Synthesis, structure and sensing behavior of hydrazone based chromogenic chemosensors for Cu2+ in aqueous environment

Inorganica Chimica Acta 450 (2016) 216–224

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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Research paper

Synthesis, structure and sensing behavior of hydrazone based chromogenic chemosensors for Cu2+ in aqueous environment Soma Mukherjee a,⇑, Shrabani Talukder a, Santanu Chowdhury a, Palash Mal a, Helen Stoeckli-Evans b a b

Department of Environmental Science, University of Kalyani, Kalyani, Nadia, 741235 West Bengal, India Institute of Physics, University of Neuchâtel, rue Emile-Argand 11, CH-2000 Neuchâtel, Switzerland

a r t i c l e

i n f o

Article history: Received 2 February 2016 Received in revised form 26 April 2016 Accepted 30 May 2016 Available online 2 June 2016 Keywords: Hydrazone Chromogenic Chemosensor Cu2+ Reversible Logic gate

a b s t r a c t Two new hydrazone based receptors, quinoline-2-carboxaldehyde-2-hydrazino-2-imidazoline hydrobromide, L1 and pyrrole-2-carboxaldehyde-2-hydrazino-2-imidazoline hydrobromide, L2 have been investigated as chromogenic and ratiometric chemosensors for rapid and selective detection of Cu2+ in aqueous medium. They interacts selectively with Cu2+ by the formation of a new absorbance peak and thereby showing distinct color change which can be discriminated directly through ‘‘naked eye”. The binding pattern and thermodynamic parameters of the receptors with Cu2+ were examined by UV–Vis studies. In presence of Cu2+, both the receptors exhibit reversible absorption change with EDTA and thus offers an interesting property of molecular ‘INHIBIT’ logic gate with Cu2+ and EDTA as chemical inputs. The detection limits (0.494 lM for L1 and 0.488 lM for L2) of both the receptors for Cu2+ is far lower than the WHO limit (31.5 lM) for drinking water. Therefore, the potential utilities of the receptors were checked for the detection of Cu2+ in real water samples. Moreover, the molecular structures of the receptors have been authenticated by single crystal X-ray diffraction study. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Hydrazones are well known in the field of coordination chemistry, supramolecular chemistry and medicinal chemistry. Extensive study reveals that heterocyclic hydrazones (˜C@NAN—) have persistently played a crucial role for creation of novel organic receptors that find application in chemical, environmental and biological sciences [1–4]. Recently, the utilization of hydrazones as molecular switches in the context of supramolecular chemistry has created immense interest. They can also undergo selective chromogenic reactions with several heavy and transition metal (HTM) ions affording complexes with special electronic, magnetic, redox, ion exchange and cytotoxic activities [5]. Some earlier reported hydrazones also provide an interesting tool for fluorometric detection of different species including Cu2+, Al3+, H+, F
etc [6–9]. The preference and versatility of hydrazone functional group can be attributed to its ease of synthesis, stability towards hydrolysis and most importantly the flexible nature of AHNAN@CHA bonds through tautomerism, which enable its integration in different applications [5,10]. The structural and

⇑ Corresponding author. E-mail addresses: (S. Mukherjee).

[email protected],

http://dx.doi.org/10.1016/j.ica.2016.05.049 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.

[email protected]

functional diversity of hydrazone thus played a pivotal role in determining the range of applications it can be involved in. On the other hand, copper being the third most abundant essential trace element in the human body, performs important roles in many fundamental physiological processes in organisms [11,12]. However, excessive copper ion concentration in human body can cause extremely negative health effects such as gastrointestinal disturbance and liver or kidney damage, renal problems and Alzheimer’s or Parkinson’s diseases [13–15]. The maximum acceptable concentration of Cu2+ ion in drinking water as recommended by the World Health Organization (WHO) is 2.0 ppm (31.5 lM) [16] and the maximum allowed level of Cu2+ ion in drinking water as set by the U.S. Environmental Protection Agency (EPA) is 20 lM [17]. Therefore, the detection of trace amounts of Cu2+ ion is not only essential but critical. Among various reported methods for the detection of Cu2+, spectrophotometric methods involving chromogenic changes are especially promising because of simplicity and high sensitivity, less laborious and simple nakedeye applications [18–22]. In continuation to our previous work [23], we report herein the synthesis and characterization of two new hydrazones based receptors consisting of imidazoline and quinoline/ pyrrole moieties. Both the receptors are capable of detecting Cu2+ ions in aqueous medium by visible colorimetric and ratiometric response via formation of in-situ prepared copper(II) chelates. The X-ray

S. Mukherjee et al. / Inorganica Chimica Acta 450 (2016) 216–224

Scheme 1. Synthetic route of the receptors.

structures of the receptors were determined by single crystal X-ray diffraction study. The receptors also exhibit molecular logic gate behavior in the absorbance mode with Cu2+ and EDTA as chemical inputs. Moreover, the receptors may be used to detect and quantify Cu2+ in water samples with low LOD (limit of detection).

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(s, 1H), 8.981 (broad, 2H), 8.501 (m, 1H), 8.367 (m, 2H), 8.068 (m, 2H), 7.822 (m, 1H), 7.675 (m, 1H), 3.766 (m, 4H). 13C NMR (400 MHz, DMSO-d6) d (ppm): 158.21, 153.24, 148.65, 147.67, 137.20, 130.68, 129.40, 128.49, 128.11, 123.34, 118.38, 43.29, 31.16. FTIR (KBr pellets, cm
1): 3408, 3238, 2898, 2152, 1976, 1832, 1666, 1596, 1506, 1430, 1367, 1325, 1290, 1205, 1144, 1123, 1073, 1001, 939, 904, 842, 785, 760, 746, 596. UV–Vis in aqueous solution (methanol 4% v/v; pH 7.0), kmax (nm) (e, M
1 cm
1): 358(33,600). L2: Yield: 92%. Mp: 85–90 °C. Elemental analysis (%) for L2 (crystals); C8H16BrN5O2 (294.17) calcd: C, 32.63; H, 5.44; N, 23.80; found: C, 32.60; H, 5.46; N, 23.44. 1H NMR (400 MHz, DMSO-d6) d (ppm): 12.016 (s, 1H), 11.234 (s, 1H), 8.492 (broad, 2H), 7.982 (s, 1H), 7.072 (s, 1H), 6.535 (s, 1H), 6.158 (s, 1H), 3.742 (m, 4H). 13 C NMR (400 MHz, DMSO-d6) d (ppm): 158.33, 139.80, 127.24, 122.92, 113.58, 109.90, 43.12, 31.16. FTIR (KBr pellets, cm
1): 3249, 2977, 1668, 1607, 1451, 1419, 1370, 1291, 1237, 1203, 1123, 1089, 1071, 1029, 931, 882, 825, 770, 742, 633, 598. UV–Vis in aqueous solution (methanol 4% v/v; pH 7.0), kmax (nm) (e, M
1 cm
1): 314 (29 200) (See Scheme 1). 2.3. X-ray crystallography

2. Experimental 2.1. Reagents Quinoline-2-carboxaldehyde, pyrrole-2-carboxaldehyde and 2-hydrazino-2-imidazoline hydrobromide were obtained commercially from Sigma Aldrich. Metal salts were all chlorides (except for FeSO47H2O) and obtained commercially from Sigma Aldrich, Merck, SRL chemical companies. Solvents (Methanol and DMSO-d6) were obtained from Sigma Aldrich. All reagents were used without further purification, unless otherwise stated. In the case of spectroscopic measurements HPLC grade solvents (Methanol and DMSO-d6) were used. 2.2. Synthesis of L1 The receptors were prepared by the same general method [23]. Details are given here for a representative case (L1). L1: The receptor was synthesized by drop wise addition of a methanolic solution (5 ml) of quinoline-2-carboxaldehyde (0.031 g, 0.2 mmol) to a methanolic solution (5 ml) of 2-hydrazino-2-imidazoline hydrobromide (0.036 g, 0.2 mmol) with stirring for 2 h at room temperature. The straw yellow colored solution was filtered and the solvent was evaporated by rotary evaporator and recrystallized from methanol. Yield: 94%. Mp >200 °C. Elemental analysis (%) for L1 (crystals); C13H18BrN5O2 (356.23) Calc.: C, 43.79; H, 5.05; N, 19.65; found: C, 43.84; H, 5.08; N, 19.68. 1H NMR (400 MHz, DMSO-d6) d (ppm): 12.665

Suitable crystals of L1 and L2 were obtained by slow evaporation from methanol. The intensity data were collected at 203 K (
70 °C) for L1 on a Stoe Mark II-Image Plate Diffraction System [24] equipped with a two-circle goniometer, and at 173 K (
100 °C) for L2 on a Stoe Mark I-Image Plate Diffraction System [24] equipped with a one-circle goniometer, using Mo Ka graphite monochromated radiation (k = 0.71073 Å). The structures were solved by direct methods using the program SHELXS [25]. The refinement and all further calculations were carried out with SHELXL-2014 [26] for L1 and SHELXL-97 for L2 [25]. The NH H atoms were located in a difference Fourier map and freely refined. The C-bound H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The H atoms of the water molecules were also located in a difference Fourier map and refined with distance restraints: OAH = 0.82(2) Å for L1 and OAH = 0.84(4) Å for L2. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. A semi-empirical absorption correction was applied using the MULABS routine in PLATON [27]. The figures (Figs. 1 and 2) were drawn using programs MERCURY [28] and PLATON [27]. Further crystallographic data and details of the refinement are given in Table 1. 2.4. Physical measurements A Perkin Elmer 2400 C Elemental Analyzer was used to collect microanalytical data (C, H, N). 1H NMR and 13C NMR spectra were recorded on Bruker 400 MHz NMR spectrometer using TMS as an

Fig. 1. A view of the molecular structure of (a) L1 and (b) L2, with atom labeling. The displacement ellipsoids are drawn at the 50% probability level.

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Fig. 2. The crystal packing of (a) L1 and (b) L2 both viewed along a axis. The hydrogen bonds are shown as dashed lines, and H atoms not involved in hydrogen bonding have been omitted for clarity.

Table 1 Crystallographic data and structure refinement for L1 and L2. Compound

L1

L2

Empirical formula Formula mass T (K) k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z q (g cm
3) k (Å) l (mm
1) F(0 0 0) Crystal size (mm) h range (°) Index ranges

C13H18BrN5O2 356.23 203 0.71073 triclinic
(No. 2) P1

C8H16BrN5O2 294.17 173 0.71073 triclinic
(No. 2) P1

6.9962(6) 9.1873(8) 12.8747(11) 92.075(7) 104.073(7) 101.850(7) 782.35(12) 2 1.512 0.71073 2.639 364 0.50  0.25  0.19 1.64–26.22
8 < h < 8,
11 < k < 11,
15 < l < 15 11 502 2843 3148/0.034 full-matrix least-squares on F2 3148/4/219 1.039 0.0216, 0.0549 0.0261, 0.0561 0.27,
0.37

6.9986(11) 9.3166(15) 10.3557(16) 105.518(18) 104.396(18) 90.548(19) 628.08(17) 2 1.555 0.71073 3.269 300 0.30  0.23  0.12 2.11–26.17
8 < h < 8,
11 < k < 11,
11 < l < 12 4998 1978 2325/0.029 full-matrix least-squares on F2 2325/4/161 1.015 0.0305, 0.0767 0.0382, 0.0801 0.52,
0.93

Reflections collected/unique Reflections collected/observed [I > 2r(I)] Reflections independent (Rint) Refinement method Data/restraints/parameters GOF on F2 R1,a wR2b [I > 2r(I)] R1,a wR2b [all data] Largest difference peak and hole (e Å
3) a b

R1 = [R||Fo|
|Fc||]/R|Fo| (based on F2). wR2 = [[Rw(|F2o
F2c |)2]/[Rw(F2o)2]]1/2 (based on F2).

internal standard. FTIR data were collected with the help of a Shimadzu FTIR 8400 spectrophotometer. The absorption spectra were measured by Shimadzu UV-1700 spectrophotometer and corrected for background due to solvent absorption. The pH values of all the solutions were measured by an Orion 4 star pH.ISE Benchtop.

UV–Vis spectrophotometer by the addition of selected cations. Each and every titration was repeated at least thrice until consistent values were obtained. Job’s continuous variation method was used for determining the binding stoichiometry. The association constants (K) were calculated by the linear Benesi–Hildebrand equation from UV–Vis (Eq. (1)) studies [8,9],

2.5. Spectroscopic studies

1=ðA
A0 Þ ¼ 1=ðA1
A0 Þ þ 1=ðA1
A0 ÞK½Mnþ  UV–Vis experiments were performed by using stock solutions of L1 and L2 (1.0  10
3 M) and different salts of cations (1.0  10
3 M) in aqueous solution (methanol 4% v/v; pH 7.0). The effect of cations on the receptors was observed through

ð1Þ

where, A0, A and A1 are the absorption intensities respectively, in the absence of, at intermediate and infinite concentration of the metal ion (Mn+).

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S. Mukherjee et al. / Inorganica Chimica Acta 450 (2016) 216–224 Table 2 Selected bond lengths (Å) and bond angles (°) for receptors L1 and L2. L1 N1AC1 N1AC8 N2AC10 N2AN3 N3AC11 N4AC11 N4AC12 N5AC11 N5AC13 C1AC10 N1AC1AC2 N1AC8AC9 N4AC11AN3 N5AC13AC12 C11AN3AN2 C2AC1AC10 C5AC4AC9 C6AC7AC8 C3AC9AC4 L2 N1AC6 N1AN2 N2AC1 N3AC2 N3AC3 N4AC6 N4AC7 N1AN2AC1 N1AC6AN5 N3AC2AC5 N4AC7AC8 C6AN4AC7 C3AC4AC5

1.3200(19) 1.3724(19) 1.275(2) 1.3714(18) 1.328(2) 1.323(2) 1.462(2) 1.336(2) 1.462(2) 1.469(2) 123.53(14) 121.69(14) 125.11(14) 103.06(13) 117.17(13) 120.97(14) 120.34(17) 120.08(16) 123.33(16)

C1AC2 C2AC3 C3AC9 C4 C5 C4 C9 C5 C6 C6 C7 C7 C8 C8 C9 C12AC13 N1AC1AC10 N2 C10 C1 N4AC11AN5 C1AN1AC8 C11AN4AC12 C3AC2AC1 C4AC5AC6 C7AC8AC9 C8AC9AC4

1.424(2) 1.362(2) 1.409(2) 1.363(3) 1.417(2) 1.404(3) 1.372(2) 1.412(2) 1.417(2) 1.535(2) 115.49(13) 119.56(13) 112.02(14) 118.39(13) 110.82(13) 118.24(15) 120.89(16) 119.63(14) 118.73(16)

115.8(2) 123.9(3) 97.9(3) 103.0(2) 110.2(2) 107.4(3)

1.327(4) 1.381(3) 1.276(4) 1.371(4) 1.361(4) 1.325(4) 1.457(4) N2AN1AC6 N2AC1AC2 N3AC3AC4 N5AC8AC7 C6AN5AC8 C2AC5AC4

N5AC8 N5AC6 C1AC2 C2AC5 C3AC4 C4AC5 C7AC8 116.8(2) 120.2(3) 108.3(3) 102.2(2) 110.0(2) 107.3(3)

N1AC8AC7 N3AC11AN5 N4AC12AC13 C10AN2AN3 C11AN5AC13 C2AC3AC9 C7AC6AC5 C3AC9AC8

118.68(14) 122.86(14) 102.80(13) 115.71(13) 109.95(13) 120.19(14) 120.33(17) 117.94(14)

N1AC6AN4 N3AC2AC1 N4AC6AN5 C2AN3AC3 C1AC2AC5

1.463(4) 1.326(4) 1.438(4) 1.371(4) 1.367(5) 1.408(4) 1.534(4) 123.9(3) 122.4(3) 112.2(3) 109.1(3) 129.8(3)

Table 3 Hydrogen bond distances (Å) and angles (°) for receptors L1 and L2. DAH  A

d(DAH)

d(H  A)

d(D  A)

<(DHA)

L1 N3AH3N  O1Wi 0.80(2) O1W H1WB  O2Wii 0.777(17) O2W H2WA  N1 0.813(16) ii N4 H4N  Br1 0.81(2) iii N5 H5N  Br1 0.81(2) ii O1W H1WA  Br1 0.788(17) O2W H2WB  Br1iv 0.814(17) C12 H12A  Br1v 0.98 C13 H13A  Br1vi 0.98 Symmetry codes: (i) x, y + 1, z; (ii)
x + 1,
y + 1,
z + 1; (iii)
x + 1,
y + 2,
z + 1;

1.92(2) 2.7174(19) 1.959(17) 2.732(2) 2.012(16) 2.8243(18) 2.80(2) 3.5417(15) 2.82(2) 3.6099(15) 2.552(18) 3.3268(16) 2.587(17) 3.3976(16) 2.93 3.7408(18) 2.92 3.7552(18) (iv) x
1, y, z; (v) x, y, z + 1; (vi) x
1, y, z + 1.

170(2) 173(3) 177(2) 153(2) 167(2) 168(2) 174(3) 141 144

L2 O1WAH1WABr1i N1AH1N  O2Wii O1WAH1WB  O2Wiii O2WAH2WABr1 N3AH3NBr1 O2WAH2WB  O1W N4AH4N  O1W1v N5AH5NBr1

2.40(3) 2.04 1.94(4) 2.42(4) 2.58 1.94(6) 2.03 2.60

177(3) 164 164(5) 174(4) 171 172(6) 170 158

0.83(3) 0.88 0.82(3) 0.84(4) 0.88 0.84(6) 0.88 0.88

3.231(2) 2.901(4) 2.739(3) 3.263(3) 3.455(3) 2.771(4) 2.900(3) 3.433(2)

Symmetry codes: (i) x + 1, y, z; (ii)
x + 1,
y + 1,
z + 1; (iii)
x + 1,
y + 1, z; (iv) x, y, z + 1.

The temperature dependence of the binding constant was studied in the temperature range of 293 K and 308 K. The thermodynamic parameters were calculated from the variable temperature UV–Vis titration data in aqueous solution (methanol 4% v/v; pH 7.0). The standard Gibb’s free energy change, DG0, the standard enthalpy change, DH0, and the standard entropy change, DS0, were calculated using Van’t Hoff’s equations (Eqs. (2)–(4)) [23].

lnðK 2 =K 1 Þ ¼
DH0 =Rð1=T 2
1=T 1 Þ

ð2Þ

DG0 ¼
RT ln K eq

ð3Þ

DG0 ¼ DH0
T DS0

ð4Þ

The detection limit was calculated spectrophotometrically using Eq. (5).

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Fig. 3. UV–Vis titration of (a) L1 (1  10
5 M) and (b) L2 (1  10
5 M) upon gradual addition of Cu+2 (0–1.4  10
5 M) in aqueous solution (methanol 4% v/v; pH 7.0) at 298 K.

Table 4 The association constant (Kass), standard Gibb’s free energy change (DG0), standard enthalpy change (DH0) and standard entropy change (DS0) for L1 and L2 with Cu2+ in aqueous solution (methanol 4% v/v; pH 7.0). Compound

Kass(Abs) (M
1)

DG0 (cal M
1)

DH0 (cal M
1)

DS0 (cal M
1 deg
1)

L1–Cu2+ L2–Cu2+

(4.84 ± 0.02)  104 (4.60 ± 0.04)  104


(6429.20 ± 0.36)
(6398.89 ± 0.45)


(878.36 ± 0.34)
(761.48 ± 0.42)

24.52 ± 0.032 24.03 ± 0.044

Fig. 4. The color changes of (a) L1 (1.0  10
5 M) and (b) L2 (1.0  10
5 M) upon addition of Cu2+ (1.4  10
5 M) and selected metal ions (1.4  10
5 M) in aqueous solution (methanol 4% v/v; pH 7.0) at 298 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Detection limit ¼ 3r=k

ð5Þ

where, r is the standard deviation of blank measurement, k is the slope of the linear calibration plot [21].

and pyrrole-2-carboxaldehyde (L2) in methanol and characterized by elemental analysis, FTIR, UV–Vis, 1H NMR, 13C NMR and single crystal X-ray diffraction studies (Figs. S1–S7). 3.2. X ray crystal structures of the receptors

3. Results and discussion 3.1. Synthesis The receptors were prepared by 1:1 condensation of 2-hydrazino2-imidazoline hydrobromide with quinoline-2-carboxaldehyde (L1)

Both the receptors L1 and L2 crystallized as their hydrobromide salts with two independent molecules of water in their asymmetric
(No. 2) for both L1 and L2 unit, having Z = 2 and space group P1 (Table 1 and Fig. 1). The receptors are relatively planar with the quinoline ring (L1) or pyrrole ring (L2) being inclined to the mean

S. Mukherjee et al. / Inorganica Chimica Acta 450 (2016) 216–224

221

Fig. 5. The absorbance change profile of (a) L1 (10 lM) with Cu2+ (1.4  10
5 M) at 453 nm and (b) L2 (10 lM) with Cu2+ (1.4  10
5 M) at 380 nm in presence of selected metal ions at a ratio of 5:1 (competing ion/ Cu+2) in aqueous solution (methanol 4% v/v; pH 7.0) at 298 K. The blue bars represent the absorbance of L1 or L2 in the presence of selected metal ions; the red bars represent the absorbance upon subsequent addition of 1.4  10
5 M of Cu+2 in the above solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Absorbance ratiometric responses of (a) L1 (10 lM) and (b) L2 (10 lM) upon addition of selected metal ions in aqueous solution (methanol 4% v/v; pH 7.0).

Scheme 2. Proposed mechanism for reversibility.

plane of the imidazoline ring by 1.68(9)° for L1 and 5.48(19)° for L2. In the crystals of both compounds, a series of NAH. . .O,Br and OAH. . .O,N,Br hydrogen bonds link the various components to form slabs parallel to the ab plane for L1 and to the ac plane for L2. Selected bond lengths (Å), bond angles (°) and hydrogen bond distances are listed in Tables 2 and 3, respectively. The crystal packing diagrams of both receptors are shown in Fig. 2.

3.3. Spectroscopic studies The receptor L1 exhibits multiple absorption bands in the UV region (230–380 nm; kmax = 358 nm) (Fig. 3a) which may be assigned to the p–p⁄ transition [23]. On addition of Cu2+ new strong absorption band appears at 453 nm. The four clear isosbestic points at 260, 300, 310 and 490 nm imply the undoubted

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confirmed by Benesi–Hildebrand (B–H) plot (Figs. S10 and S11). The association constants (Kass) in aqueous solution (methanol 4% v/v; pH 7.0) were estimated from the linear Benesi–Hildebrand expression (Eq. (1)) using UV–Vis titration data (Table 4). The thermodynamic parameters (DG0, DH0 and DS0) were calculated from Van’t Hoff’s equations (Eqs. (2)–(4)) and listed in Table 4. 3.5. Selectivity of the receptors towards Cu2+and ratiometric analysis

Fig. 7. Reversible changes in the absorbance of L1 at 453 nm (10 lM) in aqueous solution (methanol 4% v/v; pH 7.0) upon sequential addition of Cu2+ and Na2EDTA.

conversion of L1 to L1–Cu2+. Similarly, L2 (kmax = 314 nm) also exhibits ratiometric change in its absorption spectra (Fig. 3b) upon addition of Cu2+. 3.4. Thermodynamics of binding Job’s Plot analysis showed 1:1 stoichiometry for in- situ prepared copper(II) chelates (Figs. S8 and S9) which was further

The selectivity of L1 and L2 towards Cu+2 over other metal ions (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Sn2+ and Pb2+) was investigated in aqueous solution (methanol 4% v/v; pH 7.0) by UV–Vis spectroscopy. It was observed that L1 (1.0  10
5 M; 298 K) in presence of Cu2+ (1.4  10
5 M) gave a characteristic absorption peak at 453 nm with a distinct color change from colorless to yellow, which can easily be observed through naked eye without any spectroscopic instrument (Fig. 4a). On the other hand, L2 (1.0  10
5 M; 298 K) in presence of Cu2+ (1.4  10
5 M) gave a characteristic absorption peak at 380 nm with a distinct color change from light pink to brownish-yellow (Fig. 4b). In contrast, there was no distinguishable change in the absorption spectra of L1 and L2 upon addition of other selected metal ions. The competition experiments were carried out by addition of Cu2+ to the solution of L1 and L2 in the presence of 5.0 equivalents of other metal ions (Fig. 5). The results indicate that both the receptors act as chromogenic chemosensors for Cu2+ and the competitive metal ions do not interfere with the naked eye detection of Cu2+. In presence of Cu+2 the characteristic absorption peaks of L1 (358 nm) and L2 (314 nm) gradually decreased and new absorption

Fig. 8. Absorption spectra of (a) L1 and (b) L2 under four different input conditions (Red: no input; Black: EDTA; Green: Cu2+ and EDTA; Blue: Cu2+). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. Truth table and circuit diagram for the INHIBIT logic gate.

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Fig. 10. The linear relationship between the absorbance of the receptors (a) L1 at 453 nm and (b) L2 at 380 nm) and concentration of Cu2+.

Table 5 Determination of Cu2+ in water samples. Cu2+ spiked (lmol/L)

Cu2+ recovered (lmol/L)

Tap water

1

L L2

3.40 ± 0.05 5.20 ± 0.04

3.20 ± 0.05 5.10 ± 0.07

94.12 98.08

River water

L1 L2

6.40 ± 0.02 8.40 ± 0.03

6.82 ± 0.13 8.75 ± 0.12

106.56 104.16

Lake water

L1 L2

4.80 ± 0.04 6.10 ± 0.03

5.16 ± 0.09 6.46 ± 0.11

107.50 105.90

Sewage waste water

L1 L2

8.30 ± 0.05 6.40 ± 0.03

8.52 ± 0.11 6.63 ± 0.10

102.65 103.59

Sample

band at 453 nm for L1 and at 380 nm for L2 appeared concomitantly. This red shift (Dkabs = 95 nm for L1 and 66 nm for L2) of the absorption band allowed a ratiometric analysis of the signaling behavior. The plots of the absorbance ratio (A453/A358 for L1 and A380/A314 for L2) versus concentration of Cu+2 and other competitive metal ions are shown in Figs. 6 and S12. These observations indicate the high selectivity of both the receptors toward Cu2+ which induce significant chromogenic and ratiometric changes.

Recovery (%)

with absorbance as output. This satisfactorily mimics an INHIBIT logic gate with Cu2+ and EDTA as chemical inputs and absorbance at 453 nm for L1 and 380 nm for L2 as output. Only in the presence of Cu2+ as chemical input, L1 and L2 exhibit characteristic absorption peaks of the copper(II) chelates (‘1’ state). While in presence of EDTA and simultaneous presence of both Cu2+ and EDTA as chemical inputs the receptors show no change in the absorption spectra (‘0’ state) (Fig. 8). The corresponding truth table and circuit diagram for INHIBIT logic gate are shown in Fig. 9.

3.6. Receptors as reversible colorimetric chemosensors for Cu2+ 3.8. Optimization of pH The reversibility of both the receptors towards Cu+2 was observed by adding Na2EDTA to the solution of the receptors and Cu2+ (methanol 4% v/v; pH 7.0) [29–31]. On addition of 1.0 equiv of EDTA, the characteristic absorbance of in-situ prepared L1–Cu2+ at 483 nm was completely disappeared and absorbance of free ligand at 358 nm restored with change in solution color from yellow to colorless. Upon addition of Cu2+ again, the yellow color and absorbance at 453 nm reappeared. A proposed reversible change mechanism is shown in Scheme 2. These observed changes were almost reversible even after several cycles with the sequential alternative addition of Cu2+ and Na2EDTA (Fig. 7). Similar, results were obtained for the receptor L2 (Fig. S13). Thus, both the receptors may be used as a reversible colorimetric chemosensors of Cu2+ in aqueous medium. 3.7. Logic gate behavior EDTA (ethylenediaminetetraacetic acid) being a good chelating agent for metal ions like Cu2+, can be used as inputs in some molecular logic function [32–34]. Here, we have introduced EDTA and Cu2+ as stimulating inputs to study the molecular logic function

It is well known that the complexation reaction or detection of a metal via formation of chelates is often pH-dependent [35,36]. So, the effect of pH was investigated during spectrophotometric determination of Cu2+. The absorbance of the receptor at 453 nm for L1 (10 lM) and at 380 nm for L2 (10 lM) in presence of Cu2+ (1.0 equiv) was measured in the pH range of 4–12 by adjusting the pH using NaH2PO4–Na2HPO4 buffer solutions. The results indicate a slight change in the absorbance values in presence of Cu2+ throughout the entire pH range (4–12) (Fig. S14) which may be helpful for real sample analysis. 3.9. Potential applicability of the receptors in analysis of Cu+2 The potential applicability of the receptors toward Cu2+ was examined in water samples [37,38]. The calibration curves were constructed for the determination of Cu2+ in aqueous solution. Both the receptors exhibited a good linear relationship between their UV–Vis intensities and Cu2+ concentration with detection limits (Eq. (5)) of 4.94  10
7 M for L1 and 4.88  10
7 M for L2 (Fig. 10). Tap and lake water samples were collected from Kalyani

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University Campus and surroundings. Waste water sample was collected from Sewage Water Treatment Plant in Kalyani and river water was collected from Ganga River (Tribeni, Hooghly, and West Bengal). The water samples thus obtained were simply filtered and spiked with standard Cu2+ solutions at different concentration levels. Further analyses were done via UV–Vis spectroscopic measurements. The results were presented in Table 5 which shows satisfactory recovery values for the drinking water samples. 4. Conclusion In conclusion, two new chromogenic chemosensors were synthesized for selective detection of Cu2+ in aqueous medium. They were characterized by FTIR, UV–Vis, 1H NMR, 13C NMR and single crystal X-ray diffraction studies. Upon interaction with Cu2+ both the receptors displayed remarkable shift in absorbance spectra with obvious color changes, which can be easily used for naked-eye detection. Being reversible chemosensors of Cu2+ in presence of EDTA, both the receptors exhibit ‘‘INHIBIT” logic gate with Cu2+ and EDTA as chemical inputs. Moreover, the receptors may be used to detect and quantify Cu2+ in real water samples with detection limit in the micro molar range. Acknowledgment University of Kalyani – India, DST-FIST (SR/FST/ESI-008/2008), DST-PURSE, New Delhi – India, are gratefully acknowledged for financial support, instrumental and infrastructural facilities. Appendix A. Supplementary material CCDC 1447698 and 1447699 contains the supplementary crystallographic data for L1 and L2, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2016.05.049. References [1] S. Kobayashi, Y. Mori, J.S. Fossey, M.M. Salter, Chem. Rev. 111 (2011) 2626. [2] S. Mukherjee, S. Chowdhury, A.K. Paul, R. Banerjee, J. Lumin. 131 (2011) 2342. [3] M. Cocco, C. Congiu, V. Lilliu, V. Onnis, Bioorg. Med. Chem. 14 (2006) 366.

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