Eu3+ chelate with phenanthroline derivative gives selective emission responses to Cu(II) ions

Article abstract of DOI:10.1016/j.jorganchem.2010.10.023

Two novel ternary europium(III) complexes (Eu-1 and 2) based on modified phenanthroline ring have been successfully synthesized and give characteristic purple-red emissions. The complex with 4-substituted carboxyl group has been proved to act as a selecti

Full text of DOI:10.1016/j.jorganchem.2010.10.023

Journal of Organometallic Chemistry 696 (2011) 829e831
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Eu3þ chelate with phenanthroline derivative gives selective emission responses
to Cu(II) ions
*
Lijun Ma, Liping Chen, Jinbing Wen, Qianming Wang , Chaoliang Tan
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel ternary europium(III) complexes (Eu-1 and 2) based on modified phenanthroline ring have
been successfully synthesized and give characteristic purpleered emissions. The complex with 4-
substituted carboxyl group has been proved to act as a selective luminescence quencher for copper(II)
ions, as shown by the “on/off” switch phenomenon and the smart responsive changes in the UVevis
Received 22 July 2010
Received in revised form
7 October 2010
Accepted 14 October 2010
region (detection limit up to 0.5
mM). The preliminary data imply that Eu-1 could be applicable as
a potential fluorescent sensor for detection of copper(II) ions.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Europium
Copper
Luminescence
Switch
Sensor
1. Introduction
electronegative group such as carboxyl moiety. Therefore, we con-
structed ternary europium(III) complex with benzoyl tri-
a
Cation complexation, in particular copper cation recognition
behavior through metaleligand bonds or hydrogen bond formation,
has attracted great interests in last decade because of its essential role
in biological or environmental system [1,2]. Among various chemical
or physical properties, the purpose of using luminescence to investi-
gate the non-covalent interactions is that emission could be consid-
ered as straightforward approach and sensitive to human eyes [3].
In recent years, the application of f elements such as Eu3þ, Tb3þ
and Sm3þ ions in signaling system has provided the possibility of
sensitizing the lanthanide luminescence via other cation or anions
[4]. The promising feature of lanthanide compounds is presently the
subject of a growing interest. In detail, their narrow line emissions
caused by forbidden transitions, which lead to high color purity of
resulted light, can extend from short wavelength like UV to visible
until near infrared region. In addition, their long-lived emission
excited states cover a wide range from micro to millisecond scale.
This area has been extensively studied by Parker, Tsukube, Bunzli,
Faulkner, Gunnlaugsson and several related researchers [4e9].
Lately, Gunnlaugsson et al. [10] reported a supramolecular phe-
nanthroline complex as a receptor for Cu(II) especially by nitrogen
atom coordination in heterocyclic ring. However, the neutral ligand is
much less effective in selectively binding copper cation than
fluoroacetonate-phenanthroline (Eu-1) for the first step, then
performed corresponding sensing experiments towards a class of
cations through the remaining COOH group. Facts substantiate that the
resulting complex gives rise to sharp luminescence changes unique to
copper ions in the presence of other tested cations (Fig.1). Tothe best of
our knowledge, here is the first report which concentrates on the
lanthanide emission change in terms of mixed heterometallic (cop-
pereligandeeuropium) complex through functional carboxyl group
interactions.
2. Experimental
All the starting materials were obtained from commercial
suppliers and used as received.
2.1. Synthesis and characterization of europium(III) complex of 2-(4-
carboxyphenyl)imidazo [4,5-f]-1,10-phenanthroline (Eu-1, see Fig. 1)
Phen-a ¼ 2-(4-carboxyphenyl)imidazo [4,5-f]-1,10-phenanthroline
Phen-b ¼ 2-(3,5-difluorophenyl)imidazo[4,5-f]-1,10-
phenanthroline
The preparation of phen-a and b used the same method as
reported in Ref. [11]. Eu(benzoyl trifluoroacetonate)3$2H2O
* Corresponding author. Tel.: þ86 20 39310258; fax: þ86 20 39310187.
E-mail address: qmwang@scnu.edu.cn (Q. Wang).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.023

830
L. Ma et al. / Journal of Organometallic Chemistry 696 (2011) 829e831
2.4. NMR experiments
1H NMR spectra of 1 (40.0 mmol Lꢁ1) in the absence and pres-
ence of CuCl2 (0.8 mmol Lꢁ1) were measured in DMSO-d6 by using
Varian NMR Systems 400 MHz spectrometer at room temperature.
b
Cu2+
c
a
O
H
e d
CF3
N
N
O
HOOC
3. Results and discussion
N
N
Eu3+
O
The total emission spectrum reveals the typical europium
spectral bands (5D0 / 7FJ transitions J ¼ 0,1,2 for peaks at 585, 591
and 617 nm respectively) and also exhibits the phenanthroline
fluorescence centered at around 420 nm (Fig. 2A) [12]. This ligand
emission, although limiting europium-based quantum efficiency,
shows a striking purple luminescence due to overlapping with
lanthanide red bands. Upon addition of Cu2þ, a rapid decrease of
Phen-a
O
CF3
O
Eu-1
O
F3C
the emission band at 420 nm occurred (saturated at 10.0 m
M Cu2þ
with a 119-fold decrease), going with a large reductions of emission
band at 593 and 617 nm, and the fluorescent color of the solution
turned from purple to colorless (fluorescent intensity of
lem ¼ 617 nm showed ca. 80% quenching at 25.0
Fig. 1. Chemical structures of europium(III) complex of 2-(4-carboxyphenyl)imidazo
[4,5-f]- 1,10-phenanthroline (Eu-1).
(Eu(BTA)3$2H2O) was prepared according to Ref. [12]. In brief, Eu-1
was prepared by the coordination of Eu(BTA)3$2H2O (20 mg,
25 mmol) and 1 (8.5 mg, 25 mmol) in methanol (8 mL). The whole
m
M Cu2þ, inset in
Fig. 2A). The fluorescent quenching effect can be compared to the
results previously obtained with Eu (III)-containing probes [13].
Due to complexation, consequent electron transfer quenching of
the phen-a S1 excited state by paramagnetic Cu2þ, and Eu-1
exhibits a luminescent switch for Cu2þ. The effective stability
constant (Ka) can be obtained based on the HildebrandeBebesi
equation [14]. The linear least-squares curve fitting of the plot with
I0/(I ꢁ I0) versus [H]ꢁ1 indicates a 1:1 binding model between the
host and the guest molecules. The binding constant of Eu-1/Cu2þ is
5.70 ꢀ 103 L molꢁ1 (Fig. 2B), revealing a medium complexation of
Eu-1 with Cu2þ. We also examined the fluorescence response of
Cd2þ, Co2þ, Cr3þ, Hg2þ and Pb2þ under the same conditions used to
test Cu2þ (Figs. S1A and S2A). The fluorescent intensity of Eu-1
resulted in less quenching, especially for the emission peak at
617 nm.
mixture was refluxed for 4 h and cooled to room temperature. The
resulting precipitate was collected and washed twice with water
and ethanol to give the titled complex (27 mg, 95%) as yellow
powder. EA found: C, 52.71; H, 2.43; N, 4.99%, Anal. Calcd for
C50H30EuF9N4O8 [¼Eu(BTA)3(1)]: C, 52.78; H, 2.66; N, 4.92%.
The preparation of Eu-2 was very similar to Eu-1 except that 2-
(3,5-difluorophenyl)imidazo[4,5-f]-1,10-phenanthroline (2) takes
place of 1. EA found: C, 52.15; H, 2.41; N, 4.91%, Anal. Calcd for
C49H28EuF11N4O6 ¼ EuBTA3-2: C, 52.09; H, 2.50; N, 4.96%.
2.2. Fluorescence titration experiments
Fluorescence emission spectra were recorded on a computer
controlled HITACHI F-2500 fluorescence spectrophotometer. A
fixed excitation wavelength at 350 nm was used in the emission
spectra. Fluorescence titration experiments were performed in
DMSO by using chloride of metal ions respectively. The concen-
tration of Eu-1 in all the fluorescent experiments is
5.0 ꢀ 10ꢁ6 mol Lꢁ1 in DMSO.
Obvious changes in the absorption spectrum of Eu-1 occurred
with the addition of Cu2þ (Fig. 3A). As the concentration of Cu2þ
varied from 0.0 to 20.0 mM, a new absorption peak at 305 nm
appeared and the absorption bands at 333 nm red shifted. The
changes of the absorption spectrum of Eu-1 signify the coordina-
tion of Cu(II) by the phen-a moiety [14]. However, the absorption
spectra of Eu-1 give no response for the addition of Cd2þ, Co2þ
,
2.3. UVevis absorption experiments
Cr3þ, Hg2þ and Pb2þ, respectively (Figs. S1B and S2B). The signifi-
cant difference of Cu2þ from these metal ions in absorption spec-
trum of Eu-1/metal ions possibly attributed to the inherent to
paramagnetic species of Cu2þ (it produces energy or electron
transfer) [15,16]. The shifts of absorption peak changed with addi-
tion of various metal ions (Fig. 3B). Therefore, such a dual response,
The measurements of UVevis absorption spectra were carried
out with a SHIMADZU UV-1700 Spectrophotometer. The concen-
tration of Eu-1 in all the UVevis experiments is 5.0 ꢀ 10ꢁ6 mol Lꢁ1
in DMSO.
Fig. 2. (A) Fluorescence emission spectra of Eu-1 (5.0
80.0, 100.0 and 120.0
m
M) with different concentrations of Cu2þ (0, 0.66, 1.3, 2.0, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5, 15.0, 20.0, 25.0, 30.0, 35.0, 40.0, 50.0, 60.0,
m
M) in DMSO. Inset shows fluorescence photographs of Eu-1 and Eu-1/Cu2þ (5.0
mM:25.0
m
M) under illumination with 365 nm light. (B) Estimation of binding
constant for Eu-1 with Cu2þ in DMSO at room temperature. The plot based on the intensity changes at 617 nm with a 1:1 binding model: I0/(I ꢁ I0) versus [Cu2þ
]
(R2 ¼ 0.9994).
ꢁ1

L. Ma et al. / Journal of Organometallic Chemistry 696 (2011) 829e831
831
0.6
0.5
0.4
0.3
0.2
0.1
0.0
15
A
B
2+
2+
2+
Cu
Cd
Co
12
9
9
3+
9
Cr
2+
Hg
2+
1
6
Pb
1
3
0
0
5
10
[M ] (10 M)
15
-6
20
25
280
300
320
340
360
Wavelength / nm
380
400
420
n+
Fig. 3. (A) UVevis spectra of europium complex (5.0
shift on the concentration of Cu2þ, Cd2þ, Co2þ, Cr3þ, Hg2þ and Pb2þ
m mM (from 1 to 9). (B) Dependence of peak
M) with different concentrations of Cu2þ 0.0, 1.0, 2.0, 4.0, 5.0, 7.5, 10.0, 15.0 and 20.0
.
distinguished by both the appearance of the new peak of 305 nm
and the red shift of peak in absorption spectra, illustrated the high
selectivity of Eu-1 to Cu2þ over the other examined metal ions. In
addition, it should be noted that high sensitivity for Cu2þ (detection
new peak and red shift of peak in absorption spectra. These studies
will provide a foundation for future developments of the fluores-
cent, spectrophotometric switches and sensors by lanthanide ions.
Our effort is aimed to exploit the luminescence sensitivity and
expand its application as fluorescent label devices used in biolog-
ical or industrial fields.
limit up to 0.5 mM) is achieved by the absorption spectra of Eu-1 in
the present study. The red shift of the maxima absorption peak can
be ascribed to be the formation of a J-pattern aggregate and copper
(II) coordination leads to the close interactions of adjacent phe-
nanthroline planes [17].
Acknowledgements
In order to further confirm the function of eCOOH group, we
prepared another europium(III) complex, 2-(3,5-fluorophenyl)
imidazo [4,5-f]-1,10-phenanthroline (Eu-2, Fig. S3), which
possesses a structure analogous to Eu-1 and only a 4-substituted
carboxyl group replaced by 3,5 substituted fluorine at phenan-
throline. The fluorescent emission bands of Eu (III) at 617 nm
present no obvious change with the addition of Cu2þ in the fluo-
rescent spectra of Eu-2 (Fig. S4A). Simultaneously, the absorption
bands exhibit no obvious shifts in UVevis absorption spectra of Eu-
2 (Fig. S4B). Therefore, it can be concluded that the 4-substituted
carboxyl group in phenanthroline plays a crucial role for the
response of Eu-1 for Cu2þ. In terms of the paramagnetism of Eu (III),
we attempted to explore the coordination mechanism of Eu-1/Cu2þ
using 1H NMR experiment of phenanthroline/Cu2þ (see Supporting
Information). The addition of Cu2þ led to the large chemical shift of
4-substituted carboxyl group in phenanthroline (from 13.694 to
12.799 ppm). The proton signals of phenanthroline ring (a and c)
were more adjacent to each other, showing that copper recognition
might induce the phenanthroline plane stacking together. Partic-
ularly, He which is in proximity to carboxyl moiety, has relative
large downfield shift (8.414e8.431, see Table 1) compared with Hd
and Hb. The fact suggests that the coordination through carboxylate
group will make the nearer aromatic protons locate in a decreasing
Lijun is grateful to the projects of the Natural Science Founda-
tion of China (No. 20903041) and Qianming thanks for the National
Natural Science Foundation of China (21002035) and start funding
of South China Normal University No. G21117.
Appendix. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2010.10.023.
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4. Conclusions
We have reported a lanthanide luminescent switch and UVevis
sensor for Cu2þ, which shows a fluorescent quenching of Eu (III) by
Cu2þ in fluorescent spectra and a high selectivity to Cu2þ over
others examined metal ions in absorption spectra. The sensor
exhibited a dual response, distinguished by both the appearance of