Rare silver bromide clustered supramolecular polypseudorotaxane: Preparation and its application in photocatalytic degradation of organic compounds

2.1 Materials and methods

According to the reported literature method, the organic cationic template, 1, 3-bis(3,5-lutidine)propanedibromide [BLP·2Br (Scheme 1)] was synthesized [25]. All chemicals used in the synthesis were of A.R. grade(≥99%) and used without further purification. Distilled water was used for all procedures.

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Scheme 1

The cationic template BLP·2Br.

Luminescent measurements of compound were conducted on the F-7000 FL spectrophotometer and the data was collected in solid state at room temperature. Elemental analyses (C, H, and N) were determined on a FLASH EA 1112 elemental analyzer. The UV-Vis diffuse reflectance spectra (DRS) was recorded on a Cary 5000 UV-Vis-NIR at the speed of 300 nm min^–1 from 800 to 200 nm. The UV–vis spectroscopic studies were measured on UV-5500 PC spectrophotometer. Crystallographic data for the compound 1 was collected on a Bruker APEX-II area-detector diffractometer equipped with graphite-monochromatized Mo-Kα radiation (λ= 0.71073Å). PXRD can be confirmed by the degree of matching between the simulated and experimental X-ray diffraction patterns. A model NETZSCHTG209 thermal analyzer was used to record simultaneous TG curves in flowing air atmosphere of 20 mL·min^–1 at a heating rate of 5°C·min^–1 in the temperature range 50–800°C using platinum crucibles.

Compound 1 was prepared by evaporation of the solvent in air. Dissolve BLP•Br ₂ 0.1 mmol (0.0304 g) in 3–5 ml MeOH and 0.1 mmol (0.0188 g) AgBr and 0.049 g KBr in 4–6 mL dimethylformamide (DMF), and the former was added dropwise to the latter, and stirred to allow the two solutions to mix well and then filtered. The filtrate is left in the air, allowing the solvent to evaporate naturally. After 7–10 days white flake crystals suitable for X-ray analysis are obtained with a yield of about 70–75%.Anal.Calc: C, 17.49; H, 2.07; N, 2.40 %; Found: C, 18.47; H, 1.89; N, 3.41%.

2.3 X-ray crystallography study

The structure was solved by direct method and expanded using Fourier techniques. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by using geometrical constraint. The structure was refined with full-matrix least-squares techniques on F² using the OLEX-2 program package [26]. Table 1 describes the crystal data of compound 1 in detail. Some of the major bond lengths and bond angles of compound 1 are listed in Table 2. The CCDC reference number is 1839494.

3.1 Structural description

Crystal structure of { ( BLP ) [ A g 8 B r 12 ] 0.5 } n(1)

Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the orthorhombic crystal system with space group Cmc2₁. The unit cell parameters were a = 22.930 (3), b = 13.9702 (16), c = 8.4088 (10) Å, and α=β=γ= 90°. As shown in Fig. 1(a), the minimum anion repeat unit of compound 1 consists of three Ag atom and four Br atoms. Each silver atom coordinates with 4 bromine atoms and 2 silver atoms to form an irregular polygonal ring structure. The bond lengths of Ag(1)-Ag(2), Ag(1)-Br(1), Ag(1)-Br(2), Ag(2) -Br(2), Ag(3) -Br(4) are 3.0493(17), 2.641(2), 2.8454(16), 2.928(3) and 2.676(3) Å. The bond angles of Br (1)-Ag (1)-Ag (2), Br (1)-Ag (1)-Br (3), Br (2)-Ag (1)-Ag (2), Br (4)-Ag (3)-Br (3) are 148.23(7), 119.72(7), 59.44(5) and 113.67(7)°. As shown in the pictures Fig. 1(b), the “wheel” components are Ag_xBr_y polyanionic, the “axle” components featuring 1, 3-bis(3, 5-lutidine)propane dication template direct the formation of the AgBr polyanionic rings. The cations form a stacking structure through hydrogen bonding, electrostatic attraction, van der Waals forces, and other weak forces and 2D anion networks. Compound 1 has perfect 2D polythreading structures as shown in Fig. 1(c). The inorganic moiety forms a ring structure from silver atoms and bromine atoms along the a-direction, and extends toward both ends to form a network Fig. 1(d). Our previous studies showed that only metal(I) pseudohalide such as MNCS facilitated to form this type of penetrating architectures [27–30]. A compound most similar to 1 is compound 2. Compounds 1 and 2 are two dimensional structures. The crystal system of compound 2 is monoclinic, and the spatial group is C2/c [4]. The least asymmetric element of the compound consists of a ligand with [Cu₂(SCN₄)]^2– Fig. 1(e).Two Cu are in different coordination environments, and the anionic part forms a large ring, in which two ligands half cross each large ring Fig. 1(f). Unlike compound 1, only one ligand passes through each ring. Comparison of some important parameters of compounds 1 and 2 (Table 3).

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Fig.1

(a) The simplified repeated unit plots of compound 1. (b) Structure showing the polyrotaxane mode of 1 viewed along the b-direction. Unnecessary H atoms were omitted for clarity. (c) Different beauteous chemical views from two orientations of compound 1. (d) Inorganic anion portion of compound 1. (e) The structure unit diagram of 2. (f) The smallest structural unit of 2.

To explore the photocatalytic activity of compound 1, we select methylene blue (MB) as a model of dye contaminant to evaluate the photocatalytic effectiveness. The experiments were performed in typical processes. A suspension containing 1 (50 mg) and 100 mL of MB (1.0×10^–5 mol·L^–1) solution was stirred in the dark for about 30 min. Then, the mixture was stirred continuously irradiation from a 500 W high-pressure xenon lamp. A sample solution (3 ml) was taken every 30 min and separated through centrifuge to remove suspended catalyst particles. After filtration, the samples were analyzed by the UV–vis spectrophotometry. By contrast, the simple photolysis experiment was also completed under the same conditions without any catalyst. The organic dye concentrations were estimated by the absorbance at 665 nm (MB). Figure 2 further confirms the MB degradation rate when using compound 1 as catalysts and without the use of catalysts. Interestingly, approximately 30% of MB was decomposed during the first hour, as can be seen from Fig. 2. This shows that the supramolecular has a good catalytic function. The results indicate that compound 1 has a high photocatalytic activity for the degradation of MB.

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Fig.2

(a) Absorption spectra of the MB solution during the decomposition reaction under xenon lamp light irradiation dealt with 1 and blank. (b) Photocatalytic degradation of MB solution under UV light irradiation with the use of compound 1 and the control experiment without any catalyst. (C₀: the initial concentration of the MB; C:the concentration of the dye at any given time).

To estimate the thermal stability of compound 1, the TG, and corresponding PXRD with different heat treatment have been carried out on the as-synthesized compound 1 samples. The TG curve is shown in Fig. 3(a).The framework of 1 remains intact up to 280°C, from which the organic components are decomposed. Compound 1 began to show weight loss for the temperature range of 280–320°C. The purity of compound 1 is confirmed by powder XRD analyses, in which the main peaks of the experimental spectra of 1 are almost consistent with its simulated spectra Fig. 3(b). The luminescent properties of compound 1 were investigated in the solid state at room temperature. The emission peaks of the compound are shown in Fig. 3(c). Compound 1 exhibits an intense emission between 400 and 550 nm (λ_max = 450 nm upon excitation at 665 nm).

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Fig.3

(a) TGA curves for 1. (b) Experimental (top) and simulated (bottom) powder X-ray diffraction patterns of 1 at 293K. (c) The emission spectra of compound 1 in the solid state at room temperature.