Anthracene-based hetero bisamide chemosensor in fluorescence sensing of monocarboxylates over monocarboxylic acids

Article abstract of DOI:10.1080/10610278.2011.566614

A fluorescent chemosensor 1 with different amide moieties as recognition elements has been designed and synthesised. The recognition behaviour of the chemosensor towards various monocarboxylates and their conjugate acids has been evaluated. Although the s

Full text of DOI:10.1080/10610278.2011.566614

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Supramolecular Chemistry
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Anthracene-based hetero bisamide chemosensor
in fluorescence sensing of monocarboxylates over
monocarboxylic acids
Kumaresh Ghosh a & Avik Ranjan Sarkar a
a Department of Chemistry , University of Kalyani , Kalyani, Nadia , 741235 , India
Published online: 03 May 2011.
To cite this article: Kumaresh Ghosh & Avik Ranjan Sarkar (2011) Anthracene-based hetero bisamide chemosensor in
fluorescence sensing of monocarboxylates over monocarboxylic acids, Supramolecular Chemistry, 23:7, 539-549, DOI:
10.1080/10610278.2011.566614
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Supramolecular Chemistry
Vol. 23, No. 7, July 2011, 539–549
Anthracene-based hetero bisamide chemosensor in fluorescence sensing of monocarboxylates
over monocarboxylic acids
Kumaresh Ghosh* and Avik Ranjan Sarkar
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, India
(Received 7 December 2010; final version received 23 February 2011)
A fluorescent chemosensor 1 with different amide moieties as recognition elements has been designed and synthesised.
The recognition behaviour of the chemosensor towards various monocarboxylates and their conjugate acids has been
evaluated. Although the sensor 1 shows significant quenching of emission of anthracene in CH3CN, it shows an increase in
emission in CHCl3 containing 2% CH3CN upon complexation of aliphatic monocarboxylates. Receptor 1 shows selectivity
for acetate, propanoate and dihydrogen phosphate over the other anions studied under different conditions. The binding
1
features have been established by H NMR, UV–vis and fluorescence spectroscopic methods.
Keywords: monocarboxylate anion recognition; PET mechanism; anthracene-based receptor; unsymmetrical bisamide
Introduction
vicinity of pyridine amide under the guidance of a flexible,
semi-rigid spacer to generate a new hetero bisamide
structure, which may serve as simple receptor for both
carboxylate and carboxylic acid on the basis of binding
complementarities. Under this situation, the pertinent
query is whether such type of receptor molecule is capable
of binding either carboxylate and carboxylic acid guests or
any one of them with measurable selectivity. This has a
strong relevance in enzyme model in which specificity in
binding of a substrate is of prime importance. In an effort
to understand this, we report here the design, synthesis and
binding properties of 1 towards monocarboxylates and
their conjugate acids in different solvents. The binding
properties of the open cleft of 1 were also evaluated for
tetrahedral-shaped anions such as H2PO24 and HSO42.
Molecular recognition of ionic substrates by designed
artificial receptors is an area of keen interest (1–3). In this
aspect, the important task is to design and construct a
receptor that possesses size, shape and functional
complementarities to a target substrate. As target
substrates, carboxylates are important due to their
biological significances (4, 5). In recent past, considerable
efforts have been made to the development of fluorescent
receptors of different architectures for both mono (6–8)
and dicarboxylates (9–12), although the fluorescent
receptors for monocarboxylate are rare (13). In designing
such receptors, various hydrogen-bonding synthons such
as urea/thiourea (14, 15) and guanidinium motifs (16) are
well explored for carboxylate ion binding, and they are
placed in the close vicinity of the different types of
fluorophores. It is of important note that the use of 3-
amidopyridinium motif for binding of carboxylate is less
explored. Jeong and Cho reported the significance of
unconventional CZH· · ·O hydrogen bond in 3-amidopyr-
idinium motif during complexation of carboxylate ions in
polar solvent (17). Then, Steed and co-workers used this
motif in several designs for complexation of spherical- and
linear-shaped anions (18). In pursuit of developing
fluorescent chemosensor, we addressed the use of this
motif in selective sensing of anions such as dihydrogen
phosphate, fluoride (19) and also dicarboxylates (20, 21) as
their tetrabutylammonium salts. Based on inspiring results
in using 3-amidopyridinium motif as a hydrogen-bonding
motif for anions, we aimed at placing this motif in the
O
O
NH
HN
N
N+
PF6–
1
*Corresponding author. Email: ghosh_k2003@yahoo.co.in
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
DOI: 10.1080/10610278.2011.566614
http://www.informaworld.com

540
K. Ghosh and A.R. Sarkar
O
O
O
O
O
O
a
b
c
1
NH
Cl–
HN
NH
HN
Cl
Cl
N
N
N
N
2
3
Scheme 1. (a) High dilution reaction with 2-amino-6-methylpyridine and 3-aminopyridine in the presence of Et3N in dry CH2Cl2
(isolated yield: 18%); (b) 9-chloromethylanthracene, reflux in dry CH3CN (isolated yield: 37%) and (c) NH4PF6, MeOH (yield: 62%).
Results and discussion
salts such as acetate, propanoate, lactate, mandelate and
benzoate (up to addition of 2 equivalent of guest).
Upon gradual addition of either carboxylic acid or
carboxylate salt to the solution of 1 in CH3CN, the
monomer emission of 1 was quenched and the quenching
was found to be significant only in the presence of
carboxylate salts (Figure 1). This marked quenching effect
on the emission of anthracene in 1 allows monocarbox-
ylates (especially aliphatic) to be distinguished from
monocarboxylic acids.
The receptor 1 was obtained according to Scheme 1.
Initially, the hetero bisamide 2 was synthesised by high-
dilution reaction of 3-aminopyridine and 2-amino-6-
methylpyridine with isophthaloyl dichloride in dry
CH2Cl2. Purification of the reaction mixture afforded 2
in 18% yield. Reaction of 2 with 9-chloromethylanthra-
cene in dry CH3CN under refluxing condition yielded the
chloride salt 3. It is to note that alkylation does not occur
on to the methylated pyridine in spite of its better
nucleophilicity. Such preferential alkylation on the
nitrogen centre of 3-amidopyridine unit in 2 is presumably
due to steric reason. However, anion exchange of the
chloride salt 3 using NH4PF6 gave the receptor 1.
To evaluate the hydrogen-bonding properties of the
receptor 1 in solution, 1H NMR, UV–vis and fluorescence
experiments were performed in CH3CN and CHCl3
containing 2% CH3CN solvents. The ability of the new
receptor 1 to sense carboxylates and their conjugate acids
was initially realised by fluorescence and UV–vis
titrations. Figure 1 shows the changes in emission of
receptor 1 (c ¼ 6.27 £ 1025 M, excitation of anthracene
fluorophore at 370 nm) upon addition of different
monocarboxylic acids and their tetrabutylammonium
During titration, no peak at higher wavelength for
excimer or exciplex was observed. It is also evident from
Figure 1 that the extent of quenching of emission of 1 is
also dependent on the steric feature of the monocarbox-
ylates. Benzoate being steric in nature than the aliphatic
monocarboxylates such as acetate, propanoate interacted
poorly in the binding site and accordingly, perturbed the
emission of 1 weakly. Due to similar reason, mandelate
and lactate also exhibited weak interactions followed by
moderate quenching of emission of anthracene in 1.
Figure 2 demonstrates the change in emission of 1 upon
addition of CH3CH2COO2 to the solution of 1.
The quenching effect during complexation is attributed
to a photo-induced electron transfer (PET) mechanism.
Lactate
(R)- Mandelic Acid
Mandelate
Benzoic acid
Benzoate
Dihydrogenphosphate
Hydrogensulphate
Myristic Acid
Myristate
350
300
250
200
150
100
50
2
1
0
Propanoic Acid
Propanoate
Acetic Acid
300
350
400
450
Wavelength (nm)
Acetate
0.0
0.1
0.2
0.3
0.4
0.5
0
(I0 - I) / I0
400
450
500
550
Wavelength (nm)
Figure 1. Fluorescence ratio [(I0 2 I)/I0] of receptor 1
(c ¼ 6.27 £ 1025 M) at 412 nm upon addition of 2 equiv. of a
particular tetrabutylammonium carboxylate and its acid in
CH3CN.
Figure 2. Change in emission of 1 (c ¼ 6.27 £ 1025 M) in
presence of increasing amounts of CH3CH2COO2 in CH3CN;
inset: change in absorbance of 1 with CH3CH2COO2 in CH3CN.

Supramolecular Chemistry
541
160
140
120
100
80
1.4
1.2
(b)
(a)
(e)
(a) Acetate
(b) Propanoate
(c) Benzoate
(d) Mandelate
(e) Myristate
(f) Hydrogensulphate
(g) Dihydrogenphosphate
(h) Lactate
1.0
0.8
0.6
0.4
0.2
0.0
NHb of 1 with Acetate
60
(c)
NHb of 1 with Propanoate
(d)
40
(f)
(h)
(g)
20
0
0
2
4
6
8
10
12
[G]/[H]
0
1
2
3
4
5
6
Figure 3. Plot of change in emission of 1 at 412 nm vs. the ratio
of guest to host concentration in CH3CN.
[G]/[H]
Figure 4. NMR titration curves for 1 with acetate and
propanoate in CD3CN.
In the interaction process, 1:1 stoichiometries of the
complexes were realised from the break of titration curves
at [G]/[H] ¼ 1 in Figure 3. In this regard, for acetate,
benzoate and dihydrogen phosphate, the titration curves
continue to decrease after saturation suggesting that as
greater excess of the guest anion is added, the 1:1 host–
guest complex is disrupted and anions begin to bind
individually to the pyridinium and pyridine amides, rather
than in a cooperative manner. Thus, in principle,
complexes of 1:1 and 2:1 (guest to host) stoichiometries
may remain in equilibrium, in solution. Other anions
except acetate, propanoate, benzoate and dihydrogen
phosphate did not show this behaviour. Thus, this
behaviour depends on the anions (22a) and also the
concentration of the host and guest used at which the
titration experiments are being monitored. It is of note that
the downward running of the titration curves for AcO2 and
CH3CH2COO2 in Figure 3 was not observed when the
titrations were carried out in 1H NMR at the concentration
range ,1023 M (Figure 4). The sharp break of the titration
curves at [G]/[H] ¼ 1 in Figure 4 indicates the formation
of stable 1:1 complexes. In this context, it is mentionable
that the titration of 1 with H2PO24 ion at ,1023 M was not
possible due to precipitation. However, the downward
running of the curve for H2PO24 ion in fluorescence
titration is either due to the presence of excess H2PO24 that
presumably causes deprotonation of the bound dihydrogen
phosphate with the resultant formation of monohydrogen
phosphate receptor complex as described by Gale et al.
(22b) or due to partial decomplexation of H2PO24 .
Similarly, partial decomplexation of AcO2 and CH3CH2-
COO2 at the concentration range 1025 M cannot be ruled
out. This is presumably another reason for downward
running of the titration curves for AcO2 and CH3CH2-
COO2 in Figure 3. However, Job plots (22c) for anions
(AcO2, CH3CH2COO2, C6H5COO2 and H2PO24 ) were
additionally performed to confirm the exact stoichi-
ometries of the complexes. Although downward running
of the titration curves for AcO2, CH3CH2COO2,
C6H5COO2 and H2PO24 after [G]/[H] ¼ 1 was noted in
Figure 3, the inflection points at the mole fraction 0.5 in
their Job plots corroborated 1:1 stoichiometry of the
complexes. Figure 5, for example, represents the Job plot
for 1 with CH3CH2COO2 in which the stoichiometry is
depicted as 1:1. For all the carboxylic acid guests, the
titration curves were almost linear and thereby suggested
weak interaction (see Supplementary Information, avail-
able online).
The fluorescence titration data were used to determine
the association constants (23) of the complexes of 1 with the
anionic guests. In the determination of the association
constants, the stoichiometry of the complexes of all the
anions was considered as 1:1. The results are summarised in
Table 1. Binding constant values for acids were difficult to
determine due to minimum change in emission upon
1.6×10–5
1.4×10–5
1.2×10–5
1.0×10–5
8.0×10–6
6.0×10–6
4.0×10–6
2.0×10–6
0.0
0.0
0.2
0.4
0.6
0.8
1.0
XG
Figure 5. Fluorescence Job plot for 1 with tetrabutylammonium
salt of propanoate at 372 nm in CH3CN.

542
K. Ghosh and A.R. Sarkar
H2PO24 , receptor 1 indicated almost no interaction in
CH3CN. Nonlinear least squares method (24a) was also
adopted to check the reliability of binding constant values,
and, for example, the values for AcO2
Table 1. Association constants (Ka) based on fluorescence and
UV–vis methods in CH3CN.
Guests
Ka (M21)a (SD)
Ka (M21)b (SD)
AcO2
1.08 £ 104 (0.11)
9.65 £ 103 (0.25)
[(K ¼ 1.06 ^ 0.09) £ 104 M21
]
and CH3CH2COO2
CH3CH2COO2
Myristate
C6H5COO2
(R)-Mandelate
H2PO24
1.25 £ 104 (0.07)
1.34 £ 104 (0.28)
[(K ¼ 1.13 ^ 0.06) £ 104 M21] determined by fluor-
escence method were found to be comparable with the
values found using linear curve fitting method (Table 1). 1H
NMR titration results for AcO2 and CH3CH2COO2 were
also used to determine the Ka (24b), and they were found to
1.12 £ 103 (0.56)
–
1.34 £ 103 (1.34)
6.08 £ 103 (0.23)
–
–
–
–
3.83 £ 103 (0.23)
HSO24
Lactate
–
–
be (1.78 ^ 0.35) £ 104 M21 and (1.81 ^ 0.26) £ 104 M21
,
Notes: SD, standard deviation; dashes indicate that the association constants were
not determined either due to minimal change or difficult to determine.
respectively.
a
The probable hydrogen-bonding structures of the
complexes of 1 with carboxylate (1A, 1B and 1C),
carboxylic acid (1D) and H2PO24 (1E) are shown in
Figure 6 based on the hydrogen-bonding complementa-
rities. Due to the flexible nature of 1, cooperative binding
mode 1A for anions may remain in equilibrium with non-
cooperative binding modes (1B and 1C). This is true for
carboxylic acids, H2PO24 and HSO24 also. Experimental
findings and the previous report (25a) revealed that
structures involving cooperative bindings (1A, 1D and 1E)
dominate over the structures with non-cooperative
bindings in the solution. In this context, binding-induced
mutual activation of the binding sites and their
Determined by fluorescence method at the wavelength of 412 nm.
b
Determined by UV–vis method at the wavelength of 372 nm.
complexation (see Supplementary Information, available
online). As given in Table 1, receptor 1 shows higher
association constant values with acetate and propanoate
compared to others. Long-chain myristate did not show any
measurable change in emission of 1. We presume that
coiling natureofthe longaliphatic chain probablyintroduces
a steric clash into the isophthaloyl diamide core of 1 for
which myristate exhibited weaker binding. Indeed, the
preference in binding of AcO2 and CH3CH2COO2 anions
over C6H5COO2 has the strong relevance in the distinction
between aliphatic and aromatic monocarboxylates. The
results further indicated the selectivity between carboxylates
and carboxylic acids (see Figure 1), although the receptor 1
can provide binding complementarity to both kinds of
guests. In spite of the binding complementarity of 1 with
1
cooperativities is recently well documented (25b). In H
NMR, downfield shifting of both the amide protons in 1
upon interaction with AcO2 and CH3CH2COO2 further
supported the cooperative mode of binding, i.e. mode 1A
(see Supplementary Information, available online).
R
-
O
O
H
O
H
O
O
N
N
O
N
N
NH
O–
HN
NH
O
NH
-
H
O
-
O
O
O
N
O
O
N
H
N
H
+
+
N
R
+
R
R
–
–
PF6
PF6
–
PF6
1A
1B
1C
O
O
O
O
N
N
H
NH
R
HN
OH
H
H
O
P
N
O
N
H
N
+
N
+
-
O
O
O
H
–
–
PF6
PF6
1D
1E
R = CH3, Et, CH3(CH3)11, PhCH(OH), Ph, CH3CH(OH)
Figure 6. Possible modes of complexation of 1 with carboxylates, carboxylic acids and H2PO24 .

Supramolecular Chemistry
543
(i) Receptor Itself (c = 1.2 x 10–4M)
(v),(vi),(vii)
(viii),(ix)
(ii) [Receptor +1 equiv. AcOH (c = 1.59 x 10–2M)] = A
(iii) [A + 1 equiv. TBA-OH(c = 1.59 x 10–2M)] = B
(iv) [B+ 60 µLTFA (c = 2.4 x 10–3M)] = C
(v) [C + 75 µLTFA] = D
(a) CH3CN; [1] = 6.27 × 10–5
(b) 4:1 CH3CN : H2O (v/v); [1] = 6.77 × 10–5
M
400
300
200
100
0
350
300
250
200
150
100
50
M
(i),(ii)
(iv)
(c) 2% CH3CN in CHCl3; [1] = 6.11 × 10–5
(d) 2% CH3CN in CHCl3; [1] = 6.11 × 10–6
M
(vi) [D+ 135 µLTFA] = E
M
(vii) [E + 135 µLTFA] = F
(viii) [F + 135 µLTFA] = G
(ix) G + 135 µLTFA
(c)
(d)
(iii)
(b)
(a)
0
400
450
500
550
600
Wavelength (nm)
400
450
500
550
600
650
Figure 7. Emission profile for sensitivity of AcO2 over AcOH
and the reversibility in the process.
Wavelength (nm)
Figure 8. Emission spectra of 1 in different solvents
(lex ¼ 370 nm).
The preference in the binding of carboxylate ion over
carboxylic acid was realised by doing a control experiment
in fluorescence. The emission intensity of 1 (c ¼ 1.20 £
1024 M) at 412 nm, which was almost unperturbed in the
presence of equivalent amount of carboxylic acid,
quenched to a considerable extent upon addition of
equivalent amount of tetrabutylammonium hydroxide
(c ¼ 1.59 £ 1022 M). Upon addition of trifluoroacetic
acid (TFA) (c ¼ 2.4 £ 1023 M) to the resulting solution
further caused an increase in emission and thereby
indicated a reversibility in the process as well as a
preference in binding towards monocarboxylate. Figure 7,
for example, demonstrates this phenomenon with AcOH.
It is mentionable that the individual addition of TFA and
tetrabutylammonium hydroxide to the receptor solution of
1 caused negligible change in emission (see Supplemen-
tary Information, available online).
CH3CN, the receptor 1 showed an enhanced selectivity
towards carboxylates and H2PO24 by exhibiting a different
emission pattern. When 1 in CHCl3 containing 2% CH3CN
was excited at 375 nm, it gave an emission at 416 nm along
with a broad peak at 510 nm, which was attributed to the
formation of intermolecular excimer between anthracene
or exciplex between pyridine and excited anthracene
(26a). The intensity of the peak at 510 nm is considerably
less when the concentration of 1 is 6.11 £ 1026 M in
CHCl3 containing 2% CH3CN. This suggested the self-
association behaviour of 1 that results in the formation of
excimer rather than exciplex (26b). In more polar solvents
such as CH3CN and CH3CN/H2O (4:1 v/v), this broad
signal at 510 nm was no longer observed. Figure 8
demonstrates this comparative view.
However, upon addition of the carboxylates to the
solution of 1 in CHCl3 containing 2% CH3CN, the
monomer emission at 416 nm underwent minimum change
The simultaneous UV–vis experiments in CH3CN
upon addition of the same guests as mentioned in Table 1
showed minor changes in the absorbance of the anthracene
peaks except for AcO2 and CH3CH2COO2. For example,
the change in absorbance of 1 upon gradual addition of
CH3CH2COO2 is represented in the inset of Figure 2. This
further indicated the strong binding of CH3CH2COO2.
From the UV–vis titration in CH3CN, the change in
absorbance of the anthracene peak at 372 nm was used to
determine the association constant (21) values (Table 1).
The trend was similar to that of fluorescence. Even the
binding constant values determined by nonlinear least
square method (24b) using UV titration results [e.g.
AcO2: K ¼ (1.01 ^ 0.12) £ 104 M21; CH3CH2COO2:
K ¼ (1.14 ^ 0.08) £ 104 M21] were comparable with the
values obtained by linear curve fitting method (Table 1).
The stoichiometries of the complexes of 1 with the guests
in the ground state were also 1:1 as confirmed by Job plots
(see Supplementary Information, available online).
Lactate
Mandelic acid
Mandelate
Hydrogensulphate
Dihydrogenphosphate
Benzoic acid
Benzoate
Myristic acid
Myristate
Propanoic acid
Propanoate
Acetic acid
Acetate
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
(I - I0)/I0
Figure 9. Fluorescence ratio [(I 2 I0)/I0] of receptor 1
(c ¼ 6.11 £ 1025 M) at 510 nm upon addition of 1 equiv. of a
particular tetrabutylammonium salt of anions in CHCl3
containing 2% CH3CN.
To increase the selectivity in binding for anions, we
managed the solvent polarity. In CHCl3 containing 2%

544
K. Ghosh and A.R. Sarkar
O
(a)
N
(b)
Lactate
O
O
N
O
N
Mandelate
NH
N
Hydrogensulphate
Dihydrogenphosphate
Benzoate
NH
N
H
H
H
N
Myristate
Propanoate
Acetate
= Carboxylates, H2PO4–, HSO4–.
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Fluorescence enhancement factor (Z)
Figure 10. Proposed intermolecular binding interactions
between 1 and anions involving (a) NH donors of pyridinium
amides and (b) NH and CH donors of pyridinium amides.
Figure 12. Fluorescence enhancement factor (Z) of 1
(c ¼ 6.11 £ 1025 M) at 510 nm upon addition of 1 equiv. of a
particular tetrabutylammonium salt of anions in CHCl3
containing 2% CH3CN.
up to 1:1 stoichiometry and then increased markedly when
particular carboxylate was added in more amounts. The
intensity of emission at 510 nm was gradually intensified
on progression of the titration and it varied with nature of
the carboxylate added. This suggested the persistence of
strong intermolecular association of 1 in 2% CH3CN in
CHCl3 even on complexation. The change in emission at
510 nm of 1 with the addition of different anions and
carboxylic acids is displayed in Figure 9. It is evident from
Figure 9 that the sensing ability of 1 in 2% CH3CN in
CHCl3 is good to moderate for acetate, propanoate,
dihydrogen phosphate and hydrogen sulphate. Carboxylic
acids merely perturbed the emission (see Supplementary
Information, available online). However, the gradual
increase in intensity of the emission at 510 nm upon
titration is explained by suggesting the different possible
binding modes in Figure 10 that presumably originates
from the guest-mediated intermolecular chelation (either
in polymeric or discrete fashion) of the receptor 1 in
solution (26b). Fluorescence Job plot confirmed 1:1
stoichiometry of the complexes in all cases (see
Supplementary Information, available online).
Figure 11 displays the change in emission of 1 during
complexation of acetate in CHCl3 containing 2% CH3CN.
The complexation-induced increase in emission of 1 in
CHCl3 containing 2% CH3CN is explained due to the
inhibition of PET process. This is reversed to the case,
observed in pure CH3CN in which PET process was
activated upon complexation. We believe that such
different behaviours are essentially due to the different
dispositions of MOs that take part in the PET process, in
solvents of different polarities.
However, we also determined the fluorescence
enhancement factor of 1 at the excimer wavelength
(510 nm) in the presence of equivalent amount of each
anion in 2% CH3CN in CHCl3. The increase in emission is
a relative indicator of binding strength. Figure 12, in this
regard, shows the plot of fluorescence enhancement factor
(Z) (26c) in the presence of the different anions. From the
plot, it is clearly understood that the response of the
receptor towards acetate, propanoate and dihydrogen
phosphate is significant.
2.0
400
1.5
350
1.0
300
250
0.5
Simultaneous UV titrations were conducted in CHCl3
containing 2% CH3CN and absorbance decreased on
complexation (see Supplementary Information, available
online). As representative, inset of Figure 11 shows the
titration spectrum of 1 with acetate. The stoichiometries of
the complexes of 1 with the anions in 2% CH3CN in
CHCl3 were also 1:1 as established by Job plots and
titration curves (see Supplementary Information, available
online). Furthermore, during UV–vis titrations of 1 with
all the monocarboxylates, clear isosbestic points were
observed. This, indeed, indicated the formation of new
species in solution.
200
0.0
300
350
400
450
150
100
50
Wavelength (nm)
0
400
450
500
550
600
650
Wavelength (nm)
Figure 11. Change in emission of 1 (c ¼ 6.11 £ 1025 M) in
presence of increasing amount of CH3COO2 in 2% CH3CN in
CHCl3; inset: change in absorbance of 1 with CH3COO2.

Supramolecular Chemistry
545
Table 2. Association constants (Ka) based on fluorescence
method in 2% CH3CN in CHCl3.
Hc
Hb
Ho'
Guests
Ka (M21)a (SD)
Ka (M21)b (SD)
Hp
Hm
Ha
(c)
Acetate
1.16 £ 104 (0.13)
1.04 £ 104 (0.32)
3.02 £ 103 (0.18)
1.59 £ 103 (0.63)
9.56 £ 103 (0.10)
8.39 £ 102 (0.04)
1.32 £ 103 (0.17)
1.96 £ 103 (0.05)
1.33 £ 104 (0.51)
Propanoate
Benzoate
Myristate
H2PO24
HSO24
Lactate
Mandelate
8.85 £ 103 (0.50)
1.03 £ 103 (0.46)
Hc
Ho'
1.30 £ 103 (0.56)
Hb
Hp
Hm
–
–
–
Ha
(b)
(a)
Ho
Hb
Hc
Ho'
1.45 £ 103 (0.64)
Ha
H
pHm
Notes: SD, standard deviation; dashes indicate that the association constant was not
determined either due to minimal change or due to greater percentage of error.
a
Determined by fluorescence method at the wavelength of 510 nm.
b
Determined by UV–vis method.
11
10
9
8
7
Figure 13. Partial 1H NMR of (a) 1 (c ¼ 7.18 £ 1023 M) and its
1:1 complexes with (b) CH3CH2COO2NþBu4 and (c)
CH3COO2 NþBu4 in CD3CN.
Binding constant values, determined (23) in 2%
CH3CN in CHCl3 (Table 2), indicate that receptor has a
clear cut selectivity for acetate, propanoate over benzoate
and even myristate, a long-chain aliphatic monocarbox-
ylate. Between dihydrogen phosphate and hydrogen
sulphate, dihydrogen phosphate is complexed more
strongly in the cavity.
did not exhibit any measurable shift. Therefore, the
possible hydrogen-bonding structures that may remain in
equilibrium in solution are shown in Figure 14.
1
In addition, H NMR of 1 with selected carboxylates
In order to check the interaction properties of 1 in
aqueous system, we performed the interaction of 1 in
CH3CN/H2O (4:1 v/v) in the presence of the same guests
but there was no measurable change in emission during the
course of titrations (see Supplementary Information,
available online).
and carboxylic acids were recorded in 2% CD3CN in
CDCl3 to understand the recognition selectivity between
carboxylates and carboxylic acids towards the cleft of 1
(see Supplementary Information, available online).
The strong interactions of 1 with the carboxylates were
Theoretical observation
1
also established from the change in H NMR of 1 in the
presence of equivalent amount of the guests. Due to
In order to understand the nature of orientation of the
binding groups around isophthaloyl spacer, we performed
DFT calculation using 6-31G (27) basis set and the
1
insolubility of 1 in CDCl3, we took H NMR of 1 in d6-
*
DMSO and observed the change in chemical shift of the
interacting protons in the presence of the guests. The
amide protons Ha and Hb appeared at 11.22 and 10.83 ppm,
respectively, and showed negligible downfield shift
(Dd ¼ 0–0.3 ppm for Ha and 0–0.07 ppm for Hb). This
negligible change in chemical shift of the amide protons is
attributed to the binding of DMSO (26d) into the open
cavity. This was verified by replacing d6-DMSO with
CD3CN, which did not interfere in the binding process.
The amide protons Ha and Hb appeared at 10.08 and
9.34 ppm, respectively, in CD3CN, and moved downfield
measurably in the presence of aliphatic monocarboxylates.
For example, Figure 13 shows the change in 1H NMR of 1
in the presence of equivalent amounts of AcO2 and
CH3CH2COO2. Interestingly, during complexation ortho
proton Ho of 1 was not found either due to broadening for
strong complexation or due to deprotonation. The
downfield shifting of the signal for Ho indicated the
complexation-induced conformational change of 1 for
which the Ho became close to the carboxylate ion for
hydrogen bonding. In the event, the para proton Hp of 1
popular b3LYP (28) functional on the structure 1 as well as
its complexes. Figure 15 shows the DFT optimised
geometry of 1 showing the charges on the different atoms.
From the DFT results, global electrophilicity index
(v; measure of electrophilic power to interact with
O
O
O
O
Hc
Hc
O
HaN
Hp
HaN
Hp
NHb
NHb
O
Hm
Hm
H
N
N
-O
Ho
R
R
O
N+
–
o–' N+
Ho'
–
Ho
PF6
PF6
Figure 14. Suggested hydrogen-bonding structures of 1 with
carboxylate in CD3CN.

546
K. Ghosh and A.R. Sarkar
receptor for anion is due to its significant global
electrophilic character as calculated theoretically.
The hydrogen bonding (NZH- - -O and CZH- - -O) and
charge–charge interactions as a whole contribute to
stabilise the complex of carboxylates over the carboxylic
acids, where the charge–charge interaction is absent.
In this regard, although it is obvious that the charged guest
has a preference over the neutral one because of
electrostatic reason, this systematic study in the present
report is quite meaningful to understand the hydrogen-
bonding behaviour as well as sensing property of a simple
hetero bisamide receptor. The sensing ability of this hetero
bisamide receptor is dependent on the polarity of the
solvent used, and in the present case CHCl3 containing 2%
CH3CN is the better choice in reporting the recognition
and sensing properties successfully for a wide range of
anions. Further progress in this direction is underway in
our laboratory.
Figure 15. DFT-optimised geometry of 1 (E ¼ 21681.19 a.u.
and dipole moment m ¼ 3.42 D).
nucleophiles; (29a,b)) of 1 was calculated, and it was
found to be 0.3389. We also optimised the complexes of 1
with AcO2, CH3CH2COO2 and H2PO24 in gas phase. In
this relation, Figure S20 (see Supplementary Information,
available online) highlights the hydrogen-bonding
schemes in which all the possible hydrogen-bond donors
are intimately involved in complexation. Interestingly, the
pyridine ring nitrogen is found to participate in the
formation of unconventional CZH- - -N hydrogen bonds
with carboxylate anions (Figure S20 of the Supplementary
Information, available online) and conventional OZH- - -
N bond with H2PO24 (Figure S20 of the Supplementary
Information, available online). In the complexes, the
pyridinium ortho proton (Ho) is also involved in hydrogen
bonding with the anions and overall supports the
experimental findings.
Experimental
Synthesis
N1-(6-methylpyridin-2-yl)-N3-(pyridin-3-yl)
isophthalamide (2)
The unsymmetrical hetero bisamide 2 was obtained by
simultaneous dropwise addition of 2-amino-6-methylpyr-
idine (0.532 g, 4.93 mmol, dissolved in 20 ml dry
CH2ZCl2) and 3-aminopyridine (0.463 g, 4.93 mmol,
dissolved in 20 ml dry CH2ZCl2) to the solution of
isophthaloyl diacid chloride (1 g, 4.92 mmol) in dry
CH2Cl2 (30 ml) at high dilution condition under nitrogen
atmosphere. Triethylamine (4.92 mmol) was added in each
amine solution during the reaction. After completion of
addition of the amines, the reaction mixture was stirred
overnight. The progress of the reaction was monitored by
thin layer chromatography (TLC). After completion of the
reaction, the solvent was evaporated and the residual mass
was extracted with CHCl3/CH3OH mixture (CHCl3:CH3-
OH ¼ 4:1; 3 £ 20 ml). The organic extract was next
washed with NaHCO3 solution (3 £ 15 ml) and dried over
anhydrous Na2SO4. The solvent was removed under
vacuum, and the residual mass was purified by silica gel
column chromatography using 4:1 ethyl acetate:petroleum
ether as eluent to afford the compound 2 (304 mg, 18%
Conclusion
In conclusion, a simple hetero bisamide receptor contain-
ing anthracene moiety in a selective position has been
synthesised as a fluorescent chemosensor for aliphatic
monocarboxylates. The receptor shows selective binding
of monocarboxylates over their conjugate acids in spite of
having binding complementarity to both carboxylate and
carboxylic acid. The results indicate that the open cleft of
the receptor can distinguish the aliphatic carboxylates such
as AcO2, CH3CH2COO2 from their acid analogues and
also other anions studied by exhibiting significant change
in emission in different solvent combinations. Analysis of
the results shows that in 2% CH3CN in CHCl3 is a better
choice of solvent to attain greater selectivity for sensing
anions, especially short-chain aliphatic monocarboxylates
AcO2, CH3CH2COO2 and also tetrahedral-shaped anion
H2PO24 from HSO24 . The high affinity of such simple
1
yield, mp 1928C). H NMR (d6-DMSO, 400 MHz): 10.49
(s, NH, 1H), 9.99 (s, NH, 1H), 9.06 (s, 1H), 8.73 (s, 1H),
8.50 (d, 1H, J ¼ 8 Hz), 8.34 (d, 2H, J ¼ 8 Hz), 8.19 (d, 1H,
J ¼ 1.2 Hz), 8.17 (d, 1H, J ¼ 1.2 Hz), 7.81 (t, 1H,
J ¼ 8 Hz), 7.60 (t, 1H, J ¼ 7.8 Hz), 7.38–7.35 (m, 1H),
7.04 (d, 1H, J ¼ 8 Hz) and 2.56 (s, 3H). 13C NMR (d6-
DMSO, 125 MHz): 166.7, 165.4, 156.7, 151.4, 144.8,
142.1, 138.5, 135.7, 134.4, 133.3, 131.3, 130, 129.1, 128.7,
127.4, 123.6, 119.2, 111.6 and 23.6. FTIR: n cm21 (KBr):
3395, 2959, 2918, 2849, 1675, 1660, 1544 and 1458. m/z

Supramolecular Chemistry
547
(ES þ ): 333.4 (M þ H)þ, 225.3 and 167.1. Analysis
calculated for C19H16N4O2: C 68.66, H 4.85 and N 16.86.
Found: C 68.52, H 4.91 and N 16.71.
UV–vis titration experiments were carried out at 258C. All
the experiments were repeated thrice to check the
reproducibility.
General method for NMR titration
1-(Anthracen-9-ylmethyl)-3-(3-(6-methylpyridin-2-
ylcarbamoyl)benzamido)pyridinium hexafluorophosphate
(V) (1)
Stock solutions of host (c ¼ 7.18 £ 1023 M) and guests
(c ¼ 4.79 £ 1022 M for AcO2 and 4.54 £ 1022 M for
CH3CH2COO2) were made in CD3CN. In each titration,
0.5 ml of the host was transferred to an NMR tube, and
the spectrum was collected. Then, guest solution was
Compound 2 (0.08 g, 0.26 mmol) was treated with 9-
chloromethyl anthracene (0.07 g, 0.32 mmol) in dry
CH3CN (30 ml), and the reaction mixture was refluxed
for 48 h to afford the chloride salt 3 (60 mg, 37% yield).
The compound 3 (0.055 g, 0.098 mmol) in MeOH (20 ml)
was subsequently treated with aqueous NH4PF6 solution to
carry out the anion exchange reaction. After heating with
stirring of the solution for 20 min, precipitate appeared.
Filtration of the precipitate followed by thorough washing
with ether afforded the receptor 1 in 67% yield (42 mg, mp
1
added to the host solution in different amounts and H
NMR spectra were collected. The chemical shifts of the
resonances corresponding to the amides of the host 1
were noted and used in the determination of binding
constant values.
1
Method for Job plot
1588C). H NMR (d6-DMSO, 400 MHz): 11.23 (s, NH,
1H), 10.85 (s, NH, 1H), 9.33 (s, 1H), 8.96 (s, 1H), 8.81 (d,
1H, J ¼ 8 Hz), 8.75 (d, 1H, J ¼ 8 Hz), 8.46 (d, 2H,
J ¼ 8 Hz), 8.43 (s, 1H), 8.27 (d, 2H, J ¼ 8 Hz), 8.24 (d,
1H, J ¼ 8 Hz), 8.13 (t, 1H, J ¼ 8 Hz), 8.04 (d, 1H,
J ¼ 8 Hz), 8.00 (d, 1H, J ¼ 8 Hz), 7.76–7.62 (m, 6H),
7.05 (d, 1H, J ¼ 8 Hz), 7.01 (s, 2H) and 2.45 (s, 3H). 13C
NMR (d6-DMSO, 125 MHz): 165.8, 165.0, 156.5, 151.2,
139.6, 138.9, 138.4, 135.2, 134.4, 134.1, 133.1, 131.7,
131.4, 131.1, 131.0, 129.5, 128.7 (1C unresolved), 128.3,
128.2, 127.5, 125.7, 123.1, 121.6, 119.2, 111.5, 56.4 and
23.4. FTIR: n cm21 (KBr): 3384, 2951, 2922, 2853, 1681,
1673, 1548 and 1454. m/z (ES þ ): 669.4 (M þ H)þ, 523.4
[(M 2 PF6)]þ, 333.5 and 225.2. Analysis calculated for
C34H27F6N4O2P: C 61.08, H 4.07 and N 8.38. Found:
C 60.92, H 4.16 and N 8.24.
In this method, the stoichiometry of the complexes was
determined using the continuous variation method (22c),
and the solutions of host and guests of equal concen-
trations were prepared using the solvents used in the
experiment. Then host and guest solutions were mixed in
different proportions maintaining a total volume of 3 ml of
the mixture. All the prepared solutions were kept for 1 h
with occasional shaking at room temperature. Then,
emission and absorbance of the solutions of different
compositions were recorded. The concentration of the
complex, i.e. [HG] was calculated using the equation
[HG] ¼ DI/I0 £ [H] or [HG] ¼ DA/A0 £ [H], where DI/I0
and DA/A0 indicate the relative emission and absorbance
intensities and [H] corresponds the concentration of pure
host. Mole fraction of the host (XH) was plotted against
concentration of the complex [HG]. In the plot, the
mole fraction of the host at which the concentration of the
host–guest complex concentration [HG] is maximum
gives the stoichiometry of the complex.
General procedure for fluorescence and UV–vis
titrations
Stock solutions of the receptor were prepared in different
solvents such as CH3CN and CHCl3 containing 2%
CH3CN in the concentration range ,1025 M. A sample of
2.5 ml receptor solution was taken in the cuvette. Stock
Determination of binding constant
solutions of guests in the concentration range ,1023
M
The measured relative fluorescence intensities [I0/(I0 2 I)]
as a function of the inverse of guest concentrations were
plotted to ascertain the binding constant values. The ratio
of the intercept to the slope gave the binding constant (Ka)
values (22). In the similar way, using the absorbance data,
the binding constant values were determined from the plot
of [A0/(A0 2 A)] versus 1/[G]. For the determination of the
binding constant values for AcO2, CH3CH2CO22,
C6H5CO22 and H2PO24 , we considered the emission and
absorbance data up to the addition of 1 equivalent amount
of the guest to the receptor solution. NMR titration results
for AcO2 and CH3CH2CO22 were used to evaluate the
binding constant values through nonlinear curve fit using
were prepared in the same solvents, and were individually
added in different amounts to the receptor solution.
For fluorescence, the solution was irradiated at the
excitation wavelength of 370 nm maintaining the exci-
tation and emission slits 12 and 10, respectively. Upon
addition of guests, the change in emission of the receptor
was noted.
Same stock solutions for receptor and guests were used
to perform the UV–vis titration experiment. Guest
solution was successively added in different amounts to
the receptor solution (2.5 ml) taken in the cuvette, and the
absorption spectra were recorded. Both fluorescence and

548
K. Ghosh and A.R. Sarkar
(6) Gunnlaugsson, T.; Davis, A.P.; Glynn, M. Chem. Commun.
2001, 2556–2557.
Origin 6.0. The working formula (23) used for this was
(7) Gunnlaugsson, T.; Davis, A.P.; Hussey, G.M.; Tierney, J.;
Glynn, M. Org. Biomol. Chem. 2004, 2, 1856–1863.
(8) (a) Gunnlaugsson, T.; Glynn, M.; Tocci, G.M.; Kruger, P.E.;
Peffer, F.M. Coord. Chem. Rev. 2006, 250, 3094–3117.
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Org. Lett. 2002, 4, 2449–2452, and reference cited therein.
(10) Qin, D.-B.; Xu, F.-B.; Wan, X.-J.; Zhao, Y.-J.; Zhang, Z.-Z.
Tetrahedron Lett. 2006, 47, 5641–5643.
dObs ¼ ðdC 2 dHÞð{ð1 þ ½Gꢀ=½Hꢀ þ 1=Ka½HꢀÞ=2}
2
2 {ð1 þ ½Gꢀ=½Hꢀ þ 1=Ka½HꢀÞ =4½Gꢀ=½Hꢀ}1=2Þ þ dH:
Computational study
Structure 1 was optimised in gas phase at DFT [6-31G
*
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Lin, C.-Y.; Yen, Y.-P. Org. Biomol. Chem. 2007, 5,
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Atlamsani, A. J. Incl. Phenom. Macrocycl. Chem. 2008, 62,
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(27) and B3LYP (28)] level, individually and in presence
of acetate, propanoate and dihydrogen phosphate anions.
The Gaussian-03 package (27) was used for the
calculations. For the structure 1, the global electrophilicity
index (v; measure of electrophilic power to interact with
the nucleophiles; (29a,b)) was determined by the equation:
v ¼ x 2/2h, where x and h are the electronegativity and
hardness, respectively. Electronegativity (x) (30) and the
hardness (h) (31) were obtained from the relations
x ¼ (EHOMO þ ELUMO)/2 and h ¼ ELUMO 2 EHOMO
,
where EHOMO and ELUMO are the energies of the highest
occupied and lowest unoccupied molecular orbitals,
respectively.
Supporting information
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Change in emission and absorption of 1 with selected
guests (anions and carboxylic acids) in CH3CN and in 2%
CH3CN in CHCl3, Job plots for 1 with the guests in
CH3CN and in 2% CH3CN in CHCl3, selected binding
constant curves, change in emission of 1 in the presence of
propanoate in CH3CN/H2O (4:1 v/v), DFT-optimised
1
geometry of the complex of 1 with H2PO24 , H NMR
studies, spectral data of 1 and 2.
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Acknowledgements
K.G. expresses his deep sense of gratitude to Professor Uday
Maitra, IISC, Bangalore, for his valuable suggestions. We thank
CSIR, New Delhi, India, for financial support. ARS thanks
University of Kalyani, West Bengal, India, for a university
research fellowship.
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