8(E)-4-[{2-(2,4-dinitrophenyl)hydrazono}benzene-1,3-diol] as a solvatochromic Schiff base and chromogenic signaling of water content by its deprotonated form in acetonitrile

Article abstract of DOI:10.1039/c4ra03249g

A simple, cost effective Schiff base (E)-4-[{2-(2,4-dinitrophenyl) hydrazono}benzene-1,3-diol] (DBH) is synthesized and characterized by various physico-chemical and spectroscopic tools along with single crystal X-ray crystallography. This is the first report which describes its solvatochromism in solution and the signaling behavior of deprotonated DBH for the determination of water content in acetonitrile through UV-visible absorption spectra. Addition of acetate ions in acetonitrile solution of DBH results in the formation of deprotonated DBH. The effect of water content on the formation of deprotonated DBH is utilized in signaling. The deprotonated DBH exhibits a pronounced chromogenic signaling behavior that can be detected by the naked eye in response to the changes in water content in acetonitrile. Prominent color changes are observed up to 2% water content in acetonitrile and limit of detection (LOD) of the deprotonated DBH for determination of the water content in acetonitrile is calculated as 0.012%.

Full text of DOI:10.1039/c4ra03249g

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PAPER
8(E)-4-[{2-(2,4-dinitrophenyl)hydrazono}benzene-
1,3-diol] as a solvatochromic Schiff base and
chromogenic signaling of water content by its
deprotonated form in acetonitrile†
Cite this: RSC Adv., 2014, 4, 27556
*
Karishma Tiwari, Monika Mishra and Vinod P. Singh
A simple, cost effective Schiff base (E)-4-[{2-(2,4-dinitrophenyl)hydrazono}benzene-1,3-diol] (DBH) is
synthesized and characterized by various physico-chemical and spectroscopic tools along with single
crystal X-ray crystallography. This is the first report which describes its solvatochromism in solution and
the signaling behavior of deprotonated DBH for the determination of water content in acetonitrile
through UV-visible absorption spectra. Addition of acetate ions in acetonitrile solution of DBH results in
the formation of deprotonated DBH. The effect of water content on the formation of deprotonated DBH
is utilized in signaling. The deprotonated DBH exhibits a pronounced chromogenic signaling behavior
that can be detected by the naked eye in response to the changes in water content in acetonitrile.
Prominent color changes are observed up to 2% water content in acetonitrile and limit of detection
(LOD) of the deprotonated DBH for determination of the water content in acetonitrile is calculated as
0.012%.
Received 10th April 2014
Accepted 9th June 2014
DOI: 10.1039/c4ra03249g
www.rsc.org/advances
alternative optical methods are quite desirable in terms of
convenience and instrument requirements.11 Selective and
1. Introduction
sensitive optical signaling systems that utilize multifunctional
dye molecules12 such as merocyanines,13 avones,14 chalcone,15
3-hydroxychromone,16 naphthalimide17 and indole derivatives18
have been developed as probes to sense the water content of
commonly used organic solvents. In this case, dye molecules
convert a chemical interaction or recognition process into an
optically detectable signal. Optical signaling is simple and
convenient compared to the traditional method. In addition,
optical signaling is possible to use in situ monitoring and even
allows naked eye detection. Another optical water sensing
system utilizes dye-anion complexes. Chang et al. reported the
dye–anion complex as a water sensor that utilized the disruptive
effect of water on complexation of dyes with anions.19 In this
system, water destroyed hydrogen bonds between dye and
anion. In addition Kang et al. utilized the deprotonation and
protonation of anion receptor for water sensing.11
Herein, for the rst time, we explored the solvatochromism
in solution of a Schiff base, (E)-4-[{2-(2,4-dinitrophenyl)hydra-
zono}benzene-1,3-diol] (DBH) and water sensing behavior of
deprotonated DBH through UV-visible absorption spectra. Our
selection of a 2,4-dihydroxy Schiff base for the present study, is
based on the reason that in such Schiff bases, one ortho hydroxy
group stabilizes the molecular conformation by forming an
intramolecular (O–H/N or O/H–N type) hydrogen bond, while
the other hydroxy group is responsible for a series of intermo-
lecular interactions between neighbors or solvent molecule. The
Solvatochromism is an important property of a solute because
most chemical reactions and all biochemical reactions in the
human body occur in solution.1 Solvent effects are related to the
nature and the extent of the solute–solvent interactions devel-
oped in the solvation shell of the solutes.2–4 The solvent effect
can be determined by solvent polarity scale or solvatochromic
parameters.5 The solvent dependent spectral shis can arise
from either non-specic (dielectric enrichment) or specic (e.g.
hydrogen-bonding) solute–solvent interactions.6
As water is the most common impurity in organic solvents,
determination and control of water contents in organic solvents
or chemical products are highly important in laboratory
chemistry and industrial processes.7,8 Karl Fisher titration9 and
gas chromatography10 have been traditionally used for the
quantitative measurement of water in organic solvents.
Although these approaches have several useful characteristics,
some disadvantages such as the requirement for skilled
personnel and specialized equipment, the incapability of per-
forming continuous monitoring, and interference from other
co-existing species limit their wider application. However,
Department of Chemistry, Banaras Hindu University, Varanasi-221005, India. E-mail:
singvp@yahoo.co.in; Tel: +919450145060
† Electronic supplementary information (ESI) available: 1H & 13C NMR, IR, Mass
spectra and crystallographic data of DBH, detection limit curve plot for water.
CCDC 978704. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4ra03249g
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intermolecular hydrogen bonding tuned the solvatochromism dilution in all the solvents (chloroform, methanol, ethanol,
of DBH has been observed in hydrogen bond acceptor (HBA) acetonitrile and DMSO). For UV-visible experiment in various
and hydrogen bond donor (HBD) type solvents. These obser- solvents, 50 mM solution of DBH was used. For anion sensing
vations were supported by TD-DFT calculations on DBH. Since through UV-visible spectral studies, 50 mM solution of DBH and
HBA type solvents can promote deprotonation of DBH by tetrabutylammonium salts of anions in acetonitrile were used.
accepting hydrogen bond from DBH, this has been investigated For water sensing, 50 mM solution of DBH + 10 equivalents of
1
by recording the UV-visible absorption and H NMR spectra of tetrabutylammonium acetate in acetonitrile and triple distilled
1
DBH in the presence of stronger HBA type molecules (acetate water were used. H NMR titration was performed by treating
ions) in DMSO solution.
10ꢀ2 M solution of DBH in DMSO-d6 with 10ꢀ1 M solution of
It has been well established that water interrupts deproto- tetrabutylammonium acetate ions in DMSO-d6 and varying the
nation/complex formation of hydrazones by anions11,19 and equivalents (0.1, 0.3, 0.5, 1.0 and 5.0) of acetate ions.
hence, the investigation of change from deprotonated DBH/
To determine the S/N ratio, the absorbance of 50 mM solution
DBH–acetate complex to DBH could be convenient signaling of DBH + 10 equivalents of tetrabutylammonium acetate ions in
system to determine the water content of organic solvent. The acetonitrile without water was measured by 10 times and the
variation of water content in the acetonitrile is sensitively standard deviation of blank measurements was determined.
visualized by ratiometry as well as naked-eye detectable color The detection was calculated as three times the standard devi-
changes. The design and synthesis of the DBH is much easier ation (s) from the blank measurement (in the absence of water)
and cost effective than the reported water sensors. In literature a divided by the slope of calibration plot between % of water and
number of papers have been published for anion sensors absorbance (eqn (1)).23
that are based on the molecular framework of hydrazones,
thioureas, and amide derivatives.20 Hydrazones of 2,4-din-
trophenylhydrazine are of great interest as anion sensors due to
their interaction with anion through hydrogen bonding or
deprotonation of hydrazones in the presence of anions in
various solvents.21,22 Although many papers have been reported
on water sensors based on dye molecules, the work on anion
receptors acting as water sensors are very few in number.
Detection limit ¼ 3s/slope
(1)
Single crystal X-ray diffraction data for the DBH was
obtained at 293(2) K, using a graphite mono-chromated Mo Ka
(l ¼ 0.71073 A) radiation source. The structure was solved by
˚
direct method (SHELXL-97) and rened against all data by full
matrix least-square on F2 using anisotropic displacement
parameters for all non-hydrogen atoms. All hydrogen atoms
were included in the renement at geometrically ideal position
and rened with a riding model.24,25 The Mercury and ORTEP-3
soware packages for windows program were used for gener-
ating structures.26,27
2. Experimental
2.1 Materials
All analytical reagent grade chemicals were obtained from the
commercial sources. 2,4-dinitrophenylhydrazine, 2,4-dihydroxy-
benzaldehyde and tetrabutylammonium salts of anions were
purchased from Sigma-Aldrich Chemicals, USA. Methanol,
ethanol and chloroform solvents were purchased from Merck
Chemicals, India. DMSO and acetonitrile of spectroscopy grade
were purchased from Spectrochem Pvt. Ltd. India and S.D. Fine
Chemicals, India, respectively.
The density functional theory using Becke's three parame-
terized Lee–Yang–Parr (B3LYP) exchange functional with 6–
311G** basis sets, in Gaussian-03 programs28 has been
employed to obtain optimized structure of DBH in gaseous
state. Based on the optimized geometries, TD-DFT calculations
were performed at the same B3LYP level to calculate the vertical
electronic transition energies.29
2.4 Synthesis of DBH
2.2 Instrumentation
The Schiff base (E)-4-[{2-(2,4-dinitrophenyl)hydrazono}benzene-
1,3-diol] (DBH), was synthesized by a slight modication of the
reported method.30 The ethanolic solution of 2,4-dini-
trophenylhydrazine (10 mmol) was reacted with ethanolic
solution of, 2,4-dihydroxybenzaldehyde (10 mmol) in a round
bottom ask. The reaction mixture was reuxed for 4 h and a
bright red crystalline product was obtained on cooling the
above solution at room temperature. The product was ltered
C, H, N contents were determined on an Exeter Analytical Inc.
1
CHN Analyzer (Model CE-440). H and 13C NMR spectra were
recorded in DMSO-d6 on a JEOL AL-300 FT-NMR multinuclear
spectrometer. Chemical shis were reported in parts per
million (ppm) using tetramethylsilane (TMS) as an internal
standard. Infrared spectra were recorded in KBr on a Perkin
Elmer FT-IR spectrophotometer in the 4000–400 cmꢀ1 region.
UV-visible spectra in various solvents were recorded on a Shi-
madzu spectrophotometer, Pharmaspec UV-1700 model. Single
crystal X-ray diffraction data for DBH was collected on an Oxford
Diffraction Gemini Diffractometer equipped with CrysAlis Pro.
¨
on a Buchner funnel and washed several times with aqueous
ethanol (5%, v/v). The pure compound was recrystallized from
hot ethanol and dried in a desiccator at room temperature. The
crystals suitable for X-ray diffraction analysis were grown by
slow evaporation of the saturated solution of DBH in DMSO.
2.4.1 Analytical data. Bright red crystalline, yield 70%. M.p.
2.3 General methods
The stock solution of DBH (10ꢀ2 M) was prepared in DMSO > 250 ꢁC, anal. calc. for C13 H10 N4 O6 (318): found C, 49.12;
which was used for UV-visible absorption studies through H, 3.14; N, 17.57; calc: C, 49.06; H, 3.17; N, 17.61%. ESI-MS:
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sharp singlet at 11.58, 10.12 and 9.95 ppm due to the presence
of –NH, –C2–OH and –C4–OH protons, respectively. The imine
proton appears as sharp singlet at 8.79 ppm and the aromatic
protons are found to be resonating in the region 8.84–6.34 ppm.
The 13C NMR (Fig. 3, ESI†) spectra shows signals at 161.37 and
158.73 ppm for C2–OH and C4–OH carbons, respectively. The
carbon signal of imine group is observed at 136.27 ppm. The
molecular ion peak observed at m/z ¼ 317.05 in ESI-MS (Fig. 4,
ESI†) corresponds to [M ꢀ H]ꢀ also validates the formation of
DBH.
Fig. 1 Structure of the Schiff base, DBH showing hydrogen bonding
interaction sites for solvent molecules.
[M ꢀ H]ꢀ, m/z, 317.05 (100% abundance); IR/cmꢀ1 (KBr): n(O–
Finally the structure of DBH was conrmed by the single
crystal X-ray diffraction study. The details concerning the data
collection and structure renement are summarized in Table 1
ESI†. Relevant geometrical data and hydrogen bonding
1
H), 3421; n(N–H), 3268; n(C]N), 1617. H NMR d ppm (DMSO-
d6): 11.58 (s, 1H, –N–H); 10.12 (s, 1H, –OH); 9.95 (s, 1H, –C4–
2
OH); 8.79 (s, 1H, –CH]N); 8.84–6.34 (Ar-H). 13C NMR (DMSO-
d6): 161.37 (C2), 158.73 (C4),147.77 (C13), 144.21 (C11), 136.27
(C7), 129.74–102.46 (aromatic carbon). The structure of the
compound was nally conrmed by single crystal X-ray
diffraction technique. On the basis of above spectral data, the
following structure for the compound has been proposed
(Fig. 1).
3. Results and discussion
3.1 Structural characterization of DBH
The Schiff base, (E)-4-[{2-(2,4-dinitrophenyl)hydrazono}
benzene-1,3-diol] (DBH) has been characterized by infrared,
NMR and ESI-mass spectroscopy, and X-ray crystallography. IR
bands (Fig. 1, ESI†) observed at 3421 and 3268 cmꢀ1 are
attributed to the –OH and –NH stretching vibrations of DBH,
respectively. The IR band appeared at 1617 cmꢀ1 clearly indi-
cates the presence of an imine (>C]N–) functionality which
must have resulted due to the condensation of 2,4-dini-
trophenyl hydrazine with 2,4-dihydroxybenzaldehyde. The 1H
NMR spectra (Fig. 2, ESI†) of DBH in DMSO-d6 exhibits three
Fig. 3 Visible color change of DBH (50 mM) in different solvents (A)
methanol, (B) ethanol, (C) chloroform, (D) acetonitrile, (E) DMSO and
(F) 30% aqueous DMSO (upper), and corresponding UV-visible
absorption spectra of DBH (50 mM) (lower).
Fig. 2 (a) ORTEP diagram of DBH$DMSO showing atom numbering
scheme with ellipsoids of 30% probability (b) Diagram showing the Fig. 4 Theoretically optimized structure of DBH (left) and energy level
hydrogen bonds (blue dashed lines) and S/O interactions (black diagram for the frontier p MOs of first electronic transition in DBH
dashed line) in DBH$DMSO.
(right).
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parameters are given in Table 2 and 3, ESI†, respectively. The acceptor basicity) parameters which are commonly used to
ORTEP diagram of DBH was shown in Fig. 2a. The molecular describe solvent hydrogen-bonding properties.6 The data given
structure of DBH also contains a DMSO molecule which was in Table 1 indicate that chloroform and acetonitrile have small b
used as a solvent for crystallization. The unit cell dimensions value (0.10–0.40), whereas, ethanol and methanol not only have
show that the crystal belongs to the monoclinic system with the larger b (0.75–0.66) but also large a value (0.86–0.98). On the
P21/c space group. Two intra-molecular hydrogen bonds, O(1)– other hand, DMSO has largest b value (0.76) and smallest a value
H/N(1) and N(2)–H/O(3) help to facilitate the planarity of (0.0). Hence, DMSO is the strongest hydrogen bond acceptor
DBH. The DMSO molecule accept a hydrogen bond at >S]O (HBA) type solvent. It is well documented in literature that in
group from H-atom of the p-hydroxy group (C4–OH) of DBH to HBA type solvents, absorption spectra of solute molecule is red
form intermolecular hydrogen bond between DMSO and DBH shied due to the more stabilization of excited state than the
(Fig. 2b). The hydrogen bond distances and angles (Table 3, ground state of solute molecule. On the other hand, in hydrogen
ESI†) show that they stabilize DBH molecule signicantly.31 The bond donor (HBD) type solvents (e.g. ethanol, methanol), the
˚
intermolecular S/O distance, S(1)/O(2) of 3.2 A, is less than absorption spectra of solute molecules are blue shied due to
their sum of Van der Walls radii indicating prominent S/O more stabilization of ground state than excited state of a mole-
interactions as shown in Fig. 2b.32
cule.33 The intermolecular hydrogen bond between DBH and
DMSO is clearly visible even in the single crystal of DBH con-
taining a DMSO molecule as solvent of crystallization (Fig. 2).
TD-DFT calculations were performed on DBH to explain the
3.2 Solvatochromism
The naked eye visible color change and change in UV-visible reason behind more stabilization of DBH by HBA type solvents.
absorption spectra of DBH (50 mM) are examined at room As shown in Fig. 4, HOMO and LUMO of the rst electronic
temperature in three different type of solvents, non polar transition (460 nm) in DBH are prominently localized on dihy-
solvent chloroform, polar aprotic solvents acetonitrile and droxy phenyl and dinitrophenyl moieties, respectively, which
DMSO, and polar protic solvents ethanol and methanol (Fig. 3). suggest that the electron density migrates away from the dihy-
DBH exhibits yellow color in acetonitrile, chloroform, methanol droxy phenyl to dinitrophenyl moiety during excitation process.
and ethanol solvents. However, it gives orange color in DMSO. Hence, formation of the hydrogen bond between DBH and HBD
Hence, it is observed that highly polar aprotic solvent (DMSO) type solvents opposes this migration and stabilizes the ground
exhibits drastic visible change in color of DBH. In another state more (blue shi). However, HBA type solvents favor the
experiment, we analyzed the effect of water on visible color of electron density migration and stabilize the excited state more,
DBH in DMSO solution (50 mM). The 30% aqueous DMSO resulting a red shi in UV-visible absorption spectra of DBH.
solution of DBH exhibits a dark yellow color. These observations
suggest that water compete with DMSO solvent in inducing aqueous DMSO is not drastically shied from the absorption
orange color of DBH. band observed at 426 nm of DBH in DMSO solution. However,
Although the absorption band observed at 416 nm in 30%
The UV-visible absorption spectra of DBH in various solvents the color of DBH in 30% aqueous DMSO becomes dark yellow as
(50 mM) show a broad absorption band in the region 396–426 observed in other solvents. Therefore, the absorption spectra of
nm (Fig. 3). DBH has an absorption maximum at 396 nm in DBH in DMSO and 30% aqueous DMSO were compared and
acetonitrile, 397 nm in chloroform, 398 nm in methanol, 401 observed that the hyperchromic shi in the region 480–590 nm
nm in ethanol, 426 nm in DMSO (with signicant hyperchromic appeared for DBH in DMSO, got diminished in 30% aqueous
shi in the region 490–590 nm) and at 416 nm in 30% aqueous DMSO. Hence, it appears that the orange color of DBH in the
DMSO. Three factors are responsible for the positive sol- DMSO solution is due to the hyperchromic shi in the region
vatochromism in various solvents (a) dielectric constant of the 480–590 nm. These observations suggest the possibility of
solvents (b) the ability of the solvent to form stronger inter- existence of two different species of DBH (absorbing distinctly
molecular H-bonds with solute molecule (c) selective solvation. at 426 nm and in the region 480–590 nm) in DMSO. The
It is observed that the absorption spectra of the DBH in DMSO absorption band observed at 426 nm may be due to the highly
(426 nm) is red shied as compared to other solvents, indi- populated hydrogen bonded DBH to DMSO. However, the
cating relatively strong host–guest interaction between the DBH hyperchromic region 480–590 nm may be due to the absorbance
and DMSO environment. In addition, it is also observed that the
spectral shi in various solvents does not change signicantly
with polarity (p* parameters) of all the employed solvents (Table
1).6 Since the absorption maxima do not correlate with the
Table 1 Solvent polarity parameters. Where, a is solvent hydrogen-
bond donor acidity, b is solvent hydrogen-bond acceptor basicity and
p* is a measure of the solvent dipolarity/polarizability
polarity (non specic interactions) of the solvents, the positive
solvatochromic effect in DMSO can be explained with the help
of specic solute–solvent interactions (e.g. hydrogen bonding).
The hydrogen bonding sites of DBH for solvent molecules are
shown in Fig. 1.
The solvent interaction with solute in terms of hydrogen
bonding, can be described by the Kamlet–Ta, a (solvent
hydrogen-bond donor acidity) and b (solvent hydrogen-bond
Solvent
a
b
p*
Water
Methanol
Ethanol
Chloroform
Acetonitrile
DMSO
1.17
0.98
0.86
0.20
0.19
0.00
0.47
0.66
0.75
0.10
0.40
0.76
1.09
0.60
0.54
0.58
0.75
1.00
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of low populated deprotonated DBH. Since, DMSO is a polar product was ltered, washed several times with diethyl ether to
and strong HBA type solvent, it may promote the deprotonation remove acetic acid and dried in a desiccator.
of acidic protons (NH, C2–OH & C4–OH) of DBH. On deproto-
3.3.1 Analytical data. Purple, yield 65%. M.p. > 250 ꢁC,
nation, a substantial delocalization of negative charge leads to a anal. calc. for C61H115N7O6 (1042): found C, 70.15; H, 11.11; N,
large red shi. Water interrupts deprotonation due to which the 9.36; calc: C, 70.27; H, 11.12; N, 9.40%. 1H NMR d ppm (DMSO-
hyperchromic shi observed in the region 480–590 nm in d6): 8.36 (1H, –CH]N); 8.39–6.28 (6H, Ar-H); 3.17–1.26 (72H,
DMSO, gets diminished in 30% aqueous DMSO.
CH2); 0.94–0.90 (36H, CH3).
Since, absorbing species of DBH in DMSO solution, in the
region 480–590 nm, are low populated, stronger HBA type
molecule (acetate ions) than DMSO is used so that their pop-
ulation may increase. Interestingly, this has been observed that
upon addition of one equivalent of acetate ions in DMSO
solution of DBH (50 mM), the hyperchromic shi in the region
480–590 nm becomes more prominent in the form of a new
band at 500 nm. In addition, the absorption band at 426 nm
disappears (Fig. 5a) and the color of the solution changes from
orange to purple. Now, 1H NMR spectra of DBH in DMSO-d6 in
the absence and presence of acetate ions were recorded to nd
out the absorbing species at 500 nm. The addition of one
equivalent of acetate ions in DMSO-d6 solution of DBH (10ꢀ2 M)
shows deprotonation of NH, C2–OH and C4–OH protons of DBH
(Fig. 5b). These observations strongly suggest that it is the
deprotonated DBH, which is absorbed at 500 nm in UV-visible
spectra and the absorbance region 480–590 nm in DMSO
solution of DBH is due to the presence of deprotonated DBH. To
verify the existence of DBH in deprotonated form, the DBH
was reacted with tetrabutylammonium acetate salt and the
compound thus obtained was characterized.
3.4 Structural characterization of tetrabutylammonium salt
of DBH
1
The tetrabutylammonium salt of DBH was characterized by H
NMR spectroscopy (Fig. 5 ESI†). The signals observed at 11.58,
10.12, and 9.95 ppm in DBH for NH, C2–OH and C4–OH
protons, respectively, are absent in its tetrabutylammonium
salt. This indicates absence of NH, C2–OH and C4–OH groups
due to deprotonation. The imine proton (8.79 ppm) and six
aromatic protons (8.84–6.34 ppm) of DBH shi upeld at 8.36
and 8.39–6.28 ppm, respectively in tetrabutylammonium salt of
DBH due to increase in electron density upon deprotonation.34
The appearance of signals in the region 3.17–1.26 and 0.94–0.90
ppm corresponds to >CH2 and –CH3 protons of three tetrabu-
tylammonium cations, respectively.
Above results suggest the formation of deprotonated DBH on
addition of acetate ions. However, its deprotonation is signi-
cantly interrupted by the addition of water. Therefore, DBH
molecule may be used for sensing of anions and water.
3.5 Sensing of anions
3.5.1 Visual sensing of anions. Firstly, visual inspection of
DBH in various solvents was done before and aer addition of
3.3 Synthesis of tetrabutylammonium salt of DBH
The tetrabutylammonium salt of DBH was synthesized by each anion salts (Fig. 6). Upon addition of 1 equivalent of
reacting 25 mL acetonitrile solutions of DBH (5 mmol, 1.59 g) acetate ions or uoride in the DMSO solution of DBH (50 mM),
and tetrabutylammonium acetate (15 mmol, 4.52 g) in a round the color changes from orange to purple and in acetonitrile, it
bottom ask in 1 : 3 molar ratio. The reaction solution was changes from yellow to red under similar experimental condi-
reuxed for 2 h and the solid product was obtained by slow tions. However, aforementioned anions are inert in inducing
evaporation of the above solution at room temperature. The any change in color of DBH in protic solvents, ethanol and
Fig. 5 (a) Diagram showing changes in UV-visible absorption spectra of DBH (50 mM) upon addition of 1 equivalent of tetrabutylammonium
acetate (b) Diagram showing changes in 1H NMR spectra of DBH (10ꢀ2 M) upon addition of different equivalents of tetrabutylammonium acetate
in DMSO-d6.
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Fig. 6 Diagram showing color changes of DBH (50 mM) in DMSO (left) and acetonitrile (right) upon addition of 1 equivalent of tetrabuty-
lammonium anions (A) free receptor, (B) bromide or iodide, (C) fluoride and (D) acetate ions.
methanol (Fig. 6, ESI†). The addition of bromide and iodide did
not result in detectable color change of DBH in any solvents.
From above experiment, it is observed that DBH senses
colorimetrically both the acetate ions and uoride in DMSO or
acetonitrile solvents. Hence, it was not fruitful to work on anion
sensing behavior of DBH due to lack of selectivity.
3.5.2 UV-visible studies. As evidenced from Fig. 7, addition
of 1 equivalent of acetate ions or uoride in acetonitrile solution
of DBH (50 mM) shows bathochromic shi (ꢂ70–100 nm) and
exhibits a broad band in the region 465–496 nm. The change in
UV-visible absorption spectra is more prominent for acetate ions
than uoride. From UV-visible titration it can be observed that as
the number of equivalents of acetate ions increase (0.5 to 5
equivalents), the absorption band appeared at 396 nm gradually
reduces and a new broad band appears at 492 nm (Fig. 8). The
appearance of an isosbestic point at 436 nm indicates that only
two species are present at equilibrium over the course of the
titration experiment. Similar to DMSO, the changes observed in
Fig. 8 The UV-visible spectra of DBH (50 mM) upon addition of 0, 0.5,
1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 and 5 equivalents of tetrabutylammonium
UV-visible absorption spectra of DBH in acetonitrile on addition
of acetate ions is due to the formation of deprotonated DBH.
acetate in acetonitrile solution.
compete with low water content. Since acetonitrile is a weak
HBA type solvent it is found appropriate to investigate water
sensing behavior of deprotonated DBH in acetonitrile. The
addition of 3% water to the deprotonated DBH (50 mM DBH + 10
3.6 Sensing of water
The deprotonated DBH cannot be utilized as water sensor in
DMSO because DMSO is a strong HBA type solvent and may
Fig. 7 Diagram showing changes in absorption spectra of DBH (50 Fig. 9 Diagram showing change in absorption spectra of deproto-
mM) upon addition of tetrabutylammonium acetate and fluoride (100 nated DBH (50 mM DBH + 10 equivalents acetate ions) upon addition of
mM), separately in acetonitrile.
3% water in acetonitrile.
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The changes in the absorption spectra of deprotonated DBH
upon addition of water content could be conveniently analyzed
by ratiometry using the ratio of the two absorbances at 492 and
398 nm. The plot of the absorbance ratio A492/A398 as a function
of the water content gives a well-correlated relationship
(Fig. 12). The ratio considerably decreases from approximately
5.7 for 100% acetonitrile to 0.33 for 2% water, and then slowly
decreases further to 0.18 for 5% water content. This has also
been observed from Fig. 12 that the ratiometric changes are
particularly prominent in the region of less than 2% water.
Therefore, a calibration curve for the determination of water
content can be drawn in this range. The relatively higher limit of
detection (LOD) for water content in acetonitrile with deproto-
nated DBH was determined to be 0.012% (Fig. 7, ESI†). For
instance, Chang et al. have reported LOD, 0.037% and 0.16% for
the detection of water content in acetonitrile by anion
receptors.19,35
Scheme 1 Signalling of water content in acetonitrile by the depro-
tonated DBH.
In order to gain further understanding of the signaling
behavior of deprotonated DBH, the effect of acetate ion
concentration on the signaling of the water content in aceto-
nitrile has been evaluated. As the concentration of acetate ion
increases from 10 to 100 equivalents, the titration prole is
affected slightly as shown in Fig. 12. An increase in the amount
Fig. 10 UV-visible absorption spectra of deprotonated DBH (50 mM
DBH + 10 equivalents acetate ions) in acetonitrile after adding different
amount of water content (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1,
1.2, 1.3, 1.5, 1.6, 2, 2.3, 2.6 3, 4, 4.6 and 5%).
equivalents acetate ions) shows UV-visible spectrum almost
identical to DBH alone (Fig. 9). This suggests that water disrupt
the formation of deprotonated DBH in acetonitrile. Hence,
deprotonated DBH could be used as a water sensor in acetoni-
trile (Scheme 1).
To investigate the systematic signaling of water content in
acetonitrile with deprotonated DBH, the changes in the
absorption spectra of deprotonated DBH (50 mM DBH + 10
equivalents acetate ions) upon addition of different amount of
water content (0.1 to 5%) in acetonitrile were measured (Fig. 10).
On increasing the water content, the absorption band observed
at 492 nm gradually reduces and a new broad band appears at
398 nm. At the same time, color of the solution progressively
changes from dark red (deprotonated DBH) to yellow (DBH).
This can be easily discerned by naked eye (Fig. 11).
Fig. 12 Effect of acetate ion concentration on UV-visible spectra of
deprotonated DBH (DBH, 50 mM) in acetonitrile with varying amount of
water.
Fig. 11 Diagram showing changes in color of deprotonated DBH (50 mM DBH + 10 equivalents acetate ions) in acetonitrile after adding different
amount of water content (A) 0%, (B) 0.5%, (C) 1%, (D) 2%, (E) 2.5%, (F) 3%, (G) 4%.
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The authors thank the Head, S.A.I.F., Indian Institute of Tech- 27 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
nology, Kanpur, India for recording the ESI-mass spectra. Two 28 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
of the authors (K.T. and M.M.) are also thankful to CSIR, New
Delhi for awarding Senior Research Fellowship.
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
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