TiO₂-Coated CsPbI₃ Quantum Dots Coupled with Polyoxometalates for On/Off Fluorescent Photoswitches

Lead halide perovskite quantum dots (QDs) have attracted significant attention because of their excellent optoelectronic properties. In this study, we focused on reversibly modulating the photoluminescence (PL) emission of perovskite QDs using a redox cluster of polyoxometalates (POMs). Three different CsPbBrxI3–x (x = 0, 0.4, and 0.7) QDs of 9.6∼12.8 nm diameter were synthesized, stabilized by TiO2 coating, and coupled with (Bu4N)4[W10O32] (tetra-n-butylammonium decatungstate: TBADT) in organic solution. The TiO2-coated CsPbBrxI3–x (CsPbBrxI3–x@TiO2 core/shell) QDs showed bright PL emissions at 705, 678, and 605 nm, which were efficiently quenched by one-electron-reduced W10O325– (POM–) via photoinduced electron transfer (PET). Particularly, the PL emission at 705 nm of CsPbI3@TiO2 QDs was most efficiently quenched by 95% via PET and Förster resonance energy transfer (FRET) because of a large spectral overlap between the QD emission and POM– absorbance. The quenching mechanism was analyzed by steady-state and time-resolved PL measurements. CsPbI3@TiO2 QDs was found to photocatalytically reduce POM to POM– by visible light. The PL emission from CsPbI3@TiO2 QDs was reversibly switched between On and Off states by alternately exposing the QD–POM system to intense visible light (PL quenching via PET and FRET) and reoxidation of POM– in ambient air (PL recovery). The obtained results open the possibility of constructing perovskite QD-based photoswitches for super-resolution imaging, optical data storage, smart display, and bioimaging.