Self-assembly of zinc polymers based on a flexible linear ligand at different pH values: Syntheses, structures and fluorescent properties

Solid State Sciences 11 (2009) 635–642

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Self-assembly of zinc polymers based on a flexible linear ligand at different pH values: Syntheses, structures and fluorescent properties Yan-Hong Xu, Ya-Qian Lan, Xin-Long Wang, Hong-Ying Zang, Kui-Zhan Shao, Yi Liao, Zhong-Min Su* Department of Chemistry, Northeast Normal University, Changchun 130024, Jilin, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 August 2008 Received in revised form 10 October 2008 Accepted 15 October 2008 Available online 29 October 2008

Five novel coordination polymers, [Zn(imbz)2]n (1), {[Zn(imbz)2]$H2O}n (2), [Zn(imbz)(m2-OH)]n (3), [Zn3(imbt)2(p-bdc)3]n (4), [Zn4(m3-OH)2(imbt)2(p-bdc)3]n (5), (imbt ¼ 40 -(imidazol-1-ylmethyl)benzonitrile, imbz ¼ 40 -(imidazol-1-ylmethyl)benzoate and p-bdc ¼ terephthalic acid) have been hydrothermally prepared through systematically changing the pH values of reaction mixture, and structurally characterized by elemental analysis, IR spectroscopy and single-crystal X-ray crystallography. Compounds 1 and 2 exhibit similar 2D (4,4) grid structures, whereas compound 2 contains a righthanded helix along b-axis. Compound 3 has a distorted diamond framework which was constructed via imbz ligands and m2-OH groups linking metal atoms. Compound 4 shows a 2D 6-connected network with trinuclear zinc clusters as secondary building units (SBUs), whereas 5 shows a distorted a-Po with tetranuclear zinc clusters as SBUs, in which p-bdc ligands act as bridges. Moreover, compounds 1–5 all exhibit strong blue photoluminescence in the solid state at room temperature. Ó 2008 Elsevier Masson SAS. All rights reserved.

Keywords: Hydrothermal reaction Linear ligand 40 -(Imidazol-1-ylmethyl)benzonitrile pH-Dependent Fluorescent property

1. Introduction The rational design and synthesis of multidimensional coordination polymers based on the interaction of metal ions with bridging organic ligands, have attracted considerable attention in recent years, not only because of their intriguing variety of architectures and topologies, but also because of their potential applications in catalysis, nonlinear optics, luminescent materials and porous materials [1–8]. The selection of appropriate ligands as ‘‘building blocks’’ plays a crucial role in manipulating the network structure of the coordination polymers. The design of coordination polymers is highly influenced by several factors such as the coordination properties of the metal ions, the functionality, length and flexibility/rigidity of the ligands, the nature of the counterions, pH value of solution and the solvent systems and so on [9–15]. Up to now, a variety of one-, two- and three-dimensional frameworks have been obtained using rigid bridging ligands such as 4,40 -bipyridine and its analogues [16–21]. These rigid polyfunctional ligands are well-known to form grids, rods, bricks, honeycombs, diamondoid nets, and other noteworthy species [22–25]. Contrary to the rigid ligands, the functional flexible linear ligands with conformational flexibility may facilitate the

* Corresponding author. Northeast Normal University, Department of Chemistry, No. 5268 Renmin Street, Changchun 130024, Jilin, China. E-mail address: [email protected] (Z.-M. Su). 1293-2558/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.10.008

formation of helixes and some novel formation of supramolecular isomers [26–30]. N- or O-donor molecules play important roles in crystal engineering because they are excellent donors for coordinating to metal ions. For example, imidazoles and their derivatives as neutral Ndonors have gained increasing importance for the preparation of metal compounds, biological systems and metalloproteins [31,32]. Moreover, it is well-known that carboxylate ligands play vital roles to construct novel MOFs in coordination chemistry and can adopt various binding modes such as terminal monodentate, chelating to one metal center, bridging bidentate in a syn–syn, syn–anti and anti– anti configurations to two metal centers, and bridging tridentate to two metal centers [33–36]. However, there are still only rare reports of ligands based on imidazole and carboxylate groups as building blocks for the construction of MOFs [37–39]. On the other hand, Zn(II) metal compounds have attracted extensive interest in recent years because they not only exhibit appealing structures but also possess photoluminescent properties [40]. Based on the above considerations, we chose and synthesized a flexible ligand 40 -(imidazol-1-ylmethyl)benzonitrile (imbt), in which the –CN groups can be hydrolyzed into carboxylates (Scheme 1), as organic ligand to construct novel coordination polymers. As expected, we obtained five new coordination polymers [Zn(imbz)2]n (1), {[Zn(imbz)2]$H2O}n (2), [Zn(imbz)(m2-OH)]n (3), [Zn3(imbt)2(p-bdc)3]n (4), [Zn4(m3-OH)2(imbt)2(p-bdc)3]n (5). Herein we report the syntheses, structures and fluorescent properties of five compounds.

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Scheme 1. Schematic view from imbt to imbz.

2. Experimental section

2.2. Syntheses of compounds

2.1. Materials and general methods

[Zn(imbz)2]n (1): a mixture of Zn(OAc)2$2H2O (0.10 mmol) and imbt (0.20 mmol) was added to 10 mL of distilled water at room temperature. When the pH value of the mixture was adjusted to about 7 with NaOH solution (1 M), the mixture was transferred into a 25 mL Teflon-lined stainless steel vessel and heated to 150  C for three days, then cooled to room temperature at a rate of 5  C/h. The colorless block crystals of compound 1 were obtained in ca. 65% (based on Zn). Anal. Calcd for C22H18N4O4Zn: C, 56.43; H, 0.21; N, 11.97%. Found: C, 56.48; H, 0.24; N, 12.01%. Selected IR data (KBr, cm1): 3739s, 3100s, 3030s, 2740s, 1675s, 1645s, 1598s, 1550s, 1515s, 1417s, 1398s, 1368s, 1293s, 1089s, 949s, 745s, 649s, 421s. {[Zn(imbz)2]$H2O}n (2): the same synthetic procedure as for 1 was used except that the pH value of the mixture was adjusted to

All reagents were purchased commercially and used without further purification. Solvents were purified according to the standard methods. Imbt was synthesized readily by the procedure reported in the literature [41]. FT-IR spectra were recorded from KBr pellets in the range of 4000–400 cm1 on a Mattson AlphaCentauri spectrometer. Elemental analyses of carbon, hydrogen and nitrogen were carried out with a Carlo Erba 1106 elemental analyzer. Thermal gravimetric (TG) analyses were performed on a Perkin–Elmer TGA instrument in flowing N2 with a heating rate of 10  C/min. The emission/excitation spectra were recorded on a Varian Cary Eclipse spectrometer.

Fig. 1. (a) Coordination environment of Zn(II) in compound 1 with the ellipsoids drawn at the 50% probability level; symmetry code: (A) x þ 1/2, y  1/2, z; (B) x þ 1/2, y þ 1/2, z; hydrogen atoms were omitted for clarity. (b) Two-dimension (4,4) grid of 1. Red, O; blue, N; gray, C; and pink, Zn. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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about 8. The colorless block crystals of compound 2 were obtained in ca. 77% (based on Zn). Anal. Calcd for C22H20N4O5Zn: C, 42.37; H, 5.68; N, 14.47%. Found: C, 42.51; H, 5.89; N, 14.82%. Selected IR data (KBr, cm1): 3861s, 3741s, 3623s, 3153s, 1677s, 1616s, 1550s, 1515s, 1386s, 1086s, 746s, 650s. [Zn(imbz)(m2-OH)]n (3): the same synthetic procedure as for 1 was used except that the pH value of the mixture was adjusted to about 9.5. The colorless block crystals of compound 3 were obtained in ca. 68% (based on Zn). Anal. Calcd for C11H9N2O3Zn: C, 46.38; H, 3.86; N, 9.83%. Found: C, 46.43; H, 3.89; N, 9.87%. Selected IR data (KBr, cm1): 3742s, 3371s, 3100s, 2740s, 1598s, 1558s, 1385s, 1257s, 1090s, 950s, 770s, 663s,594s, 549s,468s. [Zn3(imbt)2(p-bdc)3]n (4): a mixture of Zn(OAc)2$2H2O (0.15 mmol), p-bdc (0.15 mmol) and imbt (0.10 mmol) was added to 10 mL of distilled water at room temperature. When the pH value of the mixture was adjusted to about 8.0 with NaOH solution (1 M), the mixture was transferred into a 25 mL Teflon-lined stainless steel vessel and heated to 130  C for three days, and then cooled to room temperature at a rate of 5  C/h. The colorless block crystals of compound 4 were obtained in ca. 82% (based on Zn). Anal. Calcd for C46H30N6O12Zn3: C, 47.91; H, 2.61; N, 7.29%. Found: C, 47.94; H, 2.65; N, 7.32%. Selected IR data (KBr, cm1): 3844s, 3741s, 3678s, 3622s, 2215s, 2106s, 1740s, 1692s, 1645s, 1547s, 1515s, 1464s, 673s.

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[Zn4(m3-OH)2(imbt)2(p-bdc)3]n (5): the same synthetic procedure as for 4 was used except that the pH value of the mixture was adjusted to about 9.5. The colorless block crystals of compound 5 were obtained in ca. 73% (based on Zn). Anal. Calcd for C46H30N6O14Zn4: C, 47.91; H, 2.61; N, 7.29%. Found: C, 47.94; H, 2.65; N, 7.32%. Selected IR data (KBr, cm1): 3739s, 3599s, 3391s, 2234s, 2158s, 1570s, 1540s, 1367s, 920s, 782s, 744s, 654s, 518s, 448s. 2.3. Crystal structure determinations Single-crystal X-ray diffraction data for compounds 1–5 were collected on a Bruker ApexII CCD diffractometer with graphitemonochromated Mo Ka radiation (l ¼ 0.71069 Å) at 293 K. The structures were solved with direct methods and refined with fullmatrix least-squares (SHELX-97) [42]. For the compounds, all the hydrogen atoms attached to carbon atoms were generated geometrically, while the hydrogen atoms attached to water molecules were not located but were included in the structure factor calculations. Anisotropic thermal parameters were used to refine all non-hydrogen atoms. The crystal data and structure refinements of compounds 1–5 are summarized in Table S1 (Supporting information). Selected bond lengths and angles of the five compounds are listed in Table S2 (Supporting information).

Fig. 2. (a) Coordination environment of Zn(II) atoms in 2 with the ellipsoids drawn at the 50% probability level, Symmetry code: (A) x þ 1, y þ 1/2, z þ 1/2; (B) x  1, y  1/2, z þ 1/2, hydrogen atoms were omitted for clarity. (b) Two-dimension (4,4) grid of compound 2 and the right-handed helical chain (green). Red, O; blue, N; gray, C; and pink, Zn. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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3. Results and discussion 3.1. Description of the structures [Zn(imbz)2]n (1): a single-crystal X-ray diffraction study reveals that 1 crystallized in the monoclinic space group Cc. As shown in Fig. 1a, the structure of 1 contains one kind of Zn(II) cation and a imbz ligand. Zn(II) ion shows a four-coordinated tetrahedral geometry which is finished by two nitrogen atoms (Zn1– N2A ¼ 2.027(5) and Zn1–N4B ¼ 2.013(5) Å) from two different imbz ligands and two oxygen atoms (Zn1–O1 ¼1.954(4) and Zn1– O3 ¼ 1.957(5) Å) from two distinct imbz ligands. The Zn–O and Zn–N bond lengths are in the normal range [43]. The bond angles range from 98.5(2) to 124.7(3) . The structure of 1 exhibits a 2D (4,4) network when taking the zinc(II) atoms as 4-connected nodes and the imbz ligands as linkers (Fig. 1b). Four zinc(II) atoms and four imbz ligands form a 44-membered rhombus ring, and such rhombus ring repeats to give an infinite 2D network. In the Zn4 rhombus, the distances between two adjacent zinc(II) atoms linked by imbz ligand are all 11.198(5) Å, and the diagonals of the Zn4 rhombus are 13.516(2) Å and 17.857(1) Å, respectively. Moreover, imidazole ring and benzene ring of one imbz ligand are not in the same plane due to the presence of –CH2– spacer, which has the dihedral angle of 70.95 . {[Zn(imbz)2]$H2O}n (2): compound 2 crystallized in the orthorhombic chiral space group P212121 and exhibits a 2D layer

coordination polymer. The coordination environment of metal center is depicted in Fig. 2a. Each Zn(II) cation is coordinated by two oxygen atoms (Zn1–O1 ¼ 2.002(3) and Zn1–O4 ¼ 1.981(3) Å) from two different imbz ligands, and two nitrogen atoms (Zn1– N1B ¼ 2.033(3) and Zn1–N3A ¼ 2.029(3) Å) from two distinct imbz ligands to construct a distorted tetrahedral geometry with the bond angles ranging from 99.19(13) to 122.19(14) . Compound 2 also exhibits a 2D (4,4) grid similar to that of compound 1, and the Zn/Zn distances linked by imbz ligand are all 11.748(29) Å, and the diagonals of the Zn4 rhombus are 14.914(5) Å and 18.216(10) Å, which are slightly longer than that in compound 1, respectively (Fig. 2b). Furthermore, the dihedral angle between imidazole ring and benzene ring of one imbz ligand is 89.67, which is 13.90 larger than that in compound 1. Interestingly, 2 contains a right-handed helical chain along the crystallographic b-axis with the pitch of 14.914(5) Å (green). Such a (4,4) grid is similar to the previously reported compounds {[CoII(imbz)2]$H2O}n [37] and [NiII(imbz)2(H2O)]n [38]. [Zn(imbz)(m2-OH)]n (3): a single-crystal X-ray diffraction study reveals that compound 3 crystallized in the orthorhombic chiral space group P212121. The coordination environment of metal center is depicted in Fig. 3a. Each Zn(II) cation is coordinated by one oxygen atom (Zn1–O1 ¼ 2.002(3) Å) and one nitrogen atom (Zn1– N1B ¼ 2.033(3) Å) from two different imbz ligands, and two m2-OH groups (Zn1–O3 ¼ 1.892(5) and Zn1–O3A ¼ 1.942(6) Å) to construct a distorted tetrahedral geometry with the bond angles varying from 99.3(2) to 120.4(2) .

Fig. 3. (a) Coordination environment of Zn(II) in compound 3 with the ellipsoids drawn at the 50% probability level; symmetry code: (A) 0.5 þ x, 0.5  y, z; (B) 2  x, 0.5 þ y, 0.5  z; hydrogen atoms were omitted for clarity. (b) Three-dimensional grid of 3 and the right-handed helical structure. Red, O; blue, N; gray, C; and pink, Zn. (c) Schematic view of the distorted diamond topology. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Two adjacent zinc atoms are interlinked by the m2-OH group to generate a dinuclear cluster with the Zn/Zn distance is 3.568(9) Å, and the dinuclear clusters are further linked by imbz ligands to form a 3D network, and the Zn/$Zn distances by imbz ligand are all 10.949(19) Å. The dihedral angle between imidazole ring and benzene ring of one imbz ligand is 85.54 , which is slightly smaller than that in compound 2. It is worth noting that the right-handed helical chain in 3 along a-axis with the pitch of 5.744(5) Å is different from 2, which consists of m2-OH groups and Zn(II) atoms (Fig. 3b). A better insight into the nature of the framework can be achieved by the application of topological approach. Each Zn(II) cation links two imbz and two m2-OH groups to form a fourconnected node, while each imbz ligand connects two Zn(II)

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cations acting as a connector. So the whole framework can be simplified to a distorted 3D diamond topology as shown in Fig. 3c. [Zn3(imbt)2(p-bdc)3]n (4): X-ray crystallography reveals that the compound 4 features a 2D framework, consisting of centrosymmetric and linear trinuclear Zn(II) building subunits. The intercluster separation of Zn1/Zn2 is 3.209(6) Å. As shown in Fig. 4a, the three zinc atoms are joined together by six carboxylate groups from six p-bdc ligands. There are two independent Zn atoms represented herein, exhibiting two different coordination geometries. The Zn1 atom lies at a symmetry center and coordinates to six carboxylate oxygen atoms (Zn1–O ¼ 2.042(2)–2.214(3) Å) from six different p-bdc ligands to form a distorted octahedral geometry with the bond angles ranging from 84.25(10) to 180.00(9) ;

Fig. 4. (a) Perspective views of the Zn(II) coordination environments; symmetry code: (A) x, y þ 1, 2  z; (B) 0.5  x, 0.5 þ y, 1.5  z; C: 0.5 þ x, 0.5  y, 0.5 þ z; hydrogen atoms were omitted for clarity. (b) Schematic view of the 2D six-connected network of (36.46.53) topology.

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whereas Zn2 exists in a highly distorted tetrahedral geometry, being ligated by three carboxylate oxygen atoms (Zn2– O ¼ 1.965(3)–1.974(3) Å) from three different p-bdc ligands and one imbz nitrogen atom (Zn2–N1 ¼1.976(3) Å), in which the bond angles range from 99.28(12) to 132.29(13) . Additionally, the p-bdc ligand exhibits two coordination modes (Supporting information, chart S1 (a) and (b)). From the topological point of view, the trinuclear zinc cluster, which links eight organic ligands (six p-bdc and two imbz ligands), is regarded as a six-connected node, the pbdc ligand is regarded as a linker, and the overall framework becomes a 2D six-connected network of (36.46.53) topology (Fig. 4b). [Zn4(m3-OH)2(imbt)2(p-bdc)3]n (5): a single-crystal X-ray diffraction study reveals that the structure of 5 contains tetranuclear zinc clusters, in which each m3-OH group interlinks three crystallographically unique zinc atoms, with non-bonding Zn/Zn distances of 3.043(1)–3.468(14) Å. Three independent Zn atoms represented herein, exhibit two different coordination geometries, as shown in Fig. 5a. The Zn1 atom is coordinated by two carboxylate oxygen

atoms (Zn1–O5 ¼ 1.922(2) and Zn1–O5A ¼ 1.922(2) Å) from two different p-bdc ligands and two m3-OH groups (Zn1–O7 ¼ 2.008(2) and Zn1–O7A ¼ 2.008(2) Å) to construct a distorted tetrahedral environment with the bond angles ranging from 89.18(12) to 120.13(9) ; whereas Zn2 exists in a distorted octahedral geometry, being ligated by four carboxylate oxygen atoms (Zn2–O ¼ 1.977(2)– 2.203(2) Å) from four different p-bdc ligands and two m3-OH groups (Zn2–O7 ¼ 2.142(2) and Zn2–O7A ¼ 2.142(2) Å), in which the bond angles are in the range from 82.32(11) to 175.39(9) ; and Zn3 is coordinated by two carboxylate oxygen atoms (Zn3–O1 ¼ 2.068(2) and Zn3–O3 ¼ 1.933(2) Å) from two distinct p-bdc ligands, one imbz nitrogen atom (Zn3–N2 ¼ 1.979(2) Å) and a m3-OH group (Zn3–O7 ¼ 1.963(2) Å) to complete a distorted tetrahedral sphere with the bond angles varying from 96.63(9) to 129.13(10) . Obviously, the average bond lengths around Zn1 and Zn3 are somewhat shorter than those of Zn2, which is consistent with the fact that bond lengths in a tetrahedral geometry are generally shorter than those in other geometries, which is comparable to the previously reported in the literature [44]. The ligand of p-bdc exhibits one coordination

Fig. 5. (a) Perspective views of the Zn(II) coordination environments, symmetry code: (A) x, y, z þ 1/2, hydrogen atoms were omitted for clarity. (b) Schematic illustrating of the distorted a-Po topology.

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mode (Supporting information, chart S1 (a)). As a result, each tetranuclear metal clusters is surrounded by eight organic ligands (six bridging p-bdc and two coordinated imbt). When we take the tetranuclear zinc cluster as a single node, this SBU becomes a sixconnected node, and the p-bdc ligand is regarded as a linker. As shown in Fig. 5b, the network can be simplified to a distorted 3D a-Po topology. 3.2. Influence of the pH values on the structures of compounds 1–5 It has been reported that synthetic conditions, such as pH value, reaction temperature, solvent, etc., are the key factors in assembling MOFs [45,46]. Compounds 1–3 were synthesized under the similar reaction conditions except for the differences of the reaction pH. As a result, two 2D compounds 1 and 2 were obtained at lower pH values. Whereas, compound 3, obtained at a relatively high pH value, shows a distorted 3D diamond-like framework with each Zn(II) center bonded by two m2-OH groups and two imbz ligands. A similar effect has been observed in compounds 4 and 5. At lower pH value, 4 shows a 2D six-connected sheet. At higher pH value, 5 was formed by m3-OH groups and bridge ligand, which exhibits a distorted 3D a-Po topology. It is noted that the –CN groups of imbt do not be hydrolyzed into carboxylate in compounds 4 and 5, and it may be attributed to the temperature of reaction system which is lower than that in compounds 1–3. Moreover, the p-bdc ligands adopt bridging bidentate and bridging monodentate modes to two metal centers in 4, while it only adopts bis-bidentate binding fashion in 5. Their structures vary from two-dimensional to threedimensional coordination polymers with an increase of the pH values. Investigation on their structural differences reveals that high pH value of the reaction facilitates the formation of m-OH group and construction of high-dimensional coordination polymer. It is clear that the pH value of the reaction system is the key factor in influencing the structures and topologies of these compounds, which indicates that the assembly process is pH-dependent. 3.3. Solid-state fluorescence spectroscopy To examine the luminescent properties of the d10 metal compounds, the solid-state fluorescence spectra of imbt, Himbz, and compounds 1–5 at room temperature are studied as shown in Fig. 6. Their emission and excitation bands are listed in Table 1. In comparison to the free ligand Himbz, the fluorescent mechanism of

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Table 1 Emission and excitation bands for imbt, Himbz and compounds 1–5. Ligand/compound

imbt

Himbz

1

2

3

4

5

lem lex

453 360

421 350

424 324

426 350

428 324

421 334

413 324

the emission maximums of compounds 1–3 is tentatively attributed to ligand-to-ligand transitions (p / p*) that are in reasonable agreement with reported examples on this class of zinc coordination polymers [47]. The emission spectra of compounds 4 and 5 show a blue-shift at 32 and 40 nm compared with the free imbt ligand, respectively, which may be assigned to ligand-to-metal charge transfer (LMCT) [48]. Moreover, solid-state carboxylate ligands, p-bdc can also exhibit fluorescent properties at room temperature reported by the literature [49], and the emission bands of these carboxylate ligands can be assigned to the p* / n transition. It is well-known that the p / p* transition is stronger than the p* / n transition, furthermore, the strong electron withdrawing group of the carboxyl group results in fluorescence quenching, so carboxylate ligands are almost have no contribution to the fluorescent emissions of compounds 4 and 5. According to the above description, the emissions of the five compounds may be assigned to p / p* transitions of Himbz ligand or ligand to metal charge transfer (LMCT). 3.4. Thermogravimetric analyses In order to characterize the compounds more fully in terms of thermal stability, their thermal behaviors were studied by TGA. The experiments were performed on samples consisting of numerous single crystals of 1–5 under N2 atmosphere with a heating rate 10  C/min are summarized in Chart S2 (Supporting information). TGA curves of 1 and 3 exhibit one main step of weight loss corresponding to the combustion of the organic groups, respectively. TGA curve of 2 exhibits two significant weight losses. The first step varies from 50  C to 138  C, which corresponds to the release of water molecules with the weight loss of 3.89% that is close to the calculated values (3.66%). The second step covers from 342  C to 500  C, which can be attributed to the combustion of the organic groups. TGA curves of 4 and 5 are similar and exhibit two significant weight losses. The first covers from 163  C to 396  C for 4 and 238  C to 420  C for 5, which assign to the release of imbt ligand, respectively. The observed weight loss of 35.22% (for 4) and 32.56% (for 5) are close to the calculated values (34.75% for 4 and 31.77% for 5), respectively. The second step covers from 464  C to 576  C for 4 and 486  C to 612  C for 5, which correspond to the combustion of the p-bdc groups. 4. Conclusion In summary, we have synthesized and characterized five coordination polymers based on imidazole derivative imbt ligand under the different pH conditions. Investigation on their structural differences reveals that the pH values of the reaction system play an important role in modulating the architecture of coordination compounds. Additionally, compounds 1–5 display fluorescent properties indicating that they may have potential applications as optical materials. Further work on the properties of this ligand and its metal compounds are still explored in our laboratory. Acknowledgments

Fig. 6. Solid-state emission and excitation spectra of imbt, Himbz and compounds 1–5 at room temperature.

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project Nos.

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20373009 and 20703008), Changjiang Scholars Program (2006), Program for Changjiang Scholars and Innovative Research Team in University (IRT0714), the National High-tech Research and Development Program (863 Program 2007AA03Z354), the Science Foundation for Young Teachers of NENU (20070309). Appendix. Supplementary Material Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.solidstatesciences. 2008.10.008.

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