Very fast electron migrations within p-doped aromatic cofacial arrays leading to three-dimensional (toroidal) η-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 (lambda) and the electronic coupling element (H-ab) that are required for the evaluation of the activation barrier (Delta G(ET)*) 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 Delta G(ET)* = 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 T-t > 0.5/R or roughly -20 degrees C, the electron-transfer dynamics become essentially barrierless since the whizzing occurs beyond the continuum of states and effectively achieves complete pi-delocalization.