Efficient Photooxidation of Sulfides with Amidated Alloxazines as Heavy-atom-free Photosensitizers

Abstract

Photooxidation utilizing visible light, especially with naturally abundant O 2 as the oxygen source, has been wellaccepted 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 ∼450 nm is blue-shifted to ∼380 nm and weakened (ε = 8.7 × 10 3 for FL to ∼6.8 × 10 3 M −1 cm −1 ), but the absorption at ∼340 nm is red-shifted to ∼350 nm and enhanced by ∼50% (from ε = 6.4 × 10 3 for FL to ∼9.9 × 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 ∼30 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.

Aerobic Photocatalysis: Oxidation of Sulfides to Sulfoxides

“…According to mechanistic experiments, both singlet oxygen and superoxide anion are involved in the oxidation mechanisms. In 2020, a similar study employing an amidated alloxazine as the photocatalyst was introduced [31b] …”

Luminescent cis-Bis(bipyridyl)ruthenium(II) Complexes with 1,2-Azolylamidino Ligands: Photophysical, Electrochemical Studies, and Photocatalytic Oxidation of Thioethers

“…The photocatalytic activity of the 1,2-azolylamidino complexes was tested in the oxidation of sulfides as a model reaction, using ambient oxygen as the oxidant. ,,− The oxidations were performed using methyl p -tolyl sulfide 7a as sulfide and 1 mol % of the Ru­(II) complex 3 or 4 in ethanol (Table ). The reaction mixture was stirred open to the air under white-light irradiation.…”

Enhancing the quantum yield of singlet oxygen: photocatalytic degradation of mustard gas simulant 2-chloroethyl ethyl sulfide catalyzed by a hybrid of polyhydroxyl aluminum cations and porphyrin anions

“…According to current reports, 10,62 1 O 2 has mild oxidizability, and can selectively oxidize HD and its simulant CEES to non-toxic sulfoxide compounds, while O 2 c À can also selectively oxidize suldes to sulfoxide. 64,65,69,70 Therefore, the hybrid used in this study accelerated the degradation of CEES by using singlet oxygen and superoxide ions produced by porphyrin as oxidants.…”