Colorimetric 'naked-eye' sensor for anions based on conformational flexible tripodal receptor

Article abstract of DOI:10.1007/s10847-010-9869-2

A new tripodal receptor for anion sensing based on amide-pyridinium as recognition site and nitro-benzene as signaling unit was designed and successfully synthesized. This receptor showed high selectivity and strong binding affinity toward AcO^-

Full text of DOI:10.1007/s10847-010-9869-2

J Incl Phenom Macrocycl Chem (2011) 70:115–119
DOI 10.1007/s10847-010-9869-2
ORIGINAL ARTICLE
Colorimetric ‘naked-eye’ sensor for anions based
on conformational flexible tripodal receptor
•
Shuang Bao Wei-tao Gong Wen-dan Chen
•
•
•
Jun-wei Ye Yuan Lin Gui-ling Ning
•
Received: 13 June 2010 / Accepted: 9 September 2010 / Published online: 25 September 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A new tripodal receptor for anion sensing based
on amide-pyridinium as recognition site and nitro-benzene
as signaling unit was designed and successfully synthe-
sized. This receptor showed high selectivity and strong
binding affinity toward AcO- over the investigated anions,
especially over H2PO4-. Addition of AcO- induced clear
color change of solution from colorless to yellow, realizing
profound effect on anion binding. Comparing with those
rigid cyclic systems (it means the preorganized systems such
as macrocycles/macrobicycles which are difficult to change
their conformation before and after complexation with
anions), flexible podands are more intriguing and significant
because they are frequently more easily synthesized and
undergo conformational change on anion binding offering
the induced fit signal transduction. Those properties form the
basis of molecular switches and switchable sensing devices.
Recently, much excellent work has been done focusing on
tripodal anion receptors with a trisubstituted trimethyl or
triethylbenzene core [5–8]. The resulting hexasubstituted
systems provide some degree of preorganization into a
conical conformation (3-up, 3-down) with all three binding
and sensing arms orientated in the same direction and thus
are able to bind anions more efficiently [7, 8].
1
the ‘‘naked-eye’’ detection. UV–Vis and H NMR experi-
ments indicated the selectivity might origin from the
synergistic effects arising from hydrogen bonding, elec-
trostatic interactions and conformational change.
Keywords Amide-pyridinium Á Tripodand Á Anion
sensing Á Conformational change
Introduction
As a part of our continuous work on developing new
anion receptors [9–12], in this paper, we report a novel
conformational flexible tripodand based on amide pyridinium
binding motif, which exhibits colorimetric ‘‘naked eye’’
sensing of AcO- in rather polar organic solvents. In com-
parison with those well-documented hydrogen donating
frameworks such as amide, urea, pyrrole, indole, ammonium,
guanidimium and imidazolium, etc. [13–19], amidepyridi-
nium-based anion receptor is relatively less investigated
[20–22] in spite of pyridinium-based anion receptors repor-
ted by Steed and coworkers exhibited excellent anion bind-
ing properties [8, 23]. The main difference between
pyridinium and amidepyridinium binding sites is from
whether the presence of intramolecular hydrogen bonding or
not. As for amidepyridinium binding site, the intramolecular
hydrogen bonding was proved to play the positive role to
preorganize the conformation of receptor [12]. When inter-
action with anions, the conformational change will give rise
to more efficient anion binding (See Fig. 1).
The design and synthesis of receptors capable of selective
binding and sensing anions remains challenging and is a
current area of active research [1–4]. Besides the well-
known roles of various binding motifs play in anion recog-
nition, the overall receptor topology has exhibited a
Electronic supplementary material The online version of this
article (doi:10.1007/s10847-010-9869-2) contains supplementary
material, which is available to authorized users.
S. Bao Á W. Gong (&) Á W. Chen Á J. Ye Á Y. Lin Á
G. Ning (&)
State Key Laboratory of Fine Chemicals,
School of Chemical Engineering, Dalian University of
Technology, 116012 Dalian, People’s Republic of China
e-mail: wtgong@dlut.edu.cn
G. Ning
e-mail: ninggl@dlut.edu.cn
123

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J Incl Phenom Macrocycl Chem (2011) 70:115–119
O
1)SOCl2
O2N
COOH
O2N
CONH
H
N
N+
2)
H2N
A-
R
N
O
N
H
2
N+
Br
R
N
H
H
A-
3
Br
Br
L1
Fig. 1 Intramolecular hydrogen bonding in amidepyrydinium
binding site
Scheme 1 The synthetic route to tripodal receptor L1
and 13C NMR spectra were recorded on a AVANCE II400
spectrometer at room temperature using with Me4Si as an
internal standard. HRMS were measured on UPLC/Q-Tof
Microa MS apparatus. The UV–Vis titration spectra were
measured on a HITACHI U-4100 spectrophotometer.
With this idea in mind, the structure of our designed
tripodal receptor is shown in Fig. 2, L1 comprises trim-
ethylsubstitued benzene as the core to control the direction
of the binding arms and nitro-benzene group is introduced
as the signal-reporting part. It should be mentioned that the
conformation of L1 would be restricted by the intramo-
lecular hydrogen bonds (IHB) between acidic proton at
a-position of pyridinium and carbonyl group, and anion-
induced conformational change via disturbing the IHB
might realize selective anion sensing, just according with
the biologically important induce-fit mechanism [24]. The
most interesting point of this mechanism is that only the
most suitable anions induce the appropriate conformational
change that will result in signal generation even if they
have no the strongest bound by the receptor. On the other
hand, the electrostatic interaction between electron-positive
pyridinium ring and anions will also be expected.
General experimental procedure for the synthesis
of the receptor L1
The synthesis of the receptor L1 is shown in Scheme 1.
Synthesis of intermediate compound 2
To a solid of 4-nitrobenzoic acid (334 mg, 2 mmol) was
added excess of SOCl2 and stirred at 70 °C for about 24 h
to give clear solution. The excess of SOCl2 was removed
under reduced pressure, and the residue was dried under
vacuum for 3 h. The obtained acid chloride was used
directly without any further treatment. To a 20 mL of dry
THF solution of 3-aminopyridine (190 mg, 2 mmol) was
added dropwise a solution of abovementioned acid chloride
in 20 ml of dry THF at 0 °C. After the solution was stirred
overnight at room temperature, the THF was removed, the
solid residue was dissolved in CH2Cl2 and the solution was
washed with water. The organic layer was separated, dried
over MgSO4 and concentrated. The product was purified by
silica gel column chromatography (3:1 CH2Cl2:AcOEt) to
give white solid. The characterized data we showed below
are consistence with those reported previously [25].
Experimental
Materials and methods
All the anions existed as their tetrabutylammonium salts
and were purchased from Alfa-Aesar Chemical Co. Other
chemical reagents were used as received without further
purification. Unless otherwise specified, all of the UV–vis
titration experiments were carried out at 298.2 0.1 K. 1H
1
Yield: 0.784 g (65%), H NMR (400 MHz, DMSO-d6):
O2N
NO2
NO2
d(ppm) 10.75 (s, 1H, NH), 8.92 (s, 1H, Py-H), 8.39–8.36
(d, J = 11.2 Hz, 2H, Ph-H), 8.41 (d, J = 6.0 Hz, 1H,
Py-H), 8.21–8.18 (d, J = 11.2 Hz 2H, Ph-H), 8.17 (d,
J = 4.4 Hz, 1H, Py-H),7.41 (m, 1H, Py-H). 13C NMR
(100 MHz, DMSO-d6): d(ppm) 164.8, 149.8, 145.5, 142.2,
140.4, 135.9, 129.8, 128.0, 124.1. HRMS (ESI?) m/z:
244.0731; calcd 244.0722.
HN
Br-
O
O
HN
HN
H
N+
O
H
Synthesis of target tripodal receptor L1
N+
N+
Br-
Br-
A mixture of 2(263 mg, 1.07 mmol) and 3 [26] (160 mg,
0.357 mmol) in dry 15 mL CH3CN was refluxed for 3.5 h,
and gradually white precipitate was formed. After cooling
Fig. 2 Structure of tripodal receptor L1
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J Incl Phenom Macrocycl Chem (2011) 70:115–119
117
to room temperature, the precipitate was filtered and
washed several times with cold CH3CN to give pure
compound L1 with bromide as counter ion.
1346, 129.5, 128.9, 128.3, 123.5, 58.9, 17.1; ESI–MS (m/z):
found [M-Br-]: 1048.1.
1
Yield: 0.338 g (80%), H NMR (400 MHz, DMSO-d6):
d(ppm) 11.37 (s, 2H, NH), 9.36 (s, 2H, Py-H), 8.90 (d,
J = 6.0 Hz,2H, Py-H), 8.68 (d, J = 8.4 Hz, 2H, Py-H),
8.30–8.27 (d, J = 8.4 Hz, 4H, Ph-H), 8.18 (m, 2H, Py-H),
8.01–7.99 (d, J = 8.4 Hz,4H, Ph-H), 6.21 (s, 4H, –CH2–),
2.45 (s, 9H, MePh); 13C NMR (100 MHz, DMSO-d6):
d(ppm) 164.4, 149.5, 144.1, 139.5, 139.1, 137.9, 135.4,
Results and discussion
UV–vis spectroscopic studies
In order to investigate the anion sensing properties of L1,
we carried out the UV–vis titration experiments by adding
a standard solution of the tetrabutylammonium salt of
anions, such as F-, Cl-, Br-, I-, HSO4-, AcO-, NO3- and
H2PO4-, to a dry CH3CN solution of the L1 (1 9
10-5 mol/L) at 298 0.1 K (Fig. 3). As shown in Fig. 3,
free L1 displayed a broad absorption band centered about
at 275 nm coming from substituted phenyl group. Among
the anions tested, only addition of AcO- and F- resulted in
a new peak centered about at 375 nm. Other anions, even
H2PO4-, did not induce any spectral response even added
in abundance. Upon addition of AcO- and F-, the maxi-
mum absorption peak of L1 was shifted from UV to visi-
ble region, which rationalized the corresponding color
change of the solution from colorless to yellow-green,
realizing a ‘‘naked-eye’’ detection of AcO- and F- in
solution (Fig. 4). The significant bathochromic shift could
be explained on the basis of the intramolecular charge
transfer (ICT) from electron-rich anion binding amide
pyridinium unit to relative electron-deficient nitro-benzene
fragment.
1.0
0.8
-
-
F
AcO
0.6
0.4
0.2
0.0
-
- -
-
-
Free,Cl ,Br ,I ,H PO ,HSO
2
4
4
250
300
350
400
450
500
550
Wavelength/nm
Fig. 3 The absorption spectra of L1 (10-5 M) after the addition of 5
equivalent of representative anions
In addition, 1:1 stoichiometry of the receptor L1 with
AcO- was confirmed by the Job plot analysis, and the
binding constant of L1 toward AcO- was calculated to be
(1.00 0.05) 9 105 M-1 from the nonlinear curve fitting
based on the detailed titration of L1 (Fig. 5). On the other
hand, the Job plot curve of the complex with F- was not
conventional as usual, which might be due to the larger
electro negativity and stronger basicity of F-, which
Fig. 4 The visible color changes of the receptor L1 in CH3CN
(10-5 M) upon additions of 5 equivalent of different anions
(a)
(b) 0.6
0.30
0.5
0.4
0.25
0.20
0.15
0.10
0.05
0.00
0.3
30 equiv.
0.2
0 equiv.
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
300
350
400
450
500
550
Wavelength/nm
Molar fraction
Fig. 5 a Job plot for the complex formation [L1] ? [AcO-] =
2 9 10-5 mol L-1 at 298.2 0.1 K. b The UV–vis spectra of the
receptor L1 (10-5 mol L-1) in CH3CN solution during the titration
with 0, 0.2, 0.4, 0.7, 0.9, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0, 5.0, 10, 15, 20, and
30 equivalent of AcO-
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J Incl Phenom Macrocycl Chem (2011) 70:115–119
1
O2N
Fig. 6 Partial H NMR spectra
of a receptor L1 only and b in
the presence of 1.0 equivalent of
AcO- in DMSO-d6
NO2
a
b
c
d
HN
O
O
e
HN
g
Hg
f
HN
N+
O
H
Br-
H
N+
Br-
N+
Br-
(b)
(a)
resulted in complicated binding manner between L1 and
F- (see supporting information).
NMR spectroscopic studies
To further investigate the nature of the receptor–anion
interactions, 1H NMR experiment was performed. As
shown in Fig. 6, apparently, the amide proton (from 11.37
to 11.50 ppm) and hydrogen proton at the a-position of
pyridinium ring (from 9.36 to 9.38 ppm) displayed a
remarkable downfield shift upon addition of 1.0 equivalent
of AcO- indicating that acidic proton Hg at a-position of
pyridinium and carbonyl group participated in hydrogen-
bonding interactions with AcO-. On the other hand, other
hydrogen protons at the pyridinium ring shifted to upfield
implying the participation of electrostatic interaction of
charged pyridinium ring in binding AcO- [12].
As mentioned before, it is interesting that the receptor L1
showed high selectivity for AcO- over H2PO4 and the
-
conformational change might play a positive role to explain
this superiority. In this case, the conformational analyses
were carried out by using 2D-COSY NMR spectra. It is well
known that, L1 might adopt ‘‘3-up or 3-down’’ conforma-
tion due to the steric effect of the methyl groups on benzene
core. In addition, as shown in Fig. 7a, before addition of
AcO-, there is clear correlation between methylene proton
Hh and protons on pyrydinium ring, such as Hd, Hg and Hf
respectively, which indicated that the pyridinium ring might
face to the center of cavity taking the twisted conformation.
On the other hand, there is no coupling between Hc with Hg
indicating the long distance between them due to the pres-
ence of intramolecular hydrogen bond of proton Hg with
Fig. 7 Partial 2D-COSY NMR spectra of a free L1 and b L1 upon
addition of 1.0 equivalent of AcO- in DMSO-d6
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Acknowledgments This research has been supported by the
National Natural Science Foundation of China (20772014 and
20923006) and the Fundamental Research Funds for the Central
Universities (1000-893116).
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