Which compound posses highest dipole moment ?

Polycyclic systems : Huckel's (4n + 2) rule applies to monocyclic systems. The rule, however, is not necessarily successful for polycyclic conjugated systems;

 

the (4n + 2) rule should be applied to the peripherat (conjugated) $\mathrm{\pi }$-electrons. The five-membered ring has five and the seven-membered ring has seven n-electrons (two $\mathrm{\pi }$-electrons are common to both rings). If one $\mathrm{\pi }$-electron is transferred from the seven - to the five - ring, each ring will now hive a closed shell of six $\mathrm{\pi }$-electrons (cf. naphthalene). In this condition, the m6lecule will have a dipolar structure, and this has been shown to be the case from dipole - moment measurements. Azulene also behaves chemically as an aromatic compound.