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.

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 pK units, expanding to as low pK values as possible with our current experimental methodology.

Dedicated to Prof. Alexandra M. Z. Slawin on the occasion of her 60 th birthday.The influence of fluorination on the acid-base properties and the capacity of structurally related 6-5 bicyclic compounds -1,3-benzodiazole 1, 1,2,3-benzotriazole 2 and 2,1,3-benzoselenadiazole 3 to σ-hole interactions, i. e. hydrogen (1 and 2) and chalcogen (3) bondings, is studied experimentally and computationally. The tetrafluorination increases the Brønsted acidity of the diazole and triazole scaffolds and the Lewis acidity of selenadiazole scaffold decreases the basicity. Increased Brønsted acidity facilitates anion binding via the formation of hydrogen bonds; particularly, tetrafluorinated derivative of 1 (compound 4) binds Cl À . Increased Lewis acidity of tetrafluorinated derivative of 3 (compound 10), however, is not enough for binding with Cl À and F À via chalcogen bonds in contrast to previously studied Te analog of 10. It is suggested that the maximum positive values of molecular electrostatic potential at the σ-holes, V S,max , can be a reasonable metric for design and synthesis of new anion receptors with selenadiazole-diazole/ triazole hybrids as a special target. Related chlorinated compounds are also discussed.