Strong emission increase of a dicarboxyterpyridene europium (III) complex in the presence of citrate and hydrogen peroxide

Article abstract of DOI:10.1016/j.ica.2005.01.015

A substituted 6,6″-dicarboxy-2,2′:6′,2″-terpyridene europium (III) complex (1) responds with an emission wavelength shift and intensity increase to the presence of citrate and hydrogen peroxide, while many oxoanions show no effect.

Full text of DOI:10.1016/j.ica.2005.01.015

Inorganica Chimica Acta 358 (2005) 2445–2448
www.elsevier.com/locate/ica
Note
Strong emission increase of a dicarboxyterpyridene europium
(III) complex in the presence of citrate and hydrogen peroxide
a,
b
b
c
Valery N. Kozhevnikov *, Christian Mandl , Stefan Miltschitzky , Axel Duerkop ,
c,
b,
*
Otto S. Wolfbeis *, Burkhard Koenig
a
Department of Chemistry, University of Ekaterinburg, Ekaterinburg, Russia
b
c
Institute of Organic Chemistry, University of Regensburg, Universitaetsstr. 31, 93040 Regensburg, 93040 Germany
Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, 93040 Regensburg, Germany
Received 17 December 2004; accepted 21 January 2005
Abstract
A substituted 6,600-dicarboxy-2,20:60,200-terpyridene europium (III) complex (1) responds with an emission wavelength shift and
intensity increase to the presence of citrate and hydrogen peroxide, while many oxoanions show no effect.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Europium complexes; Luminescent probes; Citrate sensing
1. Introduction
from the primary or secondary coordination sphere.
This is accompanied by changes in luminescence inten-
sity, especially of the hypersensitive D0 ! F2 transi-
tion at 615 nm, and by a change of luminescence
decay time.
5
7
Luminescent probes are important analytical tools in
chemistry, biochemistry or medicinal diagnostics [1].
Lanthanide complexes are of particular interest, because
their long emission decay times and large Stokes shifts
allow signal detection in the presence of short lived
background luminescence [2]. The chelating ligand usu-
ally acts as a photo antenna transferring absorbed en-
ergy to the lanthanide ion. In addition, the ligand
largely determines the complex properties, such as solu-
bility, hydrophobicity, number of labile coordination
sites or allows complex immobilization. The binding of
the analyte to the Lewis-acidic lanthanide ion often re-
sults in a repulsion of labile coordinated water molecules
Europium complexes with derivatives of 2,20:6,20-ter-
pyridine [3] or N-(isothiocyanatobenzyl)diethylenetria-
mine-N,N0,N00,N000-tetrakis(acetic acid) [4] have been
used for protein labeling. Complexes of cryptates [5]
are well established for the development of fluorescent
time-resolved bioassays. The cyberfluor system was
introduced for similar purposes, but employs 4,7-di-
(chlorosulfophenyl)-1,10-phenanthroline-2,9-dicarbox-
ylic acid as the light harvesting ligand for Eu3+ [6].
Recent applications use complexes of modified
1,4,7,10-tetraazacyclododecane (cyclen) as probes for
oxoanions [7] and the europium tetracycline complex
for the detection of hydrogen peroxide [8,9].
*
Corresponding authors. Tel.: +49 0941 94314576; fax: +49 0941
9431717.
We report here the structure, emission properties and
specific response to citrate and hydrogen peroxide of ter-
pyridene europium (III) complex 1 (see Scheme 1).
E-mail addresses: kozhevnikov_v@htf.ustu.ru (V.N. Kozhevni-
kov), Otto.Wolfbeis@chemie.uni-regensburg.de (O. Wolfbeis),
Burkhard.Koenig@chemie.uni-regensburg.de (B. Koenig).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.01.015

2446
V.N. Kozhevnikov et al. / Inorganica Chimica Acta 358 (2005) 2445–2448
anol to give 3 (330 mg, 85%). MS (FAB): C35H25
N3O4Eu = 703 [M+].
Crystal data of 1: C39H36N3O8Eu M = 826.70, tri-
-
N
N
O
N
O
˚
clinic, a = 9.5362(5), b = 11.2777(6), c = 17.7004(10) A,
Eu3+
3
˚
ꢀ
U = 1751.01(18) A , T = 173 K, space group P1, Z = 1,
l(Mo Ka), 8597 reflections measured, 6551 unique
(Rint = 0.0423) which were used in all calculations. The
final wR(F2) was 0.0934 (all data).
O
O
O
O H
O
OH
H
1
Scheme 1. Structure of complex 1.
3. Results and discussion
2. Experimental
The ligand, 2,6-bis[5-phenyl-6-carboxy-3,4-cyclo-
pentenopyridyl-2]-pyridine, is efficiently prepared in
gram amounts by a recently published procedure (see
Scheme 2) [11,12]. The synthetic route allows a wide var-
iation of the pyridine substitution pattern. Complex 1
was obtained by reaction of the ligand with Eu(AcO)3 ·
6H2O in ethanol–methanol solution and was character-
ized by mass spectrometric and X-ray analysis. The crys-
tal structure analysis (see Fig. 1) reveals the presence of
one acetate, one ethanol and one water molecule in the
europium coordination sphere in the solid state. Bond
lengths and angles are typical.
Compound 2 was prepared as described previously
[10].
2,6-Bis(6-carboxy-5-phenyl-3 4-cyclopentenopyridyl-
2)-pyridine (3): 2,6-Bis(6-cyano-5-phenyl-3,4-cyclopent-
enopyridyl-2)-pyridine
2
(668 mg, 1.3 mmol) was
dissolved in concentrated sulfuric acid (3 ml) and stirred
at 100 ꢁC for 3 h. Water (3 ml) was carefully added, the
reaction mixture was stirred at 110 ꢁC for 24 h and then
poured into ice. An aqueous solution of potassium
hydroxide 5 M was added until basic pH and the result-
ing solution was treated with dichloromethane
(2 · 30 ml). The aqueous layer was acidified with acetic
acid and extracted with dichloromethane (5 · 30 ml).
The combined organic phase was dried over sodium sul-
fate, the solvent was removed in vacuum and the residue
was recrystallized from DMSO to give compound 3
(430 mg, 60%); m.p. 205–206 ꢁC. 1H NMR (DMSO-
d6): d2.02 (m, 4H, CH2), 2.83 (t, 4H, 7.5 Hz, CH2),
3.40 (t, 4 H, 7.5 Hz, CH2), 7.39–7.51 (m, 10H, Ph),
8.16 (dd, x1H, 7.2 Hz, 7.2 Hz, pyr), 8.26 (d, 2H,
7.2 Hz, pyr). MS (FAB): C35H27N3O4 = 553 [M+]. Anal.
calc. for C35H27N3O4 (553.6): C, 75.93; H, 4.92; N, 7.59.
Found: C, 75.64; H, 4.85; N, 7.42%.
The absorption spectrum with a maximum at 337 nm
(pH 6.9, 4-morpholino propane sulfonic acid, MOPS
buffer) is only slightly affected by adding citrate or phos-
phate. Upon addition of H2O2, a significant increase of
Synthesis of the Eu chelate 1. A suspension of ligand 3
(260 mg, 0.47 mmol) in ethanol (10 ml) was added to an
aqueous methanol solution (50%, 10 ml) of Eu(AcO)3
(232 mg, 0.7 mmol). The reaction mixture was heated
under reflux for 5 min dissolving ligand 3 completely.
The solution was very slowly evaporated over 2 weeks
reducing the volume to approximately 7 ml leading to
crystals of 1 suitable for X-ray analysis. The crystals
were filtered off and washed with a small amount of eth-
Fig. 1. Structure of complex 1 in the crystal.
O
O
O
N
O
N
CN
CN
Eu(OAc)3,
EtOH
N
N
H2SO4, H2O
100oC
N
N
1
85 %
60 %
2
3
Scheme 2. Synthesis of complex 1.

V.N. Kozhevnikov et al. / Inorganica Chimica Acta 358 (2005) 2445–2448
2447
1,2
1,0
0,8
0,6
0,4
0,2
0,0
10
8
6
4
2
3
2
1
4
1
3
4
2
0
300
320
340
360
380
300
350
400 575
600
625
wavelength / nm
wavelength / nm
Fig. 2. Absorption spectra of Eu3+ complex 1 (60 lmol/L in MOPS
buffer) in the absence of analyte (1), in the presence of H2O2 (100 lmol/
L) (2), in the presence of citrate (100 lmol/L) (3), and in the presence
of phosphate (100 lmol/L) (4).
Fig. 3. Emission and excitation (615 nm) spectra of the Eu3+ complex
1 (60 lmol/L in MOPS buffer), in the absence of analyte (1), in the
presence of H2O2 (100 mmol/L) (2), in the presence of citrate
(100 lmol/L) (3), and in the presence of phosphate (100 lmol/L) (4).
that only the second equivalent of added citrate is
changing the emission properties of the europium com-
plex. However, in the absence of structural data no
molecular mechanism of the binding process or struc-
ture of the analyte-bound complex can be derived.
In summary, we report the synthesis, structure and
emission property changes in the presence of citrate of
a new europium (III) terpyridine complex, which may
be of interest for applications as a molecular probe.
the absorbance at shorter wavelength is observed. (see
Fig. 2)
The complex shows a weak emission at 615 nm (exci-
tation wavelength 320 nm) in buffered aqueous solution.
Upon addition of citrate, a strong increase of emission
intensity is observed (Fig. 3, trace 3). Mono- or dicar-
boxylic acids of equal concentration and anions, such
as sulfate, nitrate, chloride, acetate or carbonate, show
no significant effect on the emission properties. The
addition of phosphate leads to a small shift of the emis-
sion wavelength, but not to an increase in emission
intensity. The presence of hydrogen peroxide induces
changes of the emission properties at concentrations
above 100 mmol/L analyte. An approx. fivefold increase
in emission intensity and a small, but significant shift in
the emission wavelength are observed. The excited state
lifetimes of 1 increase in the presence of citrate or H2O2,
which indicates that emission quenching water mole-
cules in the europium coordination sphere are displaced.
The different changes in emission properties of 1 induced
by citrate or H2O2 may allow ratiometric measurements
Acknowledgements
V.N.K. thanks the Russian Foundation for Basic Re-
search (No. 04-03-96106) and the Alexander von Hum-
boldt foundation for support.
Appendix A. Supplementary data
CIF file of crystal structure of 1; isothermal calori-
metric titration data of 1 with citrate. Supplementary
data associated with this article can be found, in the on-
line version at doi:10.1016/j.ica.2005.01.015.
1
for their distinction.
Isothermal calorimetric titration of complex 1 with
citrate in buffered solution (MOPS, pH 6.5) at 30 ꢁC re-
vealed two binding processes with affinities of 2 · 107
and 2 · 104 l/mol, respectively. The first process is
clearly driven by entropy, while the second one shows
a negative binding enthalpy (DH = ꢀ3600 cal/mol). This
suggests the stepwise formation of a 2:1 citrate – 1 com-
plex and correlation with fluorescence titrations show
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