Considering the significant quenchability of PCN-224 that relies on a fluorescent dye we expected much stronger sensitivity for phosphorescent Pt(II)PCN-224 and Pd(II)PCN-224. Experimental results (Fig. ) show that this is indeed the case. In fact, in case of the former more that 5-fold decrease of luminescence intensity is observed at 0.2 kPa pO2, whereas Pd(II)PCN-224 shows 3-fold quenching at only 2 Pa of oxygen. The limit of detection (LOD) was estimated with the blank value method (blank value + 3 x standard deviation). The LOD of Pt(II)PCN-224 was estimated to be 1 Pa and the LOD of Pd(II)PCN-224 was found to be 0.015 Pa. The materials based on Pt(II) and Pd(II) porphyrins thus cover trace and ultra-trace range of oxygen concentrations. Since the Stern-Volmer constant KSV is proportional to the luminescence decay time of the indicator dye (eq. 1), it is interesting to compare the bimolecular quenching constant kq = KSV/0 that reflects how efficient the diffusion of oxygen in the porous material is. The bimolecular quenching constant kq was calculated using the KSV values obtained from the luminescence intensity plots. The kq constants were calculated to 37000, 3900, and 6700 Pa -1 s -1 for PCN-224, Pt(II)PCN-224 and Pd(II)PCN-224, respectively (Table ). The bimolecular quenching constant is about one order of magnitude higher for the MOF based on fluorescent free-base porphyrin compared to the phosphorescent MOFs. This is in good agreement with the expectations of kq approaching diffusion controlled limit (kdiff) for fluorescent dyes (quenching of the excited singlet state) and 1/9 kdiff for phosphorescent dyes (quenching of the excited triplet state) due to contribution of the spin-statistical factor. For comparison, the kq constant for H2TMCPP embedded into PTMSP was calculated to be 7200 Pa -1 s -1 , that is about 5-fold lower than that for PCN-224 indicating that oxygen permeability of the MOF is significantly higher than that of PTMSP. Although the arrangement of the porphyrin molecules in PCN-224 may be particularly advantageous for efficient oxygen quenching, such property is not truly unique and may be characteristic for many other luminescent MOFs. In fact, kq constant for the MOF MAFx-11 is 19000 Pa -1 s -1 which is about 2-fold lower than for PCN-224. Table summarizes the oxygen-sensing properties of various reported luminescent MOFs as well as selected porous and non-porous materials based on porphyrin indicators embedded in highly oxygen-permeable matrices. It should be noted that the selection of materials was limited to those which allowed for the calculation of quenching constant kq, i.e. contain the data on the luminescence decay time in the absence of the quencher. As can be seen, the bimolecular quenching constant kq for most materials is much lower than that obtained for PCN-224, with the exception of MAF-X11 and H2TCPP adsorbed on other porous materials such as silica-gel and aluminium oxide. Comparison of the trace sensors based on Pt(II) and Pd(II) porphyrins reveals that these indicators immobilized in porous matrices (silica-gel) and highly oxygen-permeable non-porous polymers (Hyflon AD60, silicone rubber, PTMSP) show the kq values in the range of 20-70 Pa -1 s -1 that is roughly 2 orders of magnitude lower than the bimolecular quenching constant obtained for Pt(II)PCN-224 and Pd(II)PCN-224. PCN-224 and its metalated analogues thus appear the most oxygen-permeable materials ever reported.