Gold-colored organic crystals of an azobenzene derivative

A new azobenzene derivative, bis[4-(3-methylbutoxy)phenyl]diazene (DC-azo), was synthesized as yellow powder through a three-step reaction. DC-azo was crystallized from a mixture of acetone/water in the form of highly insulating gold-colored crystals with a specular reflectance of 21% over the wavelength range of 520-800 nm. The CIELAB color space of the crystals indicated that the crystals were slightly more reddish and less yellowish than metallic Au. The crystals retained their gold coloration under dry conditions and were stable up to 383 K. The X-ray diffraction (XRD) measurements estimated the long-range periodical thickness in the crystal to be 1.12 nm, thereby indicating that the DC-azo molecules were oriented such that the azobenzene moieties showed a tilt angle of 32.2° against the crystal surface. The gold-colored crystals of DC-azo could be a potential substitute for the conventional metallic paints.