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An Oregon State University researcher develops durable, reddish magentas inspired by lunar mineralogy and ancient Egyptian chemistry. The new pigments can be used as energy-efficient coatings for vehicles and buildings.
The pigments are based on divalent chromium - Cr2+. They are the first to use it as a chromophore. Chromophores are the parts of a molecule that determine color by reflecting some wavelengths of light while absorbing others.
Chromium-based Magenta Pigments
Divalent chromium has the same number of unpaired electrons as trivalent manganese. Trivalent manganese is the chromophore responsible for the intense color of YInMn blue, which Subramanian’s team discovered years ago. The Shepherd Color Company licensed YInMn blue for use in a wide range of coatings and plastics, and it also inspired a new Crayola crayon color - Bluetiful.
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To date, no earth-based mineral has been reported to contain chromium in the divalent state as one of the components,” said Mas Subramanian, the Milton Harris professor of Materials Science in the OSU College of Science. “
However, the analysis of lunar mineral samples collected from Apollo missions showed the occurrence of chromium in the divalent state.”
The study was inspired by the divalent copper that serves as a chromophore in Egyptian blue, which is the world’s first known synthetic pigment and dates to more than 5,000 years ago.
The researchers replaced the divalent copper in Egyptian blue with divalent chromium, leading to durable, reddish magenta pigments. To stabilize the divalent chromium on Earth, researchers maintained high temperatures, almost 2,500 degrees Fahrenheit, under high vacuum during the synthesis that started from chromium metal, chromium trioxide and other chemicals.
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Most of the magenta-colored pigments used today are organic chemicals and suffer from stability issues when exposed to ultraviolet rays and heat from the sun because they can break down organic chemical bonds,” continued Subramanian. “
Inorganic magenta pigments are rare, and most require a significant amount of cobalt salts. These are hazardous to both humans and the environment.”
Thermally & Chemically Inert Pigments
The magenta pigments developed by OSU researchers are thermally and chemically inert because of their high preparation temperature and remain unaltered structurally and optically upon exposure to acid and alkali.
In addition, unlike pigments that contain cobalt, the chromium-based magenta pigments are highly reflective of heat from the sun. This means they have a cooling property that would lead to energy savings for cars and structures coated in them.
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Most pigments are discovered by chance,” explained Subramanian. “
The reason is because the origin of the color of a material depends not only on the chemical composition. But also, on the intricate arrangement of atoms in the crystal structure. So, someone has to make the material first in a laboratory, then study its crystal structure thoroughly to explain the color.”
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Despite recent advances in quantum mechanical theories and computational methods, predicting a crystal structure that will produce an intense inorganic pigment of a desired color is still elusive,” stated Subramanian.
Utilizing Divalent Chromium as Chromophore
The NSF funding for the just-published study was a special grant earmarked for high-risk, high-reward research. The grant is known by the acronym EAGER, which stands for Early Concepts Grants for Exploratory Research.
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We got lucky the first time with YInMn blue, and now we are coming up with some fundamental chemical and crystal structural design principles to rationally create new pigments,” noted Subramanian.
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Determining the key structural ingredients required for making vivid colors should allow for shorter times between pigment discoveries. Science doesn't always follow a prescribed path, but we’re exploring pigments with divalent chromium as a chromophore in diverse coordination environments in crystal structures of various inorganic compounds,” added Subramanian.
Source: Oregon State University