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

ABSTRACT The success of near‐infrared (NIR) to visible upconversion (UC) and NIR downshifting photoluminescence (PL) is mainly due to 4 f ‐4 f energy transfer (ET) interactions of lanthanide ion pairs. This concept was later extended to 3 d ‐4 f ion pairs, where Cr 3+ enabled UC PL via intra‐configurational spin‐flip (ICSF) transition. Interestingly, recent studies reveal that heavier transition‐metal ions such as Mo 3+ (4 d ) can exhibit multiple ICSF emissions spanning the NIR‐I and NIR‐II regions. This finding raises a key question: can a 4 d ‐4 f 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 Mo 3+ /Er 3+ ‐codoped Cs 2 NaInCl 6 double perovskite, exhibiting tunable NIR‐II downshifting emissions at 1095 nm (Mo 3+ ) and 1540 nm (Er 3+ ). Both emissions can be achieved by excitation of either Mo 3+ d ‐electrons or Er 3+ f ‐electrons through efficient 4 d ‐4 f ET, as established by temperature‐dependent (300–6.3 K) PL. Furthermore, this 4 d ‐4 f interaction enables the demonstration of two‐photon UC PL from Mo 3+ d ‐electrons at 700 nm upon Er 3+ f ‐electrons excitation at 980 nm. This finding establishes 4 d ‐4 f interaction as a strategy to simultaneously tune downshifting and UC pathways, opening a new opportunity to tailor optical and optoelectronic materials.