Synthesis and characterization of phosphorescent cyclometalated iridium complexes

Article abstract of DOI:10.1021/ic0008969

The preparation, photophysics, and solid state structures of octahedral organometallic Ir complexes with several different cyclometalated ligands are reported. IrC1₃·nH₂O cleanly cyclometalates a number of different compounds (i.e., 2-phenylpyridine, 2-(p-tolyl)pyridine, benzoquinoline, 2-phenylbenzothiazole, 2-(1-naphthyl)benzothiazole, and 2-phenylquinoline), forming the corresponding chloride-bridged dimers, C∧N₂Ir(μ-C1)₂IrC∧N₂ (C∧Nis a cyclometalated ligand) in good yield. These chloride-bridged dimers react with acetyl acetone (acacH) and other bidentate, monoanionic ligands such as picolinic acid (picH) and N-methylsalicylimine (salH), to give monomeric C∧N₂Ir(LX) complexes (LX = acac, pic, sal). The emission spectra of these complexes are largely governed by the nature of the cyclometalating ligand, leading to λ_max values from 510 to 606 nm for the complexes reported here. The strong spin-orbit coupling of iridium mixes the formally forbidden ³MLCT and ³π-π*transitions with the allowed ¹MLCT, leading to a strong phosphorescence with good quantum efficiencies (0.1-0.4) and room temperature lifetimes in the microsecond regime. The emission spectra of the C∧N₂Ir(LX) complexes are surprisingly similar to the fac-IrC∧N₃ complex of the same ligand, even though the structures of the two complexes are markedly different. The crystal structures of two of the C∧N₂Ir(acac) complexes (i.e., C∧N = ppy and tpy) have been determined. Both complexes show cis-C,C′, trans-N,N′ disposition of the two cyclometalated ligands, similar to the structures reported for other complexes with a C∧N₂Ir fragment. NMR data (¹H and ¹³C) support a similar structure for all of the C∧N₂Ir(LX) complexes. Close intermolecular contacts in both (ppy)₂Ir(acac) and (tpy)₂Ir(acac) lead to significantly red shifted emission spectra for crystalline samples of the ppy and tpy complexes relative to their solution spectra.

Full text of DOI:10.1021/ic0008969

1704
Inorg. Chem. 2001, 40, 1704-1711
Synthesis and Characterization of Phosphorescent Cyclometalated Iridium Complexes
Sergey Lamansky, Peter Djurovich, Drew Murphy, Feras Abdel-Razzaq, Raymond Kwong,
Irina Tsyba, Manfred Bortz, Becky Mui, Robert Bau, and Mark E. Thompson*
Department of Chemistry, University of Southern California, Los Angeles, California 90089
ReceiVed August 8, 2000
The preparation, photophysics, and solid state structures of octahedral organometallic Ir complexes with several
different cyclometalated ligands are reported. IrCl3‚nH2O cleanly cyclometalates a number of different compounds
(i.e., 2-phenylpyridine, 2-(p-tolyl)pyridine, benzoquinoline, 2-phenylbenzothiazole, 2-(1-naphthyl)benzothiazole,
and 2-phenylquinoline), forming the corresponding chloride-bridged dimers, C∧N2Ir(µ-Cl)2IrC∧N2 (C∧Nis a
cyclometalated ligand) in good yield. These chloride-bridged dimers react with acetyl acetone (acacH) and other
bidentate, monoanionic ligands such as picolinic acid (picH) and N-methylsalicylimine (salH), to give monomeric
C∧N2Ir(LX) complexes (LX ) acac, pic, sal). The emission spectra of these complexes are largely governed by
the nature of the cyclometalating ligand, leading to λmax values from 510 to 606 nm for the complexes reported
3
3
here. The strong spin-orbit coupling of iridium mixes the formally forbidden MLCT and π-π* transitions
1
with the allowed MLCT, leading to a strong phosphorescence with good quantum efficiencies (0.1-0.4) and
room temperature lifetimes in the microsecond regime. The emission spectra of the C∧N2Ir(LX) complexes are
surprisingly similar to the fac-IrC∧N3 complex of the same ligand, even though the structures of the two complexes
are markedly different. The crystal structures of two of the C∧N2Ir(acac) complexes (i.e., C∧N ) ppy and tpy)
have been determined. Both complexes show cis-C,C′, trans-N,N′ disposition of the two cyclometalated ligands,
similar to the structures reported for other complexes with a “C∧N2Ir” fragment. NMR data (1H and 13C) support
a similar structure for all of the C∧N2Ir(LX) complexes. Close intermolecular contacts in both (ppy)2Ir(acac) and
(tpy)2Ir(acac) lead to significantly red shifted emission spectra for crystalline samples of the ppy and tpy complexes
relative to their solution spectra.
Introduction
chelate complexes of Rh and Ir have been prepared with diimine
ligands as well as cyclometalated ligands, such as 2-phenyl-
The photophysics of octahedral 4d6 and 5d6 complexes has
been studied extensively.1 These complexes, particularly those
prepared with Ru2+ and Os2+, have been used extensively in
photocatalysis and photoelectrochemistry.2 The reason these d6
complexes are attractive in photochemical applications is that
they have long-lived excited states and good photoluminescence
efficiencies. The majority of the studies have been focused on
the photophysical properties of octahedral metal-diimine
complexes Ru2+ and Os2+, with ligands such as the bipyridine
and phenanthroline.3 More recently a number of groups have
investigated the isoelectronic Rh3+ and Ir3+ complexes.4 Tris-
pyidinato-C2,N (ppy).5 Several tris-cyclometalated complexes
of Rh and Ir have been reported, e.g., fac-M(ppy)3, fac-M(2-
(2-thienyl)pyridine)3.6 The cyclometalated ligands are mono-
anionic, thus forming neutral metal tris-chelate complexes,
which are isoelectronic to the cationic Ru and Os complexes.
When both cyclometalated and diimine ligands are used,
monocationic complexes can be obtained, e.g., (ppy)2M(L′)+
(L′ ) 2,2′-bipyridine, phenanthroline).7
Both neutral and cationic tris-chelate complexes of Rh and
Ir show excited state lifetimes in the microsecond regime, as
expected for a high-spin excited state. The Ir complexes show
intense phosphorescence at room temperature, while the Rh
complexes give measurable emission only at low temperatures.
The stronger spin-orbit coupling expected for Ir relative to Rh
significantly mixes singlet and triplet excited states for Ir, largely
removing the spin-forbidden nature of the phosphorescent
transitions, leading to efficient phosphorescence for these
complexes [e.g., φphos(fac-Ir(ppy)3) ) 0.4].9 The phosphorescent
lifetimes for these Ir complexes are relatively long (τ ≈ µs),4-7
compared to fluorescent lifetimes, which are typically in the
* Author to whom correspondence should be addressed. E-mail: met@
usc.edu.
(1) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood: Chichester, U.K., 1991. (b) Balzani, V.; Credi, A.;
Scandola, F. In Transition Metals in Supramolecular Chemistry;
Fabbrizzi, L., Poggi, A., Eds.; Kluwer: Dordrecht, The Netherlands,
1994; p1. (c) Lehn, J.-M. Supramolecular ChemistrysConcepts and
Properties; VCH: Weinheim, Germany, 1995. (d) Bignozzi, C. A.;
Schoonover, J. R.; Scandola, F. Prog. Inorg. Chem. 1997, 44, 1.
(2) (a) Kalyanasundaran, K. Coord. Chem. ReV. 1982, 46, 159. (b) Chin,
K.-F.; Cheung, K.-K.; Yip, H.-K.; Mak, T. C. W.; Che, C. M. J. Chem.
Soc., Dalton Trans. 1995, 4, 657-665. (c) Sonoyama, N.; Karasawa,
O.; Kaizu, Y. J. Chem. Soc., Faraday Trans. 1995, 91, 437. (d) Tan-
Sien-Hee, L.; Mesmaeker, A. K.-D. J. Chem. Soc., Dalton Trans. 1994,
24, 3651-3658. (e) Kalyanasundaram, K.; Gratzel, M. Coord. Chem.
ReV. 1998, 177, 347-414.
(3) (a) Anderson, P. A.; Anderson, R. F.; Furue, M.; Junk, P. C.; Keene,
F. R.; Patterson, B. T.; Yeomans, B. D. Inorg. Chem. 2000, 39, 2721-
2728. (b) Li, C.; Hoffman, M. Z. Inorg. Chem. 1998, 37, 830-832.
(c) Berg-Brennan, C.; Subramanian, P.; Absi, M.; Stern, C.; Hupp, J.
T. Inorg. Chem. 1996, 35, 3719-3722. (d) Kawanishi, Y.; Kitamura,
N.; Tazuke, S. Inorg. Chem. 1989, 28, 2968-2975.
(4) (a) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 759. (b) Shaw, J. R.; Sadler, G. S.; Wacholtz, W. F.;
Ryu, C. K.; Schmehl, R. H. New J. Chem. 1996, 20, 749.
(5) Dedeian, K.; Djurovich, P. I.; Garces, F. O.; Carlson, G.; Watts, R. J.
Inorg. Chem. 1991, 30, 1685-1687.
(6) Colombo, M. G.; Brunold, T. C.; Riedener, T.; Gu¨del, H. U. Inorg.
Chem. 1994, 33, 545-550.
(7) (a) Ohsawa, Y.; Sprouse, S.; King, K. A.; DeArmond, M. K.; Hanck,
K. W.; Watts, R. J. J. Phys. Chem. 1987, 91, 1047-1058. (b) Wilde,
A. P.; King, K. A.; Watts, R. J. J. Phys. Chem. 1991, 95, 629-634.
10.1021/ic0008969 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/01/2001

Phosphorescent Cyclometalated Iridium Complexes
Inorganic Chemistry, Vol. 40, No. 7, 2001 1705
nanosecond regime.8 However, the Ir phosphor lifetimes are
significantly shorter than phosphorescent lifetimes of common
organic luminophores, which can range from many milliseconds
to minutes (common for a spin-forbidden transition).8 The
significant decrease in the lifetime of the triplet excited state
for the Ir complexes relative to the triplet excited state of organic
molecules is a direct consequence of the strong spin-orbit
coupling of the Ir center. The electronic transitions responsible
for luminescence in these cyclometalated complexes have been
assigned to mixture of MLCT and ligand-centered transitions.7b,9
An interesting application for these organometallic Ir complexes
is their use as phosphors in organic light emitting diodes
(OLEDs). The excited states generated in electroluminescence
are trapped at the phosphor, where strong spin-orbit coupling
leads to efficient phosphorescence at room temperature and
efficient utilization of these excited states.10 OLEDs prepared
with these heavy metal complexes are the most efficient OLEDs
reported to date,10 with efficiencies of greater than 80%
(photons/electrons) reported for Ir(ppy)3 based OLEDs.11
Unfortunately, many of the cyclometalating ligands that could
be incorporated into Ir-based phosphors do not give tris-ligand
complexes by the reported synthetic methods. Herein we
describe the synthesis as well as spectroscopic and structural
characterization of a class of highly phosphorescent Ir com-
plexes. These complexes have two cyclometalated ligands and
a single bidentate, monoanionic ancillary ligand, making the
complexes neutral and sublimable. The emission color from the
complex is dependent on the choice of cyclometalating ligand,
ranging from green to red. The structures of two of these
complexes have been determined and strongly resemble that of
the chloride-bridged dimers prepared with the same cyclo-
metalated ligands, i.e., C∧N2Ir(µ-Cl)2IrC∧N2.
Cyclometalated Ir(III) µ-chloro-bridged dimers of general formula
C∧N2Ir(µ-Cl)IrC∧N2 were synthesized by the method reported by
Nonoyama, which involves refluxing IrCl3‚nH2O (Next Chimica) with
2-2.5 equiv of cyclometalating ligand in a 3:1 mixture of 2-methoxy-
ethanol (Aldrich Sigma) and water.15
Synthesis of (<ul;1>C∧N)2Ir(acac) Complexes. General Proce-
dure. [(C∧N)2IrCl]2 complex (0.078 mmol), 0.2 mmol of 2,4-
pentanedione (Aldrich Sigma), and 85-90 mg of sodium carbonate
were refluxed under inert gas atmosphere in 2-ethoxyethanol for 12-
15 h. After cooling to room temperature, the colored precipitate was
filtered off and was washed with water, followed by 2 portions of ether
and hexane. The crude product was flash chromatographed using a
silica/dichloromethane column to yield ca. 75-90% of the pure
(C∧N)2Ir(acac) after solvent evaporation and drying.
(ppy)2Ir(acac): iridium(III) bis(2-phenylpyridinato-N,C2′) acetylac-
1
etonate (yield 83%). H NMR (360 MHz, acetone-d6), ppm: 8.55 (d,
2H, J 5.8 Hz), 8.07 (d, 2H, J 7.9 Hz), 7.91 (t, 2H, J 7.4 Hz), 7.63 (d,
2H, J 7.9 Hz), 7.32 (t, 2H, J 7.4 Hz), 6.74 (t, 2H, J 7.4 Hz), 6.59 (t,
2H,J 5.8 Hz), 6.21 (d, 2H, J 7.4 Hz), 5.26 (s, 1H), 1.69 (s, 6H). Anal.
Found: C 54.20, H 3.92, N 4.71. Calcd: C 54.08, H3.87, N4.67.
(tpy)2Ir(acac): iridium(III) bis(2-(4-tolyl)pyridinato-N,C2′) acetyl-
acetonate (yield 75%). 1H NMR (500 MHz, CDCl3), δ, ppm: 8.45 (d,
2H, J 5.1 Hz), 7.77 (d, 2H, J 8.3 Hz), 7.68 (dt, 2H, 7.4, 1.4 Hz), 7.42
(d, 2H, J 8.3 Hz), 7.07 (ddd, 2H, J 6.9, 6.5, 1.4 Hz), 6.61 (d, 2H, J 7.8
Hz), 6.04 (s, 2H), 5.17 (s, 1H), 1.54 (s, 6H). Anal. Found: C 55.35, H
4.40, N 4.52. Calcd: C 55.49, H 4.43, N 4.46.
(bzq)2Ir(acac): iridium(III) bis(7,8-benzoquinolinato-N,C3′) acetyl-
1
acetonate (yield 90%). H NMR (360 MHz, acetone-d6), ppm: 8.96
(d, 2H, J 4.8 Hz), 8.49 (d, 2H, J 7.9 Hz), 7.80 (6H,6,8, m), 7.26 (d,
2H, J 7.9 Hz), 6.89 (t, 2H, J 7.4 Hz), 6.21 (t, 2H, J 7.9 Hz), 5.33 (s,
1H), 1.69 (s, 6H). Anal. Found: C 58.15, H 3.89, N 4.37. Calcd: C
58.08, H 3.81, N 4.23.
(bt)2Ir(acac): iridium(III) bis(2-phenylbenzothiozolato-N,C2′) acetyl-
1
acetonate. H NMR (360 MHz, DMSO-d6), ppm: 8.25 (m, 2H), 7.93
(m, 2H), 7.74 (dd, 2H, J 8.0, 2.0 Hz), 7.55 (m, 4H), 6.85 (td, 2H, J
7.6, 1.0 Hz), 6.60 (td, 2H, J 7.6, 1.0 Hz), 6.20 (dt, 2H, 7.5, 0.6 Hz),
5.12 (s, 1H), 1.71 (s, 6H). Anal. Found: C 52.08, H 3.30, N 3.47.
Calcd: C 52.23, H 3.39, N 3.93.
Experimental Section
UV-visible spectra were measured on an Aviv model 14DS
spectrophotometer (a re-engineered Cary 14 spectrophotometer). Pho-
toluminescent spectra were measured with a Photon Technology
International fluorimeter. Quantum efficiency measurements were
carried out at room temperature in a 2-MeTHF solution (degassed by
several freeze-pump-thaw cycles). Degassed solutions of fac-Ir(ppy)3
were used as a reference. Steady state emission experiments at room
temperature were performed on a PTI QuantaMaster model C-60
spectrofluorimeter. Phosphorescence lifetime measurements were per-
formed on the same fluorimeter equipped with a microsecond Xe flash
lamp. NMR spectra were recorded on Bruker AC 250 MHz, AM 360
MHz, or AMX 500 MHz instruments. Solid probe MS spectra were
taken with Hewlett-Packard GC/MS instrument with electron impact
ionization and model 5873 mass sensitive detector. High-resolution mass
spectrometry was done at Frick Chemical Laboratory at Princeton
University. The Microanalysis Laboratory at the University of Illinois,
Urbana-Campaign, performed all elemental analyses.
(bt)2Ir(pico): iridium(III) bis(2-phenylbenzothiozolato-N,C2′) picoli-
1
nate. H NMR (360 MHz, DMSO-d6), ppm: 8.24 (m, 3H), 8.09 (td,
1H, J 7.5, 1.7 Hz), 7.99 (d, 1H, J 7.7 Hz), 7.91 (dd, 1H, J 7.8, 0.8 Hz),
7.83 (m, 2H), 7.70 (m, 1H), 7.49 (m, 2H), 7.43 (t, 1H, J 4.5 Hz), 7.13
(t, 1H, J 7.9 Hz), 7.00 (td, 1H, J 7.7, 1.0 Hz), 6.95 (td, 1H, J 7.3, 1.2
Hz), 6.78 (td, 1H, J 7.3, 0.9 Hz), 6.73 (td, 1H, J 7.5, 1.2 Hz), 6.41 (d,
2H, J 7.7 Hz), 6.15 (d, 1H, J 7.7 Hz), 6.00 (d, 1H, J 8.3 Hz). Anal.
Found: C 52.44, H 2.72, N 5.35. Calcd: C 52.30, H 2.74, N 5.72.
(bt)2Ir(sal): iridium(III) bis(2-phenylbenzothiozolato-N,C2′) (N-me-
thylsalicylimine-N,O). 1H NMR (360 MHz, acetone-d6), ppm: 8.60 (d,
1H, J 8.4 Hz), 8.15 (d, 1H, J 8.4 Hz), 8.09 (m, 1H), 7.86 (d, 1H, J 8.0
Hz), 7.78 (d, 1H, J 7.6 Hz), 7.70 (d, 1H, J 7.6 Hz), 7.41-7.52 (m,
3H), 7.31 (t, 1H, J 8.4 Hz), 6.99-7.05 (m, 2H), 6.83-6.89 (m, 2H),
6.61-6.67 (m, 2H), 6.43-6.49 (m, 2H), 6.27 (d, 1H, J 7.6 Hz), 6.18
(t, 1H, J 7.6 Hz), 5.61 (s, 1H), 3.13 (s, 3H). Anal. Found: C 53.62, H
3.44, N 4.85. Calcd: C 54.67, H 3.24, N 5.63.
(bsn)2Ir(acac): iridium(III) bis(2-(1-naphthyl)benzothiazolato-N,C2′)
acetylacetonate. 1H NMR (250 MHz, CDCl3), ppm: 8.56 (d, 2H, J 7.8
Hz), 8.09 (d, 2H, J 7.9 Hz), 7.99 (d, 2H, J 7.5 Hz), 7.60 (m, 4H), 7.43
(m, 4H), 7.31 (t, 2H, J 7.8 Hz), 6.99 (d, 2H, J 8.2 Hz), 6.55 (d, 2H, J
8.5 Hz), 5.07 (s, 1H), 1.72 (s, 6H). Anal. Found: C 57.69, H 3.35, N
3.45. Calcd: C 56.56, H 3.51, N 3.03.
All procedures involving IrCl3‚H2O or any other Ir(III) species were
carried out in inert gas atmosphere despite the air stability of the
compounds, the main concern being their oxidative and thermal stability
of intermediate complexes at the high temperatures used in the reactions.
(8) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/
Cummings Publishing Co., Inc.; Menlo Park, California, 1978. Murov,
S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry; Marcel
Dekker: New York, 1993.
(9) (a) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem.
Soc. 1984, 106, 6647-6653. (b) Crosby, G. A. J. Chim. Phys. 1967,
64, 160.
(10) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) Baldo,
M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S.
R. Appl. Phys. Lett. 1999, 75, 4. (c) Thompson, M. E.; Burrows, P.
E.; Forrest, S. R. Curr. Opin. Solid State Mater. Sci. 1999, 4, 369.
(11) Adachi, C.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett., in press.
(pq)2Ir(acac): iridium(III) bis(2-phenylquinolyl-N,C2′) acetylaceto-
1
nate (yield ca. 95%). H NMR (360 MHz, DMSO-d6), ppm: 8.51 (d,
2H, 8 Hz), 8.37 (m, 4H), 8.01 (m, 4H), 7.55 (m, 4H), 6.88 (tm, 2H, 5
(12) XSCANS system of data collection solution software: Siemens
Analytical Instruments, Madison, Wisconsin.
(13) G. M. Sheldrick, SHELX system of programs for crystallographic
analysis, University of Go¨ttingen, Germany.
(14) Program DIFABS for empirical absorption corrections: Walker, N.;
Stuart, D. Acta Crystallogr. 1983, A39, 158.
(15) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767.

1706 Inorganic Chemistry, Vol. 40, No. 7, 2001
Lamansky et al.
Hz), 6.53 (td, 2H, 7 Hz, 1 Hz), 6.29 (dd, 2H, 8 Hz, 1 Hz), 4.70 (s, 1H),
2.08 (s, 6H).
Table 1. Crystal Data and Summary of Intensity Data Collection
and Structure Refinement
fac-Ir(ppy)3: iridium(III) fac-tris(2-phenylpyridinato-N,C2′). Iridium-
(III) bis(2-phenylpyridinato-N,C2′) acetylacetonate (0.20 mmol) and
phenylpyridine (0.5 mmol) were refluxed for 15 h in 10 mL of glycerol
to give a yellow suspension upon cooling. The crude product was
isolated by filtration, washed with hexane and ether, dried in a vacuum,
and zone sublimed (sample sublimed in a temperature gradient of 300-
270 °C) to give a green-yellow product (yield 85%). Analytical data
matches that reported for fac-Ir(ppy)3.5
Ir(bzq)3: iridium(III) fac-tris(7,8-benzoquinolinate-N,C3′). This com-
plex was prepared by two different methods, method A and method B.
Method A. This complex was prepared from Ir(acac)3 by a procedure
analogous to the one reported for Ir(ppy)3.5 Ir(acac)3 (50 mg) and 7,8-
benzoquinoline (113 mg) were dissolved in 5 mL of glycerol and
refluxed for 12-15 h under an inert atmosphere. After cooling, the
product was filtered off and washed with several 10 mL portions of
hexanes, ether, and acetone. The crude product was sublimed to give
a 32% yield of orange Ir(bzq)3.The Ir(bzq)3 product from this reaction
is a mixture of fac- and mer-isomers. Several wash cycles (acetone
and dichloromethane) cause significant enrichment of the mixture in
fac-product but still does not allow isolation of a pure facial complex.
1H NMR (500 MHz, CD2Cl2), ppm: 8.31 (d, J 8.1 Hz), 8.19 (d, J 8.0
Hz), 8.12 (d, J 10.0 Hz), 8.03 (m), 7.9 (m), 7.6 (m), 7,47 (t, J 7.5 Hz),
7.39 (t, J 7.4 Hz), 7.22 (m), 7.14 (m), 7.07 (m), 6.96 (d, J 8.0 Hz),
6.80 (d, J 8.1 Hz), 6.57 (d, J 7.9 Hz). Anal. Found: C 63.55, H 3.51,
N 5.32. Calcd: C 64.45, H 3.33, N 5.78.
(ppy)2Ir(acac)
C27H23IrN2O2
599.67
173
0.71073
(tpy)2Ir(acac)
C29H26IrN2O2
626.72
173
0.71073
triclinic
P1h (No. 2)
empirical formula
fw
temp, K
wavelength, Å
cryst syst
space group
unit cell dimens
a (Å)
orthorhombic
Pbcn (No. 60)
13.171(3)
10.086(3)
16.613(3)
90
90
90
9.697(2)
11.616(1)
12.371(1)
64.340(6)
76.153(9)
87.211(10)
1217.2(4)
2
1.710
5.51
614
2.0-25.0
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
vol, Å3
2206.9(8)
4
Z
density (calcd), g/cm3 1.805
abs coeff, mm-1
F(000)
θ range for data
collection, deg
index ranges
6.08
1168
2.5-22.5
0 e h e 14,
0 e k e 10,
0 e l e 17
2176
-9 e h e 9,
-12 e k e 12,
0 e l e 14
4663
reflns collected
indep reflns
refinement meth
1410 [R(int) ) 0.074] 3745 [R(int) ) 0.0233]
full-matrix least-
full-matrix least-
squares on F2
3745/0/309
0.725
Method B. (bzq)2Ir(acac) (0.16 mmol) and 7,8-benzoquinoline (0.5
mmol) were refluxed for 15 h in 10 mL of glycerol to give a yellow-
orange suspension upon cooling. The crude product was isolated by
filtration, washed with hexane and ether, dried in a vacuum, and zone
sublimed (sample sublimed in a temperature gradient of 300-270 °C)
to give an orange product, a mixture of fac- and mer-isomers (total
squares on F2
data/restraints/params 1409/0/71
GOF on F2
final R index
[I > 2σ(I)]
R index (all data)
1.129
0.0678
0.0270
0.0870
0.0336
1
yield 75%). H NMR and elemental analysis characterization match
those listed for Ir(bzq)3 prepared by method A.
agreement factors is given in Table 1. In both cases the Ir centers are
octahedrally coordinated by the three chelating ligands, with the N
atoms in a trans configuration, and a 2-fold rotation axis passes through
the Ir atom and the acac ligand.
Electrochemistry. Cyclic voltammograms (CV) were recorded on
an EG&G potentiostat/galvanostat model 283 in degassed dichloro-
methane solutions using 0.1 M tetra(n-butyl)ammonium hexafluoro-
phosphate as a supporting electrolyte and a Ag/AgCl reference electrode.
The Cp2Fe/Cp2Fe+ redox couple was used as a secondary, internal
reference. All studied complexes showed reversible oxidation waves
with half-wave potentials of 870, 820, 860, 930, and 1000 mV for
(ppy)2Ir(acac), (tpy)2Ir(acac), (bzq)2Ir(acac), (bsn)2Ir(acac), and (bt)2Ir-
(acac) vs Ag/AgCl, respectively.
Crystallography. Diffraction data for (tpy)2Ir(acac) and (ppy)2Ir-
(acac) were collected at T ) -100 °C on a Siemens P4 diffractometer
with Mo KR radiation (λ ) 0.71073 A). The cell dimensions and
orientation matrix for data collection were obtained from a least-squares
refinement of the setting angles of at least 25 accurately centered
reflections. Data collection in the 2θ scan mode, cell refinement, and
data reduction were carried out with the use of the program XSCANS12
and resulted in data sets of 2176 reflections for (ppy)2Ir(acac) and 4663
reflections for (tpy)2Ir(acac). Pertinent crystal and unit cell parameters
are given in Table 1. The (ppy)2Ir(acac) crystal used in the X-ray
diffraction study was of lower quality than the (tpy)2Ir(acac) crystal.
We attempted to grow better crystals of (ppy)2Ir(acac), but could not
get better crystals than the one used here. As a result the (ppy)2Ir-
(acac) crystal only gave useful data out to 2θ ) 45°, while the (tpy)2Ir-
(acac) diffracted strongly to well beyond the 50° cutoff used here. The
lower quality of the (ppy)2Ir(acac) crystal is reflected in the compara-
tively higher R factor for the (ppy)2Ir(acac) structure.
Results and Discussion
Synthesis and Characterization of C∧N2Ir(LX) Com-
plexes. Iridium chloride reacts with phenylpyridine in refluxing
2-ethoxyethanol to give a cyclometalated complex.15 The
product of this reaction is a chloride-bridged dimer, (ppy)2Ir-
(µ-Cl)2Ir(ppy)2, ppy ) 2-phenylpyridinato-C2,N, eq 1. This
cyclometalation reaction leads to a similar chloride-bridged
dimer for other substituted pyridyl or heterocyclic ligands as
well, i.e., 2-phenylbenzothiazole (bt), benzoquinoline (bzq), and
donor- or acceptor-substituted phenylpyridines.9b,16 Crystal-
lographic studies of the ppy complex show that the Ir center in
(ppy)2Ir(µ-Cl)2Ir(ppy)2 is octahedrally coordinated by the two
bridging chlorides and two cyclometalated ppy ligands, with
the heterocyclic rings of the ligands in a trans configuration,16a,17
as illustrated in Figure 1b. Crystal structures have not been
reported for the other chloride-bridged dimers, but NMR data
suggests a structure similar to that of (ppy)2Ir(µ-Cl)2Ir(ppy)2.16b,18
The structures were solved by direct methods using the SHELXS13
package of computer programs. Both structures were refined by full-
matrix least-squares methods based on F2 using SHELX9313 and
corrected for absorption by the program DIFABS.14 Calculated hydrogen
positions were input and refined in a riding manner along with their
attached carbons. Due to the limited data set for (ppy)2Ir(acac), the Ir
was refined anisotropically and all other atom positions were refined
isotropically. For (tpy)2Ir(acac), the non-hydrogen atom positions were
refined anisotropically and the hydrogen positions were refined
isotropically. A summary of the refinement details and the resulting
We have found this cyclometalation reaction to be a general
one, giving good yields of the corresponding C∧N2Ir(µ-

Phosphorescent Cyclometalated Iridium Complexes
Inorganic Chemistry, Vol. 40, No. 7, 2001 1707
bipyridine), the phosphorescence efficiency increases markedly.
The weak phosphorescence of the chloride-bridged dimers has
been attributed to a monomer-dimer equilibrium that occurs
in solution.16c The tris-chelate complexes are attractive in a
number of applications due to their high phosphorescence
efficiencies and microsecond lifetimes; however, we have found
it very difficult to make the complexes for many of the
cyclometalated ligands of interest. While we were able to
prepare the chloride-bridged dimers for all of the C∧N ligands
shown in Figure 1, the preparation of tris-chelates, i.e., IrC∧N3,
has only been possible for ppy, tpy, and bzq ligands.
The chloride-bridged dimers can be converted to emissive,
monomeric complexes by treating the dimer with a bidentate,
anionic ancillary ligand (LX), such as a â-diketonate, 2-picolinic
acid, or an N-alkylsalicylimine, eq 2. These reactions give C∧N2-
Ir(LX) in high yield, typically greater than 80%. The majority
of the C∧N2Ir(LX) complexes prepared are emissive at room
temperature, with λmax values between 510 and 610 nm, Table
2. The complexes are stable in air and can be sublimed in a
vacuum without decomposition.
C∧N2Ir(µ-Cl)2IrC∧N2 + 2LXH f C∧N2Ir(LX) + 2HCl
(2)
Analysis by cyclic voltammetry shows that the C∧N2Ir(acac)
complexes all undergo a reversible one-electron oxidation;
however, no reduction processes were observed in CH2Cl2. The
oxidation potentials of these complexes vary between 0.8 and
1.0 V (vs Ag/AgCl), Table 2. The complexes with pyridyl type
ligands (i.e., ppy, tpy, and bzq) are easier to oxidize than the
complexes with benzothiazolyl based ligands (i.e., bt and bsn).
The oxidation potentials of (ppy)2Ir(acac) and (tpy)2Ir(acac) are
ca. 300 mV less positive than the values reported for the
corresponding cationic (ppy)2Ir(L′)+ and (tpy)2Ir(L′)+ complexes,
where L′ ) 2,2′-bipyridine or 1,10-phenanthroline.7a,16a The
oxidation potentials of the ppy and tpy complexes are compa-
rable to those found in IrC∧N3 complexes with the same ligands,
showing that the metal-localized HOMOs are at very similar
energies.
The C∧N2Ir(acac) complexes can be used to prepare IrC∧N3.
Treating (ppy)2Ir(acac) with phenylpyridine in refluxing glycerol
gives Ir(ppy)3 in greater than 75% yield. The overall yield of
Ir(ppy)3 (based on IrCl3‚nH2O) by this route is greater than 60%,
compared to less than 20% when Ir(acac)3 is used as the
intermediate.20 Similar yields have been achieved for the
synthesis of Ir(bzq)3 and Ir(2-(2-thienyl)pyridine)3 via the C∧N2-
Ir(acac) complexes. Only C∧N2Ir(acac) complexes of phenyl-
pyridines, benzoquinoline, and thienylpyridine form tris com-
plexes when reacted with excess C∧N ligand. To the best of
our knowledge this is the first report of a tris-cyclometalated
complex of benzoquinoline, Ir(bzq)3. Based on molecular
modeling there are no obvious steric interactions that would
prevent the formation tris complexes from bt, bsn, or pq ligands.
However, other factors such as imine basicity could play a
controlling role in stabilizing intermediates in the mechanism
of tris complex formation.
Figure 1. C∧N2Ir(LX) complexes. The C∧N2Ir fragments are shown
in part a, with the acronym used for each C∧N ligand listed under the
fragment. The coordination geometries of C∧N2Ir(µ-Cl)2IrC∧N2 and
C∧N2Ir(acac) complexes are shown in part b. The identities of the LX
ligands are shown in part c.
Cl)2IrC∧N2 dimers (C∧N ) cyclometalated ligand) for a number
of ligands. The ligands examined here are shown in Figure 1.
The reactions to form the dimers proceed under fairly mild
conditions, giving good yields of the organometallic complexes,
typically greater than 75%. All of the ligands used here have a
heterocyclic nitrogen and an sp2-hybridized carbon positioned
such that the cyclometalation reaction leads to a five-membered
ring. The NMR spectra of these complexes are consistent with
the heterocyclic rings of the C,N ligands being in a trans
disposition, as shown in Figure 1b. The chloride-bridged
complexes form a racemic mixture of ∆∆/ΛΛ dimers that are
1
characterized by pronounced downfield shifts in the H NMR
resonance of heterocyclic protons nearest to the bridging
chlorides.19
C∧N2Ir(µ-Cl)2IrC∧N2 complexes show strong MLCT- and
ligand-based absorption bands. These dimers are not strongly
emissive at room temperature; however, they phosphoresce
strongly in frozen glasses at 77 K. When the dimer is dissociated
into monomeric Ir complexes, either by preparing an IrC∧N3
complex or a cationic species, e.g., (ppy)2Ir(bpy)+ (bpy ) 2,2′-
Photophysical Properties of C∧N2Ir(acac) Compounds.
The absorption spectra for (tpy)2Ir(acac) and (bzq)2Ir(acac) are
characteristic of C∧N2Ir(LX) complexes, Figure 2. Intense bands
are observed in the ultraviolet part of the spectra, between 250
(16) (a) Garces, F. O.; King, K. A.; Watts, R. J. Inorg. Chem. 1988, 27,
3464-3471. (b) Carlson, G. A.; Djurovich, P. I.; Watts, R. J Inorg.
Chem. 1993, 32, 4483-4484.
(17) Garces, F. O.; Dedian, K.; Keder, N. L.; Watts, R. J. Acta Crystallogr.
1993, C49, 1117.
1
and 350 nm, which can be assigned to the allowed (π-π*)
(18) (a) Yin, C. C.; Deeming, A. J. J. Chem. Soc., Dalton Trans. 1975,
2091. (b) Nord, G.; Hazell, A. C.; Hazell, R. G.; Farver, O. Inorg.
Chem. 1983, 22, 3429-3434. (c) Spellane, P. J.; Watts, R. J.; Curtis,
C. J. Inorg. Chem. 1983, 22, 4060-4062.
(20) The reported yield of Ir(acac)3 from IrCl3‚nH2O is 35% (Collins, J.
E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J.; Geiger, W. E.;
Rieger, A. L.; Rieger, P. H. Organometallics 1995, 14, 1232-1238),
and the yield of IrC∧N3 from Ir(acac)3 is 40%.5
(19) Garces, F. O.; Watts, R. J. J. Magn. Reson. Chem. 1993, 31, 529.

1708 Inorganic Chemistry, Vol. 40, No. 7, 2001
Lamansky et al.
Table 2. Photophysical and Electrochemical Data for C∧N2Ir(LX) Complexes
lifetime (µsec)R
absorbance
emission
quantum
efficiency*
oxidation potential
mV vs Ag/AgCl
C∧N
LX
λ (log ꢀ)
λ
max (nm)
77 K
298 K
ppy
acac
260 (4.5), 345 (3.8), 412 (3.4),
460 (3.3), 497 (3.0)
270 (4.5), 370 (3.7), 410 (3.5),
460 (3.4), 495 (3.0)
260 (4.6), 360 (3.9), 470 (3.3),
500 (3.2)
269 (4.6), 313 (4.4), 327 (4.5), 408 (3.8),
447 (3.8), 493 (3.4), 540 (3.0)
206 (4.6), 326 (4.6), 355 (4.1),
388 (3.9), 437 (3.8), 475 (3.7)
276 (4.6), 325 (4.3), 450 (3.7),
540 (2.9)
274 (4.7), 300 (4.6), 345 (4.5),
427 (4.0), 476 (4.0), 506 (3.9)
268 (5.0), 349 (4.4), 433 (3.9),
467 (3.9), 553 (3.6)
516
3.2
1.6
0.34
0.31
0.27
0.26
0.37
0.22
0.22
0.10
870
tpy
bzq
bt
acac
acac
acac
pic
512
548
557
541
562
606
597
4.5
3.1
4.5
1.8
2.3
1.4
1.8
2.0
820
23.3
4.4
860
1000
bt
3.6
bt
sal
3.1
bsn
pq
acac
acac
2.5
930
a Lifetime and quantum efficiency measurements were made in 2-MeTHF, except for the 77 K lifetime of (ppy)2Ir(acac), which was carried out
in a CH2Cl2 frozen glass. The lifetimes have error bars of (10%, and the quantum efficiencies are (20%.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
transitions of the C∧N ligands. The energies and extinction
(ppy)2Ir(acac) and (tpy)2Ir(acac)
coefficients for these bands correlate well with the transitions
(ppy)2Ir(acac) (tpy)2Ir(acac)
observed for the free hydrocarbons, i.e., tolylpyridine and
benzoquinoline.6,16 Somewhat weaker bands are observed at
Bond Distances (Å)
Ir(1)-N(1)
2.010(9)
2.010(9)
2.003(9)
2.003(9)
2.146(6)
2.146(6)
2.040(5)
2.023(5)
1.985(7)
1.982(6)
2.161(4)
2.136(4)
lower energies (λmax > 400 nm). In other cyclometalated
complexes these bands have been assigned to singlet and triplet
MLCT transitions, and the same assignment is likely here.6,16
In the spectra for (tpy)2Ir(acac), two MLCT bands are clearly
resolved at 410 and 460 nm, with extinction coefficients of 3500
and 2500 M-1 cm-1, respectively. The formally spin forbidden
3MLCT at 460 nm gains intensity by mixing with the higher
lying 1MLCT transition through the strong spin-orbit coupling
of Ir. This mixing is strong enough in these Ir complexes that
the formally spin forbidden 3MLCT has an extinction coefficient
that is more than half that of the spin-allowed 1MLCT. In some
cases, the 1MLCT and 3MLCT are not clearly resolved, as seen
in (bzq)2Ir(acac), where a broad band is observed between 430
and 520 nm. The spectra observed for C∧N2Ir(LX) complexes
are very similar to those of the C∧N2Ir(µ-Cl)2Ir C∧N2 and
C∧N2Ir(bpy)+ complexes with the same C∧N ligand,7a,16b
suggesting that the dominant absorptions in the C∧N2Ir(LX)
are due to the “C∧N2Ir” fragment, which has the same structure
in all three types of complexes, Vide infra.
All of the C∧N2Ir(LX) complexes reported here give phos-
phorescence quantum yields between 0.1 and 0.4, and lumi-
nescence lifetimes between 1 and 5 µs for degassed solutions
at room temperature, Figure 2. These photophysical parameters
are very similar to those of the fac-IrC∧N3 complexes. The
emission spectra of the C∧N2Ir(acac) complexes match those
of the fac-IrC∧N3 complexes prepared with the same C∧N
ligands for ppy-, tpy-, and bzq-based complexes, Figure 2. It
has been shown that emission from Ir(ppy)3 comes from MLCT
transitions.6,21 Considering that the absorption spectra of the ppy
and bzq complexes reported here are very similar to their IrC∧N3
and C∧N2Ir(L′)+ analogues, it is not surprising that the emission
spectra of C∧N2Ir(acac) and IrC∧N3 complexes are similar,
since all of the ppy, tpy, and bzq complexes presumably emit
from MLCT states of the same energy.
Ir(1)-N(1)*/Ir(1)-N(2)
Ir(1)-C(11)/Ir(1)-C(12)
Ir(1)-C(11)*/Ir(1)-C(24)
Ir(1)-O(1)/Ir(1)-O(2)
Ir(1)-O(1)*/Ir(1)-O(1)
Angles (deg)
N(1)-Ir(1)-N(1)*/N(1)-Ir(1)-N(2)
N(1)-Ir(1)-C(11)/N(1)-Ir(1)-C(12)
N(1)*-Ir(1)-C(11)*/N(2)-Ir(1)-C(24)
N(1)-Ir(1)-O(1) /N(1)-Ir(1)-O(2)
C(11)*-Ir(1)- O(1) /C(24)-Ir(1)- O(2)
O(1)-Ir(1)-O(1)* /O(12)-Ir(1)-O(1)
176.3(4)
176.2(2)
81.7(4)
81.7(4)
94.5(3)
87.5(3)
90.0(3)
80.4(2)
80.3(2)
94.2(2)
91.3(2)
88.2(2)
ppy and tpy ligands give green emission, bzq and bt ligands
give yellow emission, pq ligands give orange emission, and the
bsn complex gives red emission. If the pyridyl group of ppy is
replaced with a 2-quinolinyl group, the emission line is shifted
by 77 nm. A similar red shift is observed if the phenyl group
of the bt ligand is replaced with an R-naphthyl group (bsn).
The spectra of C∧N2Ir(LX) complexes show very little LX
ligand character in the excited state. For example, (bt)2Ir(LX)
(LX ) acac, pic, and sal) give very similar spectra, Figure 2c,
with only minor shifts as a function of the LX ligand. Vibronic
fine structure is observed in the (bt)2Ir(LX) emission spectra,
consistent with a significant ligand 3(π-π*) contribution to the
phosphorescence, as observed in related IrC∧N3 complexes.6
Structures of C∧N2Ir(LX) Complexes. Molecular plots of
(ppy)2Ir(acac) and (tpy)2Ir(acac) are shown in Figure 3a and
Figure 3b, respectively. Crystallographic data are given in Table
1, and selected bond lengths (Å) and angles (deg) for these
complexes are presented in the Table 3. Crystals of (ppy)2Ir-
(acac) were of lower quality than those of (tpy)2Ir(acac), which
is reflected in the poorer agreement factor for the (ppy)2Ir(acac)
complex. The molecular structures and solid state packing
determined for the two complexes are very similar. The tpy and
ppy complexes have an octahedral coordination geometry around
Ir, retaining the cis-C,C trans-N,N chelate disposition of the
chloride-bridged precursor complex, i.e., (tpy)2Ir(µ-Cl)2Ir(tpy)2.
The Ir-C bonds of these complexes (Ir-Cav ) 1.993(8) Å)
are shorter than the Ir-N bonds (Ir-Nav ) 2.021(7) Å). These
Ir-C bond lengths are similar to values of 2.02(2)-2.13(6) Å
Changes in the C∧Nligand in a C∧N2Ir(LX) complex
typically have a marked effect on the phosphorescence spectrum,
while changing the LX ligand leads to a relatively minor shift.
(21) Colombo, M. G.; Hauser, A.; Gu¨del, H. U. Inorg. Chem. 1993, 32,
3088-3092.
17
reported for the (tpy)2Ir(µ-Cl)2Ir(tpy)2 and fac-Ir(tpy)3 com-

Phosphorescent Cyclometalated Iridium Complexes
Inorganic Chemistry, Vol. 40, No. 7, 2001 1709
Figure 2. Absorption spectra for C∧N2Ir(LX) and photoluminescence spectra for C∧N2Ir(LX) and IrC∧N3 complexes. Spectra for (tpy)2Ir(acac)
and fac-Ir(ppy)3 are shown in part a. The absorption and emission spectra for (tpy)2Ir(acac) and (ppy)2Ir(acac) are identical. The absorption spectrum
for (bzq)2Ir(acac) as well as the emission spectra for (bzq)2Ir(acac) and Ir(bzq)3 are shown in part b. Spectra for (bt)2Ir(LX), LX ) acac, pic, sal,
are shown in part c. Samples were in degassed 2-MeTHF solutions, measured at room temperature.

1710 Inorganic Chemistry, Vol. 40, No. 7, 2001
Lamansky et al.
Figure 4. Crystal packing for (tpy)2Ir(acac) (top) and (ppy)2Ir(acac)
(bottom). The molecule in the foreground is shown in black. The π-π
spacing between the arylpyridine (tpy or ppy) ligands of adjacent
molecules is 3.4 and 3.5 Å for the tpy and ppy complexes, respectively.
reflect the large trans influence of the phenyl groups. All other
bond lengths and angles within the chelate ligands are typical
for cyclometalated ppy17,22,23 and acac ligands bound to
Ir(III).25-27
The crystal packing of (tpy)2Ir(acac) shows a nearest neighbor
molecule related by a C2 rotation that positions the pyridyl rings
over the aryl rings of adjacent molecules, as shown in Figure 4
(top). The aromatic ring systems of the dyads have a significant
degree of π-π overlap with a ca. 3.4 Å face-face separation
between the adjacent ring planes. A similar dyad formation also
occurs in the crystal packing of (ppy)2Ir(acac), albeit with a
lesser degree of ppy ligand overlap relative to tpy ligands and
a separation of ca. 3.5 Å between the aromatic ring systems,
Figure 4 (bottom). On the basis of the higher degree of spatial
overlap and closer face-face spacing of the tpy ligands, relative
to the ppy ligands, we expect the π-π interaction to be greater
for (tpy)2Ir(acac).
Figure 3. ORTEP drawings of (ppy)Ir2(acac) (a) and (tpy)Ir2(acac)
(b). The thermal ellipsoids for both images represent a 50% probability
limit.
plexes as well those given for other mononuclear complexes
with the “ppy2Ir” fragment.22,23 The Ir-N bond lengths also
fall within the range of values given for complexes with a “ppy2-
Ir” fragment and a trans-N,N′ disposition of pyridyl ligands
(1.98(1)-2.07(1) Å). The Ir-N bonds are shorter than the value
of 2.132 Å observed in fac-Ir(tpy)3, which has all pyridyl groups
trans to phenyl groups, consistent with the strong trans influence
of phenyl groups. The Ir-O bond lengths of 2.146(6) and
2.161(4) Å are longer than the mean Ir-O value of 2.088 Å
reported in the Cambridge Crystallographic Database24 and
The distance separating the aromatic rings in the solid state
dimers is well within range for electronic interaction between
the adjacent π systems.28 Consequently, despite having nearly
(24) Allen, F. H.; Davies, J. E.; Galloy, J. J.; Johnson, O.; Kennard, O.;
Macrae, C. F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J. M.; Watson,
D. G. J. Chem. Inf. Comput. Sci. 1991, 31, 187.
(25) Esteruelas, M. A.; Lahoz, F. J.; On˜ate, E.; Oro, L. A.; Rodr´ıguez, L.
Organometallics 1996, 15, 823.
(22) Urban, R.; Kra¨mer, R.; Mihan, S.; Polborn, K.; Wagner, B.; Beck,
W. J. Organomet. Chem. 1996, 517, 191.
(23) Neve, F.; Crispini, A. Eur. J. Inorg. Chem. 2000, 1039.
(26) Papenfuhs, B.; Mahr, N.; Werner, H. Organometallics 1993, 12, 4244.
(27) Bezman, S. A.; Bird, P. H.; Fraser, A. R.; Osborn, J. A. Inorg. Chem.
1980, 19, 3755.

Phosphorescent Cyclometalated Iridium Complexes
Inorganic Chemistry, Vol. 40, No. 7, 2001 1711
the influence of π stacking on the emission properties of
octahedral complexes is less well documented.34 While the
luminescence from solid (ppy)2Ir(acac) is characteristic of
excimer emission, the structured luminescence from (tpy)2-
Ir(acac) suggests a different mechanism, perhaps originating
from ground state dimers which act as luminescent traps.
Conclusion
Iridium tris-chelates have high phosphorescence efficiency
and microsecond lifetimes, which make them ideal for OLED
applications.10 However, the synthesis of a large variety of these
complexes is difficult due to steric and electronic effects of the
ligands. Here it has been shown that a facile route to a variety
of phosphorescent compounds exists through chloride-bridged
dimers. These complexes have the general formula C∧N2-
Ir(LX), where C∧N is a cyclometalating ligand and LX is a
monoanionic bidentate ligand. These complexes give good
phosphorescence efficiencies at room temperature and have
luminescent lifetimes of 1-5 µs. The absorption and emission
spectra of the C∧N2Ir(LX) complexes are very similar to the
spectra of the chloride-bridged dimers and mononuclear cationic
(C∧N2Ir(bpy)+) complexes leading to the conclusion that the
emission is mainly due to the “C∧N2Ir” fragment. We have
recently prepared organic LEDs with these complexes as
emissive dopants and achieved quantum efficiencies comparable
to those of devices with Ir(ppy)3 dopants. This LED work will
be reported elsewhere.35
Figure 5. Solution (CH2Cl2) and solid state photoluminescence spectra
of (ppy)Ir2(acac) and (tpy)Ir2(acac). The spectra were recorded at room
temperature; however, cooling the samples to 77 K does not signifi-
cantly alter the spectra.
identical solution absorption and emission properties, the two
complexes exhibit pronounced differences in their solid state
luminescence spectra (Figure 5). Powdered samples of both
complexes are an indistinguishable yellow color. Powdered
(ppy)2Ir(acac) luminesces yellow-green with a spectrum that is
broader and red-shifted (λmax ) 540 nm) relative to the solution
spectrum, while powdered (tpy)2Ir(acac) exhibits an orange
emission that is more structured than that of (ppy)2Ir(acac) and
has a maximum at 575 nm. The shift to lower emission energy
for solid (tpy)2Ir(acac) versus solid (ppy)2Ir(acac) correlates with
the greater degree of π overlap present in the dimeric units of
the former complex.29 Face to face π-π interactions between
aromatic molecules in the solid state often lead to a broad and
featureless luminescence that is red-shifted from the emission
that occurs in dilute solution and is typically ascribed to excimer
formation.30 Excimer formation has been reported in several
examples of square planar Pt(diimine) complexes;31-33 however,
Acknowledgment. We thank Universal Display Corporation,
the Defense Advanced Research Projects Agency, and the
National Science Foundation for financial support of this work.
Supporting Information Available: Crystal data, structure refine-
ment, atomic coordinates, bond distances, bond angles, and anisotropic
displacement parameters ((tpy)2Ir(acac) only) for (ppy)2Ir(acac) and
(tpy)2Ir(acac) and the indexed powder patterns for (ppy)2Ir(acac) and
(tpy)2Ir(acac). This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0008969
(30) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-
science: New York, 1970.
(31) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. Inorg. Chem.
1999, 38, 4262.
(32) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer,
W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591
(33) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529.
(34) Alcock, N. W.; Barker, P. R.; Haider, J. M.; Hannon, M. J.; Painting,
C. L.; Pikramenou, Z.; Plummer, E. A.; Rissanen, K.; Saarenketo, P.
J. Chem. Soc., Dalton Trans. 2000, 1447.
(28) Hunter, C. A.; Sander, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.
(29) An alternate explanation involves the formation of aggregates in the
powered sample, which are not present in solution or in the crystals
examined by X-ray diffraction. The powder X-ray diffraction patterns
for the ppy and tpy complexes exclude this possibility. Both complexes
give well-resolved powder patterns, which can be indexed to the same
unit cell found for single crystals of the same complexes. The measured
powder patterns and the fits to the single-crystal unit cell parameters
are given in the Supporting Information. Spectroscopic analysis of
solutions of the complexes (NMR, UV-vis, and emission) match those
expected for (ppy)2Ir(acac) and (tpy)2Ir(acac).
(35) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Adachi,
C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Submitted.