Very Fast Electron Migrations within p-Doped Aromatic Cofacial Arrays Leading to Three-Dimensional (Toroidal) π-Delocalization

The charge-resonance phenomenon originally identified by Badger and Brocklehurst lies at the
core of the basic understanding of electron movement and delocalization that is possible within p-doped
aromatic (face-to-face) arrays. To this end, we now utilize a series of different aryl-donor groups (Ar) around
a central platform to precisely evaluate the intramolecular electron movement among these tethered redox
centers. As such, the unique charge-resonance (intervalence) absorption bands observed upon the one-electron oxidation or p-doping of various hexaarylbenzenoid arrays (Ar6C6) provide quantitative measures
of the reorganization energy (λ) and the electronic coupling element (Hab) that are required for the evaluation
of the activation barrier (ΔGET*) for electron-transfer self-exchange according to Marcus−Hush theory. The
extensive search for viable redox centers is considerably aided by the application of a voltammetric criterion
that has led in this study to Ar = N,N-dialkyl-p-anilinyl, in which exceptionally low barriers are shown to lie
in the range ΔGET* = 0.3−0.7 kcal mol-1 for very fast electron hopping or peregrination around the hexagonal
circuit among six equivalent Ar sites. Therefore, at transition temperatures Tt > 0.5/R or roughly −20 °C,
the electron-transfer dynamics become essentially barrierless since the whizzing occurs beyond the
continuum of states and effectively achieves complete π-delocalization.