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|Section2= |Section3= |Section4= }} Azulene is an organic compound and an isomer of naphthalene. Whereas naphthalene is colourless, azulene is dark blue. Its name is derived from the Spanish word azul, meaning "blue". Two terpenoids, vetivazulene (4,8-dimethyl-2-isopropylazulene) and guaiazulene (1,4-dimethyl-7-isopropylazulene), that feature the azulene skeleton are found in nature as constituents of pigments in mushrooms, guaiac wood oil, and some marine invertebrates. Azulene has a long history, dating back to the 15th century as the azure-blue chromophore obtained by steam distillation of German chamomile. The chromophore was discovered in yarrow and wormwood and named in 1863 by Septimus Piesse. Its structure was first reported by Lavoslav Ružička, followed by its organic synthesis in 1937 by Placidus Plattner. == Structure and bonding== Azulene is usually viewed as resulting from fusion of cyclopentadiene and cycloheptatriene rings. Like naphthalene and cyclodecapentaene, it is a 10 pi electron system. It exhibits aromatic properties: (i) the peripheral bonds have similar lengths and (ii) it undergoes Friedel-Crafts-like substitutions. The stability gain from aromaticity is estimated to be half that of naphthalene. Its dipole moment is 1.08 D, in contrast with naphthalene, which has a dipole moment of zero. This polarity can be explained by regarding azulene as the fusion of the aromatic 6 π-electron cyclopentadienyl anion and aromatic 6 π-electron tropylium cation. In order to achieve the stable aromatic sextet in both rings, one electron from the seven-membered ring is transferred to the five-membered ring. Reactivity studies confirm that seven-membered ring is electrophilic and the five-membered ring is nucleophilic. The dipolar nature of the ground state is reflected in its deep colour, which is unusual for small unsaturated aromatic compounds. Another notable feature of azulene is that it violates Kasha's rule by exhibiting fluorescence from an upper-excited state (S2 → S0). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Azulene」の詳細全文を読む スポンサード リンク
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