Azulene-based conjugated polymers: Unique seven-membered ring connectivity leading to stimuli-responsiveness

A novel family of stimuli-responsible polyazulenes connected through the seven-membered ring, in contrast to traditional five-membered connectivity, have been prepared by Yamamoto-, Sonogashira-, and Stille-coupling of 4,7-dibromoazulenes. Polymer absorption can be tuned by structural modification of the conjugated backbones with UV-vis and EPR studies indicating that the seven-membered ring connection of the azulene unit is effective for the generation and stabilization of azulenium cations. The degree of conjugation and the reversibility of optical properties upon protonation and deprotonation is sensitive to the steric environment of the seven-membered ring and can be tuned by both substitution and incorporation of linkers between the azulene units.