Dicyanoaromatic radical anions as mixed valence species

Abstract

The reduction of nine symmetric dicyanoaromatic radical anions by sodium amalgam in the presence of cryptand[2.2.2] was studied using cyclic voltammetry and using optical and electron paramagnetic resonance (EPR) spectroscopies in two aprotic solvents. All radicals are charge‐delocalized (Class III) mixed valence species, as shown by the weak solvatochromism of their low‐energy optical bands and by the vibrational structure exhibited by most of the bands. The maximum of the low‐energy optical transition decreases logarithmically with the number of bonds (n) between cyano groups in a series of p‐phenylene‐bridged radicals, from p‐phenyl (n = 5) to p‐quaterphenyl (n = 17), although the energy of the latter lies higher than the value predicted by the linear regression. The energy of this band decreases linearly with the cos2 value of the torsion angle between phenyl rings in the spectra of biphenyl‐4,4′‐dicarbonitrile radical anion and their methyl‐substituted derivatives. The fact that charge delocalization is maintained in radicals with non‐Kekulé bridges, with unusually large bridges and with bridges with highly twisted biphenyl systems suggests that cyano charge‐bearing units have small reorganization energies and induce high electronic couplings through the bridges. Copyright © 2012 John Wiley & Sons, Ltd.