Water‐Triggered Domino‐Like Phase Transition in a Molecular Ferroelectric

ABSTRACT Domino‐like phase transitions offer a powerful route to propagate structural changes through local stimuli, yet their coupling with ferroelectricity in molecular materials has remained elusive. Here we show that water enables a domino‐like phase transition in the hybrid copper halide Gua 3 Cu 2 I 5 (Gua + = CN 3 H 6 + ), providing non‐thermal access to a polar molecular ferroelectric. We find that the polar α phase cannot be reached through conventional thermal pathways but instead emerges through a hierarchical transition mechanism. In this mechanism, water first induces a surface solid–liquid–solid conversion via localized dissolution and reprecipitation, which subsequently triggers an interface‐propagated domino‐like transformation from the non‐polar β phase into the ferroelectric α phase in the bulk. This process can be directly visualized through real‐time luminescence contrast and is accompanied by robust, switchable ferroelectric polarization at room temperature. By identifying water as an active mediator rather than a passive environment, our results establish a new pathway for accessing polar phases beyond thermal equilibrium and provide a conceptual framework for coupling domino‐like phase transitions with ferroelectric functionality in molecular materials.