Direct evidence for distinct colour origins in ROY polymorphs

Area of Science:

• Solid-state chemistry
• Materials science
• Crystallography

Background:

• ROY is a highly polymorphic organic material known for its varied crystal structures and colors.
• The origin of color in ROY polymorphs is generally attributed to molecular conformation.
• The orange needle (ON) polymorph of ROY is considered a potential exception to this rule.

Purpose of the Study:

• To experimentally determine the origin of color in the ON polymorph of ROY.
• To investigate the effect of pressure on the ON polymorph's color and crystal structure.
• To rationalize the observed color changes using theoretical calculations.

Main Methods:

• High-pressure, single-crystal X-ray diffraction measurements.
• Hybrid density functional theory (DFT) calculations for band structure analysis.
• Analysis of intermolecular interactions, specifically π⋯π stacking.

Main Results:

• Direct experimental evidence shows color origin in ON is intermolecular, not conformational.
• The ON polymorph exhibits a reversible pale orange to dark red color change between ambient pressure and 4.18 GPa.
• Band structure calculations accurately account for color variations in five ROY polymorphs, highlighting ON's outlier behavior due to pressure-modulated π⋯π stacking.

Conclusions:

• Intermolecular interactions, particularly π⋯π stacking, are the primary drivers of color in the ON polymorph of ROY.
• Pressure significantly influences these intermolecular interactions, leading to observable color changes.
• This study provides a comprehensive understanding of color origins in polymorphic organic materials.