Azulene - an interesting blue organic compound

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I was reading an article about some interesting features of azulene, a bicyclic organic compound that is apparently present in some blue mushrooms and sea creatures. So, I decided to learn more out of curiosity.

Azulene is usually viewed as resulting from fusion of cyclopentadiene and cycloheptatriene rings. Like naphthalene and cyclodecapentaene, it is a 10 pi electron system.
https://en.wikipedia.org/wiki/Azulene

It is different than indole - https://en.wikipedia.org/wiki/Indole

Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha’s Rule​

https://pubs.acs.org/doi/10.1021/jacs.3c07625

Knowledge of how the renegade azulene can flout the rules may help scientists harness its energy for solar power tech, according to Popular Mechanics.
. I'm not going to link to the article, since the comment seems overly speculative for now. Suffice it to say that azulene has attracted some attention in the chemistry research area.
 
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Surprising that this is blue. Usually conjugated systems of this size would be yellow/orange. Just goes to show that intuition sometimes leads to spurious conclusions.
 

What is azulene and why is it unique?

Azulene is a unique organic compound known for its distinct deep blue color. It is a non-benzenoid aromatic hydrocarbon consisting of fused five- and seven-membered rings. Unlike most aromatic compounds which are typically colorless, azulene's structure results in significant electronic delocalization that gives it a vibrant blue hue. This unusual coloration in organic compounds is due to its inverted electron distribution compared to its isomer naphthalene.

What are the primary uses of azulene?

Azulene is primarily used in cosmetics and medicinal products due to its anti-inflammatory, soothing, and antioxidant properties. It is commonly found in skincare products like creams and lotions meant to calm irritated skin. Additionally, azulene is used in the formulation of dyes and pigments for its strong and stable blue color.

How is azulene synthesized?

Azulene is typically synthesized through a variety of organic reactions. One common method involves the cyclization and dehydrogenation of acyclic precursors like guaiazulene. Another method is the Diels-Alder reaction, followed by dehydrogenation. The synthesis can be challenging due to the instability of the seven-membered ring, making it an interesting subject for research in organic chemistry.

What are the challenges in working with azulene?

Working with azulene presents several challenges. Its synthesis can be complex and requires careful handling due to the compound's reactive nature. Additionally, azulene's low solubility in water and most organic solvents can complicate its use in various applications. Researchers and chemists must also manage its stability under different conditions to prevent degradation or color loss.

How does the structure of azulene affect its properties?

The structure of azulene, which includes a five-membered ring fused to a seven-membered ring, significantly affects its chemical and physical properties. This unusual structure leads to a dipole moment, which is rare in aromatic hydrocarbons and results in higher reactivity and lower symmetry compared to more common aromatic compounds like benzene. The electronic effects of this structure are also responsible for azulene's characteristic blue color, as they alter the way it absorbs light.

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