Pressure Modulates Emission of Cs ₂ WCl ₆ in the NIR‐II Window

Abstract Second near‐infrared (NIR‐II) fluorescent materials hold great potential for a range of applications in biomedical imaging and optical sensing. Nonetheless, their application is limited by short wavelength, low photoluminescence efficiency, and narrow emission spectrum. Here, an emission tuning from the first near‐infrared (NIR‐I) region to the NIR‐II window in the 0D lead‐free vacancy‐ordered double perovskite Cs 2 WCl 6 is successfully achieved through pressure engineering. Upon application of external pressure, a 6.9‐fold emission enhancement and a redshift as large as 227 nm are observed. Pressure enhances structural rigidity, thereby reducing electron‐phonon coupling, reducing non‐radiative losses, and optimizing the emission performance of vibration‐coupled d‐d 1 T 2g / 1 E g → 3 T 1g transitions. Furthermore, the energy gap between the 3 T 1g and 1 T 2g / 1 E g progressively decreases under pressure, W─Cl bond contraction amplifies crystal field splitting, and the multistate emission balance modulation jointly drives emission redshift. This work demonstrates the robust pressure strategy to achieve emission in the NIR‐II window and provides deep insights into the underlying mechanism between structural evolution and optical properties.