YInMn blue - 200 Years in the making: New intense inorganic pigments based on chromophores in trigonal bipyramidal coordination

Abstract

Materials with various colors have always fascinated humans from prehistory to the present. They bring joy to our daily life by making the world a more beautiful place to live in and influence our feelings. The important factor that decides the color of a material is its interaction with light in the visible range (400–700 nm) sensitive to human eye. Transition metal cations engender color because they have electronic transition energies that resonate with frequencies in the visible range. In general, transition metal chromophores with a coordination environment of less symmetry and more mixing between the p and d orbitals will have a greater probability of producing intense colors due to the relaxation of selection rules for the d-d transitions. This review chronicles the historical importance of inorganic pigments, reviews the origin of color in solids and highlights recent advancements in designing color pigments. Although fundamentals of the color science about gemstones and minerals are qualitatively well-understood, it is difficult to predict the color of inorganic solids before they are prepared experimentally. In spite of recent advancements in quantum mechanical theories and computational methods, predicting a crystal lattice that will produce an intense inorganic pigment of a desired color is still elusive. Serendipity often plays a role in the discovery of commercially important pigments and dyes such as Prussian Blue, Perkin's aniline dyes and recently YInMn Blue. We conclude that designing durable, nontoxic and multifunctional inorganic pigments with brilliant colors is quite challenging, and a deeper understanding of the structure-optical property relationship in this respect can be helpful.