Experimental and theoretical anion binding studies on coumarin linked thiourea and urea molecules

Article abstract of DOI:10.1016/j.molstruc.2011.08.004

A series of coumarin linked thiourea and urea molecules 1-5 have been designed and synthesized. Thiourea-based monotopic receptors 1-4 showed selective sensing of F^- and C₆H₅COO^- by exhibiting significant change

Full text of DOI:10.1016/j.molstruc.2011.08.004

Journal of Molecular Structure 1004 (2011) 193–203
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Experimental and theoretical anion binding studies on coumarin linked
thiourea and urea molecules
a
b
c
Kumaresh Ghosh a, , Suman Adhikari , Roland Fröhlich , Ioannis D. Petsalakis ,
⇑
Giannoula Theodorakopoulos c,
⇑
a Department of Chemistry, University of Kalyani, Kalyani, Nadia 741 235, India
b Organisch-Chemisches Institut, Universität Münster, Corrensstrabe 40, D-48149 Münster, Germany
c Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 116 35, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2011
Received in revised form 31 July 2011
Accepted 1 August 2011
Available online 23 August 2011
A series of coumarin linked thiourea and urea molecules 1–5 have been designed and synthesized. Thio-
urea-based monotopic receptors 1–4 showed selective sensing of Fꢀ and C6H5COOꢀ by exhibiting signif-
icant change in emission as well as change in color. The ditopic receptor 5, on the other hand,
fluorometrically distinguished isomeric aromatic dicarboxylates in polar solvent DMSO. Theoretical cal-
culations have been carried out on systems 1–4 and their complexes with C6H5COOꢀ and Fꢀ anions in the
presence of CH3CN solvent. Calculations have been carried out to determine the geometry and bonding of
the complexes in the ground electronic state as well as on the excited electronic states relevant to the
absorption spectra and emission spectra in an effort to obtain information on the spectral changes
accompanying complexation of the receptors 1–4 with the two anions.
Keywords:
Coumarin-based urea and thiourea
Fluoride recognition
Benzoate recognition
Dicarboxylate recognition
Anion binding
Ó 2011 Elsevier B.V. All rights reserved.
DFT results
1. Introduction
reported prior to our attempt that exhibit ideal PET behavior and
color changes as signaling event, detectable by the naked eye, upon
The development of simple molecular systems that can recog-
nize and sense anions selectively through visible, electrochemical
and optical responses has received considerable interest in recent
years because of the important roles played by anions in biological,
industrial and environmental processes [1]. Molecules with func-
tional groups such as amides [2], ureas/thioureas [3], guanidinium
[4] and ammonium [5] derivatives have proven to be particularly
effective in this regard, as they are capable of binding anions using
directional hydrogen bonding interactions. However, attachment
of such functional groups with a suitable chromophoric part either
covalently or intermolecularly provides a complete receptor mod-
ule that can intimate binding information either by a color change,
fluorescence or both. In an effort to develop fluorescent anion
receptors, we previously reported coumarin-based chemosensors
1 and 2 for the selective recognition of anions [6] employing the
criteria of PET sensing using the ‘fluorophore–spacer–receptor’ mod-
el developed by de Silva for the detection of cations [1b]. Although
several PET sensors for anions are known [7], no such thiourea-
linked coumarin systems employing neutral anion receptors, was
binding of anions. In this full account, we report here our extended
approach to the synthesis of other coumarin-based receptors 3–5
of which 5 acts as ditopic receptor and their molecular recognition
properties towards varieties of anions such as halides, HSOꢀ4 ,
H2POꢀ, as well as mono- and dicarboxylates in organic solvent.
4
Theoretical calculations have been carried out on systems 1–4
and their complexes with C6H5COOꢀ and Fꢀ anions in the presence
of CH3CN solvent. Calculations to a smaller extent have been car-
ried out on receptor 5 as well. The purpose of the calculations
was (i) to determine the geometry and bonding of the complexes
in the ground electronic state and (ii) to calculate the excited elec-
tronic states relevant to the absorption spectra and emission spec-
tra in an effort to obtain information on the spectral changes
accompanying complexation of the receptors 1–4 with the two
anions.
2. Experimental section
Chemosensors 1 and 2 were easily synthesized [6] in good
yields from readily available starting materials, following Scheme
1 in the Supplementary information. Chemosensors 3–5 were ob-
tained in good yields, following Scheme 2. The 6-aminocoumarin
⇑
Corresponding authors. Fax: +91 3325828282 (K. Ghosh).
E-mail addresses: ghosh_k2003@yahoo.co.in (K. Ghosh), idpet@eie.gr
(I.D. Petsalakis), ithe@eie.gr (G. Theodorakopoulos).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.08.004

194
K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
O
O2N
O
S
S
O2N
S
N
H
N
H
N
H
N
H
N
H
N
H
O
O
O
O
O
1
2
3
O
O
O
O
O
S
O
O
N
H
N
H
N
H
N
H
N
N
H
H
5
4
1714, 1622, 1572, 1538, 1494, 1290, Mass (EI): 297.2 [M + H]+,
288.2, 263.1, C16H14N2O2S: calcd. C 64.41, H 4.73, N 9.39; found C
64.32, H 4.69, N 9.33.
H2N
,
i
(yield: 60%)
ii
70%
-nitrophenyl
p
3
4
R=
5
, R= phenyl(yield: 70%)
O
O
2.3. 1,10-(1,3-Phenylene)bis(3-(2-oxo-2H-chromen-6-yl)urea (5)
Scheme 2. Reagents and conditions: (i) RNCS in dry THF, (ii) phenyl-1,3-diisocy-
anate in dry THF.
To a stirred solution of 6-aminocoumarin (0.150 g, 0.93 mmol)
in dry THF (20 mL) was added phenyl 1, 3 diisocyanate (0.074 g,
0.46 mmol) at room temperature. The reaction mixture was stirred
for 24 h at room temperature. After completion of reaction, the sol-
vent was removed under vacuo, the crude product obtained was
recrystallised from CHCl3 to give 5 as white solid (0.314 g, yield:
70%); mp > 300 °C; 1H NMR (400 MHz, d6-DMSO): d 8.82 (s, 2H,
urea NH), 8.78 (s, 2H, urea NH), 8.07 (d, 2H, J = 8 Hz), 7.90 (d, 2H,
J = 6.8 Hz), 7.74 (s, 1H), 7.57 (dd, 2H, J1 = 8 Hz, J2 = 2.4 Hz), 7.35
(d, 2H, J = 8 Hz), 7.18 (t, 1H, J = 8 Hz), 7.07 (d, 2H, J = 8 Hz), 6.48
(d, 2H, J = 8 Hz), 13C NMR (125 MHz, d6-DMSO): d 160.1, 152.4,
148.6, 144.3, 139.9, 136.1, 129.0, 122.5, 118.7, 116.6, 116.5,
was coupled with different isothiocyanates in dry THF under inert
atmosphere at room temperature to afford 3 and 4 as pale yellow
solids. Use of phenyl-1,3-diisocyanate in the coupling reaction
gave the ditopic chemosensor 5.
Structures of the receptors were established from the analysis
of 1H NMR, 13C, mass, FT-IR spectral data and also by solving single
crystal X-ray structures in case of 2 and 4.
2.1. 1-(4-Nitrophenyl)-3-((2-oxo-2H-chromen-6-yl))thiourea (3)
116.4, 111.8. 108.0, FT-IR:
m
cmꢀ1 (KBr): 3383, 1715, 1691, 1679,
To a stirred solution of 6-aminocoumarin (0.1 g, 0.62 mmol) in
dry THF (15 mL) containing few drops of dry DMF, was added p-
nitrophenyl isothiocyanate (0.123 g, 0.68 mmol) at room tempera-
ture. The reaction mixture was stirred overnight at room tempera-
ture. After completion of reaction, the solvent was evaporated and
the residue was purified by column chromatography using 4%
CH3OH in CHCl3 as eluent to give 3 as yellowish powder, (0.127 g,
yield: 60%); mp 197–199 °C; 1H NMR (400 MHz, d6-DMSO): d 9.69
(s, 1H, urea NH), 9.45 (s, 1H, urea NH), 8.11 (d, 2H, J = 8 Hz), 7.90
(d, 2H, J = 8 Hz), 7.78 (s, 1H), 7.64 (1 d, H, J = 8 Hz), 7.46 (d, 1H,
J = 8 Hz), 7.23 (d, 1H, J = 8 Hz) 6.33 (d, 1H, J = 8 Hz), 13C NMR
(125 MHz, d6-DMSO): d 179.7, 159.7, 150.6, 145.9, 143.9, 142.3,
1544, 1438, 1185, Mass (EI): 505.0 [M + Na]+, [M + H]+ 483.3,
353.8, C26H22N4O6: calcd. C 64.19, H 4.56, N 11.52; found C
64.10, H 4.49, N 11.48.
3. Results and discussion
The anion binding properties of thiourea-based monotopic
receptors 1–4 were investigated by observing the changes in their
emission, absorption spectra in CH3CN and 1H NMR in CDCl3.
3.1. UV–vis, fluorescence and crystallographic studies
135.1, 128.3, 124.2, 123.2, 121.6, 118.5, 116.3, 112.2, FT-IR:
m
cmꢀ1
(KBr): 3275, 2923, 1720, 1635, 1464, 1174, Mass (EI): 342.2
[M + H]+, 311.1, 260.5, 247.1, C16H13N3O4S: calcd. C 55.97, H 3.82,
N 12.24; found C 55.90, H 3.74, N 12.17.
In the earlier report [6] we demonstrated the anion recognition
properties of 1 and 2 in CH3CN (containing 0.08% DMSO for homoge-
neity of the solution). UV titration of 1 (c = 5.63 ꢁ 10ꢀ5 M), which
exhibited a broad strong absorption band at 330 nm due to the cou-
marin moiety, was carried out with anions such as tetrabutylammo-
nium fluoride, bromide, iodide, hydrogen sulfate, dihydrogen
phosphate and benzoate. Upon addition of fluoride (Fꢀ), the inten-
sity of the absorption peak at 330 nm was remarkably reduced with
simultaneous growth of a new peak at 455 nm and the almost color-
less solution turned yellow brown [6]. The change in absorbance of 1
at 330 nm with the concentration of Fꢀ was almost linear [6]. In the
case of benzoate, the absorption peak at 330 nm shifted to 348 nm
2.2. 1-((2-Oxo-2H-chromen-6-yl)-3-phenylthiourea (4)
To a stirred solution of 6-aminocoumarin (0.100 g, 0.57 mmol) in
dry THF (15 mL) was added phenyl isothiocyanate (0.08 mL,
0.66 mmol) at room temperature. The reaction mixture was stirred
overnight at room temperature. After completion of reaction, the
solvent was evaporatedand the residue was chromatographed using
50% ethyl acetate in benzene as eluent to give 4 as white crystalline
product (0.110 g, yield: 70%); mp 158–160 °C; 1H NMR (400 MHz,
d6-DMSO): d 9.94 (s, 2H, urea NH), 8.08 (d, 1H, J = 8 Hz), 7.81 (br s,
1H), 7.64 (d, 1H, J = 8 Hz), 7.47 (d, 2H, J = 8 Hz), 7.39–7.32 (m, 3H),
7.15 (t, 1H, J = 8 Hz), 6.50 (d, 1H, J = 8 Hz), 13C NMR (125 MHz, d6-
DMSO): d 180.3, 160.4, 151.2, 143.2, 137.6, 135.0, 129.0, 128.6,
(Dk = 18 nm) with a concomitant decrease in intensity of the
absorption and the solution turned light green in color [6]. The
change in absorbance of 1 at 330 nm with the concentration of ben-
zoatewas alsoalmost linear as that of 1 withFꢀ. The presenceof isos-
bestic points during titration with both Fꢀ and C6H5COOꢀ revealed
the formation of 1:1 complexes. No significant change in absorption
126.0, 124.5, 123.5, 118.5, 116.7, 116.6, FT-IR:
m
cmꢀ1 (KBr): 3269,

K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
195
25
20
15
10
5
0
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. Change in fluorescence spectra for 3 (c = 5.86 ꢁ 10ꢀ5 M) in CH3CN upon
Fig. 3. Color changes observed: (a) receptor 3, (b) on addition of benzoate and (c)
on addition of fluoride.
addition of tetrabutylammonium fluoride.
20
15
10
5
2.0
1.5
1.0
0.5
0.0
0
350
250
300
350
400
450
500
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 4. Change in UV–vis spectra for 3 (c = 5.86 ꢁ 10ꢀ5 M) in CH3CN upon addition
Fig. 2. Change in fluorescence spectra for 3 (c = 5.86 ꢁ 10ꢀ5 M) in CH3CN upon
of tetrabutylammonium fluoride.
addition of tetrabutylammonium benzoate.
C6H5COOꢀ were significant but smaller compared to 1. The fluores-
cence emissions at 369 nm (kex = 320 nm) were ca. 12 and 19%
‘switched off’ for C6H5COOꢀ and Fꢀ, respectively [6]. The smaller de-
gree of quenching of emission in 2 was ascribed to a weak hydrogen
bonding interaction between the less acidic thiourea protons and
anions. This is reflected in their binding constant values (Ka: for
or a noticeable colorꢀchange was observed for 1 with other anions
such as Brꢀ, Iꢀ, H2PO and HSOꢀ4 .
4
The fluorescence emission spectra of 1 (c = 4.51 ꢁ 10ꢀ4 M)
showed a broad band at 420 nm when excited at 380 nm (red edge
excitation). With the addition of anions such as C6H5COOꢀ and Fꢀ
as tetrabutylammonium salts, the emissions were ca. 88% and
99.6% ‘switched off ’or quenched, due to the formation of anion-
receptor hydrogen bonded complexes [6]. During the titration there
were no other observable changes in the emission spectra Addition
1.Fꢀ 5.78 ꢁ 103 Mꢀ1
,
for 2.Fꢀ 2.26 ꢁ 103 Mꢀ1
,
for 1.C6H5COꢀ
2
2.02 ꢁ 104 Mꢀ1 and for 2.C6H5COꢀ 1.04 ꢁ 104 Mꢀ1), which were
2
measured by following the change in absorbance as a function of
the concentration of the anions [8]. The association constants for
both the receptors with benzoate are greater than Fꢀ. This is purely
due to strong hydrogen bonding interactions instead of deprotona-
tion as observed in the case of fluoride, established by 1H NMR [6].
In the present work we examined the binding interactions of 3
and 4 towards the same anions as considered for 1 and 2. In both 3
and 4, thiourea-binding sites are directly attached to the fluorescent
probe. Interestingly, these two coumarin-based thiourea receptors
behaved differently from 1 and 2 with respect to the selectivity
and sensitivity toward anions. Receptor 3 exhibited the emission
at 434 nm when excited at 330 nm in CH3CN. Upon addition of the
of H2POꢀ and HSOꢀ4 , hardly affected the emission spectra suggesting
4
their weak interactions with the receptor site. The spherical ions
such as Brꢀ and Iꢀ did not cause any significant quenching of emis-
sion (see Stern–Volmer plot in Supporting information), thereby rul-
ing out quenching by the heavy atom effect. Addition of Fꢀ,
C6H5COOꢀ, Brꢀ, Iꢀ, H2POꢀ and HSOꢀ4 as tetrabutylammonium salts
4
to a solution of 2 in CH3CN (containing 0.08% DMSO) resulted in a
minorchange in the UV–vis spectrum of receptor 2 and did not result
in any new peaks at higher wavelengths or color changes of the solu-
tion. The fluorescence changes of 2 upon addition of Fꢀ and

196
K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
80
70
60
50
40
30
20
10
0
2.0
1.5
1.0
0.5
0.0
350
400
450
500
550
600
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 7. Change in fluorescence spectra for 4 (c = 5.4 ꢁ 10ꢀ5 M) in CH3CN upon
Fig. 5. Change in UV–vis spectra for 3 (c = 5.86 ꢁ 10ꢀ5 M) in CH3CN upon addition
addition of tetrabutylammonium fluoride.
of tetrabutylammonium benzoate.
70
60
50
40
30
20
10
0
2.0
1.5
1.0
0.5
0.0
250
300
350
400
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 6. Change in UV–vis spectra for 4 (c = 5.4 ꢁ 10ꢀ5 M) in CH3CN upon addition of
Fig. 8. Change in fluorescence spectra for 4 (c = 5.4 ꢁ 10ꢀ5 M) in CH3CN upon
tetrabutylammonium fluoride.
addition of tetrabutylammonium benzoate.
anions the emission of 3 was perturbed to the different extents. Ex-
cept for Fꢀ and C6H5COOꢀ ions, the emission of 3 was changed neg-
ligibly. Figs. 1 and 2 display the change in emission of 3 in the
presence of Fꢀ and C6H5COOꢀ, respectively. In the presence of
increasing concentrations of Fꢀ, the intensity of emission centered
at 437 nm as a broad doublet decreased gradually with simulta-
neous growth of a new peak at 503 nm accompanying an isosbestic
point at 464 nm (Fig. 1). During the course of titration, the color of
the solution of 3 was changed from colorless to light red in the pres-
ence of Fꢀ (Fig. 3). As shown in Fig. 4, the solution of 3
(c = 5.86 ꢁ 10ꢀ5 M) has the original absorption peak at 335 nm,
which is due to the coumarin moiety. The absorption peak at
335 nm decreases, whereas the absorption peak at 380 nm increases
by Fꢀ titration, accompanying the formation of an isosbestic point at
364 nm. This signified the 1:1 binding interaction. As the corre-
sponding result of visual inspection, the addition of C6H5COOꢀ led
to similar spectral change bꢀut the change was small (Fig. 5). How-
ever, as the H2POꢀ4 , Brꢀ, HSO4 and Iꢀ were titrated into 3, the spectra
hardly changed even when the anions were in excess.
strated by Gale and coworkers [9], Fabbrizzi et al. and others
[10]), one would have expected that such a deprotonation should
occur with the addition of the 2 equiv of Fꢀ to 3. However, this does
not seem to be the case and only upon addition of a large excess of
Fꢀ this deprotonation occurs. Such deprotonation would give rise
to the formation of HFꢀ2 [11] with concomitant changes in the
absorption spectra, which would be shifted to longer wavelengths
[12]. Indeed this was found to be the case for 3 at high Fꢀ concen-
trations. Hence, we can conclude that for Fꢀ, the binding occurs
only through hydrogen bonding within a certain concentration
range and deprotonation occurs when it remains in excess in solu-
tion (established by 1H NMR study also; see latter).
Receptor 4 interacted with the anions weakly due to less acidic
character of thiourea protons in 4 compared to 3. In both fluores-
cence and UV, the spectral changes of 4 were observed in CH3CN only
in the presence of Fꢀ. For other anions the changes were minor. Fig. 6
represents the change in absorption spectra of 4 in CH3CN upon
increasing addition of Fꢀ. Figs. 7 and 8 demonstrate the change in
emission of 4 in CH3CN during titration with Fꢀ and C6H5COOꢀ,
respectively. The binding selectivity of 3 and 4 was evaluated in
Given the fact that Fꢀ is a strong Lewis base and can deproto-
nate one or more of the N–H protons of the receptor (as demon-

K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
197
S1
N7
C6
C20
C19
C8
C11
C18
C16
C1
C10
N9
C17
O1
C12
C2
C5
C4
C14
C13
C3
O15
(a)
(b)
Fig. 9. (a) XP plot with atom numbering scheme and (b) packing view of 2.
C5
C6
S2
C7
O8
C4
C19
C17
C14
C16
C2
C9
N1
C15
C18
N3
C13
C12
O9
C10
C11
(a)
(b)
Fig. 10. (a) XP plot with atom numbering scheme and (b) packing view of 4.
the ground state by determining the association constant values
using the UV titration data [8] and we were able only to determine
with Fꢀ (Ka: for 3.Fꢀ 4.51 ꢁ 103 Mꢀ1, for 2.Fꢀ 1.22 ꢁ 103 Mꢀ1).
Guest induced quenching of emission of all the receptors is
attributed to the activation of PET process occurring in between
the binding site and fluorophore probe. The non-covalent interac-
tions altogether enhance the efficiency of the PET process. The high
degree of fluorescence quenching is believed to result from the
increase in the reduction potential of the thiourea receptor moie-
ties after anion recognition. This increases the rate of electron
transfer from the HOMO of the thiourea-anion complex to the
coumarin-excited state and encourages the PET process. It appears
that the deprotonated species LHꢀ/L2ꢀ, being more electron rich
compared to the hydrogen-bonded complex with benzoate, acti-
vates the PET process more efficiently and shows greater quench-
ing. Thus among the monotopic receptors 1–4, receptors 1, 2 which
are different from 3 and 4 due to the presence of the –CH2-group in
between the binding site and the coumarin probe, are promising.
The presence of the –CH2-group controls the orientation of the
thiourea binding site around the coumarin for which 1 and 2 be-
have differently from 3 and 4 in emission. To get an insight into
this we solved the single crystal structures of 2 and 4 although
the behavior of the molecules in solution is expected to be differ-
ent. In the solid state it is found that compound 2 follows a differ-
ent packing structure from 4. The thiourea functional group in both
structures is involved in intermolecular hydrogen bonding through
the anti form and assembles the molecules in such a way that the
disposition of the coumarin moiety varies. Figs. 9 and 10 represent
the packing views of compounds 2 and 4, respectively along with
their XP plots. (Thermal ellipsoids are shown on the 50% probabil-
ity level.) Crystal data are shown in the Supporting information.
The anti form of the thiourea group in some cases is found to be
dominated over the syn form (cf. urea) [13] and accordingly, the
possibility of the anti form of the thiourea in the binding of anions
in solution cannot be ruled out. However, the maximum number of
hydrogen bond formation of the thiourea receptors via syn form
with the anions in solution is possibly a reason for their conforma-
tional preference over the anti form. This is quite understandable
from the complexation induced downfield shifting of the thiourea
protons in 1H NMR (Supporting information). The situation is
meaningless when the anions are present with the receptors in
the solution in excess. In this circumstance, the most acidic thio-
urea proton is deprotonated and the preference of the syn or anti
form of the thiourea moiety in binding of anions becomes
irrelevant.
During interaction the color change in the host solutions of 1
and 3 is ascribed to the charge-transfer interactions between the
electron-rich thiourea–anion complex donor unit and the elec-
tron-deficient p-nitrophenyl moiety [14].
3.2. 1H NMR study
To understand the binding events further, 1H NMR experiments
were carried out in CDCl3 like that of 1 and 2 [6]. In the present
case, the large downfield chemical shift of the thiourea protons
of 3 in the presence of Fꢀ and C6H5COOꢀ also indicated the strong

198
K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
800
600
400
200
0
400
450
500
550
600
650
Wavelength (nm)
Fig. 13. Change in fluorescence spectra for 5 (c = 4.15 ꢁ 10ꢀ5 M) in DMSO upon
addition of tetrabutylammonium pimelate.
600
500
400
300
200
100
0
Fig. 11. Partial 1H NMR (400 MHz) of 5 in DMSO-d6 and its 1:1 complexes with
different dicarboxylates.
800
700
600
500
400
300
200
100
0
400
450
500
550
600
Wavelength (nm)
Fig. 14. Change in fluorescence spectra for 5 (c = 4.15 ꢁ 10ꢀ5 M) in DMSO upon
addition of tetrabutylammonium succinate.
O
O
O
O
O
H
N
H
N
O
N
H
N
H
O
400
450
500
550
600
650
O
Mode B
O
O
Wavelength (nm)
Fig. 12. Change in fluorescence spectra for 5 (c = 4.15 ꢁ 10ꢀ5 M) in DMSO upon
addition of tetrabutylammonium isophthalate.
O
O
O
O
O
O
hydrogen bonded interaction (Supporting information). Upon com-
plexation of Fꢀ, the thiourea protons of 4 were also deprotonated in
CDCl3 like the cases of 1 and 3 with Fꢀ.
Since urea/thiourea functionalities bind carboxylate anion
involving directed hydrogen bonds, we intended to extend the
scope of dicarboxylate ion recognition employing our designed
and synthesized ditopic coumarin-receptor 5.
N
N
H
N
N
H
H
H
O
Mode A
O
O
O
Fig. 15. Possible modes of complexation of dicarboxylates with 5.

K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
199
Table 1
DMSO) such as different dicarboxylates, was not greatly affected.
This indicated poor interaction of 5 with the anions in the ground
state. Only in presence of tetrabutylammonium salts of pimelic and
isophthalic acids, the absorbance at 353 nm was changed accom-
panying isosbestic points at 371 and 374 nm, respectively (Sup-
porting information). In comparison, the dicarboxylates such as
phthalate, and terephthalate, which are isomeric to isophthalate
did not change the absorbance of 5 significantly.
Association constants of receptor 5 with anions in DMSO deter-
mined by UV method.
Guest anionsa
Receptor 5 (Ka in Mꢀ1
)
c
C6H5COOꢀ
Isophthalate
Pimelate
–
b
2.51 ꢁ 103
2.605 ꢁ 103
b
AcO-
–
Terephthalate
Phthalate
Succinate
8.21 ꢁ 102
In the absence of anions, 5 (c = 4.2 ꢁ 10ꢀ5 M) showed a charac-
teristic emission band at 455 nm when excited at 350 nm. Upon
successive addition of guests, the intensity of the emission band
gradually decreased to different extents for various dicarboxylates
without producing any significant spectral change (i.e. no spectral
shift or formation of new emission band). The degree of quenching
in emission varied with the nature of the dicarboxylates as evident
from the Stern–Volmer plot (Fig. S3).
b
–
–
b
a
b
c
Anions were used as their tetrabutylammonium salts.
The changes were too small to calculate precisely.
Errors in Ka were 610%.
Table 2
Among the different dicarboxylates studied, only isophthalate
and pimelate brought marked change in emission spectra. Figs.
12 and 13 represent the change in emission of 5 upon gradual addi-
tion of isophthalate and pimelate, respectively. The insignificant
change of emission of 5 upon titration with succinate is displayed
in Fig. 14. The stoichiometry of the complexes was confirmed from
the break of the titration curves at [G]/[H] = 1 (Supporting informa-
tion). The stoichiometry of complexes was additionally confirmed
by Job’s plots. Surprisingly, receptor 5 achieved 1:1 complexation
with all the dicarboxylates studied in the present case irrespective
of their nature. This is presumed to be due to the flexible nature of
the m-xylene spacer in 5, for which the dimensionally fitted dicar-
boxylates are involved in the formation of discrete 1:1 complexes
in the mode A and overlong dicarboxylates induce dynamic 1:1
supramolecular structures via the mode B (Fig. 15). Both forms A
and B remain equilibrium in solution. Reversibility of the complex-
ation was confirmed by the addition of methanol into the DMSO
solutions of receptor 5-isophthalate and receptor 5-pimelate. To
realize the binding potencies of 5 with the anions we used both
emission and absorption data for the determination of binding
constant values [8]. As can be seen from Tables 1 and 2, the open
cleft of 5 shows moderate affinities for isophthalate and pimelate.
Other dicarboxylates such as phthalate and terephthalate, which
are isomeric to isophthalate interacted weakly and showed lower
binding constant values. The binding constant values determined
by UV and fluorescence methods are of similar trend.
Association constants of receptor 5 with anions in DMSO deter-
mined by fluorescence method.
c
Guest anionsa
Receptor 5 (Ka in Mꢀ1
)
b
C6H5COOꢀ
Isophthalate
Pimelate
–
5.27 ꢁ 103
4.33 ꢁ 103
b
AcO-
–
Terephthalate
Phthalate
Succinate
8.11 ꢁ 102
7.55 ꢁ 102
b
–
a
b
c
Anions were used as their tetrabutylammonium salts.
The changes were too small to calculate precisely.
Errors in Ka were 610%.
3.3. Complexation studies on ditopic receptor 5
Prior to the UV–vis and fluorescence studies, we recorded 1H
NMR of 5 in the absence and presence of the different dicarboxyl-
ates. Fig. 11 represents the spectral changes of 5 in the presence of
equivalent amount of different dicarboxylates in d6-DMSO. It is
evident from Fig. 11 that the urea protons in 5 moved downfield
upon complexation. In addition, the isophthaloyl peri proton (Hh)
and the coumarin ring proton (He) also underwent downfield shift
during interaction. Such downfield shifting of the interacting pro-
tons was indicative of short hydrogen bonding between the urea
motif of 5 and the dicarboxylates.
The effect on the absorption and emission spectra of the urea-
based molecule 5 was recorded in the presence of anions, using tet-
rabutylammonium salts in DMSO. The absorbance spectrum of 5 in
presence of increasing concentrations of the guest anions (in
4. Details of the calculations
Geometry optimizations for the receptors 1–4 and the corre-
sponding complexes with benzoate anion and with fluoride anion
a
b
Receptor 1: N-H (a) 1.01279 Å, N-H (b) 1.01306 Å
Receptor 2: N-H (a) 1.01312 Å, N-H (b) 1.01244 Å
Receptor 3: N-H (a) 1.01323 Å, N-H (b) 1.01321 Å.
Receptor 4 : N-H (a) 1.01311 Å, N-H (b) 1.01291 Å.
Fig. 16. Optimized geometries of the receptors showing the amide N–H bond lengths.

200
K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
a
b
1 with benzoate
(binding energy 0.67 ev;
without BSSE it is 0.72 ev)
Ground state
Excited state
(a) O…H 1.72644, N-H 1.04791
(b) O…H 1.75593, N-H 1.04297
(a) O…H 1.61288, N-H 1.06342
(b) O…H 1.68202, N-H 1.04921
2 with benzoate
(binding energy 0.58 ev;
without BSSE it is 0.63 ev)
Ground state
Excited state
(a) O…H 1.78312, N-H 1.03925
(b) O…H 1.77301, N-H 1.04229
(a) O…H 1.74396, N-H 1.04069
(b) O…H 1.73671, N-H 1.04271
3 with benzoate
(binding energy 0.71 ev;
without BSSE it is 0.76 ev)
Ground state
Excited state
(a) O…H 1.71700, N-H 1.04886
(b) O…H 1.71797, N-H 1.04882
(a) O…H 1.60777, N-H 1.06419
(b) O…H 1.63354, N-H 1.05792
with benzoate
4
(binding energy 0.55 ev;
without BSSE it is 0.68 ev)
Ground state
Excited state
(a) O…H 1.75297, N-H 1.04437
(b) O…H 1.74025, N-H 1.04501
(a) O…H 1.68335, N-H 1.04984
(b) O…H 1.60904, N-H 1.06478
Fig. 17. Optimum geometries of ground and the first excited electronic states of the complexes of receptors with benzoate.
have been carried out by density functional theory (DFT) [15] cal-
culations employing the CAM-B3LYP functional [16] and the
6-31+G(d, p) basis set. At the optimum geometries, time dependent
density functional theory (TDDFT) [17] calculations were carried

K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
201
Table 3
Absorption and emission maxima k and oscillator strengths (f) of the three low-energy strongest transitions in receptors 1–4 and the complexes formed with Fꢀ and with
benzoate anion, obtained by TDDFT/CAM-B3LYP calculations.
Complex
Absorption
Emission
k (nm), f
k (nm), f
k (nm), f
k (nm), f
k (nm), f
k (nm), f
(1)
331, 0.638
361, 0.819
370, 1.002
295, 0.369
230, 0.476
299, 0.532
332, 0.140
361, 0.895
359, 1.318
292, 0.070
321, 0.177
325, 0.258
322, 0.114
330, 0.029
331, 0.001
265, 0.424
267, 0.352
271, 0.915
321, 0.457
346, 0.079
320, 0.204
272, 0.494
285, 0.504
289, 1.591
295, 0.484
299, 0.521
298, 0.507
261, 0.266
259, 0.319
266, 0.179
288, 0.331
312, 0.208
300, 0.265
255, 0.121
275, 0.293
266, 0.508
485, 0.104
504, 0.170
–
365, 0.017
377, 0.132
–
287, 0.339
291, 0.336
–
363, 0.153
382, 0.209
324, 0.247
299, 0.772
301, 0.800
305, 1.611
315, 0.509
334, 0.539
–
271, 0.005
275, 0.063
–
317, 0.501
343, 0.545
317, 0.208
286, 0.725
299, 0.358
294, 0.464
(1)+benzoate
(1)+Fꢀ
a
(2)
345, 0.422
349, 0.397
(2)+benzoate
(2)+Fꢀ
–
a
(3)
483, 0.094
517, 0.123
416, 1.382
387, 0.003
406, 0.186
401, 0.230
(3)+benzoate
(3)+Fꢀ
(4)
(4)+benzoate
(4)+Fꢀ
a
Excited-state geometry optimization did not converge.
moiety, and this indicates increased sensing capacity for these
receptors [20]. As shown in the molecular structure of the com-
plexes with benzoate anion, binding is through dual hydrogen
bonding, resulting in elongation of the two N–H bonds of the
receptors in the complexes. There are some small variations in
the minimum energy geometry of the excited state, compared to
the ground state minimum, such as smaller Oꢂ ꢂ ꢂH distances and
larger N–H distances, indicating ‘‘tighter’’ hydrogen bonding in
the excited-state geometries.
Fig. 18. DFT optimized geometry of 5.
The optimum ground-state geometries of receptors 1–4 with
fluoride anion were also calculated (Fig. S6), and (1)+Fꢀ and
(2)+Fꢀ are not planar while (3)+Fꢀ and (4)+Fꢀ are planar structures.
out for the six lowest excited states of singlet spin multiplicity. In
this way information relevant to the low-energy absorption spec-
trum is obtained. Subsequently, geometry optimization was car-
ried out for the first excited state, using TDDFT, resulting in
information regarding the emission spectra of the systems of inter-
est. All the calculations were carried out with the aid of Gaussian
09 suite of programs [18], and the solvent was included using
the PCM model [19].
4.2. Excited state energies, absorption and emission spectra
In Table 3, the calculated absorption and emission maxima are
listed for the four receptors and the corresponding four complexes.
In all cases a red shift is found in the spectra of the complexes with
respect to the spectra of the corresponding receptors. The absorp-
tion and emission spectra, as given by Gaussian 09, are listed in
Fig. S7 (Supporting information), where the vertical lines corre-
spond to the calculated levels and oscillator strengths. As shown
in Table 3 and Fig. S7, for receptor 1, the observed broad peak at
330 nm is well reproduced by the theoretical calculations while
the corresponding maxima in the complexes are calculated at
361 nm for 1.benzoate (experimental at 348 nm) and at 370 nm
for 1.Fꢀ (experimental at 455 nm). In general, the calculations
are in good agreement with experiment regarding the absorption
maxima of the receptors alone and less so for those of the com-
plexes, but they are in general agreement regarding the observed
4.1. Results on the optimum geometries
The optimum geometries of the receptors are as given in Fig. 16
(with the bond lengths in Angstrom units and a and b as indicated
for receptor 1). The optimum geometries of ground and the first ex-
cited electronic states of the complexes with benzoate anion are
listed in Fig. 17, where in all cases a and b are as indicated for
the case of receptor 1.
The results of the calculations show higher binding energies for
complexes of receptors 1 and 3, i.e. those containing the NO2
(a) Absorption
(b) Emission
Fig. 19. Low-energy absorption and emission spectra of receptor 5.

202
K. Ghosh et al. / Journal of Molecular Structure 1004 (2011) 193–203
5. Conclusions
In conclusion, ‘fluorophore–spacer–receptor’ based receptors 1–2
and ‘fluorophore–receptor’ based receptors 3–4 have been designed
and synthesized. The receptors are comprised of thiourea binding
site and coumarin fluorophore. Receptor 1 shows strong binding
to benzoate over fluoride ion by exhibiting the changes in fluores-
cence and color. Receptors 3 and 4, which are ‘fluorophore–receptor’
model based, are not promising compared to 1 and 2 in the sensing
of anions. In this context, work to develop the practical colorimet-
ric coumarin – based sensors for recognition of the anions espe-
cially Fꢀ and carboxylates in aqueous solution is underway in the
laboratory. However, we have also represented the use of couma-
rin linked urea in devising ditopic receptor 5 that are more sensible
to the dicarboxylates in organic solvent. The interaction study re-
veals that receptor 5 is capable of distinguish isomeric aromatic
dicarboxylates with moderate binding constant values. Further
investigations into steric and electronic effects in similar pre-orga-
nized hosts or receptors are ongoing. The results of the theoretical
calculations show differences in the geometries of the complexes,
depending on the receptor and the anion and differences in the
binding energies, indicating that both the presence of the nitro-
phenyl group in receptors 1 and 3 (but not in 2 and 4) as well as
the existence of the spacer group receptors 1 and 2 (but not in 3
and 4) affect the efficiency of anion recognition by the different
receptors.
Fig. 20. Optimized geometry of the complex of 5 succinate.
red shifts. The calculated emission peaks are in variable agreement
with the experimental: for receptor 1 the observed peak at 420 nm
(when excited at 380 nm) is calculated at 485 nm for the receptor
alone and at 504 nm for 1 + benzoate. For receptor 2 with experi-
mental peak at 369 nm the calculated is at 345 for the receptor
alone and at 349 nm for 2 + benzoate. The reduction in intensity
is not predicted for 1 + benzoate while it is reproduced for 2 + ben-
zoate (cf. Table 3). For receptor 3, the observed fluorescence peak
at 437 nm is calculated at 483 nm and 517 nm for 3 + benzoate,
while for receptor 4 the observed peak at 425 nm is calculated at
387 nm and at 406 nm for 4 + benzoate. For both receptors 3 and
4 the calculated transition probability in the complexes is not re-
duced with respect to the receptors alone.
Acknowledgment
The excited states of the receptors with significant transition
probability retain their character in the corresponding complexes,
i.e. the excitations involve mainly orbitals of the receptor mole-
cules. In Fig. S8, electron density plots are given for the lowest
unoccupied orbital (LUMO) and the 4 highest occupied (HOMO,
HOMO-1, HOMO-2 and HOMO-3) calculated for the complex of
receptor 1 with benzoate anion, at the ground state geometry
and at the geometry of the lowest excited state. These orbitals
are involved in the excitations contributing to the excited states
corresponding to the absorption and emission maxima. As shown,
the contribution of the benzoate orbitals is minimal and mainly
for the HOMO-3 orbital. Similar pictures are obtained for the
other complexes as well, regarding the orbitals involved in the
relevant excitations, and as a result it may be concluded that
there is no significant photoinduced electron transfer involving
the benzoate, although there is a degree of PET within the differ-
ent parts of the receptor molecules, for example in the excitations
K.G. thanks CSIR, New Delhi, India for financial support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2011.08.004.
Synthesis of 1 and 2, Stern–Volmer plots of 1, F- induced change
in emission of 2, Stern-Volmer plot of 5, Crystal data of 2 and 4, 1H
NMR of 3 in absence and presence of Fꢀ and C6H5COOꢀ, Optimum
ground-state geometries of receptors 1–4 with Fꢀ, Absorption and
emission spectra of 1–4 and their complexes with benzoate and
F-,Electron density plot for the complex 1.C6H5COOꢀ.
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