Search

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 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 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, pKa values correlate well with the Hammett parameters σpara. Furthermore, for most compounds, electrochemical reduction is significantly facilitated by protonation. Extensive theoretical and NMR studies strongly support the postulated protonation at carbon. [ABSTRACT FROM AUTHOR]

The basicity scale of very weak bases has been set up in 1,2-dichloroethane to give, for the first time, reliable quantitative insights into the basic properties of weak bases in a low-polarity solvent. The scale contains 30 compounds, including anilines; phosphanes, and carbonyl bases, such as esters and amides, linked by 53 relative basicity measurements. The scale spans more than 12 p Kip units, expanding to as low p Kip values as possible with our current experimental methodology. [ABSTRACT FROM AUTHOR]