Coordinative and electrostatic forces in action: From the design of differential chromogenic anion sensors to selective carboxylate recognition

Article abstract of DOI:10.1039/b314997h

A new family of differential chromogenic anion chemosensors is described based on anilinopyridine-metal cation coordinative signalling ensembles.

Full text of DOI:10.1039/b314997h

Coordinative and electrostatic forces in action: from the design of
differential chromogenic anion sensors to selective carboxylate
recognition†
Beatriz García-Acosta,a Xavier Albiach-Martí,a Eduardo García,b Luis Gil,b Ramón
Martínez-Máñez,*a Knut Rurack,*c Félix Sancenóna and Juan Sotoa
a GDDS, Dpto de Química, Universidad Politécnica de Valencia, Camino de Vera s/n, Valencia 46071,
Spain. E-mail: rmaez@qim.upv.es; Fax: +34 96 3879349; Tel: +34 96 3877343
b Group of Hibrid Microelectronics, Dpto de Ingeniería Electrónica, Universidad Politécnica de Valencia,
Camino de Vera s/n, Valencia 46071, Spain. E-mail: lgil@eln.upv.es; Fax: +34 96 3876009
c
Div. I.3, Bundesanstalt für Materialforschung und – prüfung (BAM), Richard-Willstätter-Strasse 11,
D-12489 Berlin, Germany. E-mail: knut.rurack@bam.de; Fax: +49 30 81045005
Received (in Cambridge, UK) 27th November 2003, Accepted 28th January 2004
First published as an Advance Article on the web 26th February 2004
A new family of differential chromogenic anion chemosensors is
described based on anilinopyridine–metal cation coordinative
signalling ensembles.
reflected in the respective spectral fingerprint of each C–M–X
complex. We envisioned that combination of a suitable C with
different metal ions M should yield a family of differential
chemosensor ensembles for the pattern recognition of anions.
With this concept in mind, we chose the CT-active anilinopyr-
idine dye 1 as the chromophore component of the ensemble
(Scheme 1, inset). In 1, the 2,6-diphenylpyridine acceptor (A) unit
promised to show the required metal ion binding features.8 Since
under the conditions employed, simple dimethylanilino donors (D)
are also known to weakly coordinate certain transition metal ions,
the donor side of 1 was equipped with a dithia dioxa crown-type
“shielding” group. This chemical modification, which was inspired
by previous studies on thia anilino-crowned D–A dyes,6 led to the
preferential coordination of aminophilic cations to the pyridine
unit. In the present studies, we thus focused on the Fe3+, Cu2+, Zn2+
and Pb2+ complexes of 1 in MeCN.9
The field of chromogenic anion receptors has been growing in
interest over the last years, however, the number of examples is still
limited.1 Here, design commonly follows the “binding site–
signalling subunit” approach where the two units are covalently
linked to give a colorimetric response upon selective anion binding.
The development of such chemosensors requires i) target-optimiza-
tion of the binding site, ii) selection of an appropriate mechanism
for signal generation and iii) choice of a suitable chromophore as
transducer of the coordination event. Although the strong point of
this approach is selectivity, it usually harbours the disadvantage of
requiring extensive and time-consuming synthetic efforts to tune
structurally complicated hosts. Recently, for certain applications
the use of differential receptors has been proposed as an alternative
concept.2 This strategy relies on a set of suitable sensors that
respond rather unspecifically yet differently toward a group of
related guests, generating signal patterns that can be analyzed by
multicomponent algorithms. Prominent examples comprise sophis-
ticated biosensor arrays, electronic noses or tongues.3 Furthermore,
in molecular biochemistry, the importance of metal ions as versatile
species that control the signalling at the molecular level has very
recently also gained considerable attention. For instance, metal–
ligand coordinative interactions allow certain metalloenzymes to
signal various species such as nitric oxide, carbon monoxide and
reactive oxygen species in a very similar fashion.4 However and
despite all these advances, the field of differential recognition
utilizing actual chromogenic sensor molecules or their complexes is
still largely unexplored.5
The different spectroscopic characteristics of the 1–M ensembles
can be rationalized as follows. By analogy with the photophysics of
a closely related anilinopyrimidine dye,10 1 also shows CT features
connected with largely planar conformations and favourable
extinction coefficients for sensitive chromogenic indication. The
maximum of the absorption band of 1 is found at 338 nm in
acetonitrile (e ≈ 23 000 M21 cm21). In the presence of Fe3+, Cu2+
,
Zn2+ and Pb2+, the centre of the band appears at 432 ± 4 nm, while
maintaining the high extinction coefficients. This increase in CT
character, due to the higher electron deficiency of the pyridine
acceptor moiety in 1–M, is responsible for the colour change of the
solution from colourless (absorption in the UV) to yellow.
Monitoring the intensity changes at 340 and 440 nm upon addition
Based on our experience with colorimetric chemosensors,6 we
developed a new design concept for chromogenic anion sensing
that utilizes metal–ligand coordinative interaction of weak metal
complexes7 of charge-transfer (CT) chromophores. The idea is
outlined in Scheme 1. The free chromophore C shows a certain CT
absorption band. Upon coordination of a metal ion M to the
acceptor (Scheme 1, C–M), the strength of the latter substantially
increases and the band is displaced to longer wavelengths. In the
final step of the sensing protocol, the anion (X) is electrostatically
attracted by the cation, forming an ion pair–complex conjugate (C–
M–X) which induces a weakening of the C–M bond. The latter
effect is fed back into the chromophore, showing again a reduced
CT character with the corresponding appearance of a band again at
shorter wavelengths. Considering the forces at play, the response of
a certain C–M pair toward a certain anion X is given by the intrinsic
chemical natures of M and X such as e.g. charge density, valency
or hard- and softness. These subtle differences should in turn be
Scheme 1 Signalling protocol for chromogenic anion recognition using D–
A chromophore–metal ion coordinative compounds. Schematic representa-
tion of bond strengths and response of the dye is shown, the CT character
increasing from white via grey to black.
† Electronic Supplementary Information (ESI) available: details on synthe-
sis, 1–M interaction, complex stability constants and fluorescence decay
measurements. See http://www.rsc.org/suppdata/cc/b3/b314997h/
774
C h e m . C o m m u n . , 2 0 0 4 , 7 7 4 – 7 7 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

of the cations gave isosbestic points, strongly suggesting the
formation of well-defined 1 : 1 complexes. Moderate logK values
with metal ions were observed (see ESI†). Preliminary studies in
the presence of certain anions revealed that the long-wavelength
absorption (at 440 nm) decreases as a function of 1–M–X
formation. To ensure that a decoloration of the 1–M solution
occurring upon anion addition is not due to simple dissociation of
the complex, further fluorescence studies were carried out.
Whereas 1 is highly emissive with a fluorescence lifetime tf of 2.86
ns, 1–Pb for instance is strongly quenched with tf = 31 ps.
Formation of a ternary conjugate can thus be assumed when the
lifetime in the presence of X differs significantly from tf of 1. This
was indeed observed as manifested in tf of 2.66 ns for 1–Pb–AcO
and similar results were also found in other 1–M–X systems
studied. In control experiments, protonated 1 (tf < 3 ps) was
deprotonated with F2 to yield again a decay time of 2.86 ns, with
the uncertainties of measurement amounting to ± 3 ps. The same
lifetime of 2.86 ns was found for 1 in the presence of a non-
interacting electrolyte (tetrabutylammonium perchlorate) at similar
concentrations.
described until today.11 Additionally, we found that other carbox-
ylates typically present in water12 such as formate, oxalate and
propionate also gave a similar response to that of acetate. To
explore the suitability of simple and weakly bound metal ion–
chromophore complexes for chromogenic anion analysis in real
samples, the colorimetric determination of acetic acid in commer-
cially available vinegar was carried out, yielding a satisfactory
agreement with an approved method.13
In conclusion, a family of metal ion–chromophore chemosensors
has been prepared and successfully employed for the differential
recognition of several small monovalent anions. One of the
differential receptors proved to act also as a specific chemosensor.
The molecular framework used in our present approach is simple,
easily tuneable and can dispense with advanced host–guest
chemistry. It is assumed that the interplay of rather weak
coordinative and electrostatic forces in combination with un-
specific ligands and metal ions harbours an enormous potential and
can open new perspectives for the future design of assays for
chromogenic anion detection. We are currently developing similar
systems working in a more suitable wavelength range capable to
address some yet irresolvable species.
To test the ability of the system in terms of differential
recognition of small monovalent anions, acetonitrile solutions of
1–M chemosensors where prepared (1 : 1 metal-to-ligand ratios; M
= Fe3+, Cu2+, Zn2+ and Pb2+) and mixed with aliquots of solutions
containing one or more equivalents of the anions F2, Cl2, Br2, I2,
NO32, acetate (AcO2) or SCN2. No well-defined selective colour
changes were found by simple visual inspection (except in one case,
see below). However, qualitative analysis of the response of the
1–M chemosensing ensembles for different anions with PCA
(Principal Component Analysis) algorithms yielded the results
summarized in Fig. 1. As can be deduced from the figure,
recognition patterns can be identified for AcO2, F2 and SCN2
This work was supported by the projects MAT2003-08568-C03
and REN2002-04237-C02-01.
Notes and references
1 P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486; Coord.
Chem. Rev., 2003, 240 (Special Issue on Anion Receptors); R. Martínez-
Máñez and F. Sancenón, Chem. Rev., 2003, 103, 4419.
2 J. J. Lavigne and E. V. Anslyn, Angew. Chem., Int. Ed., 2001, 40,
3118.
3 H. T. Nagle, S. S. Schiffman and R. Gutierrez-Osuna, IEEE Spectrum,
1998, 22; K. J. Albert, N. S. Lewis, C. L. Schauer, G. A. Sotzing, S. E.
Stitzel, T. P. Vaid and D. R. Walt, Chem. Rev., 2000, 100, 2595; A. Riul,
Jr., R. R. Malmegrim, F. J. Fonseca and L. H. C. Mattoso, Artif. Organs,
2003, 27, 469.
4 C. E. Cooper, R. P. Patel, P. S. Brookes and V. M. Darley-Usmar,
Trends Biochem. Sci., 2002, 27, 489.
5 S. C. McCleskey, M. J. Griffin, S. E. Schneider, J. T. McDevitt and E.
V. Anslyn, J. Am. Chem. Soc., 2003, 125, 1114; S. L. Wiskur, P. N.
Floriano, E. V. Anslyn and J. T. McDevitt, Angew. Chem., Int. Ed.,
2003, 42, 2070.
2
whereas the proximity between scores for Cl2, Br2, I2 and NO3
indicates a too high similarity in colour variation with the present
family of 1–M chemosensors. It is remarkable to note that a truly
selective bleaching was found for 1–Pb and AcO2 with this kind of
sensor design (Fig. 2). Selective chromogenic recognition of
carboxylates is rare and to the best of our knowledge, only one
acetate-selective molecular chromogenic chemosensor has been
6 K. Rurack, J. L. Bricks, G. Reck, R. Radeglia and U. Resch-Genger, J.
Phys. Chem. A, 2000, 104, 3087; A. B. Descalzo, R. Martínez-Máñez,
R. Radeglia, K. Rurack and J. Soto, J. Am. Chem. Soc., 2003, 125,
3418.
7 Weak coordination refers to complexes with logK values typical for
monodentate ligands involving heterocyclic or anilino-type nitrogen
donor atoms; cf. R. M. Smith and A. E. Martell, Critical Stability
Constants, Plenum, New York, 1975, vol. 2, pp. 36.
8 In terms of complex stability (cf. R. D. Hancock and A. E. Martell,
Chem. Rev., 1989, 89, 1875; M. Kurihara, T. Kawashima and K.
Ozutsumi, Z. Naturforsch., 2000, B55, 277) as well as the desired
stoichiometry (cf. J. L. Bricks, J. L. Slominskii, M. A. Kudinova, A. I.
Tolmachev, K. Rurack, U. Resch-Genger and W. Rettig, J. Photochem.
Photobiol. A Chem., 2000, 132, 193), the bulky phenyl groups
facilitating the formation of 1 : 1 complexes.
9 The choice of a weakly solvating medium (acetonitrile with up to 5 vol%
water) and the initial counteranion of 1–M (perchlorate) are also
important prerequisites to allow the desired interplay of coordination,
ion association, and the formation of ternary conjugates.
10 J. Herbich and J. Waluk, Chem. Phys., 1994, 188, 247.
11 R. Kato, S. Nishizawa, T. Hayashita and N. Teramae, Tetrahedron Lett.,
2001, 42, 5053. Most of the other carboxylate chemosensors reported so
far rely on hydrogen bonding interactions and usually also display
colour changes with other basic anions.
Fig. 1 Principal component analysis (PCA) score plot for the anions F2,
Cl2, Br2, I2, NO32, AcO2 and SCN2 using 1–M (M = Fe3+, Cu2+, Zn2+
and Pb2+) chemosensing ensembles. Data shown from three different trials
using 1 : 1 or 1 : 2 1–M to anion ratios. PC axes are calculated to lie along
lines of diminishing levels of variance in the data set.
12 Y. Zeng and J. Liu, Appl. Geochem., 2000, 15, 13.
13 Calibration curves were obtained by addition of increasing amounts of
AcO2 to MeCN–H2O (98:2 v/v) solutions of 1–Pb. An aliquot of
vinegar was buffered at pH 7, bleached with charcoal, filtered and added
to the 1–Pb solutions. The acetic acid concentration in the vinegar
sample was thus determined to 5.4 g per 100 mL, which was in
agreement with the results of a conventional method (5.2 g per 100 mL).
Metrohm Appl. Bull. 084-E, Titrimetric analysis of vinegar, 2002.
Fig. 2 Photograph showing the color observed in 1–Pb acetonitrile solutions
upon addition of equimolar amounts of certain monovalent anions (from left
to right); no anion, F2, Cl2, Br2, NO32, AcO2, SCN2.
C h e m . C o m m u n . , 2 0 0 4 , 7 7 4 – 7 7 5
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