Photoluminescence color tuning of phosphorescent bis-cyclometalated iridium(III) complexes by ancillary ligand replacement

Article abstract of DOI:10.1016/j.dyepig.2012.05.026

A series of bis-cyclometalated iridium(III) complexes bearing various types of 1,3-diketonate ancillary ligands were prepared, and their photoluminescent (PL) properties were investigated, especially focusing on the emission color tuning. When the dipivaloylmethanate ancillary ligand (O^O-1a) was replaced by conjugated 1,3-diketonates such as 1,3-bis(3,4-dibutoxyphenyl)propane-1,3- dionate (O^O-1b) and 1,3-bis(4-(dibenzo[b,d]furan-4-yl)phenyl)propane-1,3- dionate (O^O-1c), the blue-emitting bis[2-(3,5-bis(trifluoromethyl)phenyl) pyridinato-N,C^2′]iridium(III) (Ir-1) and bluish green-emitting bis[2-phenylpyridinato-N,C^2′]iridium(III) (Ir-2) complexes exhibited significantly red-shifted phosphorescence in solution (λ_PL; 474-604 nm and 521-661 nm for Ir-1a-c and Ir-2a-c, respectively: the subscripts a, b, and c corresponding to O^O-1a-c). On the other hand, the ancillary ligand replacement was less effective on tuning the emission color of the green-emitting bis[2-(2,4-bis(trifluoromethyl)phenyl) pyridinato-N,C^2′]iridium(III) complex (λ_PL; 541-566 nm for Ir-3a-c), and no PL color change was observed for the red-emitting bis[1-(dibenzo[b,d]furan-4-yl)isoquinolinato-N, C^3′]iridium(III) complex (Ir-4). The X-ray crystallographic analysis for Ir-1a-c revealed that these complexes adopt cis-C,C and trans-N,N configuration, indicating that the coordination geometry around the iridium center is not a main factor for the emission spectral differences caused by the O^O ancillary ligands. Furthermore, the ancillary ligand effect on the PL properties of the present iridium(III) complexes is independent on solvent polarity and concentrations. Taking these results into consideration, the ancillary ligand effect should be attributed to the electronic structures of the complexes, and the inter-ligand energy transfer from the triplet metal-to-ligand charge transfer level at the cyclometalated ligand (³MLCT_C^N) to the ancillary ligand-related triplet level (³LC_O^O or ³MLCT_O^O) should play a significant role in the triplet exciton formation. The PL properties of Ir-1-Ir-4 in PMMA films were also investigated, and the remarkable blue shifts of λ_PLs due to rigidochromism were observed for Ir-1b,c and Ir-2b,c that have the aromatic O^O ancillary ligands. It was found that some of the complexes developed here exhibited larger Φ_PLs in PMMA films than in toluene solutions.

Full text of DOI:10.1016/j.dyepig.2012.05.026

Dyes and Pigments 95 (2012) 695e705
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photoluminescence color tuning of phosphorescent bis-cyclometalated
iridium(III) complexes by ancillary ligand replacement
,
a
a
b
Shigeru Ikawa a, Shigeyuki Yagi a , Takeshi Maeda , Hiroyuki Nakazumi , Hideki Fujiwara ,
*
Yoshiaki Sakurai c
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
b Department of Chemistry, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
c Environment and Chemistry Department, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of bis-cyclometalated iridium(III) complexes bearing various types of 1,3-diketonate ancillary
ligands were prepared, and their photoluminescent (PL) properties were investigated, especially focusing
on the emission color tuning. When the dipivaloylmethanate ancillary ligand (O^O-1a) was replaced by
conjugated 1,3-diketonates such as 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate (O^O-1b) and 1,3-
Received 27 November 2011
Received in revised form
12 April 2012
Accepted 14 May 2012
Available online 9 July 2012
bis(4-(dibenzo[b,d]furan-4-yl)phenyl)propane-1,3-dionate (O^O-1c), the blue-emitting bis[2-(3,5-
0
bis(trifluoromethyl)phenyl)pyridinato-N,C2 ]iridium(III) (Ir-1) and bluish green-emitting bis[2-
0
phenylpyridinato-N,C2 ]iridium(III) (Ir-2) complexes exhibited significantly red-shifted phosphores-
Keywords:
Bis-cyclometalated iridium(III) complex
Ancillary ligand
Cyclometalated ligand
Organic light-emitting diode
Phosphorescent material
Rigidochromism
cence in solution (lPL; 474e604 nm and 521e661 nm for Ir-1aec and Ir-2aec, respectively: the
subscripts a, b, and c corresponding to O^O-1aec). On the other hand, the ancillary ligand replacement
was less effective on tuning the emission color of the green-emitting bis[2-(2,4-bis(trifluoromethyl)
0
phenyl)pyridinato-N,C2 ]iridium(III) complex (lPL; 541e566 nm for Ir-3aec), and n0o PL color change was
observed for the red-emitting bis[1-(dibenzo[b,d]furan-4-yl)isoquinolinato-N,C3 ]iridium(III) complex
(Ir-4). The X-ray crystallographic analysis for Ir-1aec revealed that these complexes adopt cis-C,C and
trans-N,N configuration, indicating that the coordination geometry around the iridium center is not
a main factor for the emission spectral differences caused by the O^O ancillary ligands. Furthermore, the
ancillary ligand effect on the PL properties of the present iridium(III) complexes is independent on
solvent polarity and concentrations. Taking these results into consideration, the ancillary ligand effect
should be attributed to the electronic structures of the complexes, and the inter-ligand energy transfer
from the triplet metal-to-ligand charge transfer level at the cyclometalated ligand (3MLCTC^N) to the
ancillary ligand-related triplet level (3LCO^O or 3MLCTO^O) should play a significant role in the triplet
exciton formation. The PL properties of Ir-1eIr-4 in PMMA films were also investigated, and the
remarkable blue shifts of lPLs due to rigidochromism were observed for Ir-1b,c and Ir-2b,c that have the
aromatic O^O ancillary ligands. It was found that some of the complexes developed here exhibited larger
FPLs in PMMA films than in toluene solutions.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
achieve the internal quantum efficiency (hint) of 100%, corresponding to
four times magnitude of the maximum hint of fluorescent OLEDs that
Organic light-emitting diodes (OLEDs) have attracted much interest
of researchers for the last few decades due to their promising applica-
tions such as flat-panel displays and solid-state lightings [1,2]. In order
to obtain high performance OLEDs, phosphorescent materials are
frequently used as emitters because combination of statistically
generated triplet excitons (75% of all) with those obtained from the
singlet excitons (25% of all) via intersystem crossing can theoretically
utilize only singlet excitons. With this respect, organometallic
complexes with heavy metal centers such as Pt(II) [3,4], Ir(III) [5e7],
Ru(II) [8], and Os(II) [9,10] have been widely studied because the strong
spin-orbit coupling gives rise to the efficient intersystem crossing from
the singlet excited state to the triplet to facilitate the generation of
triplet excitons. Especially, cyclometalated iridium(III) complexes have
been intensely developed since the pioneering work on the phospho-
rescent OLED was reported by Thompson and Forrest [11]. Homoleptic
tris-cyclometalated (Ir(C^N)3) and heteroleptic bis-cyclometalated
(Ir(C^N)2(LX)) iridium(III) complexes are representative examples,
* Corresponding author: Tel.: þ81 72 254 9324; fax: þ81 72 254 9910.
E-mail address: yagi@chem.osakafu-u.ac.jp (S. Yagi).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.05.026

696
S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
where C^N and LX represent an arylpyridine-type cyclometalated
ligand and an anionic ancillary ligand such as 1,3-diketonate, respec-
tively [6,12,13]. The cyclometalated ligands bring about the strong
CHD2CN (1.95 ppm) as internal standards for CDCl3 and CD3CN,
respectively. Matrix-assisted laser desorption/ionization time-of-
flight (MALDI-TOF) mass spectra were measured on a Shimadzu-
Kratos AXIMA-CFR PLUS TOF mass spectrometer using sinapinic
acid as a matrix. Elemental analyses were carried out on a J-Science
MICRO CORDER JM10 analyzer.
The preparation of 2-(3,5-bis(trifluoromethyl)phenyl)pyridine
(HC^N-1), 2-(2,4-bis(trifluoromethyl)phenyl)pyridine (HC^N-3),
and 1-(dibenzo[b,d]furan-4-yl)isoquinoline (HC^N-4) were previ-
ously reported [20,24]. 2-Phenylpyridine (HC^N-2) was purchased
from Tokyo Chemical Industry Co., Ltd. For the preparation of 1,3-
bis(4-(dibenzo[b,d]furan-4-yl)phenyl)propane-1,3-dione (HO^O-
1c), 4-iodoacetophenone and ethyl 4-iodobenzoate were
purchased from Wako Pure Chemical Industries, Ltd. (Dibenzo[b,d]
furan-4-yl)boronic acid was purchased from Sigma-Aldrich Co. As
ligand field influence on the metal center via the dep orbital overlap.
Thus, the energy gap of the metal-centered excited states (ded*)
increase to suppress the non-radiative decay, and strong phosphores-
cence is obtained from the triplet metal-to-ligand charge transfer
(MLCT) state [3,14,15]. In addition, it has been well demonstrated that
the skeletal variation as well as the modification by substituent groups
in the C^N ligands affords effective color tuning of phosphorescence, so
various types of C^N ligands have been enthusiastically investigated to
obtain desired colors of phosphorescence [6,16e19].
We previously developed highly emissive red phosphorescent
(C^N)2Ir(O^O) complexes (O^O; 1,3-diketonate ancillary ligand)
[20]. The employment of a
[b,d]furan-4-yl)isoquinoline yielded pure red phosphorescence
lPL ¼ 640 nm, in toluene at 298 K). When 1,3-bis(3,4-
p-extended C^N ligand such as 1-(dibenzo
a
catalyst for the Suzuki-Miyaura cross-coupling reaction,
(
Pd(PPh3)4 was purchased from Wako Pure Chemical Industries, Ltd,
and used just as obtained. Dipivaroylmethane (HO^O-1a) was
purchased from Tokyo Chemical Industry Co., Ltd. The preparation
of 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dione (HO^O-1b) was
previously reported [25]. For the preparation of Ir-1eIr-4, 2-
ethoxyethanol was purchased from Wako Pure Chemical
Industries, Ltd.
dibutoxyphenyl)propane-1,3-dionate was employed as an O^O
ancillary ligand, a larger photoluminescence (PL) quantum yield was
obtained (FPL ¼ 0.61, in toluene at 298 K) in comparison with the
dipivaloylmethanate derivative (FPL ¼ 0.55, in toluene at 298 K),
although the emission color of these complexes is independent on the
O^O ligand. However, in the course of our investigation of the ancillary
ligand effect on photoluminescent properties of bis-cyclometalated
Ir(III) complexes, the 1,3-diphenylpropane-1,3-dionate-type ligands
significantly affect the PL color of the complexes when they are
combined with some C^N ligands. Taking it into consideration that the
O^O ancillary ligand has received less attention to tune and improve
phosphorescent properties [21e23], the systematic investigation to
elucidate how the ancillary ligand determine the emission properties
should lead to understanding the emission mechanism of the bis-
cyclometalated Ir(III) complex as well as designing new types of
Ir(III)-based phosphorescent emitters for OLEDs.
2.2. Preparation of materials
2.2.1. Synthesis of 1-[4-(dibenzo[b,d]furan-4-yl)phenyl]ethanone (1)
To
a mixture of 4-iodoacetophenone (1.36 g, 5.52 mmol),
(dibenzo[b,d]furan-4-yl)boronic acid (1.02 g, 4.81 mmol) and
Pd(PPh3)4 (197 mg, 0.170 mmol) in 1,2-dimethoxyethane (50 mL)
was added sodium carbonate (1.35 g, 12.7 mmol) in water (5 mL).
Then, the mixture was heated at 70 ꢀC for 15 h under nitrogen. After
cooling, the solvent was removed on a rotary evaporator. The
residue was dissolved in chloroform (200 mL), and the solution was
washed with water (150 mL ꢁ 2) and sat. brine (150 mL), and then
dried over anhydrous MgSO4. The solvent was removed on a rotary
evaporator, and the residue was purified by silica gel column
chromatography using chloroform as eluent. The obtained solid
was further washed with hexane to afford a white solid of 1 (1.22 g,
In this study, we show how a series of O^O ancillary ligands
affect PL properties of (C^N)2Ir(O^O) complexes bearing four types
of C^N ligands (Ir-1eIr-4, Fig. 1). We employ three types of O^O
ancillary ligands that have different electronic structures from each
other (O^O-1aeO^O-1c, Fig. 1).
2. Experimental section
4.26 mmol, 89%); 1H NMR (400 MHz, CDCl3)
d 2.68 (s, 3H), 7,38 (dt,
J ¼ 0.9 and 7.8 Hz, 1H), 7.45 (t, J ¼ 7.8 Hz, 1H), 7.49 (dt, J ¼ 0.9 and
7.8 Hz, 1H), 7.61 (d, J ¼ 7.8 Hz, 1H), 7.64 (dd, J ¼ 0.9 and 7.8 Hz, 1H),
7.98e8.01 (m, 2H), 8.03 (d, J ¼ 8.3 Hz, 2H), 8.13 (d, J ¼ 8.3 Hz, 2H);
MALDI-TOF MS m/z 287 ([M þ H]þ). Anal. Calcd for C20H14O2: C,
83.90; H, 4.93. Found: C, 83.55; H, 4.99.
2.1. General procedures
1H NMR spectra were obtained on
(400 MHz) spectrometer, using TMS (0.00 ppm) and residual
a Jeol JNM-ECX400
Fig. 1. The structures of bis-cyclometalated iridium(III) complexes Ir-1eIr-4, each of which is classified to three categories (aec) with respect to the diketonate ancillary ligand.

S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
697
2.2.2. Synthesis of ethyl 4-(dibenzo[b,d]furan-4-yl)benzoate (2)
To a mixture of ethyl 4-iodobenzoate (1.26 g, 4.56 mmol),
(dibenzo[b,d]furan-4-yl)boronic acid (1.09 g, 5.14 mmol), and
Pd(PPh3)4 (153 mg, 0.132 mmol) in a solvent mixture of toluene
(100 mL) and ethanol (10 mL) was added sodium carbonate
(0.806 g, 7.60 mmol) in water (4 mL). Then, the mixture was heated
at 80 ꢀC for 20 h under nitrogen. After cooling, ethyl acetate
(100 mL) and water (100 mL) were added, and the insoluble
materials were removed by filtration. The filtrate was divided into
two layers, and the upper organic layer was separated using
a separation funnel. The lower was extracted with ethyl acetate
(50 mL ꢁ 2). All the organic layers were combined, washed with
water (200 mL ꢁ 2) and sat. brine (200 mL), and dried over anhy-
drous MgSO4. The solvent was removed on a rotary evaporator, and
the residue was purified by silica gel column chromatography using
dichloromethane/hexane (2:1, v/v) as eluent to afford a white solid
water and sat. brine, and dried over anhydrous Na2SO4. The solvent
was removed on a rotary evaporator, and the residue was purified
by alumina column chromatography. Further purification by
recrystallization from chloroform/hexane gave a crystal of the
(C^N)2Ir(O^O) complex. The preparation of Ir-1a [26], Ir-2a [6], Ir-
4a [20], and Ir-4b [20] were prepared according to the reported
procedures.
0
2.2.5.1. Bis[2-(3,5-bis(trifluoromethyl)phenyl)pyridinato-N,C2 ]iridiu-
m(III) [1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate-O,O] (Ir-
1b). A mixture of [(C^N-1)2Ir(m-Cl)]2 (184 mg, 0.114 mmol), HO^O-
1b (110 mg, 0.214 mmol), and Na2CO3 (96 mg, 0.905 mmol) in 2-
ethoxyethanol (33 mL) was heated at 100 ꢀC for 1.5 h under
nitrogen. Subsequent isolation and purification were carried out
according to the general procedure. The column chromatography
was eluted with dichloromethane; yield, 80% (220 mg,
of 2 (1.25 g, 3.95 mmol, 87%); 1H NMR (400 MHz, CDCl3)
d
1.43 (t,
0.171 mmol); 1H NMR (400 MHz, CDCl3)
d 0.93e0.98 (m, 12H),
J ¼ 7.2 Hz, 3H), 4.43 (q, J ¼ 7.2 Hz, 2H), 7,37 (dt, J ¼ 1.2 and 7.8 Hz,
1H), 7.45 (t, J ¼ 7.8 Hz, 1H), 7.48 (dt, J ¼ 1.2 and 7.8 Hz, 1H), 7.60 (d,
J ¼ 7.8 Hz, 1H), 7.64 (dd, J ¼ 1.2 and 7.8 Hz, 1H), 7.97e8.01 (m, 4H),
8.21 (d, J ¼ 8.3 Hz, 2H); MALDI-TOF MS m/z 317 ([M þ H]þ). Anal.
Calcd for C21H16O3: C, 79.73; H, 5.10. Found: C, 79.78; H, 5.15.
1.40e1.49 (m, 8H), 1,69e1.81 (m, 8H), 3.82 (t, J ¼ 6.4 Hz, 4H), 3.98
(m, 4H), 6.32 (s, 1H), 6.74 (d, J ¼ 8.2 Hz, 2H), 6.94 (t, J ¼ 7.8 Hz, 2H),
7.05 (d, J ¼ 1.8 Hz, 2H), 7.21 (dd, J ¼ 1.8 and 7.8 Hz, 2H), 7.56 (s, 2H),
7.74 (dt, J ¼ 1.8 and 7.8 Hz, 2H), 7.96 (d, J ¼ 8.2 Hz, 2H), 8.11 (s, 2H),
8.20 (d, J ¼ 7.8 Hz, 2H); MALDI-TOF MS m/z 1284 ([M]þ). Anal. Calcd
for C57H55F12IrN2O6: C, 53.31; H, 4.32; N, 2.18. Found: C, 53.31; H,
4.35; N, 2.30.
2.2.3. Synthesis of 1,3-bis[4-(dibenzo[b,d]furan-4-yl)phenyl]
propane-1,3-dione (HO^O-1c)
0
To a mixture of 2 (1.46 g, 4.62 mmol) and sodium hydride (60%
oil dispersion, 845 mg, 14.1 mmol) in THF (7.2 mL) was added
dropwise a solution of 1 (1.24 g, 4.33 mmol) in THF (7.2 mL) over
2 h with a dropping funnel. Then, the mixture was heated at 60 ꢀC
for 15 h under nitrogen. After cooling, water (30 mL) added to the
reaction mixture on an ice bath, and the mixture was acidified to
pH 3 with 1.2 M HClaq. Then, chloroform (200 mL) was added, and
the organic layer was separated using a separation funnel. The
aqueous layer was further extracted with chloroform (100 mL ꢁ 3).
All the organic layers were combined, washed with sat. NaHCO3
(300 mL ꢁ 3), H2O (300 mL ꢁ 2), and sat. brine (300 mL ꢁ 2), and
dried over anhydrous MgSO4. The solvent was removed on a rotary
evaporator, and the residue was purified by recrystallization from
chloroform to afford a yellow solid of HO^O-1c in 39% (950 mg,
2.2.5.2. Bis[2-(3,5-bis(trifluoromethyl)phenyl)pyridinato-N,C2 ]iri-
dium(III)
[1,3-bis(4-(dibenzo[b,d]furan-4-yl)phenyl)propane-1,3-
-Cl)]2 (300 mg,
dionate-O,O] (Ir-1c). A mixture of [(C^N-1)2Ir(
m
0.186 mmol), HO^O-1c (142 mg, 0.255 mmol), and Na2CO3 (193 mg,
1.82 mmol) in 2-ethoxyethanol (50 mL) was heated at 100 ꢀC for
6 h under nitrogen. Subsequent isolation and purification were
carried out according to the general procedure. The column chro-
matography was eluted with dichloromethane; yield, 38% (128 mg,
0.0964 mmol); 1H NMR (400 MHz, CDCl3)
d 6.66 (s, 1H), 7.02 (t,
J ¼ 6.4 Hz, 2H), 7,37 (dt, J ¼ 1.4 and 7.8 Hz, 2H), 7.43 (t, J ¼ 7.8 Hz,
2H), 7.48 (dt, J ¼ 1.4 and 7.8 Hz, 2H), 7.57e7.59 (m, 4H), 7.61 (d,
J ¼ 1.8 Hz, 2H), 7.78e7.82 (m, 6H), 7.88 (d, J ¼ 8.3 Hz, 4H), 7.98 (m,
4H), 8.05 (d, J ¼ 7.8 Hz, 2H), 8.18 (d, J ¼ 1.8 Hz, 2H), 8.30 (d,
J ¼ 6.4 Hz, 2H); MALDI-TOF MS m/z 1328 ([M]þ). Anal. Calcd for
C65H35F12IrN2O4: C, 58.78; H, 2.66; N, 2.11. Found: C, 58.81; H, 3.07;
N, 2.17.
1.71 mmol); 1H NMR (400 MHz, CD3CN)
d 3.52 (br s, 2H), 7.41 (t,
J ¼ 7.8 Hz, 2H), 7.47e7.54 (m, 4H), 7.62 (d, J ¼ 7.8 Hz, 2H), 7.71 (dd,
J ¼ 1.4 and 7.8 Hz, 2H), 8.03 (d, J ¼ 8.6 Hz, 4H), 8.08e8.11 (m, 4H),
8.15 (d, J ¼ 8.6 Hz, 4H); MALDI-TOF MS m/z 557 ([M þ H]þ). Anal.
Calcd for C39H24O4: C, 84.16; H, 4.35. Found: C, 83.87; H, 4.40.
0
2.2.5.3. Bis[2-phenylpyridinato-N,C2 ]iridium(III)
[1,3-bis(3,4-
dibutoxyphenyl)propane-1,3-dionate-O,O] (Ir-2b). A mixture of
[(C^N-2)2Ir( -Cl)]2 (500 mg, 0.466 mmol), HO^O-1b (469 mg,
m
2.2.4. Synthesis of
Cl)]2; the general procedure
m
-chloro-bridged iridium(III) dimers [(C^N)2Ir(
m
-
0.914 mmol), and Na2CO3 (433 mg, 4.09 mmol) in 2-ethoxyethanol
(65 mL) was heated at 100 ꢀC for 5 h under nitrogen. Subsequent
isolation and purification were carried out according to the general
procedure. The column chromatography was eluted with
dichloromethane/hexane (1:2, v/v); yield, 70% (648 mg,
These compounds were prepared according to the conventional
procedure [12,24]. To a solution of the cyclometalated ligand HC^N
(2.5 mmol) in 2-ethoxyethanol (50 mL) was added a solution of
IrCl3$3H2O (1.2 mmol) in water (15 mL), and the mixture was
heated on an oil bath for 24 h, where the bath temperature was
kept at 100 ꢀC. After cooling, water (300 mL) was added, and the
precipitate was collected by filtration. The obtained precipitate was
washed with ethanol (20 mL) and hexane (20 mL) to afford the
chloro-bridged iridium(III) dimer [(C^N)2Ir( -Cl)]2. These materials
were highly insoluble, and thus, used in the next reaction without
further purification.
0.640 mmol); 1H NMR (400 MHz, CDCl3)
d 0.91e0.96 (m, 12H),
1.37e1.50 (m, 8H), 1,67e1.80 (m, 8H), 3.89 (m, 4H), 3.98 (t,
J ¼ 6.4 Hz, 4H), 6.38 (d, J ¼ 7.8 Hz, 2H), 6.40 (s, 1H), 6.71 (t, J ¼ 7.8 Hz,
2H), 6.75 (d, J ¼ 8.2 Hz, 2H), 6.82 (t, J ¼ 7.8 Hz, 2H), 7.02 (t, J ¼ 6.0 Hz,
2H), 7.29 (d, J ¼ 1.8 Hz, 2H), 7.38 (dd, J ¼ 1.4 and 8.2 Hz, 2H), 7.56 (d,
J ¼ 7.8 Hz, 2H), 7.66 (t, J ¼ 1.4 and 8.2 Hz, 2H), 7.83 (d, J ¼ 8.2 Hz, 2H),
8.59 (d, J ¼ 6.0 Hz, 2H); MALDI-TOF MS m/z 1012 ([M]þ). Anal. Calcd
for C53H59IrN2O6: C, 62.89; H, 5.87; N, 2.77. Found: C, 62.87; H, 5.79;
N, 2.75.
m-
m
2.2.5. Synthesis of (C^N)2Ir(O^O); the general procedure
0
A mixture of [(C^N)2Ir(
m-Cl)]2, 1,3-diketone HO^O (1.5e2 eq of
2.2.5.4. Bis[2-phenylpyridinato-N,C2 ]iridium(III) [1,3-bis(4-(dibenzo
[(C^N)2Ir( -Cl)]2), and Na2CO3 (9e10 eq of [(C^N)2Ir(
m
m
-Cl)]2) in 2-
[b,d]furan-4-yl)phenyl)propane-1,3-dionate-O,O] (Ir-2c). A mixture
of [(C^N-2)2Ir(m-Cl)]2 (268 mg, 0.250 mmol), HO^O-1c (200 mg,
ethoxyethanol was heated at 100 ꢀC under nitrogen. After cooling,
the solvent was removed on a rotary evaporator. The residue was
dissolved in dichloromethane, and the solution was washed with
0.359 mmol), and Na2CO3 (234 mg, 2.21 mmol) in 2-ethoxyethanol
(35 mL) was heated at 100 ꢀC for 3.5 h under nitrogen. Subsequent

698
S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
isolation and purification were carried out according to the general
procedure. The column chromatography was eluted with
dichloromethane; yield, 38% (144 mg, 0.136 mmol); 1H NMR
according to the general procedure. The column chromatography
was eluted with dichloromethane/hexane (1:2, v/v); yield, 34%
(225 mg, 0.168 mmol); 1H NMR (400 MHz, CDCl3)
d 6.50 (d,
(400 MHz, CDCl3)
d
6.42 (dd, J ¼ 0.9 and 7.3 Hz, 2H), 6.77e6.79 (m,
J ¼ 7.8 Hz, 2H), 6.80 (s, 1H), 7.24e7.31 (m, 2H, masked by residual
CHCl3), 7.33e7.39 (m, 8H), 7.43 (dt, J ¼ 1.4 and 7.8 Hz, 2H), 7.51e7.54
(m, 6H), 7.58 (d, J ¼ 6.4, 2H), 7.73e7.80 (m, 10H), 7.90e7.96 (m, 10H),
8.67 (d, J ¼ 6.4 Hz, 2H), 9.20 (dd, J ¼ 1.4 and 7.8 Hz, 2H); MALDI-TOF
MS m/z 1336 ([M]þ). Anal. Calcd for C81H47IrN2O6: C, 72.79; H, 3.54;
N, 2.10. Found: C, 72.41; H, 3.79; N, 2.12.
3H), 6.89 (dt, J ¼ 1.1 and 7.8 Hz, 2H), 7.09 (dt, J ¼ 1.4 and 7.8 Hz, 2H),
7.36 (dt, J ¼ 1.1 and 7.3 Hz, 2H), 7.41 (t, J ¼ 7.3 Hz, 2H), 7.46 (dt, J ¼ 1.1
and 7.3 Hz, 2H), 7.56e7.63 (m, 6H), 7.72 (dt, J ¼ 1.4 and 7.8 Hz, 2H),
7.85e7.90 (m, 6H), 7.95 (dd, J ¼ 1.4 and 7.8 Hz, 2H), 7.98 (d,
J ¼ 7.8 Hz, 2H), 8.03 (d, J ¼ 8.7 Hz, 4H), 8.68 (dd, J ¼ 0.9 and 6.0 Hz,
2H); MALDI-TOF MS m/z 1056 ([M]þ). Anal. Calcd for C61H39IrN2O4:
C, 69.37; H, 3.72; N, 2.65. Found: C, 69.43; H, 4.11; N, 2.56.
2.3. X-ray structure determination
0
2.2.5.5. Bis[2-(2,4-bis(trifluoromethyl)phenyl)pyridinato-N,C2 ]iri-
dium(III) [2,2,6,6-tetramethylheptane-3,5-dionate-O,O] (Ir-3a). A
mixture of [(C^N-3)2Ir(m-Cl)]2 (177 mg, 0.109 mmol), HO^O-1a
Diffraction data for Ir-1aeIr-1c were collected on a Rigaku
RAXIS RAPID imaging plate area detector with graphite mono-
chromated Mo-K
a
radiation (
l
¼ 0.71075 Å). The cell parameters for
(41 mg, 0.222 mmol), and Na2CO3 (95 mg, 0.896 mmol) in 2-
ethoxyethanol (33 mL) was heated at 100 ꢀC for 2 h under
nitrogen. Subsequent isolation and purification were carried out
according to the general procedure. The column chromatography
was eluted with chloroform/hexane (1:2, v/v); yield, 71% (150 mg,
Ir-1aeIr-1c were collected at a temperature of 23 ꢂ 1 ꢀC to
a maximum 2q
value of 55.0ꢀ. The structures were solved by direct
methods using the SIR2002 program for Ir-1a [27] and the SIR92
program for Ir-1b and Ir-1c [28], and expanded using the DIRDIF99
[29] program. Anisotropic temperature factors were applied to the
non-hydrogen atoms. The calculated positions of the hydrogen
atoms were refined using the riding model. All calculations were
performed using the CrystalStructure 3.8 [30] software packages of
computer programs. The crystal data and refinement details of the
crystal structure determination are given in Table 1.
0.157 mmol); 1H NMR (400 MHz, CDCl3)
d 0.86 (s, 18H), 5.51 (s, 1H),
6,65 (s, 2H), 7.25e7.26 (m, 2H, masked by residual CHCl3), 7.49 (s,
2H), 7.86 (dt, J ¼ 1.4 and 7.8 Hz, 2H), 8.36e8.41 (m, 4H); MALDI-TOF
MS m/z 956 ([M]þ). Anal. Calcd for C37H31F12IrN2O2: C, 46.49; H,
3.27; N, 2.93. Found: C, 46.48; H, 3.52; N, 2.97.
0
2.2.5.6. Bis[2-(2,4-bis(trifluoromethyl)phenyl)pyridinate-N,C2 ]iri-
dium(III) [1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate-O,O] (Ir-
2.4. Photophysical characterization
3b). A mixture of [(C^N-3)2Ir(m-Cl)]2 (177 mg, 0.109 mmol), HO^O-
UVevis absorption and photoluminescence (PL) spectra were
measured on a Shimadzu UV-3100 and a Jasco FP-6600 spectro-
photometer, respectively, using a quartz cell. PL lifetimes were
obtained on a Horiba Jobin Yvon FluoroCube spectroanalyzer using
a 390 nm nanosecond-order LED light source. PL quantum yields
were obtained on a Hamamatsu Photonics C9920 PL quantum yield
measurement system. The sample solutions for optical and pho-
tophysical measurements were deaerated by argon bubbling fol-
lowed by complete sealing, and the analyses were carried out just
after preparation of the samples.
1b (109 mg, 0.213 mmol), and Na2CO3 (95 mg, 0.896 mmol) in 2-
ethoxyethanol (33 mL) was heated at 100 ꢀC for 2.5 h under
nitrogen. Subsequent isolation and purification were carried out
according to the general procedure. The column chromatography
was eluted with chloroform/hexane (2:1, v/v); yield, 60% (163 mg,
0.127 mmol); 1H NMR (400 MHz, CDCl3)
d
0.91 (t, J ¼ 6.9 Hz, 6H),
0.94 (t, J ¼ 6.9 Hz, 6H),1.36e1.51 (m, 8H),1.70 (quint, J ¼ 6.9 Hz, 4H),
1.77 (quint, J ¼ 6.9 Hz, 4H), 3.80e3.86 (m, 4H), 3.99 (t, J ¼ 6.9 Hz,
4H), 6.47 (s, 1H), 6.63 (s, 2H), 6.78 (d, J ¼ 8.7 Hz, 2H), 7.17 (d,
J ¼ 2.3 Hz, 2H), 7.22 (t, J ¼ 6.4 Hz, 2H), 7.34 (dd, J ¼ 1.8 and 8.7 Hz,
2H), 7.52 (s, 2H), 7.86 (t, J ¼ 7.3 Hz, 2H), 8.39 (d, J ¼ 8.7 Hz, 2H), 8.62
(d, J ¼ 6.4 Hz, 2H); MALDI-TOF MS m/z 1284 ([M]þ). Anal. Calcd for
C57H56F12IrN2O6: C, 53.31; H, 4.32; N, 2.18. Found: C, 53.60; H, 4.34;
N, 2.26.
Table 1
Crystal data and structure refinement for Ir-1aeIr-1c.
Compound
Ir-1a
Ir-1b
Ir-1c
0
2.2.5.7. Bis[2-(2,4-bis(trifluoromethyl)phenyl)pyridinate-N,C2 ]iridiu-
Formula
C37H31N2O2F12Ir C57H55N2O6F12Ir C65H35N2O4F12Ir
Formula weight
Crystal system
Space group
Lattice parameters
a (Å)
955.86
Triclinic
P-1(#2)
1284.27
Triclinic
P-1(#2)
1328.20
Triclinic
P-1(#2)
m(III) [1,3-bis(4-(dibenzo[b,d]furan-4-yl)phenyl)propane-1,3-dionate-
O,O](Ir-3c). A mixture of [(C^N-3)2Ir(m-Cl)]2 (130 mg, 0.0804 mmol),
HO^O-1c (70 mg, 0.126 mmol), and Na2CO3 (95 mg, 0.896 mmol) in
2-ethoxyethanol (15 mL) was heated at 100 ꢀC for 3 h under
nitrogen. Subsequent isolation and purification were carried out
according to the general procedure. The column chromatography
was eluted with dichloromethane/hexane (1:1, v/v); yield, 44%
11.3363 (5)
13.2534 (6)
14.7442 (7)
81.9281 (14)
69.3969 (15)
69.9899 (15)
1947.86 (15)
2
13.2698 (5)
13.4972 (6)
15.8762 (6)
88.6908 (14)
78.3164 (12)
89.1659 (12)
2783.72 (18)
2
13.6888 (12)
14.4881 (12)
17.0849 (14)
110.5895 (18)
92.7006 (17)
105.3759 (19)
3021.2 (4)
2
b (Å)
c (Å)
a
b
g
(o)
(o)
(o)
(74.0 mg, 0.0557 mmol); 1H NMR (400 MHz, CDCl3)
d 6.67 (s, 2H),
V (Å3)
6.83 (s, 1H), 7.28e7.31 (m, 2H), 7.36 (dt, J ¼ 1.1 and 7.8 Hz, 2H),
7.41e7.49 (m, 4H), 7.56e7.59 (m, 6H), 7.89e7.93 (m, 6H), 7.95e8.00
(m, 8H), 8.45 (d, J ¼ 8.3 Hz, 2H), 8.73 (d, J ¼ 6.0 Hz, 2H); MALDI-TOF
MS m/z 1328 ([M]þ). Anal. Calcd for C65H35F12IrN2O4: C, 58.78; H,
2.66; N, 2.11. Found: C, 58.55; H, 3.08; N, 2.16.
Z
Dcalc (g cmꢃ3
)
1.630
35.278
1.532
24.954
1.460
23.003
m
(cmꢃ1
)
F (000)
T (K)
936.00
296
1288.00
296
1312.00
296
No. of unique
8721 (0.037)
12,614 (0.039)
13,320 (0.098)
0
reflections (Rint
)
2.2.5.8. Bis[1-(dibenzo[b,d]furan-4-yl)isoquinolinato-N,C3 ]iridium(III)
No. of reflections
6684
9640
6225
[1,3-bis(4-(dibenzo[b,d]furan-4-yl)phenyl)propane-1,3-dionate-O,O]
(Ir-4c). A mixture of [(C^N-4)2Ir(m-Cl)]2 (500 mg, 0.306 mmol),
HO^O-1c (276 mg, 0.496 mmol), and Na2CO3 (270 mg, 2.55 mmol) in
2-ethoxyethanol (45 mL) was heated at 100 ꢀC for 5 h under
nitrogen. Subsequent isolation and purification were carried out
used (I > 2.00
R1; wR2
GOF indicator
s(I))
0.0326; 0.0418
0.981
0.0342; 0.0385
1.086
0.0692; 0.0861
1.004
R1 ¼ SjjFoj ꢃ jFcjj/SjFoj.
wR2 ¼ [S
u
(jFoj ꢃ jFcj)2/S
u .
F2o]1/2

S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
699
3. Results and discussion
twisted from the
p-conjugation of 1,3-diphenylpropane-1,3-
dionate skeleton. This result indicates that the
p-extension is
3.1. Synthesis and structures
likely to be partially prevented due to the crystal packing forces.
All bis-cyclometalated iridium(III) complexes (Ir-1eIr-4) were
prepared according to the conventional procedure reported by
3.2. Optical properties
Thompson and coworkers [12]. That is, the preparation of the m-chloro-
bridged iridium(III) dimers followed by the ligand exchange with the
corresponding diketonates yielded the bis-cyclometalated iridium(III)
complexes. For the C^N ligands, 2-phenylpyridine (HC^N-2) was
commercially available, and HC^N-1, HC^N-3 and HC^N-4 were
prepared according to the reported procedures [20,24]. The prepara-
tion of the O^O ancillary ligand HO^O-1b was previously reported [25].
In this study, HO^O-1c was newly synthesized, as shown in Scheme 1.
All the new compounds prepared here were identified by 1H NMR and
MALDI-TOF mass spectra as well as elemental analyses.
The ligand configuration of the present bis-cyclometalated iri-
dium(III) complexes was investigated from the X-ray crystallo-
graphic analyses of Ir-1aeIr-1c. The single crystals were obtained
by slow diffusion of chloroform solutions of the complexes to
hexane. In Fig. 2 are shown the ORTEP drawings of the structures of
Ir-1aeIr-1c, and the crystal data are summarized in Table 1. The
selected bond lengths and angles around the iridium center are also
summarized in Table 2.
Fig. 2 shows that each of Ir-1aeIr-1c has a six-coordinated
structure with cis-C,C and trans-N,N configuration as is observed
for bis-cyclometalated iridium(III) complexes so far reported
[12,16,31,32]. The C-Ir and N-Ir bond lengths as well as the C-Ir-C, C-
Ir-N and N-Ir-N bond angles are almost the same among these three
complexes (within 0.041 Å for the bond lengths and within 1.53ꢀ
for the bond angles), indicating that the coordination geometry
around the Ir(III) center is almost independent on the O^O ligand.
For Ir-1b, the constituent atoms included in the 1,3-
diphenylpropane-1,3-dionate skeleton exhibited deviation within
0.196 Å from the mean plane, showing that the O^O ligand in Ir-1b
is extensively conjugated from one phenyl ring to the other. For Ir-
UVevis absorption and PL spectra of Ir-1eIr-4 were obtained in
toluene at 298 K. These spectra are shown in Figs. 3 and 4, and the
spectral data are also summarized in Table 3. All the spectra were
taken for deaerated samples, kept away from luminescence
quenching by oxygen.
As shown in Fig. 3a, Ir-1a exhibits the light absorption from the
near UV to the visible region up to 490 nm. The absorption bands at
<350 nm are assigned to the spin-allowed ligand-centered
pe
p*
transitions at the C^N ligand (1LCC^N transitions), and the next
absorption band at 384 nm is assigned to the spin-allowed MLCT
transition from Ir(III) to the C^N ligand (1MLCTC^N transition). The
lowest-energy absorption band at 466 nm is assignable to the spin-
forbidden 3MLCT transition (Ir(III) to C^N, 3MLCTC^N transition). The
relatively large molar absorption coefficient (419 Mꢃ1 cmꢃ1 at
466 nm), comparable with that of the 1MLCTC^N transition, indicates
effective mixing of the 3MLCTC^N level with the 1MLCTC^N due to the
strong spin-orbit coupling caused by the Ir(III) center. On the other
hand, the relatively intense light absorption of Ir-1b and Ir-1c was
observed in comparison with that of Ir-1a, obviously showing the
overlap of the 1LCC^N and 1MLCTC^N transition bands with the p*
pe
transition band at the p
-conjugated O^O ligand (1LCO^O transition).
For these complexes, the 3MLCTC^N bands were also observed at ca.
465 nm as shoulder peaks. It is worthy to note that the onset of the
absorption spectrum is red-shifted according to the extension of the
p-conjugation of the O^O ancillary ligand from Ir-1a to Ir-1c, indi-
cating that the lowest tripletenergylevel(T1) is loweredusing the
p
-
conjugated O^O ligands. Thus, one can see that the aromatic O^O
ligands, O^O-1b and O^O-1c, participate in the determination of T1:
The 3LCO^O and 3MLCTO^O levels should be considerably lowered
upon employment of the aromatic ancillary ligands.
1c, the
p-planes of dibenzo[b,d]furan moieties are considerably
Scheme 1. Reagents and conditions: (i) 4-iodoacetophenone, Pd(PPh3)4, Na2CO3aq, 1,2-dimethoxyethane, 70 ꢀC; (ii) ethyl 4-iodobenzoate, Pd(PPh3)4, Na2CO3aq, toluene, EtOH, 80 ꢀC;
(iii) NaH, THF, 60 ꢀC; (iv) IrCl3$3H2O, 2-ethoxyethanol, 100 ꢀC; (v) HO^O-1, Na2CO3, 2-ethoxyethanol, 100 ꢀC.

700
S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
Fig. 2. The ORTEP drawings of (a) Ir-1a, (b) Ir-1b, and (c) Ir-1c. The hydrogen atoms are omitted for clarity. The frame format of the geometry around the Ir(III) center is also
illustrated in (d).
Similar spectral features are found for the electronic absorption
spectra of Ir-2aeIr-2c (Fig. 3b), and the 3MLCTC^N transition band is
slightly red-shifted to ca. 470 nm. The onset of the absorption
spectrum is also red-shifted for Ir-2b and Ir-2c. This indicates that
the O^O ancillary ligand contributes to determination of T1 in the
series of Ir-2. For Ir-3 and Ir-4, the 3MLCTC^N transition bands are
further red-shifted (Fig. 3c,d); ca. 480 nm for Ir-3 and ca. 560 nm
for Ir-4. In both series, the spectral onset is almost constant (ca.
540 nm for Ir-3 and ca. 650 nm for Ir-4), independent on the O^O
ancillary ligand. Thus, it is likely that the O^O ligands do not have
any influences on T1 in the cases of Ir-3 and Ir-4. The most red-
shifted 3MLCTC^N transition and spectral onset of Ir-4 are obvi-
ously caused by the extensively conjugated C^N ligand.
In the PL spectrum of Ir-1a (Fig. 4a), the PL maximum (lPL) was
observed at 474 nm, showing that Ir-1a emits blue in solution. The
structured spectrum indicates that the considerable contribution of
the 3LCC^N level to PL [33]. In the case of Ir-1b, the lPL was observed
at 558 nm, shifted by 84 nm from that of Ir-1a. This red shift
brought about the PL color shift to yellow (Fig. 4a, inset). In the case
of Ir-1c, the employment of the more conjugated aromatic O^O
ligand (O^O-1c) gave rise to a further red shift of the PL spectrum
(
lPL ¼ 604 nm). The solution of Ir-1c emitted reddish orange, and
thus it is clearly noted that the O^O ancillary ligands with
p
-
extended structures significantly affect the photoluminescent
properties of the C^N-1-based bis-cyclometalated complexes. The
PL spectra of Ir-1b and Ir-1c are less structured and significantly
broadened, indicating that the electronic character of the triplet
energy levels of these complexes are different from that of Ir-1a.
3.3. Photoluminescent and photophysical properties
The detailed emission mechanisms are discussed below. The sPLs of
Ir-1aeIr-1c were determined as 0.97
m
s
(
c2 ¼ 1.03), 1.00
ms
As shown in Fig. 4, the PL spectra of Ir-1eIr-4 were obtained in
toluene at 298 K under deaerated conditions. PL lifetimes (sPLs) and
PL quantum yields (FPLs) of these complexes were also obtained,
and these data are summarized in Table 3.
(
c2 ¼1.06), and 0.21
m
s (c2 ¼ 1.09), respectively, each of which was
well fitted to single-exponential decay. Such short phosphores-
cence lifetimes are effective to suppress triplet-triplet annihilation
in phosphorescent OLEDs [34]. The FPLs of Ir-1aeIr-1c were also
determined as 0.91, 0.31, and 0.083 (in toluene at 298 K), respec-
tively, significantly decreasing as the lPL is red-shifted. The PL
spectral profiles of Ir-2 also varied with the O^O ancillary ligands
(Fig. 4b), as discussed for Ir-1. The lPLs of Ir-2aeIr-2c were observed
at 521, 633, and 661 nm, respectively, significantly red-shifted from
bluish green to deep red when the aromatic O^O ligands were
Table 2
Selected bond distances and angles around the Ir(III) center for Ir-1aeIr-1c.
Ir-1a
Ir-1b
Ir-1c
Distance (Å)
Angle (ꢀ)
IreO1
IreO2
IreN1
IreC1
IreN2
IreC2
O1eIreO2
N1eIreC1
N2eIreC2
2.088 (4)
2.098 (2)
2.037 (3)
2.011 (3)
2.020 (3)
2.008 (9)
88.94 (13)
80.57 (15)
81.32 (19)
2.104 (2)
2,131 (2)
2.038 (3)
2.049 (3)
2.033 (3)
2.033 (3)
88.87 (9)
80.31 (14)
80.84 (15)
2.098 (9)
2.104 (7)
2.039 (9)
2.032 (12)
2.004 (9)
2.016 (12)
90.2 (3)
employed. The sPLs of Ir-2aeIr-2c were determined as 0.29 ms
(
c2 ¼1.04), 0.02
m
s (c2 ¼1.10), and 0.52
m
s (c2 ¼1.08), respectively,
well fitted to single-exponential decay. Although the sPL of Ir-2b
was somewhat short, the energy gap more than 1.0 eV between the
spin-allowed transition and the lPL clearly indicates that the
80.7 (4)
81.7 (4)

S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
701
Fig. 3. UVevis absorption spectra of (a) Ir-1, (b) Ir-2, (c) Ir-3, and (d) Ir-4 in deaerated toluene at 298 K.
observed emission is phosphorescence. The FPL of Ir-2aeIr-2c were
determined as 0.14, 0.006, and 0.003, respectively, significantly
decreasing as the lPL is red-shifted.
On the other hand, different tendencies of PL spectral behavior
from Ir-1 and Ir-2 were observed for Ir-3 and Ir-4. In Fig. 4c are
shown the PL spectra of Ir-3aeIr-3c. Only slight red shifts were
observed for the PL spectra even when the aromatic O^O ancillary
ligands (lPLs of Ir-3aeIr-3c; 541, 543, and 566 nm, respectively).
The PL color shifts are less remarkable, varying from green to
yellow (Fig. 4c, inset). The PL spectrum of Ir-3b is similar to that of
Ir-3a, indicating that the triplet energy levels of these complexes
are almost the same in spite of the ligand replacement from O^O-1a
to O^O-1b. The
profiles, ranging from 1.09
s
PLs of Ir-3aeIr-3c show single-exponential decay
m
s to 1.25 s with
m
c
2 values of 1.00e1.03.
Contrary to Ir-1 and Ir-2, each of Ir-3aeIr-3c exhibits a relatively
high FPL (0.43, 0.51, and 0.53 for Ir-3aeIr-3c, respectively),
Fig. 4. Photoluminescence spectra of (a) Ir-1, (b) Ir-2, (c) Ir-3, and (d) Ir-4 in deaerated toluene at 298 K.

702
S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
Table 3
Table 4
UVevis absorption and photoluminescence spectral data of Ir-1eIr-4 in toluene at
Photoluminescence spectral data in various solvents at 298 K.a
298 K.
b
Solvent
εr
lPL (nm)
Compd UVevisa
Photoluminescencea
,
b
,b
Ir-1a
Ir-1b
Ir-1c
labs (nm), ε (Mꢃ1 cmꢃ1
)
lPL (nm) sPL s (c
(
m
2)) FPL
Toluene
2.43
473
474
474
474
474
557
558
560
559
558
604
607
607
609
609
Ir-1a
Ir-1b
Ir-1c
342 (7700), 384 (5800),
466 (419)
345 (26,400), 385 (27,400),
463 [sh] (881)
342 (40,700), 383 (24,600),
467 [sh] (1360)
341 (7470), 467 (2550)
348 (28,500), 409 [sh] (13,900),
466 [sh] (4120)
474, 506 0.97 (1.03) 0.91c
558 1.00 (1.06) 0.31
604 0.21 (1,09) 0.083
Chloroform
1,2-Dichloroethane
Ethanol
4.89
10.74
25.00
36.00
Acetonitrile
a
The concentration of the complex was adjusted to 10 mM.
b
εr: relative permittivity.
Ir-2a
Ir-2b
521 0.29 (1.04) 0.14
633 0.02 (1.10) 0.006
between 0.1e100 mM. Therefore, from the results of the verification
experiments, the O^O ligand effect on PL seen in Ir-1 and Ir-2 should
be attributed tothe differences in theirown electronic structures, i.e.
their T1 characters.
Here we show the most possible mechanism of the PL behavior
of the present bis-cyclometalated iridium(III) complexes upon
employment of O^O-1aeO^O-1c as ancillary ligands (Fig. 5). Upon
the 1MLCT excitation from the Ir(III) center to the C^N ligand in the
singlet manifold, highly efficient intersystem crossing (ISC) to the
3MLCTC^N level occurs due to the strong spin-orbit coupling. The
exciton at the 3MLCTC^N level might be transferred to the emissive
Ir-2c
Ir-3a
Ir-3b
Ir-3c
Ir-4a
Ir-4b
Ir-4c
344 (41,600), 407 (16,000),
466 [sh] (2480)
361 (5330), 393 (4390),
479 (2560)
336 (19,400), 383 (19,500),
480 (2800)
333 (29,400), 392 (17,200),
\482 [sh] (2940)
338 (39,400), 421 (6560), 493 (5460),
557 [sh] (3160)
342 (51,400), 492 (5420),
558 [sh] (3440)
338 (53,700), 489 (6010),
560 [sh] (3270)
661 0.52 (1.08) 0.003
541 1.14 (1.03) 0.43
543 1,25 (1.00) 0.51
566 1.09 (1.00) 0.53
639d 1.04 (1.00)d 0.55d
640d 1.07 (1.00)d 0.61d
637 0.41 (1.00) 0.20
a
Obtained in toluene solution at 298 K. [sh]; shoulder peak.
b
c
[Ir(III) complex] ¼ 10
mM.
Reported previously in ref. [26].
Reported previously in ref. [20].
d
regardless of the O^O ancillary ligand. It is interesting that the PL
spectra of Ir-4aeIr-4c (Fig. 4d) are almost the same lPL
(
;
637e640 nm), indicating that the O^O ancillary ligands do not
affect the PL spectral profiles of these complexes. As shown in the
inset in Fig. 4d, the emission color of Ir-4 is pure red. The sPLs of Ir-
4aeIr-4c fitted to single-exponential decay were 1.04 ms
(
c2 ¼ 1.00), 1.07
m
s (c2 ¼1.00), and 0.41
m
s (c2 ¼ 1.00), respectively.
The FPLs of Ir-4aeIr-4c were determined as 0.55, 0.61, and 0.20 (in
toluene at 298 K), respectively. The relatively small FPL of Ir-4c
indicates that the O^O ancillary ligand O^O-1c affects the photo-
physical properties at some extent.
3.4. Ancillary ligand effect on photoluminescence behavior
As shown above, the replacement of aromatic O^O ancillary
ligand from O^O-1a to O^O-1b significantly affects the PL colors of
bis-cyclometalated complexes Ir-1 and Ir-2. Here we discuss the
mechanism of the ancillary ligand effect on the PL properties of Ir-
1eIr-4. In general, several factors should be taken into consider-
ation when PL spectral changes upon ligand replacement are dis-
cussed. The first factor is the configurational difference among Ir-
1aeIr-1c [7,35]. As discussed in the X-ray crystallographic analysis,
however, all these complexes have the same geometric structure,
the six-coordinated cis-C,C and trans-N,N configuration. Thus, the
configuration should not be a factor for the spectral differences
caused by the O^O ancillary ligands. The second is the solvent effect
on the PL spectra [36]. In order to verify this factor, the PL spectra of
Ir-1aeIr-1c were obtained in solvents with different polarity. The
results are summarized inTable 4. As a result, no remarkable spectral
differences were observed in solvents used here, indicating that the
solvent polarity should not afford drastic PL spectral differences in
the present series of bis-cyclometalated iridium(III) complexes. The
third is the generation of the excimer-based emission upon
extending the p-conjugation in the O^O ancillary ligand from O^O-
1a to O^O-1b and O^O-1c [37,38]. However, no spectral changes
were observed when the concentrations of the complexes varied
Fig. 5. Possible mechanisms of the O^O ligand effects on PL for (a) Ir-1, (b) Ir-3 and (c)
Ir-4.

S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
703
Fig. 6. Photoluminescence spectra in PMMA films doped with (a) Ir-1, (b) Ir-2, (c) Ir-3, and (d) Ir-4. In a sample film, the ratio of PMMA to the iridium complex was adjusted to 100/4
(wt/wt).
O^O-related triplet state (3LCO^O and/or 3MLCTO^O) when it is
placed below the 3MLCTC^N level (inter-ligand energy transfer,
ILET). In the aliphatic diketonate ancillary ligand O^O-1a, the
3LCO^O and 3MLCTO^O levels should be unstabilized and enough
high to prevent the exciton transfer from the 3MLCTC^N level
(Fig. 5a, path A), and thus the observed PL is predominantly based
on the radiative decay from the 3MLCTC^N level. On the other hand,
the 3LCO^O and 3MLCTO^O levels of the aromatic O^O ancillary
ligands such as O^O-1b and O^O-1c are expected to be lower than
that of O^O-1a because of the increasing stability by the extended
for Ir-1eIr-4. Poly(methyl methacrylate) (PMMA) was chosen as
a matrix polymer because the absence of near UV-to-visible absorp-
tion allows us to estimate the intrinsic FPLs of the iridium complexes.
The sample films were fabricated by a spin-coating method. In
a sample film, the ratio of PMMA to the iridium complex was adjusted
to 100/4 (wt/wt). The obtained PL spectra are shown in Fig. 6, and all
the data are summarized in Table 5. In the series of Ir-1 (Fig. 6a), few
spectral changes were observed for Ir-1a in comparison with the
spectrum in solution shown in Fig. 4a, whereas significant blue shifts
of lPL by 32 and 64 nmwere observed for Ir-1b and Ir-1c, respectively.
In the series of Ir-2, similar behavior was observed, where Ir-2b and
Ir-2c exhibited blueshifts by 81and 38 nm, respectively, inthePMMA
films. These blue shifts should be due to so-called rigidochromism
[39-41] that is likely to be caused by the aromatic O^O ligands. In the
case of Ir-1a and Ir-2a, the C^N ligand mainly contributes to the T1
level, asshowninFig. 5, andthusthevectorialchangeofthemolecular
dipole is relatively small upon photo-excitation. On the other hand,
the T1 levels of the iridium complexes with aromatic O^O ancillary
p-conjugation. This is clearly indicated by the red-shifted UVevis
onsets of Ir-1b and Ir-1c, as discussed in Section 3.2. Therefore, as
the 3MLCTC^N level is relatively high, the O^O-related triplet levels
are placed below the 3MLCTC^N level. As a result, the ILET occurs
(Fig. 5a, path B), yielding the red-shift of the PL spectrum [21]. In Ir-
2, the PL spectral differences caused by the ancillary ligands are
attributed to the similar reason. In the case of red emissive Ir-4, any
PL red shifts were not observed when the ancillary ligand was
replaced from O^O-1a to O^O-1b and O^O-1c. This can be
explained by the low-lying 3MLCTC^N level caused by the
p-
extended C^N ligand of Ir-4. The ILET from the 3MLCTC^N level to the
3LCO^O (or 3MLCTO^O) level is a less probable process, and the
phosphorescent emission is based on the radiative decay from the
3MLCTC^N level (Fig. 5c). According to this mechanism, the 3LCO^O
(or 3MLCTO^O) level of Ir-3b is likely to lie at an energy level
comparable to the 3MLCTC^N level, and thus any remarkable PL
color changes do not occur even when O^O-1a is replaced by O^O-
1b (Fig. 5b, path A). In the case of Ir-3c, the O^O-related triplet level
should be slightly lower than the 3MLCTC^N level, and the red-shift
of PL is modest upon replacing O^O-1a to O^O-1c (Fig. 5b, path B).
Table 5
,b
Photoluminescence spectral data of Ir-1eIr-4 in PMMA films.a
Compd
lPL (nm)
FPL
Ir-1a
Ir-1b
Ir-1c
Ir-2a
Ir-2b
Ir-2c
Ir-3a
Ir-3b
Ir-3c
Ir-4a
Ir-4b
Ir-4c
473, 505
526
0.51
0.61
0.40
0.33
0.058
0.038
0.49
0.47
0.29
0.25
0.25
0.11
540
518
552
623
538
540
536
632
3.5. Photoluminescent properties in polymer thin films
635
630
a
From the viewpoint of application to solid-state electrolumines-
centdevices,thePLpropertiesinpolymerthinfilmswereinvestigated
Obtained in PMMA films at 298 K.
The ratio of PMMA/Ir(III) complex was adjusted to 100/4 (wt/wt).
b

704
S. Ikawa et al. / Dyes and Pigments 95 (2012) 695e705
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ligands such as Ir-1b,c and Ir-2b,c are based on the O^O ligands, that
should bring about drastic vectorial changes of the molecular dipoles
upon photo-excitation. As the PMMA molecules around the iridium
complexes do not change their dipoles in the solid state, so the O^O-
related T1 levels of Ir-1b,c and Ir-2b,c should be significantly unsta-
bilized in the PMMA films. Therefore, these complexes exhibit
remarkable rigidochromic behavior. It is interesting that Ir-1 and Ir-2
afford larger FPLs in PMMA matrices than in toluene solutions, except
for Ir-1a. Especially, theFPL of Ir-1c wasgreatlyenlargedfrom 0.083to
0.40. ThesePLspectralfeaturesgive greatadvantages inapplication to
OLEDs.
In the series of Ir-3 and Ir-4, the ancillary ligands do not affect
their PL spectra in the PMMA films, except for Ir-3a, because their
T1 levels are based on considerable contributions from the C^N
ligands rather than the O^O ligands, as shown in Fig. 5b and Fig. 5c,
and thus any rigidochromic behavior was not observed. In the case
of Ir-3a, one can see that the 3MLCTC^N level is set at the T1 level due
to destabilization of the O^O-related triplet level, and thus the PL
spectrum of Ir-3a is almost similar to those of Ir-3b and Ir-3c. The
FPLs of Ir-3 and Ir-4 in PMMA films are not more than those in
toluene solutions although the FPL of Ir-3a is exceptionally
enhanced from 0.43 to 0.49.
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[17] Tsuboyama A, Iwawaki H, Furugori M, Mukaide T, Kamatani J, Igawa S, et al.
Homoleptic cyclometalated iridium complexes with highly efficient red
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A trinuclear bright red luminophore containing cyclometallated Ir(III) motifs.
Chemical Communications 2011;47:10836e8.
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4. Conclusions
Photoluminescent properties of bis-cyclometalated iridium(III)
complexes bearing four types of C^N ligands Ir-1eIr-4 were investi-
gated, especially focusingonthe influence of the O^O ancillaryligands
on the PL color tuning. In the case of Ir-1 and Ir-2, PL spectra were
significantly red-shifted by employing the aromatic O^O ancillary
ligands (Ir-1; from 474 to 604 nm, Ir-2; from 521 to 661 nm), and the
PL color was tuned from blue to deep red. In the case of Ir-3, the red
shifts by the aromatic O^O ancillary ligands were quite modest (from
541 to 566 nm), and for red emissive Ir-4, any O^O ligand effects on
the PL color were not found. These results can be explained by the
relationship between the 3MLCTC^N and O^O-related triplet levels. In
general, the 3MLCTC^N level is T1 in typical bis-cyclometalated iri-
dium(III) complexes with aliphatic O^O ligands such as acetylaceto-
nate and dipivaloylmethanate, whereas the employment of the
aromatic O^O ancillary ligands lowers the 3LCO^O and/or 3MLCT levels
below the 3MLCTC^N level. As a result, the PL spectrum is red-shifted
by the ILET from the 3MLCTC^N to the O^O-related T1 level. Thus, the
color tuning by the O^O ancillary ligand is possible. We also investi-
gated the PL properties of Ir-1eIr-4 in PMMA films. For Ir-1 and Ir-2,
the complexes with the aromatic ancillary ligands O^O-1b and O^O-
1c showed remarkable rigidochromic behavior yielding considerable
blue shifts of lPLs in comparison with those in solutions. These results
should becaused by destabilizationof theO^O-related T1 levels. Some
of the present iridium complexes exhibited the enhancement of their
FPLs in the PMMA films. We are currently investigating the electro-
phosphorescence behavior of these complexes by fabricating
polymer-basedOLEDs(PLEDs), and theO^O ligand effectsonthePLED
performance will be discussed elsewhere.
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b-diketonate
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