![]() In the solid flavins, π-stacking interactions of the molecules lead to a charge transfer state with low oscillator strength resulting in aggregation-caused quenching (ACQ) with low lifetimes and quantum yields. Theoretical calculations using plane-wave and QM/MM methods are in good correspondence with the experimental results and explain the electronic structures as well as the photophysical properties of crystalline MIA and the composites. Both the fluorescence lifetime and quantum yield of decrease with the flavin loading in MIL-53 due to the formation of various J-aggregates. The fluorescence quantum yield ( Φ F) of the composites is about half of the solution but is significantly higher compared to the solid flavin dyes. The fluorescence-weighted lifetime of of 4 ± 1 ns also corresponds to those in solution but is significantly prolonged compared to the solid flavin dyes with less than 1 ns, thereby confirming the concept of “solid solutions” for composites. Time-resolved spectroscopy showed that multi-exponential fluorescence lifetimes were needed to describe the decays. As compared to the flavins in solution, the fluorescence spectrum of these composites is broadened at the bathocromic side especially for MIA. ![]() The maximum amount of flavin dye incorporation is 3.9 wt% for (Al) and 1.5 wt% for (Al), 0.85 wt% for and 5.2 wt% for For the high incorporation yields the probability to have more than one dye molecule in a pore volume is significant. We used a post-synthetic, diffusion-based incorporation into microcrystalline MIL-53 powders with one-dimensional (1D) pores and an in-situ approach during the synthesis of MOF-5 with its 3D channel network. The flavin derivatives 10-methyl-isoalloxazine (MIA) and 6-fluoro-10-methyl-isoalloxazine (6F-MIA) were incorporated in two alternative metal-organic frameworks, (MOFs) MIL-53(Al) and MOF-5.
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