Oregon State University is developing purple pigments inspired by lunar mineralogy

A team of researchers from Oregon State UniversityA team of scientists led by chemistry professor Mas Subramanian has successfully developed long-lasting reddish-purple pigments inspired by lunar mineralogy and ancient Egyptian chemistry. The study, funded by the National Science Foundation, was published in the journal Materials Chemistry.

The new pigments, which can be used as energy-efficient coatings for vehicles and buildings, are based on divalent chromium, Cr2+, and are the first pigments to be used as a pigment carrier. Chromphores are parts of a molecule that determine color by reflecting some wavelengths of light while absorbing others.

Professor Subramanian explained that so far, no terrestrial mineral has been reported to contain chromium in the divalent state as one of the components. However, analysis of lunar mineral samples collected from the Apollo missions showed the presence of chromium in the divalent state.

Divalent chromium has the same number of unpaired electrons as trivalent manganese, and is the chromophore responsible for the intense blue color YInMn, the pigment discovered by Subramanian's team 15 years ago. Shepherd Color Company has licensed YInMn blue for use in a wide range of coatings and plastics.

When YInMn blue was discovered, researchers were experimenting with new materials that could be used in electronic applications, mixing manganese oxide, which is black, with other chemicals, then heating them in an oven to approximately 2,400 degrees Fahrenheit. It turned out that one of the samples obtained was bright blue, called YInMn blue because of the elements it consists of: yttrium, indium and manganese. This was the first discovery of a blue dye in two centuries, and marked a major advance in safety, durability, and viability.

In the new study, Subramanian, co-researcher Jun Lee, and graduate student Anjali Verma were inspired by the divalent copper that acts as a chromophore in Egyptian blue, the world's first known synthetic pigment dating back more than 5,000 years.

The researchers replaced divalent copper in Egyptian blue with divalent chromium, resulting in long-lasting purple-red pigments. To stabilize divalent chromium on Earth, researchers maintained high temperatures, nearly 2,500 degrees Fahrenheit, in a high vacuum during the synthesis of metallic chromium, chromium trioxide and other chemicals.

“Most purple pigments used today are organic chemicals and suffer from stability problems when exposed to UV rays and solar heat because they can break organic chemical bonds,” Subramanian said. “Inorganic purple dyes are rare and most require a large amount of cobalt salts that are hazardous to both humans and the environment.”

The purple dyes developed by researchers at Ohio State University are thermally and chemically inert due to their high preparation temperature, and remain structurally and optically unchanged upon exposure to acids and alkalis, the researchers note.

Additionally, unlike cobalt-containing pigments, chromium-based purple pigments highly reflect the sun's heat, meaning they have a cooling property that will generate energy savings for cars and structures painted with them.

“Most pigments are discovered by chance,” Subramanian explained. The reason is that the origin of the color of a substance depends not only on the chemical composition but also on the complex arrangement of atoms in the crystal structure. So someone has to first make the material in a laboratory and then carefully study its crystal structure to interpret the color. “Despite recent progress in quantum mechanical theories and computational methods, it remains difficult to predict the crystal structure that will produce an intense inorganic pigment of the desired color,” he added.

“We got lucky the first time with YInMn blue, and are now devising some basic chemical and crystallographic structural design principles to rationally create new pigments. Identifying the key structural components needed to create vivid colors should allow shorter periods between discoveries.” “Science does not always follow a set path, but we are exploring pigments containing divalent chromium as a pigment carrier in different coordination environments in the crystal structures of various inorganic compounds,” Subramanian said.