Porphyrins, commonly known as the pigments of life, give rise to the vivid colors associated with blood and green leaves. They are large aromatic species whose colors result from their electronic prop..
Porphyrins, commonly known as the pigments of life, give rise to the vivid colors associated with blood and green leaves. They are large aromatic species whose colors result from their electronic properties. Smaller aromatic compounds are usually colorless but azulenes, a two-membered aromatic ring structure, are commonly deep blue or purple. The image shows the unique colors of some azulene derivatives. The azulene crystals (foreground) sparkle and exhibit a dark indigo color, while a solution in dichloromethane (left) has a distinctive purple color. We are investigating the synthesis of porphyrin-like molecules that incorporate azulene rings. An intermediate was prepared by adding two pyrrole rings to form an azulitripyrrane. The middle beaker contains a solution of this intermediate and shows a striking change in color from the original purple to blue. Further reaction with a pyrrole dialdehyde afforded an azuliporphyrin. The Erlenmeyer flask on the far right contains a dilute solution of the azuliporphyrin and this shows a distinctive shift from blue to a deep olive green color. These results demonstrate how the properties of the azulene ring system can be dramatically altered and thereby produce compounds with radically differing coloration.