Flavin Dibromide as an Efficient Sensitizer for Photooxidation of Sulfides

Flavin derivatives (FLs) are the building blocks and functional groups within many enzymes that absorb strongly in the visible light region and are redox cofactors in a large number of biological processes. We directly attached Br atoms into the conjugated framework of FL to afford FL dibromide (DBFL) and expected the heavy atom effect of Br to facilitate the intersystem crossing of excited FLs to reach the triplet states for efficient sensitization of O-2. Compared with FL (epsilon = 1.01 x 10(4) M-1 cm(-1) at 441 nm), DBFL shows stronger absorption in the visible range (epsilon = 1.90 x 10(4) M-1 cm(-1) at 450 nm). The singlet oxygen quantum yield of DBFL is enhanced from 55.3% in FL to 92.2% at the expense of decreased luminance quantum yield from 37.7% in FL to 5.5%, confirming that a large portion of the excited DBFL molecules evolves into triplet excited states. Both FL and DBFL were used in photosensitized oxidation of various sulfides to afford corresponding sulfoxides. DBFL exhibits a two-fivefold performance enhancement with respect to FL in sensitizing O-2 for photocatalytic oxidation. In addition, the oxidation of sulfides with DBFL was found efficient and led exclusively to sulfoxides, with no secondary oxidation products observed. Mechanistic investigations showed that both singlet oxygen and superoxide anion radical are formed as reactive oxygen species. The findings pave the way for design and application of novel organic sensitizers for photocatalytic oxidation.