‘Naked-eye’ colorimetric/fluorimetric detection of F⁻ ions by biologically active 3-((1H-indol-3-yl)methyl)-4-hydroxy-2H-chromen-2-one derivatives

Abstract

An improved synthetic protocol has been developed for the construction of highly functionalized heterologous alkyl and benzyl indolyl-coumarin derivatives through a rapid, catalyst-free and solvent-free one-pot three-component reaction of indole, aldehyde and 4-hydroxycoumarin. The synthesized compounds display a high selectivity and sensitivity towards F⁻ ions. The detection of F⁻ ions was accompanied by fluorescence quenching with noticeable colour change, which is evident from the Stern–Volmer plot. The Stern–Volmer plot shows an association constant of 6.2625 × 10⁴ M⁻¹ with a lowest detection limit (LOD) of 0.50 μM for F⁻ ions. The interaction mechanism was studied by ¹H-NMR titration and also supported by advanced TD-DFT calculations. The designed chemosensor has been utilized as a mini colorimetric kit for F⁻ ions on test paper strips and is also capable of detecting F⁻ ions in commercially available toothpaste and mouth rinser. The chemosensors also exhibited good antifungal and antibacterial activity.

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1. Introduction

The synthesis of biologically and pharmaceutically effective sensory molecules remains key objectives of current research in view of their potential applications in biology, medical and sensing materials. Coumarin and indole derivatives play an important role in human life due to their valuable biological, medical and optical properties.¹ Therefore, a conjugation of indole and coumarin is expected to be more efficient than the individual forms because of their several applications such as optical brighteners,² photosensitizers,³ fluorescent and laser dyes⁴ as well as additives in food, perfumes, cosmetics and pharmaceuticals.⁵ The 4-hydroxycoumarin-indole based receptor brings two fluorescent chromophores into remarkable proximity, possessing an enolic-OH group and indolyl-NH group (hydrogen bond donor position due to which the electronic properties of the receptor is changed allowing the subsequent detection of anions), which leads to a highly sensitive chemosensor.⁶ In the past two decades, several reports have been published on the synthesis of α-benzylaminocoumarins and benzylindolyl coumarin derivatives.^7–12 Despite the intrinsic worth of these methods, some of them suffer from several limitations such as long reaction time, use of catalysts, toxic reagents, carcinogenic solvents, limited substrate scope, low to moderate yields, tedious work-up procedure and purification steps. Therefore, it is a challenging task to develop a novel, efficient, economical and eco-friendly approach for the synthesis of relevant heterologous indolyl-coumarin scaffolds.

In spite of the gargantuan potential interest, there is no data available in the literature describing the optical properties of α-benzylaminocoumarins and benzylindolyl coumarin derivatives. Although a very few reports have been published describing the optical behaviour of bis(coumarin)methylene.^13–15 Anions play a noteworthy role in a wide range of biological, environmental, medicinal and chemical processes. Among various anions, F⁻ ion is one of the most essential ions in the human body with its significant applicability in dental care and potential use for the treatment of osteoporosis, psychiatric and hypnotics.¹⁶ However, excess F⁻ ion is also harmful to human health. It is easily absorbed and causes acute gastric, fluorosis, cancer and kidney problems.^17,18 According to World Health Organization (WHO) F⁻ ion in drinking water is considered beneficial at level of about 5.3 × 10⁻³ mol L⁻¹ but harmful once it exceeds 7.9 × 10⁻³ mol L⁻¹.^19–21 Thus, it is necessary to add appropriate concentration of F⁻ ion in drinking water.²² At present, a large population in the world still drink water containing inappropriate level of F⁻ ion which leads to several types of pathologies. In India alone about seven regions of West Bengal (WB) are predicted to be endemic for fluorosis and almost 66 million common people in these regions are at high risk of F⁻ contamination.²³ Therefore, considering these obvious health concerns and to minimize the possibility of excessive F⁻ ion intake, a simple, economical and selective assay is highly desirable for practical purposes.²⁴ Several efforts have been made to monitor F⁻ ion by traditional analytical methods such as ion chromatography²⁵ and ion-selective electrodes.²⁶ However, the major disadvantages of these techniques are that they are time consuming and require costly instruments. Alternatively, the fluorimetric and colorimetric techniques have attracted more attention over other available techniques for detecting cations and anions due to high selectivity, quick response, real-time detection and cost effectiveness.^27,28 In literature, a great deals of excellent colorimetric and fluorescent sensors for F⁻ ion have been reported,^29–31 while, most of these sensors have complicated structure and synthesized by multi-step procedures.^32–34 The development of chemosensor for the selective and sensitive detection of F⁻ ions is crucial. Therefore, design and development of synthetic receptors and chemosensor for the selective and sensitive detection of F⁻ ion have garnered considerable attention for researchers due to toxicity of the F⁻ ion.

In continuation of our research interest on the synthesis of various functionalized molecules, acid catalysis.^35–37 Furthermore, suitable supramolecular sensing probe for cations have been developed and recently published.³⁸ Influenced by the results of these studies, herein we synthesized heterologous indole-4-hydroxycoumarin derivatives via one-pot three component reactions of substituted 4-hydroxycoumarin, aldehyde and indole at 50–60 °C under grinding and solvent-free conditions (Scheme 1). The current method is significantly improved from earlier described protocols in terms of applicability of wide range of aldehyde, better yield, shorter reaction time, simplistic workup and essentially no need of column chromatography. The synthesized compounds selectively detect F⁻ ion by naked eye, exhibit intense hypochromic shift in absorption and significant fluorescence quenching in emission spectra upon interaction with F⁻ ion in acetonitrile. Interestingly, synthesized series of compounds also exhibited significantly antifungal and antibacterial response.

Scheme 1 One pot green synthesis of alkyl/benzylindolyl coumarin derivatives.

2. Experimental details

2.1 Reagents and instrumentation

All common reagents and solvents were of AR grade, purchased from Sigma Aldrich and Himedia, India and used as received otherwise mentioned. ¹H-NMR and ¹³C-NMR spectra of compounds were recorded in DMSO-d₆ on a Brüker (500 MHz) spectrometer and chemical shifts were reported as part per million (ppm) in δ scale downfield from TMS (as internal standard). The following abbreviations were used to explain the multiplicities: br = broad, s = singlet, d = doublet, dd = double of doublet, dt = doublet of triplet, t = triplet, q = quartet, m = multiplet. The UV-vis absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer and the fluorescence emission spectra were recorded on a Shimadzu RF-5301 PC Spectrofluorophotometer with a 3.0 cm standard quartz cell. Melting points were recorded on Optimelt automated melting point system. Quantum yield were obtained by using FLS 980 Fluorescence spectrometer (Edinburgh Instruments). The IR spectra were recorded on a Nicolet 6700 FTIR Thermoscientific in the range 4000–400 cm⁻¹ with KBr pellet. Fluorescence life time spectra were obtained by using HORIBA Jobin Yvon, Fluorocube Fluorescence Lifetime System.

2.2 General procedure for the synthesis compounds 4a–4n

A mixture of 4-hydroxycoumarin 1 (1.0 mmol), aldehyde 2 (1.0 mmol) and indole 3 (1.0 mmol) was taken in mortar and mixed thoroughly with pestle and ground at 50–60 °C. The reaction was monitored by thin layer chromatography. After completion, the reaction mixture was cooled to room temperature and washed with ethanol and dried to obtain pure desired product.

3. Results and discussion

3.1 Synthesis, substrate scope and characterization of compounds 4a–4n

In the design of anion sensor, we were interested in developing heterologous scaffold bearing –OH and –NH groups in the same molecule; a 4-hydroxycoumarin moiety as enolic scaffold and an indole as an –NH entity to detect anions through deprotonation mechanism. At the outset 4-hydroxycoumarin 1a, benzaldehyde 2a and indole 3a were chosen as the model substrate to optimize the reaction conditions. In a preliminary attempt, the reaction was performed in the presence of 20 mol% of I₂ at room temperature, the proposed reaction not proceed under these conditions to give the desired product 4a (Table 1, entry 1). When reaction was carried out at elevated temperature (40–100 °C), the desired product 4a was obtained in 20% yield along with the traces of side product 5a (Table 1, entry 2–4). Furthermore, when the reaction was carried out in absence of I₂ under solvent free condition at room temperature for 1 h, the reaction underwent smoothly to give the desired product 4a with 50% yield along with a significant amount of 5a (40%) (Table 1, entry 5). In order to improve the yield of heterodimeric product 4a, reaction was explored at different temperatures under neat condition, the optimum 88% yield was obtained at 50 °C (Table 1, entry 7). Thus the reaction at 50 °C under solvent-free conditions emerged as the optimum for subsequent studies. After having the optimized reaction conditions, we further investigated the substrate scope and generality of the reaction. A number of structurally diverse aldehydes, substituted indole and 4-hydroxycoumarin are used for this purpose. There were no significant effects on the reaction yield by substituents on indole and 4-hydroxycoumarin. However, substituents on aldehydes exhibited marginal effects on both yield as well as in reaction rate. The aldehyde bearing electron withdrawing substituent afforded the product 4g with 85% yield, whereas aldehyde having electron rich group like 4-methyl and 3-methoxy afforded the product with relatively better yield (Table 2, entry 5 and 6). The assigned structures of the products 4a–4n were well characterized by using analytical tools such as ¹H, ¹³C-NMR and IR spectroscopy (S1–S26†).

Table 1 Optimization of reaction condition for the synthesis of benzylindolyl coumarin 4a^a

Entry	Catalyst	Temperature (°C)	Yield^b (%)
			4a	5a
a The reaction was carried out using 4-hydroxycoumarin 1 (1.0 mmol), aldehyde 2 (1.0 mmol) and indole 3 (1.0 mmol) in mortar and mixed thoroughly with pestle.b Isolated yield.c Catalyst 20 mol% used.d Reactants decomposed.
1	I2^c	RT	—	40
2	I2^c	40	Trace	35
3	I2^c	50	20	Trace
4	I2^c	100	—^d	—^d
5	—	RT	50	40
6	—	30	60	10
7	—	50	88	5
8	—	60	88	5
9	—	100	—^d	—^d

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Table 2 Summarized results of substrate scope under optimized reaction conditions^a

S. No.	Comp.	R1	R2	R3	R4	Time (min)	Yield^b (%)
a The reaction was carried out using 4-hydroxycoumarin 1 (1.0 mmol), aldehyde 2 (1.0 mmol) and indole 3 (1.0 mmol) in mortar and mixed thoroughly with pestle and ground at 50–60 °C.b Isolated yield.
1	4a	H	Phenyl	H	H	5	88
2	4b	H	4-Bromophenyl	H	H	5	85
3	4c	H	4-Chlorophenyl	H	H	5	86
4	4d	H	2-Chlorophenyl	H	H	5	85
5	4e	H	4-Methylphenyl	H	H	8	90
6	4f	H	3-Methoxyphenyl	H	H	10	88
7	4g	H	4-Nitrophenyl	H	H	5	85
8	4h	H	4-Fluorophenyl	H	H	9	88
9	4i	H	Ethyl	H	H	5	90
10	4j	H	Ethyl	H	OCH3	6	90
11	4k	CH3	Ethyl	CH3	H	6	88
12	4l	H	4-Bromophenyl	CH3	H	8	80
13	4m	H	4-Chlorophenyl	CH3	H	8	82
14	4n	H	2-Chlorophenyl	CH3	H	8	81

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3.1.1 3-((1H-Indol-3-yl)(phenyl)methyl)-4-hydroxy-2H-chromen-2-one (4a). Yield: 323 mg (88%) as white solid; mp = 191 °C; IR ν_max (KBr, cm⁻¹): 3417, 3345, 2940, 2830, 1689, 1621, 1563, 1417, 1278, 943, 758; ¹H-NMR (DMSO-d₆, 500 MHz) δ (ppm): 11.6 (s, broad, D₂O exchangeable, 1H), 10.9 (s, D₂O exchangeable, 1H), 8.02 (d, J = 7.5 Hz, 1H), 7.60 (t, J = 7.0 Hz, 1H), 7.37–7.28 (m, 7H), 7.10–7.03 (m, 4H), 6.90 (t, J = 7.5 Hz, 1H); 6.08 (s, 1H); ¹³C-NMR [125 MHz (DMSO-d₆)] δ (ppm): 162.2, 160.7, 152.6, 143.1, 136.4, 132.3, 128.6, 128.2, 127.7, 126.1, 124.7, 124.3, 123.9, 121.3, 118.9, 118.7, 116.7, 116.6, 114.6, 111.9, 108.9, 37.5.

3.2 Anion sensing: colorimetric and fluorimetric studies

The UV-vis absorption and fluorescence emission spectra of all of the compounds were investigated in acetonitrile and the results are summarised in Table S2, ESI.† Out of the 14 compounds 4a–4n, for the complete study of optical behaviour, 4a was selected. The chemosensor 4a (1.0 × 10⁻⁵ M) exhibited a strong absorption band centred at 485 nm in acetonitrile which can be assigned to the π–π* transition (Fig. 2a). Upon gradual addition of F⁻ ion (TBAF, 0–20 equiv.) to the above solution of 4a, the absorption band at 485 nm regularly decreased and a new absorption band appeared at 415 nm (blue shift of 70 nm) (Fig. 3a). A clear isosbestic point was observed at 455 nm, indicating the strong interaction between 4a and F⁻ ion. The absorption intensity at 415 nm reaches a plateau after the addition of exactly 20 equivalent of F⁻ ion and there is no significant enhancement of the absorption intensity on further addition of F⁻ ion. Moreover, a light orange colour obtained after the addition of F⁻ ion to the solution of 4a which was initially colourless. This colour change could be easily observed via “naked eye” (Fig. 1). On the contrary, absorption spectrum of 4a was also studied in the presence of various anions (20 equiv.) such as I⁻, Br⁻, OAc⁻, Cl⁻, ClO₄⁻, BF₄⁻, HSO₄⁻, NO₃⁻ and PF₆⁻ (tetrabutylammonium salt), however, no considerable change in the colour of the solution as well in the UV-vis spectra was observed.

Fig. 1 (a) Colourimetric and (b) fluorimetric naked-eye detection of 4a in the absence and presence of different anions (20 equiv.) in CH₃CN.

Fig. 2 (a) Absorption and (b) fluorescence emission spectra of 4a (1.0 × 10⁻⁵ M) in the absence and presence of 20 equiv. of different anions in CH₃CN. λ_ex = 455 nm.

As shown in Fig. 3b, the selectivity of 4a to F⁻ ion was also investigated by fluorescence emission spectral titration in acetonitrile. The chemosensor 4a exhibited an intense emission band at 507 nm (absolute quantum yield, ϕ_f = 0.47), upon the addition of F⁻ ion (0–20 equiv.) to the solution of 4a (10 μM) fluorescence intensity at 507 nm decreases gradually and shifted to 534 nm (red shift of 27 nm, absolute quantum yield, ϕ_f = 0.049) (Fig. 3b). Finally the reaction gets saturated with F⁻ ion and demonstrated a remarkable quenching effect. The result suggested that 4a can act as a “switched on–off” sensor for F⁻ ion particularly. As mentioned earlier also, selectivity of chemosensor 4a towards different anions (I⁻, Br⁻, OAc⁻, Cl⁻, ClO₄⁻, BF₄⁻, HSO₄⁻, NO₃⁻ and PF₆⁻) was also investigated which displayed no change in the optical properties. Moreover, no significant fluorescence quenching was observed upon the addition of any other anion. Hence, individual response of 4a against these anions revealed a remarkable selectivity only for F⁻ ion binding (Fig. 2b). The selective response of 4a towards F⁻ ion was evaluated by adding 5 equiv. of various anions and result showed that there was no interference from other anionic species (Fig. 4a).

Fig. 3 (a) Absorption and (b) fluorescence emission spectral responses of 4a (1.0 × 10⁻⁵ M) towards varying F⁻ ion concentrations (0 to 20 × 10⁻⁵ M) in CH₃CN. λ_ex = 455 nm.

Fig. 4 (a) Interference studies of F⁻ with 4a (1 : 5), black bars represent the emission intensity of [4a–other anions] system and red bars show [4a–F⁻–other anions] system (b) Jobs plot shows 1 : 1 stoichiometry in CH₃CN.

The time resolved fluorescence study of chemosensor 4a was also analysed in the absence and presence of F⁻ ions, obtained at excitation frequency of 455 nm. As shown in Fig. 5b, the fluorescence lifetime decreased from 4.65 ns to 2.47 ns after the addition of F⁻ ions, indicates strong interaction of receptor 4a with F⁻ ions. The binding stoichiometry was obtained from the Job's plot measurement on the basis of fluorescence. The maximum fluorescence intensity appeared at 0.5 mole fraction which clearly expressing 1 : 1 binding stoichiometry of 4a with F⁻ ion (Fig. 4b).³⁹ The association constant was calculated by using S.V. plot which was found to be 6.2625 × 10⁴ M⁻¹ (Fig. 5a) with 0.50 μM detection limit (Fig. 5c).^40–42

Fig. 5 (a) Stern–Volmer plot (b) life time decay plot (c) limit of detection for binding of F⁻ ion with chemosensor 4a.

The obvious hypsochromic shift in the absorption band from 485 to 415 nm and remarkable quenching in fluorescence spectra suggests that the π-conjugation was inhibited by the addition of F⁻ ion to 4a which may be due to the deprotonation of –OH and –NH protons. To test the deprotonation possibility of –OH and –NH protons as per proposed mechanism, absorption and emission titration of 4a was also performed with a relatively strong base [Bu₄N]OH, which leads to deprotonation. Similar changes in absorption and emission spectra were obtained as those observed with F⁻ ion (Fig. S29, ESI†). The similar anion sensing properties of 4a were also observed in the more polar solvent DMSO. It was reported as the protic solvents⁴³ such as water would compete with the anions for the bonding sites of chemosensor and could therefore disturb their hydrogen bonding interactions. To investigate the applicability of chemosensor 4a for F⁻ ion detection in drinking water similar experiments were carried in DMSO–H₂O media and was observed that the receptor was able to demonstrate noticeable changes in both absorption and fluorescence spectra when water content was not more than 10% (v/v) in DMSO (Fig. S30, ESI†). Thus the chemosensor could be applied for F⁻ ion detection in environmental water samples and biological samples (Scheme 2).

Scheme 2 Proposed sensing mechanism of 4a through deprotonation in the presence of F⁻ ion.

3.3 Nature of interaction between chemosensor 4a and F⁻ ions

To gain an insight about the mechanism of interaction between 4a and F⁻ ion, ¹H-NMR titration experiment was performed in DMSO-d₆. The ¹H-NMR spectra of 4a show NMR peaks as singlet at δ 11.65, 10.91 and 6.09 ppm which are attributable to the –OH, –NH and –CH proton, respectively and all aromatic protons appeared within the region δ 6.89–8.03 ppm. Upon the addition of 0.5 equiv. of [Bu₄N]F to the solution of 4a in DMSO-d₆, the most noticeable change was observed in –OH proton. The singlet at δ 11.65 ppm corresponding to –OH proton completely disappeared due to deprotonation. This was confirmed by the appearance of broad signal corresponding to HF₂⁻ at around 16.0 ppm in presence of excess F⁻ ions.⁴⁴ Alternatively a slight upfield shift of –NH proton was noticed (from δ 10.91 to 10.84 ppm), reflecting the increase in electron density (Fig. 6). Thus, the ¹H-NMR experiment clearly supported the hypothesis that the F⁻ ions preferably interacts with the receptor –OH rather than –NH. We also carried out computational DFT and TD-DFT studies in gas phase, to further appreciate the binding procedures of F⁻ ion with chemosensor 4a.⁴⁵ The DFT and TD-DFT calculations were performed for the optimized structure of chemosensor 4a and 4a + F⁻ to the B3LYP/6-31G(d) level using Gaussian 03 program. DFT calculation results showed that the –OH bond length was changed by 0.975 Å to 1.446 Å after addition of F⁻ ions. So, one conclude from these data deprotonation of the –OH protons of chemosensor 4a. TD-DFT calculations were performed, to interpret further the absorption properties of 4a and 4a + F⁻ complexes. As shown in Table S1, Fig. S34 and S35 ESI,† the lowest energy transitions of 4a arise from the HOMO → LUMO, HOMO−2 → LUMO, HOMO−3 → LUMO and HOMO−4 → LUMO whereas in the 4a + F⁻ originated from HOMO → LUMO and HOMO−1 → LUMO. We also calculated the UV-Vis absorption spectra of chemosensor 4a and 4a + F⁻ in gaseous state by TD-DFT. The absorption spectra indicates a systematic blue shift from λ_max = 478 nm to λ_max = 321 nm, in chemosensor 4a when binding with F⁻ ion. The blue shift in absorption maxima of chemosensor 4a after binding with F⁻ ion could be understand in terms of increases energy gap between HOMO and LUMO of 4a + F⁻ (Fig. 7). For chemosensor 4a, the HOMO was distributed on the indole scaffold while the LUMO was distributed on the coumarin moiety with energy deference ΔE = 3.432 eV. Whereas, for 4a + F⁻, the HOMO was distributed on the whole indole and coumarin system while LUMO was distributed on coumarin scaffold with energy deference ΔE = 4.220 eV. This clearly indicates the sensing of the fluoride ion. Therefore, the interaction of F⁻ ion with –OH of chemosensor 4a was result of deprotonation due to charge transfer, which also support the blue shift in the UV-Vis. absorption spectra of 4a with F⁻ ion and naked eye colour change.

Fig. 6 ¹H-NMR titrations of 4a with F⁻ ion in DMSO-d₆.

Fig. 7 DFT optimised structure and HOMO–LUMO band gap of chemosensor 4a and 4a + F⁻.

4. Application

4.1 Visual colour changes on test papers

To investigate the potential application of 4a as an anion sensor in solid state, we prepared a test kit using Whatman-40 filter paper coated with acetonitrile solution of 4a (10 μM) followed by drying in air. The colour of the test strip changed to bright orange only with F⁻ ions supporting the practical applicability of the receptor 4a (Fig. 8). This experiment shore up that compound 4a has the potential to detect F⁻ ions in solid state.

Fig. 8 Colour change of test paper containing 4a (10 μM) with different anion.

4.2 Ocular colorimetric fluoride detection from mouth rinser and toothpaste

Inspired by the results of test kit, we were moved to test its capacity in detecting F⁻ ions from commercially available toothpaste (131 mg in 1.0 g) and mouth rinser (0.01% w/v). The chemosensor 4a (1.0 × 10⁻⁵ M) can effectively sense fluoride from mouth rinser and toothpaste by appearance of light orange colour to the colourless solution (Fig. 9). We have directly used commercially available mouth rinser and in case of toothpaste a 10 mL 10⁻⁶ (M) water solution was prepared to conduct F⁻ ions detection.

Fig. 9 F⁻ ion detection; 4a before and after exposure to commercially available toothpaste and mouth rinser.

4.3 Biological evaluation

Synthesized compounds were evaluated for their in vitro antibacterial activity against Gram-positive bacteria (B. subtilis and S. aureus), Gram-negative bacteria (E. coli and S. Flexneri) and antifungal potential against Candida albicans fungal strain. The results are summarized in Table 3.

Table 3 Minimum inhibitory concentration (MIC, mg mL⁻¹) of synthesized compounds 4a–n against bacterial and fungal strains

	Gram-positive		Gram-negative		Fungus
	B. subtilis	S. aureus	E. coli	S. flexneri	C. albicans
4a	>100	>100	>100	50	25
4b	25	25	12.5	50	3.12
4c	50	25	>100	12.5	50
4d	3.12	0.78	1.56	3.12	6.25
4e	0.39	12.5	50	12.5	12.5
4f	1.56	25	25	12.5	12.5
4g	>100	12.5	0.78	3.12	25
4h	6.25	25	12.5	6.25	25
4i	50	3.12	0.39	3.12	25
4j	6.25	6.25	0.39	6.25	0.39
4k	12.5	3.12	1.56	>100	3.12
4l	3.12	1.56	0.78	3.12	12.5
4m	6.25	1.56	1.56	12.5	25
4n	1.56	>100	>100	50	3.12
Ampicillin	0.39	0.78	1.56	0.78	—
Cefadroxil	3.12	1.56	0.78	1.56	—
Fluconazole	—	—	—	—	3.12

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4.3.1 Antibacterial activity. The compounds 4d, 4e, 4f, 4l, and 4n (MIC: 3.12, 0.39, 1.56, 3.12 and 1.56 mg mL⁻¹ respectively) show excellent antibacterial activity against B. subtilis compared to the standard drugs. Compound 4e (MIC: 0.39 mg mL⁻¹) and, compounds 4d and 4l (MIC: 3.12 mg mL⁻¹ each) exhibit equal antibacterial activity than positive controls against B. subtilis. In case of S. aureus, the compounds show equal or lower activity than Ampicillin and Cefadroxil drugs. Compound 4d (MIC: 0.78 mg mL⁻¹) and, compounds 4l and 4m (MIC: 1.56 mg mL⁻¹) were found equally potent as the standard drugs against S. aureus. However, compounds 4i and 4j (MIC: 0.39 mg mL⁻¹) show better activity than positive controls against E. coli while compound 4l (MIC: 0.78 mg mL⁻¹) and, compounds 4d and 4m (MIC: 1.56 mg mL⁻¹) show equal activity to the reference drugs. Compounds 4d, 4g, 4i and 4l show higher antibacterial activity (MIC: 3.12 mg mL⁻¹) compared to Ampicillin and Cefadroxil against Shigella flexneri. Thus, the electron releasing group (CH₃, OCH₃ and 2-methyl indole moiety) containing compounds such as 4e, 4f and 4n has shown excellent (MIC: 0.39 mg mL⁻¹) to good (MIC: 1.56 mg mL⁻¹) activity against B. subtilis. Against S. aureus also, the electron donating group (2-methyl indole moiety and 4-substituted phenyl) carrying compounds (4l and 4m) exhibit equal (MIC: 1.56 mg mL⁻¹) antibacterial activity compared to Cefadroxil. Against E. coli, compounds 4d, 4g, 4i, 4j, 4k, 4l and 4m show excellent activity compared to Ampicillin and Cifadroxil regardless of presence of electron withdrawing or donating group. Against Shigella flexneri, all compounds exhibit lower activity compared to the standard drugs without depending on the presence of any functional group.

4.3.2 Antifungal activity. Compounds were subjected to MIC (minimum inhibitory concentration) determination by microdilution method⁴⁶ and the results are summarized in Table 3. The standard reference drugs Fluconazole was selected as positive control in antifungal activity assay. Compound 4j exhibits excellent antifungal activity (MIC: 0.39 mg mL⁻¹) against C. albicans.

Moreover, compounds 4b, 4k and 4n show similar antifungal activity (MIC: 3.12 mg mL⁻¹) as Fluconazole against C. albicans while others show lower activity. Thus, compound 4j (carrying electron releasing group –OCH₃ on the indole moiety) shows excellent antifungal activity against C. albicans compared to standard drug.

5. Conclusions

In conclusion, an economical, eco-friendly, simple and efficient method was developed for the synthesis of indole–coumarin heterocycle adducts. The products were obtained in high purity with easy work up procedure, short reaction time and excellent yields under solvent free and grinding conditions.

The series of chemosensor 4a–4n displayed a highly selectivity and sensitivity towards F⁻ ion. Significant blue shift in absorption spectra and remarkable fluorescence quenching was observed in the presence of F⁻ ion, while other anions show no significant changes in optical properties. Chemosensor 4a also exhibited very low detection limit of 0.50 μM for F⁻ ions. The experimental results are proved by ¹H-NMR titration and computational DFT and TD-TFT calculations. Naked-eyes detection on the basis of colour change for fluoride ion was realized successfully in aqueous solution.

Furthermore, compounds 4a–4n exhibited good biological activity, particularly compounds 4e, 4f and 4i exposed excellent biological activity against Gram-positive bacteria, while compounds 4j and 4k exhibited highest activity towards Gram-negative bacteria, even as compound 4k exhibited the highest activity against pathogenic fungi C. albicans in the MIC method.

Acknowledgements

We thank Ministry of Human Resource and Development (MHRD), New Delhi, India [grant number MHR01-23-200-428], for financial supports.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24597h

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