Metal-organic frameworks, MOFs are porous coordination polymers which contain metal cations and organic “linkers”. Aluminum MOFs, Al-MOFs show structural versatility, exceptional stability, and affinity to many molecules. Both stoichiometric and sub-stoichiometric complexes of Al-MOFs with molecules were reported, which are of interest to sensing, (photo)catalysis, opto-electronics, separations. Most Al-MOFs show fluorescence in near-UV or visible range, however their “conventional” fluorescence spectra are featureless and poorly resolved. Studies of MOFs and their complexes are limited by lack of knowledge of their intra-molecular and intermolecular interactions upon absorption of light. We present a novel powerful method of solid-state synchronous fluorescence spectroscopy, SS-SFS. It allows obtaining the significantly narrowed spectral peaks with unprecedented line-shaped resonances, even at room temperature. The SS-SFS spectra are collected when the excitation λexc and emission λemiss wavelengths are scanned at constant offset Δλ = λemiss - λexc. The unique advantages of SS-SFS are illustrated by analysis of the benchmark Al-MOF Basolite A100 and its complexes. Due to its excellent spectroscopic resolution, SS-SFS allows assignments of multiple pathways of absorption/emission of light which are not available through other methods, and their accurate energies. In Basolite A100, the first transition is a resonance absorption by linker at 308 nm. The second one is absorption by pairs of linkers aka “excimers” at 387 nm via ligand-to-ligand charge transfer, and the third transition is by linker assemblies (bandgap) at 355 nm. High-resolution SS-SFS was validated through the accurate measurements of bandgap in the MOF. Next, the stoichiometric complexes of Basolite A100 with water and dimethyl sulfoxide DMSO were studied. In complex with water, the “excimer” emission is significantly enhanced in the SS-SFS spectra, consistently with lattice “breathing” observed by powder XRD. The formation of complex with DMSO is accompanied by increase of bandgap by 0.11 eV. Quenching the sharp resonance in SS-SFS due to linker permits identification of binding site, consistently with the complementary FTIR spectra. The SS-SFS shows an excellent potential to become the method of choice in the spectroscopic characterization of solid-state inorganic substances, coordination compounds, and hybrid organic-inorganic materials.

High-resolution solid-state synchronous fluorescence spectroscopy of metal-organic frameworks and their complexes

ACS Spring Meeting 2021

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