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.