Dyes & Color

By Dr. Klaus L.E. Kaiser —— Bio and Archives May 13, 2013

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Dyes and pigments are everywhere. They make us see things in many colors. The blue of your jeans is just one dye of many.

Some History

In ancient times (say Neanderthals’ time), the available dyes were mostly minerals, such as “ocher,” rust, carbon (black) and similar pigments. Pictographs made with them some tens of thousands of years ago have survived quite well to this date. Later on, just a few thousand years ago, some organisms-derived organic dyes became widely known. They include Indigo (the blue dye in “blue jeans”) from the plant Indigofera tinctoria, Tyrian Purple (a purple dye, the Roman imperial purple) from the snail Murex sp., Cochineal (a red dye) from the aphid-like insect Sternorrhyncha sp. among many others.

The evolution of chemistry changed it all. Beginning with the elucidation of the chemical structures of some of the natural dyes above, chemists found ways to duplicate those en masse in laboratories and create derivatives and novel dye structures on the basis of then-recognized simple principles. Fig. 1, showing the chemical structure of indigo helps to understand that principle. All the “aromatic bonds” (indicated by a pair of parallel lines spaced by a single line) create a large molecule with a “conjugated-bond” structure. Such conjugated-double bond arrangements have optical absorption bands in the visible spectrum of light and hence are dyes. Fig. 1. The chemical structure of the indigo (blue) dye from the Indigo plant. Just a (small) change of the chemical structure causes a shift in the material’s spectrum to make it a purple dye. This compound has two additional bromine atoms, as indicated by “Br” in Fig. 2. Fig. 2. The chemical structure of the Tyrian Purple (purple-red) dye from the Murex snail. Today, such synthetic dyes number now in many tens of thousands. Your (color) photocopier or printer, your newspaper and every glossy magazine you look at makes liberal use of such inventions. Without the glitzy color, the new car ads just wouldn’t have the same sex appeal!

The World of Color

Just twenty years ago, most newspapers were strictly black and white without any color pictures whatsoever. These days, in the same papers you’ll find full page color-ads and pictures throughout. While we humans enjoy a world of color, others on Earth are only able to perceive dark and light. For example, deer and moose do not have color vision. Instead, their dark and light perception far exceeds ours. That allows them to move through dense forests in the middle of the night without much difficulty. Yet other species perceive entirely different “colors,” namely parts of the electromagnetic wave spectrum we cannot see at all; the infrared spectrum belongs to that. Many insects, certain snakes and other critters have organs which can “see” infrared radiation as it is emanating from warm bodies. We only feel that radiation as warmth hitting our skin when sitting near a camp fire or similar heat source, but we cannot see it with our eyes. Without our ability to perceive color, the world would appear more bleak, just in shades of light and dark. Aren’t you glad that chemistry invented some fabulous additions to the world of color?