Causes of Color: Molecular Orbitals and Charge Transfer

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay


Organic Compounds

Most plant colors owe their hues to molecular orbitals
Most of our dyes are also molecular orbital colors. We first learned to do this in the mid 19th century, but it was hit or miss until we discovered the mechanisms that make it possible.

Organic compounds with at least one double bond take the suffix -ene. Compounds with chains of multiple bonds are termed polyenes. Short chains absorb outside the visible range but as the chain gets longer, the absorption peak shifts into the visible. Beta carotene, a major natural and artificial colorant, has a long chain of -CH=CH- units, as does crosin, the pigment in saffron. and rhodopsin, or visual purple, the pugment in the retina.

Among the molecules with ring structures, two of the most important are hemoglobin, with an iron atom in the center of a complex ring, and chlorophyll, a structurally similar molecule with a magnesium atom. Magnesium isn't a color forming ion, and iron might contribute some color, but the colors of both molecules are due to their molecular orbitals.

Incidentally, the "blood" that appears in the middle of a rare steak, or on the plate, is not really blood but myoglobin, a molecule distantly related to hemoclobin.

Note to chefs: I don't care if it's myoglobin, hemoglobin, chlorophyll or Love Potion Number Nine, seeing meat sitting in a puddle is disgusting and serving it that way is beyond unprofessional.

Charge Transfer

Charge transfer involves dislodging an electron from one atom and moving it to another. Pretty much any combination of atoms can be involved in charge transfer colors.

Cation-Cation

Although titanium is a transition metal, it's not a very common source of color in itself because its ions mostly lack unpaired electrons. But as an impurity, it can be a common source of color in charge transfer. One very common cause of color is charge transfer between Ti and Fe. It produces the spectacular blue of sapphire and the much more mellow blue of kyanite. In both cases the charge transfer.

Cation - Ligand (Anion)

Some of the complex anions, notably the chromates, owe their strong colors to charge transfer between the cation and the surrounding anions.

Crocoite, lead chromate, is rare because it's a combination of two elements that don't frequently occur together, but poroduces spectacular colors when they do.

By far the most important case, in terms of its significance in nature, is the charge transfer between oxygen and iron in ferric oxide, or hematite. Iron is the only abundant color-forming element. Titanium doesn't produce much color, and manganese would be a very distant second.

The red, brown and yellow colors, and even the beige dolomite at bottom right, are charge transfer colors between Fe+++ and oxygen.

The black and dark green colors in the top two rows are charge transfer colors between Fe++ and Fe+++.

Ligand-Ligand (Anion-Anion)

The best known examples of these colors are the blue, sulfur-containing silicates like lazurite.

Note that most cases of charge transfer involve ions capable of multiple ionic states.


Kurt Nassau and the Fifteen Causes of Color
Atoms and Light
Causes of Color: Incandescence and Simple Excitations
Causes of Color: Energy Bands
Causes of Color: Ligand Fields
Causes of Color: Physical Optics

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Created 22 April 2013, Last Update 29 May 2020