Synthesis and Characterization of Four Novel Supramolecular

Synthesis and Characterization of Four Novel Supramolecular Compounds Based on Metal Zinc and Cadmium Jing Lu, Kui Zhao, Qian-Rong Fang, Ji-Qing Xu,* Jie-Hui Yu, Xiao Zhang, Hai-Ying Bie, and Tie-Gang Wang

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 1091-1098

College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130023, China Received October 26, 2004

ABSTRACT: Four zinc(II) or cadmium(II) supramolecular compounds, namely, [Zn(ox)(Him)]n (1), [Zn(bta)2]n (2), [Zn(H2O)4(na)2]n (3), and [Cd2(ox)(OH)2]n (4) (ox ) oxalate anion, Him ) imidazole, bta ) benzotriazole anion, and na ) nicotinic anion), have been synthesized. Among them, 1 is constructed to be a 3-D supramolecular framework by covalent and noncovalent interactions, 2 containing two bta coordination modes displays a 2-D layer structure linked by covalent bonds, the 3-D network of 3 is mainly constructed by strong hydrogen bonds, while 4 exhibits a 3-D framework with 1-D channels only constructed by covalent interactions. The different metal centers and ligand dimensions result in the different structures although they were obtained from similar reaction systems. All of these compounds display strong fluorescence emissions in the blue region, which may be assigned to charge transfer between metal and ligand (for 1, 3, and 4) and the π f π* transition of bta (for 2). Introduction Supramolecular chemistry based on metal-ion-directed assembly of organic molecular building blocks is receiving increasing attention owing to the potential discovery of novel functional materials, which are expected to find use in the areas of catalysis, optics, sensors, magnetism, and molecular recognition.1-10 The discovery of new supramolecular frameworks using the principles of crystal engineering provides an important opportunity to explore assembly and structural diversity in the solid state.11-15 Therefore, one of the most necessary tasks of current research is the fabrication of a supramolecular self-assembly organized by covalent or noncovalent interaction. In the construction of new supramolecular frameworks, the design of ligands is often a useful way of manipulating the structures. Bior multidentate ligands containing N- or O-donors are usually used to bind metal centers to form covalent high-dimensional arrangements. Additionally, self-assembly by H-bonding, π-π stacking, and van der Waals interactions is also an important process in the formation of noncovalent supramolecular frameworks.16-20 During the past decade, many supramolecular compounds with interesting structures have been synthesized and characterized, such as the 1-D chains (heliced, zigzag, line, etc.), 2-D layers (honeycomb, square, rectangular network, etc.), and 3-D open frameworks. In this work, we chose oxalic acid as O-donor due to its strong ability to bind metals and adopt many kinds coordination modes (Scheme 1).21-25 At the same time, a series of N-heterocyclic organic ligands as N-donors were also applied to prepare supramolecular compounds based on mixed ligands. Four compounds constructed by covalent or noncovalent interactions or both were obtained: [Zn(ox)(Him)]n (1), [Zn(bta)2]n (2), [Zn(na)2(H2O)4]n (3), and [Cd2(ox)(OH)2]n (4) (ox ) oxalate anion, * E-mail: [email protected]. Fax: +86-431-8499158.

Scheme 1. The Coordination Modes of ox Ligands

Him ) imidazole, bta ) benzotriazole anion, and na ) nicotinic anion), which display strong fluorescent emissions at room temperature. Experimental Section Materials. All the chemicals were obtained from commercial sources and used without further purification. Infrared spectra were recorded as KBr pellets by using a Perkin-Elmer Spectrum One FT-IR spectrometer in the 400-4000 cm-1 region. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 2400 element analyzer. Fluorescent data were collected on an Edingburgh FS900 instruments. Synthesis of [Zn(ox)(Him)]n (1). Compound 1 was synthesized hydrothermally in poly(tetrafluoroethylene)-lined stainless steel autoclaves under autogenous pressure. The reaction of Zn(OAc)2‚2H2O, oxalic acid, imidazole, and H2O in a molar ratio of 1:1:1:450 was allowed to proceed at 170 °C for 5 days (pH ) 5); then the reactant mixture was cooled at room temperature to give a 75% yield (based on Zn) of 1 as colorless crystals, which were collected by mechanical isolation and washed with water. C5H4ZnN2O4 (221.47): calcd C 27.12, H 1.82, N 12.65; found C 27.56, H 1.79, N 12.84%. IR (KBr pellet): 3161 (m), 1650 (s), 1606 (s), 1377 (s), 1308 (s), 1143 (m), 1078 (s), 955 (m), 841 (m), 798 (s), 756 (m), 657 (s), 620 (m), 521 (m), 504 (m).

10.1021/cg049637a CCC: $30.25 © 2005 American Chemical Society Published on Web 02/10/2005

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Figure 1. (a) ORTEP representation of the local geometry around the Zn center in 1 (50% probability ellipsoids); (b) the 2-D layer constructed by ox ligands (Him are omitted for clarity); (c) perspective view of hydrogen bonds between the 2-D layers. Table 1. Crystal Data Collection and Structure Refinement for 1-4 empirical formula formula weight temp (K) crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Fcalc (g/cm3) F(000) data/restraints/params GOF on F2 final R indices [I > 2σ(I)] largest diff peak and hole (e Å-3)

1

2

3

4

C5H4N2O4Zn 221.47 293(2) monoclinic P21/c 8.4292(6) 9.4026(7) 8.2760(6) 93.1560(10) 654.93(8) 4 2.246 440 1445/0/109 1.057 R1 ) 0.0285 wR2 ) 0.0707 0.399; -1.011

C12H8N6Zn 301.61 293(2) monoclinic P21/n 10.3405(9) 10.0948(6) 12.1549(6) 109.317(4) 1197.36(14) 4 1.673 608 2959/0/172 0.859 R1 ) 0.0395 wR2 ) 0.0593 0.475; -0.517

C12H16N2O8Zn 381.64 293(2) monoclinic c2/m 14.22(3) 6.890(12) 8.437(15) 117.93(3) 730(2) 2 1.996 436 588/0/68 1.051 R1 ) 0.0593 wR2 ) 0.1055 1.087; -0.841

C2H2Cd2O6 346.84 293(2) monoclinic P21/n 7.3915(6) 5.9789(5) 12.3309(6) 92.885(2) 544.25(7) 4 4.233 632 927/0/92 1.134 R1 ) 0.0252 wR2 ) 0.0544 0.839; -1.488

Synthesis of [Zn(bta)2]n (2). Compound 2 was prepared in the same way as 1 using Hbta instead of Him. The pH value was adjusted to 5. Colorless crystals of 2 were obtained (57% yield based on Zn). C12H8ZnN6 (301.61): calcd C 47.79, H 2.67, N 27.86; found C 47.65, H 2.51, N 28.01%. IR (KBr pellet): 2920 (s), 1633 (s), 1489 (m), 1449 (m), 1398 (s), 1274 (m), 1225 (m), 1186 (m), 1159 (m), 754 (m), 739 (m). Synthesis of [Zn(na)2(H2O)4]n (3). Similar to the synthesis of 1, the hydrothermal reaction of Zn(OAc)2‚2H2O, oxalic acid, na, and H2O in molar ratio of 1:1:1:450 was performed at 170 °C for 5 days (pH ) 5). After the reactant cooled at room temperature, the mixture was filtrated, and the filtrate was allowed to stand in air at room temperature for 2 days, yielding

colorless crystals (43% yield based on Zn). C12H16ZnN4O8 (381.64): calcd C 37.77, H 4.19, N 7.34; found C 37.69, H 4.23, N 6.36%. IR (KBr pellet): 3429 (s), 1614 (s), 1568 (m), 1387 (s), 1053 (m), 764 (m), 700 (m), 641 (m). Synthesis of [Cd2(ox)(OH)2]n (4). Compound 4 was prepared in the same way as 1 using CdCl2‚2.5H2O instead of Zn(OAc)2‚2H2O. The pH value was also adjusted to 5. Colorless crystals of 4 were obtained (25% yield based on Zn). C2H2Cd2O6 (346.84): calcd C 6.92, H 0.58; found C 7.05, H 0.60. IR (KBr pellet): 3454 (s), 3425 (s), 1608 (vs), 1362 (m), 1311 (vs), 906 (m), 811 (m), 763 (m), 493 (m). X-ray Crystallography. X-ray single-crystal data collections for 1-4 were carried out at 293 K on a Siemens SMART

Four Novel Zn and Cd Supramolecular Compounds

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Table 2. The Selected Bond Lengths (Å) and Angles (deg) of Compounds 1-4a 1 Zn(1)-N(1) Zn(1)-O(2) Zn(1)-O(3) N(1)-Zn(1)-O(4) O(4)-Zn(1)-O(2) O(4)-Zn(1)-O(1) N(1)-Zn(1)-O(3) O(2)-Zn(1)-O(3) N(1)-Zn(1)-O(3)#1 O(2)-Zn(1)-O(3)#1 O(3)-Zn(1)-O(3)#1

2.0224(16) 2.0737(12) 2.2921(12) 99.23(6) 89.70(5) 169.87(5) 94.67(6) 84.35(5) 96.68(6) 85.96(5) 165.24(6)

Zn(1)-N(6) Zn(1)-N(2)#2 N(6)-Zn(1)-N(4)#1 N(4)#1-Zn(1)-N(2)#2 N(4)#1-Zn(1)-N(1)

1.976(2) 2.006(3) 121.97(9) 105.44(10) 107.18(10)

Zn(1)-O(1W) O(1W)#1-Zn(1)-O(1W) O(1W)-Zn(1)-O(1W)#2 O(1W)#2-Zn(1)-N(1)

2.114(4) 90.8(2) 89.2(2) 88.99(17)

Cd(1)-O(5) Cd(1)-O(6)#1 Cd(1)-O(6)#2 Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(1)#3 Cd(2)-O(5)#4 O(5)-Cd(1)-O(6)#1 O(5)-Cd(1)-O(6)#2 O(6)#1-Cd(1)-O(6)#2 O(5)-Cd(1)-O(1) O(6)#1-Cd(1)-O(1) O(6)#2-Cd(1)-O(1) O(5)-Cd(1)-O(2) O(6)#1-Cd(1)-O(2) O(6)#2-Cd(1)-O(2) O(1)-Cd(1)-O(2) O(5)-Cd(1)-O(1)#3 O(6)#1-Cd(1)-O(1)#3 O(6)#2-Cd(1)-O(1)#3 O(1)-Cd(1)-O(1)#3 O(2)-Cd(1)-O(1)#3 O(5)#4-Cd(2)-O(6) O(5)#4-Cd(2)-O(5) O(6)-Cd(2)-O(5) O(5)#4-Cd(2)-O(3)#5

2.191(2) 2.215(2) 2.255(2) 2.384(3) 2.394(3) 2.515(2) 2.227(2) 116.75(8) 106.04(10) 122.76(4) 153.49(8) 75.17(9) 83.23(9) 86.10(10) 129.86(9) 87.17(10) 69.32(9) 78.08(8) 81.37(8) 71.91(8) 128.29(4) 148.77(9) 112.02(8) 80.34(9) 167.61(9) 134.64(9)

Zn(1)-O(4) Zn(1)-O(1) Zn(1)-O(3)#1 N(1)-Zn(1)-O(2) N(1)-Zn(1)-O(1) O(2)-Zn(1)-O(1) O(4)-Zn(1)-O(3) O(1)-Zn(1)-O(3) O(4)-Zn(1)-O(3)#1 O(1)-Zn(1)-O(3)#1

2.0449(13) 2.0774(13) 2.4102(12) 170.57(6) 90.36(6) 80.57(5) 76.76(5) 99.35(5) 92.08(5) 90.01(5)

Zn(1)-N(4)#1 Zn(1)-N(1) N(6)-Zn(1)-N(2)#2 N(6)-Zn(1)-N(1) N(2)#2-Zn(1)-N(1)

1.989(2) 2.012(2) 106.15(10) 106.97(10) 108.59(10)

Zn(1)-N(1) O(1W)#1-Zn(1)-O(1W)#2 O(1W)-Zn(1)-N(1) N(1)-Zn(1)-N(1)#3

2.173(5) 180.000(1) 91.01(17) 180.000(1)

Cd(2)-O(6) Cd(2)-O(5) Cd(2)-O(3)#5 Cd(2)-O(4)#5 Cd(2)-O(4)#6 Cd(2)-Cd(2)#4

2.248(2) 2.308(2) 2.361(3) 2.363(3) 2.459(3) 3.4662(6)

O(6)-Cd(2)-O(3)#5 O(5)-Cd(2)-O(3)#5 O(5)#4-Cd(2)-O(4)#5 O(6)-Cd(2)-O(4)#5 O(5)-Cd(2)-O(4)#5 O(3)#5-Cd(2)-O(4)#5 O(5)#4-Cd(2)-O(4)#6 O(6)-Cd(2)-O(4)#6 O(5)-Cd(2)-O(4)#6 O(3)#5-Cd(2)-O(4)#6 O(4)#5-Cd(2)-O(4)#6 Cd(1)-O(1)-Cd(1)#7 Cd(2)#5-O(4)-Cd(2)#8 Cd(1)-O(5)-Cd(2)#4 Cd(1)-O(5)-Cd(2) Cd(2)#4-O(5)-Cd(2) Cd(1)#1-O(6)-Cd(2) Cd(1)#1-O(6)-Cd(1)#9 Cd(2)-O(6)-Cd(1)#9

86.39(10) 83.96(10) 147.38(9) 85.98(9) 83.51(9) 70.68(9) 83.37(9) 81.91(9) 100.98(9) 141.62(9) 72.12(9) 96.63(8) 107.88(9) 118.44(10) 112.17(11) 99.66(9) 119.52(10) 109.91(9) 107.00(11)

2

3

4

a Symmetry codes: (1) #1 x, -y + 1/ , z + 1/ ; (2) #1 -x, y - 1/ , -z + 1/ ; #2 -x, -y + 2, -z; (3) #1 x, -y, z; #2 -x, y, -z - 2; #3 -x, -y, 2 2 2 2 -z - 2; (4) #1 -x, -y + 1, -z; #2 x + 1/2, -y + 1/2, z - 1/2; #3 -x + 1/2, y - 1/2, -z - 1/2; #4 -x, -y, -z; #5 -x + 1, -y + 1, -z; #6 x - 1, 1 1 1 1 1 y, z; #7 -x + /2, y + /2, -z - /2; #8 x + 1, y, z; #9 x - /2, -y + /2, z + 1/2.

system equipped with a CCD detector with Mo KR radiation at 0.710 73 Å. Empirical absorption corrections were applied using SADABS. The structures were solved by conventional methods and refined with full-matrix least-squares technique using SHELXS-97 and SHELXP-97 programs, respectively. Anisotropic thermal parameters were applied to all nonhydrogen atoms; the organic hydrogen atoms were generated geometrically. No attempt was made to locate the hydrogen atoms of water for 3. The crystallographic data for 1-4 are given in Table 1, and the selected bond lengths and angles are listed in Table 2.

Results and Discussion Syntheses. In the past decade, the hydrothermal reaction has been developed extensively because it can offer a powerful synthetic route to prepare polymeric solids with better crystals than traditional solution

techniques such as evaporation, diffusion and cooling, etc. In most cases, water not only is a solvent but also acts as a competing ligand in these hydrothermal reactions. An interesting phenomenon can be observed whereby many coordination polymers constructed from polycarboxylate ligands contain water molecules as terminal or bridging ligands in the coordination sphere of the metal. It is therefore possible to obtain metal polycarboxylates with novel coordination architectures of diverse sizes and shapes by use of other suitable compounds with different steric demands and ligating capabilities as the competing bridging ligand to substitute the water ligands mentioned above. Taking account of the potential of these approaches, we have embarked on a program aimed at using mixed ligands containing O- and N-donors to prepare supramolecular frameworks.

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Scheme 2. The Reaction Scheme for 1-4

Scheme 3. The Coordination Modes of Benzotriazole Ligand

So a series of N-hetercyclic ligands as N-donors and oxalic acid as O-donor were applied. And the reactions were performed under the same pH value, because the deprotonation of polycarboxylates is very important for the metal polycaboxylate coordination polymers.26 As described in the Experimental Section, although the synthesis conditions are very similar, only a small difference in the steric demands of the N-donor reactants results in very different supramolecular architectures (Scheme 2). Compound 1 is constructed by the mixed ligands; however, there is no ox ligand binding to Zn centers for 2 and 3. The reason is probably that the bta and na ligands are preferred to coordinate to the Zn atom under these reaction conditions and the steric demands of them are so large that there is no space for an ox ligand. Compound 4 is different from compounds 1-3; it is the ox but not the Him that binds to the Cd center. Compound 4 can also be obtained when Him was substituted by Hbta or Hna, which suggests that Cd(II) inclines to coordinate to ox in this reaction coordination. Compound 3 can also obtained without oxalic acid as reported by Z. Vargova´ et al.,27 but the synthesis of compounds 2 and 4 cannot be performed in the absence of ox or N-hetercyclic ligands, even though none of them are found as ligands or guests in the resulting supramolecular architectures. This suggests that the ox (for 4) and N-hetercyclic ligands (for 2) play a template or mineralizer role in the construction of the resulting coordination structures. X-ray Single-Crystal Structure of 1. A singlecrystal X-ray diffraction study reveals that 1 adopts a 3-D supramolecular structure. The Zn center is sixcoordinated by one N atom from Him and five O atoms from three ox ligands (Figure 1a). The ox ligands adopt two different coordination modes: one of them adopts a chelate bis-bidentate linkage (mode d, Scheme 1), while the other two adopt a chelate and bridging bisbidentate linkage (mode g, Scheme 1). As shown in Figure 2a, two kinds of ox (mode d and g) connect the Zn centers into a 2-D layer along the bc plane, which is different from the other 2-D metal-ox layer structures reported previously,25,28-32 in which most of them are constructed by only one ox coordination mode (mode d) and possess various pores, and only one example25 was reported in which a 2-D Cd-ox layer is performed by two ox modes (d and f). In compound 1, due to coordination of ox to Zn increasing the ligand conformational rigidity, the two kinds of ox are both almost planar

Figure 2. The 2-D network representation of 2: (a) viewed along the bc plane (benzolate rings are omitted for clarity); (b) viewed along the ab direction. Dark gray represents Zn atoms; medium gray represents N atoms; light gray represents C atoms.

conformation with the mean deviation of 0.0008 Å for mode d and 0.0020 Å for mode g, while the dihedral angle of the two planes is 86.9°. As shown in Figure 1c, the Him ligands alternatively extrude above and below the [Zn(ox)]n layer. The dihedral angle of the intralayer adjacent Him ligands is 49.6°, which suggests that there is no π-π interaction between them. It is interesting that there is also no π-π stacking between the interlayer Him rings with dihedral angle 50.3°, which probably results from the stronger hydrogen bond interactions existing between the layers, which is performed by the H atom attached to N2 from Him and O2 from the mode D ox with the N2-O2 distance of 2.817 Å and NHO angle of 167.13°. Thus, the 3-D supramolecular structure of 1 is constructed by covalent and noncovalent interactions. X-ray Single-Crystal Structure of 2. X-ray singlecrystal diffraction study reveals that 2 adopts a complicated 2-D supramolecular network. As shown in Figure 2a, the Zn center is four-coordinated by N atoms from four bta molecules in which two of them are in µ1,2bridging linkages and the other two are µ1,3-bridging linkages. It is reported9,33-35 that benzotriazole ligand can adopt three coordination modes: η1-, µ1,2-, and µ1,2,3(Scheme 3), and the mixed modes can also exist in one compound: η1- and µ1,2- or η1-, µ1,2-, and µ1,2,3-. So the µ1,3- is a novel coordination mode, and 2 is of course the first compound containing µ1,2- and µ1,3-bridging bta coordination modes. In 2, the adjacent two Zn atoms are connected by two µ1,2-bta’s to form a six-membered ring (Zn2N4) with a



Figure 3. (a) The 1-D hydrogen chain along the c axis of 3; (b) the 2-D hydrogen bond layer parallel to the ab plane; (c) a ball-and-stick representation of the 3-D network of 3 (ball ) [Zn(na)2(H2O)4] unit; stick ) hydrogen bond).

Figure 4. (a) ORTEP representation of the local geometry around the Cd center in 4 (50% probability thermal ellipsoids); (b) the 3-D [Cd2(ox)]n network along the b axis (OH groups are omitted for clarity); (c) the 2-D corrugated sheet of [Cd(OH)2]n (ox units are omitted for clarity); (d) the 3-D framework of compound 4. Dark gray represent Cd atoms; medium gray represents O atoms; light gray represents C atoms.

Zn-Zn distance 3.685 Å. And the [Zn2(µ1,2-bta)2] building blocks are connected to a 2-D network along the bc plane by µ1,3-bta ligands, in which there are two groups

Zn2N4 building blocks (A and B groups) with a dihedral angle of 49.1° (mean deviation lf 0.0434 Å for A and B). All of the aromatic rings are extrude out of the 2-D

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Figure 5. Fluorescent spectra of 1 (a), 2 (b), 3 (c), and 4 (d) measured at room temperature.

network (shown in Figure 2b), among which the µ1,3bta planes are parallel, while the two groups µ1,2-bta planes, attached to two groups Zn2N4 configurate the 46° dihedral angles. So, the 2-D supramolecular network is mainly constructed by covalent interactions. X-ray Single-Crystal Structure of 3. X-ray singlecrystal study reveals that supramolecular compound 3 is constructed mainly by hydrogen bonds. The coordination circumstance around Zn atom has been described in detail in ref 27. As shown in Figure 3, each water molecule offers two hydrogen atoms and each carboxylate oxygen atom accepts two hydrogen atoms; that is to say each [Zn(na)2(H2O)4] unit is connected to other units by 16 hydrogen bonds, which can be classified into two groups. One group is the carboxylate oxygen atom O1 accepting two hydrogen atoms attached to two water molecules of one adjacent unit with the Ow-O1 distance of 2.696 Å (Figure 3a). So the [Zn(na)2(H2O)4] units are linked to a 1-D supramolecular chain along the c axis by this group of hydrogen bonds. The other group consists of the carboxylate oxygen atom O2 and two water molecules coming from two adjacent units with an Ow-O1 distance of 2.697 Å. As shown in Figure 3b, through hydrogen bonds, the water molecules of each unit and the carboxylate atoms O2 of adjacent units are connected to each other by strong hydrogen bonds, and the 2-D network parallel ab plane is formed. As mentioned above, each water molecule can form a 1-D hydrogen bond chain with O1 along the c axis and a 2-D hydrogen bond network with O2 parallel to the ab plane. So the 3-D hydrogen bond supramolecular net-

work is constructed. Figure 3c shows the 3-D topology structure of 3, in which each [Zn(na)2(H2O)4] unit is connected to the six adjacent units. X-ray Single-Crystal Structure of 4. X-ray singlecrystal study reveals that the 3-D supramolecular compound 4 is constructed by covalent bonds, which has been discussed in detail in communication format.36 The coordination circumstance around the Cd atom and the crystal packing structure are displayed in Figure 4. Fluorescence Properties. The fluorescence properties of 1-4 were measured at room temperature, and the fluorescent spectra are displayed in Figure 5. It is well-known that the lowest excited state of imidazole is an n f π* transition that displays no photoluminescence, and the ox ligand exhibits very weak fluorescence emission. So the Him and ox ligands show no contribution to the broad emission band at 368-388 nm (λex ) 320 nm) for 1 (Figure 5a), which should be assigned to the charge transfer between the metal and ligand. The diagram of energy levels for 1 can be drawn according to the fluorescence properties (shown in Figure 6a). When excited at 320 nm, the electron transfers from ground-state E0 to excited-state E3; then it nonradiatively decays to E2 and E1 energy levels, from which it comes back to ground-state E0 with light energy emission (368 nm and 388 nm). It is verified by the fluorescence spectrum that the chances of nonradiative decay from E3 to E2 are similar to that of nonradiative decay from E3 to E1. As shown in Figure 5b, the fluorescence emission band of free Hbta is mainly located at 350 nm with shoulders at 330, 340, and 406 nm (λex ) 300 nm).



transfer between metal and ligand of compounds 1, 2, and 4 are similar, while compound 2 exhibits very different fluorescence emission, which should be assigned to π f π* transfer of the bta ligand probably because the luminescence originated by charge transfer between the metal and ligand is too weak compared with that originated by the π f π* transfer of bta. Conclusions

Figure 6. The diagram of energy levels for compounds 1 (a), 2 (b), 3 (c), and 4 (d).

Because the emission band of 2 is similar to that of the free Hbta ligand with a slight red shift, it should be assigned to the π f π* transition of the bta ligand. The emission wavelength of 2 undergoes a red shift probably because the Hbta ligand is deprotonated and the energy gap between the π* and π molecular orbitals of bta decreases. Due to the increase in the rigidity of the ligand on forming the polymeric compound and reduction of the nonradiative decay of the intraligand (π f π*) excited state, the emission intensity of 2 increases and is approximately 1.4 times larger than that of free Hbta. Similar enhancements have also been found in polypyridyl cadmium(II) and polypyridyl zinc(II) polymers.37-40 The diagram of energy levels for 2 is displayed in Figure 6b. The electron located at excitedstate E5 can nonradiatively decay to E4, E3, E2, and E1, while the ratio of decay to E2 is the largest, so the main emission band of 2 is at 356 nm. Compound 3 displays two strong fluorescence emission peaks at 382 and 400 nm with a shoulder at 367 nm when excited at 340 nm (Figure 5c), which are assigned to the charge transfer between metal and ligand because there is no fluorescence emission of free na ligand measured. While when excited wavelength is changed to 320 nm, the emission peak of 3 is at 367 nm with a shoulder 382 nm. So the diagram of energy levels for 3 can be drawn in Figure 6c. The electron is excited at 340 nm from the ground state to excited-state E4; then it nonradiatively decays to E3, E2, and E1, from which it comes back to ground state with light energy emission. The chances of nonradiative decay from E4 to E2 and E1 are similar and larger than that of decay to E3. When excited at 320 nm, the excited-state electron (E5) nonradiatively decays to E3 and E2, in which the ratio of E3 is larger. For compound 4, the fluorescence emission spectrum (Figure 5d) has been studied in ref 36. The diagram of energy levels of 4 is shown in Figure 6d. When excited at 360 nm, 4 exhibits a main emission band at 423 nm with a shoulder 380 nm. So the excited electron at E5 inclines to nonradiatively decay to E2 and E3, while the excited electron at E4 inclines to nonradiatively decay to E1. In summary, all of these compounds display photoluminescent properties in the blue region at room temperature. The emission bands assigned to be charge

Although the four supramolecular compounds 1-4 were synthesized from a similar reaction system, the structures of them are very different because of the different metal center and steric demands of ligands. The four supramolecular compounds constructed by covalent or noncovalent bonds or both all display strong fluorescence properties in the blue region and may find widespread applications in blue light-emitting devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20271021 and 20333070). Supporting Information Available: Four X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG049637A

Synthesis and Characterization of Four Novel Supramolecular

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