Probing the efficiency of thermal and photochemical bond homolysis in functionalized nanostructured SBA-15 silicas

Abstract : The formation of radical species in solution can be triggered through thermal or photochemical activation of a suitable precursor. This EPR study aimed to establish if these two activation modes were as effective in mesoporous silicas as in solution. First, a calibration system was devised and validated to reliably quantify the radical formation. Alkoxyamines were selected to generate stable nitroxyl radicals upon heating or irradiation. Thanks to direct synthesis by the sol-gel process, these precursors were selectively located on the pore surface or in the framework of mesoporous silicas. The thermal or photo-chemical activation of the functionalized materials showed that nitroxides were formed in yields comparable to those observed in solution. No significant differences were observed with the implementation of these activation modes between the solution and the mesoporous silicas. Moreover, the thermal experiments enabled to measure the C-O bond dissociation constants of alkoxyamines covalently anchored to a nanostructured silica, their values were in the same order of magnitude than those determined in solution.