This blog is my attempt to reconnect with the world of chemistry. I have a PhD in Inorganic Chemistry and make a living doing research for a large company in Michigan. As times have changed, that company has changed its focus and I no longer have as much chance to do the basic, fundamental research which I most enjoy. Through this blog, I am hoping to recapture the magic which I felt during my graduate (and undergraduate) days in college. Expect topics on chemistry and alchemy along with some non-chemistry related items which I think might be interesting.

"The chymists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasure among smoke and vapour, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I would change places with the Persian King."

Johann Joachim Becher (phlogistonist)
Acta Laboratorii Chymica Monacensis, seu Physica Subterranea, (1669).

Friday, March 6, 2009

Colorful Chemistry

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 root
Carmine - obtained from the bodies of cochineal insects
Indigo – obtained from the indigo plant
Tyrian 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.

Starting in the 1800s, many of these natural dyes were replaced with aniline based compounds produced from coal tar. Some of these are considered safe enough to eat, which is why Blue No. 2 (indigotine) is found both in your blue jeans and in your blue M&Ms. Not all of today’s dyes are synthetic. Carmine, which is still obtained from insects, is still used to impart a reddish color to some foods in the US, which is creating a bit of an uproar. Not surprisingly, the food industry is not enthused about telling consumers that some of their products are made from bugs.

The blue food coloring referred to as Brilliant Blue FCF (or Blue No. 1) has a noticeable side effect of which most parents are aware. Green poop. I remember the first time my six month old son presented me with such a gift. Unfortunately, he was suffering from some unknown intestinal disorder at the time which already had us a little worried. The only reason I didn’t immediately panic was that the bright Kelly green color was so artificial looking that it was hard to believe it was physiological in origin. Apparently purple goldfish crackers have Blue No. 1 in them.

3 comments:

Ψ*Ψ said...

Dye chemistry is probably my favorite kind! Especially because many dyes are also useful as semiconductors if correctly functionalized. ;)

John Fetzer said...

The dyestuffs drove a lot of the development in organic syntheses. Making better dyes or synthesizing naturally rare ones were the goals.

I got into the polycyclic aromatic hydrocarbons PAHs) and found that they are highly colored. There are even blue, green, or purple ones, the absorbances are above 500 nm. They have molar absorptivities greater than 10,000 and they fluoresce in different colors than their visual one - higher wavelengths.

Yeah, it goes back to basic impressions. My other choice might have been flavor chemistry for analogous reasons.

Anonymous said...

AAH!! soz but im so annoyed im only 12 and me and my friend are working together on STS (science talent search) and ths years theme was chemistry and different topics about that and i thought colorful chemistry ment like awesome experiments. :( lol im so upset!