Optical resolution of chiral inorganic crystals achieved through alcohol addition

A research group consisting of Assistant Professor Yusuke Inomata from the Faculty of Advanced Science and Technology, Graduate Student Suwan Yamada from the Graduate School of Science and Technology, and Professor Tetsuya Kida from the Institute of Industrial Nanomaterials at Kumamoto University, has discovered that optical resolution of cesium copper chloride (CsCuCl₃), a chiral inorganic crystal, can be achieved by adding alcohols during crystallization. Their findings were published in Crystal Growth & Design.

While chirality is a widely known concept in organic molecules, chiral substances also exist among inorganic crystals. CsCuCl₃ is known as a chiral inorganic crystal with both right-handed and left-handed forms, and chiral functional properties derived from its structure are expected. However, the challenge has been that right-handed and left-handed domains coexist during crystallization, forming racemic twin crystals.

Quartz (SiO₂), a naturally occurring mineral, is also a chiral inorganic crystal, but it has different morphologies and twin crystal structures depending on the location where it is found. This is thought to be due to differences in crystal growth conditions such as crystallization temperature and the presence of impurities. Conversely, this indicates that the chirality of inorganic crystals can be controlled by crystal growth conditions.

The research group had previously clarified that crystallization temperature is important for optical resolution of CsCuCl₃. In this study, they found that when organic solvents such as alcohols are added during crystallization, cesium copper chloride crystallizes separately into right-handed and left-handed forms, enabling optical resolution.

Since CsCuCl₃ is a water-soluble compound, it was crystallized by solvent evaporation from aqueous solution (crystallization temperature 35℃). With water alone, racemic twin crystals with mixed right-handed and left-handed domains were obtained, but when organic solvents were added, the crystals separated into right-handed and left-handed forms during crystallization. Among organic solvents, alcohols were particularly effective for optical resolution, with 1-pentanol and ethylene glycol showing significant effects. When the researchers compared crystal morphologies, they found that the addition of alcohols suppressed c-axis growth of the crystals, resulting in different growth faces. These differences in crystal growth processes are thought to be related to whether racemic twin crystals form or crystals with single chirality are obtained.

Furthermore, when an increased amount of organic solvent was added, the achiral phase Cs₃Cu₃Cl₈(OH) was formed. When crystallization was conducted using this as a seed crystal, chiral cesium copper chloride precipitated on the Cs₃Cu₃Cl₈(OH) crystals. This result indicates that a chiral phase is not necessarily required for the formation of chiral crystals.

This study identified one methodology for obtaining chiral inorganic crystals. This technique may potentially be applicable to the optical resolution of chiral inorganic crystals other than CsCuCl₃. It is expected to serve as a method for freely obtaining chiral inorganic crystals.

Journal Information
Publication: Crystal Growth & Design
Title: Solvent and Achiral Crystalline Phase-Induced Chiral Resolution of CsCuCl₃
DOI: 10.1021/acs.cgd.5c01350