English

• 0.   Introduction

  Coumarin is an excellent fluorescent compound with large Stokes shift, visible light excitation and emission, tunable light wavelength, two-photon effect, stable chemical properties, biocompatibility and high fluorescent rate^[1-2]. Different positions of coumarin ring can be replaced by different groups, which can obtain different absorption and fluorescent emission wave-lengths, show different colors, and derive derivatives with strong fluorescence^[3-5]. In particular, coumarin modified with amino groups at positions 3 and 6 can react with aldehydes to prepare Schiff base compounds. Schiff base derivatives containing N and O donor site and fluorophore are appealing to tools for designing optical probes for metal ions^[6-9]. It is already established that Schiff base alone is weakly or nonfluorescent as the fluorescence is quenched due to the free rotation around C=N bond^[10-15]. Recently, numerous papers described the specific detection of various metal ions through the chelation of Schiff base probes^[16-18]. Coumarin, as an excellent chromophore, can be invoked as a fluorescent sensor to detect cations, anions and biologically related species^[19-22]. Meanwhile, coumarin Schiff base can form a variety of metal complexes because of their abundant coordinating atoms and the ability to participate in coordination is outstanding^[23-26]. Herein, three coumarin Schiff bases ligands and their corresponding complexes, [Zn(L¹)₂] (1) (HL¹=6-((4-diethylamino-2-hydroxy-benzylidene)-amino)-benzopyran-2-one), [Co(L²)₂] (2) (HL²=6-((4-me-thoxy-2-hydroxy-benzylidene)-amino)-benzopyran-2-one) and [Ni₂(L³)₂(CH₃OH)₄] (3) (H₂L³=4-hydroxy-3-((4-methoxy-2-hydroxy-benzylidene)-amino)-benzopyran-2-one), were synthesized and characterized. In addition, the photochemical properties of HL¹, HL², H₂L3 and their corresponding complexes 1, 2 and 3 were discussed. Hg²⁺ can be selectively identified in DMF/H₂O (4:1, V/V) solution by HL¹ based on UV-Vis spectrum, and HL² can detect Zn²⁺ in DMSO/H₂O (4:1, V/V) solution by UV-Vis spectrum and the fluorescence spectrum.

  1.   Experimental

  1.1   Material

  4 -hydroxyl coumarin and coumarin from Alfa Aesar were used without further purification. 6-amino-coumarin and 3-amino-4-hydroxy-coumarin were synthesized according to an analogous method reported earlier^[27-28]. The other reagent and solvent were analytical grade from Tianjin Chemical Reagent Factory, and were used without further purification.

  1.2   Instruments and methods

  C, H and N analysis carried out with a GmbH VariuoEL V3.00 automatic elemental analyzer. FT-IR spectra were recorded on a VERTEX70 FT-IR spectro-photometer with samples prepared as KBr (400~4 000 cm^-1) pellets. UV-Vis absorption spectra were recorded on a Hitachi UV -3900 spectrometer. Luminescence spectra in solution were recorded on a Hitachi F-7000 spectrometer. X-ray single crystal structures were determined on a Bruker Smart 1000 CCD area detectors. Melting points were measured by a X-4 microscopic melting point apparatus made by Beijing Taike Instrument Limited Company and were uncorrected.

  1.3   Syntheses of ligands HL¹, HL² and H₂L³

  The synthetic routes of ligands HL¹, HL² and H₂L³ are shown in Scheme 1.

  Scheme 1

  

  Scheme 1.  Synthetic routes of HL¹, HL² and H₂L³

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  1.3.1   Synthesis of HL¹ and HL²

  6-amino coumarin was synthesized depending on an analogous method reported previously in literature^[27]. Firstly, coumarin (1.46 g, 0.010 mol) and sodium nitrate (5.20 g) were placed in a flask, and 5.0 mL of H₂O was added to the flask. The mixture was stirred thoroughly until the solid was completely dissolved. Concentrated sulfuric acid (12.0 mL) was slowly added dropwise into the solution under an ice-water bath, and the temperature was kept below 25 ℃. After stirred at room temperature for 4.0 h, the solution was poured into crushed ice, and a light-yellow solid was precipitated. After filtering, 6-nitrocoumarin was obtained by recrystallization with glacial acetic acid. Secondly, 3.5 mL water, 0.86 g iron powder, 0.3 mL glacial acetic acid and an ethanol solution of 6-nitrocoumarin (0.60 g, 3.14 mmol) was added and refluxed at 95 ℃ for 1.5 h. The solution was cooled slightly, then 20.0 mL warm saturated sodium carbonate solution was added. The mixture was filtered while it was hot. Yellow solid 6-aminocoumarin was obtained by vacuum filtration after the filtrate was cooled. Yield: 67.8%, m.p. 172~173 ℃. In the end, 6-amino coumarin (1.00 g, 9.50 mmol) and 4-diethylamino-salicylaldehyde (1.20 g, 6.01 mmol) were placed in a flask, followed by 5.0 mL anhydrous ethanol and 2~3 drops of glacial acetic acid. The mixture was subjected to reflux at 75 ℃ for 6.0 h and cooled to room temperature, and bright yellow solid precipitates were obtained. HL¹ was obtained by filtering. Yield: 72.5%, m.p. 155~ 156 ℃. Anal. Calcd. for C₁₈H₁₆N₂O₃(%): C, 70.12; H, 5.23; N, 9.09. Found(%): C, 70.21; H, 5.19; N, 9.02.

  Ligand HL² was synthesized by a method analogous to that of HL¹ except substituting 4-diethylamino-salicylaldehyde with 4-methoxy-salicylaldehyde. Yield: 77.4%. m. p. 187~188 ℃. Anal. Calcd. for C₁₇H₁₃NO₄ (%): C, 69.15; H, 4.44; N, 4.74. Found(%): C, 69.19; H, 4.45; N, 4.72.

  1.3.2   Synthesis of H₂L³

  3-amino-4-hydroxy-coumarin (1.23 g, 6.90 mmol) and 4-methoxy-salicylaldehyde (1.00 g, 6.60 mmol) were placed in a flask, then 15.0 mL absolute ethanol and 1~2 drops of acetic acid were added to the flask. The mixture was subjected to reflux at 70 ℃ for 6.0 h. After returning to room temperature, the yellow precipitate was filtered and dried to obtain H₂L³. Yield: 78.6%, m. p. 125~126 ℃. Anal. Calcd. for C₁₇H₁₃NO₅ (%): C, 65.59; H, 4.21; N, 4.50; Found(%): C, 65.60; H, 4.22, N, 4.54.

  1.4   Synthesis of complexes 1, 2 and 3

  1.4.1   Synthesis of complex 1

  A colorless methanol solution (3.0 mL) of zinc(Ⅱ) acetate dihydrate (3.0 mg, 0.014 mmol) was added dropwise to a dichloromethane solution (3.0 mL) of HL¹ (4.6 mg, 0.014 mmol) at room temperature. The mixing solution turned yellow immediately and was kept at room temperature for about one month. Clear light colorless single crystals of complex 1 suitable for X-ray structural determination were obtained. Anal. Calcd. for C₄₀H₃₈N₄O₆Zn(%): C, 65.26; H, 5.20; N, 7.61. Found (%): C, 66.58; H, 5.36; N, 7.85.

  1.4.2   Synthesis of complexes 2 and 3

  The complexes 2 and 3 were prepared by the same method as that of the complex 1 except changing the solvent ratios and metal ions. The solvent ratios of complexes 2 and 3 were adjusted from methanol/dichloromethane (3:3, V/V) to methanol/dichloromethane (5: 5, V/V), and the metal salts were adjusted to the corresponding cobalt(Ⅱ) acetate tetrahydrate and nickel(Ⅱ) acetate tetrachloride. Complex 2: Anal. Calcd. for C₃₄H₂₄N₂O₈Co(%): C, 63.07; H, 3.74; N, 4.33. Found (%): C, 64.80; H, 3.63; N, 4.52. Complex 3: Anal. Calcd. for C₃₈H₃₈N₂Ni₂O₁₄(%): C, 52.82; H, 4.43; N, 3.24. Found(%): C, 53.83; H, 4.32; N, 3.46.

  1.5   Crystal structure determinations of complexes 1, 2 and 3

  The single crystals of dimensions 0.27 mm×0.23 mm×0.21 mm (1), 0.29 mm×0.27 mm×0.25 mm (2) and 0.24 mm×0.22 mm×0.21 mm (3) were used to determine the crystal structures by X-ray diffraction tech-nique on Bruker SMART 1000 CCD area-detector diffractometer. The reflections were collected using graph-ite-monochromatized Mo Kα radiation (λ =0.071 073 nm) at 291, 295 and 293 K, respectively. The SMART and SAINT software packages^[29] were used for data collection and reduction, respectively. Absorption corrections based on multi-scans using the SADABS soft-ware^[30] were applied. The structures were solved by direct methods and refined by full-matrix least-squares against F² using the SHELXL program^[31]. All non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated and isotropic fixed in the final refinement. Details of the crystal parameters, data collection, and refinements for complexes 1~3 are summarized in Table 1.

  Table 1

  

  Table 1.  Crystal data and structure refinement for complexes 1~3

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  Complex	1	2	3
  Empirical formula	C40H36N4O6Zn	C34H24CoN2O8	C38H38N2Ni2O14
  Formula weight	734.10	647.48	864.12
  Crystal system	Monoclinic	Triclinic	Monoclinic
  Space group	C2/c	P1	P21/n
  a/nm	2.571 4(3)	0.931 4(5)	0.967 0(1)
  b/nm	0.982 9(6)	1.231 3(3)	0.791 6(1)
  c/nm	1.380 0(1)	1.302 1(4)	2.398 1(2)
  α/(°)		83.68(6)
  β/(°)	93.90(1)	75.80(6)	99.96(9)
  γ/(°)		82.38(7)
  V/nm3	3.479 9(6)	1.430 2(1)	1.807 9(3)
  Z	4	2	2
  μ/mm-1	0.760	0.659	1.117
  F(000)	1 528.0	666.0	896.0
  θ range/(°)	3.3~26.0	3.3~25.027	3.3~25.027
  Limiting indices	-31 ≤ h ≤ 29,	-9 ≤ h ≤ 11,	-11 ≤ h ≤ 11,
  	-12 ≤ k ≤ 7,	-15 ≤ k ≤ 15,	-9 ≤ k ≤ 5,
  	9 ≤ l ≤ 17	-1 6 ≤ l ≤ 16	-29 ≤ l ≤ 27
  Reflection collected, unique	6 930, 1 485 (Rint=0.057 6)	11 299, 4 680 (Rint=0.052 6)	6 239, 3 193 (Rint=0.135)
  Data, restraint, parameter	3 404, 6, 233	11 299, 0, 409	3 193, 6, 262
  GOF on F2	1.039	0.819	0.951
  R1, wR2 [I > 2σ(I)]	0.086 0, 0.193 6	0.068 9, 0.138 7	0.084 2, 0.118 4
  Largest diff. peak and hole/(e·nm-3)	448 and -299	470 and -440	790 and -410

  CCDC: 2034542, 1; 2034547, 2; 2034548, 3.

  2.   Results and discussion

  2.1   Crystal structures of complexes 1, 2 and 3

  The molecular structure of complexes 1, 2 and 3 are depicted in Fig. 1. Selected bond lengths and angles for complexes 1~3 are listed in Table S1 (Supporting information). X-ray crystallographic analysis shows that complexes 1 and 2 have similar mononuclear crystal structures. The crystals of complexes 1 and 2 are solved as monoclinic space group C2/c and triclinic space group P1, respectively. Complexes 1 and 2 all consist of one central ion (Zn²⁺ for 1 and Co²⁺ for 2) and two units ((L¹)^- for 1 and (L²)^- for 2). The central metal (Zn²⁺ and Co²⁺) of complexes 1 and 2 are all four-coordinated by two deprotonated hydroxyl oxygen atoms (O3, O3A in 1 and O3, O7 in 2) and two nitrogen atoms (N1, N1A in 1 and N1, N2 in 2) from the ligand units ((L¹)^- and (L²)^-, respectively), which constitute the [Zn(L¹)₂] (1) and [Co(L²)₂] (2) moieties, respectively (Fig. 1a~1d). Therefore, the coordination geometry around the central Zn²⁺ and Co²⁺ of complexes 1 and 2 all can be described as a tetrahedron space configurations.

  Figure 1

  

  Figure 1.  Molecular structure of complexes 1 (a), 2 (c) and 3 (e) showing 30% probability displacement ellipsoids for Zn²⁺ (b), Co²⁺ (d), Ni²⁺ (f)

  Symmetry codes: A: -x, y, 0.5-z for 1; B: 1-x, -y, 1-z for complex 3

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  The Schiff base ligand H₂L³ moiety of complex 3 contains phenolate-O atoms that connect two neighboring nickel units by way of two μ-O bridges to form binuclear structures, which crystallizes in monoclinic system P2₁/n space group, and the asymmetry unit consists one Ni²⁺ ion, one (L³)^2- ligand and two coordinated methanol molecules (Fig. 1e). In complex 3, Ni²⁺ center is characterized by a twisted octahedron coordination geometry, with the basal donor atoms coming from the phenolate-O (O4), coumarin's hydroxyl-O (O1), and imine-N (N1) atoms of the Schiff base ligand, the symmetry-related phenolate-O (O4B) and the two coordinated methanol-O (O6, O7) (Fig. 1f). The length of Ni—O and Ni—N falls in a range of 0.200 2~0.216 9 nm and 0.200 4 nm (Table S1), which are within the ranges reported for other similar nickel complexes.

  2.2   IR spectra of HL¹, HL², H₂L³ and their corresponding complexes 1, 2, 3

  The FT-IR spectra of the ligands HL¹, HL² and H₂L³ and their corresponding complexes 1, 2 and 3 exhibited various bands in the 400~4 000 cm^-1 region, and the important FT-IR bands are given in Table 2. The broad O—H group stretching bands at 3 419, 3 442 and 3 439 cm^-1 for the free ligands HL¹, HL² and H₂L³, respectively, disappeared in complexes 1, 2 and 3, which indicates that the oxygen atoms in the phenolic hydroxyl groups are completely deprotonated and coordinated to the metal ion^[32-35]. Whereas, the stretching bands at 3 483 cm^-1 in complex 3 are attributed to the stretching vibrations of the O—H group of coordinated methanol^[36-37]. The Ar—O stretching bands at 1 240, 1 207 and 1 259 cm^-1 of complexes 1, 2 and 3 shifted toward lower frequencies by ca. 13, 15 and 24 cm^-1, respectively, compared with that of free ligands HL¹, HL² and H₂L³ at 1 253, 1 222 and 1 293 cm^-1, respectively. The lower frequency of the Ar—O stretching shift indicates that M—O bond is formed between the metal ions and the oxygen atoms of the phenolic groups^[38-40]. In addition, the free ligand HL¹, HL² and H₂L³ exhibited characteristic band of stretching vibration of C=N group at 1 624, 1 644 and 1 674 cm^-1, respectively, and their corresponding complexes 1, 2 and 3 were observed at 1 611, 1 644 and 1 692 cm^-1, respectively^[41-42]. Compared with ligand HL¹ and HL², the C=N stretching frequency of complexes 1 and 2 shifted to a lower frequency by ca. 13 and 8 cm^-1, respectively. While complex 3 shifted to a higher fre-quency by ca. 18 cm^-1. It is indicated that the C=N bond frequencies decreased or increased due to the coordination bond between the metal atom and the amino nitrogen lone pair^[43-44]. The FT-IR spectrum of complex 1 showed ν(M—N) and ν(M—O) vibration frequencies at 615 and 483 cm^-1 (or 505 and 467 cm^-1 for complex 2, 582 and 459 cm^-1 for complex 3). These assignments are consistent with the frequency values in literature^[44-45].

  Table 2

  

  Table 2.  Important IR bands of free ligands HL¹, HL², H₂L³ and their corresponding complexes 1, 2, 3 cm^-1

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  Compound	n(O—H)	n(C=N)	n(Ar—O)	n(M—N)	n(M—O)
  HL1	3 419	1 624	1 253	—	—
  Complex 1	—	1 611	1 240	615	483
  HL2	3 442	1 644	1 222	—	—
  Complex 2	—	1 636	1 207	485	467
  H2L3	3 439	1 674	1 283	—	—
  Complex 3	3 483	1 692	1 259	482	459

  2.3   UV-Vis absorption spectra analyses

  The absorption spectra of ligands HL¹, HL², H₂L³ and their corresponding complexes 1, 2, 3 in 50 μmol· L^-1 DMSO solution are shown in Fig. 2. As shown in Fig. 2, the typical absorption peaks of HL¹ (HL² or H₂L³) at ca. 276 nm (ca. 285 nm or ca. 282 and 314 nm) was clearly observed. The peaks can be attributed respectively to the π-π* transitions of the benzene ring^[46-47]. Whereas the peak at 382 nm (342 or 411 nm) may be attributed to π-π * transition of the C=N group in the ligand^[48-49]. Upon coordination of Zn²⁺ ion with HL¹ (Co²⁺ ion with HL² or Ni²⁺ ion with H₂L³), the absorption peaks had evidently changed to 277 nm in complex 1 (287 nm in complex 2 or 277 and 312 nm in complex 3). Compared with free ligands HL¹ at 276 nm (285 nm in HL² or 282 and 314 nm in H ₂ L3), the absorption peak was red-shifted by about 1 nm in complex 1 (2 nm in complex 2), and 282 and 314 nm were slightly shifted hypochromic to 277 and 312 nm in complex 3. Additionally, another absorption peak at 382 nm of ligand HL¹ (342 nm of HL² or 411 nm of H₂L³) disappeared in the UV-Vis absorption spectrum of complex 1 (complex 2 or complex 3), which indicates that the imine nitrogen atoms of the ligands HL¹ (HL² or H₂L³) have coordinated to the metal Zn²⁺ (Co²⁺ or Ni²⁺) ions. The new peak at 467 nm of complex 3 is assigned to transition of ligand to metal charge-transfer^[50].

  Figure 2

  

  Figure 2.  UV-Vis spectra of (a) HL¹ and its Zn²⁺ complex 1, (b) HL² and its Co²⁺ complex 2, (c) H₂L³ and its Ni²⁺ complex 3

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  The UV-Vis specific responses of HL¹ or HL² to partial common cations (Ag⁺, Ba²⁺, Ca²⁺, Cd²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Mg²⁺, Mn²⁺, Hg²⁺ and Zn²⁺) were carried out in DMF/H₂O (4:1, V/V) or DMSO/H₂O (4:1, V/V) solutions (c=50 μmol·L^-1). The UV-Vis spectra of HL¹ solution had a significant blue - shift from 383 to 348 nm when Hg²⁺ ion was added, and the other measured ions could not lead to apparent changes of the spectrum, except for Hg²⁺ ion and the self-absorption of Cu²⁺ and Fe³⁺ ions (Fig. 3a). In order to study the variation of absorption peak intensity with time in UV-Vis spectra of HL¹, the UV-Vis absorption intensity was measured at 1 min interval after the addition of Hg²⁺. It can be seen from Fig. 3b that the UV-Vis peak position of HL¹ solu-tion was significantly red-shifted from 383 to 424 nm after adding Hg²⁺. With the passage of time, the peak at 424 nm gradually disappeared and the peak intensity at 348 nm gradually increased, and remained stable after 18 min. As for HL², the UV-Vis spectra of HL² solution had a significant red-shift from 342 to 382 nm when the Zn²⁺ ion was added, and the other tested ions could not lead to obvious changes of spectra except for Zn²⁺ ion and the self-absorption of Cu²⁺ and Fe³⁺ ions (Fig. 4a). In the natural light, the color changes from yellow (or colorless) to colorless (or yellow) after adding Hg²⁺ (or Zn²⁺) to the DMF/H₂O (4:1, V/V) (or DMSO/ H₂O (4:1, V/V)) solution of HL¹ (or HL²), and the color changes of other ions except Fe³⁺ are not obvious, as shown in Fig. 3d and Fig. 4c. The results showed that the HL¹ and HL² represented excellent selectivity for Hg²⁺ and Zn²⁺, respectively.

  Figure 3

  

  Figure 3.  (a) UV-Vis spectra of HL¹ upon addition of different cations (50 eq.) in DMF/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) at room temperature; (b) UV-Vis absorption intensity of HL¹ varying with time; (c) UV-Vis spectra of HL¹ in DMF/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) by adding 0~1.15 eq. of Hg²⁺; (d) Discoloration photos of HL¹ in DMF/H₂O (4:1, V/V) solution after adding metal cations under natural light

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  Figure 4

  

  Figure 4.  UV-Vis spectra of HL² upon addition of different cations (50 eq.) in DMSO/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) at room temperature; (b) UV-Vis spectra of HL² in DMSO/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) by adding 0~0.6 eq. of Zn²⁺, (c) Discoloration photos of HL² in DMSO/H₂O (4:1, V/V) solution after adding metal cations under natural light

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  The identification characteristics of HL¹ (or HL²) as Hg²⁺ (or Zn²⁺) probe were confirmed by UV-Vis titration in DMF/H₂O (4:1, V/V) (or DMSO/H₂O (4:1, V/V)) solution (Fig. 3c and 4b). The absorption band of HL¹ (or HL²) decreased at 383 nm (or 342 nm), and a new absorption band appeared gradually at 348 nm (or 382 nm) when the concentration of Hg²⁺ (or Zn²⁺) increased from 0 to 1.15 (or 0.6) eq., which signs the deproton-ation of hydroxyl moiety and coordination with the Hg²⁺ (or Zn²⁺) ions. The change in absorbance of HL¹ (or HL²) at 348 nm (382 nm) had a linear relationship with Hg²⁺ (or Zn²⁺) ion. In the low concentration range, the detection limit of 7.45 μmol·L^-1 (or 6.10 μmol·L^-1) for Hg²⁺ (or Zn²⁺) was calculated (Fig. S7 and Fig. S8) ^[51-54]. The modified Benesi - Hildebrand formula was used to calculate LOD (limit of detection) and LOQ (limit of quantitation) ^[55] : LOD=3σ/S, LOQ=10σ/S (N=20). As shown in Table S4, we compared probe HL¹ and HL² of our work with the previously reported Hg²⁺ and Zn²⁺ probes. The probes in the table all have lower detection limits and are applied in relatively less toxic solvents. The detection limit of HL¹ (or HL²) is similar with previously reported detection limit, and the obtained LOD value is below the value (76 μmol·L^-1 for Hg²⁺ and 46 μmol·L^-1 for Zn²⁺) approved by World Health Organization (WHO)^[56-57].

  2.4   Emission spectra analyses

  The emission spectra of free ligands HL¹, HL², H₂L³ and its corresponding complexes 1, 2, 3 in 50 μmol·L^-1 DMSO solution at room temperature are shown in Fig. 5. As shown in Fig. 5a and 5b, the free ligand HL¹ (or HL²) exhibited a relatively strong emis-sion peak at ca. 546 nm (or 498 nm) and a weaker emission peak at 458 nm (or 384 nm) upon excitation at 380 nm (or 340 nm), which could be assigned to the intra-ligand π*-π transition^[58-59]. Compared with the free ligand HL¹ (or HL²), the emission intensity of complex 1 (or 2) decreased significantly at 546 nm (or 498 nm), but increased at 458 nm (or 384 nm), indicating that the introduction of M²⁺ ions (Zn²⁺ or Co²⁺) affected the fluorescence properties. These transitions may be related to the coordination of ligand HL¹ or HL² and Zn²⁺ or Co²⁺ ions, which allows the ligand to develop towards a more stable complex^[60]. Both H₂L3 and its complex 3 showed strong fluorescence emission at 482 and 396 nm with excitation at 300 nm, respectively (Fig. 5c). Compared with H₂L³, the fluorescence intensity of complex 3 reduced slightly and blue-shifted ca. 86 nm, which might be related to the coordination of the N and O atom to the metal ions, and allows H₂L3 to develop towards a more stable complex.

  Figure 5

  

  Figure 5.  Emission spectra of (a) HL¹ and complex 1 (λ_ex=380 nm), (b) HL² and complex 2 (λ_ex=340 nm), (c) H₂L3 and complex 3 (λ_ex=300 nm) in DMSO solutions at room temperature

  c=50 μmol·L^-1

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  Under the excitation wavelength of 380 nm, the fluorescence response of HL² to various metal ions (Ba²⁺, Ni²⁺, Pb²⁺, Cu²⁺, Cd²⁺, Co²⁺, Cr³⁺, Mn²⁺, Hg²⁺, Ca²⁺, Sn²⁺ and Zn²⁺) was studied. The DMSO/H₂O (4:1, V/V) solution of HL² (50 μmol·L^-1) was added to 50 eq. of the solution with above metal ions (10 mmol·L^-1). As shown in Fig. 6a, the experimental results showed that when Zn²⁺ ion was added, the fluorescence intensity of HL² solution changed significantly, and a new strong fluorescence emission peak appeared at 464 nm, which may be caused by the π*-π transition in the molecules. After the addition of other metal cations, the fluorescence intensity of HL² solution did not change, which indicates that HL² has a single and efficient recognition of Zn²⁺ ion.

  Figure 6

  

  Figure 6.  Fluorescence emission spectra of HL² upon addition of different cations (50 eq.) in DMSO/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) at room temperature; (b) Fluorescence titration spectra of HL² with increasing concentration of Zn²⁺ (0~0.5 eq.) in DMSO/H₂O (4:1, V/V) solution

  λ_ex=380 nm

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  Fluorescence titration experiments were used to detect the binding affinity of HL² to Zn²⁺ ion. The Zn²⁺ ion solution was gradually added to the DMSO/H₂O (4:1, V/V) solution of HL². With the increase of the amount of Zn²⁺ ion, the fluorescence emission intensity of HL² gradually increased at 464 nm until the amount of Zn²⁺ ion reached 0.5 eq. After adding Zn²⁺, the fluorescence intensity did not change any more, as shown in Fig. 6b. It is worth noting that the change in fluorescence intensity of HL² at 464 nm had a linear relationship with Zn²⁺ ion in a range of 0 ~0.5 eq. The experimental results show that the coordination ratio of HL² to Zn²⁺ is 2:1.

  In addition, the binding constant of Zn²⁺ ion to HL² was determined by Hill's equation to be K_sv=8.91× 104 L·mol^-1 (Fig.S9a). The calculated LOD (2.91 μmol· L^-1) and LOQ (0.808 μmol·L^-1) of HL² for Zn²⁺ ions were calculated from the titration spectra of HL² containing Zn²⁺, as shown in Fig. S9b. The detection limit was also lower than the WHO approved LOD value.

  3.   Conclusions

  Two mononuclear complexes 1, 2 and one binuclear complex 3 based on coumarin Schiff base ligands (HL¹, HL² and H₂L³) were synthesized and structural characterized. The crystal structure analyses of complexes 1 and 2 illustrated that the Zn²⁺ and Co²⁺ are all four-coordinated distorted tetrahedrons, and the Ni²⁺ in complex 3 are six-coordinated distorted octahedrons. Meanwhile, the optical properties of complexes 1, 2 and 3 indicate that the fluorescence and UV-Vis varia-tions of HL¹, HL² and H₂L³ are due to the coordination of Zn²⁺, Co²⁺ and Ni²⁺, respectively. UV-Vis studies demonstrate that the free ligands HL¹ (in DMF/H₂O (4:1, V/V) solution) and HL² (in DMSO/H₂O (4:1, V/V) solution) could selectively recognize Hg²⁺ and Zn²⁺, respectively. The detection limits were calculated to be 7.45 and 6.10 μmol·L^-1, respectively. Fluorescence studies indicate that HL² exhibits selective recognition ability of Zn²⁺ against other common cationic including Ag⁺, Ba²⁺, Ca²⁺, Cd²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Mg²⁺, Mn²⁺ and Hg²⁺ in DMSO/H₂O (4:1, V/V). The sensing capability of HL² for Zn²⁺ against other common cationic with a low limit of 2.91 μmol·L^-1 renders it a candidate probe for Zn²⁺ detection.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Synthetic routes of HL¹, HL² and H₂L³

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  Figure 1  Molecular structure of complexes 1 (a), 2 (c) and 3 (e) showing 30% probability displacement ellipsoids for Zn²⁺ (b), Co²⁺ (d), Ni²⁺ (f)

  Symmetry codes: A: -x, y, 0.5-z for 1; B: 1-x, -y, 1-z for complex 3

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  Figure 2  UV-Vis spectra of (a) HL¹ and its Zn²⁺ complex 1, (b) HL² and its Co²⁺ complex 2, (c) H₂L³ and its Ni²⁺ complex 3

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  Figure 3  (a) UV-Vis spectra of HL¹ upon addition of different cations (50 eq.) in DMF/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) at room temperature; (b) UV-Vis absorption intensity of HL¹ varying with time; (c) UV-Vis spectra of HL¹ in DMF/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) by adding 0~1.15 eq. of Hg²⁺; (d) Discoloration photos of HL¹ in DMF/H₂O (4:1, V/V) solution after adding metal cations under natural light

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  Figure 4  UV-Vis spectra of HL² upon addition of different cations (50 eq.) in DMSO/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) at room temperature; (b) UV-Vis spectra of HL² in DMSO/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) by adding 0~0.6 eq. of Zn²⁺, (c) Discoloration photos of HL² in DMSO/H₂O (4:1, V/V) solution after adding metal cations under natural light

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  Figure 5  Emission spectra of (a) HL¹ and complex 1 (λ_ex=380 nm), (b) HL² and complex 2 (λ_ex=340 nm), (c) H₂L3 and complex 3 (λ_ex=300 nm) in DMSO solutions at room temperature

  c=50 μmol·L^-1

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  Figure 6  Fluorescence emission spectra of HL² upon addition of different cations (50 eq.) in DMSO/H₂O (4:1, V/V) solution (c=50 μmol·L^-1) at room temperature; (b) Fluorescence titration spectra of HL² with increasing concentration of Zn²⁺ (0~0.5 eq.) in DMSO/H₂O (4:1, V/V) solution

  λ_ex=380 nm

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  Table 1.  Crystal data and structure refinement for complexes 1~3

  Complex	1	2	3
  Empirical formula	C40H36N4O6Zn	C34H24CoN2O8	C38H38N2Ni2O14
  Formula weight	734.10	647.48	864.12
  Crystal system	Monoclinic	Triclinic	Monoclinic
  Space group	C2/c	P1	P21/n
  a/nm	2.571 4(3)	0.931 4(5)	0.967 0(1)
  b/nm	0.982 9(6)	1.231 3(3)	0.791 6(1)
  c/nm	1.380 0(1)	1.302 1(4)	2.398 1(2)
  α/(°)		83.68(6)
  β/(°)	93.90(1)	75.80(6)	99.96(9)
  γ/(°)		82.38(7)
  V/nm3	3.479 9(6)	1.430 2(1)	1.807 9(3)
  Z	4	2	2
  μ/mm-1	0.760	0.659	1.117
  F(000)	1 528.0	666.0	896.0
  θ range/(°)	3.3~26.0	3.3~25.027	3.3~25.027
  Limiting indices	-31 ≤ h ≤ 29,	-9 ≤ h ≤ 11,	-11 ≤ h ≤ 11,
  	-12 ≤ k ≤ 7,	-15 ≤ k ≤ 15,	-9 ≤ k ≤ 5,
  	9 ≤ l ≤ 17	-1 6 ≤ l ≤ 16	-29 ≤ l ≤ 27
  Reflection collected, unique	6 930, 1 485 (Rint=0.057 6)	11 299, 4 680 (Rint=0.052 6)	6 239, 3 193 (Rint=0.135)
  Data, restraint, parameter	3 404, 6, 233	11 299, 0, 409	3 193, 6, 262
  GOF on F2	1.039	0.819	0.951
  R1, wR2 [I > 2σ(I)]	0.086 0, 0.193 6	0.068 9, 0.138 7	0.084 2, 0.118 4
  Largest diff. peak and hole/(e·nm-3)	448 and -299	470 and -440	790 and -410

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  Table 2.  Important IR bands of free ligands HL¹, HL², H₂L³ and their corresponding complexes 1, 2, 3 cm^-1

  Compound	n(O—H)	n(C=N)	n(Ar—O)	n(M—N)	n(M—O)
  HL1	3 419	1 624	1 253	—	—
  Complex 1	—	1 611	1 240	615	483
  HL2	3 442	1 644	1 222	—	—
  Complex 2	—	1 636	1 207	485	467
  H2L3	3 439	1 674	1 283	—	—
  Complex 3	3 483	1 692	1 259	482	459

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