Synthesis of 1,6′-Bi- and 1,6′:3,6′′-Terazulenes from 1-Pyridyl- and 1,3-Di(pyridyl)azulenes by the Ziegler–Hafner Method

The synthesis of 1,6′-biazulenes 3a and 3b and 1,6′:3,6′′-terazulenes 7a and 7b was established by one-pot reactions via the Zincke-type ring-opening of the corresponding pyridinium salts with diethylamine, followed by the reaction with cyclopentadiene in the presence of sodium methoxide. The intramolecular charge-transfer characters between azulene rings were investigated by UV–vis spectroscopy and theoretical calculations.