Metal-organic frameworks (MOFs) are coordination polymers with metal sites and organic “linkers”. Aluminum MOFs (Al-MOFs) show versatility, exceptional stability, and ability to interact with many molecules. Most Al-MOFs are fluorescent in near-UV or visible range, but their “conventional” fluorescence spectra are poorly resolved. Recently, stoichiometric complexes of Al-MOFs were reported which are of interest to optical sensing, (photo)catalysis, opto-electronics, separations. Studies of Al-MOFs, other fluorescent MOFs, and their complexes are limited by lack of knowledge of intra-molecular and intermolecular interactions, and opto-electronic properties. For compounds in solution, synchronous fluorescence spectra (SFS) show very narrow, line-shaped peaks. The SFS are obtained when the excitation λexc and emission λemiss wavelengths are scanned at constant Δλ = λemiss - λexc. Studies of inorganic compounds by SFS are very rare. We describe an outstanding capability of solid-state synchronous fluorescence spectroscopy (SS-SFS) to detect optical transitions in the benchmark Al-MOF Basolite A100 and its complexes. Due to excellent resolution vs. “conventional” fluorescence spectroscopy, the SS-SFS allows to assign multiple pathways of absorption/emission of light which are not available by other methods. The first transition is a resonance absorption by linkers at 308 nm and red-shifted emission. The second one is absorption by pairs of linkers (“excimers”) at 387 nm with ligand-to-ligand charge transfer (LLCT) and visible emission. The third one is due to assemblies of linkers (bandgap) absorbing at 355 nm. Further, we report new, high-resolution SS-SFS spectra based on systematic changes in Δλ while λexc is an X axis. The high-resolution SS-SFS was validated by accurate measurements of bandgap in Basolite A100. Next, we describe the SS-SFS of stoichiometric complexes of Basolite A100 with water and dimethyl sulfoxide (DMSO). Formation of complex with water triggers change in “excimers” emission, in accord with lattice change observed by XRD. When the complex with DMSO is formed, the increase in the bandgap is 0.11 eV. Quenching the line-shaped peak of linker (“monomer”) allows identification of DMSO binding site, consistently with FTIR data. The SS-SFS and high-resolution SS-SFS show excellent promise to become methods of choice in characterization of solid-state inorganic substances, coordination compounds, and various hybrid “organic-inorganic” materials.

Advances in high-resolution solid-state synchronous fluorescence spectroscopy: Revealing multiple absorption/emission transitions in MOFs as coordination polymers, and their complexes

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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