Reversed Quantum‐Well Engineering Unlocks Large Ultraviolet Chiral Nonlinear Optical Response in a Water‐Resistant 2D Hybrid Fluorozirconate

Two‐dimensional (2D) chiral organic–inorganic hybrid metal halides (OIHMHs) are promising for chiroptical applications. However, conventional Ge/Sn/Pb‐based systems remain limited by narrow bandgaps and poor moisture stability. In this work, we report the first 2D chiral hybrid fluorozirconate, ( R/S ‐MBA)ZrF 5 (MBA = methylbenzylammonium), addressing these limitations through reversed quantum‐well engineering enabled by a high‐valent Zr─F framework. It features a unique reversed Type‐I quantum‐well electronic structure, where the [ZrF 5 ] inorganic layer acts as a dielectric barrier and the band‐edge states are localized on the organic MBA cations. This structure yields a wide bandgap of 4.60 eV, a short UV cutoff edge of 265 nm, and a high laser‐induced damage threshold exceeding 1251.58 GW/cm 2 , providing a broad transparency window toward the UV region. Furthermore, ( R/S ‐MBA)ZrF 5 exhibits a large SHG circular dichroism (SHG‐CD) response with an anisotropy factor of 1.05. More importantly, it overcomes the typical moisture‐sensitivity of OIHMHs, maintaining its structural stability and SHG‐CD response after a 7‐day water immersion. Structural analysis and theoretical calculations reveal this unusual water resistance is mainly attributed to the robust Zr─F framework and an effective cavity volume of 3.6%. This work highlights reversed quantum‐well engineering as a novel way to synthesize water‐stable and wide bandgap chiral nonlinear optical materials.