Photosensitized Oxygenation of Sulfides within an Amphiphilic Dendrimer Containing a Benzophenone Core

Abstract

Photosensitized oxygenation of sulfides within amphiphilic dendrimers, G n [n(generation) = 1−3], consisting of a benzophenone (BZP) sensitizing core, apolar interior based on n-undecane spacer, and polar dendron exterior based on 2,2-bis(hydroxymethyl)propionic acid, has been investigated in O₂-saturated methanol. Sulfoxide formation occurring via reaction of O₂ with triplet excited-state sulfide (³sulfide*), which is formed by a triplet energy transfer (TET) from photoformed ³BZP* to sulfide, was accelerated by the dendric sensitizers, where G2 showed the highest yields of alkylaryl and dialkyl sulfoxides. Laser photolysis studies revealed that enhanced access of sulfide to the ³BZP* core inside the apolar microenvironment accelerates the TET to sulfide, whereas prompt migration of polar sulfoxide to the polar outer shell of the dendrimer suppresses a competitive TET to sulfoxide, thus resulting in effective ³sulfide* formation. Another notable feature of the dendric sensitizer appears in oxygenation of diaryl sulfide, which is promoted by a persulfoxide intermediate formed by photooxygenation of dialkyl sulfide; photoirradiation of a mixture of diethyl sulfide (1a) and diphenyl sulfide (4a) with G2 gave 17-fold higher diphenyl sulfoxide (4b) yield than that obtained with unmodified BZP. The apolar microenvironment within the dendric sensitizer encapsulates a large quantity of 4a, which is oxidized effectively by the persulfoxide of 1a, thus resulting in high 4b yield. The BZP core within the dendric sensitizer is stable even by photoexcitation in protic solvent, suggesting potential utilities of this dendric system for effective and selective photosensitized oxygenation of sulfides.