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Optical absorption spectra of three dinitroaromatic radical anions, 2,7-dinitro-9,9-dimethylfluorene ([1.sup.*-]), 4,4'dinitrobiphenyl ([2.sup.*-]), and 4,4'-dinitrotolane ([3.sup.*-]) in solvents THF, HMPA, and MeCN show both an unresolved broad band characteristic of charge-localized mixed valence species (Robin-Day classification Class II), and a vibronically structured band of the delocalized species (Class III). With decreasing solvent reorganization energy, a greater portion of the compounds become charge-delocalized. In particular, [1.sup.*-] with the greatest coupling, is almost entirely composed of the delocalized species in all solvents. An intense Raman mode is used to identify the charge-bearing unit as the nitrobenzene moiety. Resonance Raman profiles are utilized to gain detailed information of vibrational modes. The out-of-phase ring lengthening stretch mode is 00 observed to be enhanced at higher excitation energies, corresponding to the absorption band of the Class II species, while the in-phase ring lengthening stretch mode, a totally symmetric vibration, is most strongly enhanced in the absorption region of the Class III species. Resonance Raman profiles support the solvent-dependent coexistence of Class II and Class III molecules of the same chemical composition. Key words: mixed valence, resonance Raman spectroscopy, dinitroaromatic, neighboring orbital model. Les spectres d'absorption optique des trois anions radicalaire dinitroaromatiques, 2,7-dinitro-9,9-dimethylfluorene ([1.sup.*-]), 4,4'-dinitrobiphenyl ([2.sup.*-]) et 4,4'-dinitrotolane ([3.sup.*-]) dans les solvants THF, HMPA et MeCN presentent une bande large non resolue caracteristique des especes chargees a valence mixte (classe II dans la classification de Robin-Day) et une structure de bande vibronique propre aux especes delocalisees (classe III). Lorsque l'energie de reorganisation du solvant diminue, une plus grande partie des composes deviennent porteuses de charges delocalisees. En particulier, f-, qui presente le couplage le plus important, est presque entierement compose des especes delocalisees dans tous les solvants. Un mode Raman intense est utilise pour determiner que l'unite porteuse de charge est la moitie nitrobenzene. On a recours a des profils de resonance Raman pour obtenir les informations detaillees de modes vibrationnels. On observe que le mode d'etirement-allongement de cycle hors phase est meilleur ameliore a des energies d'excitation elevees, qui correspondent a la bande d'absorption des especes de la classe II, tandis que le mode d'etirement-allongement de cycle en phase, une vibration totalement symetrique, est tres fortement ameliore dans la region d'absorption des especes de la classe III. Les profils de resonance Raman favorisent la coexistence, dependante du solvant, des molecules des classes II et III de composition chimique identique. [Traduit par la Redaction] Mots-cles : valence mixte, spectroscopie Raman de resonance, dinitroaromatique, modele des orbitales voisines., Introduction Mixed valence molecules have two or more equivalent sites that can have different oxidation states in either the ground or excited state. (1-4) These systems are often denoted as [...]

The electronic absorption spectrumof 9,9-dimethyl-2,7-dinitrofluoreneradical anion in HMPA displays both a NIR intervalence charge transferand a visible excited state mixed valence transition. These transitionscontain a similar vibronic progression resulting from molecular orbitalsthat are common to both transitions. Vibrational frequency and intensitydata are acquired from the resonance Raman spectrum and used to calculatea best fit for the absorption spectrum. The normal coordinate distortionsare analyzed in terms of the electronic changes for both transitionsto explain their similarity. The Raman scattering intensity decreasesat lower excitation wavelength as a result of Raman de-enhancementcaused by interference between neighboring excited states. [ABSTRACT FROM AUTHOR]