Optical isotherms as a fundamental characterization method for gas sensing with luminescent MOFs by comparison of open and dense frameworks

Abstract

Optical isotherms as a novel fundamental characterization method for MOF sensors are presented by a combination of simultaneous monitoring of sorption processes of different analyte gases (N₂, Ar, CO₂, and O₂) together with in situ photoluminescence spectroscopy. Thereby, a direct correlation of both properties, luminescence and adsorption, is achieved, which provides a direct quantitative access to the effect of the MOF–analyte interaction on the photoluminescence of a MOF system. In addition, changes in equilibration time of the sorption process, temperature dependence and cyclic repetition can be systematically investigated. Thereby, optical isotherms establish a frame of reference for MOF luminescence sorption sensors. The MET MOF system (MET = metal triazolate) was chosen as a porous model candidate. A strong intensity increase of the photoluminescence of the MET-type MOF ³_∞[Zn(Tz)₂], (Tz⁻ = 1,2,3-triazolate), was achieved by introduction of Mn²⁺ as a luminescence activator. Statistical replacement of Zn²⁺ with Mn²⁺ in ³_∞[Zn_0.9Mn_0.1(Tz)₂] retains the original structural microporosity. The obtained optical isotherms were further compared to results from a non-porous, luminescent coordination polymer ³_∞[Sr_0.95Eu_0.05(Im)₂] in order to elaborate the novel characterization concept in a broader context, showing that this concept is a fundamental step to achieve quantitative read-out of the sensing signal for both, porous and dense systems.