Nitro-substituted 3,3′-bis(indolyl)methane derivatives as anion receptors: Electron-withdrawing effect and tunability of anion binding properties

Article abstract of DOI:10.1016/j.saa.2010.12.004

A series of nitro-substituted 3,3′-bis-indolyl phenylmethane derivatives were synthesized and their anion binding properties were investigated in detail. The introduction of the electron-withdrawing nitro group into indole unit and/or meso-phenyl ring, which leads to the increased acidity of indole NH and meso-position CH proton, has a positive effect on anion binding. The nitro-substituted bis(indolyl)methane receptors exhibited selective colorimetric sensing of F^- anion, as revealed by the notable color and spectral changes, rationally due to the deprotonation of the indole NH of the receptor. Meanwhile, the additive introduction of the nitro substituents on the meso-phenyl ring of bis(indolyl)methane can lead to the deprotonation of the meso-position CH and further induce an irreversible oxidation process obtaining bis(indolyl)methene product in the F^- anion sensing system.

Full text of DOI:10.1016/j.saa.2010.12.004

Spectrochimica Acta Part A 78 (2011) 726–731
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Nitro-substituted 3,3ꢀ-bis(indolyl)methane derivatives as anion receptors:
Electron-withdrawing effect and tunability of anion binding properties
Litao Wanga,c, Wei Weia,c, Yong Guoa, Jian Xua, Shijun Shaoa,b,∗
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 State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
c 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
A series of nitro-substituted 3,3ꢀ-bis-indolyl phenylmethane derivatives were synthesized and their anion
binding properties were investigated in detail. The introduction of the electron-withdrawing nitro group
into indole unit and/or meso-phenyl ring, which leads to the increased acidity of indole NH and meso-
position CH proton, has a positive effect on anion binding. The nitro-substituted bis(indolyl)methane
receptors exhibited selective colorimetric sensing of F− anion, as revealed by the notable color and spec-
tral changes, rationally due to the deprotonation of the indole NH of the receptor. Meanwhile, the additive
introduction of the nitro substituents on the meso-phenyl ring of bis(indolyl)methane can lead to the
deprotonation of the meso-position CH and further induce an irreversible oxidation process obtaining
bis(indolyl)methene product in the F− anion sensing system.
Article history:
Received 10 September 2010
Received in revised form
23 November 2010
Accepted 2 December 2010
Keywords:
Anion receptor
Indolylmethane
Nitro-substitution
Deprotonation
Oxidation process
© 2010 Elsevier B.V. All rights reserved.
studied by Ito and coworkers using 1H NMR titration techniques
[38]. Generally, under the spectrophotometric titration concen-
1. Introduction
tration condition, the simple 3,3ꢀ-bis(indolyl)methane receptors
The binding and detection of anionic substrates by artificial
receptors is currently an area of significant interest because they
play a fundamental and important role in chemical and biologi-
cal processes [1–5]. The neutral anion receptors containing urea
[6–10], thiourea [11–14], amide [15–17], phenol [18–20], pyrrole
moieties [21–23] and so on, which can provide one or more hydro-
gen bond donor sites for selective binding anions, have been widely
reported. In the last years, indoles, as an important class of building
blocks based on hydrogen bond donor, have been integrated into a
variety of anion receptor and sensor frameworks as either function-
alized indoles, biindoles or indolocarbazoles [24–33], which exhibit
high binding affinities and selectivities for specific anionic guests
[34–36].
The conjugated bis(indolyl)methene receptor for the co−lori-
metric detection of either F− in aprotic solvent or HSO4 in
water-containing medium was previously used in our lab [37],
and the reversible deprotonation/protonation of receptor induced
the changes in color and absorption spectra. Practically, the pre-
cursor bis(indolyl)methanes, containing 2- and 3-linked indole
groups, can also act as molecular receptors for selective bind-
ing anions through hydrogen bond interactions, which have been
were shown to bind anions, including more basic F−, AcO−,
−
H2PO4 anions with very weak responses in the absorption spec-
tra, which indicated the lower affinity of the indole NH as the
hydrogen bond donor for anions. Thus improving anion bind-
ing and sensing ability of bis(indolyl)methane-based receptor is
strongly desired. A simple and familiar approach is to introduce
the electron-withdrawing group into the 3,3ꢀ-bis(indolyl)methane
skeleton, which leads to an increased acidity of indole NH or
meso-position CH. As a result, we found that the introduction
of the nitro group has a positive effect on anion binding behav-
ior of 3,3ꢀ-bis(indolyl)methane, and the nitro derivatives were
found to exhibit selective colorimetric sensing of F− anion, due
to deprotonation of the indole NH and/or meso-position CH,
as well as the formation of bis(indolyl)thene product. Herein,
we would like to present our studies on the anion binding
properties of the nitro-substituted 3,3ꢀ-bis(indolyl)methane recep-
tors.
2. Experimental
2.1. Materials and methods
∗
All reagents for synthesis were obtained commercially and used
without further purification. The tetra-n-butylammonium fluo-
Corresponding author. Tel.: +86 931 4968209; fax: +86 931 8277088.
E-mail address: sjshao@licp.cas.cn (S. Shao).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.12.004

L. Wang et al. / Spectrochimica Acta Part A 78 (2011) 726–731
727
Scheme 1. Synthesis and molecular structures of compounds 1–5.
ride was purchased from Fluka, the other tetra-n-butylammonium
(Bu4N+) salts of different anions were purchased from Alfa Aesar.
CH3CN use the chromatographically pure. 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 mea-
sured 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 (1 cm quartz cell) at
room temperature.
(400 MHz, DMSO-d6), (ppm): 10.82 (s, 2H), 7.35 (dd, 4H, J = 5.6, 7.6),
7.26 (td, 4H, J = 8, 8), 7.16 (t, 1H, J = 7.2), 7.03 (t, 2H, J = 7.2), 6.85 (t,
2H, J = 7.2), 6.82 (s, 2H), 5.83 (s, 1H, Ar–CH). MS(ESI): m/z 340.1
([M + NH4]+).
2.2.1.2. Compound 1b. Yield = 88%. mp: 232–233 ◦C. IR (KBr) 3462,
3423, 3383, 3054, 1589, 1503, 1448, 1338, 1221, 1096, 1009,
743 cm−1 1H NMR (400 MHz, DMSO-d6), (ppm): 10.92 (s, 2H), 8.15
.
(d, 2H, J = 8.8), 7.61 (d, 2H, J = 8.8), 7.37(d, 2H, J = 8.4), 7.29 (d, 2H,
J = 8), 7.06 (t, 2H, J = 7.2), 6.90 (s, 2H), 6.89 (t, 2H, J = 7.6), 6.03 (s, 1H,
Ar–CH). MS(ESI): m/z 390.2 ([M + Na]+).
2.2. General experimental procedure for the synthesis of the
receptors 1–5
2.2.1.3. Compound 1c. Yield = 65%. mp: >300 ◦C. IR (KBr) 3303,
3026, 2856, 1625, 1509, 1476, 1319, 1091, 740 cm−1 1H NMR
.
(400 MHz, DMSO-d6), (ppm): 11.66 (s, 2H), 8.30 (d, 2H, J = 2.4), 7.95
(dd, 2H, J = 2.4, 2), 7.53 (d, 2H, J = 9.2), 7.39 (d, 1H, J = 7.2), 7.31 (t,
2H, J = 7.6), 7.21 (t, 2H, J = 7.6), 7.12 (s, 2H), 6.19 (s, 1H, Ar–CH).
13C NMR ı: 143.80, 140.25, 139.85, 128.54, 127.66, 126.48, 125.80,
120.56, 116.71, 112.15, 38.51. HRMS (ESI) m/z ([M + NH4]+) calcd.
for C23H16N4O4: 430.1510; found, 430.1509.
2.2.1. Synthesis of the 3,3ꢀ-bis(indolyl)methane compounds
1a–1f
Typical to 1a, compounds 1a were prepared according to a lit-
erature method [39–41]. KHSO4 (0.17 g, 1.25 mmol) was added
to a mixture of indole (0.30 g, 2.5 mmol) and benzaldehyde
(0.13 g, 1.25 mmol) in dry methanol (10 mL), and the reaction
was stirred at room temperature for 2 h. Then water (10 mL)
was added to quench the reaction, and the aqueous phase was
extracted with CH2Cl2 (3 × 10 mL). The organic phase was dried
with anhydrous MgSO4, and purified by column chromatogra-
phy and eluted with ethyl acetate and petroleum ether mixture
to afford the product (white solid). Similarly, the compounds
1b were synthesized following the above procedure [40]. The
compounds 1c–1f were prepared under inert atmosphere reflux-
ing for 12 h, and then the reaction mixture was cooled to
room temperature. Precipitate formed was filtered and washed
with CH3OH, then recrystallization with acetone/H2O. Moreover,
the compound 1e was obtained by the silica column chro-
matograph (EtOAc/petroleum ether (bp 60–90 ◦C), 2:1 v/v) (see
Scheme 1).
2.2.1.4. Compound 1d. Yield = 45%. mp: 287–289 ◦C. IR (KBr) 3292,
3101, 2846, 1629, 1513, 1480, 1327, 1092, 740 cm−1 1H NMR
.
(400 MHz, DMSO-d6), (ppm): 11.78(s, 2H), 8.38 (d, 2H, J = 2), 8.20
(d, 2H, J = 8.8), 7.99 (dd, 2H, J = 2, 2.4), 7.66 (d, 2H, J = 8.8), 7.56 (d,
2H, J = 8.8), 7.22 (s, 2H), 6.48 (s, 1H, Ar–CH). MS(ESI): m/z 456.5
([M–H]−).
2.2.1.5. Compound 1e. Yield = 62%. mp: 191–192 ◦C. IR (KBr) 3403,
3093, 1629, 1522, 1475, 1335, 1091, 736 cm−1 1H NMR (400 MHz,
.
DMSO-d6), (ppm): 11.84 (s, 2H), 8.80 (d, 1H, J = 2.4), 8.41 (dd, 1H,
J = 2.4, 2.4), 8.34 (d, 2H, J = 2), 8.00 (dd, 2H, J = 2.4, 2.4), 7.57 (d,
3H, J = 9.2), 7.19 (s, 2H), 6.78 (s, 1H, Ar–CH). 13C NMR ␦: 148.77,
146.36, 143.40, 140.55, 139.82, 139.65, 131.98, 128.60, 128.43,
127.36, 125.38, 120.14, 117.54, 116.99, 115.88, 112.32, 34.18. HRMS
(ESI) m/z ([M + NH4]+) calcd. for C23H14N6O8: 520.1211; found,
520.1216.
2.2.1.1. Compound 1a. Yield = 90%. mp: 128–130 ◦C. IR (KBr) 3407,
3054, 1597, 1458, 1417, 1331, 1213, 1088, 1009, 743 cm−1 1H NMR
.

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L. Wang et al. / Spectrochimica Acta Part A 78 (2011) 726–731
2.2.1.6. Compound 1f. Yield = 42%. mp: 292–293 ◦C. IR (KBr) 3326,
2988, 2949, 1623, 1513, 1466, 1318, 1066, 744 cm−1 1H NMR
.
(400 MHz, DMSO-d6), (ppm): 11.73(s, 2H), 7.93 (dd, 2H, J = 2.4, 2),
7.89 (s, 2H), 7.56 (d, 2H, J = 9.2), 7.33 (d, 4H, J = 4.4), 7.26 (m, 1H),
7.18 (s, 2H), 2.31(s, 3H). 13C NMR ı: 146.92, 140.29, 139.87, 128.09,
127.42, 126.32, 125.10, 124.85, 117.33, 116.27, 112.30, 42.99, 29.81.
HRMS (ESI) m/z ([M + Na]+) calcd. for C24H18N4O4: 449.1220; found,
449.1213.
2.2.2. Synthesis of the 3,3ꢀ-bis(indolyl)methane compounds
4a–4c
Compound 3 was synthesized according to the literature
method [42,43], which was used directly in next step.
Typical to 4a, N-(n-butyl)-5-nitroindole (0.44 g, 2 mmol) was
dissolved in 10 mL CH3OH. To this solution was added phenzalde-
hyde (0.10 g, 1 mmol), then KHSO4 (0.14 g, 1 mmol) was added and
stirred under inert atmosphere refluxing for 12 h. Then the reac-
tion mixture was cooled to room temperature. Precipitate formed
was filtered and washed with CH3OH, then recrystallized from
acetone/H2O and gained 0.26 g. Similarly, the compounds 4b were
synthesized following the above procedure. The compound 4c was
obtained by the silica column chromatograph (EtOAc–petroleum
ether (bp 60–90 ◦C), 1:3, v/v) (see Scheme 1).
2.2.2.1. Compound 4a. Yield = 49%. mp: 148–150 ◦C. IR (KBr) 3406,
3120, 3061, 2951, 2929, 2871, 1608, 1579, 1512, 1483, 1446, 1402,
1329, 1102, 1058, 896, 742 cm−1 1H NMR (400 MHz, DMSO-d6),
.
(ppm): 8.31 (s, 2H), 8.00 (dd, 2H, J = 2.4, 2.4), 7.67 (d, 2H, J = 9.2),
7.42 (d, 2H, J = 7.2), 7.35 (t, 2H, J = 7.6), 7.25 (t, 1H, J = 7.2), 7.19 (s,
2H), 6.21 (s, 1H, Ar-CH), 4.20 (m, 4H), 1.68 (m, 4H), 1.17 (m, 4H), 0.82
(t, 6H, J = 7.2). 13C NMR ı: 143.29, 140.13, 139.24, 130.82, 128.50,
128.15, 126.48, 125.98, 119.90, 116.52, 110.54, 45.59, 38.49, 31.82,
19.32, 13.39. HRMS (ESI) m/z ([M + NH4]+) calcd. for C31H32N4O4:
542.2762; found, 542.2758.
2.2.2.2. Compound 4b. Yield = 55%. mp: 181–182 ◦C. IR (KBr) 3435,
Fig. 1. Changes in UV–vis spectra of 1c recorded in CH3CN (5 × 10−5 M) after addi-
tion of (a) 100 equiv of various anions and (b) 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 25, 30,
3106, 3066, 2957, 2917, 2863, 1615, 1513, 1482, 1450, 1324, 1191,
40, 60, 80 and 100 equiv of F−
.
1098, 1058, 902, 807, 736 cm−1 1H NMR (400 MHz, DMSO-d6),
.
(ppm): 8.34 (d, 2H, J = 2), 8.22 (d, 2H, J = 8.8), 8.00 (dd, 2H, J = 2.4,
2.4), 7.69 (d, 2H, J = 9.2), 7.65 (d, 2H, J = 8.8), 7.25 (s, 2H), 6.46 (s,
1H, Ar–CH), 4.19 (m, 4H), 1.68 (m, 4H), 1.15 (m, 4H), 0.80 (t, 6H,
J = 7.2). 13C NMR ı: 151.28, 146.20, 140.28, 139.21, 131.17, 129.40,
125.80, 123.84, 118.64, 116.69, 116.31, 110.67, 45.66, 38.02, 31.79,
19.31, 13.38. HRMS (ESI) m/z ([M + NH4]+) calcd. for C31H31N5O6:
587.2613; found, 587.2609.
7.04 (s, 2H). 13C NMR ı: 179.98, 172.00, 150.64, 149.26, 146.82,
144.03, 140.47, 139.94, 136.67, 133.59, 131.51, 129.03, 128.80,
124.93, 121.90, 119.24, 117.90, 116.78, 113.59, 112.31, 101.65.
HRMS (ESI) m/z ([M + H]+) calcd. for C23H12N6O8: 501.0789; found,
501.0797.
3. Results and discussion
2.2.2.3. Compound 4c. Yield = 60%. mp: 172–174 ◦C. IR (KBr) 3439,
3097, 2962, 2930, 2874, 1600, 1509, 1330, 1102, 739 cm−1
.
1H NMR
The anion binding and sensing properties of these
bis(indolyl)methanes have been studied by using UV–vis spec-
(400 MHz, DMSO-d6), (ppm): 8.81 (d, 1H, J = 2.4), 8.46 (dd, 1H, J = 2.4,
2.4), 8.30 (d, 2H, J = 2), 8.01 (dd, 2H, J = 2, 2.4), 7.70 (d, 2H, J = 8.8),
7.57 (d, 1H, J = 8.8), 7.26 (s, 2H), 6.79 (s, 1H, Ar–CH), 4.19 (m, 4H),
1.68 (m, 4H), 1.15 (m, 4H), 0.80 (t, 6H, J = 7.2). 13C NMR ı: 148.88,
146.49, 143.02, 140.52, 139.36, 131.99, 127.49, 125.67, 120.24,
116.88, 110.86, 45.74, 34.39, 31.76, 19.29, 13.42. HRMS (ESI) m/z
([M + NH4]+) calcd. for C31H30N6O8: 632.2463; found, 632.2459.
troscopic techniques. As above-mentioned, in the presence of
100 equiv of various anions tested (F−, AcO−, H2PO4–, HSO4
,
–
ClO4–, Cl–, Br–, and I–, as their tetrabutylammonium salts) in
CH3CN, the simple model compound 1a showed negligible
responses in the absorption spectra. Similar results were also
observed in the case of compound 1b (see Fig. S1), which indicates
that only introduction of a nitro group into the meso-phenyl ring
of the 3,3ꢀ-bis(indolyl)methane 1a failed to improve the affinity of
indole NH as hydrogen bond donor sites for anions.
2.2.3. Synthesis of the 3,3ꢀ-bis(indolyl)methene compound 5
Compound 1e (0.25 g, 0.5 mmol) was dissolved in acetone
(10 mL), a solution of DDQ (0.11 g, 0.5 mmol) in acetone was drop-
wise and slowly added to the solution. This reaction was allowed
for 3 h and gave a dark red precipitate, which was filtered, washed
with acetone and gained 0.12 g.
Differently, the introduction of the electron-withdrawing nitro
group into indole unit of the 3,3ꢀ-bis(indolyl)methane has a posi-
tive effect on anion binding. The UV–vis spectrum of receptor 1c
in CH3CN (5 × 10−5 M) exhibits two strong absorption bands at
270 nm and 325 nm, which are assigned to ␲–␲* transitions of the
nitroindole moiety. The results of titration experiments carried out
by using the above-mentioned set of anions, as shown in Fig. 1a,
indicated that only F− anion gave rise to a remarkable spectral
Yield = 48%. mp: (>300 ◦C). IR (KBr): 3416, 3098, 2217, 1534,
1439, 1395, 1343, 1270, 1189, 1108, 1050, 830, 734 cm−1 1H NMR
.
(400 MHz, DMSO-d6) ı: 11.82 (s, 1H, NH), 8.63 (s, 1H), 8.37 (d, 2H,
J = 2), 8.31 (dd, 1H, J = 2, 2), 8.01 (dd, 2H, J = 2, 2), 7.59 (d, 3H, J = 8.8),

L. Wang et al. / Spectrochimica Acta Part A 78 (2011) 726–731
729
Fig. 2. Partial 1H NMR titration spectra of 1c (10−2 M) with F− from 0 to 10 equiv
and F− alone (bottom) (DMSO-d6 400 MHz).
Fig. 3. Spectral changes of the interaction system of 1e (5 × 10−5 M) and 100 equiv
of F− during 20–120 min. Inset: the spectral intensity changes observed at 410 nm
and 564 nm.
change with the effect that the solution instantaneously changed
color from colorless to orange-yellow (Fig. S2), which allow the
potential for “naked eye” detection. As for AcO− and H2PO4−, only
slight bathochromic shifts of the band at 325 nm were observed as
a result of the formation of hydrogen-bonding complex, and the
other anions tested induced the negligible responses. On stepwise
addition of F− anion to the solution of 1c, the clear spectral evo-
lution was observed (Fig. 1b). With increasing concentration of F−,
the absorption band at 270 nm decreased and the absorption band
at 325 nm increased gradually along with a slight blue shift, at the
same time, a new strong absorption band at 410 nm and less strong
shoulder peak at 500 nm continuously increases in intensity, which
was responsible for the solution color change.
Titration with the strong base [Me4N]OH (tetramethylammo-
nium hydroxide), which definitely leads to deprotonation, also
induced the same color and spectral changes of 1c as those observed
with F− anions (Fig. S3). The new absorption band that develops at
410 nm pertains to the negative charge delocalization of the depro-
tonated nitroindole moiety, which was also further confirmed by
Bronsted acid-base reaction of compound 2 (5-nitroindole) with
[Me4N]OH (Fig. S4), whereas, no obvious shoulder peak at 500 nm
was observed in the case of compound 2. On the other hand, the
interaction of compound 1f and F− showed a completely simi-
lar color and spectral changes process to the case of 1c (Fig. S5).
The control experiment result suggests that the formation of the
absorption between 450 and 600 nm region does not result from
either the deprotonation of the meso-position CH of 1c or further
partial oxidation of 1c to respective bis(indolyl)methene.
the presence of an excess of fluoride anion, the above two processes
can be shown in Scheme 2. Further experiment indicated that the
F−-induced deprotonation process of 1c is fully reversible, as indi-
cated by the fact that on progressive addition of water (Fig. S6), the
orange-yellow CH3CN solution turns colorless and the absorption
spectra almost exactly coincided with the spectrum of receptor 1c.
Next, the anion binding and sensing properties of nitro-
substituted 3,3ꢀ-bis(indolyl)methane derivatives 1d and 1e, bearing
mono- or di-nitro substituents on the meso-phenyl ring, were stud-
ied. The receptors also exhibited selective binding towards F− over
other anions tested in CH3CN and DMSO. The spectral and color
changes from colorless to orange-yellow are completely similar to
the case of receptor 1c (Figs. S7 and S8), meanwhile, the processes
can be fully reversed by addition of water. Nevertheless, it is note-
worthy that the sensing system of 1d or 1e is unstable, as revealed
by the succeeding changes in color and absorption spectra. Fig. 3
showed dynamic spectral changes of the sensing system of 1e with
the time, the absorption band at 410 nm and the shoulder peak
at 500 nm began to decrease after standing about 25 min, while a
new absorption band appeared at 564 nm along with the increase in
intensity, and this process reached the stable state within 120 min,
with the effect that the solution changed color from orange-yellow
to purple-red. A similar, but slower process could also be observed
in the sensing system of 1d.
The reversibility of the process was examined by adding an
excess of water to the system of 1e, in result, the absorption
spectrum cannot revert to the original spectrum of 1e (Fig. 4). Inter-
estingly, the formed spectral profile with a strong absorption band
at 455 nm mainly coincided with that of the bis(indolyl)methene
compound 5, an oxidation product of 1e. Control experiment
showed that addition of F− to compound 5 in CH3CN caused the
same absorption band at 564 nm, due to the deprotonation of 5,
and the deprotonation process is also fully reversible. The results
indicated that, in the sensing system of 1e, a further irreversible
oxidation process occurred, which resulted in the formation of cor-
responding bis(indolyl)methene product.
The interaction of receptor 1c with F− was carried out by using
1H NMR titration experiments in DMSO-d6, and the results of the
stepwise addition of F− anion to a solution of receptor 1c were
shown in Fig. 2. The signal of indole NH proton, which in the free
receptor appears at ı 11.64 ppm, disappeared on addition of 1 equiv
ofF−. The signalof [HF2]− at 16.1 ppmwas obtained afteraddition of
10 equiv of F−, which was consistent with the other group reported
[44–47]. The observations reveal that F− anion induced the depro-
tonation of the indole NH proton, which increases the electron
density of the indole unit, thus causing a shielding effect. Corre-
spondingly, the proton signals of indole ring and meso-position CH
underwent slight upfield shifts with increasing the concentration
of F−. However, the NMR titration experiments mainly reveal the
interaction of the host/guest under the high concentration condi-
tion, and merely act as aids in proof to explain the phenomenon of
the UV–vis spectra [12].
As control experiments, the interaction of the compound 4a–c
with various anions was also studied by using UV–vis spectroscopic
techniques. Addition of 100 equiv of the above-mentioned set of
anions to the solution of 4a–c (5 × 10−5 M) in CH3CN, compound
4a could not give any response in absorption spectra and com-
pound 4b has a low energy spectra change at 500 nm only to F−
(Fig. S9). Nevertheless, compound 4c showed the appearance of a
new and strong absorption band at 500 nm in the presence of F−
(Fig. 5), which should be assigned to n–␲* electron transitions of
3,3ꢀ-bis(indolyl)methane skeleton as a result of the formation of the
Above results suggested that the interaction between receptor
1c and F− anion involves mixed processes: initial hydrogen-bonded
complex formation at low anion-to-receptor ratios, this then being
followed by the deprotonation of the indole NH of receptor 1c in

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L. Wang et al. / Spectrochimica Acta Part A 78 (2011) 726–731
Scheme 2. The probable deprotonation process of receptor 1c with F−
.
1/(A − A0) against 1/[F−]2 shows a liner relationship (Fig. S10), indi-
cating that the stoichiometric ratio of the host/guest is 1:2, which
confirmed the speculation of Scheme 2. The equilibrium constants
of receptors with F− were calculated to be 4.85 × 105 M−2 for
1c, 9.94 × 105 M−2 for 1d, 1.32 × 106 M−2 for 1e, respectively. The
results shows that the introduction of nitro group into meso-phenyl
enhance the affinity of the receptors for F−.
4. Conclusions
We herein described the anions binding properties of a series
of nitro-substituted 3,3ꢀ-bis(indolyl)methane derivatives. By intro-
ducing the electron-withdrawing nitro group into the indole units
and meso-phenyl ring of 3,3ꢀ-bis(indolyl)methane skeleton that
recognizes anions through hydrogen bonding, we have realized
a complete selectivity in colorimetric sensing of F− anion in
comparison with other anions tested. The sensing selectivity is
mainly attributed to the deprotonation of the indole NH units. The
nitro substituents on the meso-phenyl ring of bis(indolyl)methane
enhanced the meso-position CH proton acidity of the receptor,
which maybe induced an irreversible oxidation process obtaining
bis(indolyl)methene product in the F− anion sensing system.
Fig. 4. UV–vis spectra of 1e and 5 recorded in CH3CN (5 × 10−5 M) induced by addi-
tion of F− and 0.5 mL H2O.
meso-position carbanion induced by F−, and the solution instanta-
neously changed color from colorless to purple-red. Compared to
compounds 4a and 4b, the acidity of the meso-position CH proton of
compound 4c, with dinitro substituents on the meso-phenyl ring, is
markedly enhanced and the meso-position CH proton is easy to be
deprotonated by F−, which can act as the binding site of chemosen-
sor for selective sensing of F− by the single CH proton based on
proton transfer process.
The observation on control experiments of compounds 4a–c
suggested that the introduction of the electron-withdrawing nitro
groups into the meso-phenyl ring can effectively enhance the acid-
ity of the meso-position CH proton of the 3,3ꢀ-bis(indolyl)methane.
Compared with receptor 1c, in the F− sensing system of receptor
1e and 1d, the meso-position CH proton acidity of the receptor can
be further increased, which potentially resulted in a further free
radical oxidation process induced by the traces of oxygen [48].
The equilibrium constants (or proton-dissociation con-
stants) of receptors 1c–e with F− were evaluated using the
Benesi–Hildebrand equation by origin software [49]. The plot of
Acknowledgement
This work was supported by the National Natural Science Foun-
dation of China (grant no. 20672121) and the Open Fund of State
Key Laboratory of Oxo Synthesis & Selective Oxidation (grant no.
OSSO2008kjk6).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2010.12.004.
References
[1] P.A. Gale, Acc. Chem. Res. 39 (2006) 465–475.
[2] J.W. Steed, Chem. Soc. Rev. 38 (2009) 506–519.
[3] R. Vilar, Eur. J. Inorg. Chem. (2008) 357–367.
[4] S. Kubik, Chem. Soc. Rev. 39 (2010) 3648–3663.
[5] L.A. Joyce, S.H. Shabbir, E.V. Anslyn, Chem. Soc. Rev. 39 (2010) 3621–3632.
[6] H. Shao, H. Lin, H.K. Lin, Spectrochim. Acta Part A 70 (2008) 682–685.
[7] P. Curinova, I. Stibor, J. Budka, J. Sykora, K. Lang, P. Lhotak, New J. Chem. 33
(2009) 612–619.
[8] S.J. Brooks, P.A. Gale, M.E. Light, Chem. Commun. (2006) 4344–4346.
[9] J.W. Steed, Chem. Soc. Rev. 39 (2010) 3686–3699.
[10] V. Amendola, L. Fabbrizzi, L. Mosca, Chem. Soc. Rev. 39 (2010) 3889–3915.
[11] E. Veale, G. Tocci, F. Pfeffer, P. Kruger, T. Gunnlaugsson, Org. Biomol. Chem. 7
(2009) 3447–3454.
[12] C. Perez-Casas, A.K. Yatsimirsky, J. Org. Chem. 73 (2008) 2275–2284.
[13] N. Morakot, W. Rakrai, S. Keawwangchai, C. Kaewtong, B. Wanno, J. Mol. Model.
16 (2010) 129–136.
[14] A.-F. Li, J.-H. Wang, F. Wang, Y.-B. Jiang, Chem. Soc. Rev. 39 (2010) 3729–3745.
[15] V. Amendola, G. Bergamaschi, A. Buttafava, L. Fabbrizzi, E. Monzani, J. Am. Chem.
Soc. 132 (2009) 147–156.
[16] S.J. Brooks, S.E. García-Garrido, M.E. Light, P.A. Cole, P.A. Gale, Chem. Eur. J. 13
(2007) 3320–3329.
[17] P.A. Gale, Chem. Soc. Rev. 39 (2010) 3746–3771.
[18] R. Sivakumar, V. Reena, N. Ananthi, M. Babu, S. Anandan, S. Velmathi, Spec-
trochim, Acta Part A 75 (2010) 1146–1151.
[19] T.F. Markle, J.M. Mayer, Angew. Chem. Int. Ed. 47 (2008) 738–740.
[20] W.T. Gong, K. Hiratani, S.S. Lee, Tetrahedron 64 (2008) 11007–11011.
Fig. 5. Changes in UV–vis spectra of 4c recorded in CH3CN (5 × 10−5 M) after addi-
tion of 100 equiv of various anions.

L. Wang et al. / Spectrochimica Acta Part A 78 (2011) 726–731
731
[21] P.A. Gale, Chem. Commun. (2005) 3761–3772.
[22] M.A. Palacios, R. Nishiyabu, M. Marquez, P. Anzenbacher, J. Am. Chem. Soc. 129
(2007) 7538–7544.
[23] D.-W. Yoon, D.E. Gross, V.M. Lynch, C.-H. Lee, P.C. Bennett, J.L. Sessler, Chem.
Commun. (2009) 1109–1111.
[24] C. Caltagirone, J.R. Hiscock, M.B. Hursthouse, M.E. Light, P.A. Gale, Chem. Eur. J.
14 (2008) 10236–10243.
[37] X.M. He, S.Z. Hu, K. Liu, Y. Guo, J. Xu, S.J. Shao, Org. Lett. 8 (2006) 333–
336.
[38] M. Nishiki, W. Oi, K. Ito, J. Inclusion. Phenom. Macrocyclic. Chem. 61 (2008)
61–69.
[39] R. Nagarajan, P.T. Perumal, Chem. Lett. 33 (2004) 288–289.
[40] R. Martinez, A. Espinosa, A. Tarraga, P. Molina, Tetrahedron 64 (2008)
2184–2191.
[25] A. Brown, K. Mullen, J. Ryu, M. Chmielewski, S. Santos, V. Felix, A. Thompson, J.
Warren, S. Pascu, P. Beer, J. Am. Chem. Soc. 131 (2009) 4937–4952.
[26] F.M. Pfeffer, K.F. Lim, K.J. Sedgwick, Org. Biomol. Chem. 5 (2007) 1795–1799.
[27] Y. Shiraishi, H. Maehara, T. Sugii, D. Wang, T. Hirai, Tetrahedron Lett. 50 (2009)
4293–4296.
[28] J.O. Yu, C.S. Browning, D.H. Farrar, Chem. Commun. (2008) 1020–1022.
[29] P. Dydio, T. Zielinski, J. Jurczak, Chem. Commun. (2009) 4560–4562.
[30] P. Dydio, T. Zielinˇıski, J. Jurczak, J. Org. Chem. 74 (2009) 1525–1530.
[31] J.L. Sessler, D.-G. Cho, V. Lynch, J. Am. Chem. Soc. 128 (2006) 16518–16519.
[32] K.J. Chang, D. Moon, M.S. Lah, K.S. Jeong, Angew. Chem. Int. Ed. 44 (2005)
7926–7929.
[41] A. Karam, J.C. Alonso, T.I. Gerganova, P. Ferreira, N. Bion, J. Barrault, F. Jerome,
Chem. Commun. (2009) 7000–7002.
[42] M.G. Ferlin, G. Chiarelotto, S. Dall’Acqua, E. Maciocco, M.P. Mascia, M.G. Pisu, G.
Biggio, Biorg. Med. Chem. 13 (2005) 3531–3541.
[43] A.K.A. Selen Gurkan, Z. Buyukbingol, A. Adejare, E. Buyukbingol, Arch. Pharm.
(Weinheim) 338 (2005) 67–73.
[44] T. Gunnlaugsson, P.E. Kruger, P. Jensen, J. Tierney, H.D.P. Ali, G.M. Hussey, J. Org.
Chem. 70 (2005) 10875–10878.
[45] Y.-M. Zhang, Q. Lin, T.-B. Wei, X.-P. Qin, Y. Li, Chem. Commun. (2009)
6074–6076.
[46] H.M. Chawla, R. Shrivastava, S.N. Sahu, New J. Chem. 32 (2008) 1999–2005.
[47] X. Peng, Y. Wu, J. Fan, M. Tian, K. Han, J. Org. Chem. 70 (2005) 10524–
10531.
[33] K.-J. Chang, B.-N. Kang, M.-H. Lee, K.-S. Jeong, J. Am. Chem. Soc. 127 (2005)
12214–12215.
[34] P.A. Gale, Chem. Commun. (2008) 4525–4540.
[35] C. Caltagirone, P.A. Gale, Chem. Soc. Rev. 38 (2009) 520–563.
[36] M. Cametti, K. Rissanen, Chem. Commun. (2009) 2809–2829.
[48] G.A. Russell, E.G. Janzen, J. Am. Chem. Soc. 89 (1967) 300–308.
[49] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc 71 (1949) 2703–2707.