Azulene

Other names: bi­cyclo[5.3.0]deca­penta­ene

C₁₀H₈

11/50 common

Fusing together the (colorless) simple hydrocarbon rings cyclopentadienide and tropylium produces the bicyclic aromatic chromophore azulene, whose vivid blue color is quite uncharacteristic for a comparatively small hydrocarbon. The physical origin of this color may in fact be understood as the result of an absorption in the red-orange region of the visible light spectrum roughly corresponding in energy to an electronic transition between a cyclopentadienide and a tropylium ring – explaining the intense hue of azulene where its structural isomer naphthalene is colorless. Azulene derivatives are among the few blue compounds found in nature, being responsible for the unique coloration of various herbs and mushrooms such as Lactarius indigo and Entoloma hochstetteri.