Editing Azulene (section)

== Structure and bonding==
[[image:Lactarius indigo 48568 edit.jpg|thumb|left|The blue color of the mushroom ''[[Lactarius indigo]]'' is due to the azulene derivative (7-isopropenyl-4-methylazulen-1-yl)methyl stearate.<ref>{{cite journal |last1=Harmon |first1=A. D. |last2=Weisgraber |first2=K. H. |last3=Weiss |first3=U. |s2cid=21207966 |year=1980 |title=Preformed azulene pigments of ''Lactarius indigo'' (Schw.) Fries (Russulaceae, Basidiomycetes) |journal=[[Experientia]] |volume=36 |pages=54–56 |doi=10.1007/BF02003967}}</ref>]]
[[File:Entoloma hochstetteri.jpg|thumb|right|The blue color of the mushroom ''[[Entoloma hochstetteri]]'' is also identified as another kind of azulene derivative: 7-acetyl-1,4-dimethylazulene.<ref>{{cite thesis |last1=Nicholas |first1=Gillian May |title=Australasian fungi: a natural product study. |date=1998 |page=56 |doi=10.26021/9162 |doi-access=free }}</ref>]]
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 [[wikt:periphery|peripheral]] bonds have similar lengths and (ii) it undergoes [[Friedel-Crafts reaction|Friedel-Crafts]]-like substitutions. The stability gain from aromaticity is estimated to be half that of naphthalene.

Its [[Molecular dipole moment|dipole moment]] is {{val|1.08|u=[[debye|D]]}},<ref>{{cite journal |last1=Anderson |first1=Arthur G. |last2=Steckler |first2=Bernard M. |year=1959 |title=Azulene. VIII. A Study of the Visible Absorption Spectra and Dipole Moments of Some 1- and 1,3-Substituted Azulenes |journal=[[Journal of the American Chemical Society]] |volume=81 |issue=18 |pages=4941–4946 |doi=10.1021/ja01527a046}}</ref> in contrast with naphthalene, which has a dipole moment of zero. This polarity can be explained by regarding azulene as the fusion of a 6 [[Pi bond|π-electron]] [[cyclopentadienyl anion]] and a 6 π-electron [[tropylium cation]]: one electron from the seven-membered ring is transferred to the five-membered ring to give each ring aromatic stability by [[Hückel's rule]]. Reactivity studies confirm that seven-membered ring is electrophilic and the five-membered ring is [[nucleophilic]].

[[File:Azulene resonance.svg|350px|center]]

The dipolar nature of the ground state is reflected in its deep colour, which is unusual for small unsaturated aromatic compounds.<ref>{{cite journal |last1=Michl |first1=Joseph |last2=Thulstrup |first2=E. W. |date=1976 |title=Why is azulene blue and anthracene white? A simple mo picture  |journal=[[Tetrahedron (journal)|Tetrahedron]] |volume=32 |issue=2 |page=205 |doi=10.1016/0040-4020(76)87002-0}}</ref> Another notable feature of azulene is that it violates [[Kasha's rule]] by exhibiting fluorescence from an upper-excited state (S<sub>2</sub> → S<sub>0</sub>).<ref>{{cite journal | last1 = Tétreault | first1 = N. | last2 = Muthyala | first2 = R. S. | last3 = Liu | first3 = R. S. H. | last4 = Steer | first4 = R.P. | year = 1999 | title = Control of the Photophysical Properties of Polyatomic Molecules by Substitution and Solvation: The Second Excited Singlet State of Azulene | journal = [[Journal of Physical Chemistry A]] | volume = 103 | issue = 15| pages = 2524–31 | bibcode = 1999JPCA..103.2524T | doi=10.1021/jp984407q}}</ref>