TEMPO radically expedites the conversion of sulfides to sulfoxides by pyrene-based metal-organic framework photocatalysis

Abstract

Metal-organic frameworks (MOFs) are well-documented for visible light photocatalysis because of their tailorable structures and tunable absorptions through organic linkers. By employing a highly conjugated linker, 4,4’,4’’,4’’’-(pyrene-1,3,6,8-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid, the optical absorption of the MOF NU-1100 is effectively tuned to visible light below 600 nm region. Under green light irradiation, NU-1100 triggers charge separation and modulates electron transfer from the linkers to the Zr6O4(OH)412+ clusters, driving the oxidation of sulfides to sulfoxides. Notably, adding a redox mediator radically expedites the oxidation of sulfides by NU-1100 photocatalysis, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and 4-carboxy-TEMPO. At least 2.7 and 5.2 times of conversions of phenyl methyl sulfide are achieved by NU-1100 photocatalysis with TEMPO and 4-carboxy-TEMPO, respectively. A series of characterizations illustrate that 4-carboxy-TEMPO is adsorbed onto the exterior surface of Zr6O4(OH)412+ clusters of NU-1100 to mediate hole transfer and achieve higher charge transfer efficiency. Mechanistic studies indicate that superoxide is the essential reactive oxygen species and that the oxidation of sulfides is driven by an electron transfer pathway. This study demonstrates the integration of redox mediators with MOFs can drive more efficient visible light photocatalytic reactions.