Color tuning associated with heteroleptic cyclometalated Ir(iii) complexes: Influence of the ancillary ligand

Article abstract of DOI:10.1039/b700998d

We report the preparation of a series of new heteroleptic Ir(iii) metal complexes chelated by two cyclometalated 1-(2,4-difluorophenyl)pyrazole ligands (dfpz)H and a third ancillary bidentate ligand (L^X). Such an intricate design lies in a core concept t

Full text of DOI:10.1039/b700998d

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PAPER
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Color tuning associated with heteroleptic cyclometalated Ir(III) complexes:
influence of the ancillary ligand†
Chau-Jiun Chang,a Cheng-Han Yang,a Kellen Chen,a Yun Chi,*a Ching-Fong Shu,*b Mei-Lin Ho,c
Yu-Shan Yehc and Pi-Tai Chou*c
Received 22nd January 2007, Accepted 28th February 2007
First published as an Advance Article on the web 23rd March 2007
DOI: 10.1039/b700998d
We report the preparation of a series of new heteroleptic Ir(III) metal complexes chelated by two
cyclometalated 1-(2,4-difluorophenyl)pyrazole ligands (dfpz)H and a third ancillary bidentate ligand
∧
(L X). Such an intricate design lies in a core concept that the cyclometalated dfpz ligands always
warrant a greater pp* gap in these series of iridium complexes. Accordingly, the lowest one-electron
∧
excitation would accommodate the p* orbital of the ancillary L X ligands, the functionalization of
∧
which is then exploited to fine-tune the phosphorescent emission wavelengths. Amongst the L X
ligands designed, three classes (series 1–3) can be categorized, and remarkable bathochromic shifts of
phosphorescence were observed by (i) replacing the 2-benzoxazol-2-yl substituent (1a) with the
2-benzothiazol-2-yl group (1b) in the phenolate complexes, (ii) converting the pyridyl group (2a) to the
pyrazolyl group (2b) and even to the isoquinolyl group (2c) in the pyrazolate complexes and (iii)
extending the p-conjugation of the benzimidazolate ligand from 3a to 3b. Single-crystal X-ray
diffraction study on complex [(dfpz)Ir(bzpz)] (2b) was conducted to confirm their general molecular
architectures. Complex 2b was also used as a representative example for fabrication of multilayered,
green-emitting phosphorescent OLEDs using the direct thermal evaporation technique.
tridentate cyclometalating Pt(II) complexes can be systematically
Introduction
varied from green-yellow to saturated red.4 Moreover, homoleptic
Organometallic complexes involving third-row transition metal
∧
∧
azolate complexes [Pt(N N)2], N NH = 2-pyridyl azole, were
reported to exhibit tunable and highly efficient phosphorescence.5
On the other hand, varying the ancillary ligands from carbonyl
to phosphine or pyridine donors in the octahedral pyridyl azolate
elements have become crucial for the fabrication of highly efficient
organic light emitting diodes (OLEDs).1 The strong spin–orbit
coupling induced by a heavy metal ion such as iridium(III)
promotes an efficient intersystem crossing from the singlet to
the triplet excited state manifold, which then facilitates strong
electroluminescence by harnessing both singlet and triplet ex-
citons after the initial charge recombination. Because internal
phosphorescence quantum efficiency (gint) as high as ∼100% can
theoretically be achieved, these heavy metal containing emitters
will be superior to their fluorescent counterparts in future OLED
applications.2 As a result, there is a continuous trend of shifting
research focusing on these heavy transition metal complexes.
Moreover, since the manufacture of a full color display requires
the use of emitters with all three primary colors, i.e. blue, green and
red, rationally tuning emission color over the entire visible range
has emerged as an important task.3 Several precedents involving
the third-row metal elements were documented; for example: by
choosing appropriate r-alkynyl ligands with different degrees of
p-conjugation, the emission of the multi-functionalized or the
3
Os(II) system led to a significant lowering of the MLCT energy
gap, giving a notable change from blue to green and then to
saturated red and near infrared (NIR) emission.6
For the related Ir(III) complexes, the systematic derivatization
of cyclometalated ligands has also allowed a lucid prediction of
their emission colors.7 It has been documented that the LUMO
and HOMO of the homoleptic, tris-cyclometalated complexes
[Ir(ppy)3], ppy = 2-phenylpyridine, are mainly located at the
pyridyl segment of the ppy ligands and the phenyl and central
metal dp segments, respectively.8 Accordingly, addition of electron
withdrawing (or releasing) groups at the pyridyl site would
display a significant decrease (or increase) of the LUMO energy
level. Conversely, the addition of electron withdrawing (donating)
substituents at the phenyl group would stabilize (destabilize)
the HOMO energy level.9 Thus, even without the assistance
of theoretical computation, such an energy tuning strategy can
qualitatively predict the phosphorescent wavelengths within a
series of functionalized complexes.
aDepartment of Chemistry, National Tsing Hua University, Hsinchu 300,
Taiwan. E-mail: ychi@mx.nthu.edu.tw; Fax: +886-3-772-0864; Tel: +886-
2-571-2956
In yet another approach, the emission of the heterolep-
∧
∧
∧
tic complexes [Ir(C N)2(L X)], such as [Ir(ppy)2(L X)] and
bDepartment of Applied Chemistry, National Chiao Tung University,
Hsinchu 300, Taiwan. E-mail: shu@cc.nctu.edu.tw; Fax: +886-3-572-3764;
Tel: +886-3-571-2121 ext. 56544
∧
[Ir(piq)2(L X)], (piq)H = 1-phenylisoquinoline, could be tuned
∧
by varying the third, ancillary ligand (L X), among which the
bidentate chelating anions such as acetylacetonate, picolinate,
N-methylsalicylimine, pyrazolate and even pyrazolyl borate have
been exploited.10 It is notable, however, that the resulting
cDepartment of Chemistry, National Taiwan University, Taipei 106, Taiwan.
E-mail: chop@ntu.edu.tw; Fax: +886-2-23695208; Tel: +886-2-33663894
† The HTML version of this article has been enhanced with colour images.
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phosphorescence wavelength of these complexes was largely
2-(2-pyridyl)benzimidazole (pbimH), 2-benzoxazol-2-yl phenol
(bopH) and 2-benzothiazol-2-yl phenol (btpH) were purchased
from Aldrich Chem. Inc.
∧
governed by the pair of orthogonal cyclometalating C N ligands,
∧
while the third L X ligand had only a secondary and relatively
smaller influence. Obviously, this is due to the fact that most of
∧
Syntheses
the L X ligands possess a significantly larger pp* energy gap and,
thus, they tend to affect solely the metal dp orbitals contributing
to the HOMO through their inherent electron withdrawing (or
donating) properties.11
Synthesis of [(dfpz)2Ir(bop)] (1a). A mixture of [(dfpz)2IrCl]2
(100 mg, 0.085 mmol), 2-benzoxazol-2-yl phenol (bopH, 50 mg,
0.21 mmol), and Na2CO3 (90 mg, 0.85 mmol) in 2-methoxyethanol
(25 mL) was heated to reflux for 4 h. An excess of water was added
after the solution was cooled to RT (room temperature). The
precipitate was collected by filtration and washed with ethanol
(10 mL), followed by diethyl ether (10 mL). Yellowish green
[(dfpz)2Ir(bop)] (1a) was obtained from column chromatography
using ethyl acetate and hexane (1 : 2) as eluent, followed by
recrystallization from CH2Cl2 and hexane at room temperature;
yield: 100 mg, 0.13 mmol, 73%.
In this paper, we report the synthesis and characterization of a
series of a new class of heteroleptic iridium complexes with for-
The orange benzothiazolyl complex [(dfpz)2Ir(btp)] (1b) was
prepared using a similar procedure, yield: 76%.
∧
mula [Ir(dfpz)2(L X)], dfpzH = 1-(2,4-difluorophenyl)pyrazole.
1
Spectral data for 1a. MS (FAB, 193Ir), m/z: 762 [M+ + 1]. H
∧
In contrast to the previously reported [Ir(ppy)2(L X)] and
NMR (400 MHz, d6-acetone, 298 K): d 8.56 (d, 1H, JHH = 2.8 Hz),
12
∧
[Ir(piq)2(L X)] systems, the exceedingly large pp* energy gap
of the dfpz ligands in these Ir(III) complexes13 allowed the third,
ancillary L X ligand to dominate the lowest lying excited state.
As a result, tuning the emission color among these complexes
can be achieved by systematically employing various distinctive
8.46 (d, 1H, JHH = 2.8 Hz), 7.92 (dd, 1H, JHH = 8.0, 1.0 Hz), 7.70
(d, 1H, JHH = 2.4 Hz), 7.63–7.58 (m, 2H), 7.29 (t, 1H, JHH
8.0 Hz), 7.20 (ddd, 1H, JHH = 5.2, 5.2, 1.6 Hz), 7.06 (t, 1H, JHH
=
=
∧
8.0 Hz), 6.77–6.66 (m, 5H), 6.61 (d, 1H, JHH = 8.4 Hz), 6.49 (t,
1H, JHH = 6.6 Hz), 5.91 (dd, 1H, JHH = 8.4, 2.4 Hz), 8.56 (dd,
1H, JHH = 8.4, 2.4 Hz). 19F NMR (470 MHz, d6-acetone, 298 K):
d −114.33 (1F), −114.34 (1F), −124.68 (1F), −125.80 (1F). Anal.
Calcd for C31H18F4IrN5O2: C, 48.94; H, 2.38; N, 9.21. Found: C,
48.51; H, 2.64: N, 9.19%.
∧
L X ligands, simplifying the design strategy and hence the
synthetic task. Our designs are somewhat analogous to those
of the Ir(III) complexes with phenylpyrazole (ppz) and neutral
diimine ligands14 and even the isoquinolinecarboxylate ligand
as the tunable phosphors.15 As for the second example, due to
the smaller energy gap of cyclometalated ppz ligands, its alkyl
substituents can still exert a weak influence over the final emission
color according to experimental work and the accompanying DFT
calculations.
1
Spectral data for 1b. MS (FAB, 193Ir), m/z: 777 [M+ + 1]. H
NMR (400 MHz, d6-acetone, 298 K): d 8.60 (d, 1H, JHH = 2.8 Hz),
8.39 (d, 1H, JHH = 2.8 Hz), 7.97 (d, 1H, JHH = 0.8 Hz), 7.95 (d,
1H, JHH = 0.8 Hz), 7.61–7.58 (m, 2H), 7.34–7.20 (m, 4H), 6.78
(t, 1H, JHH = 2.4 Hz), 6.71 (t, 1H, JHH = 2.4 Hz), 6.70–6.53
(m, 4H), 6.03 (dd, 1H, JHH = 8.8, 2.4 Hz), 5.42 (dd, 1H, JHH
=
8.8, 2.4 Hz). 19F NMR (470 MHz, d6-acetone, 298 K): d −114.67
(1F), −114.82 (1F), −124.94 (1F), −125.65 (1F). Anal. Calcd for
C31H18F4IrN5OS: C, 47.93; H, 2.34; N, 9.02. Found: C, 48.06; H,
2.59: N, 8.99%.
Experimental
General information and materials
All reactions were performed under a nitrogen atmosphere us-
ing anhydrous solvents or solvents treated with an appropriate
drying reagent. Mass spectra were obtained on a JEOL SX-
102A instrument operating in electron impact (EI) mode or fast
atom bombardment (FAB) mode. 1H and 19F NMR spectra
were recorded on Varian Mercury-400 or INOVA-500 instru-
ments. Elemental analyses were conducted at the NSC Regional
Instrumentation Center at National Chiao Tung University.
The chelating azolate ligands: 3-tert-butyl-5-(2-pyridyl)pyrazole
(bppzH), 3-tert-butyl-5-(2-pyrazyl)pyrazole (bzpzH), 3-tert-butyl-
5-(2-isoquinolyl)pyrazole (bqpzH) and 3-trifluoromethyl-5-(2-
pyridyl) pyrazole (fppzH), were prepared according to the lit-
erature procedures,16 while 1-(2,4-difluorophenyl)pyrazole (df-
pzH) was prepared from reaction of 2,4-difluorophenyl hy-
drazine hydrochloride with 1,1,3,3-tetramethoxypropane, and
was directly used for the preparation of chloride-bridged
dimer [(dfpz)2IrCl]2 according to the literature.13,17 The imi-
dazole ligands, namely: 2-(2-quinolinyl)benzimidazole (qbimH)
was obtained from condensation of quinoline-2-carboxylic acid
with 1,2-phenylenediamine;18 while other bidentate ligands:
Synthesis of [(dfpz)2Ir(bppz)] (2a). A mixture of [(dfpz)2IrCl]2
(100 mg, 0.09 mmol), 3-tert-butyl-5-(2-pyridyl) pyrazole (bppzH,
40 mg, 0.21 mmol), and Na2CO3 (90 mg, 0.85 mmol) in 2-
methoxyethanol (20 mL) was heated to reflux for 4 h. An
excess of water was added after the solution was cooled to RT
(room temperature). The precipitate was collected by filtration
and washed with ethanol (10 mL), followed by diethyl ether
(10 mL). Pure samples of [(dfpz)2Ir(bppz)] (2a) were obtained from
column chromatography using ethyl acetate and hexane as eluent,
followed by recrystallization from CH2Cl2 and hexane at room
temperature; yield: 90 mg, 0.12 mmol, 70%. The related complexes
[(dfpz)2Ir(bzpz)] (2b), [(dfpz)2Ir(bqpz)] (2c) and [(dfpz)2Ir(fppz)]
(2d) were prepared using similar procedures, yield: 65–70%.
1
Spectral data for 2a. MS (FAB, 193Ir), m/z: 752 [M+ + 1]. H
NMR (400 MHz, CDCl3, 298 K): d 8.29 (d, 1H, JHH = 2.8 Hz),
8.22 (d, 1H, JHH = 3.2 Hz), 7.82 (d, 1H, JHH = 5.6 Hz), 7.68–
7.65 (m, 2H), 6.92 (s, 1H), 6.81 (d, 1H, JHH = 2.0 Hz), 6.65 (s,
1H), 6.57–6.50 (m, 3H), 6.45–6.42 (m, 2H), 5.76 (dd, 1H, JHH
=
8.0, 2.4 Hz), 5.70 (dd, 1H, JHH = 8.0, 2.4 Hz), 1.31 (s, 9H). 19F
1882 | Dalton Trans., 2007, 1881–1890
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NMR (470 MHz, d6-acetone, 298 K): d −113.80 (1F), −115.39
(1F), −124.91 (1F), −126.37 (1F). Anal. Calcd for C30H24F4IrN7:
C, 47.99; H, 3.22; N, 13.06. Found: C, 47.56; H, 3.62: N, 12.98%.
7.00 (d, 1H, JHH = 2.0 Hz), 6.84 (t, 1H, JHH = 8.0 Hz), 6.70 (ddd,
1H, JHH = 11.6, 9.0, 2.6 Hz), 6.57 (ddd, 1H, JHH = 11.6, 9.0,
2.6 Hz), 6.47 (d, 1H, JHH = 2.4 Hz), 6.41 (dd, 1H, JHH = 2.8,
1
Spectral data for 2b. MS (FAB, 193Ir), m/z: 754 [M+ + 1]. H
2.4 Hz), 6.29 (dd, 1H, JHH = 2.8, 2.4 Hz), 6.07 (d, 1H, JHH
8.4 Hz), 5.77 (dd, 1H, JHH = 8.74, 2.8 Hz), 5.62 (dd, 1H, JHH
=
=
NMR (400 MHz, d6-acetone, 298 K): d 9.13 (s, 1H), 8.50 (d,
2H, JHH = 16.0 Hz), 8.21 (d, 1H, JHH = 3.6 Hz), 7.91 (d, 1H,
8.4, 2.8 Hz). 19F NMR (470 MHz, CDCl3, 298 K): d −112.29 (2F),
−123.57 (1F), −124.25 (1F). Anal. Calcd for C34H20F4IrN7: C,
51.38; H, 2.54; N, 12.34. Found: C, 51.34; H, 2.96: N, 12.15%.
JHH = 3.6 Hz), 7.40 (s, 1H), 6.79–6.65 (m, 6H), 5.87 (d, 1H, JHH
=
8.4 Hz), 5.71 (d, 1H, JHH = 8.4 Hz), 1.31 (s, 9H). Anal. Calcd for
C29H23F4IrN8: C, 46.33; H, 3.08; N, 14.91. Found: C, 46.44; H,
3.56: N, 14.95%.
X-Ray diffraction study
1
Spectral data for 2c. MS (FAB, 193Ir), m/z: 802 [M+ + 1]. H
Single crystal X-ray analysis was measured on a Bruker SMART
NMR (400 MHz, d6-acetone, 298 K): d 9.02 (d, 1H, JHH = 7.5 Hz),
8.49 (d, 1H, JHH = 3.0 Hz), 8.45 (d, 1H, JHH = 3.0 Hz), 7.96 (dd,
1H, JHH = 7.0, 2.5 Hz), 7.92 (d, 1H, JHH = 10.5 Hz), 7.85–7.80 (m,
2H), 7.50 (d, 1H, JHH = 3.0 Hz), 7.12 (d, 1H, JHH = 2.0 Hz), 7.11
Apex CCD diffractometer using k(Mo-Ka) radiation (k =
˚
0.71073 A). The data collection was executed using the SMART
program. Cell refinement and data reduction were made by
the SAINT program. The structure was determined using the
SHELXTL/PC program and refined using full-matrix least
squares. All non-hydrogen atoms were refined anisotropically,
whereas hydrogen atoms were placed at the calculated positions
and included in the final stage of refinements with fixed param-
eters. Crystallographic refinement parameters of complex 2b are
summarized in Table 1.
(s, 1H), 6.79–6.74 (m, 1H), 6.68–6.62 (m, 3H), 6.57 (d, 1H, JHH
2.0 Hz), 5.90 (dd, 1H, JHH = 8.3, 2.3 Hz), 5.77 (dd, 1H, JHH
=
=
8.3, 2.3Hz). 19F NMR (470 MHz, d6-acetone, 298 K): d −113.90
(1F), −115.41 (1F), −124.85 (1F), −126.42 (1F). Anal. Calcd for
C34H26F4IrN7: C, 50.99; H, 3.27; N, 12.24. Found: C, 50.94; H,
3.67: N, 12.14%.
Spectral data for 2d. MS (FAB), m/z: 764 [M+]. 1H NMR
(500 MHz, CDCl3, 294 K): d 8.23 (dd, JHF = 16.3, 3.5 Hz, 2H), 7.87
(d, JHH = 5.5 Hz, 1H), 7.76–7.80 (m, 2H), 7.06 (t, JHH = 5.8 Hz,
1H), 6.96 (s, 1H), 6.86 (d, JHH = 1.5 Hz, 1H), 6.76 (d, JHH = 2.0 Hz,
1H), 6.51–6.44 (m, 4H), 5.76 (dd, JHH = 6.5, 2.5 Hz, 1H), 5.68 (dd,
JHH = 8.5, 2.0 Hz, 1H). 19F NMR (470 MHz, CDCl3, 294 K): d
−125.6 (1F), −124.8 (1F), −113.8 (1F), −113.2 (1F), −60.3 (s,
3F). Anal. Calcd for C27H15F7IrN7: N, 12.86; C, 42.52; H, 1.98.
Found: N, 12.89; C, 42.28; H, 2.05%.
CCDC reference numbers 634398.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b700998d
Spectral measurement
Steady-state absorption and emission spectra were recorded
with a Hitachi (U-3310) spectrophotometer and an Edinburgh
(FS920) fluorimeter, respectively. For the emission with peak
wavelength <550 nm, quinine sulfate with an emission yield
of U ∼ 0.55 (kmax ∼ 460 nm) in 1.0 N H2SO4 served as
a standard to calculate the emission quantum yield.19 On
the other hand, DCM (4-(dicyanomethylene)-2-methyl-6-(para-
dimethylaminostyryl)-4H-pyran, kmax = 615 nm, Exciton) in
methanol, with quantum yield of ∼0.43, served as the standard for
measuring the quantum yield of the emission band of ≥550 nm.20
Lifetime studies were performed with an Edinburgh FL 900
photon-counting system with a hydrogen-filled/or a nitrogen
Synthesis of [(dfpz)2Ir(pbim)] (3a). A mixture of [(dfpz)2IrCl]2
(200 mg, 0.17 mmol), 2-pyridyl benzimidazole (pbimH, 80 mg,
0.42 mmol), and Na2CO3 (180 mg, 1.7 mmol) in 2-methoxyethanol
(35 mL) was heated to reflux for 4 h. An excess of water was
added after the solution was cooled to RT (room temperature).
The precipitate was collected by filtration and washed with
ethanol (10 mL), followed by diethyl ether (10 mL). Yellowish
green [(dfpz)2Ir(pbim)] (3a) was obtained from column chro-
matography using ethyl acetate as eluent, followed by recrystal-
lization from CH2Cl2 and hexane at room temperature; yield:
180 mg, 0.24 mmol, 71%. The related 2-quinolinyl complex
[(dfpz)2Ir(qbim)] 3b was obtained using similar procedures.
Table 1 X-Ray structural data for complex 2b
Formula
M
C29H23F4IrN8
751.75
Monoclinic
P21/c
150(2)
10.4704(4)
21.4926(8)
12.0294(5)
90.799(1)
2706.79(18)
4
1
Spectral data for 3a. MS (FAB, 193Ir), m/z: 746 [M+ + 1]. H
Crystal system
Space group
T/K
NMR (400 MHz d6-acetone, 298 K): d 8.53 (d, 1H, JHH = 3.0 Hz),
8.48 (d, 1H, JHH = 3.0 Hz), 8.45 (d, 1H, JHH = 7.0 Hz), 8.08–8.05
(m, 2H), 7.57 (d, 1H, JHH = 8.0 Hz), 7.36 (dd, 1H, JHH = 6.5,
5.3 Hz), 7.20 (d, 1H, JHH = 2.0 Hz), 6.95 (dd, 1H, JHH = 8.5,
6.5 Hz), 6.88–6.80 (m, 3H), 6.72 (dd, 1H, JHH = 7.5, 6.5 Hz), 6.66–
6.63 (m, 2H), 6.30 (d, 1H, JHH = 8.5 Hz), 5.96 (dd, 1H, JHH = 7.5,
2.0 Hz), 5.89 (dd, 1H, JHH = 7.5, 2.0 Hz). 19F NMR (470 MHz,
d6-acetone, 298 K): d −113.78 (1F), −114.49 (1F), −124.60 (1F),
−125.29 (1F). Anal. Calcd for C30H18F4IrN7: C, 48.38; H, 2.44; N,
13.17. Found: C, 48.07; H, 2.66: N, 13.06%.
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
Dc/g cm−3
1.845
1464
4.996
F(000)
l(Mo-Ka)/mm−1
Crystal size/mm
h, k, l ranges
0.20 × 0.13 × 0.10
−13 to 13, −27 to 26, −15 to 14
0.6349, 0.4348
6207/0/413
1.148
0.0384, 0.0730
1.935, −2.128
1
Spectral data for 3b. MS (FAB, 193Ir), m/z: 795 [M+ + 1]. H
Transmission: max, min.
Data/restraints/parameters
GOF on F2
R1, wR2 with I > 2r(I)
D-map, max, min/e A
NMR (400 MHz, CDCl3, 298 K): d 8.83 (d, 1H, JHH = 8.4 Hz),
8.33–8.32 (m, 2H), 8.22 (d, 1H, JHH = 8.8 Hz), 8.18 (d, 1H, JHH
3.0 Hz), 7.81 (dd, 2H, JHH = 13.6, 8.4 Hz), 7.44 (t, 1H, JHH
=
=
−3
˚
7.6 Hz), 7.31 (t, 1H, JHH = 8.4 Hz), 7.12 (t, 1H, JHH = 8.0 Hz),
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lamp as the excitation source. Data were analyzed using the
nonlinear least squares procedure in combination with an iterative
convolution method. The emission decays were analyzed by the
sum of exponential functions, which allows partial removal of the
instrument time broadening and consequently renders a temporal
resolution of ∼200 ps.
iridium complex [(dfpz)2IrCl]2. Three parent heteroleptic irid-
ium complexes, [(dfpz)2Ir(bop)] (1a), [(dfpz)2Ir(bppz)] (2a) and
[(dfpz)2Ir(pbim)] (3a), possessing respectively 2-benzothiazol-2-
yl phenolate, 2-pyridylpyrazolate and 2-pyridylbenzimidazolate,
as well as other functionalized derivatives, were prepared from
the direct reaction of [(dfpz)2IrCl]2 with each of the designated
ancillary ligands in the presence of Na2CO3 as proton scavenger.
Chart 1 summarizes their structural drawings as well as their
numbering codes. It is notable that all these iridium complexes are
moderately soluble in chlorinated solvents. However, they show
good thermal stability, as supported by negligible degradation in
the solution phase over a long period of time. Detailed character-
izations were carried out using routine FAB mass spectrometry,
1H and 19F NMR spectroscopies and elemental analysis, among
which complex 2b was further identified using single crystal X-ray
analysis to establish their exact solid state structure.
Computational methodology
Calculations on the electronic ground state of the studied com-
plexes were carried out using B3LYP density functional theory.21
A
“double-f” quality basis set consisting of Hay and Wadt’s effective
core potentials (LANL2DZ)22 was employed for the Ir atom,
and a 6-31G* basis, for H, C, N, F and O atoms. A relativistic
effective core potential (ECP) replaced the inner core electrons
of Ir(III), leaving the outer core (5s25p6) electrons and the 5d6
valence electrons. Time-dependent DFT (TDDFT) calculations
using the B3LYP functional were then performed on the basis of
the structural optimized geometries. Typically, the lowest 10 triplet
and 10 singlet roots of the nonhermitian eigenvalue equations were
obtained to determine the vertical excitation energies. Oscillator
strengths were deduced from the dipole transition matrix elements
(for singlet states only). The ground-state B3LYP and excited-state
TDDFT calculations were carried out using Gaussian03.23
Electrochemical measurement
Cyclic voltammetry (CV) measurements were performed using
a BAS 100 B/W electrochemical analyzer. The oxidation and
reduction measurements were recorded using Pt wire and an
Au disk coated with Hg as working electrodes, respectively,
in anhydrous CH2Cl2 and anhydrous THF containing 0.1 M
(TBA)PF6 as the supporting electrolyte, at a scan rate of 50 mV s−1.
The potentials were measured against an Ag/Ag+ (0.01 M AgNO3)
reference electrode with ferrocene as the internal standard.
Chart 1
Fabrication of OLEDs
As depicted in Fig. 1, complex 2b reveals a distorted octahedral
geometry around the Ir atom with two cyclometalated dfpz ligands
and one anionic 2-pyrazylpyrazolate (bzpzH) ligand. All three
chelate ligands show small bite angles of 76.8–80.5◦, which then
induce minor structural distortion. The dfpz ligands adopt a
mutually eclipsed configuration with the nitrogen atoms N(5)
and N(7) residing at the trans locations, and the Ir–N distances
Dopant emitter 2b, in five different concentrations from 7 to 100%,
were fabricated for the phosphorescent OLEDs. Our devices have
the conventional multilayer configuration: ITO/NPB (40 nm)/2b
dopant in CBP (30 nm)/BCP(10 nm)/AlQ3(30 nm)/Mg : Ag
(55 nm); where NPB (1,4-bis(1-naphthylphenylamino)biphenyl)
served as the hole transport layer, while CBP (4,4ꢀ-bis-
(9-carbazolyl)biphenyl), BCP (2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline) and AlQ3 (tris-(8-hydroxyquinolinyl) aluminium)
functioned as the host matrix, hole blocking layer and electron
transport layer, respectively. The cathode Mg : Ag alloy (10 : 1) was
deposited onto the AlQ3 layer by coevaporation. An additional
layer of Ag (100 nm) was deposited onto the cathode as a protective
coating. The current–voltage–luminance was measured at ambient
conditions with a Keithley 2400 source meter and a Newport
1835C optical meter equipped with an 818ST silicon photodiode.
˚
lie between 1.998(4) and 2.005(4) A. The cyclometalated carbon
atoms C(12) and C(21) are mutually cis on the iridium and show
˚
marginally longer distances 2.007(4) and 2.004(5) A. The third,
anionic pzpz ligand displays notably elongated Ir–N distances of
˚
2.143(4) and 2.102(4) A vs. those of the trans-orientated Ir–N
distances of the fppz ligands. Such an observation is believed to
be caused by the stronger Ir–C bonding interaction of the dfpz
ligands, which eventually weakens the Ir–N bonds at their trans-
disposition. Moreover, the gross ligand arrangement is similar to
several reported examples, including other cyclometalated Ir(III)
complexes with pyridyl azolate serving as the ancillary ligand,24
Results and discussion
˚
except that the Ir–N(pz) distance in 2b (2.102(4) A) is slightly
˚
longer than the Ir–N(pz) distance (2.096(3) A) in the analogous
Synthesis and characterization
CF3 substituted complex 2d,25 this result being attributed to the
electron donating tert-butyl group vs. the electron deficient CF3
group in fppz ligand.
Treatment of the cyclometalating ligands (dfpz)H with IrCl3·nH2O
in refluxing ethoxyethanol afforded the respective chloride bridged
1884 | Dalton Trans., 2007, 1881–1890
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Fig. 1 ORTEP diagram of 2b with thermal ellipsoids shown at 30%
probability level; methyl groups of the tert-butyl substituent are disordered
unequally over two orientations, only one of which is shown. Selected bond
˚
lengths (A): Ir–N(1) = 2.143(4), Ir–N(3) = 2.102(4), Ir–N(5) = 1.998(4),
Ir–N(7) = 2.005(4), Ir–C(12) = 2.007(4), Ir–C(21) = 2.004(5); bond angles
(◦): N(1)–Ir–N(3) = 76.8(1), N(5)–Ir–C(12) = 80.5(2), N(7)–Ir–C(21) =
80.4(2).
Photophysical data
The absorption and luminescence spectra were recorded for all
samples in CH2Cl2. The numerical data are summarized in Table 2
and the associated spectra are depicted in Fig. 2. According to
the structure of the chromophores, and hence the photophysical
∧
properties, the L X ligands can be categorized into three classes,
namely 1a–b, 2a–d and 3a–b. As shown in Fig. 2a, the lowest
energy absorption peaks of complexes 1a and 1b occurred in the
region 416–441 nm (>5000 M−1 cm−1), which can be ascribed
to a typical spin-allowed 1pp* transition mixed with a small
proportion of 1MLCT transition. This hypothesis was confirmed
by the observation of a similar spectral profile of the pp* transition
maximized at 375 and 400 nm for the uncoordinated bop and btp
anions, respectively. Concomitantly, the emission peaks of 1a and
1b occurred at kmax = 522 and 574 nm, respectively, with a quantum
efficiency of 0.82 for 1b (s ∼ 7.6 ls) being substantially greater
than that of 1a (U = 0.29, s ∼ 10.7 ls). The moderate separation
of the 0–0 onsets between emission signal and the lowest energy
absorption band, in combination with a broad spectral feature,
Fig. 2 UV/Vis absorption and emission spectra of complexes 1a and 1b
(a), complexes 2a–2d and complexes 3a and 3b (c) in CH2Cl2 at room
temperature.
leads us to conclude that the phosphorescence originates primarily
3
from the pp state together, in part, with contribution from the
3MLCT transition. The radiative lifetime of 1b (9.3 ls), calculated
using the equation srad = sobs/U, is also notably shorter than that
3
of 1a (36.9 ls), indicating an increased proportion of MLCT
character for the benzothiazolyl derivative 1b. Qualitatively, this
delineation is inconsistent with the diminished vibronic coupling
Table 2 Electrochemical data and photophysical properties of all iridium complexesa
Entry
kabs /nm (e × 10−3/M−1 cm−1
)
kem/nm
U (%)
sobs/ls
sr/ls
E1/2ox/V (DEp/mV)
E1/2red/V (DEp/mV)
1a
1b
2a
2b
2c
2d
3a
3b
345 (4.4), 416 (7.3)
362 (4.9), 441 (5.3)
272 (23.6), 302 (15.9), 346 (6.5), 374 (3.1)
276 (21.2), 302 (16.9), 363 (7.0), 433 (1.7)
252 (24.1), 302 (14.6), 358 (6.4), 416 (3.3)
267 (22.2), 290 (13.1), 349 (2.7), 372 (0.6)
251 (23.2), 303 (9.9), 347 (10.9)
522
574
490
558
562, 596
440, 455, 478
516
29
82
6.0
72.0
100
0.67
81
10.7
7.6
0.79
3.0
20.2
0.042
28.0
14.7
36.9
9.3
13.2
4.2
20.2
6.3
34.6
18.4
0.55 (irr)
0.52 (irr)
0.72 (120)
0.78 (100)
0.72 (100)
0.98 (100)
0.73 (irr)
0.71 (irr)
−2.03 (100)
−1.82 (120)
−3.04 (irr)
−2.27 (75)
−2.35 (90)
−2.72 (irr)
−1.90 (110)
−1.53 (120)
255 (43.2), 303 (21.7), 385 (20.5)
555
80
a All photophysical data were recorded in degassed CH2Cl2 solution using three freeze–pump–thaw cycles. sobs and sr denote the observed and radiative
decay time, respectively. E1/2 refers to [(Epa + Epc)/2] where Epa and Epc are the anodic and cathodic peak potentials referenced to the Fc/Fc+ couple.
DEp = |Epa − Epc| was reported in mV, and the oxidation and reduction experiments were conducted in CH2Cl2 and THF solution, respectively.
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observed in the emission profile of 1b (see Fig. 2a, cf. 1a). Further
support of this viewpoint is given by the electrochemical analyses
(vide infra).
Theoretical approaches
Further insights into the photophysical properties of these com-
plexes are gained from a theoretical approach (TD-B3LYP/6-
31G* and LANL2DZ//B3LYP/6-31G* and LANL2DZ, see
Experimental section). For some representative complexes, the
features of the frontier orbitals mainly involved in the electronic
transitions are depicted in Fig. 3, while the descriptions and the
energy gap of each transition are listed in Table 3. As listed in
Table 3, for complexes 1a–3b, the lowest singlet (S1) and triplet
(T1) states are mainly contributed to from the HOMO → LUMO
transition. In addition, the calculated excitation energies for the
lowest singlet (S1) and triplet (T1) states are in very good agreement
with the measured absorption and emission energies, respectively.
We thus believed that the theoretical level adopted here is suitable
for interpreting the photophysical properties of the studied Ir(III)
complexes in a qualitative manner.
The absorption and emission spectra of the pyridylpyrazo-
late derivatives 2a–2d are depicted in Fig. 2b. For 2a and
2d, the corresponding absorption spectra are characterized by
an intense ligand localized pp* transition in the UV region
<400 nm, the results of which are consistent with the higher
energy gap of all the chromophores, i.e. cyclometalated 1-(2,4-
difluorophenyl)pyrazoles and pyridylpyrazolate ligands incorpo-
rated into their coordination sphere. In sharp contrast, both
complexes 2b and 2c showed strong absorptions in the region
1
>400 nm, which originated mainly from the spin-allowed pp*
transitions of the bzpz and bqpz ligands. In comparison to
2a, introducing an extra nitrogen atom forming pyrazine (in
2b), or isoquinoline fragments to extend the p conjugation (in
2c) lowered the gap of the S0–S1 transition. For the respective
phosphorescence spectra, although complex 2d exhibited the
highest energy emission in the blue region (440–478 nm), its
quantum yield turned out to be the lowest among this class
of complexes. Such an observation can be attributed to the
deliberate increase of the ligand energy gaps, for which the frontier
orbitals attributed to the lowest triplet state may spread out to
all ligated fragments possessing p chromophores, resulting in a
mixing of various types of electronic transitions such as intraligand
pp* charge transfer (ILCT) and even ligand-to-ligand charge
transfer (LLCT) transitions.25,26 Further support of this viewpoint
is given in the section of theoretical approaches. The net result
is the weakening of all metal–ligand bonds, i.e. a possible loose
molecular framework, giving rise to an enhanced radiationless
deactivation process. Conversely, other complexes 2a–2c displayed
bright emission in the region 490–596 nm with substantially higher
quantum efficiencies (see Table 2). For instance, complexes 2b
and 2c exhibited an emission quantum efficiency as high as 72%
and ∼100%, respectively. Interestingly, however, despite a similar
emission peak wavelength between 2b and 2c, the associated
vibronic fine structures of the phosphorescence for 2c suggests
3
the emission originates primarily from the pp(ILCT) manifold,
together with a somewhat reduced contribution from the 3MLCT
transition. This viewpoint is firmly supported by its substantially
longer radiative lifetime (20.2 ls) relative to that of 2b (4.2 ls, see
Table 2).
Finally, as shown in Fig 2c, the third class of the benzimidazolate
complexes 3a and 3b gave the lowest energy absorption band
maxima at 347 and 385 nm and emission peak wavelength
at 516 and 555 nm, respectively. Table 2 lists their respective
photophysical data in solution phase at room temperature. The
observed lifetimes of ca. 28 and 14.7 ls for 3a and 3b, together
with the quantum yields of 0.81 and 0.80, respectively, in CH2Cl2,
deduce a radiative lifetime of 34.6 for 3a and 18.4 ls for 3b.
The short radiative lifetime, in combination with relatively broad,
structureless phosphorescence, manifests a typical transition in-
corporating 3pp*/3MLCT mixing character. In comparison to 3a
with a pyridyl group at the benzimidazolate chelate, complex 3b
bearing a 2-quinolinyl group reveals an ∼40 nm bathochromic
shift in the emission wavelength, the result of which can be
promptly rationalized by a decrease of the LUMO energy level
resulting from the increase of p-conjugation in the 2-quinolinyl
group (versus the pyridyl moiety in 3a).
Fig. 3 Selected frontier orbitals of representative complexes 1a, 2a, 2d
and 3a.
Complexes 1a and 1b (not shown here) reveal a similar frontier
orbital configuration. The transitions for the lowest-lying exicted
states (S1 and T1) are mainly contributed to the intra-ligand
pp* charge transfer, that is, shifting from the phenolate to the
benzoxazolyl (1a) (or benzothiazolyl in 1b) moieties, accompanied
by the Ir(dp) → p* (benzoxazolyl in 1a, or benzothiazolyl in 1b)
MLCT. Consistent with the experimental results, the calculated
S0–S1 enegy gap of 1b is red shifted by as much as ∼2400 cm−1 with
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Table 3 Calculated energy levels of the lower lying transitions of com-
plexes 1a–3b
comparison to 3a, the ∼40 nm bathochromic shift in the emission
wavelength of 3b can be fully justified.
Assignment
k/nm
f
Electrochemistry
1a
1b
2a
S1
T1
S1
T1
S1
T1
HOMO → LUMO (+85%)
HOMO → LUMO (+108%)
HOMO → LUMO (+86%)
HOMO → LUMO (+106%)
HOMO → LUMO (+88%)
HOMO → LUMO (+80%)
HOMO − 2 → LUMO (+18%)
HOMO → LUMO + 2 (+9%)
HOMO → LUMO (+88%)
HOMO → LUMO (+87%)
HOMO − 2 → LUMO (21%)
HOMO → LUMO (+80%)
HOMO − 2 → LUMO (+9%)
HOMO → LUMO (+90%)
HOMO − 2 → LUMO (+20%)
HOMO → LUMO (+81%)
HOMO → LUMO (+65%)
HOMO − 1 → LUMO (+23%)
HOMO → LUMO (+90%)
HOMO → LUMO (+75%)
HOMO − 1 → LUMO (+23%)
HOMO → LUMO (+91%)
HOMO → LUMO (+64%)
HOMO − 1 → LUMO (+37%)
HOMO → LUMO+3 (+6%)
412.3
536.7
457.7
580.4
374.2
457.9
0.1249
∼0
The electrochemical behavior of these iridium complexes was
investigated using cyclic voltammetry; the data are also listed in
Table 2. It is notable that the oxidation potential of complexes
1a and 1b (0.52–0.55 V) is significantly lower than those of
the pyrazolate complexes 2a–2d (0.72–0.98 V) and imidazolate
complexes 3a–3b with potentials spanning the 0.71–0.73 V range.
As revealed by previous electrochemical studies27 and theoretical
calculations,8 the one-electron oxidation of such d6 complexes
would mainly occur at the metal site, together with a minor
contribution from the surrounding chelates. Thus, the lowered
oxidation potential of 1a and 1b is attributed to the phenolate
ligand, which destabilized the metal d orbitals via donation of
the lone pair of the oxygen atom. Furthermore, in comparison to
that of 1a (0.55 V), the lower oxidation potential in 1b (0.52 V)
may imply a lift of the dp orbital, rationalizing its greater 3MLCT
contribution (vide supra). Conversely, complexes 2a–2d and 3a–3b
possess only the pyrazolate or the imidazolate ligands, in which
no lone pair donation is available; thus the oxidation potentials
are relatively higher than those of the complexes 1a and 1b.
Upon switching to the reduction sweep in THF, one reversible,
lower energy reduction process was detected for all of them,
except for the complexes 2a and 2d, which turned out to be
irreversible. These data are in good agreement with the greatest
blue shift for their emission signals, showing the possession of the
highest destabilized LUMO among all complexes. Moreover, the
reduction potentials of iridium complexes within each subgroup
also showed good correlation with the nature of the third, ancillary
0.0744
∼0
0.0308
∼0
2b
2c
S1
T1
421.1
518.2
0.0203
∼0
S1
423.8
562.8
0.0776
T1
∼0
2d
3a
S1
T1
365.8
424.2
0.0209
∼0
S1
T1
406.5
503.3
0.0206
∼0
3b
S1
T1
458.8
551.0
0.0130
∼0
respect to that of 1a. The result may be rationalized by the fact that
the sulfur atom (benzothiazolyl moiety in 1b) is more polarizable
than the oxygen atom (benzoxazolyl moiety in 1a), resulting in an
increase of the conjugative effect and hence a smaller pp* energy
gap. Similar frontier-orbital configurations were found among
2a–2c (2b and 2c not shown here). The electron densities of the
HOMO are mainly located on the central Ir metal atom and the
∧
L X ligand. For instance, substitution of the bop ligand in
1a by the more electron donating btp ligand in 1b markedly
decreased the reduction potential (−2.03 to −1.82 V) as well as
the oxidative potential (0.55 to 0.52 V). It should be noted that the
variation in the reduction potentials was much greater than the
difference observed for the oxidation potentials. This result firmly
supports the original concept that the reduction mainly occurred
∧
pyrazolate fragment of the bidentate ancillary ligand (L X), while
those of the LUMO are primarily distributed over another moiety
of the same ligand. The results clearly indicate that the lowest-
lying singlet (S1) and triplet states (T1) are composed of MLCT in
combination with a pp* transition (ILCT) in character. In sharp
contrast, for complex 2d, only a small portion of electron density
in the HOMO locates at the pyrazolate fragment. Instead, it is
mainly distributed on the difluorophenyl moieties. The result can
be rationalized by the addition of a strong electron withdrawing
group, i.e. CF3, in the pyrazolate fragment, resulting in a significant
decrease of the p orbital energy in the pyrazolate moiety, which
no longer serves as a major contribution to the HOMO in 2d.
The switch of transition from ILCT (2a–2c) to LLCT (2d) should
lead to the spreading of electron densities over the molecular
framework and hence a possible looser structure (vice supra)
that induces an efficient radiationless transition, consistent with
the lowest emission quantum efficieny for 2a among complexes
2a–2d.
∧
at the ancillary L X ligand. In view of the oxidation potentials,
comparing the same class of L X ligands such as 2a–2d and 3a–
3b, except for 2d, the smaller variation in oxidation potential was
due to a less pronounced inductive effect that transmitted to the
central metal atom, confirming that both reduction and oxidation
∧
∧
reactions mainly occurred at the L X ligand and the adjacent
iridium metal site, respectively. Conversely, the significantly higher
oxidation potential (0.98 V, see Table 2) in 2d manifests its different
type of ligand contribution to the HOMO, i.e. the difluorophenyl
rather than the pyrazolate moiety in 2d concluded from the
theoretical approach.
Electroluminescent devices
Also depicted in Fig. 3, the lowest lying states (S1 and T1) for
both 3a and 3b (not shown here) can be described as a typical
MLCT mixed with p (benzoimidazolate) → p* (pyridyl in 3a, 2-
quinolinyl in 3b) ILCT. Apparently, the replacement of pyridyl
(3a) by the 2-quinolinyl group (3b) leads to a decrease of the
LUMO energy level due to the increase of p-conjugation in the
2-quinolinyl group (versus the pyridyl moiety in 3a). As a result, in
To demonstrate their capabilities in exhibiting decent electro-
luminescent properties, complex 2b was selected as the repre-
sentative example as it exhibited a relatively shorter radiative
lifetime in solution. It should be noted that, despite its unit
emission quantum efficiency, 2c was not picked as a prototype
simply due to its relatively much longer radiative lifetime (vide
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Table 4 Performance characteristics for ITO/NPB/CBP : x wt% 2b/BCP/AlQ3/Mg : Ag devices
x conc. (%) Max. lum.a/cd m−2 Quantum efficiencyb (%) Luminous efficiencyb/cd A−1 Power efficiencyb/lm W−1 kmax/nm (C.I.E. coordinatesc)
7%
14%
28%
50%
100%
60 036 (14.5)
33 865 (16.0)
33 760 (16.0)
10 298 (14.5)
15 485 (16.0)
6.11 (5.03)
6.05 (4.56)
5.69 (4.10)
2.01 (1.48)
2.21 (1.82)
23.2 (19.1)
22.8 (17.3)
21.2 (15.1)
7.5 (5.5)
9.6 (6.3)
7.7 (4.6)
7.1 (4.1)
2.4 (1.5)
2.2 (1.5)
526 (0.34, 0.61)
528 (0.32, 0.62)
530 (0.33, 0.61)
536 (0.32, 0.62)
536 (0.37, 0.58)
8.1 (6.7)
a Values in the parentheses are the applied driving voltage (in V). b Data collected at 20 mA cm−2, while values in parentheses are the data collected at
100 mA cm−2 c Measured at the driving voltage of 9 V.
.
Fig. 4 (a) Current density vs. applied voltage for the 2b-doped devices, (b) EL spectra of all devices, (c) external quantum efficiency vs. current density,
and (d) luminescence vs. current density.
supra). The devices incorporating 2b are prepared using thermal
evaporation and their structures consist of the multilayer config-
uration ITO/NPB (40 nm)/CBP : x wt% of 2b (30 nm)/BCP
(10 nm)/AlQ3(30 nm)/Mg : Ag (100 nm), where NPB, CBP, BCP
and AlQ3 stand for 1,4-bis(1-naphthylphenylamino)biphenyl, 4,4ꢀ-
N.Nꢀ-dicarbazolyl-1,1ꢀ-biphenyl, 2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline, and tris(8-hydroxyquinolinato)aluminium(III) re-
spectively; whilst the BCP layer acts as a hole blocker to
prevent any emission from AlQ3. The crucial device performance
characteristics are collected in Table 4, showing the systematic
trend that varied according to the five dopant concentrations
from 7% to 100%. Bright greenish yellow emission was observed
for all the concentrations applied, even for the one with a pure
layer of the Ir(III) emitter. Fig. 4a shows the current–voltage
curves of the electroluminescent devices for 2b at various doping
concentrations. The driving voltages of these devices increase with
dopant concentrations, consistent with earlier reports that iridium
complexes behave as carrier traps in devices.28 Concomitantly, a
small red shifting of the EL emission was observed with increasing
dopant concentrations, e.g. from kmax = 526 nm for the 7% device
to 536 nm for the device with neat dopant (Fig. 4b). This effect is
presumably attributed to the change of the medium polarity.29
Among various dopant concentration, the best device perfor-
mance was achieved at 7 wt% (Fig. 4c), which rendered a turn-on
voltage of 4 V (at 1 cd m−2) and peak EQE of 6.11 at 5.03 V, reached
the maximum brightness of 60 036 cd m−2 at a driving voltage of
14.5 V and with C.I.E. coordinates of 0.34, 0.61 at 9 V. Like other
phosphorescent emitters, the efficiencies also witnessed a drop with
increasing driving voltage.30 At a driving current of 20 mA cm−2,
the external quantum efficiency is 6.11% and luminous efficiency
is 23.19 cd A−1, whereas at 100 mA cm−2 the efficiencies remain
at 5.03% and 19.09 cd A−1 respectively. However, this efficiency
roll-off at high current densities is much less pronounced than
those reported for the triplet-state emitters.31 The key difference is
plausibly due to the short radiative lifetime of complex 2b, which
significantly reduces the triplet–triplet annihilation.15,32
As a result of concentration quenching, raising the doping
concentration leads to a fall off in device efficiency.33 Relatively
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bright luminescence of over 10 000 cd m−2 was observed for other
concentrations applied (≤28%), albeit with significant decrease of
intensity once the concentration was increased over 28% (Fig. 4d),
which is evidence of the device using 50% of 2b as dopant, showing
the peak EQE of 2.01 at 1.48 V, maximum brightness of 10 298 cd
m−2 at a driving voltage of 14.5 V and with C.I.E. coordinates
of 0.32, 0.62 at 9 V. Upon further increasing of the dopant
concentration to 100%, the device exhibits a comparable peak
EQE of 2.21 at 1.82 V, while the maximum brightness is lowered
to 15485 cd/m2 at 16.0 V and showing C.I.E. coordinates of (0.37,
0.58) at 9 V.
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Zuppiroli and M. Graetzel, J. Am. Chem. Soc., 2003, 125, 8790; (b) F.-
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Conclusions
In summary, we report the preparation of a series of new het-
eroleptic Ir(III) metal complexes chelated by two cyclometalated 1-
(2,4-difluorophenyl)pyrazole ligands (dfpz)H and a third ancillary
∧
bidantate ligand (L X). This intricate design lies in a core concept
that the cyclometalated dfpz ligands ensure a greater pp* gap in
these series of iridium complexes. Accordingly, the lowest one-
electron excitation would accommodate the p* orbital of the
∧
ancillary L X ligands. The experimental results firmly support the
original concept in that reduction mainly occurred at the ancillary
∧
L X ligand; whereas the smaller variation of oxidation potential
was due to a less pronounced inductive effect that transmitted
to the central metal atom. Except for 2d, the redox potentials of
the entire series of complexes are consistent with the delineation
that the reduction and oxidation reactions mainly occurr at the
∧
L X ligand and the adjacent iridium metal site, respectively. The
feasibility of OLED application is given by complex 2b, which was
used as a representative example for fabrication of the multilay-
ered, green-emitting phosphorescent OLEDs using direct thermal
evaporation techniques. From the viewpoint of strategic design,
this series of heteroleptic cyclometalated Ir(III) complexes establish
a simple and straightforward methodology for color tuning via
incorporation of the facile variation of ancillary ligands only.
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