Spin‐Flip Upconversion Luminescence and Tunable Downshifting Near‐Infrared Emissions via 4 d ‐4 f Interaction in Doped Halide Perovskite

The success of near-infrared (NIR) to visible upconversion (UC) and NIR downshifting photoluminescence (PL) is mainly due to 4f-4f energy transfer (ET) interactions of lanthanide ion pairs. This concept was later extended to 3d-4f ion pairs, where Cr3+ enabled UC PL via intra-configurational spin-flip (ICSF) transition. Interestingly, recent studies reveal that heavier transition-metal ions such as Mo3+ (4d) can exhibit multiple ICSF emissions spanning the NIR-I and NIR-II regions. This finding raises a key question: can a 4d-4f ion pair be designed to achieve multimode downshifting NIR and UC PL by combining f-f and ICSF d-d transitions? Herein, we report the first synthesis of Mo3+/Er3+ -codoped Cs2NaInCl6 double perovskite, exhibiting tunable NIR-II downshifting emissions at 1095 nm (Mo3+) and 1540 nm (Er3+). Both emissions can be achieved by excitation of either Mo3+ d-electrons or Er3+ f-electrons through efficient 4d-4f ET, as established by temperature-dependent (300-6.3 K) PL. Furthermore, this 4d-4f interaction enables the demonstration of two-photon UC PL from Mo3+ d-electrons at 700 nm upon Er3+ f-electrons excitation at 980 nm. This finding establishes 4d-4f interaction as a strategy to simultaneously tune downshifting and UC pathways, opening a new opportunity to tailor optical and optoelectronic materials.