This study aims at finding fluorinated flavin derivatives with modified intersystem crossing (ISC) and fluorescence properties. In total, photophysical properties of 8 derivatives were investigated computationally using combined density functional theory and multireference configuration interaction methods. On top of a screening procedure, the excited‐state decay mechanisms of selected chromophores were investigated in detail. Kinetic schemes including fluorescence, ISC as well as internal conversion (IC) channels were set up to unravel the complex excited‐state decay kinetics. We find two chromophores that exhibit promising properties with respect to fluorescence microscopy. Distinctive stabilization of the bright S1 ππ* state results in absorption in the green wavelength region and emission of (infra‐)red light. The 1(πHπL)⤳3(πH–1πL) ISC channel of the flavin chromophore was found to be deactivated upon both modifications, but nonradiative deactivation of the fluorescence by IC appears to be a problem. Alternative modifications of the pteridine dione moiety were found to result in a marked stabilization of nπ* states along with activation of El‐Sayed allowed ISC channels. For the latter two compounds, we predict fluorescence to be quenched by ISC followed by efficient population of the long‐lived T1 state via IC.

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.

The Front Cover illustrates a full strike in fluorescence! Double fluorination of isoalloxazines at positions 7 and 8, to which quantum chemistry has pointed at, gives compounds with the highest fluorescence quantum yields reported so far for its class. The mallet shows the power of fluorination for the modulation of intersystem crossing when aimed delicately. The corresponding difluorinated flavins show even higher quantum yields when using deuterated solvents. Cover design by Marie Glasewald. More information can be found in the Research Article by Claus A. M. Seidel, Peter Gilch, Constantin Czekelius and co‐workers.