Fluorescence of A100 MOF and Adsorption of Water, Indole, and Naphthalene on A100 by the Spectroscopic, Kinetic, and DFT Studies

Metal–organic frameworks (MOFs) are promising materials for adsorption and separations. It is important to understand the details of chemical bonding between the adsorbate and structural units in the MOFs. In A100 MOF, the near-UV–visible fluorescence is found to be the intralinker fluorescence. Naphthalene and indole form the stoichiometric “host-guest” π–π adsorption complexes with A100 that contain one adsorbate molecule per two BDC linkers, and adsorption of indole causes a strong quenching of the intralinker fluorescence. The excitation wavelength dependent steady-state fluorescence spectra, the nanosecond time-resolved fluorescence spectra, and DFT calculations indicate the strong π–π interactions between adsorbed indole and naphthalene and aromatic ring of the BDC linker, as well as hydrogen bonding between adsorbed indole and COO group of the linker. Activated A100 adsorbs up to four water molecules per BDC linker. Kinetic study of adsorption of naphthalene and indole from n -alkane on hydrated A100 yields the preferential adsorption of indole as determined by the in-situ time-dependent fluorescence spectroscopy and complementary ex-situ UV–vis absorption spectroscopy.