Recognition of anions by bis urea based fluorescent receptors

Article abstract of DOI:10.2174/157017810791514779

Three bis urea based acyclic receptors with fixed spacer and varying size of substituents have been synthesized and their binding behavior with different anions are reported along with the selectivity of these receptors towards anions. The fluorescence quenching of the receptors 2 and 3 on binding with anions is also shown. The receptor 1 binds better with Y-shaped AcO-ion whereas receptor 3 binds well with Cl-ion.

Full text of DOI:10.2174/157017810791514779

Letters in Organic Chemistry, 2010, 7, 399-405
399
Recognition of Anions by bis Urea based Fluorescent Receptors
Shyamaprosad Goswami* and Subrata Jana#
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah-711 103, India
Received February 04, 2009: Revised February 24, 2010: Accepted March 09, 2010
Abstract: Three bis urea based acyclic receptors with fixed spacer and varying size of substituents have been synthesized
and their binding behavior with different anions are reported along with the selectivity of these receptors towards anions.
The fluorescence quenching of the receptors 2 and 3 on binding with anions is also shown. The receptor 1 binds better
with Y-shaped AcO- ion whereas receptor 3 binds well with Cl- ion.
Keywords: Anion recognition, fluorescence sensing, urea and acyclic receptors.
1. INTRODUCTION
receptor 1, two 3-methyl phenyl groups have been attached
with the angular bis urea moiety. But in case of the receptors
2 and 3, naphthyl and pyrenyl groups have been attached
with a common moiety. So the receptors are synthesized
with increasing size of the attached aryl groups. These also
influence the photo-physical behavior of the receptors upon
addition of the guest anions. On account of high sensitivity
and simplicity, fluorescent chemosensor can be effectively
used as a tool to analyze and measure the amount of anions
as well as to clarify their function in living system. Thus, the
design and synthesis of fluorescent devices for the
recognition of anions are currently of importance in chemical
trace element detection [12,13].
Anions play an important role in different biological
processes [1-5]. This phenomenon has inspired scientific
community towards the synthesis and study of the
complexation behavior of anions by different type of
artificial receptors [6]. As different anions are varied in sizes,
it is a challenging problem to design synthetic receptors for
anions.
Anion binding compounds could be used to stabilize and
increase the bioavailability of anionic drugs, along similar
lines to those already applied to cyclodextrin neutral drug
inclusion complexes [7]. Selective binding of anionic species
is of environmental interest with respect to developing new
waste remediation methods for toxic anions, such as
selenate, arsenate or radioactive pertechnetate. There is also
profound interest in producing sulfate selective hosts for
removing sulfate from nitrate-rich radioactive waste, because
vitrification of low-activity nuclear waste performed by
producing glass logs from evaporated waste solutions is
adversely affected by the presence of sulfate [8].
This common spacer arrange in an angular fashion by
which two urea groups may be directed inwards for the
recognition of anions. So anions may bind with the receptors
either in 1:2 or 1:1 stoichiometry depending upon their shape
and size. Smaller anions prefer to make 1:2 but larger ones
1:1 complexes as expected (Fig. 2).
2. SYNTHESIS
Anions have larger radii and a greater variety of
geometries than common cations. This characteristic requires
complexity in the three-dimensional structure of anion
receptors. Recent reviews describe anion recognition [9,10],
some with specific emphasis on the use of amine [9d], amide
[9c], urea [9c], thiourea [9c], pyrrole [9e], guanidinium [9f],
and metal coordination-Lewis acid groups [9g]. Cyclic
peptides are another class of anion sensors which have been
studied recently [11].
1-Aminopyrene has been synthesised following the
reported procedure (Scheme 1) [14]. The receptors have been
synthesised by a single step procedure where aromatic
amines are coupled with commercially available
diisocyanate in dry CH2Cl2 at room temperature under
anhydrous condition (Scheme 2). The designed compounds
(receptors) have been purified by prolonged washing with
CHCl3, CHCl3-MeOH (1:1) and finally with MeOH.
Among different groups, substituted urea based receptors
[9c] have been widely used for the recognition of anions.
The two N-H groups have been arranged to the same
direction and are available for the binding of anions. In this
work, receptors have been synthesized having two
substituted urea moiety, which are further separated by the
common spacer 4-methylene phenyl group (Fig. 1). In
3. BINDING STUDIES
Binding behavior of the anions with the receptors was
studied by UV-vis and fluorescence titration methods in
DMSO. Among the receptors, 2 and 3 have the potential
fluorescence properties due to the presence of the
fluorophores, naphthalene and pyrene moieties, which are
quenched during titration.
*Address correspondence to this author at the Department of Chemistry,
Bengal Engineering and Science University, Shibpur, Howrah-711 103,
India; Tel: +91 33 2668 4561-3; Fax: + 91 33 2668 2916;
E-mail: spgoswamical@yahoo.com
UV-vis Studies
#Presently at Department of Chemistry, University of Utah, Salt Lake City,
USA.
The titration experiments were carried out by taking
solution of receptors 1 (7.76 x 10-5 M), 2 (5.97 x 10-5 M) and
1570-1786/10 $55.00+.00
© 2010 Bentham Science Publishers Ltd.

400 Letters in Organic Chemistry, 2010, Vol. 7, No. 5
Goswami and Jana
O
O
N
O
O
N
N
N
N
N
N
H
H
N
H
H
H
H
H
H
Receptor 1
Receptor 2
O
O
N
N
N
N
H
H
H
H
Receptor 3
Fig. (1). Acyclic receptors for the recognition of anions.
O
O
O
O
N
H
N
H
N
N
N
N
H
N
R
N
H
R
H
R
R
H
H
H
X-
X-
X-
ꢀ(b)
Fig. (2). Probable mode of binding of anions (a) 1:2 mode for small anion and (b) 1:1 mode for large anion.
NH2
(a)
NO2
[15]. The receptor 1 shows strong absorbance at 274 nm
(ꢀmax), which gradually decreases upon addition of the guest
solution during titration. The nature of the absorbance
spectra for F-, Cl-, Br- and I- is almost identical without any
blue or red shift. But a very weak increase of absorbance at
~280 nm was observed in case of AcO- in UV-vis titration
spectra. In this case change of absorbance is not regular
which affects the results of compexation studies. The
association constants of the receptor 1 with Cl- and AcO- are
closer and higher than other anions. Highest binding constant
is observed in case of Y-shaped AcO- ion, which may be
attributed by the locking of the flexibility of receptor 1.
Interestingly, Br- ion shows lowest binding constant value
among the other anions used in our study. The binding
nature in cases of other two receptors is slightly different.
(i)
(ii)
Scheme 1. Reagents & Conditions: (i) Cu(NO2)2, Ac2O, AcOEt, 55
°C, 21 h, 65%; (ii) SnCl2.2H2O, AcOEt, 80 °C, 15 h, 89%.
3 (4.68 x 10-5 M) in DMSO. The solutions of the guest
substrates were prepared in the concentration of 1 mM. Each
titration was performed using 2 mL stock solution of
receptors and the solution of guest substrates by judicious
choice. The binding constants for all the anions with
receptors 1-3 were calculated and summarized in Table 1
In case of titration with receptor 2 (5.97 x 10-5 M), strong
absorbance at 306 nm (ꢀmax) is gradually decreased upon
O
O
(i)
R
NH2
N
N
+
N
N
H
OCN
NCO
H
R
H
R
H
R = -C7H7
-C10H7
R = -C7H7(Receptor 1)
-C10H7(Receptor 2)
-C16H9(Receptor 3)
-C16H9
Scheme 2. Reagents & conditions: (i) dry CH2Cl2, r.t., 3-5 h, 80-90%.

Recognition of Anions
Letters in Organic Chemistry, 2010, Vol. 7, No. 5
401
(i)
(ii)
(iii)
Fig. (3). UV-vis titration spectra with F- ion of the receptors: (i) receptor 1, (ii) receptor 2 and (iii) receptor 3.
Table 1. Association Constants [Ka(M-1)]a and Free Energy Changes [ꢀG(Kcal/mol)] at 25 °C for Compounds 1-3, Determined by
UV-vis Titration Method in DMSO
Receptors
1
2
3
Guestsb
Ka
ꢀG
Ka
ꢀG
Ka
ꢀG
F-
Cl-
Br-
I-
8.78 x 103
8.01 x 104
1.94 x 103
2.63 x 104
9.53 x 104
-5.4
-6.7
-4.5
-6.0
-6.8
1.20 x 103
3.68 x 103
5.12 x 101
9.56 x 102
1.24 x 103
-4.2
-4.9
-2.3
-4.1
-4.2
2.29 x 103
3.31 x 103
7.05 x 102
1.19 x 103
1.43 x 103
-4.6
-4.8
-3.9
-4.2
-4.3
AcO-
aBased on K11. For receptor 1: ꢁmax= 273 nm; receptor 2: ꢁmax= 306 nm; receptor 3: ꢁmax= 356 nm. All the errors are ±10%.
bAll the anions were used in the form of their tetrabutylammonium salts.
Table 2. Fluorescence Quenching Upon Addition of the Guest
Anions to Receptors 2 and 3
addition of the anions. The changing pattern of absorbance
(Fig. 3(ii)) is almost identical with different magnitude. The
binding constants of F-, Cl- and AcO- are of the same order
with highest value for Cl-. In case of Br-, the binding
constant is lowest as for the receptor 1. Due to less flexibility
compared to receptor 1, Cl- ion binds more effectively with
receptor 2 than AcO- ion.
The binding of anions with receptor 3 (4.68 x 10-5 M) by
UV-vis method is almost close with the results, which were
obtained with the receptor 2. In receptor 3, strong absorption
at 356 nm (ꢀmax) gradually decreased (Fig. 3(iii)) upon
gradual addition of the guest solution. In this case, highest
binding constant value was observed for Cl- ion and the
lowest binding constant was observed for Br- ion like that
obtained in the previous two cases. All the binding constants
are presented in Table 1.
Guests
% Fluorescence Quenching
Receptor 2 Receptor 3
F-
Cl-
12.3
11.4
8.6
24.0
16.1
17.8
16.7
16.5
Br-
I-
11.6
12.9
AcO-
In case of receptor 2, maximum and minimum quenching
effects were observed for AcO- and Br- ions respectively.
But in case of receptor 3, maximum quenching of the
emission intensity was observed for F- ion. Besides F- ion,
quenching is almost the same in magnitude for all the other
ions in case of receptor 3.
Fluorescence Studies
Binding behavior of the receptors 2 and 3 was also
studied by monitoring fluorescence emission. As the
receptors 2 and 3 contain two naphthyl and pyrenyl groups
attached to the common spacer, the receptors showed strong
emission spectra (Figs. 4 and 5). The fluorescence intensity
was quenched upon addition of the increasing amount of
guests. The fluorescence was ‘switched off’’ depending upon
the host-guest interactions (Table 2). The association
constants were determined from the of fluorescence titration
method (Table 3) [16].
The binding behavior of the anions with the receptors 2
and 3 is fairly close. In both cases, anions bind in 1:2
(host:guest) ratio. But the change in fluorescence intensity
for the receptor 2 is more pronounced than that for the
receptor 3. The association constants are summarized in the
Table 3. In both cases, association constant for Cl- ion is the
highest as found by fluorescence titration method (Table 3).
But the lowest binding constants were found in case of AcO-
and I- ions for the receptors 2 and 3 respectively.

402 Letters in Organic Chemistry, 2010, Vol. 7, No. 5
Goswami and Jana
(i)
(ii)
Fig. (4). Fluorescence (i) titration spectra of receptor 2 with F- ion and (ii) titration curves for receptor 2 with anions by fluorescence titration
method in DMSO.
(i)
(ii)
Fig. (5). Fluorescence (i) titration spectra of receptor 3 with F- ion and (ii) titration curves for receptor 3 with anions by fluorescence titration
method in DMSO.
Table 3. Association Constants [Ka(M-1)]a and Free Energy Changes [ꢀG(Kcal/mol)] at 25 °C for the Receptors 2 and 3,
Determined by Fluorescence Quenching in DMSO
Receptors
2
3
Guestsb
Ka
ꢀG
Ka
ꢀG
F-
Cl-
3.52 x104
3.73 x104
3.34 x104
1.72 x104
4.32 x103
-6.20
-6.23
-6.17
-5.77
-4.96
3.06 x104
9.37 x104
3.26 x104
1.01 x104
2.88 x104
-6.12
-6.78
-6.15
-5.46
-6.08
Br-
I-
AcO-
aBased on K11. All the errors are within ±10%. For receptor 2: ꢁmax(ex) = 306 nm, ꢁmax(em) = 377 nm, emission slit width = 2.5 nm, excitation slit width = 14.0 nm, Scan rate = 500
nm/min. For receptor 3: ꢁmax(ex) = 356 nm, ꢁmax(em) = 402 nm, emission slit width = 2.5 nm, excitation slit width = 14.0 nm, Scan rate = 500 nm/min.
bAll the anions were used in the form of their tetrabutylammonium salts.
4. DISCUSSION
fluorescence methods. The association constant for AcO- ion
with receptor 1 is the highest amongst all the ions used for
the study in the UV-vis method (Table 1). But with
increasing the size of the aryl groups attached with the
common spacer, the binding pattern has been changed. The
From the titration spectra, it is observed that this type of
acyclic receptors bind with the anions of varying size and
shape in mainly 1:2 (host:guest) fashion. The association
constants were consistent as determined by UV-vis and

Recognition of Anions
Letters in Organic Chemistry, 2010, Vol. 7, No. 5
403
receptors 2 and 3 bind Cl- ion more strongly than other ions.
Similar observation for the binding behavior of the receptors
2 and 3 towards Cl- ion was noted in the fluorescence
titration method. This type of binding behavior may be
attributed due to the formation of pseudo cyclic arrangement
of the receptors 2 and 3, where ꢀ- ꢀ interaction in solution
phase may play a crucial role. These arrangements of the
receptors containing higher number of fused phenyl rings
may properly fit the cavity for the Cl- ion. However, no such
appreciable spectral change was observed either in case of
UV-vis or fluorescence methods. So with changing the size
of attached aryl groups with simple substituted urea based
receptors, the selectivity towards the recognition of Cl- ion
by the receptors 2 and 3 increases.
1,1'-(4,4'-methylenebis(4,1-phenylene))bis(3-m-tolylurea)
(Receptor 1)
Mp. > 250 ºC.
1H NMR (DMSO-d6, 400 MHz): ꢁ (ppm): 8.55 (s, 2H),
8.52 (s, 2H), 7.35 (d, 4H, J = 7.7 Hz), 7.28 (s, 2H), 7.20 (d,
2H, J = 7.1 Hz), 7.12 (d, 2H, J = 10.4 Hz), 7.11 (d, 4H, J =
7.5 Hz), 6.77 (d, 2H, J = 6.7 Hz), 3.81 (s, 2H), 2.26 (s, 6H).
13C NMR (DMSO-d6, 100 MHz): ꢁ (ppm): 152.50,
139.66, 137.90, 137.60, 134.98, 128.89, 128.58, 122.45,
118.61, 118.32, 115.29, benzylic carbon probably merged
with solvent peak, 21.21.
FT-IR (KBr): 3319, 2930, 2856, 1773, 1650, 1548, 1509,
1235, 808 cm-1.
Mass (HRMS-ESI): Calculated for C29H28N4O2Na is
487.2104; found 487.2099 (M+Na)+.
5. EXPERIMENTAL
General
Anal. Calcd for C29H28N4O2; C, 74.98; H, 6.08; N 12.06.
Found: C, 75.12; H, 6.10; N 11.95.
All the melting points were determined on a hot-coil
1
stage melting point apparatus and are uncorrected. H NMR
1,1'-(4,4'-methylenebis(4,1-phenylene))bis(3-(naphthalen-
1-yl)urea) (Receptor 2)
and 13C NMR spectra were recorded on 400 MHz
spectrometers. For NMR spectra DMSO-d6 was used as
solvents using TMS as an internal standard. Chemical shifts
are expressed in ꢀ units and 1H–1H, 1H–C coupling constants
in Hz. IR spectra were recorded using KBr discs.
Mp. 220-25 ºC (dec).
1H NMR (DMSO-d6, 400 MHz): ꢁ (ppm): 9.00 (bs, 2H),
8.73 (bs, 2H), 8.11 (d, 2H, J = 8.3 Hz), 8.01 (d, 2H, J = 7.6
Hz), 7.92 (d, 2H, J = 8.0 Hz), 7.61 (t, 3H, J = 9.3 Hz), 7.55
(t, 3H, J = 6.2 Hz), 7.47 (d, 2H, J = 7.9 Hz), 7.42 (d, 4H, J =
8.3 Hz), 7.15 (d, 4H, J = 8.3 Hz), 3.82 (s, 2H).
13C NMR (DMSO-d6, 100 MHz): ꢁ (ppm): 152.95,
137.69, 135.11, 134.38, 133.73, 129.03, 128.45, 125.90,
125.87, 125.70, 122.84, 121.30, 118.35, 117.35, benzylic
carbon probably merged with solvent peak.
General Procedure for UV-vis Titrations
Stock solutions of the receptors 1, 2 and 3 were taken in
the order of concentration of 1 x 10-5 M in DMSO. Anions
were dissolved in DMSO in the order of 1 x 10-3
M
concentration. Then the guest solution is added to the
receptor solution (taking 2 mL in the UV-cell) and
continuous decrease of absorbance in UV spectra was
recorded each time.
FT-IR (KBr): 3283, 3046, 1777, 1639, 1550, 1241, 787
cm-1.
Mass (HRMS-ESI): Calculated for C35H28N4O2Na is
559.2104; found 559.2099 (M+Na)+.
General Procedure for Fluorescence Titration
Stock solutions of the receptors 2 and 3 were taken in the
Anal. Calcd for C35H28N4O2; C, 78.33; H, 5.26; N 10.44.
Found: C, 78.30; H, 5.30; N 10.50.
order of 1 x 10-5 M and 1 x 10-6 M respectively in DMSO.
Anions were dissolved in DMSO in order of 1 x 10-4
M
1,1'-(4,4'-methylenebis(4,1-phenylene))bis(3-(pyren-1-yl)
urea) (Receptor 3)
concentration. Then the guest solution is added to the
receptor solution (taking 2 mL in the cell) and continuous
decrease of intensity of emission spectra was recorded each
time. Titration curves were determined by plotting ꢁI vs
[G]/[H] (ꢁI = I0 –I; I0 and I are the initial and final intensity
of the receptor solution after each addition during titration).
Mp. > 250 ºC.
1H NMR (DMSO-d6, 400 MHz): ꢁ (ppm): 9.18 (bs, 2H),
9.12 (bs, 2H), 8.64 (d, 2H, J = 8.4 Hz), 8.37 (d, 2H, J = 9.4
Hz), 8.26 (d, 4H, J = 5.1 Hz), 8.25 (d, 4H, J = 4.0 Hz), 8.12
(d, 2H, J = 9.0 Hz), 8.06 (d, 2H, J = 9.1 Hz), 8.05 (t, 2H, J =
8.5 Hz), 7.49 (d, 4H, J = 8.4 Hz), 7.21 (d, 4H, J = 8.44 Hz),
3.89 (s, 2H).
General Procedure for the Synthesis of Substituted Urea
Based Receptors
13C NMR (DMSO-d6, 100 MHz): ꢁ (ppm): 153.03,
137.66, 135.21, 133.15, 131.11, 130.59, 129.07, 127.36,
126.92, 126.58, 126.36, 125.61, 125.35, 124.95, 124.53,
124.46, 124.20, 121.03, 120.99, 119.87, 118.47, benzylic
carbon probably merged with solvent peak.
FT-IR (KBr): 3278, 1777, 1639, 1536,1245, 841 cm-1.
Mass (ESI): m/z (%): 685 [(M+H)+, 2], 680 [(M-4H)+,
12%].
Methylenedi-p-phenyl diisocyanate (1 eqv.) was taken in
25 mL r.b and was stirred for 20 min with dry CH2Cl2 (5
mL). The required aryl amine (2 eqv.) was dissolved in dry
CH2Cl2 (5 mL) and added to the isocyanate solution slowly
for a few minutes. The reaction mixture was stirred for
another 2-3 h at room temperature and the precipitate was
filtered out and washed with CHCl3, CHCl3 + MeOH (1:1)
and then with MeOH. Finally the compound was obtained by
drying well under reduced pressure and the yield of the
compounds varies from 80-90%.

404 Letters in Organic Chemistry, 2010, Vol. 7, No. 5
Goswami and Jana
(g) Gale, P.A. Structural and molecular recognition studies with
acyclic anion receptors. Acc. Chem. Res., 2006, 39, 465-475. (h)
Vickers, M.S.; Beer P.D. Anion templated assembly of
mechanically interlocked structures. Chem. Soc. Rev., 2007, 36,
211-225.
(a) Stadler-Szoke, A.; Szejtli, J. A nitroglicerin beta-ciklodextrin
zárványkomplex. Acta Pharm. Hung., 1979, 49, 30-34. (b) Szejtli,
J.; Gerloczy, A.; Szente, L.; Banky-Elod, E.; Sebestyen, G.;
Fonagy, A.; Kurcz, M. Farmakonok felszívódásának fokozása
ciklodextrin zárványkomplex képzéssel. Acta Pharm. Hung., 1979,
49, 207-221.
Anal. Calcd for C47H32N4O2; C, 82.43; H, 4.71; N 8.18.
Found: C, 82.37; H, 4.75; N 8.13.
CONCLUSION
[7]
In this work, binding as well as spectral changes of the
receptors with anions were studied. The shape and size of the
receptors have been changed to make the receptor more
selective for a particular ion amongst the array of ions. Here
receptor 1 is more suitable for the binding towards Y-shaped
AcO- ion than the other receptors. Similarly receptors 2 and
3 are more suitable for the recognition of spherical Cl- ion
compared to the other ions. This work is now further
extended in our laboratory to the recognition of organic
dicarboxylate anions [17] of varying chain lengths. From the
results it is evident that larger substituent in arm of the urea
moiety directs the selectivity in anion recognition.
[8]
[9]
Sessler, J.L.; Katayev, E.; Pantos, G.D.; Ustynyuk, Y.A. Synthesis
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ACKNOWLEDGEMENT
We wish to express our appreciation to the CSIR and
DST, Govt. of India for financial support. S. J. thanks the
CSIR, Govt. of India for a research fellowship during
doctoral study. We also thank Dr. Avijit Kumar Adak and
Professor Thomas Schrader for their help for some of the
spectral data.
[10]
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