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Increasing the Fluorescence Quantum Yield and Lifetime of the Flavin Chromophore by Rational Design

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Rational design: Both 7,8-difluoro- and 9-fluoromethylisoalloxazine as well as the 7,8-difluorinated riboflavin analogue were synthesized and their photophysical properties (fluorescence, intersystem crossing, internal conversion, and water mediated quenching) are reported. We found very good agreement between previous quantum chemical predictions and experimental results making rational design of new fluorescent probes feasible.


Abstract

A previous quantum chemical study (M. Bracker et al., Phys. Chem. Chem. Phys. 2019, 21, 9912–9923) on the excited state properties of fluorinated derivatives of the flavin chromophore promised an increased fluorescence performance of the derivative 7,8-difluoro-10-methyl-isoalloxazine (7,8-dF-MIA). Here, we describe the synthesis of 7,8-dF-MIA, its ribityl derivative, and for reason of comparison 9-F-MIA. The compounds dissolved in water (H2O and D2O) were characterized by steady state, time resolved, and fluorescence correlation spectroscopy. The experiments confirm the increase of the fluorescence quantum yield of 7,8-dF-MIA (0.42 in H2O) compared to MIA (0.22) predicted by quantum chemistry. The anticipated reduction of the fluorescence quantum yield for 9-F-MIA is also confirmed experimentally. The quantum chemical computations as well as the spectroscopic observations attribute the increased fluorescence quantum yield of 7,8-dF-MIA predominantly to a decrease of the rate constant of intersystem crossing. Switching from H2O to D2O as a solvent is shown to increase fluorescence quantum yields (0.53 for 7,8-dF-MIA) and lifetimes of all fluorinated MIA derivatives. This can be attributed to a Förster type energy transfer from the excited chromophore to vibrational overtones of water and further water-mediated deactivation processes.

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