The present invention concerns materials consisting in a porous and structured network, this network being at least in part formed by Si atoms, or Si atoms and metal atoms, linked to each other by oxy bridges, comprising organic molecules which include at least one nitroxyl radical -NO· and which are covalently bonded to the network via siloxy bonds, the amount of nitroxyl radical -NO· ranging from 30 to 120 nmol per square meter of material characterized in that : Q > 0.4 and 4s < T1a < T1b < 800s, with Q, T1a * and T1b being obtained by the method defined in claim 1.