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 [C 6 H 12 NO][FeCl 4 (H 2 O)] 0.5 [FeCl 4 ] 0.5 ·0.5H 2 O ( 1 ) and [C 6 H 12 NO][FeCl 4 ] ( 2 ). Compound 1 crystallizes in the centrosymmetric space group P 2 1 / m . Upon the loss of both coordinated and guest water molecules, cooperative rotational and translational rearrangements of the [FeCl 4 ] − anions and [C 6 H 12 NO] + cations occur, accompanied by reconstruction of the hydrogen-bonding network. This transformation breaks the inversion symmetry of 1, yielding compound 2, which crystallizes in the noncentrosymmetric space group Cm . 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 2 exhibits a proton conductivity 2 orders of magnitude lower than that of 1, demonstrating a pronounced switching behavior in proton transport.