Enhancement of Push-Pull Properties of Pentafulvene and Pentafulvalene Derivatives by Protonation at Carbon

We report visible color changes and new intense, bathochromically shifted bands in electronic absorption spectra that reach into the near-infrared region (up to 862 nm) upon protonation of nine pentafulvene and expanded pentafulvalene derivatives. This phenomenon can only be explained by the formation of carbocations with highly delocalized charges. Solution pK(a) values in organic solvents were determined, making use of the method of relative basicity measurements. All seven 6-phenylpentafulvenes are weak bases, and pK(a) values of the protonated forms range from 0.92 to 10.29 in acetonitrile and from 1.3 to 5.9 in 1,2-dichloroethane. For 6-phenylfulvenes with varying para-substituents on the phenyl ring, pK(a) values correlate well with the Hammett parameters sigma(para). Furthermore, for most compounds, electrochemical reduction is significantly facilitated by protonation. Extensive theoretical and NMR studies strongly support the postulated protonation at carbon.