Spectroscopic and Theoretical Study on Electronically Modified Chromophores in LOV Domains: 8‐Bromo‐ and 8‐Trifluoromethyl‐Substituted Flavins

Two chemically synthesized flavin derivatives, 8‐trifluoromethyl‐ and 8‐bromoriboflavin (8‐CF₃RF and 8‐BrRF), were photochemically characterized in H₂O and studied spectroscopically after incorporation into the LOV domain of the blue light photoreceptor YtvA from Bacillus subtilis. The spectroscopic studies were paralleled by high‐level quantum chemical calculations. In solution, 8‐BrRF showed a remarkably high triplet quantum yield (0.97, parent compound riboflavin, RF: 0.6) and a small fluorescence quantum yield (0.07, RF: 0.27). For 8‐CF₃RF, the triplet yield was 0.12, and the fluorescence quantum yield was 0.7. The high triplet yield of 8‐BrRF is due to the bromine heavy atom effect causing a stronger spin–orbit coupling. Theoretical calculations reveal that the decreased triplet yield of 8‐CF₃RF is due to a smaller charge transfer and a less favorable energetic position of T₂, required for intersystem crossing from S₁ to T₁, as an effect of the electron‐withdrawing CF₃ group. The reconstitution of the LOV domain with the new flavins resulted in the typical LOV photochemistry, consisting of triplet state formation and covalent binding of the chromophore, followed by a thermal recovery of the parent state, albeit with different kinetics and photophysical properties.