Photoinduced Charge-Transfer Processes on MOF-5 Nanoparticles: Elucidating Differences between Metal-Organic Frameworks and Semiconductor Metal Oxides

Abstract

Metal-organic frameworks (MOFs), which exhibit large surface area and porosity retention upon solvent removal, have attracted considerable attention due to their elegant topology and potential applications in separation, gas storage, nonlinear optics, and catalysis. We report herein comprehensive studies on the nature of luminescence transitions in MOF-5 nanoparticles, which are referred to as MOF-5_n, and the interfacial charge transfer from the photoexcited MOF-5_n to various organic compounds. The time-resolved diffuse reflectance (TDR) and fluorescence spectroscopies were combined in order to clarify the photoinduced one-electron oxidation processes of organic compounds on the MOF-5_n. First, to identify the nature of the luminescence transitions in MOF-5, the temperature dependences of the spectral characteristics were elucidated and compared with those of ZnO nanoparticles. The quenching of MOF-5 emission by several substrates (S), such as aromatic sulfides and amines, was then investigated using steady-state and time-resolved fluorescence spectroscopies. The one-electron oxidation reaction of S during the 355 nm laser flash photolysis of MOF-5_n in acetonitrile was directly examined using TDR spectroscopy, and it was revealed that MOF-5 has a much higher oxidation reaction efficiency than that of P-25 TiO2 powder, which is the most common photocatalyst. The experimental data were rationalized in terms of the Marcus theory on the electron transfer reactions. Moreover, the influence of adsorbed water on the reaction processes was examined because the MOF crystal morphology is affected by exposure to water during synthesis or after evacuation. Consequently, the present study provides an excellent opportunity to understand the difference in the photoinduced charge-transfer processes between MOFs and semiconductors.