Deciphering colour mechanisms in co-crystals and salts containing violuric acid and chosen l-amino acids†

The understanding of colour origins in single-component materials is well established, however comprehending the factors that influence colour in multi-component crystalline phases remains a complex endeavor. This study addresses the challenge of predicting absorption properties in solid-state materials, crucial for designing systems for specific applications. We focused on six multi-component crystals based on violuric acid (VA) and selected L-amino acids, chosen for their chemical and structural diversity. This approach allowed us to explore various factors affecting absorption properties, including co-crystal and salt formation, hydrogen bonding, π–π interactions, amino acid side chain conformations, and solvent inclusion. Techniques such as X-ray diffraction, ¹H NMR spectroscopy, and UV-Vis spectroscopy were employed for detailed analysis of the structure and optical properties of the studied systems. The intermolecular interactions were scrutinized through the topology of electron density, Hirshfeld surface analysis, and the non-covalent interaction (NCI) index. UV-Vis titrations of crystallization solutions were also conducted to determine the binding constants of salts. This comprehensive study aims to enhance our understanding of absorption properties in multi-component materials.