Enhancement of Push-Pull Properties of Pentafulvene and Pentafulvalene Derivatives by Protonation at Carbon
Abstract
We report visible color changes and new intense, bathochromically shifted bands in 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 pKa values in organic solvents were determined, making use of the method of relative basicity measurements. All seven 6-phenylpentafulvenes are weak bases, and pKa values range from 0.92 to 10.29 in acetonitrile and from 1.77 to 6.46 in 1,2-dichloroethane. For the 6-phenylfulvenes with varying para-substituents on the phenyl ring, pKa values correlate well with the Hammett parameters para. Furthermore, for most compounds, electrochemical reduction is significantly facilitated by protonation. Extensive theoretical and NMR studies strongly support the postulated protonation at carbon.
Domains
Organic chemistry
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