Functionalized periodic mesoporous organosilicas: from metal free catalysis to sensing

Abstract

In this work a widely usable post-modification route for periodic mesoporous organosilicas (PMOs) was developed. Using the developed method, two diverse ligands, picolinic acid (Pic) and 4,4′,4′′,4′′′-porphyrin-5,10,15,20-tetrayltetrabenzoic acid (Porph), were successfully covalently coupled onto the PMO material and well-characterized. Both obtained materials show high BET surface areas (565 m² g⁻¹ for Pic@PMO, 483 m² g⁻¹ for Porph@PMO and 548 m² g⁻¹ for the unmodified PMO) and pore sizes (5.1 nm). The materials were subsequently tested for their catalytic activity in the cycloaddition of epoxide and CO₂, a frequently studied carbon capture and utilization reaction. Interestingly, both materials showed very good reactivity (with conversions of up to 90%) as metal free heterogeneous catalysts and proved to be perfectly stable in recyclability and aging tests. Moreover, by (co-)grafting Eu³⁺ and Tb³⁺ ions onto Pic@PMO and Yb³⁺ ions onto Porph@PMO a strong emission was observed in the visible and near-infrared (NIR) range, respectively. Eu,Tb@Pic@PMO showed potential for use as a temperature sensor in the physiological range (a maximum S_r value of 2.11 %K⁻¹ was obtained at 273 K), while Yb@Porph@PMO could efficiently be excited within the human tissue penetrating window showing characteristic Yb³⁺ luminescence (with decay times of around 10 μs). These findings prove that simple modifications of this PMO can provide smart materials for very diverse applications.