Reversible Color Switching of an Organic Crystal Induced by Organic Solvent Vapors

5-Aminoisophthalic acid (AIPA) was found to undergo reversible color change on exposure to solvent vapors, which is called vapochromism, via pseudopolymorphic transformations between the pale-pink hemihydrate and the yellow anhydrous crystals. By exposing methanol, ethanol, or acetonitrile vapor on the hemihydrate crystal, the hemihydrate crystal transformed into a novel anhydrous crystal and the anhydrous crystal returned into the hemihydrate crystal under the existence of water vapor. X-ray structural analysis revealed that the AIPA molecule is in a zwitterionic form in the hemihydrate crystal whereas it is in a nonionic form in the anhydrous phase. Theoretical calculations revealed that the isomerization between nonionic and zwitterionic forms causes a solid-state color difference.