Luminescence of the three MOFs was found to be quenched by molecular oxygen (Fig. ). Whereas such behaviour was expected for the MOFs based on phosphorescent Pd(II) and Pt(II) porphyrins it is very unusual in case of PCN-224 formed by a fluorescent free-base porphyrin. In fact, fluorescent dyes are usually seen as "immune" to oxygen quenching due to their short emission lifetimes. Luminescence quenching by molecular oxygen is of dynamic nature and the Stern-Volmer equation ( ) is used to adequately describe the quenching dependency in case of dissolved dyes. In these conditions, the decrease of the luminescence intensity is identical to the decrease of the decay time and both show linear dependency on the concentration of the quencher [O2].