Unraveling the Origin of Mn(II) Near‐Infrared Luminescence via Crystal Field Engineering

Elucidating the intrinsic origin of near‐infrared (NIR) luminescence in Mn 2+ ‐activated materials remains a long‐standing contentious issue. Herein, we systematically clarify this origin for the first time, resolving the long‐standing controversy over Mn 2+ NIR emission properties. Specifically, precise modulation of the HCl/ethanol volume ratio in the reaction medium enables the controlled‐continuous phase transition of manganese‐based halide perovskites, yielding crystalline phases with distinct photoluminescent properties: CsMnCl 3 emits deep‐red light at 660 nm, whereas Cs 2 MnCl 4 ·2H 2 O exhibits efficient NIR luminescence at 780 nm with exceptional thermal stability. More strikingly, Cs 2 MnCl 4 ·2H 2 O exhibits a pronounced anti‐thermal quenching effect, with its relative photoluminescence (PL) intensity rising to about 300% of that at room temperature at 420 K. Through systematic structural characterization, local coordination environment analysis, crystal field parameter quantification, and first‐principles calculations, we established a clear structure‐optical property relationship that effectively eliminates interference from local crystal field variations induced by host lattice differences—a critical limitation confounding previous studies. This work establishes a reliable, tunable experimental platform, laying a solid foundation for elucidating the intrinsic mechanism of the Mn 2+ ‐based NIR luminescence. Furthermore, the anti‐thermal quenching behavior enhances its high‐temperature optical performance, demonstrating great potential for high‐temperature imaging and advanced anti‐counterfeiting applications.