Highly phosphorescent iridium complexes containing both tridentate bis(benzimidazolyl)-benzene or -pyridine and bidentate phenylpyridine: Synthesis, photophysical properties, and theoretical study of Ir-bis(benzimidazolyl)benzene complex

Inorg. Chem. 2006, 45, 8907−8921

Highly Phosphorescent Iridium Complexes Containing Both Tridentate

Bis(benzimidazolyl)-benzene or -pyridine and Bidentate Phenylpyridine:

Synthesis, Photophysical Properties, and Theoretical Study of

Ir-Bis(benzimidazolyl)benzene Complex

Shinya Obara,^†Masumi Itabashi,^†Fumio Okuda,^§Satoru Tamaki,^†Yoshiaki Tanabe,^†Youichi Ishii,^†

Koichi Nozaki,*^,‡and Masa-aki Haga*^,†

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo UniVersity, 1-13-27,

Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan, Department of Chemistry, Graduate School of

Science, Osaka UniVersity, 1-16, Machikaneyama, Toyonaka, Osaka 560-0043, Japan, and

Central Research Laboratories, Idemitsu Kosan Co., Ltd., 1280 Kami-Izumi, Sodegaura,

Chiba 299-293, Japan

Received May 10, 2006

+

Novel mixed-ligand Ir(III) complexes, [Ir(L)(N

CCPh; n 0 or 1), were synthesized, where N

(N-phenylbenzimidazolyl)benzene (Phbib), N

∧

C)X]ⁿ(L

)

N

∧

C

∧

N or N

∧

N

∧

N; X ) Cl, Br, I, CN, CH₃CN, or

−

)

∧

C

)

∧

N

bis(N-methylbenzimidazolyl)benzene (Mebib) and bis-

∧

N

∧

N

bis(N-methylbenzimidazolyl)pyridine (Mebip), and N

∧

C

)

phenylpyridine (ppy) derivatives. The X-ray crystal structures of [Ir(Phbib)(ppy)Cl] and [Ir(Mebib)(mppy)Cl] [mppy

5-methyl-2-(2

carbon atom in Me- or Phbib, while the coordinating carbon atom in ppy occupies the trans position of Cl. [Ir-

(Mebip)(ppy)Cl]⁺showed a quasireversible Ir(III/IV) oxidation wave at

1.05 V, while the Ir complexes, [Ir(Mebib)-

(ppy)Cl], were oxidized at C bond in [Ir(Mebib)(ppy)Cl] induces

0.42 V versus Fc/Fc⁺. The introduction of an Ir

a large potential shift of 0.63 V in a negative direction. Further, the oxidation potential of [Ir(Mebib)(Rppy)X] was

altered by the substitution of R, R , and X groups. Compared to the oxidation potential, the first reduction potential

revealed an almost constant value at 2.36 to 2.46 V for [Ir(L)(ppy)Cl] (L Mebib and Phbib) and 1.52 V for

[Ir(Mebip)(ppy)Cl. The UV vis spectra of [Ir(Mebib)(R-ppy)X] show a clear singlet metal-to-ligand charge-transfer

transition around 407 425 nm and a triplet metal-to-ligand charge-transfer transition at 498 523 nm. [Ir(Mebip)-

(ppy)Cl]⁺emits at 610 nm with a luminescent quantum yield of Φ ) 0.16 at room temperature. The phosphorescence

of [Ir(Mebib)(ppy)X] was observed at 526 nm for X CN and 555 nm for X Cl with the high luminescent

quantum yields, Φ ) 0.77 0.86, at room temperature. [Ir(Phbib)(ppy)Cl] shows the emission at 559 nm with a

′

-pyridyl)phenyl] indicate that the nitrogen atom of the ppy ligand is located trans to the coordinating

+

−

′

−

)

−

∼

)

∼

luminescent quantum yield of Φ ) 0.95, which is an unprecedentedly high value compared to those of other

emissive metal complexes. Compared to the luminescent quantum yields of the Ir(ppy)₂(L) derivatives and [Ir-

(Mebip)(ppy)Cl]⁺, the neutral Ir complexes, [Ir(L)(R-ppy)X] (L

)

Me- or Phbib), reveal very high quantum yields and

large radiative rate constants (k_r) ranging from 3.4

× ×

10⁵to 5.5 10⁵s^-1. The density functional theory calculation

suggests that these Ir complexes possess dominantly metal-to-ligand charge-transfer and halide-to-ligand charge-

transfer excited states. The mechanism for a high phosphorescence yield in [Ir(bib)(ppy)X] is discussed herein

from the perspective of the theoretical consideration of radiative rate constants using perturbation theory and a

one-center spin−orbit coupling approximation.

Introduction

emitting diodes (OLEDs).^1,2It has recently been demon-

strated that luminescent cyclometalated Ir(III) complexes

such as Ir(ppy)₃and Ir(ppy)₂(acac) (ppy ) phenylpyridinate

and acac ) acetylacetonate) may serve as triplet-emitting

dopants for constructing OLEDs.^3-13A significant research

The design of luminescent inorganic compounds continues

to attract great interest in the development of organic light-

* Authors to whom correspondence should be addressesd. Phone: 81

3-3817-1908. Fax: 81 3-3817-1895. E-mail: mhaga@chem.chuo-u.ac.jp

(M.-a.H.) and nozaki@ch.wani.osaka-u.ac.jp (K.N.).

^†Chuo University.

(1) Baldo, M. A.; O’Brien, D. F.; Shoustrikov, A.; Sibley, S.; Thompson,

M. E.; Forrest, S. R. Nature 1998, 395, 151.

(2) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403,

750.

^‡Osaka University.

^§Idemitsu Kosan Co., Ltd.

10.1021/ic060796o CCC: $33.50

Published on Web 09/29/2006

Inorganic Chemistry, Vol. 45, No. 22, 2006 8907

Obara et al.

Chart 1. Examples of Ir Complexes with Tridentate Ligands

Chart 2. Chemical Structures of Ligands, Their Ir Complexes, and

effort has been made to improve the internal quantum

efficiency of OLEDs by use of Ir(III) cyclometalated

complexes with high luminescence quantum yields. As a

consequence, many bis- and tris-cyclometalated complexes

of Ir(III) with relatively high quantum efficiency (∼0.5) have

been synthesized. Recently, [Ir(ppy)₂(X)₂]^-(X ) CN, NCS,

and NCO) have been reported to exhibit unprecedentedly

high quantum yields (∼1);¹⁴however, these results are

puzzling because of the inability of other groups, including

us, to reproduce the data. On the other hand, only limited

examples of luminescent Ir(III) complexes with tridentate

ligands are known (Chart 1).^15-20

Their Numbering^a

The octahedral Ir complexes can be classified in categories

by the coordination atoms around the Ir ion, that is, Ir(N)₆

type for [Ir(tpy)₂]³⁺, Ir(N)₅X for [Ir(tpy)(bpy)Cl]²⁺, Ir(N)₄-

(C)₂for [Ir(bpy)(ppy)₂]⁺, Ir(N)₃(C)₃for [Ir(ppy)₃], Ir(N)₂-

(C)₂(O)₂for [Ir(ppy)₂(acac)], Ir(N)₃(C)₂O for [Ir(ppy)₂(pic)],

and so forth. The coordination environment around the Ir-

(III) ion strongly affects the Ir 5d orbital energy, and the

excited-state properties of Ir complexes are governed by the

orbital mixing between Ir 5d and ligand π* orbitals. In

particular, the introduction of an Ir-C bond leads to a

stronger ligand field on the Ir ion, which induces the

destabilization of Ir 5d orbitals; as a result, there is larger

^aThe abbreviation of ligands and the proton-numbering system is also

shown for the assignment of H NMR spectra.

1

state mixing of the metal-to-ligand charge-transfer (MLCT)

excited state with ligand-centered ones. We have recently

synthesized the Ir(N)₆- and Ir(N)₅(C)-type Ir complexes, [Ir-

(Mebip)₂]³⁺and [Ir(Mebip)(Mebib)]²⁺[Mebip ) bis(N-

methyl-2-benzimidazolyl)pyridine and Mebib ) bis(N-

methyl-2-benzimidazolyl)benzene].^21,22The mixed-ligand

complex, [Ir(Mebip)(Mebib)]²⁺, showed a higher lumines-

cence quantum yield than the Ir(N)₆-type complex, [Ir-

(Mebip)₂]³⁺. Furthermore, the comparison of the emission

quantum yields for [Ir(Mebip)₂]³⁺, [Ir(Mebip)(Mebib)]²⁺, and

[Ir(Mebip)(bpy)Cl]²⁺revealed that the mixed-ligand coor-

dination environments of the Ir ion surrounded by both

tridentate and bidentate ligands lead to higher-emission

quantum yields.

Therefore, the judicious selection of coordination atoms

and an appropriate ligand π-orbital system hold the key to

facilitating the development of highly emissive Ir complexes.

We report herein the synthesis and photophysical properties

of highly luminescent Ir(N)₃(C)₂(X)-type Ir(III) complexes

containing both tridentate N∧C∧N and bidentate N∧C

ligands, as shown in Chart 2. For comparison, the corre-

sponding Ir(N)₄(C)(X)-type complex, [Ir(Mebip)(ppy)X]⁺,

was also synthesized.

(3) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-

E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J.

Am. Chem. Soc. 2001, 123, 4304-4312.

(4) Huang, W. S.; Lin, J. T.; Chien, C. H.; Tao, Y. T.; Sun, A. S.; Wen,

Y. S. Chem. Mater. 2004, 16, 2480.

(5) Jenmifer, B. M.; Rayabarapu, D. K.; Cheng, C. H. AdV. Mater. 2004,

16, 2480.

(6) Chen, T. M.; Laskar, I. R. Chem. Mater. 2004, 16, 111.

(7) Cheng, C. H.; Sun, P. P.; Duan, J. P. AdV. Mater. 2003, 15, 224.

(8) Sun, I. W.; Tai, C. C.; Yang, C. H. J. Mater. Chem. 2004, 14, 947.

(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, 12971-12979.

(10) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba,

I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003,

125, 7377-7387.

(11) Grushin, V. V.; Herron, N.; LastName, D. L.; Marshell, W. J.; Petrov,

V. A.; Wang, Y. Chem. Commun. 2001, 1494.

(12) 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.

(13) Hwang, F.-M.; Chen, H.-Y.; Chen, P.-S.; Liu, C.-S.; Chi, Y.; Shu,

C.-F.; Wu, F.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H. Inorg. Chem.

2005, 44, 1344.

(14) Nazeeruddin, M. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.;

Zuppiroli, L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790-8797.

(15) Collin, J. P.; Dixon, I. M.; Sauvage, J. P.; Williams, J. A. G.;

Barigelletti, F.; Flamigni, L. J. Am. Chem. Soc. 1999, 121, 5009.

(16) Lo, K. K. W.; Chung, C. K.; Ng, D. C. M.; Zhu, N. New. J. Chem.

2002, 26, 81.

Experimental Section

Materials. Isophthalic acid (Aldrich), NH₄PF₆(Wako), 1-phenyl-

2-(trimethylsilyl)acethylene (Aldrich), Pd(PPh₃)₄(Kanto), 2,4-

difluorophenylbronic acid (Aldrich), pyridine dicarboxylic acid

(17) Yoshikawa, N.; Matumura-Inoue, T. Anal. Sci. 2003, 19, 761.

(18) Polson, M.; Fracasso, S.; Bertolasi, V.; Ravaglia, M.; Scandola, F.

Inorg. Chem. 2004, 43, 1950.

(19) Wilkinson, J. A.; Goeta, A. E.; Foster, C. E.; Williams, J. A. G. Inorg.

Chem. 2004, 43, 6513.

(20) Neve, F.; Crispini, A.; Serroni, S.; Loiseau, F.; Campagna, S. Inorg.

Chem. 2001, 40, 1093.

(21) Yutaka, T.; Obara, S.; Ogawa, S.; Nozaki, K.; Ikeda, N.; Ohno, T.;

Ishii, Y.; Sakai, K.; Haga, M.-a. Inorg. Chem. 2005, 44, 4737-4746.

(22) Yutaka, T.; Obara, S.; Haga, M.; Yokoyama, Y.; Sakai, K. Acta

Crystallogr., Sect. E 2005, 61, m1357-m1359.

8908 Inorganic Chemistry, Vol. 45, No. 22, 2006

Highly Phosphorescent Iridium Complexes

(Aldrich), N-methyl-1,2-phenylenediamine (Aldrich), 2-(p-tolyl)-

pyridine (mppy) (Aldrich), and IrCl₃‚nH₂O (Heraeus) were used

as received. The other synthetic reagents were purchased from

Kanto Kagaku and used as received. For the photophysical

measurements, dichloromethane, acetonitrile, N,N-dimethylforma-

mide (spectroscopic grade), methanol, and ethanol (HPLC grade)

were purchased from Kanto Kagaku and used as received. For the

electrochemical measurements, N,N-dimethylformamide for non-

aquesous titrimetry (Kanto) was used as supplied, and tetra-n-

butylammonium tetrafluoroborate (Nacalai) was purified by

recrystallization from EtOH. Ir(Mebip)Cl₃, 2-(4′,6′-difluoro-

phenyl)pyridine (dfppy), Mebib, Mebip,²³and 2,6-bis(1-phenyl-

benzimidazol-2-yl)benzene (Phbib)⁷were prepared according to the

literature.

Synthesis. [Ir(Mebib)Cl₂]₂. IrCl₃‚4H₂O (0.10 g, 0.27 mmol) and

Mebib (0.183 g, 0.54 mmol) were refluxed in methanol (15 mL)

for 5 h. After cooling to room temperature, the resulting precipitate

was collected and washed with methanol and then ether. This

complex is insoluble in common organic solvents and was therefore

used without purification for further preparation. Yield: 0.13 g

(82%). Anal. Calcd for [C₂₂H₁₇N₄IrCl₂]₂: C, 44.00; H, 2.85; N, 9.33.

Found: C, 43.98; H, 3.00; N, 9.72. MALDI-MS: m/z 1201.91

(1201.09 required for [C₄₄H₃₅N₈Ir₂Cl₄]⁺).

[Ir(Mebib)(ppy)I]. The synthetic procedure was the same as that

used for [Ir(Mebib)(ppy)Br], substituting NaBr for NaI. Yield: 50%.

Anal. Calcd for C₃₃H₂₅N₅IrI: C, 48.89; H, 3.11; N, 8.64. Found:

C, 48.41; H, 3.30; N, 8.85. ¹H NMR (CDCl₃): δ 4.23 (s, 6H, N-Me),

5.61 (d, 1H, J ) 6.9 Hz, H_1′), 6.20 (d, 1H, J ) 8.0 Hz, H₁), 6.42

(t, 1H, J ) 8.3 Hz, H_2′), 6.54 (t, 1H, J ) 8.0 Hz, H_3′), 6.85 (t, 2H,

J ) 8.3 Hz, H₂), 7.09 (t, 2H, J ) 7.7 Hz, H₃), 7.17 (2H, m, H₄),

7.31 (t, 1H, J ) 7.7 Hz, H₆), 7.39 (d, 1H, J ) 9.2 Hz, H_4′), 7.52

(t, 1H, J ) 7.2 Hz, H_7′), 7.94 (d, 2H, J ) 7.4 Hz, H₅), 8.03 (m,

2H, H_5′,6′), 11.01 (d, 1H, J ) 5.2 Hz, H_8′).

[Ir(Mebib)(ppy)(CN)]. [Ir(Mebib)(ppy)Cl] (0.05 g, 0.07 mmol)

and KCN (0.0091 g, 0.14 mmol) were heated by microwave

irradiation (650 W) in ethylene glycol (10 cm³) for 5 min. After

the mixture was cooled to room temperature, the addition of water

resulted in the formation of a yellow precipitate, which was

collected by filtration and washed with water and ether. The

purification was performed by recrystallization from CH₂Cl₂-

hexane. Yield: 0.03 g (61%). Anal. Calcd for C₃₄H₂₅N₆Ir: C, 57.53;

1

H, 3.55; N, 11.84. Found: C, 57.75; H, 3.33; N, 11.45. H NMR

(CDCl₃): δ 4.20 (s, 6H, N-Me), 5.79 (d, 1H, J ) 6.3 Hz, H_1′),

6.19 (d, 2H, J ) 8.0 Hz, H₁), 6.48 (t, 1H, J ) 7.5 Hz, H_2′), 6.60

(t, 1H, J ) 6.3 Hz, H_3′), 6.86 (t, 2H, J ) 8.0 Hz, H₂), 7.09 (d, 2H,

J ) 8.0 Hz, H₃), 7.19 (t, 2H, J ) 8.0 Hz, H₆), 7.35 (t, 1H, J ) 7.5

Hz, H_5′), 7.47 (d, 1H, J ) 8.0 Hz, H_7′), 7.91 (d, 2H, J ) 7.5 Hz,

H₅), 8.03 (d, 2H, J ) 4.5 Hz, H_5′,6′), 10.48 (d, 1H, J ) 5.7 Hz,

H_1′). FT-IR (KBr): 2102.6 cm^-1[ν(CN)]. MALDI MS: m/z 710.07

(710.18 required for [C₃₄H₂₅N₆Ir]⁺).

[Ir(Phbib)Cl₂]₂. This complex was synthesized in a manner

similar to that for [Ir(Mebib)Cl₂]₂, except that bis(N-phenylbenz-

imidazolyl)benzene (Phbib) was used instead of Mebib. A pale

yellow precipitate was obtained. This complex was used as a starting

compound for the next reaction without further purification.

Yield: 66%.

[Ir(Mebib)(dfppy)Cl]. The mixture of [Ir(Mebib)Cl₂]₂(0.1 g,

0.084 mmol) and dfppy (0033 g, 0.17 mmol) in glycerol (10 cm³)

was irradiated with a microwave (650 W, multimode) for 4 min

under a nitrogen atmosphere. After the mixture cooled to room

temperature, water (50 cm³) was added to give the yellow-orange

precipitate. The precipitate was collected by filtration and washed

with hexane and ether. Purification was performed by the diffusion

of ether to a CH₂Cl₂solution of the complex. Yield: 0.046 g (32%).

Anal. Calcd for C₃₃H₂₃N₅IrClF₂: C, 52.48; H, 3.07; N, 9.27.

[Ir(Mebib)(ppy)Cl]. The mixture of [Ir(Mebib)Cl₂]₂(0.1 g, 0.084

mmol) and ppy (0.46 g, 1.4 mmol) in glycerol (30 mL) was

irradiated in a microwave (650 W, multimode) for 6 min. After

the mixture cooled to room temperature, water (50 cm³) was added

to give a yellow-orange precipitate. After the precipitate was filtered

and washed with hexane and ether, an orange precipitate was

obtained. Yield: 0.03 g (23%). Anal. Calcd for C₃₃H₂₅N₅IrCl: C,

1

55.11; H, 3.50; N, 9.74. Found: C, 54.86; H, 3.60; N, 9.85. H

Found: C, 52.12; H, 3.43; N, 9.55. H NMR (CDCl₃): δ 4.25 (s,

NMR (CDCl₃): δ 4.29 (s, 6H, N-Me), 5.84 (d, 1H, J ) 7.0 Hz,

H_1′), 6.26 (d, 2H, J ) 8.0 Hz, H₁), 6.47 (t, 1H, J ) 6.9 Hz, H_2′),

6.60 (t, 1H, J ) 7.5 Hz, H_3′), 6.91 (t, 2H, J ) 7.5 Hz, H₂), 7.15 (t,

2H, J ) 7.5 Hz, H₃), 7.24 (d, 2H, J ) 8.0 Hz, H₄), 7.39 (t, 1H, J

) 8.0 Hz, H₆), 7.50 (d, 1H, J ) 8.0 Hz, H_4′), 7.64 (t, 1H, J ) 7.0

Hz, H_7′), 7.97 (d, 2H, J ) 7.5 Hz, H₅), 8.10 (m, 2H, H_5′,6′), 10.61

(d, 1H, J ) 5.7 Hz, H_8′). ESI MS: m/z 719.405 (719.143 required

for [C₃₃H₂₅N₅IrCl]⁺).

6H, N-Me), 5.27 (d, 1H, J ) 9.5 Hz, H_1′), 6.02 (t, 1H, J ) 10.5

Hz, H_3′), 6.12 (d, 2H, J ) 8.6 Hz, H₁), 6.88 (t, 2H, J ) 7.5 Hz,

H₂), 7.12 (t, 2H, J ) 7.5 Hz, H₃), 7.21 (d, 2H, J ) 8.0 Hz, H₄),

7.34 (t, 1H, J ) 8.0 Hz, H₆), 7.58 (t, 1H, J ) 7.5 Hz, H₇′), 7.91 (d,

2H, J ) 7.5 Hz, H₅), 8.04 (t, 1H, J ) 8.6 Hz, H_6′), 8.41 (d, 1H, J

) 8.6 Hz, H_5′), 10.57 (d, 1H, J ) 5.7 Hz, H_8′). ¹⁹F NMR (CDCl₃):

δ -110.0, -110.6. ESI MS: m/z 755.411 (755.123 required for

[C₃₃H₂₃N₅F₂IrCl]⁺).

[Ir(Mebib)(ppy)Br]. [Ir(Mebib)(ppy)Cl] (0.05 g, 0.07 mmol)

and NaBr (0.0215 g, 0.35 mmol) were heated by microwave

irradiation (650 W) in ethylene glycol (20 cm³) for 6 min. After

the mixture was cooled to room temperature, the addition of water

resulted in the formation of an orange precipitate, which was

collected by filtration and washed with water and ether. The

purification was performed by recrystallization from CH₂Cl₂-

hexane. Yield: 0.034 g (64%). Anal. Calcd for C₃₃H₂₅N₅IrBr: C,

[Ir(Mebib)(mppy)Cl]‚2CH₂Cl₂. The synthetic procedure was

the same as that used for the [Ir(Mebib)(dfppy)Cl] analogue,

substituting dfppy for mppy. Orange crystals suitable for X-ray

analysis were obtained by the diffusion of ether to a CH₂Cl₂solution

of the complex. Yield: 32%. Anal. Calcd for C₃₄H₂₇N₅IrCl‚2CH₂-

Cl₂: C, 47.88; H, 3.46; N, 7.75. Found: C, 47.65; H, 3.20; N,

7.89. ¹H NMR (CDCl₃): δ 1.85 (s, 3H, Ph-Me), 4.29 (s, 6H, N-Me),

5.62 (s, 1H, H_1′), 6.26 (d, 2H, J ) 8.0 Hz, H₁), 6.40 (d, 1H, J )

6.3 Hz, H_3′), 6.90 (t, 2H, J ) 7.5 Hz, H₂), 7.13 (t, 2H, J ) 8.0 Hz,

H₃), 7.22 (t, 2H, J ) 8.0 Hz, H₄), 7.37 (t, 2H, J ) 7.0 Hz, H₆),

7.58 (t, 1H, J ) 7.0 Hz, H_7′), 7.96 (d, 2H, J ) 7.5 Hz, H₅), 8.04

(m, 2H, H_5′,6′), 10.56 (d, 1H, J ) 5.1 Hz, H_8′). ESI MS: m/z 733.175

(733.155 required for [C₃₄H₂₇N₅IrCl]⁺).

1

51.90; H, 3.30; N, 9.17. Found: C, 51.86; H, 3.60; N, 9.35. H

NMR (CDCl₃): δ 4.29 (s, 6H, N-Me), 5.79 (d, 1H, J ) 7.4 Hz,

H_1′), 6.26 (d, 2H, J ) 8.6 Hz, H₁), 6.47 (t, 1H, J ) 7.4 Hz, H_2′),

6.60 (t, 1H, J ) 7.4 Hz, H_3′), 6.91 (t, 2H, J ) 7.7 Hz, H₂), 7.15 (t,

2H, J ) 7.4 Hz, H₃), 7.23 (m, 2H, H₄), 7.38 (t, 1H, J ) 7.7 Hz,

H₆), 7.49 (d, 1H, J ) 7.4 Hz, H_4′), 7.62 (t, 1H, J ) 6.9 Hz, H_7′),

7.98 (d, 2H, J ) 7.4 Hz, H₅), 8.11 (m, 2H, H_5′,6′), 10.81 (d, 1H, J

) 6.3 Hz, H_8′).

[Ir(Mebib)(dfppy)CN]. The complex was synthesized in a

manner similar to that for [Ir(Mebib)(ppy)CN]. Yield: 72%. Anal.

Calcd for C₃₄H₂₃N₆IrF₂: C, 54.76; H, 3.11; N, 11.27. Found: C,

54.42; H, 3.50; N, 11.52. ¹H NMR (CDCl₃): δ 4.30 (s, 6H, N-Me),

5.31 (d, 1H, J ) 8.2 Hz, H_1′), 6.09 (t, 1H, J ) 10.0 Hz, H_3′), 6.18

(23) Addison, A. W.; Burke, P. J. J. Heterocycl. Chem. 1981, 18, 803-

805.

Inorganic Chemistry, Vol. 45, No. 22, 2006 8909

Obara et al.

(d, 1H, J ) 8.0 Hz, H₁), 6.95 (t, 2H, J ) 7.5 Hz, H₂), 7.19 (t, 2H,

J ) 7.5 Hz, H₃), 7.29 (d, 2H, J ) 8.0 Hz, H₄), 7.42 (t, 1H, J ) 8.0

Hz, H₆), 7.57 (t, 1H, J ) 7.2 Hz, H_7′), 7.98 (d, 2H, J ) 7.5 Hz,

H₅), 8.12 (t, 1H, J ) 7.5 Hz, H_6′), 8.45 (d, 1H, J ) 9.8 Hz, H_5′),

10.55 (d, 1H, J ) 4.6 Hz, H_8′). FT-IR (KBr): 2105.8 cm^-1[ν-

(CN)]. MALDI MS: m/z 745.95 (746.16 required for [C₃₄H₂₃N₆F₂-

Ir]⁺).

[Ir(Mebib)(ppy)(CH₃CN)](OTf)‚Et₂O. [Ir(Mebib)(ppy)Cl] (0.05

g, 0.07 mmol) and AgOTf (0.036 g, 0.14 mmol) in acetonitrile (20

cm³) were refluxed for 3 h. After the mixture was cooled to room

temperature, the solution was filtered through Celite. The resulting

yellow filtrate was concentrated in vacuo to one-fourth of its original

volume, and the addition of ether afforded the precipitation. The

purification was performed by recrystallization from CH₂Cl₂-ether.

Yield: 92%. Anal. Calcd for C₃₆H₂₈N₆IrF₃SO₃‚Et₂O: C, 50.68; H,

4.04; N, 8.86. Found: C, 50.26; H, 4.41; N, 8.51. ¹H NMR

(CDCl₃): δ 4.26 (s, 6H, N-Me), 5.64 (d, 1H, J ) 8.0 Hz, H_1′),

6.06 (d, 2H, J ) 8.0 Hz, H₁), 6.42 (t, 1H, J ) 8.0 Hz, H_2′), 6.58

(t, 1H, J ) 8.0 Hz, H_3′), 6.93 (t, 2H, J ) 7.5 Hz, H₂), 7.20 (t, 2H,

J ) 7.5 Hz, H₃), 7.46 (d, 2H, J ) 8.0 Hz, H₄), 7.50 (t, 1H, J ) 8.0

Hz, H_4′), 7.71 (m, 1H, H_7′), 8.11 (d, 2H, J ) 8.0 Hz, H₅), 8.22 (d,

2H, J ) 4.5 Hz, H_5′,6′), 9.75 (d, 1H, J ) 5.7 Hz, H_8′). ESI MS:

m/z 725.132 (725.201 required for [C₃₅H₂₈N₆Ir]⁺).

[Ir(Mebib)(mppy)(CH₃CN)](OTf). The complex was synthe-

sized in a manner similar to that for [Ir(Mebib)(ppy)(CH₃CN)](OTf),

except that [Ir(Mebib)(mppy)Cl] (0.051 g, 0.07 mmol) was used

as a starting compound. Yield: 88%. Anal. Calcd for C₃₇H₃₀N₆-

IrF₃SO₃: C, 50.05; H, 3.40; N, 9.46. Found: C, 49.63; H, 3.90; N,

1

10.07. H NMR (CDCl₃): δ 1.79 (s, 3H, Ph-Me), 4.37 (s, 6H,

N-Me), 5.58 (s, 1H, H_1′), 6.16 (d, 2H, J ) 8.6 Hz, H₁), 6.49 (t, 2H,

J ) 8.0 Hz, H₂′), 7.03 (t, 2H, J ) 7.5 Hz, H₂), 7.30 (t, 2H, J ) 7.5

Hz, H₃), 7.48 (d, 1H, J ) 7.5 Hz, H_4′), 7.56 (d, 2H, J ) 8.0 Hz,

H₄), 7.61 (t, 1H, J ) 8.0 Hz, H₆), 7.76 (t, 1H, J ) 6.0 Hz, H_7′),

8.20 (d, 2H, J ) 8.0 Hz, H₅), 8.28 (m, 2H, H_5′,6′), 9.79 (d, 1H, J )

4.0 Hz, H_8′). ESI MS: m/z 739.124 (739.216 required for [C₃₆H₃₀N₆-

Ir]⁺).

[Ir(Phbib)(ppy)(CH₃CN)](OTf). The complex was synthesized

in a manner similar to that for [Ir(Mebib)(ppy)(CH₃CN)](OTf),

except [Ir(Phbib)(ppy)Cl] was used as a starting compound.

Yield: 30%. Anal. Calcd for C₄₆H₃₂N₆IrF₃SO₃: C, 55.36; H, 3.23;

1

N, 8.42. Found: C, 55.00; H, 3.40; N, 8.77. H NMR (CD₃CN):

[Ir(Mebib)(mppy)(CCPh)]. The mixture of KF (20 mg, 0.339

mmol) and Me₃Si-CCPh (20 mg, 0.113 mmol) in methanol (10

cm³) was stirred for 30 min at room temperature under a nitrogen

atmosphere. To the solution was added [Ir(Mebib)(mppy)(CH₃CN)]-

(OTf) (0.05 g, 0.056 mmol), and the solution was refluxed for 2 h.

After the solution cooled to room temperature, a yellow precipitate

was formed, which was collected by filtration and washed with

methanol and ether. The complex was recrystallized from CH₂-

Cl₂-ether. Yield: 54%. Anal. Calcd for C₄₂H₃₂N₅Ir: C, 63.14; H,

4.04; N, 8.76. Found: C, 62.96; H, 3.72; N, 9.02. ¹H NMR

(CDCl₃): δ 1.72 (s, 3H, Ph-Me), 4.31 (s, 6H, N-Me), 5.60 (s, 1H,

H_1′), 6.19 (d, 2H, J ) 8.0 Hz, H₁), 6.38 (d, 1H, J ) 8.0 Hz, H_2′),

6.78 (d, 2H, J ) 6.9 Hz, C-Ph), 6.85 (t, 1H, J ) 7.5 Hz, C-Ph),

6.92 (m, 3H, J ) 8.0 Hz, C-Ph, H₂), 7.19 (t, 2H, J ) 7.5 Hz, H₃),

7.45 (m, 4H, H_6,4,4′), 7.63 (q, 1H, H_7′), 8.12 (d, 2H, J ) 4.6 Hz,

δ 5.71 (d, 1H, J ) 7.4 Hz, H_1′), 6.24 (d, 2H, J ) 8.0 Hz, H₁), 6.55

(t, 1H, J ) 6.9 Hz, H_2′), 6.71-6.69 (m, 3H, H_2,6), 6.89 (t, 1H, J )

8.0 Hz, H_3′), 7.07-7.06 (m, 4H, H_2,5), 7.20 (t, 2H, J ) 7.7 Hz,

H₃), 7.58 (d, 2H, J ) 7.4 Hz, N-Ph), 7.63 (d, 1H, J ) 7.4 Hz, H_4′),

7.76-7.69 (m, 8H, N-Ph), 7.84 (t, 1H, J ) 5.4 Hz, H_7′), 8.32 (dt,

2H, J ) 6.9 and 3.4 Hz, H_5′,6′), 9.90 (d, 1H, J ) 5.2 Hz, H_8′). ESI

MS: m/z 861.06 (861.02 required for [C₄₆H₃₂N₆Ir]⁺).

[Ir(Mebip)(ppy)Cl](PF₆)‚2CH₃CN. A mixture of [Ir(Mebip)-

Cl₃] (0.1 g, 0.16 mmol) and ppy (0.03 g, 0.19 mmol) in glycerol

(30 mL) was irradiated in a microwave (650 W, multimode) for

12 min under a nitrogen atmosphere. After the mixture cooled to

room temperature, a saturated aqueous solution of NH₄PF₆(20 cm³)

was added to give an orange-red precipitate. The resulting

precipitate was filtered and washed with hexane, CHCl₃, and ether.

Single crystals were grown from a slow evaporation of ether into

a CH₃CN solution of the complex. The crystals lost solvated Et₂O

in vacuo. Yield: 0.03 g (61%). Anal. Calcd for C₃₆H₃₁ClF₆N₈IrP:

C, 45.59; H, 3.29; N, 11.82. Found: C, 45.94; H, 3.10; N, 11.65.

¹H NMR (DMSO-d₆): δ 4.59 (s, 6H, N-Me), 5.90 (d, 2H, J ) 8.6

Hz, H₁), 6.08 (d, 1H, J ) 7.5 Hz, H_1′), 6.73 (t, 1H, J ) 6.9 Hz,

H_2′), 6.82 (t, 1H, J ) 7.5 Hz, H_3′), 7.20 (t, 2H, J ) 8.0 Hz, H₂),

7.51 (t, 2H, J ) 7.7 Hz, H₃), 7.85 (d, 2H, J ) 8.0 Hz, H_4′), 7.93

(m, 3H, H_4,7′), 8.41 (t, 1H, J ) 8.0 Hz, H_6′), 8.50 (t, 1H, J ) 8.3

Hz, H₅), 8.57 (d, 1H, J ) 8.0 Hz, H_5′), 8.82 (d, 2H, J ) 8.6 Hz,

H₆), 10.35 (d, 1H, J ) 4.5 Hz, H_8′). ESI MS: m/z 721.52 (721.15

required for [C₃₂H₂₅N₆IrCl]⁺).

H

_5′,6′), 8.15 (d, 2H, J ) 8.0 Hz, H₅), 10.69 (d, 1H, J ) 5.7 Hz,

H_8′). ESI MS: m/z 799.564 (799.229 required for [C₄₂H₃₂N₅Ir]⁺).

[Ir(Phbib)(ppy)Cl]. This complex was synthesized in a manner

similar to that for [Ir(Mebib)(ppy)Cl], except that Phbib was used

instead of Mebib. The crude compound was purified by column

chromatography on silica with CH₂Cl₂-methanol (25:1 v/v) and

recrystallized from CH₂Cl₂-benzene-hexane. Red crystals were

obtained. Yield: 24%. Anal. Calcd for C₄₃H₃₁N₅IrClO: C, 59.95;

1

H, 3.63; N, 8.10. Found: C, 59.93; H, 3.46; N, 7.60. H NMR

(CDCl₃-d): δ 5.92 (d, 1H, J ) 7.4 Hz, H_1′), 6.34 (d, 2H, J ) 8.6

Hz, H₁), 6.55 (t, 1H, J ) 7.4 Hz, H_2′), 6.68 (t, 1H, J ) 7.4 Hz,

H_3′), 6.74 (m, 3H, H_4,6), 6.95 (dt, 4H, J ) 7.3 and 3.6 Hz, H_2,5),

7.07 (t, 2H, J ) 7.4 Hz, H₃), 7.53 (t, 2H, J ) 3.1 Hz, N-Ph), 7.58

(d, 1H, J ) 8.0 Hz, H_4′), 7.60-7.68 (m, 8H, J ) 3.7 and 1.9 Hz,

N-Ph), 7.70 (t, 1H, J ) 6.3 Hz, H_7′), 8.12 (t, 1H, J ) 7.5 Hz, H_6′),

8.17 (d, 1H, J ) 8.0 Hz, H_5′), 10.71 (d, 1H, J ) 5.7 Hz, H_8′).

Physical Measurements. ¹H NMR, ESI MS, MALDI-TOF MS,

and UV-vis spectra were recorded with a Mercury 300 (Varian)

or JNM-ECA 500 (JEOL) spectrometer, a Micromass electrospray

LCT, and Shimadzu MALDI-TOF AXIMA-CFR mass spectrom-

eter, and a Hewlett-Packard 8453 UV-vis spectrophotometer,

1

[Ir(Phbib)(ppy)CN]‚2H₂O. The complex was obtained by a

metathesis reaction of [Ir(Phbib)(ppy)Cl] with KCN under condi-

tions similar to those for [Ir(Mebib)(ppy)(CN)]. A yellow powder

was obtained. Yield: 21.0%. Anal. Calcd for C₄₄H₃₃N₆IrO₂: C,

respectively. For the assignment of H NMR spectra, the proton-

numbering system in Chart 1 is used in this paper. Electrochemical

measurements were carried out in a standard one-compartment cell

under N₂gas, equipped with a BAS platinum (φ 1.6 mm) or a BAS

glassy-carbon (φ 3 mm) working electrode, a platinum wire counter

electrode, and a Ag/Ag⁺reference electrode with an ALS/CH model

660A electrochemical analyzer. The reference electrode was Ag/

AgNO₃(0.01 M in 0.1 M TBABF₄CH₃CN), abbreviated as Ag/

Ag⁺. The ferrocene/ferrocenium (Fc/Fc⁺) oxidation process was

used as an internal reference standard, and all potentials are reported

versus Fc/Fc⁺. The E_1/2values for the Fc/Fc⁺couple were +0.04

1

60.74; H, 3.82; N, 9.66. Found: C, 61.03; H, 3.46; N, 9.85. H

NMR (CDCl₃-d): δ 5.92 (d, 1H, J ) 7.4 Hz, H_1′), 6.35 (d, 2H, J

) 7.4 Hz, H₁), 6.63 (t, 1H, J ) 7.2 Hz, H₂′), 6.72-6.78 (m, 4H,

H_3′,4,6), 6.97 (dt, 4H, J ) 7.4 and 3.7 Hz, H_2,5), 7.09 (t, 2H, J ) 8.0

Hz, H₃), 7.50 (t, 2H, J ) 6.0 Hz, N-Ph), 7.60 (d, 1H, J ) 8.0 Hz,

H_4′), 7.63-7.72 (m, 9H, H_7′, N-Ph), 8.16 (m, 2H, H_5′,6′), 10.66 (d,

1H, J ) 5.7 Hz, H_8′).

8910 Inorganic Chemistry, Vol. 45, No. 22, 2006

Highly Phosphorescent Iridium Complexes

V versus Ag/Ag⁺, which was found to be 0.33 V more negative

than that versus a saturated calomel electrode. Emission spectra

were recorded using a grating monochromator (Triax 1900, Jobin

Yvon) with a CCD image sensor (S7031, Hamamatsu). The spectral

sensitivity of the spectrofluorometer was corrected using a bromine

lamp (IPD 100V 500WCS, Ushio). A sample solution in a 1 mm

quartz cell was deoxygenated and excited using an LD-excited solid-

state Nd:YAG laser (355 nm, 440 ps, 13 kHz, 6 mW, JDS

Uniphase). The emission quantum yields of the Ir(III) compounds

were determined using 9,10-diphenylanthracene (DPA; Φ ) 0.91)

as a reference.²⁴Depolarized light was used in all cases to determine

emission quantum yields. The error in the emission quantum yields

is determined primarily by the accuracy of the sensitivity correction

in the spectral range from 450 to 800 nm, which is (5%. The

Huang-Rhys factors of accepting modes were determined by a

one-mode Franck-Condon analysis of emission spectra using eq

1 in ref 26b.^25,26

For the determination of emission lifetimes, the deoxygenated

sample solution was photoexcited using a pulsed Nd:YAG laser.

The light from the sample was monochromated using a grating

monochromator (H-20, Jobin Yvon) and converted into current

signals by a photomultiplier tube (R3896, Hamamatsu). The

transient signals for 10 240∼2560 shots were accumulated on a

digitizing oscilloscope (HP 54520 Hewlett-Packard) to get the decay

profile of the emission intensity, which was fit to a single-

exponential function with convolution of the instrumental response

function of the measuring system. The time resolution of the system

was 2 ns.. The measurements at 77 K were performed using a

cylindrical quartz cell (1 mm inner diameter) and a liquid-nitrogen

Dewar.

For the computations with ADF, an uncontracted triple-ú Slater-

type orbital basis set with one polarization function was used

(denoted as tzp). The core orbitals (C and N, 1s; Cl, 1s2p; Ir, 1s4d)

were obtained as solutions of an all-electron calculation on the

isolated atom and kept frozen during the self-consistent field

calculation of the molecule. The zero-order regular approximation^36-38

was used for relativistic corrections. Both the local spin density

approximation with the parametrization of Vosko, Wilk, and Nusair

(VWN)³⁹and the generalized gradient approximated correlation

functional proposed by Perdew and Wang (PW91) were employed.

Drawing of the molecular orbitals was performed using MOLE-

KEL.^40,41

X-ray Diffraction Studies for 5‚2CH₂Cl₂, 9‚C₆H₁₄, 11OTf‚

Et₂O, 13OTf, and 14PF₆‚2CH₃CN‚0.5Et₂O. Red block crystals

of [Ir(Mebib)(mppy)Cl]‚2CH₂Cl₂(5‚2CH₂Cl₂), yellow block crys-

tals of [Ir(Mebib)(ppy)(CH₃CN)]OTf‚Et₂O (11OTf‚Et₂O), yellow

plate crystals of [Ir(Phbib)(ppy)(CH₃CN)]OTf (13OTf), orange

block crystals of [Ir(Phbib)(ppy)Cl]‚C₆H₁₄(9‚C₆H₁₄), and orange

blockcrystalsof[Ir(Mebip)(ppy)Cl]PF₆‚2CH₃CN‚0.5Et₂O(14PF₆‚2CH₃-

CN‚0.5Et₂O) were obtained by the slow diffusion of diethyl ether

or n-hexane into the corresponding dichloromethane or acetonitrile

solutions. Diffraction data for 5‚2CH₂Cl₂, 11OTf‚Et₂O, 13OTf, 9‚

C₆H₁₄, and 14PF₆‚2CH₃CN‚0.5Et₂O were collected on a Rigaku

Mercury CCD area detector with graphite monochromated Mo KR

radiation (λ ) 0.710 69 Å) at -150 ( 1 °C for the 2θ range of

5-55°. Intensity data were corrected for numerical absorptions

(NUMABS⁴²for 14PF₆‚2CH₃CN‚0.5Et₂O) or empirical absorptions

(REQAB⁴³for 5‚2CH₂Cl₂, 11OTf‚Et₂O, 9‚C₆H₁₄, and 13OTf) and

for Lorentz and polarization effects. Corrections for secondary

extinction were further applied for 14PF₆‚2CH₃CN‚0.5Et₂O (coef-

ficient, 0.162 05), for 13OTf‚Et₂O (coefficient, 30.82), and for 9‚

C₆H₁₄(coefficient, 148.07).⁴⁴The structure solution and refinements

were carried out using the CrystalStructure package.^45,46The

positions of the non-hydrogen atoms were determined by heavy-

atom Patterson methods (SHELX-97⁴⁷for 14PF₆‚2CH₃CN‚0.5Et₂O

and 5‚2CH₂Cl₂; PATTY⁴⁸for 11OTf‚Et₂O) or direct methods

Density functional theory (DFT) calculations were performed

with the Gaussian 98, revision A11.3, suite (G98)²⁷and with the

Amsterdam Density Functional program package (ADF, version

2002).^28-30The basis functions used in the computations with G98

were Dunning-Hay’s split-valence double-ú for C, H, and N atoms

(D95) and the Hay-Wadt double-ú with a Los Alamos relativistic

effective core potential for heavy atoms (LANL2DZ).^31,32Becke’s

stylethree-parameterhybridfunctionalofB3PW91wasemployed.^33-35

(35) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.

(36) Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99,

4957.

(37) Lenthe, E.; Ehlers, A. E.; Baerends, E. J. J. Chem. Phys. 1999, 110,

8943.

(38) Lenthe, E. V.; Leeuwen, R. V.; Baerends, E. J.; Snijders; J. G. J.

Quantum. Chem. 1996, 57, 281.

(39) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.

(40) Flukiger, P.; Luthi, H. P.; Portmann, S.; Weber, J. Swiss Center for

Scietific Computing, Manno, Switzerland, 2000-2002.

(41) Portmann, S.; Luthi, H. P. Chimia 2000, 54, 766.

(42) Higashi, T. NUMABS: Numerical Absorption Correction; Rigaku

Corp.: Tokyo, Japan, 1999.

(24) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,

2nd ed.; Marcel Dekker: New York, 1993.

(25) Kober, E. M.; Casper, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.

Chem. 1986, 90, 3722-3734.

(26) Islam, A.; Ikeda, N.; Nozaki, K.; Ohno, T. J. Photochem. Photobiol.,

A 1997, 106, 61-66.

(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,

M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.;

Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,

A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,

V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;

Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;

Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;

Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;

Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.

L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,

A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;

Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,

E. S.; Pople, J. A. Gaussian 98, version A11.3; Gaussian Inc.:

Pittsburgh, PA, 1998.

(28) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1993, 2, 42.

(29) Boerrigter, P. M.; Velde, G. t.; Baerends, E. J. Int. J. Quantum Chem.

1988, 33, 87.

(30) Velde, G. t.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84.

(31) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;

Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3.

(32) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270; 284; 299.

(33) Perdew, J. P. Phys. ReV. B: Condens. Matter Mater. Phys. 1986, 33,

8822.

(43) Jacobson, R. A. REQAB: PriVate Communication to Rigaku Corp.;

Rigaku Corp.: Tokyo, Japan, 1998.

(44) Larson, A. C. In Crystallographic Computing; Proceedings of an

International Summer School Organized by The Commission on

Crystallographic Computing of the International Union of Crystal-

lography, August 4-11, 1969, Ottawa, Canada; Ahmed, F. R., Hall,

S. R., Huber, C. P., Eds.; Munksgaard: Copenhagen, Denmark, 1970;

pp 291-295.

(45) CrystalStructure 3.60: Single Crystal Structure Analysis Software;

Rigaku Corp: Tokyo, Japan; MSC: The Woodlands, TX, 2000-2004.

(46) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.

CRYSTALS Issue 10; Chemical Crystallography Laboratory: Oxford,

U. K., 1996.

(47) Sheldrick, G. M. SHELX-97: Program for the Refinement of Crystal

Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

(48) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Garcia-

Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. PATTY: The

DIRDIF Program System; Technical Report of the Crystallography

Laboratory; University of Nijmengen: Nijmengen, The Netherlands,

1992.

(34) Perdew, J. P.; Burke, K.; Wang, Y. Phys. ReV. B: Condens. Matter

Mater. Phys. 1996, 54, 16533.

Inorganic Chemistry, Vol. 45, No. 22, 2006 8911

Obara et al.

Table 1. X-ray Crystallographic Data for 5‚2CH₂Cl₂, 9‚C₆H₁₄, 11OTf‚Et₂O, 13OTf, and 14PF₆‚2MeCN‚0.5Et₂O

5‚2CH₂Cl₂

9‚C₆H₁₄

11OTf‚Et₂O

13OTf

14PF₆‚2MeCN‚0.5Et₂O

chemical formula

fw

C₃₆H₃₁Cl₅IrN₅

903.16

C₄₉H₄₃ClIrN₅

929.59

C₄₀H₃₈F₃IrN₆O₄S

948.05

C₄₆H₃₂F₃IrN₆O₃S

998.07

C₃₈H₃₆ClF₆IrN₈O_0.5

985.39

P

cryst color, habit

dimensions of crystal

cryst syst

space group

a, Å

red, block

0.30 × 0.15 × 0.10

monoclinic

P2₁/c

orange, block

0.90 × 0.80 × 0.80

monoclinic

P2₁/c

yellow, block

0.50 × 0.40 × 0.20

triclinic

P1

8.94(2)

yellow, plate

0.50 × 0.35 × 0.15

monoclinic

C2/c

orange, block

0.20 × 0.20 × 0.20

monoclinic

P2₁/c

14.360(6)

10.53(2)

26.59(3)

11.156(2)

b, Å

14.339(6)

20.44(3)

13.21(2)

10.36(1)

16.503(3)

c, Å

17.123(7)

18.15(5)

16.71(3)

29.29(3)

21.440(4)

R, deg

â, deg

γ, deg

V, Å³

78.19(8)

79.99(11)

82.88(9)

1894.9(50)

2

1.661

36.53

0.5335-1.0000

17204

107.460(6)

99.04(6)

99.76(4)

102.924(3)

3363.3(24)

4

1.783

44.13

0.6326-1.0000

12694

3859.1(14)

4

1.600

35.82

0.4692-1.0000

34574

7951.8(14)

8

1.667

34.85

0.5335-1.0000

36271

3847.2(13)

4

1.701

36.62

0.4129-0.5559

28274

Z

F

_calcd, g cm^-3

µ, cm^-1

range of trans. factors

no. rflns measured

no. unique rflns

R_int

6562

0.090

9713

0.078

8112

0.047

11397

0.056

8668

0.068

no. rflns used

no. params refined

R [I > 3 σ(I)]^a

2548 [I > 3σ(I)]

5407 [I > 3σ(I)]

6951 [I > 3σ(I)]

7401 [I > 3σ(I)]

3052 [I > 2σ(I)]

430

519

535

573

534

0.055 [I > 3σ(I)]

0.056 [I > 3σ(I)]

1.002

0.062 [I > 3σ(I)]

0.066 [I > 3σ(I)]

1.000

0.040 [I > 3σ(I)]

0.041 [I > 3σ(I)]

1.000

0.048 [I > 3σ(I)]

0.050 [I > 3σ(I)]

1.000

0.031 [I > 2σ(I)]

0.033 [I > 2σ(I)]

1.000

R

_w[I > 3 σ(I)]^b

GOF [I > 3 σ(I)]^c

2

^aR ) ∑||F_o| - |F_c||/∑|F_o|. ^bR_w) [∑w(|F_o| - |F_c|)²/∑wF_o]^1/2, w ) [pF₀+ qσ(F₀) + 0.5]^-1[p ) 0 (11OTf, 12‚C₆H₁₄, 14PF₆‚2MeCN‚0.5Et₂O),

0.0007 (5‚2CH₂Cl₂), 0.0001 (11OTf‚Et₂O); q ) 0.185 (14PF₆‚2MeCN‚0.5Et₂O), 1.35 (5‚2CH₂Cl₂), 2.98 (11OTf‚Et₂O), 2.075 (9‚C₆H₁₄), 2.033 (13OTf)].

^cGOF ) [∑w(|F_o| - |F_c|)²/(N_obs- N_params)]^1/2

.

(SHELX-97⁴⁷for 9‚C₆H₁₄; SIR-97⁴⁹for 11OTf) and subsequent

Fourier syntheses (DIRDIF99).⁵⁰For 14PF₆‚2CH₃CN‚0.5Et₂O, the

C(10) and C(17) atoms were refined with isotropic parameters,

while the solvating Et₂O molecule positioned on the center of

symmetry was modeled over two positions with occupancies of

50%. For 5‚2CH₂Cl₂, the C(2), C(5), C(10), C(16), and C(18) atoms

were refined with isotropic parameters. For 9‚C₆H₁₄, the carbon

atoms of n-hexane were refined with isotropic parameters. All of

the other non-hydrogen atoms were refined on F_o[I > 2σ(I) for

14PF₆‚2CH₃CN‚0.5Et₂O; I > 3σ(I) for the others] by full-matrix

least-squares techniques with anisotropic thermal parameters, while

all of the hydrogen atoms were placed at the calculated positions

(C-H distance of 0.95 Å) with fixed isotropic parameters. The

atomic scattering factors were taken from ref 51. Anomalous

dispersion effects were included in F_c.⁵²The values of ∆f′ and ∆f′′

were taken from refs 53 and 54. The maximum and minimum

residual peaks on the final difference Fourier maps were 0.63/-

0.63, 0.78/-0.88, 1.14/-1.47, 2.66/-2.43, and 2.27/-1.73 for

14PF₆‚2CH₃CN‚0.5Et₂O, 5‚2CH₂Cl₂, 11OTf‚Et₂O, 9‚C₆H₁₄, and

13OTf, respectively. Details of the X-ray diffraction study are

summarized in Table 1, and selected bond lengths, bond angles,

and selected torsion angles between planes are listed in Tables 1S

and 2S in the Supporting Information.

Results and Discussion

Syntheses and Characterizations. The reaction of Mebib

or Phbib with IrCl₃‚4H₂O in methanol gave an insoluble

yellow precipitate (P), of which MALDI-TOF mass spec-

trometry provides the mass peak corresponding to [Ir₂(L)₂-

Cl₄]⁺(L ) Mebib or Phbib). Because the compound was

insoluble in common organic solvents and water, this

compound P was used as a starting material without further

characterization. The corresponding Rh(III) complexes have

been synthesized by the reaction of ligand L with RhCl₃‚

3H₂O, in which the insoluble product might be formulated

as [Rh₂(L)₂X₄] (L ) Hbib and Mebib; X ) Cl, Br, and I) by

1

FAB-MS and H NMR for [Rh₂(Hbib)₂Cl₄] in DMSO-d₆.⁵⁵

New mixed-ligand Ir(III) complexes with the coordination

environment of [Ir(L)(N∧C)Cl] (L ) N∧N∧N and N∧C∧N)

were prepared by the reaction of compound P with the N∧C

ligand under microwave irradiation conditions in glycerol,

where Mebip, Me- or Phbib, and ppy were used as N∧N∧N,

N∧C∧N, and N∧C, respectively (see Chart 1). As for [Ir-

(L)(N∧C)Cl], two geometrical isomers are possible because

N∧C is coordinated to Ir in an asymmetrical manner; that

is, two isomers are shown in Chart 3 as an example of [Ir-

(Mebib)(ppy)Cl].

(49) Altomare, A.; Burla, M.; Camalli, M.; Cascarano, G.; Giacovazzo,

C.; Guagliardi, A.; Moliterni, A.; Polidori, G.; Spagna, R. J. Appl.

Crystallogr. 1999, 32, 115-119.

(50) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de

Gelder, R.; Israel, R.; Smits, J. M. M. DIRDIF99: The DIRDIF-99

Program System; Technical Report of the Crystallography Laboratory;

University of Nijmegen: Nijmengen, The Netherlands, 1999.

(51) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-

lography; Ibers, J. A., Hamilton, W. C., Eds.; Kynoch Press:

Birmingham, England, 1974; Vol. IV, Table 2.2 A.

(52) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781.

(53) Creagh, D. C.; McAuley, W. J. International Tables for X-ray

Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic Publish-

ers: Boston, MA, 1992; Vol. C, Table 4.2.6.8.

When the reaction took place in ethylene glycol under

reflux conditions for 15 h, the reaction product gave a

complicated NMR spectrum, which arose from the formation

of a mixture of the two isomers. However, upon microwave

(54) Creagh, D. C.; Hubbell, J. H. International Tables for X-ray Crystal-

lography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Boston,

MA, 1992; Vol. C, Table 4.2.4.3.

(55) Gayathri, V.; Leelamani, E. G.; Gowda, N. M. N.; Reddy, G. K. N.

Polyhedron 1999, 18, 2351.

8912 Inorganic Chemistry, Vol. 45, No. 22, 2006

Highly Phosphorescent Iridium Complexes

Figure 1. Time-dependent ¹H NMR spectra of [Ir(Mebib)(ppy)Cl] in CD₃-

CN-d₃. Time after dissolution: (1) 0 h, (2) 17 h, and (3) 62 h. Isolated

sample of [Ir(Mebib)(ppy)MeCN](OTf) in CD₃CN-d₃(4).

Chart 3. Two Possible Geometrical Isomers of [Ir(Mebib)(ppy)Cl]

Figure 2. Crystal structures of 5‚2CH₂Cl₂, 9‚C₆H₁₄, 11OTf‚Et₂O, 13OTf,

and 14PF₆‚2MeCN‚0.5Et₂O.

irradiation conditions (6∼12 min irradiation at 650 W) in

glycerol, one geometrical isomer was selectively formed. To

increase the selectivity, the longer microwave irradiation time

and the use of a solvent such as glycerol with a higher boiling

point were required for the preparation of [Ir(Mebip)(N∧C)-

Cl], compared to that of [Ir(Mebib)(N∧C)Cl]. The pure

isomer was obtained by recrystallization from CH₂Cl₂-

hexane. Characterization was carefully done by elemental

during the course of the reaction. The time profile of the

change in absorbance follows first-order kinetics. In contrast,

[Ir(Phbib)(ppy)Cl] (9) and [Ir(Mebip)(ppy)Cl]⁺(14) are less

reactive in CD₃CN under the same conditions and the same

time domain as those for 1 and show no Cl^-substitution

reaction with CD₃CN at room temperature. The sharp

contrast for the reactivity difference between 1 and 14 is

attributable to the electron richness of 1 compared to 14.

[Ir(R³bib)(N∧C)Cl] (R³) Me or Ph) reacted with Ag(OTf)

in CH₃CN to give the cationic complex [Ir(R³bib)(N∧C)-

(CH₃CN)]⁺separately. Further, the reaction of [Ir(Mebib)-

(ppy)(CH₃CN)]⁺(9) with phenylacetylene in the presence

of potassium fluoride provided a neutral complex, [Ir-

(Mebib)(ppy)(CCPh)] (8). All of the Ir(III) complexes were

characterized by ¹H NMR, ESI- and MALDI-TOF MS, and

elemental analysis. Single crystals of 5, 9, 11, 13, and 14

suitable for X-ray crystallographic analysis were obtained

by recrystallization from CH₂Cl₂-hexane or CH₃CN-ether,

some of which included the solvent.

Crystal Structures of 5‚2CH₂Cl₂, 9‚C₆H₁₄, 11OTf‚Et₂O,

13OTf, and 14PF₆‚2MeCN‚0.5Et₂O. The X-ray crystal

structures of 5‚2CH₂Cl₂, 9‚C₆H₁₄, 11OTf‚Et₂O, 13OTf, and

14PF₆‚2MeCN‚0.5Et₂O are shown in Figure 2. The selected

bond distances, bond angles, and torsion angles are provided

in Tables 2, 1S, and 2S (1S and 2S as Supporting Informa-

tion). The coordination geometries of Ir(III) centers for all

four complexes were found to be distorted octahedral. The

torsion angles are provided in Table 2S, showing that three

rings formed by bib tridentate coordination are almost

coplanar and that distortion from the planes is relatively small

1

analysis, H NMR, and ESI or MALDI-TOF mass spectra.

The metathesis reaction of [Ir(Mebib)(N∧C)Cl] with an

anionic ligand X (X ) Br, I, and CN^-) in ethylene glycol

under microwave irradiation gave the corresponding complex

[Ir(Mebib)(N∧C)(X)] (X ) Br, I, and CN) quantitatively.

A

¹H NMR chemical shift of the H_8′proton on the

coordinated ppy of [Ir(Mebib)(ppy)(X)] (X ) Cl, Br, I, and

CN) around δ 10.2∼10.7 ppm is a good indicator of the

anion exchange; that is, the chemical shift reveals the

downfield shift in the order X ) CN > Cl > Br > I, which

is consistent with the increase in the deshielding effect of

the Ir-X bond on neighboring H_8′protons. The Ir complexes

adopt the structure A shown in Chart 3, as confirmed by the

X-ray crystallographic analysis described later.

When [Ir(Mebib)(ppy)Cl] was kept in a CD₃CN solution,

a new set of ¹H signals gradually appeared (t_1/2∼ 20 h; see

1

Figure 1). These new H signals are consistent with those

of [Ir(Mebib)(ppy)(CH₃CN)](OTf) (11), which was sepa-

rately synthesized and isolated from the substitution reaction

of coordinated Cl with CH₃CN in 1. Furthermore, when the

reaction of [Ir(Mebib)(ppy)Cl] (1) in CH₃CN was monitored

by the absorption spectra, the isosbestic points were observed

Inorganic Chemistry, Vol. 45, No. 22, 2006 8913

Obara et al.

Table 2. Selected Bond Lengths (Å) for Complexes 5‚2CH₂Cl₂, 9‚C₆H₁₄, 11OTf‚Et₂O, 13OTf, and 14PF₆‚2MeCN‚0.5Et₂O

11 13

5

9

14

Ir(1)-N(1)

Ir(1)-N(2)

Ir(1)-C(1)

Ir(1)-N(3)

Ir(1)-C(2)

Ir(1)-Cl(1)

2.07(1)

2.04(1)

1.92(2)

2.14(2)

2.00(1)

2.487(4)

Ir(1)-N(1)

Ir(1)-N(2)

Ir(1)-C(1)

Ir(1)-N(3)

Ir(1)-C(2)

Ir(1)-Cl(1)

2.058(8)

2.088(9)

1.92(1)

2.15(1)

1.99(1)

2.474(3)

Ir(1)-N(1)

Ir(1)-N(2)

Ir(1)-C(1)

Ir(1)-N(3)

Ir(1)-C(2)

Ir(1)-N(6)

N(6)-C(34)

2.053(4)

2.050(4)

1.945(6)

2.157(4)

2.013(5)

2.113(5)

1.136(7)

1.472(8)

Ir(1)-N(1)

Ir(1)-N(2)

Ir(1)-C(1)

Ir(1)-N(3)

Ir(1)-C(2)

Ir(1)-N(6)

N(6)-C(44)

2.029(5)

2.059(5)

1.965(6)

2.155(5)

2.007(6)

2.122(5)

1.138(8)

1.468(9)

Ir(1)-N(1)

Ir(1)-N(2)

Ir(1)-N(4)

Ir(1)-N(3)

Ir(1)-C(2)

2.037(8)

2.02(1)

1.972(7)

2.059(7)

2.00(1)

Ir(1)-Cl(1)

2.475(3)

C(34)-C(35)

C(44)-C(45)

the range of 1.90∼1.95 Å.^59,60Further, the Ir-C_bibbond

length in [Ir(Mebip)(Mebib)]²⁺was found to be 2.018 Å,²¹

which is similar to the values reported for Ir(ppy)₃deriva-

tives. Therefore, the Ir-C_bibbond lengths appear to be

influenced by not only the steric requirement for the

tridentate accommodation of Mebib but also electronic

effects. In [Ir(Phbib)(ppy)Cl], the N-phenyl groups adopt a

tilted orientation ∼66° to the tridentate Phbib plane (see

Table 2S, Supporting Information). In 14, the Mebip ligand

is coordinated to the Ir atom in a tridentate manner, and the

Ir-C bond in the Ir-ppy moiety occupies the trans position

of the Ir-Cl bond. The Ir-N_ppybond length is 2.059(7) Å.

The Ir-Cl and Ir-C_ppybond lengths were found to be 2.475-

(3) and 2.00(1) Å, respectively, with a trans influence of

the Ir-C_ppybonds being observed.

Electrochemistry. Electrochemical data for all of the new

Ir(III)-Mebib complexes are summarized in Table 3,

together with the UV spectroscopic data. Complex 14 in CH₃-

CN shows a quasireversible Ir(III/IV) oxidation wave at

+1.05 V versus Fc^+/0and two reduction waves at -1.52

and -1.91 V versus Fc^+/0as reversible and irreversible

processes, respectively. Similarly, a reversible Ir(III/IV)

oxidation of complex 1 in N,N′-dimethyl formamide (DMF)

occurred at +0.42 V versus Fc^+/0, while two quasireversible

and irreversible reduction processes were observed at -2.42

and -2.80 V versus Fc^+/0respectively for Mebib and ppy

reductions. The introduction of an Ir-C bond into Ir(N)₃-

(C)₂X-type complex 1 induces a large negative shift (∼0.6

V) in the Ir(III/IV) oxidation potential, compared to Ir(N)₄-

(C)X-type complex 14. The Ir(III/IV) oxidation potential is

shifted to the negative direction in the order [Ir(Mebib)-

(dfppy)CN] (7) (+0.77 V) > [Ir(Mebib)(ppy)CN] (4) (+0.65

V) > [Ir(Mebib)(dfppy)Cl] (6) (+0.57 V) > [Ir(Mebib)(ppy)-

Cl] (1) (+0.42 V) > [Ir(Mebib)(mppy)Cl] (5) (+0.40 V),

although the first reduction potentials are almost unchanged

for all of these complexes (∼-2.40 V vs Fc^+/0). The

substitution of X from Cl to CH₃CN on [Ir(Mebib)(ppy)X]ⁿ⁺

induces a large positive shift in the oxidation potential.

UV-vis Absorption Spectra. The UV-vis spectrum of

1 in CH₂Cl₂shows the MLCT bands at 425, 472, and 523

nm in addition to the ππ* transition at 244 nm for ppy and

299 and 360 nm for the Mebib ligand (see Figure 3). Upon

the substitution of Cl by CN or ppy by dfppy, a shorter

wavelength shift of the MLCT band was observed (Figure

Chart 4. Tridentate Pincer Ligand Systems

except in 4 and 5. The X-ray structure of 5 reveals that the

Ir-C_bibbond in the tridentate Ir(Mebib) moiety is located

trans to the nitrogen atom in the mppy ligand, while the other

Ir-C_ppybond in the Ir(mppy) moiety is trans to the Ir-Cl

bond, confirming the structure of the isomers as isomer A

in Chart 3. For the structure of 11, the CH₃CN ligand

occupied a trans position in relation to the coordinating

carbon atom in ppy, as shown in Figure 2. In four complexes,

5, 9, 11, 13, two Ir-C bonds were located at the cis position.

The cyclometalated Ir-C bonds in the mixed-ligand Ir(bib)-

(ppy) system induce a strong trans influence on the bond

length; that is, the bond lengths of the central Ir-C_bibbond

are 1.92(2) and 1.945(6) Å for 5 and 9, while the Ir-N_ppy

bonds, which are located trans to the Ir-C_bibbond, are 2.14-

(2) and 2.157(4) Å, respectively (Table 2). These Ir-N_ppy

bond lengths are longer than those of Ir-C_ppy[2.00(1) and

2.013(5) Å], which are within the range of typical Ir-C bond

lengths. Furthermore, the Ir-Cl bond length at the position

trans to the Ir-C_ppybond in 5 [2.487(4) Å] is longer than

that seen in [Ir(tpy)(dmbpy)Cl]²⁺(tpy ) 2,2′:6′,2′′-terpyri-

dine, dmbpy ) 4,4′-dimethyl-2,2′-bipyridine) (2.357 Å).⁵⁶

These results indicate that the strong σ donation in the Ir-C

bond results in the elongation of Ir-N and Ir-Cl bonds at

the trans position. A similar trans influence of Ir-C bonds

has been observed in the X-ray crystallographic structures

of fac- and mer-Ir(tppy)₃[tppy ) 2-(p-tolyl)pyridine].¹⁰In

particular, the Ir-C_bibbond lengths are relatively short

compared to those of other Ir-C bonds reported in fac-Ir-

(tppy)₃derivatives (average Ir-C ) 2.018 Å). The two fused

five-membered coordination modes in Ir(N∧C∧N) com-

plexes are relevant to the “pincer”-ligated metal complexes

such as [Ir(P∧C∧P)H₂], [Pt(N∧C∧N)Cl], and [Pd(N∧C∧N)l]

(see Chart 4), in which the Ir-C bond distances range from

2.00 to 2.09 Å^57,58and the Pd or Pt-C bond distances are in

(56) Yoshikawa, N.; Sakamoto, J.; Kanehisa, N.; Kai, Y.; Matsumura-Inoue,

T.; Takashima, H.; Tsukahara, K. Acta Crystallogr., Sect. E 2003,

59, m830.

(57) Gottker-Schnetmann, I.; LastName, W. P.; Brookhart, M. Organo-

metallics 2004, 23, 1766.

(59) Gagliado, M.; Rodriguez, G.; Dam, H. H.; Lutz, M.; Spek, A. L.;

Havenith, R. W. A.; Coppo, P.; De Cola, L.; Hartl, F.; van Klink, G.

P. M.; van Koten, G. Inorg. Chem. 2006, 45, 2143.

(58) Zhang, X.; Emge, T. J.; Ghost, R.; Goldman, A. S. J. Am. Chem. Soc.

2005, 127, 8250.

(60) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weistein, J. A.; Wilson,

C. Inorg. Chem. 2003, 42, 8609-8611.

8914 Inorganic Chemistry, Vol. 45, No. 22, 2006

Highly Phosphorescent Iridium Complexes

Table 3. Absorption Spectral and Electrochemical Data for Ir(Mebib)(R-ppy)X at Room Temperature^a,b

λ_abs

E_1/2^ox(∆E_p)^cor E_Pa

E_1/2^red(∆E_p)^cor E_Pc

complex

nm (ꢀ × 10³/M^-1cm^-1

)

V vs Fc⁺/Fc (mV)

0.42 (60)

V vs Fc⁺/Fc (mV)

[Ir(Mebib)(ppy)Cl] (1)

244 (50.0), 299 (38.7), 360 (7.7), 404 (14.0),

425 (15.6), 472(3.1), 523(0.84)

-2.46 (66),^d-2.80^e

-2.45 (68),^d-2.82^e

-2.44 (70),^d-2.81^e

-2.42 (66),^d-2.79^e

-2.46 (65),^d-2.82^e

-2.40 (65),^d-2.79^e

-2.37 (66),^d-2.70^e

-2.49 (65),^d-2.90^e

[Ir(Mebib)(ppy)Br] (2)

[Ir(Mebib)(ppy)I] (3)

244 (40.0), 300 (29.4), 361 (5.44), 403 (11.4),

424 (12.9), 470sh (2.64), 531 (0.50)

250sh (53.5), 267sh (42.4), 299 (38.5), 359 (8.66),

404 (15.1), 423 (15.9), 474sh (3.49),531 (0.88)

242 (54.9), 299 (37.8), 353 (7.9), 392 (15.8),

413 (19.0), 506 (0.35)

236 (50.0), 300 (44.6), 361 (6.5), 405 (13.5),

426 (15.2), 473 (3.0), 526 (0.75)

245 (61.1), 299 (45.6), 355 (8.7), 397 (17.4),

418 (18.7), 455 (3.7), 514 (0.50)

0.46 (60)

0.45 (60)

0.65 (59)

0.40 (70)

0.57 (60)

0.77 (58)

0.61 (65)

[Ir(Mebib)(ppy)CN] (4)

[Ir(Mebib)(mppy)Cl] (5)

[Ir(Mebib)(dfppy)Cl] (6)

[Ir(Mebib)(dfppy)CN] (7)

[Ir(Mebib)(mppy)(CCPh)] (8)

242 (52.4), 298 (35.0), 349 (7.7), 386 (15.2),

407 (17.6), 498 (0.30)

236 (68.3), 272(56.2), 298 (63.7), 360 (6.9),

419 (12.9), 437 (15.9), 482 (3.4), 530 (0.88)

301 (39.3), 406 (15.3), 429 (17.1), 527 (0.52)

298 (43.0), 393 (18.1), 414 (21.5), 510 (0.38)

247 (50.5), 298 (37.2), 380 (17.9), 398 (18.2),

496 (0.36)

245 (65.4), 296 (50.1), 379 (22.2), 396 (23.1),

490 (0.27)

245sh (64.9), 297 (32.8), 381 (17.3), 400 (17.4),

498 (0.61)

[Ir(Phbib)(ppy)Cl] (9)

0.52 (60)

0.78 (60)

0.81 (65)

-2.36 (68),^d-2.78^e

-2.24 (60),^d-2.69^e

-2.32,^e-2.57,^e-2.75^e

[Ir(Phbib)(ppy)CN] (10)

[Ir(Mebib)(ppy)(MeCN)]⁺(11)^b

[Ir(Mebib)(mppy)(MeCN)]⁺(12)

[Ir(Phbib)(ppy)(MeCN)]⁺(13)

[Ir(Mebip)(ppy)Cl](PF₆) (14)

f

242 (38.2), 313 (27.2), 348 (23.9), 510 (0.42),

575 (0.14)

1.05^e

-1.52 (68),^d-1.91^e

^aUV-vis spectra were measured in CH₂Cl₂and electrochemical data in DMF unless otherwise noted. ^bElectrochemical measurement was done in CH₃CN.

^cPeak-to-peak separation. ^dQuasireversible process. ^eIrreversible process. ^fNot measured.

Figure 3. UV spectra of Ir complexes 1 (red) and 4 (black) in CH₂Cl₂and 14 (black, dashed line) in CH₃CN at room temperature.

3). In contrast, the UV absorption maxima of [Ir(Mebip)-

(ppy)X] are unaltered by the substitution of X ) Cl to Br or

I, which is unexpected. Further, the alternation of methyl to

a phenyl group on the benzimidazole moiety leads to a

slightly longer wavelength shift on the MLCT band, while

the spectral shapes are almost identical. The band around

510∼530 nm can be assigned as a triplet MLCT band,

compared to other reported Ir(ppy) complexes. This MLCT

band for [Ir(Mebib)(R-ppy)X] is shifted to a shorter wave-

length in the order 5 > 1 > 6 > 4 > 7, and at the same

time, the intensity of the band is decreasing in this order

(Table 3).

Emission Spectra. Figure 4 shows the emission spectra

of 1 in Ar-saturated CH₂Cl₂at 295 K and CH₂Cl₂-toluene

(1:1 v/v) at 77 K. The complex exhibits a strong emission

band with a vibrational structure at around 555 nm with a

lifetime of 1.20 µs at room temperature. In a glass matrix at

77 K, the emission spectra of all of the Ir(III)-Mebib and

Ir(III)-Phbib complexes exhibited almost the same vibra-

tional structures with a small sideband at about 1500 cm^-1

lower, while for the Ir-Mebip complex 14, a vibrational

sideband appeared approximately 1360 cm^-1lower with a

relatively high intensity. Emission maxima of 1 were tuned

by the substitution of ppy or Cl; that is, the maxima were

shifted to a shorter wavelength in the order 1 ∼ 5 > 6 > 4

> 7 (Figure 5 and Table 4). A strong relationship between

UV spectral data and electrochemical data has been re-

ported.⁶¹In the Ir-Mebib complexes, plots of emission

maxima versus the redox potential are shown in Figure 6.

The Ir(III/IV) oxidation potential shows a good linear

relationship with the emission maxima; in contrast, the

reduction potential appears to remain almost constant upon

changes in the emission maxima. This linear relationship

strongly indicates that the excited state of [Ir(L)(N∧C)X]

complexes has considerable MLCT character.

The emission maxima of the Ir complexes shift to higher

energy (600∼675 cm^-1) on going from room temperature

to 77 K. A relatively large rigidochromic shift indicates a

(61) Vlcek, A. A.; Dodsworth, E. S.; Pietro, W. J.; Lever, A. B. P. Inorg.

Chem. 1995, 34, 1906 and references therein.

Inorganic Chemistry, Vol. 45, No. 22, 2006 8915

Obara et al.

Figure 4. Emission spectra of complexes 1 (a) and 9 (b) in Ar-saturated CH₂Cl₂at room temperature (red) and in CH₂Cl₂-toluene (1:1 v/v) (red) at 77

K (blue) and 14 (c) in Ar-saturated CH₃CN at room temperature (red) and in DMF-EtOH-MeOH (1:5:5 v/v) at 77 K (blue).

s^-1for [Ir(Mebib)(ppy)CN] to 5.5 × 10⁵s^-1for [Ir(Mebib)-

(mppy)Cl], while those of the nonradiative rate constant (k_nr)

are from 0.6 × 10⁵s^-1for [Ir(Mebib)(dfppy)CN] to 1.6 ×

10⁵s^-1for [Ir(Mebib)(mppy)Cl]. The k_rvalues are greater

than those reported for [Ir(ppy)₃] (2.0 × 10⁵s^-1) and [Ir-

(ppy)₂(acac)] (2.1 × 10⁵s^-1). The substitution of the ligand

L from Mebip to Mebib in [Ir(L)(ppy)Cl] leads to a shorter

wavelength shift of emission maxima and an increasing

quantum yield. On the other hand, the emission maxima of

[Ir(Mebib)(ppy)X] are unaltered by the substitution of Xd

Cl to Br or I, which is reflected in the UV absorption spectra.

As for the complexes [Ir(Mebib)(ppy)(CH₃CN)]⁺and [Ir-

(Mebib)(mppy)(CH₃CN)]⁺, the substitution of Cl with CH₃-

Figure 5. Emission spectra of Ir complexes 7 (blue line), 4 (green), 6

(orange), and 1 (red) in CH₂Cl₂-toluene (1:1 v/v) at 77 K.

CN leads to a drastic decrease in the luminescent quantum

yield (Φ ) 0.24 and 0.22, see Table 4), while a high quantum

yield is maintained for the complex [Ir(Mebib)(mppy)-

(CCPh)] prepared by the replacement of Cl in the complex.

Plots of k_rand ln k_nrvalues versus emission energy are shown

in Figures 7a and b. Although radiation theory predicts that

the k_rvalue is proportional to the cube of the emission

greater polarity of the excited state. A similar rigidochromic

effect has been previously reported for [Ir(ppy)₃] derivatives

and emissive metal complexes.^9,62Surprisingly, all of the

present neutral Ir(III) complexes [Ir(L)(R-ppy)X] (L ) Mebib

and Phbib; R ) H, CH₃, and F; X ) Cl and CN) exhibited

a high luminescent quantum yield (Φ ) 0.63∼0.95), with

energy,⁶³the k_rvalues of these Ir complexes fall as the

[Ir(Phbib)(ppy)Cl] having the highest yield of Φ ) 0.95.

emission energy increases (Figure 7a). However, employing

Several Ir complexes containing bidentate ppy derivatives

Mebip instead of Mebib leads to a decrease in the emission

have been reported to have high quantum yields; for example,

energy, and the k_rvalues decrease considerably. The varia-

tions in the radiative rate with tridentate ligands or X are

relevant to the phosphorescence mechanism of these Ir

complexes, and evaluations of k_rvalues based on DFT

calculations are described later.

[Ir(tppy)₃] and [Ir(ppbi)₂(acac)] have luminescent quantum

yields of 0.54 and 0.78, respectively.^3,10However, variations

in the substituent of ppy leads to large changes in the

quantum yield. In the present system, the luminescent

quantum yields were high even if the substituent of ppy was

changed. The values of the radiative rate constant (k_r) for

the [Ir(Mebib)(R-ppy)X] complexes range from 3.9 × 10⁵

On the basis of the linear relationship of ln k_nrversus the

emission energy (Figure 7b), the present Ir system closely

(62) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949-

1960.

(63) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814.

8916 Inorganic Chemistry, Vol. 45, No. 22, 2006

Highly Phosphorescent Iridium Complexes

Table 4. Photophysical Properties of [Ir(Mebib)(Rppy)X] in Ar-Saturated CH₂Cl₂

complex

λ

_em/nm

77 K^b

S_M, ν/cm^-1

77 K

τ/ms

Φ^a

k_r

k_nr

295 K

555

295 K

77 K^b

10⁵s^-1

[Ir(Mebib)(ppy)Cl](1)

535, 580, 633

0.85, 1490

1.78

(1.61)^c

1.73

1.83

2.17

1.40

1.63

2.50

1.46

1.55

2.02

0.96

0.79

2.00

2.22

(2.84)^c

2.28

2.50

2.28

2.26

2.70

2.33

2.17

2.45

2.82

2.86

13.5

0.78

(0.59)^c

0.87

0.81

0.84

0.77

0.86

0.85

0.75

0.95

0.89

0.24

0.22

0.16

4.4 ( 0.30

(3.7 ( 0.35)^c

5.0 ( 0.26

4.4 ( 0.25

3.9 ( 0.26

5.5 ( 0.36

5.3 ( 0.33

3.4 ( 0.20

5.1 ( 0.30

6.1 ( 0.29

4.4 ( 0.24

2.5 ( 0.52

2.9 ( 0.75

0.80 ( 0.25

1.2 ( 0.30

(2.6 ( 0.35)^c

0.8 ( 0.26

1.0 ( 0.25

0.7 ( 0.26

1.6 ( 0.36

0.9 ( 0.33

0.6 ( 0.20

1.7 ( 0.30

0.3 ( 0.29

0.5 ( 0.24

7.9 ( 0.52

9.9 ( 0.75

4.20 ( 0.25

[Ir(Mebib)(ppy)Br](2)

[Ir(Mebib)(ppy)I](3)

[Ir(Mebib)(ppy)CN](4)

553

526

556

540

517

563

562

533

513

512

610

532, 579, 632

533, 579, 632

510,552,600

536,582,636

521, 565,616

501, 542, 590

542, 590, 645

541, 588, 645

515, 558, 608

520, 564, 614

519, 562, 614

578

0.78, 1490

0.76, 1480

0.84, 1510

0.77, 1490

0.82, 1480

0.82, 1490

0.74, 1520

0.74, 1490

0.80, 1490

0.83, 1510

0.85, 1500

1.05, 1360

[Ir(Mebib)(mppy)Cl](5)

[Ir(Mebib)(dfppy)Cl] (6)

[Ir(Mebib)(dfppy)CN] (7)

[Ir(Mebib)(mppy)(CCPh)] (8)

[Ir(Phbib)(ppy)Cl] (9)

[Ir(Phbib)(ppy)CN](10)

[Ir(Mebib)(ppy)(MeCN)]⁺(11)

[Ir(Mebib)(mppy)(MeCN)]⁺(12)

[Ir(Mebip)(ppy)Cl](PF₆) (14)

^aThe emission quantum yields of the Ir(III) compounds were determined using DPA () 0.91)²⁴as a reference. The error in the yields is (5%. ^bIn

CH₂Cl₂-toluene (1:1 v/v). ^cIn Ar-saturated CH₃CN.

Figure 6. Plots of emission energies vs redox potentials (the number in

the plots corresponds to the complex numbering).

Figure 8. Frontier molecular orbitals of complex 5, which were obtained

by DFT calculation at the B3PW91/LANL2DZ level.

fashion, while the lowest unoccupied molecular orbital

(LUMO) and LUMO+1 originate from Mebib π* and ppy

π* orbitals, respectively.

The energy differences between the HOMOs and occupied

Figure 7. Plots of k_rvalues vs emission energy (a) and plots of ln k_nrvs

emission energy (b).

orbitals with a larger contribution of Ir 5d are one of the

important factors determining the k_rvalues of phosphores-

cence.^65,66Orbital energy levels for complexes 5, 11, and

14 are depicted in Figure 9 together with their orbital

parentage. The differences in orbital energy between HOMO

and HOMO-1 are 0.20 eV for 5, 0.33 eV for 11, and 0.50

follows the energy gap law,⁶⁴except for X ) CH₃CN. The

Ir complexes with CH₃CN show somewhat shorter lifetimes

at room temperature, while the lifetimes at 77 K are

comparable to those of the other Ir-Mebib complexes,

suggesting the presence of additional decay pathways involv-

3

ing thermally accessible excited states such as via dd.

eV for 14, reflecting orbital interactions with the ligands.

DFT Calculations. To shed light on the characterization

The Ir d_xyorbital in these Ir complexes is the most

of the emitting states in the present iridium complexes, DFT

destabilized among the Ir t_2gorbitals because of π donation

calculations were carried out for the Ir complexes with

from both a phenyl anion in ppy and imidazole moieties in

available X-ray structures. Frontier molecular orbitals (MOs)

Mebib or Mebip and thus contributes to the HOMO.

of complex 5 obtained at the B3PW91/LAN2DZ level are

HOMO-1 of these Ir complexes is composed of the Ir d_yz

shown in Figure 8. For complex 5, both the highest occupied

orbital, which is destabilized by mixing with π orbitals of a

molecular orbital (HOMO) and HOMO-1 are formed from

a combination of Ir 5d and Cl 2p orbitals in an antibonding

(65) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem.

2003, 42, 6366.

(64) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.

Chem. 1986, 90, 3722.

(66) Siddique, Z. A.; Ohno, T.; Nozaki, K.; Tsubomura, T. Inorg. Chem.

2004, 43, 663.

Inorganic Chemistry, Vol. 45, No. 22, 2006 8917

Obara et al.

The replacement of Cl by CH₃CN with weak π-donating

ability does not give remarkable change in k_r, as shown in

Table 3.

Mechanism of High Phosphorescence Yield in [Ir-

(Mebib)(ppy)Cl]. The phosphorescence of transition metal

complexes originates from the mixing of emissive singlet

states into the lowest triplet state due to spin-orbit coupling

of the d electrons. The transition dipole moments of the

singlet states are therefore the origin of the radiative

capability of the lowest triplet state. The radiative rate

constant k_rof phosphorescence can be calculated from the

transition dipole moment of the perturbed triplet state using

the following expression:

16π³× 10⁶E³n³

2

k_r)

|M|

(1)

3hꢀ₀

Figure 9. MO levels calculated at the B3PW91/LANL2DZ level for X-ray

structures of [Ir(Mebib)(mppy)Cl] (5) (a), [Ir(Mebib)(ppy)CH₃CN]⁺(11)

(b), and [Ir(Mebip)(ppy)Cl]⁺(14) (c). The molecule is oriented so that the

z axis is the C₂axis of the tridentate ligand and the y axis is parallel with

the Ir-X bonding.

where E is the emission energy and n, h, and ꢀ_oare the

refractive index of the medium, Planck’s constant, and the

vacuum permittivity, respectively. According to the first-

order perturbation theory, the transition dipole moment from

the triplet state to the ground state M_Tis given by

phenyl moiety in Mebib involving 5 and 11 or the pyridyl

moiety in Mebip of 14; however, the Ir d_yzorbital of 14 is

less destabilized because of the weaker π-donating ability

of Mebip, and thus, the energy difference between HOMO

and HOMO-1 is much larger than in the Ir-Mebib com-

plexes. As shown in Figure 9, the energy gap in 5 is smaller

than that for 11 because Ir d_xyand d_yzorbitals are considerably

destabilized by the antibonding interaction with the Cl 2p_x

and 2p_zorbitals, respectively.

Ψ_T|H_SO|Ψ_Sn

M )

M_Sn

(2)

∑

T

³E_T- ¹E_S

n

where Ψs and Es are eigenfunctions and eigenvalues of the

Hamiltonian without spin-orbit coupling (H_SO), respectively.

M_Sis the transition dipole moment from the nth singlet state

n

to the ground state. Thus, the radiative rate constant of the

phosphorescence is dominated by three factors: spin-orbit

coupling matrix element, the energy gap between T₁and S_n,

and Ms.

The LUMO of these Ir-Mebib complexes is the π*

orbitals of Mebib, which is consistent with the observed

invariance of the first reduction potentials in the Ir-Mebib

complexes. Further, the π* orbitals of ppy are located at a

higher energy than those of Mebib. For complex 14, the

LUMO and LUMO+1 are localized on the 2-pyridylbenz-

imidazolyl moiety (pbim) in Mebip, which is probably due

to the deconstructive interaction between the π* orbitals of

two pbim moieties through the phenylene group. Conse-

quently, the promoted electron in MLCT states of Ir-Mebip

complexes is not delocalized over the Mebip ligand but

instead tends to be localized on one of the pbim moieties,

which is probably responsible for the pronounced vibrational

sideband in the emission spectra of complex 14 (Figure 4c).

Time-dependent DFT calculations revealed that the elec-

tronic configuration of the lowest triplet MLCT state involves

primarily the HOMO and LUMO (Tables 3S, 4S, and 5S in

the Supporting Information), and therefore, the emitting state

consists of the combination of Ir dπ to Mebib π* ligand

charge transfer (MLCT) and Cl 3p to Mebib π* (XLCT) in

the Ir complexes with the Cl anion. In particular, the orbital

mixing in an antibonding fashion between Ir dπ and Cl 2p

orbitals plays an important role in controlling the MLCT

characters in these complexes, which can determine the

excited-state properties such as radiative and nonradiative

rate constants and quantum yields. Concerning the complex

having CH₃CN as X, the lowest state is characterized by a

mixture of MLCT and ππ* of the ppy and Mebip ligands.

The radiative rate of ³ππ* in Rh(III) compounds has been

proposed to be the result of two entirely different mechan-

isms.^67-69In one mechanism, the increase in the radiative

character induced by heavy metal is ascribed to a direct

3

1

spin-orbit coupling between ππ* and dπ* () MLCT)

electronic configurations.⁶⁷This hypothesized mechanism,

however, does have drawbacks. Because the spin-orbit

3

1

coupling between ππ* and dπ* involves only the two-

center integrals on the metal center, the magnitudes of the

spin-orbit coupling are usually quite small. Therefore, the

spin-orbit coupling between ³ππ* and ¹dπ* does not seem

to be responsible for the large effect of the heavy metal.

Accordingly, Miki et al. have proposed a new mechanism

3

in which ππ* is mixed with dπ* by a configurational

3

1

interaction because the mixing between dπ* and dπ* is

induced effectively by one-center spin-orbit coupling.^68,69

Thus, the magnitude of the spin-orbit coupling element in

3

eq 2 increases as the contribution of the dπ* electronic

configuration in the lowest triplet state increases. In an

(67) Komada, Y.; Yamauchi, S.; Hirota, N. J. Phys. Chem. 1986, 90, 6425.

(68) Miki, H.; Shimada, M.; Azumi, T.; Brozik, J. A.; Crosby, G. A. J.

Phys. Chem. 1993, 97, 11175.

(69) (a) Azumi, T.; Miki, H. Top. Curr. Chem. 1997, 191, 1. (b) Solomon,

E. I.; Lever, A. B. P. Inorganic Electronic Structure and Spectroscopy;

John Wiley & Sons: New York, 1999; Vol. I, Chapter 1.

8918 Inorganic Chemistry, Vol. 45, No. 22, 2006

Highly Phosphorescent Iridium Complexes

Table 5. Data Used for the Evaluation of Phosphorescence Radiative Rate Constants

Ir(Mebib)(mppy)Cl

[Ir(Mebib)(ppy)CH₃CN]⁺

(11)

[Ir(Mebip)(ppy)Cl⁺

(5)

(14)

T₁

S_n

E/eV^a

C_dxy

1.97

0.63

2.62

0.58

1.72

0.56

b

E/eV^a

2.49 (S₂)

0.58

0.074

3.43 (S₃)

0.48

0.227

2.50 (S₃)

0.44

0.024

b

C_dyz

f ^a

∆E (S_n-T₁)/ cm^-1

SOC (S_n-T₁)/cm^{-1 c}

k_r/s^-1(n ) 1.42)^d

k_r,obsd/s^-1(CH₂Cl₂)

4190

6530

6290

809

617

546

1.0 × 10⁶

1.3 × 10⁶

4.6 × 10⁴

4.4 × 10⁵

2.5 × 10⁵

8.0 × 10⁴

^aObtained from the calculation at the TD-B3PW91/LANL2DZ level. ^bCoefficient of the natural atomic orbital of Ir 5d in HOMO (T₁) or HOMO-1 (S_n)

obtained from NBO analysis. ^cAbsolute value of the spin-orbit coupling matrix element calculated from eq 4. ^dValue calculated using eq 3.

alternative interpretation using recent MO theory, the elec-

tronic state is often described by a linear combination of

electronic transitions between delocalized MOs constructed

by a combination between metal-centered and ligand-

centered orbitals. Using the MO model, the coupling element

in eq 2 increases with the increase in the contribution of d

orbitals in the MOs ascribed to the lowest triplet state.

Because the DFT calculation for the present Ir complexes

indicates a greater contribution of the Ir 5d orbital in the

HOMO, as shown in Figures 8 and 9, one-center spin-orbit

rescence mechanism is then confirmed by evaluating the k_r

value using a crude approximation of the one-center spin-

orbit coupling element. From the definition of oscillator

2

strength in vacuo f ) 8π²mE|M| /3he²and eq 2, eq 1 can be

transformed as

3

n³E_TS|H_SO|T

f_S

2

k_r(T₁) ≈

×

(3)

(

)

1.5 E_S- E_T

E_S

where the emission energy E and matrix element are in cm^-1.

Among the spin-orbit coupling elements between S₂(d_yzfπ*)

and triplet sublevels of T₁(d_xyfπ*), that is, T_1,x, T_1,y, and

T_1,z, only the element involving T_1,yhas a nonzero value.

When the element between spin-orbitals involving ú () d_xy)

and ê () d_yz) given in eq 9 in ref 69a or eq 67 in ref 69b,

the one-center spin-orbit coupling element can be evaluated

as

3

1

coupling between dπ* and dπ* is considered to be the

predominant mechanism for the intense phosphorescence of

the complexes.

Theoretical consideration of spin-orbit coupling places

1

further restrictions on the electronic configuration of dπ*,

which can be coupled with ³dπ* in the lowest triplet

state.^65,66,70Both ³dπ* and ¹dπ* should involve the same π*

orbital because spin-orbit couplings of π electrons are

negligibly small. Furthermore, as any atomic angular mo-

menta in low-symmetry transition metal complexes are

quenched, the spin-orbit coupling of the d electron works

between the dπ* configurations involving different d orbitals.

Consequently, under the assumption of one-center spin-orbit

integrals, the ³dπ* electronic configuration in the lowest state

is effectively coupled with the specific ¹dπ* configurations

involving the d orbital with a different orientation and having

a common π* orbital.^65,66,70

i

2

S₂|H_SO|T_1,y) ( C_dxyd^R_xy|H_SO|C_dyzd_y^â_z

-

i

C_dxyd^â_xy|H_SO|C_dyzd^R_yz) ) ú_Ir-5dC_dxyC_dyz(4)

2

where ú_Ir-5dis the one-electron spin-orbit coupling constant

of the 5d electron in iridium and C_dxyand C_dxzare the

coefficients of the 5d orbital involved in HOMO and HOMO-

1, respectively. Using theoretical values of ú_5d-Ir) 4430

cm^-1for the Ir(III) ion,^70,71∆E (S₂-T₁) ) 4190 cm^-1, and

f ) 0.074 obtained by TDDFT and the coefficients of natural

atomic orbitals obtained using an NBO 5.0 program⁷²of C_dxy

) 0.63, C_dxz) 0.58, and n ) 1.42 (CH₂Cl₂), eq 3 gives 1.0

× 10⁶s^-1as the k_rvalue of complex 5. For complex 11, a

similar analysis indicates that spin-orbit coupling with S₃

is the origin of the radiative probability of the T₁state, and

the k_rvalue is calculated as 1.3 × 10⁶s^-1(Table 5).

The temperature-dependent density functional theory (TD-

DFT) calculation of complex 5 at the B3PW91/LANL2DZ

level indicates that the lowest triplet state (T₁) is the HOMO

f LUMO electronic configuration in which the donor 5d_xy

orbital interacts with the accepting Mebib π* in a pseudo-δ

bonding fashion. As described above, spin-orbit coupling

of the Ir 5d electron effectively mixes 5d_xyf LUMO in T₁

with the other MLCTs from different d orbitals to LUMO.

A rather smaller k_rvalue of 4.6 × 10⁴s^-1is predicted for

the Ir-Mebip complex 14. Thus, it turns out that the

evaluation based on one-center spin-orbit coupling almost

reproduces the magnitude of k_rvalues of the present Ir

Thus, T₁is mixed with 5d_yzf LUMO in S₂and 5d_xz

f

LUMO in S₆(see Figure 9 and Table 3S). Because the

oscillator strengths (f) of S₂and S₆are 0.074 and 0.009,

respectively, the intense phosphorescence of complex 5

originates from the mixing between S₂and T₁because of

the spin-orbit coupling of Ir 5d electrons. This phospho-

(71) Fraga, S.; Saxena, K. M. S.; Karwowski, J. Handbook of Atomic Data.

Physical Sciences Data; Elsevier: Amsterdam, The Netherlands, 1976;

Vol. 5.

(72) Weinhold, F.; Landis, C. R. Chem. Ed.: Res. Practice Eur. 2001, 2,

(70) Nozaki, K. J. Chin. Chem. Soc. 2006, 53, 101.

91.

Inorganic Chemistry, Vol. 45, No. 22, 2006 8919

Obara et al.

M_CT) φ_d|µ|φ_π*+ λ_a( φ_π*|µ|φ_π*- φ_d|µ|φ_d) +

λ_bφ_π|µ|φ_π*+ λ_cφ_π′*|µ|φ_π*+ λ_fφ_d|µφ_p+ λ_gφ_d|µ|φ_p*

(6)

complexes. The relatively large deviation for complex 11

can be ascribed to the oversimplifications of spin-orbit

interactions: a more reliable evaluation would require the

consideration of spin-orbit interactions with many excited

states and geometrical changes of emitting states.⁷⁰Neverthe-

less, the present simplified analysis can rationalize the

observed variation of k_rvalues. As shown in Table 4, the

replacement of Cl by CH₃CN does not cause a large change

in k_r. HOMO-1 in the Cl complex 5 is composed of Ir 5d

and Cl 3p orbitals, while HOMO-1 in the CH₃CN complex

11 is produced from Ir 5d and Mebib π orbitals. The larger

contribution of Mebib π orbitals in HOMO-1 in complex

11 gives rise to the stronger oscillator strength of S₃

(HOMO-1 f LUMO) from which the T₁state borrows its

intensity. On the other hand, the larger contribution of Mebib

orbitals in HOMO-1 increases the magnitude of the exchange

integral in S₃and thereby increases the energy gap ∆E(S₃-

T₁), which compensates for the increase in the oscillator

strength of S₃. Consequently, the resulting k_rvalue of

phosphorescence is almost the same as that of complex 5.

In the Mebip complex (14), because the LUMO is

localized on the pbim moiety, as described above, it mixes

only weakly with HOMO-1 with C_ssymmetry, which is

responsible for the rather small oscillator strength of S₃

(HOMO-1 f LUMO). Moreover, the energy gap ∆E(S₃-

T₁) is larger than that for complex 5 because of a larger

energy difference between HOMO and HOMO-1 (Figure 9).

Both of these effects remarkably reduce the k_rvalue of

phosphorescence in the Mebip complex.

The TDDFT calculation indicates that the transition dipole

moments of S₂in 5 and S₃in 11 are oriented along the x

axis, not along the CT direction, that is, the C₂axis of the

Ir-Mebib subunit. The origin of the transition dipole moment

of MLCT in a mononuclear complex has been studied by

Day and Sanders.⁷³When a transition metal ion is coordi-

nated to ligands, the d orbitals interact with the π* orbitals

to form the donor and acceptor orbitals ascribed to CT

transitions. Mixing of the other metal-centered or ligand-

centered orbitals will lead to other sources of intensity. The

donor and the acceptor orbitals ascribed to the CT transition

are then given by

where µ denotes a dipole operator. The first term is the

contact CT term, which is usually negligible. The second is

the product of a mixing coefficient and the dipole moment

of the transferred charge, often called the transfer term. The

transfer term should be the origin of the intensity of the

MLCT transitions between the d and π* orbitals with a large

resonance integral, and therefore, the transition dipole

moment is oriented along the CT direction. The intense

MLCT absorption in tris(2,2′-bipyridine) complexes of Fe-

(II), Ru(II), and Os(II) with a D₃point group is ascribed to

the transfer term in e f e transitions.^74-76Recent theoretical

investigation of the phosphorescence in [fac-Ir(ppy)₃] and

[Ru(bpy)₃]²⁺has revealed that the lowest triplet state borrows

1

the transfer term of the dπ* (b₁f b₁) transition in the

pseudo-C_2Vpoint group.⁷⁰

The other four terms in eq 6 are the moments borrowed

from interunit transitions. The third term is the moment

borrowed from ¹ππ* due to d-orbital delocalization over the

ligands. The fourth term is borrowed from the transition

between π* and π′*. The latter is the unoccupied orbital

mixed into the d orbital through π back-donation. The fifth

and sixth terms are the moments of Laporte-allowed metal-

centered transition d T p. The intensity-borrowing mecha-

nism could possibly contribute to the CT intensity when the

resonance integral between d and π* is very small because

of their unfavorable overlapping. In particular, when the d

and π* orbitals interact in a π-bonding fashion but are

different in symmetry, the d orbital is mixed with π and π′*,

for which the symmetry is different from that of the accepting

π* orbital. In such complexes, the CT band may borrow

intensity from the π f π* transition or from the transition

between unoccupied ligand orbitals π′* T π*. The intensity

of the e f a₂transition in [Ru(bpy)₃]²⁺with the D₃point

group is considered to borrow from the transition between

unoccupied ligand orbitals e f a₂.⁷⁶

The contributions of parent fragment orbitals in each

frontier MO of Ir(Mebib)(mppy)Cl (5) were calculated using

the ADF program package and are summarized in Table 6.

Because the contributions were calculated at the pure DFT

level, they are somewhat different from those determined

by the hybrid B3PW91 calculations shown in Figure 9 and

Table 5; in particular, the contributions of Cl 3p in HOMO

and HOMO-1 seem to be rather large compared with those

at the hybrid DFT level. The HOMO is characterized

primarily by antibonding between Cl 3p_xand Ir 5d_xy, with a

small contribution of the occupied 31st MO in mppy (ppy-

31) and the 61st MO in Mebib (bib-61) and of the

unoccupied 64th MO in Mebib (bib-64*). The HOMO-1 is

antibonding between Cl 3p_zand Ir 5d_yzwith small contribu-

tions of bib-62, bib-65*, and bib-68*. While the LUMO is

formed from bib-64*, the LUMO+1 is from ppy-33*. The

transition dipole moment of the CT transition GS T S₂

cannot be accounted for by the transfer term because the

donor d_yzand the acceptor bib-64* orbital are different in

φ_D) φ_d+ λ_aφ_π*+ λ_bφ_π+ λ_cφ_π′*

φ_A) φ_π*- λ_aφ_d+ λ_fφ_p+ λ_gφ_p*(5)

where φ_dand φ_pare the metal-centered d and p orbitals,

respectively, and π and π′* are ligand-centered occupied or

unoccupied orbitals, respectively, other than π*. λ is the

mixing coefficient. The mixing coefficients increase with

increasing resonance integrals between orbitals. Normaliza-

tion factors are neglected for simplification. Ignoring higher-

order terms, the transition dipole moment (M) of the CT

transition is thus given by

(73) Day, P.; Sanders, N. J. Chem. Soc. A 1967, 1530.

(74) Daul, C.; Schlaepfer, W. J. Chem. Soc., Dalton Trans. 1988, 393.

(75) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967.

(76) Ceulemans, A.; Vanquickenborne, L. G. J. Am. Chem. Soc. 1981, 103,

2238.

8920 Inorganic Chemistry, Vol. 45, No. 22, 2006

Highly Phosphorescent Iridium Complexes

Table 6. Fragment Analysis for the Frontier Molecular Orbitals in

Ir(bib)(mppy)Cl at X-ray Structure Using the ADF 2002 Program

Package^a

transition between unoccupied orbitals bib-65* f bib-64*

also produces a large transition dipole moment. The direction

of the dipole moment of both intra-bib transitions is along

the inter-bim direction, which is the x axis, consistent with

the results obtained by TDDFT.

MO

major contribution of fragment orbital (%)

Unoccupied

LUMO+2

LUMO+1

LUMO

bib-65* (59), ppy-34* (24), 5d_yz(4.0), bib-68* (2.1)

ppy-33* (80), ppy-34* (9.2), ppy-32 (3.6), 5d_xz(2.7)

bib-64* (82), bib-63 (11), 5d_xy(2.7)

Consequently, the theoretical consideration based on DFT

calculations revealed that the intense phosphorescence of the

Ir-Mebib complexes is accounted for in terms of the

effective mixing between the 5d_xyf LUMO configuration

in the lowest triplet state with 5d_yzf LUMO in singlet states

through spin-orbit coupling. In addition, the intensity of the

singlet dπ* transition originates from the very unique

electronic structure of Mebib, which allows for an electronic

interaction between the spatially separated bim moieties.

Occupied

HOMO

3p_x(48), 5d_xy(33), ppy-31 (6.9), bib-61 (5.3),

bib-64* (1.4)

HOMO-1

HOMO-2

3p_z(60), 5d_yz(23), bib-62 (3.8), bib-68* (1.4),

bib-65* (1.3)

3p_x(29), bib-61 (19), 5d_xz(16), 5d_xy(13),

ppy-31 (6.0), bib-60 (4.1), ppy-30 (2.5),

bib-57 (2.3), bib-58 (2.0)

^aThe * denotes unoccupied orbital.

Conclusion

In the present study, we achieved the selective synthesis

of novel mixed-ligand Ir complexes of the general formula

[Ir(N∧C∧N)(ppy)X] (N∧C∧N ) Me- or Phbib; X ) Cl,

Br, I, -CCPh, or CN), which have high luminescent quantum

yields, Φ ) 0.77∼0.95. The X-ray structures of [Ir(Phbib)-

(ppy)Cl] and [Ir(Mebib)(mppy)Cl] ′reveal that the nitrogen

atom in the ppy ligand is located at a position trans to the

central carbon atom in N∧C∧N (Me- or Phbib), while the

coordinating carbon atom in ppy occupies a trans position

in relation to Cl. Upon a change of substituent on ppy or X,

the Ir(III/IV) oxidation potential exhibits a strong dependence

on the ligand substitution, although the reduction potentials

remain almost constant. These findings suggest that the

excited state involves a large contribution of MLCT char-

acter, which is supported by DFT calculations. Therefore,

the present Ir(bib) complexes provide new insight into the

molecular design of a new type of highly emissive phos-

phorescent Ir complexes. Investigation of Ir(III) complexes

with regard to this concept is now in progress.

Acknowledgment. M.H. gratefully acknowledges finan-

cial support from the Institute of Science and Engineering

at Chuo University, the Ministry of Education, Science,

Sports, and Culture (Monkasho) for a Grant-in-Aid for

Scientific Research (No. 15310076 and 16074215 “Chem-

istry of Coordination Space”). K. N. gratefully acknowledges

Monkasho for a Grant-in-Aid for Scientific Research

(16550056).

Figure 10. Molecular orbitals involved in the interligand transition ascribed

to the intensity of the S₂transition in Ir(Mebib)(mppy)(Cl).

symmetry. The CT transition in S₂is therefore borrowed

intensity from interunit transitions. Because HOMO-1 has

the contribution of both the occupied bib-62 and unoccupied

bib-65*, the CT transition can borrow intensity from both

bib-62 f bib-64* and bib-65* f bib-64* intra-Mebib

transitions. As shown in Figure 10, bib-64* contains the π*

orbitals on two benzimidazoyl (bim) moieties with out-of-

phase combination, while the π orbitals are combined in an

in-phase fashion in bib-62. Therefore, the electronic transition

bib-62 f bib-64* generates a strong electronic oscillator

between two π systems of bim moieties approximately 10

Å apart. Similarly, bib-65* involves an in-phase interaction

between the π* orbitals of bim moieties; therefore, the

Supporting Information Available: CIF files of 5‚2CH₂Cl₂,

9‚C₆H₁₄, 11OTf‚Et₂O, 13OTf, and 14PF₆‚2MeCN‚0.5Et₂O, and

Tables 1S and 2S give their selected bond distances and angles

and the torsion angles between planes. Tables 3S, 4S, and 5S

summarize the excitation energy, oscillator strength (f), and major

electronic configurations for Ir(Mebib)(mppy)Cl (5), Ir(Mebib)-

(ppy)(CH₃CN)⁺(11), and Ir(Mebip)(ppy)Cl⁺(14). This material

is available free of charge via the Internet at http://pubs.acs.org.

IC060796O

Inorganic Chemistry, Vol. 45, No. 22, 2006 8921

Article Doi

Highly phosphorescent iridium complexes containing both tridentate bis(benzimidazolyl)-benzene or -pyridine and bidentate phenylpyridine: Synthesis, photophysical properties, and theoretical study of Ir-bis(benzimidazolyl)benzene complex

DOI: 10.1021/ic060796o

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ic060796o

Full text of DOI:10.1021/ic060796o

Products guided by the article

R&D Labs maybe for 915129-65-6

Relevant to this article

Hot Product