All reagents and solvents with analytic grade for the synthesis in this work were purchased from Energy Chemical Corporation and Beijing HWRK Chemical Co.,Ltd in China and used as received.

Diffraction data were collected by using a computer-controlled XCalibur E CCD diffractometer(Agilent, America) with graphite-monochromated Mo Kα radiation( λ_{Mo Kα}=0.071073 nm) at T=293.2 K. The phase purity and the structural integrity of experimental samples were determined by MiniFlex2 X-ray diffractometer(XRD, Rigaku, Japan) using Cu Kα( λ=0.1542 nm) radiation. The diffractometer data were recorded for 2 θ values from 3° to 50° at a scanning rate of 1°/min. Thermogravimetric analysis(TGA, Netzsch, Germany) was carried out on a Netzsch STA449C equipped with a platinum pan at a heating rate of 15 ℃/min in N₂ atmosphere.

A mixture of Zn(NO₃)₂·6H₂O(0.1600 g, 0.54 mmol), pyrazine(0.0300 g, 0.37 mmol), 5-amino-1 H-tetrazole(atz, 0.0500 g, 0.59 mmol), thiophene-2,5-dicarboxylic acid(thb, 0.0560 g, 0.33 mmol), tetramethylammonium bromide(0.0300 g, 0.20 mmol), methanol(3 mL) and N, N-dimethylacetamide(DMA, 3 mL) in a 23 mL Teflon-lined stainless steel vessel was heated at 120 ℃ for 36 h, and then cooled to room temperature. The resulting transparent colorless crystals(TTF-8) were obtained, washed with acetone, and dried at room temperature. TTF-9 was obtained by the similar method as described for TTF-8 except for using 4,4'-oxybisbenzoic acid(obb, 0.0800 g, 0.31 mmol) instead of thiophene-2,5-dicarboxylic acid.

Tetragonal, M=553.1, a=b=2.15065(16) nm, c=1.02633(9) nm, V=4.7471(7) nm³, T=293(2) K, space group P4(2)mc, Z=4, 8440 reflections measured, 3862 independent reflections( R_int=0.1128). The final R₁ values were 0.1128( I>2 σ( I)). The final wR( F²) values were 0.2765( I>2 σ( I)). The goodness of fit on F² was 1.072.

Monoclinic, M=639.21, a=1.02628(4) nm, b=3.5575(4) nm, c=1.82706(15) nm, V=5.5614(8) nm³, T=293(2) K, space group P2₁/c, Z=4, 12063 reflections measured, 8132 independent reflections( R_int=0.0515). The final R₁ value was 0.0771( I>2 σ( I)). The final wR( F²) value was 0.1872( I>2 σ(I)). The goodness of fit on F² was 1.027.

Colorless crystals of TTF-8 and TTF-9 were solvothermally prepared, respectively. With the purpose of achieving the charge-balancing of compounds, N(CH₃ )4+is chosen as the counter-ion, which is very important for successfully obtaining TTF-8 and TTF-9. Their structures were characterized by single-crystal X-ray diffraction. Both of them exhibit anionic porous frameworks with large free voids occupied by the charge-balancing N(CH₃ )4+cations and structurally disordered solvent molecules. The phase purity and thermal stability of TTF-8 and TTF-9 were measured by powder X-ray diffraction(PXRD) and TGA, respectively(see Fig.S1~Fig.S4 in Supporting Information).

Fig.1

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	Fig.1 The coordination environment of the zinc atom and bridging mode( A), the building block( B) and view of the 3D framework( C) of TTF-8. Hydrogen atoms were omitted for clarity

TTF-8 crystallizes in a tetragonal space group of P4₂mc. In the structure of TTF-8, Zn(Ⅱ) ion adopts a tetrahedral coordination mode and is connected by three atz ligands and one thb ligand(Fig.1 A). The deprotonated atz ligand just uses two N donors to bridge two Zn(Ⅱ) sites, and the thb linker bridges two Zn centers. Six Zn centers are bridged by six atz ligands to generate a 6-membered ring, which is further linked by atz ligand to form a channel unit(Fig.1 B). Such Zn-atz channels are further linked by the thb ligands into a three-dimensional framework(Fig.1 C). It is worth noting that three types of channels are fabricated with vertex-sharing fashion to form the three-dimensional open framework of TTF-8.

By employing obb to replace thb, another zeolitic framework TTF-9 is obtained under a similar reaction condition. Comparatively, the structure of TTF-9 shows low symmetry in monoclinic P2₁/c. TTF-9 exhibits the similar coodination environment of TTF-8, where Zn(Ⅱ) ion is 4-connected by three atz ligands and one obb ligand to form tetrahedron geometry(Fig.2 A). The deprotonated atz and obb ligand act as linear linkers to bridge one Zn1 and Zn2(Fig.2 B). If obb linkers serve as the pillars, atz ligands adopt a μ₂-1,4 bridging mode to link two Zn atoms and resulted in a wavy honeycomb-like Zn-atz layer along the b-axis(Fig.2 C). As shown inFig.2 D, the adjacent layers are bridged by auxiliary obb linkers along the b-axis, which produces a three-dimensional network.

Fig.2

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	Fig.2 The coordination environment of the zinc atom( A), bridging mode of ligands( B), layer unit( C) and view of the 3D framework( D) of TTF-9. Hydrogen atoms were omitted for clarity

It is obvious that there are many structural differences between TTF-8 and TTF-9. Although their coordination environment and bridging mode are similar, further analysis reveals that the Zn-atz-Zn angles are entirely different. The Zn-atz-Zn angles in TTF-8 are 112.7°, 112.7° and 113.7°, respectively, which lead to a one-dimensional channel as building block(Fig.3 A and Fig.3 C). Different Zn-atz-Zn angles(109.2°, 109.9° and 114.2°) in TTF-9 bring a layer as unit(Fig.3 B and 3 D). The thb and obb link these two types of units, respectively, which produce two different structural features with the same topology. Both TTF framework topologies are identified as the 4-connected net with the symbol BCT(vertex symbol:4·6⁵) by reducing each Zn site as the 4-connected node(Fig.3 E and Fig.3 F).

Fig.3

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	Fig.3 The angle among Zn atoms bridged by atz in TTF-8( A) and TTF-9( B); the building block of TTF-8( C) and TTF-9( D); the BCT topology of TTF-8( E) and TTF-9( F)

Investigating the performance of absorbing I₂ was finished by means of immersing 100 mg of single crystals of TTF-8 and TTF-9 in a cyclohexane solution of I₂(0.1 mol/L), respectively. The dark red solutions of I₂ faded slowly to pale brown with the colour of crystals changing from colorless to dark brown(Fig.4 A and Fig.S5 A in Supporting Information). The mass of TTF-8 and TTF-9 after loading iodine increased by ca. mass fraction 23.3% and 18.1%, respectively. As depicted inFig.4 B(and Fig.S5 B in Supporting Information), when the compounds with encapsulated I₂ were put into the ethanol(EtOH), the color of EtOH extract changed from colorless to pale brown, which shows that I₂ sorption is reversible. The releasing process of I₂ was further investigated by the following method with a non-aqueous solution. Very little I₂-loaded single crystals of TTF-8 or TTF-9 were placed in 9 mL ethanol, and the iodine content was estimated by UV/Vis spectroscopy at room temperature(Fig.4 C and Fig.S5 C in Supporting Information). The absorbance of I₂ extracted into ethanol at 288 nm and 360 nm normally increases within 50 minutes. The dynamic equilibrium of the release and adsorption of I₂ is reached about 1 hour(Fig.S6 and Fig.S7 in Supporting Information)

Fig.4

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	Fig.4 ( A)Photos of the iodine recovery process with 100 mg of crystals of TTF-8 soaked in cyclohexane solution of I2(0.1 mol/L, 1.5 mL); ( B)iodine releasing process of TTF-8(10 mg) soaked in 1.5 mL of EtOH; ( C)temporal evolution of UV/vis absorption spectra for the I2 releasing from TTF-8 in 9 mL of EtOH