Simple naked-eye ratiometric and colorimetric receptor for anions based on azo dye featuring with benzimidazole unit

Article abstract of DOI:10.1016/j.tetlet.2015.01.128

A simple, tailor made receptor 2 based on azo dye featuring with benzimidazole unit with hybrid -OH and -NH binding sites was synthesized and characterized. The addition of CN^-, AcO^-, F^-, and H₂PO₄

Full text of DOI:10.1016/j.tetlet.2015.01.128

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Simple naked-eye ratiometric and colorimetric receptor for anions
based on azo dye featuring with benzimidazole unit
⇑
Navneet Kaur , Gargi Dhaka, Jasvinder Singh
Department of Chemistry, Panjab University, Chandigarh 160014, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple, tailor made receptor 2 based on azo dye featuring with benzimidazole unit with hybrid –OH
and –NH binding sites was synthesized and characterized. The addition of CNÀ, AcOÀ, FÀ, and H2POÀ4
in acetonitrile solution of 2 resulted in a large bathochromic shift from 315 nm to 470 nm allowing naked
eye color change from colorless to orange. Anion sensing characteristics were determined using visual
inspection, UV–vis and 1H NMR spectroscopy pointing toward the improved chromogenic ability after
introduction of AN@NA group in the azo dye 2. The mechanism of anion binding with receptor 2 showed
1:2 (L/Anion) and the association constants were found in the order of AcOÀ > CNÀ > FÀ > H2PO4À.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 24 December 2014
Revised 14 January 2015
Accepted 19 January 2015
Available online xxxx
Keywords:
Azo-dye
Naked eye
Colorimetric
UV–vis
Anion sensing
Introduction
not only released by industries but are also the degradation prod-
ucts (hydrolysis) of chemical warfare agents such as sarin, soman,
The development of receptors which undergo a color change on
addition of an analyte has attained great significance in the area of
supramolecular chemistry.1 The colorimetric-based sensing has
shown unique merit, as it provides naked-eye detection of the ana-
lyte without resorting to any expensive equipment.2 In general, the
design of such receptors is based on the receptor-chromophore
general binomial, which involves the binding of a specific analyte
with receptor sites and a chromophore responsible for translating
the binding of receptor–analyte into an optical signal.3 In colori-
metric receptors, due to its interaction with the analyte, batho-
chromic or hypsochromic shift of absorption spectra or visual
color change is affected by the respective increase or decrease in
electron densities on the chromophore moiety.4
Owing to the ubiquity of anions and their importance as agri-
cultural fertilizers and industrial raw materials, the development
of anion sensing receptors received considerable attention.5 In
recent years, the development of chromogenic anion sensors is
mainly based on the anion induced polarization/deprotonation
(in extreme cases) of OH in case of appropriately substituted phe-
nols and NH in case of appropriately substituted anilides and aryl
ureas.6–8 Anions particularly fluoride, cyanide, and phosphate are
and tabun, making them harmful to the environment as well as to
human health.9 Among various anions, cyanide is an extremely
toxic detrimental anion causing poisoning in biology; environ-
ment, and even relatively small amounts of this species are lethal
to humans.10 Moreover, raw materials for synthetic fibers, resins,
herbicides, and the gold-extraction process releases cyanide into
the environment as a toxic contaminant. So, the colorimetric sen-
sors for determination of anions are big challenges for investigators
and they attract much interest.
It is well known that azo colorants are the largest chemical class
of industrial colorants. They represent a robust group of synthetic
dyes, which cover the full range of shades of colors. Their ease of
preparation and economy of the reaction resulted in a large num-
ber of dyes.11 Azo dyes possessing an aromatic heterocyclic moiety
increased the chromophoric strength and thus, have been reviewed
comprehensively.12 Benzimidazole and its derivatives have been
studied in anion and cation recognition systems that display color
changes or fluorescence quenching or enhancement upon
binding.13
This paper describes the design and synthesis of a new and sim-
ple anion receptor 2-(1-H-benzoimidazol-2-yl)-4-(pyridine-3-
ylazo)-phenol (2) containing both azo dye and benzimidazole
and photophysical study of its anion recognition behaviors. In
addition, the sensing processes can be realized by the ‘naked eye’
determination as it has a remarkable color response.
⇑
Corresponding author. Tel.: +91 172 2534430; fax: +91 172 2545074.
E-mail addresses: neet_chem@yahoo.co.in, neet_chem@pu.ac.in (N. Kaur).
http://dx.doi.org/10.1016/j.tetlet.2015.01.128
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kaur, N.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.01.128

2
N. Kaur et al. / Tetrahedron Letters xxx (2015) xxx–xxx
Results and discussions
Synthesis of receptor 2
The preparation of receptor 2 based on the azo dye of benzimid-
azole was carried out by diazo-coupling reaction as shown in
Scheme 1. The chemical structure of the newly synthesized chemo-
sensor was confirmed by 1H NMR, 13C NMR, and mass spectral data.
Figure 2. Naked-eye images of 1 in CH3CN upon the addition of various anions
(100 equiv).
Sensing properties of receptor 1
excess, presumably due to weaker and insignificant interactions
with the receptor.
To study the role of azo functionality in receptor 2, the sensing
abilities of 1 were primary investigated by adding various anions
such as CNÀ, FÀ, ClÀ, BrÀ, IÀ, AcOÀ, H2PO4À, and HSOÀ4 (tetrabutyl
ammonium was used as a counter anion) to its CH3CN solution
UV–vis spectral studies
(10 lM). The addition of 5 equiv of these anions did not cause
The qualitative anion sensing behavior of 2 was determined by
recording the absorption spectra. In the UV–vis spectra of 2
(10 lM) in acetonitrile media, the receptor peak is characterized
by the appearance of a band centered at 315 nm witha shoulder near
365 nm. Upon interaction with CNÀ, FÀ, AcOÀ, and H2PO4À ions, a new
intramolecular charge transfer band has been observed at 470 nm.
UV–vis titrations were next carried out with increasing concentra-
tions of CNÀ, FÀ, AcOÀ, and H2POÀ4 anions. Upon addition of CNÀ ions
any change in the absorption spectrum; however, further addition
of these anions up to 50 equiv resulted in the emergence of a very
weak charge transfer band at 374 nm with CNÀ and FÀ ions (Fig. 1),
but the spectral changes were too small to calculate the corre-
sponding association constants. Earlier results show that the addi-
tion of 5 equiv of FÀ ions to a solution of receptor 1 in DMSO caused
only a small bathochromic shift of 11 nm. Whereas, other anions
did not induce any significant changes in the absorption spectra.14
However, by changing the polarity of the solvent from DMSO to
CH3CN, the small bathochromic shift observed with the addition
of 5 equiv of FÀ ion gets diminished and requires higher equiva-
lents of anions to cause only a small red-shift.
to a solution of receptor 2 (10 lM) in CH3CN, a new band starts
emerging at 470 nm with the addition of only 0.2 equiv of CNÀ ions.
However, after adding 3 equiv of CNÀ ions, a sharp and gradual
increase in intensity of the new band has been observed along with
gradual decrease in absorbance of the 315 nm band (Fig. 3).
Colorimetric signaling of (2)
0 µM
CN-
(a)
As the presence of the AN@NA group improves the chromo-
genic ability of the chemosensors, therefore, after introducing the
azo functionality in the receptor 2, both the colorimetric and
absorption changes were greatly enhanced in CH3CN itself. In
60 µM
naked-eye experiment (Fig. 2), receptor 2 (10 lM) responded with
a dramatic color change from colorless to dark yellow with the
addition of 50 equiv of CNÀ, FÀ, AcOÀ, and H2POÀ4 , which indicates
the formation of host–guest complex between these anions and 2.
However, no obvious color change was observed in the case of ClÀ,
BrÀ, IÀ, and HSOÀ4 ions, even when these anions were added in
N
N
N
Wavelength (nm)
H
N
H
N
H2N
(i) HCl
(ii) NaNO2, 0-50C
N
N
N
HO
HO
(b)
2
1
315 nm
Scheme 1. Synthetic procedure for receptor 2.
0.2
0.1
0
1
AcO-
Br-
470 nm
Cl-
318
CN-
F-
330
374
µ
H2PO4-
Anions
I-
HSO4-
Figure 3. Evolution of the UV–vis spectrum of receptor 2 (10 lM in CH3CN) during
Figure 1. UV–vis absorbance of 1 (10
CH3CN.
lM) in the presence of 100 equiv of anions in
the titration of the CNÀ ion; (b) plot of absorbances at 315 and 470 nm vs the
concentration of CNÀ added.
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N. Kaur et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
FÀ > H2POÀ4 . This could be due to more stabilization of negative
charge on the acetate oxygen as compared to the negative charge
on the benzimidazole nitrogen. Thus intermolecular proton trans-
fer occurs from phenyl OH to the acetate ion rather than proton
transfer to the benzimidazole nitrogen.16 The association constant
values of receptor 2 with other anions, that is, CNÀ, H2PO4À, and
HSOÀ4 follows the order of basicity of anions.
0.2
0.1
0
2
O
Ac
CN
F
4
470
315
O
4
P
O
H2
HS
Br
Cl
1H NMR titrations
n-
I
Anions (A
)
Figure 4. UV–vis absorbance of 2 (10 lM) in the presence of 100 equiv of anions in
CH3CN.
To further elucidate the nature of the intermolecular interac-
tions between anions and receptor 2, as an example, 1H NMR spec-
tral changes upon the addition of CNÀ as their tetrabutyl
ammonium salts to the DMSO-d6 solution of 2 (5 Â 10À2 mol LÀ1
)
Table 1
, ) of receptor 2 with different anions and the
MÀ2
were investigated. The formation of the hydrogen bond (deprotona-
tion in extreme cases) between the binding sites of the host and the
anion may give rise to two effects: (i) through-bond effects, which
increase the electron density of the benzene ring and promote
upfield shifts in 1H NMR spectrum, and (ii) through-space effects,
which polarize the C–H bond in proximity to the hydrogen bond,
create the partial positive charge on the proton and cause downfield
shifts.17 In Figure 5, the broad band at d 13.91 has been assigned to –
OH and –NH groups. Addition of 0.5 equiv of CNÀ results in disap-
pearance of this broad band. Moreover, all the aromatic protons
especially phenyl protons of rings possessing the –OH group pres-
ent at d 7.21–7.28 as multiplets clearly split into a doublet at d
6.70–6.73 and a singlet at d 7.05 and give an upfield shift, indicative
of the increase in the electron density of the phenyl ring owing to
the through-bond effects. Thus, the results of 1H NMR titration
and UV–vis titrations, both, point to the occurrence of tautomeric
equilibrium during the anion recognition process as shown in
Scheme 2.
Association constants (Kass
respective number of equivalents required for achieving saturation in CH3CN
Anions
Number of equiv required for saturation
Kass
CNÀ
5
3
15
50
9.54 Â 109
5.01 Â 1010
8.8 Â 108
AcOÀ
FÀ
H2POÀ4
1.03 Â 107
The absorbance increases nearly thirtyfold at 470 nm which is
responsible for the generation of a yellow color after the addition
of CNÀ into the solution of the receptor 2 (Fig. 3b). After addition
of 5.0 equiv of tetrabutylammonium cyanide, it reaches a satura-
tion level. However, the presence of a protic solvent such as H2O
will compete with anions for binding sites. This will disturb the
H-bond interactions between the receptor and the anionic guest
and will lead to a reversal of the visual color and spectral change.
Similar effects were observed in the UV–vis spectra of 2 upon
the addition of AcOÀ, FÀ, and H2POÀ4 ions (Figs. S1–S3). Addition
of these three ions resulted in the appearance of a new red shifted
band at 470 nm; however, their higher equivalents are required for
achieving saturation. In the case of weak basic ions such as ClÀ, BrÀ,
IÀ, and HSOÀ4 , only nominal changes are observed in the UV–vis
spectra (Fig. 4).
Possible sensing mechanism
The appearance of a new red-shifted band with the addition of
anions (CNÀ, AcOÀ, FÀ and H2POÀ4 ) to a solution of receptor 2 may
be attributed to the possible hydrogen bonding interaction and/or
the deprotonation of the –OH and imidazole-NH groups, rather
than ESIPT (excited state intramolecular proton transfer) with the
benzimidazole nitrogen atom, which favored the intramolecular
charge transfer (ICT) transition within the whole molecule. How-
ever, 1H NMR titration of receptor 2 with CNÀ reveals that depro-
tonation of both –OH and –NH occurs upon CNÀ addition (Fig. 5).
Prior to coordination with anions, the azophenol isomer of receptor
2 dominates in solution which attributes to the 315 nm band. Upon
interaction with anions, deprotonation of the –OH group occurs,
which will reinforce the formation of hydrazone via azophenol to
quinone-hydrazone tautomerization (Scheme 2).18 This will result
The sigmoidal shape of the titration curve observed in 2 with
CNÀ is remarkable and characteristic of cooperative binding. The
best fit corresponds to a stoichiometry of 1:2 (1igand/anion) and
the stability constant is 9.54 Â 109 MÀ2 15
.
Thus, in accordance with
the 1:2 stoichiometry, the possible binding mode between 2
toward CNÀ has been proposed in Scheme 2. The respective stabil-
ity constants and number of equivalents required for achieving sat-
uration with various anions are tabulated in Table 1. The stability
constant values reveal that the AcOÀ ion binds most strongly and
interaction with various anions is in the order of AcOÀ > CNÀ
>
N
N
N
N
NH
N
A
A
H
O
O
H
H
H2O
N
HN
N
N
A
azophenol
hydrazone
-
CN-, AcO-, F-, H2PO4
A
=
Figure 5. Partial 1H NMR titration of 2 (5 Â 10À2 M) in DMSO-d6 with [BuN4]CN.
Scheme 2. Proposed binding mode of receptor 2 with anions in solution.
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4
N. Kaur et al. / Tetrahedron Letters xxx (2015) xxx–xxx
in a band at longer wavelength, that is, 470 nm allowing for visual
naked-eye detection.
calcd for C18H13N5O: (relative abundance (%), assignment) = 316
[53.5, (M+1)+]. Elemental analysis calcd for C18H13N5O: C, 68.56;
H, 4.16; N, 22.21. Found: C, 68.38; H, 4.28; N, 22.40.
Conclusions
Acknowledgments
In summary, we have synthesized a simple azo-dye 2 featuring
with benzimidazole group. The presence of AN@NA group in
receptor 2 improves the chromogenic and sensing ability toward
CNÀ, FÀ, AcOÀ, and H2PO4À in CH3CN. Presence of these anions
resulted in a naked-eye color change from light yellow to dark yel-
low. Other weak basic ions such as ClÀ, BrÀ, IÀ, and HSO4À did not
cause any changes in the UV–vis spectra. In particular, a tauto-
meric equilibrium might have occurred during the anion recogni-
tion whose different electronic properties are responsible for the
observed color and spectral changes.
The authors are greatly thankful to the SAIF, Panjab University
Chandigarh for recording the NMR and Mass spectra and are grate-
ful to the DST (Grant no. SR/FT/CS-36/2011) and UGC (Grant no.
AB2/12/3115) for financial assistance.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.01.
128.
Experimental
References and notes
Synthesis of 2-(1H-benzoimidazol-2-yl)-phenol (1)
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The receptor 1 was synthesized according to the literature
reported.19 A mixture of o-hydroxy benzaldehyde (4 mmol), o-
phenylenediamine (4.5 mmol) and PEG-400 (0.1 ml) were taken
together and heated in a microwave oven at 110 °C, 600 W for
10 min. After completion of the reaction (tlc), the product was
directly recrystalized from hot methanol to afford pure 1. Light
4. Kumar, S.; Kaur, N. Supramol. Chem. 2006, 18, 137–140.
pale yellow solid, yield 82%, mp 142 °C, IR (cmÀ1): 3318 (
mO–H, H-
5. (a) Gale, P. A. Acc. Chem. Res. 2006, 39, 465–475; (b) Sung, O. K.; Rowshan, A. B.;
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bonded), 3232 N–H), 3055
(
m
(
mAromatic C–H). 1H NMR (CDCl3,
400 MHz) d (ppm): 6.94 (dd, J1 = 11.8, J2 = 6.0 Hz, 2H, C4,Ar-H),
7.23 (dd, J1 = 5.16, J2 = 2.88 Hz, 2H, Ar-H), 7.32 (t, J = 5.85 Hz, Ar-
H), 7.60 (s, 2H, Ar-H), 7.98 (d, J = 5.88 Hz, 1H, Ar-H), 13.11 (s, 2H,
–OH phenolic ring, –NH– benzimidazole ring). 13C NMR (CDCl3,
100 MHz) d (ppm): 112.5, 117.0, 118.8, 123.1, 125.9, 131.4,
151.6, 158.0 (aromatic carbons). Mass: m/z calcd for C13H10N2O
(relative abundance (%), assignment) = 211 [100, (M+1)+]. Elemen-
tal analysis calcd for for C13H10N2O: C, 74.27; H, 4.79; N, 13.33.
Found: C, 74.35; H, 4.58; N, 13.42.
8. (a) Kato, R.; Nishizawa, S.; Hayashita, T.; Teramae, N. Tetrahedron Lett. 2001, 42,
5053–5056; (b) Miyaji, H.; Collinson, S. R.; Prokes, I.; Tucker, J. H. R. Chem.
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Synthesis of 2-(1-H-benzoimidazol-2-yl)-4-(pyridine-3-ylazo)-
phenol (2)
10. (a) Kulig, K. W. Cyanide Toxicity; U.S. Department of Health and Human
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4220.
3-Aminopyridine (0.94 g, 0.01 mol) was suspended in 30 ml of
water at 0–5 °C. Then 1.0 ml of 6.0 M HCl was added to the mix-
ture. After 15 min, 10.0 ml aq solution of NaNO2 (0.076 g) along
with a solution of receptor 1 in THF was added. Then the pH was
adjusted to 8–9 with 2 M NaOH solution. Mixture was stirred for
2 h and then neutralized with 1 N HCl solution. The precipitates
were filtered and washed with deionized water. The crude product
was purified by silica column chromatography to yield pure 2.
Orange solid, yield 50%, mp 182 °C, IR (cmÀ1): 3355 (mO–H
H-bonded (broad)), 3060 ( C–H, aromatic), 1634 ( C@N), 1386,
1424 (
AN@NA). 1H NMR (CDCl3+DMSO-d6, 400 MHz) d (ppm):
, mN–H,
m
m
m
7.22 (d, J = 6.66, 1H, Ar-H), 7.26 (dd, J1 = 4.47, J2 = 2.25 Hz, 2H, Ar-
H), 7.58 (dd, J1 = 6.0, J2 = 3.5 Hz, 1H, Ar-H), 7.66 (dd, J1 = 4.2,
J2 = 2.4 Hz, 2HAr-H), 7.94 (d, J = 6.63, 1H, Ar-H),8.12 (d,
J = 6.09 Hz, 1H, Ar-H), 8.68 (d, J = 2.82 Hz, 1H, Ar-H), 8.86 (s, 1H,
Ar-H), 9.05 (s, 1H, Ar-H), 13.9 (br s, 2H, –OH phenolic ring, –NH–
benzimidazole ring) (Fig. S4). 13C NMR (CDCl3+DMSO-d6,
100 MHz) d (ppm): 112.8, 118.0, 122.9, 123.1, 124.1, 124.9,
126.6, 144.9, 145.8, 147.4, 150.7, 150.9, 161.8 (Fig. S5). Mass: m/z
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Please cite this article in press as: Kaur, N.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.01.128