Polycycilic systems : Huckel's `(4n+2)`n rule applies to monocyclic system. The rule, however , is not necessarily succesful for polycyclic conjugated system,
The (4n+2) rule should be applied to the peripheral (conjugated ) `pi` electrons . The five-membered ring has five and the seven-membered ring has seven `pi` electrons (two `pi` electrons are common to both rings ) . If one `pi` electron is transferred from the seven -to the five. ring , each will now have a closed shell of six `pi` electrons (cf. naphtalene) . In this condition, the molecule will have a dipolar structure, and this has been shwon to be the case from dipole -moment measurements. Azulene also behaves chemically as an aromatic compound.
