Crystal engineering of amphiphilic organic dye for metallic coloration

Self-assembly control of supramolecular dyes is a challenging research field to generate programmed hierarchical superstructures with various shapes and dimensions that are closely related to their physical properties. The highly ordered thin film prepared by recrystallizing an azobenzene-based amphiphilic molecule (A1GC) provides a gold-like color with a brilliant metallic luster. To understand the origin of the gold-colored crystals, morphological observations and diffraction analyses are performed. It is found that A1GC forms dimers by an intermolecular hydrogen bonding of hydroxyl groups and constructs layered crystal structures by a lateral molecular recognition of nanophase separated building blocks. During the recrystallization process, large flat plate crystals with smooth surfaces are formed by combining small powder crystals. To build the structure, morphology, and property relationship, the gold-colored A1GC crystal is further characterized by using reflection spectra and color space. The maximum reflectance of the gold-colored A1GC crystal is 16% in the wavelength range of 500-800 nm. Without using conventional metallic paints, organic paints can create a brilliant metallic luster by the combination of chemical functionalities and hierarchical superstructures through a bottom-up self-assembly strategy. By using this crystal engineering of amphiphilic supramolecular organic dyes, we successfully demonstrated three-dimensional objects and paintable materials with metallic gold color.