Synthesis and Characterization of Face-to-Face Arylsulfanyl and Phenoxyl Radicals Embedded in Mesoporous Silicas

Abstract

Recent applications of bi-, oligo-, or polyradicals to spin sciences have generated considerable interest in the design of new bi-/oligoradical organic molecules. Nevertheless, studies of physicochemical properties open shell systems are generally based on stable homonuclear organic radicals, which represent only a small portion of the existing radicals. In this context, diazene precursors of heteroatomic radicals were incorporated into the SBA-15 silica framework. Photolysis at 360 nm of the diazene moiety resulted in the formation of two face-to-face oxygen- and sulfur-centered radicals. These systems were characterized by X-, Q-, and W-band EPR spectroscopy by comparing them to silicas functionalized with either face-to-face sulfur-centered radicals or face-to-face oxygen-centered radicals. These EPR studies allowed measuring their half-life as well as their relaxation times (T1 and T2). The properties of silicas functionalized with sulfur- and oxygen-centered radicals fall between those measured for the two reference systems functionalized with only sulfur- or oxygen-centered radicals. These nanostructured silicas functionalized by two radicals of different nature are new potential candidates as a polarizing agent for DNP NMR.