Two Schiff base ligands for distinguishing Zn^II/Cd^II sensing—effect of substituent on fluorescent sensing

Abstract

Two Schiff base ligands (HL¹, HL²) were conveniently synthesised by one-step condensation between pyridine 2-ylmethanamine and 3-ethoxy-2-hydroxybenzenaldhyde (for HL¹) or salicylaldehyde (for HL²) as fluorescent sensors for distinguishing sensing of Zn²⁺ or Cd²⁺. Both of the two fluorescent sensors present very weak emission at 463 nm (for HL¹) or 453 nm (for HL²). For HL¹, upon addition of Zn²⁺, the fluorescence intensity of HL¹ enhanced and gradually red shifted to 493 nm with a green emission while addition of Cd²⁺ only induced enhancement of fluorescent intensity at 463 nm. For HL², only addition of Zn²⁺ induced enhancement of fluorescence intensity, presenting a high Zn²⁺/Cd²⁺ selectivity. A Zn²⁺-induced red shift in fluorescent spectra of HL¹ could be attributed to twisted intramolecular charge transfer (TICT) from the interaction between the Zn²⁺ ion and in situ formed ligand L^1′ with the twisted structure in compound 1, which is absent in compound 2. The Zn²⁺/Cd²⁺ selectivity of fluorescent response for HL² correlates with the Cd–HL² and Zn–HL² coordination bond distances. Obviously, introduction of ethoxyl groups onto the benzene ring as an electron-donating group facilitates the Zn-induced in situ dimerization of HL¹ into new ligand L^1′ with a twisted molecular structure, further resulting in a red shift of the fluorescence spectra.

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Introduction

Fluorescent sensors for metal ions are some of the most sought-after detection methods for metal ions owing to their convenient use, high selectivity, potential use in living cells and discernible response with color variation under UV irradiation or with the naked eye.¹ Whether in protein-bound or free form, Zn²⁺ is a type of abundant ion among the trace metal ions in the human body and is related to many crucial biological processes including nervous system function, enzyme regulation, gene expression, biological catalysis and Zn(II)-disorder-related diseases.² Zn²⁺ is also environmentally important because increased levels of Zn²⁺ in water can cause environmental problems including poor soil microbial activity causing phytotoxic effects or smelly water.³ Some fluorescent sensors have been developed to investigate the distribution of mobile Zn²⁺ in living cells over the last decades which are mainly based on metal complexation and complexation-based photoinduced electron-transfer (PET), chelation-enhanced-fluorescence (CHEF) effects or internal charge-transfer (ICT) mechanisms.⁴ Twisted intramolecular charge transfer (TICT) based on a twisted molecular structure is among the types of ICT and is widely employed in fluorescent ion sensing due to its spectral shifts and color variation originating from locally emission (LE) band and TICT emission band.^1c,5 Theoretically, introduction of electron donating groups (e.g. –OR, –R) and electron withdrawing groups (e.g. –NO₂, –CN) onto the moiety of the organic sensor, may generate TICT upon coordination of the ligand with metal ions, based on which metal ions sensing could be achieved.^5,6 On the other hand, the detection of Zn²⁺ is often related to Cd²⁺ because of their similar electron configurations and similar response to the same fluorescent sensor, making one of the biggest challenges in constructing Zn²⁺ fluorescent chemosensor.⁷ Moreover, because of cadmium's accumulation in the food chain and its great toxicity to human's body,⁸ it is desirable to develop some analytical methods for cadmium detection in the environment or living cells. Therefore, it is necessary to develop fluorescent sensor for distinguishing sensing of Zn²⁺ and Cd²⁺.

Based on the above considerations, we designed and synthesized two Schiff base ligands HL¹ and HL² for distinguishing sensing of Zn²⁺ and Cd²⁺. Ethoxyl group as electron donating group was introduced onto HL¹ endowing it with TICT property and subsequent spectral shift upon reaction with Zn²⁺. HL¹ and HL² were conveniently obtained through one-step condensation (Scheme 1) between pyridine 2-ylmethanamine and 3-ethoxy-2-hydroxybenzenaldhyde (for HL¹) or salicylaldehyde (for HL²). Compared with other previously reported Zn²⁺ organic sensor,⁴ HL¹ and HL² present distinct advantages including its convenient synthesis and tridentate donor set of N₂O capable of strongly chelating to Zn²⁺ or Cd²⁺ ions through forming five/six membered ring with common side, providing a binding site between metal ions and sensor. For HL², together with the pyridine ring as electron withdrawing group,⁹ introduction of ethoxyl group as electron donating group onto the benzene ring facilitates the internal charge transfer of the sensor and further presents Zn-induced TICT and spectral red shift, which does not exist in the system of HL² owing to absence of ethoxyl group. However, without ethoxyl group as hindrance effect, HL² shows greater sensitivity towards sensing Zn²⁺ with a much lower detection limit (Scheme 2).

Scheme 1 Synthesis of HL¹ and HL².

Scheme 2 Effect of substituent group on fluorescent sensing of Zn²⁺ and Cd²⁺ based on Schiff base complexes 1–4.

Experimental

General procedures

All chemicals were of analytical reagent grade and were used as received without any further purification. Elemental analysis for C, H, and N were performed on a Perkin-Elmer 2400 analyzer. IR spectra were recorded with a Perkin-Elmer Fourier transform infrared spectrophotometer with samples prepared as KBr disks in the 4000–400 cm⁻¹ range. UV-vis absorption spectra were recorded on a Shimadzu UV1800 UV-vis Spectrophotometer. Fluorescent spectra were recorded on a Hitachi F-2500 spectrometer with the excitation slit as 5 nm and emission slit as 5 nm.

X-ray crystallography

Crystal data collections were performed at 298 K on a Bruker Smart Apex II diffractometer with graphite mono-chromated Mo Kα radiation (λ = 0.71073 Å) for four compounds 1–4. Absorption corrections were applied by using the multi-scan program SADABS.¹⁰ Structural solutions and full-matrix least-squares refinements based on F² were performed with the SHELXS-97 (ref. 11) and SHELXL-97 (ref. 12) program packages, respectively. All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms on organic motifs were placed at calculated positions. Details of the crystal parameters, data collections, and refinements for compounds 1–4 are summarized in Table S1.† Selected bond lengths and angles of compounds 1–4 are shown in Table S2.†

Synthesis of HL¹ and HL²

The Schiff-base ligand 2-ethoxy-6((pyridin-2-ylmethylimino)-methyl)phenol (HL¹) was synthesized according to corresponding reference.¹³ 2-((Pyridin-2-ylmethylimino)methyl)-phenol (HL²) was prepared according to related reference.¹⁴

Synthesis of compounds 1–4

[Zn₄(L¹)(L^1′)Cl₅] (1). After refluxing at 60 °C for 1 hour, to a mixed solution of 3-ethoxy-2-hydroxybenzenaldhyde (0.2 mmol, 33.2 mg) and 2-pyridin-2-ylmethanamine (0.2 mmol 21 μL) in MeOH (20 mL) was added ZnCl₂ (0.2 mmol, 27 mg). The resulting clear yellow solution was stirred for 1 more hour at room temperature and then filtered. After 2 weeks, pale yellow needle-shaped crystals of compound [Zn₄(L¹)(L^1′)Cl₅] (1) was obtained after slow diffusion of petroleum ether into the resulted clear solution. Yield (based on ZnCl₂): 45 mg (75%). IR (KBr cm⁻¹): 3417(br, vs), 2980(w), 2894(w), 1631(s), 1604(m), 1556(m), 1487(m), 1460(m), 1427(w), 1388(m), 1273(m), 1246(w), 1217(s), 1110(w), 1047(s), 991(w), 904(w), 842(w), 783(s), 738(m), 663(m), 561(s), 468(m). Elemental analysis calcd (%) for C₄₅H₄₄Cl₅N₆O₆Zn₄: C, 44.90; H, 3.68; N, 6.98; found: C, 44.70; H, 3.69; N, 7.01.

[Cd₂(L¹)Cl₂(DMSO)]₂ (2). The procedure was the same as that for 1 using CdCl₂·2.5H₂O (0.2 mmol 46 mg) instead of ZnCl₂, and drops of DMSO were added to dissolve the resulting precipitate after stirring. After 1 week, pale yellow needle-shaped crystals of [Cd₂(L¹)Cl₂(DMSO)]₂ (2) were obtained after slow diffusion of petroleum ether into the resulted clear solution. Yield (based on CdCl₂): 29.6 mg (60%). IR (KBr cm⁻¹): 3478 (br, vs), 3304 (w), 2970(w), 1635(vs), 1558(vs), 1420(m), 1413(s), 1303(w), 1280(w), 1234(w), 1209(s), 1151(w), 1078(m), 1041(m), 906(w), 767(m), 746(m), 642(m), 524(w), 443(w). Elemental analysis calcd (%) for C₃₄H₄₂Cd₄Cl₆N₄O₆S₂: C, 30.72; H, 3.18; N, 4.22; found: C, 30.80; H, 3.13; N, 4.26.

[Zn(L²)Cl]₂ (3). The procedure was the same as that for 1 using salicylaldehyde (21 μL, 0.2 mmol) instead of 3-ethoxy-2-hydroxybenzenaldhyde. Pale yellow single crystals of 3 were formed in three days. Yield (based on ZnCl₂): 51.1 mg (82%). IR (KBr cm⁻¹): 3439(s), 1637(s), 1595(w), 1552(m), 1469(s), 1413(m), 1350(w), 1290(s), 1197(w), 1145(w), 1064(w), 1001(w), 906(w), 837(w), 761(s), 661(w), 582(w), 439(w). Elemental analysis calcd (%) for C₂₆H₂₂Cl₂N₄O₂Zn₂: C, 50.03; H, 3.55; N, 8.98; found: C, 50.18; H, 3.59; N, 9.13.

{[Cd₄L²₆]²⁺·[CdCl₄]²⁻·CH₃OH}₂·H₂O (4). The procedure was the same as that for 2 using salicylaldehyde (21 μL, 0.2 mmol) instead of 3-ethoxy-2-hydroxybenzenaldhyde. Pale yellow single crystals of 4 were formed in two weeks. Yield (based on CdCl₂): 35.2 mg (44%). IR (KBr cm⁻¹): 3443(s), 2980(m), 1634(s), 1598(w), 1562(m), 1467(s), 1403(m), 1355(w), 1290(s), 1199(w), 1155(w), 1074(w), 1081(w), 918(w), 844(w), 766(s), 677(w), 562(w), 449(w). Elemental analysis calcd (%) for C₁₅₈H₁₄₂Cd₁₀Cl₈ N₂₄O₁₅: C, 47.15; H, 3.56; N, 8.35; found: C, 47.14; H, 3.57; N, 8.34.

Results and discussion

Fluorescence/UV absorption spectra and titration of HL¹

The fluorescence response of receptor HL¹ towards Zn²⁺ was studied in 0.10 mM ethanol. Upon excitation at 350 nm, HL¹ in ethanol presented dark blue emission at 463 nm which could be attributed to the intraligand π–π* transition.¹⁵ With stepwise addition of ZnCl₂ solution in ethanol with concentration ranging from 0.1 equiv. to 2.0 equiv., fluorescent emission of HL¹ gradually red-shifted to 493 nm with their fluorescent intensity at 493 nm enhanced linearly with the Zn²⁺ concentration and saturated when [Zn²⁺] reaches 1.33 equiv. (Fig. 1). Interaction between Zn²⁺ and HL¹ was also studied by UV absorption spectra (Fig. S1†). Concomitant addition of ZnCl₂ into HL¹ in ethanol causes a new absorption peak at 380 nm with intensity increased while that at 220 nm, 264 nm, 334, 432 nm decreased with four isosbetic points at 249, 276, 303, 347 nm. The binding ratio between Zn²⁺ and ligand was estimated to be 4 : 3 as confirmed by its job plot (Fig. S2†) and crystal structure. The limit of detection (LOD) was measured to be 1.11 × 10⁻⁶ M (R² = 0.999) with a linearity range between 1 × 10⁻⁵ M and 9 × 10⁻⁵ M (LOD = 3σ/slope) (Fig. S3†).

Fig. 1 Fluorescence emission spectra of HL¹ upon addition of ZnCl₂ in ethanol. λ_ex = 350 nm at room temperature [HL¹] = 0.10 mM; [Zn²⁺] = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.131, 0.132, 0.133, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20 mM (inset: corresponding ZnCl₂ titration profile according to the fluorescence intensity, indicating 4 : 3 stoichiometry for Zn²⁺/HL¹).

The selectivity of HL¹ to Zn²⁺ was examined by fluorescence titration of HL¹ with various metal ions (Fig. 2). The fluorescence intensity of HL¹ was slightly quenched with some cations such as Ni²⁺, Cu²⁺, Co²⁺, and Fe³⁺, Mn²⁺. Other cations such as Na⁺, K⁺, Li⁺, Mg²⁺, Ba²⁺, Ca²⁺, Hg²⁺, Fe²⁺, Pb²⁺, Al³⁺ and Cr³⁺ did not cause any significant changes, showing selective CHEF & spectral red shift in the presence of Zn²⁺. Nevertheless, interaction of Cd²⁺ with HL¹ also presents fluorescence spectral response which is relatively weaker compared with that of Zn²⁺. Concomitant addition of CdCl₂ into HL¹ in ethanol induced enhancement of the ligand-centered emission (Fig. 3) with binding ratio between Cd²⁺ and HL¹ as 2 : 1 confirmed by job plot (Fig. S4†) and crystallography with a limit of detection as 9.2 × 10⁻⁶ M (R² = 0.999) with a linearity range between 2 × 10⁻⁵ to 1.8 × 10⁻⁴ M (LOD = 3σ/slope) (Fig. S5†). UV absorption titration also demonstrates interaction between Cd²⁺ and HL¹ (Fig. S6†). The selectivity of HL¹ on Zn²⁺ over Cd²⁺ was determined as 4.8 calculated from ratio of I_Zn/I_Cd (2 equiv.).

Fig. 2 Fluorescence emission spectra of HL¹ in the presence of different ions such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Co²⁺, Al³⁺, Cr³⁺, Ni²⁺, Fe²⁺, Fe³⁺, Zn²⁺, Cd²⁺, Mn²⁺, Hg²⁺, and Pb²⁺ (metal ions as their Cl⁻ and Hg²⁺, Pb²⁺ as their NO₃⁻ salts) in ethanol. λ_ex = 350 nm, [HL¹] = 0.1 mM, and [Mⁿ⁺] = 0.2 mM.

Fig. 3 Fluorescence emission spectra of HL¹ upon addition of CdCl₂ in ethanol. λ_ex = 350 nm at room temperature [HL¹] = 0.10 mM; [Cd²⁺] = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.24, 0.26, 0.28, 0.30, 0.32, 0.34, 0.36, 0.38, 0.40 mM (inset: corresponding CdCl₂ titration profile according to the fluorescence intensity, indicating 2 : 1 stoichiometry for Cd²⁺/HL¹).

To further evaluate selectivity of HL¹ towards Zn²⁺, the interference of series of metal ions on detection of Zn²⁺ was examined. Seeing from Fig. 4, in the presence of various metal ions including Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe³⁺, Hg²⁺, Mg²⁺ and Cd²⁺, the emission intensity of HL¹–Zn remain hardly perturbed, while in the vicinity of other involved metals, it is slightly quenched but is still clearly detectable, indicating a high selectivity of HL¹ on Zn²⁺.

Fig. 4 Selectivity of HL¹ for Zn²⁺ in the presence of other metal ions in ethanol. λ_ex = 350 nm. Black bars represent the addition of an excess of the appropriate metal ion (0.5 mM for Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe³⁺, and Cu²⁺ and 0.3 mM for all other metal ions) to a 0.1 M solution of HL¹. Green bars represent the subsequent addition of 0.1 mM ZnCl₂ to the solution.

Crystal structure of Zn₄L¹L^1′Cl₅ (1) and [Cd₂(L¹)Cl₂-(DMSO)]₂ (2) and proposed mechanism of HL¹ sensing Zn²⁺/Cd²⁺

Reaction of HL¹ with Zn²⁺ induced part of HL¹ to undergo [3 + 2] cycloaddition¹⁶ generating a dimerized form of L^1′ (Scheme S1†) and compound 1 was formed. Crystal analysis revealed that compound 1 crystallized in triclinic P1̄ space group and each discrete tetranuclear unit comprised of four Zn²⁺ ions, one dimerized ligand L^1′, one original Schiff base ligand L¹ and five chlorides. As shown in Fig. 5, out of the four Zn ions, Zn1 was penta-coordinated to one pyridine nitrogen atom (N2), one amide nitrogen (N4), two hydroxyl oxygen atoms (O3, O4) all from L^1′, while Zn2 was four-coordinated to one hydroxyl oxygen atom of L^1′ and three chlorides (one μ₂-bridged Cl2 and the other two terminal coordinated Cl1, Cl5). And Zn3 was penta-coordinated to one hydroxyl oxygen atom (O3) of L^1′, two N atoms (N1, N2), one O atom (O1) of L¹ and one μ₂-bridged chlorides (Cl2), connecting L¹ and L^1′ together while Zn4 was quadra-coordinated to two nitrogen atoms (N5 N6) from L^1′ and two terminal chlorides (Cl3, Cl4). Moreover, π⋯π stacking between benzene ring and pyridine ring helps to stabilize structure of compound 1 (Fig. 5b).

Fig. 5 (a) Molecular structure of compound Zn₄(L¹)(L^1′)Cl₅ (1) with partial atomic labels and (b) twisted conformation of the dimerized form of L^1′ accounting for the TICT and spectral red-shift.

When reacted with CdCl₂, HL¹ generated compound [Cd₂(L¹)Cl₂(DMSO)]₂ (2), which crystallized in triclinic, P1̄ space group presenting a centrosymmetric discrete tetra-nuclear structure (Fig. 6). Its asymmetric unit contained two Cd²⁺ ions, one deprotonated (L¹)⁻ anion, three chlorides and one coordinated DMSO molecule. Cd1 was penta-coordinated to one pyridine nitrogen atom (N1), one azomethine amide nitrogen atom (N2), one hydroxyl oxygen atom (O1) and two chlorides (Cl1, Cl1, one μ₂-bridged and the other terminal coordinated), exhibiting a distorted square pyramid coordination geometry. The other Cd2 ion is hexa-coordinated with two μ₂-bridged chlorides (Cl2, Cl3), one hydroxyl oxygen atom (O1), one ethoxyl oxygen atom (O2) and one oxygen atom (O3) from the DMSO molecule, showing distorted octahedral coordination geometry. Moreover, by μ₂-bridging of two chlorides (Cl3, Cl3a), two asymmetric units were connected to assemble a tetranuclear unit of compound 2.

Fig. 6 Molecular structure of tetranuclear compound [Cd₂(L¹)Cl₂-(DMSO)]₂ (2) with partial atomic labels. Asymmetry code: (a) 1 − x, 2 − y, 1 − z.

Emission spectra of compounds 1–2 in ethanol also accord with emission spectra generated from titration, further confirming that the products formed upon titration are compounds 1 and 2 (Fig. S7†). Hence, mechanism for Cd²⁺ and Zn²⁺ sensing of HL¹ could be proposed referring to the structural analysis of compound 1 and 2. Compound 1 was obtained through reaction of ZnCl₂ with HL¹, in which HL¹ as 1,3-dipole undergoes Zn-induced dimerization through [3 + 2] cycloaddition, generating a five-membered ring between two mole of HL¹ (Scheme S1†). Viewing from the top of the five-membered ring, it is not difficult to find that L^1′ was fixed to present a twisted molecular structure, in which the pyridine ring and benzene ring was almost twisted to be orthogonal. Such twisted structure induced a twisted-intramolecular charge transfer state upon excitation which narrowed down the energy gap between HOMO and LUMO, generating an emission peak of longer wavelength at 493 nm (Fig. 7).^5a,17 Unlike compound 1, in compound 2, coordinated L¹ presents a planar structure with the angle between the pyridine ring and benzene ring as nearly 0° (Fig. 7). Therefore, reaction of Cd²⁺ with HL¹ presented locally excited (LE) emission at 463 nm.

Fig. 7 Diagram presenting planar structure of ligand L¹ in compound 2 and twisted structure of L^1′ in compound 1 accounting for locally excited (LE) emission at 463 nm and TICT emission at 493 nm, respectively.

Furthermore, fluorescent intensity enhancement of HL¹ upon interaction with Zn²⁺ or Cd²⁺ should be ascribed to the CHFF (chelation enhanced fluorescence), inhabitation of C=N isomerization^9,18 and weaken of PET⁹ (photo induced charge transfer) process in the ligand. On the other hand, different coordination structure of compounds 1 and 2 are probably related to the difference in radius between Cd²⁺ (0.96 Å) and Zn²⁺ (0.74 Å), which also affects the extend of the CHEF effect on the chromophores. Moreover, the greater heavy atom quenching effect of Cd²⁺ also conduces to weaker spectral response of HL¹ on Cd²⁺ than that of Zn²⁺.

Fluorescence/UV absorption spectra and titration of HL²

Twisted molecular structure of compound 1 could be ascribed to introduction of ethoxyl group which facilitates internal charge transfer of HL¹. In order to explore the effect of ethoxyl group on its spectral response to Zn²⁺ and Cd²⁺, we choose HL² absent of an ethoxyl group (Scheme 1) to examine its spectral response towards Zn²⁺/Cd²⁺ and found that HL² has high selectivity on Zn²⁺ sensing with much more sensitivity.

The fluorescent spectral response of HL² toward Zn²⁺ was so sensitive compared to that of HL¹ that the concentration of examined HL² was diminished as 10 μM with the [Zn²⁺] ranging from 0–20 equiv. in ethanol. Unlike that of HL¹ with an ethoxyl group, addition of ZnCl₂ into HL² in ethanol only induced enhancement of ligand-centered luminescence at 453 nm without any shift of emission peak and reached saturation when the ratio amounted to 1 : 1 (Fig. 8). The UV absorption of HL² also responds to titration of Zn²⁺ with a new absorption peak at 373 nm while that at 258, 319 nm decreased with four isosbetic points at 248, 269, 338, 291 nm (Fig. S8†). The binding ratio between Zn²⁺ and HL² was determined to be 1 : 1 which was further confirmed by both job plot (Fig. S9†) and crystal structure (Fig. 10). More sensitivity of HL² on Zn²⁺ could be supported by the relatively low limit of detection as 7 × 10⁻⁸ M ranging (Fig. S10,† LOD = 3σ/slope, R² = 0.997) with a linearity range between 1 × 10⁻⁶ M and 9 × 10⁻⁶ M.

Fig. 8 Fluorescence emission spectra of HL² upon addition of ZnCl₂ in ethanol. λ_ex = 350 nm at room temperature [HL²] = 10 μM; [Zn²⁺] = 0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0 μM.

Likewise, selectivity of HL² towards Zn²⁺ was examined by fluorescence of HL² (Fig. 9) in the presence of equimolar metal ions including Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Cr³⁺, Hg²⁺, and Fe³⁺ as their Cl⁻ salts (Hg²⁺ as its NO₃⁻ salt). The results revealed that Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺caused slight quenching of fluorescence of HL² while Cd²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Cr³⁺, and Fe³⁺ induced slight fluorescence enhancement, indicating that HL² has high selectivity towards Zn²⁺. Specifically, the selectivity of HL² on Zn²⁺ over Cd²⁺ was calculated as 30 using ratio of I_Zn/I_Cd, much higher than that of HL¹. Furthermore, selectivity of HL² on Zn²⁺ was studied by fluorescence respond of Zn²⁺ in the presence of other competing metal ions including Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Cr³⁺, Hg²⁺, and Fe³⁺, results of which revealed that these metal ions have negligible disturbance on Zn–HL² fluorescence, further indicating high selectivity of HL² over Zn²⁺ (Fig. 10).

Fig. 9 Fluorescence emission spectra of HL² in the presence of different ions such as Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Cr³⁺, Hg²⁺, and Fe³⁺ (metal ions as their Cl⁻ salts and Hg²⁺ as NO₃⁻ salts) in ethanol. λ_ex = 330 nm, [HL²] = 10 μM, and [Mⁿ⁺] = 10 μM.

Fig. 10 Selectivity of HL² for Zn²⁺ in the presence of other metal ions in ethanol. λ_ex = 350 nm. Black bars represent the addition of an excess of the appropriate metal ion (50 μM for Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe³⁺, Cu²⁺ and 30 μM for all other metal ions) to a 10 μM solution of HL². Blue bars represent the subsequent addition of 0.1 mM ZnCl₂ to the solution.

Crystal structure of (ZnL²Cl)₂ (3) and [Cd₄L²₆]·[CdCl₄]·CH₃OH (4) and proposed mechanism for Zn²⁺ sensing of HL¹

Single crystal analysis revealed that compound 3 crystallized in monoclinic P2₁/n space group and showed a centrosymmetric binuclear structure composed of two Zn²⁺ ions, two deprotonated (L²)⁻ anions and two chlorides. In this discrete unit, each Zn1 was penta-coordinated to two nitrogen atoms (N1, N2), one hydroxyl oxygen atom (O1) from one ligand, one hydroxyl oxygen atom (O1a) from the other ligand and one chloride (Cl1) (Fig. 11). Upon coordination, N₂O donor set of L² strongly chelated to Zn²⁺ and two hydroxyl oxygen atoms adopted μ₂-coordination modes to bridge two Zn^II ions together and formed the dinuclear structure.

Fig. 11 Molecular structure of dinuclear compound [Zn(L²)Cl]₂ (3) with atomic labels. Asymmetry code: (a) 1 − x, −y, −z.

Replacing ZnCl₂ with CdCl₂, HL² was deprotonated and coordinated to generate compound {[Cd₄L²₆]·[CdCl₄]⁻·CH₃OH}₂·H₂O (4) at room temperature. The single crystal analysis revealed that compound 4 crystallized in monoclinic P2/c space group and possessed two anionic mononuclear [CdCl₄]²⁻ units, two lattice methanol molecules, one water molecule and two independent tetranuclear cationic [Cd₄L²₆]²⁺ units associated with intermolecular hydrogen bonds C–H⋯Cl. It could be seen from Fig. 12 that two tetranuclear [Cd₄L²₆]²⁺ cationic moieties containing one mirror symmetry are structurally identical, each comprising of four Cd²⁺ ions and six deprotonated ligand (L²)⁻. Four Cd²⁺ ions in each unit are hexa-coordinated to donor set N4O2 from two adjacent ligand (L²)⁻ for three cadmium ions (Cd2, Cd3, Cd3a/Cd5, Cd6, Cd6a) at the vertices of an equilateral triangle or donor set O6 from six ligand (L²)⁻ for one central cadmium ion (Cd1/Cd4), presenting the distorted octahedral coordination geometry. While each mononuclear anion unit contained one Cd²⁺ ion and four coordinated chlorides, presenting one tetrahedral coordination geometry. Together with lattice methanol and water molecules, cationic [Cd₄L²₆]²⁺ and anionic [CdCl₄]²⁻ moieties were further connected each other by hydrogen bonds C–H⋯Cl(O) into the 2-D supramolecular layer (Fig. S11†).

Fig. 12 Structures of cationic [Cd₄(L²)₆]²⁺ and anionic [CdCl₄]²⁻ units in compound 4 with partial atomic labels, and hydrogen atoms, the lattice water and methanol molecules are omitted for clarify. Symmetric code: (a): 1 − x, y, 1.5 − z, (b): −x, y, 1.5 − z.

Luminescence spectrum of compound 3 in ethanol was also obtained to attest that the product upon titration of Zn²⁺ into HL² is compound 3. (Fig. S†) Similar to HL¹, Zn-induced fluorescent intensity enhancement of ligand HL² could be explained by CHFF, PET, and C=N isomerization-related process as mentioned above. Even though upon reaction of HL² and CdCl₂, crystal of compound 4 can be obtained, yet instant fluorescent response cannot be detected upon titration of Cd²⁺ salts (anion as Cl⁻, NO₃⁻, ClO₄⁻, or OAc⁻) into HL². This negative response could be possibly attributed to larger radius and therefore less affinity to HL² of Cd²⁺ compared to that of Zn²⁺, which could be supported by the difference of Cd–N, Cd–O distances and Zn–N, Zn–O distances in compound 3 and 4 (Table 1).¹⁹ On the other hand, despite several reports of coordination compounds based on HL² and other metals²⁰ such as Fe(III), V(III), Cu(II), and Ni(II), yet high fluorescent sensing selectivity of HL² towards Zn²⁺ can be also rationalized ascribing to paramagnetism of other metals which causes fluorescence quenching.

Table 1 Information of HL¹ and HL² as fluorescent sensors for Zn²⁺ and/or Cd²⁺

Sensor	Metal	Binding ratio (M^2+ : L)	Limit of detection	IZn/ICd (1 equiv.)	Mean of metal–N distances (Å)	Mean of metal–O distances (Å)	Sensing mechanism
HL^1	Zn^2+	4 : 3	1.1 × 10^−6 M	4.8	2.098	2.020	TITC,^1c CHEF, PET^9 C=N isomerization^9,18
	Cd^2+	2 : 1	9.2 × 10^−6 M		2.315	2.302	CHEF, PET, C=N isomerization
HL^2	Zn^2+	1 : 1	4.7 × 10^−8 M	30	2.070	2.108	CHEF, PET, C=N isomerization
	Cd^2+	2 : 3	—		2.357	2.289	—

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Conclusions

Ligands (HL¹, HL²) as fluorescent sensors towards Zn²⁺ and/or Cd²⁺ and their metal ions sensing properties were investigated, through which we demonstrated the effect of ethoxyl substituent on Zn²⁺ fluorescent sensing. Their spectral difference in sensing Zn²⁺ or Cd²⁺ could be attributed to ethoxyl group which facilitated Zn^II-induced twisted intramolecular charge transfer. Such results may give insight into how to develop fluorescent sensors for discriminating ion pairs.

For HL¹ with an ethoxyl substituent on the benzene ring, interaction of the ligand with Zn²⁺ induced twisted molecular structure and causes fluorescent red shift and fluorescent enhancement, while interaction of Cd²⁺ with HL¹ merely induced fluorescence enhancement retaining the original emission peak. Though HL¹ spectrally responds to both Zn²⁺ and Cd²⁺, these two metal ions can be discriminated by HL¹ via two coordination conformations.

For HL² absent of the ethoxyl group, it has exclusive response towards Zn²⁺ over other metal ions including Cd²⁺, showing that HL² could also be a selective fluorescent sensor for Zn²⁺. Unlike HL¹, no spectral shift was detected upon interaction of HL² with Zn²⁺, which could be ascribed to absence of twisted molecular structure in Zn–HL² compound 3. Negative response of HL² towards Cd²⁺ correlates with longer Cd–N, Cd–O bond distances compared with that of Zn–N, Zn–O bond lengths in compounds 3 and 4, indicating less affinity of Cd²⁺ towards HL². On the other hand, without ethoxyl group as hindrance effect, HL² shows greater sensitivity towards sensing Zn²⁺ with a much lower detection limit for Zn²⁺.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China (Grant no. 91122008, 21071056 and 21471061), Research Fund for the Doctoral Program of Higher Education of China (20124407110007), Guangdong Province of higher school science and technology innovation key project (cxzd1113), and Foundation for High-level Talents in Higher Education of Guangdong, China (C10301). The authors also thank Prof. Keith Man-chung Wong for offering valuable suggestions during composition of this essay.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1003835–1003838, 1015499–1015501 and 1015694. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra00987a

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