Guaiazulene

Technical Specifications

Product Description & Scientific Applications

Guaiazulene (1,4-dimethyl-7-isopropylazulene) is a naturally occurring alkyl-substituted azulene, built on the fused five- and seven-membered ring system of the azulene chromophore and supplied as a research-grade organic building block. It is available at far lower cost than parent azulene.

Electronic structure of the azulene chromophore. The azulene chromophore is a non-alternant aromatic hydrocarbon. Its π-electron density is distributed unevenly across the two rings, so the highest occupied and lowest unoccupied molecular orbitals occupy different regions and overlap little. The S₀–S₁ gap is therefore unusually small: the transition falls in the visible, and the compound is blue — unlike its colourless, alternant isomer naphthalene. The parent azulene has a permanent dipole moment of 0.80–1.08 D depending on the method of determination, rare for a hydrocarbon.

A polarised, aromatic-ion framework. Electron density shifts from the seven-membered ring toward the five-membered ring, so the chromophore is described as a six-π-electron tropylium cation fused to a six-π-electron cyclopentadienyl anion within one neutral molecule. Both rings thereby attain aromatic stabilisation, and the charge separation makes the five-membered ring the nucleophilic site.

Anti-Kasha photophysics. The azulene chromophore violates Kasha's rule. The gap between the second and first excited singlet states is about 14000 cm⁻¹ in the parent, which suppresses internal conversion from S₂ and produces fluorescence from S₂ rather than S₁. That anomalous upper-state emission is a reason the framework is studied in photophysics.

Feedstock for substituted azulenes. Azulenes are potent nucleophiles for electrophilic aromatic substitution, the 1- and 3-positions being the most reactive. In guaiazulene C-1 already bears a methyl, so C-3 is the free electrophilic site, while the acidity of the protons on the C-4 methyl group provides a separate, regioselective route: the guaiazulene C-4 aldehyde, a building block for C-4-alkenylated and further derivatives. The inactive C-2 position can be reached through a boryl intermediate. Reaction with cationic iron-carbonyl diene complexes couples guaiazulene at C-3 in yields to 98%; decomplexation and onward chemistry give 1-substituted azulenes.

Precursor for functional materials. Oxidative polymerisation with earth-abundant iron oxidants converts guaiazulene to poly(guaiazulene), a low-bandgap conjugated polymer for optoelectronic applications.

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