Phosphorescent Bis-Cyclometalated Iridium Complexes
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4305
of isoelectronic Rh3+ and Ir3+ complexes, with both diimine
and cyclometalated ligands, such as 2-phenylpyidinato-C2,N
(ppy).5 The cyclometalated ligands are formally monoanionic
and can thus be used to prepare neutral tris-ligand complexes,
which are isoelectronic with the cationic trisdiimine complexes
of Ru and Os, e.g., fac-M(ppy)3,6 fac-M(2-(R-thiopheneyl)-
pyridine)3 (fac ) facial).7 The d6 Ir complexes show intense
phosphorescence at room temperature, while the Rh complexes
give measurable emission only at low temperatures, consistent
with stronger spin-orbit coupling of Ir relative to Rh. The
electronic transitions responsible for luminescence in these
complexes have been assigned to a mixture of metal-to-ligand
electron-hole recombination are trapped at the phosphor, where
strong spin-orbit coupling leads to singlet-triplet state mixing
and, hence, efficient phosphorescent emission at room temper-
ature. Both singlet and triplet excited states can be trapped at
the phosphor. OLEDs prepared with these heavy metal com-
plexes are the most efficient OLEDs reported to date, with
internal quantum efficiencies exceeding 75% (photons/electrons)
(>15% external efficiency).12 Furthermore, OLEDs have been
prepared with C∧N2Ir(LX) phosphor dopants, giving efficient
green, yellow, or red emission. The external quantum efficien-
cies for these devices vary from 5% to nearly 20%. In this work,
we explore the photophysical and electroluminescent properties
of a series of Ir complexes used as efficient phosphorescent
dopants in OLEDs. We demonstrate that, by optimizing the
molecular structure of C∧N2Ir(LX) dopants and the energy-
transfer process, exceedingly high external and power efficien-
cies can be obtained in the green to red spectral region.
3
charge-transfer (MLCT) and (π-π*) ligand states.8 We have
recently found that highly emissive Ir complexes can be formed
with two cyclometalated ligands (abbreviated hereafter as C∧N)
and a single monoanionic, bidentate ancillary ligand (LX).9 The
emission colors from these complexes are strongly dependent
on the choice of cyclometalating ligand, ranging from green to
red, with room-temperature lifetimes from 1 to 14 µs. The
photophysical properties of these C∧N2Ir(LX) complexes are
similar to those observed for the tris-cyclometalated complexes,
and will be discussed below.
Heavy metal complexes, particularly those containing Pt and
Ir, can serve as efficient phosphors in organic light emitting
devices.10 In these devices, holes and electrons are injected into
opposite surfaces of a planar multilayer organic thin film. The
holes and electrons migrate through the thin film, to a material
interface, where they recombine to form radiative excited states,
or excitons. This electrically generated exciton can be either a
singlet or a triplet. Both theoretical predictions and experimental
measurements give a singlet/triplet ratio for these excitons of 1
to 3.11 Fluorescent materials typically used to fabricate organic
light emitting diodes (OLEDs) do not give detectable triplet
emission (i.e., phosphorescence), nor is there evidence for
significant intersystem crossing between the triplet and singlet
manifolds at room temperature. The singlet/triplet ratio thus
implies a limitation of 25% for the internal quantum efficiency
for OLEDs based on fluorescence. By doping OLEDs with
heavy metal phosphors, we have shown that the singlet-triplet
limitation can be eliminated.10 The excited states generated by
Experimental Section
Synthesis. All synthetic procedures involving IrCl3‚H2O and other
Ir(III) species were carried out in inert gas atmosphere despite the air
stability of the compounds, the main concern being the oxidative
stability of intermediate complexes at the high temperatures used in
the reactions. NMR spectra were recorded on Bruker AMX 360- or
500-MHz instruments. High-resolution mass spectrometry was carried
out by the mass specroscopy facility at the Frick Chemistry Laboratory,
Princeton University. Elemental analyses (C, H, N) were carried out
by standard combustion analysis by the Microanalysis Laboratory at
the University of Illinois, UrbanasChampagne.
Cyclometalated Ir(III) µ-chloro-bridged dimers of a general formula
C∧N2Ir(µ-Cl)2IrC∧N2 were synthesized according to the Nonoyama
route, by refluxing IrCl3‚nH2O (Next Chimica) with 2-2.5 equiv of
cyclometalating ligand in a 3:1 mixture of 2-ethoxyethanol (Aldrich
Sigma) and water.13
C∧N2Ir(acac), C∧N ) ppy, tpy, bzq, bt, Rbsn, and pq were prepared
as described previously.9
Synthesis of (C∧N)2Ir(acac) Complexes. General Procedure. The
chloro-bridged dimer complex (0.08 mmol), 0.2 mmol of acetyl acetone,
and 85-90 mg of sodium carbonate were refluxed in an inert
atmosphere in 2-ethoxyethanol for 12-15 h. After cooling to room
temperature, a colored precipitate was filtered off and washed with
water, hexane, and ether. The crude product was flash chromatographed
on a silica column with dichloromethane mobile phase to yield ∼75-
90% of the pure C∧N2Ir(acac), after solvent evaporation and drying.
thp2Ir(acac): Iridium(III) bis(2-(2′-thienyl)pyridinato-N,C3′) (acetyl
acetonate) (yield 83%). 1H NMR (360 MHz, acetone-d6): δ, ppm 8.41
(d, 2H, J 5.8 Hz), 7.79 (td, 2H, J 7.9, 1.6 Hz), 7.56 (d, 2H, J 7.9 Hz),
7.22 (d, 2H, J 4.7 Hz), 7.11 (td, 2H, J 6.3, 1.6 Hz), 6.09 (d, 2H, J 4.7
Hz), 5.29 (s, 1H), 1.72 (s, 6H). Anal. Found C 45.33, H 3.00, N 4.81.
Calcd C 45.16, H 3.13, N 4.58.
(4) (a) Anderson, P. A.; Anderson, R. F.; Furue, M.; Junk, P. C.; Keene,
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2001, 40, 1704-1711.
btp2Ir(acac): Iridium(III) bis(2-(2′-benzothienyl)pyridinato-N,C3′)
1
(acetylacetonate) (yield 72%). H NMR (360 MHz, acdtone-d6): δ,
ppm 8.39 (d, 2H, J 5.9 Hz), 7.80 (t, 2H, J 7.9 Hz), 7.77 (d, 2H, J 8.0
Hz), 7.68 (d, 2H, J 8.0 Hz), 7.25 (t, 2H, J 7.0 Hz), 7.10 (t, 2H, J 7.1
Hz), 6.82 (t, 2H, J 8.0 Hz), 6.40 (d, 2H, J 7.3 Hz), 5.70 (s, 1H), 1.90
(s, 6H). Anal. Found C 52.51, H 3.29, N 4.01. Calcd C 52.30, H 3.26,
N 3.94.
dpo2Ir(acac): Iridium(III) bis(2,4-diphenyloxazolato-1,3-N,C2′) (acetyl
acetonate) (yield 93%).1H NMR (360 MHz, CDCl3): δ, ppm 7.79 (d,
4H, J 7.4 Hz), 7.53 (d, 2H, J 7.9 Hz), 7.49 (m, 6H), 7.45 (s, 2H), 7.40
(t, 2H, J 7.4 Hz), 6.84 (t, 2H, J 7.4 Hz), 6.76 (t, 2H, J 7.4 Hz), 6.62 (d,
2H, J 7.9 Hz), 5.25 (s, 1H), 1.86 (s, 6H). Anal. Found C 56.35, H
3.67, N 3.89. Calcd C 57.44, H 3.72, N. 3.83.
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C62Ir(acac): Iridium(III) bis(3-(2-benzothiazolyl)-7-(diethylamino)-
2H-1-benzopyran-2-onato-N′,C4) (acetyl acetonate) (yield 59%). 1H
(12) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl.
Phys. Lett. 2000, 78, 1704.
(13) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767-768.