Have You Seen this Color Before?

Blue dyes have been around since the days of crushing lapis lazuli: a silicate based inorganic mineral. But now, a new set of compounds, which was the subject of a recent article in the New York Times, has been added to a list of blue inorganic pigments which has not been altered in the last 200 years. The Y-In-Mn containing oxides, discovered by a team of materials science researchers headed by Mas Subramanian at Oregon State University, was an unexpected product of a high temperature reaction involving constituent metal oxides. This addition of an inorganic synthetic pigment presents an environmentally benign alternative to the current pigments used in industry for creating blue paints.

 The new compounds, with a general formula YIn_1-xMn_xO₃, were synthesized back in 2009 from a high temperature reaction of Y₂O₃, In₂O₃, and Mn₂O₃ and shows a range of blue hue depending, on the level of Mn content, i.e. the larger ratio of manganese in the compound, the more intense the blue color becomes. This is the first example of a manganese containing compound being involved in a synthetic dye. So, the question is, what is special about these compounds such that they have this characteristic blue color?

As it turns out, the compound highly absorbs light in the green and red regions of the electromagnetic spectrum, meanwhile there is little absorbance in the blue region. As a result, the compound appears blue to the eye. The absorbance in the red-green region is highly dependent on the manganese content, though, with higher Mn concentrations yielding greater and broader absorbance. Thus, the compounds with a lower Mn content appear to be a more navy-blue meanwhile those with higher amounts of Mn take on a vibrant blue. And this color remains highly stable, according to the researchers, with no indications that the color will degrade. This pigment was appropriately named after the element it contains (YInMn Blue) and was approved for industrial use in coatings and plastics in 2017 by the Environmental Protection Agency.

This series of compounds represents the first synthetic inorganic pigment that has been synthesized in 200 years and is a revolutionary finding for the paint/coating industry. Pigments like Cobalt Blue, Prussian Blue, and ultramarine, amongst others, have been the go to inorganic pigments for getting a blue color. But each of these have historically had drawbacks that this new YInMn Blue does not. Cobalt Blue, for instance, is a toxicity hazard being that in contains  cobalt. Prussian Blue is unstable under acidic conditions. And, not only is ultramarine unstable under heat, but also, the manufacturing of the pigment causes the release of harmful emissions into the environment. Thus, YInMn Blue provides the blue color without the potential toxicity, instability, and harmful emissions while also having a tuneability for its color. In other words, this new synthetic pigment provides a better alternative.

This YInMn oxide is now available for purchase by artists everywhere as a paint and was entered into the Forbes Pigment Collection at Harvard University. The compound's brilliant blue color has even attracted Crayola where a crayon shade is being devised with the name "Bleutiful." This surely provides an example of the broad impacts of chemistry as this discovery left its mark on the paint and coating industry all the way to children who are excited to draw with this new shade of blue. 

Sources:

https://www.nytimes.com/2021/02/05/style/blue-pigment-YInMn.html

Smith, A.; Mizoguchi, H.; Delaney, K.; Spaldin, N.; Sleight, A.; Subramanian, M. Mn3+In Trigonal Bipyramidal Coordination: A New Blue Chromophore. Journal of the American Chemical Society 2009, 131 (47), 17084-17086.