Anion binding and sensing properties of bis(3-indolyl)methene derivatives based on proton transfer process

Article abstract of DOI:10.2174/157017811794557796

A series of the bis(3-indolyl)methene derivatives were synthesized and their anion binding and sensing properties in CH₃CN or mixed CH ₃CN/H₂O solution have been investigated in detail by UV-vis spectroscopic techniques. The deprotonation/protonation of the bis(3-indolyl)methene receptor is responsible for the dramatic color and spectral changes. The introduction of the electron-donating or withdrawing group into different moiety of the bis(3- indolyl)methene skeleton, which tunes the acidity of the H-bond donor moiety or the basicity of the H-bond acceptor moiety, has a positive effect on the selectivity and sensitivity of such "proton-transfer" chemosensors for anions.

Full text of DOI:10.2174/157017811794557796

60
Letters in Organic Chemistry, 2011, 8, 60-65
Anion Binding and Sensing Properties of Bis(3-indolyl)methene
Derivatives Based on Proton Transfer Process
Litao Wanga,b, Xiaoming Hea,b, Yong Guoa, Jian Xua and Shijun Shao*,a
aKey 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, P. R. China
bGraduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China
Received October 25, 2010: Revised December 16, 2010: Accepted December 21, 2010
Abstract: A series of the bis(3-indolyl)methene derivatives were synthesized and their anion binding and sensing
properties in CH3CN or mixed CH3CN/H2O solution have been investigated in detail by UV-vis spectroscopic techniques.
The deprotonation/protonation of the bis(3-indolyl)methene receptor is responsible for the dramatic color and spectral
changes. The introduction of the electron-donating or withdrawing group into different moiety of the bis(3-
indolyl)methene skeleton, which tunes the acidity of the H-bond donor moiety or the basicity of the H-bond acceptor
moiety, has a positive effect on the selectivity and sensitivity of such “proton-transfer” chemosensors for anions.
Keywords: Anion recognition, bis(3-indolyl)methene, colorimetric chemosensor, hydrogen bond, proton transfer, tunable
selectivity and sensitivity.
1. INTRODUCTION
anion binding. In an effort to further investigate its potential
ability for anion binding and sensing, a series of the bis(3-
indolyl)methene derivatives were designed by introducing
the electron-withdrawing nitro group or electron-donating
methoxy group into the different moiety of the
bis(indolyl)methene skeleton, which can tune the acidity of
the H-bond donor moiety or the basicity of the H-bond
acceptor moiety. Here in, the anion recognition and sensing
behaviors of the derivatives were evaluated in detail using
the UV-vis spectroscopic techniques.
Anion recognition and sensing is an important research
area in supramolecular chemistry, mainly because of the
understanding that anions play key roles in biology,
medicine, catalysis and environmental sciences [1]. The
development of the electroneutral receptors and
chemosensors for anions continues to provide a challenge for
supramolecular chemists and has been attracting much
attention in recent years [2]. More and more efforts have
been devoted to develop novel receptors for anion
recognition and sensing [3]. Most of the known colorimetric
chemosensors are based on the synthetic receptors generally
consisting of two parts: binding site and signal-reporting
group (chromophore), either covalently attached or
intermolecularly linked. Generally, the binding sites are the
H-bond donor groups which in most cases contain the NH
fragment of the carboxyamides, sulfonamides, urea, thiourea
and pyrroles [4]. In the last years, indoles, as an important
class of hydrogen bond donor-based building blocks, have
been designed for a variety of anion receptors and sensors
due to the high binding affinity and selectivity for specific
anionic guests [5].
2. RESULTS AND DISCUSSION
The meso-phenyl bis(3-indolyl)methene derivatives were
successfully synthesized by treating the bis(3-indolyl)
methane precursors with DDQ in CH3CN, as shown in
Scheme 1. According to a literature method [7], the bis(3-
indolyl)methane compounds were prepared by the conden-
sation of indole derivatives with corresponding benzalde-
hyde in CH3OH.
The bis(3-indolyl)methene derivatives usually display the
characteristic absorption band at 424-438 nm regions, which
is assigned to the ꢁ-ꢁ* transition of the conjugated bisindole
skeleton and make the color of the solution yellow. As for
receptors 1a, 1c and 1e, the weak shoulder peaks at 500 nm
and 590 nm (Fig. S1), which disappeared when adding a
small quantity of polar protic solvent such as CH3OH or H2O
to the solution, can be presumably attributed to its
intermolecular hydrogen bond interaction.
Previously, we reported the bis(3-indolyl)methene
compound 1a that can be used as anion receptor for the
colorimetric detection of either Fꢀ and AcOꢀ in organic
aprotic solution or HSO4ꢀ in water-containing medium
through the reversible proton transfer signaling mode [6].
The conjugated bis(3-indolyl)methene skeleton can not only
act as a color-reporting group but also provide an acidic H-
bond donor moiety and a basic H-bond acceptor moiety for
The anion binding and sensing properties of receptors 1b
and 1c, containing a nitro group or methoxy group in the
meso-phenyl ring of bis(3-indolyl)methene skeleton, were
first studied by UV-vis spectroscopic measurements. Similar
to the case of receptor 1a [6], receptor 1b or 1c was found to
exhibit highly selective binding properties towards fluoride
*Address correspondence to this author at the 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, P. R. China; Tel: +86-931-
4968209; Fax: +86-931-8277088; E-mail: sjshao@licp.cas.cn
1570-1786/11 $58.00+.00
© 2011 Bentham Science Publishers Ltd.

Anion Binding and Sensing Properties
Letters in Organic Chemistry, 2011, Vol. 8, No. 1
61
R2
R2
R2
R1
R1
R1
R1
R1
KHSO4
DDQ
rt, CH3OH
N
H
HN
2a R1=H, R2=H
2b R1=H, R2=NO2
2c R1=H, R2=OCH3
2d R1=NO2, R2=H
2e R1=OCH3, R2=H
NH
HN
1a R1=H, R2=H
1b R1=H, R2=NO2
1c R1=H, R2=OCH3
1d R1=NO2, R2=H
1e R1=OCH3, R2=H
N
O
Scheme 1. The synthesis of compounds 1 and 2.
Fig. (1). Changes in UV–vis spectra of 1b (a) and 1c (b) (5ꢀ10-5 M) recorded in CH3CN after addition of 25 equiv of various anions (Fꢁ,
AcOꢁ, H2PO4ꢁ, HSO4ꢁ, ClO4ꢁ, Clꢁ, Brꢁ, and Iꢁ, as their Bu4N+ salts).
anions in CH3CN, as revealed by the marked Fꢁ-induced
changes in the absorption spectra (Fig. 1). The new
absorption band indicative of the deprotonated form of
receptor 1b or 1c appeared at 531 nm and 515 nm,
respectively, while the solution color all turned from yellow
to red. Careful evaluation of titration experiments carried out
by Fꢁ anion revealed the process from H-bond interaction to
deprotonation (Fig. S2, S3). Similar, but less remarkable
spectral changes could be also observed in the presence of a
large excess of AcOꢁ (Fig. S4, S5), ascribed to the
incomplete deprotonation of receptor 1b or 1c. As for
H2PO4ꢁ, only a slight red shift of the band at about 430 nm,
due to the formation of a hydrogen-bonded complex, was
observed. However, the other anions tested induced the
negligible responses. The above results showed that the
introduction of electron-withdrawing or electron-donating
group into the meso-phenyl of bis(3-indolyl)methene 1a has
no significant effect on its selectivity and affinity towards
anions in CH3CN (see Table 1).
488 nm, respectively (Fig. 2), with the effect that the
solution instantaneously changed color from yellow to pink.
Moreover, receptor 1c was found to display a similar, but
less remarkable spectral and color change upon addition of
H2PO4ꢁ, which indicated that the introduction of methoxy
group into meso-phenyl ring of bis(3-indolyl)methene
skeleton increased the basicity of the H-bond acceptor site of
receptor 1c to a certain extent.
As expected, the increased affinity and sensitivity
towards anions has been achieved by the introduction of
electron-withdrawing substituents into the indole ring of the
bis(3-indolyl)methene skeleton. Due to the increase in the
acidity of indole NH caused by the nitro group, receptor 1d
exhibited selective binding and colorimetric sensing towards
Fꢁ, AcOꢁ and H2PO4ꢁ over other anions tested, and AcOꢁ and
H2PO4ꢁ induced the same spectral and color changes as Fꢁ
did (Fig. 3a, S6). Compared with receptor 1a whose
equilibrium constant was determined in CH3CN, receptor 1d
gives much higher binding affinity for these anions (see
Table 1).
On the other hand, in mixed CH3CN/H2O (4:1, v/v)
solution, receptors 1b and 1c also showed the same sensing
properties for HSO4ꢁ over other anions tested as that of
receptor 1a [6], and the new absorption band pertained to the
protonated form of receptor 1b or 1c appeared at 517 nm and
As shown in (Fig. 3), upon addition of Fꢁ from 0 to 0.5
equiv, the absorption band at 438 nm of receptor 1d slightly
increased and shifted to 434 nm, which is assigned to the
formation of the initial H-bond complex. With further

62 Letters in Organic Chemistry, 2011, Vol. 8, No. 1
Wang et al.
Fig. (2). Changes in UV–vis spectra of 1b (a) and 1c (b) (5ꢀ10-5 M) recorded in CH3CN/H2O (4:1, v/v) after addition of 25 equiv of various
anions.
Fig. (3). Changes in UV–vis spectra of 1d recorded in CH3CN (5ꢀ10-5 M) after addition of (a) 25 equiv of various anions; (b) 0, 0.1, 0.2, 0.3,
0.4, 0.5 equiv of Fꢁ; (c) 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 2, 2.5, 3 equiv of Fꢁ.
increasing in Fꢁ concentration, the band at 434 nm decreased
gradually along with a new red-shifted band at 552 nm
evolving and reaching a maximum upon addition of 3 equiv
of Fꢁ (Fig. 3c), which is ascribed to the internal charge
transfer (ICT) process of the deprotonated bis(3-
indolyl)methene skeleton. The same deprotonation process
of receptor 1d was also induced by addition of over 0.5
decreased in water-containing system. Compared with that of
receptor 1a, a less remarkable spectral change was observed
upon addition of HSO4ꢁ to the CH3CN/H2O solution of
receptor 1d (Fig. S10), giving a faint color change from
yellow to baby pink.
In order to improve the protonation of bis(3-
indolyl)methene receptor in water-containing system, the
basicity of H-bond acceptor site can be increased. It has been
achieved by the introduction of electon-donating substituents
into bis(3-indolyl)methene skeleton. The methoxy-bearing
derivatives 1c and 1e displayed the increased affinity and
sensitivity for HSO4ꢁ and H2PO4ꢁ in CH3CN/H2O. Compared
with receptor 1c which showed a moderate sensing
sensitivity for H2PO4ꢁ (Fig. 2b), receptor 1e exhibited the
almost same spectral and color response for H2PO4ꢁ as that
for HSO4ꢁ (Fig. 4a), due to the distinctly increased basicity
of H-bond acceptor site caused by methoxy group in indole
ring. On stepwise addition of HSO4ꢁ, the clear spectral
evolution was observed (Fig. 4b), the absorption band at 434
nm decreased and a new strong absorption band at 498 nm,
due to the protonated receptor [H2L]+, increased gradually
and reached its limiting value after addition of about 5 equiv
of HSO4ꢁ. Moreover, the titration process of the HSO4ꢁ was
carried out in mixed CH3CN/H2O (1:1, v/v) solution (Fig.
equiv of AcO– or 1 equiv of H2PO4 anions (Fig. S7, S8).
–
Moreover, a slight decrease in intensity of the absorption
band at 438 nm of receptor 1d along with a slight red shift
was observed after adding an excess of Clꢁ (Fig. S9), which
corresponded to the formation of a hydrogen-bonded
complex as a result of the increased acidity of indole NH.
The above results show significant differences in anion
binding and colorimetric sensing properties between receptor
1d and receptor 1a or 1b. The electron-withdrawing nitro
group on indole ring can notably enhance the proton acidity
of the indole NH and promote the occurrence of the
deprotonation, therefore, the increased affinity and
sensitivity towards anions could be expected, but the anion
selectivity decreased. On the other hand, the introduction of
nitro group into indole unit can decrease the electron density
of indole N of the H-bond acceptor site, and in consequence,
the sensing sensitivity of receptor 1d towards H+ may be

Anion Binding and Sensing Properties
Letters in Organic Chemistry, 2011, Vol. 8, No. 1
63
Fig. (4). Changes in UV–vis spectra of 1e (5ꢁ10-5 M) after addition of: (a) 25 equiv of various anions in CH3CN/H2O (4:1, v/v); (b) 0-100
equiv of HSO4ꢀ in CH3CN/H2O (4:1, v/v); and (c) 0-10 equiv of HSO4ꢀ in CH3CN/H2O (1:1, v/v).
4c), the spectral changes reached its limiting value after
addition of about 1 equiv of HSO4ꢀ. Thus it can be seen that
the higher the water content, the more sensitivity, which is
related to the ionization degree of the HSO4ꢀ.
The equilibrium constants (where Ka and Kd are the
association constant and disassociation constant,
respectively) of the receptors 1a-1e with anions were
evaluated through nonlinear least squares fitting by origin
software according to 1:1 stoichiometry [9], and the results
are summarized in Table 1.
On the other hand, in CH3CN solution, receptor 1e
displayed selective recognition and colorimetric sensing of
Fꢀ over other anions tested (Fig. S11), which was completely
similar to the case of receptor 1a. The result indicated that
the introduction of electron-donating methoxy groups into
indole rings of bis(3-indolyl)methene 1a has no significant
effect on its selectivity and affinity towards anions in
CH3CN.
A further insight into Table 1 reveals that the highest
affinity of receptor 1d is displayed for Fꢀ, AcOꢀ, H2PO4ꢀ and
Clꢀ, due to the increase in the acidity of the indole NH
proton caused by the electron-withdrawing nitro group on
the indole moiety. In addition, the order of the affinity of
receptors 1a-1e towards anions is: Fꢀ > AcOꢀ > H2PO4ꢀ > Clꢀ
>> Brꢀ, Iꢀ, HSO4ꢀ, and ClO4ꢀ, which is consistent with their
basicity in CH3CN.
The deprotonation/protonation of the bis(3-indolyl)
methene receptors 1b-1e is also reversible, as described in
the case of receptor 1a [6]. The interaction of receptors 1a-
1e with anions in CH3CN involves the mixed process: initial
hydrogen-bonded complex formation at low anion-receptor
ratios, then followed by the deprotonation of the indole NH
proton in presence of an excess of Fꢀ or AcOꢀ. The two
processes can be described by the following equilibrium (1)
and (2) [4a, 8]:
3. EXPERIMENTAL SECTION
General
1H NMR spectra were recorded on a Bruker AV400
instrument at 400 MHz with TMS as an internal standard.
ESI-MS measurements were carried out using a Waters
ZQ4000 mass spectrometer. IR spectra were measured using
a Thermo Nicolet NEXUS TM spectrometer as KBr disks.
Melting points were detected on a PHMK 05 micro melting
point apparatus and are uncorrected. UV-vis spectra were
performed on a PerkinElmer Lamda 35 spectrophotometer
Ka
…
ꢀꢀꢀꢁ
LH + Xꢀ
[LH X]ꢀ
(1)
(2)
ꢂꢀꢀꢀ
Kd
…
ꢀꢀꢀꢁ
[LH X]ꢀ + Xꢀ
Lꢀ + [HX ]ꢀ
2
ꢂꢀꢀꢀ
Table 1. Equilibrium Constants [K(M-1)] for the Compounds 1a-1e with Various Anions in CH3CN at 25 ºC
Receptors
Anions ꢀ
1a
1b
1c
1d
1e
Fꢀ/Kd
AcOꢀ/Kd
H2PO4ꢀ/Ka
Clꢀ/Ka
3.40ꢁ103±180
7.56ꢁ102±29
5.47ꢁ102±24
3.04ꢁ103±296
3.80ꢁ103±567
(2.30±0.5)ꢁ104
2.81ꢁ103±347
1.75ꢁ103±68
5.65ꢁ102±18
(1.43±0.5)ꢁ104
6.14ꢁ102±25
6.86ꢁ102±46
4.28ꢁ102±26
(8.44±1.5)ꢁ103 b
5.31ꢁ102±17
a
-
-
-
-
-
-
-
-
-
-
-
5.70ꢁ102±26
-
-
-
-
-
Brꢀ/Ka
-
-
-
-
-
-
-
-
Iꢀ/Ka
HSO4ꢀ/Ka
ClO4ꢀ/Ka
athe spectral change is little and the affinity constant cannot be determined. b The dissociation constant Kd.

64
Letters in Organic Chemistry, 2011, Vol. 8, No. 1
Wang et al.
(1-cm quartz cell) at room temperature. The tetra-n-
butylammonium fluoride was purchased from Fluka, the
other tetra-n-butylammonium (Bu4N+) salts of different
anions were purchased from Alfa Aesar. CH3CN was used
the chromatographically pure.
1H), 6.81(d, 2H), 6.72(d, 2H), 6.68(dd, 2H), 5.73(s, 1H),
3.58(s, 6H). MS(ESI): m/z 381.4 ([M-H]-).
Experimental Procedure for Compounds 1a-1e
Typical to 1a. Compound 2a (0.17 g, 0.5 mmol) was
dissolved in acetonitrile (8 mL), DDQ (0.14 g, 0.6 mmol)
solution of acetonitrile was dropwise and slowly added to the
solution. This reaction was allowed for 30 min and given the
dark red precipitate, which was filtered, washed with
CH3CN, and recrystallized from mixed ethanol/water
solution. Similarly, the compounds 1b, 1c and 1e were
synthesized following the above procedure, as for the
compound 1d was prepared in acetone solvent.
Experimental Procedure for compounds 2a-2e
Typical to 2a. KHSO4 (0.17g, 1.25 mmol) was added to
a mixture of indole (0.30g, 2.5 mmol) and benzaldehyde
(0.13g, 1.25 mmol) in dry methanol (10 mL), and the
reaction was stirred for 2 h at room temperature. Then water
(10 mL) was added to quench the reaction, and the aqueous
phase was extracted with CH2Cl2 (3ꢀ10 mL). The organic
phase was dried with anhydrous MgSO4, and purified by
column chromatography and eluted with ethyl acetate and
petroleum ether mixture to afford the product 2a (white
solid). Similarly, the compounds 2b, 2c and 2e were
synthesized following the above procedure. As for the
compound 2d was prepared under inert atmosphere refluxing
for 12h, then the reaction mixture was cooled to room
temperature. Precipitate formed was filtered and washed
with CH3OH, then recrystallization with acetone/H2O.
Compound 1a
Yield=37%. Mp: 242-243 ºC. IR (KBr) 3384, 3108,
3058, 2792, 2546, 2201, 1956, 1583, 1520, 1481, 1409,
1240, 1176, 1122, 996, 833, 747. 1H NMR (400MHz,
DMSO–d6), (ppm): 8.33 (s, 2H), 7.74 (t, 1H), 7.60 (m, 6H),
7.27 (t, 2H), 7.00 (t, 2H), 6.65 (d, 2H). HRMS (ESI) m/z
([M+H]+) calcd for C23H16N2: 321.1386; found, 321.1383.
Compound 1b
Compound 2a
Yield=25%. Mp > 300 ºC. IR (KBr) 3384, 3236, 3110,
3069, 2974, 2925, 2593, 2201, 1942, 1582, 1523, 1478,
1408, 1347, 1239, 1172, 1116, 998, 823, 760. 1H NMR
(400MHz, DMSO–d6), (ppm): 10.92 (s, 1H), 8.43 (d, 2H),
8.31 (d, 2H), 7.82 (d, 2H), 7.60 (d,2H), 7.28 (t, 2H), 7.00 (t,
2H), 6.67 (d, 2H). MS(ESI): m/z 366.2 ([M+H]+).
Yield=90%. Mp: 128-130 ºC. IR (KBr) 3407, 3054,
1597, 1458, 1417, 1331, 1213, 1088, 1009, 743 cm–1. H
1
NMR (400MHz, DMSO–d6), (ppm): 10.82 (s, 2H), 7.35 (t,
4H), 7.26 (t, 4H), 7.16 (t, 1H), 7.03 (t, 2H), 6.85 (t, 2H), 6.82
(s, 2H), 5.83 (s, 1H, Ar–CH). MS(ESI): m/z 340.1
([M+NH4]+).
Compound 1c
Compound 2b
Yield=45%. Mp: 232-234 ºC. IR (KBr) 3416, 3110,
2540, 2199, 1604, 1508, 1479, 1408, 1370, 1173, 1122,
1025, 996, 891, 810, 753. H NMR (400MHz, DMSO–d6),
(ppm): 8.27 (s, 2H), 7.61 (d, 2H), 7.55 (d, 2H), 7.28 (t, 2H),
7.18 (d, 2H), 7.06 (t, 2H), 6.80 (d, 2H), 3.92 (s, 3H).
MS(ESI): m/z 351.3 ([M+H]+).
Yield=88%. Mp: 232–233 ºC. IR (KBr) 3462, 3423,
3383, 3054, 1589, 1503, 1448, 1338, 1221, 1096, 1009, 743
cm–1. 1H NMR (400MHz, DMSO–d6), (ppm): 10.92 (s, 2H),
8.15 (d, 2H), 7.61 (d, 2H), 7.37(d, 2H), 7.29 (d, 2H), 7.06 (t,
2H), 6.90 (s, 2H), 6.89 (t, 2H), 6.03 (s, 1H, Ar–CH).
MS(ESI): m/z 390.2 ([M+Na]+).
1
Compound 1d
Compound 2c
Yield=35%. Mp > 300 ºC. IR (KBr) 3097, 3049, 2552,
2205, 1966, 1623, 1588, 1536, 1475, 1444, 1398, 1336,
1272, 1241, 1174, 1105, 994, 888, 829, 760. 1H NMR
(400MHz, DMSO–d6), (ppm): 8.34(d,1H), 7.98(d,2H),
7.56(d,2H), 7.47(d,2H), 7.37(t,2H), 7.29(t,2H), 6.94(s,2H).
HRMS (ESI) m/z ([M+H]+) calcd for C23H14N4O4: 411.1088;
found, 411.1081.
Yield=40%. Mp: 187-189 ºC.IR (KBr) 3394, 3054, 2946,
2831, 1610, 1509, 1454, 1418, 1335, 1245, 1171, 1093,
1009, 784, 741 cm–1 1H NMR (400MHz, CDCl3), (ppm):
7.88 (brs, 2H), 7.35 (m, 4H), 7.24 (s, 2H), 7.15 (t, 2H), 6.99
(t, 2H), 6.80 (d, 2H), 6.63 (d, 2H), 5.82 (s, 1H), 3.76 (s, 3H).
MS(ESI): m/z 352.1 ([M+H]+).
Compound 1e
Compound 2d
Yield=32%. Mp: 239-241 ºC. IR (KBr) 3423, 3104,
3049, 2986, 2929, 2827, 2546, 2198, 1968, 1591, 1525,
1459, 1404, 1365, 1334, 1285, 1223, 1181, 1112, 1031, 993,
Yield=65%. Mp: >300 ºC IR (KBr) 3303, 3026, 2856,
1625, 1509, 1476, 1319, 1091, 740 cm–1. 1H NMR
(400MHz, DMSO–d6), (ppm): 11.66 (s, 2H), 8.30 (d, 2H),
7.95 (dd, 2H), 7.53 (d, 2H), 7.39 (d, 1H), 7.31 (t, 2H), 7.21
(t, 2H), 7.12 (s, 2H), 6.19 (s, 1H, Ar–CH). HRMS (ESI) m/z
([M+NH4]+) calcd for C23H16N4O4: 430.1510; found,
430.1509.
1
835, 754. H NMR (400MHz, DMSO–d6), (ppm): 8.34(s,
2H), 7.77(t, 1H), 7.66(t, 2H), 7.58(d, 2H), 7.52(d, 2H),
6.92(dd, 2H), 6.06(s, 2H), 3.41(s, 6H). HRMS (ESI) m/z
([M+H]+) calcd for C25H20N2O2: 381.1598; found, 381.1594.
Compound 2e
CONCLUSION
Yield=89%. Mp: 215-217 ºC. IR (KBr) 3389, 3310,
2934, 2824, 1623, 1577, 1482, 1443, 1280, 1208, 1168,
1019, 925, 807, 721. 1H NMR (400MHz, DMSO–d6), (ppm):
10.66(s, 2H), 7.35(d, 2H), 7.27(d, 2H), 7.22(d, 2H), 7.16(t,
In this work, several of nitro- or methoxy-substituted
bis(3-indolyl)methene derivatives were synthesized, and the
derivatives show significant differences in anion binding and

Anion Binding and Sensing Properties
Letters in Organic Chemistry, 2011, Vol. 8, No. 1
65
S. Recognition of anions by bis urea based fluorescent receptors.
Lett. Org. Chem., 2010, 7, 399; (c) You, J.-M.; Jeong, H.; Seo, H.;
colorimetric sensing properties. The electron-withdrawing or
electron-donating group at the para position of meso-phenyl
ring have no significant effect on its selectivity and affinity
towards anions in CH3CN. However, the presence of
electron-withdrawing nitro groups in indole rings of the
bis(3-indolyl)methene skeleton can notably enhance the
anion binding affinity and sensing sensitivity due to the
increased acidity of the H-bond donor moiety, and the nitro-
bearing receptor displays indistinguishable spectra and color
response towards Fꢀ, AcOꢀ and H2PO4ꢀ in CH3CN.
Moreover, the introduction of electron-donating methoxy
groups into bis(3-indolyl)methene skeleton can increase the
basicity of H-bond acceptor moiety and enhanced the
sensitivity for H+ in water-containing system. The anion-
induced deprotonation/protonation of the H-bond
donor/acceptor moiety modulates the internal charge transfer
state of the bis(3-indolyl)methene skeleton and gives rise to
the dramatic color and spectral changes.
Jeon, S.
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ACKNOWLEDGEMENT
This work was supported by the National Natural Science
Foundation of China (Grant No. 20672121).
SUPPLEMENTARY MATERIAL
Supplementary material is available on the publishers
Web site along with the published article.
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