Reversible Hydration-Induced Ionic Conductivity Switching Accompanied by Large Desorption Enthalpy in Sodium Salts of 1,2,3-Triazole-Fused p-Benzoquinone

Abstract

Stimuli-responsive molecular solids that exhibit multifunctionality are highly sought after for advanced sensing and thermal energy storage applications. However, introducing guest molecules such as water into dense π-conjugated crystal lattices reversibly, while simultaneously achieving significant property modulation, remains a challenge. Herein, we demonstrate that the stepwise substitution of protons with sodium ions in 1,2,3-triazole-fused p-benzoquinone (H₂BTBQ) enables the control of flexibility and hydrophilicity of the crystal lattice, leading to reversible hydration-induced functional switching. While the neutral form H₂BTBQ is anhydrous and insulating, the sodium salts NaHBTBQ and Na₂BTBQ form stable hydrates. Notably, Na₂BTBQ exhibits a reversible two-step water sorption behavior involving four water molecules per formula unit. Thermal analyses revealed that the dehydration of Na₂BTBQ·4H₂O is accompanied by a large endothermic enthalpy change (50.45 and 57.05 kJ (mol-H₂O)⁻¹), which significantly exceeds the heat of vaporization of bulk water, indicating its potential as a solid-state cooling material. Furthermore, alternating current (AC) impedance spectroscopy demonstrated that water adsorption triggers a drastic switching of ionic conductivity in Na₂BTBQ, originating from hydrated sodium ion migration. This work provides a crystal engineering strategy to realize functional materials combining large latent heat storage and dielectric switching via reversible hydration in dense organic salt crystals.