Fluoride-selective colorimetric sensors based on hydrazone functionality

Article abstract of DOI:10.1246/cl.2004.850

Hydrazone derivatives 2, 3, 5, 6 with H-bond donor and color-reporting chromophore were synthesized to be used as chemosensors for selective detecting anionic species via naked eye and absorption spectroscopy. The results revealed that these compounds exh

Full text of DOI:10.1246/cl.2004.850

850
Chemistry Letters Vol.33, No.7 (2004)
Fluoride-selective Colorimetric Sensors Based on Hydrazone Functionality
Lili Zhou, Xiaohong Zhang,ꢀ and Shikang Wuꢀ
Nano-Organic Photoelectronic Labsoratory, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing, 10010, P. R. China
(Received March 29, 2004; CL-040337)
Hydrazone derivatives 2, 3, 5, 6 with H-bond donor and col-
of 100 equivalents of diverse anions in acetonitrile, respectively
(anions were added in the form of tetrabutylammonium salts).
The most remarkable effect was observed when Fꢁ was added.
The colors of both compounds 2 and 3 solutions changed from
yellow to purple upon addition of Fꢁ. For compound 2, the peak
at 396 nm disappeared while a new peak appeared at 562 nm in
absorption spectra, no significant color change was observed
upon adꢁdition of H2PO4ꢁ, Clꢁ, Brꢁ, Iꢁ, HSO4ꢁ, NO3ꢁ, AcOꢁ,
or ClO4 anions. This hints that there is no interaction between
compound 2 and anionic species listed above except for Fꢁ.
However, the color of compound 3 solution changed from yel-
low to dark yellow upon addition of H2PO4ꢁ, while it was sim-
ilar to compound 2 when other anionic species were added. The
or-reporting chromophore were synthesized to be used as che-
mosensors for selective detecting anionic species via naked
eye and absorption spectroscopy. The results revealed that these
compounds exhibited good selectivity to fluoride anion (Fꢁ). In
order to clarify the mechanism of interaction between anionic
species and sensors, compound 4 was synthesized for compara-
tive study.
The design and synthesis of systems that are capable of sens-
ing various biologically and/or chemically negative charged
species is an area of the current interest.1 This is due to the cru-
cial roles anions play in biological processes, medicine, cataly-
sis, and molecular assembly.2 Moreover, various anions are be-
lieved to have deleterious effect on the environment.3 Redox and
photoactive sensor molecules have been developed to coordinate
with anions and report their presence via changes of physical
properties (such as redox potential, fluorescence, or phosphores-
cence) of hosts.4 A more attractive approach in this field involves
the construction of colorimetric anion sensors5 which have con-
siderable advantages over others,6 because it can be carried out
without potentiostat or spectrometer. However, few cases in
such system have been reported so far.7 For example, aminoan-
thraquinone has been used to monitor various anions in organic
solution through the color changes with poor selectivity.8 Re-
cently, we discovered that hydrazone function group could act
as powerful colorimetric sensors selective to Fꢁ anions over oth-
er anionic species and its derivatives are very easy to be synthe-
sized in high yield by reacting p-nitrophenylhydrazine with sub-
stituted benzaldehyde in ethanol.
ꢁ
absorption spectra also indicate that the addition of H2PO4 can
induce significant bathochromic shift, though less dramatic than
that induced by addition of Fꢁ. These results indicate that com-
pounds 2 and 3 (especially, compound 2) could be used as a nak-
ed-eye sensor for effective recognition of Fꢁ.
–
F–
F–
H2PO4
b
0.25
0.20
0.15
0.10
0.05
0.00
a
0.15
0.10
0.05
0.00
400
500
600
700
400
500
600
700
λ
/nm
λ
/nm
Figure 2. UV–vis absorption spectra of compound 2(a), 3(b)
(1 ꢂ 10ꢁ5 mol/L) after addition of 100 equiv. of anions (Fꢁ,
H2PO4ꢁ, Clꢁ, AcOꢁ, HSO4ꢁ, Brꢁ, NO3ꢁ, ClO4ꢁ, and Iꢁ) in
acetonitrile, respectively.
The structures of the six compounds studied as colorimetric
sensors for detecting anionic species are listed in Figure 1. Be-
cause there exist several different binding sites in these mole-
cules, it is necessary to clarify where the key site is for the rec-
ognition of anions.
In order to clarify the interaction between sensing molecules
and anionic species, a series of similar compounds with differen-
tial functionalities were synthesized (seen in Figure 1). Com-
pared to compound 2, compound 3 possesses a hydroxy group
at para-position of phenyl group. The specific ability of com-
pound 2 to recognize the Fꢁ and the recognition ability of com-
NO2
NO2
NO2
NO2
NO2
pound 3 to both Fꢁ and H2PO4 imply that hydrazone function
ꢁ
NH
N
N
group prefers to interact with Fꢁ. In order to determine whether
the hydrogen on carbon or on nitrogen of hydrazone function
group participates in the Fꢁ recognition, compound 5 and 6 in
which a methyl group replaces the hydrogen on carbon were syn-
thesized. The color of compounds 5 and 6 solutions similarly
changed from yellow to purple when Fꢁ was introduced. This
hints that the hydrogen on nitrogen is response for the recogni-
tion of Fꢁ. To further verify the role of the hydrogen on nitrogen
played on the recognition of Fꢁ, compound 4, a pyrazolin deriv-
ative in which an alkyl group replaces both hydrogens of hydra-
zone function group were synthesized and investigated upon ad-
dition of anions. It is very interesting to note that the addition of
NH
NH
NH
NO2
N
N
N
C
N
CH
CH3
C
CH3
CH
OH
OH
OH
OH
1
2
3
4
5
6
Figure 1. The molecular structures of the hydrazone com-
pounds.
The color of the solution changed when the effective inter-
action between sensors and anions occurred. Figure 2 displays
the UV–vis absorption spectra of compound 2, 3 upon addition
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.7 (2004)
851
ꢁ
ꢁ
either Fꢁ or H2PO4 induces a little color change and the appa-
bonding between H2PO4 and H donor of hydrazone deriva-
tives. This is further confirmed from results that no significant
color change was observed in compound 2 (without OH) solu-
tion upon addition of H2PO4ꢁ, while color change occured when
rent color change requires the large amount of anion (the absorp-
tion peak, originally at 405 nm, shifts to 438 and 422 nm upon
addition of 100 equivalents of tetrabutylammonium fluoride
and dihydrogen phosphate, respectively). These results corrobo-
rate that the hydrogen on nitrogen in this series of hydrazone de-
rivatives is the specific site for the recognition of Fꢁ. Unfortu-
nately, in our case, attempts to acquire complex stability
constants from titration absorption experiments were unsuccess-
ful. The absorption bands of these hydrazone derivatives kept
shifting to the red when anions were added (data not shown).
This hints that the interactions between hydrazone derivatives
and anions are very complicated. However competitive assays
indicate that the system is selective to Fꢁ. For instance, the ad-
dition of a mixture of Fꢁ, H2PO4ꢁ, Clꢁ, AcOꢁ, HSO4ꢁ, Brꢁ,
NO3ꢁ, ClO4ꢁ, and Iꢁ to the solutions of compounds 2, 5, or 6
in acetonitile resulted in the absorbance shift in UV–vis absorp-
tion spectra similar to that obtained from the addition of Fꢁ.
In order to understand the mechanism of the interaction be-
tween anionic species and sensors, the binding properties of re-
ceptor 2 to different anions were further investigated by using 1H
NMR experiments in DMSO-d6. The complexation-induced
chemical shifts can be used to infer a quite detailed picture of
ꢁ
H2PO4 was added into compound 3 (with OH) solution.
In these compounds, the hydrazone function group is the
most important part for the recognition of anionic species. It
can be illustrated from the interaction between the control com-
pound 1 and different anions. There is no significant color
change observed upon addition of 100 equivalents diverse
anions. It displays that the hydrazone function group is necessary
for the color-reporting resulted from its sensitivity to anions.
In summary, a new family of easy-to-prepare reagents used
as anionic sensors has been synthesized and studied. It was found
that compounds 2, 5, 6 are highly selective to Fꢁ, in concomitant
with color change and compound 3 can interacts with both
H2PO4ꢁ and Fꢁ. The present study demonstrates that hydrazone
derivatives can be used as powerful and effective colorimetric
anion chemosensor.
We thank the Major State Research Development Program
of China (Grant NO. G2000078100) and the Chinese Academy
of Sciences for financial Support.
1
anions binding. Although the H NMR spectrum of 2 shown a
signal at 8.15 ppm for the NH proton, this signal became broad
and undetectable upon addition of 5 equivalence of Fꢁ anion.
However, the addition of Fꢁ led to significant downfield shifts
(up to 0.52 ppm) for the signals arising from the protons of the
phenyl group. In contrast, no shift was observed for receptor 2
upon addition of other anions, suggesting that the interaction be-
tween receptor 2 and other anions is energetically unfavorable.
These findings indicate that the proton on nitrogen is strongly
perturbed by added Fꢁ.9 In addition, treating 2 with increasing
amount of n-Bu4NþOHꢁ (1.0 M solution in methanol) in
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Published on the web (Advance View) June 14, 2004; DOI 10.1246/cl.2004.850