Lower rim 1,3-di{bis(2-picolyl)}amide derivative of calix[4]arene (L) as ratiometric primary sensor toward Ag+ and the complex of Ag + as secondary sensor toward cys: Experimental, computational, and microscopy studies and INHIBIT logic gate properties of L

Article abstract of DOI:10.1021/jo901676s

(Figure Presented) Astructurally characterized lower rim 1,3-di{bis(2-picolyl)}amide derivative of calix[4]arene (L) exhibits high selectivity toward Ag⁺ by forming a 1:1 complex, among nine other biologically important metal ions, viz., Na⁺, K⁺, Mg ²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Co ²⁺, Ni²⁺, and Zn²⁺, as studied by fluorescence, absorption, and ¹H NMR spectroscopy. The 1:1 complex formed between L and Ag⁺ has been further proven on the basis of ESI mass spectrometry and has been shown to have an association constant, K_a, of 11117 ± 190 M^-1 based on fluorescence data. L acts as a primary ratiometric sensor toward Ag⁺ by switch-on fluorescence and exhibits a lowest detectable concentration of 450 ppb. DFT computational studies carried out in mimicking the formation of a 1:1 complex between L and Ag ⁺ resulted in a tetrahedral complex wherein the nitrogens of all four pyridyl moieties present on both arms are being coordinated. Whereas these pyridyls are located farther apart in the crystal structure, appropriate dihedral changes are induced in the arms in the presence of silver ion in order to form a coordination complex. Even the nanostructural features obtained in TEM clearly differentiates L from its Ag⁺ complex. The in situ prepared silver complex of L detects Cys ratiometrically among the naturally occurring amino acids to a lowest concentration of 514 ppb by releasing L from the complex followed by formation of the cysteine complex of Ag⁺. These were demonstrated on the basis of emission, absorption, ¹H NMR, and ESI mass spectra. The INH logic gate has also been generated by choosing Ag ⁺ and Cys as input and by monitoring the output signal at 445 nm that originates from the excimer emission of L in the presence of Ag⁺. Thus L is a potential primary sensor toward Ag⁺and is a secondary sensor toward Cys. 2009 American Chemical Society.

Full text of DOI:10.1021/jo901676s

pubs.acs.org/joc
Lower Rim 1,3-Di{bis(2-picolyl)}amide Derivative of Calix[4]arene (L) as
Ratiometric Primary Sensor toward Ag+ and the Complex of Ag+ as
Secondary Sensor toward Cys: Experimental, Computational, and
Microscopy Studies and INHIBIT Logic Gate Properties of L‡
Roymon Joseph, Balaji Ramanujam, Amitabha Acharya, and Chebrolu P. Rao*
Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay,
Mumbai 400 076, India
cprao@iitb.ac.in
Received July 31, 2009
A structurally characterized lower rim 1,3-di{bis(2-picolyl)}amide derivative of calix[4]arene (L) exhibits
high selectivity toward Ag+ by forming a 1:1 complex, among nine other biologically important metal
ions, viz., Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, and Zn2+, as studied by fluorescence,
absorption, and 1H NMR spectroscopy. The 1:1 complex formed between L and Ag+ has been further
proven on the basis of ESI mass spectrometry and has been shown to have an association constant, Ka, of
11117 ( 190 M-1 based on fluorescence data. L acts as a primary ratiometric sensor toward Ag+ by
switch-on fluorescence and exhibits a lowest detectable concentration of 450 ppb. DFT computational
studies carried out in mimicking the formation of a 1:1 complex between L and Ag+ resulted in a
tetrahedral complex wherein the nitrogens of all four pyridyl moieties present on both arms are being
coordinated. Whereas these pyridyls are located farther apart in the crystal structure, appropriate
dihedral changes are induced in the arms in the presence of silver ion in order to form a coordination
complex. Even the nanostructural features obtained in TEM clearly differentiates L from its Ag+
complex. The in situ prepared silver complex of L detects Cys ratiometrically among the naturally
occurring amino acids to a lowest concentration of 514 ppb by releasing L from the complex followed by
formation of the cysteine complex of Ag+. These were demonstrated on the basis of emission, absorption,
1H NMR, andESI mass spectra. TheINH logicgate has also been generated by choosing Ag+ and Cys as
input and by monitoring the output signal at 445 nm that originates from the excimer emission of L in the
presence of Ag+. Thus L is a potential primary sensor toward Ag+ and is a secondary sensor toward Cys.
Introduction
enzymes, where Ag+ and Cys are not exceptions. Though
the Ag+ is not a biologically essential element, its complexes
are used in medicine and agriculture.1 The antimicrobial
Recognition of metal ions and amino acids are of prime
interest in the context of metalloproteins and metallo-
(1) (a) Jung, W. K.; Koo, H. C.; Kim, K. W.; Shin, S.; Kim, S. H.; Park,
Y. H. Appl. Environ. Microbiol. 2008, 74, 2171. (b) Silver, S.; Phung, L. T.;
Silver, G. J. Ind. Microbiol. Biotechnol. 2006, 33, 627.
‡ Dedicated to Professor Padmanabhan Balaram on the occasion of his 60th
birthday.
DOI: 10.1021/jo901676s
r
Published on Web 10/09/2009
J. Org. Chem. 2009, 74, 8181–8190 8181
2009 American Chemical Society

Joseph et al.
JOCArticle
action of silver mainly takes place through its interactions
with sulfur in the case of amino acids and peptides,2 and Ag+
ions are inactive toward other amino acids. A side effect of
silver originating from its prolonged use is irreversible
darkening of the skin and mucous membrane.3 Accumula-
tion of silver by microbial species could also act as a source
for this element.4 Among the 20 naturally occurring amino
acids, Cys plays an important role in living cells.5 Both the
deficiency and excess accumulation of Cys are detrimental to
life.6 Therefore, the detection and sensing of silver and Cys,
by a single molecular system with dual functionality, are
certainly challenging and are intriguing to the current re-
searchers. Owing to the ubiquitous nature of calix[4]arenes7
by possessing hydrophilic and hydrophobic characters to-
gether in the same structure, these molecules can act as good
mimics of enzymes8 and also provide a suitable platform for
building appropriate binding cores. There are some reports
in the literature for the selective recognition of silver either by
recognition by calix[4]arene derivatives is rather limited, and
the recognitions were mainly carried out by mass spectro-
scopy,12 calorimetry,13 1H NMR studies,13b,c,14 and HPLC;15
detection by fluorescence spectroscopy16 is scarce. Coleman
et al. reported several p-sulfonatocalix[4]arenes for amino
acid recognition and were able to establish the structure of
these calixarene amino acid complexes even in the solid
state.17 There are few reports available in the literature in
which amino acid recognition was achieved by chemo sen-
sing ensemble.12b,18 Though there are several receptors to
recognize Agþ and Cys individually, to the best of our
knowledge, there has been no report of any calix[4]arene
molecular system that would detect selectively Agþ followed
by Cys by acting as primary as well as secondary sensor. Thus
the present paper reports the synthesis, characterization, and
ratiometric silver-sensing properties of a lower rim functio-
nalized calix[4]arene possessing dipicolyl moiety connected
through an amide linkage (L) and the recognition of Cys by
the corresponding silver complex. Thus L acts as a primary
sensor toward Agþ and as a secondary sensor toward Cys
and thereby exhibits INH logic gate properties. The sensor
property of L toward Agþ has been proven to be due to the
formation of the complex on the basis of different experi-
mental and computational studies. Experimental evidence
has also been provided for the release of L from the reaction
of Cys with the silver complex.
1
potentiometry,9 by H NMR,10 or by fluorescence techni-
que11 using calix[4]arene derivatives. Similarly, amino acid
(2) Mcdonnell, G.; Russell, D. A. Clin. Microbiol. Rev. 1999, 12, 147.
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Results and Discussion
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M.; Sandberg, M.; Lehmann, A. Brain Res. 1995, 705, 65.
Receptor Molecule L. The receptor molecule, L, has been
synthesized by four known steps starting from p-tert-butyl
calix[4]arene as given in Scheme 1,19 (see also Experimental
Section). The acid chloride derivative 4 was prepared by
reacting 3 with SOCl2, followed by coupling with bis(2-picolyl)-
amine to result in the receptor molecule, L. All of these
molecules including L were characterized satisfactorily by 1H
NMR, 13C NMR, ESI MS, FTIR, and elemental analysis. The
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Y. L.; Reinaud, O. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 6831.
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41, 1044. (c) Cacciapaglia, R.; Casnati, A.; Mandolini, L.; Reinhoudt, D. N.;
Salvio, R.; Sartori, A.; Ungaro, R. J. Am. Chem. Soc. 2006, 128, 12322.
(d) Molenveld, P.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Org. Chem. 1999,
64, 6337. (e) Molenveld, P.; Stikvoort, W. M. G.; Kooijman, H.; Spek, A. L.;
Engbersen, J. F. J.; Reinhoudt, D. N. J. Org. Chem. 1999, 64, 3896.
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Weng, L.; Chen, L.; Leng, X.; Ju, H.; He, X.; Zhang, Z.-Z. J. Chem. Soc.,
Perkin Trans. 2 2001, 545.
1
cone conformation of L has been confirmed by H NMR
spectroscopy and also by the structure determined on the basis
of single crystal X-ray diffraction, as reported in this paper.
Crystal Structure of L. The ligand L was crystallized by slow
diffusion of diethyl ether into a solution containing L in a mix-
ture of methanol and chloroform, and the structure was deter-
mined by single crystal XRD (Supporting Information, S01).
(10) (a) Creaven, B. S.; Deasy, M.; Flood, P. M.; McGinley, J.; Murray,
B. A. Inorg. Chem. Commun. 2008, 11, 1215. (b) Wong, M. S.; Xia, P. F.;
Zhang, X. L.; Lo, P. K.; Cheng, Y.-K.; Yeung, K.-T.; Guo, X.; Shuang, S.
J. Org. Chem. 2005, 70, 2816. (c) Struck, O.; Chrisstoffels, L. A. J.;
Lugtenberg, R. J. W.; Verboom, W.; van Hummel, G. J.; Harkema, S.;
Reinhoudt, D. N. J. Org. Chem. 1997, 62, 2487. (d) Kim, S. K.; Lee, J. K.;
Lee, S. H.; Lim, M. S.; Lee, S. W.; Sim, W.; Kim, J. S. J. Org. Chem. 2004, 69,
2877. (e) Wong, M. S.; Xia, P. F.; Lo, P. K.; Sun, X. H.; Wong, W. Y.;
Shuang, S. J. Org. Chem. 2006, 71, 940. (f) Zeng, X.; Sun, H.; Chen, L.; Leng,
X.; Xu, F.; Li, Q.; He, X.; Zhang, W.; Zhang, Z.-Z. Org. Biomol. Chem. 2003,
1, 1073. (g) Kim, J. S.; Yang, S. H.; Rim, J. A.; Kim, J. Y.; Vicens, J.; Shinkai,
S. Tetrahedron Lett. 2001, 42, 8047. (h) Budka, J.; Lhotak, P.; Stibor, I.;
Michlova, V.; Sykora, J.; Cisarova, I. Tetrahedron Lett. 2002, 43, 2857.
(11) Kim, J. S.; Noh, K. H.; Lee, S. H.; Kim, S. K.; Kim, S. K.; Yoon, J.
J. Org. Chem. 2003, 68, 597.
(14) (a) Douteau-Guevel, N.; Coleman, A. W.; Morel, J.-P.; Morel-
Desrosiers, N. J. Phys. Org. Chem. 1998, 11, 693. (b) Da Silva, E.; Coleman,
A. W. Tetrahedron 2003, 59, 7357. (c) Arena, G.; Contino, A.; Gulino, F. G.;
Magri, A.; Sansone, F.; Sciotto, D.; Ungaro, R. Tetrahedron Lett. 1999, 40,
1597. (d) Frish, L.; Sansone, F.; Casnati, A.; Ungaro, R.; Cohen, Y. J. Org.
Chem. 2000, 65, 5026. (e) Casnati, A.; Fabbi, M.; Pelizzi, N.; Pochini, A.;
Sansone, F.; Ungaro, R. Bioorg. Med. Chem. Lett. 1996, 6, 2699. (f) Baldini,
L.; Casnati, L.; Sansone, F.; Ungaro, R. Chem. Soc. Rev. 2007, 36, 254.
(g) Ikeda, A.; Shinkai, S. Chem. Rev. 1997, 97, 1713. (e) Casnati, A.; Sansone,
F.; Ungaro, R. Acc. Chem. Res. 2003, 36, 246.
(15) Kalchenko, O. I.; Da Silva, E.; Coleman, A. W. J. Inclusion Phenom.
Macrocyclic Chem. 2002, 43, 305.
(16) (a) Lu, J. Q.; Zhang, L.; Sun, T. Q.; Wang, G. X.; Wu, L. Y. Chin.
Chem. Lett. 2006, 17, 575. (b) Li, H.; Wang, X. Photochem. Photobiol. Sci.
2008, 7, 694.
(12) (a) Stone, M. M.; Franz, A. H.; Lebrilla, C. B. J. Am. Soc. Mass
Spectrom. 2002, 13, 964. (b) Perret, F.; Morel-Desrosiers, N.; Coleman,
A. W. J. Supramol. Chem. 2002, 2, 533.
(17) (a) Perret, F.; Lazar, A. N.; Coleman, A. W. Chem. Commun. 2006,
ꢀ
2425. (b) Selkti, M.; Coleman, A. W.; Nicolis, I.; Douteau-Guevel, N.;
(13) (a) Zielenkiewicz, W.; Marcinowicz, A.; Cherenok, S.; Kalchenko,
V. I.; Poznanski, J. Supramol. Chem. 2006, 18, 167. (b) Arena, G.; Casnati,
A.; Contino, A.; Magri, A.; Sansone, F.; Sciotto, D.; Ungaro, R. Org.
Villain, F.; Tomas, A.; de Rango, C. Chem. Commun. 2000, 161. (c) Lazar,
A. N.; Danylyuk, O.; Suwinska, K.; Coleman, A. W. J. Mol. Struct. 2006,
825, 20.
ꢀ
Biomol. Chem. 2006, 4, 243. (c) Douteau-Guevel, N.; Perret, F.; Coleman,
A. W.; Morel, J.-P.; Morel-Desrosiers, N. J. Chem. Soc., Perkin Trans. 2
(18) Ait-Haddou, H.; Wiskur, S. L.; Lynch, V. M.; Anslyn, E. V. J. Am.
Chem. Soc. 2001, 123, 11296.
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2002, 524. (d) Douteau-Guevel, N.; Coleman, A. W.; Morel, J.-P. ; Morel-
Desrosiers, N. J. Chem. Soc., Perkin Trans. 2 1999, 629.
(19) Joseph, R.; Ramanujam, B.; Acharya, A.; Rao, C. P. Tetrahedron
Lett. 2009, 50, 2735.
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SCHEME 1. Synthesis of Lower Rim Calix[4]arene-1,3-di-derivatives, L, and Its XRD Structurea
aReagents and conditions: (a) bromoethylacetate/K2CO3/acetone; (b) NaOH/C2H5OH, reflux; (c) SOCl2/benzene, reflux; (d) bis(2-picolyl)amine/Et3N/
THF. R = tert-butyl.
FIGURE 1. From the crystal structure of L: (a) dimer formed by the arms of L (i.e., CA AC type), (b) dimer formed by the hydrophobic
3 3 3
arene cavity (i.e., AC---CA type), and (c) lattice structure of L where methanols are shown in green. “A” is the arms portion, and “C” is the arene
cavity.
In the asymmetric unit cell of the centrosymmetric triclinic
system, two methanol molecules were crystallized along with
bond data are given in Supporting Information, S01. However,
the two arms are disposed in space farther apart by exhibiting
˚
inter-arm N N distances ranging from 5.1 to 8.9 A and intra-
one molecule of L. The two O-H O intramolecular hydrogen
3 3 3
3 3 3
˚
˚
bonds {H O, O O distances (A) and O-H O angle
arm N N distances ranging from 4.9 to 5.4 A. Hence, L needs
3 3 3
3 3 3
3 3 3
3 3 3
(deg) are 1.820, 2.844, 169 and 2.014, 2.784, 147} observed at the
lower rim fixes the calix[4]arene in a cone conformation. Because
of the presence of a hydrogen bond between amide CO and
to undergo some conformational changes if the arms are to be
involved in binding to a metal ion through coordination.
Lattice Structure. In the lattice, L forms a dimer through
the interactions extended between the arms of both partners
in a head-to-head fashion. These are through π-π interac-
tions extended between the pyridyl moieties of similar type of
arms of the two partners (Figure 1a). Such head-to-head
dimers form a supramolecular column through arene-arene
interactions extended between the hydrophobic cavities of
the two neighbor dimers via the tert-butyl groups, where
the shortest distance between the tert-butyl carbons being
˚
˚
phenolic OH (2.362 A, 3.010 A, 131°), one of the amide CO is
projected inside the cavity of calix[4]arene (Arm 2, Scheme 1),
while lack of the hydrogen bonding makes the other amide CO
to point outside the calix[4]arene cavity (Arm 1, Scheme 1). The
nitrogen atoms present in the pyridyl moiety generates possible
binding cores in L (based on arms, N4 and N4O2; arms þ
phenolic OH, O4 and N4O6). Among these, the nitrogen-rich
pyridyl core is best suited for transition metal ions.19,20 The
details of the structure determination and the refinement as well
as some important bond lengths, bond angles, and hydrogen
˚
∼3.45 A (Figure 1b). Thus the column is a repetition of
---CA AC---CA AC---CA AC---CA , where “C”
3 3 3
3 3 3
3 3 3
3 3 3
represents the hydrophobic cavity portion possessing
the tert-butyl groups and “A” represents the arms portion.
These columns are further connected through weak H-bond
(20) Joseph, R.; Ramanujam, B.; Pal, H.; Rao, C. P. Tetrahedron Lett.
2008, 49, 6257.
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FIGURE 2. (a) Fluorescence spectral traces of L (10 μM) during titration with Agþ; (b) histogram representing the relative fluorescence
intensity (I/I0) observed at 445 nm; (c) plot of relative fluorescence intensity ratio, viz., I445/I315 vs [Agþ]/[L] mole ratio.
FIGURE 3. Absorption spectral titration of L with Agþ: (a) spectral traces observed during the titration in the region 230-350 nm; (b) plots
of absorbance versus mole ratio of [Agþ]/[L] for different bands (b = 282, O = 268, 9 = 261, and 2 = 255 nm); and (c) Job’s plot of nm versus
A*nm, where nm is the mole fraction of the metal ion added and A is the absorbance.
interactions offered by the solvent methanol molecules present
in the lattice resulting in the structure shown in Figure 1c.
A. Primary Sensor Properties of L in the Selective Recogni-
tion of Agþ. Fluorescence Titration Studies. Calix[4]arene
derivative L has been studied for its interaction with Agþ by
exciting the solutions at 285 nm and by following the
emission bands observed at 315 and 445 nm, which shows
fluorescence quenching and enhancement, respectively, as
the concentration of added Agþ increases. This resulted in
the formation of an iso-emissive point at 375 nm (Figure 2a).
Moreover the ligand L showed a fluorescence enhancement
of ∼25-fold during the titration with Agþ (Figure 2b). None
Fe2þ, Co2þ, Ni2þ, Cu2þ, and Zn2þ, showed fluorescence
changes except Cu2þ, which shows changes only in the
315 nm band, suggesting that the changes observed in this
band alone can be used for selective recognition of Cu2þ as
reported by us recently,19 while Agþ can be selectively
recognized by ratiometric variation of the fluorescence in-
tensity, viz., [I445/I315] as reported in this paper (Figure 2c).
The new emission band at 445 nm originates from the
excimer formation of pyridyl moieties in the excited state,
suggesting that the silver ion brings the pyridyl moieties
together during the metal ion binding. Such excimer formation
has been reported in the literature for a tetra-derivative of
calix[4]arene containing pyridyl moieties at its lower rim.21 The
binding constant of L with Agþ was calculated by the Benesi-
Hildebrand equation, and the corresponding association con-
stant Ka was found to be 11117 ( 190 M-1. The minimum
concentration at which L can detect Agþ has been found to be
450 ppb (Supporting Information, S02).
carried out between L and Agþ. The ligand L exhibited four
absorption bands centered at 261, 285, 255, and 268 nm; all
of these bands showed an increase in the absorbance upon
addition of Agþ (Figure 3a); and saturation in absorbance
was observed at g2 equiv (Figure 3b). The stoichiometry of
the complex formed between L and Agþ has been derived
to be 1:1 on the basis of a Job’s plot (Figure 3c), and the
Ka value was found to be 11941 M-1
.
Electrospray Mass Spectrometry. ESI MS spectrum ob-
tained during the titration of L with Agþ results in a 100%
molecular ion peak at m/z = 1235.5 corresponding to a 1:1
complex. The isotopic peak pattern observed for Agþ closely
resembles that of the calculated one, supporting the presence
of metal ion and hence the complex formation (Figure 4).
of the other Mnþ ions, viz., Naþ, Kþ, Mg2þ, Ca2þ, Mn2þ
,
FIGURE 4. ESI MS spectrum obtained in the titration of L with
Agþ. Insets: (a) expanded peak of the complex; (b) calculated iso-
topic pattern for the complex of [LþAgþ].
Absorption Titration Studies. To get further support for
the metal ion binding, absorption titration has also been
1H NMR Titration of L with Agþ. Interaction between L and
Agþ was studied by NMR spectroscopy in DMSO-d6. During
the titration, the concentration of L was kept constant and the
added [Agþ] mole ratio was varied, viz., 0.5, 1, 1.5, 2.0, 2.5, 3.5,
(21) Souchon, V.; Maisonneuve, S.; David, O.; Leray, I.; Xie, J.; Valeur,
B. Photochem. Photobiol. Sci. 2008, 7, 1323.
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FIGURE 5. 1H NMR spectra measured during the titration of L with different mole ratios of Agþ (in DMSO-d6): (a) 0; (b) 0.5; (c) 1.0; (d) 1.5;
(e) 2.0.
1
and 4.5 equiv (Figure 5). H NMR studies showed minimal to
marginal changes in the δ values of pyridyl, phenolic-OH, and
bridged -CH2 protons (Figure 6). Absence of any changes ob-
served in the chemical shift of aromatic and/or tert-butyl protons
rules out any interaction between the arene and Agþ. If the Agþ
were to be interacting with the arene cavity of the calixarene, the
aromatic protons would show corresponding shifts as reported in
the literature by Shinkai and co-workers.22 The downfield shift
observed in the pyridyl and the arm -NCH2 protons is suggestive of
the binding of Agþ in the pyridyl core through nitrogens. The
upfield shift observed in the bridged -CH2 and the phenolic -OH
groups of calix[4]arene are suggestive of the breakage and/or
weakening of lower rim H-bond interactions to result in conforma-
tional changes as reported for calix[4]arene derivatives in the
literature in the presence of Agþ ion10f (Figure 6).
Competitive Metal Ion Titrations. To find out whether
L can detect Agþ selectively even in the presence of other
metal ions, competitive metal ion titrations were carried out.
A 1:5 mole ratio mixture of L and Mnþ were titrated against
different equivalents of Agþ, and the corresponding fluore-
scence emission spectra were recorded. It was found that Agþ
can replace all the other Mnþ ions except Cu2þ, as can be seen
from Figure 7. Thus, while Agþ ion can be detected quanti-
tatively in the presence of a number of biologically relevant
Mnþ ions except Cu2þ, the Cu2þ indeed can be quantified by
L in the presence of all of these.
FIGURE 7. Relative fluorescence intensity of L upon addition of
3 equiv of Agþ in the presence of 5 equiv of Mnþ ions at (a) 315 and
(b) 445 nm.
Control Molecules. To substantiate the role of the calix-
[4]arene platform and the presence of a nitrogen core in the
recognition process of L, several related molecular systems,
viz., L1, L2, L3, and L4, shown in Figure 8 have been
subjected to similar titration studies by emission spectro-
scopy. These were chosen for the following reasons: (a) L1
FIGURE 6. Metal-ion-induced shift (Δδ = δL - δLþAgþ) observed
with different protons of L at various mole ratios of [Agþ]/[L] as
taken from Figure 5. Symbols are the same as in Figure. 5.
(22) Ikeda, A.; Tsuzuki, H.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2
1994, 2073.
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FIGURE 8. Schematic representation of the control molecules, L1, L2, L3, and L4. R = tert-butyl.
contains one methyl pyridyl moiety instead of the two
present in L; (b) the binding core present in L2 has one
additional CH2 group but one less pyridyl group, providing a
different binding core compared to that of L; (c) the pyridyl
moieties in L have been replaced by noncoordinating benzyl
moieties in L3; (d) the calix[4]arene platform present in L has
been replaced by a single strand in L4 while retaining the
binding arm.
Synthesis and characterization details of the control mole-
cules, L1 and L3, have been discussed by us earlier.19 L2 was
FIGURE 9. Plot of relative fluorescence intensity (I/I0) versus mole
ratio of Agþ added to the control molecules: L1 (9), L2 (Δ), L3 (O),
and L4 (f).
prepared by coupling a diacid chloride derivative of calix-
[4]arene, 4 (given in Scheme 1), with 2-amino ethyl pyridine,
and the product formed was purified and characterized by
spectral techniques (Supporting Information, S03). A single
strand derivative having p-tert-butyl phenol instead of calix-
[4]arene has been synthesized by coupling the acid chloride
derivative of p-tert-butyl phenol with bis(2-picolyl)amine
moiety as shown in Scheme 2 (Supporting Information,
S03). The product formed has been purified by silica gel
MS studies, the nature of Agþ coordination with L was
studied by computational studies using the Gaussian03
software package.23 The initial geometry for L was taken
from the single crystal XRD structure. As the number of
atoms present in L was too large, L0 was generated from L by
replacing the tert-butyl groups present at the upper rim with
hydrogens. Such changes made in calixarene systems to
reduce the computational times exhibited no influence
on the conformation of the arm as shown by us earlier.20,24
L0 is chemically and structurally similar to L and hence was
used in the calculations to reduce the computational times.
The geometry optimizations for L and L0 were done in a
cascade fashion starting from semiempirical PM3 follo-
wed by ab initio HF to DFT B3LYP by using various
basis sets, viz., PM3 f HF/STO-3G f HF/3-21G f HF/
6-31G f B3LYP/STO-3G f B3LYP/3-21G f B3LYP/
6-31G. The optimization at the DFT level showed con-
siderable change in the dihedral angles of the arms when
a
SCHEME 2. Synthesis of L4
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci,
B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada,
M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain,
M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski,
J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M.W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02;
Gaussian, Inc.: Wallingford, CT, 2004.
aReagents and conditions: (g) bromoethylacetate/K2CO3/acetone;
(h) KOH/C2H5OH/water, reflux; (i) SOCl2/benzene, reflux; and
(j) bis(2-picolyl)amine/Et3N/THF. R = tert-butyl.
column chromatography and was characterized by different
analytical and spectral techniques.
The control molecules, L1, L2, L3, and L4, have been
studied for their binding ability toward Agþ ion, and it was
found that none of these show any fluorescence change,
indicating their noninteractive nature with Agþ ion
(Figure 9).
Computational Modeling of the Agþ Complex of L. As the
stiochiometry of the complex formed between Agþ and L
was found to be 1:1 based on emission, absorption, and ESI
(24) Joseph, R.; Ramanujam, B.; Acharya, A.; Khutia, A.; Rao, C. P.
J. Org. Chem. 2008, 73, 5745.
8186 J. Org. Chem. Vol. 74, No. 21, 2009

Joseph et al.
JOCArticle
FIGURE 10. (a) Molecular structure of L from single crystal XRD, (b) L optimized through B3LYP/6-31G, (c) L0 optimized through B3LYP/
0
ꢀ
˚
6-31G, and (d) DFT optimized structure of [Ag-L ]. The bond distance (A) and bond angles (deg) in the coordination sphere are Ag-N2 =
0
0
0
0
ꢀ
˚
2.517, Ag-N3 = 2.326, Ag-N2 = 2.517, Ag-N3 = 2.326 A and N2-Ag-N3 = 109.7, N2-Ag-N2 = 98.9, N2-Ag-N3 = 91.8,
N3-Ag-N20 = 91.8, N3-Ag-N30 = 147.1, N20-Ag-N30 = 109.7°.
FIGURE 11. (a) Fluorescence spectral traces obtained during the titration of [LþAgþ] with cysteine, (b) plot of relative fluorescence intensity
(I445/I315) versus mole ratio of cysteine, and (c) histogram representing the effect of various amino acids on the fluorescence behavior of
[LþAgþ]. Filled columns = 445 nm, open columns = 315 nm.
compared to the crystal structure (Supporting Information
S04). As a result of this, the inter-arm N N distances are
shortened from 5.1-8.9 A to 4.7-5.5 A, and intra-arm
B. Secondary Sensor Behavior of [LþAgþ] and Selective
Recognition of Cys among Naturally Occurring Amino Acids.
Since L recognizes Agþ ratiometrically, the utility of [LþAgþ]
complex toward selective recognition of amino acid has been
studied so that this complex can act as a secondary sensor.
Fluorescence Titration. The chemosensing ensemble used
in these titrations was prepared in situ by mixing L and Agþ
in a 1:2 ratio. Out of 20 amino acids studied for their
interaction, only Trp and Tyr did not yield interpretable
results owing to their strong emission that overlaps with the
emission of L. The chemosensing mixture was titrated with
all of the remaining naturally occurring 18 amino acids,
where the fluorescence spectrum of [LþAgþ] is altered only
in the presence of Cys and not with the other 17 amino acids.
During the titration of [LþAgþ] with Cys, the emission band
at 315 nm enhances and that observed at 445 nm quenches.
This is exactly reverse to what happens when L is titrated
with Agþ, as reported in this paper (Figure 2), indicating that
the Agþ is being removed from the complex by Cys to release
free L (Figure 11) and thus the recognition of Cys by
[LþAgþ] complex as a secondary sensor. The release of
L in the titration of Cys is followed by the formation of
Cys complex of Agþ as it can form a stable complex with the
thiol functionality. The necessity of -SH function and the
formation of a Cys complex of Agþ were further proven on
the basis of the emission studies performed on control
molecular systems as reported in this paper. The detec-
tion limit of Cys by [LþAgþ] complex has been found to
be 514 ppb (Supporting Information, S05).
3 3 3
˚
˚
˚
˚
N
N distances are shortened from 4.9-5.4 A to 3.73 A;
3 3 3
thus the arms are well poised for metal ion binding in the
optimized L. The conformational changes of the arms are
exactly same in both L and L0 (Figure 10, Supporting
Information S04), and hence their coordination features
are exactly same.
To study the binding characteristics of Agþ, a starting
model was generated by taking the DFT optimized L0 and
placing the silver ion well above the core of pyridyl moieties
at a non-interacting distance. This model was then optimized
initially using HF/3-21G level of calculations, and the output
structure from this was taken as input for DFT calcula-
tions performed using B3LYP with SDDAll basis set for
Agþ and 6-31þG(d) for all other atoms in the complex.
The optimized complex of Agþ with L0 at this stage showed
a distorted tetrahedral geometry around Agþ where all
four pyridyl nitrogens are bonded to the central ion with
˚
Ag-N distances of 2.326 and 2.517 A (Figure 10, Support-
ing Information S04). These Ag-N distances are compar-
able with the experimental ones reported in the literature.25
However, the geometry about Agþ is significantly distor-
ted by exhibiting coordination angles in the range 91.8-
147.1°.
(25) Munakata, M.; Wen, M.; Suenaga, Y.; Kuroda-Sowa, T.; Maekawa,
M.; Anahata, M. Polyhedron 2001, 20, 2321.
J. Org. Chem. Vol. 74, No. 21, 2009 8187

Joseph et al.
JOCArticle
Titration with Control and Analogous Molecules. Since
fluorescence changes occur only in the presence of Cys and
not with the other amino acids, the role of the -SH function in
Agþ binding was further confirmed by studying the fluore-
scence response of [LþAgþ] with different sulfur-containing
systems, viz., cysteine ethyl ester, cysteamine, glutathio-
nine reduced (GSH), and glutathionine oxidized (GSSG)
(Figure 12). All of these, except GSSG, exhibited changes in
the fluorescence owing to the presence of the -SH function-
ality, whereas in GSSG the -SH is oxidized to -S-S-. Even
Met does not show changes in the fluorescence owing to the
presence of R-S-Me and not the -SH function. This experi-
ment clearly suggests that Agþ mainly interacts through the
-SH function of cysteine, and hence the [LþAgþ] chemo-
ensemble acts as a sensor for the thiol functionality.
[LþAgþ] with Cys could not be continued beyond 1 equiv
owing to precipitation (Supporting Information, S07); there-
fore, the detailed titrations were carried out using cysteamine
instead of Cys, as the fluorescence behavior of both of these was
found to be same. During the titration, as the concentration
of cysteamine increases, the proton signals of bridged-CH2,
-OCH2, -NCH2, pyridyl, and lower rim phenolic-OH are shifted
and moved toward simple L (Supporting Information, S07).
Quantitative changes observed in the chemical shift can be seen
from the plots given in Figure 14. Thus it has been found that
cysteamine removes Agþ from the binding core of calix[4]arene
using its -SH function, resulting in the recovery of simple L from
the [LþAgþ]. On the other hand the formation of the Agþ
complex with Cys has been identified based on ESI MS.
FIGURE 14. (a) Cysteamine-induced chemical shifts Δδ (Δδ =
δ(LþAgþþcysteamine - δ(LþAgþ)) observed with different protons of
L at various mole ratios of [cysteamine]/[LþAgþ] as noted in
Supporting Information, S07. For symbols refer to Figure 5.
FIGURE 12. Histogram representing the enhancement and quench-
ing fold exhibited by sulfur-containing (viz., -SH and -S-S-) molecules,
where A = cysteine ethyl ester, B = cysteamine, C = glutathionine
reduced (GSH), and D = glutathionine oxidized (GSSG). Filled and
open columns correspond to 445 and 315 nm emission bands, respec-
tively.
TEM Studies. To determine whether the release of L from
the reaction of [LþAgþ] with Cys is reflected in their
nanostructural features, TEM studies were performed with
L, [LþAgþ], and {[LþAgþ]þCys}, and the corresponding
micrographs and particle size distributions are shown in
Figure 15. The micrographs show spherical particles in the
case of L. The smaller size particles are in the range of
90-150 nm, whereas the bigger clusters have diameters of
180-270 nm. The interconnectivity present in these particles
is similar to that observed in case of the complexation of
Hg2þ with amide-linked benzimidazole-1,3-di-derivative of
calixarene as reported by us recently.24 When AgClO4 is
added to L, the size of the particles is reduced drastically to
15-35 nm and the shape is changed to nonspherical, indicat-
ing the complexation of L with Agþ where the complex
possessing silver ions exhibits dark spots. Thus, both the size
and shape of the particles differ between L and [LþAgþ],
suggesting that it is possible to identify the complex forma-
tion between L and Agþ by using nanostructural features.
ESI MS Titration of [LþAgþ] with Cys. To provide further
proof for the release of L and the formation of a Cys complex
of Agþ, ESI MS titration was carried out. The spectrum
exhibited two major m/z peaks (Figure 13). Whereas the peak
observed at 240.2 is assignable to [2Cys þ 2Ag þ Na - H]2þ
,
that observed at 1128 is assignable to [L þ H]þ. The first peak
is from the Agþ complex of Cys, and the second peak is from
simple L. All this clearly supports the release of L as well as
the formation of the Agþ complex of Cys, i.e., {[LþAgþ] þ
Cys f L þ [CysþAgþ]}. Even the absorption spectral studies
clearly demonstrate the release of L when Cys is added to
[LþAgþ] (Supporting Information, S06).
FIGURE 13. ESI mass spectrum obtained in the titration of
[LþAgþ] with Cys.
1H NMR Titration of [LþAgþ] with Cysteine and Cystea-
mine. The release of L and the formation of the complex of Agþ
FIGURE 15. TEM images of (a) L, (b) [LþAgþ], and (c) {[LþAgþ]þ
Cys} and their particle size distribution plots (bottom). Number of
particles analyzed ranges from 100 to 150 in each case.
1
have been further proven through H NMR titration carried
out between [LþAgþ] and Cys/cysteamine. The titration of
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Joseph et al.
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However, the micrographs also show background images
corresponding to the unreacted AgClO4. Upon addition of
Cys to the complex, the micrographs are filled with particles
and the calix[4]arene platform in the recognition of Agþ has
been demonstrated by comparing the fluorescence results
obtained from several control molecules.
arising from L, [LþAgþ], AgClO4, and [Cys Agþ] complex.
Whereas the fluorescence and absorption spectroscopy
provided information for the formation of 1:1 complex
between Agþ and L, viz., [LAgþ], the ESI MS confirmed
the same unambiguously by exhibiting an isotopic peak
pattern for the presence of silver in the complex. The complex
formation between L and Agþ has been further followed by
3
Therefore, it is difficult to clearly distinguish the particles of
L alone from all of these because the particles containing the
Agþ dominate as dark spots, and hence the lighter spots
expected from L cannot be easily distinguished as these are
buried in the background. (Supporting Information S08)
Dynamic Light Scattering (DLS) Studies. In DLS, all of
the species have shown solvo-dynamic diameters of more
than 10 μm. The simple ligand L is shown to have an average
diameter much higher than that of its [LþAgþ] complex as
well as that observed from the reaction mixture of {[LþAgþ]
þ Cys}. The trend observed in solvo-dynamic diameters is in
line with the particle size distribution of the individual
species observed in TEM (Supporting Information, S09).
C. INH Logic Gate Properties of L with Agþ and Cys. In
the present case, the logic gate properties of L have been
studied using two input signals as Agþ and Cys, respectively,
by monitoring the emission of L at 445 nm (Figure 16). From
the perspective of logic functions, the emission of L has been
determined only by the presence of single input, viz., Agþ,
and not with the cysteine, as Cys is insensitive to L. This
means that the emission of L is switched on only in the
presence of Agþ (25 fold fluorescence enhancement of
445 nm band of L) and in the absence of Cys. Under the
conditions where Agþ is absent and Cys is present, no
emission is observed in the 445 nm band of L (Supporting
Information, S10). Similarly in the presence of 2 equiv of
Agþ and Cys, the fluorescence emission of L at 445 nm was
quenched and no significant output signal was observed.
From these studies, it has been found that L can be used as a
INHIBIT (INH) logic gate toward Agþ in the absence of Cys
by observing the emission at 445 nm. The truth table and the
pictorial representation for the corresponding INH are given
in Figure 16.
1
measuring H NMR spectra as a function of added silver
perchlorate concentration. Comparison of the changes ob-
served in the chemical shifts of different protons, in terms of
downfield and upfield, suggests Agþ binding in the pyridyl-
nitrogen core (N4 core) associated with some conformational
changes in the calixarene platform. It has been demonstrated
by transmission electron microscopy that the nanostructural
features of L and its complex of Agþ differ substantially to
differentiate the complex from that of simple L.
Therefore, the silver-binding capabilities of the nitrogen-
binding core of pyridyl moieties available in L has been
examined by computational calculations at the DFT level.
The arms are found to be quite farther apart in the crystal
structure of L as can be judged from the intra- and inter-arm
N
N distances. Therefore, the binding of metal ions can
3 3 3
take place only when considerable dihedral changes take
place particularly in the arms of L. Indeed the computational
calculations performed on L (or its shorter version, L0) in the
presence of Agþ clearly demonstrated substantial changes in
the dihedral angles. Comparison of the orientation of the
arms and the binding cores present in the crystal structure of
L, the optimized structure of L, and its Agþ complex clearly
suggest that the arm undergoes conformational changes
along with some rotation about the pyridyl moiety so as to
bring all four pyridyl nitrogens in a manner to bind to the
central Agþ in a tetrahedral fashion (Figure 17). The tetra-
hedral geometry observed about Agþ is highly distorted as
can be judged on the basis of the wide range (91.8-147.1°) of
coordination sphere angles observed.
FIGURE 16. (a) INH logic gate represented using a conventional
gate notation; an active output signal is obtained when Agþ = 1 and
Cys = 0. (b) Truth table for the INH logic gate; Agþ and Cys are
inputs to the system; I445 is the output signal of L at 445 nm.
Conclusions and Correlations
A structurally characterized lower rim 1,3 diamide-bis-
(2-picolyl)-derivative of calix[4]arene (L) exhibits high selec-
tivity toward Agþ. The selectivity has been demonstrated by
fluorescence, absorption, and ESI MS spectroscopy. Inter-
action of Agþ with L enhances the fluorescence emission at
445 nm and quenches the fluorescence at 315 nm, thus
exhibiting a ratiometric behavior. L is sensitive and selective
toward Agþ over nine other biologically important ions
FIGURE 17. Space-filling model for (a) molecular structure of
L from XRD, (b) L0 optimized at DFT (B3LYP/6-31G), and
(c) [L0þAgþ] optimized at DFT {B3LYP with mixed basis set
SDDAll(Ag) and 6-31þG(d) for other atoms}. Agþ is in cyan.
The in situ prepared silver complex of L, viz., [LþAgþ], was
able to detect Cys ratio-metrically in a manner exactly reverse
to what happens when Agþ is added to L in fluorescence
spectroscopy. Thus, upon addition of Cys to [LþAgþ], the
445 nm band intensity decreases and that of the 315 nm band
increases, suggesting the release of L from the silver complex.
The release of L has also been further proven on the basis of
NMR titration. The present studies also demonstrate the
studied, viz., Naþ, Kþ, Mg2þ, Ca2þ, Mn2þ, Fe2þ, Co2þ
,
Ni2þ, and Zn2þ, as demonstrated by individual as well as
competitive metal ion titrations. L can detect up to 450 ppb
by switch-on fluorescence at 445 nm; thus L is a ratiometric
primary sensor for Agþ. The necessity of the pyridyl moieties
J. Org. Chem. Vol. 74, No. 21, 2009 8189

Joseph et al.
JOCArticle
necessity of the -SH function in removing the Agþ from its
complex of L based on several analogous and control mole-
cules. The removal of Agþ by cysteine (or other -SH molecules)
and the formation of a Agþ complex of Cys has been proven on
the basis of ESI mass spectrometry. Thus the in situ generated
[LþAgþ] complex selectively recognizes Cys among the natu-
rally occurring amino acids to a lowest concentration of
514 ppb, and hence L acts as secondary sensor toward Cys.
The characteristic changes observed in the fluorescence spec-
troscopy during the sensing of Agþ as well Cys are represented
schematically in Scheme 3 to express the primary and second-
ary sensor properties of L.
solid. Yield (35%, 0.52 g). Anal. Calcd for C72H82N6O6
3
C2H5OH: C, 75.74; H, 7.56; N, 7.16. Found: C, 75.18; H, 7.34;
N, 7.34. FTIR (KBr, cm-1): 1641 (νCdO), 3394 (νOH). 1H NMR
(CDCl3, δ ppm): 0.93 (s, 18H, C(CH3)3), 1.27 (s, 18H, C(CH3)3),
3.27 (d, 4H, Ar-CH2-Ar, J = 13.14 Hz), 4.34 (d, 4H, Ar-CH2-
Ar, J = 13.14 Hz), 4.71, 4.94 (s, 8H, NCH2), 4.97 (s, 4H, OCH2),
6.76 (s, 4H, Ar-H), 7.02 (s, 4H, Ar-H), 7.04 (t, 2H, Py-H,
J = 6.40 Hz), 7.13 (t, 2H, Py-H, J = 6.42 Hz), 7.25 - 7.27
(m, 2H, Py-H), 7.40 (d, 2H, Py-H, J = 7.90 Hz), 7.51-7.58
(m, 6H, Py-H and OH), 8.41 (d, 2H, Py-H, J = 4.88 Hz) 8.52
(d, 2H, Py-H, J = 4.88 Hz). 13C NMR (CDCl3, 100 MHz
δ ppm): 31.1, 31.8 (C(CH3)3), 31.9 (Ar-CH2-Ar), 33.9, 34.0
(C(CH3)3), 51.4, 52.2 (NCH2), 74.5 (OCH2CO), 122.2, 122.3,
122.5, 125.1, 125.7, 127.9, 132.7, 136.7, 136.9, 141.3, 147.1,
148.9, 149.9, 150.8, 156.4, 157.3 (Py-C and calix-Ar-C), 169.2
(CdO). m/z (ES-MS) 1127.78 ([M]þ 70%), 1128.80 ([M þ H]þ
40%,).
SCHEME 3. Schematic Representation of the Primary and
Secondary Sensing Properties of L
Solutions for Spectroscopy. The fluorescence and absorption
titrations were carried out in methanol. The concentration of
L and its reference molecules were kept constant at 10 μM for the
fluorescence titrations and at 20 μM for the absorption titra-
tions. The amino acids were dissolved in 500 μL of water and
diluted further using methanol to get the required concen-
tration. 1H NMR titrations, viz., Agþ and cysteamine with
L (0.011 M), were carried out in DMSO-d6 (400 μL) except for
the cysteine titration, where the amino acid was dissolved in
D2O (200 μL).
Solutions and Sample Preparation for TEM and DLS Studies.
Ligand L and silver perchlorate solutions were made in metha-
nol. Cys solution was made by dissolving the amino acid in
∼500 μL deionized water and then the volume was made up by
methanol. The final concentration of the solutions was kept
∼6 ꢀ 10-4 M. TEM and DLS studies of L were carried out by
using a solution of 50 μL in 3.0 mL of methanol, which results in
10 μM as also used in case of fluorescence studies. For the silver
complexation studies 50 μL of L was mixed with 100 μL of Agþ.
Similarly for the reaction with Cys, 100 μL of Cys was added to
the silver complex. Different sample blocks for TEM were made
by placing 20-30 μL of the sonicated (15 min) sample solution
on the copper grid and allowing it to dry in air at room
temperature.
The INH logic gate has been generated by choosing Agþ
and Cys as inputs and by monitoring the output signal at
445 nm that originated from the excimer emission of L in the
presence of Agþ. Thus L is a potential primary sensor toward
Agþ and is a secondary sensor toward Cys, and both act
ratiometrically.
Experimental Section
All the perchlorate salts, viz., Mn(ClO4)2 6H2O, Fe(ClO4)2
xH2O, Co(ClO4)2 6H2O, Ni(ClO4)2 6H2O, Cu(ClO4)2 6H2O,
3
3
3
3
3
Zn(ClO4)2 6H2O, Ag(ClO4) xH2O, NaClO4 H2O, KClO4,
3
3
3
Ca(ClO4)2 4H2O, and Mg(ClO4)2 6H2O, were procured from
3
3
Sigma Aldrich Chemical Co., USA. All solvents used were dried
and distilled by usual procedures immediately before use. Dis-
tilled and deionized water was used in the studies. The synthesis
procedures and the characterization data of all molecules used in
this paper are given in Supporting Information S03. Original
NMR spectra of all these molecules are given in Supporting
Information S03. The details of the instruments used for all
studies are discussed in Supporting Information S11.
Acknowledgment. C.P.R. acknowledges the financial sup-
port by DST, CSIR, and BRNS-DAE. R.J. thanks UGC,
and A.A. thanks CSIR for their research fellowships. We
thank SAIF and CRNTS, IIT Bombay for TEM and DLS
measurements.
Ligand L. A suspension of bis(2-picolyl)amine (0.59 g, 2.96
mmol) and Et3N (0.55 g, 5.43 mmol) was stirred in dry THF
(30 mL) under argon atmosphere. Diacid chloride 4 (1.08 g, 1.35
mmol) in dry THF (30 mL) was added dropwise to this reaction
mixture. Immediately, a yellowish precipitate was formed, and
stirring was continued for 48 h at room temperature. After
filtration, the filtrate was concentrated to dryness. A yellow
solid was obtained that was extracted with CHCl3 and washed
with water and then with brine, and the organic layer was dried
with anhydrous MgSO4. Filtrate was concentrated to dryness
and purified by column chromatography to give L as white
Supporting Information Available: Single crystal X-ray
structural information (S01), minimum detection limit (S02
and S05), synthesis and characterization (S03), computational
data (S04), absorption data (S06), 1H NMR titration data (S07),
TEM and DLS data (S08 and S09), fluorescence titration data
(S10), and instrumental details (S11). This material is available
free of charge via the Internet at http://pubs.acs.org.
8190 J. Org. Chem. Vol. 74, No. 21, 2009