A known strategy to obtain highly oxygen-sensitive materials is to immobilize the indicator on the surface of a porous material such as silica-gel or porous aluminium oxide. In such materials the indicator molecules are exposed to the gas that more or less freely diffuses in the channels. As can be seen, the fluorescence of H2TCPP adsorbed on the surface of porous silica-gel is indeed significantly quenched by molecular oxygen (Fig. ). The Stern-Volmer constant of 0.19 kPa -1 is however lower that for PCN-224 despite that the fluorescence decay time is higher for H2TCPP (0 of 10 ns) than for the porphyrin in the MOF (0 = 6.7 ns).