Pyrene-based simple new hetero bis amide pyridinium salt for selective sensing of benzoate and hydrogen sulphate

Article abstract of DOI:10.1080/10610278.2010.514911

A new pyrene-based hetero bis amide pyridinium salt 1 has been designed and synthesised. The hetero bis amide salt 1 selectively recognises benzoate over a range of aliphatic monocarboxylates in CHCl₃ containing 2% CH ₃CN by showing

Full text of DOI:10.1080/10610278.2010.514911

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Supramolecular Chemistry
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Pyrene-based simple new hetero bis amide pyridinium
salt for selective sensing of benzoate and hydrogen
sulphate
Kumaresh Ghosh a & Avik Ranjan Sarkar a
a Department of Chemistry, University of Kalyani, Nadia, Kalyani, 741235, India
Published online: 10 May 2011.
To cite this article: Kumaresh Ghosh & Avik Ranjan Sarkar (2011) Pyrene-based simple new hetero bis amide pyridinium
salt for selective sensing of benzoate and hydrogen sulphate, Supramolecular Chemistry, 23:5, 365-371, DOI:
10.1080/10610278.2010.514911
To link to this article: http://dx.doi.org/10.1080/10610278.2010.514911
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Supramolecular Chemistry
Vol. 23, No. 5, May 2011, 365–371
Pyrene-based simple new hetero bis amide pyridinium salt for selective sensing of benzoate and
hydrogen sulphate
Kumaresh Ghosh* and Avik Ranjan Sarkar
Department of Chemistry, University of Kalyani, Nadia, Kalyani 741235, India
(Received 8 March 2010; final version received 7 August 2010)
A new pyrene-based hetero bis amide pyridinium salt 1 has been designed and synthesised. The hetero bis amide salt 1
selectively recognises benzoate over a range of aliphatic monocarboxylates in CHCl3 containing 2% CH3CN by showing
concomitant increase in emission of pyrene. The flexible cleft of 1 also showed selective sensing of tetrahedral-shaped
HSO24 ion over H2PO24 ion. The recognition property of 1 was evaluated by 1H NMR, UV–vis and fluorescence studies.
Keywords: benzoate recognition; hydrogen sulphate recognition; pyrene-based receptor; pyridinium salt; anion recognition
The rational design of molecules that exhibit selective
recognition of anion by showing change in fluorescence
has gained much interest over the past decade (1–4).
Anion-induced changes in fluorescence appear to be
particularly attractive in anion recognition due to the
simplicity and high detection limit of fluorescence (5–8).
In devising these sensors, various functional sites with
hydrogen bond donors and acceptors are usually installed
in close proximity to the fluorophore. Moreover, to gain
selectivity in the recognition process by this class of
receptors, size and shape matching between the receptor
and substrate is crucial. In addition, anions are prone to
interact through either electrostatic or hydrogen bond
interactions and thus the incorporation of positively
charged groups [ammonium (9), guanidinium (10),
pyridinium (11), benzimidazolium (12–14) and imidazo-
lium (15)] along with neutral hydrogen bond donors
[amide (16), urea (17), pyrrole (18), sulphonamide (19),
etc.] into the designed receptor is fundamentally important.
Among the different positively charged species, the
pyridinium motif is less explored. To investigate the
significance of unconventional C-H- - -O hydrogen bonds
in highly polar solvents for carboxylate ion recognition,
Jeong and Cho (20), for the first time, reported the use of a
pyridinium salt. Later on, this pyridinium motif was used
by Steed and co-workers (21) in the synthesis of tripodal-
shaped receptor for selective sensing of Cl2 ion and also by
Gong and Hiratani (22) in the synthesis of tripodal-shaped
sensor for dihydrogen phosphate. During the course of our
work on anion binding, we placed this pyridinium motif
onto the different scaffolds for fluorometric assessment of
specific anion (23). In this paper, we report a new hetero bis
amide receptor 1, in which the pyridinium motif and the
pyrene fluorophore are attached to semi-rigid isophthaloyl
group via amide linkages. The receptor is found to be
efficient in selective sensing of benzoate over a range of
aliphatic monocarboxylates in CHCl3 containing 2%
CH3CN involving hydrogen bonding, charge–charge and
p-stacking interactions altogether. Tetrahedral-shaped
anions such as H2PO24 , HSO24 are also sensed moderately
by the flexible cleft of 1.
The hetero bis amide salt 1 was obtained according to
Scheme 1. The hetero bis amide 2 was, initially,
synthesised in 45% yield by high-dilution reaction of
two different amines with isophthaloyl dichloride in dry
CH2Cl2. Subsequent reaction of 2 with n-butyl bromide in
dry CH3CN under refluxing condition afforded 1 in 77%
yield. The compound 1 was characterised by 1H, 13C NMR
and mass analysis.1
Prior to the investigation of the recognition properties
of 1 in solution, we performed the optimisation of the
different conformations of the structure 1 by MM2
method2 (Supplementary Data). In principle, the hetero
bis amide 1 can exhibit different conformations depending
on the orientation of the amides (in–in, in–out and out–
out) in solution (24). Among the several conformations,
one in–in (E ¼ 223.0 kcal/mol) and one out–out (E ¼ 2
27.1 kcal/mol) conformers are energetically stable and they
are close in energy values (Supplementary Data, Figure
S8A). Between these two, in–in conformer (E ¼ 2
23.0 kcal/mol) which has the possibility to complex the
sterically fitted guest favourably involving the maximum
number of hydrogen bonds, was further optimised using
density functional theory (25) calculations with the B3LYP
functional (26, 27) and the basis set 6-311G , provided by
**
Gaussian 03 (28) (Figure 1). As can be seen from Figure 1,
*Corresponding author. Email: ghosh_k2003@yahoo.co.in
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
DOI: 10.1080/10610278.2010.514911
http://www.informaworld.com

366
K. Ghosh and A.R. Sarkar
O
O
O
O
ii
O
i
O
iii
1
NH
HN
NH
HN
Cl
Cl
Br–
N+
N
2
3
Scheme 1. (i) Dropwise addition of 1-aminopyrene and 3-aminopyridine under high-dilution condition, Et3N, dry CH2Cl2; (ii) n-butyl
bromide in dry CH3CN, heating with stirring, 48 h and (iii) NH4PF6/aq. MeOH.
the amides are not perfectly in one plane. Pyrene ring is
also slightly twisted from the molecular plane and provides
a space for the inclusion of anionic guests. To have an idea
about the electrophilic character of the receptor, we
calculated the global electrophilicity index (v ¼ 0.7946)
of 1 (29). The value as determined indicates that the
receptor 1 has a significant electrophilic character for the
complexation of anionic guests.
with no other spectral changes being observed (i.e. no
spectral shifts or formation of new emission bands)
(Figure 3). Other aromatic carboxylates such as electron-
rich 4-BuOC6H4CO22 and electron-deficient pyridine-3-
carboxylate upon interaction with 1 also increased
emission but to a lesser extent compared to benzoate
(Figure 2). Under similar condition, aliphatic mono-
carboxylates such as acetate, propanoate, long chain
myristate, etc. did not perturb the emission of 1 so
markedly (Supplementary Data). The emission of 1 was
found to be totally unperturbed in the presence of more
steric carboxylate salt such as salt of ibuprofen
(Supplementary Data), where the phenyl ring of ibuprofen
can experience p-stacking interaction with pyrene. Due to
similar steric reason, rac-lactate, mandelate also interacted
weakly. Interestingly, NO23 ion being planar like
carboxylate ion, was non-interacting in the binding
process. We, additionally, investigated the interaction of
1 with carboxylic acids such as acetic, propanoic, myristic,
benzoic, ibuprofen, rac-lactic and (R)-mendalic acids,
which weakly perturbed the emission of 1 (Supplementary
Data). In comparison to the planar carboxylate ions,
tetrahedral-shaped anion, HSO24 only increased the
emission (Supplementary Data) significantly. In the
presence of 1 equiv. amount of H2PO24 , the emission of
Anion-binding properties of 1 with the different
anions were investigated in CHCl3 containing 2% CH3CN
by 1H NMR, UV–vis and fluorescence spectroscopic
methods. The fluorescence emission spectra of 1
(c ¼ 6.11 £ 1025 M) were obtained by excitation of the
pyrene fluorophore at 340 nm. Figure 2 shows the
fluorescence emission changes in compound 1 upon
addition of C6H5CO22 , 4-BuOC6H4CO22 , CH3CO22 ,
CH3CH2CO22 , myristate, salt of ibuprofen, pyridine-3-
carboxylate, (R)-mandelate, rac-lactate, H2PO24 , HSO42,
ClO24 and NO23 (10 equiv., tetrabutylammonium salts). As
shown in Figure 2, there was a unique change in emission
of 1 in the presence of C6H5CO22 and HSO42 ions. In the
absence of the guests, the fluorescence emission spectra
consisted of a structured and broad band centred at
430 nm, when excited at 340 nm. Upon addition of
C6H5COO2, the intensity of this band gradually increased
O
O
Hp
N+
f
NHb Hc aHN
c
b
a
Ho′
Ho
d
–
PF6
e
1
Figure 1. Density functional theory-optimised structure of 1 (E ¼ 21589.33 au; atomic charges: a ¼ 0.2485, b ¼ 0.0735, c ¼ 0.2314,
d ¼ 0.1707, e ¼ 0.1669, f ¼ 0.1902, Nþ ¼ 20.3792).

Supramolecular Chemistry
367
350
300
250
200
150
100
Rec-Lactate
(R)-Mandelate
Nitrate
Perchlorate
Hydrogen sulphate
Dihydrogen phosphate
Pyridine-3-carboxylate
4-Butoxy benzoate
Salt of ibuprofen
Benzoate
Myristate
Propanoate
Acetate
0
2
4
6
8
10 12 14
50
∆I/I0
0
400
450
500
550
600
Figure 2. Fluorescence ratio (I0 2 I/I0) of receptor 1 at 407 nm
upon addition of 10 equiv. of a particular guest in CHCl3
containing 2% CH3CN (c ¼ 6.11 £ 1025 M).
Wavelength (nm)
Figure 4. Change in emission of 1 (c ¼ 6.11 £ 1025 M) upon
gradual addition of H2PO24 ions in CHCl3 containing 2%
CH3CN.
1 increased to a small extent. Further addition shifted the
band to the longer wavelength with a broad emission at
,500 nm (Figure 4). This broad emission, presumably, is
due to the formation of intermolecular excimer upon
complexation of H2PO24 . All the anions except H2PO24
bind in 1:1 stoichiometric fashion, confirmed by Job’s
plots (30). Figure 5, for example, demonstrates the Job plot
for 1 with C6H5CO22 . Even, the linear nature of the
fluorescence titration curves for all the anions except
H2PO24 in Figure 6 corroborates the 1:1 stoichiometry of
the complexes in solution. Receptor 1 binds H2PO42
initially in 1:1 stoichiometry and then attains a 2:1
(host:guest) stoichiometry when excess concentration of
the guest is added. The fluorescence enhancing effect of 1
upon complexation of anions can be due to the
deactivation of photo-induced electron transfer (PET)
process occurring in between the binding site and excited
state of pyrene. The large fluorescence enhancing effect of
1 in the presence of C6H5CO22 over the aliphatic
monocarboxylates in the present study is due to the
combining effect of hydrogen bonding (both conventional
and unconventional), charge–charge and p-stacking
interactions for which the PET process is effectively
stopped. In relation to this, the different equilibrium
binding modes (A and B) of C6H5CO22 ion into the
diamide core of 1 are suggested in Figure 7. Molecular
modelling was performed on these two different modes
and interestingly, mode A was found to be favourable over
mode B (Supplementary Data). However, similar binding
involving no p-stacking interaction is plausible with
aliphatic monocarboxylates. Thus, the contribution of this
weak force in the present case discriminates benzoate from
3.0
700
2.5
1.0×10–4
8.0×10–5
6.0×10–5
4.0×10–5
600
2.0
500
1.5
1.0
400
0.5
300
0.0
250
300
350
400
450
200
100
0
Wavelength (nm)
2.0×10–5
400
450
500
550
600
Wavelength (nm)
0.0
Figure 3. Change in emission of 1 (c ¼ 6.11 £ 1025 M) upon
gradual addition of benzoate ions in CHCl3 containing 2%
CH3CN; inset: UV–vis titration spectra of 1 (c ¼ 6.11 £ 1025 M)
upon titration with benzoate (10 equiv.) in CHCl3 containing 2%
CH3CN.
0.0
0.2
0.4
0.6
XG
0.8
1.0
Figure 5. Fluorescence Job’s plots of 1 with C6H5CO22 at
407 nm in CHCl3 containing 2% CH3CN.

368
K. Ghosh and A.R. Sarkar
(a) Acetate
(b) Propanoate
(c) Myristate
550
500
450
400
350
300
250
200
150
100
50
(d) Benzoate
(e) H2PO4–
(f) HSO4–
(g) R-Mandelate
(h) Rac-Lactate
(i) Perchlorate
(j) Pyridine-3-carboxylate
(k) Salt of Ibuprofen
(l) 4-butoxybenzoate
(m) NO3–
(a),(b),(c),(g)
(h),(i),(j),(k)
(m)
(d)
(f)
(l)
(e)
0
0
2
4
6
8
10
[G]/[H]
Figure 6. Plot of change in emission of 1 at 407 nm vs. the ratio of guest-to-host concentration.
O
O
O
O
–
NHb
O–
aHN
PF6
+
O
NHb
aHN
(
)
3 N
O
H
N+
PF6
-
–
O
H
A
B
Figure 7. Suggested mode of binding of C6H5CO22 into the cleft of 1.
aliphatic monocarboxylates. This has a strong relevance
in the distinction between aromatic and aliphatic
monocarboxylates. On the other hand, steric fit of the
tetrahedral-shaped HSO24 ion over H2PO42 and ClO42 (less
basic compared to organic carboxylates) into the non-
The change in absorbance of 1 upon titration with
C6H5CO22 ions is displayed in the inset of Figure 3.
Table 1. Binding constant values for 1 in CHCl3 containing 2%
CH3CN.
Guest
Ka (M21 a
)
Ka (M21 b
)
planar diamide cleft of 1 induced a marked change in
the emission, although there was no role of p-stacking
interaction.
c
c
c
Acetate
Propanoate
Myristate
Benzoate
4-Butoxybenzoate
Pyridine-3-carboxylate
Ibuprofen salt
Nitrate
Dihydrogen phosphate
Hydrogen sulphate
Perchlorate
2.49 £ 102
1.91 £ 102
1.67 £ 102
To look into the ground state interaction properties,
UV–vis titrations of 1 with all the anions in CHCl3
containing 2% CH3CN were performed. The absorption
spectrum of 1, consisting of bands at 268, 278 and 344 nm,
was affected upon addition of all the anions studied. Upon
complexation, the intensity of the absorption bands
decreased marginally up to the addition of 1 equiv. amount
of each anion. Excess addition caused substantial decrease
in intensity. The stoichiometries of complexes in the ground
state were also found to be 1:1, confirmed by UV Job’s
plots (Supplementary Data) as well as from the break of
the titration curves at [G]/[H] ¼ 1 (Supplementary Data).
3.46 £ 103
1.32 £ 103
8.69 £ 102
c
5.48 £ 102
c
c
c
c
c
c
4.79 £ 102
1.23 £ 103
1.82 £ 102
2.72 £ 102
3.37 £ 102
1.87 £ 103
c
c
Rac-Lactate
(R)-Mandelate
c
a Determined by the fluorescence method at the wavelength of 407 nm.
b Determined by UV–vis method at the wavelength of 344 nm.
c Association constants were not determined due to minimum and
irregular changes.

Supramolecular Chemistry
369
greater selectivity for aromatic monocarboxylates over the
aliphatic monocarboxylates and in the present case
especially for C6H5CO22 ion accompanying with a higher
binding constant value. The electron-rich and electron-
deficient aromatic monocarboxylates such as 4-BuOC6-
H4CO22 and pyridine-3-carboxylate, respectively, exhib-
ited moderate binding constant values and they were found
to be less in magnitude than C6H5CO22 . We determined
the binding constant value for C6H5CO22 by the NMR
titration method (32) at the concentration of ,1023 M of 1
and it was found to be 3.99 £ 103 M21 which is higher
than the value 1.32 £ 103 M21 determined by the UV
method at the concentration range of ,1025 M. In
addition, the binding constant values in Table 1 reveal
that the selectivity of 1 is found to be slightly higher in the
excited state possibly for more polar character of the
receptor in the excited state. Based on steric fit, HSO24 also
shows a greater binding constant value like C6H5CO22 ion.
In this context, it is mentionable that it was difficult to
determine the binding constant values for the carboxylic
acids with 1 due to a minor change in emission as well as
absorption of 1.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
NHa
0
2
4
6
8
[G]/[H]
Figure 8. Titration curve for 1 (c ¼ 1.86 £ 1023 M) with
tetrabutylammonium salt of benzoate.
The interaction of 1 with C6H5CO22 and HSO42 was
1
further established by H NMR. Pyrene amide proton Ha
(Dd ¼ 0.84 ppm) of 1 underwent a downfield chemical
shift upon complexation of benzoate (Figure 8). The
pyridinium amide proton Hb became broad and difficult to
identify. Moreover, in the interaction process, the
pyridinium amide arm of 1 was found to be deeply
involved in the binding of C6H5CO22 as proved by large
downfield chemical shift of the pyridinium ortho proton Ho
(Dd ¼ 0.92 ppm) (for titration spectra see Supplementary
Data). During interaction, the isophthaloyl peri proton Hc
also moved to the downfield direction. The protons Ho, Ho’
and Hp of the pyridinium ring were identified from the
analysis of COSY spectrum of 1 (Supplementary data). On
the other hand, as can be seen from Figure 9, AcO2 being
smaller in size as well as more basic than C6H5CO22 , also
showed a downfield shift of the amide protons
(DdH ¼ 0.20 ppm; DdH ¼ 0.26 ppm). The ortho proton
Figure 9. Partial 1H NMR of (a) 1 (c ¼ 1.86 £ 1023 M) and its
1:þ1 complexes with (b) C6H5COO2 NþBu4, (c) CH3COO2
N Bu4 and (d) HSO42 NþBu4 in CDCl3 containing 2% CD3CN
(see labelled structure 1).
In order to understand the binding selectivity of 1 for
the anions, fluorescence and UV–vis titration data were
used to calculate the binding constant values (Table 1)
(31). It is evident from Table 1 that receptor 1 exhibits
a
b
Ho vanished, presumably, due to deprotonation, and the
O
O
O
O
NHb
H
Hc
aHN
NHb
aHN
O
O
O
O
N
S
+
PF6–
N+
S
O–
H
PF6–
OH
O–
OH
C
D
Figure 10. Possible modes of binding of HSO24 into the cleft of 1.

370
K. Ghosh and A.R. Sarkar
J ¼ 8 Hz), 4.67 (t, 2H, J ¼ 8 Hz), 1.94–1.91 (m, 2H), 1.37–
1.31 (m, 2H), 0.93 (t, 3H, J ¼ 7.20 Hz); 13C NMR (d6-
DMSO, 100 MHz): d 165.9, 165.7, 139.4 (one carbon
unresolved), 135.8, 135.2, 134.9, 133.5, 131.7, 131.5, 131.2,
130.7, 130.4, 129.1, 128.05, 127.7, 127.25, 127.20, 127.02,
126.4, 125.6, 125.4, 125.1, 125.04, 124.9, 124.8, 124.3,
123.7, 122.8, 61.1, 32.7, 18.7, 13.3; FTIR: n cm21 (KBr):
3397, 1680, 1635, 1654, 1539, 1555, 1504, 1462; m/z (ESþ):
498.3 (M 2 PF26 2 2)þ.
proton assigned as Ho’ moved to the downfield direction
(Dd ¼ 0.94 ppm) effectively. This was also true for HSO42
where the amide protons underwent appreciable downfield
chemical shift (DdH ¼ 0.42 ppm; DdH ¼ 0.48 ppm) but
a
b
pyridinium ortho proton (Ho) was almost positionally
unaffected. The Ho’ proton moved to the downfield
direction weakly suggesting equilibrium binding structures
C/D, shown in Figure 10. The Hp proton did not show any
measurable shift in the interaction process.
2. Energy optimisation was performed using CS Chem 3D
version 10.0.
In conclusion, we have designed and synthesised a
simple hetero bis amide salt 1, which shows unique
recognition and sensing properties. The open cleft of 1
discriminates the aromatic monocarboxylates from the
aliphatic ones. Results demonstrate that the hetero bis amide
salt 1 selectively recognises benzoate from other aromatic
and aliphatic monocarboxylates examined in the present
study, by exhibiting large binding-induced fluorescence
enhancing effect. In addition, tetrahedral-shaped HSO24 is
also sensed with moderate binding constant value based on
steric complementarity and is differentiated from other
tetrahedral-shaped anions studied. The high affinity and
selectivity of 1 for benzoate are due to the combined effects
of semi-rigid structures, charge–charge interactions, invol-
vement of both N-H-- -O and C-H- - -O hydrogen bonds and
more specifically p-stacking interaction. Further progress in
this direction is underway in our laboratory.
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benzoate and hydrogen sulphate, MM2-optimised geome-
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Acknowledgements
We thank the CSIR [01 (2240)/08-EMRII], Government of
India for financial support. A.R.S. thanks the University of
Kalyani for providing a university research fellowship. K.G.
thanks the DST and UGC, Government of India for providing
facilities in the department under FIST and SAP programs,
respectively.
Notes
1. Mp 1248C; 1H NMR (d6-DMSO 400 MHz): d 11.50 (s, NH,
1H), 11.03 (s, NH, 1H), 9.59 (s, 1H), 8.82–8.80 (m, 2H),
8.71 (d, 1H, J ¼ 8 Hz), 8.49 (d, 1H, J ¼ 8 Hz), 8.37–8.27
(m, 4H), 8.24–8.19 (m, 5H), 8.16–8.09 (m, 2H), 7.85 (t, 1H,
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