Simple indole-based colorimetric sensors with electron-withdrawing chromophores: Tuning selectivity in anion sensing

Article abstract of DOI:10.1016/j.jfluchem.2011.08.001

Three simple colorimetric anion sensors (1, 2, and 3) containing anthrone, 1,3-indanedione, and malononitrile as signaling chromophores and an indole binding site have been designed and synthesized. The introduction of electron-withdrawing groups can not only provide chromogenic signal output, but also tune the sensitivity and selectivity of indole-based anion sensors by electron push-pull features. Their anion binding and sensing properties were investigated in detail by dramatic color changes, UV-vis absorption, and ¹H NMR. As results revealed, sensor 1 showed high selectivity for F^- over AcO^- and H₂PO₄^- with a distinct change in color due to the deprotonation of indole NH group. The excellent selectivity of 1 for F^- can be attributed to the fitness in the acidity of its NH-group, which is tuned to be able to distinguish the subtle difference in the affinity of F^-, AcO^-, and H ₂PO₄^- to NH proton.

Full text of DOI:10.1016/j.jfluchem.2011.08.001

Journal of Fluorine Chemistry 132 (2011) 973–977
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Simple indole-based colorimetric sensors with electron-withdrawing
chromophores: Tuning selectivity in anion sensing
Yongjun Lv a,b, Yong Guo a, Jian Xu a, Shijun Shao a,
*
a Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, PR China
b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Three simple colorimetric anion sensors (1, 2, and 3) containing anthrone, 1,3-indanedione, and
malononitrile as signaling chromophores and an indole binding site have been designed and
synthesized. The introduction of electron-withdrawing groups can not only provide chromogenic
signal output, but also tune the sensitivity and selectivity of indole-based anion sensors by electron
push–pull features. Their anion binding and sensing properties were investigated in detail by dramatic
color changes, UV–vis absorption, and 1H NMR. As results revealed, sensor 1 showed high selectivity for
Received 20 May 2011
Accepted 2 August 2011
Available online 9 August 2011
Keywords:
Anion binding
Indole
F
ꢀ over AcOꢀ and H2PO4ꢀ with a distinct change in color due to the deprotonation of indole NH group.
The excellent selectivity of 1 for Fꢀ can be attributed to the fitness in the acidity of its NH-group, which is
tuned to be able to distinguish the subtle difference in the affinity of Fꢀ, AcOꢀ, and H2PO4ꢀ to NH proton.
Crown Copyright ß 2011 Published by Elsevier B.V. All rights reserved.
Chromogenic sensor
Deprotonation
Spectroscopy
1. Introduction
and familiar method is to introduce an electron-withdrawing
group into the skeleton of indole motieties. Thus, we adopted the
In view of crucial roles anions play in biological, pathological,
and industrial processes, the development of simple and sensitive
anion sensors continues to be a research area of considerable
interest [1]. In this stance, artificial sensors capable of binding and
sensing anions by an optical signal are being increasingly
appreciated [2]. Of particular interest is the design of anion-
induced colorimetric sensors, which allow direct naked-eye
detection of anions with no equipment required [3]. Thus many
synthetic anion sensors moieties have been widely reported, which
can effectively respond anion-induced changes in color and optical
spectra based on hydrogen bonds or deprotonation [4]. Unfortu-
nately, a few colorimetric anion sensors are able to differentiate
selectively between anionic substrates of similar ꢀbasicity and
surface charge density such as Fꢀ, AcOꢀ, and H2PO4 [4,5].
In recent years, indoles, as an important class of building blocks
for anion binding and sensing, have been integrated into a variety
of anion sensors and sensors to detect specific anionic guests [6]. In
our lab, we previously investigated indole-based derivatives as
molecular sensors for special anionic species based on hydrogen
bonds or deprotonation mechanism [7]. Generally, indole-based
sensors were shown to bind anions with weak responses, which
indicated the lower affinity of the indole NH as a hydrogen bond
donor for anions. To improve their anion sensing abilities, a simple
nitro group to enhance the acidity of indole NH and obtained a
positive effect on anion binding behaviors [8].
Herein, we have designed and synthesized three simple indole-
based colorimetric anion sensors 1, 2, and 3 containing anthrone,
1,3-indanedione, and malononitrile as electron-withdrawing and
chromogenic groups, respectively (Scheme 1). We found that the
introduction of electron-withdrawing groups has a positive effect
on anion binding behavior of indole, aꢀnd the selectivity of indole
derivatives for Fꢀ, AcOꢀ, and H2PO4 is interrelated with the
electron-withdrawing ability of chromophores. In this paper, we
would like to present our studies on the anion binding properties of
three indole-based colorimetric sensors.
2. Results and discussion
2.1. UV–vis absorption study
As shown in Fig. 1, the anion binding properties of 1, 2, and 3
were firstly studied by observing changes in color and UV–vis
absorption in CH3CN. The anions tested were added as tetra-
butylammonium salts to the CH3CN solutions of 1–3 (10
mM). In
the presence of various anions (Fꢀ, AcOꢀ, H2PO4ꢀ, Clꢀ, Brꢀ, Iꢀ,
ClO4ꢀ, and HSO4ꢀ), 1 only responded to the most electronegative
Fꢀ by significant color changes from pale yellow to light blue. In the
case of 2 and 3, moderate basic AcOꢀ or H2PO4ꢀ could also produce
the similar color changes to Fꢀ from pale yellow to golden and from
colorless to pale yellow, respectively. This observation indicated
* Corresponding authors. Tel.: +86 931 4968209; fax: +86 931 8277088.
E-mail addresses: guoyong@licp.cas.cn (Y. Guo), sjshao@licp.cas.cn (S. Shao).
0022-1139/$ – see front matter. Crown Copyright ß 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2011.08.001

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Y.L. Lv et al. / Journal of Fluorine Chemistry 132 (2011) 973–977
that 2 and 3, compared to 1, showed more enhanced acidity of
indole NH, which resulted from the stronger electron-withdrawing
ability of 1,3-indanedione and malononitrile than that of anthrone
moiety. In contrast, the addition of other anions tested resulted in
no change in color. UV-vis spectroscopy experiments revealed
Fig. 2. The variations of UV–vis absorption spectra of sensor 1 (10 (M, CH3CN) after
addition from 0 to 30 equiv. of Fꢀ. Inset: nonlinear curve fitting as a function of [Fꢀ].
large red sifts of spectra of sensors 1–3 upon addition of anions
corresponding to changes in color. Such large bathochromic shifts
can be attributed to a charge transfer from the indole NH proton
being interacted with anions to the electron-withdrawing
chromophore [9]. The titration experiments were conducted to
provide a further insight into sensor-anion interaction in detail.
As for 1 in Fig. 2, with stepwise addition of Fꢀ to 1 (10
CH3CN, the main absorption peak at 430 (
= 16,380 Mꢀ1 cmꢀ1) nm
was gradually decreased and red-shifted to 440 nm
mM) in
e
(
(
e
e
= 11,800 Mꢀ1 cmꢀ1), while
= 21,860 Mꢀ1 cmꢀ1) evolved and reached its limiting value after
a
new band at 593 nm
the addition of 26 equiv. of Fꢀ. A clear isobestic point at 473 nm
indicated the interaction between 1 and Fꢀ in a 1:1 stoichiometry.
Two Fꢀ-induced absorption bands at 440 nm and 593 nm may be
assigned to the deprotonationed form of anthrol tautomer (1a and
1b Fig. 3) in the presence of the most electronegative Fꢀ with
delocalization of the negative charge from the nitrogen atom to the
oxygen atom [10]. This was confirmed by the Brønsted acid–base
reaction of 1 with a strong base [Me4N]OH (Fig. 1). The equilibrium
constant of 1 for Fꢀ was calculated to be 7.28 ꢁ 104 Mꢀ1 according
to Eq. (1), as shown in the inset of Fig. 2. The stoichiometry
between 1 and Fꢀ was proved by UV-vis Job plot, giving a sensor/
fluoride stoichiometry of 1:1 (Fig. 4). The resulting bind curves
were analyzed by nonlinear regression methods, and the associa-
tion constants are listed in Table 1.
As expected, 2 and 3 displayed significant absorption responses
not only for the mꢀost basic Fꢀ (Figs. 5 and 6), but for moderate basic
AcOꢀ and H2PO4 (Figs. S1–S4). This is presumably due to the
strong electron-withdrawing nature of 1,3-indandione and mal-
ononitrile groups, which increased the acidity of the indole NH
proton. During the absorption titration of Fꢀ, AcOꢀ, and H2PO4
ꢀ
with a solution of 2, the band at 437 nm (
e
e
= 48,900 Mꢀ1 cmꢀ1
= 56,600 Mꢀ1 cmꢀ1
)
)
disappeared and two new bands at 458 nm (
Table 1
Equilibrium constants of sensors 1, 2, and 3 with Fꢀ, AcOꢀ, and H2PO4ꢀ in CH3CN as
determined from UV–vis absorption titrations.
Aniona
Sensor 1 (Ka in Mꢀ1
)
Sensor 2 (Ka in Mꢀ1
)
Sensor 3 (Ka in Mꢀ1
)
Fꢀ
7.46 ꢁ 103
2.67 ꢁ 105
2.24 ꢁ 105
3.44 ꢁ 105
4.73 ꢁ 105
3.99 ꢁ 105
3.67 ꢁ 105
b
AcOꢀ
ꢀ
b
H2PO4
a
Fig. 1. UV–vis absorption spectra of sensors 1 (a), 2 (b), and 3 (c) (10 (M, CH3CN) in
the presence of 30 equiv. of various anions. Inset: color changes ofꢀsensors 1 (a), 2
Anions are used as their tetrabutylammonium salts.
b
The changes in the absorption spectra are too small to calculate the equilibrium
constants precisely.
(b), and 3 (c) in the presence of 30 equiv. of Fꢀ, AcOꢀ, and H2PO4
.

Y.L. Lv et al. / Journal of Fluorine Chemistry 132 (2011) 973–977
975
Scheme 1. The synthetic routes of sensors 1, 2, and 3.
Fig. 3. The probably deprotonationed structures of sensors 1, 2, and 3 after interaction with corresponding anions.
and 476 nm (
sible for simultaneous color changes. Similar results were also
observed for in the presence of putative anions, which
e
= 77,400 Mꢀ1 cmꢀ1) developed, which was respon-
indole NH proton signals in 1–3 appeared at 11.67, 12.76, and
12.79 ppm in DMSO-d6, respectively (indole NH signals appeared
at approximately 10.1 ppm). The observed order was consistent
with the Fꢀ binding affinities observed for sensors 1–3. As
mentioned above, we concluded that the significant changes in
color and absorption spectra might be resulted from the
deprotonation of chromophore-modified indole NH. This depro-
tonation was also examined in the 1H NMR titration spectra using
sensor 1 with Fꢀ anion in DMSO-d6. As depicted in Fig. 7, the signal
of indole NH proton at 11.67 ppm disappeared after the addition of
0.4 equiv of Fꢀ anion. With the gradual addition of Fꢀ a new singlet
signal developed at 16.10 ppm and then split into a 1:2:1 triplet
signal. The appearance of new signals can be ascribed to the FHFꢀ
dimer [11], which demonstrated the occurrence of the deprotona-
tion of the NH group of the indole ring. With the deprotonation of 1,
charge delocalized on the entire conjugated system, which shifted
the vinyl CH and anthra rings upfield further. This deprotonation
process indeed supported the conclusion from the UV–vis spectra
changes observed in the presence of [Me4N]OH.
3
demonstrated the existence of deprotonationed forms of corre-
sponding tautomers (Fig. 3). The addition of [Me4N]OH, which
definitely led to deprotonation, also induced the same color and
spectral changes of both sensors as those observed with Fꢀ, AcOꢀ,
and H2PO4ꢀ anions (Fig. 1). Moreover, equilibrium constants for 2
and 3 with Fꢀ, AcOꢀ, and H2PO4ꢀ were also given in Table 1 (Figs. 4
and 5 and S1–S4 inset). As described in Table 1, the order of Fꢀ
affinities of sensors is 1 < 2 ꢂ 3, which is consistent with the
electron-withdrawing nature of chromophores. These groups
increase the acidity of indole NH proton, which, in turn, enhance
the availability of NH moiety for deprotonation and affinity of
sensors toward anions.
2.2. 1H NMR spectra study
This trend in sensor-anion affinity, as reflected by the
equilibrium constants, was also observed with the chemical shifts
of NH proton signals of sensors 1–3 in 1H NMR. The chromogenic
Additionally, the reversibility of the deprotonation process was
examined by adding water to the system of sensors and anions in

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Y.L. Lv et al. / Journal of Fluorine Chemistry 132 (2011) 973–977
Fig. 7. Partial 1H NMR spectra of sensor 1 in the presence of 0, 0.4, 1.0, 2.0, and 8.0
equiv. of Fꢀ
.
UV–is spectra (Fig. S5). We found that the absorption spectrum
completely reverted to the original spectrum for 1 in CH3CN
containing 0.5% water, while 2 and 3 could tolerate trace water in
the event of anion binding and sensing due to the strong acidity of
their indole NH groups.
Fig. 4. A job plot of sensor 1 with Fꢀ in CH3CN, [1] + [Fꢀ] = 10 (M.
3. Conclusions
We herein have presented three simple indole-based colori-
metric anion sensors using electron-pulling chromophores to tune
the anion selectivity and the output signal. The selectivity and
sensitivity of anion sensors is found to be interrelated with the
electron-withdrawing ability of chromophores constituting the
anion binding sites. Sensors 1–3 show increased affinities and
enhanced binding constants for anions along with the increased
electron-withdrawing nature of chromogenic groups. The push–
pull feature renders the anion binding site indole NH more acidic
and thus more available for hydrogen bonds or deprotonation. We
believe that the introduction of reasonable chromophores allows
the design of a variety of sensors for selective binding and sensing
targeted substrates.
Fig. 5. The variations of UV–vis absorption spectra of sensor 2 (10 (M, CH3CN) after
addition from 0 to 2.7 equiv. of Fꢀ. Inset: nonlinear curve fitting as a function of [Fꢀ].
4. Experimental
4.1. Materials and methods
CH3CN was HPLC grade and used directly, and other chemical
materials were analytical pure and obtained from commercial
companies. 1H NMR and 13C NMR spectra were determined on a
Varian INOVA 400 MHz spectrometer. ESI-MS studies were carried
out using a Waters Micromass ZQ-4000 Spectrometer. UV–vis
absorption spectra were recorded on a Perkin-Elmer Lambda 35
spectrometer. Elemental (C, H, N) analyses were made on a Vario-
EL. The binding ability of sensors 1, 2 and 3 for anions (as
tetrabutylammonium salts) was investigated by UV–vis absorp-
tion spectroscopy in CH3CN solution using a constant host
concentration (10
mM) and increasing concentrations of anions.
The change in absorbance of three sensors was plotted against
anion concentration and fitted by Formulae (1) as described by
Conners [12].
X ¼ X0
8
<
9
=
"
#
1=2
ꢀ
ꢁ
2 ꢀ 4cHcG
(1)
1
1
þ 0:5De cH þ cG
þ
ꢀ
cH þ cG
þ
:
;
Ks
Ks
Fig. 6. The variations of UV–vis absorption spectra of sensor 3 (10 (M, CH3CN) after
addition from 0 to 2.6 equiv. of Fꢀ. Inset: nonlinear curve fitting as a function of [Fꢀ].

Y.L. Lv et al. / Journal of Fluorine Chemistry 132 (2011) 973–977
977
4.2. Synthesis of sensors
133.42, 126.73, 123.93, 122.56, 119.04, 115.97, 115.91, 113.08,
110.99, 69.06. Anal. Calcd. for sensor 3 (C12H7N3): C, 74.60; H, 3.65;
N, 21.75. Found: C, 74.62; H, 3.61; N, 21.72.
Sensors 1, 2, and 3 were prepared from the condensation of
indolylaldehyde with anthrone, 1,3-indandione, and malonitrile,
respectively, as depicted in Scheme 1 [13].
Acknowledgements
4.2.1. 10-(Indol-3-yl)-methylene-anthrone-9 (1)
This work was supported by the National Natural Science
Foundation of China (Grant No. 20972170 to S-J. Shao) and by the
Natural Science Foundation of Gansu province (No. 096RJ2A033 to
Y. Guo).
Indole-3-carboxaldehyde (0.73 g, 5 mmol) and anthrone
(0.97 g, 5 mmol) were refluxed in a mixture of toluene (80 mL)
and glacial acetic acid (0.5 mL). Any water formed during the
reaction was removed azeotropically by heating in a Dean-Stark
apparatus overnight. Solvents were evaporated under reduced
pressure, and the crude product was purified by column
chromatography with dichloromethane as an eluent to obtain 1
as a red solid (0.92 g, 57% yield). Mp: 247–248 8C. ESI-MS: m/z
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jfluchem.2011.08.001.
322.3 [M+H]+. 1H NMR (400 MHz, DMSO-d6),
d (ppm): 11.67 (s, 1H,
NH), 8.42 (d, 1H, VinylH, J = 8.00 Hz), 8.16 (d, 2H, AranthraH,
J = 7.20 Hz), 8.07 (s, 1H, CHNH), 8.04 (d, 1H, ArindolylH, J = 7.20 Hz),
7.76 (m, 2H, AranthraH), 7.45 (m, 4H, AranthraH), 7.11 (m, 2H,
ArindolylH), 6.93 (t, 1H, ArindolylH, J = 7.20 Hz). 13C NMR (100 MHz,
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