Development of colorimetric receptors for selective discrimination between isomeric dicarboxylate anions

Article abstract of DOI:10.1016/j.tetlet.2006.08.010

Three new chromogenic receptors (1, 2, and 3) containing p-nitrophenyl or p-nitronaphthyl group appended to the thiourea units or containing p-nitrophenyl group appended to the urea moiety were synthesized and characterized. Upon addition of a series of i

Full text of DOI:10.1016/j.tetlet.2006.08.010

Tetrahedron Letters 47 (2006) 7357–7361
Development of colorimetric receptors for selective
discrimination between isomeric dicarboxylate anions
Yao-Pin Yen* and Kao-Wai Ho
Department of Applied Chemistry, Providence University, 200 Chungchi Road, Sha-Lu, Taichung Hsien, 433, Taiwan
Received 26 June 2006; revised 2 August 2006; accepted 4 August 2006
Available online 24 August 2006
Abstract—Three new chromogenic receptors (1, 2, and 3) containing p-nitrophenyl or p-nitronaphthyl group appended to the thio-
urea units or containing p-nitrophenyl group appended to the urea moiety were synthesized and characterized. Upon addition of a
series of isomeric dicarboxylate anions to receptor 1 in DMSO/H2O (80:20 v/v), the appearance of the solution of receptor 1 with
maleate or phthalate showed color changes from blue to green or blue to dark-green, respectively, which those can be detected by
naked eye at parts per million. Similar experiments were repeated using 2, the solution showed a distinct color change from blue to
pink only when 2 is formed as a complex with maleate. Whereas, the addition of the same isomeric dicarboxylate anions to receptor
3, did not induce any color change. Thus, for unique color change, both receptors 1 and 2 can act as optical chemosensors for rec-
ognition of maleate versus fumarate. In addition, the receptor 1 can also be a colorimetric receptor for selective discrimination
between aromatic isomeric dicarboxylate anions.
ꢀ 2006 Elsevier Ltd. All rights reserved.
COO-
Construction of colorimetric chemosensors for a specific
anion is a particular attractive research area. However,
most of the chemosensors have been developed for the
colorimetric sensing of inorganic anions1 whereas very
few have been designed for recognition of organic
anions.2 Development of chromogenic reagents for such
species remains a challenge. A colorimetric sensor for
anions can be built following the binding site-signaling
unit approach by attaching an appropriate chromo-
phore to a specific anion receptor.3 Urea and thiourea
subunits are currently used in the design of neutral
receptors for anions, owing to their ability to act as
H-bond donors,4 and many ligands containing either
one or two of these groups have been reported to be
excellent sensors for dicarboxylate anions.5 During
recent years, we have been studying the synthesis of
colorimetric chemosensors for dicarboxylate anions
and their possible application in sensing.6 Now we
would like to report the preparation of new chromo-
genic receptors 1–3 and their utility in the selective
colorimetric discrimination between certain organic iso-
mers (cis/trans and ortho/meta/para dicarboxylates)
(Chart 1). Differentiation of geometric isomers is, in
-OOC
fumarate
COO-
COO-
-OOC
maleate
COO-
-OOC
COO-
-OOC
-OOC
isophthalate
phthalate
terephthalate
Chart 1.
general, a difficult task because of their rather similar
chemical and physical properties. To the best of our
knowledge, only few examples have been published.7
The interest in selective sensors able to distinguish male-
ate versus fumarate is not only related to p-diastereoiso-
mer recognition but is also due to the different biological
behavior of these anions. In fact, whereas fumarate is
generated in the Krebs cycle, maleate is a well known
inhibitor of this cycle and its implication in different
kidney diseases has been widely described.8 Moreover,
the interest to selectively discriminate between the three
phthalic acid isomers (ortho, meta, and para) is due
*
Corresponding author. Tel.: +886 4 2632 8001x15218; fax: +886 4
2632 7554; e-mail: ypyen@pu.edu.tw
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.08.010

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Y.-P. Yen, K.-W. Ho / Tetrahedron Letters 47 (2006) 7357–7361
to ortho-phthalate being a high-production-volume
synthetic chemical and also due to ubiquitous environ-
mental contaminant. The potential health risk associ-
ated with exposure to it has been increasingly
concerned.9
In this work, a chromogenic unit, an anthraquinone was
chosen as a scaffold to link the recognition units
(Scheme 1). The introduction of binding sites at 1- and
4-positions of 1,4-diaminoanthraquinone through an
ethylene spacer form a convergent binding site for a
feasible complexation with target species. The p-nitro-
phenyl or p-nitronaphthyl fragment which is linked to
the thiourea moiety was chosen as chromophore to
provide spectral sensing character upon complexation
with anions. In spite of lacking electronic conjugation
between the thiourea and anthraquionone moiety, the
receptors 1–3 showed UV–vis spectral changes on
complexation with anions.
Figure 1. Family of spectra taken in the course of the titration of a
5 · 10À5 M DMSO/H2O (80:20 v/v) solution in 1 with a standard
solution of maleate at 25 ꢁC titration profiles (insert) indicate the
formation of a 1:1 complex.
The receptors 1–3 were synthesized by the reaction of
p-nitrophenyl isothiocyanate or p-nitronaphthyl isothio-
cyanate or p-nitrophenyl isocyanate with 1,4-di-
(2-aminoethylamino)-anthraquinone (4) in high yields
(Scheme 1).10 All of these compounds were character-
1
ized by H NMR, 13C NMR, IR, and HRMS.
The colorimetric selective sensing ability of the receptors
1–3 with maleate and fumarate anions in DMSO/H2O
(80:20 v/v) was monitored by UV–vis absorption and
by ‘naked eye’ observation. The anions were added as
tetrabutylammonium salts to the DMSO/H2O (80:20
v/v) solutions of the receptors 1–3 (5 · 10À5 M). Figure
1a shows that the UV–vis absorption spectra of a mix-
ture of receptor 1 with different concentrations of male-
ate in DMSO/H2O (80:20 v/v). When the concentration
of maleate was increased, a new absorption band at
480 nm was substantially enhanced, while the intensity
of absorption at 362 nm decreased correspondingly.
Interestingly, the color of the solution of receptor 1
was changed from blue to green (Fig. 2), which could
be easily observed by the naked eyes. A clear isobestic
point was observed at 393 nm. This result demonstrates
that a complex formation of 1 with maleate anion takes
place via hydrogen bonding electrostatic interactions.
Figure 2. Effect of anions (as (C4H9)4 N+ salt) on color changes of 1 in
DMSO/H2O (80:20 v/v) (5 · 10À5 M) after the addition of 2 equiv of
anion: (a) 1 only; (b) 1+maleate; (c) 1+fumarate; (d) 1+phthalate; (e)
1+isophthalate and (f) 1+terephthalate.
The formation of these hydrogen bonds affects the elec-
tronic properties of the chromophore, resulting in a
color change with a subsequent new charge transfer
interaction between the electron donor nitrogen atom
of thiourea unit and the electron deficient 4-nitrophenyl
moiety.11 Judging from the titrations, the strong binding
of maleate allowed the Job’s plot method12 (as shown
in the inset of Fig. 1) to be used in the determination
O
O
O
O
HO
O
OH
O
HN
NH
i
ii
HN
NH
NHb
HN
X
X
H2N
NH2
NHa
R
HN
R
4
1. X = S, R = p-NO2-phenyl (Yield : 81%)
2. X = S, R = p-NO2-naphthyl (Yield : 91%)
3. X = O, R = p-NO2-phenyl (Yield : 58%)
Scheme 1. Reagents and conditions: (i) ethylenediamine, 50 ꢁC, 2 h, 37.2% and (ii) R-isothiocyanate or R-isocyanate, THF, reflux, 18 h.

Y.-P. Yen, K.-W. Ho / Tetrahedron Letters 47 (2006) 7357–7361
7359
of the binding stoichiometry, which was found to be a
1:1 host-to-anion complexation with an association con-
stant (1.09 0.78) · 104 MÀ1. This also showed that for
receptor 1, two thiourea functionalities simply act as
cooperative binding sites. In contrast, similar experi-
ments with corresponding fumarate salts were repeated
and no significant changes in spectra were observed in
the UV–vis absorption. The solution remains blue color
(Fig. 2). Thus, it indicates that the receptor 1 is weakly
binding or not interacting significantly with fumarate
in this solvent medium. Apparently, receptor 1 has a un-
ique color change and higher selectivity for maleate than
fumarate. The different color observed with maleate and
fumarate can be related to the receptor stereochemistry
that gives rise to different geometries depending on the
anion stereochemistry. Thus, the maleate anion with
its cis configuration perfectly fits into the complex induc-
ing a conformation change in the receptor. By contrast,
the fumarate anion with a trans disposition of carboxyl-
ate moieties does not induce changes in the ligand con-
formation and only a small increase of the UV–vis
absorption is observed. The proposed conformational
structure for the complex formed between receptor 1
and the maleate anion is shown in Figure 3.
Figure 4. UV–visible changes of 1 operated in DMSO/H2O (80:20 v/v)
(5 · 10À5 M) after the addition of 2 equiv of anion: (a) 1 only; (b)
1+isophthalate; (c) 1+terephthalate and (d) 1+phthalate.
Parallel investigations were carried out with a series of
other isomeric dicarboxylate anions (phthalate, isophth-
alate, and terephthalate). A similar phenomena of UV–
vis absorptions are observed in Figure 4. Spectrum (a)
was measured in the absence of anions where 1 has a
UV–vis spectrum with kmax at 362, 596, and 643 nm.
As shown in spectra (b) and (c), 1 exhibits negligible
perturbations upon addition of 2 equiv of isophthalate
and terephthalate anions, respectively. By contrast, a
significant change is observed in the presence of phtha-
late anion. As shown in spectrum (d), the CT absorption
band appears at 480 nm and the solution color changes
from blue to dark-green color (Fig. 2). It is apparent
that 1 has a unique color change and higher selectivity
for phthalate anion than other isomeric anions. The
selectivity of 1 for recognition of these anions can be
rationalized on the basis of the chain length and the
geometry of the anionic species.
Figure 5. Family of spectra taken in the course of the titration of a
5 · 10À5 M DMSO/H2O (80:20 v/v) solution in 2 with a standard
solution of maleate at 25 ꢁC titration profiles (insert) indicate the
formation of a 1:1 complex.
two thiourea subunits and four oxygen atoms of dicarb-
oxylate ions: in particular, electron density is transferred
on the thiourea moiety, which makes the intensity of the
dipole increase and shifts the charge-transfer band to
longer wavelength. These changes are accompanied by
a color change from a blue solution to pink color
(Fig. 6), visible to the naked eye. The changes in the
absorbance as a function of the concentration of male-
ate added can be fitted to a 1:1 binding equilibrium
model, giving association constant in Table 1.12 On
the contrary, however, after addition of fumarate, no
significant changes in spectra were observed in the
UV–vis absorption. The solution remains blue color
(Fig. 6). This result demonstrates that the receptor 2
can form a complex with maleate. Similarly, a dark-
red color can be observed while adding a series of
aromatic isomeric dicarboxylate anions (phthalate, iso-
phthalate, and terephthalate) into the solution of 2 in
DMSO/H2O (8:2 v/v), but the color changes were not
When these measurements were repeated using 2, similar
behavior was observed. Upon addition of different
concentrations of maleate to receptor 2 in DMSO/
H2O (80:20 v/v), the initial absorption peak at 382 nm
was gradually decreased and a new absorption band ap-
peared with a maximum absorption at 523 nm (Fig. 5).
This red-shift is ascribed to the occurrence of H-bond
interactions involving the four N–H fragments of the
Figure 3. Possible binding model of 1 with maleate anion.

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Y.-P. Yen, K.-W. Ho / Tetrahedron Letters 47 (2006) 7357–7361
Hb) changed from 10.26 to 10.90 (Dd = 0.64 ppm),
8.51 to 8.90 (Dd = 0.39 ppm), respectively (Fig. 7). The
larger downfield shifts indicate the formation of two
hydrogen bondings between Ha, Hb and maleate. These
results show that receptor 1 and maleate form a 1:1 stoi-
chiometry complex via hydrogen-bonding interaction
between thiourea and carboxyl groups. In contrast,
when 1 formed a complex with fumarate, the proton
chemical shifts of thiourea (Ha, Hb) changed from
10.28 to 10.42 (Dd = 0.14 ppm), 8.52 to 8.67
(Dd = 0.15 ppm), respectively (Fig. 8). These smaller
downfield shifts indicate the formation of two weak
hydrogen bondings between Ha, Hb and fumarate.
Figure 6. Effect of anions (as (C4H9)4 N+ salt) on color changes of 2 in
DMSO/H2O (80:20 v/v) (5 · 10À5 M) after the addition of 2 equiv of
anion: (a) 2 only; (b) 2+maleate; (c) 2+fumarate; (d) 2+phthalate; (e)
2+isophthalate and (f) 2+terephthalate.
In conclusion, the new colorimetric anion receptors 1–3
were synthesized in high yields. Among them, both 1
and 2 have higher selectivity for maleate than fumarate
and there are distinct color changes that can be observed
by the naked-eyes. Besides that, receptor 1 can also form
a complex with phthalate which results in a distinct col-
or change. Thus, both the receptors 1 and 2 can act as
optical chemosensors for recognition of maleate versus
fumarate. And the receptor 1 can also be a colorimetric
receptor for selective discrimination between aromatic
isomeric dicarboxylate anions.
distinct (Fig. 6). Apparently, the chromogenic reagent 2
is unable to be used for discrimination between the
isomeric aromatic dicarboxylate anions.
In order to gain a clear picture of how thiourea or urea
unit affects the binding property of 1, a UV–vis study
was conducted on the control compound 3. In a manner
similar to 1, 3 showed three absorption bands at 359,
596, and 643 nm. Upon gradual increase of the concen-
trations of maleate or phthalate or other isomeric an-
ions, no significant changes but only small increase of
the absorptions in UV–vis spectra and no color change
were observed. This weak binding could be explained
by the strongly electronegative oxygen atom of the urea
subunit which poorly contributes to the charge-transfer
transition. Thiourea is a much stronger protonic acid
than urea (pKa = 21.1 and 26.9, respectively, in
DMSO).13 The Job’s plots of these isomeric anions in
DMSO/H2O (80:20 v/v) showed a 1:1 binding stoichio-
metry, and the association constants were calculated by
the Benesi–Hildebrand equation12 and are listed in Ta-
ble 1. The values of the association constant are smaller
than those of receptor 1 or 2. Based on these results, the
receptors 1 and 2 can provide suitable chromophores
and binding sites for maleate anions.
Figure 7. 1H NMR (400 MHz) spectra of sensor 1 (10 mM) in DMSO-
d6: (a) Sensor 1 only; (b) 1+0.5 equiv of tetrabutylammonium maleate
and (c) 1+1.0 equiv of tetrabutylammonium maleate.
To investigate the interaction between receptor and cis/
trans isomeric anions further, we also monitored the
changes in the 1H NMR spectra of 1 or 2 upon addition
of maleate and fumarate anions. Addition of 1 equiv of
the tetrabutylammonium salts of maleate to 1 or 2 in
DMSO-d6 caused remarkable downfield shifts of the
1
Figure 8. 1H NMR (400 MHz) spectra of sensor 1 (10 mM) in DMSO-
d6: (a) Sensor 1 only (b) 1+1.0 equiv of tetrabutylammonium fumarate.
NH resonances in the H NMR. In the case of 1 with
maleate, the proton chemical shifts of thiourea (Ha,
Table 1. Association constants Ka (MÀ1) of receptors 1, 2 and 3 with guest anions
Anion
Receptor 1 K (MÀ1 a
)
Rb
Receptor 2 K (MÀ1 a
)
R
Receptor 3 K (MÀ1 a
)
R
Maleatec
(1.09 0.78) · 104
(6.65 1.22) · 102
(9.13 0.77) · 103
(1.58 0.24) · 103
(3.17 0.16) · 103
0.9905
0.9926
0.99
0.9932
0.9923
(1.34 0.26) · 104
(6.05 0.37) · 102
(1.30 0.68) · 104
(1.07 0.09) · 104
(1.17 0.02) · 104
0.9915
0.9912
0.9909
0.9953
0.9923
(4.26 1.44) · 102
(1.28 0.61) · 102
(5.50 1.24) · 102
(3.34 0.17) · 102
(4.26 0.36) · 102
0.9944
0.9964
0.9933
0.9971
0.9912
Fumaratec
Phthalated
Isopthalatec
Terephthalatec
a The data were calculated from UV–visible titration in DMSO/H2O (80:20 v/v).
b The data values of R were obtained by the results of nonlinear curve fitting.
c The anions were use as their tetrabutylammonium salts.
d The anions were use as their sodium salts.

Y.-P. Yen, K.-W. Ho / Tetrahedron Letters 47 (2006) 7357–7361
7361
6. Yen, Y.-P.; Ho, K.-W. Tetrahedron Lett 2006, 47, 1193–
1196.
Acknowledgements
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S. Chem. Commun. 2006, 761–763.
We thank the National Science Council of the Republic of
China for financial support (NSC 94-2113-M-126-002).
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10. Data for Compound 1: Yield: 81%. Mp 213–214 ꢁC. 1H
NMR (400 MHz, DMSO-d6): d: 3.74–3.76 (m, 8H), 7.71
(s, 2H), 7.75–7.77 (m, 4H), 7.80–7.82 (m, 2H), 8.14–8.16
(m, 4H), 8.25–8.27 (m, 2H), 8.49 (br s, 2H), 10.24 (br s,
2H), 10.90 (br s, 2H). 13C NMR (100 MHz, DMSO-d6): d
40.6, 44.1, 109.0, 120.9, 124.7, 125.9, 132.6, 134.0, 142.1,
146.2, 146.3, 180.7, 181.1. IR (KBr): m = 3329, 3067, 2933,
2858, 2356, 2335, 1639, 1598, 1511, 1326 cmÀ1. UV
(DMSO): 362 nm (e = 3504), 596 nm (e = 9370), 643 nm
(e = 8635). HRMS (FAB): calcd for C32H28N8O6S2 [M+]
684.1576; found 684.1584. Compound 2: Yield: 91 %. Mp
1
206–207 ꢁC. H NMR (400 MHz, DMSO-d6): d 3.72–3.75
(m, 8H), 7.67–7.69 (m, 4H), 7.76–7.83 (m, 6H), 8.10 (d,
J = 8.4 Hz, 2H), 8.26–8.31 (m, 6H), 8.41 (d, J = 8.4 Hz,
2H), 10.13 (br s, 2H), 10.92 (br s, 2H). 13C NMR
(100 MHz, DMSO-d6): d 30.9, 40.8, 44.3, 108.9, 122.4,
122.9, 123.9, 124.8, 125.6, 125.9, 127.7, 129.5, 130.0, 132.7,
134.0, 141.0, 143.4, 146.4, 181.1, 182.2. IR (KBr):
m = 3293, 3068, 2930, 2853, 2330, 1639, 1568, 1506, 1311,
1265, 1168, 1045, 1025, 830. cmÀ1. UV (DMSO): 382 nm
(e = 2510), 532 nm (e = 7493), 597 nm (e = 2662), 644 nm
(e = 1442). HRMS (FAB): calcd for C40H32N8O6S2 [M+]
784.1890; found 784.1891. Compound 3: Yield 58%. Mp
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1
210–211 ꢁC. H NMR (400 MHz, DMSO-d6): d 3.38–3.39
(m, 4H), 3.59–3.60 (m, 4H), 6.69 (s, 2H), 7.60–7.62 (m,
4H), 7.78 (s, 2H), 8.09–8.11 (m, 4H), 8.20 (br s, 2H), 8.49
(br s, 2H), 10.24 (br s, 2H). 13C NMR (100 MHz, DMSO-
d6): d 42.1, 108.9, 117.1, 124.7, 125.3, 125.9, 132.6, 134.0,
140.6, 147.3, 155.0, 181.0. IR (KBr): m = 3313, 3113, 3062,
2929, 2857, 2432, 1643, 1592, 1556, 1505, 1326, 1300, 1239,
1239, 1172, 1111, 1044, 1018 cmÀ1. UV (DMSO): 359 nm
(e = 2817), 596 nm (e = 7691), 643 nm (e = 6782). HRMS
(FAB): calcd for C32H28N8O8 [M+] 652.2030: found
652.2025.
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