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.

研究发现，5-氨基间苯二甲酸（5-Aminoisophthalic acid, AIPA）暴露于溶剂蒸气时会发生可逆的颜色变化，该现象被称为蒸气致变色（vapochromism），其变化过程通过浅粉色半水合物（hemihydrate）与黄色无水晶体（anhydrous crystals）之间的假多晶型（pseudopolymorphic）转变实现。将甲醇、乙醇或乙腈蒸气施加于该半水合物晶体表面时，其可转变为一种新型无水晶体；而在水蒸气环境中，该无水晶体又可回复为半水合物晶体。X射线结构分析（X-ray structural analysis）结果显示，AIPA分子在半水合物晶体中以两性离子形式（zwitterionic form）存在，而在无水相（anhydrous phase）中则呈现非离子形式。理论计算（Theoretical calculations）结果表明，非离子形式与两性离子形式之间的异构化（isomerization）是引发固态颜色差异的根本原因。