Luminescent Porphyrinic Metal-Organic Frameworks for Oxygen Sensing: Correlation of Nanostructure and Sensitivity

Oxygen sensing capabilities of a series of luminescent zirconium-based porphyrinic metal-organic frameworks (MOFs) (PCN-222, PCN-223, and PCN-224) have been investigated. Design of experiments allowed identification of the most sensitive material, a PCN-224 MOF. In this material, luminescence in air is quenched by up to 7.3-fold with a bimolecular quenching constant kq of 56,000 Pa-1 s-1, which is significantly above the previously reported benchmark material of the same MOF type (kq of 34,000 Pa-1 s-1). The oxygen sensitivity increases in the order PCN-222 < PCN-223 < PCN-224 with effective Stern-Volmer (SV) constants in the range of 0.13 kPa-1 for PCN-222, 0.18 kPa-1 for PCN-223 and up to 0.45 kPa-1 for PCN-224-type materials. This effect is attributed to the increase in the porphyrin-porphyrin separation in these nanostructured materials that results in the increase in the fluorescence decay time and thus the probability of the quenching event. Defects in the nanostructure also are important and explain variations in the sensitivity within the MOFs of the same type. Among the investigated materials, those belonging to the PCN-223 type were found to be the most robust in terms of oxygen sensing capabilities with only little variation in the SV constants. The analogues of the PCN MOFs that utilize a Pt(II) porphyrin as a building block were also prepared. It is shown that the same synthetic protocols established for the metal-free systems can be used for the metal complex. Analogous to the MOFs based on the metal-free porphyrin, the oxygen sensitivity increases from Pt(II)PCN-222 (KSV 30 kPa-1) via Pt(II)PCN-223 (KSV 40 kPa-1) to Pt(II)PCN-224 (KSV up to 87 kPa-1).