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Analysis of optical spectra of a pseudo-para-dinitro[2.2]paracyclophane radical anion using Marcus--Hush theory reveals that its off diagonal coupling element, Hab, is large enough, relative to its reorganization energy, to change it from a localized class II compound to a delocalized class III compound by changing solvents. The optical spectra, along with frontier orbital analysis, show that the assumption that Eop = 2Hab for class III compounds is not true in this case. [ABSTRACT FROM AUTHOR]

Analyses the Class III intervalence shown by delocalized dinitroaromatic radical anions in their near-infra red (IR) spectra. Account of electronic interactions that dinitroaromatic radical anions have; Overview of vertical excitation energies that were estimated from vibrational components; Comparison of IV charge-transfer band interpretation for localized and delocalized compounds; Comparison of the absorption spectra of the 1, 4-PH, 2, 6-AN-bridged dinitroaromatic radical anions;Summary of vertical excitation energy values obtained from Fitting Optical Spectra of dinitroaromatic radical anions.

A model for the quantitative treatment of molecular systems possessing mixed valence excited states is introduced and used to explain observed spectroscopic consequences. The specific example studied in this paper is 1,4-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)-2,3,5,6-tetramethylbenzene-1,4diyl dication. The lowest energy excited state of this molecule arises from a transition from the ground state where one positive charge is associated with each of the hydrazine units, to an excited state where both charges are associated with one of the hydrazine units, that is, a Hy-to-Hy charge transfer. The resulting excited state is a Class II mixed valence molecule. The electronic emission and absorption spectra, and resonance Raman spectra, of this molecule are reported. The lowest energy absorption band is asymmetric with a weak low-energy shoulder and an intense higher energy peak. Emission is observed at low temperature. The details of the absorption and emission spectra are calculated for the coupled surfaces by using the time-dependent theory of spectroscopy. The calculations are carried out in the diabatic basis, but the nuclear kinetic energy is explicitly included and the calculations are exact quantum calculations of the model Hamiltonian. Because the transition involves the transfer of an electron from the hydrazine on one side of the molecule to the hydrazine on the other side and vice versa, the two transitions are antiparallel and the transition dipole moments have opposite signs. Upon transformation to the adiabatic basis, the dipole moment for the transition to the highest energy adiabatic surface is nonzero, but that for the transition to the lowest surface changes sign at the origin. The energy separation between the two components of the absorption spectrum is twice the coupling between the diabatic basis states. The bandwidths of the electronic spectra are caused by progressions in totally symmetric modes as well as progressions in the modes along the coupled coordinate. The totally symmetric modes are modeled as displaced harmonic oscillators; the frequencies and displacements are determined from resonance Raman spectra. The absorption, emission, and Raman spectra are fit simultaneously with one parameter set. The coupling in the excited electronic state H[SUBah][SUPex] is 2000 cm[SUP-1]. Excited-state mixed valence is expected to be an important contributor to the electronic spectra of many organic and inorganic compounds. The energy separations and relative intensities enable the excited-state properties to be calculated as shown in this paper, and the spectra provide new information for probing and understanding coupling in mixed valence systems. [ABSTRACT FROM AUTHOR]