Currently, lanthanide (Ln 3 + )-doped near-infrared (NIR)-emitting double perovskites (DPs) suffer from low photoluminescence quantum yield (PLQY). Herein, we develop a new class of NIR-emitting DPs based on Ln 3 + -doped Cs 2 (Na/Ag)BiCl 6 . Benefiting from the Na + -induced breakdown of local site symmetry in the Cs 2 AgBiCl 6 DPs, effective NIR emissions of Ln 3 + are realized through Bi 3 + sensitization. Specifically, 7.3-fold and 362.9-fold enhanced NIR emissions of Yb 3 + and Er 3 + are achieved in Cs 2 Ag 0.2 Na 0.8 BiCl 6 DPs relative to those in Na-free Cs 2 AgBiCl 6 counterparts, respectively. The optimal absolute NIR PLQYs for Yb 3 + and Er 3 + in Cs 2 Ag 0.2 Na 0.8 BiCl 6 DPs are determined to be 19.0 % and 4.3 %, respectively. Raman spectroscopy and first-principles density functional theory calculations verify the sublattice distortion in Cs 2 (Na/Ag)BiCl 6 DPs via Na + doping. These findings provide fundamental insights into the design of efficient NIR-emitting Ln 3 + -doped DPs for versatile optoelectronic applications.

Lanthanide (Ln3+)‐doped upconversion (UC) nanoprobes, which have drawn extensive attention for various bioapplications, usually suffer from small absorption cross‐sections and weak luminescence intensity of Ln3+ ions. Herein, we report the controlled synthesis of a new class of Ln3+‐doped UC nanoprobes based on CsLu2F7:Yb/Er nanocrystals (NCs), which can effectively increase the intersystem crossing (ISC) efficiency from singlet excited state to triplet excited state of IR808 up to 99.3 % through the heavy atom effect. By virtue of the efficient triplet sensitization of IR808, the optimal UC luminescence (UCL) intensity of IR808‐modified CsLu2F7:Yb/Er NCs is enhanced by 1309 times upon excitation at 808 nm. Benefiting from the intense dye‐triplet‐sensitized UCL, the nanoprobes are demonstrated for sensitive assay of extracellular and intracellular hypochlorite with an 808‐nm/980‐nm dual excited ratiometric strategy.