Synthesis, Oxygen-Sensing Properties, and Photodegradation of PCN-224-Type Metal–Organic Frameworks Based on Partly Fluorinated Tetrakis(4-carboxyphenyl)porphyrins

Porphyrin-based metal–organic frameworks (MOFs) are promising candidates for applications involving photoexcitation, such as optical oxygen sensing and photocatalysis. The introduction of electron-withdrawing substituents into the chromophore is an effective way to enhance and improve the photostability of conventional oxygen-sensing materials, but this approach has not yet been studied for MOFs. Herein, we report the synthesis of organic linkers based on meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP), which contains eight fluorine atoms in the meso- or β-positions, and their incorporation into Zr-based MOFs. Similar to TCPP, fluorinated metal-free porphyrins and their Pt(II) complexes form PCN-224-type MOFs. These MOFs show room-temperature fluorescence (in case of nonmetalated linkers) and phosphorescence (with platinated linkers), which is quenched by molecular oxygen. The photophysical and oxygen-sensing properties of MOFs composed of fluorinated and unfluorinated porphyrinic linkers are similar, and no enhancement of photostability is observed. Exposure of MOFs to dimethylformamide vapor strongly accelerates their photobleaching, indicating that this solvent, which is considered essential for the synthesis of Zr-based MOFs, plays a pivotal role in their photodegradation. Studying the photodegradation of linkers in solution revealed much faster bleaching of linkers with free carboxylic acid groups than that of the corresponding methyl esters, suggesting that defects in MOFs resulting in noncoordinated COOH groups may also contribute to photobleaching.