Tuning the saturated red emission: Synthesis, electrochemistry and photophysics of 2-arylquinoline based iridium(iii) complexes and their application in OLEDs

Article abstract of DOI:10.1039/b605783g

A series of novel iridium(iii) complexes with two 2-arylquinoline derivatives as cyclometalated ligands and one monoanionic ligand, such as acetylacetonate (acac), N,N′-diethyldithiocarbamate (Et₂dtc) and O,O′-diethyldithiophosphate (Et₂dtp), as ancillary ligands have been synthesized and structurally characterized by ¹H NMR, MS and elemental analysis (EA). The cyclic voltammetry, absorption, emission and electroluminescence properties of these complexes were systematically investigated. Through extending π-conjugation, introducing electron-donating groups in the ligand frame, or changing the ancillary ligands, the HOMO energy levels of the iridium(iii) complexes can be tuned, while their LUMO levels remain little affected; in consequence, the emission wavelengths of the iridium(iii) complexes can be tuned in the range 606-653 nm. The highly efficient organic light-emitting diodes (OLEDs) with saturated red emission have been demonstrated. A maximum current efficiency of 10.79 cd A^-1, at a current density of 0.74 mA cm^-2, with an emission wavelength of 616 nm and Commisioon Internationale de L'Eclairage (CIE) coordinates of (0.65, 0.35), which are very close to the National Television System Comittee (NSTC) standard red emission, have been achieved when using complex (DPQ) ₂Ir(acac) as a phosphor dopant. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605783g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Tuning the saturated red emission: synthesis, electrochemistry and
photophysics of 2-arylquinoline based iridium(III) complexes and their
application in OLEDs{
Lianqing Chen,a Han You,b Chuluo Yang,*a Xiaowei Zhang,a Jingui Qin*a and Dongge Ma*b
Received 24th April 2006, Accepted 27th June 2006
First published as an Advance Article on the web 14th July 2006
DOI: 10.1039/b605783g
A series of novel iridium(III) complexes with two 2-arylquinoline derivatives as cyclometalated
ligands and one monoanionic ligand, such as acetylacetonate (acac), N,N9-diethyldithiocarbamate
(Et2dtc) and O,O9-diethyldithiophosphate (Et2dtp), as ancillary ligands have been synthesized and
structurally characterized by 1H NMR, MS and elemental analysis (EA). The cyclic voltammetry,
absorption, emission and electroluminescence properties of these complexes were systematically
investigated. Through extending p-conjugation, introducing electron-donating groups in the
ligand frame, or changing the ancillary ligands, the HOMO energy levels of the iridium(III)
complexes can be tuned, while their LUMO levels remain little affected; in consequence, the
emission wavelengths of the iridium(III) complexes can be tuned in the range 606–653 nm. The
highly efficient organic light-emitting diodes (OLEDs) with saturated red emission have been
demonstrated. A maximum current efficiency of 10.79 cd A21, at a current density of
0.74 mA cm22, with an emission wavelength of 616 nm and Commisioon Internationale de
L’Eclairage (CIE) coordinates of (0.65, 0.35), which are very close to the National Television
System Comittee (NSTC) standard red emission, have been achieved when using complex
(DPQ)2Ir(acac) as a phosphor dopant.
saturated and pure red-emitting iridium complexes are still
Introduction
scarce because their luminescent quantum yields tend to be
intrinsically low.6
In the past two decades, organic light-emitting diodes
(OLEDs) have attracted a great deal of attention as a very
promising technology for flat panel displays.1 Recently, heavy
attention in optoelectronic materials due to their high electron
Quinoline based compounds have received considerable
affinities.7 For example, aluminium tris(8-hydroxyquinolate)
metal complexes were doped into charge-transporting hosts
as efficient phosphorescent emitters because of their high
(AlQ3) is usually applied as an electron-transporting, electron-
emitting and host material in doped OLED systems.4c,5b–f,6a–c
emission efficiency compared with the conventional fluores-
cent OLEDs.2 The strong spin–orbit coupling, caused by the
In this article, we systematically designed and synthesized a
heavy metal atom, makes the intersystem cross from the
series of new 2-arylquinoline based iridium complexes by
singlet to the triplet excited state more efficiently. In theory,
extending the p-electron delocalization of the aromatic motif,
the mixing of the singlet and triplet excited states can lead to
or applying the electron-donating aromatic thiophene in the
an internal phosphorescence quantum efficiency as high as
ligand frame, toward the saturated red emission. In addition,
100%.3 Especially, the iridium(III) complexes with cyclo-
the two dithiolates as ancillary ligands were used to tune the
metalated ligands show intense phosphorescence at room
saturated red emission. The electrochemical, photophysical
temperature and behave as one of the most promising
phosphorescent dyes in OLEDs.4 For full-color display
applications, red-, green-, and blue-emitting materials that
have sufficient luminous efficiency and proper chromaticity
must be developed.5 Although highly efficient OLED devices
using iridium complexes with emission covering the whole
visible region have been reported over the past several years,
and electrophosphorescent properties of these iridium com-
plexes will be discussed.
Experimental
General information
1H NMR and 31P NMR spectra were recorded on a Varian
Mercury VX-300 MHz spectrometer. Elemental analyses of
carbon, hydrogen, and nitrogen were performed on a Carlo
Erba 1106 microanalyzer. Mass spectra (FAB-MS) were
determined on a VJ-ZAB-3F Mass Spectrometer. Cyclic
voltammetry (CV) was carried out in nitrogen-purged anhy-
drous THF, or dichloromethane, at room temperature with a
CHI voltammetric analyzer. Tetrabutylammonium hexa-
fluorophosphate (TBAPF6) (0.1 M) was used as a supporting
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan 430072, China.
E-mail: clyang@whu.edu.cn; Fax: 86-27-68756757; Tel: 86-27-68756757
bState Key Laboratory of Polymer Chemistry and Physics, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China. E-mail: mdg1014@ciac.jl.cn
{ Electronic supplementary information (ESI) available: cyclic vol-
tammograms for complexes 1, 2 and 3 and CIE coordinate plot for
OLED devices. See DOI: 10.1039/b605783g
3332 | J. Mater. Chem., 2006, 16, 3332–3339
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electrolyte. The conventional three-electrode configuration
consists of a platinum working electrode, a platinum wire
auxiliary electrode, and an Ag wire pseudo-reference electrode
with ferrocenium/ferrocene (Fc+/Fc) as the internal standard.
product was directly used for next step without further
purification.2b,4c
Synthesis of 1, 2, 3 and 4: the dimers (0.075 mmol),
acetylacetone (0.25 mmol) and anhydrous sodium carbonate
(Na2CO3, 1.0 mmol) were dissolved in 2-ethoxyethanol (10 ml).
The mixture was refluxed under argon for 15–18 h. After
cooling to room temperature, a small quantity of water was
added. The resulting red precipitate was collected by filtration,
washed with water, ethanol and hexane, and dried under
vacuum. The crude product was purified by column chroma-
tography on silica gel with CH2Cl2–petroleum ether (1 : 1 to
1 : 3) as eluent.
Bis(6-bromo-2,4-diphenylquinolyl-N,C2)iridium(acetylaceto-
nate) [Ir(DPQ)2(acac)] 1: red solid, yield: 62%. 1H NMR
(CDCl3, 300 MHz) d: 8.44 (d, J = 9.3 Hz, 2H), 8.01 (s, 2H),
7.99 (s, 2H), 7.82 (d, J = 7.5 Hz, 2H), 7.66–7.62 (m, 10H), 7.48
(d, J = 9.6 Hz, 2H), 6.85 (t, J = 7.5 Hz, 2H), 6.67 (d, J = 6.9 Hz,
2H), 6.56 (d, J = 7.5 Hz, 2H), 4.76 (s, 1H), 1.56 (s, 6H). Anal.
calcd for C47H33O2N2Br2Ir: C 55.90, H 3.29, N 2.77%; found:
C 55.69, H 3.41, N 2.55%. MS (FAB): m/z: 1010 (M+).
Bis[6-bromo-2-(naphth-1-yl)-4-phenylquinolyl-N,C2]iridium-
(acetylacetonate) [Ir(NPQ)2(acac)] 2: deep red solid, yield:
59%. 1H NMR (CDCl3, 300 MHz) d: 8.68 (d, J = 8.4 Hz, 2H),
8.62 (s, 2H), 8.22 (d, J = 8.7 Hz, 2H), 7.98 (s, 2H), 7.70–7.65
(m, 12H), 7.52 (t, J = 6.9 Hz, 2H), 7.41 (d, J = 9.9 Hz, 2H),
7.32 (t, J = 6.6 Hz, 2H), 7.06 (d, J = 7.8 Hz, 2H), 6.79 (d, J =
7.5 Hz, 2H), 4.68 (s, 1H), 1.57 (s, 6H). Anal. calcd for
C55H37O2N2Br2Ir: C 59.52, H 3.36, N 2.52%; found: C 59.33,
H 3.42, N 2.77%. MS (FAB): m/z: 1110 (M+).
Cyclic voltammograms were obtained at scan rate of 50 mV s21
.
Formal potentials are calculated as the average of the cyclic
voltammetric anodic and cathodic peaks. UV-Vis absorption
spectra were recorded on a Shimadzu 160A spectrophoto-
meter. Photoluminescence (PL) spectra were performed on
Perkin-Elmer LS 55 luminescence spectrophotometer.
All reactions and manipulations were carried out under an
argon atmosphere. All silica gel column chromatography was
performed with use of silica gel (200–300 mesh). (2-Amino-5-
bromophenyl)phenylmethanone,8 1-naphthalen-1-ylethanone,9
1-(5-methylthiophen-2-yl)ethanone,10 1-(7-bromo-9,99-dihexyl-
9H-fluoren-2-yl)ethanone,11 sodium N,N9-diethyldithiocarba-
mate (Et2dtc)12 and potassium O,O9-diethyldithiophosphate
(Et2dtp)13 were prepared following published procedures. All
other chemicals were purchased and used without further
purification.
Synthesis of 2-arylquinoline ligands8,14
(2-Amino-5-bromophenyl)phenylmethanone (10 mmol) and
the corresponding ketones (10 mmol) were added to 0.1 ml of
concentrated sulfuric acid and 10 ml of glacial acetic acid. The
mixture had been refluxed for 24 h before being cooled to
room temperature. Then the mixture was poured slowly into
an ice-cold solution prepared from 15 ml of concentrated
ammonium hydroxide and 40 ml of water. The precipitate was
collected and purified by column chromatography.
Bis[6-bromo-2-(7-bromo-9,99-dihexyl-9H-fluoren-2-yl)-4-
phenylquinolyl-N,C2]iridium(acetylacetonate) [Ir(FPQ)2(acac)]
1
3: red solid, yield: 50%. H NMR (CDCl3, 300 MHz) d: 8.32
6-Bromo-2,4-diphenylquinoline (DPQ): white solid, yield:
64%. 1H NMR (300 MHz, CDCl3) d: 8.18 (d, J = 6.6 Hz, 2H),
8.10 (d, J = 9.0 Hz, 1H), 8.03 (d, J = 6.3 Hz, 1H), 7.78–7.83 (m,
2H), 7.47–7.51 (m, 8H).
(d, J = 9.3 Hz, 2H), 7.99 (s, 4H), 7.90 (s, 2H), 7.58–7.67 (m,
8H), 7.33 (d, J = 8.4 Hz, 2H), 7.27 (s, 2H), 7.09 (d, J =
6.6 Hz, 2H), 6.89 (d, J = 7.8 Hz, 2H), 6.89 (s, 2H), 6.65 (s,
2H), 4.68 (s, 1H), 1.88–1.91 (m, 8 H), 1.18 (s, 6H), 1.04 (br,
10 H), 0.88 (br, 10 H), 0.72–0.76 (m, 12 H), 0.54–0.58 (m,
12H). Anal. calcd for C85H87O2N2Br4Ir: C 60.75, H 5.21, N
1.67%; found: C 60.46, H 5.36, N 1.70%.
6-Bromo-2-naphthalen-1-yl-4-phenylquinoline
(NPQ):
1
white solid, yield: 61%. H NMR (300 MHz, CDCl3) d: 8.20
(d, J =1.5 Hz, 1H), 8.17 (s, 1H), 8.11 (d, J = 9.0 Hz, 1H), 8.04
(d, J = 2.1 Hz, 2H), 7.84 (s, 1H ), 7.81 (d, J = 2.1 Hz, 1H),
7.58–7.48 (m, 9H).
Bis[6-bromo-2-(5-methylthiophen-2-yl)-4-phenylquinolyl-
N,C2]iridium(acetylacetonate) [Ir(TPQ)2(acac)] 4: red solid,
yield: 51%. 1H NMR (CDCl3, 300 MHz) d: 8.34 (d, J = 8.1 Hz,
2H), 8.15 (d, J = 4.5 Hz, 2H), 7.78–7.82 (m, 3H), 7.71 (d, J =
7.5 Hz, 1H), 7.56–7.67 (m, 6H), 7.26–7.38 (m, 4H), 5.99 (s,
2H), 4.92 (s, 1H), 2.38 (s, 6H), 1.68 (s, 6H). Anal. calcd for
C45H33O2N2S2Br2Ir: C 51.47, H 3.17, N 2.67%; found: C
51.26, H 3.43, N 2.65%. MS (FAB): m/z: 1051 (M+).
6-Bromo-2(7-bromo-9,99-dihexyl-9H-fluoren-2-yl)-4-phenyl-
quinoline (FPQ): yellowish oil, yield: 57%. 1H NMR
(300 MHz, CDCl3) d: 8.35 (d, J = 8.4 Hz, 2H), 8.21 (s,
1H), 7.89–7.93 (m, 2H), 7.81(d, J = 7.5 Hz, 1H), 7.74–7.76
(m, 1H), 7.58–7.63 (m, 5H), 7.30–7.41 (m, 3H), 1.98–2.09
(m, 4H), 1.02–1.18 (m, 16H), 0.75 (t, J = 6.9 Hz, 6H).
6-Bromo-2(5-methylthiophen-2-yl)-4-phenylquinoline (TPQ):
yellowish solid, yield: 66%. 1H NMR (300 MHz, CDCl3) d:
7.93–7.98 (m, 2H), 7.73 (d, J = 6.3 Hz, 1H), 7.66 (s, 1H),
7.50–7.57 (m, 6H), 6.80–6.81 (m, 1H), 2.56 (s, 3H).
Synthesis of 5 and 6: the two complexes were synthesized
according to the same method as 1–4, but using KEt2dtp or
NaEt2dtc to replace acetylacetone, respectively.
Bis[6-bromo-2(5-methylthiophen-2-yl)4-phenylquinolyl-N,C2]
iridium(N,N9-diethyldithiocarbamate) [(TPQ)2Ir(Et2dtc)] 5:
red solid, yield: 71%. 1H NMR (CDCl3, 300 MHz) d: 9.80
(d, J = 9.3 Hz, 2H), 8.36 (d, J = 9.3 Hz, 2H), 8.01 (d, J =
2.1 Hz, 2H), 7.42–7.71 (m, 10H), 6.85 (d, J = 6.9 Hz, 2H), 5.91
(s, 2H), 3.84–4.10 (m, 2H), 3.57–3.68 (m, 2H), 2.34 (s, 6H),
1.01 (t, J = 7.2 Hz, 6H). Anal. calcd for C45H36N3S4Br2Ir: C
49.18, H 3.30, N 3.82%; found: C 49.33, H 3.16, N 3.91%. MS
(FAB): m/z: 1099 (M+).
Synthesis of Ir(III) complexes
Cyclometalated chloride-bridged dimers: iridium trichloride
hydrate (0.352 g, 1.0 mmol), combined with 2.5 equiv. of
2-arylquinoline derivatives, were dissolved in a mixture of
2-ethoxyethanol (30 ml) and water (10 ml), and then refluxed
for 24 h.15 The solution was cooled to room temperature,
and the resulting red precipitate was collected by filtration,
and washed with water and ethanol. After drying, the crude
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Bis[6-bromo-2(5-methylthiophen-2-yl)4-phenylquinolyl-N,C2]
iridium(O,O9-diethyldithiophosphate) [(TPQ)2Ir(Et2dtp)] 6:
red solid, yield: 65%. 1H NMR (CDCl3, 300 MHz) d: 9.82
(d, J = 8.4 Hz, 2H), 8.42 (d, J = 8.7 Hz, 2H), 8.03 (s, 2H), 7.46–
7.68 (m, 8H), 7.41 (s, 2H), 6.85 (d, J = 6.9 Hz, 2H), 5.92 (s,
2H), 3.61–3.69 (m, 4H), 2.36 (s, 6H), 0.98 (t, J = 6.9 Hz, 6H).
Anal. calcd for C44H36O2N2S4Br2PIr: C 46.52, H 3.19, N
2.47%; found: C 46.48, H 3.22, N 2.53%.
photodiode. The EL spectra were measured by a JY SPEX
CCD3000 spectrometer.
Results and discussion
Synthesis and characterization
Four 2-arylquinoline derived ligands, DPQ, NPQ, FPQ
and TPQ, were conveniently synthesized by using the acid-
catalyzed Friedla¨nder condensation reaction8,14,16 of (2-amino-
5-bromophenyl)phenylmethanone with the corresponding
acetyl compounds. Cyclometalated Ir(III) m-chloro-bridged
dimers were synthesized by iridium trichloride hydrate
with an excess of 2-arylquinoline derivatives (2.5 times).15
The iridium complexes were prepared by the dimer with the
corresponding ancillary ligands in 50–70% yield (Scheme 1).
The chloride-bridged diiridium complexes can be easily
converted to mononuclear iridium complexes by replacing
the two bridging chlorides with bidentate monoanionic
ancillary ligands. Elemental analysis of these complexes is
consistent with the expected formulation of their structures;
OLED fabrication and measurement
Organic layers were fabricated by high-vacuum thermal
evaporation onto pre-cleaned indium tin oxide (ITO,
30 V/square). The glass substrates were sequentially cleaned
with detergent, deionized water, ethanol, acetone and chloro-
form in an ultrasonic bath, and heated in an ultra-infrared dry
oven. In a vacuum chamber with a pressure of ,1 6 1024 Pa,
40 nm of {4,49-bis[N-(1-naphthyl-N-phenylamino)biphenyl]}
(NPB) as the hole-transporting layer (HTL), 30 nm of Ir
complexes doped with 4,49-biscarbazolybiphenyl (CBP) as
the emitting layer, 10 nm of 2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline (BCP) as a hole- and exciton-blocking layer,
30 nm of AlQ3 as the electron-transporting layer (ETL), and a
cathode composed of 1 nm of lithium fluoride and 100 nm of
aluminium were sequentially deposited onto the substrate to
construct the device. The current-voltage-brightness (I-V-B) of
the electroluminescent (EL) devices was measured at ambient
conditions with a Keithley 2400 source meter and a Keithley
2000 source multimeter equipped with a calibrated silicon
1
the mass spectra give corresponding molecular ion peaks. H
NMR shows well-resolved multiplets for the 2-arylquinoline
moiety. The chemical shift for the –CH– moiety in the
acetylacetonate (acac) ancillary ligand is 4.76 (1), 4.68 (2),
4.68 (3) and 4.92 (4) ppm, which shift to the high-field
compared to other analogues, including 2-arylpyridine
(ca. 5.15 ppm),5a–d 2-arylbenzothiazole (ca. 5.12 ppm)5f and
2-arylbenzoimidazole (ca. 5.13 ppm)4b as cyclometalated
Scheme 1 Synthesis of Ir complexes.
3334 | J. Mater. Chem., 2006, 16, 3332–3339
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Table 1 Electrochemical data for the iridium complexes
a,b
b,c
d
e
b
Complex
E1/2ox/V
E1/2red/V
Eonsetox/V
Eonsetred/V
HOMO/eV
LUMO/eV
DE/V
1
2
3
4
5
6
a
0.32
0.21
0.10
0.24
0.33
0.38
22.44, 22.26
22.45, 22.23
22.43, 22.24
22.46, 22.25
22.41, 22.21
22.42, 22.22
0.18
0.07
0.02
0.06
0.13
0.19
21.95
21.97
21.93
21.94
21.98
21.96
24.98
24.87
24.82
24.86
24.93
24.99
22.85
22.83
22.87
22.86
22.82
22.84
2.13
2.04
1.95
2.00
2.11
2.15
The oxidation potential was measured in CH2Cl2 solution at a concentration of 1 6 1023 M with a scan rate of 50 mV s21
.
d
Potential
b
values are reported versus Fc+/Fc. The reduction potential was measured in THF solution at a concentration of 1 6 1023 M. Determined
c
e
from the onset oxidation potential. Determined from the onset reduction potential.
ligands. The maximum high-field chemical shift in the
aromatic region for the (TPQ)2Ir(L^X) complexes appear at
5.99 (4), 5.91(5) and 5.92 (6) ppm, assigned for the protons
ortho to the metalated carbon atom of the thiophene ring. 31P
NMR of complex 6 shows a single peak at 100.6 ppm.
Thompson and co-workers have systematically investigated
the influence of ancillary ligands on the HOMO energy of bis-
cyclometalated iridium complexes and found that the ancillary
ligands with stronger ligand field strength could stabilize the
HOMO level.20 Consistent with this, the dithiolate ligands with
stronger ligand field strength than that of acetylacetone result
in higher oxidation potentials and lower HOMO energy levels
in complexes than those of acetylacetone (Table 1).
Electrochemical properties
Cyclic voltammetry was used to investigate the electrochemical
behavior of these iridium complexes. The redox potentials,
measured relative to an internal ferrocenium/ferrocene
reference (Fc+/Fc), are listed in Table 1. During the cathodic
scan in THF at 298 K, all complexes undergo two reversible
reduction processes with potentials ranging from 22.46 to
22.21 V, corresponding to the reduction of two cyclo-
metalated ligands of these complexes. These are in accordance
with reported bis-cyclometalated iridium complexes. Upon the
anodic sweep in CH2Cl2 at 298 K, each of these complexes
Photophysical propeties
Fig. 1 shows the absorption and photoluminescence spectra of
(C^N)2Ir(acac) complexes (i.e. 1, 2 and 3) and (TPQ)2Ir(L^X)
undergoes
a reversible one-electron oxidation with the
oxidative potential falling within the range 0.10–0.38 V.
On the basis of the onset potentials of oxidation and
reduction, the HOMO and LUMO energy levels of these
iridium complexes, relative to the energy levels of ferrocene
(4.80 eV under vacuum), were estimated.17 As revealed
previously by electrochemistry and theoretical calculations of
cyclometalated Ir(III) complexes, reduction is generally con-
sidered to occur mainly on the heterocyclic portion of the
cyclometalated C^N ligands, whereas oxidation processes
largely involve the Ir–aryl center.2c,18 Consistent with this
conclusion, these complexes have almost equal LUMO energy
levels due to the same pyridyl based frame, which makes the
reduction potentials of these complexes stay in a narrow
range, whereas their HOMO level systematically changes
with the structures of 2-arylquinoline ligands and the ligand
field strength of the ancillary ligands. In comparison with
(DPQ)2Ir(acac) (1), the HOMO levels of Ir(NPQ)2(acac) (2)
and Ir(FPQ)2(acac) (3), in which a naphthalenyl and fluorenyl
group, respectively, replace the phenyl in DPQ, systematically
increase from 24.98 to 24.82 V, attributed to the extended
p conjugated aromatic motif of the ligand frame.19 With
(DPQ)2Ir(acac) (1) as a reference, the complex Ir(TPQ)2(acac)
(4), in which a thiophene group replaces the phenyl in DPQ,
reveals an additional 0.12 V in the HOMO level due to the
more electron-donating ability of the thiophene ring. For
the series of complexes (TPQ)2Ir(L^X) (i.e. 4, 5 and 6), the
oxidative potentials span a wide range from 0.24 to 0.38 V due
to the different ligand field strength of the ancillary ligands.
Fig. 1 Absorption and PL spectra of complexes (C^N)2Ir(acac) (top)
and (TPQ)2Ir(L^X) (bottom) in CH2Cl2 solution.
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Table 2 Photophysical data for the iridium complexes
UV-Vis labs(log e)/nm
PL lmax/nm
Complex
Solutiona
Filmb
Solutiona
Filmb
613
638
653
634
615
609
Wfc (%)
15.3
13.2
10.3
9.8
1
2
3
4
5
6
a
221(3.8), 258(4.0), 364(3.2)
407(3.0), 461(2.9), 523(2.1)
218(3.7), 241(4.0), 372(3.5)
434(3.3), 469(3.1), 531(2.7)
218(3.8), 239(4.0), 373(3.8)
438(3.5), 498(3.0), 532(2.9)
242(4.0), 311(3.9), 363(3.6)
434(3.4), 495(3.0), 538(2.8)
241(4.0), 308(3.9), 367(3.7)
426(3.5), 492(3.2), 536(2.8)
242(4.0), 304(3.9), 360(3.8)
438(3.3), 498(3.1), 535(2.8)
b
228, 263, 386
421, 478, 524
225, 246, 381
436, 472, 536
226, 249, 383
442, 482, 534
245, 317, 368
436, 498, 535
243, 312, 371
431, 489, 541
244, 315, 369
440, 495, 538
c
611
636
649
631
612
606
8.4
3.2
In CH2Cl2 solution at 298 K. In PMMA film (3% weight ratio). Quantum yield was measured in CH2Cl2 solution relative to quinine
bisulfate (1 6 1025 M in 1.0 M H2SO4).
complexes (i.e. 4, 5 and 6) in CH2Cl2 solution. The absorption
spectra of these complexes in PMMA [poly(methyl methacryl-
ate)] film at 3% weight ratio at 298 K were also measured.
Each of the complexes exhibits similar absorption behavior
whether in solution or in PMMA film. Absorption data along
with molar extinction coefficients of all complexes are listed in
Table 2. By comparing with the absorption spectra of free
ligands, the intense absorptions in the ultraviolet region of the
spectra, between 250 and 350 nm, can be easily assigned to
the spin allowed p–p* transitions from cyclometalated C^N
ligands. Similar to the previous works,2b,4c,21 the weak absorp-
tion bands in the range of 350–400 and 400–550 nm are
According to previous reports,4c,5b,22 phosphorescence
emissions from the 3MLCT state are broad and featureless,
while those from the ligand-centered 3p–p* excited state
usually exhibit vibronic progressions. As shown in Fig. 1, the
emission spectra of all the complexes exhibit no vibronic fine
structure, implying that the lowest excited triplet states of
these complexes are likely to be dominated by the 3MLCT
excited state.
Electroluminescent properties
To illustrate the electroluminescent properties of these com-
plexes, typical OLED devices using complexes 1–4 as dopants
in the emitting layer have been fabricated. The device consists
of multilayer films with the configuration ITO/NPB (40 nm)/
CBP + dopant (30 nm)/BCP (10 nm)/AlQ3 (30 nm)/LiF(1 nm)/
Al (100 nm) (Fig. 2). Complexes 1–4 were doped into the CBP
as the emissive layer at a concentration of 3–5%, in accordance
1
3
ascribed to the S0 A MLCT and S0 A MLCT transitions,
respectively. The comparable intensity of the S0
A
1MLCT
3
3
and S0 A MLCT transitions suggests that the S0 A MLCT
transition is allowed by significant singlet–triplet mixing due to
spin–orbit coupling.
The PL spectra of these complexes were measured in CH2Cl2
solution and in PMMA film (3% weight ratio) at 298 K. All
complexes show strong red luminescence both in the solid
state and organic solutions upon irradiation with UV-light at
ambient temperature.
The emission wavelengths of these complexes fall in the
range 606–653 nm, depending on the structures of the cyclo-
metalating and ancillary ligands. Compared to the complex
(PQ)2Ir(acac) (PQ = phenylquinoline) with an emission wave-
length at 599 nm,7c (DPQ)2Ir(acac), with one more phenyl
group at the 4-position of the quinoline, exhibits a 12 nm
bathochromic shift. Ir(NPQ)2(acac) and Ir(FPQ)2(acac), which
bear a naphthalenyl and fluorenyl group, respectively, reveal
an additional 25 and 38 nm red shift in the emission maxima
with respect to (DPQ)2Ir(acac), attributed to the extended
p-conjugation raising the HOMO energy level. (TPQ)2Ir(acac)
shows a 20 nm red shift in the emission maxima due to the
more electron-donating ability of the thiophene ring relative to
the phenyl ring. In contrast, hypsochromic shifts of 19 nm
for 5, and 25 nm for 6, relative to their analogue 4, take place
when dithiolate ancillary ligands replace acetylacetone. The
above alteration of emission wavelength is in good correlation
with the variation of the energy gap evaluated from the results
of cyclic voltammetry (vide supra).
Fig. 2 The configuration of the OLED device, and the molecular
structures of the compounds used in the device.
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Fig. 5 Brightness–voltage (L–V) curves for the devices using com-
plexes 1, 2, 3 and 4 as dopants at 3% dopant level.
Fig. 3 EL spectra of devices at a driving voltage of 8 V using
complexes 1, 2, 3 and 4 as dopants. EL spectra of 1 dependent on the
driving voltage (inset).
with the doping concentration of the complex in PMMA film
in the photoluminescence tests. The EL spectra show that
emission peaks are very close in appearance to the photo-
luminescence spectra of the complexes in PMMA film,
indicating that the EL emission of the device originates from
the triplet excited states of the phosphors (Fig. 3). The EL
emission intensity of Ir(DPQ)2(acac) shows a systematic
decrease with decreasing driving voltage, but the EL emission
peaks are independent of the driving voltage (inset in Fig. 3).
Figs. 4 and 5 show the current density–voltage curves and the
luminance–voltage characteristics of the devices, respectively.
The current efficiency (gc) and power efficiency (gp) as
functions of current density (J) for the devices are displayed
in Figs. 6 and 7, respectively. The performance data for the
devices are summarized in Table 3.
Fig. 6 Current efficiency–current density (gc–J) curves of the OLED
devices using complexes 1, 2, 3 and 4 as dopants at 3% dopant level.
Among these four phosphors, 1 shows better EL perfor-
mance than 2–4. The device based on complex 1 at 3% dopant
concentration shows a maximum brightness of 5107 cd m22
at 26.8 V, a maximum current efficiency of 10.79 cd A21 at a
current density of 0.74 mA cm22, and a maximum power
efficiency of 3.44 lm W21. The devices based on complexes 2–4
show a maximum current efficiency of 2.00 cd A21 at a current
density of 0.61 mA cm22 for 2, 1.64 cd A21 at a current density
of 0.71 mA cm22 for 3, and 2.14 cd A21 at 1.82 mA cm22 for 4,
respectively. The better EL performance of 1 may be attributed
Fig. 4 Current density–voltage (J–V) curves for the devices using
complexes 1, 2, 3 and 4 as dopants at 3% dopant level.
Fig. 7 Power efficiency–current density (gp–J) curves of the OLED
devices using complexes 1, 2, 3 and 4 as dopants at 3% dopant level.
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Table 3 EL performance of devices with the structure ITO/NPB (40 nm)/CBP + dopant (30 nm)/BCP (10 nm)/AlQ3 (30 nm)/LiF (1 nm)/Al
(100 nm)
Dopant phosphor
Ir(DPQ)2(acac)
Ir(NPQ)2(acac)
Ir(FPQ)2(acac)
Ir(TPQ)2(acac)
Wt%
3
5107
26.8
7.41
4.47
10.79
0.74
1.42
0.65
3.44
5.7
3
726
3
1216
21.1
3
904
5
814
22.1
0.32
0.15
0.44
0.10
0.29
0.14
1.01
6.7
Lmax/cd m22
Voltage/Va
gc/cd A21 b
gc/cd A21 c
35.7
1.31
0.70
2.00
0.61
0.22
0.07
0.70
6.5
21.2
1.33
0.85
2.14
1.82
1.36
0.90
1.61
5.5
0.88
0.64
1.64
0.71
0.21
0.12
0.76
6.1
g
c max/cd A21
J/mA cm22 d
gp/lm W21 b
gp/lm W21 c
g
p max/lm W21
VON/Ve
CIE (x, y)
ELmax/nmf
a
(0.65, 0.35)
616
(0.70, 0.30)
650
(0.74, 0.26)
656
Recorded at 100 mA cm22
(0.68, 0.32)
641
Voltage at the maximum brightness. Recorded at 20 mA cm22
.
. Recorded at a driving voltage of 8 V.
.
Current density at the maximum current
b
c
d
efficiency. Recorded at a brightness of 1 cd m22
e
f
to its hypsochromic shift of the emission maximum, in which
the relatively large energy gaps are less perturbed by
nonradiative processes. All the devices show a gradual
decrease in luminescence efficiency with increasing current
density, arising from increasing triplet–triplet annihilation of
the phosphor-bound excitons.23
China, and the Hubei Province Science Fund for Distinguished
Young Scholars (No. 2003ABB008).
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Conclusions
In summary, we have synthesized and characterized six new
iridium(III) complexes with 2-arylquinoline derivatives as
cyclometalated C^N ligands and b-diketonate or dithiolate as
ancillary ligands. Through extending p-conjugation, increasing
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The alteration of emission wavelength coincides well with the
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electrochemical analysis. Highly efficient OLEDs with
saturated and pure red emission were demonstrated. A
maximum current efficiency of 10.79 cd A21 at a current
density of 0.74 mA cm22 along with CIE coordinates of
(0.65, 0.35) was achieved with (DPQ)2Ir(acac) as the phosphor
dopant. These complexes are promising candidates for efficient
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We are grateful for financial support from the National
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