Spectroscopic properties of the receptors based on 2- And 3-linked tri(indolyl)methanes for anion binding

Article abstract of DOI:10.1016/j.saa.2010.06.036

The anion binding properties of two title tri(indolyl)methane compounds have been evaluated by using UV-vis absorption and fluorescence spectroscopy. Different from 3-linked receptor which exhibited faint even no obvious spectral response to anions, 2-linked receptor can bind F^-, AcO^- and H₂PO₄^-, with the bathochromic shifts of the UV absorption and fluorescence quenching. This selectivity will be attributed to suitable steric array of NH binding sites of 2-linked receptor, which helps to form multiple hydrogen bonding interactions with anions.

Full text of DOI:10.1016/j.saa.2010.06.036

Spectrochimica Acta Part A 77 (2010) 620–624
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic properties of the receptors based on 2- and 3-linked
tri(indolyl)methanes for anion binding
Wei Weia,b, Yong Guo , Jian Xu , Shijun Shao
a
a
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
Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The anion binding properties of two title tri(indolyl)methane compounds have been evaluated by using
UV–vis absorption and fluorescence spectroscopy. Different from 3-linked receptor which exhibited faint
even no obvious spectral response to anions, 2-linked receptor can bind F , AcO and H2PO4 , with the
bathochromic shifts of the UV absorption and fluorescence quenching. This selectivity will be attributed
to suitable steric array of NH binding sites of 2-linked receptor, which helps to form multiple hydrogen
bonding interactions with anions.
Received 12 February 2010
Received in revised form 1 June 2010
Accepted 21 June 2010
−
−
−
Keywords:
Anion recognition
Anion binding
Indolylmethane
Hydrogen bonding
Spectroscopy
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
the steric array of the NH binding sites of 2-linked receptors helps
to bind anions.
At present, anion recognition and sensing still have been a
In view of these facts and as a part of our work on anion recog-
nition of the receptors based on indole group [15,18,19], herein
we compared the binding properties of receptors 1 and 2 based
on tri(indolyl)methane with anions by UV and fluorescence spec-
troscopy in dilute solution. The results indicated that 2-linked
hotspot of supramolecular chemistry research because it has played
an important role in biology, chemical, pharmacy and environ-
mental science [1,2]. Improving the sensitivity and selectivity of
anion receptors continues to be an extensive research area [3–8].
Indole has been regarded as an important class of hydrogen bond
donor groups [9]. Recently, these neutral and highly effective anion
receptors have attracted much attention because they can provide
more hydrogen bonding points for anion and induce high binding
properties [10–14].
−
−
−
receptor has a high selectivity for F , AcO and H PO4 , and it can
2
bind the three anions through multiple hydrogen bonding interac-
tions. In contrast, 3-linked receptor has almost no obvious spectral
response to anions in the same conditions.
In our previous works, we observed the changes in colors and
2
. Experimental
ꢀ
−
absorption spectra of 3,3 -bis(indolyl)methene receptor with F
−
ꢀ
and HSO4 [15], whereas, 3,3 -bis(indolyl)methane has almost
no obvious spectral response to anions. Actually, indolylmethane
receptors can bind anions by hydrogen bonding interactions, which
2-Linked receptor 1 was prepared from sulfuric acid-catalyzed
reactions of 3-methylindole with triethyl orthoformate, as
described in the literature [20]. 3-Linked receptor 2 was prepared
from condensation of indole with indolylaldehyde in MeOH (see
Scheme 1).
has been investigated by Ito and co-workers [16,17] by using 1
H
NMR titration techniques. Indolylmethane receptors they reported
−
−
−
−
−
−
1
can show the response to Cl , Br , I , HSO and NO3 in H NMR
4
−
−
spectroscopy, but more basic anions, such as F , AcO or H PO4 ,
2
2.1. Tri-(3-methylindol-2-yl) methane (receptor 1)
are not mentioned. Meanwhile, their results exhibited that asso-
ciation constants of 3-linked receptors and anions were obviously
much smaller than that of 2-linked receptors. It is possibly because
Triethyl orthoformate (10 mmol) was added dropwise to
a stirred solution of 3-methylindole (30 mmol) in 100 mL of
methanol. Concentrated sulfuric acid (10 drops) was added
and the resulting solution was stirred for 24 h at room tem-
perature and in the N2 atmosphere. The resulting precipitate
was isolated by filtration and washed with cold methanol.
∗ Corresponding author. Tel.: +86 931 4968209; fax: +86 931 8277088.
E-mail address: sjshao@licp.cas.cn (S. Shao).
1
386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.06.036

W. Wei et al. / Spectrochimica Acta Part A 77 (2010) 620–624
621
Scheme 1. The synthetic routes of receptors 1 and 2.
−
5
Fig. 1. UV–vis absorption spectra of receptor 1 (2 × 10 M, MeCN) in the presence
of 200 equiv. of various anions.
Recrystallization from hot methanol afforded light green powder
of 1.
+
1
ESI-mass: m/z 402.6 (M−H) . H NMR (400 MHz, DMSO-d ), ı
6
(
ppm): 10.46 (s, 3H, N–H), 7.44–7.42 (d, 3H), 7.32–7.30 (d, 3H),
7
.04–6.94 (m, 6H), 6.24 (s, 1H, C–H), 2.07 (s, 9H, Me–H). Yield: 47.4%.
Anal. calcd. for receptor 1 (C28H25N ): C, 83.34; H, 6.24; N, 10.42.
3
Found: C, 83.23; H, 6.28; N, 10.36.
2.2. Tri-(indol-3-yl) methane (receptor 2)
Hydrosulfate of potassium (1.25 mmol) was added to a stirred
solution of indole (2.5 mmol) and 3-indolyaldehyde (1.25 mmol)
in 10 mL of methanol. The resulting solution was stirred for 2 h
at room temperature and then quenched by 10 mL of water and
extracted by 10 mL of CH Cl2 for three times. Then the organic
2
phases were mixed and dried by Na SO , filtration, evaporation,
2
4
and then afforded light red needle crystal of 2.
+
−1
FAB-mass: m/z 361 (M) . IR (KBr, cm ): 3396, 3055, 2795,
1
1
600, 1543, 1455, 1419, 1337, 1217, 1088, 1007, 798, 744. H NMR
(
400 MHz, DMSO-d ), ı: 10.71 (s, 3H, N–H), 7.37 (d, 3H), 7.30 (d,
6
3
3
1
H), 6.99 (t, 3H), 6.91 (t, 3H), 6.83 (s, 3H), 6.03 (s, 1H, C–H). Yield:
5%. Anal. calcd. for receptor 2 (C25H19N ): C, 83.07; H, 5.30; N,
−5
3
Fig. 2. UV–vis absorption spectra of receptor 2 (2 × 10 M, MeCN) in the presence
1.63. Found: C, 82.65; H, 5.33; N, 11.56.
. Results and discussion
of 200 equiv. of various anions.
3
3.1. UV–vis spectral studies
The UV–vis absorption spectra of receptor 1 in MeCN in the
presence of various anions (200 equiv. added as the tetrabuty-
lammonium salts) showed that receptor 1 has the remarkable
−
−
−
recognition selectivity of F , AcO and H PO , but has not obvious
2
4
−
−
−
−
spectral response tootheranions such as Cl , Br , HSO andClO4 ,
4
as shown in Fig. 1. In contrast, Fig. 2 shows no obvious spectral
changes of receptor 2 and various anions.
Titration experiments for receptor 1 with anion were performed
by UV–vis spectroscopy in MeCN for investigating the binding abil-
ities of receptor 1 with anions. As shown in Fig. 3, the absorption
spectra of solution of receptor 1 in the course of the titration with
−
different concentrations of F in MeCN have been observed. With
−
gradual addition of F , the intensity of absorption peak decreased
at 230 nm, and two new absorption peaks were observed at 222
and 235 nm, while the intensity of absorption peak enhanced at
2
95 nm and it was particularly necessary to notice that a discernible
Fig. 3. The variations of UV–vis absorption spectra of receptor
(
1 in MeCN
bathochromic shift occurred from 292 to 295 nm. Meanwhile, four
obvious isobestic points appeared at 223, 233, 251 and 283 nm.
1 × 10−5 M) after addition from 0 to 3 equiv. of F .
−

6
22
W. Wei et al. / Spectrochimica Acta Part A 77 (2010) 620–624
−
−
Scheme 2. The probable binding modes of receptor 1 with F and AcO .
Fig. 4. The variations of UV–vis absorption spectra of receptor
(
1 in MeCN
1 × 10− M) after addition from 0 to 13 equiv. of AcO .
5
−
In addition, the addition of the protonic solvent MeOH can
−
−
destroy hydrogen bonding interactions of receptor 1 with F , AcO
−
and H PO4 , and reversed into the molecular structure of receptor
2
Similar variations of the absorption spectra of receptor 1 with
1
. Therefore, the possible binding modes of receptor 1 with fluoride
−
−
addition of AcO and H PO4 were observed, respectively (see
2
and acetate ions are illustrated in Scheme 2.
Figs. 4 and 5). The formation of hydrogen bonding interactions
−
−
−
between receptor 1 and F , AcO or H PO4 was the best reason
2
3.2. Fluorescence spectral studies
for inducing these variations.
Further calculation of the above titration experiments data by
non-linear fitting curve [21] gave the association constants of
The variations of molecular structure of receptor 1 in the course
−
−
−
5
of binding with anions were also studied by fluorescence spec-
troscopy. The fluorescence spectra indicated that the addition of
receptor 1 with F , AcO and H PO4 which were 3.36 × 10 ,
2
5
5
2
.88 × 10 and 2.56 × 10 L/mol, respectively. The order of the con-
−
−
−
the three anions such as F , AcO and H PO4 could quench the
2
stants for anions can be rationally explained based on the anion
structures. Because the size of spherical fluoride ion may be suit-
able for the steric array of NH binding sites of receptor 1 and it was
bound with receptor 1 preferentially. Acetate ion was planar trian-
gle and has two oxygen atoms, so it may be bound with receptor 1 in
a limited size. Dihydrophosphate ion has a stereo tetrahedral struc-
ture, and it may be bound by complicated structure. In addition,
the job plot also confirmed the formation of the 1:1 stoichiometry
of complex between receptor 1 and anions by hydrogen bonding
interactions. However, for 3-linked receptor 2, the three NH groups
may be too far and unsuitable to match the anions, so it exhibit faint
even no obvious spectral response to anions.
emission signal of receptor 1, as shown in Fig. 6, but the obvious
−
−
−
−
variations with other anions such as Cl , Br , HSO
and ClO4
4
have not been observed. In contrast, as shown in Fig. 7, receptor 2
showed almost no obvious spectral variations with anions.
Similar to UV–vis titration, the fluorescence titration experi-
ments of receptor 1 for binding anion were also obtained from a
−
MeCN solution of receptor 1 in the absence and presence of F ,
−
−
AcO or H PO4 . Fig. 8 shows the obvious fluorescence quench-
ing of receptor 1 with the gradual addition of F . Meanwhile,
2
−
the similar variations of receptor 1 with the gradual addition of
−
−
AcO or H PO4 were observed, respectively (see Figs. 9 and 10).
2
The linear relationship of 1/I − I vs 1/[Anion] obtained in the
0
Fig. 5. The variations of UV–vis absorption spectra of receptor
1
in MeCN
Fig. 6. Fluorescence spectra of receptor 1 (10 ␮M, ꢀex = 283 nm, MeCN) in the pres-
ence of 100 equiv. of various anions.
1 × 10− M) after addition from 0 to 47 equiv. of H2PO4
5
−
.
(

W. Wei et al. / Spectrochimica Acta Part A 77 (2010) 620–624
623
Fig. 7. Fluorescence spectra of receptor 2 (10 ␮M, ꢀex = 283 nm, MeCN) in the pres-
ence of 100 equiv. of various anions.
Fig. 9. The quenching effect of fluorescence spectra of receptor 1 in MeCN
−
(
5 ␮M, ꢀex = 283 nm) after addition from 0 to 582 equiv. of AcO . The inset is
−
a Benesi–Hildebrand plot of receptor 1 with AcO in MeCN (5 ␮M), in which
the reciprocal of the differences in fluorescence intensity at 356 nm was plotted
Benesi–Hildebrand plot [22,23] indicated the formation of 1:1 com-
−
−9
against the reciprocal concentrations of AcO (y = a + bx = 0.00191 + 8.5846 × 10 x,
−
−
plex. The binding constants for receptor 1 with F , AcO and
5
Ka = a/b = 2.22 × 10 L/mol, R = 0.9981).
−
5
5
5
H PO were 3.39 × 10 , 2.22 × 10 and 1.66 × 10 L/mol, respec-
2
4
tively.
The fluorescence spectral results were likely due to the facts as
follows. Hydrogen bonding interactions of receptor 1 with anions
destroyed the previous planarity of receptor 1 and cause the obvi-
ous fluorescence quenching effects of receptor 1. It revealed the
formations of hydrogen bond interactions of receptor 1 with anions.
The fluorescence quenching effect of receptor 1 with the triangle
acetate ions was more severe possibly because the interactions of
receptor 1 with acetate ions can severely destroy the planarity of
receptor 1. These fluorescence results are in agreement with the
above binding mode of receptor 1 and anions.
−
In addition, the binding ability of the receptor 1 for F is evident
1
in H NMR spectra in DMSO-d , as shown in Fig. 11. The addition of
6
0.2 and 1 mol equiv. of tetrabutylammonium fluoride to receptor 1
in DMSO-d6 caused the drastic decrease of signal for NH protons
at 10.46 ppm. Meanwhile, the signal of NH protons was broadened,
Fig. 10. The quenching effect of fluorescence spectra of receptor 1 in MeCN
−
(
5 ␮M, ꢀex = 283 nm) after addition from 0 to 672 equiv. of H2PO4 . The inset is a
−
Benesi–Hildebrand plot of receptor 1 with H2PO4 in MeCN (5 ␮M), in which the
reciprocal of the differences in fluorescence intensity at 356 nm was plotted against
−
−8
the reciprocal concentrations of H2PO4
Ka = a/b = 1.66 × 10 L/mol, R = 0.9995).
(y = a + bx = 0.00465 + 2.8068 × 10 x,
5
Fig. 8. The quenching effect of fluorescence spectra of receptor 1 in MeCN
−
(
5 ␮M, ꢀex = 283 nm) after addition from 0 to 740 equiv. of F . The inset is a
−
Benesi–Hildebrand plot of receptor 1 with F in MeCN (5 ␮M), in which the
reciprocal of the differences in fluorescence intensity at 356 nm was plotted
−
−8
Fig. 11. 1H NMR spectra of receptor 1 in the presence of 0.2 and 1 equiv. of F−.
against the reciprocal concentrations of F (y = a + bx = 0.00734 + 2.16591 × 10 x,
5
Ka = a/b = 3.39 × 10 L/mol, R = 0.9891).

6
24
W. Wei et al. / Spectrochimica Acta Part A 77 (2010) 620–624
and a large downfield from 10.46 to 12.60 ppm can be observed.
However, the signal of CH proton at 6.24 ppm was almost invari-
able. These results indicated that hydrogen bonding interactions
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[
−
−
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Cl , Br , HSO
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4
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3
Acknowledgement
[
(
We gratefully acknowledge the support from the National Nat-
ural Science Foundation of China (no. 20672121).
[
1