A green-emitting iridium complex used for sensitizing europium ion with high quantum yield

Article abstract of DOI:10.1016/j.ica.2017.01.019

A green-emitting iridium(III) complex Ir(dfppy)₂(cbphen) (dfppy?=?2-(4′,6′-difluoro-phenyl)pyridine, cbphen?=?4-carboxylate-phenanthroline) was synthesized and fully characterized. The single crystal data confirm its Ir(C^N)₂(N^N) structure with the carboxylate group on phenanthroline stretching freely which could be able to combine with europium(III) ion. The complex was reacted with EuCl₃·6H₂O and Eu(TTA)₃·2H₂O to provide bimetallic complexes. Through photophysical studies, characteristic emission from Eu^IIIion was obtained, and meanwhile the green emission from iridium(III) center was almost quenched completely, suggesting this iridium(III) complex is a good sensitizer for Eu^IIIions. It is well worth noticing that by introducing the iridium(III) complex as a chromophore, the excitation wavelength of Eu^IIIion has been extended to the visible range (470?nm). Besides, due to the efficient energy transfer, the quantum yield of Ir₃-Eu was nearly equal to that of Eu(TTA)₃·2H₂O.

Full text of DOI:10.1016/j.ica.2017.01.019

Accepted Manuscript
Research paper
A Green-Emitting Iridium Complex Used for Sensitizing Europium Ion with
High Quantum Yield
Weili Jiang, Chenming Hong, Huibo Wei, Zongkai Wu, Zuqiang Bian, Chunhui
Huang
PII:
S0020-1693(16)30960-4
http://dx.doi.org/10.1016/j.ica.2017.01.019
ICA 17412
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
29 November 2016
20 January 2017
21 January 2017
Please cite this article as: W. Jiang, C. Hong, H. Wei, Z. Wu, Z. Bian, C. Huang, A Green-Emitting Iridium Complex
Used for Sensitizing Europium Ion with High Quantum Yield, Inorganica Chimica Acta (2017), doi: http://
dx.doi.org/10.1016/j.ica.2017.01.019
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A Green-Emitting Iridium Complex Used for Sensitizing
Europium Ion with High Quantum Yield
1
Weili Jiang , Chenming Hong , Huibo Wei , Zongkai Wu , Zuqiang Bian * and Chunhui Huang
,2,*
1
1
1
1,
1
1
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials
Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University,
Beijing, 100871, China
2
Current address: State Key Laboratory of Heavy Oil, Institute of New Energy, China University of
Petroleum, Beijing, 102249, PR China
*
Corresponding
authors,
E-mail:
jiangweilipku@163.com;
bianzq@pku.edu.cn;
Tel:
(
+86)10-6275-3544. Fax: (+86)10-6275-7156
A green-emitting iridium(III) complex Ir(dfppy)
cbphen = 4-carboxylate-phenanthroline) was synthesized and fully characterized. The single crystal data
confirm its Ir(C^N) (N^N) structure with the carboxylate group on phenanthroline stretching freely
which could be able to combine with europium(III) ion. The complex was reacted with EuCl ·6H O and
Eu(TTA) ·2H O to provide bimetallic complexes. Through photophysical studies, characteristic
2
(cbphen) (dfppy = 2-(4’,6’-difluoro-phenyl)pyridine,
2
3
2
3
2
III
emission from Eu ion was obtained, and meanwhile the green emission from iridium(III) center was
III
almost quenched completely, suggesting this iridium(III) complex is a good sensitizer for these Eu
ions. It is well worth noticing that by introducing the iridium(III) complex as a chromophore, the
III
excitation wavelength of Eu ion has been extended to the visible range (470 nm). Besides, due to the
efficient energy transfer, the quantum yield of of Ir
3
-Eu was nearly equal to that of Eu(TTA)
3
·2H O.
2
Keywords: iridium(III) complex; europium(III) ion; bimetallic complex; visible light excitation;
lanthanide; sensitize

1
. Introduction
Lanthanide complexes as a brilliant group of luminescent materials due to their special f-f transition
with narrow line-like emissions, and long luminescent lifetimes [1, 2], have received considerable
attention due to their potential use in organic light-emitting diodes (OLED) [3-10], analytical probes
[
11-16], and biological sensing and imaging [17-24]. Because of the low absorption coefficient of
lanthanides resulting from the f-f forbidden transitions, intense luminescence of lanthanide is usually
achieved from indirect excitation of light harvesting chromophores (termed the ‘antenna effect’) [25-27].
Although abundant antennas have been reported for decades, new sensitizers are still craved for a deep
understanding of the lanthanide photophysics [28]. The traditional antennas are usually pure organic
molecules with absorption in the UV region, which does not match well with the lanthanide energy level,
III
III
III
III
especially for the red-emitted Eu ions and near infrared lanthanide ions (Nd , Er , Yb ). It does not
only cause a waste in energy, but will also increase the damage to the organisms when the materials are
applied in labeling. Many efforts are made to expand the excitation of lanthanide ions, such as Wang’s
great improvement in the singlet pathway for the sensitization of europium(III) luminescence [29].
III
II
I
II
II
Another well-known way is introducing transition-metal-based ligands (Ir , Ru , Re , Pt , Pd , etc.) as
antennas because these organometallic sensitizers usually have high triplet quantum yields as well as
long-lived triplet excited state, which will promote energy transfer to the lanthanide centers [30-32].
III
Especially, cyclometalated iridium(III) complexes are intriguing in sensitizing Ln ions due to their
tunable MLCT (metal-to-ligand charge-transfer) transitions and high quantum efficiency [33-35], which
makes it not so difficult to achieve energy match between the donor and lanthanide acceptor. The first
iridium(III) complexes used as an antenna was reported by De Cola and co-workers using
5
-(2-pyridinyl)-1,2,4-triazole-3-carboxylic acid as a bridging ligand for sensitizing europium(III) ion

[
36]. Due to the partial energy transfer, white emission was detected resulting from the mixture of the
III
III
cyan emission (Ir center) and red emission (Eu ion).
III
III
III
III
Our group has made a lot of progress in studying Ir -Ln dyads, including Ir -Eu [37-40], and
III
III
Ir -Ln (Ln=Nd, Er, Yb) [41-43] complexes. Among them, the most brilliant iridium(III) complex,
Ir(dfppy)
2
(phen5f), with 4,4,5,5,5-pentafluoro-1-(1′,10′-phenanthrolin-2′-yl)pentane-1,3-dione (Hphen5f)
III
III
as the bridging ligand, was shown to achieve effective energy transfer from Ir complex to Eu center
[
37]. During our research, we found that for the cyclometalated iridium complex, the cyclometalating
III
ligand (2,4-difluoro)phenylpyridine (dfppy) is more suitable for sensitizing Eu ions than ppy
III
(
1-phenylpyridine) [39]. The energy level of the Ir complex is considered to be the most important
factor affecting the luminescence efficiency [43]. And complete energy transfer will occur only when
3
III
the triplet energy level of the linking ligand was lower than the MLCT energy level of the Ir moiety
5
III
III
III
and higher than the D level of Eu ion [38]. Furthermore, for Ir -Eu bimetallic complexes, the
J
discrepancy in their quantum yields can be explained by the nonradiative relaxation of the excited
III
energy of Eu ion [40].
However, among the lots of iridium(III) complexes reported for sensitizing europium(III) ion, few
of them show both an obvious luminescence emission at room temperature and suitable energy levels to
achieve total energy transfer, even for Ir(dfppy)
2
(phen5f) [37] or its analogs Ir(dfppy)
2
(phen3f),
Ir(dfppy)
2
(bpy5f), Ir(dfppy)
2
(bpy3f) [40]. Luminescent iridium(III) complexes with suitable energy
III
level to sensitize Eu ion are desirable because there will be an obviously visible change in
luminescence outputs, which is particularly welcome in analytical probes field [44]. Therefore, with
dfppy as the cyclometalating ligand, we modulated the ancillary ligand phen5f to cbphen (cbphen =
4
-carboxylate-phenanthroline), to synthesize a novel iridium(III) complex, Ir(dfppy) (cbphen). The
2

iridium(III) complex was characterized with well-defined X-ray structure to sensitize the red
III
luminescence of Eu ion. Photophysical results show that it has a suitable triplet energy level to match
III
III
III
the excited states of Eu ion and intense red emission of the Ir -Eu bimetallic complexes was found
with high quantum yield.
2
2
. Experimental
.1. Materials and methods
All reagents and solvents were analytical grade. 2-(4’,6’-Difluoro-phenyl)pyridine (dfppy) [45],
phenanthroline-4-carboxylic acid (Hcbphen) [46] and Eu(TTA)
according to previous literatures. Cyclometalated iridium(III) dimer (dfppy)
synthesized through the Nomoyama route, by refluxing IrCl ·3H O with 2.2 equiv of dfppy in
3
·2H
2
O [47] were prepared and purified
2
Ir(µ-Cl) Ir(dfppy) was
2
2
3
2
2
-ethoxyethanol mixed with water (3:1) for 24 h [48].
1
The H NMR spectra were measured on an ARX-400 NMR (Bruker) spectrometer. The chemical
shift data of each compound were reported in ppm units with tetramethylsilane as the internal reference.
Elemental analysis (C, H, N) was performed on an Elementar Vario MICRO CUBE. High-resolution
mass spectra were measured on a Bruker Apex IV FTMS. The X-ray diffraction data were collected on a
Rigaku MicroMax-007 CCD diffractometer using graphite-monochromated Mo/KR (λ = 0.71073 Å)
radiation. Full crystallographic data are provided in CIF file as Supporting Information and are available
from the Cambridge Crystallographic Data Centre (CCDC-764289). A Shimadzu UV-3100 spectrometer
was used to detect the UV–vis absorption spectra. The photoluminescence (PL) spectra and lifetime data
were obtained on an Edinburgh Analytical Instruments FLS920 spectrometer. The solvents employed in
photophysical experiments were spectroscopic grade. Luminescence quantum yields (Φ) in different
solvents were measured at room temperature with an aerated aqueous solution of [Ru(bpy)
3
]Cl
2
(Φ =
r

0
.028) as a standard [49].
2
.2. Synthesis
Ir(dfppy)
OH (15 ml) was mixed with a solution of (dfppy)
CO (1 mmol) was added to the reaction mixture which was then heated to reflux under
stirring for 4 h. After cooled to room temperature, the solvent was removed under reduced pressure. The
2
(cbphen) The suspension of phenanthroline-4-carboxylic acid (Hcbphen, 0.44 mmol) in
CH
3
2
Ir(µ-Cl) Ir(dfppy) (0.2 mmol) in CH Cl (15 ml).
2
2
2
2
Excessive Na
2
3
residue was then purified through a silica column with CH
2
Cl
2
/CH
3
OH (1:1) for the elution and a
1
green-yellow solid was collected as pure product with a yield of 81%. H NMR (400MHz, CDCl
3
): δ
9
8
1
.11 (1H, d, J = 9.0 Hz), 8.78 (1H, d, J = 8.1 Hz), 8.30 (2H, d, J = 8.4 Hz), 8.23 (1H, d, J = 4.2 Hz),
.19-8.14 (3H, m), 7.89-7.86 (1H, td, J = 3.1 Hz, J = 6.6 Hz, J = 13.2 Hz), 7.75 (2H, t, J = 7.9 Hz, J =
5.7 Hz), 7.36 (1H, d, J = 4.9 Hz), 7.19 (1H, d, J = 5.5 Hz), 6.93 (1H, t, J = 6.2 Hz, J = 12.4 Hz), 6.87
(
1H, t, J = 6.2 Hz, J = 12.4 Hz), 6.63-6.57 (2H, m), 5.82-5.79 (1H, dd, J = 2.3 Hz, J = 8.2 Hz),
5
6
(
.75-5.73 (1H, dd, J = 2.3 Hz, J = 8.2 Hz). Anal. Found for C35
H
19IrF
4
N
4
O
2
·H
2
O: C, 51.49; H, 2.69; N,
, 797.1152
+
.92. Calcd: C, 51.66; H, 2.60; N, 6.88. HRMS (ESI , CH
3
OH) calcd for C35
H
20IrF
4
N
4
O
2
+
[M+H] ); found, 797.1145.
Ir(dfppy)
2
(cbphen) was refluxed with EuCl
3
·6H
2
O (for Ir
3
-Eu) or Eu(TTA)
3
·2H
2
O (for Ir-Eu) in
EtOH for about 6 h. After cooled to room temperature, the solvent was removed under reduced pressure
III
III
and the solid left was recrystallized with CH
2
Cl /n-hexane to offer the final Ir -Eu bimetallic
2
complexes.
{
[Ir(dfppy)
2
(cbphen)]
·2H
OH) calcd for C105
3
ClEu}Cl
2
(Ir
3
-Eu)
Yield
84%.
Anal.
Found
for
C
105
H
57Cl
3
EuF12Ir
3
N
12
O
6
2
O: C, 46.44; H, 2.92; N, 6.10. Calcd: C, 46.71; H, 2.35; N, 6.23. HRMS
+
+
(
ESI , CH
3
H
57Cl
2
EuF12Ir
3
N
12
6
O , 2610.1820 ( [M-Cl] ); found, 2610.2421.

Ir(dfppy)
2
(cbphen)Eu(TTA)
3
(Ir-Eu) Yield 90%. Anal. Found for C59
H
31EuF13IrN
4
O
8
S
3
·H O: C,
2
+
4
3.44; H, 2.15; N, 3.60. Calcd: C, 43.49; H, 2.04; N, 3.44. HRMS (ESI , CH
3
OH) calcd for
+
C
51
H
27EuF10IrN
4
6
O S
2
, 1389.0031 ( [M-TTA] ); found, 1389.0047.
3
3
. Results and discussion
.1. Synthesis and characterization
III
III
Scheme 1 shows the general synthesis route of the iridium(III) complex and Ir -Eu bimetallic
complexes concerned in this work. The purchased 4-methyl-1,10-phenanthroline was oxidized by SeO
in solvent of 1,4-dioxane mixed with water (96:4, v/v). The obtained 1,10-
phenanthroline-4-carbaldehyde was further oxidized by 80% HNO to form the ligand,
,10-phenanthroline-4-carboxylic acid (Hcbphen). HNO with a concentration of lower than 70% could
seriously reduce the yield. The ligand Hcbphen was refluxed with the cyclometalated dimers
2
a
3
1
3
(
dfppy)
Na CO
reacting Ir(dfppy)
2
Ir(µ-Cl)
to give the complex Ir(dfppy)
(cbphen) with corresponding EuCl
2
Ir(dfppy)
2
in a mixture of CH
3
OH and CH
2
Cl (1:1, v/v) in the presence of excessive
2
III
III
2
3
2
(cbphen). The Ir -Eu bimetallic complexes were obtained by
2
3
·6H
2
O (for Ir
3
-Eu) or Eu(TTA)
3
·2H O (for Ir-Eu)
2
1
in EtOH. All the products were characterized through H NMR, high-resolution mass spectrometry and
elemental analysis.
CHO
0% HNO3
COOH
[Ir(dfppy) (u-Cl)]
EuCl3 6H2O
SeO2
N
N
N
8
2
2
Ir
relux
F
or Eu(TTA)3 .2H O
N
N
1
,4-dioxane, reflux
N
N
N
N
2
2
COO
F
Ir(dfppy)2(cbphen)
O
EuCl 2Cl
O
Eu(TTA)3
O
O
N
N
N
N
N
Ir
F
Ir
F
N
2
3
2
F
Ir3-Eu
F
Ir-Eu
III
III
Scheme 1 Synthetic procedure of the Ir(dfppy)
2
(cbphen) and Ir -Eu bimetallic complexes

3
.2. X-ray crystal structure of Ir(dfppy)
The single crystal of Ir(dfppy) (cbphen) was obtained by slowly evaporating solvent from a
Cl /acetone/n-hexane solution of Ir(dfppy) (cbphen). X-ray crystallographic structure is shown in
2
(cbphen)
2
CH
2
2
2
Fig. 1 and crystallographic data are listed in Table 1. Just as we expected, the two nitrogen atoms on
Hcbphen coordinate well with the centered iridium(III) ion because of the strong affinity of Ir to N,
while the carboxyl group unfolds freely. The hydrogen atom on the carboxyl is lost to meet the need of
electroneutrality of the molecule. Due to the electronic delocalization, the two bond lengths of
O(1)-C(35) (1.240 Å) and O(2)-C(35) (1.205 Å) are averaged, though the former is still a little longer
because of the attraction of the H atom on C(27) to O(1). The bond lengths between Ir and N on cbphen
group are about 2.151 Å and 2.144 Å for Ir(1)-N(3) and Ir(1)-N(4) respectively, while the bond lengths
between Ir and N on dfppy are about 2.040 Å and 2.054 Å for Ir(1)-N(1) and Ir(1)-N(2) respectively. So
the distance between Ir and cbphen is longer than that between Ir and dfppy because of the trans effect
of the C donor in dfppy (bonds trans to C atoms are longer than bonds trans to N atoms due to the sigma
donation of the carbons) [50, 51]. In the octahedral geometry of Ir center, N atoms from C^N ligands
adopt trans-configuration which was popular in many studies of Ir(C^N) (LX) complexes. The angles
2
for N(1)-Ir(1)-C(11), C(22)-Ir(1)-N(2) and N(4)-Ir(1)-N(3) all deviate from 90°, while those for
III
C(11)-Ir(1)-N(3), C(22)-Ir(1)-N(4) and N(1)-Ir(1)-N(2) deviate from 180 °, meaning the Ir ion is
centered at a distorted octahedron.

Fig. 1 (A) X-ray crystal structure and (B) molecule packing of Ir(dfppy)
2
(cbphen), with thermal ellipsoids drawn at the
30% probability level. H atoms and solvent molecules were omitted for clarity.
2
Table 1 Crystal data and structure refinement for Ir(dfppy) (cbphen)
complex
empirical formula
crystal system
space group
a (Å)
Ir(dfppy)
2
(cbphen)
C
35
H
19
F
4
IrN O
4 2
Triclinic
P
1
8.6184(17)
10.262(2)
17.387(3)
104.93(3)
97.47(3)
102.02(3)
1425.5(5)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
3
V (Å )
Z
-
3
ρ (calcd) (g·cm )
F(000)
1.854
772
θ range (deg)
2.47-25.02
0.803
2
GOF on F
Final R indices
R1 = 0.0576, wR2 = 0.1455
[
I>2sigma(I)]
R indices (all data)
R1 = 0.0608, wR2 = 0.1487
2
Table 2 Selected bond lengths (Å) and angles (°) of Ir(dfppy) (cbphen)

Ir(1)-N(1)
Ir(1)-N(2)
Ir(1)-N(3)
Ir(1)-N(4)
Ir(1)-C(11)
Ir(1)-C(22)
O(1)-C(35)
2.040(8)
2.054(7)
2.151(7)
2.144(7)
2.031(9)
2.023(9)
O(2)-C(35)
1.205(11)
80.5(3)
N(1)-Ir(1)-C(11)
C(22)-Ir(1)-N(2)
N(4)-Ir(1)-N(3)
C(11)-Ir(1)-N(3)
C(22)-Ir(1)-N(4)
80.5(3)
77.0(3)
176.1(3)
172.2(3)
171.7(3)
1.240(11) N(1)-Ir(1)-N(2)
3
.3. Photophysical properties of Ir(dfppy)
2
(cbphen)
The absorption and photoluminescence spectra of Ir(dfppy)
2
(cbphen) in different solvents are
shown in Fig. 2. The intense absorption below 310 nm is assigned to the spin-allowed π-π* transition of
the organic ligands. From 310 to 390 nm, weak absorption bands were detected, attributed to the
1
1
MLCT (metal-to-ligand-charge-transfer) of the complex and LLCT (ligand-to-ligand-charge-transfer)
between different ligands. Furthermore, there is also absorption in the visible region (> 390 nm), which
3
3
may be resulted from the MLCT and LLCT of the complex, suggesting a potential in the long-wave
excitation [35, 51].
2
Fig. 2 UV-vis absorption and photoluminescence spectra of Ir(dfppy) (cbphen) in different solvents
The complex shows typical emission of cyclometalated Ir(dfppy) (L) in EtOH glass at 77 K [52].
2

Two intense emission bands at 474 nm and 507 nm indicate the triplet levels of the complex are T =
1
-
1
-1
2
1097 cm (τ = 9.75 µs), T
Ir(dfppy) (phen5f) as well as the Ir(dfppy) moiety [37], which is suitable for sensitizing Eu ion ( D
19000 cm , D
band at 512 nm was observed. It is because at low temperature, the transition from T
restricted, both the emissions from the two states were detected; while at room temperature, the fast
reorganization of solvent molecules enhanced the non-radiative transition from T to T , so only the
emission from the lower vibrational level of T (512 nm) was observed [53, 54].
The emission of the complex at room temperature in CH Cl centers at 520 nm, a little red shift to
2
=19724 cm , respectively. This triplet energy is also similar to
2
+
III
5
2
1
-1
5
-1
=
0
= 17500 cm ). When the temperature rose up to room temperature, a strongly broad
1
to T was partly
2
1
2
2
2
2
that in EtOH (512 nm), which is contrary to the previous studies that λmax gets longer with the increase
in polarity of solvents [53]. In order to explain such a phenomenon and study the excitation states of the
complex, TD-DFT analysis was carried out with the result shown in Fig. 3. It suggests that the HOMO
-
orbital mainly localizes on the –COO group, and the LUMO is mainly on the phenanthroline of cbphen
group. The abnormal distribution of HOMO from other Ir(C^N) (N^N) complexes may be caused by the
2
-
strong electron-withdrawing ability of –COO group and the electron-deficient phenanthroline, which
III
-
facilitates the electron density flowing from Ir ion to –COO . Thus, the electron density on Ir is
-
lowered to HOMO-3 and –COO becomes to be the main distribution area of electrons.
-
In EtOH, the H atom on the EtOH will bind with –COO to form H bond, which will slightly
decrease the electron density of HOMO, then enlarging the energy gap. The luminescence lifetime in
EtOH and CH
2
Cl are 339.2 ns and 392.2 ns, which also indicates the H bond quenches the emission a
2
little more in EtOH. The submicrosecond lifetime reveals the phosphorescent property of the emissions.
Furthermore, the red shift of the emission in solid state suggests an obvious π-π stacking in the solid

state, which is also confirmed by the X-ray structure shown in Fig. 1B.
2
Fig. 3 HOMOs and LUMOs distributions for Ir(dfppy) (cbphen)
The calculated excitation states in the complex were listed in Table 3. We can see all the emissions
3
-
in EtOH and CH
2
Cl
2
at room temperature can be attributed to the LLCT from –COO to dfppy, while
3
the higher energy emission at 77 K involves the MLCT (Ir→phen) and π→π* transition of phen. That
3
is to say, when the ancillary ligand was modulated from phen5f to cbphen, the enhanced LLCT
transition results in the strong luminescence of the Ir(dfppy) (cbphen). On the other hand, the shorter C
2
chain of cbphen than phen5f and its analogs will effectively reduce the luminescence quenching caused
by the fast vibration.
Table 3 The calculated excitation states in Ir(dfppy)
2
(cbphen)
λ
5
4
cal, nm
E
cal, eV
Electron transition orbitals
HOMO-2→LUMO+2 (0.70)
HOMO-2→LUMO+3 (0.70)
Excitation state
3
14
2.41
2.49
LLCT
3
99
LLCT

HOMO-3→LUMO (0.47)
3
3
453
2.73
MLCT+ LC
HOMO-3→LUMO+1 (0.39)
III
III
3
.4. Photophysical properties of Ir -Eu bimetallic complexes
III
In order to examine the sensitization performance that the iridium(III) complex played on Eu ion,
III
III
the photophysical properties of Ir -Eu bimetallic complexes together with Ir(dfppy)
2
(cbphen) and
Eu(TTA)
3
·2H
2
O were studied in detail. From Fig. 4 it can be found that Eu(TTA)
3
·2H O shows no
2
3
absorption signal in the region > 400 nm. The MLCT in Ir(dfppy)
2
(cbphen) made the two bimetallic
complexes with longer-wavelength absorption property. The absorption of Ir-Eu is nearly the overlay of
Ir(dfppy)
Ir(dfppy)
2
2
(cbphen) and Eu(TTA)
(cbphen), meaning same transition bands exist in the two complexes; while the molar
-Eu is remarkably larger, since there are three Ir(dfppy) (cbphen)
3
·2H
2
O. As to Ir -Eu, the shape of its absorption spectrum is similar to
3
extinction coefficient (ε) of Ir
3
2
moieties in one Ir -Eu molecule.
3
2 2
Fig. 4 UV-vis absorption spectra of the complexes in CH Cl at room temperature.
Fig. 5 demonstrates that the excitatiocn spectra of Eu(TTA)
3
·2H
2
O and Ir-Eu are similar, but the
III
latter shows obvious long-wavelength excitation property because of the introduction of Ir complex.
For the both bimetallic complexes, the excitation window was expanded to about 470 nm.

2 2
Fig. 5 Excitation spectra of the complexes in CH Cl at room temperature.
Fig. 6 shows the luminescence titration spectra of EuCl
3
·6H
2
O into Ir(dfppy)
2
(cbphen) in EtOH. It
III
is found that the green emission of Ir(dfppy)
2
(cbphen) quenched gradually as the addition of Eu ion,
III
5
7
togerther with the emission of Eu ion at 615 nm ( D
0
→ F
2
) rising, indicating energy transfer from
III
III
Ir(dfppy)
2
(cbphen) to Eu ion took place. However, the emission of Eu fell after a maximum value,
III
with the 615 nm emission shape changed, implying a multi coordination type between Ir complex and
III
III
III
Eu . At the beginning of the titration, enough Ir complexes can sensitize each Eu ion well, so the
III
III
emission at 615 nm turned strong. As the amount of Eu ions increased, there are not enough Ir
III
complexes to coordinate with every Eu ion, so after a maximum value the luminescence intensity fell
due to the weak sensitization and quenching effect made by the water and EtOH molecules.

3 2 2
Fig. 6 Luminescence titration spectra of EuCl ·6H O into Ir(dfppy) (cbphen) in EtOH
When Ir-Eu and Ir -Eu were excited at 360 nm (Fig. 7A), both of them emitted strong red
3
III
luminescence of Eu , while the green emission of Ir(dfppy)
2
(cbphen) was nearly quenched completely.
III
That is to say, the energy transferred from iridium(III) complex to the Eu ion effectively. Moreover,
III
III
longer-wavelength excitation of Eu was studied for the three Eu complexes with an excitation
5
7
wavelength of 470 nm (Fig. 7B). The ratio of the intensity of the peak near 613 nm ( D
three complexes is 12.9 : 33.0 : 1 (Ir -Eu : Ir-Eu : Eu(TTA) ·2H O). The measured quantum yield of
Ir -Eu (10%) was nearly equal to that of Eu(TTA) ·2H O (11%) (Table 4), suggesting the
Ir(dfppy) (cbphen) can sensitize Eu ion well; while Ir-Eu has a high quantum yield of 44%,
quadruples that of Eu(TTA) ·2H O. Silva et al. measured the quantum yields of Eu(TTA) ·2H O and
Eu(TTA) phen (phen = 1,10-phenanthroline), which are 23% and 69% by experiment respectively [55].
0
→ F
2
) for the
3
3
2
3
3
2
III
2
3
2
3
2
3
It is clear that the introduction of both Ir(dfppy) (cbphen) and phen can improve the luminescence
2
III
intensity of Eu
Ir(dfppy)
of 16807 cm [56], lower than the D
complexes are summarized in Fig. 8.
ion because of the removement of coordinated water. Nevertheless, the
III
2
(cbphen) works better than phen, as phen is not good at sensitizing Eu ion with a triplet level
-1
5
III
III
0
level of Eu . The energy transfer processes in these Eu

III
III
III
III
Fig. 7 (A) Emission spectra of Ir -Eu bimetallic complexes (λex = 360 nm); (B) Emission spectra of Ir -Eu
bimetallic complexes and Eu(TTA) ·2H O (λex = 470 nm)
3
2
III
III
For the three Eu complexes, the lifetime for the Eu emission is 344 µs for Eu(TTA)
3
·2H
2
O, 424
µs for Ir
3
-Eu and 428 µs for Ir-Eu (Table 4), implying the larger iridium(III) complex is helpful to
III
prevent the solvent molecule quenching the Eu emission. Fig. 9 exhibits the photographs of the
III
III
solutions of mononuclear iridium(IIII) complexes and europium(IIII) compounds as well as Ir -Eu
bimetallic complexes under excitation with a UV lamp at 365 nm, from which an obvious luminescence
change can be observed.
III
Table 4 Luminescent intensity and lifetime for the Eu complexes
complex
quantum yield Lifetime (µs)
Maximum emission intensity ( λex = 470 nm)
Eu(TTA)
Ir -Eu
3
·2H
2
O
0.11
344
1
0.10
424
428
-
11.6
3
Ir-Eu
0.44
33.3
Eu(TTA)
Eu(TTA)
3
3
·2H
2
O*
0.23 (0.29)
0.69 (0.63)
-
-
phen*
-

*
reported data in Ref [52]; the data in parenthesis are calculated values in the literature.
III
Fig. 8 The energy transfer process in Eu complexes (S
1 1
: the lowest excited singlet state, T : the lowest excited triplet
state)
III
III
Fig. 9 The photographs of the solutions of Ir and Eu complexes under excitation with a UV lamp at 365 nm
4
. Conclusions
We designed and synthesized a novel green-emitting iridium(III) complex Ir(dfppy) (cbphen) with a
2
III
suitable triplet energy level to sensitize Eu ion. After molecular structure modulation, this complex
exhibits a much stronger luminescence at room temperature than our previous Ir(dfppy) (phen5f)
2

III
III
III
complex. The Ir -Eu bimetallic complexes show intense characteristic emission from Eu ion, and
the green emission from iridium(III) center was almost quenched completely. The quantum yields are
0
.11 for Eu(TTA)
3
·2H
2
O, 0.10 for Ir
3
-Eu and 0.44 for Ir-Eu, respectively, confirming
III
Ir(dfppy)
2
(cbphen) is a good sensitizer for Eu ions. By the introduction of the iridium complex, the
III
excitation wavelength of Eu ion was extended to visible range (470 nm).
Acknowledgments
This work was supported by the National Key Basic Research Program of China (No.
2
2
014CB643802) and the National Natural Science Foundation of China (NNSFC, Nos. 21371012,
1321001) for financial support.
Appendix A. Supplementary material
CCDC 764289 contains the supplementary crystallographic data for Ir(dfppy)
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Highlights
>
2
A green-emitting iridium(III) complex Ir(dfppy) (cbphen) was synthesized and fully characterized with
X-ray single crystal structure.
The iridium(III) complex was applied in sensitizing europium ion in both titration experiment and in
>
III
Ir -Eu bimetallic complexes.
III
3
+
Only intense characteristic emission from Eu ion was obtained with high quantum yield.
>
>
3
Excitation wavelength of Eu ions in Ir -Eu bimetallic complexes was extended to visible range (470
+
III
III
nm).