CHEMICALS WORLD

I suspect that one of the reasons I chose Inorganic chemistry as my major was due to the colorful chemistry of transition metal complexes. Organic compounds, at least the ones I saw during my first several years in college, were almost always white. And in those instances where some color was present, usually a rather boring pale yellow or brown, it was often due to the presence of impurities.Unfortunately, the electronic transitions responsible for most of the transition metal colors are d-d transitions, which are generally forbidden under the rules of quantum mechanics, so it often requires fairly concentrated solutions to generate rich colors. There are the occasional exceptions – e.g. MnO4-, whose color is due to a quantum mechanically allowed electronic charge transition (the electron jumps from a metal orbital to an oxygen orbital) -- but generally, the extinction coefficients of most inorganic molecules are low.So it’s rather ironic that a majority of the most deeply colored compounds are organic molecules. No forbidden electronic transitions here, just conjugated systems that can be tailored to absorb just about any wavelength of light in the visible and UV spectrum. This property has led to their use as dyes for over 4000 years. A list of the early dyes would include:Alizarin – produced by the madder rootCarmine - obtained from the bodies of cochineal insectsIndigo – obtained from the indigo plantTyrian Purple – a brominated version of indigo, obtained from the Murex (a type of shellfish) in minute amounts, so quite expensive. Only affordable to the uber-wealthy, it eventually became seen as a symbol of royalty (thus the saying “born to the purple”). In Roman times, it was a capital offense to wear it if you were not a noble. Exposure of the dye to alkali turns it crimson, producing the color worn by Cardinals in the Catholic church.