Fluorescent Chemosensors for Selective Recognition of Fluoride

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Bisthiocarbohydrazones as Colorimetric and “Turn on” Fluorescent Chemosensors for Selective Recognition of Fluoride Saravana Loganathan Ashok Kumar,† Ramalingam Tamilarasan,† Moorthy Saravana Kumar,† and Anandram Sreekanth*,† †

Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, Tamilnadu, India

bS Supporting Information ABSTRACT: Bisthiocarbohydrazone derivatives of two heterocyclic ketones were designed and synthesized to selectively sense fluoride ion from a biologically competing solvent dimethylsulfoxide (DMSO). The selective recognition of fluoride is clearly visible to the naked eye with a distinct color change. The recognition mechanism has been investigated by UV
visible, fluorescence spectroscopy, and 1H NMR titration experiments. From the Benesi
Hidebrand equation and Job's plot, it was inferred that both compounds bind to fluoride with a 1:1 stoichiometry. 1H NMR titration data indicates deprotonation of the NH protons by fluoride as a prominent step in the recognition.

1. INTRODUCTION Anion sensors have been of recent interest due to the important role many anions play in the environmental and health related aspects.1 Among these the detection of anions like F
and AcO
have become more important as their presence in the environment is very critical and cause many hazards. Even though fluoride is an important ingredient in many pharmaceutical products, their concentration in drinking water in excess can cause severe fluorosis, thyroid activity depression, etc.; hence, it is important to detect fluoride in water. Despite many reports of fluoride sensors, there are still milestones to be achieved mainly in the biochemically competing solvents like alcohols, dimethylformamide (DMF) and dimethylsulfoxide (DMSO) where many of the reported sensors do not perform well.2
5 Most of the receptors reported previously act via H
bonding induced π delocalization or NH deprotonation which may be the only mechanism by which the recognition of fluoride can be justified. Among many of the sensors reported only few are “turn on” sensors while most of them exhibit “turn off” mode during recognition and very few have the advantage of a dual mode of detection i.e. colorimetric and fluorescent sensing respectively.6,7 Thiocarbohydrazone are thiourea derivatives having unique structural properties and chemical properties and have recently gained interest in the anion sensing.8 They are easily synthesized in high yields and are found to be highly specific with low detection limits. We have designed and synthesized two different chemosensors compounds for fluoride 1,5-bis(2-acetylthiophene) thiocarbohydrazone (C1)9 and 1,5-bis(2-acetylfuran) thiocarbohydrazone (C2), (Figure 1) which function as dual mode sensors toward fluoride with much lower detection limits in biologically competing solvents like DMF and DMSO. 2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 2-Acetylthiophene (Alfa), 2-acetylfuran (Alfa), thiocarbohydrazide (Aldrich), and tetra-n-butylammonium (TBA) salts (Aldrich) were stored in vacuum desiccators containing self-indicating silica-gel and were used as received. r 2011 American Chemical Society

Analytical grade DMSO and other solvents (Merck) were used without further purification. 1H NMR and 13C NMR spectra were recorded on a Bruker AVANCE III, 500 MHz in DMSO-d6 and CDCl3 at 298 K with TMS as an internal standard. FT
IR spectra were measured on a Perkin
Elmer FT
IR spectrometer using KBr pellets. UV
visible and Fluorescence spectra were recorded in 1 cm path length quartz cell on PG Instrument T90+ spectrophotometer and JASCO FP-6200 spectrofluorophotometer, respectively. Elemental analyses of all compounds (C1 and C2) were carried out using Elementar Vario EL III CHNS. 2.2. Synthesis of Thiocarbohydrazone Compounds (C1 and C2). General Procedure. Thiocarbohydrazide (0.106 g, 1 equiv, 1 mmol) was dissolved in hot methanol (30 mL) and was added to the appropriate ketones [2-acetylthiophene (0.223 g, 2 equiv, 2.1 mmol), 2-acetylfuran (0.231 g, 2 equiv, 2.1 mmol),] dissolved in methanol (10 mL), after adding a drop of HCl, the reaction mixture was gently refluxed for 1 h. Precipitated product was filtered, washed with methanol and ether, recrystallized from chloroform, and dried over P4O10 in vacuum. C1 (pale yellow). Yield: 74%. M.P. 205
207 °C. Anal. calc. for C13H14N4S3: C, 48.53; H, 4.74; N, 17.26; S, 29.94. Found: C, 48.42; H, 4.38; N, 17.37; S, 29.83%. IR data (KBr, cm
1) 1535 (CdN). 1H NMR (500 MHz, DMSO-d6, δ ppm): 2.40 (6H, s, CH3), 7.12 (2H, t, Ar), 7.55 (2H, d, Ar), 7.64 (2H, d, Ar), 11.35 (1H, s, N
NH), 10.77 (1H, s, N
NH). 13C NMR (125 MHz, DMSO-d6): 174.80 (1C, CdS), 144.88 (1C, CdN), 143.26, 129.50, 128.77, 128.19 (4C, Ar), 14.7 (1C, CH3). C2 (pale yellow). Yield: 68%. M.P. 202
204 °C. Anal. calc. for C13H14N4O2S1: C, 53.67; H, 4.96; N, 19.52; S, 11.06. Found: C, 53.78; H, 4.86; N, 19.30; S, 11.04%. IR data (KBr, cm
1) 1605 (CdN). 1H NMR (500 MHz, CDCl3, δ ppm) 2.25 (6H, s, CH3), 6.50 (2H, t, Ar), 6.57 (2H, d, Ar), 7.69 (2H, d, Ar), 10.68 Received: June 26, 2011 Accepted: September 29, 2011 Revised: September 20, 2011 Published: September 29, 2011 12379

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ARTICLE

(1H, s, N
NH), 8.59 (1H, s, N
NH). 13C NMR (125 MHz, CDCl3): 179.52 (1C, CdS), 144.26 (1C, CdN), 151.11, 112.00, 111.94, 111.10 (4C, Ar), 12.33 (1C, CH3). 2.3. UV
visible and Fluorescence Titrations. The stock solutions of C1 and C2 (5  10
5 mol L
1) were prepared in DMSO. The TBA salts (F
, Cl
, Br
, I
, ClO4
, H2PO4
, and AcO
) solutions were prepared at a concentration of 1  10
2 mol L
1 in DMSO. Different equivalents of TBA salts (F
, Cl
, Br
, I
, ClO4
, H2PO4
, and AcO
) were added to compounds and their corresponding UV
visible and fluorescence spectra were recorded at 298 K. 2.4. 1H NMR Titration. C1 (1.0  10
2 mol L
1 in DMSO-d6) was titrated with fluoride anion (as tetrabutylammonium salts) by addition of increasing equivalents of anion in DMSO-d6.

ketone methyl group, respectively. Compounds (C1 and C2) heterocyclic aromatic ring protons appeared around δ 6.5
7.7 ppm accounting for two doublet protons and one triplet proton. The 13C NMR spectrum of C1 and C2 showed well-defined peaks at 174.80 and 179.52 ppm respectively, assignable to thione carbon (CdS). Azomethine group (
CdN) peaks appeared at 144.88 and 144.26 respectively for C1 and C2. The signals corresponding to quaternary carbons are absent in DEPT 135 spectrum. 3.1. Colorimetric Analysis and UV
Visible Spectral Studies. The interaction of the compounds (C1 and C2) with various anions (F
, Cl
, Br
, I
, ClO4
, H2PO4
, and AcO
) was investigated in DMSO solution through colorimetric analysis. Noticeable and appreciable color changes were observed when the compounds (C1 and C2) were treated with the fluoride anions, imparting an immediate deep yellow color. Acetate anion hardly produces any visible color change in interaction with the compounds (C1 and C2); however, they are detectable in the UV
visible spectrum. Addition of other anions such as Cl
, Br
, I
, ClO4
, and H2PO4
did not show any visible color change (shown in Figure S1 of the Supporting Information) and a spectral change shown in Figure 2. In order to deduce the anion sensing ability of the compounds (C1 and C2) with fluoride and acetate at 298 K, titrations were carried out in DMSO and were monitored by means of UV
visible spectroscopy. The experiment was performed by preparing 5  10
5 mol L
1 solution of compounds (C1 and C2) in DMSO followed by the addition of tetrabutylammonium fluoride solution in different concentrations. The spectral profile is as shown in Figure 3. C1 and C2 shows absorption peaks at 260 (C1) and 270 nm (C2) attributable to the π f π* transitions. While a second absorption at 340 (C1) and 330 nm (C2) was observed which was attributed to the n f π* transitions. Upon the addition of fluoride anion to compounds, a prominent change was observed in the absorption spectra due to complexation between the host
guest molecules as proposed in Figure 4. Excess addition of fluoride results in a hyperchromic shift in the π f π* transitions and also results in the formation of a new peak in yellow region of spectra at 472 (C1) and 455 nm (C2), respectively. This may be due to the newly formed deprotonated species. The newly formed band in the visible region also exhibit the hyperchromic shift along with the addition of excess of fluoride. The n f π* bands show a bathochromic shift as this is the transition which is expected to change during the strong hydrogen-bonding interactions between the fluoride ion with C1 and C2, respectively. In both cases, the spectra changed without clear isosbestic point implying the formation of higher order complexes with excess of fluoride in analogy to the formation of [HF2
] species.11
14 C1 shows better recognition toward fluoride anion, which may be due to the influence of enhanced aromaticity of the thiophene compared to furan ring. With the addition of acetate anion to the compounds (C1 and C2), a new peak was observed at 425 (C1) and 403 (C2) nm respectively but with less intensity (see Figure S2 of the Supporting Information). However, the hyperchromic shift and the bathochromic shift for the π f π* and n f π*

3. RESULTS AND DISCUSSION The structural formulas of compounds (C1 and C2) are shown in Figure 1. They were well-characterized by elemental analyses, FT-IR, and 1H and 13C NMR-DEPT 135 spectra. The elemental analysis data where in good agreement with the calculated values, and all compounds (C1 and C2) were readily soluble in DMF and DMSO. IR spectrum of the compounds (C1 and C2) showed characteristic stretching frequencies at 1535 (C1) and 1605 (C2) cm
1 attributed to ν(CdN) indicative of the newly formed azomethine group and bands at 1214 (C1) and 1232 (C2) cm
1 assigned to ν(CdS) indicating that the thione form is dominating in the solid state. The characteristic spectral data are listed in Table 1. 1H NMR spectrum of C1 showed two singlets at δ 11.35 and δ 10.77 ppm assignable to NH protons which disappears with the addition of D2O. Similarly for C2, the corresponding two NH protons disappeared at δ 10.68 and δ 8.59, respectively. Hydrogen bonding interactions decreases the electron density around the proton, and hence shifting of resonance absorption to a lower field.10 C1 and C2 showed singlet protons at δ 2.40 and δ 2.25 ppm corresponding to the

Figure 1. Structural formulas of the compounds (C1 and C2).

Table 1. Characteristic Spectral Propertiesa FT-IR data (cm
1)

UV-visible (nm) π
π*

a

n
π*

νCdN

νCdS

s

s

1

13

H NMR (ppm)

ν N
H

C NMR (ppm)

NH

NH

CdN

CdS

CH3

br

C1

260

340

1531

1214

3281

11.35

10.77

143.26

174.80

14.70

C2

270

330

1605m

1232vs

3260br

10.68

8.59

144.26

179.52

12.33

m = medium, s = strong, br = broad, vs = very strong. 12380

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Figure 2. (a) Absorption spectra for C1 (5  10
5 mol L
1) in DMSO solution with addition of different anions (5 equiv) as their TBA salts. (b) Absorption spectra for C2 (5  10
5 mol L
1) in DMSO solution with addition of different anions (5 equiv) as their TBA salts 298 ( 0.1 K.

transitions respectively were not significant indicating that both C1 and C2 can be utilized selectively for fluoride recognition. Continuous variation method was used to determine the stoichiometric ratio of the compounds (C1 and C2) to the fluoride anion guest (Job's plot; see Figure S3 of the Supporting Information). A Job's plot of the C1 and C2 with fluoride anion in DMSO shows the maxima at a molar fraction15 of 0.5. This indicates that the C1 and C2 bind with the fluoride anion in a 1:1 ratio during the initial stages of the reaction. Considering this result along with the other data, the binding mode of C1 and C2 to the fluoride anion is proposed as shown in Figure 4. The binding stoichiometry of C1 and C2 were calculated in accordance with the Benesi
Hildebrand equation,16 (see Figure S4 and S5 of the Supporting Information) which was given as follows: " # 1 1 1 ¼ þ 1 A
A0 A∞
A0 K½F
0 where A0 is the absorbance of free compound, A is the absorbance with a specific fluoride concentration, A∞ is the absorbance with excess amount of F
, K is the association constant (M
1), and [F
]0 is the concentration of F
added (M). The plot of 1/(A
A0) against 1/[F
]0 shows a linear relationship (R = 0.99), indicating that compounds associates with fluoride

and acetate in a 1:1 stoichiometry. The association constant, K, between compounds C1 and C2 and F
was determined from the ratio of intercept/slope to be 2.91  105 (C1) and 5.5  104 M
1 (C2), respectively, from which it is inferred that C1 has more affinity toward fluoride anion. The association constant, K, for compounds C1 and C2 with acetate anion were found to be 7.69  104 (C1) and 1.21  104 M
1 (C2), from which it is inferred that compounds C1 and C2 have more affinity toward fluoride anion compared to acetate anion. The spectra of compounds C1 and C2 did not show any change with the addition of other anion (Cl
, Br
, I
, ClO4
, and H2PO4
); hence association constants cannot be determined by using the spectra of both compounds C1 and C2 with other anions. 3.2. Fluorescence Spectral Studies. The emission spectrum of both the compound viz., C1 and C2, were monitored in identical conditions which gave ample information regarding the “turn on” behavior of the compounds C1 and C2. For both the compounds C1 and C2, excitation wavelength was optimized and chosen as 375 nm. There are clear observations that the socalled fluorescence enhancement comes by the removal of n f π* transitions which normally mask the π f π* transition which are mainly responsible for the emission behavior.17 Hence the recognition of a molecule does not simply involve a color change, but also involves serious electronic changes within the molecule affecting the HOMO and LUMO interactions considerably. Figure 5 shows the 12381

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Figure 3. (a) Absorption spectra for C1 (5  10
5 mol L
1) in DMSO solution with sequential addition of fluoride anion (0 to 5 equiv). (b) Absorption spectra for C2 (5  10
5 mol L
1) in DMSO solution with sequential addition of fluoride anion (0 to 5 equiv) at 298 ( 0.1 K.

Figure 4. Proposed binding mode of compounds (C1 and C2) with fluoride anion in solution.

emission spectrum of compounds C1 and C2 with increasing concentrations of fluoride ion. There was a change in the fluorescence

spectrum upon addition of initial increments of fluoride. However, a dramatic change began to occur as the ratio of fluoride ion to the compounds C1 and C2 reached 5 equiv, with emission maximum about 412 (C1) and 434 (C2) nm. C1 show maximum sensitivity to the fluoride anion, which may be due to the presence of thiophene moeity which is more aromatic hence having a better π delocalization. As seen in Figure 5, there is a strong enhancement in the emission intensity upon the addition of fluoride. 3.3. 1H NMR Titration. 1H NMR titration of C1 with the fluoride gave a clear idea into the nature of host
guest interaction. Titration experiments were conducted in DMSO-d6. The 1 H NMR spectrum of C1 shows two NH protons at δ 11.35 and δ 10.77 ppm, respectively. The changes happening in the spectrum of C1 during the addition of fluoride anion (see in Figure S6 the Supporting Information). Upon addition of 0.5 equiv of fluoride ions, two NH protons are slightly shifted to upfield with their relative intensity decreased, due to the formation of hydrogen bonding between fluoride anion and NH proton.8 Upon further addition of 1 equiv of fluoride anion, the two NH peak showed severe broadening. On addition of an excess fluoride, the resonance signal corresponding to the two NH protons almost disappeared. Consequently, the result of 1 H NMR titration, hence provides a supporting proof for the 12382

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’ ACKNOWLEDGMENT A.S. acknowledges the DAE
BRNS, Department of Atomic Energy (No. 2010/20/37C/10/BRNS/2494) for a young scientist award. S.L.A. acknowledges MHRD, Govt of India, for a research fellowship. A.S. and R.T. acknowledge the SERC, Department of Science and Technology (DST-No. SR/S1/IC36/2008) for a research grant and for a research fellowship. A.S. is thankful to Dr. S. Abraham John, Associate Professor, Gandhigram Rural University, Tamilnadu, India, for photoluminescence (PL) measurements. ’ REFERENCES

Figure 5. (a) Fluorescence spectra (λex = 375 nm) for C1 (5  10
5 mol L
1) in DMSO solution with the addition (0, 4, 4.5, and 5 equiv) of fluoride anion. (b) Fluorescence spectra (λex = 375 nm) for C2 (5  10
5 mol L
1) in DMSO solution with the addition (0, 4, 4.5, and 5 equiv) of fluoride anion at 298 ( 0.1 K.

proposed binding mode of the compounds (C1 and C2) with the incoming fluoride anion.8

4. CONCLUSION Thiocarbohydrazone based schiff bases were synthesized and studied for their sensing properties against different anions. It has been found that they were highly and selectively detect fluoride even in a biologically competing solvent like DMSO. ’ ASSOCIATED CONTENT

bS

Supporting Information. Color change of compounds (C1 and C2) on addition of anions, absorption spectra titration with acetate anion, Job's plot for complexation with fluoride anion, Benesi
Hildebrand plot, and 1H NMR titration with fluoride anion. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: +91 431 2503642. Fax: +91 431 2500133.

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