WANG Wen-Qian CHEN Jun WANG Shuai-Hua WU Shao-Fan②

a (College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou 350117, China) b (Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China) c (School of Ecological Environment and Urban Construction, Fujian University of Technology, Fuzhou 350118, China)

ABSTRACT We have designed and synthesized a new luminescent coordination polymer [Zn2(NO3)(NCP)3(H2O)3]n·2nH2O (1, HNCP = 2-(2-carboxyphenyl) imidazo-[4,5-f]-1,10-phenanthroline) under hydrothermal conditions, which has been structurally characterized by single-crystal X-ray diffraction analyses. 1 crystallizes in monoclinic, space group P21/n, with a = 13.7748(3), b = 19.2651(4), c = 19.9543(4) Å, β = 95.339(2)º, V = 5272.35(19) Å3, C60H39.73N13O13.33Zn2, Mr = 1286.80, Dc = 1.621 g/cm3, Z = 4, μ(MoKa) = 2.118, F(000) = 2629, the final R = 0.0598 and wR = 0.1483. In 1, the organic ligand NCP- displays two different bridging modes to connect adjacent Zn(II) ions into a 1D chain along the c-direction. Photoluminescent analyses reveal that 1 exhibits a strong green emission with a fluorescent lifetime of 5.57 ns. The first-principle calculation results show that the luminescence mainly originates from ligand-centered charge transition.

Keywords: hydrothermal method, crystal structure, luminescence, theoretical calculation;

1 INTRODUCTION

Coordination polymers (CPs) are assembled by metal atoms or metal clusters as connecting nodes, and organic bridging ligands as linkers[1]. The synthesis of new CPs has received considerable attention in recent years. CPs as a new type of structurally diversified and unique characteristic materials in catalysis[2-4], sensing[5], gas storage and separation[6,7]luminescence[8-10], and supercapacitor[11]. Meanwhile, CPs also exhibit even more unexpected applications in luminescent performance, with deeper mechanism research, such as metal-centered charge transfer, metal-to-ligand charge transfer, ligand-centered charge transfer and ligand-to-metal charge transfer[12-14]. Generally, luminescent coordination polymers choose rare-earth metal, but recently, CPs based on nonluminescent zinc ions have attracted particular attention for their low expense and biocompatibility[15-17]. Zn(II) ion as d10metal ion possesses various coordination numbers, and can coordinate with O and N atoms[18-20]. At the same time, structural and functional design is also a research feature of CPs, and large conjugated ligands form excellent fluorescent luminescent CPs[21]. In our research, we chose a large conjugated organic molecule 2-(2-carboxyphenyl) imidazo (4,5-f) (1,10) phenanthroline (HNPC) as the ligand because it possesses richer electronic behaviors and are usually used to construct luminescent compounds[22]. Consequently, we design and synthesize a new Zn(II) coordination polymer [Zn2(NO3)(NCP)3(H2O)3]n·2nH2O by hydrothermal reaction. Pertinent crystal data and structure refinement results for 1 are summarized in Table S1, and selected bond lengths and bond angles are listed in Table S2. Structural analysis shows that NCP-in 1 displays two different bridging fashions (Scheme 1). In addition, the luminescent property of 1 was investigated in the solid state at room temperature. Moreover, the coordina- tion polymer has green fluorescence under 380 nm light excitation, and we use the first-principle calculation to reveal the emitting mechanism of 1.

Scheme 1. Structural formula of HNCP and two bridging modes of NCP- ligand in 1

2 EXPERIMENTAL

2. 1 Materials and instruments

All chemicals and solvents were used without further purification. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku Miniflex 600 diffractometer using CuKα radiation at 40 kV and 40 mA in the range of 5≤2θ≤60°. The FT-IR spectra were obtained on a Perkin- Elmer spectrum in the range of 4000～400 cm-1using KBr disks. The thermogravimetric analysis (TGA) was deter- mined by NETZSCH STA 449F3 Jupiter thermo-gravi- metric analyzer, with the sample heated in an alumina crucible in a nitrogen atmosphere at a heating rate of 10 K/min. Luminescence excitation and emission spectra of the samples were using a 500 W Xenon lamp as excitation source obtained on an Edinburgh FLS1000 spectro- photometer.

2. 2 Synthesis

A mixture of ligand HNCP (15 mg, 0.15 mmol), Zn(NO3)2·6H2O (44.6 mg, 0.15 mmol) and KOH (5.6 mg, 0.1 mmol) in distilled water (10 mL) was sealed into a 25 mL poly(tetrafluoroethylene)-lined stainless-steel con- tainer under autogenous pressure and then heated at 140 °C for 3 days and cooled to 30 °C at a rate of 2.5 °C·h-1. Orange block crystals suitable for X-ray analyses were obtained, washed with distilled ethanol and dried in air. Yield: 30% (based on HNCP) for 1. Anal. Calcd. for C60H39.73N13O13.33Zn2: C, 56.00; H, 3.11; N, 14.15%. Found: C, 56.36; N, 3.13; H, 14.25%. IR (KBr pellet, cm-1): 3515 m, 3415 s, 3075 m, 1615 s, 1556 w, 1510 m, 1375 s, 1247 w, 1141 w, 1083 m, 1029 s, 970 m, 931 m, 735 s, 648 m.

2. 3 Single-crystal structure determination

Single crystals of 1 suitable for X-ray analyses were stuck to a fiberglass. Data collections were performed on a Rigaku Saturn-70 CCD diffractometer at 293 K. All diffractometers were equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The intensity data sets were collected with the ω scan technique and reduced by CrystalClear software. A total of 46809 reflections were collected in the range of 1.74≤θ≤27.50°, of which 11740 were independent (Rint= 0.0756) and 8714 were observed (I > 2σ(I)). The structures were solved by direct methods with the SHELXTL (version 5) crystallographic software package and refined by full-matrix least-squares refinement on F2. Non-hydrogen atoms were located by difference Fourier maps and subjected to anisotropic refinement. The hydrogen atoms of water molecules except those belonging to disordered ones were located in difference Fourier syntheses and refined with O-H distances restrained to a target value of 0.85 Å and Uiso(H) = 1.5Ueq(O), and other hydrogen atoms were added according to theoretical models. The final R = 0.0598, wR = 0.1483 (w = 1/[σ2(Fo2) + (0.0731P)2+ 9.1842P], where P = (Fo2+ 2Fc2)/3), (Δ/σ)max= 0.001, S = 1.112, (Δρ)max= 1.978 and (Δρ)min= -0.879 e·Å-3. Selected bond lengths and bond angles are listed in Table 1.

3 RESULTS AND DISCUSSION

3. 1 Structure description

X-ray crystallographic analysis shows that 1 crystallizes in monoclinic space group P21/n and features a 1D chain structure. Each asymmetric unit contains two crystallo- graphically independent Zn(II) centers (Zn(1) and Zn(2)), three NCP-ligands (labeled as LI, LII, LIII), two coordinated water molecules and three lattice water molecules. The coordination geometry around the Zn(1) ion can be described as a distorted square pyramidal geometry. As illustrated in Fig. 1, one oxygen atom (O(5W)) from water molecule, two nitrogen atoms (N(11A), N(12A)) and another oxygen atom from two symmetry-related of LIligands constitute the equatorial plane, while the axial position is filled by a carboxylate O(21) atom from LII, whereas the Zn(2) center is six-coordinated and resides in a distorted octahedral environment with four nitrogen atoms (N(21), N(22), N(31), N(32)) and two oxygen atoms (O(31B), O(32B)) from two crystallographically independent NCP-ligands (LIIand LIII). The N(22), N(31), O(31B) and O(32B) atoms constitute the equatorial plane, while N(21) and N(32) atoms occupy the apical positions with the axial angle N(21)-Zn(2)-N(32) of 169.36(9)° deviating from the ideal angle of 180°. All the Zn-N and Zn-O distances of 1.808(5)～2.405(11) and 1.765(4)～2.251(2) Å are in the normal ranges, respectively[23,24]. Two different bridging styles of NCP-ligands have been observed in 1: the LIand LIIligands display similar coordination mode: µ2-η2η1, while the LIIligand adopts a µ2-η2η2mode (Scheme 1). The LIand LIIligands connect adjacent Zn(1) and Zn(2) ions into a tetranuclear zinc(II) cluster (Fig. 2(b)). Then, the paratactic LIIIligands, acting as double linkers, bridge neighboring zinc(II) clusters to generate a 1D chain along the c-direction (Fig. 2(c)). As the NCP-ligand owns large conjugate system, π···π interactions exist between adjacent organic ligands in the 1D chains of 1. There exist intrachain hydrogen bonds among the NCP-ligand, two water molecules and NO3-ion with the H-band distance of 2.665(10)～2.897(4) Å (Fig. S1). These chains stack together in an ···ABAB··· fashion along the b axis (Fig. S2).

Table 1. Selected Bond Lengths (Å) and Bond Angles (º) for 1

Fig. 1. Asymmetric unit and coordination geometry of Zn(II) ions in 1. Symmetry codes: A: -x + 1, -y + 1, -z + 1, B: -x + 1, -y + 1, -z + 2. Hydrogen atoms have been omitted for clarity

Fig. 2. (a) Three crystallographically independent NCP- ligands (labeled as LI, LII and LIII). (b) Tetranuclear zinc(II) cluster. (c) 1D chain of 1 along the c-direction. Hydrogen atoms have been omitted for clarity

3. 2 Powder X-ray diffraction and thermal analysis

The experimental powdered X-ray diffraction (PXRD) pattern of 1 agrees well with the simulated one based on the single-crystal X-ray data (Fig. S3), indicating that 1 is in a pure phase. Thermogravimetric analyses (TGA) curves (Fig. S4) show that there is no obvious weight loss below 100 °C. With continuous heating, the weight loss of 2.8% from 100 to 250 °C corresponded to the release of two water molecules. Further heating led to the decomposition of organic ligands, and the loss of other three water molecules.

3. 3 Luminescent properties

In order to decrease the influence of self-aggregation, the luminescence of free ligand HNCP was investigated in degassed dichloromethane at room temperature (Fig. 3(a)). The HNPC shows a broad emission centred at ～400 nm upon the excitation wavelenghth of 270 nm, and the overall emission is located in the blue region, as illustrated by the CIE (CIE = Commission International de l’Echairage) chromaticity coordinates (0.198, 0.147) of the emission spectra in Fig. 3(b). Such emissions observed in the free ligand HNPC can be assigned to the typical ligand-centered transitions. The solid-state luminescence of polymer 1 was also investigated at room temperature. As depicted in Fig. 3(a), Polymer 1 displys a dominant emission band at ～500 nm when excited by 380 nm light. The overall emission is located in the green region with CIE chromaticity coordinate of (0.242, 0.385). Compared with free ligand, emission band of 1 displays significant red-shift, which might be attributed to different charge transfer in HNCP and 1. Moreover, the emission decay time of 1 is 5.57 ns (λem= 500 nm, λex= 380 nm) (Fig. S5), indicating that 1 has fluorescence charac- teristic.

3. 4 Theoretical calculation

To understand better the photoluminescent mechanism, the optimized structure of 1 was theoretically calculated by evaluation of the density of states (DOS). The calculation was performed with the CASTEP code based on density functional theory with a plane-wave expansion of the wave functions. The calculated absorption edge value of 1 is 2.34 eV, which is similar with the measured one (2.77 eV) from optical absorption spectra (Fig. S6). As illustrated in Fig. 4, the top of the valence bands (VBs) mainly results from the p-π orbitals of NCP-and a limited contribution of d orbitals of Zn(II) ions, while the CBs between energy 2 and 4 eV is almost contributed by p-π* antibonding orbitals of the NCP-ligands. Theoretically speaking, ligand-to-ligand (LLCT) and metal-to-lignad charge transfer (MLCT) coexists in the absorption transition of 1. Therefore, the emission peak at ～500 nm should be mainly attributed to the interligand π-π* transition of NCPˉ ligand, in accordance with the other NCP--based complexes[25]. The contribution of MLCT is comparatively limited. Both experimental and theoretical results consistently support the idea that the interligand charge transition in 1 presents a dominating contribution. This may be the reason why the emission band of 1 displays significant red-shift compared with HNCP ligands.

Fig. 3. (a) Emission spectra of 1 (λex = 380 nm) in the solid state and HNCP (λex = 270 nm) in dichloromethane. (b) CIE chromaticity diagram of 1 and HNCP

Fig. 4. Ttotal and partial DOS of 1 with the position of the Ferm level set at 0 eV

4 CONCLUSION

In summary, we have successfully prepared a novel 1D NCP--based Zn(II) coordination polymer [Zn2(NO3)(NCP)3(H2O)3]n·2nH2O (1) using a hydrothermal method. Interestingly, the organic ligand NCP-displays two different bridging modes to connect adjacent Zn(II) ions into a 1D chain along the c-direction. Furthermore, photolumine- scent spectra demonstrate that 1 might be a good candidate for an efficient green emission material.

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