A luminescent anion sensor based on a europium hybrid complex

Full text of DOI:10.1021/ja0118688

12694
J. Am. Chem. Soc. 2001, 123, 12694-12695
A Luminescent Anion Sensor Based on a Europium
Hybrid Complex
Scheme 1
1
,1
1
Marco Montalti, Luca Prodi,* Nelsi Zaccheroni,
2
2
,2
Lo ¨ı c Charbonni e` re, Laurent Douce, and Raymond Ziessel*
Laboratoire de Chimie Mol e´ culaire
ECPM, 25 rue Becquerel, BP 08
7087 Strasbourg Cedex 2, France
6
ReceiVed August 1, 2001
Over the past few decades, considerable attention has been
devoted to the design of luminescent labels and sensors, since
they offer the real possibility to solve critical analytical problems
in growing fields of social and economical importance.3,4 For
example, the widespread use of nitrogen- and phosphorus-rich
fertilizers in agriculture has ensured that nitrates and phosphates
are major pollutants in soils and wastewater. Although nitrate is
not considered toxic to humans, its metabolites may be carcino-
-
The relevant europium complexes [Eu‚1]X
3
(X ) NO
) were prepared by mixing stoichiometric amounts of 1
with the corresponding Eu-salt in a CH Cl /MeOH mixture for
the nitrate or in CH CN for the triflate salt. On the basis of mass
3
or
-
3 3
CF SO
2
2
3
spectroscopy, it was established that both complexes possess a
stoichiometry of one ligand to one Eu, an assignment supported
by elemental analysis.
IR spectra of all complexes in the solid state displayed features
characteristic of the ligand (νCdC, νCdN, νPdO) and of the anion
employed. For nitrate, the splitting between the two highest
5
gens. Most of the sensors used to analyze nitrates in food,
fertilizers, or plant tissues are based on the principle of ion
6
exchange and lack sensitivity. Undoubtedly the development of
highly selective ionophores able to monitor anions at the
subnanomolar range remains a major goal for contemporary
analytical science. In this context, an ever-growing interest
revolves around the design of luminescent lanthanide labels
-
frequency bands attributed to the NO
3
vibrations (∆ν ) 170
-
1
cm ) is in keeping with a bidentate chelating mode of ap-
proximate C2V local symmetry. Coordination of the P(O) to the
europium is substantiated by splitting of the PdO absorption band
7
possessing excited-state lifetimes in the ms range. Among such
-
1
systems, the Ln(III) texaphyrins exhibit significant affinities for
oxyanions but suffer from poor luminescence properties.8
In seeking to design an improved anion sensor we have
capitalized on the realization that the PdO group binds tightly
to lanthanide cations.9 Furthermore, it is known that bipy
fragments act as effective photon antennae for Ln cations. Putting
together these two ideas, we have synthesized the bis-bipyridine-
phenylphosphine oxide ligand 1,1 which appears to possess all
the features to be a good luminescent anion sensor when
complexed with lanthanide cations. The pentadentate ligand in
fact ensures that the first coordination sphere of the Ln cation is
incompletely saturated, thereby leaving vacant coordination sites
for interaction with the incoming anion. A schematic representa-
tion of the ligand and the coordination pocket available for
lanthanide binding is sketched in Scheme 1.
with an average shift of 15 cm toward lower energy. For the
triflate complex, this shift is increased to 75 cm . Stretching
vibrations corresponding to triflate anions (νs(SO2) ) 1029 cm
and νas(SO3) ) 638 cm ) point to coordination in the first
coordination sphere, while bands at 573 and 517 cm (νEu-O
also indicate the presence of coordinated water in the solid state.
SO in acetonitrile
(Figure 1a) is dominated by transitions centered on the bipy
moieties, since the phenyl chromophore linked to the P-atom has
a much lower molar absorption coefficient. Complexation of Eu
cations shifts the lowest-energy absorption band from λmax ) 291
nm (ꢀ ) 28 000 M cm ) in the free ligand to 312 nm (ꢀ )
17 450 M cm ) for the complex. Such a variation in the πfπ*
transitions centered on the bipy moieties is indicative of coordina-
tion to Eu-center and could be used to determine a stability
constant (log â ) 5.8 ( 0.5 M for the formation of a 1/1 ligand
to Eu stoechiometry). Interestingly, this bathochromic shift
becomes less pronounced in solvents with increasing coordinative
-1
-
1
-
1
-1
)
The absorption spectrum of [Eu‚1](CF
3
3 3
)
0
3+
-
1
-1
-1
-1
-1
(1) Dipartimento di Chimica “G. Ciamician”, Universit a` di Bologna and
11
Via Selmi 2, 40126 Bologna, Italy, Fax: 39-051-2089456, E-mail: lprodi@
ciam.unibo.it.
-1
-1
(
2) Laboratoire de Chimie Mol e´ culaire, EÄ cole de Chimie, Polym e` res,
affinity such as methanol (λmax ) 294 nm; ꢀ ) 21 400 M cm )
or water (λmax ) 292 nm; ꢀ ) 22 000 M cm ). Upon excitation
into the absorption bands of the ligand in acetonitrile at 20 °C,
Mat e´ riaux, 25 rue Becquerel, BP 08, 67087 Strasbourg Cedex 2, France.
Fax: 33-390-242689. E-mail: ziessel@chimie.u-strasbg.fr.
-1
-1
(
3) (a) Fluorescent Chemosensors for Ion and Molecule Recognition;
1
2
Czarnik, A. W., Ed.; American Chemical Society: Washington, DC, 1992.
3 3 3
the [Eu‚1](CF SO ) complex luminesces weakly (Φ ) 0.026,
(
b) Chemosensors for Ion and Molecule Recognition; Desvergne, J.-P., Czarnik,
A. W., Eds.; NATO Advanced Study Institute Series; Kluwer Academic
Publisher: Dordrecht, 1997.
τ ) 0.80 ms) (Figure 1b). The luminescence excitation spectrum
is very similar to the absorption spectrum, confirming that energy
transfer from the ligand to the metal ion occurs. Addition of
tetrabutylammonium triflate (100 equiv) has no observable effect
on either absorption or luminescence spectra, in keeping with a
weak interaction of the triflate anion with the Eu center in
(
4) (a) Mayer, A.; Neuenhofer, S. Angew. Chem., Int. Ed. Engl. 1994, 33,
1
044. (b) Fabbrizzi, L.; Poggi, A. Chem. Soc. ReV. 1995, 197. (c) De Silva,
A. P.; Gunaratne, Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.;
Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515. (d) Prodi, L.;
Bolletta, F.; Montalti, M.; Zaccheroni, N. Coord. Chem. ReV. 2000, 205, 59.
(
5) Ellis, G.; Adatia, I.; Yazdanpanah, M.; Makela S. K. Clin. Biochem.
13
solution.
1
998, 31, 195.
Addition of tetrabutylammonium nitrate (Figure 1b), chloride,
fluoride, or acetate to an acetonitrile solution of [Eu‚1](CF SO )
3 3 3
(
6) B u¨ hlmann, P.; Pretsch, E.; Bakker, E. Chem. ReV. 1998, 98, 1593.
7) (a) Carnall, W. T. Handbook on the Physics and Chemistry of Rare
(
Earth; Gschneidner, K. A., Jr., Eyring, L., Eds.; North-Holland Publishing
Co.: Amsterdam, 1979; Vol. 3, p 171. (b) Sabbatini, N.; Guardigli, M.; Manet,
I.; Ziessel, R. In Calixarenes 2001; Asfari, Z., B o¨ hmer, V., Harrowfield, J.,
Vicens, J., Eds.; Kluver Academic Publishers: Dordrecht, The Netherlands,
(11) Association constant was obtained in anhydrous CH
3 6
CN/TBAPF 0.01
M by monitoring changes in the UV-vis absorption spectra of ligand solutions
-3
-5
-4
(5 × 10 to 10 M) titrated by increasing amounts of [Eu(OTf)
3
] (10 to
-5
2
001; p 583 and references therein.
8) (a) Lisowski, J.; Sessler, J. L.; Mody, T. D. Inorg. Chem., 1995, 34,
336. (b) Lisowski, J.; Sessler, J. L.; Lynch, V.; Mody, T. D. J. Am. Chem.
5 × 10 M). The spectral changes were fitted to the following equation 1 +
3+ 3+
(
Eu h [Eu‚1] using the Specfit software (see ref 14).
4
(12) Luminescence quantum yields (uncertainty ( 15%) were determined
2
+
Soc. 1995, 117, 2273.
9) Baaden, M.; Berny, F.; Boehme, C.; Muzet, N.; Schurhammer, R.;
Wipff, G. J. Alloys Compd. 2000, 303, 104.
10) Douce, L.; Charbonni e` re, L.; Cesario, M.; Ziessel, R. New J. Chem.
001, 25, 1024.
using [Ru(bpy)
3
]
as standard (Φ ) 0.028 in aerated water), Nakamaru, K.
(
Bull. Chem. Soc. Jpn. 1982, 55, 2697.
(13) B u¨ nzli, J.-C.; Milicic-Tang, A. Handbook on the Physics and Chemistry
of Rare Earths; Gschneidner, K. A., Jr.; Eyring L., Eds.; North-Holland
Publishing Co.: Amsterdam, 1995; Vol. 21, p 305.
(
2
1
0.1021/ja0118688 CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/20/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 50, 2001 12695
Table 1. Cumulative Association Constants for Complexes
Formation with One, Two, and Three Anions in Acetonitrile
Solution at 25 °C, Starting from the [Eu‚1](OTf)
3
Complex
Log â
14.4 ( 0.7
Log â Log â
1
2
3
-
-
NO
3
6.1 ( 0.3
6.9 ( 0.2
7.8 ( 0.7
6.9 ( 0.6
11.0 ( 0.2
12.0 ( 0.2
15.5 ( 1.1
13.2 ( 0.8
-
Cl
<14.5
-
F
21.2 ( 1.2
19.1 ( 1.1
AcO
Evolution of the emission spectrum was also analyzed during
the titration, confirming the luminescence enhancement (Figure
1b). The changes observed upon addition of nitrate and, in
particular, the different structure of the more symmetry-sensitive
5
7
5
7
0 2 0 4
transitions ( D f F and D f F ) indicates that this anion
coordinates to europium. As such, the anion must replace
acetonitrile, water, or triflate molecules bound at the metal center.
A similar effect can be ascribed to the other anions, which are
also expected to be inserted in the first coordination sphere.
Similar modulations of lanthanide luminescence have been
2
-
-
16
observed in the presence of CO
Taken together, these findings are consistent with the entry of
3 3
, HCO , and organic anions.
-
-
-
-
anions such as F , AcO , Cl , NO
3
to the first coordination
sphere of the [Eu‚1](CF SO complex giving rise to drastic
3
3 3
)
changes in the luminescence intensity. An appreciation of the
above features led us to consider that the first anion coordinates
to [Eu‚1] to produce a dicationic species. This is corroborated
Figure 1. (a) Evolution of the UV-vis absorption spectrum of [Eu‚1]-
-
5
(OTf)
3
(5.36 × 10 M) in acetonitrile upon addition of increasing
3+
amounts of tetrabutylammonium nitrate. (b) Emission spectra of [Eu‚1]-
by the fact that very slight modification of the UV-vis spectra
is observed during the onset of the titration experiments. Addition
of the second equivalent of anion has a more drastic influence
on the absorption spectrum. This is in keeping with a concomitant
coordination of the second anion with dissociation of a single
bipy arm, leading to a monocationic species. It is worth noting
that the increase of luminescence quantum yield and that of
excited state lifetime are not parallel, indicating that the coordi-
nated anion influences the efficiency of the energy-transfer
processes from the ligand to the metal ion. Studies for correlating
the structure of the various species to the efficiency of such
processes are actually in progress in our laboratories. Finally,
addition of the third anion has a more spectacular effect with an
additional 30 nm hypsochromic shift of the most intense absorp-
tion band due to disassociation of the second bipy arm and
formation of a neutral species. This process, as expected, typically
leads to a decrease of the luminescence intensity.
-
5
(
OTf)
TBANO
electrolyte TBAPF
of NO equivalents.
3
(7.45 × 10 M) in the presence of 0, 0.6, 1.0, and 2.0 equiv of
, showing the increase of the D f F transitions. Supporting
, 0.01 M. Inset: Increase of Iem at 614 nm vs number
5
7
3
0
J
6
3
greatly changed its photophysical properties. The most striking
effect was observed for nitrate, where the emission yield increased
by a factor of 11-fold upon addition of 2 equiv of salt (respectively
5
, 1.8, and 1.5 for chlorides, acetates, and fluoride). Under these
conditions, the excited-state lifetime and emission quantum yield
reached values similar to that obtained with an authentic sample
of [Eu‚1](NO ) (Φ ) 0.30, τ ) 1.45 ms). Furthermore, it is worth
3 3
noting that the structure of the luminescence profile changes
during the titration with nitrate or with the other anions that
progressively replace the loosely bound solvent molecules.
From the variation in the absorption spectra of an acetonitrile
3 3 3
solution of [Eu‚1](CF SO ) on addition of anion (Figure 1a) it
We have therefore demonstrated that the present system
registers anion binding by an increase in luminescence yield and
lifetime. This provides the possibility to employ such complexes
as luminescent sensors for anions; systems with this ability are
14
was possible to calculate cumulative association constants for
the formation of complexes with one, two, or three anions.
2
+
[
(Eu‚1‚X) ]
17
3
+
-
2+
much less common than cation sensors and of great practical
[
Eu‚1] + X h [Eu‚1‚X]
â1 )
3+
-
importance. This new system is particularly useful relative to other
molecules because it allows detection of traces of halides and
nitrates by luminescence monitoring at an europium center, whose
long excited-state lifetime allows the enhancement of the signal-
to-noise ratio with the use of time-resolved spectroscopy. The
design of second-generation sensors, able to perform in coordinat-
ing solvents, might be achieved by rational design of locking bipy
[
(Eu‚1) ] × [X ]
+
[
(Eu‚1‚X ) ]
3
+
-
+
2
[
Eu‚1] + 2 X h [Eu‚1‚X ] â2 )
2
3+
- 2
[
(Eu‚1) ] × [X ]
(Eu‚1‚X )]
[
3
+
-
3
[
Eu‚1] + 3 X h [Eu‚1‚X ] â3 )
3
3+
- 3
18
[
(Eu‚1) ] × [X ]
subunits such as the use of anionic bipy fragments.
Supporting Information Available: Experimental details for the
synthesis of the ligand and complexes, characterization of all new
The cumulative association constants with increasing number
of anions are gathered in Table 1. Except for chloride, three
binding constants could be determined for each system, while
compounds. Absorption and luminescence spectrum of the [Eu‚1](CF -
3
SO
)
3 3
3 3
and [Eu‚1](NO ) complexes in acetonitrile and experimental data
-
the affinity of the Eu-complex for anions follows the trend: F
for UV-vis titration experiments for each anion are also provided (11
pages, print/PDF). See any current masthead page for ordering information
and Web access instructions.
-
-
-
>
AcO > Cl > NO
3
. The association constants are very high
in all cases and significantly different from those found by
species in DMF
This discovery is in keeping with the pronounced
π-acidic character of the hybrid ligand 1.
(3-n)+
JA0118688
calorimetric titration for formation of [EuCl
n
]
solution.1
3,15
(16) Bruce, J. I.; Dickins, R. S.; Govenlock, L. J.; Gunnlaugsson, T.;
Lopinski, S.; Lowe, M. P.; Parker, D.; Peacock, R. D.; Perry, J. J. B.; Aime,
S.; Botta, M. J. Am. Chem. Soc. 2000, 122, 9674.
(
14) (a) Binstead R. A. SPECFIT; Spectrum Software Associates: Chapell
(17) Harriman, A.; Hissler, M.; Jost, P.; Wipff, G.; Ziessel, R. J. Am. Chem.
Soc. 1999, 121, 14.
Hill, NC, 1996. (b) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberb u¨ hler A. D.
Talanta 1985, 32, 257.
(18) Charbonni e` re, L.; Ziessel, R.; Guardigli, M.; Roda, A.; Sabbatini, N.;
Cesario, M. J. Am. Chem. Soc. 2001, 123, 2436.
(15) Ishiguro, S.; Takahashi, R. Inorg. Chem. 1991, 30, 1854.