English

• 1. INTRODUCTION

  In the past decades, the design of coordination polymers has been well developed, not only owing to their amusing variety of topological structures but also to their potential applications as photoelectric materials, photoluminescent materials, semiconductor, and so on^[1-6]. The design and synthesis functional complexes with special structures and expected properties have always been the focus of scientific researchers. Why organic ligands such as nitrogen-containing heterocyclic and nitrogen-containing heterocyclic carboxylic acids have always been the focus of researches in chemistry and materials sciences? Clearly, these organic ligands have many kinds due to the strong coordination ability of nitrogen/oxygen atoms and various coordination modes, and can be directionally designed with a variety of metal atoms or ions to coordinate compound materials with unique structures, and polynitrogen organic ligands^[7-10].

  Quinolinecarboxylate ligands, with nitrogen and carboxyl oxygen atoms contained in, are easy to coordinate with metal ions. Under different pH conditions and common ligands, the coordination modes of carboxyl oxygen atoms are variable and can exhibit various structures and unique properties^{[11, 12]}. Based on this, we are interested in the crystal engineering of transition metal Zn(Ⅱ) compounds with 3-hydroxy-2-methylquinoline-4-carboxylic acid (HL) and 2, 2΄-bipyridine (bipy) as the mixed ligands. In this article, we report the solvothermal synthesis, X-ray crystal structure, photoluminescent and semiconductor properties, as well as time-dependent density functional theory (TDDFT) calculations for the novel zinc(Ⅱ) complex, [ZnL(bipy)(H₂O)]·H₂O (1, HL = 3-hydroxy-2-methylquinoline-4-carboxylic acid, and bipy = 2, 2΄-bipyridine), which is an isolated mononuclear (0-D) structure.

  2. EXPERIMENTAL

  2.1 General procedure

  The reagents and chemicals for the synthesis of the title compound were of analytical reagent grade, commercially available and applied without further purification. Infrared spectra were obtained with a PE Spectrum-One Fourier transform infrared (FT-IR). Elemental microanalyses of carbon, hydrogen and nitrogen were performed on an Elementar Vario EL elemental analyzer. ¹H NMR spectra were measured on a Bruker Avance 400MHz instrument. Photoluminescence measurements were performed on a F97XP photoluminescence spectrometer. Solid-state UV/Vis reflectance spectroscopy was carried out with a TU1901 UV/Vis spectrometer equipped with an integrating sphere. TDDFT investigations were carried out by means of the Gaussian 09 suite of program packages^[13].

  2.2 Synthesis of 3-hydroxy-2-methylquinoline-4-carboxylic acid (HL)

  The ligand HL was prepared according to the literature^{[14, 15]}, as shown in Scheme 1.

  Scheme 1

  

  Scheme 1.  Synthetic route of ligand HL

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  2.2.1   Synthesis of isatin

  Indigo (131 g, 0.5 mol), K₂Cr₂O₇ (74 g, 1.0 mol) and distilled water (200 mL) were added into three flasks of 500 mL and stirred. After cooling, dilute H₂SO₄ (10%, 250 mL) was added and kept stirring at 43 ℃ for 1.5 h. The mixture was diluted with twice its volume of distilled water, filtered off, dissolved by 10% NaOH solution, filtered again, acidified with 10% HCl to pH = 7 and refiltered. Yield: 116 g (90%); m.p.: 210 ℃; H RMS m/z (ESI) calcd. for C₈H₅NO₂ ([M+H]⁺) 147.0320, found 147.0826.

  2.2.2   Synthesis of HL

  Isatin (73.5 g, 0.5 mol) and NaOH (20 g, 0.5 mol) were dissolved into a sufficient amount of distilled water and filtered. The filtrate and NaOH (20 g, 0.5 mol) were added into chloroacetone (92 g, 1.0 mol), and hydrochloric acid was added dropwise to adjust pH to 7 followed by filtration. Yield: 96 g (95%); m.p. 225 ℃; H RMS m/z (ESI) calcd. for C₁₁H₉NO₃ ([M+H]⁺) 203.0582, found 203.0548. ¹H NMR (400MHz, DMSO) δ 9.15(s, 1H), δ 7.93(d, J = 8.0Hz 1H), δ 7.64(t, J = 8.0 Hz, 1H), δ 7.60~7.52(m, 2H), 2.70(s, 3H).

  2.3 Synthesis of [ZnL(bipy)(H₂O)]⋅(H₂O) (1)

  The title complex (1) was synthesized by mixing HL (0.5 mmol, 101.5 mg), bipy (0.5 mmol, 78 mg), Zn(CH₃COO)₂·2H₂O (0.5 mmol, 209.5 mg) and 10 mL distilled water into a 25 mL Teflon-lined stainless-steel autoclave. The autoclave was heated to 105 ℃ in an oven and kept there for 7 days, then let to cool down to room temperature. Light yellow block crystals were obtained and used to collect the single-crystal X-ray data. Yield: 149.1 mg (65% based on HL). IR (KBr, cm^–1): 3442(vs), 1630(w), 1599(w), 1577(m), 1561(s), 1447(m), 1444(vs), 1351(s), 770(vs), 748(w), 632(w), 497(w). Anal. Calcd. for C₂₁H₁₉N₃O₅Zn: C, 54.98; H, 4.17; N, 9.16. Found: C, 55.02; H, 4.13; N, 9.17%.

  2.4 X-ray structure determination

  The single-crystal data of the title compound were collected on a SuperNova charge-coupled device (CCD) X-ray diffractometer with a suitable single crystal (dimensions of 0.31mm × 0.14mm × 0.12mm). The crystal was kept at 20(2) ℃ during data collection. Using Olex2^[16], the structure was solved with the ShelXT^[17] structure solution program using Intrinsic Phasing and refined with the ShelXL^[18] refinement package using Least-Squares minimisation. All of the non-hydrogen atoms were generated based on the subsequent Fourier difference maps and refined anisotropically. The hydrogen atoms were located theoretically and ride on their parent atoms. Reflections measured are 12116; the final R = 0.0468 for 276 parameters and 4671 observed reflections with I > 2σ(I) and wR = 0.0845, index ranges are –13≤h≤7, –2≤k≤17, –27≤l≤28, S = 1.019, (Δρ)_max = 0.29 and (Δρ)_min = –0.52 e/ų. The selected bond lengths and bond angles for the crystal structure are displayed in Table 1. The hydrogen bonding interactions are presented in Table 2.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(3)	1.9370(2)		Zn(1)–O(4)	2.0034(2)		Zn(1)–O(2)	2.0043(2)
  Zn(1)–N(3)	2.112(2)		Zn(1)–N(2)	2.149(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(3)–Zn(1)–O(4)	117.89(8)		O(3)–Zn(1)–O(2)	90.07(8)		O(3)–Zn(1)–N(3)	131.87(9)
  O(3)–Zn(1)–N(2)	88.45(8)		O(4)–Zn(1)–O(2)	96.35(9)		O(4)–Zn(1)–N(3)	109.46(9)
  O(4)–Zn(1)–N(2)	98.13(9)		O(2)–Zn(1)–N(3)	92.96(9)		O(2)–Zn(1)–N(2)	164.32(9)
  N(3)–Zn(1)–N(2)	76.57(9)

  Table 2

  

  Table 2.  Hydrogen Bond Lengths (Å) and Bond Angles (°)

  DownLoad: CSV

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
  O(4)–H(4)···O(5)	0.88	1.85	2.684(3)	158
  O(5)–H(5)···O(5)ⅰ	0.85	1.92	2.739(3)	162
  O(4)–H(4)···O(1)ⅱ	0.88	1.82	2.682(3)	164
  O(5)–H(5)···N(1)ⅲ	0.85	2.00	2.843(3)	174
  C(7)–H(7)···O(1)	0.93	2.28	2.868(3)	121
  Symmetry codes: ⅰ: 1+x, y, 1–z; ⅱ: 1/2+x, 1/2–y; ⅲ: 1/2+x, y, 3/2–z

  3. RESULTS AND DISCUSSION

  Single-crystal X-ray diffraction measurement revealed that the title compound (1) crystallizes in space group Pbca of the orthorhombic system. In the crystal structure of 1, the metal Zn atom is sitting at the inversion center. The Zn(Ⅱ) ion is pentacoordinated by HL, bipy and water molecule, yielding a rectangular pyramidal geometry (Fig. 1). The bond distances of Zn(1)–O(2), Zn(1)–O(3) and Zn(1)–O(4) are 2.0043(2), 1.9370(2) and 2.0034(2) Å, respectively, while those of Zn(1)–N(2) and Zn(1)–N(3) are 2.149(2) and 2.112(2) Å. These are comparable with that reported in the references^[19-21]. Quinolinecarboxylate (L^–) and bipy act as the bidentate ligand and water molecule as the monodentate ligand coordinated to the zinc metal center, and two such ligands occupy both the axial positions. The intramolecular hydrogen bond can be found between the carbon atom, carboxyl oxygen atoms, coordination water and lattice water molecules (C(7)–H(7)···O(1), O(4)–H(4)···O(5)). Some intermolecular hydrogen bonds like O(5)–H(5)···O(1)^ⅰ existing between the lattice water and the another carboxyl oxygen atom, O(4)– H(4)···O(1)^ⅱ occurring between the coordination water and another carboxyl oxygen atom and O(5)–H(5)···N(1)^ⅲ appearing between the lattice water and another N atom of quinoline moiety interconnect the molecules together to form a three-dimensional supramolecular structure, as presented in Figs. 2 and 3. In the complex, there are strong offset face-to-face π···π stacking interaction between Cg(1)···Cg(4)^ⅰ, Cg(2)···Cg(2)^ⅱ and Cg(3···Cg(3)^ⅲ (Cg(1) are C(6)/C(7)/C(8)/C(9)/C(10)/C(11), Cg(2) are N(3)/C(12)/C(13)/C(14)/C(15)/C(16), Cg(3) are N(2)/C(21)/C(20)/C(19)/C(18)/C(17), Cg(4) are N(1)/C(4)/C(3)/C(2)/C(6)/C(11)) and C(6)~C(11) ring centroids; i = –1/2+x, y, 3/2–x; ii = 1–x, 1–y, 1–z; iii = 2–x, 1–y, 1–z). The centroid-centroid distance of Cg(1)···Cg(4)^ⅰ is 3.810 Å with the shift distance being 1.349 Å and the twist angle of 177.135°. The centroid-centroid distance of Cg(2)···Cg(2)^ⅱ is 3.843 Å with the shift distance to be 1.357 Å and the twist angle being 0°. The centroid-centroid distance of Cg(3)···Cg(3)^ⅲ is 3.841 Å with the shift distance in 1.657 Å and the twist angle of 0°. These π···π stacking interactions yield the two-dimensional supramolecular layers along the ac-axes plane, then complete a crystal packing via van de Waals forces, as presented in Fig. 4.

  Figure 1

  

  Figure 1.  An ORTEP drawing of the title compound with 50% thermal ellipsoids

  DownLoad: Full-Size Img PowerPoint

  Figure 2

  

  Figure 2.  Hydrogen bond diagram of compound 1. Hydrogen atoms not involved in the motif shown were removed for clarity. Symmetric codes: i: –1/2+x, +y, 3/2–x; ii: 1–x, 1–y, 1–z; iii: 2–x, 1–y, 1–z

  DownLoad: Full-Size Img PowerPoint

  Figure 3

  

  Figure 3.  Packing diagram of compound 1 with the dashed lines representing the hydrogen bonding interactions. Symmetric codes: i: 1+x, y, 1–z; ii: 1/2+x, 1/2–y, z; iii: 1/2+x, y, 3/2–z

  DownLoad: Full-Size Img PowerPoint

  Figure 4

  

  Figure 4.  π⋅⋅⋅π stacking interaction diagram of compound 1

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  In order to reveal the potential photoluminescent properties of complex 1, the photoluminescence spectra were determined for solid state samples of 1 at room temperature, and the results are present in Fig. 5. It is obvious that the photoluminescent spectrum of 1 displayed an effective energy absorption in a wavelength range of 250~310 nm. Upon 448 nm emission, the excitation spectrum showed a band at 275 nm. Upon excitation at 275 nm, the emission spectrum was characterized by a sharp band at 448 nm in the blue region. The emission band of complex 1 located in the blue violet light region with the CIE1931 chromaticity coordinate (0.1745, 0.0051) (Fig. 6). As a result, complex 1 is a potential blue photoluminescent material^[22].

  Figure 5

  

  Figure 5.  Solid-state photoluminescence spectra of complex 1 (red curve: excitation; green curve: emission); inset: solid-state photoluminescence spectrum of HL

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  Figure 6

  

  Figure 6.  CIE chromaticity diagram and chromaticity coordinates of the emission spectrum of complex 1

  DownLoad: Full-Size Img PowerPoint

  In order to reveal the nature of the photoluminescence emission of complex 1, we truncated ground state geometry from its single-crystal X-ray diffraction data set without optimization and carried out its theoretical calculation in light of the time-dependent density functional theory (TDDFT) based on this ground state geometry. The TDDFT calculations were performed using the B3LYP function^{[23, 24]} with basis set of SDD for Zn and 6-31G* for C, H, O, N and carried out by means of the Gaussian09 program^[13]. The characteristics of HOMO (the highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) of the title complex 1 are shown in Fig. 7. It is easy to find out that the electron-density distribution of HOMO was totally resided at the coordinating π-orbital of carboxylic acid HL with an energy of –0.14325 Hartree; while, the electron-density population of LUMO locates at π-orbital of ligand bipy and the energy LUMO was calculated to be –0.10334 Hartree. The energy difference between LUMO and HOMO is 0.03991 Hartree, and this is small enough to allow the charge transfer from HOMO to LUMO. In light of this observation, it is proposed that the essence of the photoluminescence of complex 1 could be assigned to the ligand-toligand charge transfer (LLCT; from the HOMO of the π-orbital of ligand HL to the LUMO of the π-orbital of ligand bipy). This calculation result is in good agreement with the experimental observations.

  Figure 7

  

  Figure 7.  HOMO and LUMO of complex 1 with isosurface of 0.003 a.u.

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  To investigate the semiconductive properties of complex 1, the solid-state UV-Vis diffuse reflectance spectra of powder sample of complex 1 were measured at room temperature, using barium sulfate as the reference for 100% reflectivity. After measuring the solid-state diffuse reflectance spectra, the data were treated with the Kubelka-Munk function known as α/S = (1 – R)²/(2R). With regard to this function, the parameter α means the absorption coefficient, S means the scattering coefficient, and R means the reflectance, which is actually wavelength independent when the size of the particle is larger than 5 μm. From the α/S vs. energy gap diagram, we can obtain the value the optical band gap, which can be extrapolated from the linear portion of the absorption edges. The solid-state UV-Vis diffuse reflectance spectrum reveals that complex 1 has a narrow optical energy band gap of 1.83 eV, as shown in Fig. 8. As a result, complex 1 is a possible candidate for narrow band gap semiconductors. The energy band gap of 1.83 eV of complex 1 is obviously larger than those of GaAs (1.4 eV), CdTe (1.5 eV), and CuInS2 (1.55 eV)^{[25, 26]}, which are well known as highly efficient band gap photovoltaic materials.

  Figure 8

  

  Figure 8.  Solid-state UV-Vis diffuse reflectance spectrum of complex 1; inset: the original image of solid-state UV-Vis diffuse reflectance spectrum

  DownLoad: Full-Size Img PowerPoint

  In summary, a novel mononuclear (0-D) zinc (Ⅱ) complex containing 3-hydroxy-2-methylquinoline-4-carboxylic acid and 2, 2΄-bipyridine ligands has been synthesized and characterized by single-crystal X-ray diffraction. Solid-state photoluminescent characterization reveals that it displays an emission in the blue region due to the ligand-to-ligand charge transfer (LLCT; from the HOMO of HL to the LUMO of bipy), as shown by the TDDFT calculation. A narrow optical band gap of 1.83 eV is determined by the solid-state UV-Vis diffuse reflectance spectrum, suggesting that complex 1 is probably a candidate for narrow band gap semiconductors.

  
  
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• 

  Scheme 1  Synthetic route of ligand HL

  下载: 全尺寸图片 幻灯片
  

  Figure 1  An ORTEP drawing of the title compound with 50% thermal ellipsoids

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Hydrogen bond diagram of compound 1. Hydrogen atoms not involved in the motif shown were removed for clarity. Symmetric codes: i: –1/2+x, +y, 3/2–x; ii: 1–x, 1–y, 1–z; iii: 2–x, 1–y, 1–z

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Packing diagram of compound 1 with the dashed lines representing the hydrogen bonding interactions. Symmetric codes: i: 1+x, y, 1–z; ii: 1/2+x, 1/2–y, z; iii: 1/2+x, y, 3/2–z

  下载: 全尺寸图片 幻灯片
  

  Figure 4  π⋅⋅⋅π stacking interaction diagram of compound 1

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  Figure 5  Solid-state photoluminescence spectra of complex 1 (red curve: excitation; green curve: emission); inset: solid-state photoluminescence spectrum of HL

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  Figure 6  CIE chromaticity diagram and chromaticity coordinates of the emission spectrum of complex 1

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  Figure 7  HOMO and LUMO of complex 1 with isosurface of 0.003 a.u.

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  Figure 8  Solid-state UV-Vis diffuse reflectance spectrum of complex 1; inset: the original image of solid-state UV-Vis diffuse reflectance spectrum

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(3)	1.9370(2)		Zn(1)–O(4)	2.0034(2)		Zn(1)–O(2)	2.0043(2)
  Zn(1)–N(3)	2.112(2)		Zn(1)–N(2)	2.149(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(3)–Zn(1)–O(4)	117.89(8)		O(3)–Zn(1)–O(2)	90.07(8)		O(3)–Zn(1)–N(3)	131.87(9)
  O(3)–Zn(1)–N(2)	88.45(8)		O(4)–Zn(1)–O(2)	96.35(9)		O(4)–Zn(1)–N(3)	109.46(9)
  O(4)–Zn(1)–N(2)	98.13(9)		O(2)–Zn(1)–N(3)	92.96(9)		O(2)–Zn(1)–N(2)	164.32(9)
  N(3)–Zn(1)–N(2)	76.57(9)

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  Table 2.  Hydrogen Bond Lengths (Å) and Bond Angles (°)

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
  O(4)–H(4)···O(5)	0.88	1.85	2.684(3)	158
  O(5)–H(5)···O(5)ⅰ	0.85	1.92	2.739(3)	162
  O(4)–H(4)···O(1)ⅱ	0.88	1.82	2.682(3)	164
  O(5)–H(5)···N(1)ⅲ	0.85	2.00	2.843(3)	174
  C(7)–H(7)···O(1)	0.93	2.28	2.868(3)	121
  Symmetry codes: ⅰ: 1+x, y, 1–z; ⅱ: 1/2+x, 1/2–y; ⅲ: 1/2+x, y, 3/2–z

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