Electronic Communication in Linear Oligo(azobenzene) Radical Anions

Abstract

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.

Multiphotochromic molecular systems

“…Moneo et al studied the delocalisation of the charge in the radical anion 78 built with three AZBs. 186 In that study, the coupling between the sites bearing a localized charge was quantified by electron paramagnetic resonance (EPR) spectroscopy, but the mechanism of photochromism of this derivative remained unexplored.…”

ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

“…Contrary to the above‐mentioned tetramethoxy and ethylene‐bridged azobenzenes, which are strongly twisted about the NN double bond, ortho ‐fluoroazobenzenes are essentially planar (see crystal structure, Figure 1 C). The planarity of this moiety is of interest for certain applications in which an efficient conjugation along the π‐system is required,15 or that involve assemblies based on π,π‐stacking. As a remarkable consequence of lowering the energy of the n orbital through the introduction of ortho ‐fluoro substituents, the Z forms of these azobenzenes are thermally very stable, with an unprecedented half‐life of 2 years for F4 ‐azobenzene (DMSO, 25 °C) 16.…”

Multiple Azoarenes Based Systems – Photoswitching, Supramolecular Chemistry and Application Prospects

“…Linear bis‐azo chromophores where photochromic units are positioned meta A‐14 , and para A‐15 to each other have also been explored as part of polymeric network to study photo‐orientation and all‐optical surface patterning (Figure 4b) [22] . Electronic coupling in linear oligoazobenzene radical anions of five bis‐azobenzene A‐16 – 20 and one tris‐azobenzene A‐21 moieties (Figure 4b) was studied by Telo and co‐workers with the help of optical and electron paramagnetic resonance (EPR) spectroscopies [23] . Compared to other organic mixed valence radical anions with similar bridges, these radical anions were found to have lower electronic coupling between the charge‐bearing azo units because of accumulation of spin density in nitrogen atoms of azo groups farthest from the bridge.…”