Phosphorescent surface-anchored metal-organic frameworks for optical sensing of oxygen

Thin metal-organic framework (MOF) films, known as surface-anchored metal-organic frameworks (SURMOFs), offer convenient handling and a variety of new applications due to the direct immobilization of porous crystalline materials on various substrates. In this study, we demonstrate the feasibility of using luminescent SURMOFs for optical sensing of oxygen both in the gas phase and in organic solvents. Luminescent SURMOFs were prepared from indium(iii) chloride and platinum(ii) tetrakis(4-carboxyphenyl)porphyrin on pre-treated transparent substrates, such as rigid glass or flexible poly(ethylene terephthalate). These SURMOFs have an average thickness of ∼310 nm and exhibit room-temperature phosphorescence (λ _max = 645-676 nm). This phosphorescence is efficiently quenched by molecular oxygen in the gas phase (I ₀/I _air ∼50, Stern-Volmer constant K _SV ∼4-13 kPa^-1). For the SURMOFs fabricated under similar conditions, the sensitivity does not correlate with the morphology (revealed by scanning electron microscopy) or degree of crystallinity (accessed by grazing-incidence wide angle X-ray scattering) of the porous films. Additionally, it has been demonstrated that the SURMOFs exhibit luminescence in toluene, which is quenched by oxygen (I ₀/I _air ∼ 2.5, K _SV ∼ 0.18 kPa^-1). While optical oxygen sensing in organic solvents remains a significant challenge, the presented approach offers a viable solution, provided that future research can mitigate fast photodegradation of the SURMOFs under continuous irradiation.