Coumarin functionalized thiocarbonohydrazones as a new class of chromofluorescent receptors for selective detection of fluoride ion

Abstract

Two chromofluorescent thiocarbonohydrazone receptors (4 and 5) functionalized with coumarin derivatives have been synthesized and evaluated for selective detection of fluoride ions. Receptor 4 selectively recognizes fluoride ions via H-bond interaction and subsequent deprotonation to elicit a distinct visual colour change from colourless to pink with a significant 20-fold enhancement in its emission intensity to “turn on” a blue fluorescent. In contrast, receptor 5 exhibited a visible colour change from colourless to deep red upon interaction with fluoride ions over other anions. Detailed analysis of the binding characteristics of these receptors with fluoride ions revealed a 1 : 1 binding stoichiometry at lower concentrations while higher concentrations led to a 1 : 2 binding stoichiometry between the receptor and fluoride ions. These experimental results were further corroborated with quantum chemical calculations.

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Introduction

Development of synthetic receptors that can sense the presence of anionic species in micro molar concentrations through a dual mode of detection comprising chromogenic as well as fluorogenic sensing has been attracting a great deal of interest in recent years.^1–4 Amongst the various anionic species, the detection of fluoride ions is of most significant because of its major role in the area of environmental, chemical and medical sciences.^5,6 Moreover the excess use of fluoride can cause dental and skeletal fluorosis while its deficiency can cause osteoporosis in human beings.^7,8 Further, fluoride ion represents a special case in comparison to other anions due to its small size and high electronegativity, where it can form strong hydrogen bond at lower concentrations while act as a sufficiently strong base at higher concentrations to promote deprotonation through typical Brønsted acid–base reactions.^9–11 Distinction between these two processes has often been blurred in the case of fluoride recognition experiments.^12,13 In spite of many reports on fluoride sensors,^14–16 there is paucity of receptor that work on dual mode detection of fluoride ion using chromogenic and fluorogenic sensing. Previous works on this concept have employed molecular receptors consist of different combinations of amide,^17–20 urea,^21–23 thiourea,^24,25 imidazole,^26,27 indole,^28–30 and pyrrole^31,32 units which coordinate and bind the fluoride ion through N–H–F hydrogen bonds. Among these majority of them exhibit fluorescent “turn-off”^{18–24,27,29–32} while a few are expressing “turn-on”^17,25,26,28 behaviors. Very recently thiocarbonohydrazone derivatives have been emerging as a promising scaffold for fluoride ion sensors.^33–38 The uniqueness of this molecule is the presence of thiourea NH protons which can act as hydrogen bond donors for anionic species and the azomethine bond that can be conjugated to a chromogenic and/or fluorogenic moiety in order to provide a photophysical response through efficient charge transfer during anion interaction. It is therefore envisaged that the conjugation of a thiocarbonohydrazide unit with a coumarin derivative can pave the way to the development of dual mode sensor where the anion interaction may trigger a colour change and fluorescent change in the sensor medium. Moreover visual detection of fluoride ion through efficient colour change and fluorescence change will offer qualitative and quantitative information without the help of any sophisticated analytical instrument. With this aim, we have designed and synthesized two coumarin functionalized thiocarbonohydrazones 4 and 5 by simple condensation reactions in high yields. These receptors have demonstrated the dual mode detection of fluoride ions though instant visual colour change and fluorescent “turn-on” behavior in a polar solvent such as dimethyl sulphoxide (DMSO). A significant UV-visible spectral change was observed by receptor 4 upon interaction with fluoride ions by the appearance of a longest wavelength absorption band at 546 nm when compared with other thiocarbonohydrazone based fluoride sensors reported so far.^33–38

Experimental section

Materials

All the solvents and reagents (analytical grade and spectroscopic grade) were obtained from Merck (India) and Spectrochem Pvt. Limited (India) and were used without purification. The required starting materials 3-acetyl-chromen-2-one (1), 2-acetyl-3H-benzo[f]chromen-3-one (2) and thiocarbonohydrazide (3) were prepared according to the literature procedures published earlier.^39,40 The anions such as F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻ and AcO⁻ were used in form of their tetrabutylammonium (TBA) salts to study the anion binding properties of receptor 4 and 5. For UV-visible and fluorescence experiments, the concentration of the stock solutions of 4 and 5 was prepared 20 μM and 10 μM respectively in UV-grade DMSO. The various equivalents of guest anions were added from the stock solutions varying from 10⁻³ to 10⁻⁴ M prepared in UV-grade DMSO.

General methods

¹H NMR was recorded on an Avance III-400 MHz Bruker spectrometer. Chemical shifts are reported in parts per million from tetramethylsilane with the solvent (DMSO-d₆: 2.5 ppm) resonance as the internal standard. Data are reported as follows: chemical shifts, multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet), coupling constant (Hz).¹³C NMR (100 MHz) spectra were recorded on an Avance III-400 MHz Bruker spectrometer in proton decoupling mode. Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance as the internal standard (DMSO-d₆: 39.51 ppm). UV-visible absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer. Fluorescence emission spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer. Time-resolved fluorescence measurements were recorded on a Horiba Jobin Yvon NanoLED System. FT-IR spectra were recorded on a Perkin Elmer RXI spectrometer. Chromatographic purification was done using 60–120 mesh silica gels (Merck). For reaction monitoring, manually coated silica gel-60 TLC plates were used.

Synthesis and characterization

The synthesis of receptors 4 and 5 could be achieved through facile synthetic protocols as depicted in Scheme 1. Condensation reaction of 1 or 2 with 3 in ethanol–water mixture (6 : 1) under reflux condition afforded the thiocarbonohydrazone derivatives 4 or 5 in quantitative yields. The spectroscopic analysis of 4 and 5 were consistent with their indicated structures (Fig. S1–S6 see ESI†). For instance the ¹H NMR spectra of 4 exhibited two singlets at 9.76 and 10.41 ppm which could be attributed to thiourea –NH protons that disappeared upon addition of D₂O. In receptor 5 the corresponding –NH signals disappeared at 9.97 and 10.40 ppm on D₂O addition. 4 and 5 each exhibited D₂O non-exchangeable singlet at 8.47 and 9.12 ppm respectively that could be assigned to =CH–Ar proton of the coumarin unit whereas another singlet at 2.21 and 2.27 ppm respectively in the high field region of 4 and 5 correspond to the ketone methyl group. The aromatic proton signals of 4 and 5 appeared in the range of 7.37–8.71 ppm. The ¹³C NMR signals at 174.4 and 179.0 ppm respectively in 4 and 5 could be attributed to the [double bond splayed left]C=S carbon while the signals at 140.5 and 142.4 could be assigned to azomethine (–C=N–) carbon in 4 and 5 respectively. The FT-IR spectrum exhibited sharp peaks at 1601 and 1240 cm⁻¹ in 4 and 1599 and 1246 cm⁻¹ in 5 which could be attributed to the stretching frequencies of C=N and C=S bonds respectively.

Scheme 1 Synthesis of receptors 4 and 5.

Results and discussion

Anion interaction study of receptor 4 and 5 by colorimetric analysis

The sensitivity of 4 and 5 against various anions (such as F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻ and AcO⁻ in the form of their tetrabutylammonium salts) was first monitored through visual and colorimetric analysis in dimethyl sulphoxide (DMSO) solvent medium. It was found that the receptor 4 exhibited a prominent and instant colour change from colourless to pink only with fluoride ions while no significant visual response was observed on treatment with Cl⁻, Br⁻, I⁻, H₂PO₄⁻ and HSO₄⁻ ions under similar conditions. Similarly receptor 5 showed a prominent colour change from colourless to deep red only in presence of fluoride ions. On the other hand interaction of 4 and 5 with AcO⁻ ions exhibited a faint colour change in comparison to that observed with fluoride ions (Fig. 1).

Fig. 1 Selectivity of 4 and 5 for fluoride ion over other anions. Colour changes of 4 (a) and 5 (b) in DMSO solution (20 μM) with the addition of 5 equivalents of various anions (as TBA salts).

UV-visible study of receptor 4 and 5 with anions

Spectrophotometric investigation of 4 and 5 upon interaction with fluoride ions exhibited a new longer wavelength absorption peak at 546 and 542 nm respectively at 20 μM concentration in their UV-visible spectrum. On the other hand no shift in the absorption maxima was observed in the presence of Cl⁻, Br⁻, I⁻, H₂PO₄⁻ and HSO₄⁻ ions as depicted in Fig. 2. However a similar absorption band at 546 and 542 nm with low intensity was observed in the UV-visible spectrum of 4 and 5 respectively upon interaction with AcO⁻ ions, which indicated a poor binding nature of this anion in comparison to that of fluoride ion (Fig. 2).

Fig. 2 Absorption spectra of 4 (a) and 5 (b) upon addition of 10 equiv. of F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻ and AcO⁻ ions (as tetrabutylammonium salts) in DMSO solution (20 μM). All anion mixture also includes fluoride ion.

To examine the selectivity of the receptors for fluoride ion, a mixture of all anions were added to the solutions of 4 and 5 which exhibited identical colour change and UV-visible absorption spectrum as that observed with fluoride ions, indicating thereby the non-interference of other anionic species during the fluoride ion detection (Fig. 1 and 2).

UV-visible titration experiment was conducted by gradual addition of a standard solution of fluoride ion (TBAF) to a 20 μM solution of 4 and 5 which indicated progressive decrease in intensity of the absorption peaks at 342 and 379 nm respectively with concurrent increase in the absorption bands centred at 546 and 542 nm in the UV-visible spectra (Fig. 3). A colour change from colourless to reddish is observed when the concentration of fluoride ion approached to one equivalent with respect to 4 and 5. Further addition of fluoride ions increased the intensity of the absorption bands at 546 and 542 nm with the appearance of pink colour and deep red colour in the receptor solution of 4 and 5 respectively. The observed colorimetric changes in receptor 4 and 5 reached its limiting value on addition of four equivalents of fluoride ions.

Fig. 3 UV-visible titration spectra of 4 (a) and 5 (b) (20 μM) with 0–10 equiv. of TBAF in DMSO. Blue lines and grey lines correspondingly represent addition of F⁻ ions from 0–1 equiv. and 2–10 equiv. Inset shows the change in absorbance at 546 nm (4) and 542 nm (5) with the addition of various equiv. of F⁻ ions.

Detailed analysis of the UV-visible titration results further indicated that addition of fluoride ions till one equivalent with respect to 4 and 5 consistently increase the absorption peak at 546 and 542 nm respectively with appearance of a clear isosbestic point at 414 nm and 435 nm as shown in Fig. 3. Addition of F⁻ ions beyond one equivalent exhibited a change in the spectral pattern with emergence of hypsochromically shifted (Δλ_max 14 nm) absorption band at 532 nm in 4 and bathochromically shifted (Δλ_max 8 nm) absorbance band at 550 nm in 5 with unclear isosbestic points in both cases. These observations can be accounted for an assumption that receptor 4 and 5 possibly form 1 : 1 complex with fluoride ion up to one equivalent through hydrogen bond interactions while beyond that it forms higher order complexes. The higher order complexes could be either 1 : 2 (receptor : fluoride) hydrogen-bonded adduct (A) or partial deprotonated species (B) on addition of a second equivalent of F⁻ ions in analogy to the formation of [HF₂⁻] species¹² as depicted in Scheme 2. Further addition of F⁻ ions beyond two equivalents seems to result in complete deprotonation of thiourea –NH protons in both 4 and 5. However distinction between these two species A and B has often been challenging in the case of fluoride recognition experiments.¹³

Scheme 2 A proposed binding mode and deprotonation of 4 in the presence of fluoride ions.

The appearance of new absorbance band at 546 and 542 nm respectively in the UV-visible spectra of receptor 4 or 5 during fluoride interaction is either due to the formation of hydrogen bonds with the thiourea –NH protons or its deprotonation by fluoride ions which eventually changes the n → π* transition bands and thereby results in a visual colour change possibly through efficient charge transfer.^{10,12,13,41,42} It was interesting to note that addition of a few drops of protic solvents (water or methanol) triggered the disappearance of observed colouration of receptor–fluoride complex with immediate regeneration of the original receptor colour in both 4 and 5 (Fig. S7 see ESI†). This suggested that the fluoride–receptor interaction did not involve any plausible covalent bond formation and that the complexation of fluoride ion and 4 or 5 was reversible in nature.

UV-visible titration experiments of 4 and 5 with acetate ions exhibited a consistent increase in the absorption bands centred at 546 and 542 nm respectively, however, with less intensity (Fig. S8 and S9 see ESI†). Unlike the case of fluoride ion, the addition of acetate ions beyond one equivalent with respect to 4 and 5 did not show any shift in the absorption bands in either case while it exhibited a clear isosbestic point at 414 nm and 435 nm respectively. This observation clearly indicated that the interaction of acetate ions with receptor molecules mainly occurs through the formation of H-bonds with the thiourea NH protons. However subsequent deprotonation of NH protons by acetate ions could not be achievable even at higher concentrations possibly due to poor basic nature of acetate ions in comparison to that of fluoride ions.

Quantitative analysis of the binding characteristics of 4 and 5 with fluoride ions when determined by Job's continuous variation plots revealed a maximum absorbance at 0.5 mole fraction of receptor to indicate a 1 : 1 binding stoichiometry for the interaction of 4 and 5 with fluoride ions during its initial period of addition (Fig. 4).

Fig. 4 Job's plot for complexation of 4 (546 nm) and 5 (542 nm) with F⁻ ions determined by UV-visible experiments in DMSO at 298 K.

The Benesi–Hildebrand plot of measured absorbance [1/(A − A₀)] at 546 nm versus 1/[F⁻] in 4 revealed a linear relationship at lower concentration of fluoride ions (0 to 1 equivalent) while at higher concentration (2 to 4 equivalents) the plot was completely nonlinear as shown Fig. 5. In contrast when a plot of [1/(A − A₀)] at 546 nm versus 1/[F⁻]² was made it revealed a linear relationship at higher concentration of fluoride ions (2 to 4 equivalent) with a correlation (R²) of 0.997 (inset of Fig. 5). These observations clearly indicated that a 1 : 1 complex stoichiometry was established between 4 and F⁻ ion during the initial stages of the addition (up to 1 equivalent) while at higher concentration beyond one equivalent a 1 : 2 receptor–fluoride adduct (A) is more predominate over the deprotonated species (B) (Scheme 2). In case of receptor 5, the Benesi–Hildebrand plot of measured absorbance [1/(A − A₀)] at 542 nm versus 1/[F⁻] and 1/[F⁻]² exhibited similar results of 1 : 1 and 1 : 2 binding stoichiometry between 5 and F⁻ ion at lower and higher concentrations respectively (Fig. S10 see ESI†). By considering the 1 : 1 complex stoichiometry, the apparent binding constants (K) of 4 and 5 in the presence of F⁻ ion were determined from the UV-visible titration data in DMSO and depicted in Table 1. From the binding constant values it was clearly understood that receptor 4 has more binding affinity for fluoride ion than 5.

Fig. 5 Benesi–Hildebrand (B–H) plot of 1/(A − A₀) at 546 nm vs. 1/[F⁻] indicating 1 : 1 binding between 4 and F⁻ ion. Inset shows the B–H plot of 1/(A − A₀) at 546 nm vs. 1/[F⁻]² indicating 1 : 2 binding between 4 and F⁻ ion.

Table 1 Binding constants from UV-visible titrations for complexes of receptors 4 and 5 with F⁻ and AcO⁻ ions in DMSO^a

Receptor	K (M^−1)
	F^−	AcO^−
a The anions were added in the form of their TBA salts.
4	3.01 × 10^5	3.41 × 10^3
5	1.02 × 10^5	1.09 × 10^4

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The Benesi–Hildebrand plot of measured absorbance [1/(A − A₀)] versus 1/[AcO⁻] in 4 (546 nm) and 5 (542 nm) revealed a linear relationship indicating 1 : 1 binding stoichiometry between the receptor and acetate ions (Fig. S11 and S12 see ESI†). The binding constants (K) of 4 and 5 in the presence of acetate ion were determined from the UV-visible titration data in DMSO and depicted in Table 1 which clearly indicated that both 4 and 5 have high binding affinity towards fluoride ion compared to acetate ion.

Fluorescence study of receptor 4 and 5 with anions

The binding event of 4 and 5 with the anionic species was also investigated using fluorescence spectroscopy by observing the changes in their emission spectra in 10 μM DMSO solution. The emission spectra were recorded from 390 to 550 nm by exciting the receptors 4 and 5 at 360 nm when the emission maxima (λ_em) were observed at 437 and 455 nm respectively. The initial emission intensity of 4 was very weak while 5 exhibited strong emission intensity possibly due to the presence of an extra aromatic ring in its molecular structure. Hence the fluorescence measurements were conducted by choosing the slit width of 5 nm and 2.5 nm for 4 and 5 respectively. Upon addition of 50 equivalents of various anions such as F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻ and AcO⁻ (as TBA salts) to a solution of 4 and 5 in DMSO resulted in remarkable enhancement of fluorescence intensity at 437 nm in 4 in the presence of fluoride ions only (Fig. S13 see ESI†). In contrast, receptor 5 exhibited a hypsochromic shift of 16 nm in its emission band that appeared at 439 nm upon interaction with fluoride ions (Fig. S14 see ESI†). The fluorescence titration experiments of 4 and 5 were conducted under similar conditions in the presence of fluoride ions. The emission intensity of 4 at 437 nm was increased significantly by 20-fold (50 equiv. F⁻) while that of 5 at 439 nm enhanced by 5-fold only (50 equiv. F⁻) after the gradual addition of fluoride ions in various equivalents (Fig. 6). These observations can be comprehended from previous report⁴³ which suggested that fluorescence “turn-on” is a result of hydrogen bonding interactions of 4 and 5 with F⁻ ion at lower concentrations while at higher concentrations it leads to deprotonation of –NH protons that possibly eliminates the n → π* transitions to facilitate the π → π* transitions mainly responsible for the emissive behavior of 4 or 5.

Fig. 6 Fluorescence titration spectra of 4 (a) and 5 (b) (10 μM) with 0–50 equiv. of TBAF in DMSO at 298 K. Inset shows change in fluorescent intensity (I/I₀) at 437 and 439 nm for 4 and 5 respectively with varying equivalents of F⁻ ion. λ_ex = 360 nm, λ_em = 437 nm (4) and 439 nm (5).

The fluorescent colour changes observed for the receptor–fluoride interaction is found to be more prominent in 4 than 5 (Fig. 7 and S15 see ESI†). The selectivity of the fluoride ion in comparison to other anions for emission enhancement in 4 and 5 may be ascribed to its greater hydrogen bonding and deprotonation ability. This finding in case of our receptors 4 and 5 is worthy to note as fluoride binding mostly causes fluorescence quenching rather than enhancement in earlier reported receptors.^{18–24,27,29–32}

Fig. 7 Fluorescent “turn on” behaviour of 4 for fluoride ion over other anions in DMSO solution (10 μM) with the addition of 5 equivalents of various anions (as TBA salts) under UV light (365 nm).

The interaction between F⁻ and receptor 4 in DMSO has also been investigated by the time-resolved fluorescence technique and the representative fluorescence decay profiles of 4 with different concentrations of F⁻ are shown in Fig. 8. The free receptor 4 exhibited three-exponential lifetime decay with average lifetime of 9.64 ns and with the addition of various concentrations of fluoride ion the average fluorescence lifetime is increased (Table 2). At higher concentration, the enhancement of average lifetime is ∼2.38 ns. These results indicated the formation of new ICT states of 4 in presence of fluoride ions.²⁴

Fig. 8 Fluorescence decay profiles of 4 (10 μM) at different concentrations of F⁻ in DMSO; λ_ex = 360 nm and λ_em = 437 nm: green colour = laser profile; black colour = 4 alone; red colour = 1.7 mM; blue colour = 3.3 mM of F⁻ ion. Residual plot of 4 alone.

Table 2 Fluorescence lifetimes of 4 in absence and in presence of fluoride ions

Sample	α1	τ1 (ns)	α2	τ2 (ns)	α3	τ3 (ns)	Avg. τ
4 only	11.54	0.84	23.33	4.14	65.13	10.54	9.64
4+F^− (1.7 mM)	15.72	0.85	31.22	4.68	53.06	12.66	11.07
4+F^− (3.3 mM)	18.59	0.77	31.55	4.35	49.86	13.79	12.02

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The UV-visible spectral change, visual colour observation, fluorescence enhancement, and binding constant of 4 was compared with those of other thiocarbonohydrazone based sensors as shown in Table 3. 4 showed the longest wavelength absorption band and highest emission enhancement with a stronger binding constant upon interaction with fluoride ion amongst the recently reported thiocarbonohydrazone based colorimetric fluoride ion sensors.

Table 3 Properties of the receptor 4 and other thiocarbonohydrazone (TCH) based colorimetric fluoride ion sensors

Key functionality	UV-visible band in nm without F^− ion	UV-visible band in nm with F^− ion (saturated equiv.)	Shift in UV-visible band, Δλmax in nm	Visible colour change	Fluorescence enhancement	Binding constant (K)	Medium of detection	Reference
a Not determined.
p-Nitrophenyl	360	495(4)	135	Colourless to reddish	ND^a	4.3 × 10^4 M^−1	MeCN	33
Indole	342	442(4)	100	Colourless to pink	ND^a	6.3 × 10^3 M^−1	MeCN–DMF	34
Thiophene	340	472(5)	132	Colourless to yellow	12-fold	2.91 × 10^5 M^−1	DMSO	35
2-Hydroxyacetophenone	348	391(3)	43	Colourless to yellow	2-fold	9.85 × 10^4 M^−1	DMSO	36
Isatin	376	515(4)	139	Colourless to reddish	ND^a	4.89 × 10^4 M^−2	MeCN	37
N,N-Diethylaminocoumarin	448	477(7.2)	29	Yellow to light brown	3-fold	0.70 × 10^4 M^−1	MeCN–DMSO	38
Acetyl coumarin	342	546(4)	204	Colourless to intense pink	20-fold	3.01 × 10^5 M^−1	DMSO	Present work

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¹H NMR study of receptor 4 with fluoride ions

The binding mode of fluoride ions with receptor 4 was further investigated by ¹H NMR titration experiments in DMSO-d₆ solution. The NMR spectrum of 4 in the presence of one equivalent of fluoride ion (Fig. S16 see ESI†) demonstrated a severe broadening and subsequent collapse of the two –NH signals at 9.76 and 10.41 ppm to a single broad peak at 10.17 ppm, which completely disappeared on addition of excess of fluoride ion (5 equivalents). Further analysis of the NMR titration spectra revealed that gradual addition of fluoride ions to receptor 4 was found to result in an upfield chemical shift in the proton signals of the coumarin unit which got saturated beyond four equivalents of F⁻ ions. This observation could be ascribed to the formation of hydrogen bonded complex between the receptor and fluoride ion at lower concentration while at higher concentration it led to deprotonation of the NH protons which resulted in an upfield shift of the coumarin signals possibly by through-bond propagation of electron density. Moreover, the appearance of a broad triplet signal at around 16.0 ppm was attributed to the formation of the HF₂⁻ species (inset of Fig. S16 see ESI†), which further supports the proposed deprotonation mechanism beyond four equivalents of fluoride addition.³³

Computational study of receptor 4 with fluoride ion

To obtain more information about the photo physical properties of receptor 4 in the presence of fluoride ions quantum chemical calculations were carried out by using the TURBOMOLE program suite.⁴⁴ The single point geometry optimizations and frontier molecular orbital properties of the concerned molecules were carried out by employing Density Functional Theory^45,46 at B3LYP⁴⁷/TZVP⁴⁸ level of theory taking the advantage of the RI^49,50 approximation which yields an acceleration by a factor of 5–10.⁵¹ The obtained optimized structures were visualized by using Gauss view programme. The optimized structures of receptor 4 and 4+F⁻ complex are shown in Fig. 9. The total energy of the 4+F⁻ complex was found to be lower (−51826.99 eV) as compared to its receptor alone (−49116.40 eV) at DMSO solvent phase which indicated more stability of the receptor–fluoride complex possibly due to the formation of a six-member ring. The bond length obtained for both N–H⋯F⁻ hydrogen bonds are found to be 1.3 Å which suggested that both the thiourea NH protons bind the fluoride ion with equal strength. The results of the quantum chemical calculations further indicated that the energy gap between the HOMO and LUMO of receptor 4 was found to be larger than that in 4+F⁻ complex (Fig. 10). This result was found to be in good agreement with the significant wavelength shift (Δλ = 204 nm) observed in the absorption spectrum of receptor 4 upon the addition of F⁻ ions.

Fig. 9 Optimized structures of receptor 4 and 4+F⁻ (1 eqvt) (colour key: white = hydrogen; grey = carbon; blue = nitrogen; red = oxygen; yellow = sulfur; cyan = fluorine).

Fig. 10 Energy diagrams of HOMO and LUMO orbital of 4 and 4+F⁻ complex calculated on the DFT level using a B3LYP/TZVP(RI) basis set.

Conclusions

In summary, two chromofluorescent thiocarbonohydrazone receptors 4 and 5 functionalized with coumarin derivatives have been synthesized through simple condensation reactions in quantitative yields. Receptor 4 and 5 selectively recognizes fluoride ions among the other anions through a distinct visual colour change from colourless to pink and deep red with the appearance of a new longer wavelength absorption peak at 546 and 542 nm respectively in their UV-visible spectra. Furthermore, addition of fluoride ions significantly enhances the emission intensity by 20-fold in receptor 4 and thereby providing fluorescent “turn-on” chemosensory response to fluoride ions. Job's plot analysis revealed a 1 : 1 binding stoichiometry between the receptor and fluoride ion. However a more detailed analysis of Benesi–Hildebrand plots obtained from the UV-visible titration experiments of 4 and 5 with fluoride ions showed a linear relationship [R² = 0.995(4), 0.991(5)] with 1 : 1 binding stoichiometry at lower concentration (0–1 equivalent) of fluoride ion whereas at higher concentration (2–5 equivalents) it exhibited a linear relationship [R² = 0.997(4), 0.992(5)] with 1 : 2 binding stoichiometry between the receptor and fluoride ions. The receptor 4 showed the largest shift in UV-visible absorption band (Δλ_max = 204 nm) and highest emission enhancement (20-fold) with a stronger binding constant (3.01 × 10⁵ M⁻¹) among the thiocarbonohydrazone based fluoride sensors reported so far. The results of the quantum chemical calculations of receptor 4 in the presence of fluoride ion indicated a decrease in total energy and lowering of HOMO–LUMO energy gap in receptor 4 upon interaction with fluoride ions which were in good agreement with the experimental data. Taken together, the newly synthesized receptors 4 and 5 are extremely good candidates for sensing fluoride ion through visual colour change and fluorescence enhancement.

Acknowledgements

S. N. S gratefully acknowledges the financial assistance received from DST, New Delhi for the Fast-Track project grant (SR/FT/CS-46/2011). S. K. P is thankful to DST, New Delhi for a project assistantship. Authors are thankful to Central Instrumentation Laboratory, GJUST, Hisar-Haryana for recording NMR spectra and UGC New Delhi for financial assistance to School of Chemistry under DRS grant. The financial assistance received from DST New Delhi under the FIST grant to School of Chemistry for the procurement of time-resolved fluorescence spectrophotometer is gratefully acknowledged. We are thankful to Dr H. Chakraborty for his useful suggestions towards the improvement of this manuscript.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis procedures of 4 and 5, ¹H, ¹³C NMR spectra and FT-IR spectra of 4 and 5, colour change, UV-visible titration spectra of 4 and 5 with acetate ions, B–H plot of 5 with fluoride ion, B–H plots of 4 and 5 with acetate ions, emission spectra of 4 and 5 with anions, fluorescence colour change of 5 with anions, partial ¹H NMR titration spectra of 4 with fluoride ions, Cartesian coordinates of 4 and 4+F⁻, detail procedure for Job's plot experiment, general method for ¹H NMR titrations experiments. See DOI: 10.1039/c6ra19647k

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