Metallic paints are used, e.g., for home decorations, cars, or art. Most of these paints owe their metallic look to flakes of aluminum, copper, zinc, or other metals. However, metal flakes tend to settle to the bottom of the can, requiring regular stirring during use. Multiple coats of metallic paint are often needed to provide good coverage. Also, when used in ink-jet printers, metallic pigments can clog ink nozzles. Nonmetallic, organic paints with a metal-like luster could solve these issues. However, so far, few candidates provide the required solubility, film-forming properties, and stability.

Katsuyoshi Hoshino, Chiba University, Japan, and colleagues have developed metal-free, organic dyes that can form films resembling gold or bronze. The team made two different versions of a chloride-doped 3-methoxythiophene oligomer (pictured), which, unlike a previously reported perchlorate-doped variant, can be dissolved in water. They subjected 3-methoxythiophene to oxidative polymerization with FeCl₃ or FeCl₃·6H₂O to obtain the oligomers. The researchers then coated glass plates with aqueous solutions of the new dyes and allowed them to dry.

The two chloride-doped oligomers produced lustrous gold- and bronze-like films, respectively. However, they each formed a non-glossy dark stain near the center. Further research is needed to determine how to avoid this effect. The paints are soluble in water but can be made water-resistant by dehydration, which makes their industrial implementation feasible. The new dyes could be used in commercial ink-jet printers.

• Oligo(3-methoxythiophene)s as Water-Soluble Dyes for Highly Lustrous Gold- and Bronze-like Metal-Effect Coatings and Printings,
  Minako Tachiki, Reo Tagawa, Katsuyoshi Hoshino,
  ACS Omega 2020.
  https://doi.org/10.1021/acsomega.0c02752