A fluoride-sensing receptor based on 2,2′-bis(indolyl)methane by dual-function of colorimetry and fluorescence

Article abstract of DOI:10.1016/j.saa.2015.04.086

A compound based on 2,2′-bis(indolyl)methane containing nitro group was studied as a new anion receptor. It could recognize selectively F^- by an increasing fluorescence signal and a visible color change from colorless to blue. The introduction of nitro group induced the spectral dual-function related to the deprotonation of N-H protons.

Full text of DOI:10.1016/j.saa.2015.04.086

Accepted Manuscript
A Fluoride’-sensing Receptor Based on 2,2’ ’-Bis(indolyl)methane by Dual’-
function of Colorimetry and Fluorescence
Wei Wei, Shi Jun Shao, Yong Guo
PII:
S1386-1425(15)00558-2
http://dx.doi.org/10.1016/j.saa.2015.04.086
SAA 13637
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
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Revised Date:
Accepted Date:
18 January 2015
29 March 2015
21 April 2015
Please cite this article as: W. Wei, S.J. Shao, Y. Guo, A Fluoride’-sensing Receptor Based on 2,2’ ’-
Bis(indolyl)methane by Dual’-function of Colorimetry and Fluorescence, Spectrochimica Acta Part A: Molecular
and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.04.086
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A Fluoride-sensing Receptor Based on 2,2’-Bis(indolyl)methane by
Dual-function of Colorimetry and Fluorescence
Wei Wei,1∗ Shi Jun Shao2 and Yong Guo2
1 Material and Chemical Engineering College, Sichuan University of Science and Engineering,
Zigong, 643000, P. R. China
2Key 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
Abstract:
A compound based on 2,2’-bis(indolyl)methane containing nitro group was studied as a new anion
receptor. It could recognize selectively F− by an increasing fluorescence signal and a visible color change
from colorless to blue. The introduction of nitro group induced the spectral dual-function related to the
deprotonation of N−H protons.
Keywords:
Anion recognition, Indolylmethane, deprotonation, Spectroscopy
∗ Corresponding author. Tel.: 86-13990046750.
E-mail address: weiw0811@163.com
1

1. Introduction
Among the development of a variety of receptors for anions, especially for fluoride ions [1-11],
indole-based anion receptors such as biindole, carbazole and indolocarbazole, have received considerable
attention in molecular recognition because indole groups, like pyrrole, donate N−H groups as hydrogen
bonding donors [12-17]. Indole-based receptors bind more easily with anions by hydrogen-bonding
interactions than pyrrole because indole contains more acidic N−H groups [18]. In this field,
biindole-based receptors have been reported as one of the feature anion receptors [19-21].
In our previous work, we have taken an interest in the research of biindole-based anion receptors such as
3-linked bis(indolyl)methane/methene [22-23]. We found that phenyl-3,3’-bis(indolyl)methene can show
favorable anion recognition properties by UV-vis spectroscopy techniques, whereas,
phenyl-3,3’-bis(indolyl)methane had faint even no obvious response to anions (See Fig. S1). Similar
results have also been observed by Ito and co-workers [24]. They investigated the anion binding properties
1
of indolylmethanes containing 2- and 3- linked indole groups by H NMR titration techniques in CDCl3.
Their research results indicated that the position of the N−H binding sites of the indolylmethane is one of
the important factors for the anion recognition properties. The stability constants for anion complexes of
3-linked receptors are obviously smaller than that of 2-linked complexes possibly because the position of
the two N−H binding sites of 2-linked receptors may be more suitable to match anions.
In view of this facts and as a part of our work on anion recognition of receptors based on 2-linked
tri(indolyl)methane we reported last time [25], herein we developed a series of receptors 1−4 based on
2,2’-bis(indolyl)methane. Remarkably, receptor 4 containing nitro group could recognize selectively F− by
2

an increasing fluorescence signal and clear color change from colorless to blue.
2. Experimental
The synthesis of receptors 1−4 is showed in Scheme 1, as described in the literature [26]. They were
prepared from sulfuric-acid-catalyzed reactions of 3-methylindole with appropriate aryl aldehyde.
2.1. General Remarks
Chemicals were purchased from commercial suppliers, and used without further purification.
Acetonitrile was chromatogram pure. Melting points were determined on a PHMK 05 microscopic
melting-point apparatus (Germany) and are uncorrected. The FTIR spectra were recorded on a Nicolet
NEXUS FTIR spectrometer in 4000-400 cm–1 region using KBr pellets. MS results were determined on
Agilent-1100 LC/MSD Trap instrument. 1H and 13C NMR spectra were recorded at 400 MHz in Inova-400
spectrometer (Varian Company) and were performed with TMS as an internal reference (1H in d6-DMSO,
13C in CDCl3). The absorbance measurements were performed on a PerkinElmer Lambda 35 UV/VIS
Spectrometer. The fluorescence measurements were performed on a PerkinElmer LS 55 Luminescence
Spectrometer.
2.2. Synthetic method
Appropriate aryl aldehyde (16 mmol) was added dropwise to a stirred solution of 3-methylindole (4.20
g, 32.0 mmol) in 30 mL of ethanol. Concentrated sulfuric acid (10 drops) was added and the reaction
solution stirred at room temperature and in the N2 atmosphere. The resulting precipitate was isolated by
filtration and washed with cold ethanol when the reaction was finished. Recrystallization from hot ethanol
afforded compounds 1−4.
3

Phenyl bis(3-methylindole-2-yl)methane (Compound 1): Yield: 57.1%. White powder. M.p.:
160-162 °C; ε 64750 L/mol−1·cm−1 (λmax 224 nm); IR (KBr, cm−1): 3439, 3384, 3022, 2914, 2856, 1596,
1
1485, 1459, 1331, 1308, 1262, 1206, 1028, 1006, 873, 745; H NMR (400 MHz, d6-DMSO): δ 10.44 (s,
2H, N−H), 7.42 (d, J = 8 Hz, 2H, Ar-H), 7.28 (m, 4H, Ar-H), 7.14 (d, J = 8 Hz, 2H, Ar-H), 7.01 (t, J = 8
Hz, 2H, Ar-H), 6.96 (t, J = 8 Hz, 2H, Ar-H), 6.03 (s, 1H, C−H), 2.15 (s, 3H, Me-H); MS (ESI): 349.5 [M −
H]−.
4-Methyloxyphenyl bis(3-methylindole-2-yl)methane (Compound 2): Yield: 85%. White powder. M.p.:
173-175 °C; ε 98080 L/mol−1·cm−1 (λmax 228 nm); IR (KBr, cm−1): 3433, 3413, 3026, 2949, 2914, 2857,
2834, 1608, 1508, 1458, 1333, 1305, 1251, 1178, 1032, 1007, 818, 745; 1H NMR (400 MHz, d6-DMSO):
δ 10.38 (s, 2H, N−H), 7.41 (d, J = 8 Hz, 2H, Ar-H), 7.28 (d, J = 8 Hz, 2H, Ar-H), 7.04 (m, 4H, Ar-H), 6.97
(t, J = 14 Hz, 2H, Ar-H), 6.92 (t, J = 12 Hz, 2H, Ar-H), 5.96 (s, 1H, C−H), 3.71 (s, 3H, OMe-H), 2.13 (s,
3H, Me-H); 13C NMR (400 MHz, CDCl3): δ 158.69, 135.14, 133.74, 131.92, 129.54, 129.48, 121.55,
119.39, 118.42, 114.29, 110.80, 108.47, 55.27, 39.99, 8.50; MS (ESI): 379.8 [M + H]+.
4-Chlorophenyl bis(3-methylindole-2-yl)methane (Compound 3): Yield: 82%. White needle crystal.
M.p.: 180-182 °C; ε 68520 L/mol−1·cm−1 (λmax 226 nm); IR (KBr, cm−1): 3430, 3379, 3046, 2916, 2856,
1
1618, 1487, 1460, 1332, 1309, 1246, 1179, 1089, 1012, 924, 858, 804, 745; H NMR (400 MHz,
d6-DMSO): δ 10.41 (s, 2H, N−H), 7.40 (m, 4H, Ar-H), 7.28 (d, J = 12 Hz, 2H, Ar-H), 7.14 (d, J = 12 Hz,
2H, Ar-H), 7.01 (t, J = 8 Hz, 2H, Ar-H), 6.95 (t, J = 8 Hz, 2H, Ar-H), 6.03 (s, 1H, C−H), 2.12 (s, 1H,
13
Me-H); C NMR (400 MHz, CDCl3): δ 138.52, 135.26, 133.17, 132.75, 129.79, 129.42, 129.15, 121.86,
119.58, 118.54, 110.89, 108.91, 40.21, 8.54; MS (ESI): 383.4 [M − H]−.
4

4-Nitrophenyl bis(3-methylindole-2-yl)methane (Compound 4): Yield: 36.6%. Yellow powder. M.p.:
240-242 °C; ε 58488 L/mol−1·cm−1 (λmax 222 nm); IR (KBr, cm−1): 3450, 3419, 3287, 3057, 2913, 2857,
1
1594, 1513, 1458, 1346, 1239, 1183, 1108, 1006, 851, 747; H NMR (400 MHz, d6-DMSO): δ 10.46 (s,
2H, N−H), 8.21 (d, J = 8 Hz, 2H, Ar-H), 7.28 (d, J = 8 Hz, 2H, Ar-H), 7.42 (m, 4H, Ar-H), 7.02 (t, J = 6
Hz, 2H, Ar-H), 6.96 (t, J = 8 Hz, 2H, Ar-H), 6.21 (s, 1H, C−H), 2.13 (s, 1H, Me-H); 13C NMR (400 MHz,
CDCl3): δ 147.72, 146.98, 135.48, 131.64, 129.34, 129.24, 124.09, 122.19, 119.74, 118.65, 110.98,
109.40, 40.67, 8.62; MS (ESI): 394.4 [M − H]−.
3. Results and discussion
3.1. UV spectral studies
The anion recognition properties of receptors 1−4 were firstly investigated by UV-vis absorption
spectroscopy techniques. The absorption spectra of receptors 1−3 have obvious bathochromic shifts on
−
adding of F−, AcO− or H2PO4 , but have not the obvious interactions of other anions (See Fig. S2). The
association constants for 1:1 stoichiometry complexes between receptors 1−3 and anions have been given
by non-linear fitting curve [27] (See Table S1). These results indicated 2,2’-bis(indolyl)methane receptors
could more easily bind with anions by double hydrogen-binding interactions than 3,3’-bis(indolyl)methane
probably because of the suitable position of the two N−H binding sites, which accordant with the previous
results studied by Ito and our laboratory.
−
Interestingly, receptor 4 containing nitro group can even recognize selectively F− over AcO−, H2PO4
and other anions by a favorable color change of colorless-to-blue, as shown in Fig. 1, but similar color
change had not been observed in receptors 1−3 upon addition of F−. The intense absorption band of
5

receptor 4 at 283 nm declined with the excessive addition of F−, while three absorption bands at 320, 410
and 593 nm appeared pertained to the solution color changed from colorless to blue (See Fig. 2). On
−
−
−
adding of other anions such as Cl−, Br−, I−, AcO−, H2PO4 , HSO4 and ClO4 , no noticeable spectra were
observed under the same conditions.
Moreover, the interactions of receptor 4 with F− were studied in detail by UV-vis spectroscopic titration
techniques, as shown in Fig. 3. On addition of F− from 0 to 4 equiv., the bands at 223 nm and 280 nm
decreased slightly, and a weak bathochromic shift was observed from 280 to 285 nm. The spectral changes
were similar to the results of receptors 1−3 with anions, which was responsible for the formation of the
complex between receptor 4 and F− (See Fig. 3a). With gradually increasing the concentration of F− from 4
to 580 equiv., the absorption bands at 223 nm and 280 nm decreased drastically and three new bands at
320, 410 and 593 nm evolved (See Fig. 3b). The absorption changes can be ascribed to the N−H
deprotonations of receptor 4 with the excessive addition of F−. However, when nitro group was introduced
into the same position of the 3,3’-bis(indolyl)methane framework, similar spectral changes were not
observed under the same conditions (See Fig. S3).
In our further studies, the reversible work of receptor 4 had been investigated by the addition of polar
solvent. The peaks at 410 and 593 nm decreased gradually and even vanished on addition of polar solvent
MeOH (See Fig. S4). The result suggested that polar solvent MeOH can destroy hydrogen bonding
interactions or deprotonation of receptor 4 with fluoride ions, and the reversible protonation of receptor 4
can also take place. Similarly, the reversible protonation for other receptors with anions can also be
observed under the same conditions.
6

3.2. Fluorescence spectral studies
The fluorescence emission spectra of the receptor 4 with 100 equiv. of various anions in MeCN were
observed in Fig. 4. Surprisingly, receptor 4 exhibited high selective recognition of F− over other anions
with a favorite increase of fluorescence intensity. With gradual adding of F−, as shown in Fig. 5, a drastic
increase of the fluorescence intensity of receptor 4 at 420 nm was observed. The results indicated that the
excessive addition of F− induced the N−H deprotonations and enhanced the conjugation and co-planarity
of the molecular structure of receptor 4. In contrast, the fluorescence intensities of receptors 1−3 decreased
obviously on adding of F− probably because the hydrogen bonding interactions of receptors 1−3 with F−
destroyed the planarity of original structure of the receptors (See Fig. S5).
3.3. 1H NMR spectral studies
1
To further investigate the interactions between receptor 4 and F−, H NMR titration experiments of
receptor 4 and F− were carried out in d6-DMSO. (See Fig. 6) The addition of 0.2 equiv. of F− caused the
drastic decreases of the signals of N−H protons of receptor 4 at 10.49 ppm and large downfield shifts to
12.72 ppm take place, and the signals almost vanished upon addition of 1 equiv. of F−, and a new proton
peak regarded as the [HF2]− peak was appeared at 16.1 ppm upon addition of 4 equiv. of F−, which was
consistent with the other groups reported [28–29]. It suggested that the excessive addition of fluoride ions
induced the deprotonation of receptor 4.
In addition, the interactions of receptors 1-3 with fluoride ions have also been evaluated by the 1H NMR
titration. The results indicated that the N−H protons of receptor 1 bind with fluoride ions by double
hydrogen bonding interactions (See Fig. S6). The signals of N−H protons of receptor 1 almost vanished
7

upon addition of 1 equiv. of F− or even excessive F−, whereas the similar proton peak of [HF2]− was not
observed at 16.1 ppm. Similarly, the variations of the signals for receptor 2 and 3 with fluoride ions can
also be observed in the same conditions. The results suggested that receptors 1-3 bound with anions by
double hydrogen bonding interactions instead of deprotonation.
4. Conclusion
In summary, we have developed a series of 2,2’-bis(indolyl)methane 1−4 as anion receptors. The anion
recognition properties of receptors 1−4 are superior to that of 3,3’-bis(indolyl)methane possibly because of
the suitable position of the two N−H binding sites of 2-linked receptors. It is notable that receptor 4, as a
new anion receptor, exhibited an excellent selectivity of F− by a dual-function of a colorless-to-blue color
change and increasing fluorescence signals in MeCN. The introduction of nitro group into
2,2’-bis(indolyl)methane framework caused a deshielding effect in the whole molecule of receptor and
activated the N−H protons. The excessive addition of F− resulted in the N−H deprotonations and probably
enhanced the conjugation and co-planarity of molecular structure of receptor 4, which displayed the
excellent spectral changes. To our best knowledge, this is the first report of anion recognition employing
2-linked bis(indoly)methane by the naked-eye detection and increasing fluorescence signals. Then more
detailed studies of the mechanism between receptor 4 and anions are under investigation.
Acknowledgements
8

We gratefully acknowledge the supporting from the National Natural Science Foundation of China (No.
20672121) and the talent introduction project of Sichuan University of Science and Engineering
(2011RC14).
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Scheme and Figures caption:
Scheme 1. Synthesis of receptors 1−4
Fig. 1. (a) Color changes of receptor 4 (6 × 10−5 M, MeCN) upon addition of 200 equiv. of various anions;
(b) Color comparison of receptors 1−4 upon addition of F− under the same conditions
Fig. 2. Absorption spectra of receptor 4 (5 × 10−5 M, MeCN) in the presence of 200 equiv. of various
anions
Fig. 3. Changes in the UV-vis absorption spectra of receptor 4 in MeCN (1 × 10−5 M) after addition of F−
from (a) 0 to 4 equiv.; (b) 4 to 580 equiv.
Fig. 4. Fluorescence emission spectra (a) and the column graph (b) of receptor 4 (1 × 10−5 M) upon
addition of 100 equiv. of various anions in MeCN (λex = 300 nm)
Fig. 5. Changes in the fluorescence spectra of receptor 4 (1 × 10−5 M) with gradual increasing of the
concentration of F− in MeCN (λex = 300 nm)
Fig. 6. The 1H NMR titrations of receptor 4 and tetrabutylammonium fluoride in DMSO-d6
11

Scheme 1. Synthesis of receptors 1−4
12

Fig. 1. (a) Color changes of receptor 4 (6 × 10−5 M, MeCN) upon addition of 200 equiv. of various anions;
(b) Color comparison of receptors 1−4 upon addition of F− under the same conditions
13

Fig. 2. Absorption spectra of receptor 4 (5 × 10−5 M, MeCN) in the presence of 200 equiv. of various
anions
14

Fig. 3. Changes in the UV-vis absorption spectra of receptor 4 in MeCN (1 × 10−5 M) after addition of F−
from (a) 0 to 4 equiv.; (b) 4 to 580 equiv.
15

Fig. 4. Fluorescence emission spectra (a) and the column graph (b) of receptor 4 (1 × 10−5 M) upon
addition of 100 equiv. of various anions in MeCN (λex = 300 nm)
16

Fig. 5. Changes in the fluorescence spectra of receptor 4 (1 × 10−5 M) with gradual increasing of the
concentration of F− in MeCN (λex = 300 nm)
17

Fig. 6. The 1H NMR titrations of receptor 4 and tetrabutylammonium fluoride in DMSO-d6
18

Graphical Abstract:
Receptor 4 based on 2,2’-bis(indolyl)methane containing nitro group can recognize selectively F− over
AcO−, H2PO4 and other anions by a favorable color change of colorless-to-blue.
−
19

Highlights:
ꢀ We reported a series of receptors based on 2,2’-bis(indolyl)methane.
ꢀ The receptor containing nitro group was studied as a perfect receptor.
ꢀ This receptor could recognize selectively F− by a spectral dual-function.
ꢀ The spectral dual-function was related to the deprotonation of N−H protons.
20