Defect structure–performance correlation in Eu3+@UiO-66: design of coordination sites for rapid optical O2 sensing

Abstract

Defect engineering offers an effective route to tailor the local coordination environment, gas transport and excited-state processes in metal–organic frameworks (MOFs). We establish a quantitative structure–property relationship linking defect-modulated porosity in UiO-66-based MOFs to the Stern–Volmer parameter for O₂ sensitivity. By systematically introducing three distinct defect types, we reveal that vacancy formation increases the accessibility of Eu³⁺ sites and accelerates O₂ diffusion, thereby enhancing dynamic quenching, while simultaneously introducing a non-negligible static contribution. An optimal defect concentration (represented by S_BET) is identified for highest O₂ gas response, arising from a balance between O₂ accesibility and the availability of active quenching sites. Phase-correlated luminescence decay measurements show a clear reduction of Eu³⁺ lifetime upon switching from N₂ to ambient air, confirming the contribution of the collisional quenching pathway. The resulting nonlinear Stern–Volmer behaviour is rationalized by an apparent quenching constant K_app,SV, incorporating both the intrinsic quenching rate, the native lifetime, and a structural factor H that reflects defect-dependent porosity. A volcano-type dependence of sensing efficiency on defect concentration further highlights the competing roles of Eu³⁺ exposure and diffusion resistance. This study provides a mechanistic framework for defect-regulated triplet quenching in porous solids and offers a general design principle for lanthanide-based porous optical oxygen sensors. The general design principle established in this study is that defect-engineered, O₂-accessible coordination environments can be used to balance mass transport and triplet-state energy transfer, thereby maximizing lanthanide-based O₂ sensing performance.