Golden organic crystals of an azobenzene derivative containing two ester bonds

Recently we found while we were synthesising a cationic surfactant that a synthetic intermediate, 4,4'-bis{1-[2-(N,N-dimethylamino)]ethoxy}azobenzene, produced gold-coloured crystals [Matsumoto et al., J. Oleo Sci., 59, (2010) 151]. Here we describe how an azobenzene derivative, bis[4-(ethoxycarbonylmethoxy)phenyl]diazene, also forms gold-coloured crystals after recrystallisation from a mixture of acetone and water. The profile of the specular reflectance as a function of wavelength for the crystals is very similar to that for metallic gold plate, indicating that the crystals have a golden colour. The maximum specular reflectance of the crystals is 16%. Atomic force microscopy observation shows that the crystals have flat surfaces. X-ray diffraction measurements demonstrate the presence of a periodic structure in the crystals, which indicates that monolayers (lamellar structure) consisting of azobenzene molecules with a thickness of 1.42 nm are stacked in the crystals. The gold-coloured crystals are tolerant of ultraviolet light irradiation. The gold-coloured crystals may be useful as a substitute for conventional metallic pigments in specific coating applications.