found that the desired ligands could be prepared in two steps. For the n-propyltriazole (28) Katritzky, A. R.; Rogovoy, B. V.; Vvedensky, V. Y.; Kovalenko, K.; Steel, P. J.; Markov, V. I.; Forood, B. Synthesis 2001, 897.

Blue Phosphorescence from Iridium(III) Complexes

derivatives the first step involved the reaction of a chloroform solution of the requisite benzoyl chloride with ethylbutyrimidate hydrochloride in the presence of triethylamine. The intermediate N-benzoylbutanimidic acid ethyl esters were then directly reacted with methylhydrazine hydrate in chloroform at room temperature to give the corresponding phenyltriazolyl ligand. Following this procedure 1-methyl-3-propyl5-phenyl-1H-[1,2,4]triazole (5) was formed from benzoyl chloride 1 in an overall yield of 59% for the two steps. 1-Methyl-3-propyl-5-(4-fluorophenyl)-1H-[1,2,4]triazole (6), 1-methyl-3-propyl-5-(2,4-difluorophenyl)-1H-[1,2,4]triazole (7), and 1-methyl-3-propyl-5-(4-trifluoromethylphenyl)-1H-[1,2,4]triazole (8) were similarly prepared from 2, 3, and 4 in overall yields of 62%, 74%, and 63%, respectively. The good yields of one regioisomer in the formation of 5-8 is due to the steric preference between the two amino groups of methylhydrazine in the final step.29 In addition to the required ligands, the 2-methyl-3-propyl-5-aryl-1H-[1,2,4]triazole regioisomers were also generally formed in yields ranging from 5 to 20%, but these could be easily separated from the desired materials by column chromatography. The different regioisomers were identified by 1H NMR using the Nuclear Overhauser Effect (NOE). For example, for 5 there was a significant NOE between the methyl protons and the ortho phenyl protons, while for its regioisomer, 2-methyl3-propyl-5-phenyl-1H-[1,2,4]triazole, there were strong NOEs observed between the methyl protons and the n-propyl protons. 1-Methyl-5-phenyl-3-trifluoromethyl-1H-[1,2,4]triazole (9) was prepared following a modified literature procedure.30 Sequential reaction of ethyltrifluoroacetate with methylhydrazine hydrate in tetrahydrofuran heated at reflux followed by treatment of the resultant intermediate with benzamidine under basic conditions gave 9 in an overall yield of 32%. The two-step procedure commonly used for the preparation of iridium(III) complexes31 was employed to synthesize the n-propyl-derivatized iridium(III) complexes [10-13]. First, each ligand was reacted with iridium(III) chloride trihydrate in a water/2-ethoxyethanol mixture heated at reflux and then the formed bis-iridium chloro-bridged dimers were reacted with an excess of the ligand in the presence of silver trifluoromethanesulfonate. Under these conditions iridium(III) complexes 10, 11, 12, and 13 were produced in yields of 83%, 89%, 73%, and 86%, respectively. These are excellent yields for this type of reaction for the formation of iridium(III) complexes. Complex 14 was synthesized in a 34% yield in a single step by treating 9 with iridium(III) acetylacetonate in glycerol heated at reflux.32 All the new complexes were assigned as facial isomers by a combination of X-ray crystallography and 1H NMR. Crystal structures of 10 and 11 (see supplementary information) showed that they were facial isomers. 12, 13, and 14 were also assigned as the facial isomers by analogy given the similarity of their 1H NMR (29) Kelarev, V. I.; Silin, M. A.; Kobrakov, K. I.; Rybina, I. I.; Korolev, V. K. Chem. Heterocyc. Compd. (English Translation of Khim. Geterotsikl. Soedin.) 2003, 39, 736. (30) Funabiki, K.; Noma, N.; Kuzuya, G.; Matsui, M.; Shibata, K. J. Chem. Res. (S). 1999, 300. (31) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767. (32) Dedeian, K.; Djurovich, P. I.; Garces, F. O.; Carlson, G.; Watts, R. J. Inorg. Chem. 1991, 30, 1685.

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spectra to those of 10 and 11. It is interesting to note that there was a difference in 1H NMR signals for the methylene protons of the n-propyl groups between the ligands and the corresponding iridium(III) complexes 10-13. For the ligands there were three sets of well-defined proton signals for the n-propyl group consistent with the three different proton environments. However, on formation of the iridium(III) complexes the proton signals on each of the n-propyl methylene groups are split into two distinctive sets, indicating that the methylene protons are diastereotopic. This would suggest that formation of the iridium(III) complex restricts rotation of the side chain of the ligands. Thermal gravimetric analysis (TGA) showed that all complexes, 10-14, exhibit good thermal stabilities. A weight loss of 5% was found in the temperature range of 318340 °C for 10, 11, 13, and 14. A slightly lower temperature (267 °C) was observed for the 5% weight loss for the bisfluorinated complex 12. Molecular Orbital Calculations. The molecular orbital densities and energies of the complexes were calculated using the Gaussian03 (Revision C.01) suite of programs,33 utilizing B3LYP34 theory and the LANL2DZ35-38 basis set as employed therein. Geometries were optimized to energy minima with a C3 point group enforced. This is the same method used for the calculation of the orbitals of Irppy3.26 The orbital surfaces were visualized with Molekel 4.339 and Mulliken population analyses were performed using AOMix.40,41 In our calculations we have included a contribution to the frontier molecular orbitals from the iridium 6d orbitals as they lie relatively close in energy to the 5d orbitals. The results of the calculations on Irppy3 and compounds 10-14 are summarized in Table 1. For Irppy3 we have calculated that its highest occupied molecular orbital (HOMO) has 53% d-orbital character with the 6d orbitals providing an 8% contribution, which is similar to that previously reported.26 The remaining HOMO character of Irppy3 is comprised of ligand π-orbitals of which 8% resides on the pyridyl ring (33) Frisch, M. J.; Trucks, F. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; 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.01; Gaussian, Inc.: Wallingford, CT, 2004. (34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (35) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1. (36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (37) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284. (38) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (39) (a) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. MOLEKEL 4.3, Swiss Center for Scientific Computing, Manno, Switzerland, 2000-2002. (b) Portmann, S.; Lu¨thi, H. P. Chimia 2000, 54, 766. (40) Gorelsky, S. I. AOMix; Program for Molecular Orbital Analysis; York University: Toronto, ON, 1997; http://www.sg-chem.net. (41) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635, 187.

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Lo et al.

Table 1. Calculated Orbital Energies and Distribution, and Excited State Energiesa calculated orbital energies (eV)

LUMO+1 LUMO HOMO HOMO-1 HOMO-2

calculated orbital contributions

LUMO

HOMO

calculated S1 excited T1 state ener- L3 gies (eV)

Ir Py or Tz Ph Ir Py or Tz Ph

Irppy3

10

11

12

13

14

-1.38 -1.47 -4.95 -5.12 -6.01

-0.98 -1.14 -4.94 -5.17 -6.12

-1.30 -1.46 -5.42 -5.67 -6.43

-1.68 -1.82 -5.87 -6.11 -6.80

-1.67 -1.83 -5.55 -5.80 -6.81

-1.54 -1.68 -5.51 -5.76 -6.68

0.2 73.5 25.9 52.8 8.2 38.9

0.1 45.3 52.3 56.9 9.4 33.0

0.0 46.7 49.2 56.6 10.7 31.8

0.0 45.5 49.2 57.0 11.3 30.9

0.1 36.8 56.1 58.6 9.7 30.8

0.1 51.3 46.1 55.0 8.5 35.9

2.72 2.51 3.00

3.05 2.79 2.99

3.18 2.89 3.07

3.29 2.97 3.10

2.92 2.66 3.00

3.11 2.84 3.16

aIr ) iridium 5d and 6d orbitals, Ph ) phenyl, Py ) pyridyl, and Tz ) triazolyl.

with the remaining 39% being on the phenyl ring. The calculations also show that the lowest unoccupied molecular orbital (LUMO) of Irppy3 has 99% ligand character with 74% of the orbital density on the pyridyl ring and 26% on the phenyl ring. These results explain why the attachment of groups to the pyridine that destabilize the LUMO and groups on the phenyl ring that stabilize the HOMO can have a strong effect on the emission color, and in particular blue-shift the spectrum. In addition, and perhaps more importantly, the fact that the HOMO has significant ligand orbital density means that the transition to the lowest excited state must necessarily have ligand character. That is, the emission of Irppy3 cannot be considered purely as a metal-to-ligand charge transfer (MLCT) transition. For complexes 10-14 the iridium d-orbitals contribute 55-59% to the HOMOs with the remaining orbital density being found on the ligand (31-36% on the phenyl ring and 8-11% on the triazole ring). This is similar to the case of Irppy3, although there is slightly higher orbital density on the iridium(III) for the phenyltriazolyl complexes. However, there are differences between Irppy3 and 10-14 in the LUMO distribution. For all the complexes there is only a small amount of iridium(III) d-orbital character in the LUMO. However, with the phenyltriazolyl complexes there is a larger proportion of the LUMO density on the phenyl ring when compared to Irppy3. For 10, 11 (fluorophenyl), and 12 (difluorophenyl) the LUMO density is slightly higher on the phenyl (49-52%) than the triazolyl (45-47%) ring. The change in LUMO distribution is greatest for 13 (trifluoromethylphenyl), which has 56% of the LUMO orbital density on the phenyl ring, in contrast to Irppy3, which only has 26% of the LUMO density on the phenyl ring. Complex 14 (trifluoromethyltriazolyl) differs from the other phenyltriazolyl complexes in having a greater orbital density on the triazolyl ring (51%). What these calculations clearly show is that the nature of the HOMO-LUMO transition is going to be significantly different not only between Irppy3 and the fac-tris(phenyltriazolyl)iridium(III) complexes but also between the group of 10, 11, and 12 that have similar molecular orbital distributions, and 13 and 14. The differences between the complexes of their HOMO and LUMO distribution and electronic transitions means that caution needs to be exercised

when determining trends in the photophysical properties of the materials. The fact that the LUMO density is less localized on the heteroaryl ring in the phenyltriazolyl complexes means that addition of a substituent onto the phenyl ring will have a strong effect not only on the HOMO but also on the LUMO. In addition, as with Irppy3 the large contribution of the ligand to the HOMO of the complex results in the excitations not being pure MLCT transitions. The calculations also confirmed our initial conjecture that when the ligand pyridyl moiety is changed to the triazole ring in the unsubstituted complexes, the color of light emission should be blue-shifted on moving from Irppy3 to 10 due to an increase in the LUMO energy. The HOMO energies of Irppy3 and 10 are essentially the same at -4.95 and -4.94 eV, respectively, while the LUMO energy of 10 at -1.14 eV is just over 0.33 eV higher in energy than that of Irppy3. Attachment of a fluorine atom on the phenyl moiety para to the triazole ring (compound 11) results in stabilization of both the LUMO and the HOMO with respect to 10, with the stabilization of the HOMO being slightly greater. The fact that both the HOMO and LUMO are stabilized can be understood from the calculation of the orbital distributions. Both the HOMO and LUMO have substantial orbital density on the phenyl ring and hence a substituent attached to the phenyl ring will strongly affect the energy of both orbital sets. This is in contrast to the case of Irppy3 where the LUMO has a much smaller amount of orbital density on the phenyl ring and hence the attachment of a substituent on the phenyl ring would be expected to have a much stronger effect on the HOMO than the LUMO. The stabilization of the HOMO and LUMO was greater for the difluorinated complex 12 with the HOMO at -5.87 eV and LUMO at -1.82 eV, and the HOMO again being stabilized to a slightly larger extent. Therefore, in the case of 10, 11, and 12 the calculations show that the HOMOLUMO energy gap increases across the series (3.80, 3.96, and 4.05 eV, respectively) and hence bluer emission should be observed in moving from 10 to 12. For 13 with the trifluoromethyl group on the phenyl ring the situation is slightly different. Although 12 and 13 have similarly stabilized LUMOs, the HOMO of 13 is less stabilized relative to 12 by 0.32 eV, leading to a smaller HOMO-LUMO energy gap (3.72 eV); hence, the emission color of 13 would be expected to be less blue than 10. In contrast, when the trifluoromethyl group moiety is attached to the triazole ring in 14, the HOMO energy does not change appreciably when compared to 13 but the LUMO is destabilized so that the HOMO-LUMO energy gap is 3.83 eV, implying that the emission color should be similar to that of 10. Finally, to help understand the nature of the emissive state, we have calculated the singlet (S1) and lowest excited triplet state (T1) energies of the complexes and the ligands (L3) with the results summarized in Table 1. Electrochemistry. We have used cyclic voltammetry to confirm the trends of the HOMO energies of the complexes determined by the calculations with the results summarized in Table 2. It should be noted that we were not able to observe chemically reversible reductions for the blue materials. The calculations show that the HOMO energies of Irppy3

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Table 2. Summary of the Photophysical and Electronic Properties of the Complexes estimated Huang-Rhys factorb complex 10 11 12 13 14

PL peaks

(nm)a

449, 479 428, 456 425, 450 462, 495 442, 470

PLQYs

(%)a

66 ( 7 27 ( 5 3(1 67 ( 7 5.5 ( 2

PL CIE (x,

y)a

(0.16, 0.20) (0.16, 0.13) (0.16, 0.12) (0.18, 0.30) (0.16, 0.16)

τ (µs) 1.08 ( 0.03 1.25 ( 0.3 0.15 ( 0.07 2.20 ( 0.05 0.15 ( 0.07

kr

(×105 s-1)

6.1 ( 0.8 2.2 ( 0.9 2.0 ( 1.6 3.0 ( 0.4 3.7 ( 3

knr

(×105 s-1)

3.1 ( 1 5.8 ( 2.8 65 ( 33 1.5 ( 0.5 63 ( 34

method a

method b

E1/2 (ox)c (V)

1.4 2.2 1.9 1.4 1.6

1.0 1.5 1.2 0.9 1.1

0.28 0.50 0.72 0.61 0.59

a Measured in toluene. b Using S ) -ln(I(0-0)/I(total)), from 77 K spectra, Figure 2, assuming linear coupling and harmonic oscillators. I(0-0) estimated by method a: 2 × integrated area from short wavelength onset to I(0-0) maximum. Method b: integrated area from short wavelength onset to minimum between I(0-1) and I(0-0). c Quoted against the ferricenium/ferrocene couple.

Figure 1. Solution absorption spectra of 10-14 (dichloromethane).

emission peak at around 510 nm.19 The blue shift in the PL spectra of 10-14 relative to Irppy3 is consistent with the molecular orbital calculations, which show larger HOMOLUMO energy gaps for the phenyltriazolyl-based complexes. The blue shift in the emission can be more easily seen by comparing the CIE coordinates of the materials. The CIE coordinates of Irppy3 are (0.28, 0.62)5 while for 10 the coordinates are (0.16, 0.20). It is interesting to note that complex 10 is bluer than the reported blue-emitting heteroleptic cyclometalated complexes based on bis[2-(2,4-difluorophenyl)pyridyl]iridium(III) with acetylacetonate (0.17, 0.34),6 tetrapyrazolylborate (0.16, 0.26),18 and pyridyltetrazolyl (0.15, 0.24) co-ligands.25 The molecular orbital calculations show that the HOMO-LUMO energy gap increases for the complexes with mono- (11) and difluorinated (12) ligand phenyl rings and this is seen in the PL spectra and reflected in the CIE coordinates. At room temperature 11 has PL peaks at 428 and 456 nm and CIE coordinates of (0.16, 0.13) with 12 having PL peaks at 425 and 450 nm and coordinates of (0.16, 0.12). These color chromaticities are sufficient to provide a fully saturated blue for full color OLED displays. Complex 13 (trifluoromethylphenyl) has CIE coordinates of (0.18, 0.30) and 14 (trifluoromethyltriazolyl) has coordinates of (0.16, 0.16) with the variation in color consistent with the HOMO-LUMO energy gap determined from the molecular orbital calculations.

and 10 are similar. The electrochemistry confirms this with the oxidation potentials of Irppy3 and 10 being 0.26 V42 and 0.28 V, respectively. An increase in stability of the HOMO of 11 (fluorophenyl) over 10, calculated to be 0.48 eV, is consistent with the electrochemistry, which shows the oxidation of 11 occurs at a potential 0.22 V more positive than that of 10. The difluorophenyl-derivatized complex 12 has an oxidation potential of 0.72 V, a shift of 0.22 V relative to 11, which again follows the trend in the calculations of a more stabilized HOMO. The oxidations of 13 (trifluoromethylphenyl) and 14 (trifluoromethyltriazolyl) are similar at 0.61 and 0.59 eV, respectively, in line with the calculations as well as being intermediate between the oxidation potentials of 11 and 12, which again is consistent with calculated trends in the HOMO energies. The electrochemistry therefore gives confidence that the calculations represent a reasonable trend in the orbital energies. Photophysical Properties. The absorption spectra of the five complexes are shown in Figure 1. In common with most iridium(III) complexes the absorption spectra can be divided into two regions: the short wavelength region below around 280 nm being due to the π-π* transitions of the ligands, with the longer wavelength absorptions being due to the “MLCT” transitions. The room-temperature photoluminescence (PL) spectra of complexes 10-14 are shown in Figures 2a and 2b. It can be clearly seen that the PL maxima are all blue-shifted when compared with Irppy3, which has an

We further probed the photophysical properties of the complexes by measuring their PL quantum yields (PLQYs) in dilute degassed toluene solutions at room temperature. Across the compounds 10-12, a large reduction in PLQY was observed from 66% for the nonfluorinated compound (10) to 27% for the monofluorinated compound (11) to 3% for the difluorinated compound (12) (Table 2). That is, for this group of materials, we observe a quenching of emission with blue shift. For materials 13 and 14, the PLQYs are 67% and 5%, respectively, also suggesting a quenching of emission with blue shift. The decrease of PLQY with movement to deeper blue phosphorescence seen for 10, 11, and 12 is similar to that observed in other blue emissive iridium(III) complexes.19,24,43 Nonetheless, it should be noted that the PLQY of 66% for 10 is excellent for a phosphorescent blue compound with CIE coordinates of (0.16, 0.20) and compares well with the PLQYs of Irppy3 and the complexes containing the 2-(2,4-difluorophenyl)pyridyl ligand.19,24,43

(42) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov, V. A.; Wang, Y. Chem. Commun. 2001, 1494.

(43) Dedeian, K.; Shi, J.; Shepherd, N.; Forsythe, E.; Morton, D. C. Inorg. Chem. 2005, 44, 4445.

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Figure 2. PL spectra in 2-methyltetrahydrofuran measured for the following temperatures and materials (the excitation wavelength in all cases is 360 nm). (a) Room temperature, 10-12. (b) Room temperature 10, 13, and 14. (c) 77 K, 10-12. (d) 77 K, 10, 13, and 14.

To develop an understanding of the factors that cause the decrease in PLQY with the blue shift in emission, we carried out a number of different measurements. The first of these was to determine the phosphorescence lifetimes (τ) of the complexes at room temperature (Table 2). For 10 and 11 (non- and monofluorinated) the measured lifetime was just above 1 µs, while for 12 (difluorinated), the lifetime was around 10 times shorter (∼0.15 µs). Complex 13 (trifluoromethylphenyl) had a slightly longer lifetime (2.2 µs) while the lifetime of 14 (trifluoromethyltriazolyl) was approximately a factor of 10 times shorter (∼0.15 µs). By combining eqs 1 and 2 below and using the measured PLQY and lifetime data at room temperature, it is possible to calculate the radiative (kr) and nonradiative (knr) decay rates and these are summarized in Table 2 for complexes 10-14. PLQY )

kr (kr + knr)

(1)

1 ) kr + knr τ

(2)

Between 10 and the fluorinated complexes 11 and 12, the radiative decay rate was found to decrease by a factor of approximately 2 (Table 2). At the same time the nonradiative rate was observed to increase substantially across the series from 10 to 12, doubling from 10 to 11 and increasing by a factor of approximately 20 from 10 to 12. For the trifluoromethyl-substituted complexes 13 and 14, the radiative decay rates were reasonably similar, while the nonradiative decay rate was approximately 40 times faster for 14 (trifluoromethyl group on the triazolyl) with respect to 13. We next investigated the activation energy for the quenching of the radiative decay for 11 and 12. The PL lifetime of complexes 11 and 12 were measured as a function of temperature between room temperature and 77 K by fitting a singleexponential decay to the PL decay as a function of time (Figure 3a shows the data for 12). The measured PL lifetime

Blue Phosphorescence from Iridium(III) Complexes

Chem. Mater., Vol. 18, No. 21, 2006 5125

Figure 4. Activation energy plot of 11 and 12 dissolved in 2-methyltetrahydrofuran.

Figure 3. (a) PL decay of 12 dissolved in 2-methyltetrahydrofuran at different temperatures. (b) PL lifetimes at different temperatures.

increases with decreasing temperature such that for monofluorinated complex 11 the lifetime doubles between 300 and 270 K (2.2 µs) and then increases gradually to 3.2 µs at 77 K. For difluorinated complex 12 the measured lifetime increases from 0.15 to 3.4 µs between 290 and 200 K, and below 200 K it is constant (Figure 3b). In analyzing these data we have assumed that the radiative decay rate is constant over the temperature range. The nonradiative decay rates extracted from the PL decay measurements were plotted in an Arrhenius plot (Figure 4) and the data between 270 and 300 K, which show the most change in lifetime, were fitted to find activation energies for quenching. For 11 and 12 the activation energy for luminescence quenching was determined to be 0.27 and 0.67 eV, respectively. Finally, the PL spectra for all the compounds were measured at both 77 K and room temperature in 2-methyltetrahydrofuran (Figures 2a-d). For all the complexes very

little shift in the highest energy peak wavelength at 77 K with respect to room temperature (6-10 nm) was observed, although at 77 K the vibrational structure in the emission spectra became more defined. In the PL spectra of the complexes the highest energy peak corresponds to the 0-0 transition with the next peak being the 0-1 transition. The energy difference between them corresponds to the energy, hω, of the dominant vibrational mode, which is also emitted in the transition due to a difference in nuclear configuration between the excited state and ground state, known as vibronic coupling. For the complexes in this study the difference in energies between the main 0-0 to 0-1 peaks corresponds to a C-C ring deformation mode. Additional structure due to different vibrational modes is also seen within the lowtemperature emission spectra for each of the complexes, suggesting that vibronic coupling is occurring to different modes in each of the complexes. The degree of vibronic coupling is often quantified by the Huang-Rhys “S” factor.44 At low temperatures this may be obtained by dividing the intensity of the (0-1) transition by the (0-0) transition. In our materials more than one vibronic mode is apparent so that the Huang-Rhys factor cannot be obtained just from the heights of the 0-1 and 0-0 peaks. Since the peaks overlap, it is not easy to estimate their individual intensities and hence we measure the combined low-temperature Huang-Rhys factor “S” using I(0-0) ) Itotal e-S. We have estimated the maximum and minimum intensity of I(0-0) and this gives two sets of Huang-Rhys factors Sa and Sb (Table 2). In materials 11 and 12, the amount of vibronic coupling from either set of estimated Huang-Rhys factors is seen to be larger than that in material 10, and likewise the amount of vibronic coupling in material 14 is larger than that in material 13. We now consider how each of the experimental results leads to an understanding of the factors that affect the emissive properties of the complexes. The observed decrease in (44) Huang, K.; Rhys, A. Proc. R. Soc. London, Ser. A 1950, 204, 406.

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radiative decay rate in going from complex 10 to 11 and 12 indicates increasing ligand character in the transition. This is consistent with the calculated decrease in energy difference between the T1 and the lowest excited-state ligand (L3) energy across these three complexes (Table 1). This change explains part of the decrease in PLQY with blue shift in our materials as slower radiative decay competes less effectively with nonradiative decay. However, particularly in material 12 with respect to 10 and in material 14 with respect to 13, the quenching with blue shift is primarily due to the increased nonradiative decay rate. This is different from previously reported work on a family of iridium(III) complexes where the PLQY is primarily determined by the radiative decay rate.45 The question that therefore arises is what causes this increase in the nonradiative decay rate in the complexes of this study with increasing emission energy. A possible source of quenching in iridium(III) complexes is excitation into unoccupied metal d-orbitals. This explanation is not supported by the molecular orbital calculations, which show that there are no low lying unoccupied molecular orbitals with substantial d-orbital character for 10, 11, and 12 with an energy difference close to the determined quenching activation energies. In addition, there are no dramatic differences in the d-orbital contribution to these higher energy orbitals. The second possible source of nonradiative decay is the conversion of the energy difference between the excited state and ground state into the energy of multiple vibrations in the ground state. To explain the observed increase in nonradiative decay with blue shift, the vibrational decay would be required to increase with emission energy. The nonradiative vibrational decay rate typically has a near exponential dependence on the energy gap, referred to as the energy gap law. This is clearly not the case in our materials, which at first sight suggests that vibrational decay is not responsible for the quenching in our materials. However, it is important to understand the limits of applicability of the energy gap law before discounting vibronic quenching of the luminescence in the complexes of this study. First, the energy gap law works best when comparing a series of materials with identical vibrational modes such as those with identical chromophoric ligands.46 For 11 and 12 extra fluorine atoms are added to the ligands, which may allow extra vibronic coupling to new modes and account for why the energy gap law does not fit. Such an effect was not observed in a family of tris(phenylisoquinolinyl)iridium(III) complexes where the nonradiative decay was reported to qualitatively fit the energy gap law, including one complex that had a fluorine atom attached to the para position of the chromophoric ligand.47 However, it is not clear whether this is relevant to our phenyltriazolyl compounds as the effect of a substituent on the vibronic coupling will depend on the molecular orbital distributions which are different for the (45) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 1713. (46) Cummings, S. D.; Eisnenberg, R. J. Am. Chem. Soc. 1996, 118, 1949. (47) 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.

Lo et al.

phenyltriazolyl and phenylisoquinolinyl iridium(III) complexes. The second key issue in determining whether vibrational quenching is occurring for the tris(phenyltriazolyl)iridium(III) complexes is whether the molecules fall into the weak coupling limit for which the energy gap law is applicable. In the model of Englman and Jortner, the weak coupling limit for which their exponential energy gap law applies corresponds to a Huang-Rhys factor that is approximately 1 for materials 11, 12, and 14, suggesting that the weak coupling limit may not be applicable. According to that model, vibrational decay may be thermally activated in the strong coupling limit with Arrhenius-type behavior at high temperatures.48 This could qualitatively explain the thermal activation of nonradiative decay observed in materials 11 and 12 (Figure 4). Small but significant differences in the vibronic coupling to different modes between complexes suggest that different thermally activated vibrational nonradiative decay paths could exist, depending on the complex in question. This is consistent with the Arrhenius plot (Figure 3), which shows different activation energies between materials 11 and 12 and could therefore account for the large differences in room-temperature nonradiative decay rate between these materials. In summary, vibrational nonradiative decay appears to be a plausible explanation for the quenching of the PL. It appears that the complexes are in the strong vibronic coupling regime, meaning the energy gap law is not followed and that thermally activated vibrational decay may account for the changes in the room-temperature nonradiative decay rate, which was observed with increasing blue shift of the emission. Conclusion We have developed a new family of iridium(III) complexes that can be easily synthesized in good yields. Roomtemperature blue phosphorescent emission was observed for all the complexes, although the PLQY decreased with increasing emission energy. Molecular orbital calculations indicated that this could be due to increased ligand triplet energy in the emissive energy state, which would decrease the radiative decay rate of the materials. Time-resolved measurements showed that while the radiative lifetimes did decrease, consistent with the molecular orbital calculations, an increase in the nonradiative decay rate was primarily responsible for the quenching of the luminescence. The mechanism for the quenching of the luminescence has been attributed to thermally activated vibrational decay. Experimental Section Synthesis of Organic Materials. Unless otherwise noted, all chemicals were obtained from commercial suppliers and used as received. Melting points were measured in a glass capillary on a Gallenkamp apparatus and are uncorrected. The 1H NMR spectra were measured in deuterated chloroform with either Bruker DPX 400 MHz, AV 400 MHz, or AV 500 MHz spectrometers. All J (48) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145.

Blue Phosphorescence from Iridium(III) Complexes values are quoted in Hertz. Microanalyses were carried out in the Inorganic Chemistry Laboratory, Oxford, UK. The UV-visible absorption spectra were recorded as solutions in HPLC-grade dichloromethane with a Perkin-Elmer UV-vis Lambda 25 spectrometer. Mass spectra were recorded on a Fisons Platform for ESI and Fisons AutoSpec for FAB. Thermal gravimetric analysis was performed on a Perkin-Elmer thermogravimetric analyzer TGA7. Light petroleum refers to the fraction of boiling point 40-60 °C. When solvent mixtures are used for chromatography over silica, the proportions are given by volume. Electrochemical Measurements. Electrochemistry was performed using an EG&G Princeton Applied Research potentiostat/ galvanostat model 263A. All measurements were made at room temperature on samples dissolved in dichloromethane with 0.1 M tetra-n-butylammonium tetrafluoroborate as the electrolyte. The sample concentration was 1 mM, and a platinum working electrode, platinum counter electrode in 0.1 M tetra-n-butylammonium tetrafluoroborate in tetrahydrofuran, and a Ag/0.1 M AgNO3 in acetonitrile reference electrode were used. The scan rate was 40 mV/s. The electrolyte was purified by recrystallization from a mixture of ethyl acetate and diethyl ether. The solutions were deoxygenated with argon. The ferricenium/ferrocene couple was used as the standard49 and the ferrocene was purified by sublimation. All potentials are quoted relative to the ferricenium/ferrocene couple. In all cases several scans were carried out to confirm the chemical reversibility of the redox processes. Photophysical Studies. For room-temperature measurements, samples were dissolved in spectroscopic-grade toluene in quartz degassing cuvettes, degassed by three freeze-pump-thaw cycles, sealed under vacuum, and warmed to nominal room temperature in a bath of tepid water. The optical density (OD) of the samples and standard were similar and small (e0.1 at g360 nm). For lowtemperature measurements, sample preparation was identical except that spectroscopic-grade anhydrous 2-methyltetrahydrofuran was used as the solvent and larger quartz degassing cuvettes were used. Photoluminescence spectra in solution were recorded using a Jobin Yvon Fluoromax 2 fluorimeter, at the highest spectral resolution, using an excitation wavelength of 360 nm. Spectra were corrected after measurement using the emission calibration obtained from measuring a calibrated lamp spectrum. PLQYs were measured by a relative method using quinine sulfate in 0.5 M sulfuric acid as a standard.50 The error in this method is estimated to be approximately 10% of the measured value. Photoluminescence transient decays were measured by a time-correlated single-photon counting (TCSPC) system. Excitation was at 393 nm by a pulsed light-emitting diode (Picoquant LDH 400) giving 10 pJ/pulse at a pulse repetition rate of 100 kHz. The emitted light was dispersed in a monochromator with an entrance slit and detected with a cooled Hamamtsu microchannel plate photomultiplier tube RU-3809U50. The average number of photons collected per pulse was 0.05 or less and the intensity at time ) tmax was 275 °C. TGA(5%) 318 °C. λmax (CH2Cl2)/nm: 248 (log /dm3 mol-1 cm-1 4.96), 262 sh (4.80), 274 sh (4.62), 296 (4.24), 348 (4.22), 376 sh (3.88), 413 sh (3.49), and 442 sh (2.86). 1H NMR (400.2 MHz, CDCl3): δ 0.69 (9 H, t, J ) 7.5, CH3), 1.09-1.22 (3 H, bm, CH2), 1.30-1.44 (3 H, bm, CH2), 1.82-1.94 (3 H, m, CH2), 2.14-2.26 (3 H, m, CH2), 4.18 (9 H, s, NCH3), 6.61 (3 H, d, J ) 7.5, phenyl H), 6.81 (3 H, dd, J ) 7.5 and J ) 7.5, phenyl H), 6.90 (3 H, dd, J ) 7.5 and J ) 7.5, phenyl H), 7.52 (3 H, d, J ) 7.5, phenyl H). 13C NMR (100.6 MHz, CDCl3): δ 13.8, 21.8, 29.0, 37.8, 119.9, 123.4, 129.6, 132.7, 137.4, 158.3, 161.4, 163.2. m/z [ESI+] 793.1 (M+). Anal. Calcd for C36H42IrN9: C, 54.5; N, 15.9; H, 5.3. Found: C, 54.5; N, 15.9; H, 5.4. Excess 5 (798 mg), which co-chromatographed with and had an identical 1H NMR to an authentic sample, was also recovered. fac-Tris[5-(4-fluorophenyl)-1-methyl-3-propyl-[1,2,4]triazolyl]iridium(III) (11). A mixture of 6 (710 mg, 3.24 mmol), iridium chloride trihydrate (457 mg, 1.30 mmol), water (5.5 mL), and 2-ethoxyethanol (16.5 mL) was heated at reflux under argon for 15 h. The mixture was allowed to cool to room temperature and water (20 mL) was added. The mixture was filtered and the residue was washed with water (3 × 10 mL). The residue was collected and dissolved in dichloromethane (∼30 mL), dried over anhydrous sodium sulfate, and filtered, and the solvent was completely removed to give the crude bis-iridium chloro-bridged dimer (897 mg). A mixture of the dimer, 6 (1.30 g, 5.93 mmol), and silver trifluoromethanesulfonate (666 mg, 2.59 mmol) was heated with an oil bath at 166 °C for 17 h under argon. The mixture was allowed to cool to room temperature and purified by column chromatography over silica gel using an ethyl acetate-light petroleum mixture (1:50 to 1:5) followed by a dichloromethane-ethyl acetate-light petroleum (1:4:20) mixture as eluent to give 11 (980 mg, 89%) as a yellow powder. mp > 275 °C. TGA(5%) 320 °C. λmax (CH2Cl2)/ nm: 245 (log /dm3 mol-1 cm-1 4.74), 258 sh (4.60), 269 sh (4.51), 295 (4.18), 331 (4.06), 359 sh (3.80), 384 sh (3.52), and 442 (2.51). 1H NMR (400.2 MHz, CDCl ): δ 0.68 (9 H, t, J ) 7.5, CH ), 3 3 1.05-1.20 (3 H, bm, CH2), 1.27-1.40 (3 H, bm, CH2), 1.78-1.90 (3 H, bm, CH2), 2.11-2.23 (3 H, bm, CH2), 4.16 (9 H, s, NCH3), 6.28 (3 H, dd, J ) 2 and 10.5, phenyl H), 6.63 (3 H, m, phenyl H), 7.48 (3 H, m, phenyl H). m/z [ESI+] 847.0 (M+). Anal. Calcd for C36H39F3IrN9: C, 51.05; N, 14.9; H, 4.6. Found: C, 51.0; N, 14.85; H, 4.7. Excess 6 (620 mg), which co-chromatographed with and

Blue Phosphorescence from Iridium(III) Complexes had an identical 1H NMR to an authentic sample, was also recovered. fac-Tris[5-(4,6-difluorophenyl)-1-methyl-3-propyl-[1,2,4]triazolyl]iridium(III) (12). A mixture of 7 (500 mg, 2.11 mmol), iridium chloride trihydrate (297 mg, 0.84 mmol), water (3 mL), and 2-ethoxyethanol (10 mL) was heated at reflux under argon for 22 h. The mixture was allowed to cool to room temperature and water (10 mL) was added. The mixture was filtered and the residue was washed with water (3 × 10 mL). The residue was dissolved in dichloromethane (∼90 mL), dried over anhydrous sodium sulfate, and filtered, and the solvent was completely removed to give the crude bis-iridium chloro-bridged dimer (590 mg). A mixture of the dimer, 7 (1.00 g, 4.21 mmol), and silver trifluoromethanesulfonate (433 mg, 1.69 mmol) was heated with an oil bath at 166 °C for 23 h under argon. The mixture was allowed to cool to room temperature and then purified by column chromatography over silica gel using a dichloromethane-light petroleum mixture (1:20 to 1:0) followed by an ethyl acetate-dichloromethane mixture (1:40 to 1:4) as eluent to give 12 (556 mg, 73%) as a pale yellow powder. mp 219-220 °C. TGA(5%) 267 °C. λmax (CH2Cl2)/nm: 248 (log /dm3 mol-1 cm-1 4.73), 258 sh (4.65), 274 sh (4.37), 324 (4.04), 354 (3.82), 379 sh (3.50), and 417 (2.48). 1H NMR (400.2 MHz, CDCl3): δ 0.71 (9 H, t, J ) 7.5, CH3), 1.13-1.28 (3 H, bm, CH2), 1.32-1.49 (3 H, bm, CH2), 1.73-1.85 (3 H, m, CH2), 2.07-2.17 (3 H, m, CH2), 4.21 (9 H, d, J ) 8, NCH3), 6.05 (3 H, m, phenyl H), 6.40 (3 H, m, phenyl H). m/z [ESI+] 901.7 (MH+). Anal. Calcd for C36H36F6IrN9: C, 48.0; N, 14.0; H, 4.0. Found: C, 47.9; N, 14.0; H, 4.0. Excess 7 (718 mg), which co-chromatographed with and had an identical 1H NMR to an authentic sample, was also recovered. fac-Tris[1-methyl-3-propyl-5-(4-trifluoromethylphenyl)-[1,2,4]triazolyl]iridium(III) (13). A mixture of 8 (900 mg, 3.34 mmol), iridium chloride trihydrate (471 mg, 1.34 mmol), water (6 mL), and 2-ethoxyethanol (18 mL) was heated at reflux under argon for 21 h. The mixture was allowed to cool to room temperature and water (30 mL) was added. The mixture was filtered and the residue was washed with water (3 × 6 mL). The residue was then dissolved in dichloromethane (40 mL), dried over anhydrous sodium sulfate, and filtered, and the solvent was completely removed to give the crude bis-iridium chloro-bridged dimer (1.02 g). A mixture of the dimer, 8 (3.35 g, 12.4 mmol), and silver trifluoromethanesulfonate (672 mg, 2.62 mmol) was heated with an oil bath at 166 °C for 14 h under argon. The mixture was allowed to cool to room temperature and then purified by column chromatography over silica using a dichloromethane-light petroleum mixture (1:40 to 1:0) as eluent

Chem. Mater., Vol. 18, No. 21, 2006 5129 to give 13 (1.15 g, 86%) as a bright yellow powder. mp > 275 °C. TGA(5%) 318 °C. λmax (CH2Cl2)/nm: 246 (log /dm3 mol-1 cm-1 4.79), 265 sh (4.61), 277 sh (4.38), 295 (4.03), 355 (3.82), 387 sh (3.82), 427 sh (3.35), and 454 sh (2.78). 1H NMR (400.2 MHz, CDCl3): δ 0.70 (9 H, t, J ) 7, CH3), 1.07-1.22 (3 H, bm, CH2), 1.29-1.45 (3 H, bm, CH2), 1.80-1.90 (3 H, m, CH2), 2.15-2.28 (3 H, m, CH2), 4.23 (9 H, s, NCH3), 6.75 (3 H, s, phenyl H), 7.18 (3 H, d, J ) 8, phenyl H), 7.58 (3 H, d, J ) 8, phenyl H). 1H NMR (125.7 MHz, CDCl3): δ 13.6, 21.4, 28.8, 37.9, 117.4 (d, J ) 13), 123.2, 123.9 (q, J ) 277), 130.9 (q, J ) 13), 132.9, 135.5, 156.7, 161.4, 162.0. m/z [FAB+] 995.2 (M+). Anal. Calcd for C39H39F9IrN9: C, 47.0; N, 12.6; H, 3.9. Found: C, 46.7; N, 12.6; H, 4.0. Excess 8 (2.05 g), which co-chromatographed with and had an identical 1H NMR to an authentic sample, was also recovered. fac-Tris(1-methyl-5-phenyl-3-trifluoromethyl-[1,2,4]-triazolyl)iridium(III) (14). A mixture of 9 (1.52 g, 6.69 mmol), iridium acetylacetonate (328 mmol), and glycerol (2.0 mL) was deoxygenated under vacuum and then back-filled with argon. The mixture was heated at reflux under a flow of nitrogen for 40 h before being cooled to room temperature. The mixture was purified by column chromatography over silica using a dichloromethanelight petroleum mixture (1:40 to 1:0) as eluent to give 14 (190 mg, 34%) as a yellow powder. mp > 275 °C. TGA(5%) 340 °C. λmax (CH2Cl2)/nm: 244 (log /dm3 mol-1 cm-1 4.75), 257 sh (4.63), 270 sh (4.40), 288 sh (3.89), 341 (4.01), 369 (3.84), 398 sh (3.55), and 432 sh (2.70). 1H NMR (400.2 MHz, CDCl3): δ 4.29 (9 H, s, NCH3), 6.63 (3 H, dd, J ) 7.5, J ) 1, phenyl H), 6.85 (3 H, ddd, J ) 8, J ) 8, J ) 1, phenyl H), 6.98 (3 H, ddd, J ) 8, J ) 8, J ) 1, phenyl H), 7.60 (3 H, dd, J ) 7.8, J ) 1, phenyl H). m/z [ESI+] 868.4 (M+). Anal. Calcd for C30H21F9IrN9: C, 41.4; N, 14.5; H, 2.4. Found: C, 41.4; N, 14.3; H, 2.5. Excess 9 (1.34 g), which co-chromatographed with and had an identical 1H NMR to an authentic sample, was also recovered.

Acknowledgment. We thank CDT Oxford Ltd., EPSRC, and SHEFC for financial support. We also thank the EPSRC National Service for Computational Chemistry Software (URL: http://www.nsccs.ac.uk). Supporting Information Available: ORTEP diagrams of 10 and 11 and tables of crystallographic data and bond lengths and angles. This material is available free of charge via the Internet at http://pubs.acs.org. CM061173B