Interpretation of Mixed-Valence Compound Optical Spectra Near the Class II/III Border: Dinitrobiphenyl and Dinitrophenanthrene Radical Anions

The optical spectra of 4,4′-dinitrobiphenyl and other similar 9-bond bridged radical anions show that these mixed-valence compounds are predominantly charge-localized in the high λS solvent MeCN, charge-delocalized in the low λS solvent HMPA, and show intermediate behavior in DMF. Hush analysis of the localized charge-transfer band in MeCN allowed the calculation of the electronic coupling between nitro groups (H ab). H ab changes with bridge structure, depending mainly on the twist angle between the two aromatic rings: H ab is higher for the planar 9,9-dimethyl-2,7-dinitrofluorene radical anion (1100 cm−1) and about one-half of this value for the more twisted 2,2′-dimethyl-4,4′-dinitrobiphenyl radical anion (540 cm−1). The reorganization energy λ decreases as H ab increases. We suggest that this is due to a decrease of the internal reorganization energy λv as the Class II/Class III borderline is approached, and that λv should be zero at the borderline. Subtracting from the experimental spectra the fraction corresponding to the delocalized part (taken as the spectrum in HMPA or THF), we get localized charge-transfer bands that show a significant cutoff effect at the low energy side, as predicted by classical Marcus−Hush theory.