Direct chiroptical correlation of dissymmetric crystal morphologies

Area of Science:

• Crystallography
• Materials Science
• Physical Chemistry

Background:

• Molecular dissymmetry in crystals, or chirality, has been studied since Pasteur.
• Chirality links macroscopic crystal form to microscopic molecular structure.
• Enantiomorphous tartrate salt crystals were historically used to demonstrate chirality.

Purpose of the Study:

• To establish a direct correlation between enantiomorphous metal-organic framework (MOF) crystals and molecular chirality.
• To investigate the relationship between crystal habit geometry and single-crystal optical activity.
• To determine the heterochirality of mirror-image MOF forms when X-ray methods are insufficient.

Main Methods:

• Single-crystal X-ray scattering was used to analyze crystal structure and space group.
• Chiroptical imaging with a complete polarimetric microscope was employed.
• Optical circular birefringence measurements were performed on MOF enantiomorphs.

Main Results:

• A direct correlation was shown between enantiomorphous MOF crystals and molecular structure chirality.
• Crystal habit geometry correlated with single-crystal optical activity in low-symmetry directions.
• X-ray scattering suggested a hexagonal, enantiomorphous space group, but could not confirm heterochirality.

Conclusions:

• Chiroptical imaging provides a method to correlate optical and morphological chirality in MOFs.
• This technique overcomes limitations of X-ray crystallography for determining MOF enantiomorph heterochirality.
• The study confirms the link between macroscopic crystal properties and molecular chirality in MOFs.