Water-Induced Reversible Structural Transition Enables Dual Cooperative Switching of Second-Harmonic Generation and Proton Conductivity

A reversible single-crystal-to-single-crystal (SCSC) dehydration-rehydration phase transition was realized between [C₆H₁₂NO][FeCl₄(H₂O)]_0.5[FeCl₄]_0.5·0.5H₂O (<b>1</b>) and [C₆H₁₂NO][FeCl₄] (<b>2</b>). Compound <b>1</b> crystallizes in the centrosymmetric space group <i>P</i>2₁/<i>m</i>. Upon the loss of both coordinated and guest water molecules, cooperative rotational and translational rearrangements of the [FeCl₄]^- anions and [C₆H₁₂NO]⁺ cations occur, accompanied by reconstruction of the hydrogen-bonding network. This transformation breaks the inversion symmetry of <b>1</b>, yielding compound <b>2</b>, which crystallizes in the noncentrosymmetric space group <i>Cm</i>. The reversible phase transition involves a distinct symmetry change and induces a switchable second-harmonic generation response. Moreover, due to the reorganization of the hydrogen-bonding network, compound <b>2</b> exhibits a proton conductivity 2 orders of magnitude lower than that of <b>1</b>, demonstrating a pronounced switching behavior in proton transport.