LI Ji SHI Lin-Shui YI Xiu-Gung, FANG Xio-Niu GUO Ting LI Yong-Xiu

a (School of Chemistry and Chemical Engineering, Jinggangshan University, Ji’an 343009, China)

b (School of Chemistry, Nanchang University, Nanchang 330000, China)

c (School of Stomatology, Nanchang University; The Key Laboratory of Oral Biomedicine, Nanchang 330000, China)

ABSTRACT A novel zinc complex [ZnL(bipy)(H2O)]?H2O with mixed ligands of 3-hydroxy- 2-methylquinoline-4-carboxylic acid (HL) and bipy (bipy = 2,2?-bipyridine) was synthesized by solvothermal reaction and its crystal structure was determined by single-crystal X-ray diffraction technique. The title compound crystallizes in the orthorhombic system of Pbca space group, and exists as an isolated mononuclear complex. The intermolecular hydrogen bonds and strong π…π stacking interactions form a three-dimensional (3-D) supramolecular network. Solid-state photoluminescence spectrum reveals that it shows an emission in the blue region of the light spectrum. Time-dependent density functional theory (TDDFT) calculations reveal that this emission can be attributed to ligand-to-ligand charge transfer (LLCT). Solid-state diffuse reflectance data show that there is a narrow optical band gap of 1.83 eV.

Keywords: crystal structure, photoluminescence, semiconductor, TDDFT, LLCT;

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 photoelec- tric materials, photoluminescent materials, semicon- ductor, 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-con- taining 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 coor- dination 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 pro- perties[11,12]. Based on this, we are interested in the crystal engineering of transition metal Zn(II) com- pounds with 3-hydroxy-2-methylquinoline-4-car- boxylic 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-hy- droxy-2-methylquinoline-4-carboxylic acid, and bipy = 2,2?-bipyridine), which is an isolated mono- nuclear (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 measure- ments were performed on a F97XP photolumine- scence 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. Synthetic route of ligand HL

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)δ9.15(s, 1H),δ7.93(d,J= 8.0Hz 1H),δ7.64(t,J= 8.0 Hz, 1H),δ7.60～7.52(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 finalR= 0.0468 for 276 parameters and 4671 observed reflections withI＞ 2σ(I) andwR= 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/?3. 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. Selected Bond Lengths (?) and Bond Angles (°)

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

3 RESULTS AND DISCUSSION

Single-crystal X-ray diffraction measurement revealed that the title compound (1) crystallizes in space groupPbcaof 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) ?, respectively, while those of Zn(1)-N(2) and Zn(1)-N(3) are 2.149(2) and 2.112(2) ?. 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)iexisting between the lattice water and the another carboxyl oxygen atom, O(4)- H(4)···O(1)iioccurring between the coordination water and another carboxyl oxygen atom and O(5)-H(5)···N(1)iiiappearing 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-faceπ···πstacking interaction between Cg(1)···Cg(4)i, Cg(2)···Cg(2)iiand 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 ofCg(1)···Cg(4)iis 3.810 ? with the shift distance being 1.349 ? and the twist angle of 177.135°. The centroid-centroid distance of Cg(2)···Cg(2)iiis 3.843 ? with the shift distance to be 1.357 ? and the twist angle being 0°. The centroid-centroid distance of Cg(3)···Cg(3)iiiis 3.841 ? with the shift distance in 1.657 ? and the twist angle of 0°. Theseπ···πstacking interactions yield the two-dimensional supramole- cular layers along theac-axes plane, then complete a crystal packingviavan de Waals forces, as presented in Fig. 4.

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

Fig. 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

Fig. 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

Fig. 4. π???π stacking interaction diagram of compound 1

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].

To investigate the semiconductive properties of complex 1, the solid-state UV-Vis diffuse reflec- tance 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 asα/S= (1 -R)2/(2R). With regard to this function, the parameterαmeans the absorption coefficient,Smeans the scattering coefficient, andRmeans the reflectance, which is actually wavelength independent when the size of the particle is larger than 5μm. From theα/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.

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

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 calcula- tion. A narrow optical band gap of 1.83 eV is determined by the solid-state UV-Vis diffuse reflec- tance spectrum, suggesting that complex 1 is pro- bably a candidate for narrow band gap semicon- ductors.