where I0 and τ0 are the luminescence intensity and luminescence decay time under oxygen-free conditions, I and τ the same parameters at a certain oxygen concentration, KSV is the Stern-Volmer constant and kq the bimolecular quenching constant. For indicator dyes immobilized into different matrices (most commonly polymers) such ideal dependency is rarely found. Instead, the dependencies are not linear and not fully identical for the luminescence intensity and the decay time. This is explained by heterogeneity of the environment in which individual indicator molecules are located that results in different oxygen quenchability. As can be seen from Fig. , the same is true for the three MOFs. Such behaviour can be due to combination of several factors including (i) defects in the MOF, (ii) different oxygen accessibility of the porphyrin molecules that are located on the surface of an individual crystal and on the inner pores, (iii) blocking of some pore channels for instance with residual solvent molecules that are trapped inside and hinder oxygen transport. Therefore, the so called "two site model" (Eq. 2) was used to fit the experimental data. The model postulates localization of the indicator in two environments of significantly different oxygen permeability and although is physically meaningful only for the luminescence intensity, it also adequately fits the decay time plots.