- March 01, 2003, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
Chance discoveries are common in the history of material science. Many discoveries have been the result of chance observations and accidents rather than of systematic search. However, in the field of discovery, chance typically favors the prepared mind. It is not enough to have accidents; everyone has those. It requires the right state of preparedness to profit from the happy accident. Too often, the ill-prepared simply will wash the failed experiment down the drain.
One of the great discoveries of the last century was phthalocyanine pigments. These were discovered during the era of dye stuff research exemplified by William Perkins' chance discovery of the first synthetic purple dye and Karl Neumann's synthesis of indigo (after a chemist accidentally broke a mercury thermometer, which led to the formation of mercuric sulphate, which catalyzed the formation of the dye).
Phthalocyanine pigments were discovered by A.G. Dandridge at Scottish Dye Ltd. (later to become part of I.C.I.). In an experiment, he noticed blue crystals on the side of a vat containing molten phthalic anhydride. These turned out to be complexes of iron. Their chemical structure was related to haem in haemogloblin and chlorophyll. With copper as the metal, the pigment, introduced commercially in 1935, was called Monastral Blue.
Copper phthalocyanine is made up of carbon, hydrogen, nitrogen, and one copper ato and exists in two crystalline forms: a red shade, called Alpha, and a green shade, which is termed Beta and is the standard form used in cyan inks for process printing.
The crystal structure of this pigment is quite extraordinary. From a top view, it resembles a snowflake having four equidistant corners with a six-edge star at each corner. The middle is an open space occupied by a copper atom. Facing the copper atom are eight nitrogen atoms neatly stitched together forming an umbrella around the copper atom. The stitching involves the interaction of “d-orbitals” in copper that neatly coordinate themselves with the electron-rich nitrogen atoms.
A side view of this pigment would show a planer flat molecule with clouds of electrons above and below the plane. This planer structure allows adjacent molecules to get close and form hard crystals.
The center of this structure is dominated by a copper ion. Copper dissolved in water has pale blue color, but if other electron-rich atoms are around, it turns a deeper blue. Nitrogen atoms form strong interactions with the electrons on the copper ion and increase the blue color.
Copper is not the only atom that can occupy this center space. There are some 66 known metal complexes of this pigment.
If we introduce chlorine or bromine and replace the hydrogens in the copper phthalocyanine structure, the blue color shifts to shades of green. Standard Pigment Green 7 is a green pigment based on chlorinated copper phthalocyanine. This pigment, as well as other green shades, was introduced in 1938.
All these phthalocyanine pigments have excellent color strength and lightfastness, and they are chemically quite inert. Clearly, these pigments represent one of the major discoveries of the last century and play an important part in today's packaging and printing of raw materials.
Perhaps the lesson to learn from these chance discoveries is this: “Don't throw away your unwanted mess too hastily.” I have pleaded my case, but my wife just doesn't appreciate I may be at the threshold of a great discovery. She would rather I clean up my office mess and come down to dinner. Oh well, there is always tomorrow.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; mailto:email@example.com