Tuning iridium(III) complexes containing 2-benzo[b]thiophen-2-yl-pyridine based ligands in the red region

Article abstract of DOI:10.1016/j.ica.2007.03.023

Synthesis and characterization of four iridium(III) complexes containing 2-benzo[b]thiophen-2-yl-pyridine based ligands are reported. The absorption, emission, electrochemistry, and thermostability of the complexes were systematically investigated. The (btmp)₂Ir(acac) (btmp = 2-benzo[b]thiophen-2-yl-4-methyl-pyridyl, acac = acetyl acetone) was characterized using X-ray crystallography. Calculation on the electronic ground state for (btmp)₂Ir(acac) was carried out using B3LYP density functional theory, HOMO levels are a mixture of Ir and btmp ligand orbitals, while the LUMO is predominantly btmp ligand based. Introduction of substituents (CH₃, CF₃) into pyridyl ring in a typical red emitter (btp)₂Ir(acac) leads to a marked decrease in the sublimation temperature, which is more suitable for OLEDs process. Electrochemical studies showed that (btmp)₂Ir(acac) has a slightly lower oxidation potential, but (btfmp)₂Ir(acac), (btfmp)₂Ir(dbm), and (btfmp)₂Ir(pic) (btfmp = 2-benzo[b]thiophen-2-yl-5-trifluoromethyl-pyridine, dbm = dibenzoylmethane, pic = 2-picolinic acid) containing CF₃ group are much difficult to oxidate than (btp)₂Ir(acac). The emission characteristics of these complexes can be tuned by either changing the substituents and their position on 2-benzo[b]thiophen-2-yl-pyridine or using different monoanionic ligands, showing emission λ_max values from 604 to 638 nm in CH₂Cl₂ solution at room temperature.

Full text of DOI:10.1016/j.ica.2007.03.023

Inorganica Chimica Acta 360 (2007) 3149–3154
www.elsevier.com/locate/ica
Tuning iridium(III) complexes containing
2-benzo[b]thiophen-2-yl-pyridine based ligands in the red region
a,
a
a
a
a
b
MaoLiang Xu *, GeYang Wang , Rui Zhou , ZhongWei An , Qun Zhou , WenLian Li
a
Xi’an Modern Chemistry Research Institute, Xi’an 710065, PR China
Key Laboratory of Excited State processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,
Changchun 130033, PR China
b
Received 5 August 2005; received in revised form 11 March 2007; accepted 11 March 2007
Available online 23 March 2007
Abstract
Synthesis and characterization of four iridium(III) complexes containing 2-benzo[b]thiophen-2-yl-pyridine based ligands are
reported. The absorption, emission, electrochemistry, and thermostability of the complexes were systematically investigated. The
(btmp)2Ir(acac) (btmp = 2-benzo[b]thiophen-2-yl-4-methyl-pyridyl, acac = acetyl acetone) was characterized using X-ray crystallogra-
phy. Calculation on the electronic ground state for (btmp)2Ir(acac) was carried out using B3LYP density functional theory, HOMO
levels are a mixture of Ir and btmp ligand orbitals, while the LUMO is predominantly btmp ligand based. Introduction of substit-
uents (CH3,CF3) into pyridyl ring in a typical red emitter (btp)2Ir(acac) leads to a marked decrease in the sublimation temperature,
which is more suitable for OLEDs process. Electrochemical studies showed that (btmp)2Ir(acac) has a slightly lower oxidation poten-
tial, but (btfmp)2Ir(acac), (btfmp)2Ir(dbm), and (btfmp)2Ir(pic) (btfmp = 2-benzo[b]thiophen-2-yl-5-trifluoromethyl-pyridine,
dbm = dibenzoylmethane, pic = 2-picolinic acid) containing CF3 group are much difficult to oxidate than (btp)2Ir(acac). The emission
characteristics of these complexes can be tuned by either changing the substituents and their position on 2-benzo[b]thiophen-2-yl-pyr-
idine or using different monoanionic ligands, showing emission kmax values from 604 to 638 nm in CH2Cl2 solution at room
temperature.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Cyclometalated iridium complex; Red phosphorescent emitting material; Single crystal structure
1. Introduction
phosphorescent dye (btp)2Ir(acac), bis(2-benzo[b]thio-
0
phen-2-yl-pyridinato-N,C3 )iridium(III) acetylacetonate,
which was employed as a dopant in devices with
Phosphorescent materials have attracted extensive atten-
tion in recent years owing to their potential advantages of
harvesting excitons from both singlet and triplet states in
the OLEDs [1–6]. Cyclometalated iridium(III) complexes
received the most wide study partly due to tuning color
emission from red to blue by using different ligands [7–9].
Many of green phosphorescent materials were obtained
and successfully used in organic electrophosphorescent
devices [10–13]. Forrest and co-workers reported a red
gex = 7% [14]. But for the red phosphorescent dye, since
the luminescence quantum yields tend to decrease with an
increase in the emission peak wavelength according to the
energy gap law [15], it is difficult to find a good perfor-
mance material for OLEDs, there are only a limited num-
ber of high emissive phosphors suitable to red devices. In
this study, we designed and synthesized four new red phos-
phorescent iridium complexes based on (btp)2Ir(acac) skel-
eton, the emission characteristics of these complexes can be
tuned by either changing the substituents and their position
on 2-benzo[b]thiophen-2-yl-pyridine or using different
monoanionic ligands.
*
Corresponding author. Tel.: +86 29 88294395; fax: +86 29 88294186.
E-mail address: maoliangxu@yahoo.com.cn (M.L. Xu).
0020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.03.023

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M.L. Xu et al. / Inorganica Chimica Acta 360 (2007) 3149–3154
Table 1
2. Experimental
Crystal data and structure refinement for (btmp)2Ir(acac)
Identification code
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Crystal system, space group
Unit cell dimensions
(btmp)2Ir(acac)
C33H27IrN2O2S2
739.89
296(2)
0.71073
2-Chloro-5-trifluoromethyl-pyridine, 2-bromo-4-methyl-
pyridine, benzo[b]thiophene-2-boronic acid, IrCl3 Æ nH2O,
Pd(PPh3)4, acetylacetone, dibenzoylmethane, and 2-picoli-
nic acid were from Aldrich, and used without further
purification. All procedures were carried out in inert gas
˚
monoclinic, P2ð1Þ=n
1
atmosphere despite the air stability of the compounds. H
˚
a (A)
11.2185(10)
16.6343(14)
16.0375(14)
90
100.813(2)
90
2939.7(4)
4, 1.672
4.717
1456
0.28 · 0.16 · 0.13
1.78–25.00
ꢀ13 6 h 6 11, ꢀ19 6 k 6 19,
ꢀ12 6 l 6 19
14746/5167 [0.0248]
99.9
NMR spectra were recorded on Bruker Avance 300 MHz
instrument. Elemental analyses were performed on VarioEL
III CHNS instrument. Absorption spectra were measured
on UV–Vis–NIR scanning spectrophotometer. Photolumi-
nescence (PL) spectrum was measured with a F-4500
Fluorescence Spectrometer. Thermogravimetric analysis
(TGA) was performed by a TGA 2950 thermal analyzer
(TA Co.) under N2 stream with a scanning rate of 10 ꢁC/
min.
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z, Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
Crystal size (mm)
h Range for data collection (ꢁ)
Limiting indices
)
2.1. Electrochemistry
Reflections collected/unique [Rint
Completeness to h = 25.00 (%)
Absorption correction
Maximum and minimum
transmission
]
Cyclic voltammetry experiments were performed with a
MCP-1 electrochemical analyzer (JiangFen co., china). All
measurements were carried out at room temperature with a
conventional three-electrode configuration consisting of a
platinum working electrode, a Pt wire auxiliary electrode,
and Ag/AgCl reference electrode. The E1/2 values were
determined as 1=2ðEap þ Epc Þ, where Epa and Epc are the ano-
dic and cathodic peak potentials, respectively. All poten-
tials reported are not corrected for the junction potential.
The solvent was CH2Cl2 and the supporting electrolyte
was 0.1 M tetra(n-butyl)ammonium hexafluorophosphate.
multi-scan
0.5791 and 0.3518
Refinement method
full-matrix least-squares on F2
5167/0/365
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
1.009
R1 = 0.0219, wR2 = 0.0484
R1 = 0.0308, wR2 = 0.0509
Largest difference in peak and hole 0.898 and ꢀ0.332
ꢀ3
˚
(e A
)
(0.68 g), toluene (60 ml), ethanol (20 ml), and 2 M Na2CO3
(60 ml) were heated to reflux for 6 h. It was then cooled,
water was added, extracted with toluene. The combined
organic layer was washed with water, dried over MgSO4,
filtered, and the filtrate was evaporated under reduced pres-
sure, the residue was chromatographed using n-hexane/
dichloromethane (1:1) as eluent to afford the white powders
in 73% yield.
2.2. X-ray crystallography
Intensity data for (btmp)2Ir(acac) were collected at
room temperature (T = 296 K) on a BRUKER SMART
APEX II CCD diffractometer with graphite-monochro-
˚
mated Mo Ka radiation (k = 0.71073 A). Data were cor-
rected for LP factors and absorption was applied by
using the SADABS program. The structure was solved by
direct methods and refined by full-matrix least-squares
refinement on F2 using the SHELXTL-97 program package.
All non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were placed at calculated positions and
refined using riding model with C–H distances in the range
Hbtmp: White powders. Yield: 73%. 1H NMR
(300 MHz, CDCl3) d (ppm): 8.42 (d, 1H), 7.78 (m, 3H),
7.52 (s, 1H), 7.27 (t, 2H), 6.92 (d, 1H), 2.31 (s, 3H). Anal.
Calc.: C, 74.67; H, 4.889; N, 6.222. Found: C, 74.66; H,
4.822; N, 5.965%.
Hbtfmp: White powders, Yield: 75%. 1H NMR
(300 MHz, CDCl3) d (ppm): 8.80 (s, 1H), 7.80 (m, 5H),
7.3 (t, 2H). Anal. Calc.: C, 60.22; H, 2.867; N, 5.018.
Found: C, 60.09; H, 2.693; N, 4.913%.
˚
0.93–0.96 A and Uiso(H) = 1.2Ueq(C) or 1.5Ueq(methyl C).
A summary of crystal data and refinement parameters is
given in Table 1.
2.3. Synthesis and characterization of ligands
2.4. Synthesis and characterization of iridium complex
2-Benzo[b]thiophen-2-yl-5-trifluoromethyl-pyridine (Hbtfmp)
[16] and 2-benzo[b]thiophen-2-y- 4-methyl-pyridine (Hbtmp)
were synthesized according to the literature procedure
[17]. A detailed description is provided only for Hbtmp.
2-Bromo-4-methyl-pyridine (4.4 g, 0.0195 mol), benzo[b]-
thiophene-2-boronic acid (3.47 g, 0.0195 mol), Pd(PPh3)4
Iridium(III) complexes were obtained in two steps using
standard procedure [18]. These iridium complexes were
prepared by a similar procedure, only the synthesis of
(btmp)2Ir(acac) is presented in detail. A mixture of 1.00 g
(2.835 mmol) of IrCl3 Æ nH2O, 1.27 g (5.670 mmol) of

M.L. Xu et al. / Inorganica Chimica Acta 360 (2007) 3149–3154
3151
Hbtmp, 80 ml of 2-ethoxyethanol, and 20 ml of water was
refluxed at 120 ꢁC for 24 h. After cooling down to room
temperature, the chloride-bridged dimer was isolated by
addition of water followed by filtration and methanol
wash. The dimer was not further characterized and was
used directly in subsequent reaction. A mixture of 1.70 g
(1.270 mmol) of the dimer, 0.32 g (3.182 mmol) of acetylac-
etone, 1.2 g Na2CO3, and 100 ml of 2-ethoxyethanol was
refluxed for 16 h. After cooling down to room temperature,
the solvent was removed by vacuum, the crude product was
washed with methanol to remove any unreacted acetylace-
tone. The red powder was afforded in 40% yield with sub-
limation in a vacuum of 100 Pa. The structures and
abbreviations of Ir complexes used in this study are shown
in Fig. 1.
7.35 (d, 2H), 7.23 (m, 4H), 7.13 (t, 2H), 6.85 (t, 2H), 6.55
(s, 1H), 6.25 (d, 2H). Anal. Calc.: C, 53.13; H, 2.574; N,
2.883. Found: C, 52.85; H, 2.529; N, 2.831%.
3. Results and discussion
3.1. Crystal structure of (btmp)2Ir(acac)
Single crystals of (btmp)2Ir(acac) were grown from sub-
limation and characterized using crystallography. The
ORTEP drawing of the complex is shown in Fig. 2.
Selected parameters of the molecular structure are listed
in Table 2. The complex has a distorted octahedral coordi-
nation geometry around iridium atom and the two nitrogen
atoms of btmp ligands exhibit cis-C–C and trans-N–N che-
˚
(btmp)2Ir(acac): bis(2-benzo[b]thiophen-2-yl-4-methyl-
late dispositions. The Ir–C bonds (Ir–Cav = 2.003(3) A) are
0
pyridinato-N,C3 )iridium(III) (acetylacetonate) (red pow-
der, yield 45%). 1H NMR (300 MHz, CDCl3) d (ppm):
8.20 (d, 2H), 7.55 (d, 2H), 7.37 (s, 2H), 6.97 (t, 2H), 6.77
(m, 4H), 6.20 (d, 2H), 5.17 (s, 1H),2.55 (s, 6H), 1.70 (s,
6H). Anal. Calc.: C, 52.93; H, 3.521; N, 3.588. Found: C,
52.67; H, 3.448; N, 3.539%.
shorter than the Ir–N bonds (Ir–Nav = 2.046(3) A). The Ir–
˚
C bonds of (btmp)2Ir(acac) are equal to that of (ppy)2Ir(a-
˚
cac) (Ir–C, 2.003(9) A), while the Ir–N bonds are longer
˚
than the value of the latter (Ir–N, 2.010(9) A), but the Ir–
˚
˚
O bonds (Ir–Oav = 2.126 A) are 0.020 A shorter than the
value of (ppy)2Ir(acac) [7]. The N(1)–Ir(1)–N(2)
(177.59(11)ꢁ), C(1)–Ir(1)–N(1) (80.00(12)ꢁ), C(18)–Ir(1)–
N(2) (80.13(13)ꢁ), N(1)–Ir(1)–O(1) (94.77(10)ꢁ), C(18)–
Ir(1)–O(1) (89.61(11)ꢁ), and O(1)–Ir(1)–O(2) (89.13(10)ꢁ)
bond angles are typical for cyclometalates and b-diketo-
nate derivatives of Ir [7,19,20]. All other features appear
to be normal.
(btfmp)2Ir(acac) [16]: bis(2-benzo[b]thiophen-2-yl-5-
0
trifluoromethyl-pyridinato-N,C3 )iridium(III) (acetylaceto-
nate) (red powder, yield 65%). 1H NMR (300 MHz,
CDCl3) d (ppm): 8.56 (s, 2H), 7.65 (d, 2H), 7.60 (t, 2H),
7.10 (m, 4H), 6.79 (t, 2H), 6.16 (d, 2H), 5.25 (s, 1H), 1.75
(s, 6H). Anal. Calc.: C, 46.85; H, 2.485; N, 3.313. Found:
C, 46.56; H, 2.455; N, 3.277%.
(btfmp)2Ir(pic): bis(2-benzo[b]thiophen-2-yl-5-trifluo-
3.2. DFT calculations
0
romethyl-pyridinato-N,C3 )iridium(III) picolinate (red
powder, 40%). 1H NMR (300 MHz, CDCl3) d (ppm):
8.79 (s, 1H), 8.44 (t, 2H), 8.17 (m, 4H), 7.96 (m, 2H),
7.71 (m, 2H), 7.46 (s, 1H), 7.26 (m, 2H), 6.98 (m, 2H),
6.15 (d, 1H), 5.91 (d, 1H). Anal. Calc.: C, 48.33; H,
2.488; N, 4.284. Found: C, 47.91; H, 2.316; N, 4.305%.
DFT calculation on the electronic ground state of
(btmp)2Ir(acac) was carried out at the B3LYP/LANL2DZ
level [21,22]. A similar approach has been used to investigate
the properties of Ir(ppy)3 and (ppy)2Ir(acac) [23]. The
HOMO and LUMO energies were determined using mini-
mized singlet geometries to approximate the ground state.
The results of the optimized structure for (btmp)2Ir(acac)
are summarized in Table 2, which are compared with
X-ray diffraction studies. The calculated Ir–Cav bond length
(btfmp)2Ir(dbm): bis(2-benzo[b]thiophen-2-yl-5-trifluo-
0
romethyl-pyridinato-N,C3 )iridium(III)
(dibenzoylme-
thane) (red powder, yield 58%). 1H NMR (300 MHz,
CDCl3) d (ppm): 8.64 (s, 2H), 7.86 (d, 2H), 7.64 (d, 8H),
CF3
N
CH3
N
O
O
O
O
Ir
Ir
S
S
2
2
(btfmp)2Ir(dbm)
(btmp)2Ir(acac)
CF
CF3
N
3
N
O
O
O
O
N
O
O
Ir
C
S
Ir
Ir
S
S
N
2
2
2
(btp)2Ir(acac)
(btfmp)2Ir(acac)
(btfmp)2Ir(pic)
Fig. 1. Iridium complexes and their abbreviations in this study.
Fig. 2. ORTEP diagram of (btmp)2Ir(acac).

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M.L. Xu et al. / Inorganica Chimica Acta 360 (2007) 3149–3154
Table 2
Table 3
Comparison of calculated bond lengths for (btmp)2Ir(acac) with exper-
imental values from X-ray diffraction
Composition (%) of HOMO and LUMO for (btmp)2Ir(acac)
MOs
Ir
Benzo[b]thiophene Pyridyl
Methyl on
pyridine
acac
Calculated
Experimental
ring
HOMO 31.2 51.5
LUMO 6.1 32.1
ring
˚
Bond lengths (A)
14.4
57.0
0.08
2.3
3.4
2.4
Ir(1)–C(7)
Ir(1)–C(21)
Ir(1)–N(1)
Ir(1)–N(2)
Ir(1)–O(1)
Ir(1)–O(2)
S(1)–C(1)
S(1)–C(8)
S(2)–C(15)
S(2)–C(22)
2.0181
2.0181
2.0675
2.0675
2.1697
2.1697
1.8172
1.8259
1.8172
1.8259
2.006(3)
1.999(3)
2.046(3)
2.047(3)
2.124(2)
2.129(2)
1.731(4)
1.745(3)
1.743(4)
1.746(4)
train sublimation equipment. Lower sublimation tempera-
ture is beneficial to materials purity and devices fabrica-
tion. The vaporization temperature was determined by a
TGA thermal analyzer under N2 stream with a scanning
rate of 10 ꢁC/min. The 10% weight loss temperatures
(DT10%) of these iridium complexes are in the range of
338–348 ꢁC, while the value of an ordinary red dye
(btp)2Ir(acac) is 368 ꢁC, indicating introduction of substit-
uents (CH3,CF3) into pyridyl ring in (btp)2Ir(acac) leading
to a marked decrease in the vaporization temperature,
which is more suitable to materials purity and devices fab-
rication [24,25].
Bond angles (ꢁ)
N(1)–Ir(1)–N(2)
N(1)–Ir(1)–C(1)
N(2)–Ir(1)–C(18)
N(1)–Ir(1)–O(1)
C(18)–Ir(1)–O(1)
O(1)–Ir(1)–O(2)
178.20
80.36
80.36
92.93
90.14
86.22
177.59(11)
80.00(12)
80.13(13)
94.77(10)
89.61(11)
89.13(10)
3.4. Cyclic voltammetric studies
˚
˚
of 2.0181 A is 0.0151 A longer than the experimental value.
˚
The Ir–Nav bond length of 2.0675 A and Ir–Oav bond length
The electrochemical properties of these complexes were
characterized using cyclic voltammetry, the redox data are
given in Table 4. All studied complexes showed reversible
oxidation waves in the range of 680–1073 mV versus Ag/
AgCl, which can be attributed to the oxidation of the irid-
ium(III). No reduction waves were measured in the com-
plexes up to ꢀ3.0 V. The oxidation wave value of
(btmp)2Ir(acac) with a electron-donating group (CH3) on
the 4-positon of pyridyl ring is 680 mV, which is lower
than that of (btp)2Ir(acac) (720 mV, in this measurement
condition), while the (btfmp)2Ir(acac), (btfmp)2Ir(dbm),
and (btfmp)2Ir(pic) containing electron-withdrawing
groups (CF3) on 5-positon of pyridyl ring have higher val-
ues (941–1073 mV) relative to that of (btp)2Ir(acac). So,
incorporating electron-donating group on the 4-positon
of pyridyl ring of ligand gives a decrease in the oxidation
potential, while introduction of electron-withdrawing
groups into the 5-positon of pyridyl ring of ligand leads
to a marked increase in value. Meanwhile, different ancil-
lary ligands also lead to changing oxidation potentials of
iridium complexes, the ligand of 2-picolinic acid contain-
ing ‘deficient electron’ atom N makes the iridium(III)
more difficult to oxidate than that of b-diketonates. This
is consistent with our DFT calculation, the HOMO orbi-
tals of (btmp)2Ir(acac) are a mixture of Ir and btmp,
incorporation of substituents on cyclometalated ligands
was shown to affect the oxidation potential of the iridium
ion.
˚
˚
of 2.1697 A are 0.0215 and 0.0437 A longer than the mea-
sured values, respectively. The calculated values for C–Ir–
N, N–Ir–N, N–Ir–O, O–Ir–O chelate angles are consistent
with the corresponding experimental values determined in
the X-ray structure of (btmp)2Ir(acac). The HOMO and
LUMO levels are shown in Fig. 3. The HOMO
(ꢀ4.806 eV) consists of a mixture of 2-benzo[b]thiophen-2-
yl ring of btmp ligands (51.5%) and Ir orbitals (31.2%), while
the LUMO (ꢀ1.631 eV) is mostly on the 2-benzo[b]thio-
phen-2-yl ring (32.1%) and pyridyl ring (57.0%) of btmp
ligands. The composition (%) of HOMO and LUMO for
(btmp)2Ir(acac) is shown in Table 3.
3.3. Thermal stability
In the materials sublimation process, the pure emitting
materials were obtained under 300–500 ꢁC in ordinary
3.5. Photophysical properties
Absorption and room-temperature emission spectra were
recorded for all of the complexes as shown in Figs. 4 and 5,
respectively. The data are summarized in Table 4. The
Fig. 3. Density functional theory calculation (DFT) of HOMO (left) and
LUMO (right) for (btmp)2Ir(acac).

M.L. Xu et al. / Inorganica Chimica Acta 360 (2007) 3149–3154
3153
Table 4
Absorption, emission, /pl, TGA, and redox properties of Ir(III) complexes
Abs, ka (nm) (loge)
kem (nm)
/
pl
Redox (mV) , E
TGA (ꢁC)e
b
c
d
ox
1=2
Complexes
(btmp)2Ir(acac)
(btfmp)2Ir(acac)
(btfmp)2Ir(pic)
(btfmp)2Ir(dbm)
(btp)2Ir(acac)
a
280 (4.60), 322 (4.46), 334 (4.45), 449 (3.84), 477 (3.94)
290 (4.57), 335 (4.49), 352 (4.45), 369 (4.36), 482 (3.90), 513 (4.00)
294 (4.54), 345 (4.44), 368 (4.31), 498 (4.01)
292 (4.68), 334 (4.52), 354 (4.49), 371 (4.44), 479 (3.83), 512 (3.92)
283 (4.62), 326 (4.45), 355 (4.35), 482 (3.92)
604
638
627
637
613
0.025
0.040
0.048
0.125
0.017
680
947
1073
941
338
341
340
348
368
720
Measured in CH2Cl2 solution at a concentration of 10ꢀ5 mol/L.
Measured in CH2Cl2 solution at 298 K.
b
c
Measured in toluene at 298 K. Excitation wavelength was 400 nm for all complexes, Ir(ppy)3 (/pl = 0.4 in toluene) was used as the reference for
quantum yield (/pl).
d
Measured in CH2Cl2 at a concentration of 10ꢀ3 M and scan rate was 80 mV sꢀ1, values are reported relative to Ag/AgCl.
Temperature listed is at a point of 10% weight loss.
e
Strong spin–orbit coupling in the iridium complexes
leads to efficient phosphorescence, these complexes exhibit
high solution phosphorescence quantum yields
(/pl = 0.025–0.125) at room temperature under air free
condition. These values are high comparative to typical
red phosphorescent materials (btp)2Ir(acac) (/pl = 0.017)
under the same measurement condition used Ir(ppy)3 as
the reference for quantum yield.
1.0
0.8
0.6
0.4
0.2
0.0
(btfmp)2Ir(pic)
(btfmp)2Ir(dbm)
(btp)2Ir(acac)
(btmp)2Ir(acac)
(btfmp)2Ir(acac)
Incorporation of a weak electron-donating group (CH3)
on the 4-position of the pyridyl of 2-benzo[b]thiophen-2-yl-
pyridine in (btp)2Ir(acac) raises the LUMO level, enlarges
the gap between HOMO and LUMO levels, and hence a
9-nm blue shift in the (btmp)2Ir(acac) is observed in com-
parison to that of (btp)2Ir(acac) (613 nm) in CH2Cl2 solu-
tion [18]. Substitution of a electron-withdrawing group
(CF3) on the 5-position of pyridyl ring results in lowering
LUMO level, so large red shifts were observed. Meanwhile
the different monoanionic ligands also affect the emission
wavelength of these complexes, the (btfmp)2Ir(pic) with
the monoanionic ligand of 2-picolinic acid shows the max-
imum emission wavelength of 627, 10 nm blue shift in com-
parison to that of (btfmp)2Ir(acac) (638 nm) and
(btfmp)2Ir(dbm) (637 nm) in CH2Cl2 solution. The emis-
sion characteristics of these complexes can be tuned by
either changing the substituents and their position on
2-benzo[b]thiophen-2-yl-pyridine or using different mono-
anionic ligands. The photophysical properties for these
iridium complexes are consistent with the result of our
DFT calculation.
300
400
500
600
Wavelength (nm)
Fig. 4. Absorption spectra for (btp)2Ir(acac), (btmp)2Ir(acac),
(btfmp)2Ir(acac), (btfmp)2Ir(pic), and (btfmp)2Ir(dbm) complexes in
CH2Cl2 solution.
(btfmp)2Ir(pic)
(btfmp)2Ir(dbm)
(btfmp)2Ir(acac)
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
(btmp) Ir(acac)
2
(btp) Ir(acac)
2
550
600
650
700
750
Wavelength (nm)
Fig. 5. Emission spectra for (btp)2Ir(acac), (btmp)2Ir(acac), (btfmp)2Ir
(acac), (btfmp)2Ir(pic), and (btfmp)2Ir(dbm) complexes in CH2Cl2 solu-
tion at room temperature.
4. Conclusion
In summary, we report four Iridium complexes contain-
ing 2-benzo[b]thiophen-2-yl-pyridine based ligands with the
maximum emission wavelength in the range of 604–638 nm
in CH2Cl2 solution. The emission characteristics of these
complexes can be tuned by either changing the substituents
and their position on 2-benzo[b]thiophen-2-yl-pyridine or
using different monoanionic ligands. These iridium com-
plexes with excellent sublimation property are easily pre-
pared, making them good candidates for the fabrication
of devices.
intense absorption bands at higher energy were assigned to
p–p* ligand-centered (LC) transitions, and low energy bands
in the range of 420–560 nm to singlet and triplet MLCT
transitions. The 1MLCT and 3MLCT were not well
resolved, which has been observed from the complex
(btp)2Ir(acac) [7,18]. A large Stokes shift between the
3MLCT absorption and emission bands was observed, so
these complexes should emit from an excited state that is
predominantly due to 3(p–p*) [18,20].

3154
M.L. Xu et al. / Inorganica Chimica Acta 360 (2007) 3149–3154
[7] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. Lee, C.
5. Supplementary material
Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, Inorg. Chem.
40 (2001) 1704.
CCDC 280190 contains the supplementary crystallo-
graphic data for (btmp)2Ir(acaac). These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
[8] Sungouk Jung, Youngjin Kang, Hyung-Sun Kim, Yun-Hi Kim,
Chang-Lyoul Lee, Jang-joo Kim, Sung-Koo Lee, Soon-Ki Kwon,
Eur. J. Inorg. Chem. 17 (2004) 3415.
[9] P. Coppo, E.A. Plummer, L.D. Cola, Chem. Commun. (2004)
1774.
[10] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R.
Forrest, Appl. Phys. Lett. 75 (1999) 4.
[11] R.R. Das, C.-L. Lee, Y.-Y. Noh, J.-J. Kim, Opt. Mater. 21 (2002)
143.
Acknowledgements
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Samuel, Appl. Phys. Lett. 80 (2002) 2645.
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This work was supported by the National Nature Sci-
ence Foundation of China under Grant No. 90201012
and the innovation research project of Xi’an Mordern
Chemistry Research Institute. The authors thank Dr.
Gao-Hong Zhai (Northwest University, PR China) and
Prof. Wen-Liang Wang (Shaanxi Normal University, PR
China) for DFT calculation and for helpful discussions.
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