Encapsulation of guest molecules having different functionalities within molecular-sieve host materials, such as zeolites and metal-organic frameworks (MOFs), is a common strategy to develop novel functional materials [1–5]. Polypyridine metal compounds have been embedded in molecular-sieve materials to fabricate photocatalytic materials. For example, photo catalytically active ruthenium tris(bipyridine) [Ru(bpy)₃] was embedded in zeolite Y to synthesize a new material with stable photochemical activity and electron transfer mechanism [6]. Ru(bpy)₃exhibits homogeneous photocatalysis under visiblelight irradiation, which induces charge transfer. However, free Ru(bpy)₃ undergoes dimerization and degradation and exhibits a short photocatalytic life. Although encapsulating Ru(bpy)₃in a zeolite solves the abovementioned issues, zeolites are chemically inert and have limited applications in photocatalysis.

MOFs are state-of-the-art nanopore host materials that are suitable alternatives to zeolites. Electron transfer in MOFs depends on the ligand and the metal ion type, facilitating diverse applications. In particular, zeolitic imidazolate frameworks (ZIFs) exhibit high-density porosity, chemical stability and the ability to capture simple hydrophobic molecules [7–11]. ZIF-71 has a rhombohedral structure with large cages (1.68 nm) interconnected through pore windows of 0.48 nm and is more hydrophobic than ZIF-8 [12].

In this study, we encapsulated Ru(bpy)₃ in ZIF-71 using a simple in situ synthesis method to obtain a new material, Ru(bpy)₃@ZIF-71. The amount of Ru(bpy)₃ loaded onto ZIF-71 was controlled by varying the concentration of Ru(bpy)₃ used during the synthesis. Subsequently, Ru(bpy)₃@ZIF-71 was used as a photocatalyst for the debromination of α-bromoacetophenone.

2.1. Materials

Zinc acetate dihydrate [Zn(CH₃COO)₂·2H₂O, 98 %], a metal ion source, was purchased from Alfa Aesar. 4,5-Dichloroimidazole (C₃-H₃Cl₂N₂, 98 %) and tris(bipyridine)ruthenium(II) chloride [Ru(bpy)₃- Cl2·6H2O, 99.95 %] were purchased from TCI. Triethanolamine [N-(CH₂CH₂OH)₃, 99 %] and 2-bromoacetophenone (C₆H₅COCH₂Br, 98 %) were purchased from Acros and used without additional purification.

2.2. Synthesis of Ru(bpy)₃@ZIF-71

To prepare the ligand solution, 4,5-dichloroimidazole (0.22 g, 1.6 mmol), the ligand of ZIF-71, and tris(bipyridine)ruthenium(II) chloride (0.01–0.05 mmol) were added to a 50 mL glass flask containing 15 mL of methanol. The solution was stirred at 25 °C. In a separate 50 mL glass flask, zinc acetate dihydrate (0.088 g, 0.4 mmol) and the metal ion source of ZIF-71 were dissolved in 15 mL of methanol. This solution was poured into the previously prepared ligand solution and stirred for 24 h at room temperature to initiate crystallization. The mixture was centrifuged, washed with methanol, and dried overnight in an oven at 60 °C to obtain Ru(bpy)₃@ZIF-71.

2.3 Photocatalytic performance test

Triethanolamine (200 µmol) and 2-bromoacetophenone (50 µmol) were added to a glass cell containing 5 mL acetonitrile. Ru(bpy)₃@ZIF- 71 (3 mg) was added to the glass cell, which was bubbled with Ar gas for 5 min and sealed using a glass cap with grease. The closed glass cell was irradiated with a blue LED lamp (1 W, 445 nm) at room temperature for 4 h.

Ru(bpy)₃ can be encapsulated in the nanopores of ZIF-71 during crystal growth as the diameter of Ru(bpy)₃ is approximately 14 Å and the pore diameter in ZIF-71 is 16.8 Å, as illustrated in Fig. 1(a) [13,14]. We used an in situ synthetic strategy to fabricate Ru(bpy)₃@ZIF-71, where a Zn²⁺ solution was mixed with a ligand solution containing dichloroimidazole and Ru(bpy)₃Cl2. The formation of Ru(bpy)₃@ZIF- 71 was confirmed by the transformation of the transparent red gel into a turbid red gel during synthesis. The Ru(bpy)₃ loading on ZIF-71 was controlled by varying the concentration of Ru(bpy)₃(x)@ZIF-71 (x = 0.01, 0.02, 0.03, 0.04, and 0.05 mmol) added to the initial synthetic gel. The scanning electron microscopy (SEM) images in Fig. 2 show that increasing the Ru(bpy)₃ content reduced the crystal size of Ru(bpy)₃@ZIF-71, even though the overall crystal morphology was maintained. The X-ray diffraction (XRD) patterns of pristine ZIF-71 and the Ru(bpy)₃@ZIF-71 series are shown in Fig. 3(a). The diffraction patterns of all Ru(bpy)₃@ZIF-7 samples were identical to those of pristine ZIF-71, indicating that the encapsulation of [Ru(bpy)₃] maintained the crystal structure of the ZIF-71 framework.

Figure 1. (a) ZIF-71 crystal structure and (b) figure showing photocatalytic conversion of α-bromoacetophenone to acetophenone using Ru(bpy)₃@ZIF-71.

Figure 2. SEM images of pristine ZIF-71 and series of Ru(bpy)₃@ZIF-71 as indicated.

Figure 3. (a) XRD patterns, (b) UV-vis diffuse reflectance spectra represented as the Kubelka–Munk function, and (c) fluorescence spectra of pristine ZIF-71 (inset) and series of Ru(bpy)₃@ZIF-71 as indicated.

Furthermore, the encapsulation of Ru(bpy)₃ in ZIF-71 was validated using diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy, and the spectra of a series of Ru(bpy)₃@ZIF-71 samples were compared with those of a Ru(bpy)₃ methanol solution [Fig. 3(b)]. The series of Ru(bpy)₃@ZIF-71 exhibited the characteristic metal-to-ligand charge-transfer (MLCT) band of Ru(bpy)₃ at 500 nm, which was identical to that of pure Ru(bpy)₃. In addition, the absorption intensity increased nearly linearly with an increase in the amount of encapsulated Ru(bpy)₃. Moreover, the fluorescence spectra of the series of Ru(bpy)₃@ZIF-71 were obtained at an excitation wavelength of 450 nm and compared with those of a pure Ru(bpy)₃ methanol solution [Fig. 3(c)]. The fluorescence enhanced with the increasing concentrations of Ru(bpy)₃, indicating that the optical properties of the Ru(bpy)₃-@ZIF-71 series were well maintained. Moreover, a series of Ru(bpy)₃-@ZIF-71 was dissolved in acetic acid to accurately quantify the concentration of Ru(bpy)₃ in ZIF-71. A molar plot of the Ru(bpy)₃@ZIF- 71 unit cell is shown in [Fig. 4(a)].

Figure 4. (a) The number of Ru(bpy)₃ in the series of Ru(bpy)₃@ZIF-71, (b) conversion yields with respect to the amount of loaded Ru(bpy)₃ in the series of Ru(bpy)₃@ZIF-71, (c) variation in the reaction time of Ru(bpy)₃(0.05)@ZIF-71, and (d) reusability test. Reaction conditions: 0.05 mmol of bromoacetophenone, 1 mg of the catalyst, 1.0 mL of CH3CN, 0.2 mmol of TEOA, 4 h of reaction time, 1 W blue LEDs (445 nm) at approximately 35 °C.

Next, we evaluated the photocatalytic performance of Ru(bpy)₃@ZIF-71 during the photocatalytic debromination of α-bromoacetophenone, as illustrated in Fig. 1(b) [15,16]. The photocatalytic reaction was performed for 4 h in the presence of triehtnaolamine (TEOA), an electron donor, using a blue LED lamp (1 W, 445 nm) whose irradiation corresponded to the MLCT band of Ru(bpy)₃. Pristine ZIF-71 was used as the control under identical conditions and did not undergo any photocatalytic reaction. However, the conversion yield of the series of Ru(bpy)₃@ZIF-71 increased with an increment in the concentration of Ru(bpy)₃, where Ru(bpy)₃(0.05)@ZIF-71 exhibited a high conversion yield of approximately 95 %, which increased with the reaction time [Figs. 4(b) and 4(c)]. Leaching of the encapsulated Ru(bpy)₃ was not observed during the reactions. Furthermore, Ru(bpy)₃(0.05)@ZIF-71 maintained its conversion yield during four cycles during the reusability test, suggesting its excellent photocatalytic stability [Fig. 4(d)].

We demonstrated that ZIF-71 is an efficient host material for encapsulating Ru(bpy)₃. A series of Ru(bpy)₃(x)@ZIF-71 catalysts were synthesized via an in situ method. Their photocatalytic activities were analyzed for the photocatalytic conversion of α-bromoacetophenone to acetophenone. Ru(bpy)₃(0.05)@ZIF-71 exhibited a high conversion yield (95 %) and excellent recycling stability over three cycles. We believe these findings will promote the development of similar host– guest photocatalytic systems where photocatalytic molecules are encapsulated in ZIF materials.

This work was supported by a Research Grant from the Pukyong National University (2021).

The authors declare no conflicts of interest.