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 O2. Compared
with FL (ε = 1.01 × 104 M–1 cm–1 at 441 nm), DBFL shows stronger absorption
in the visible range (ε = 1.90 × 104 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 twofivefold performance enhancement with respect
to FL in sensitizing O2 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.
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