Adsorption of naphthalene and indole on F300 MOF in liquid phase by the complementary spectroscopic, kinetic and DFT studies

Metal-organic frameworks (MOFs) are the promising functional materials for adsorption in liquid phase. It is important to understand the details of chemical bonding between the aromatic and heteroaromatic adsorbates and major structural units in the MOF sorbent-metal coordinatively unsaturated sites (CUS) and organic linkers. In this paper, we report the mechanistic studies of adsorption of naphthalene and indole on F300 Basolite MOF by two complementary spectroscopic methods and density functional theory (DFT) calculations. Fluorescence spectra, the near-UV/visible diffuse reflectance spectroscopy (near-UV/VIS DRS) and DFT calculations suggest that naphthalene forms an adsorption complex with F300 MOF where naphthalene is quantum confined within the cavity of the F300, weakly electronically bound to the Fe(III) CUS, and strongly dispersively stabilized by side interactions with the benzene rings of the linker of F300. On the other hand, indole forms an adsorption complex with F300 MOF in which indole is electronically bound to the Fe(III) CUS and dispersively stabilized by the side interactions with benzene rings of the linker. Coordination bonds between indole and F300 MOF in the adsorption complex are detected by geometry optimization using the DFT method, and electronic spectra are calculated by the time-dependent-DFT method. The direct spectroscopic proof of the formation of adsorption complex with coordination bonds between indole and F300 MOF is provided by the complementary near-UV/VIS DRS spectroscopy (new absorption bands at 460-660 nm), the wavelength-dependent fluorescence spectroscopy (new fluorescence bands at 410 and 430 nm) and by the time-dependent fluorescence spectroscopy.