In 2009, a researcher at Oregon State University achieved a breakthrough in color history by creating a vibrant blue pigment. Now, the same researcher has developed long-lasting, reddish magentas that were inspired by lunar mineralogy and ancient Egyptian chemistry. Mas Subramanian, a distinguished professor of chemistry, led the study, which was funded by the National Science Foundation and the findings were reported in the journal Chemistry of Materials. The new pigments are a result of collaboration with OSU colleagues.The new coatings, suitable for vehicles and buildings, are made with divalent chromium, Cr2+, which is the first of its kind to be used as a chromophore. Chromophores are the parts of a molecule that determine color by reflecting some wavelengths of light while absorbing others. According to Subramanian, the Milton Harris Professor of Materials Science at OSU College of Science, no earth-based mineral has been found to contain divalent chromium as one of its components. However, the analysis of lunar mineral samples collected from Apollo missions has revealed the occurrence of chromium in the divalent state.
In 2009, Oregon State University scientist Mas Subramanian and his team accidentally discovered a vibrant blue pigment that they named YInMn blue. This new pigment is the result of a combination of yttrium, indium, and manganese oxides, with the manganese ions in a highly unusual +3 oxidation state. This unusual oxidation state of manganese is what gives YInMn blue its intense color.
The discovery of YInMn blue led to its licensing by the Shepherd Color Company for use in a variety of coatings and plastics. Additionally, it inspired the creation of a new Crayola crayon color called Bluetiful.
YInMn blue was the result of researchers’ experiments with new materials that could be used in electronics applications. They mixed manganese oxide, which is typically black in color, with other chemicals, and heated them in a furnace to nearly 2,400 degrees Fahrenheit.
The researchers discovered a stunning blue pigment called YInMn blue, which was named after the elements yttrium, indium, and manganese. This pigment was the first blue pigment discovery in 200 years and represented a major advancement in terms of safety, durability, and vividness. The discovery was a significant breakthrough.
The researchers drew inspiration from the divalent copper found in Egyptian blue, which is the world’s oldest known synthetic pigment dating back over 5,000 years. In their new study, Subramanian, research associate Jun Li, and graduate student Anjali Verma replaced the divalent copper in Egyptian blue with a different component.The process involves the use of divalent chromium, which produces long-lasting, reddish magenta pigments. To ensure the stability of the divalent chromium on Earth, the researchers subjected the synthesis to high temperatures of almost 2,500 degrees Fahrenheit, under high vacuum conditions. The process began with chromium metal, chromium trioxide, and other chemicals.
According to Subramanian, “Today, most magenta-colored pigments are made from organic chemicals, but they are not very stable when exposed to ultraviolet rays and heat from the sun because the organic chemical bonds can break down. Inorganic magenta pigments are not commonly found, and most of them require a large amount of hazardous cobalt salts.”
The magenta pigments created by OSU scientists are resistant to both heat and chemicals due to their high preparation temperature, and they maintain their structure and color when exposed to acid and alkali, according to the researchers.
Furthermore, unlike pigments containing cobalt, the chromium-based magenta pigments are highly reflective of solar heat, providing a cooling effect that could potentially save energy for vehicles and buildings coated with them.
Subramanian explained, “Most pigments are discovered by accident because the source of a material’s color is often not known.”
The color of a pigment not only depends on its chemical composition but also on the arrangement of atoms in its crystal structure. Therefore, in order to understand the color of a pigment, the material must first be created in a laboratory and then its crystal structure thoroughly studied. Despite recent advances in quantum mechanical theories and computational methods, it is still difficult to predict a crystal structure that will produce a specific color for an inorganic pigment, the researcher added. The discovery of YInMn blue was a stroke of luck, and now efforts are being made to establish fundamental chemical and crystal structural design principles to systematically create new pigments.The speaker mentioned that identifying the essential elements needed to create vibrant colors could lead to faster pigment discoveries. They emphasized that while science does not always follow a set path, they are currently researching pigments with divalent chromium as a chromophore in various coordination environments in the crystal structures of different inorganic compounds.
Reference:
Anjali Verma, Jun Li, M. A. Subramanian. “Cr2+ in Square Planar Coordination: Durable and Intense Magenta Pigments Inspired by Lunar Mineralogy”. Chemistry of Materials, 2024; DOI: [link]The article can be accessed at the following link: http://dx.doi.org/10.1021/acs.chemmater.4c00253.