Abstract
Photooxidation utilizing visible light, especially with naturally abundant O-2 as the oxygen source, has been well-accepted as a sustainable and efficient procedure in organic synthesis. To ensure the intersystem crossing and triplet quantum yield for efficient photosensitization, we prepared amidated alloxazines (AAs) and investigated their photophysical properties and performance as ...
Abstract
Photooxidation utilizing visible light, especially with naturally abundant O-2 as the oxygen source, has been well-accepted as a sustainable and efficient procedure in organic synthesis. To ensure the intersystem crossing and triplet quantum yield for efficient photosensitization, we prepared amidated alloxazines (AAs) and investigated their photophysical properties and performance as heavy-atom-free triplet photosensitizers and compared with those of flavin (FL) and riboflavin tetraacetate (RFTA). Because of the difference in the framework structure of AAs and FL and the introduction of carbonyl moiety, the absorption of FL at similar to 450 nm is blue-shifted to similar to 380 nm and weakened (epsilon = 8.7 x 10(3) for FL to similar to 6.8 x 10(3) M-1 cm(-1)), but the absorption at similar to 340 nm is red-shifted to similar to 350 nm and enhanced by similar to 50% (from epsilon = 6.4 x 10(3) for FL to similar to 9.9 x 10(3) M-1 cm(-1)) in AAs. The intersystem crossing rates from the S-1 to T-1 are also enhanced in these AAs derivatives, while the fluorescence quantum yield decreases from similar to 30 to similar to 7% for FL and AAs, respectively, making the triplet excited state lifetime and the singlet oxygen quantum yield of AAs at least comparable to those of FL and RFTA. We examined the performance of these heave-atom-free chromophores in the photooxidation of sulfides to afford sulfoxides. In accordance with the prolonged triplet excited state lifetime and enhanced triplet quantum yield, 2-5-fold performance enhancements were observed for AAs in the photooxidation of sulfides with respect to FL. We proposed that the key reactive oxygen species of AA-sensitized photooxidation are singlet oxygen and superoxide radical anion based on mechanistic investigations. The research highlights the superior performance of AAs in photocatalysis and would be helpful to rationalize the design of efficient heavy-atom-free organic photocatalysts.
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