Charge localization in bis(dioxaborine) radical anions

The mixed valence (MV) radical anions of several bis(dioxaborines) with aromatic bridges of different length were studied by Vis/NIR spectroscopy, cyclic voltammetry, and theoretical calculations. The phenyl-bridged (1), the biphenyl-bridged (2), and bithiophene-bridged (5) radical anions show intense low-energy intervalence bands with vibrational structure typical of charge delocalized mixed valence species in the range of solvents studied. However, by subtracting from the experimental spectra of 2⁻ in MeCN the fraction corresponding to the delocalized part (taken as the spectrum in tetrahydrofuran [THF]), we get a localized charge-transfer bands that show a significant cutoff effect at the low-energy side, as predicted by classical Marcus–Hush theory. In the radical anions with three aromatic rings on the bridge, the localization of the charge changes with solvent. These radicals are predominantly charge-localized in the high λ_S solvent MeCN, charge-delocalized in the low λ_S solvent THF, and show both type of intervalence bands in DMF. Experimental results and theoretical calculations show that the electronic coupling between dioxaborine units in these three-ring bridged radical anions increases with the number of thiophene rings on the bridge.