Synthesis and Optical/Electronic Properties of Imitation-copper Crystals Based on Low-molecular Azobenzene Derivatives

Three imitation-copper compounds were synthesized from low-molecular azobenzene derivatives: 4-(3-methylbutoxy)-4′-cyanoazobenzene (NC-azo-DC), 4-(3-methylbutoxy)-4′-nitroazobenzene (NO2-azo-DC), and 1,4-bis[4′-(3-methylbutoxy)phenylazo]benzene (Bis-DCazo). Each compound crystallized with large plate-like planar faces. Copper-colored luster was attained using filtration to accumulate the planar crystals. Specular reflection spectroscopy and calculations indicate the reddish color was enhanced by extending π-conjugation of the azobenzene phenyl rings. These imitation-copper compounds were facilely synthesized and metal free, and did not exhibit electronic conductivity, making them promising substitutes for conventional metal-based lustrous pigments and dyes.