Synthesis of a dihydroquinoline based merocyanine as a 'naked eye' and 'fluorogenic' sensor for hydrazine hydrate in aqueous medium and hydrazine gas

Article abstract of DOI:10.1039/c4ra02426e

1,2,2,4-Tetramethyldihydroquinoline (TMDQ) is formylated at the 6-position and the resulting quinoline aldehyde is condensed with 3-methyl-1-phenyl-1H- pyrazol-5(4H)-one to obtain a merocyanine 6-pyrazolinyl TMDQ. The merocyanine is a selective and sensitive sensor for hydrazine hydrate in aqueous ethanolic medium and also for hydrazine gas. The merocyanine detects hydrazine hydrate selectively in less than 5 s over other amines, anions, and metals. The detection can be done simply by the naked eye and quantification can be done by absorbance/fluorescence spectroscopy. Hydrazine hydrate as well as hydrazine gas can be detected by the TLC plate technique.

Full text of DOI:10.1039/c4ra02426e

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Synthesis of a dihydroquinoline based merocyanine
as a ‘naked eye’ and ‘fluorogenic’ sensor for
hydrazine hydrate in aqueous medium and
hydrazine gas†
Cite this: RSC Adv., 2014, 4, 30712
a
b
a
*
K. Vijay, C. Nandi and Shriniwas D. Samant
1,2,2,4-Tetramethyldihydroquinoline (TMDQ) is formylated at the 6-position and the resulting quinoline
aldehyde is condensed with 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one to obtain a merocyanine 6-
pyrazolinyl TMDQ. The merocyanine is a selective and sensitive sensor for hydrazine hydrate in aqueous
ethanolic medium and also for hydrazine gas. The merocyanine detects hydrazine hydrate selectively in
less than 5 s over other amines, anions, and metals. The detection can be done simply by the naked eye
and quantification can be done by absorbance/fluorescence spectroscopy. Hydrazine hydrate as well as
hydrazine gas can be detected by the TLC plate technique.
Received 19th March 2014
Accepted 3rd July 2014
DOI: 10.1039/c4ra02426e
www.rsc.org/advances
people started using cyanines for diverse applications.15 Mer-
ocyanines are a class of cyanines which have various applica-
tions.16 Merocyanines can be used as probes and markers in
chemical analysis, biology and medical applications, non-linear
optics, and organic semiconductors.16 Many cyanines contain
the quinoline moiety.17
Introduction
Hydrazine is an important industrial chemical used widely in
rockets and missiles as fuel,1 as a reducing agent,2 and as an
antioxidant in nuclear and electrical power plants.1 It has a vital
function in the pharmaceutical and chemical industries as a
catalyst, for dyes in textiles, and as an inhibitor.3 Despite these
many applications, hydrazine hydrate is extremely toxic.
Hydrazine is most likely a carcinogen and its threshold limit
value (TLV) is 10 ppb.4 It can induce damage of the central
nervous system (CNS), nose irritation, and temporary blind-
ness.5 Hence, designing a reliable and rapid detector that can
provide on-site and real-time recognition of hydrazine is very
desirable.
There are various detection methods for sensing toxic
chemicals,18 such as naked eye detection, spectrophotometry,19
spectrouorimetry,20 electrochemistry,21 GC,22 and HPLC.23
Each method has its own pros and cons with respect to porta-
bility, analysis time, sensitivity, and economy. However, a
sensor which can be used in multiple detection methods can
overcome these drawbacks and thus is advantageous. For
example, ‘naked eye’ sensors particularly show potential
because the alteration in colour can easily be observed without
any equipment and hence are more economic. On the other
hand, UV instrument can permit the quantication by the
measurement of the absorption intensity. Further, if a sensor
shows uorescence, it is an added benet. For example, the
analyte may be determined with more accuracy in a spectro-
uorimeter than in an UV instrument. In addition, uorescence
can be used in biological applications like cell imaging.
Merocyanines have a potential as sensors and many of them
contain a quinoline nucleus. This prompted us to synthesize a
merocyanine dye with dihydroquinoline joined to a heterocyclic
ring through a conjugation extending linker. In this paper
we would like to report the synthesis of (E)-3-methyl-1-phenyl-
4-((1,2,2,4-tetramethyl-1,2-dihydroquinolin-6-yl)methylene)-1H-
pyrazol-5(4H)-one which belongs to the class of merocyanines
and is a conjugate of 1,2,2,4-tetramethyldihydroquinoline and
pyrazolone. The merocyanine can effectively and competently
sense hydrazine hydrate. It can act as a rapid and selective
There are several reports on sensing of hydrazine.6–13 But very
few reports of ‘naked eye’ and ‘uorescent’ probes for hydrazine
hydrate can be found in the literature. Zhao reported a hydra-
zine sensor which contains expensive gold metal.11 Further, the
sensor does not show any uorescence properties. Very recently,
a hydrazine sensor which shows uorescence was reported.13
A
similar molecule was reported by Cui almost at the same time.14
However a probe which can be used either as a ‘naked eye’
detector, or in ‘UV-vis absorption’, as well as in ‘uorescence
technique’ is denitely a superior one.
Cyanines were initially used as developers in photography.
Because of their conjugated structure and useful properties,
aDepartment of Chemistry, Institute of Chemical Technology, Matunga,
Mumbai-400019, India. E-mail: samantsd@yahoo.com
bR & D, NOCIL Limited, C 37, TTC Industrial Area, Navi Mumbai-400705, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02426e
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naked-eye, uorescent, as well as absorbance sensor for
hydrazine hydrate in an aqueous alcoholic medium. It is easy to
synthesize, eco-friendly, sensitive, and selective. The sensing
does not involve any complicated buffer-making procedure, and
can be done simply in an ethanol–water system. To the best of
our knowledge, this is the rst report of a merocyanine acting as
a hydrazine sensor in an aqueous alcoholic medium and also
for hydrazine gas.
Fig. 1 Visual colour change in 7 (16.16 mM, in ethanol–water (8 : 2))
when hydrazine hydrate is added at different concentrations (0, 2, 3,
and 4 mM, (a)–(d) respectively).
Results and discussion
1,2,2,4-Tetramethyl-1,2-dihydroquinoline (1) was prepared by
the condensation of aniline with acetone.24 1 was N-methylated
by methyl iodide in DMF in the presence of potassium
carbonate to obtain 2. 2 was formylated using DMF/POCl3
according to the reported procedure to 1,2,2,4-tetramethyl-1,2-
dihydroquinoline-6-carbaldehyde (3).24 Phenyl hydrazine was
condensed with ethyl acetoacetate to obtain 3-methyl-1-phenyl-
1H-pyrazol-5(4H)-ones (6) as per the reported procedure.25 3 and
6 were condensed in the presence of TEA in ethanol to obtain 7
in 82% yield (Scheme 1). The structure of 7 was conrmed by 1H
NMR and 13C NMR as well as by ESI-MS. It was an intensely
orange coloured powder.
Water is the best solvent for any sensor. 7 was insoluble in
water, but soluble completely in ethanol–water (8 : 2 v/v).
Hence, all studies were done in ethanol–water (8 : 2 v/v). The
UV-vis spectrum showed lmax at 503 nm. When increasing
concentration of hydrazine hydrate was put in to a xed
concentration of 7, the absorbance of the dye decreased.
Hydrazine hydrate at different concentrations was added to
16.16 mM (concentration which obeyed Beer–Lambert's law) of
probe-7, (Fig. 1). This showed that with increasing concentra-
tion of hydrazine hydrate the red color of dye decreased linearly
and nally vanished. This visual response was instantaneous
(<5 seconds). Thus, 7 is a ‘naked-eye’ colorimetric sensor.
Moreover, the probe was not uorescent initially, but aer
addition of hydrazine hydrate, it showed uorescence at
432 nm.
Scheme 2 Proposed reaction of hydrazine with probe-7.
carbonyl group in pyrazolone and cleave the molecule to form
hydrazone 8 and pyrazolone 6 (Scheme 2). This leads to a
change in the absorbance of the probe. To conrm this, the
probe-7 was mixed with 5 eq. of hydrazine hydrate in ethanol
and stirred for 1 h at room temperature and the reaction was
monitored by TLC which showed the formation of a new
compound. This compound was matching with hydrazone of
quinoline aldehyde 3. The reaction was worked up and puried
by ash column chromatography to get an off white to light
yellow coloured product. The structure was further conrmed
by 1H, 13C NMR.
UV-Vis study
The probe sensed hydrazine hydrate very rapidly within few
seconds and a visible response was seen. In order to follow the
reaction between probe-7 and hydrazine hydrate, the absor-
bance of 7 at 503 nm was recorded with respect to time aer
mixing hydrazine hydrate. Using 160 ꢀ 10ꢁ5 M hydrazine
hydrate (concentration where the absorbance is almost negli-
gible) and 16.16 mM of 7, absorbance was measured with respect
to time (Fig. 2). The initial response was very rapid. The reaction
was complete aer 90 min.
The colour of the merocyanine is due to extended conjuga-
tion from quinoline ring to pyrazoline ring through the exo-
cyclic double bond on the pyrazoline ring. Hydrazine hydrate
may attack the linker double bond that is conjugated to the
UV-Vis spectrum of 7 was recorded with increasing concen-
tration of hydrazine hydrate (Fig. 3a and b). Addition of
hydrazine to the solution of 7 led to a decrease in the absor-
bance, which vanished aer addition of about 99 equiv. (160 ꢀ
10ꢁ5 M) of hydrazine hydrate. All the spectra were recorded 90
min aer the addition of hydrazine hydrate. In the range of 2 ꢀ
10ꢁ5 to 180 ꢀ 10ꢁ5 M of hydrazine hydrate for 16.16 mM of 7, the
change in absorption was inversely proportional to the
concentration of hydrazine. Thus, 7 acted as an efficient ‘turn-
off absorbance’ probe for the selective detection of hydrazine
hydrate in an aqueous alcoholic medium. The experiments were
repeated for three times to check the reproducibility of the
results.
Scheme
1 Synthesis of (E)-3-methyl-1-phenyl-4-((1,2,2,4-tetra-
methyl-1,2-dihydroquinolin-6-yl)methylene)-1H-pyrazol-5(4H)-one.
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Selectivity of 7
Some metals may form complex with the probe and interfere
with the sensing process. In fact anions and amines also may
interfere. Hence the selectivity of the probe towards hydrazine
hydrate was studied with respect to various common metals,
anions, and neutral molecules. For 1 equivalent of probe-7, 50
equivalents of interfering anions/cations/neutral molecules
were added separately and the absorption spectra was recorded
(ESI†). All these spectra were recorded 90 minutes aer the
addition of anions/cations/neutral molecules. The graph
showing the extinction ratio at 503 nm was plotted (Fig. 4a–c).
Hydrazine hydrate (50 equivalents/80 ꢀ 10ꢁ5 M) only is found to
affect the absorption of 7 to a signicant extent. Thus probe-7
showed excellent selectivity towards hydrazine hydrate.
Fig. 2 Plot of absorbance at 503 nm with respect to time of a system
containing 160 ꢀ 10ꢁ5 M hydrazine hydrate and 16.16 mM of 7.
Fluorescence study
A graph of extinction ratio (A0
ꢁ A503)/A0
versus hydra-
503
An interesting feature of probe-7 was that aer addition of
hydrazine hydrate is UV absorption was ‘turned off’. When the
resulting solution was kept in an UV chamber at 254 nm,
hydrazine hydrate ‘turned on’ the emission feature of the probe.
Hence the uorescence study of the probe was carried out. The
uorescence spectra of probe-7 and 7 in the presence of
increasing concentrations of hydrazine hydrate are given in
Fig. 5a. The action of hydrazine with probe-7 turns-on the
emission feature and shows peak at 432 nm. Fluorescence
spectral changes were estimated aer 90 minutes quantitatively
503
zine hydrate concentration was plotted (Fig. 3c and d). This
showed good sensitivity of the probe upon addition of hydra-
zine hydrate. The extinction ratio showed a good linear rela-
tionship with concentration of hydrazine hydrate between 1.23
to 61.9 equivalents (i.e., 2 to 100 ꢀ 10ꢁ5 M) (Co-efficient of
determination (R2) given in the graph). The detection limit26
of probe-7 for hydrazine hydrate was estimated to be of 16 ꢀ
10ꢁ7 M (ESI†).
Fig. 3 (a) UV-Vis absorption spectra of the 7 upon the addition of hydrazine hydrate from 2 to 20 ꢀ 10ꢁ5 M (b) UV-Vis absorption spectra of the 7
upon the addition of hydrazine hydrate from 20 to 180 ꢀ 10ꢁ5 M. (c) Plot of (A0503 ꢁ A503)/A0503 vs. hydrazine hydrate concentration from 0 to
20.0 ꢀ 10ꢁ5 M. (d) Plot of (A0503 ꢁ A503)/A0503 vs. hydrazine hydrate concentration from 20 to 180 ꢀ 10ꢁ5 M; error bars for extinction ratio are
shown using standard deviations from the mean.
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Fig. 4 (a) UV response of probe-7 (16.16 mM) to 50 equivalents of various metals and hydrazine. (b) UV response of probe-7 (16.16 mM) to 50
equivalents of various anions and hydrazine. (c) UV response of probe-7 (16.16 mM) to 50 equivalents of various neutral molecules.
with increasing concentrations of hydrazine (20–400 ꢀ 10ꢁ5 M) probe with respect to other common interfering anions, cations,
in ethanol–water system (8 : 2) (Fig. 5b). The uorescence and related amino molecules also was checked. This study
intensity reached a point of negligible deviation upon the showed very high selectivity of the probe towards hydrazine
addition of 99 equiv. (160.0 ꢀ 10ꢁ5 M) of hydrazine. The ratio- hydrate among others (ESI†).
metric changes under uorescence were same as those observed
in UV-vis studies. A linear relationship was found between the
emission intensity at 432 nm and the hydrazine concentration
over 20–140 ꢀ 10ꢁ5 M range. The uorescence response of the
NMR study
1H NMR and 13C NMR were used for characterizing the product.
The 1H NMR of the probe-7 and that of product-8 on reaction of
7 with hydrazine hydrate were recorded. From the spectra it
appears that the pyrazolone part was entirely removed in 8.
1
(Fig. 6). H NMR of product-8 shows loss of methyl (d ¼ 2.33)
and phenyl groups of pyrazolone unit. The proton of probe-7 at
8.47 (]CH–C) is shied to 8.58 (N]CH–C) in the product-8.
1
Further when the H NMR spectrum of 8 was compared with
Fig. 5 (a) Fluorescence spectra of probe-7 (16.16 mM) with increasing
concentrations of hydrazine hydrate. (b) Variation of Fl intensity of
probe-7 (16.16 mM) at 432 nm with increasing concentrations of
hydrazine hydrate; error bars for extinction ratio are shown using
standard deviations from the mean.
Fig. 6 1H NMR spectra in CDCl3 of probe-7 and product-8.
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that of 3, the only difference was found on the peak position of
aldehydic H of 3 (d ¼ 9.8) and the proton of imine (d ¼ 8.58).
TLC plate detection techniques
Detection of hydrazine hydrate by TLC plates. Probe-7 was
dissolved in ethyl acetate to get a 5 mM solution. Thin layer
chromatography (TLC) plates were dipped into it and kept for 30
min at room temperature for drying. These plates were
immersed into solutions of different concentrations of hydra-
zine hydrate in ethanol–water (8 : 2) medium and then exposed
to air to get rid of the solvent. A minimum of 1 mM of hydrazine
hydrate was easily recognizable by naked eye in this TLC plate
technique. This is similar as observed in naked eye detection in
aqueous ethanolic medium in Fig. 1. Hence the detection limit
of hydrazine hydrate by TLC plate technique is 1 mM. In Fig. 7,
the complete colour change from orange to colourless is shown.
Detection of hydrazine gas. The probe could rapidly and
effectively sense hydrazine gas. This experiment was carried out
using a TLC plate coated with probe-7. A 5 mM solution of the
probe was prepared in ethyl acetate. A TLC plate was dipped
into it and dried at room temperature for 30 min. The TLC plate
was kept in a vial. Excess of hydrazine gas was passed into the
vial for 5 s. The colour of the TLC plate changed from orange to
colourless within 5 s (Fig. 8).
Fig. 9 HOMO–LUMO levels and optimized structures of probe-7 and
its reduced product from Gaussian 03.
DFT study
the FMOs of probe-7 and product-8 are shown in Fig. 9. The
HOMO–LUMO orbital distribution is spread from the quinoline
moiety to the pyrazolone moiety. There is a planarity over the
major chromophore extending from quinoline to pyrazolone
unit. The HOMO–LUMO energy gap in the probe-7 is less than
that of the product-8 because of extended conjugation. This
observation is matching with the fact that the probe is coloured
and the product is not.
In order to throw more light on the actual chromo/uorophore,
DFT studies were done on 7 and the possible reduced product-8.
DFT study was done using Gaussian 03 program.27 We per-
formed B3LYP exchange correlation functional under 6-311G+,
p, d basis set and calculated HOMO and LUMO levels. The
optimized geometries and calculated electron distributions in
Conclusion
A merocyanine probe is described that provides very good
sensitivity for detecting hydrazine without needing any
advanced, complex readout equipment or complex buffer
making procedure. The synthesis of the probe is very easy. The
detection limit of the probe for hydrazine hydrate is 1.6 mM. The
gaseous hydrazine is detected by the probe within 5 seconds.
The TLC plate technique provides a more practical and sensitive
detection method.
Fig. 7 Visible colour changes of probe-7 (5.0 mM)-coated on TLC
plates after exposure to different concentrations (0%, 0.5%, 2.0%, 3.0%,
and 6.0%) of hydrazine hydrate.
Acknowledgements
The authors are thankful to IIRBS, Kerala, India and GJUST,
Punjab, India for providing NMR spectra. KV is thankful for
CSIR, New Delhi, India for a fellowship.
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Fig. 8 TLC plate before hydrazine gas passing, and after excess
hydrazine gas passing for 5 seconds.
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