LU Sheng-Nan SHI Feng-Xiang WU Wen-Shi QUAN Zhi-Long

(College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, China)

ABSTRACT Three new metal-organic complexes, namely [Zn(POA)2(H2O)2] (1), [Zn(POA)2(H2O)2]·2H2O (2) and [Zn(POA)2]n (3), have been synthesized by organic ligand 2-carboxylic acid-4-nitropyridine-1-oxide (POA) and zinc(II) ions. The structures of complexes 1～3 are characterized by single-crystal X-ray analysis, XRD powder diffraction analysis, infrared spectroscopy and thermal stability analysis method. Complex 1 belongs to monoclinic system, space group C2/c with a = 22.8215(15), b = 7.5613(16), c = 10.048(3) Å, β = 109.47°, V = 1634.7(6) Å3, Z = 4, F(000) = 944, Dc = 1.900 g/cm3, C12H10N4O12Zn, Mr = 467.61 and μ = 1.584 mm-1. The whole molecule presents “V” shape. Complex 2 is a centrosymmetric structure in triclinic system with space group P1: a = 7.4728(5), b = 7.6825(6), c = 8.5184(6) Å, α = 65.975(2), β = 79.87(2), γ = 89.855(2)°, V = 4384.1(5) Å3, Z = 1, F(000) = 256, Dc = 1.908 g/cm3, C12H14N4O14Zn, Mr = 503.64 and μ = 1.492 mm-1. Complex 3 is a one-dimensional chain structure belonging to monoclinic system and space group P21/c with a = 4.9456(6), b = 12.5322(14), c = 11.2514(13) Å, β = 97.313(11)°, V = 6916.8(14) Å3, Z = 2, F(000) = 432, Dc = 2.072 g/cm3, C12H6N4O10Zn, Mr = 431.58, and μ = 1.852 mm-1. In three complexes, six oxygen atoms from the surrounding coordination atoms form a ZnO6 distorted octahedral coordination geometry around the zinc ions. Meanwhile, fluorescent properties of the three complexes were investigated at room temperature. The fluorescence spectroscopic analysis demonstrated that the ligand POA shows red-shift after coordinating with the zinc(II) ions.

Keywords: 2-carboxylic acid-4-nitropyridine-1-oxide, zinc complexes, crystal structures,

1 INTRODUCTION

In recent years, organic ligands and metal ions self-assembly design and synthesis of complexes with special functions have drawn great attention. These materials have not only obtained various novel structures, but also have been applied to the fields of catalysts[1], gas adsorption and separation[2,3], magnets[4,5], fluorescence[6,7], biological[8], hydrogen storage[9], and so forth. Pyridine is an important aromatic six-membered heterocyclic. The lone pair electrons on the ring that can be combined with protons does not participate in the formation of a conjugated system on the ring. Therefore, pyridine has a certain alkalinity, and is often used as ligands to construct complexes with metal ions. On account of this property, pyridine ligands have a wide range of applications, such as fluorescence materials[10,11], pesticides[12], molecular reservoirs[13], catalysts[14], life sciences[15], and so on.

In the previous work, our group reported the crystal material 2-carboxylic acid-4-nitropyridine- 1-oxide (POA)[16]with the second-order nonlinear optical (NLO). The structure is modified by introducing a substituent to a different position of the pyridine. It is well-known that -COOH is a planar configuration and has a plurality of coordination sites. If the 4-nitropyridine-1-oxide in the 2 position is substituted by -COOH, the molecular coplane will be formed by the -COOH group with 4-nitropyridine- 1-oxide and the steric interaction will be avoided between -COOH and -NO2. Furthermore, it is possible that the molecular conjugation will be enhanced and crystal centrosymmetry will be removed by adding -COOH in the 2-position of 4-nitropyridine-1-oxide. The properties of excited states were calculated by using the time-dependent density functional theory (TDDFT). The Kurtz powder SHG measurement shows that POA exhibits powder SHG efficiency approximately 4.6 and 9.8 times as large as that of KH2PO4(KDP). It is an excellent second-order nonlinear optical material.

As part of this series of research work, we report the synthesis, crystal structure and fluorescence properties of three compounds [Zn(POA)2(H2O)2] (1), [Zn(POA)2(H2O)2]·2H2O (2) and [Zn(POA)2]n(3) with different structures in which POA serves as a ligand with transition metal Zn(II) ions by different methods.

2 EXPERIMENTAL

2. 1 General procedures

All the reagents and chemicals in this work were of analytical reagent grade, commercially obtained and used without further purification. Elemental analyses (C, H and N) were performed on an Elmentar Vario EL elemental analyzer. Powder X-ray diffraction (PXRD) patterns were carried out on a Rigaku MiniFlex 600 using CuKα(λ= 1.54056 Å) at a rate of 5 ℃/min in the range of 5°～50°. IR spectra were measured on a NICOLET iS50 of a Fourier transform infrared spectrometer using KBr pellets with the frequency wavelength range of 4000～400 cm-1. TG measurements were performed under a flow of argon atmosphere from room temperature to 1000 ℃ at a heating rate of 10 ℃/min by using a Shimadzu DTG-60H thermo- gravimetric analyzer. Single-crystal data for the three complexes were collected on an Agilent GeminiE single-crystal diffractometer from Agilent, Poland. The fluore- scence spectra were performed on an Edinburgh FS5 fluorescence spectrometer in the solid state at room temperature.

2. 2 Synthesis of POA

2-Carboxylic acid-4-nitropyridine-1-oxide (POA) was synthesized according to literature[16]. The preparation process is shown in Scheme 1 in 68% yield. m.p.: 148 ℃. Anal. Calcd. for C6H4N2O5(%): C, 39.14; H, 2.17; N, 15.21. Found (%): C, 39.18; H, 2.23; N, 15.15. IR (KBr, cm-1): 3121 (w), 3096 (w), 2655 (w), 1724 (s), 1615 (m), 1540 (m), 1497 (m), 1437 (m), 1352 (s), 1281 (s), 845 (s), 774 (s).

Scheme 1. Preparation of ligand POA

2. 3 Synthesis of 1

Complex 1 was prepared by mixing Zn(Ac)2·2H2O (1 mmol, 219 mg), POA (2 mmol, 368 mg) and 10 mL distilled water into a 25 mL Teflon-lined stainless-steel autoclave. The autoclave was heated at 413 K for 2 days and then powered off.When the autoclave was naturally cooled down to room temperature, orange block-like crystals were obtained and used for the collection of single-crystal X-ray data. The yield was 58% based on POA. Anal. Calcd. for C12H10N4O12Zn (%): C, 30.82; H, 2.14; N, 11.98. Found (%): C, 31.57; H, 2.12; N, 12.08. IR (KBr, cm-1): 3445 (s), 3108 (w), 3076 (w), 1624 (m), 1529 (m), 1459 (m), 1431 (m), 1347 (s), 1262 (s), 857 (s), 840 (s), 749 (s).

2. 4 Synthesis of 2

Zn(Ac)2·2H2O (1.1 mmol, 241 mg) and POA (2.2 mmol, 405 mg) in distilled water (20 mL)were refluxed for 8 h and then cooled to room tempera- ture.The precipitates were filtered and the filtrate was transferred to the flask that was naturally volatilized in the air. Two days later, the yellow block-like crystals were obtained and used for the collection of single-crystal X-ray data. The yield was 45% based on POA. Anal. Calcd. for C12H14N4O14Zn (%): C, 28.61; H, 2.78; N, 11.13. Found (%): C, 28.54; H, 2.61; N, 11.75. IR (KBr, cm-1): 3601 (s), 3508 (m), 3354 (s), 3154 (w), 3092 (w), 1647 (m), 1533 (m), 1463 (m), 1351 (s), 1235 (s), 857 (s), 842 (s), 752 (s).

2. 5 Synthesis of 3

Zn(Ac)2·2H2O (5 mmol, 1095 mg) was dissolved in 15 mL of absolute methanol solution and POA (2 mmol, 368 mg) was dissolved in 10 mL of acetone, then they were mixed and stirred for 3 h. The precipitates were filtered and naturally evaporated for about 3 days to obtain brown columnar-like crystals, and used for the collection of single-crystal X-ray data. The yield was 65% based on POA. Anal. Calcd. for C12H6N4O10Zn (%): C, 33.39; H, 1.39; N, 12.98. Found (%): C, 33.14; H, 1.46; N, 12.57. IR (KBr, cm-1): 3154 (w), 3093 (w), 1655 (m), 1533 (s), 1463 (m), 1355 (s), 1235 (s), 857 (s), 841 (s), 751 (s), 715 (s).

2. 6 X-ray structural determination

The single crystals of complexes 1 (0.03mm × 0.04mm × 0.05mm), 2 (0.15mm × 0.20mm × 0.30mm) and 3 (0.11mm × 0.13mm × 0.14mm) were selected for X-ray diffraction analysis. The data were collected on an Agilent GeminiE single-crystal diffractometer equipped with a graphite-monochro- matic MoKαradiation (λ= 0.71073 Å) by using aφ-ωscan mode at 293(2) K. The empirical absorption was applied to the intensity data. A total of 7766 reflections were collected in the range of 3.38＜θ＜27.47° (-28≤h≤29, -9≤k≤9, -13≤l≤12), of which 1873 were independent (Rint= 0.0277) and 1632 were observed withI＞ 2σ(I) for 1. A total of 2492 reflections were collected in the range of 2.67＜θ＜25.81° (-9≤h≤9, -8≤k≤9, -7≤l≤10), of which 1654 were independent (Rint= 0.0162) and 1602 were observed withI＞ 2σ(I) for 2. A total of 2340 reflections were collected in the range of 3.65＜θ＜25.00° (-5≤h≤5, -9≤k≤14, -13≤l≤8), of which 1207 were independent (Rint= 0.0432) and 1013 were observed withI＞ 2σ(I) for 3. All the structures were solved by direct methods withSHELXTL-97program[17]and refined by full-matrix least-squares method onF2with anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms were theoretically added. For complex 1, the finalR= 0.0321,wR= 0.0913 (w= 1/[σ2(Fo2) + (0.0741P)2+ 0.1407P], whereP= (Fo2+ 2Fc2)/3), (Δ/σ)max= 0.000,S= 0.965, (Δρ)max= 0.625 and (Δρ)min= -0.239 e/Å3. For 2, the finalR= 0.0295,wR= 0.0760 (w= 1/[σ2(Fo2) + (0.0372P)2+ 0.3778P], whereP= (Fo2+ 2Fc2)/3), (Δ/σ)max= 0.000,S= 1.087, (Δρ)max= 0.278 and (Δρ)min= -0.384 e/Å3. For complex 3, the finalR= 0.0409,wR= 0.0980 (w= 1/[σ2(Fo2) + (0.0563P)2+ 0.0000P], whereP= (Fo2+ 2Fc2)/3), (Δ/σ)max= 0.004,S= 1.023, (Δρ)max= 0.654 and (Δρ)min= -0.613 e/Å3. The selected bond distances and bond angles are presented in Tables 1～3, while the hydrogen bonding interactions are shown in Tables 4 and 5.

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

Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for Complex 2

Table 3. Selected Bond Lengths (Å) and Bond Angles (°) for Complex 3

Table 4. Hydrogen Bonding Lengths (Å) and Bond Angles (°) for Complex 1

Table 5. Hydrogen Bonding Lengths (Å) and Bond Angles (°) for Complex 2

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of complex 1

The single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in monoclinic system, space groupC2/cwithZ= 4, and the crystal structure is shown in Fig. 1a. The selected bond lengths and bond angles are listed in Table 1. Complex 1 is composed of one Zn(II) ion, two deprotonated POA ligands, and two coordination water molecules. The entire molecular configuration likes a butterfly, in which two POA ligands are the wings of a butterfly.

The Zn(II) ion is located on theC2symmetry axis andcoordinated with two oxygen atoms O(6) and O(6)iof the two coordination water molecules, respectively. In addition to the carboxyl O(2) and O(2)iof a pair of POA ligands, the oxygen atoms O(1) and O(1)ion the nitrogen of two pyridine rings are coordinated in a chelating form to form a distorted octahedral configuration.

O(1), O(2), O(1)iand O(6) form an octahedral equatorial plane. ∠O(1)-Zn(1)-O(2) = 79.97(6)°, ∠O(2)-Zn(1)-O(1)i= 97.71(6)°, ∠O(1)i-Zn(1)- O(6) = 94.57(7)° and ∠O(6)-Zn(1)-O(1) = 87.48(7)°, with their sum to be 359.73°, which is close to 360°. The distance deviationRmsbetween the O(1), O(2), O(1)i, O(6) and Zn(1) atoms from the least-squares plane is 0.0544 Å. Zn(II) ion is almost coplanar with the equatorial plane. O(2)Iiand O(6)iare located at the two poles of the equator. The bond angle of O(2)i-Zn(1)-O(6)iis 165.58(6)°, deviating from the poles by 14.42°.

The Zn-O bonds of the complex are 2.0426(17) and 2.1375(15) Å, which belong to normal Zn-O bond lengths[18,19]. The Zn(1)-O(2) bond length is 2.1375(15) Å, and O(2) is derived from monodentate chelated acetate.The C(6)-O(2) bond length of carbonyl is 1.26 Å, which is slightly longer than the carbonyl bond length (1.206(4) Å)[16]before coordination. The double bond becomes longer, indicating that the O(2) of the carbonyl is involved in coordination. The bond length of N(1)-O(1) is 1.311(2) Å, which is slightly shorter than the bond length before coordination (1.318(4) Å). The ligand forms a conjugated system with Zn(II) ion. The bond angles of O(2)-Zn(1)-O(2)iand N(2)-Zn(1)-N(2)iare 82.11(9)° and 98.213(29)°, respectively, indicating that the overall configuration of complex 1 is V-shaped.

As shown in Fig. 1b, there is an obvious intermolecular hydrogen bond between the coor- dination water and the carboxyl of POA in the molecule (Table 4).The oxygen atom O(6) of coordination water in molecule I generates hydrogen bond (O(6)∙∙∙O(3)ii0.2733 nm 2.733 Å, symmetry code:ii-x,y-1, -z+1/2) with the oxygen atom O(3)iion the carboxyl in molecule II. These intermolecular hydrogen bonds connect molecules into a one- dimensional chain structure along theb-axis.

Fig. 1. Structure of complex 1: (a) Molecular structure; (b) 1D structure connected by hydrogen bonds. Ellipsoid probability level: 30%. Symmetry codes: i -x, y, -z+1/2, ii -x, y-1, -z+1/2

3. 2 Crystal structure of complex 2

Single-crystal X-ray diffraction analysis reveals that complex 2 crystallizes in triclinic system, space groupP1 withZ= 1, and the crystal structure is shown in Fig. 2a. The selected bond lengths and bond angles are listed in Table 2. The asymmetric unit of complex 2 contains half Zn(II) ion, one deprotonated POA ligand, one coordination water and one crystal water molecules. Different from 1, complex 2 is a symmetrical chair structure. The two deprotonated POA ligands take Zn( Ⅱ) ion as the symmetric center. The dihedral angle of the plane formed by two pyridine rings is 0.00(0.13)°, and the two pyridine rings are parallel to each other. The octahedral configuration of ZnO6is composed of two carboxyl oxygen atoms (O(2), O(2)i) on two deprotonated POA ligands, together with oxygen atoms O(1) and O(1)ion the nitrogen of two pyridine rings and two coordination water molecules. O(1), O(2), O(1)iand O(2)iform the equatorial plane of the octahedron.∠O(1)-Zn(1)-O(2) = 83.63(7)°, ∠O(2)-Zn(1)-O(1)i= 96.37(7)°, ∠O(1)i-Zn(1)- O(2)i= 83.63(6)° and ∠O(2)i-Zn(1)-O(1) = 96.37(6)°, with the sum being 360°. Distance deviationRmsis 0.0000 Å between O(1), O(2), O(1)i, O(2)Iiand Zn(1) atoms from the least-squares plane. Zn(II), O(1), O(2), O(1)Iiand O(2)iatoms are coplanar. O(6) and O(6)iare at the poles of the equator. The bond angles of O(6)i-Zn(1)-O(1), O(6)i-Zn(1)-O(2), O(6)i-Zn(1)-O(1)Iiand O(6)i- Zn(1)-O(2)ibetween the polar axis and the equatorial plane are 91.55(7)°, 87.08(7)°, 88.45(7)° and 92.92(7)°, respectively. This indicates that the polar axis is not completely perpendicular to the equatorial plane, and the octahedron is slightly distorted. The angle between the polar axis and the equatorial plane (∠O(6)-Zn(1)-O(6)i= 180.0°) forms a right angle.

The bond lengths of Zn-O are in a wide range of 2.0406(15)～2.1355(17) Å. O(2) is derived from a monodentate chelated acetate, where the Zn(1)-O(2) bond length is 2.0406(15) Å, which is shorter than the Zn(1)-O(2) bond length of complex 1. Moreover, the bond angles of O(2)-Zn(1)-O(2)iand N(2)- Zn(1)-N(2)iare 180.00(6)° and 180.00°, indicating that complex 2 is a central symmetric structure. The bond length of carbonyl C(6)-O(2) is 1.249(3) Å, which is also slightly longer than uncoordinated (1.206(4) Å)[16]. The N(1)-O(1) bond length is 1.308(2) Å, which is slightly shorter than the bond length before coordination (1.318(4) Å).

As shown in Fig. 2b, complex 2 contains coor- dinated water molecules, crystal water molecules, carboxyl, and nitro, and these cause intermolecular hydrogen bonding of complex 2 (Table 5). The oxygen atoms in coordinated water and the carboxyl oxygen atoms of the neighboring complex molecules (O(6)∙∙∙O(3)ii2.92 Å, symmetry codes:iix,y+1,z) as well as the crystal water oxygen atoms (O(6)∙∙∙O(0AA) 2.768 Å) for hydrogen bonds. O(6) of coordination water generate hydrogen bonds (O(6)∙∙∙O(3)ii2.92 Å, symmetry code:iix,y+1,z) with O(3)iion the carboxyl on adjacent molecule and (O(6)∙∙∙O(0AA) 2.768 Å) with O(0AA) on the crystal water. The oxygen atom in the crystal water acts as a hydrogen donor and two nitroxide atoms in the adjacent complex molecules (O(0AA)∙∙∙O(4)iii3.174 Å, O(0AA)∙∙∙O(5)iii3.31 Å, symmetry codes:iii-x+1, -y, -z+1) and the carboxyl oxygen atoms of adjacent molecules (O(0AA)∙∙∙O(3)iv2.818 Å, symmetry codes:iv-x+2, -y+1, -z) form hydrogen bonds, which are joined to form a two-dimensional network structure.

Fig. 2. Structure of complex 2: (a) molecular structure; (b) 2D structure connected by hydrogen bonds Ellipsoid probability level: 50%. Symmetry codes: i -x+2, -y+1, -z+1; ii x, y+1, z; iii -x+1, -y, -z+1; iv -x+2, -y+1, -z

3. 3 Crystal structure of complex 3

The single-crystal X-ray diffraction analysis shows that complex 3 is a one-dimensional chain coordina- tion polymer based on [ZnⅡ(POA)2]. It crystallizes in monoclinic system with space groupP21/c, and its asymmetric unit is shown in Fig. 3a. The Zn(II) ion coordinates with the surrounding six oxygen atoms to form a ZnO6octahedron. Among them, two carboxyl oxygens (O(2), O(2)ii) from one unit, and O(1) and O(1)ion the nitrogen atom of two pyridine rings constitute an octahedral equatorial plane. The other two carboxyl oxygen atoms (O(3)iii, O(3)iv) from adjacent units are in the axial positions of the octahedron. O(1), O(2), O(1)iiand O(2)iiform the equatorial plane of the octahedron. ∠O(1)-Zn(1)- O(2) = 88.61(9)°, ∠O(2)-Zn(1)-O(1)ii= 91.39(9)°, ∠O(1)ii-Zn(1)-O(2)ii= 88.61(9)° and ∠O(2)ii- Zn(1)-O(1) = 91.39(9)°, and the sum of them is 360°. Distance deviationRmsis 0.0000 Å between O(1), O(2), O(1)iiand O(2)iifrom the least-squares plane. The Zn(II), O(1), O(2), O(1)iiand O(2)iiatoms are coplanar. The angles of O(3)iii-Zn(1)-O(2)ii, O(3)iii-Zn(1)-O(1)ii, O(3)iii-Zn(1)-O(2) and O(3)iii- Zn(1)-O(1) between the polar axis and the equatorial plane are 88.15(9)°, 93.86(9)°, 91.85(9)° and 86.14(9)°, respectively. The polar axis is not perpen- dicular to the equator, but slightly warped. The angle between the polar axis and the equatorial plane (∠O(3)iii-Zn(1)-O(3)iv= 180.0°) forms a right angle.

In the octahedral geometry, the Zn-O bond lengths are between 2.025(2) and 2.190(2) Å. O(2) and O(3)iiiare from two different units of acetate. The Zn(1)-O(2) bond is 2.025(2) Å, which is shorter than Zn(1)-O(2) in complex 2. The bond of Zn(1)-O(3)iiiis 2.190(2) Å and longer than Zn(1)-O(2) of 2, both belonging to the normal Zn-O bond. Moreover, ∠O(2)-Zn(1)-O(2)i= 180.00(12)° and ∠N(2)- Zn(1)-N(2)i= 180.00°, and the monomer also has a central symmetrical structure. The carbonyl C(6)-O(2) bond length is 1.247(4) Å and C(6)-O(3)iiis 1.254(4) Å, which are also slightly longer than the uncoordinated ones (1.206(4) Å)[16].

As shown in Fig. 3b, the Zn(II) ions are connected by two carboxyl oxygen atoms on the deprotonated POA ligands, and the [ZnⅡ(POA)2]2+units are connected along thea-axis into a one-dimensional chain structure. Wherein, the central Zn(II) ion is collinear, and two adjacent pyridine rings of different basic units form two conjugate planes. The dihedral angle of the two conjugate planes is 0.00(0.21)°. The distance between the two conjugate planes is 3.3535 Å. It can be seen that after the formation of the coordination polymer, the deprotonated POA ligands are parallel to each other, and thus the one-dimen- sional chain constitutes a "stepped" linear chain. The three-dimensional stacked structure composed of one-dimensional chains is a parallel structure of each two molecules in the projection direction of thea-axis (Fig. 3c).

Fig. 3. Structure of complex 3: (a) Part of the molecular structure; (b) 1D chain structure; (c) 3D supramolecular structure. Hydrogen atoms are omitted for clarity; Ellipsoid probability level: 50%; Symmetry codes: i x+1, y, z; ii -x, -y+1, -z; iii x-1, y, z; iv -x+1, -y+1, -z

3. 4 Thermal analysis and PXRD patterns

Thermogravimetric analyses (TGA) were carried out under Ar atmosphere to examine the thermal stability of complexes 1～3 (Fig. 4). The TG curve of complex 1 indicates that the first weight loss of 8.13% from 135 to 171 ℃ corresponds to the removal of two coordinate water molecules (Calcd. 7.70%). The decomposition of the residue occurs in the range of 182～540 ℃, and the curve is close to flat. The remaining weight of 17.58% should be the final product ZnO (Calcd. 17.41%). The TG curve of complex 2 is also mainly divided into two segments. The first weight loss of 13.91% occurs in the range of 102～151 ℃, indicating the removal of two crystal water molecules and two coordination water molecules (Calcd. 14.30%). Then the TG curve dropped rapidly with obvious weight loss, and became stable until 542 ℃. The final residue may be ZnO (Obsd. 15.55%, Calcd. 16.16%). The TG curve of complex 3 indicates that the weight loss of 82.01% takes place between 145 and 603 ℃ due to the collapse of the one-dimensional chain skeleton. Above 603 ℃, the curve tends to be flat. The sample is decomposed completely, and the final residue may be ZnO (Obsd. 17.73%, Calcd. 18.86%).

Fig. 4. TG curves of complexes 1～3

X-ray power diffraction (PXRD) patterns of 1～3 were recorded at room temperature. As shown in Fig. 5, the peak positions of simulated and experimental patterns are in good agreement with each other, suggesting the phase purity of the products. The differences in intensity may be due to the preferred orientation of the crystalline powder samples.

Fig. 5. PXRD patterns of complexes 1～3

3. 5 Fluorescence properties

The solid fluorescence spectra of 1～3 at room temperature are shown in Fig. 6. POA displays one strong peak at maximum 340 nm under excitation at 237 nm.Using a wavelength of 300 nm as the excitation light, complexes 1, 2 and 3 show maximum fluorescence emission peaks at 446, 469 and 456 nm, respectively. The wavelengths of the maximum emission peaks of the three complexes are significantly red-shifted (106, 129, 116 nm, respectively) compared to POA.This may be the carbonyl bond length on the carboxyl becoming shorter after the ligand forms complexes with the Zn(II) ions, and the N←O bond becomes shorter due to the resonance donating effect.The complexes form a conjugated system, and the delocalized electron increases. On the other hand, the ligand-to-metal-change-transfer (LMCT) occurs[20,21], and the rigidity of the complex is enhanced, the rotation of the molecule is limited, and the energy loss is reduced, thereby making the emission band obviously red-shifted.

Fig. 6. Fluorescence emission spectra of the ligand and complexes 1～3 (POA: λex = 237 nm; 1, 2 and 3: λex = 300 nm)

4 CONCLUSION

In this paper, 2-carboxylic acid-4-nitropyridine- 1-oxide(POA) was self-assembled with Zn(II) ions by different assembly methods to obtain three com- plexes with different structures: [Zn(POA)2(H2O)2] (1), [Zn(POA)2(H2O)2]·2H2O (2) and [Zn(POA)2]n(3). The structures of complexes 1～3 are determined by single-crystal X-ray analysis, XRD powder diffraction analysis, infrared spectroscopy and thermal stability analysis method. The fluorescent properties of the three complexes were investigated at room temperature. Fluorescence analysis showed that the complexes formed conjugated system and the ligand-to-metal-change-transfer (LMCT) occurred. The coordination of POA and Zn(II) ions have a large red shift (106, 129, 116 nm, respectively). These three complexes have good fluorescence properties and will become potential fluorescent materials.