Detection of copper(II) and aluminium(III) by a new bis-benzimidazole Schiff base in aqueous media via distinct routes

Abstract

The synthesis is described of a new bis-benzimidazole appended Schiff base ligand [2-(3,5-bis(1H-benz-imidazol-2-yl)-phenyliminomethyl)phenol] (H₃L) followed by its thorough characterization by elemental analyses, spectroscopic studies (FT-IR, ¹H, ¹³C NMR, ESI-MS, UV/vis, fluorescence) and X-ray single crystal analyses. The excellent selectivity of H₃L towards Cu²⁺ and Al³⁺ via distinct responses in mixed aqueous conditions was established by spectroscopic studies. The selective detection of Cu²⁺ due to the creation of complex 1 in which Cu²⁺ occupied only the salen-type N₂O₂ coordination site and the strong binding with Al³⁺ through both benzimidazole and salen moieties to yield complex 2 was confirmed by various studies. The disparate detection of these cations by H₃L was substantiated by comparative studies, performed under analogous conditions, of the precursor compound 3,5-bis(1H-benzimidazol-2-yl)-aniline (BBA). The distinct interaction of H₃L with Cu²⁺ and Al³⁺ and the sensing mechanisms were investigated in detail by spectroscopic studies and were supported by theoretical studies (Density Functional Theory). It has been clearly shown that the use of appropriate substituents may facilitate the design and development of probes suitable for the real time detection of more than one analyte.

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Introduction

The development of efficient receptors for the sensing and recognition of environmentally and biologically important metals has been an active area of contemporary research over the past decade.¹ Aluminium (Al) and copper (Cu) are not highly toxic and are extremely useful to human beings. They are used extensively in many industries and domestically, as well as being important in fundamental biochemical reactions.² But, it has only recently been realized that they impose adverse effects on the environment and on human beings. Aluminium is the most abundant metal on the earth’s crust which, in its soluble form, has a lethal effect on the flora growing in ∼40% of the world’s acidic soil.³ Vital sources of Al exposure to human beings are cooking and storage utensils used for food stuffs and drinks, manufacturing and processing units for cosmetics, medicines, and food.⁴ Although, it is not very toxic and finds wide applications in our daily life, its upsurge in the body may cause grave health issues viz. Parkinson’s disease, neural and memory impairment, osteoporosis, and dialysis encephalopathy.^5–8 Likewise, copper is an essential element (3^rd after Fe and Zn) for the human body and plays a crucial role in various physiological and homeostatic processes.⁹ Despite being less harmful, an excess of copper in the body poses serious health problems.¹⁰ It may cause gastrointestinal problems, is hazardous to the liver and kidneys, and may cause neurodegenerative disorders like Menkes,¹¹ Alzheimer’s,¹² Wilson’s¹³ and Prions-induced diseases.¹⁴ Therefore, considering the incredible significance of these metal ions to human beings there is immense current interest in their detection.

Lately, directional efforts have been made towards the development of molecular sensors based on colorimetric/fluorescence methods¹⁵ employing photoinduced electron transfer (PET),¹⁶ internal charge transfer (ICT),¹⁷ chelation enhanced fluorescence (CHEF)¹⁸ and deprotonation mechanisms (ESIPT)¹⁹ for the detection of Cu²⁺ and Al³⁺. Being non-destructive, rapid, easy to carry out, and highly sensitive, these methods are advantageous over methods like atomic absorption,²⁰ inductively coupled plasma mass and emission spectroscopy,²¹ and voltammetry.^22,23 However, the previously developed sensors, had limitations vis-à-vis solubility, real time efficiency, bulky structures as well as the expensive and complicated synthesis of the probe materials. Therefore, it has been a challenging task to develop a multi-analyte detection technique which follows an easy and straightforward sensing approach whilst eliminating the limitations of previously developed systems. In this direction, a novel Schiff base [2-(3,5-bis(1H-benzimidazole-2-yl)-phenyliminomethyl)phenol, (H₃L)] possessing a regular N,O-binding site on a phenyl ring (salen) and two independent benzimidazole –NH units has been designed and synthesized starting from a newly designed precursor 3,5-bis(1H-benzimidazol-2-yl)aniline (BBA).

The strategic dissimilarity of the functional groups (N,O and –NH) controls the reactivity of the probe (H₃L) to achieve the distinct binding modes for Cu²⁺ and Al³⁺ which are reflected by diverse photophysical changes. The present work deals with the exploration of a multi-analyte detection system by rational detailing of the sensing interactions in the competitive milieu of two dissimilar sites. In this contribution we describe the synthesis and characterization of H₃L and its potential application in the selective detection of Cu²⁺ and Al³⁺ via distinct routes attained through the strategic choice of different functional groups. To our knowledge, this is the first time that this approach has been applied to develop a probe that accomplishes the selective detection of Cu²⁺ and Al³⁺ with admirable cost efficiency.

Experimental section

Solvents and reagents were procured from commercial sources (Sigma Aldrich, Merck and SD Fine Chem. Ltd). Reagent grade chemicals were used throughout and solvents were thoroughly dried and distilled prior to their use following standard literature procedures.²⁴ HPLC-grade solvents were used for spectroscopic studies. General details of the experimental procedures are given in the ESI.†

Synthesis of precursor, 3,5-bis(1H-benzimidazol-2-yl)aniline (BBA)

A round bottom (250 mL) flask containing a mixture of 5-aminoisophthalic acid (1.81 g, 10.0 mmol) and o-phenylenediamine (2.16 g, 20.0 mmol) was charged with ∼25 g of polyphosphoric acid (PPA). The mixture was thoroughly blended to an homogeneous paste using a glass rod and heated with vigorous stirring at ∼220 °C for 24 h in a sand bath. The ensuing reaction mixture was poured into 500 mL of water while moderately hot and neutralized with an excess of saturated aqueous solution of NaHCO₃. The resulting precipitate was filtered, washed several times with water and dried in air. It was re-dissolved in acetone, dried using anhydrous Na₂SO₄ and filtered. The filtrate upon removal of solvent under reduced pressure afforded the desired product as a dark brown solid. Yield: 2.2 g, 67%. Anal. calcd (%) for C₂₀H₁₅N₅: C, 73.91; H, 4.62; N, 21.58; found C, 73.83; H, 4.65; N, 21.52%. IR (KBr pellets, cm⁻¹): 3191 (br, N–H_str, ArNH₂), 1626–1608 (s, ν_str C=C_asymm), 1474, (s, ν_str hetero-ring), 740 (vs, ν_breathing hetero-ring). ¹H NMR (300 MHz, dmso-d₆, δ_H, ppm): δ 12.91 (s, 2H, H8̲a̲ and 8̲b̲), 8.12 (s, 1H, H1̲), 7.63 (2H, H2̲ and H3̲), 7.52–7.47 (broad, 4H, H4̲a̲, 4̲b̲, H7̲a̲ and 7̲b̲), 7.23–7.18 (m, 4H, H5̲a̲, 5̲b̲, H6̲a̲ and 6̲b̲), 5.56 (s, 2H, H9̲). ¹³C NMR (75.45 MHz, dmso-d₆, δ_C, ppm): δ 151.6, 149.4, 135.0, 131.4, 125.2, 122.4, 121.5, 118.6, 113.1, 111.2, 105.6. ESI-MS (relative intensity), calcd, found (m/z): 326.1406, 326.1401 [(M + H)⁺, 45%]. UV/vis [c, 1.0 × 10⁻⁵ M; MeOH/H₂O, 9 : 1, v/v MeOH/H₂O; λ_max, nm (ε, M⁻¹ cm⁻¹)]: 345 (7.75 × 10³), 300 (4.10 × 10⁴).

Synthesis of probe, 2-(3,5-bis(1H-benzimidazol-2-yl)phenyl-imino-methyl)phenol (H₃L)

To a methanolic solution (10 mL) of salicylaldehyde (0.53 mL, 5.0 mmol) a hot solution of BBA (1.62 g, 5.0 mmol) in the same solvent (50 mL) was added and the reaction mixture was heated under reflux overnight. After cooling to room temperature it afforded a brown-red crystalline solid which was separated by filtration, washed with cold methanol and diethyl ether. Crystals suitable for X-ray single crystal analyses were obtained within 2 days from DCM : MeOH (50/50, v/v) solution. Yield: 1.7 g, 79%. Anal. calcd (%) for C₂₇H₁₉N₅O: C, 66.85; H, 5.61; N, 4.10; found: C, 66.79; H, 5.59; N, 4.02%. IR (KBr pellets, cm⁻¹): 1617–1607 (s, ν_str C=C_asymm), 1574 (s, ν_str C=N), 1464 (s, ν_str hetero-ring), 733 (vs, ν_breathing hetero-ring). ¹H NMR (300 MHz, dmso-d₆, δ_H, ppm): δ 13.20 (s, 2H, H8̲a̲ and 8̲b̲), 12.90 (s, 1H, H1̲4̲), 9.22 (s, 1H, H9̲), 8.99 (s, 1H, H1̲), 8.30 (s, 2H, H2̲ and H3̲), 7.80 (d, J = 7.5 Hz, 1H, H10̲), 7.72 (d, J = 7.2 Hz, 2H, H4̲a̲ and 4̲b̲), 7.58 (d, J = 7.2 Hz, 2H, H7̲a̲ and 7̲b̲), 7.47 (t, J = 7.8 Hz, 1H, H1̲2̲), 7.29–7.20 (m, 4H, H5̲a̲, 5̲b̲, H6̲a̲ and 6̲b̲), 7.04 (m, 2H, H1̲1̲ and H1̲3̲). ¹³C NMR (75.45 MHz, dmso-d₆, δ_C, ppm): δ 164.7, 164.6, 160.9, 160.5, 151.8, 150.5, 149.8, 149.7, 136.7, 134.1, 132.8, 132.3, 131.6, 129.8, 123.1, 122.9, 122.4, 120.5, 119.8, 119.7, 119.5, 117.4, 117.0, 113.5, 113.1. ESI-MS (relative intensity), calcd, found (m/z): 430.1668, 430.1666 [(M + H)⁺, 25%]. UV/vis [c, 1.0 × 10⁻⁵ M; 9 : 1 (v/v) MeOH/H₂O; pH ∼ 7.2; λ_max, nm (ε, M⁻¹ cm⁻¹)]: 355 (1.00 × 10⁴), 305 (4.24 × 10⁴).

Results and discussion

Synthesis and characterization

The condensation of 5-aminoisophthalic acid with o-phenylenediamine (1 : 2) in polyphosphoric acid (PPA) at ∼220 °C under vigorous stirring afforded the precursor 3,5-bis(1H-benzimidazol-2-yl)-aniline (BBA) with ∼67% yield (Fig. S1, ESI†). BBA reacted with salicylaldehyde in methanol at a ratio of 1 : 1 under refluxing conditions to afford H₃L with a reasonably good yield (79%) (Fig. S2, ESI†). A simple strategy adopted for the preparation of BBA and H₃L is illustrated below in Scheme 1. The characterization of both the precursor BBA and ligand H₃L was achieved by carrying out elemental analyses and spectroscopic studies (IR, NMR, UV/vis, fluorescence, ESI-MS) (Fig. S3, ESI†), and the structure of H₃L was authenticated by X-ray single crystal analysis.

Scheme 1 Schematic representation for the synthesis of BBA and H₃L.

The ¹H NMR spectrum of BBA was expected to exhibit singlets due to benzimidazole (–NH) and aromatic amine (–NH₂) along with resonances due to aromatic and benzimidazole ring protons. As expected, it displayed singlets at δ 12.91 (2H; H8̲a̲ and 8̲b̲) and 5.56 ppm (2H; H9̲) assignable to –NH and –NH₂ protons. Aromatic protons other than the benzimidazole ring resonated as singlets at δ 8.12 (1H; H1̲) and 7.63 ppm (2H; H2̲ and H3̲) while the benzimidazole ring protons appeared at δ 7.52–7.47 (broad, 4H; H4̲a̲, H4̲b̲, H7̲a̲ and H7̲b̲) and 7.23–7.18 ppm (m, 4H; H5̲a̲, H5̲b̲, H6̲a̲ and H6̲b̲) (Fig. S1a, ESI†). Likewise, the ¹H NMR spectrum of H₃L was expected to show signals due to –NH and –OH protons associated with benzimidazole and salen moieties in the downfield side at a 2 : 1 ratio. Expectedly, the –NH and –OH protons resonated in the downfield side at δ 13.20 (2H, H8̲a̲,̲b̲) and 12.90 ppm (1H, H1̲4̲) at a ratio of ∼2 : 1 [with respect to the signal at δ 9.22 ppm (s, 1H, H9̲) due to aldimine (–CH=N–)]. Assignment of the –NH proton resonance for H₃L in the extreme downfield side was supported by an analogous signal (vide supra) of the precursor BBA. Further, doublets at δ 7.72 and 7.58 ppm may be attributed to benzimidazole protons H4̲a̲, 4̲b̲ (adjacent to –NH–) and H7̲a̲, 7̲b̲ (adjacent to ring =N–) while a broad multiplet at δ 7.294–7.209 ppm, may be attributed to H5̲a̲, 5̲b̲, H6̲a̲ and 6̲b̲ protons (Fig. S2a, inset, ESI†). The phenyl ring protons excluding benzimidazole and salen, resonated at δ 8.99 (1H, H1̲) and 8.30 (2H, H2̲ and H3̲) ppm as distinct singlets. On the other hand, resonances pertaining to salen ring protons appeared at δ 7.80 (d, 1H, H1̲0̲), 7.47 (t, 1H, H1̲2̲) and 7.04 (m, 2H, H1̲1̲ and H1̲3̲) ppm (Fig. S2a, ESI†). Thus, ¹H NMR spectral data affirmed the formation and formulation of BBA and H₃L. ¹³C NMR spectral data further supported their formations (Fig. S1b and S2b, ESI†).

The composition of BBA and H₃L was further substantiated by ESI-mass spectrometric studies. ESI-MS of these compounds exhibited molecular ion peaks at m/z 326.1401 [(M + H)⁺, 45%] and 430.1666 [(M + H)⁺, 25%], respectively which strongly supported their proposed formulations (Fig. S3, ESI†).

Crystal structure of H₃L

The probe H₃L crystallizes in a tetragonal system with an I4₁/acd space group (Fig. S4a, ESI†). Selected crystallographic refinement parameters are summarized in Table 1 and the pertinent view along with the partial atom numbering scheme is depicted in Fig. 1a. The crystal structure of H₃L displays a non-planar molecule wherein two benzimidazole rings lie in different planes and are mutually disposed at 43.57° (Fig. S4b, ESI†). This type of bending may significantly affect the electron transfer phenomena (ESIPT, PET etc.) responsible for changes in the fluorescence properties, particularly in the presence of a conjugated system (salen moiety).²⁵ The structure revealed a crystallographic disorder at the phenolate ring which flipped around the aldimine nitrogen. Therefore, structural optimization was also carried out using density functional theory (DFT) to support these findings (Fig. 1b).

Table 1 Crystal data and structure refinement parameters for H₃L

	H3L
Identification code	SHELXL
Empirical formula	C29H21N5O3
fw	487.508
Temperature (K)	293(2)
Wavelength (Å)	1.54184
Crystal system	Tetragonal
Space group	I41/acd
Unit cell dimensions
a (Å)	28.2864(11)
b (Å)	28.2864(11)
c (Å)	13.3571(7)
α (deg)	90
β (deg)	90
γ (deg)	90
Volume (Å^3)	10 687.3(10)
Z	16
Density (cald) (mg m^−3)	1.212
Absorbance coefficient (mm^−1)	0.658
F(000)	4064.0
Crystal size (mm^3)	0.30 × 0.15 × 0.04
θ max for data collection (deg)	66.270°
Data completeness	99.60%
Absorbance correction	Multi-scan
Refinement method	Full-matrix least-squares on F^2
Data/restraints/parameter	2335/31/185
GOF on F^2	1.044
Final R indices [I > 2σ(I)]	R1 = 0.0789, wR2 = 0.2152
R indices (all data)	R1 = 0.0931, wR2 = 0.2301

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Fig. 1 (a) ORTEP view of H₃L (50% ellipsoid probability and hydrogen atoms omitted). (b) DFT optimized structure of H₃L supporting the crystal structure.

The molecular arrangements of subsequent layers in the crystallographic ‘ab’ plane extended along the ‘c’ axis to define an anti-parallel arrangement (Fig. S5a, ESI†). Various bond distances and angles fall within the acceptable range and are comparable with other closely related systems (Table S1, ESI†).²⁶ Furthermore, the crystal lattice exhibited one methanol as a free solvent which is involved in H-bonding interactions (two) with benzimidazole nitrogen. The respective distances lie within the reported range (∼2.699 and 2.744 Å; Fig. S5b, ESI†).²⁷

UV/vis spectroscopic studies

The electronic absorption spectra of H₃L (c, 1.0 × 10⁻⁵ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2) exhibit a weak and a prominent band at 355 and 305 nm, respectively. The low energy (LE) band at 355 (ε, 1.00 × 10⁴ M⁻¹ cm⁻¹) and the high energy (HE) band at 305 nm (ε, 4.24 × 10⁴ M⁻¹ cm⁻¹) have been tentatively assigned to n–π* and π–π* transitions. To explore its interaction with metal ions, a solution of H₃L was treated with various cations viz., Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, Al³⁺ and Pb²⁺ (nitrate salts; 10.0 equiv.). It has been observed that the spectral features of H₃L exhibited substantial changes only with Cu²⁺ and Al³⁺, while the other tested cations resulted in negligible variation (Fig. 2a). The addition of Cu²⁺ (10.0 equiv.) to a solution of H₃L led to the emergence of a new band at ∼394 nm (ε, 6.9 × 10³ M⁻¹ cm⁻¹) with loss of the band at ∼355 nm. In addition, an insignificant hypso- (Δλ, ∼2 nm) and hyperchromic shift (Δε, 3.10 × 10² M⁻¹ cm⁻¹) was also noted for the HE band. Conversely, the addition of Al³⁺ (10.0 equiv.) to a solution of H₃L resulted in the emergence of a new band at 370 nm (ε, 7.30 × 10³ M⁻¹ cm⁻¹) with a decrease in the optical density for the LE band. The HE band displayed a substantial hypo- and hypsochromic shift (Δλ, 7 nm; Δε, 2.80 × 10³ M⁻¹ cm⁻¹). Also, a small but noticeable hump emerged at ∼276 nm (ε, 2.98 × 10⁴ M⁻¹ cm⁻¹) which was a visible difference from the spectral features of H₃L + Cu²⁺ (Fig. 2b).

Fig. 2 (a) Absorption spectra of H₃L (c, 1.0 × 10⁻⁵ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2) and in the presence of various metal ions (10.0 equiv.). (b) Comparative absorption spectra of H₃L in the presence of Cu²⁺ (10.0 equiv.) and Al³⁺ (10.0 equiv.) in MeOH/H₂O (9 : 1, v/v). Inset shows the color difference.

The observed differences in the spectral features of H₃L upon the addition of Cu²⁺ and Al³⁺ simply demonstrated their significant interaction with the salen and/or benzimidazole moieties of the probe. The considerable change in the LE band (λ, 355 nm) accompanied by the appearance of a new band at 394 nm and the negligible effect on the HE band in the presence of Cu²⁺ most probably occurs due to the formation of complex 1 via azomethine nitrogen and phenolate oxygen in N₂O₂ fashion [Cu^II–salen complex].²⁸ In contrast, the disparity shown by both the LE and HE bands in the presence of Al³⁺ may be attributed to its interaction through both the aldimine and benzimidazole moieties.²⁹ Based on these differences it has been suggested that Cu²⁺ and Al³⁺ may adopt distinct binding modes with H₃L.

Although these suggestions are consistent with the spectroscopic annotations, to confirm this viewpoint precursor BBA (c, 1.0 × 10⁻⁵ M) was treated with Cu²⁺ and Al³⁺ under analogous conditions. Since, BBA is short of an N,O- chelating site and has only two benzimidazole moieties along with a free amine group, a comparative absorption study using BBA as the probe may be quite informative and throw light on the competing interaction of Cu²⁺/Al³⁺ with a benzimidazole moiety. This study demonstrated that there is hardly any change in the features of the absorption spectrum of BBA upon addition of Cu²⁺ (∼20.0 equiv.), whilst there is a considerable change upon the addition of Al³⁺ (∼20.0 equiv.). This suggests that Al³⁺ indeed interacts with the benzimidazole moiety, however, a comparatively large amount of Al³⁺ was required to exhibit the said changes resulting in a lower sensitivity for BBA (Fig. S6, ESI†). From this study it has been substantiated that Cu²⁺ binds only with salen while Al³⁺ binds through both the benzimidazole and salen moieties of H₃L and exhibits a dissimilar interaction mode. Thus, we conclude that the probe has dual selectivity towards Cu²⁺ and Al³⁺ in mixed aqueous media.

To gain a deep insight into the mechanism of interaction, UV/vis titration studies were undertaken. The addition of Cu²⁺ (0.1 equiv., 1.0 × 10⁻³ M; 3.0 μL) to a solution of H₃L (c, 1.0 × 10⁻⁵ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2) displayed hypochromism for the band at 355 nm (ε, 9.3 × 10³ M⁻¹ cm⁻¹) with the emergence of a new band at 394 nm (ε, 2.5 × 10³ M⁻¹ cm⁻¹). Further, saturation was achieved by addition of ∼1.0 equiv. of Cu²⁺, which resulted in a significant decrease in the optical density for the band at 355 nm along with a final increase in the optical density of the new band at 394 nm (ε, 6.9 × 10³ M⁻¹ cm⁻¹). The spectral variations accompanied by the clear isosbestic point (λ, 368 nm) indicated the ratiometric conversion of H₃L to a third species (other than H₃L and Cu²⁺). This species may be Cu^II–salen complex (1) which may be responsible for sensing (Fig. 3a).³⁰

Fig. 3 (a) Absorption titration of H₃L (c, 1.0 × 10⁻⁵ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2) in the presence of Cu²⁺ (∼1.0 equiv.; c, 1 mM) (b) Interfering effect of the tested metal ions (10.0 equiv.) in the presence of H₃L + Cu²⁺ (∼1.0 equiv.) at rt.

Similarly, in the titration involving H₃L with Al³⁺ (c, 1.0 × 10⁻⁵ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2) saturation occurred upon the addition of ∼8.0 equiv. of the metal ion. A prominent band was observed at ∼370 nm which signified a hypso- and hypochromic shift (Δλ, 7 nm; ε, 7.3 × 10³ M⁻¹ cm⁻¹) for the HE band (Fig. 4a). The moderately high amount of Al³⁺ required relative to Cu²⁺ may be due to the larger number of Al³⁺ ions involved in the interaction with H₃L. Here, the final species at saturation (limit of quantification ∼1.0–0.5 to ∼1.0–8.0 equiv. of H₃L/Al³⁺) can be considered to adopt an intricate structure (2) rather than a simple Al^III–salen complex (3). The formation of 3 or any species parallel to it may not be sterically stable due to the bulkiness of the benzimidazole groups. Furthermore, a hump at ∼276 nm exhibited by H₃L + Al³⁺ suggested that Al³⁺ is involved in some other interactions relative to that occurring in H₃L + Cu²⁺. These additional aspects can essentially be related to the interaction between Al³⁺ and the benzimidazole moieties provided that Al³⁺ most likely binds with the benzimidazole groups of different H₃L molecules.

Fig. 4 Absorption titration for H₃L (c, 1.0 × 10⁻⁵ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2) in the presence of Al³⁺ (∼8.0 equiv.; c, 1 mM) at rt. (b) Interfering effect of the tested metal ions (10.0 equiv.) in the presence of H₃L + Al³⁺ (∼8.0 equiv.) at room temperature.

This interpretation of the interactions occuring between H₃L and analytes (Cu²⁺ and Al³⁺) confirmed that the mode of interaction of H₃L with Cu²⁺ and Al³⁺ in aqueous media is discrete due to the presence of two different interaction sites which is a necessary criterion for dual selective detection.

To explore the suitability of H₃L as a selective chemosensor for Cu²⁺ and Al³⁺, solutions containing H₃L + Cu²⁺ (∼1.0 equiv.) and H₃L + Al³⁺ (∼8.0 equiv.) were treated with interfering cations. The changes which occurred were negligible (Fig. 3b and 4b). Moreover, the mutual interference of Al³⁺ and Cu²⁺ was also verified by the insignificant changes (Fig. S7, ESI†) which were observed following the addition of Al³⁺ (∼10.0 equiv.) to H₃L + Cu²⁺ (1.0 equiv.) and Cu²⁺ (∼10.0 equiv.) to a solution of H₃L + Al³⁺ (8.0 equiv.). However, the addition of a large excess of Al³⁺ (+50.0 equiv. approx.) to a solution of H₃L + Cu²⁺ (1.0 equiv.) + Al³⁺ (10.0 equiv.) resulted in some changes near the HE band (λ, ∼300 nm). This may be due to interaction between Al³⁺ and the free benzimidazole moiety which indirectly supports the binding of Cu²⁺ with only the salen core. On the other hand, no change was observed in the spectra of H₃L + Al³⁺ (8.0 equiv.) in the presence of other analytes which confirmed that binding occurs between Al³⁺ and salen as well as the benzimidazole moieties.

Fluorescence spectroscopic studies

The binding behaviour of H₃L and its precursor BBA with Cu²⁺/Al³⁺ was investigated by fluorescence spectral studies. Upon excitation at 300 nm, a weak band (fluorescence-off) at ∼415 nm was observed for H₃L (c, 1.0 × 10⁻⁶ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2), whereas in the case of BBA (c, 1.0 × 10⁻⁶ M; MeOH/H₂O, 9 : 1, v/v) a strong band was observed at the same wavelength (λ_em, ∼415 nm) with a large Stokes shift of ∼115 nm (Fig. S8, ESI†). The salen core probably enhances the process of photoinduced electron transfer (PET) due to increased conjugation which significantly diminishes the fluorescence intensity for H₃L relative to BBA. The effect was also studied of various metal ions (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, Al³⁺ and Pb²⁺; nitrate salts; 10.0 equiv.) on the fluorescence intensity of H₃L. It was observed that only Cu²⁺ and Al³⁺ produce a significant change in the spectra while other metal ions were ineffective (Fig. 5a). The addition of Cu²⁺ (10.0 equiv.) to a solution of H₃L resulted in a decrease in fluorescence intensity for the band at ∼415 nm with the emergence of a new weak band at ∼485 nm. This is quite interesting as Cu²⁺ is known to be a fluorescence quencher which commonly induces a “fluorescence-off” signal.³¹

Fig. 5 (a) Fluorescence spectra of H₃L (c, 1.0 × 10⁻⁶ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2) in the presence of various metal ions (10.0 equiv.). (b) Fluorescence titration of H₃L [c, 1.0 × 10⁻⁵ M; MeOH/H₂O, 9 : 1 v/v] in the presence of Cu²⁺ (∼1.0 equiv.).

To gain a better understanding of the Cu²⁺ induced spectral changes, fluorescence titration studies were performed using 1.0 × 10⁻⁵ M solution of H₃L in the presence of Cu²⁺ (clear changes were not observed with 1.0 × 10⁻⁶ M solution of H₃L). A gradual decrease in the intensity of the band at ∼415 nm upon the addition of Cu²⁺ with the emergence of a new band at ∼485 nm was demonstrated. Saturation was attained by addition of up to ∼1.0 equiv. of Cu²⁺ (Fig. 5b). The observed changes can be ascribed to the continuous consumption of H₃L leading to the creation of complex 1 which is detectable to the naked eye under UV radiation (λ, 365 nm) (Fig. 5b, inset). The formation of 1 was also confirmed by ESI-MS of the isolated species (H₃L + Cu²⁺, ∼1.0 equiv.). A molecular ion peak for 1 was observed at m/z 920.2478 [calcd 920.2397, (M + H)⁺] (Fig. S15a, ESI†).

From the fluorescence studies that were carried out on H₃L (c, 1.0 × 10⁻⁶ M; MeOH/H₂O, 9 : 1, v/v; pH ∼ 7.2) it was shown that H₃L displays an excellent selectivity for Al³⁺ (10.0 equiv.) relative to the tested metal ions (10.0 equiv.) including Cu²⁺ and a new strong band (λ_em, 466 nm; Fig. 5a) was observed. Here, distinct fluorescence signals due to the two types of interactions between H₃L and Al³⁺ were not observed. Thus, the pertinent role of benzimidazole and aldimine moieties towards Al³⁺ was investigated by titration studies. In the titration experiments carried out using H₃L (c, 1.0 × 10⁻⁶ M) a small but gradual decrease in the fluorescence intensity for the band at 415 nm was observed with the emergence of a new band and isosemissive point at 466 and ∼425 nm, respectively. The intensity of the new band was greatly enhanced (∼20 folds) and attained saturation upon the addition of ∼8.0 equiv. of Al³⁺ (Fig. 6a). Noticeably, the enhancement in intensity for the band at 466 nm was considerably high relative to the decrease in the intensity of the band at 415 nm. Rather, it appeared that the band at 415 nm attained a steady intensity after a significant decrease until the complementary band (466 nm) acquired saturation. This remarkable feature was ascribed to binary spectral behaviour (ratiometric change followed by a segment of constant intensity). This was a strong indication that interaction had occurred between both benzimidazole and salen moieties with Al³⁺ leading to an enormous increase in the fluorescence intensity. The binary spectral behaviour was further confirmed by titration studies at a slightly higher concentration (c, 1.0 × 10⁻⁵ M) as spectral changes were clearly visible at this concentration particularly in the isosemissive region and corroborated well with the unusual spectral behaviour (Fig. S9, ESI†). This type of spectral conduct undoubtedly advocated the occurrence of two types of interaction between H₃L and Al³⁺, involving salen (N,O) and benzimidazole (–NH) moieties.

Fig. 6 (a) Fluorescence titration of H₃L [c, 1.0 × 10⁻⁶ M; MeOH/H₂O, 9 : 1, v/v] vs. Al³⁺ (∼8.0 equiv.). (b) The effect of the interference of the tested metal ions on the saturated fluorescence spectrum of H₃L + Al³⁺.

To validate the above conclusions, the precursor BBA (c, 1.0 × 10⁻⁶ M) was treated with Al³⁺ as it was expected to show changes exclusively due to the benzimidazole moieties. A new band at ∼465 nm was present in the fluorescence spectrum of BBA (λ_ex, 300; λ_em, 415 nm) in the presence of a large excess of Al³⁺ (∼50 equiv.). This band was similar to the one obtained for H₃L + Al³⁺ (λ_em, 466 nm), but with a negligible increase in the fluorescence intensity (Fig. S10, ESI†). This not only signifies the importance of the aldimine moiety in the enhancement of fluorescence by H₃L in the presence of Al³⁺ (fluorescence-on response) but also, confirmed the involvement of the benzimidazole moiety in the interaction with the metal centre. Further, the ultimate spectral features of BBA + Al³⁺ (∼50 equiv.) resembled those of H₃L + Al³⁺ (∼8.0 equiv.) to some extent. Thus at this instance, one can imagine that H₃L may undergo hydrolytic cleavage in the presence of Al³⁺ to generate BBA (amine counterpart of H₃L) in the solution itself. But, this presumption was discarded on the basis of ¹H NMR titration and ESI-MS studies (vide infra) as these provided apt evidence regarding the existence of the aldimine linkage after the partial/complete addition of Al³⁺. Additionally, since a large excess of Al³⁺ (∼50 equiv.) is required to bring about the spectral changes of BBA, it can be specified that BBA + Al³⁺ (∼50 equiv.) results in a different species (relative to H₃L + Al³⁺, 8.0 equiv.; 2). In conclusion, the comparative fluorescence studies on H₃L and BBA proved that there are distinct binding modes for H₃L with Cu²⁺ and Al³⁺. The studies also offered a reasonable explanation regarding the role of ‘binary fluorescence spectral behaviour’ in the fluorescence ‘turn-on’ detection by H₃L particularly for Al³⁺.

Further, to explore the applicability of the probe as a selective chemosensor, an interference study was performed (H₃L; c, 1.0 × 10⁻⁶ M). Successive addition of the interfering cations (10.0 equiv.) to a solution of H₃L + Cu²⁺ (∼1.0 equiv.) and H₃L + Al³⁺ (∼8.0 equiv.) exhibited insignificant changes (Fig. 6b). Interference was examined more precisely by measuring the background effect of the individual cations. The fluorescence intensity of H₃L (λ_ex, 300 nm; λ_em, 466 nm) was measured by addition of individual cations (10.0 equiv.) followed by the addition of Al³⁺ (10.0 equiv.; bar diagram, Fig. 7). These results not only supported the excellent selectivity of H₃L towards Al³⁺ over the tested metal ions/Cu²⁺, but also, demonstrated the insignificant background effect of individual cations on H₃L.

Fig. 7 Fluorescence response of H₃L (c, 1.0 × 10⁻⁶ M; MeOH/H₂O, 9 : 1, v/v) showing the background effects of various metal ions. The blue blocks represent the fluorescence intensity of free H₃L, the brown cylindrical blocks represent the intensity in the presence of the tested metal ions (10.0 equiv.), and the green cylindrical bars represent the intensity after the addition of Al³⁺ (10.0 equiv.) to the same solution at room temperature (λ_em = 465 nm).

Job’s plot analyses revealed a 2 : 1 stoichiometry for H₃L/Cu²⁺ and a 1 : 1 stoichiometry for H₃L/Al³⁺ (Fig. S11, ESI†). The former one is consistent with the above discussions however, the latter (1 : 1 stoichiometric ratio) may be in contradiction with the formation of a distinct monomeric species from H₃L–Al³⁺ interactions. Instead, the 1 : 1 ratio indicates that in complex (2) Al³⁺ binds with the salen core and only one benzimidazole moiety from H₃L. Further, if the coordinated Al³⁺ adopts an octahedral geometry, the remaining three coordination sites are most probably occupied by NO₃⁻ and H₂O molecules. Consequently, a polymeric species is likely formed which may involve Al³⁺ and/or some uniform distinct species in equilibrium with the polymeric species. The 1 : 1 stoichiometry may also be supported by the ¹H NMR titration between H₃L and Al³⁺ wherein only ∼1.0 equiv. of Al³⁺ was sufficient to complete the loss of the –NH and –OH protons (vide infra). The association constant for H₃L in the presence of Al³⁺ was estimated following the Benesi–Hildebrand method (K_Al = 1.8 × 10⁵ mol⁻¹: Fig. S12, ESI†).³² It indicated a pretty good sensitivity of H₃L for Al³⁺ and is in agreement with earlier reports.³³ Furthermore, the limits of detection (LOD) of Cu²⁺ and Al³⁺ using H₃L were also estimated and were found to be 4.2 × 10⁻⁸ M and 8.1 × 10⁻⁷ M, respectively. These merely defined the lowest amount of Cu²⁺ and Al³⁺ that can be precisely measured using H₃L (c, 1.0 × 10⁻⁵ M), and are in the order of 10⁻⁷ M and 10⁻⁶ M, respectively which is good enough for practical applications.

In the light of the above discussions we conclude that H₃L exhibits dissimilar fluorescence changes in the presence of Cu²⁺ and Al³⁺ due to the formation of 1 and 2, respectively through discrete interactions involving the benzimidazole and salen moieties. These interactions significantly alter the electron transfer phenomenon in the system and induce a substantial change in the fluorescence intensity, especially upon the addition of Al³⁺. The annotations from the fluorescence spectral studies are consistent with the results obtained from UV/vis studies and confirm the usability of H₃L as a dual selective chemosensor for Cu²⁺ and Al³⁺ (Fig. 8).

Fig. 8 The changes observed in (a) color and (b) fluorescence of c, 1.0 × 10⁻⁵ M [MeOH/H₂O, 9 : 1, v/v] solutions of H₃L with 10.0 equiv. of the tested cations.

Justification for pH sensitivity

The probe H₃L was expected to show pH dependent photophysical behaviour due to the presence of labile protons on the benzimidazole units. The nitrate salts of Cu²⁺ and Al³⁺ are acidic in nature; therefore to establish the potential use of H₃L as an efficient chemosensor it was necessary to examine its photophysical response solely against pH variation. Also, the aldimine bond of H₃L should be stable within the pH changes during experiments. In the photophysical studies of H₃L vs. Cu²⁺, the solution exhibited a change in pH from ∼7.2 to 6.2 whereas for H₃L vs. Al³⁺, the pH changed from ∼7.2 to 6.5. To gain a deep insight into the photophysical response of H₃L with varying pH in the said range, titration studies were undertaken using 0.1 M HCl. The aliquot addition of HCl in the absorption titration experiment resulted in a gradual decrease in pH from ∼7.2 to 2.8 without a significant change in optical density and the position of the absorption bands for H₃L up to pH ∼4.0. However, a decrease in the optical density of the HE band was observed below pH ∼ 4.0 which may be attributed to protonation of the benzimidazole moiety in H₃L (Fig. S13a, ESI†). On the other hand, fluorescence titration using acid under analogous conditions revealed an increase in the intensity of the characteristic band at 415 nm without the emergence of any new band as observed in the case of H₃L + Al³⁺ (λ, 466 nm) or H₃L + Cu²⁺ (λ, 485 nm) (Fig. S13b, ESI†). An increase in intensity was tentatively assigned to a decrease in the excited state intra-ligand proton transfer (ESIPT), which most likely occurred due to protonation of the benzimidazole moiety of H₃L. Thus, some photophysical changes of H₃L in the presence of HCl cannot be ruled out, however these are undoubtedly irrelevant and disparate from those observed for H₃L + Cu²⁺/Al³⁺.³⁴ It accounts for the solitary effect of acidic pH on H₃L and reinforced its applicability as an efficient chemosensor in the pH range ∼7.2–4.0. In addition, the stability of the probe (H₃L) was supported by ¹H NMR and ESI-MS studies which strongly suggested that the aldimine moiety remains intact both in 1 and 2.

¹H NMR spectroscopic study

To follow the interactions between H₃L and Al³⁺, ¹H NMR titrations were undertaken. In a typical experiment, deuterated (D₂O) Al³⁺ (nitrate salt) was added to a solution of H₃L (4.29 mg, 0.01 mmol) in dmso-d₆. The ¹H NMR spectrum of H₃L in dmso-d₆ shows sharp signals due to –NH (2H; H8̲a̲ and H8̲b̲, δ 13.20 ppm) and –OH (1H; H1̲4̲, δ 12.90 ppm) protons in 2 : 1 ratio, thus any interaction involving these groups can be easily followed. The addition of Al³⁺ (0.25 equiv.) to a solution of H₃L resulted in a radical loss of signal associated with –NH and a small decrease in the intensity for the –OH proton (Fig. 9). Evidently, the disappearance of the –NH proton before –OH represents an unusual behaviour as phenolate is expected to be highly reactive toward M⁺. To highlight the direct involvement of benzimidazole an expanded view between δ 7.00–8.00 ppm is depicted in Fig. S14, ESI.† It shows that doublets assigned to H4̲a̲, 4̲b and H7̲a̲, 7̲b̲ appearing at δ 7.72 and 7.58 ppm undergo significant up- and downfield shifts of about Δδ ∼0.07 ppm, in the presence of Al³⁺ (0.25 equiv.) which merged into one signal. Besides, a very small but significant downfield shift (Δδ ∼ 0.010 ppm) has been noted for the multiplet at δ 7.294–7.209 ppm (H5̲a̲, 5̲b̲, H6̲a̲ and 6̲b̲). Significant shifts in the resonances due to benzimidazole ring protons clearly demonstrates the instant interaction of Al³⁺ with the –NH moieties. Further additions of Al³⁺ (0.5–1.0 equiv.) apparently demonstrated obvious chemical shifts and signal broadening in the aromatic region (Fig. S14a, ESI†). On the other hand, the pertinent involvement of the salen core was simultaneously signified by a gradual decrease in the intensity of the –OH signal with an upfield shift (Δδ = 0.13 ppm) for the aldimine signal (1H; H9̲, δ 9.22 ppm). The inferior reactivity of the phenolate to benzimidazole may also be attributed to intramolecular H-bonding which most likely prevents the salen unit from interacting with the analyte (Al³⁺) prior to the benzimidazole moiety. The titration study thus significantly indicated that the –OH proton persists for a longer time in solution than the –NH proton which, in turn, supports the immediate binding of Al³⁺ to benzimidazole over the salen unit.

Fig. 9 Overlay of the ¹H NMR titration experiment of H₃L with Al³⁺ showing the prior loss of the –NH signal to the –OH signal and significant shifts of the benzimidazole ring signals verifying Al³⁺–NH interactions.

The contribution of –NH was substantiated by the ¹H NMR spectra for BBA in the presence of Al³⁺ (∼10 equiv.), which displayed a significant downfield shift for benzimidazole aromatic signals with the complete loss of the –NH signal (Fig. S14b, ESI†). Moreover, significant support of the benzimidazole–Al³⁺ interaction was provided by monitoring the hydrogen–deuterium exchange in H₃L (dmso-d₆) in the presence of blank D₂O (trace amount) which displayed a much smaller loss of the signal for –NH than for –OH (Fig. S15, ESI†). Under biased deuterium exchange conditions it significantly describes the requisite interaction of Al³⁺ with –NH of benzimidazole because in the H₃L vs. Al³⁺ titration, the signal for –NH disappeared prior to that for –OH which should not occur if deuterium exchange dominates.

Thus, the ¹H NMR titration study of H₃L/Al³⁺ enabled us to elucidate which particular binding site is involved prior to the other site in a competitive environment. Besides this, although transformations were achieved using different analyte ratios, the study provided irrefutable evidence regarding the interaction of both benzimidazole and salen moieties with Al³⁺ and demonstrated that the former moiety is favoured over the latter. In addition, the study clearly demonstrates the different efficiency of the two distinct groups on the same probe towards the analyte which may be useful in the development of efficient multi-analyte detection tools in the future. Conversely, a well resolved ¹H NMR spectrum could not be obtained for Cu²⁺ titration studies due to the paramagnetic Cu²⁺(d⁹) system.³⁵

Mass spectral studies

The presence and position of various peaks in the ESI-MS of BBA and H₃L authenticated their proposed formulations. In their spectra molecular ion peaks appeared at m/z 326.1401 [(BBA + H)⁺, 45%; calcd m/z 326.1406] and 430.1666 [(H₃L + H)⁺, 25%; calcd m/z 430.1668], respectively (Fig. S3, ESI†). The composition of the species isolated from the reactions between Cu²⁺/Al³⁺ and H₃L were also verified by ESI-MS studies. The ESI-MS of H₃L + Cu²⁺ displayed a peak at m/z 920.2478 [{2(H₂L) + Cu + H}⁺; calcd m/z 920.2397] in ∼20% relative abundance with respect to H₃L which conspicuously signified the molecular ion peak for complex 1. On the other hand, ESI-MS for H₃L + Al³⁺ exhibited a peak at m/z 977.4097 [(2L + NO₃ + 2H₂O + Al)⁺] in ∼15% abundance relative to BBA [m/z 651.2749; {2(BBA) + H}⁺] which clearly suggested the formation of a species parallel to 2. Notably, their isotopic patterns matched well with the simulated one (Fig. S16, ESI†). From the available mass spectral data including other investigations, it is clear that H₃L appreciably interacted with Cu²⁺ only through salen unit to form complex 1. On the other hand, with Al³⁺ it created complex 2 having an intricate structure where both benzimidazole and salen moieties are coordinated with the metal centre along with one NO₃⁻ and two H₂O molecules indicating most likely a hexa-coordinated geometry. Thus, we conclude that H₃L interacts with Cu²⁺ and Al³⁺ in a dissimilar manner and can be utilized as a dual selective chemosensor for these ions in mixed aqueous media.

Density functional theory optimizations

The structures of the end products isolated from the reactions between H₃L and Cu²⁺/Al³⁺ (1 and 2, respectively) were verified by DFT. The optimized structure of 1 displayed a tetra coordinated Cu^II consisting of two units of the deprotonated probe bonded through their salen units leading to a tetrahedral geometry about the metal centre. On the other hand, it was difficult to design a model structure for 2 due to its intricate polymeric architecture. The model for 2 could not disobey the spectral observations and had to also meet the theoretical stipulations. Therefore, a configuration with hexa-coordinated Al^III containing two H₃L, one NO₃⁻ and two H₂O molecules (along with ESI-MS and Job’s analysis of H₃L + Al³⁺) was chosen for structural optimizations. The resultant optimized structure revealed that the bonded salen core and benzimidazole units belong to different H₃L and assume three coordination sites about the metal centre in order to achieve a 1 : 1 binding mode (which complies with Job’s analysis and ¹H NMR studies). This adopted an octahedral geometry in the presence of appropriately coordinated NO₃⁻ and H₂O molecules and supported the above mentioned spectral observations without any significant infringement (Fig. S17, ESI†). The occurrence of the coordinated NO₃⁻ was also verified by the FT-IR spectrum of complex 2 which displayed bands at ∼1440 and ∼1313 cm⁻¹ due to asymmetric vibrations of the nitrate ion in the mono coordinated form.³⁶ The efficacy of the structural optimizations for 1 and 2 demonstrated that these are the most likely formed species in the presence of Cu²⁺ and Al³⁺, respectively. Also, attempts to optimize model structures other than 2 (viz. complex 3) failed. Thus, the end species formed after sensing interactions had been substantiated by DFT studies, in turn, confirmed the viability of H₃L as an efficient chemosensor for Cu²⁺ and Al³⁺.

Tentative mechanistic routes for the detection of Cu²⁺ and Al³⁺ were worked out on the basis of the results from the above spectral and theoretical studies. An aldimine chelated complex 1 having pendent benzimidazole arms displaying significant photophysical changes was created in the presence of Cu²⁺. However, with Al³⁺, both benzimidazole and salen moieties of H₃L are involved in interactions leading to the formation of complex 2. Spectral (photophysical and ¹H NMR) studies including binary fluorescence spectral behaviour and comparative studies on the precursor (BBA) clearly confirmed the involvement of both the functionalities (benzimidazole and salen) with Al³⁺. The creation of the end products 1 and 2 was substantiated by ESI-MS and their structures were verified by DFT. Furthermore, the formation of complexes 1 and 2 was also verified by obtaining PXRD patterns for H₃L, 1 and 2 in the range 2Ө, 5–60°. Fairly different diffraction patterns were observed for 1 and 2 than for H₃L which clearly indicate their formation (Fig. S18, ESI†). Therefore, the involvement of both functionalities has been reasonably attested and it has been established that H₃L can be used as an efficient probe for the detection of Cu²⁺ and Al³⁺ in mixed aqueous media at room temperature (Fig. 10).

Fig. 10 A proposed theme showing the synthesis of the probe (H₃L) which accomplishes the detection of Cu²⁺ and Al³⁺ by the formation of products 1 and 2, respectively, through distinct mechanistic pathways.

Conclusion

In summary, in this work a new bis-benzimidazole appended Schiff base ligand (H₃L) has been synthesized and thoroughly characterized by various spectral techniques. H₃L exhibits selectivity for Cu²⁺ and Al³⁺ in mixed aqueous media owing to the presence of two different binding sites. Thorough spectral investigations of H₃L in the presence of Cu²⁺/Al³⁺ have categorically shown that Cu²⁺ forms a complex 1 via salen units in a 1 : 2 ratio (verified by Job’s plot analysis), while benzimidazole remains uncoordinated. Conversely, Al³⁺ interacts with both benzimidazole and salen moieties in a 1 : 1 ratio. The addition of Al³⁺ to a solution of H₃L resulted in an enormous increase in the fluorescence intensity (20 fold) and the formation of complex 2, which was substantiated by spectral studies and supported by DFT. H₃L has been found to be very selective and sensitive towards Cu²⁺ and Al³⁺ and showed insignificant effects due to other cations. This type of dual selective behavior accomplishing the detection of two metal ions of extreme biological interest has been accredited to the strategic design of a probe material with more than one interaction site of unequal potential. The consequence of this is to permit the exploration of new avenues in the field of multi-analyte detection via the design of molecules with more than one substituent.

Acknowledgements

We thank the Department of Science and Technology, New Delhi, India, for financial assistance through the Scheme SR/S1/IC-25/2011 (DSP). A. K. thanks the University Grants Commission, New Delhi, India, for a Senior Research Fellowship [R/Dev./Sch.-(UGC-JRF-SRF)/S-01:29.2.29]. We are also thankful to the Head, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi (U.P.), India, for extending the laboratory facilities.

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Footnote

† Electronic supplementary information (ESI) available: ¹H, ¹³C NMR spectra, ESI-MS, UV/vis and fluorescence spectra, crystal data file in CIF format and log files of DFT optimized structures. CCDC 1039505. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra18566a

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