Synthesis of colorimetric sensors for isomeric dicarboxylate anions: Selective discrimination between maleate and fumarate

Article abstract of DOI:10.1039/b710695e

Four new colorimetric receptors (1-4) were synthesized and characterized. Upon addition of maleate to receptor 1 in DMSO, the appearance of the solution of receptor 1 showed a color change from dark-blue to dark-red, which can be detected by the naked eye at parts per million. Similar experiments were repeated using receptors 2-4; the solution showed a distinct color change from blue to violet for receptor 2 and from blue-green to purple for both receptors 3 and 4, when they are formed as complexes with maleate. The striking color changes are thought to be due to the deprotonation of the thiourea moiety of the 4-nitronaphthyl chromophore. Whereas, in the addition of fumarate to receptors 1-4, the color of the solution changed from dark-blue to bright yellow for receptor 1 and did not induce any color change for receptors 2-4. Thus, for a distinct color change, receptors 1-4 can act as optical chemosensors for recognition of maleate versus fumarate. Especially, only receptor 1 has a unique color change for the recognition of fumarate, accordingly it can be used for detection of the fumarate anion. In this research it was also found that the performance of the receptor is highly dependent on the substituent group on the phenyl ring; a stronger electron-withdrawing group resulted in a receptor with a higher binding constant with the maleate anion. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710695e

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis of colorimetric sensors for isomeric dicarboxylate anions: selective
discrimination between maleate and fumarate†
Yu-Ping Tseng, Guan-Min Tu, Chia-Hung Lin, Chi-Tong Chang, Chi-Yung Lin and Yao-Pin Yen*
Received 17th July 2007, Accepted 18th September 2007
First published as an Advance Article on the web 26th September 2007
DOI: 10.1039/b710695e
Four new colorimetric receptors (1–4) were synthesized and characterized. Upon addition of maleate to
receptor 1 in DMSO, the appearance of the solution of receptor 1 showed a color change from
dark-blue to dark-red, which can be detected by the naked eye at parts per million. Similar experiments
were repeated using receptors 2–4; the solution showed a distinct color change from blue to violet for
receptor 2 and from blue-green to purple for both receptors 3 and 4, when they are formed as
complexes with maleate. The striking color changes are thought to be due to the deprotonation of the
thiourea moiety of the 4-nitronaphthyl chromophore. Whereas, in the addition of fumarate to receptors
1–4, the color of the solution changed from dark-blue to bright yellow for receptor 1 and did not induce
any color change for receptors 2–4. Thus, for a distinct color change, receptors 1–4 can act as optical
chemosensors for recognition of maleate versus fumarate. Especially, only receptor 1 has a unique color
change for the recognition of fumarate, accordingly it can be used for detection of the fumarate anion.
In this research it was also found that the performance of the receptor is highly dependent on the
substituent group on the phenyl ring; a stronger electron-withdrawing group resulted in a receptor with
a higher binding constant with the maleate anion.
have designed four novel colorimetric sensors. They were based
Introduction
on anthraquinone skeleton bearing thiourea groups through an
Anions, especially dicarboxylates, play an important role in
ethylene spacer (Scheme 1). The host structure, featuring binding
chemical and biochemical processes and their recognition and
sites at the 1- and 4-positions of 1,4-diaminoanthraquinone
sensing by artificial chemosensors has been a focus of interest for
through an ethylene spacer to form a convergent binding site,
chemists in the past decades.1 The recognition of anionic species
provides a feasible complexation with target species. The 4-
1
is generally based on electrochemical, H NMR and fluorescent
nitronaphthyl is linked to one of the thiourea moieties. The other
methods through changes in redox potential, chemical shift and
chromophores such as 4-trifluoromethylphenyl, 4-nitrophenyl, 4-
fluorescence, respectively.2 In recent years, a new method based on
cyanophenyl and phenyl groups are appended to the other branch
the change in color of an anionic sensor has been developed. The
of the thiourea moiety, respectively. These chromophores would
strategy to prepare colorimetric anion sensors is the binding site-
provide spectral sensing character upon complexation with anions.
signaling unit approach in which an appropriate chromophore
is attached to a specific anion receptor.3 These chromophores
In spite of lacking electronic conjugation between the thiourea
and the anthraquionone moiety, the sensors 1–4 showed UV–vis
may contain electron-withdrawing groups that enhance the acidity
spectral changes on complexation with anions.
of the anion binding subunit. 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
Their utility in the selective colorimetric discrimination be-
tween certain organic isomers (cis/trans and ortho/meta/para
dicarboxylates) (Chart 1) has been investigated. Differentiation
containing either one or two of these groups have been reported to
of geometric isomers is, in general, a difficult task because of
be excellent sensors for dicarboxylate anions.5 During recent years,
their rather similar chemical and physical properties. To the best
of our knowledge, only few examples have been published.6b,7
The interest in selective sensors that are able to distinguish
maleate versus fumarate is not only related to p-diastereoisomer
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.7b,8 Moreover, the interest to selectively discriminate
between the three phthalic acid isomers (ortho, meta, and para)
is due to the ortho-phthalate being a high-production-volume
synthetic chemical and ubiquitous environmental contaminant.
The potential health risk associated with exposure to it has been
increasingly of concern.9
we have been studying the synthesis of colorimetric chemosensors
for dicarboxylate anions and their possible application in sensing.6
As an extension of our previous work and in order to see how
the different electron-withdrawing substituents in one of the two
binding sites can influence the binding properties and modulate
the spectral behaviors and color changes in host–guest systems, we
Department of Applied Chemistry, Providence University, 200 Chungchi
Road, Sha-Lu, Taichung Hsien 433, Taiwan. E-mail: ypyen@pu.edu.tw;
Fax: +886-4-26328001; Tel: +886-4-26327554
† Electronic supplementary information (ESI) available: Color and UV
spectral changes for titration of 1–2 with hydroxide. 2D Noesy spectrum
of 1 and NMR titration data of 2 with maleate and fumarate. See DOI:
10.1039/b710695e
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Scheme 1 Reagents and conditions: (i) ethylenediamine, 50 ◦C, 2 h, 37%; (ii) 4-nitronaphthylisothiocyanate, THF, reflux, 18 h, and (iii) 4-R-phenyl-
isothiocyanate, THF, reflux, 18 h.
Chart 1
Results and discussion
Preparation of sensors 1–4 is depicted in Scheme 1. A synthetic
intermediate, 1,4-di-(2-aminoethylamino)anthraquinone (5) was
prepared from 2,3-dihydro-9,10-dihydroxy-1,4-anthracenedione.10
Fig. 1 A series of spectra taken over the course of the titration of a
5 × 10−5 M DMSO solution in 1 with a standard solution of maleate
at 25 ◦C. The titration profile (insert) indicates the formation of a 1 : 1
complex.
Reaction of
5 with 0.7 equivalents of 4-nitronaphthy-
lisothiocyanate in THF gave 6 in 42% yield. Subsequently, reaction
of 6 with the corresponding isothiocyanates in THF afforded
the bisthiourea derivatives 1–4 in moderate yields. All of these
1
compounds were characterized by H NMR, 13C NMR, IR and
HRMS.
Anion binding studies
The colorimetric selective sensing ability of the receptors 1–4 with
maleate and fumarate anions in DMSO was monitored by UV–
vis absorption and by naked eye observation. The anions were
added as tetrabutylammonium salts to the DMSO solutions of
the receptors 1–4 (5 × 10−5 M). Receptor 1 displays four weak
absorption bands at 385, 521, 596 and 643 nm, respectively, in
DMSO. The interaction of receptor 1 with maleate anion was
investigated in detail through the UV–vis spectroscopic titration,
and complicated spectral behaviors were observed (Fig. 1). Upon
addition of maleate to receptor 1 in DMSO, the intensity of
the absorption peak at 385 nm gradually decreased, while the
band at 521 nm evolved and reached its limiting value after the
addition of 2.0 equivalents of maleate (Fig. 1). Interestingly, the
color of the solution of receptor 1 was changed from dark-blue
to dark-red, visible to the naked eye (Fig. 2). A clear isobestic
point at 422 nm indicated the shifting of a well-defined binding
Fig. 2 Color changes of complex 1 upon addition of various anions in
DMSO: (a) 1 only; (b) 1 + 2.0 equivalents of maleate; (c) 1 + 2.0 equivalents
of fumarate.
equilibrium in the solution by addition of maleate. The changes
in the absorbance as a function of the concentration of maleate
added can be fitted to a 1 : 1 binding equilibrium model, giving the
association constants shown in Table 1.11 The band that develops at
521 nm is thought to be the monodeprotonated receptor L1− (1 =
L1H), which was confirmed by the Brønsted acid–base reaction of
adding 1 equivalent of strong base [n-Bu4N]OH (cf. SI-1, see ESI†).
The spectral behavior revealed that deprotonation of the NH
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Table 1 Association constants Ka/M of receptors 1–4 with maleate and
the through-bond propagation onto the naphthyl framework of the
electronic charge generated on N–H deprotonation. In addition,
the other signals of thioureas (N–H) were also found to undergo
downfield shifts when maleate was added. The results implied that
once the complex is formed by the receptor 1 with maleate, the
monodeprotonation of the receptor occurs. To provide support
for the supposition, the intermolecular N–H–O hydrogen bonded
distances were calculated at the HF/6-31G (d) level using ab
initio calculations (Fig. 4). Four protons of thioureas are directed
toward anion ligands but each hydrogen-bond distance is different
as shown in Table 2. Among them, only the proton (H4) that
is connected to the 4-nitronaphthyl group has a much shorter
fumrate anions
Anion
Receptor
K/Ma
Rb
Maleatec
1
2
3
4
1
2
3
4
(6.85 0.03) × 103
(1.14 0.02) × 104
(7.57 0.03) × 103
(6.12 0.02) × 103
(1.43 0.03) × 103
(1.52 0.02) × 103
(1.47 0.03) × 103
(1.38 0.02) × 103
0.9974
0.9958
0.9943
0.9927
0.9943
0.9988
0.9947
0.9957
Fumratec
a The data were calculated from UV–vis titrations in DMSO. b The data
values of R were obtained by the results of nonlinear curve fitting. c The
anions were used as their tetrabutylammonium salts.
distance to the carboxylic group than a typical hydrogen-bond
15
˚
distance, which ranges between 1.86 and 2.16 A.
In contrast, a similar experiment was carried out with fumarate
anion and a different UV–vis spectral behavior was observed.
Upon addition of fumarate anion, the intensity of the absorption
band at 385 nm was slightly increased while the band at 521 nm
was gradually decreased and blue-shifted to 472 nm, and a tailing
absorption at 400 nm appeared (Fig. 5). The blue shift was proba-
bly due to recognition of fumarate, the anion induced twisting of
the two thiourea moieties out of the plane of the 4-nitronaphthyl
and 4-trifluoromethylphenyl chromophores, respectively. During
the process, the most pronounced effect is the fumarate anion
induced color change from dark-blue to bright-yellow (Fig. 2).
The dramatic color change might be the first reported example
for the recognition of fumarate by a colorimetric sensor. In order
to investigate the notion that N–H deprotonation effects were or
fragment by maleate is responsible for the drastic color change, as
a result of a charge transfer interaction between the nitrogen atom
of the thiourea unit and the electron deficient 4-nitronaphthyl
moiety. Such a deprotonation was related to the acidity of the H-
bond donor site.12 The deprotonation of receptor 1 with maleate
1
was corroborated by H NMR titration experiments carried out
in DMSO-d6 (Fig. 3). It was found that the proton signal of
N–H4 (d = 10.14 ppm), which is closer to the 4-nitronaphthyl
group (signals of N–H protons were assigned by referring to
the 2D NOSEY spectrum of 1) (cf. SI-2, see ESI†), underwent
downfield shifts with increasing maleate concentration. The N–
H4 peak disappeared after addition of 1.0 equivalent of maleate,
whilst a new signal was observed at d = 20.16 ppm. This suggests
the formation of a [HM]− (M = maleate anion) species.13 The
monodeprotonation is also signaled by the significant upfield shift
of the protons of the naphthyl group.14 Such an effect derives from
Fig. 5 A series of spectra taken over the course of the titration of a
5 × 10−5 M DMSO solution in 1 with a standard solution of fumarate
at 25 ◦C. The titration profile (insert) indicates the formation of a 1 : 1
complex.
Fig. 3 1HNMR (400 Hz) spectra of sensor 1 (10 mM) in DMSO-d6 upon
addition of various quantities of maleate: (a) 0 eq.; (b) 0.5 eq.; (c) 1.0 eq.
Fig. 4 Optimized geometries from ab initio HF/6-31G(D) calculations.
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Table 2 Receptor distancesa of NH · · · L− hydrogen bonds from ab initio HF/6–31G(D) calculations
Receptor
Maleateb
Compound
H(1) · · · L−
H(2) · · · L−
H(3) · · · L−
H(4) · · · L−
1
2
3
4
1
2
3
4
1.9555(O1)
1.9385(O1)
1.9455(O1)
1.9625(O1)
1.7805(O1)
1.7635(O1)
1.7705(O1)
1.7935(O1)
1.8585(O2)
1.8385(O2)
1.8475(O2)
1.8755(O2)
1.8765(O2)
1.8815(O2)
1.8935(O2)
1.9435(O2)
2.1555(O3)
2.1595(O3)
2.1575(O3)
2.1465(O3)
1.7815(O3)
1.7875(O3)
1.7865(O3)
1.7805(O3)
0.9715(O4)
0.9715(O4)
0.9715(O4)
0.9705(O4)
1.9145(O4)
1.8885(O4)
1.8845(O4)
1.8805(O4)
Fumrateb
a
The unit of computed distances is A. b Four oxygen atoms (O1, O2, O3 and O4) of the guest form hydrogen bonds with the receptors where O1 is
˚
hydrogen-bonded to H1 and O2 to H2 and O3 to H3 and O4 to H4.
were not contributing to the anion-induced effects, the 1H NMR
spectral analyses were carried out in DMSO-d6. A notable feature
of these titrations is that the proton signals of thioureas underwent
less downfield shifts when 1 formed a complex with fumarate. After
the addition of 1 equivalent of fumarate, the signal of N–H4 was
found to shift from 10.14 to 10.83 ppm (Fig. 6). This relative
small downfield shift indicates that the complex is formed through
multiple hydrogen bonds and is inconsistent with a deprotonation
process between receptor and fumarate. This rationale was also
supported by the ab initio calculation that showed the proton
(H4) that is connected to the 4-nitronaphthyl group has a typical
hydrogen-bond distance to the carboxylic group (Table 2). Judging
from the UV–vis titrations, the binding of fumarate allowed the
Job’s plot method11 (as shown in the inset of Fig. 5) to be used in
the determination of the binding stoichiometry, which was found
to be a 1 : 1 host–guest complexation. The association constant
was calculated and is shown in Table 1. The comparison of the
UV–vis absorption spectra of the complex 1 upon addition of
either maleate or fumarate anions is shown in Fig. 7. Apparently,
the receptor 1 has demonstrated higher sensitivity and selectivity
recognition for maleate over fumarate in DMSO.
Fig. 7 UV–vis spectra change of 1 operated in DMSO (5.0 × 10−5M)
after addition of 2.0 equivalents of anions: (a) 1 only; (b) 1 + fumarate;
(c) 1 + maleate.
In order to investigate how the different substituents on
the phenyl ring of the other branch of the thiourea group
can influence the anion binding and sensing properties, the 4-
trifluoromethylphenyl group was replaced by a more electron
withdrawing group, a 4-nitrophenyl group; the receptor 2 was
examined. Among the sensors illustrated in Scheme 1, compound
2 is expected to show the strongest binding with maleate, due to
the enhanced acidity of the thiourea protons. With progressive
addition of maleate to receptor 2, the intensity of the absorption
peak at 360 nm was gradually decreased and a new band at 523 nm
concomitantly evolved and reached its limiting value after the
addition of 2.0 equivalents of maleate. A clear isobestic point at
Fig. 6 1HNMR (400 Hz) spectra of 1 (10 mM) in DMSO-d6 upon
addition of various quantities of fumarate: (a) 0 eq.; (b) 0.5 eq.; (c) 1.0 eq.
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423 nm was observed (Fig. 9). Such a significant change of the
UV–vis spectra of receptor 2 upon titration with maleate can be
attributed to the deprotonation of the thiourea proton, similar
in the intensity of the peak at 360 nm (cf. SI-5, see ESI†). However,
in this process, no noticeable color change was observed and
the solution remained blue (Fig. 8). Thus, it indicates that the
receptor 2 is weakly binding or not interacting significantly with
fumarate in this solvent medium. The interaction of receptor 2 with
−
to that mentioned above. The monodeprotonated receptor L2
(2 = L2H), is responsible for the absorption at 523 nm. This
1
was similarly confirmed by the reaction of adding strong base
fumarate was corroborated by H NMR titration experiments.
1
[n-Bu4N]OH (cf. SI-3, see ESI†) and also corroborated by H
It was found that when receptor 2 formed a complex with
fumarate, the proton signal of N–H4 underwent downfield shifts
with increasing fumarate concentration from 10.28 to 11.50 ppm
(cf. SI-6, see ESI†). By the same token, this relative small downfield
shift indicates the formation of weak hydrogen bonding instead
of deprotonation between fumarate and receptor. This is also
supported by the ab initio calculation (Table 2). Since receptor 2
has a unique color change and higher selectivity for maleate than
fumarate, it can act as an optical chemosensor for recognition of
maleate versus fumatate. 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 inducing a conformation change
in the receptor. By contrast, the fumarate anion with a trans
disposition of carboxylate moieties does not induce changes in
the ligand conformation and only a small increase of the UV–vis
absorption is observed. The proposed conformational structure
for the complex formed between receptor 2 and the maleate anion
is shown in Fig. 10. Besides that, the basicity of the anion may also
play an important role for recognition. Since the maleate dianion
is more basic than the fumarate dianion (maleic acid, pKa1 : 5.0,
pKa2 : 18.8; fumaric acid, pKa1 : 9.0, pKa2 : 11.0 in DMSO),16
thus the deprotonation of N–H4 will occur preferentially for the
maleate anion.
NMR titration of receptor 2 with maleate in which the peak of
[HM]− (M = maleate anion) appeared at 20.22 ppm (cf. SI-4, see
ESI†). These changes are accompanied by a different color change
from a blue solution to a violet color (Fig. 8). Judging from the
UV–vis titrations, the Job’s plot method showed the formation of
a 1 : 1 stoichiochemistry complex of 2 with maleate (as shown
in the inset of Fig. 9). The association constant was calculated11
and listed in Table 1. To elucidate the interaction between the
maleate anion and the receptor 2, the ab initio calculation of the
[2 · maleate] complex was taken. It clearly shows only the proton
(H4) has a much shorter distance to the carboxylic group than a
typical hydrogen-bond distance (Table 2). Based on these results
it can be concluded that both the N–H4 monodeprotonation on
the 4-nitronaphthyl thiourea unit and the hydrogen-bond induced
p-delocalization on the other 4-nitrophenyl thiourea moiety are
believed to be responsible for signaling the binding event.
Fig. 8 Color changes of complex 2 upon addition of various anions in
DMSO: (a) 2 only; (b) 2 + 2.0 equiv. of maleate; (c) 2 + 2.0 equiv. of
fumarate.
Fig. 10 Possible binding model of 2 with maleate anion.
A weaker or a non-electron-withdrawing substituent on the
phenyl moiety will decrease the acidity of the thiourea protons.
This is illustrated by the receptor 3 (R = CN) and the receptor
4 (R = H). The UV–vis absorption spectral profiles of 3 and
4 with addition of maleate anion are similar to that of 1 with
maleate anion (cf. SI-7 and SI-8, see ESI†). It appeared that both
the receptors 3 and 4 have the same propensity; deprotonation
of the NH group occurred upon addition of maleate anion. This
is similarly proved by the titration of 3 and 4 with [n-Bu4N]OH
Fig. 9 A series of spectra taken over the course of the titration of a
5 × 10−5 M DMSO solution in 2 with a standard solution of maleate
at 25 ◦C. The titration profile (insert) indicates the formation of a 1 : 1
complex.
On the contrary, upon addition of different concentrations of
fumarate to the solution of receptor 2, the initial weak shoulder
peak at 552 nm was blue-shifted to 527 nm with a small decrease
1
and by H NMR titration experiments, respectively. During the
titration process, the color of the solution changes from an initial
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Experimental
General
The chemical reagents were purchased from Acros or Aldrich
Corporation and utilized as received, unless indicated otherwise.
All solvents were purified by standard procedures. Melting points
were measured on a Yanaco MP-S3 melting-point apparatus. The
infrared spectra were performed on a Perkin Elmer System 2000
FT-IR spectrophotometer. UV–Vis spectra were measured on a
Cary 300 spectrometer. All NMR spectra were measured on a
Bruker spectrometer at 400 (1H) and 100 MHz (13C) with DMSO-
d6 as solvent. High-resolution mass spectra were measured with a
Finnigan/Thermo Quest MAT 95XL instrument.
Fig. 11 Color changes of complex 3 upon addition of various anions in
DMSO: (a) 3 only; (b) 3 + 2.0 equiv. of maleate; (c) 3 + 2.0 equiv. of
fumarate.
blue-green color to a purple color (Fig. 11). An apparent 1 : 1
binding constant of each receptor was determined and listed in
Table 1.
Preparation of tetrabutylammonium salts
To a stirred solution of a dicarboxylic acid (2.5 mmol) in dry
methanol (5 mL), 2.0 equiv. of a 1.0 M solution of tetrabutylam-
monium hydroxide in methanol (5 mL) was added. The resulting
mixture was stirred for 2 h at room temperature. The solvent
was evaporated in vacuo and dried over P2O5. The resulting
tetrabutylammonium salt was stored under anhydrous conditions
before use.
Consistent with the result of receptor 2, titration of 3 or 4
with fumarate anion also gave an unnoticeable color change.
The color of the solution still remains the original blue-green
color (Fig. 11). The weak hydrogen-bonding between 3 or 4 with
fumarate was corroborated by the 1H NMR titration experiments
and the hydrogen-bonded distances were reflected in the ab initio
calculations (Table 2). Due to the unique color change and higher
selectivity for maleate than fumarate anion of 3 and 4, they can
be used as optical chemosensors for recognition of maleate versus
fumarate.
The binding of 1–4 receptors with three aromatic isomeric dicar-
boxylate anions (ortho/meta/para-phthalate) were also studied.
Unfortunately, no distinct color change was observed (cf. S1–
12–S1–14, see ESI†). Therefore, the chromogenic reagents 1–4
cannot be used for discrimination between these three aromatic
isomers.
1-(2-Aminoethylamino)-4-(4-nitronaphthylthiourelyene-ethene-
amino)anthraquinone (6)
To
a stirred solution of 1,4-di-(2-aminoethylamino)anthra-
quinone8 (0.30 g, 0.90 mmol) in THF (50 mL), 4-nitrona-
phthylisothiocyanate (0.14 g, 0.60 mmol) in THF (50 mL) was
added at room temperature. The resulting mixture was stirred
and heated to reflux for 18 h. After cooling to room temperature,
the solution was concentrated in vacuum. The crude product was
washed with CH2Cl2 several times to afford the pure 6. Yield:
0.22 g (42%), as a blue solid. mp: 125–128 ◦C. 1H NMR (400 MHz,
DMSO-d6): d 3.73–3.76 (m, 8H), 7.64–7.72 (m, 2H), 7.74–7.84 (m,
4H), 8.09 (d, 2H, J = 8.4 Hz), 8.22–8.32 (m, 6H), 8.42 (d, 2H,
J = 8.4 Hz), 10.93 (br s, 2H). 13C NMR (DMSO-d6): d 30.8, 40.7,
44.2, 108.8, 122.5, 122.8, 123.8, 124.7, 125.5, 125.8, 127.6, 129.4,
130.1, 132.8, 134.1, 141.1, 143.5, 146.3, 181.2, 182.1. FT-IR (KBr):
3293, 3068, 2930, 2853, 2331, 1560, 1506, 1311, 1265, 1168, 1045,
1025, 830, 769, 718, 601 cm−1. UV (DMSO): 372 nm (e = 10 270),
596 nm (e = 17 200), 641 nm (e = 19 175). HRMS (FAB) calcd for
C29H26N6O4S [M+] 554.1736; found 554.1731.
Conclusions
In conclusion, a class of easily prepared sensitive colorimetric
receptors was synthesized, and the recognition of isomeric dicar-
boxylate anions was also studied. Among them, receptors 1–4
show good sensitivity and selectivity for discrimination of maleate
versus fumarate by drastic color changes. Thus, receptors 1–4
can be used as optical chemosensors for recognition of maleate
versus fumarate anion. Among them, the receptor 1 has also a
unique color change for recognition of fumarate, accordingly it
can be used for detection of the fumarate anion. It was also found
that the performance of the sensor is highly dependent on the
substituent on the phenyl ring; a stronger electron-withdrawing
group (i.e. NO2 > CN > CF3 > H) resulted in a sensor with
a higher binding constant with maleate. From the results of the
1-(4-Nitronaphthylthiourelyene-ethene-amino)-4-(4-trifluoro-
methylphenyl-thiourelyene-ethene-amino)anthraquinone (1)
To a stirred solution of 6 (0.30 g, 0.54 mmol) in THF (50 mL), 4-
trifluoromethylphenylisothiocyanate (0.22 g, 1.08 mmol) in THF
(50 mL) was added at room temperature. The resulting mixture
was stirred and heated to reflux for 22 h. After cooling to room
temperature, the solution was concentrated in vacuum. The crude
product was washed with CH2Cl2 several times to afford the pure
1
titrations with [n-Bu4N]OH, H NMR titration experiments and
the ab initio calculations, it was clearly demonstrated that a NH
deprotonation occurred for the receptors in the presence of the
maleate anion and on the other hand, only a multiple hydrogen-
bonded complex was formed in the presence of the fumarate anion.
Further work will continue to develop the practical colorimetric
sensors for recognition of isomeric dicarboxylates that are effective
in aqueous solution.
◦
1
1. Yield: 0.15 g (37%), mp: 195–196 C. H NMR (400 MHz,
DMSO-d6): d 3.70–3.82 (m, 8H), 7.63–7.67 (m, 2H), 7.70–7.76 (m,
2H), 7.80–7.83 (m, 4H), 8.08 (d, 2H, J = 8.4 Hz), 8.26–8.30 (m,
6H), 8.40 (d, 2H, J = 8.8 Hz), 10.10 (br s, 2H), 10.90 (br s, 2H). 13
C
NMR (DMSO-d6): d 40.4, 44.0, 108.6, 122.1, 122.6, 123.6, 124.5,
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Org. Biomol. Chem., 2007, 5, 3592–3598 | 3597
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125.3, 125.6, 127.3, 129.2, 129.7, 132.4, 133.7, 140.6, 143.1, 146.1,
180.7, 181.8. FT-IR (KBr): 3269, 2332, 1518, 1323, 1267, 1166,
1065, 1021, 831, 767, 729, 669 cm−1. UV (DMSO): 521 nm (e =
21 344), 596 nm (e = 10 253), 643 nm (e = 10 626). HRMS (FAB)
calcd for C37H30F3N7O4S2 [M+] 757.1744; found 757.1756.
Acknowledgements
We thank the National Science Council of the Republic of China
for financial support (NSC 95-2113-M-126-002) and the National
Center for High-Performance Computation.
1-(4-Nitronaphthylthiourelyene-ethene-amino)-4-(4-
nitrophenylthiourelyene-ethene-amino)anthraquinone (2)
References
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4-nitrophenylisothiocyanate in place of 4-trifluoromethylphenyl-
◦
1
isothiocyanate. Yield: 0.16 g (41%), mp: 197–199 C. H NMR
(400 MHz, DMSO-d6): d 3.74–3.76 (m, 8H), 7.69–7.71 (m, 2H),
7.75–7.82 (m, 7H), 8.09–8.15 (m, 4H), 8.25–8.31 (m, 3H), 8.54
(br s, 2H), 10.28 (br s, 2H), 10.91 (br s, 2H). 13C NMR (DMSO-
d6): d 40.6, 40.8, 44.0, 108.9, 109.0, 114.7, 120.9, 122.4, 123.9,
124.7, 125.9, 127.6, 129.9, 132.6, 134.0, 142.1, 143.4, 146.2, 180.8,
181.0, 181.1, 182.2. FT-IR (KBr): 3263, 3053, 2935, 2858, 2341,
1639, 1593, 1568, 1506, 1322, 1250 cm−1. UV (DMSO): 360 nm
(e = 24 925), 596 nm (e = 15 195), 643 nm (e = 17 544). HRMS
(FAB) calcd for C36H30N8O6S2 [M+] 734.1730; found 734.1719.
1-(4-Nitronaphthylthiourelyene-ethene-amino)-4-(4-
cyanophenylthiourelyene-ethene-amino)anthraquinone(3)
A
similar procedure to the synthesis of 1 was carried
out using 4-cyanophenylisothiocyanate in place of 4-trifluoro-
methylphenylisothiocyanate. Yield: 0.15 g (36%), mp: 197–198 ◦C.
1H NMR (400 MHz, DMSO-d6): d 3.73–3.76 (m, 8H), 7.66–7.68
(m, 2H), 7.75–7.82 (m, 6H), 8.11 (d, 2H, J = 8.4 Hz), 8.26–8.30
(m, 6H), 8.42 (d, 2H, J = 8.8 Hz), 10.14 (br s, 2H), 10.91 (br s,
2H). 13C NMR (DMSO-d6): d 39.6, 39.8, 44.0, 108.8, 122.5, 122.8,
123.8, 124.7, 125.5, 125.8, 127.6, 129.4, 130.1, 132.8, 134.1, 141.1,
143.5, 146.3, 181.2, 182.1. FT-IR (KBr): 3295, 3064, 2928, 2854,
2335, 1570, 1510, 1310, 1264, 1170 cm−1. UV (DMSO): 524 nm
(e = 10 320), 596 nm (e = 11 160), 643 nm (e = 13 540). HRMS
(FAB) calcd for C37H30N8O4S2 [M+] 714.8153; found 714.8157.
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1-(4-Nitronaphthylthiourelyene-ethene-amino)-4-(phenyl-
thiourelyene-ethene-amino)anthraquinone (4)
A similar procedure to the synthesis of 1 was carried out
9 G. Latini, Clin. Chim. Acta, 2005, 361, 20.
using phthylisothiocyanate in place of 4-trifluoromethylphenyl-
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◦
1
isothiocyanate. Yield: 0.23 g (59%), mp: 206–207 C. H NMR
(400 MHz, DMSO-d6): d 3.72–3.75 (m, 8H), 7.60–7.64 (m, 1H),
7.67–7.73 (m, 2H), 7.76–7.86 (m, 6H), 8.10 (d, 2H, J = 8.4 Hz),
8.26–8.34 (m, 6H), 8.41 (d, 2H, J = 8.4 Hz), 10.13 (br s, 2H),
10.92 (br s, 2H). 13C NMR (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. FT-IR (KBr): 3293, 3068,
2930, 2853, 2330, 1639, 1568, 1506, 1311, 1265, 1168, 1045, 1025,
830 cm−1. UV (DMSO): 382 nm (e = 10 438), 523 nm (e = 8768),
596 nm (e = 17 290), 643 nm (e = 19 294). HRMS (FAB) calcd for
C36H31N7O4S2 [M+] 689.1789; found 689.1792.
3598 | Org. Biomol. Chem., 2007, 5, 3592–3598
This journal is
The Royal Society of Chemistry 2007
©