Efficient red-emitting cyclometaiated iridium(III) complexes containing lepidine-based ligands

Article abstract of DOI:10.1021/ic050385s

Heteroleptic cyclometalated iridium(III) complexes featuring lepidine-based ligands and acetyl acetone auxiliary ligand are synthesized. Multiple lowest energy absorption bands are observed for these complexes indicating substantial mixing of the singlet

Full text of DOI:10.1021/ic050385s

Inorg. Chem. 2005, 44, 5677−5685
Efficient Red-Emitting Cyclometalated Iridium(III) Complexes Containing
Lepidine-Based Ligands
K. R. Justin Thomas,† Marappan Velusamy,† Jiann T. Lin,*,†,‡ Chin-Hsiung Chien,† Yu-Tai Tao,*,†
Yuh S. Wen,† Ya-Hui Hu,§ and Pi-Tai Chou§
Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan, Department of Chemistry,
National Central UniVersity, 320 Chungli, Taiwan, and Department of Chemistry, National
Taiwan UniVersity, Taipei, Taiwan
Received March 15, 2005
Heteroleptic cyclometalated iridium(III) complexes featuring lepidine-based ligands and acetyl acetone auxiliary
ligand are synthesized. Multiple lowest energy absorption bands are observed for these complexes indicating
substantial mixing of the singlet and triplet levels. All the complexes emit orange or red color in dichloromethane
solutions with lifetimes in the range 1.6−3.7 µs. The emission in the complexes probably originates from the
3MLCT state. The complexes are applied as emitting guests in LED devices of the structure ITO/HTL(BPAPF or
NPB)/6% Ir in CBP/BCP/Alq3/LiF/Al. They exhibit excellent device characteristics with an orange to red EL profile.
Introduction
external efficiencies.4 The research on iridium(III) complexes
with relevance to their application in LED is focused on two
themes. First, designing new cyclometalating ligands or
auxiliary ligands has received immense attention in a vein
to tune the emission color of the resulting complexes. The
iridium(III) complexes known to date largely remained as
green or yellow emitters, and pure blue- and red-emitting
complexes are scarce. Only recently a few red-emitting
isoquinoline,9 pyrazine,10 or pyrimidine11 containing iridum-
(III) complexes have been reported.12,13 The second aspect
that gained focus these days involves the identification of
suitable host materials for iridium(III) complexes. Carba-
Considerable progress has been made in the design and
synthesis of molecular materials suitable for light-emitting
diodes that are regarded as potential replacements in future
display equipments.1,2 Organic materials that possess the
functions hole and electron transport, appropriate emission
characteristics, and amorphous properties are demonstrated
to be useful for the fabrication of light-emitting diodes. The
excitons formed in a device operation are composed of 25%
singlets and 75% triplets.3 This limits the efficiency of the
devices that contain organic materials as emitter to 25%. In
view of this, triplet emitters, particularly the cyclometalated
iridium(III) complexes, are considered attractive candidates
for electroluminescent devices as they enhance the possibility
of reaping both singlet and triplet excitons formed during
the device operation.4-8 This might eventually lead to higher
efficiencies. Thompson et al. first applied the iridium(III)
complexes in LED applications and recorded excellent
(4) (a) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl.
Phys. 2001, 90, 5048. (b) 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.
(5) Huang, W. S.; Lin, J. T.; Chien, C. H.; Tao, Y.-T.; Sun, S. S.; Wen,
Y. S. Chem. Mater. 2004, 16, 2480.
(6) (a) Yang, C. H.; Fang, K. H.; Chen C. H.; Sun, I. W. Chem. Commun.
2004, 2232. (b) Coppo, P.; Plummer, E. A.; De Cola, L. Chem.
Commun. 2004, 1774.
* Authors to whom correspondence should be addressed. E-mail:
jtlin@chem.sinica.edu.tw (J.T.L.). Fax: 886-2-27831237 (J.T.L.).
† Academia Sinica.
(7) Nazeeruddin, M. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.;
Zuppiroli, L.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 8790.
(8) Slinker, J. D.; Gorodetsky, A. A.; Lowry, M. S.; Wang, J. J.; Parker,
S.; Rohl, R.; Bernhard, S.; Malliaras, G. G. J. Am. Chem. Soc. 2004,
126, 2763.
‡ National Central University.
§ National Taiwan University.
(1) Muller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn,
V.; Rudati, P.; Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K.
Nature 2003, 421, 829.
(2) Kulkarni, P.; Tonozola, C. J.; Babel, A.; Janekhe, S. A. Chem. Mater.
2004, 16, 4556.
(3) Baldo, M. A.; O’Brien, D. F.; Thompson, M. E.; Forrest, S. R. Phys.
ReV. B 1999, 60, 14422.
(9) (a) Jiang, C. Y.; Yang, W.; Peng, J. B.; Xiao, S.; Cao, Y. AdV. Mater.
2004, 16, 537. (b) Yang, C. H.; Tai, C. C.; Sun, I. W. J. Mater. Chem.
2004, 14, 947.
(10) Duan, J. P.; Sun, P. P.; Cheng, C. H. AdV. Mater. 2003, 15, 224.
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Y.; Kavitha, J.; Chi, Y.; Shu, C. F.; Tseng, Y. H.; Chien, C. H. Appl.
Phys. Lett. 2004, 85, 1619.
10.1021/ic050385s CCC: $30.25
Published on Web 07/06/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 16, 2005 5677

Thomas et al.
zole14 and triazine15 based amorphous materials have been
suggested to possess suitable triplet energies to serve as hosts
for these potential triplet emitters. A biphenyl-bridged
dicarbazole, 4,4′-N,N′′-dicarbazolebiphenyl (CBP), is widely
used as a host of the fabrication of iridium complex based
electroluminescent devices. Besides the usage in LEDs, the
cyclometalated iridium(III) complexes have attracted great
interest in recent years owing to their use as biological
labeling reagents,16 oxygen sensors,17 and intermediates in
catalytic C-H activation reactions.18
Schlenk techniques. Solvents were dried by standard procedures.
All column chromatography was performed under N2 with the use
of silica gel (230-400 mesh, Macherey-Nagel GmbH & Co.) as
the stationary phase in a column of 30 cm long and 2.0 cm diameter.
1
The H NMR spectra were measured by using Bruker AC300 or
AMX400 spectrometer. Mass spectra were recorded on a JMS-
700 double focusing mass spectrometer (JEOL, Tokyo, Japan).
Elemental analyses were performed on a Perkin-Elmer 2400 CHN
analyzer. Cyclic voltammetry experiments were performed with a
BAS-100 electrochemical analyzer. All measurements were carried
out at room temperature with a conventional three-electrode
configuration consisting of a glassy carbon working electrode, a
In this paper, we adventure on the first theme and report
a series of lepidine-based iridium(III) complexes that emit
either orange or red color both in steady-state emission and
electroluminescence. Though isoquinoline-based iridium(III)
complexes have been reported recently,9 a systematic study
of the cyclometalated iridium(III) complexes derived from
the quinoline motif is missing.19 In this paper we develop a
new clean and facile synthetic protocol for the synthesis of
a series of lepidine-based ligands using the Suzuki protocol.
We attach a variety of aromatic segments on the lepidine
nucleus to arrive at a series of ligands. The complexes
prepared using these ligands are strongly emissive at room
temperature. We also have undertaken crystal structure
analysis for three complexes, and a comparative evaluation
of the bonding parameters is also attempted. Finally, the
usefulness of these complexes as triplet emitters in elec-
troluminescent devices was examined by applying them as
guests in CBP hosts in two different multilayered device
architectures. All the devices led to bright orange or red color
emission.
platinum wire auxiliary electrode, and a nonaqueous Ag/AgNO3
a
reference electrode. The E1/2 values were determined as 1/2(Ep
c
+
Ep ), where Epa and Epc are the anodic and cathodic peak potentials,
respectively. The potentials are quoted against the ferrocene internal
standard. The solvent in all experiments was dichloromethane, and
the supporting electrolyte was 0.1 M tetrabutylammonium hexaflu-
orophosphate. Electronic absorption spectra were obtained on a
Perkin-Elmer Lambda 900 UV-vis-NIR spectrophotometer for
dichloromethane solutions. Emission spectra were recorded in
deoxygenated toluene solution at 298 K with an Jobin Yvon SPEX
Fluorolog-3 spectrofluorometer. The emission spectra were collected
on samples with o.d. < 0.1 at the excitation wavelength. The spectra
were corrected for instrumental response. UV-visible spectra were
checked before and after irradiation to monitor possible sample
degradation. Emission maxima were reproducible to within 2 nm.
Luminescence quantum yields (Φem) were calculated using Cou-
marin 1 as primary standard (Φem ) 0.99 in ethyl acetate) and Ir-
(ppy)3 (Φem ) 0.40 in toluene) as secondary reference.20 Lumi-
nescence quantum yields were taken as the average of three separate
determinations and were reproducible to within 10%. The lifetime
was measured with the laser photolysis technique in which the third
harmonic of an Nd:YAG laser (8 ns, Continuum Surlite II) was
used as the excitation source, coupled with a fast response
photomultiplier (Hamamatsu model R5509-72) operated at -80 °C.
Typically an average of 512 shots were acquired for each measure-
ment.
Experimental Section
General Information. All reactions and manipulations were
carried out under N2 with the use of standard inert atmosphere and
(12) For homoleptic red-emitting iridium complexes, see: 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.
Preparation of the Ligands. The ligands were prepared by
essentially following the same procedure so an illustrative example
is provided below for BuL.
(13) For red-emitting iridium complexes grafted on polymers, see: (a)
Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby,
C. E.; Kohler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc.
2004, 126, 7041. (b) Chen, X. W.; Liao, J. L.; Liang, Y. M.; Ahmed,
M. O.; Tseng, H. E.; Chen, S. A. J. Am. Chem. Soc. 2003, 125, 636.
(14) (a) van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.;
Langeveld, B. M. W.; Rothe, C.; Monkman, A.; Bach, I.; Stossel, P.;
Brunner, K. J. Am. Chem. Soc. 2004, 126, 7718. (b) Brunner, K.; van
Dijken, A.; Borner, H.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.;
Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126, 6035.
(15) Inomata, H.; Goushi, K.; Masuko, T.; Konno, T.; Imai, T.; Sasabe,
H.; Brown, J. J.; Adachi, C. Chem. Mater. 2004, 16, 1285.
(16) (a) Lo, K. K. W.; Chan, J. S. W.; Lui, L. H.; Chung, C. K.
Organometallics 2004, 23, 3108. (b) Lo, K. K. W.; Chung, C. K.;
Lee, T. K. M.; Lui, L. H.; Tsang, K. H. K.; Zhu, N. Y. Inorg. Chem.
2003, 42, 6886.
(17) (a) DeRosa, M. C.; Hodgson, D. J.; Enright, G. D.; Dawson, B.; Evans,
C. E. B.; Crutchley, R. J. J. Am. Chem. Soc. 2004, 126, 7619. (b)
DeRosa, M. C.; Mosher, P. J.; Yap, G. P. A.; Focsaneanu, K. S.;
Crutchley, R. J.; Evans, C. E. B. Inorg. Chem. 2003, 42, 4864. (c)
Gao, R. M.; Ho, D. G.; Hernandez, B.; Selke, M.; Murphy, D.;
Djurovich, P. I.; Thompson, M. E. J. Am. Chem. Soc. 2002, 124,
14828.
2-(4-tert-Butylphenyl)-4-methylquinoline (BuL). A mixture of
2-chlorolepidine (1.78 g, 10 mmol), 4-tert-butylphenylboronic acid
(2.67 g, 15 mmol), Pd(PPh3)4 (1.16 g, 1 mmol, 10 mol %),
anhydrous sodium carbonate (1.59 g, 15 mmol), tetrahydrofuran
(20 mL), toluene (20 mL), and water (5 mL) was heated under
nitrogen atmosphere at 80 °C for 24 h. At the end of the time, the
reaction mixture was diluted with an additional 20 mL of water
and the organic layer separated was collected. It was dried to leave
a colorless liquid, adsorbed on silica gel, and purified by column
chromatography using hexane/dichloromethane mixture (1:1). A
1
colorless liquid solidified on standing. Yield: 2.48 g (90%). H
NMR (δ, CDCl3): 1.39 (s, 9 H), 2.73 (s, 3 H), 7.49-7.57 (m, 3
H), 7.67-7.73 (m, 2 H), 7.97 (d, J ) 7.8 Hz, 1 H), 8.07-8.10 (m,
2 H), 8.18 (d, J ) 8.2 Hz, 1 H). HRMS: calcd for C20H22N (M +
H), 276.1752; found, 276.1753.
2-(4-(Trifluoromethyl)phenyl)-4-methylquinoline (CF3L). A
1
colorless liquid solidified on standing. Yield: 82%. H NMR (δ,
(18) (a) Oxgaard, J.; Muller, R. P.; Goddard, W. A.; Periana, R. A. J. Am.
Chem. Soc. 2004, 126, 352. (b) Periana, R. A.; Liu, X. Y.; Bhalla, G.
C. Chem. Commun. 2002, 3000.
CDCl3): 2.74 (s, 3 H), 7.53-7.57 (m, 1 H), 7.67-7.75 (m, 4 H),
7.97 (d, J ) 8.2 Hz, 1 H), 8.17 (d, J ) 8.2 Hz, 1 H), 8.24 (d, J )
(19) While the manuscript was under preparation, a paper describing the
iridium complexes of 4-phenylquinoline-based ligands has been
appeared. See: Wu, F.-I.; Su, H.-J.; Shu, S.-F.; Luo, L.; Diau, W.-G.;
Cheng, C.-H.; Duan, J.-P.; Lee, G.-H. J. Mater. Chem. 2005, 15, 1035.
(20) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani,
J.; Iawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.;
Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971.
5678 Inorganic Chemistry, Vol. 44, No. 16, 2005

Red-Emitting Iridium(III) Complexes
7.7 Hz, 2 H). HRMS: calcd for C17H13NF3 (M + H): 288.1000;
found, 288.0992.
C41H41IrN4O2: C, 60.50; H, 5.08; N, 6.88. Found: C, 59.75; H,
4.82; N, 6.69.
Ir(NPh2L)2(acac) (4). A red solid formed. Yield: 65%. 1H NMR
(δ, CDCl3): 1.51 (s, 6 H), 2.74 (s, 6 H), 4.65 (s, 1 H), 6.11 (s, 2
H), 6.51 (d, J ) 7.5 Hz, 2 H), 6.76-6.87 (m, 20 H), 7.43-7.56
(m, 8 H), 7.80 (d, J ) 8.2 Hz, 2 H), 8.43 (d, J ) 8.2 Hz, 2 H).
FAB MS: m/z 1062.0 (M+). Anal. Calcd for C61H49IrN4O2: C,
68.97; H, 4.65; N, 5.27. Found: C, 69.07; H, 4.47; N, 5.09.
Ir(NapL)2(acac) (5). Dark brown crystals formed. Yield: 88%.
1H NMR (δ, CDCl3): 1.43 (s, 6 H), 2.94 (s, 6 H), 4.54 (s, 1 H),
6.73 (d, J ) 8.2 Hz, 2 H), 6.96 (d, J ) 8.4 Hz, 2 H), 7.25-7.37
(m, 4 H), 7.44-7.53 (m, 4 H), 7.64 (d, J ) 8.2 Hz, 2 H), 7.94 (dd,
J ) 8.2, 1.2 Hz, 2 H), 8.30 (d, J ) 8.2 Hz, 2 H), 8.52 (s, 2 H),
8.69 (d, J ) 8.7 Hz, 2 H). FAB MS: m/z 828.0 (M+). Anal. Calcd
for C45H35IrN2O2: C, 65.28; H, 4.26; N, 3.38. Found: 65.09; H,
4.11; N, 3.18.
Ir(PhenL)2(acac) (6). A dark brown powder formed. Yield:
84%. 1H NMR (δ, CDCl3): 1.51 (s, 6 H), 2.81 (s, 6 H), 4.60 (s, 1
H), 6.29-6.31 (m, 4 H), 6.81-6.87 (m, 2 H), 7.00-7.06 (m, 2 H),
7.12 (t, J ) 7.9 Hz, 2 H), 7.33 (d, J ) 8.2 Hz, 2 H), 7.56(t, J ) 7.5
Hz, 2 H), 7.68-7.76 (m, 4 H), 8.29 (d, J ) 8.2 Hz, 2 H), 8.62-
8.66 (m, 4 H), 8.87 (d, J ) 8.2 Hz, 2 H). FAB MS: m/z 928.0
(M+). Anal. Calcd for C53H39IrN2O2: C, 68.59; H, 4.24; N, 3.02.
Found: C, 68.51; H, 3.97, N, 2.92.
Structural Determination of Ir(CF3L)2(acac) (2), Ir(NMe2L)2-
(acac) (3), and (NapL)2Ir(acac) (5). Crystals of 2 (red plate,
dimensions 0.32 × 0.22 × 0.16 mm), 3, (red plate, dimensions
0.36 × 0.20 × 0.18 mm), and 5 (dark red prism, dimensions 0.22
× 0.18 × 0.16 mm) were grown from a dichloromethane solution
layered with hexane at room temperature. Relevant crystal data are
summarized in Tables S1, S4, and S7 (Supporting Information).
The space groups triclinic P1h (for 2) and monoclinic P21/n (for 3
and 5) were determined from systematic absence of specific
reflections; successful refinement of the structure confirmed the
space group assignment. Direct methods were used to locate the Ir
atom, whereas subsequent cycles of least-squares refinements and
difference Fourier maps were used to locate the remaining non-
hydrogen atoms. Hydrogen atoms were placed at calculated
positions. Fluorine positions in one of the CF3 groups in compound
2 were found to be disordered and modeled with fractional
occupancies. All calculations were performed using the SHELX
software package.
LEDs Fabrication and Measurements. Compound BCP (2.9-
dimethyl-4,7-diphenyl-1,10-phenanthroline) was purchased from
Aldrich and used as received. Compounds Alq3 (tris(8-hydroxy-
quinolinato)aluminum),21 NPB (4,4′-bis{N-(1-naphthyl-N-pheny-
lamino)biphenyl}),22 CBP (4,4′-N,N′-dicarbazolebiphenyl),22 and
BPAPF (9,9-bis{4-[bis(p-biphenyl)aminophenyl}}fluorene)23 were
synthesized according to literature procedures and were sublimed
twice prior to use. Prepatterned ITO substrates with an effective
individual device area of 3.14 mm2 were cleaned as described in a
previous report.24 A 40-nm-thick film of BPAPF or NPB was
deposited first as the hole transport layer (HTL). The light-emitting
layer (30 nm) was then deposited by coevaporating a CBP host
and a phosphorescent dopant (∼6% dopant concentration), with
both deposition rates being controlled with two independent quartz
crystal oscillators. A 10-nm-thick BCP as a hole and exciton
N,N-Dimethyl-4-(4-methylquinolin-2-yl)benzenamine (NMe2L).
1
A pale yellow solid formed. Yield: 64%. H NMR (δ, CDCl3):
2.72 (s, 3 H), 3.03 (s, 6 H), 6.80-6.84 (m, 2 H), 7.42-7.48 (m, 1
H), 7.63-7.68 (m, 2 H), 7.93 (d, J ) 7.8 Hz, 1 H), 8.07-8.13 (m,
3 H). HRMS: calcd for C18H19N2 (M + H), 263.1548; found,
263.1547.
N-(4-(4-Methylquinolin-2-yl)phenyl)-N-phenylbenzenamine
1
(NPh2L). A pale yellow solid formed. Yield: 79%. H NMR (δ,
CDCl3): 2.74 (s, 3 H), 7.06 (t, J ) 7.3 Hz, 2 H), 7.15-7.21 (m, 6
H), 7.26-7.31 (m, 4 H), 7.48-7.53 (m, 1 H), 7.66-7.72 (m, 2 H),
7.97 (d, J ) 8.2 Hz, 1 H), 8.00-8.04 (m, 2 H), 8.15 (d, J ) 8.2
Hz, 1 H). HRMS: calcd for C28H23N2 (M + H), 387.1861; found,
387.1855.
4-Methyl-2-(naphthalen-1-yl)quinoline (NapL). Yield: 76%.
A colorless solid formed. 1H NMR (δ, CDCl3): 2.78 (s, 3 H), 7.42-
7.53 (m, 2 H), 7.55-7.63 (m, 3 H), 7.68-7.78 (m, 2H), 7.90-
7.94 (m, 2 H), 8.05-8.12 (m, 2 H), 8.24 (d, J ) 8.2 Hz, 1 H).
HRMS: calcd for C20H16N (M + H), 270.1283; found, 270.1278.
4-Methyl-2-(phenanthren-10-yl)quinoline (PhenL). A colorless
1
solid formed. Yield: 71%. H NMR (δ, CDCl3): 2.80 (s, 3 H),
7.52-7.71 (m, 6 H), 7.75-7.80 (m, 1 H), 7.93-7.96 (m, 2 H),
8.07-8.11 (m, 2 H), 8.26 (d, J ) 8.2 Hz, 1 H), 8.73 (d, J ) 8.2
Hz, 1 H), 8.78 (d, J ) 8.2 Hz, 1 H). HRMS: calcd for C24H18N
(M + H), 320.1439; found, 320.1439.
Preparation of the Complexes (1-6). All the cyclometalated
iridium(III) complexes were prepared by essentially following the
same procedure so an illustrative example is provided below for 1.
Ir(BuL)2(acac) (1). A mixture of 2-(4-tert-butylphenyl)-4-
methylquinoline (BuL) (0.826 g, 3 mmol), anhydrous IrCl3 (299
mg, 1 mmol), 2-methoxyethanol (6 mL), and distilled water (3 mL)
was heated at reflux for 12 h. After cooling, the brown precipitate
formed was filtered out and washed thoroughly with methanol (50
mL), diethyl ether (50 mL), and hexane (20 mL). The dried chloro-
bridged dimer was suspended in 2-methoxyethanol (10 mL) and
treated with acetyl acetone (200 mg, 2 mmol) and anhydrous Na2-
CO3 (315 mg, 3 mmol). The reaction mixture was stirred at 80 °C
for 8 h. The insoluble products were filtered out, washed with water
(10 mL), methanol (10 mL), and diethyl ether (20 mL), and dried.
They were dissolved again in dichloromethane (20 mL) and
adsorbed on silica gel and rapidly purified by column chromatog-
raphy using hexane/dichloromethane mixture (3:2). The red solid
obtained was recrystallized from a dichloromethane/hexane mixture.
Red needles formed. Yield: 685 mg (82%). 1H NMR (δ, CDCl3):
0.86 (s, 18 H), 1.50 (s, 6 H), 2.90 (s, 6 H), 4.64 (s, 1 H), 6.49 (s,
2 H), 6.91 (d, J ) 7.3 Hz, 2 H), 7.32-7.37 (m, 2 H), 7.41-7.45
(m, 2 H), 6.71 (d, J ) 8.2 Hz, 2 H), 7.85 (s, 2 H), 7.90 (d, J ) 8.2
Hz, 2 H), 8.47 (d, J ) 8.7 Hz, 2 H). FAB MS: m/z 840.1 (M+).
Anal. Calcd for C45H47IrN2O2: C, 64.34; H, 5.64; N, 3.33. Found:
C, 64.35; H, 5.58; N, 3.15.
Ir(CF3L)2(acac) (2). A red microcrystalline solid formed.
1
Yield: 77%. H NMR (δ, CDCl3): 1.46 (s, 6 H), 2.95 (s, 6 H),
4.59 (s, 1 H), 6.69 (s, 2 H), 7.17 (d, J ) 8.8 Hz, 2 H), 7.48-7.54
(m, 4 H), 7.87 (d, J ) 8.2 Hz, 2 H), 7.94-7.97 (m, 4 H), 8.35 (d,
J ) 8.7 Hz, 2 H). FAB MS: m/z 863.9 (M+). Anal. Calcd for
C39H29F6IrN2O2: C, 54.22; H, 3.38; N, 3.24. Found: C, 54.19; H,
3.12; N, 3.0.
1
Ir(NMe2L)2(acac) (3). Red crystals formed. Yield: 56%. H
(21) Chen, C. H.; Shi, J. Coord. Chem. ReV. 1998, 171, 161.
(22) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater. 1998, 10,
2235.
(23) Ko, C.-W.; Tao, Y.-T. Synth. Met. 2002, 126, 37.
(24) Balasubramaniam, E.; Tao, Y. T.; Danel, A.; Tomasik, P. Chem. Mater.
2000, 12, 2788.
NMR (δ, CDCl3): 1.47 (s, 6 H), 2.49 (s, 12 H), 2.82 (s, 6 H), 4.68
(s, 1 H), 5.83 (s, 2 H), 6.31 (d, J ) 7.5 Hz, 2 H), 7.31-7.38 (m,
4 H), 7.59 (d, J ) 8.2 Hz, 2 H), 7.68 (s, 2 H), 7.80-7.83 (m, 2 H),
8.52-8.55 (m, 2 H). FAB MS: m/z 814.1 (M+). Anal. Calcd for
Inorganic Chemistry, Vol. 44, No. 16, 2005 5679

Thomas et al.
Scheme 1. Synthesis of the Complexes
blocking layer (HBL) and 30-nm-thick Alq3 as an electron transport
layer were then deposited sequentially. Finally, a thin layer of LiF
(5 Å) followed by aluminum (500 Å) was deposited as the cathode,
which was capped with 1000 Å of silver. I-V curves were measured
on a Keithley 2400 Source meter in ambient environment. Light
intensity was measured with a Newport 1835 optical meter.
Figure 1. Structure of the complexes.
Results and Discussion
Synthesis of the Ligands and the Complexes. The Suzuki
coupling reactions between commercially available 2-chlo-
rolepidine and the corresponding aryl boronic acids afforded
the ligands required for this study (Scheme 1). C-C-coupling
reactions involving Suzuki protocol had not been realized
earlier for 2-chlorolepidine. However, Negishi coupling
reactions were successfully demonstrated recently on 2-chlo-
rolepidine to arrive at bipyridine-like chelating ligands.25
Facile reactivity of chlorolepidine is similar to that observed
for chloropyridine in analogous reactions.26 Complexes
depicted in Figure 1 were obtained in moderate to good yields
from the above ligands by a conventional two-step sequence
(Scheme 1). In the first step a chloro-bridged dimer was
formed by the reaction of the ligand with IrCl3. This dimer
was cleaved by treatment with acetylacetone in the presence
of Na2CO3 to produce the monomeric heteroleptic complexes.
The complexes were characterized by NMR and mass
spectra, by elemental analyses, and in the case of 2, 3, and
5 by single-crystal X-ray diffraction. In the complexes, the
acetylacetonate CH protons appear in the region 4.54-4.68
ppm as a sharp singlet. For the complexes possessing
arylpyridine cyclometalating ligands this signal is reported
to appear at >5.1 ppm.4b In the complex Ir(PQ)2(acac) [where
PQ ) 2-phenylquinoline], the signal attributable to the
acetylacetonate CH proton is located at 4.7 ppm.4b All these
facts clearly point that in quinoline-based cyclometalating
iridium complexes the acetylacetonate CH protons are more
shielded due to the larger electron density on Ir center. This
obviously is the result of weaker ligand field strength of
Figure 2. ORTEP diagram of the complex Ir(CF3L)2(acac) (2).
quinoline that inhibits effective back-donation. It is interest-
ing to note here that the closely analogous isoquinoline
systems led to low field signals ca. 5.2 ppm.9b The above-
mentioned shielding effect is also manifested on the chemical
shift of the acetylacetonate CH3 protons. In arylpridine
complexes they are observed above 1.6 ppm, while in the
present complexes they resonate between 1.43 and 1.51 ppm.
Crystal Structures of the Complexes Ir(CF3L)2(acac)
(2), Ir(NMe2L)2(acac) (3), and Ir(NapL)2(acac) (5). ORTEP
plot of the compounds 2, 3, and 5 are displayed in Figures
2-4, respectively, and the coordination geometrical param-
eters are listed in Table 1. In general, the iridium(III) center
adopts a distorted octahedral geometry with the cis-O,O, cis-
C,C, and trans-N,N chelate disposition, and the angles of
the trans ligands at the metal center range between 173.1(3)
and 177.6(2) Å. Incidentally, the limiting trans angles (N-
Ir-N and C-Ir-O) were observed for the naphthyl-
substituted complex, 5. The shortest coordination distances
(25) Fang, Y. Q.; Hanan, G. S. Synlett 2003, 852.
(26) Tagata, T.; Nishida, M. J. Org. Chem. 2003, 68, 9412.
5680 Inorganic Chemistry, Vol. 44, No. 16, 2005

Red-Emitting Iridium(III) Complexes
Table 1. Coordination Bonding Parameters (Å, deg) Observed in Complexes 2, 3, and 5
compd
Ir-C
Ir-N
Ir-O
C-Ir-C
N-Ir-N
O-Ir-O
C-Ir-O
N-Ir-O
C-Ir-N
Ir(CF3L)2(acac) (2)
1.975(12)
1.976(12)
2.082(11)
2.054(11)
2.161(8)
2.177(9)
93.6(5)
175.5(3)
86.6(4)
90.8(4)
89.1(4)
175.0(3)
176.6(4)
174.4(2)
91.3(2)
176.77(19)
88.8(2)
177.2(3)
87.7(3)
101.8(4)
101.1(4)
80.4(4)
95.7(5)
97.9(5)
80.9(5)
80.0(5)
81.0(2)
98.0(2)
96.5(2)
79.6(2)
80.0(3)
98.2(3)
99.0(3)
79.7(3)
82.6(4)
Ir(NMe2L)2(acac) (3)
Ir(NapL)2(acac) (5)
1.965(5)
1.982(6)
2.068(5)
2.077(5)
2.170(4)
2.171(4)
94.0(2)
95.1(3)
176.35(19)
177.6(2)
85.91(15)
85.6(2)
99.89(18)
82.93(17)
101.74(18)
80.85(18)
100.2(2)
81.9(2)
1.982(8)
1.989(8)
2.049(7)
2.061(6)
2.145(6)
2.155(5)
173.1(3)
91.6(3)
100.4(2)
80.8(2)
the O-Ir-O angles decreases from 2 to 5. Two sets of
N-Ir-O, C-Ir-O, and C-Ir-N angles were observed in
the complexes. The larger N-Ir-O and C-Ir-O angles
shrink slightly on moving from 2 to 5, and the reverse is
true for the larger C-Ir-N angles. The smaller angles
associated with these bonds do not show a significant trend.
A comparison of the structural parameters observed for
these complexes with the previously known related com-
plexes is noteworthy. The selected bond lengths and angles
for a few recently reported cyclometalated iridium(III)
acetylacetonate complexes are listed in Table 2. On inspec-
tion of the bonding parameters a few points emerge: (i)
Among the complexes, 2 and 3 possess the shorter Ir-C
bonds. This indicates a stronger σ-donation from these
substituted phenyl ligands. (ii) Similarly the complexes 2
and 3 show relatively longer Ir-N and Ir-O bonds. Weaker
Ir-O bonds result from the trans influence exerted by the
aryl ligands, while the elongated Ir-N bonds may suggest
weaker basicity of quinoline when compared to pyridine.
Comparatively longer Ir-N bonds are also observed for the
complexes Ir(btpy-Br)2(acac)13a and Ir(CF3-pbtz)2(acac)29
probably due to the same reason. (iii) The complexes 2, 3,
and 5 exhibit narrower bite angles (86° vs 88-90°) for the
acetylacetone chelate. This may be attributed to the steric
effect of the bulkier quinoline motif which forces the
acetylacetone to confine to the smaller space. This is also
supported by the distinctively distorted N-Ir-O angles
observed for the above three complexes.
Figure 3. ORTEP diagram of the complex Ir(NMe2L)2(acac) (3).
Photophysical Properties. Absorption spectra of the
complexes and the ligands were measured in dichlo-
romethane solutions at 298 K. Figure 5 show a comparison
of absorption spectra of complexes 1, 4, and 6, and the
pertinent data are gathered in Table 3. Except the NMe2
(NMe2L) and NPh2 (NPh2L) substituted ligands, all the
ligands display absorption spectra in the range 250-350 nm
(see Figure S2 in the Supporting Information). The ligands
NMe2L and NPh2L display red-shifted absorption profiles
peaking at ca. 375 nm. On the contrary, the absorption spectra
of the complexes are dominated by multiple bands originating
from ligand-centered π-π* transitions and MLCT transitions
(Table 3). The bands that appear above 400 nm are believed
to be arising from charge-transfer transitions. The observed
Figure 4. ORTEP plot of the complex Ir(NapL)2(acac) (5).
are associated with the Ir-C bonds with the longest being
Ir-O bonds. The average Ir-O bond length observed in the
complexes [2.163(6) Å] is considerably longer than the mean
Ir-O value of 2.088 Å reported in the Cambridge Crystal-
lographic Database and reflects the pronounced trans influ-
ence of the aromatic rings. Among the complexes, naphthyl
derivative (5) possesses the relatively shorter Ir-O distances
[2.150(6) vs 2.170(6) Å] indicative of comparatively weaker
transophobic behavior.27 In the three complexes, the C-Ir-C
and N-Ir-N bond angles assume the order 2 < 3 < 5, while
(28) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong,
R.; Tsyba, I.; Bortz, M.; Mmui, B.; Bau, R.; Thompson, M. E. Inorg.
Chem. 2001, 40, 1704.
(27) Vicente, J.; Arcas, A.; Bautista, D.; de Arellano, M. C. R. J.
Organomet. Chem. 2002, 663, 164.
(29) Laskar, I. R.; Chen. T. M. Chem. Mater. 2004, 16, 111.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5681

Thomas et al.
Table 2. Comparison of Average Bond Lengths (Å) and Angles (deg) in Iridium(III) Acetylacetonate Complexes
complex
Ir-C
Ir-N
Ir-O
C-Ir-C
N-Ir-N
O-Ir-O
C-Ir-O
N-Ir-O
C-Ir-N
Ir(CF3L)2(acac) (2)
1.976(12)
2.068(11)
2.169(9)
93.8(4)
175.5(3)
86.6(4)
175.8(4)
90.0(4)
175.6(2)
90.1(2)
175.2(3)
89.7(3)
175.6(3)
87.5(3)
173.9(2)
89.5(2)
171.3(7)
89.7(7)
176.1(2)
88.9(2)
174.6(1)
89.5(1)
101.5(4)
81.5(4)
100.8(2)
81.9(2)
100.3(2)
81.4(2)
94.5(3)
88.1(3)
94.3(2)
88.3(2)
93.2(6)
87.0(6)
96.6(2)
87.1(2)
95.1(1)
90.2(1)
94.76(6)
82.96(6)
96.8(5)
80.5(5)
97.3(2)
80.3(2)
98.6(3)
79.9(3)
95.8(4)
81.7(4)
97.2(2)
80.4(2)
101.7(8)
79.7(8)
96.2(2)
80.1(2)
94.8(2)
80.1(1)
101.93(7)
80.25(7)
Ir(NMe2L)2(acac) (3)
Ir(NapL)2(acac) (5)
Ir(ppy)2(acac)28
1.974(6)
1.986(8)
2.003(9)
1.984(6)
2.01(2)
2.073(5)
2.055(7)
2.010(9)
2.032(5)
2.071(17)
2.055(4)
2.038(3)
2.044(1)
2.171(4)
2.150(6)
2.146(6)
2.149(4)
2.136(15)
2.136(3)
2.145(3)
2.133(1)
94.0(2)
95.1(3)
95.2(5)
93.4(2)
92.1(8)
93.5(2)
93.4(2)
95.92(7)
176.4(2)
177.6(2)
176.3(4)
176.2(2)
177.6(7)
174.7(2)
172.7(2)
176.56(5)
85.9(2)
85.6(2)
90.0(3)
88.2(2)
89.8(5)
88.9(1)
88.0(2)
87.68(6)
Ir(tpy)2(acac)28
Ir(btpy-Br)2(acac)13a
Ir(CF3-pbtz)2(acac)29
Ir(fbi)2(acac)5
1.990(5)
2.006(4)
1.986(2)
Ir(cbtz)2 (acac)17a
173.91(6)
88.36(7)
Table 3. Physical Parameters for the Complexes
complex
λ
abs/nma
λ
em/nma
τ, µsa
Φfb
Eox/mVa,c
HOMO/eVd
LUMO/eVe
∆E/eVe
1
2
3
4
5
6
345, 420, 465, 505, 545
317, 430, 460, 500, 545
345, 398
366, 405, 502 sh
310, 374, 487, 528 sh
310, 377, 508, 535 sh
599
591
598
593
634
663
1.6
1.9
3.7
2.4
1.8
3.3
0.17
0.36
0.10
0.16
0.18
0.29
266 (68)
564 (69)
128 (72), 403 (86)
237 (83), 544 (68)
312 (70)
5.066
5.364
4.928
5.037
5.112
4.998
2.908
3.187
2.789
2.916
3.027
2.930
2.158
2.177
2.139
2.121
2.085
2.068
198 (85)
a Measured in dichloromethane solution. Concentration for UV measurements: ∼2.0 × 10-5 M. b For emission measurements, toluene solutions with o.d.
< 0.1 at excitation wavelength (350 nm) were used. Approximately concentration: 10-6-10-7 M. c Scan rate: 100 mV/s. Electrolyte: 0.1 M
tetrabutylammonium hexafluorophosphate. Concentration: 2.5 × 10-4 M. The potentials are quoted against the internal ferrocene standard. d Deduced form
the equation HOMO ) 4.8 + Eox. e Calculated from the optical edge and the relation ∆E ) HOMO - LUMO.
Ir. In the present complexes, probably the spin-orbit
coupling is enhanced by the presence of closely lying π-π*
and MLCT states and the heavy atom effect.30 It is interesting
to note the changes in the molar extinction coefficient of
the low-energy MLCT band that in complex 5 and 6 is two
times higher than that of the complex 1 which in turn is 0.5
times higher than that of the CF3 complex 2. Electron
deficient coligands were found to influence the position and
extinction coefficient of the MLCT transitions in cyclo-
metalated iridium(III) complexes. This was identified as a
unique pathway for blue-shifting the emission profile.7 In
our complexes, the observed trend in optical density and peak
maxima may reflect the trend in the electron-donating
capability of the cyclometalating aryl moieties.
All the complexes reported here exhibit moderate to
intense phosphorescence at room temperature in degassed
toluene solutions with the lifetimes in the range 1.6-3.7 µs.
Figure 5. Absorption and emission spectra of the selected complexes,
Ir(BuL)2(acac) (1), Ir(NPh2L)2(acac) (4), and Ir(PhenL)2(acac) (6), recorded
in CH2Cl2 solutions.
These lifetimes are indicative of strong spin-orbit coupling
leading to intersystem crossing from the singlet to the triplet
state.30 Hence, we believe that in these complexes the
MLCT transitions are weaker, however, clearly resolved for
emission originates from the triplet states. It seems that the
cyclometalating aromatic segment plays an important role
in positioning the emission profile. The red-shifted emission
observed for naphthyl and phenanthrenyl complexes may be
associated with the comparatively weaker ligand field
strength of the aryl units. It is interesting to note here that
thienyl and benzothiophenyl complexes displayed batho-
the complexes 1, 2, 5, and 6. In case of 3 and 4 the red-
shifted ligand-based transitions probably overlay the weaker
MLCT transitions. For 1 and 2 at least four MLCT transitions
are noticed, indicating the presence of multiple close lying
MLCT states in these complexes (see for instance Figure
5). For the naphthyl and phenanthrenyl complexes (5 and 6)
the MLCT transitions appear distinctively above 450 nm with
appreciable extinction coefficients (>15 000 M-1 cm-1) and
(30) (a) Wang, Y.; Herron, N.; Grushin, V. V.; LeCloux, D.; Petrov, V.
Appl. Phys. Lett. 2001, 79, 449. (b) Garces, F. O.; King, K. A.; Watts,
R. J. Inorg. Chem. 1998, 27, 3464.
3
suggest substantial mixing of spin forbidden MLCT and
1
higher lying MLCT transitions by spin-orbit coupling of
5682 Inorganic Chemistry, Vol. 44, No. 16, 2005

Red-Emitting Iridium(III) Complexes
between the t2g and eg orbitals, leading to an enhanced gap
between the eg and LUMO of the ligand. (c) Close-lying
π-π* and MLCT states together with the heavy atom effect
enhance the spin-orbit coupling. Moderate quantum ef-
ficiencies realized for our complexes is noteworthy and may
be ascribed to the combination of one more factors enumer-
ated above. Comparatively low phosphorescence yield
observed for dimethylamino-substituted complex 3 may be
attributed to the intramolecular electron-transfer quenching
of the excited state by dimethylamino units. This speculation
is reasonable as this complex possesses two oxidation couples
with lowest oxidation potentials in the series (Table 3).
Considering low quantum efficiency for the NMe2-substituted
complex 3, it is expected to display smaller excited-state
lifetime. Contrary to this belief, we observed a long lifetime
for this complex in this series. This may be ascribed to the
predominant 3IL (π f π* (NMe2L-) character of the excited
state, although the mixing of MLCT cannot be excluded.32
Figure 6. Cyclic voltammograms of complexes Ir(BuL)2(acac) (1), Ir-
(CF3L)2(acac) (2), and Ir(NPh2L)2(acac) (4) recorded in dichloromethane.
Scan rate: 100 mV/s.
chromic shifts in the emissions when compared to the
corresponding phenyl analogues.31 Moderate to high quantum
efficiencies were observed for the complexes. Normally
orthometalated iridium complexes are known to have the
highest triplet quantum yields due to several factors:7 (a)
Iridium normally undergoes large d-orbital splitting compared
to other metals in the series. (b) The strong ligand field
strength of the aromatic anion ligand increases the energy
Electrochemical Studies. All the complexes exhibit a one-
electron reversible oxidation couple at relatively low positive
potentials (Table 3). None of the ligands display oxidation
waves in this potential region (see Figure S2 in the
Supporting Information). Thus, we attribute this low positive
oxidation potential to the Ir centered oxidation. Additional
reversible oxidation couple is also noticed in the complexes
Figure 7. Device configurations and structure of the materials.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5683

Thomas et al.
Table 4. Electroluminescence Data for the Devicesa
3 and 4, which originates from the amine unit. The
corresponding ligands NMe2L and NPh2L also displayed
oxidation waves at 0.45 V (irreversible) and 0.53 V (revers-
ible, ∆Ep ) 76 mV), respectively. Changes in the electron-
donating and electron-withdrawing nature of the ligands may
result in the variation in the electronic properties at the metal
center, which in turn will be manifested on the redox
propensity of the complexes.7,19 It is interesting to compare
the complexes 1-4 that contain tert-butyl, CF3, and amino
functionality, respectively, in the phenyl ring. The electron-
withdrawing CF3 group significantly shifts the Ir couple
positively (∆E1/2 ) 298 mV) in complex 2, while a negative
shift (∆E1/2 ) 138 and 29 mV for 3 and 4) for the electron-
donating amino substituents is seen in 3 and 4 when
compared to 1 (Figure 6). The larger positive shift for the
CF3-substituted ligand-containing complex 2 arises from the
diminished electron density on Ir center due to the electron-
withdrawing effect of CF3. Observed oxidation potentials
suggest that the phenanthrenyl unit is more electron-rich
when compared to the tert-butyl- and diphenylamino-
substituted phenyl ligands (compare complex 6 vs 1 and 4).
In complexes 1-6, no reversible reduction waves due to the
ligands were observed in dichloromethane. However, sig-
nificant reduction currents above -1.8 V are visible in the
cyclic voltammogram, which were not analyzed in detail.
com-
plex (fwhm)
λ
EL/nm
CIE
x, y
VON
/
Lmax
cd m-2 cd m-2
/
L100
/
V100
V
/
ηb/
%
ηcb/
ηp
/
b
b
b
V
cd A-1 lmW-1
1
2
3
4
5
6
598 (60) 0.59, 0.39 3.5 61 750 19 500 11.86 8.84 19.61 5.20
598 (62) 0.60, 0.39 4.0 72 130 26 260 12.36 12.11 26.37 6.71
586 (68) 0.56, 0.43 5.0 38 200 11 800 11.70 4.73 11.92 3.21
586 (70) 0.57, 0.43 4.5 38 560 12 250 10.72 5.02 12.32 3.61
600 (66) 0.61, 0.39 4.5 19 050 13 150 13.87 6.85 13.22 3.00
602 (66) 0.61, 0.39 5.0 14 380 5 080c 12.98c 13.15c 25.00c 6.02c
596 (62) 0.59, 0.40 4.5 19 150 13 000 13.91 6.05 13.05 2.95
594 (62) 0.60, 0.40 4.5 16 850 16 060 14.87 7.63 16.16 3.42
638 (50) 0.69, 0.30 4.0 13 000 5 200 12.31 7.92 5.22 1.33
638 (52) 0.69, 0.30 4.0 7 350 6 000 14.53 9.06 6.00 1.30
664 (52) 0.68, 0.29 4.5 2 900 2 300 14.37 9.32 2.31 0.50
664 (52) 0.71, 0.29 4.5 2 330 2 320 14.98 9.78 2.32 0.49
a For each compound the first column gives the data for device I and
the second column is for device II. b At current density 100 mA/cm2. c At
current density 20 mA/cm2.
Light-Emitting Diodes. The complexes described here
possess red-shifted emission and moderate to good triplet
quantum efficiencies. In view of the recent surge in the search
of suitable triplet dopants for the fabrication of light-emitting
diodes, we have also examined the possibility of using these
complexes as emitting guests in LED devices. Two types of
electroluminescent devices of the configurations ITO/NPB
(40 nm)/6% 1-6 in CBP (30 nm)/BCP (10 nm)/Alq3 (30
nm)/LiF/Al (device I) and ITO/BPAPF (40 nm)/6% 1-6 in
CBP (30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF/Al (device II)
were fabricated (Figure 7). We have used CBP as the host
for the iridium complexes because of its proven earlier
performance as host for iridium complexes and theoretical
confirmation of favorable triplet energy.33 It is necessary to
introduce a hole-blocking layer (HBL), BCP, into our device
composition to confine the recombination zone inside the
doped CBP layer; without the HBL the recombination takes
place inside the Alq3 layer (emission maximum at 530 nm
wavelength). This indicates that with HBL the emission takes
place in CBP just near the CBP/BCP interface as the electron
are entering CBP. We have also examined the influence of
the hole-transporting layer by replacing NPB with a fluorene
derivative, BPAPF, which possesses excellent thermal prop-
erties and hole injection capabilities. Because of these
meticulous designs, all the devices led to intense orange or
red emission originating from the iridium complexes. The
device characteristics are listed in Table 4, and the I-V-L
trends are displayed in Figures 8-10. The close resemblance
between the PL and EL spectra indicates the absence of
aggregation or π-stacking up to 6% doping level.
Figure 8. I-V-L characteristics of the devices fabricated using Ir(BuL)2-
(acac) (1) and Ir(CF3L)2(acac) (2).
Figure 9. I-V-L characteristics of the devices fabricated using Ir-
(NMe2L)2(acac) (3) and Ir(NPh2L)2(acac) (4).
In general device I of the complexes displays higher
current densities than that of device II with the exception of
the compound 2. On the contrary, device II appears to have
(32) (a) Yersin, H. Top. Curr. Chem. 2004, 241, 1. (b) Lo, K. K.-W.; Chan,
J. S.-W.; Chung, C.-K.; Tsang, V. W.-H.; Zhu, N. Inorg. Chim. Acta
2004, 357, 3109. (c) Hay, P. J. J Phys. Chem. A 2002, 106, 1634.
(33) (a) D’Andrade, B. W.; Baldo, M. A.; Adachi, C.; Brooks, J.;
Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 1045. (b)
Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M.
A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082.
(c) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403,
750.
(31) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson
M. E.; Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622.
5684 Inorganic Chemistry, Vol. 44, No. 16, 2005

Red-Emitting Iridium(III) Complexes
Figure 10. I-V-L characteristics of the devices fabricated using
Ir(NapL)2(acac) (5) and Ir(PhenL)2(acac) (6).
better performance than device I. It is pointed out by
Thompson et al. that the iridium complexes also contribute
to the transport of charges. Therefore, there is better balance
of electron and hole transport rates inside the emitting layer
of device II. The CIE coordinates observed for the devices
I are plotted in Figure 11. Devices II also display identical
CIE patterns. Most of the devices appear in the red region,
while the diodes derived from 1, 3, and 4 are close to the
NTSC standards. The performance of the saturated red-
emitting devices derived from the complexes 3 and 5 are
promising. Another characteristic of phosphorescent dye
doped OLEDs described here is that the maximum of the
external quantum efficiency (QE) is located at low voltages
(9-10 V) and that for higher voltages a strong decrease in
the QE is seen. This kind of decrease is discussed as arising
from a triplet-triplet annihilation effect.34
Figure 11. CIE 1931 coordinates plot for device I.
rameters were compared with the related cyclometalated
iridium(III) acetylacetonate complexes. The optical properties
of the complexes were critically analyzed and related to the
ligand field strength of the aryl anion ligands involved. The
redox propensity of the iridium complexes were influenced
by the electron-donating substituents present on the cyclo-
metalating aryl unit. Bright red-emitting electroluminescent
devices were constructed using the iridium(III) complexes
as doped emitters. Variation of the dopant concentration to
achieve optimized and stabilized electroluminescent proper-
ties in devices for this class of complexes will be studied in
the future.
Summary
Acknowledgment. We thank the Academia Sinica and
National Science Council for financial support.
In summary, we have synthesized and characterized a new
series of lepidine-based iridium(III) complexes that are bright
red emitters and suitable for red electroluminescent devices.
Crystal structures of the three complexes were performed
by single-crystal X-ray diffraction, and the structural pa-
Supporting Information Available: Crystallographic informa-
tion files for complexes 2, 3, and 5, absorption spectra of the ligands,
and CV traces of the ligands. This material is available free of
charge via the Internet at http://pubs.acs.org.
(34) Baldo, M. A.; Adachi, C.; Forrest, S. R. Phys. ReV. B 2000, 62, 10967.
IC050385S
Inorganic Chemistry, Vol. 44, No. 16, 2005 5685