Vapochromism associated with the changes in the molecular arrangement of organic crystals

Abstract

Exposure of the yellow anhydrate crystals of 4′-alkoxy-4-azachalcones to water vapor gave red hydrate crystals. On the other hand, dehydration of the hydrate phase by heating recovered the anhydrate yellow phase. A comparison of the crystal structures of the anhydrate and hydrate crystals confirmed that the molecular arrangement is significantly changed during the hydration–dehydration processes: the changes in the orientation mode from head-to-tail to head-to-head and the displacement of chloride ions and water molecules are involved in the crystal transformations. PXRD studies clarified the reversibility of the crystal transformations occurring in the hydration–dehydration processes.