Molecular vibrations in solids are related to electronic processes (charge photoexcitation and relaxation). This is of interest to learn from vibronically resolved electronic spectra about interactions of functional groups in nanoporous solids, e.g. metal-organic frameworks (MOFs) with small-molecule adsorbates. First, we report vibronically resolved solid-state “conventional” fluorescence spectra of naphthalene. The distinct 0-0, 0-1, 0-2, 0-3 and 0-4 transitions are observed in the near-UV range under ambient conditions and assigned to specific vibrations within isolated molecules. Excimers of naphthalene show a well-resolved vibronic structure in the near-UV/visible range. Naphthalene forms the stoichiometric complex with MOF Basolite F300. Herein, we use naphthalene as the nanocavity-confined fluorescent ‘‘self-probe’’ showing a) loss of excimer emission, b) decrease of area ratio A(0-2)/A(0-1) of vibronic peaks, and c) non-polar environment. The complementary studies were by UV-Vis DRS and DFT calculations. However, “conventional” fluorescence spectra of most solids are broad and diffuse, so vibronic structure is not observed. For compounds in solution, strong peak narrowing is achieved by synchronous fluorescence spectroscopy (SFS). In SFS, the photoexcitation λexc and emission λemiss wavelengths are simultaneously scanned at constant Δλ = λemiss - λexc. Studies of solids by SFS are very rare. Herein, we present a novel experimental method, solid-state synchronous fluorescence spectroscopy for studies of molecular interactions in MOFs and their complexes. Basolite A100, which is similar to the benchmark MIL-53(Al), shows unprecedented, sharp resonances in the fluorescence spectra due to 1,4-benzenedicarboxylate (BDC) linker. Basolite A100 forms the stoichiometric sorption complex with an archetypal polar molecule of dimethyl sulfoxide (DMSO). Peaks of the sorption complex are blue shifted due to the interactions of BDC linker with DMSO. The complementary XRD and differential scanning calorimetry provide an additional mechanistic insight. The solid-state SFS can be useful in learning about molecular interactions in the nanomaterials beyond MOFs and their complexes.

Studies of molecular interactions by vibronically resolved solid-state fluorescence spectroscopy: From small molecules to MOFs

2019 Spring National Meeting

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