A R T I C L E S
Tsuboyama et al.
Ltd. Phosphorescence quantum yields were determined at room
temperature with the use of an N2-saturated toluene solution of 10-5
M fac-Ir(ppy)3 as a reference. Phosphorescence spectra at 77 K were
measured in a toluene-ethanol-methanol (5:4:1) mixed solvent.
Emission lifetimes were measured by a Hamamatsu Photonics C4334
streakscope with excitation light (λ ) 337.1 nm) from an N2 laser
(LN120C from Laser Photonics). UV-visible spectra were recorded
on a Shimadzu UV3100S spectrophotometer. Thermogravimetric and
differential thermal analysis (TG/DTA) was performed by a TG-DTA
2000S thermal analyzer (MAC Science Co.) under N2 stream with a
scanning rate of 10 °C/min.
(ppy)3 (fac-tris[2-phenylpyridinato]Ir(III)) and investigated its
photophysical and photochemical properties. Ir(ppy)3 in de-
gassed toluene shows very strong phosphorescence (λmax ) 514
nm, phosphorescence yield Φp ) 0.4) emitted from the
predominantly 3MLCT (metal-to-ligand charge transfer) excited
state at room temperature. Afterward, Colombo et al.12 reported
that fac-Ir(thpy)3 (fac-tris[2-(thiophen-2-yl)pyridinato]Ir(III))
3
exhibits strong phosphorescence from the predominantly π-
π* excited state.
In 1999, Baldo et al.16 made an organic light-emitting diode
(OLED) with fac-Ir(ppy)3 as a green phosphorescent dopant.
The OLED device can utilize all of the electrogenerated singlet
and triplet excitons, leading to ca. 100% internal quantum
efficiency (ηin). More recently, a green device with ηin of nearly
100% has also been reported17 with the use of a heteroleptic
complex, (ppy)2Ir(acac) (bis[2-phenylpyridinato]Ir(III)(acetyl-
acetonate)), as a phosphorescent dopant. Interestingly, Adachi
et al.18 made a red-phosphorescent OLED with the use of
(btpy)2Ir(acac) (bis[2-(benzo[b]thiophen-2-yl)pyridinato]Ir(III)-
(acetylacetonate)) as a dopant. The red OLED provided pure-
red electroluminescence [CIE coordinates (x, y) ) (0.68, 0.32)]
with ηex ) 7%.
Crystals of Ir(piq)3 suitable for X-ray analysis were obtained from
chloroform solutions at room temperature. Diffraction data were
collected at 93 K on a Rigaku RAXIS-RAPID imaging plate
diffractometer equipped with graphite-monochromated Mo KR radiation
(λ ) 0.71069 Å). The crystal structure of Ir(piq)3 was solved by the
direct method using SIR88. The crystal structure was refined by the
full-matrix least-squares method on F2. All non-hydrogen atoms were
refined anisotropically, while hydrogen atoms were refined isotropically.
All analyses were performed by the crystallographic software package
CrystalStructure. A summary of the refinement details and the resulting
agreement factor is given in the Supporting Information. The iridium
center of Ir(piq)3 is octahedrally coordinated by three bidentate ligands
with a facial structure and a 3-fold rotational axis passing through the
iridium atom.
To fabricate highly efficient red-emissive OLEDs, it is
necessary to search for red-emissive metal complexes with large
luminescence quantum yields. However, it is not easy to find
such a red-emissive complex, since the luminescence quantum
yields tend to decrease with an increase in the emission peak
wavelength according to the energy gap law.19
The purpose of the present study is the molecular design of
a highly efficient red-phosphorescent complex suitable for red
OLED devices. In addition to high phosphorescence efficiency,
the complex should possess high thermal stability for device
fabrication and stable device performance. We particularly
focused our attention on designing metal complexes that provide
red emission from an MLCT excited state.
OLED devices were fabricated by the conventional vacuum deposi-
tion method under pressures of less than 10-4 Pa. The devices were
made on an indium-tin oxide (ITO) film (15 Ω/cm2, thickness 120
nm, from Nippon Sheet Glass Co.) with a 3.14 mm2 round-patterned
area. Voltage-current properties of devices were measured on a
handmade circuit with an operational amplifier and a load resistance,
a computer-controlled arbitrary waveform generator Yokogawa AG1200,
and a digital oscilloscope Tektronix TDS420. Luminance of the OLEDs
was measured with a luminance colorimeter BM-7 from Topcon Ltd.
Material Preparations
Ligand Synthesis. Reagent-grade 5-methylthiophene-2-boronic acid,
thiophene-2-boronic acid, benzo[b]thiophene-2-boronic acid, phenyl-
boronic acid, 2,5-dibromopyridine, 2-chloro-5-trifluoromethylpyridine,
2-bromopyridine, and isoquinoline-N-oxide were purchased from
Aldrich Chemical Co. or Tokyo Kasei Kogyo Co. 1-Chloroisoquinoline
and 2-iodo-9,9-dimethylfluorene were prepared according to previous
papers.20,21
Experimental Section
1H NMR spectra were recorded on a Bruker DPX-400 NMR
spectrometer operating at 400 MHz. Elemental analysis was carried
out with an elemental analyzer Vario EL CHNOS from Elementar Co.
Photoluminescence spectra were recorded on a Hitachi F4500
fluorescence spectrometer. Spectral data were corrected with the use
of emission spectra of Ir(ppy)3 and Ir(piq)3 (tris[1-phenylisoquinolinato-
C2,N]iridium(III)) measured by a spectroradiometer SR1 from Topcon
All the ligands studied here were prepared by the modified Suzuki
coupling reactions.22,23 Tetrakis(triphenylphosphine)palladium and aque-
ous sodium carbonate were used as a catalyst and a base, respectively,
in all the coupling reaction. Scheme 1 outlines some examples of the
synthetic processes of the ligands. Detailed procedures of the prepara-
(13) (a) Nord, G.; Hazell, A. C.; Harzell, R. G.; Farver, O. Inorg. Chem. 1983,
22, 3429-3434. (b) Garces, F. O.; King, K. A.; Watts, R. J. Inorg. Chem.
1988, 27, 3464-3471. (c) Wilde, A. P.; Watts, R. J. J. Phys. Chem. 1991,
95, 622-629. (d) Wilde, A. P.; King, K. A.; Watts, R. J. J. Phys. Chem.
1991, 95, 629-634. (e) Urban, R.; Kra¨mer, R.; Mihan, S.; Polborn, K.;
Wagner, B.; Beck, W. J. Organomet. Chem. 1996, 517, 191-200. (f)
Colombo, M. G.; Gu¨del, H. U. Inorg. Chem. 1993, 32, 3081-3087. (g)
Colombo, M. G.; Hauser, A.; Gu¨del, H. U. Inorg. Chem. 1993, 32, 3088-
3092. (h) Neve, F.; Crispini, A.; Campagna, S.; Serroni, S. Inorg. Chem.
1999, 38, 2250-2258. (i) Neve, F.; Crispini, A.; Loiseau, F.; Campagna,
S. J. Chem. Soc., Dalton Trans. 2000, 1399-1401. (j) Neve, F.; Crispini,
A.; Serroni, S.; Loiseau, F.; Campagna, S. Inorg. Chem. 2001, 40, 1093-
1101.
1
tion, the H NMR spectra, and chemical analysis data of the ligands
are given in the Supporting Information.
Synthesis of Complexes. General Procedure. Complexes were
synthesized according to a previous paper.14 Tris(acetylacetonato)-
iridium(III), Ir(acac)3 (0.5 g, 1.0 mmol), and cyclometalating ligand
(ca. 5 mmol) were dissolved in 50 mL of glycerol. The solution was
refluxed under nitrogen stream for 6 h. After completion of the reaction,
1 M aqueous hydrochloric acid was added to the solution, resulting in
precipitation of the product. Then the poduct was filtered, washed with
water, and dried at 100 °C in vacuo. The purification of the product
was made by silica gel column chromatography with CHCl3 as an
(14) Dedeian, K.; Djurovich, P. I.; Garces, F. O.; Carlson, G.; Watts, R. J. Inorg.
Chem. 1991, 30, 1685-1687.
(15) Spellane, P.; Watts, R. J.; Vogler, A. Inorg. Chem. 1993, 32, 5633-5636
(16) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest,
S. R. Appl. Phys. Lett. 1999, 75 (1), 4-6.
(17) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys.
2001, 90 (10), 5048-5051.
(20) Shirota, Y.; Kinoshita, M.; Noda, T.; Okumoto, K.; Ohara, T. J. Am. Chem.
Soc. 2000, 122, 11021-11022.
(18) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.;
Kwong, R. C. Appl. Phys. Lett. 2001, 78 (11), 1622-1624.
(21) Zhang, H.; Kwong, F. Y.; Tian, Y.; Chan, K. S. J. Org. Chem. 1998, 63,
6886-6890.
(19) (a) Meyer, T. J. Pure Appl. Chem. 1986, 58 (9), 1193-1206. (b) Cummings,
S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949-1960. (c) Casper,
J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952-957.
(22) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
(23) Lohse, O.; Thevenin, P.; Waldvogel, E. Synlett 1999, 1, 45-48.
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12972 J. AM. CHEM. SOC. VOL. 125, NO. 42, 2003