Water‐Mediated Reversible Crystalline Phase Transformation Enables Color Switching and Circularly Polarized Luminescence Handedness Inversion

Stimuli-responsive circularly polarized luminescence (CPL) modulation is highly sought for adaptive chiroptical systems, yet achieving reversible CPL handedness inversion in crystalline solids remains challenging. Here, we report organic ionic crystals composed of luminescent cations and chiral anions that undergo crystal-to-crystal transformations through hydration/dehydration cycling. These transformations induce in situ switching between yellow and red emission, accompanied by complete inversion of CPL handedness. Structural analyses reveal that reversible changes in lattice hydration reorganize the supramolecular architecture from a double-helical hydrogen-bonded framework to a single-helical architecture with opposite helicity, while simultaneously reshaping chromophore packing. Theoretical investigations further show that this structural reconfiguration alters intermolecular electronic delocalization and reorients the electric and magnetic transition dipole moments, with the angle between them crossing the critical 90° threshold, thereby accounting for the inversion of both circular dichroism and CPL responses. The reversible optical response is robust over multiple switching cycles, and can be spatially patterned to generate multidomain CPL heterostructures, highlighting its potential for multi-parameter photonic encoding. This work offers fundamental insight into hydration-regulated chiral transfer in molecular crystals, and establishes a design platform for adaptive chiroptical materials.