A controlled supramolecular approach toward cation-specific chemosensors: Alkaline earth metal ion-driven exciton signaling in squaraine tethered podands

Article abstract of DOI:10.1021/ja0393776

Three different squaraine tethered bichromophoric podands 3a-c with one, two, and three oxygen atoms in the podand chain and an analogous monochromophore 4a were synthesized and characterized. Among these, the bichromophores 3a-c showed high selectivity toward alkaline earth metal cations, particularly to Mg²⁺ and Ca²⁺ ions, whereas they were optically silent toward alkali metal ions. From the absorption and emission changes as well as from the Job plots, it is established that Mg²⁺ ions form 1:1 folded complexes with 3a and 3b whereas Ca²⁺ ions prefer to form 1:2 sandwich dimers. However, 3c invariably forms weak 1:1 complexes with Mg²⁺, Ca²⁺, and Sr²⁺ ions. The signal output in all of these cases was achieved by the formation of a sharp blue-shifted absorption and strong quenching of the emission of 3a-c. The signal transduction is achieved by the exciton interaction of the face-to-face stacked squaraine chromophores of the cation complex, which is a novel approach of specific cation sensing. The observed cation-induced changes in the optical properties are analogous to those of the "H" aggregates of squaraine dyes. Interestingly, a monochromophore 4a despite its binding, as evident from ¹H NMR studies, remained optically silent toward Mg²⁺ and Ca²⁺ ions. While the behavior of 4a toward Mg²⁺ ion is understood, its optical silence toward Ca²⁺ ion is rationalized to the preferential formation of a "Head-Tail-Tail-Head" arrangement in which exciton coupling is not possible. The present study is different from other known reports on chemosensors in the sense that cation-specific supramolecular host-guest complexation has been exploited for controlling chromophore interaction via cation-steered exciton coupling as the mode of signaling.

Full text of DOI:10.1021/ja0393776

Published on Web 04/28/2004
A Controlled Supramolecular Approach toward
Cation-Specific Chemosensors: Alkaline Earth Metal
Ion-Driven Exciton Signaling in Squaraine Tethered Podands
Easwaran Arunkumar, Parayali Chithra, and Ayyappanpillai Ajayaghosh*
Contribution from the Photosciences and Photonics DiVision, Regional Research Laboratory
(CSIR), TriVandrum - 695019, India
Received November 2, 2003; E-mail: aajayaghosh@rediffmail.com
Abstract: Three different squaraine tethered bichromophoric podands 3a-c with one, two, and three oxygen
atoms in the podand chain and an analogous monochromophore 4a were synthesized and characterized.
Among these, the bichromophores 3a-c showed high selectivity toward alkaline earth metal cations,
particularly to Mg2+ and Ca2+ ions, whereas they were optically silent toward alkali metal ions. From the
absorption and emission changes as well as from the Job plots, it is established that Mg2+ ions form 1:1
folded complexes with 3a and 3b whereas Ca2+ ions prefer to form 1:2 sandwich dimers. However, 3c
invariably forms weak 1:1 complexes with Mg2+, Ca2+, and Sr2+ ions. The signal output in all of these
cases was achieved by the formation of a sharp blue-shifted absorption and strong quenching of the emission
of 3a-c. The signal transduction is achieved by the exciton interaction of the face-to-face stacked squaraine
chromophores of the cation complex, which is a novel approach of specific cation sensing. The observed
cation-induced changes in the optical properties are analogous to those of the “H” aggregates of squaraine
dyes. Interestingly, a monochromophore 4a despite its binding, as evident from 1H NMR studies, remained
optically silent toward Mg2+ and Ca2+ ions. While the behavior of 4a toward Mg2+ ion is understood, its
optical silence toward Ca2+ ion is rationalized to the preferential formation of a “Head-Tail-Tail-Head”
arrangement in which exciton coupling is not possible. The present study is different from other known
reports on chemosensors in the sense that cation-specific supramolecular host-guest complexation has
been exploited for controlling chromophore interaction via cation-steered exciton coupling as the mode of
signaling.
Introduction
There are several reports in the literature which pertain to
chemosensors that selectively detect alkaline earth metal ions.4
Supramolecular host-guest interaction and molecular rec-
ognition, particularly cation binding, is a subject of considerable
interest due to its implications in many fields such as chemistry,
biology, medicine, and environmental studies.1 Such interactions
will become useful when the host molecules are able to express
the recognition event in the form of a measurable signal,
invoking a change in one or more properties of the system such
as redox potentials, absorption, or emission characteristics. The
design of such molecular systems, chemosensors, the usual
configuration of which consists of a macrocyclic receptor
(ionophore) unit, integrated to an organic chromophore (fluo-
rophore), is a topic of considerable interest.2 Among various
chemosensors, the design of cation-specific sensors for the
selective detection of biologically important cations such as Na+,
K+, Mg2+, Ca2+, and Zn2+ is of extreme importance.3 In this
context, designing chemosensors for the specific recognition
between alkali and alkaline earth metal cations is a challenging
task.
Even though macrocyclic receptors are the widely used cation
binding sites in these systems, reports on chromophore-linked
pseudomacrocycle- or podand-based chemosensors are relatively
few.5,6 For example, Nakamura et al. have reported several
anthracene- or pyrene-linked podands, which specifically detect
alkaline earth metal ions.6 In addition, there are a few recent
(2) (a) Valeur, B. In Molecular Luminescence Spectroscopy: Part 3; Schulman,
S. G., Ed.; Wiley: New York, 1993. (b) Valuer, B. In Topics in
Fluorescence Spectroscopy; Lakowicz, J. R., Ed.; Plenum Press: New York,
1994; Vol. 4. (c) Valeur, B.; Badaoui, F.; Bardez, E.; Bourson, J.; Boutin,
P.; Chatelain, A.; Devol, I.; Larrey, B.; Lefevre, J. P.; Soulet, A. In
Chemosensors of Ion and Molecular Recognition; Desvergne, J.-P., Czarnik,
A. W., Eds.; NATO ASI Series; Kluwer: Dordrecht, 1997. (d) Fabbrizzi,
L.; Poggi, A. Chem. Soc. ReV. 1995, 197. (e) de Silva, A. P.; Gunaratne,
H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher,
J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515. (f) Valeur, B.; Leray, I.
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42. (b) Grandini, P.; Mancin, F.; Tecilla, P.; Scrimin, P.; Tonellato, U.
Angew. Chem., Int. Ed. 1999, 38, 3061. (c) Prodi, L.; Bargossi, C.; Montalti,
M.; Zaccheroni, N.; Su, N.; Bradshaw, J. S.; Izatt, R. M.; Savage, P. B. J.
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Wiley-VCH: Weinheim, 1995. (b) Gokel, G. W. Molecular recognition
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Cation-Specific Chemosensors
A R T I C L E S
Figure 1. Schematic representation of a chemosensor based on a cation-driven, exciton interaction in a dye-linked podand.
reports which pertain to macrocycle- and pseudomacrocycle-
based ion recognition systems in combination with the supramo-
lecular approach.7 Although none of these latter systems have
been projected as chemosensors, they are interesting from the
viewpoint of specific supramolecular recognition-induced al-
losteric effects,7a chirality transfer,7b and photochemistry.7c
Signal transduction in most of the chemosensors is achieved
either by photoinduced electron,5e-g,8 energy,5f or charge-transfer
processes,5a-c,9 by excimer formation,4h,6,10 or by conformational
restrictions.11 Therefore, the design of cation-specific chemosen-
sors, which work on alternate ways of signal transduction, is
of fundamental importance. This has prompted us to think of
exciton interaction in organic dye aggregates as a potential
signaling process of a specific cation binding event. The idea
involves a cation-induced folding or dimerization of dye-linked
podands to form complexes akin to organic dye aggregates in
which exciton interaction in the bound complex produces
measurable changes in optical and photophysical properties
(Figure 1). We speculated that if face-to-face interactions of
organic dyes similar to those of an “H” aggregate can be induced
by a metal ion, the binding event could be expressed in the
form of a measurable optical signal. For this purpose, we have
selected squaraine dyes as the signal transducing chromophore
for several reasons.
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A R T I C L E S
Arunkumar et al.
Scheme 1
cently, Liang et al. have reported that squaraines, which are
linked to flexible alkyl chains in a bichromophoric fashion, show
exciton coupled spectral changes, depending upon the length
of the alkyl chain.20 However, this property of squaraine dyes
has never before been exploited in the designing of chemosen-
sors, until recently when we reported the Ca2+ ion-specific
exciton interaction in a squaraine foldamer.21 In the present
study, we illustrate a supramolecular approach in combination
with cation-driven exciton interaction for the signaling of a
specific cation binding, where the mode of complexation and
chromophore interaction are controlled by the nature of the host
and guest molecules.
Table 1. Absorption and Emission Parameters for Compounds
3a-c and 4a in Acetonitrile
absorption
ꢀ (M-1 cm-1
emissiona
compound
λmax (nm)
)
λ
max (nm)
Φfb
3a
652
618
587 (sh)
644
120 226
664
663
0.008
3b
134 896
0.014
625
3c
4a
630
637
138 038
229 086
654
664
0.023
0.06
a Solutions were excited at 570 nm, and emission was monitored in the
region 600-750 nm for estimating Φf. b Fluorescence quantum yields were
determined using 4,4-[bis-(N,N-dimethylamino)phenyl]squaraine dye as the
standard (Φf ) 0.7 in CHCl3), error limit (5%.
Results and Discussion
Synthesis and Characterization of Squaraine Bichro-
mophores 3a-c and the Monochromophore 4a. The bis-
aniline derivatives 1a-c and the squaric acid derivatives 2a
and 2b were prepared as per reported procedures.22,23 Reaction
of 1a-c with the semisquaraine derivative 2a or 2b in a 1:2
stoichiometry, under reflux conditions in 2-propanol using
tributyl orthoformate as catalyst, resulted in the formation of
the bichromophores 3a-c in 20-25% yields along with small
amounts of the monochromophores 4a-c (Scheme 1). In a
separate experiment, the monochromophore 4a was synthesized
in 32% yield by a controlled reaction between 1a and 2b in a
1:1 stoichiometry. FT-IR spectra of 3a-c and 4a showed a sharp
band at 1590 cm-1, characteristic of the C-O- stretching of
respectively. Due to the symmetry of the charge-delocalized
zwitterionic structures of 3a-c, the aromatic protons appeared
1
as two sets of four protons each at δ 6.7 and 8.3 ppm. The H
NMR spectrum of 4a in CDCl3 showed two characteristic
singlets at δ 3.1 and 2.86 ppm, corresponding to three protons
each of the two -NCH3 groups. The -NCH2 and -OCH2
protons appeared as multiplets around δ 3.5-3.75 ppm. In the
aromatic region, five different signals could be seen. Signals at
δ 8.2 and 6.7 ppm, which correspond to four protons, are
assigned to the squaraine moiety. Signals at δ 7.2 and 6.7 ppm,
corresponding to two protons each, and the signal at δ 6.6 ppm
corresponding to a single proton are assigned to the aniline
moiety. 13C NMR spectra of 3a-c and 4a, high-resolution mass
spectra, and the elemental analyses data were in agreement with
their assigned structures. The high-resolution mass spectra of
3a-c showed the characteristic [M]+ ion peak, whereas the
MALDI-TOF mass spectra showed the [M + H]+ ion peak
along with [M + Na]+ and [M + K]+ peaks.
1
the zwitterionic squaraine dyes. H NMR spectra of 3a-c in
CDCl3 showed resonance signals corresponding to -NCH3,
-OCH2, and -NCH2 protons at δ 3.1, 3.54, and 3.65 ppm,
(20) Liang, K.; Farahat, M. S.; Perlstein, J.; Law, K.-Y.; Whitten, D. G. J. Am.
Chem. Soc. 1997, 119, 830.
(21) Ajayaghosh, A.; Arunkumar, E.; Daub, J. Angew. Chem., Int. Ed. 2002,
41, 1766.
Absorption and Emission Properties of 3a-c and 4a. The
absorption and emission maxima and the fluorescence quantum
(22) Robello, D. R. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 1.
(23) Keil, D.; Hartmann, H. Dyes Pigm. 2001, 49, 161.
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Cation-Specific Chemosensors
A R T I C L E S
Figure 2. Changes in the (a) absorption and (b) emission spectra of 3a
(3.6 µM) in acetonitrile with the addition of Mg(ClO4)2. The arrows indicate
the changes that result from progressively increasing the concentration of
Mg(ClO4)2. Insets show variation of (a) absorbance at 548 nm and (b)
fluorescence intensity at 664 upon increasing the ratio [Mg(ClO4)2]/[3a].
Figure 3. Changes in (a) absorption and (b) emission spectra of 3a (3.6
µM) in acetonitrile with the addition of Ca(ClO4)2. The arrows indicate the
changes that result from progressively increasing the concentration of
Ca(ClO4)2. Insets show variation of (a) absorbance at 548 nm and (b)
fluorescence intensity at 664 nm with the ratio [Ca(ClO4)2]/[3a].
yields of 3a-c and 4a in acetonitrile are presented in Table 1.
The bichromophore 3a showed an intense absorption maximum
at 652 nm with two strong shoulders around 620 and 585 nm
(Figure 2a). The fluorescence emission spectrum of 3a in
acetonitrile (3.6 × 10-6 M) showed a maximum at 664 nm with
a low quantum yield of 0.008 (Figure 2b). The absorption
spectrum of 3b having two oxygen atoms in the podand chain
exhibited two close maxima at 644 and 625 nm with a weak
shoulder around 590 nm. The emission maximum of 3b was at
663 nm with a quantum yield of 0.014. Interestingly, 3c showed
a sharp spectrum with absorption maximum at 630 nm and an
emission maximum at 654 nm. The fluorescence quantum yield
of 3c is 0.023, which is higher than those of 3a and 3b. The
absorption and emission maxima of 4a in acetonitrile occurred
at 637 and 664 nm, respectively, with a fluorescence emission
quantum yield of 0.06, which is almost 8 times higher than that
of 3a. The differences in the spectral pattern and the quan-
tum yields of 3a-c are attributed to the oxyethylene chain-
dependent chromophore interaction, in analogy to the report by
Liang et al.20
Cation Binding Properties of 3a-c: Changes in the
Absorption and Emission Spectra. Changes in the absorption
and emission spectra of 3a in acetonitrile (3.6 × 10-6 M) upon
the addition of aliquots of Mg(ClO4)2 are shown in Figure 2.
With an increasing amount of Mg(ClO4)2, the intensity of the
absorption maximum at 652 nm is decreased with the concomi-
tant growth of a new sharp band at 548 nm, through an isosbestic
point at 570 nm. The inset of Figure 2a shows a plot of the
absorbance at 548 nm against the ratio of the concentrations of
Mg(ClO4)2 and 3a, which gets saturated when the concentration
of the metal salt equals that of 3a indicating a 1:1 complexation
between the two. Similarly, upon addition of Mg(ClO4)2, the
intensity of the emission band at 664 nm (Φf ) 0.008) gradually
decreased with a marginal blue-shift (8 nm) when excited at
570 nm (Figure 2b). The quenching of the emission gets
saturated (Φf ) 0.004) when the concentration of Mg(ClO4)2
reached nearly that of 3a, indicating a 1:1 stoichiometry of
binding (Figure 2b, inset).
Changes in the absorption and emission spectra of 3a (3.6 ×
10-6 M) induced by the addition of Ca(ClO4)2 in acetonitrile
are shown in Figure 3, which are identical as in the case of
Mg(ClO4)2. However, in the case of Ca(ClO4)2, the plot of
absorbance at 548 nm reached a maximum when the ratio of
the concentrations of the metal salt and 3a reached a value of
0.5 (Figure 3a, inset). A same effect is observed when the ratio
of the fluorescence intensities (I/I0) was plotted against the ratio
of concentration of Ca(ClO4)2 and 3a, indicating a 2:1 stoichi-
ometry for the complexation of 3a with Ca2+ ion. Similar
changes in the absorption and emission spectra could be obtained
when Sr(ClO4)2 was added to an acetonitrile solution of 3a.
However, addition of Ba(ClO4)2 to 3a showed weak changes,
indicating a very loose interaction between the two. Interestingly,
addition of LiClO4, NaClO4, and KClO4 did not induce any
change in the absorption or emission properties of 3a, revealing
its high specificity toward alkaline earth metal cations. These
observations are justified by a comparison of the plots of the
fluorescence quantum yields (Φf) against the ratio of the concen-
trations of metal salt to the bichromophore 3a (Figure 4).
To determine the role of the podand chain length on the cation
sensing properties, the bichromophores 3b and 3c were subjected
to detailed metal ion titration studies. Changes in the absorption
9
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A R T I C L E S
Arunkumar et al.
shown in Figure 6. In the case of Mg2+ binding, the maximum
absorbance of the complex is obtained when the mole fraction
of the host (3a) has reached 0.5, which is a signature of a 1:1
stoichiometry (Figure 6a). However, for Ca2+, the maximum
absorbance at 548 nm could be seen when the mole fraction of
3a was at 0.66, which is characteristic of a host-guest binding
in a 2:1 stoichiometry (Figure 6b). Similar results were obtained
for 3b. The Benesi-Hildebrand26 plot for Mg2+ binding showed
a good linear relationship of a 1:1 complexation with a binding
constant of 6.15 × 105 M-1 for 3a and 1.04 × 106 M-1 for 3b.
It must be noted here that in acetonitrile (ꢀ ) 38.8) ambiguity
exists regarding the complete dissociation of the metal salts.
However, for bivalent metal cations in micromolar concentra-
tions, we assume the complete dissociation of the metal salts
and the resultant complexes are not ion paired.27
Figure 4. Plot of Φf versus the ratio of [metal salt] to [3a] illustrating the
variation of fluorescence quantum yield of the bichromophore with various
metal cations such as Ca2+ (9), Mg2+ (b), Sr2+ (2), Ba2+ (1), Na+ (5),
K+ (6), and Li+ ([).
1H NMR Binding Studies of 3a with Ca(ClO4)2 and Mg-
(ClO4)2. The effect of the addition of Mg(ClO4)2 and Ca(ClO4)2
1
to the H NMR spectrum of 3a in a mixture of 30% CD3CN
and 70% CDCl3 is shown in Figure 7. After the addition of an
equivalent amount of Mg(ClO4)2 or Ca(ClO4)2, the well-resolved
resonance signals of 3a (Figure 7a) became broad and shifted
toward upfield. For example, the signals at δ 8.3 and 6.8 ppm,
which correspond to the squaraine aromatic protons, became
very broad between δ 7.7-9.0 and δ 5.5-7.0 ppm, respectively.
Similarly, signals corresponding to -NCH3, -OCH2, and
-NCH2- protons at δ 3.1, 3.54, and 3.65 ppm, respectively,
were shifted toward lower δ values and merged to form broad
signals. A considerable shift could also be noticed for the water
signal at δ 2.1 ppm, toward low field after the addition of metal
perchlorates, indicating the involvements of water molecules
in the metal ion coordination processes. Interestingly, addition
of LiClO4, NaClO4, or KClO4 to 3a did not show any change
in the NMR spectrum, revealing no interaction between these
cations with 3a, which is in agreement with the absorption and
emission spectral studies.
Figure 5. A 3D plot showing the selectivity and sensitivity of 3a-c toward
various metal cations.
Exciton Coupled Signaling of the Cation Binding Events.
From the detailed analysis of the changes in the absorption and
emission properties, and from the Job plots, it is clear that 3a
and 3b form 1:1 complexes with Mg2+ ion whereas Ca2+ and
Sr2+ ions prefer to form 1:2 complexes (Schemes 2 and 3). Mg2+
ion induces the folding of 3a to form a foldamer, 5, in a 1:1
stoichiometry, whereas Ca2+ ion prefers to form a 1:2 sandwich
dimer, 6, with 3a. The difference in the absorption and emission
properties of the cation complexes 5 and 6 from that of 3a can
be rationalized on the basis of the exciton interaction in the
face-to-face stacked squaraine chromophores of the former. It
is interesting to mention here that the cation-induced changes
in the optical properties of 3a-c are similar to those of the
“H” aggregates of analogous squaraine dyes.19
and emission spectra of 3b upon gradual addition of Mg(ClO4)2
and Ca(ClO4)2 are presented in the Supporting Information.
Although these changes of 3b are similar to those of 3a, the
former showed better selectivity and sensitivity toward Ca2+
ion. Addition of Sr(ClO4)2 and Ba(ClO4)2 showed weak
response, whereas LiClO4, NaClO4, and KClO4 had hardly any
effect. The bichromophore 3b also formed a 1:1 complex with
Mg2+, whereas Ca2+ and Sr2+ ions showed a 1:2 complexation
mode as in the case of 3a. In contrast to the behavior of 3a and
3b, the bichromophore 3c having a long podand chain (three
oxygen atoms) invariably showed a 1:1 binding with Mg(ClO4)2,
Ca(ClO4)2, and Sr(ClO4)2. However, in these cases, the response
toward the metal cations was relatively weaker when compared
to those of 3a and 3b. A 3D plot of the percentage of
fluorescence quantum yields of 3a-c against various metal
cations reveals the effect of the podand chain on the selectivity
and sensitivity on cation binding (Figure 5). From the fluores-
cence quenching data, the maximum sensitivity and selectivity
are obtained for 3b. In all cases, the sensitivity was in the order
In analogy to the exciton theory of dye aggregation, the
cation-induced folding or dimerization of 3a-c results in the
splitting of the excited-state energy levels of the complex into
two, one of which is higher in energy and the other lower in
(25) A Job plot is extensively used to find the complexation mode of the host-
guest interaction. For examples, see: (a) Specht, A.; Bernard, P.; Goeldner,
M.; Peng, L. Angew. Chem., Int. Ed. 2002, 41, 4706. (b) Huang, F.; Gibson,
H. W.; Bryant, W. S.; Nagvekar, D. S.; Fronczek, F. R. J. Am. Chem. Soc.
2003, 125, 9367. (c) Huang, F.; Fronczek, F. R.; Gibson, H. W. Chem.
Commun. 2003, 1480.
(26) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
(27) We thank an anonymous reviewer for the comments on salt dissociation
and ion pairing. The reported binding constants are based on the assumption
that the concentration of the free ions is equal to the total salt concentration.
Ca2+ > Mg2+ > Sr2+ > Ba2+
.
The difference in the complexation modes of 3a-c with Mg2+
and Ca2+ was confirmed by the corresponding Job plots.24,25
For example, the Job plots of 3a with Mg2+ and Ca2+ ions are
(24) (a) Job, P. Ann. Chem. 1928, 9, 113. (b) Gil, V. M. S.; Oliveira, N. C. J.
Chem. Educ. 1990, 67, 473.
9
6594 J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004

Cation-Specific Chemosensors
A R T I C L E S
Figure 6. Job plots for the binding of 3a with Mg2+ and Ca2+ ions: (a) 1:1 complexation with Mg2+; (b) 2:1 complexation with Ca2+
.
be shifted to higher energy, resulting in a blue-shifted absorption.
Consequently, the emission of the bound complexes is weak
because the internal conversion from the upper excited state to
the lower one is very fast and the emission from the lower
excited state is theoretically forbidden.
Cation Binding Property of the Monochromophore 4a.
In light of the cation binding behavior of the bichromophores
3a-c, we speculated that a monochromophore 4a might be able
to distinguish between Mg2+ and Ca2+ ions. The rationale for
this speculation was that signaling would not be possible in the
1:1 folded complex of 4a with Mg2+ where exciton coupling is
not possible. However, signaling will be possible in a 1:2
complex of 4a with Ca2+, in the event of a face-to-face stacking
of the squaraine chromophore. However, 4a was optically silent
toward both Mg2+ and Ca2+ ions. While this is understood for
Mg2+ ions, the silence of Ca2+ ions against 4a was surprising,
which is rationalized by considering the two statistical com-
plexation modes of 4a with Ca2+ ions as shown in Scheme 4.
The “Head-Tail-Head-Tail” (H-T-H-T) arrangement 7 is ex-
pected to give an exciton coupled blue-shifted spectrum with
Ca2+ ion, whereas the “Head-Tail-Tail-Head” (H-T-T-H) ar-
rangement 8 should be optically silent. Because the absorption
or emission spectra of 4a did not change upon addition of
Ca(ClO4)2, it is reasonable to assign the “H-T-T-H” assembly
8 for the 2:1 complex of 4a with Ca2+ ion in which the two
squaraine chromophores are positioned away from each other,
thereby preventing exciton interaction between the chro-
mophores.
Figure 7. 1H NMR spectra in 30% CD3CN-70% CDCl3 of 3a (a) before
addition of metal perchlorates (2.3 mM), (b) with 2.3 mM Mg(ClO4)2, and
(c) with 2.3 mM Ca(ClO4)2.
energy.28 The spectral shift (∆ν) associated with the electronic
transition in such cases will be determined by the tilt angle R
of the transition dipole moments of the cation-complexed
chromophores with the line of centers, which is given by the
equation,
In the context of the above observations, it is important to
∆ν (dimer) ) h-1 m2 (1 - 3 cos2 R)/r2
(1)
see that 4a is optically silent toward Mg2+ and Ca2+ ions despite
1
their binding to 4a. This is proved by the H NMR studies of
where h is Planck’s constant, r is the separation of the molecular
centers, and m2 is the dipole moment of the dye monomer.29
From this relationship, it can be seen that when R becomes
greater than 54°44′, the value of ∆ν changes to positive,
indicating a hypsochromic shift, and reaches a maximum when
R becomes 90°. In the case of the folded complex 5 and the
sandwich dimer 6, the squaraine chromophores are face-to-face
stacked as in the case of “H” aggregates where R tends to
approach 90°. As a result, the excited state of the complex will
4a in CD3CN in the presence of Mg2+ and Ca2+ ions (Figure
8). In both cases, considerable changes to the resonance signals
could be noticed upon addition of the metal perchlorates. Signals
corresponding to the aromatic protons of the squaraine moiety
at δ 6.87 and δ 8.2 ppm became broad and weak. The signal at
δ 3.1 ppm corresponding to the N-CH3 protons, attached to
the squaraine moiety, is shifted upfield and merged with the
N-CH3 protons of the aniline moiety, which appeared at δ 2.86
ppm. The multiplets at δ 3.44-3.65 ppm corresponding to the
-OCH2 and -NCH2 protons underwent considerable shift
toward high field, particularly the -NCH2 protons. It is
interesting to note that the resonance signals at δ 6.6-6.68 and
δ 7.14 ppm, which correspond to the aromatic protons of the
aniline moiety of 4a, did not show many changes in the presence
of Ca(ClO4)2 whereas addition of Mg(ClO4)2 induced consider-
(28) (a) Kasha, M.; Rawls, H. R.; EI-Bayoumi, M. A. Pure Appl. Chem. 1965,
11, 371. (b) Davydov, A. S. Theory of Molecular Excitons; Plenum Press:
New York, 1971. (c) McRae, E. G.; Kasha, M. Physical Process in
Radiation Biology; Academic Press: New York, 1964; p 17. (d) Hoch-
strasser, R. M.; Kasha, M. Photochem. Photobiol. 1964, 3, 317. (e) Czikkely,
V.; Forsterling, H.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (f) Bucher,
H.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 183.
(29) Emerson, E. S.; Conlin, M. A.; Rosenoff, A. E.; Norland, K. S.; Rodriguez,
H.; Chin, D.; Bird, G. R. J. Phys. Chem. 1967, 71, 2396.
9
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A R T I C L E S
Arunkumar et al.
Scheme 2. Mg2+ Ion Induced Folding of 3a to a 1:1 Folded Dimer
Scheme 3. Ca2+ Ion Binding of 3a To Form a 2:1 Sandwich Dimer
able broadening. These observations indicate that, in the 1:1
folded complex of 4a with Mg2+, the aniline moiety experiences
considerable interaction with the squaraine moiety whereas in
3a-c and the monochromophore 4a with Mg2+ and Ca2+ ions.
From the absorption and emission changes and from the Job
plots, it is clear that 3a and 3b bind with Mg2+ ion to form
intramolecular 1:1 foldamers whereas Ca2+ ion prefers to bind
in a 1:2 stoichiometry leading to intermolecular sandwich
dimers. However, the bichromophore 3c invariably forms 1:1
folded complexes with Mg2+ and Ca2+ ions. However, addition
of LiClO4, NaClO4, or KClO4 did not bring any difference in
the absorption or emission properties of 3a-c, making them
selective toward alkaline earth metal ions. Interestingly, the
monochromophore 4a is optically silent toward Mg2+ and Ca2+
ions. While this is clear in the case of Mg2+ ion, the optical
silence of 4a toward Ca2+ ion is rationalized on the basis of
the preferential complexation in a “H-T-T-H” arrangement
where exciton coupling of the chromophore is not allowed. The
present study is different from other reports on chemosensors
in the sense that the cation-specific supramolecular host-guest
interaction has been exploited for controlling and expressing
ion recognition via chromophore interaction, which in turn
the case of the “H-T-T-H” sandwich dimer of 4a with Ca2+
,
the aniline moieties seem to be slightly displaced from the
squaraine chromophore. Thus, while the NMR experiments
clearly support the binding of Ca2+ to 4a, the absorption and
emission studies strongly favor the “H-T-T-H” arrangement for
the complex where there is no possibility for exciton interaction
as shown in Scheme 4, which justify its optical silence toward
the Ca2+ ion.
Conclusions
In the present study, we have illustrated a rational supramo-
lecular approach in controlling and expressing the binding of
specific cations, invoking exciton interaction as the mode of
optical signaling, which is different from other known signaling
mechanisms. The results obtained from the absorption, emission,
1
and H NMR studies revealed the difference in the supramo-
lecular control of the binding modes of the bichromophores
9
6596 J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004

Cation-Specific Chemosensors
A R T I C L E S
Figure 8. 1H NMR spectral changes (300 MHz, 300 K) of 4a (3.52 mM) in CD3CN (a) before addition of metal perchlorates, (b) after addition of Ca(ClO4)2
(3.52 mM), and (c) after addition of Mg(ClO4)2 (3.52 mM).
Scheme 4. Possible Complexation Modes of 4a with Ca(ClO4)2
recorded on a Nicolet Impact 400D infrared spectrophotometer. 1H and
becomes the key process in the signal transduction. The present
13C NMR spectra were measured on a 300 MHz Bruker Avance DPX
approach successfully could translate changes in the photo-
spectrometer using TMS as internal standard. High-resolution mass
physical properties associated with a dye aggregation to a
spectra were recorded in a Finnigan MAT 95 instrument using xenon
measurable signal, making use of the cation-specific supramo-
as ionization gas. Matrix-assisted laser desorption ionization time-of-
lecular organization of chromophore-linked podands. In addition,
flight (MALDI-TOF) mass spectra were obtained on a Perseptive
the present study demonstrates how organic dye aggregation, a
Biosystems Voyager DE-Pro MALDI-TOF mass spectrometer. El-
well-studied phenomenon with respect to organic semiconduct-
emental analyses were done using a Perkin-Elmer series-II 2400 CHN
ing devices30 and supramolecular chemistry,31 becomes impor-
tant in the design of cation selective chemosensors.
analyzer. The emission spectra were measured on a Spex-Fluorolog
F112X spectrofluorimeter. Fluorescence quantum yields were deter-
mined in spectroscopic grade CH3CN using optically matching solutions
Experimental Section
Unless otherwise stated, all starting materials and reagents were
(30) Zollinger, H. Color Chemistry. Synthesis, Properties and Applications of
purchased from commercial suppliers and used without further purifica-
tion. The solvents used were purified and dried by standard methods
prior to use. Melting points were determined with a Mel-Temp-II
melting point apparatus and are uncorrected. FT-IR spectra were
Organic Dyes and Pigments; VCH: Weinheim, 1991.
(31) (a) Wu¨rthner, F.; Yao, S. Angew. Chem., Int. Ed. 2000, 39, 1978. (b)
Wu¨rthner, F.; Yao, S.; Beginn, U. Angew. Chem., Int. Ed. 2003, 42, 3247.
(c) Wu¨rthner, F.; Yao, S.; Debaerdemaeker, T.; Wortmann, R. J. Am. Chem.
Soc. 2002, 124, 9431 and references therein.
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A R T I C L E S
Arunkumar et al.
of 4,4-[bis-(N,N-dimethylamino)phenyl]-squaraine dye (Φf ) 0.70 in
chloroform) as standard. The stability constants (Ks) were determined
from the absorption spectral changes using the equation,
N, 6.10. HRMS-FAB: [M]+ calcd for C56H70N4O6, 895.5374; found,
895.5385.
3c. Yield: 18%. mp 265-268 °C. FT-IR (KBr): ν ) 1588, 1483,
1
1365, 1176, 1120, 938, 833, 784 cm-1. H NMR (300 MHz, CDCl3,
A0
ꢀL
TMS): δ 3.14-3.16 (m, 18H, -NCH3), 3.54-3.65 (m, 16H, -NCH2,
and -OCH2), 6.73-6.78 (dd, J ) 1.35, 9.3 Hz, 8H, aromatic), 8.3-
8.36 (dd, J ) 2.25, 9.1 Hz, 8H, aromatic). 13C NMR: δ 39.66, 40.29,
52.38, 68.53, 70.52, 70.58, 70.63, 70.89, 112.35, 112.53, 119.86, 120.2,
133.17, 154.77, 154.98, 183.36. MALDI-TOF MS (MW ) 770.922):
m/z ) 771.09 [M + H]+. Anal. Calcd for C46H50N4O7: C, 71.67; H,
6.54; N, 7.27. Found: C, 71.36; H, 6.78; N, 7.06.
1
)
+ 1
(
)
ꢀL - ꢀML
Ks[M]
(A0 - A)
where ꢀL and ꢀML are the molar extinction coefficients of the ligand
and the complex, respectively. The quantity A0/(A0 - A) is plotted versus
1/[M], and the stability constant is then obtained from the ratio intercept/
slope.
Preparation of the Monochromophore 4a. To a 100 mL round-
bottomed flask containing 50 mL of 2-propanol were added 1a (100
mg, 0.35 mmol), 2b (105 mg, 0.35 mmol), and tributyl orthoformate
(1 mL). After being refluxed for 12 h, the hot reaction mixture was
filtered, and the solid was washed with 2-propanol until the filtrate
was almost colorless. Column chromatography (chloroform/methanol,
95:5) of the crude product over silica gel (100-200 mesh) gave the
pure product 4a. Yield: 32%. mp 112-114 °C. FT-IR (KBr): ν )
1580, 1351, 1270, 1169, 1108, 784 cm-1. 1H NMR (300 MHz, CDCl3,
TMS): δ 0.96-1.01 (t, J ) 7.26 Hz, 6H, -CH3), 1.25-1.43 (m, 8H,
-CH2), 2.92 (s, 3H, -NCH3), 3.13 (s, 3H, -NCH3), 3.41-3.65 (m,
12H, -NCH2, and -OCH2), 6.7-6.77 (m, 7H, aromatic), 7.18-7.25
(t, J ) 7.9 Hz, 2H, aromatic), 8.36-8.39 (dd, J ) 2.35, 9.35 Hz, 4H,
aromatic) ppm. 13C NMR (75 MHz, CDCl3): δ 13.8, 20.2, 29.6, 38.9,
39.5, 51.2, 52.3, 52.4, 68.7, 69.1, 112.1, 112.2, 112.3, 116.3, 119.4,
122.03, 129.1, 132.8, 133.5, 153.7, 183.4 ppm. MALDI-TOF MS (MW
) 567.7): m/z ) 568.3 [M + H]+. Anal. Calcd for C56H70N4O6: C,
76.16; H, 7.99; N, 7.40. Found: C, 75.84; H, 8.19; N, 7.54.
General Procedure for the Metal Ion Binding Studies. Metal
perchlorate solutions were prepared in spectroscopic grade acetonitrile.
The bichromophores 3a-c were dissolved in acetonitrile under soni-
cation and warming in a water bath. Metal ion titrations were carried
out by adding small volumes (1-5 µL) of the metal salt solutions (10-6
M in CH3CN, 4 mL) in a quartz cuvette. After the addition of metal
perchlorate solution to the cuvette using a microliter syringe, the solution
was shaken well and was left to stand for 1 min before recording the
absorption and emission spectra of the metal complexed dye.
General Procedure for the Preparation of the Bichromophores
3a-c. To a 100 mL round-bottomed flask containing 50 mL of
2-propanol were added the appropriate bisaniline derivative (0.35
mmol), N,N-(dialkylamino)phenyl-4-hydroxy cyclobut-3-ene-1,2-dione
(0.86 mmol), and tributyl orthoformate (1 mL). The reaction mixture
was then refluxed for 20 h. The hot reaction mixture was filtered, and
the solid was washed with 2-propanol until the filtrate was almost
colorless. Column chromatography (chloroform/methanol, 9:1) of the
crude product over silica gel (100-200 mesh) gave the pure product.
Yields, melting points, and spectral details of each product are given
below.
3a. Yield: 25%. mp 230-232 °C. FT-IR (KBr): ν ) 1590, 1481,
1398, 1351, 1175, 1113, 984, 917, 839, 787 cm-1. 1H NMR (300 MHz,
CDCl3, TMS): δ 0.96-1.01 (t, J ) 7.28 Hz, 12H, -CH3), 1.36-1.6
(m, 16H, -CH2), 3.10 (s, 6H, -NCH3), 3.41-3.46 (m, 8H, -NCH2),
3.65-3.67 (m, 8H, -NCH2, -OCH2), 6.70-6.77 (dd, J ) 1.23, 9.3
Hz, 8H, aromatic), 8.35-8.39 (dd, J ) 2.4, 9.2 Hz, 8H, aromatic) ppm.
13C NMR (75 MHz, CDCl3): δ 13.8, 20.2, 29.6, 39.5, 51.2, 52.3, 69,
112.3, 112.4, 119.5, 120.2, 132.8, 133.6, 153.8, 183.3, 187.5, 189.3
ppm. HRMS-FAB: [M]+ calcd for C54H66N4O5, 851.5111; found,
851.5103.
Acknowledgment. This paper is dedicated to Prof. M. V.
George on the occasion of his 75th birthday. We thank the
Department of Science and Technology, Government of India
for financial support (DST/SF/C6/99-2000). We are grateful to
Prof. Jo¨rg Daub, Institute of Organic Chemistry, University of
Regensburg for the HRMS data and for fruitful discussions. E.A.
and P.C. thank CSIR, Government of India for a research
fellowship. This is Contribution No. 180 from RRLT-PPD.
Supporting Information Available: Synthetic details of
1a-c, 2a, and 2b, changes in the absorption and emission
properties of 3a with Sr(ClO4)2, Ba(ClO4)2, LiClO4, NaClO4,
and KClO4, 3b with Mg(ClO4)2, Ca(ClO4)2, Sr(ClO4)2, and
Ba(ClO4)2, and 3c with Mg(ClO4)2, Ca(ClO4)2, Sr(ClO4)2, and
Ba(ClO4)2, plots of variation of quantum yield with [Mn+], Job
plots of 3b with Mg2+ and Ca2+ ions, and Benesi-Hildebrand
plots of 3a and 3b with Mg2+ ions. This material is available
free of charge via the Internet at http://pubs.acs.org.
3b. Yield: 27%. mp 236-238 °C. FT-IR (KBr): ν ) 1589, 1460,
1390, 1367, 1181, 1129, 875 cm-1 1H NMR (300 MHz, CDCl3,
.
TMS): δ 0.907-0.99 (t, J ) 7.27 Hz, 12H, -CH3), 1.37-1.63 (m,
16H, -CH2), 3.14 (s, 6H, -NCH3), 3.42-3.64 (m, 20H, -NCH2, and
-OCH2), 6.69-6.77 (dd, J ) 1.4, 9.32 Hz, 8H, aromatic), 8.33-8.36
(dd, J ) 2.3, 9.25 Hz, 8H, aromatic) ppm. 13C NMR (75 MHz,
CDCl3): δ 13.8, 20.2, 29.4, 39.5, 51.2, 52.3, 68.7, 70.93, 112.4, 112.9,
119.5, 120, 132.8, 133.5, 153.8, 183.4, 187.6, 189.2 ppm. Anal. Calcd
for C56H70N4O6: C, 75.14; H, 7.88; N, 6.26. Found: C, 74.96; H, 7.96;
JA0393776
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6598 J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004