The efficient and selective photocatalytic conversion of chemicals with visible light and naturally abundant resources has long been desired, but this requires finely designed sensitizers that are capable of converting light into chemical energy for the required energy/electron transfer, bond formation and scission, etc. Inspired by flavin (FL) based enzymes that are capable of initiating many redox reactions in biological systems in visible light, FL and naphthalimide chromophores were fused together as a heavy-atom-free triplet photosensitizer (NI-FL). It is expected that the extended conjugation within NI-FL may benefit absorption in the visible light range, promoting the intersystem crossing to triplet excited states for efficient chemoselective conversions. The absorption and emission maximum of NI-FL are redshifted by ∼40 nm and the absorption is more than doubled (1.53 × 104 M-1 cm-1 at 484 nm) with respect to FL (7.5 × 103 M-1 cm-1 at 439 nm), and a long-lived triplet excited state of intramolecular electron transfer nature was captured (τT = 45.3 μs). The performances of FL and NI-FL in the photooxidation of sulfides in air were also examined. Apart from nearly quantitative selectivity for sulfoxide, NI-FL demonstrates a 0.5-5 fold enhanced performance with respect to FL and the conversion proceeds through radical intermediates formed by electron transfer at excited states with substrates. Mechanistic investigation satisfactorily explained the observed photophysical properties and the dominant role of radical intermediates in the NI-FL catalyzed photooxidation. The findings may help to understand photooxidation sensitized by FL derivatives and benefit the design of novel efficient photosensitizers.