Synthesis, crystal structure and photoluminescence of a cyclometalated iridium(III) coumarin complex

Article abstract of DOI:10.1007/s10895-013-1214-x

In this paper a new cyclometalated iridium(III) coumarin complex, Ir(III)bis(3-(2-benzothiazolyl)coumarinato N,C⁴)(acetylacetonate) (Ir(L)₂(acac)), was synthesized and characterized. X-ray crystallography demonstrated that the iridium(III) ion is hexacoordinated by two C atoms and two N atoms from 3-(2-benzothiazolyl)coumarinato ligands and two O atoms from acac ligand, displaying distorted octahedral coordination geometry. The Ir(L)₂(acac) complex has good thermal stability with less than 2 % weight-reduction occurring at 300 C, and exhibits strong reddish orange emission. The results shown that Ir(L)₂(acac) is useful for fabrication organic light-emitting diodes.

Full text of DOI:10.1007/s10895-013-1214-x

J Fluoresc
DOI 10.1007/s10895-013-1214-x
ORIGINAL PAPER
Synthesis, Crystal Structure and Photoluminescence
of a Cyclometalated Iridium(III) Coumarin Complex
Tianzhi Yu & Chengcheng Zhang & Yuling Zhao &
ShaoQiang Guo & Peng Liu & Wentao Li & Duowang Fan
Received: 6 January 2013 /Accepted: 7 March 2013
#
Springer Science+Business Media New York 2013
Abstract In this paper a new cyclometalated iridium(III) cou-
marin complex, Ir(III)bis(3-(2-benzothiazolyl)coumarinato N,
C4)(acetylacetonate) (Ir(L)2(acac)), was synthesized and char-
acterized. X-ray crystallography demonstrated that the
iridium(III) ion is hexacoordinated by two C atoms and two
N atoms from 3-(2-benzothiazolyl)coumarinato ligands and
two O atoms from acac ligand, displaying distorted octahedral
coordination geometry. The Ir(L)2(acac) complex has good
thermal stability with less than 2 % weight-reduction occurring
at 300 °C, and exhibits strong reddish orange emission. The
results shown that Ir(L)2(acac) is useful for fabrication organic
light-emitting diodes.
100 % [5–7]. Of these transition metal complexes,
cyclometalated iridium complexes are particularly important
due to their relatively short excited-state lifetime, high
photoluminescence efficiencies and excellent color tuning
[8–12]. The color of the emission from cyclometalated Ir(III)
complexes can be tuned through the design and synthesis of
novel cyclometalating ligands [11, 13]. Increasing the π-
conjugation length or introducing fused heteroaromatic rings
into the cyclometalating ligand caused a red shift in the
emission [14, 15], whereas lowering the highest occupied
molecular orbital (HOMO) by adding an the electron-
withdrawing group or raising the triplet state energy
by use of a ligand with a strong ligand strength resulted
in a blue shift in the emission [16, 17].
.
.
Keywords Iridium(III) complex Coumarin derivative
.
Crystal structure Photoluminescence
Coumarin derivatives have super thermal stability and
outstanding optical properties including an extended spec-
tral response, high quantum yields, superior photostability,
which are commonly used as efficient laser dyes fluorescing
in the blue-green spectral region [18, 19]. Coumarin de-
rivatives can realize easy tuning of energy gaps of the
corresponding Ir(III) complexes due to the synthetic possi-
bilities of a large variety of coumarin derivatives, the class
of coumarin ligands could be a promising candidate for the
preparation of iridiun(III) complexes for a variety of pho-
tonic applications, such as optical sensing technology [20,
21] and OLED technology [22–24]. Cyclometalated
iridium(III) coumarin complexes represent new type of
phosphorescence materials for organic lighting emitting di-
odes (OLEDs), which possess efficient visible absorption,
higher quantum yields and higher brightnesses.
Introduction
Research on organic light-emitting diodes (OLEDs) has
attracted tremendous interest in the last two decades due to
their potential applications in low-cost, full-color, flat-panel
displays and portable electronic devices [1–4]. Phosphores-
cent organic light-emitting devices (PhOLEDs) based on
transition metal complexes have attracted considerable at-
tentions because they can harvest singlet and triplet exci-
tons, enabling internal quantum efficiencies approaching
:
:
:
T. Yu (*) C. Zhang P. Liu D. Fan
Key Laboratory of Opto-Electronic Technology and Intelligent
Control (Ministry of Education), Lanzhou Jiaotong University,
Lanzhou 730070, China
In recent years, we have been interested in the synthesis
and photoluminescent and electroluminescent study of some
new coumarin derivatives [25–27] and some coumarin-
based iridium(III) complexes [28, 29]. In this work, a new
cyclometalated iridium(III) coumarin complex, Ir(III)
bis(3-(2-benzothiazolyl)coumarinato N,C4)(acetylacetonate)
e-mail: yutianzhi@hotmail.com
:
:
:
:
C. Zhang Y. Zhao S. Guo P. Liu W. Li
School of Chemical and Biological Engineering, Lanzhou Jiaotong
University, Lanzhou 730070, China

J Fluoresc
(Ir(L)2(acac)), was synthesized and characterized. The X-ray
crystal structure, the UV–vis absorption and luminescent
properties and the thermal stability of Ir(L)2(acac) were also
investigated.
dissolved in 1-butanol (50 mL). The mixture solution was
refluxed for 24 h. After the reaction mixture was cooled to
room temperature, the yellow solid was filtered, and washed
with water. Monitoring by TLC, a large amount of blue
fluorescent product and a less amount of green fluorescent
product appeared in TLC. The crude was purified by chro-
matography on silica gel using ethyl acetate/petroleum ether
(1:8, v/v) as the eluent to obtain the blue fluorescent prod-
uct. Yield 58 %; m.p.: 213–215 °C (Lit: 216–217 °C) [30].
1H NMR (CDCl3, δ): 9.09 (s, 1H), 8.11 (d, J=8.0 Hz, 1H),
7.98 (d, J=8.0 Hz, 1H), 7.74 (dd, J=8.0 Hz, J=2.4 Hz, 1H),
7.66 (td, J=8.0 Hz, J=1.4 Hz, 1H), 7.54 (td, J=8.0 Hz, J=1.
4 Hz, 1H), 7.46-7.38 (m, 3H). IR (KBr, cm−1): 3048, 1716
(C = O), 1607, 1557 (C = N). Anal. Calcd. For C16H9NO2S:
C, 68.80, H, 3.25, N, 5.01. Found: 68.85, H, 3.22, N, 5.04.
Experimental
Materials and Methods
Salicylaldehyde, o-aminothiophenol, ethyl cyanoacetate and
benzoic acid were purchased from AstaTech (Chengdu)
Pharmaceutical Co., Ltd. (China). IrCl3·nH2O (iridium con-
tent > 60.0 %) was bought from Shanxi Kaida Chemical Co.
Ltd. (China) and used without further purification.
Acetylacetone was purchased from Shanghai Jingchun
Reagent Co. Ltd. (China). All other chemicals were
analytical grade reagent.
IR spectra (400 – 4,000 cm-1) were measured on a
Shimadzu IRPrestige-21 FT-IR spectrophotometer. 1H NMR
spectra were obtained on Unity Varian-500 MHz. C, H, and N
analyses were obtained using an Elemental Vario-EL automat-
ic elemental analysis instrument. Melting points were deter-
mined on an X-4 microscopic melting point apparatus
(Beijing Taike Instrument Limited Company). UV–vis ab-
sorption and photoluminescent spectra were recorded on a
Shimadzu UV-2550 spectrometer and on a Perkin-Elmer LS-
55 spectrometer, respectively. The thermogravimetry analysis
((TGA) was recorded on a Shimadzu DT-40 thermal analysis
instrument.
Ir(L)2(acac) The cyclometalated Ir (III) μ-chloro-bridged
dimer of (L)2Ir(μ-Cl)2Ir(L)2 was synthesized by refluxing
IrCl3·nH2O (1.00 g, 3.12 mmol) with 2.5 equiv. of
3-(benzothiazol-2-yl)coumarin (2.179 g, 7.80 mmol) in a
3:1 mixture of 2-ethoxyethanol and water (30 mL) under
nitrogen at 135 °C for 24 h. The mixture was cooled to room
temperature, and 100 mL of water was added. The rufous
precipitate was collected by filtration, and washed twice
with water and then methanol/water mixture (1:2, v/v).
The solid was pumped dry to give the crude (L)2Ir(μ-
Cl)2Ir(L)2. The crude dimer was directly used for subse-
quent preparation of Ir(L)2(acac). The chloro-bridged dimer
(0.4 g, 0.255 mmol), acetylacetone (0.230 g, 2.297 mmol)
and sodium carbonate (0.244 g, 2.297 mmol) were refluxed
in dichloroethane under nitrogen atmosphere for 12 h. After
cooling, a small quantity of water was added. The mixture
was extracted with dichloromethane (50 mL×3). The organ-
ic phase was washed with water (2×50 mL) and dried over
anhydrous MgSO4. After filtering, the filtrate was evaporat-
ed to dryness under reduced pressure. The crude was puri-
fied by chromatography on silica gel using ethyl
acetate/petroleum ether (1:6, v/v) as the eluent to give red
powdery Ir(L)2(acac) in 65.5 % yield. 1H NMR (CDCl3, δ):
7.96 (d, 2H, J=7.6 Hz, Aryl-H), 7.57 (d, 2H, J=8.8 Hz,
Synthesis and Characterization of Ir(III) Complex
(Ir(L)2(acac))
The synthetic routes were shown in Scheme 1.
3-(benzothiazol-2-yl)coumarin (L) Under nitrogen atmo-
sphere, a mixture of salicylaldehyde (2.93 g, 24 mmol),
ethyl cyanoacetate (2.72 g, 24 mmol), o-aminothiophenol
(3.00 g, 24 mmol) and benzoic acid (1.465 g, 12 mmol) was
Scheme 1 Synthetic routes to
COOH
the ligand L and iridium(III)
complex Ir(L)2(acac)
O
L
O
N
O
NH2
SH
CHO
NC
+
+
S
OC2H5
n-Butanol
110¡æ
OH
O
O
O
S
O
S
O
Cl
Cl
.
O
nH2O
IrCl3
O
O
O
O
O
O
Ir
Ir
Ir
N
N
S
N
S
N
2
2
2
Ir(L)2(acac)
(L)2Ir(¦Ì-Cl)2(L)2

J Fluoresc
Table 2 Selected bond distances (Å) and angles (o) for Ir(L)2(acac).
Aryl-H), 7.40 (t, 2H, J=7.2 Hz, Aryl-H), 7.28–7.20 (m, 6H,
Aryl-H), 6.55 (t, 2H, J=6.8 Hz, Aryl-H), 6.22 (d, 2H, J=8.
4 Hz, Aryl-H), 5.30 (s, 1H, -CH=C-), 1.69 (s, 6H, -CH3).
Anal. Calcd. For C37H23N2O6S2Ir: C, 52.41, H, 2.73, N, 3.
30. Found: 52.73, H, 2.80, N, 3.26.
Bond distances (Å)
Ir(1)–C(16)
1.987(4)
2.048(4)
2.110(3)
Ir(1)–C(32)
Ir(1)–N(2)
Ir(1)–O(6)
1.999(4)
2.057(4)
2.111(3)
Ir(1)–N(1)
Ir(1)–O(5)
Angles (o)
Crystallography
C(16)–Ir(1)–C(32)
C(16)–Ir(1)–N(2)
C(16)–Ir(1)–O(5)
C(32)–Ir(1)–N(2)
C(32)–Ir(1)–O(5)
N(1)–Ir(1)–O(5)
N(2)–Ir(1)–O(5)
O(5)–Ir(1)–O(6)
98.68(17)
100.71(16)
172.18(14)
80.56(17)
87.50(15)
93.71(13)
84.96(13)
88.66(13)
C(16)–Ir(1)–N(1)
C(16)–Ir(1)–O(6)
C(32)–Ir(1)–N(1)
C(32)–Ir(1)–O(6)
N(1)–Ir(1)–N(2)
N(1)–Ir(1)–O(6)
N(2)–Ir(1)–O(6)
80.43(16)
85.52(15)
101.53(16)
174.24(15)
177.49(14)
83.01(13)
94.82(14)
Suitable single crystal of the complex was obtained by
slow evaporation of the chloroform solution at room
temperature. The diffraction data were collected with a
Bruker Smart Apex CCD area detector with graphite-
monochromatized Mo-Kα radiation (λ=0.71073 Å) at
295(2) K. The structure was solved by using the pro-
gram SHELXL and Fourier difference techniques, and
refined by full-matrix least-squares method on F2. All
hydrogen atoms were added theoretically. The crystal
and experimental data of Ir(L)2(acac) are shown in
Table 1. The selected bond lengths and bond angles of
Ir(L)2(acac) are listed in Table 2.
basis set. The structural energy of the complex Ir(L)2(acac)
was calculated at B3LYP/6-31 G(d) levels. The structure
optimization and energy calculations were performed with
the GAUSSIAN 98 program.
Quantum Chemical Calculation
The structure of the complex Ir(L)2(acac) was optimized by
density functional theory (DFT) using a B3LYP/6-31 G(d)
Results and Discussion
X-ray Crystal Structure of Ir(L)2(acac)
Table 1 Crystallographic and experimental data for Ir(L)2(acac)·CHCl3
The crystal structure and packing diagram of Ir(L)2(acac)
are given in Figs. 1 and 2, respectively.
Empirical formula
Formula weight
C38H24Cl3IrN2O6S2
967.26
The structure of Ir(L)2(acac) was measured by X-ray
crystallography. The crystal structure of Ir(L)2(acac)
consists of one Ir(L)2(acac) molecule and one crystalliz-
ing CHCl3 molecule. So the crystal structural formula of
Ir(L)2(acac) was described as Ir(L)2(acac)·CHCl3. The
Temperature (K)
295(2)
Wavelength (Ǻ)
0.71073
Crystal system, space group
Unit cell dimensions
Triclinic, P-1
a=9.2606(4) Ǻ α=85.5540(10)o
b=13.2592(6) Ǻ β=75.5800(10)o
c=15.0875(7) Ǻ γ=88.1350(10)o
1788.64(14), 2
Volume (Ǻ3), Z
Calculated density (g/cm3)
Absorption coefficient (mm−1
F(000)
1.796
)
4.124
948
Crystal size (mm)
0.29×0.21×0.13
θ range for data collection (o)
1.54 to 26.02
Limiting indices
−10≤h≤11, −16≤k≤16, −18≤l≤16
9703/6880 (Rint=0.0141)
Semi-empirical from equivalents
0.6162 and 0.3809
Full-matrix least-squares on F2
6880/0/471
Reflections collected/unique
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
1.025
R1=0.0322, wR2=0.0811
R1=0.0353, wR2=0.0832
Largest diff. peak and hole (eǺ-3) 2.678 and −1.744
Fig. 1 Crystal structure of Ir(L)2(acac). Hydrogen atoms and solvent
molecule are removed for clarity

J Fluoresc
Fig. 2 Packing diagram of
Ir(L)2(acac) along b-axis..
Hydrogen atoms and solvent
molecule are removed for
clarity
crystal of Ir(L)2(acac) belongs to the triclinic space
group P-1, a=9.2606(4) Ǻ b=13.2592(6) Ǻ, c=15.
0875(7) Ǻ, α=85.5540(10)o, β=75.5800(10)o, γ=88.
1350(10)o, U=1788.64(14) Å3, Z=2, Dc=1.796 g/cm3,
μ=4.124 mm−1.
ligand L exhibits two intense absorption bands at 227 and
365 nm, and one weak absorption band at 253 nm. The
absorption band at 365 nm can be attributed to the charge
transfer type π-π* transition from the HOMO located at the
electron-donating coumarin moiety to the LUMO located at
electron-accepting benzothiazole moiety. The absorption
bands at 227 and 253 nm should come from the locally
excited π-π* transitions at the coumarin moiety [32, 33].
In UV–vis absorption spectrum of Ir(L)2(acac), there are six
obvious absorption bands at 227, 286, 354, 412, 436 and
464 nm, in which the absorption bands below 400 nm could
be ascribed to spin-allowed π→π* transition of the
ligands, and the bands at the longer wavelengths (412,
436 and 464 nm) can be assigned to both spin-orbit
As shown in Fig. 1, it is shown that the iridium(III)
ion in Ir(L)2(acac) is hexacoordinated by two C atoms
and two N atoms from 3-(benzothiazol-2-yl)coumarin
ligands and two O atoms from acetylacetonate ligand,
displaying a distorted octahedral coordination geometry.
Two C atoms and two N atoms from 3-(benzothiazol-2-
yl)coumarin ligands exhibit cis-C-C and trans-N,N che-
late mode, and two O atoms from acetylacetonate ligand
locate cis-O-O chelate disposition. The Ir-C bonds (1.
987(4) and 1.999(4) Å) are shorter than the Ir-N bonds
(2.048(4) and 2.057(4) Å). Two Ir-O bonds are nearly
equal, they are 2.110(3) and 2.111(3) Å, respectively,
indicating that the delocalized π bond formed when
acetylacetonate ligand coordinated to Ir atom. And the
Ir-O bonds are shorter than those of (ppy)Ir(acac) (2.
146(4) Å) and (tpy)Ir(acac) (2.161(4) Å) [31]. In addi-
tion, the benzothiazolyl rings are not coplanar with the
coumarin rings in two 3-(benzothiazol-2-yl)coumarin li-
gands, the dihedral angles are 14.06° and 18.05°,
respectively.
3
3
coupling enhanced (π→π*) and spin-forbidden MLCT
transitions [34–36].
The ligand L is photoactive and exhibits bright blue
emission with peak at 452 nm in dilute solution when
excited at 365 nm (Fig. 3). The ligand L is a typical D-A
molecule in which the coumarin ring acts as an electron-
donor (D) and the benzothiazole moiety acts as an electron-
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
UV (L)
UV (Ir(L) (acac))
2
PL (L)
PL (Ir(L) (acac))
2
The packing diagram for unit cell of Ir(L)2(acac)
along b-axis was shown in Fig. 2, it was found that
the π-π interaction of two adjacent Ir(L)2(acac) mole-
cules through respective 3-(benzothiazol-2-yl)coumarin
ligands favors to stack along b-axis.
300
400
500
600
700
800
UV–vis Absorption and Photoluminescence Spectra
Wavelength (nm)
The UV–vis absorption and photoluminescence spectra of
the ligand L and the complex Ir(L)2(acac) were measured in
diluted dichloromethane solutions, as shown in Fig. 3. The
Fig. 3 Normalized UV–vis absorption and photoluminescence spectra
of ligand L and the complex Ir(L)2(acac) in diluted dichloromethane
solutions. (C=1×10−6mol/L)

J Fluoresc
acceptor (A), π-conjugation through the donor to the accep-
tor forms a strong donor-acceptor system. Addition of a
benzothiazole ring to coumarin ring at the C3 position
caused a 22 nm red-shift in λem with respect to the emission
of the unsubstituted parent coumarin [32]. As shown in Fig.
3, by excitation at 464 nm the complex Ir(L)2(acac) in
dichloromethane solution exhibits reddish orange emission
with a maximum main peak (569 nm) and a shoulder peak
(623 nm), the Commission Internationale de L’Eclairage
(CIE) coordinates is (0.53, 0.47).
100
80
60
40
20
0
100
200
300
400
500
600
700
o
Temperature ( C)
Quantum Chemical Calculations
Fig. 5 Thermogravimetric analysis (TGA) of the complex Ir(L)2(acac)
in nitrogen atmosphere (heating rate 10 °C/min)
Frontier molecular orbitals play an important role in
electric and optical properties for a materials [37]. Com-
pared with the X-ray data of the complex Ir(L)2(acac),
its structure was optimized by density functional theory
(DFT) using a B3LYP/6-31 G(d) basis set. It was shown
that the DFT optimized structure is in good agreement
with the X-ray crystal structure. The electronic distribu-
tions and energy levels of HOMO and LUMO orbitals
for Ir(L)2(acac) obtained from time-dependent DFT (TD-
DFT) calculation is shown in Fig. 4. It can be seen that
the orbital energy levels of HOMO (highest occupied
molecular orbital) and LUMO (lowest unoccupied mo-
lecular orbital) of the complex are −6.18 and −2.31 eV,
respectively. The energy gap between HOMO and
LUMO is 3.87 eV.
From Fig. 4, the electron density in the HOMO of the
complex resides significantly on the cyclometalating ligands
as well as the core Ir(III) atom. The HOMO is followed, in
order of decreasing energy, by two orbitals (HOMO-1 and
HOMO-2) showing a increasing contribution from ancillary
ligand acac except the contributions coming from the
cyclometalating ligand L and the core Ir(III) atom. The electron
density in LUMO locates mainly on a cyclometalating ligand
and the core Ir(III) atom, and the electron density in LUMO + 1
locates mainly on the core Ir(III) atom and another
cyclometalating ligand. The LUMO + 2 is mainly a π* bond-
ing combination of the acac ligand with small metal character.
Thermal Property of the Complex
Thermogravimetric analysis (TGA) measurement was
performed in flowing drying nitrogen atmosphere at the
heating rate of 10 °C/min on approximately 3.484 mg sam-
ple. The result of TGA measurement of Ir(L)2(acac) is
shown in Fig. 5. It shows good thermal stability of the
complex up to 300 °C ( 2 % weight loss). At 337 and
370 °C, there are two sharp weight losses in the TGA curve,
it shows that the complex undergoes two large-stage decom-
position processes. The complex meets with the thermal
stability requirement of fabrication of OLED luminescence
application.
Fig. 4 Frontier molecular
orbitals of the complex
Ir(L)2(acac)
HOMO (-6.18 eV)
HOMO-1 (-6.36 eV)
HOMO-2 (-6.58 eV)
LUMO+2 (-1.26 eV)
LUMO (-2.31 eV)
LUMO+1 (-2.22 eV)

J Fluoresc
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Conclusion
A new iridium(III) complex containing coumarin derivative
as a cyclometalated ligand, Ir(L)2(acac), was synthesized
and confirmed by means of element analysis, H NMR,
1
and single crystal X-ray crystallography. The electronic
distributions and energy levels of HOMO and LUMO or-
bitals for Ir(L)2(acac) obtained from time-dependent DFT
(TD-DFT) calculation, and the energy gap between HOMO
and LUMO is 3.87 eV. The complex has high thermal
stability, and exhibites bright reddish orange emission with
a maximum main peak (569 nm) and a shoulder peak
(623 nm), which is suggested to be a phosphorescent
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Acknowledgments This work was supported by the National
Natural Science Foundations of China (61166003 and 60776006),
and also supported by the Program for Changjiang Scholars and
Innovative Research Team in University (IRT0629).
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