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

Recent applications of bi-, oligo-, or polyradicals tospin scienceshave generated considerable interest in the design of new bi-/oligoradicalorganic molecules. Nevertheless, studies of physicochemical propertiesopen shell systems are generally based on stable homonuclear organicradicals, which represent only a small portion of the existing radicals.In this context, diazene precursors of heteroatomic radicals wereincorporated into the SBA-15 silica framework. Photolysis at 360 nmof the diazene moiety resulted in the formation of two face-to-faceoxygen- and sulfur-centered radicals. These systems were characterizedby X-, Q-, and W-band EPR spectroscopy by comparing them to silicasfunctionalized with either face-to-face sulfur-centered radicals orface-to-face oxygen-centered radicals. These EPR studies allowed measuringtheir half-life as well as their relaxation times (T (1) and T (2)). The propertiesof silicas functionalized with sulfur- and oxygen-centered radicalsfall between those measured for the two reference systems functionalizedwith only sulfur- or oxygen-centered radicals. These nanostructuredsilicas functionalized by two radicals of different nature are newpotential candidates as a polarizing agent for DNP NMR.