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

Abstract

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%.

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1. Introduction

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 developed 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 shifts can arise from either non-specific (dielectric enrichment) or specific (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 performing continuous monitoring, and interference from other co-existing species limit their wider application. However, alternative optical methods are quite desirable in terms of convenience and instrument requirements.¹¹ Selective and sensitive optical signaling systems that utilize multifunctional dye molecules¹² such as merocyanines,¹³ flavones,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 first time, we explored the solvatochromism in solution of a Schiff base, (E)-4-[{2-(2,4-dinitrophenyl)hydrazono}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 intermolecular interactions between neighbors or solvent molecule. The intermolecular hydrogen bonding tuned the solvatochromism of DBH has been observed in hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) type solvents. These observations were supported by TD-DFT calculations on DBH. Since HBA type solvents can promote deprotonation of DBH by accepting hydrogen bond from DBH, this has been investigated by recording the UV-visible absorption and ¹H NMR spectra of DBH in the presence of stronger HBA type molecules (acetate ions) in DMSO solution.

It has been well established that water interrupts deprotonation/complex formation of hydrazones by anions^11,19 and hence, the investigation of change from deprotonated DBH/DBH–acetate complex to DBH could be convenient signaling system to determine the water content of organic solvent. The variation of water content in the acetonitrile is sensitively visualized by ratiometry as well as naked-eye detectable color changes. The design and synthesis of the DBH is much easier and cost effective than the reported water sensors. In literature a number of papers have been published for anion sensors that are based on the molecular framework of hydrazones, thioureas, and amide derivatives.²⁰ Hydrazones of 2,4-dintrophenylhydrazine 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.

2. Experimental

2.1 Materials

All analytical reagent grade chemicals were obtained from the commercial sources. 2,4-dinitrophenylhydrazine, 2,4-dihydroxybenzaldehyde 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.

2.2 Instrumentation

C, H, N contents were determined on an Exeter Analytical Inc. CHN Analyzer (Model CE-440). ¹H and ¹³C NMR spectra were recorded in DMSO-d₆ on a JEOL AL-300 FT-NMR multinuclear spectrometer. Chemical shifts 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 cm⁻¹ region. UV-visible spectra in various solvents were recorded on a Shimadzu 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.

2.3 General methods

The stock solution of DBH (10⁻² M) was prepared in DMSO which was used for UV-visible absorption studies through dilution in all the solvents (chloroform, methanol, ethanol, acetonitrile and DMSO). For UV-visible experiment in various solvents, 50 μM solution of DBH was used. For anion sensing through UV-visible spectral studies, 50 μM solution of DBH and tetrabutylammonium salts of anions in acetonitrile were used. For water sensing, 50 μM solution of DBH + 10 equivalents of tetrabutylammonium acetate in acetonitrile and triple distilled water were used. ¹H NMR titration was performed by treating 10⁻² M solution of DBH in DMSO-d₆ with 10⁻¹ M solution of tetrabutylammonium acetate ions in DMSO-d₆ and varying the equivalents (0.1, 0.3, 0.5, 1.0 and 5.0) of acetate ions.

To determine the S/N ratio, the absorbance of 50 μM solution of DBH + 10 equivalents of tetrabutylammonium acetate ions in acetonitrile without water was measured by 10 times and the standard deviation of blank measurements was determined. The detection was calculated as three times the standard deviation (σ) from the blank measurement (in the absence of water) divided by the slope of calibration plot between % of water and absorbance (eqn (1)).²³

Detection limit = 3σ/slope (1)

Single crystal X-ray diffraction data for the DBH was obtained at 293(2) K, using a graphite mono-chromated Mo Kα (λ = 0.71073 Å) radiation source. The structure was solved by direct method (SHELXL-97) and refined 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 refinement at geometrically ideal position and refined with a riding model.^24,25 The Mercury and ORTEP-3 software packages for windows program were used for generating structures.^26,27

The density functional theory using Becke's three parameterized 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

The Schiff base (E)-4-[{2-(2,4-dinitrophenyl)hydrazono}benzene-1,3-diol] (DBH), was synthesized by a slight modification of the reported method.³⁰ The ethanolic solution of 2,4-dinitrophenylhydrazine (10 mmol) was reacted with ethanolic solution of, 2,4-dihydroxybenzaldehyde (10 mmol) in a round bottom flask. The reaction mixture was refluxed for 4 h and a bright red crystalline product was obtained on cooling the above solution at room temperature. The product was filtered on a Büchner 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. > 250 °C, anal. calc. for C₁₃ H₁₀ N₄ O₆ (318): found C, 49.12; H, 3.14; N, 17.57; calc: C, 49.06; H, 3.17; N, 17.61%. ESI-MS: [M − H]⁻, m/z, 317.05 (100% abundance); IR/cm⁻¹ (KBr): ν(O–H), 3421; ν(N–H), 3268; ν(C=N), 1617. ¹H NMR δ ppm (DMSO-d₆): 11.58 (s, 1H, –N–H); 10.12 (s, 1H, ₂–OH); 9.95 (s, 1H, –C₄–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 finally confirmed 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).

Fig. 1 Structure of the Schiff base, DBH showing hydrogen bonding interaction sites for solvent molecules.

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 cm⁻¹ are attributed to the –OH and –NH stretching vibrations of DBH, respectively. The IR band appeared at 1617 cm⁻¹ clearly indicates the presence of an imine (>C=N–) functionality which must have resulted due to the condensation of 2,4-dinitrophenyl hydrazine with 2,4-dihydroxybenzaldehyde. The ¹H NMR spectra (Fig. 2, ESI†) of DBH in DMSO-d₆ exhibits three 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 = 317.05 in ESI-MS (Fig. 4, ESI†) corresponds to [M − H]⁻ also validates the formation of DBH.

Fig. 2 (a) ORTEP diagram of DBH·DMSO showing atom numbering scheme with ellipsoids of 30% probability (b) Diagram showing the hydrogen bonds (blue dashed lines) and S⋯O interactions (black dashed line) in DBH·DMSO.

Fig. 3 Visible color change of DBH (50 μM) 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 μM) (lower).

Fig. 4 Theoretically optimized structure of DBH (left) and energy level diagram for the frontier π MOs of first electronic transition in DBH (right).

Finally the structure of DBH was confirmed by the single crystal X-ray diffraction study. The details concerning the data collection and structure refinement are summarized in Table 1 ESI†. Relevant geometrical data and hydrogen bonding parameters are given in Table 2 and 3, ESI†, respectively. The ORTEP diagram of DBH was shown in Fig. 2a. The molecular structure of DBH also contains a DMSO molecule which was used as a solvent for crystallization. The unit cell dimensions show that the crystal belongs to the monoclinic system with the P21/c space group. Two intra-molecular hydrogen bonds, O(1)–H⋯N(1) and N(2)–H⋯O(3) help to facilitate the planarity of DBH. The DMSO molecule accept a hydrogen bond at >S=O group from H-atom of the p-hydroxy group (C₄–OH) of DBH to form intermolecular hydrogen bond between DMSO and DBH (Fig. 2b). The hydrogen bond distances and angles (Table 3, ESI†) show that they stabilize DBH molecule significantly.³¹ The intermolecular S⋯O distance, S(1)⋯O(2) of 3.2 Å, is less than their sum of Van der Walls radii indicating prominent S⋯O interactions as shown in Fig. 2b.³²

3.2 Solvatochromism

The naked eye visible color change and change in UV-visible absorption spectra of DBH (50 μM) are examined at room temperature in three different type of solvents, non polar solvent chloroform, polar aprotic solvents acetonitrile and DMSO, and polar protic solvents ethanol and methanol (Fig. 3). DBH exhibits yellow color in acetonitrile, chloroform, methanol and ethanol solvents. However, it gives orange color in DMSO. Hence, it is observed that highly polar aprotic solvent (DMSO) exhibits drastic visible change in color of DBH. In another experiment, we analyzed the effect of water on visible color of DBH in DMSO solution (50 μM). The 30% aqueous DMSO solution of DBH exhibits a dark yellow color. These observations suggest that water compete with DMSO solvent in inducing orange color of DBH.

The UV-visible absorption spectra of DBH in various solvents (50 μM) show a broad absorption band in the region 396–426 nm (Fig. 3). DBH has an absorption maximum at 396 nm in acetonitrile, 397 nm in chloroform, 398 nm in methanol, 401 nm in ethanol, 426 nm in DMSO (with significant hyperchromic shift in the region 490–590 nm) and at 416 nm in 30% aqueous DMSO. Three factors are responsible for the positive solvatochromism in various solvents (a) dielectric constant of the solvents (b) the ability of the solvent to form stronger intermolecular H-bonds with solute molecule (c) selective solvation. It is observed that the absorption spectra of the DBH in DMSO (426 nm) is red shifted as compared to other solvents, indicating relatively strong host–guest interaction between the DBH and DMSO environment. In addition, it is also observed that the spectral shift in various solvents does not change significantly with polarity (π* parameters) of all the employed solvents (Table 1).⁶ Since the absorption maxima do not correlate with the polarity (non specific interactions) of the solvents, the positive solvatochromic effect in DMSO can be explained with the help of specific 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–Taft, α (solvent hydrogen-bond donor acidity) and β (solvent hydrogen-bond acceptor basicity) parameters which are commonly used to describe solvent hydrogen-bonding properties.⁶ The data given in Table 1 indicate that chloroform and acetonitrile have small β value (0.10–0.40), whereas, ethanol and methanol not only have larger β (0.75–0.66) but also large α value (0.86–0.98). On the other hand, DMSO has largest β value (0.76) and smallest α value (0.0). Hence, DMSO is the strongest hydrogen bond acceptor (HBA) type solvent. It is well documented in literature that in HBA type solvents, absorption spectra of solute molecule is red shifted due to the more stabilization of excited state than the ground state of solute molecule. On the other hand, in hydrogen bond donor (HBD) type solvents (e.g. ethanol, methanol), the absorption spectra of solute molecules are blue shifted due to more stabilization of ground state than excited state of a molecule.³³ The intermolecular hydrogen bond between DBH and DMSO is clearly visible even in the single crystal of DBH containing a DMSO molecule as solvent of crystallization (Fig. 2).

Table 1 Solvent polarity parameters. Where, α is solvent hydrogen-bond donor acidity, β is solvent hydrogen-bond acceptor basicity and π* is a measure of the solvent dipolarity/polarizability

Solvent	α	β	π*
Water	1.17	0.47	1.09
Methanol	0.98	0.66	0.60
Ethanol	0.86	0.75	0.54
Chloroform	0.20	0.10	0.58
Acetonitrile	0.19	0.40	0.75
DMSO	0.00	0.76	1.00

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TD-DFT calculations were performed on DBH to explain the reason behind more stabilization of DBH by HBA type solvents. As shown in Fig. 4, HOMO and LUMO of the first electronic transition (460 nm) in DBH are prominently localized on dihydroxy phenyl and dinitrophenyl moieties, respectively, which suggest that the electron density migrates away from the dihydroxy phenyl to dinitrophenyl moiety during excitation process. Hence, formation of the hydrogen bond between DBH and HBD type solvents opposes this migration and stabilizes the ground state more (blue shift). However, HBA type solvents favor the electron density migration and stabilize the excited state more, resulting a red shift in UV-visible absorption spectra of DBH.

Although the absorption band observed at 416 nm in 30% aqueous DMSO is not drastically shifted from the absorption band observed at 426 nm of DBH in DMSO solution. However, the color of DBH in 30% aqueous DMSO becomes dark yellow as observed in other solvents. Therefore, the absorption spectra of DBH in DMSO and 30% aqueous DMSO were compared and observed that the hyperchromic shift in the region 480–590 nm appeared for DBH in DMSO, got diminished in 30% aqueous DMSO. Hence, it appears that the orange color of DBH in the DMSO solution is due to the hyperchromic shift in the region 480–590 nm. These observations suggest the possibility of existence of two different species of DBH (absorbing distinctly at 426 nm and in the region 480–590 nm) in DMSO. The absorption band observed at 426 nm may be due to the highly populated hydrogen bonded DBH to DMSO. However, the hyperchromic region 480–590 nm may be due to the absorbance of low populated deprotonated DBH. Since, DMSO is a polar and strong HBA type solvent, it may promote the deprotonation of acidic protons (NH, C₂–OH & C₄–OH) of DBH. On deprotonation, a substantial delocalization of negative charge leads to a large red shift. Water interrupts deprotonation due to which the hyperchromic shift observed in the region 480–590 nm in DMSO, gets diminished in 30% aqueous DMSO.

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 population may increase. Interestingly, this has been observed that upon addition of one equivalent of acetate ions in DMSO solution of DBH (50 μM), the hyperchromic shift 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 find out the absorbing species at 500 nm. The addition of one equivalent of acetate ions in DMSO-d₆ solution of DBH (10⁻² 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.

Fig. 5 (a) Diagram showing changes in UV-visible absorption spectra of DBH (50 μM) upon addition of 1 equivalent of tetrabutylammonium acetate (b) Diagram showing changes in ¹H NMR spectra of DBH (10⁻² M) upon addition of different equivalents of tetrabutylammonium acetate in DMSO-d₆.

3.3 Synthesis of tetrabutylammonium salt of DBH

The tetrabutylammonium salt of DBH was synthesized by reacting 25 mL acetonitrile solutions of DBH (5 mmol, 1.59 g) and tetrabutylammonium acetate (15 mmol, 4.52 g) in a round bottom flask in 1 : 3 molar ratio. The reaction solution was refluxed for 2 h and the solid product was obtained by slow evaporation of the above solution at room temperature. The product was filtered, washed several times with diethyl ether to remove acetic acid and dried in a desiccator.

3.3.1 Analytical data. Purple, yield 65%. M.p. > 250 °C, anal. calc. for C₆₁H₁₁₅N₇O₆ (1042): found C, 70.15; H, 11.11; N, 9.36; calc: C, 70.27; H, 11.12; N, 9.40%. ¹H NMR δ ppm (DMSO-d₆): 8.36 (1H, –CH=N); 8.39–6.28 (6H, Ar-H); 3.17–1.26 (72H, CH₂); 0.94–0.90 (36H, CH₃).

3.4 Structural characterization of tetrabutylammonium salt of DBH

The tetrabutylammonium salt of DBH was characterized by ¹H 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 shift upfield 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 tetrabutylammonium cations, respectively.

Above results suggest the formation of deprotonated DBH on addition of acetate ions. However, its deprotonation is significantly 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 after addition of each anion salts (Fig. 6). Upon addition of 1 equivalent of acetate ions or fluoride in the DMSO solution of DBH (50 μM), the color changes from orange to purple and in acetonitrile, it changes from yellow to red under similar experimental conditions. However, aforementioned anions are inert in inducing any change in color of DBH in protic solvents, ethanol and methanol (Fig. 6, ESI†). The addition of bromide and iodide did not result in detectable color change of DBH in any solvents.

Fig. 6 Diagram showing color changes of DBH (50 μM) in DMSO (left) and acetonitrile (right) upon addition of 1 equivalent of tetrabutylammonium anions (A) free receptor, (B) bromide or iodide, (C) fluoride and (D) acetate ions.

From above experiment, it is observed that DBH senses colorimetrically both the acetate ions and fluoride 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 fluoride in acetonitrile solution of DBH (50 μM) shows bathochromic shift (∼70–100 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 fluoride. 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 UV-visible absorption spectra of DBH in acetonitrile on addition of acetate ions is due to the formation of deprotonated DBH.

Fig. 7 Diagram showing changes in absorption spectra of DBH (50 μM) upon addition of tetrabutylammonium acetate and fluoride (100 μM), separately in acetonitrile.

Fig. 8 The UV-visible spectra of DBH (50 μM) upon addition of 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 and 5 equivalents of tetrabutylammonium acetate in acetonitrile solution.

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 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 μM DBH + 10 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 acetonitrile (Scheme 1).

Fig. 9 Diagram showing change in absorption spectra of deprotonated DBH (50 μM DBH + 10 equivalents acetate ions) upon addition of 3% water in acetonitrile.

Scheme 1 Signalling of water content in acetonitrile by the deprotonated DBH.

To investigate the systematic signaling of water content in acetonitrile with deprotonated DBH, the changes in the absorption spectra of deprotonated DBH (50 μM 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. 10 UV-visible absorption spectra of deprotonated DBH (50 μM 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%).

Fig. 11 Diagram showing changes in color of deprotonated DBH (50 μM 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%.

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 deprotonated 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

Fig. 12 Effect of acetate ion concentration on UV-visible spectra of deprotonated DBH (DBH, 50 μM) in acetonitrile with varying amount of water.

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 acetonitrile has been evaluated. As the concentration of acetate ion increases from 10 to 100 equivalents, the titration profile is affected slightly as shown in Fig. 12. An increase in the amount of acetate ion, gives a slightly less sensitive response towards the changes in water content in acetonitrile. Above observation indicates that the present water signaling system is based upon the effect of water on the formation of deprotonated DBH. The higher amount of acetate ions may compete with water in formation of deprotonated DBH.

4. Conclusion

In this paper, the solvatochromism in solution of a simple, cost effective Schiff base (E)-4-[{2-(2,4-dinitrophenyl)hydrazono}benzene-1,3-diol] (DBH) and water sensing behavior of deprotonated DBH in acetonitrile have been described. The anions (acetate ions and fluoride) and DMSO are found to promote deprotonation of DBH. On addition of acetate ions in acetonitrile solution of DBH, the color changes from yellow to dark red due the formation of deprotonated DBH. The interference of water in the formation of deprotonated DBH is utilized for water sensing in acetonitrile. The deprotonated DBH exhibits a pronounced chromogenic signaling behavior that can be detected by naked eye in response to the changes in water content in acetonitrile. The changes in UV-visible absorption spectra are successfully analyzed by ratiometry as well as by shifts in the absorption maximum and signals well for less than 2% water in acetonitrile. The limit of detection (0.012%) for water content in acetonitrile is much higher than the reported elsewhere. The anion receptor, DBH can be useful as a convenient colorimetric and ratiometric probe for analyzing the water content in acetonitrile.

Acknowledgements

The authors thank the Head, S.A.I.F., Indian Institute of Technology, Kanpur, India for recording the ESI-mass spectra. Two of the authors (K.T. and M.M.) are also thankful to CSIR, New Delhi for awarding Senior Research Fellowship.

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Footnote

† 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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