Porphyrin based metal–organic frameworks: highly sensitive materials for optical sensing of oxygen in gas phase†
The optical oxygen sensing capabilities of the porphyrin-based metal–organic frameworks, PCN-224, Pt(II)PCN-224 and Pd(II)PCN-224 were investigated. The bimolecular quenching constants (kq) of 37 000 (PCN-224), 6700 (Pd(II)PCN-224) and 3900 Pa−1 s−1 (Pt(II)PCN-224) were found and reveal an exceptionally high oxygen-permeability for these materials. Fast gas transport within the network, large pore sizes, and electronic and spatial isolation of the porphyrin indicator in the framework are held responsible for the unprecedentedly high kq values. PCN-224 shows 6.7 ns fluorescence lifetime and the fluorescence in air is quenched 4.2-fold. The metal–organic frameworks based on phosphorescent Pt(II) and Pd(II) porphyrins possess significantly longer decay times of 18.6 and 390 μs, respectively, and are suited to detect oxygen in trace and ultra-trace ranges with limits of detection of 1 and 0.015 Pa, respectively. Apart from free-standing crystals, also metal–organic frameworks supported on different fibrous substrates (poly(acrylonitrile) nanofibers, glass fibres), and flat substrates (TLC silica-gel, poly(amide) filter) were prepared in order to provide oxygen sensor materials of practical use. Electrospun and thermally treated poly(acrylonitrile) nanofibers were found to be particularly favourable and the resulting composite material exhibited the same sensitivity as the free crystals. All sensing materials show reversible cross-talk to humidity at levels up to 53% relative humidity but demonstrate a drastic decrease of oxygen sensitivity at high humidity levels and when exposed to water. A decrease of the quenching constant with rising temperature indicates an important role of surface-adsorbed oxygen in the quenching process.