Poly(cyclopropenone)s: Formal Inclusion of the Smallest Hückel Aromatic into π-Conjugated Polymers

The synthesis of precursors to π-conjugated cyclopropenium polymers is described. Monomers for chemical and electrochemical manipulation are easily prepared through electrophilic substitution of in situ generated cyclopropenium cations that are then hydrolyzed to the respective cyclopropenones. The unusually strong dipole moment associated with the cyclopropenone renders this core formally aromatic, an electronic structure that becomes more important within individual monomers upon protonation of the carbonyl function with trifluoroacetic acid or alkylation with triethyloxonium salts. The electronic properties of cyclopropenone polymers in their pristine states and after acidification are discussed along with conjugated carbonyl-containing polymers that are also acid sensitive but without the added element of aromaticity. We find that the increased contributions of cyclopropenium cation aromaticity restrict the quinoidal charge carriers due to the energetically less favorable proposition of disrupting the local aromatic stabilization.