The activation energy for intramolecular electron transfer in radical anions of 2,7-dinitrodibenzodioxin and 2,8-dinitrodibenzodioxin, obtained by simulation of their temperature-dependent EPR spectra, are well predicted by the values calculated by the two-state Marcus-Hush model from the optical charge-transfer bands using quartic-adjusted energy surfaces. The electronic coupling is higher in the 2,8-dinitrodibenzodioxin (H(ab) = 485 cm(-1)) than in the 2,7-dinitrodibenzodioxin radical anion (H(ab) = 250 cm(-1)), but for each solvent the reorganization energy, taken as the maximum of the optical band, is only slightly higher in the latter. These values are consistent with the fact that the reaction is faster in the 2,8-dinitrodibenzodioxin radical anion isomer, as determined by EPR spectroscopy. The pre-exponential factors obtained combining the EPR-derived rate constants and the activation energies calculated from the optical bands fit well the theoretical (modified) nonadiabatic values in the less viscous solvents. However, for the more viscous solvents, the trend of the pre-exponential values with solvent can only be explained if dynamical solvent effects increasingly influence their value. The influence of solvent dynamics in the 2,8-dinitrodibenzodioxin radical anion starts in the less viscous solvents DMF and DMSO, but in the 2,7-dinitrodibenzodioxin isomer this is only fully evident for the more viscous PhCN and HMPA. The influence of solvent dynamics is higher in the radical with the lowest activation barrier.

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.

The radical anions of five bis(azobenzene) and one tris(azobenzene) compounds were studied by optical and electron paramagnetic resonance (EPR) spectroscopies in polar aprotic solvents. The radicals with planar or almost-planar bridges are charge-delocalized mixed-valence species. Localization of charge occurs only on radicals with highly twisted biphenyl bridges. The electronic coupling between the azobenzene charge-bearing units, calculated as half the energy of the intervalence band for the charge-delocalized and by the Hush equation for the charge-localized radicals, decreases with the distance and torsion angle between azobenzene units. These radicals have smaller electronic couplings between charge-bearing units than other mixed-valence organic radicals with similar bridges. However, the application of a three-stage model to the tris(azobenzene) radical anion intervalence band yields an electronic coupling between consecutive azobenzenes that is higher than in any of the bis(azobenzene) radicals studied.