Coupled states in dinitrofluorene: relationships between ground state and excited state mixed valence.

The electronic absorption spectrum of 9,9-dimethyl-2,7-dinitrofluorene radical anion in HMPA displays both a NIR intervalence charge transfer and a visible excited state mixed valence transition. These transitions contain a similar vibronic progression resulting from molecular orbitals that are common to both transitions. Vibrational frequency and intensity data are acquired from the resonance Raman spectrum and used to calculate a best fit for the absorption spectrum. The normal coordinate distortions are analyzed in terms of the electronic changes for both transitions to explain their similarity. The Raman scattering intensity decreases at lower excitation wavelength as a result of Raman de-enhancement caused by interference between neighboring excited states.

Oxyallyl exposed: an open-shell singlet with picosecond lifetimes in solution but persistent in crystals of a cyclobutanedione precursor.

Photoinduced decarbonylation of 2,4-bis(spirocyclohexyl)-1,3-cyclobutanedione 1 in the crystalline solid state resulted in formation of a deep blue transient with λ(max) = 550 nm and a half-life of 42 min at 298 K, identified as kinetically stabilized oxyallyl. Support for an open-shell singlet species was obtained by spectroscopic analysis and (4/4) CASSCF calculations with the 6-31+G(d) basis set and multireference MP2 corrections. The electronic spectrum of the singlet biradical, confirmed by femtosecond pump-probe studies in solution, was matched by coupled cluster calculations with single and double corrections.

Solvent effects on the coexistence of localized and delocalized 4,4'-dinitrotolane radical anion by resonance Raman spectroscopy.

The resonance Raman spectrum of the simple alkyne bridge in 4,4'-dinitrotolane radical anion shows two distinct bands, providing proof of the solvent-dependent coexistence of charge-localized and -delocalized species. The Raman spectra of normal modes primarily involving the charge-bearing -PhNO(2) units also support the coexistence of two solvent-dependent electronic species. The temperature dependence of the spectra of the bridging unit shows an inverse relationship between the solvent reorganization energy (lambda(s)) and the temperature.

Isolation of dysprosium and yttrium complexes of a three-electron reduction product in the activation of dinitrogen, the (N2)3- radical.

DyI(2) reacts with 2 equiv of KOAr (OAr = OC(6)H(3)(CMe(3))(2)-2,6) under nitrogen to form not only the (N(2))(2-) complex, [(ArO)(2)(THF)(2)Dy](2)(mu-eta(2):eta(2)-N(2)), 1, but also complexes of similar formula with an added potassium ion, [(ArO)(2)(THF)Dy](2)(mu-eta(2):eta(2)-N(2))[K(THF)(6)], 2, and [(ArO)(2)(THF)Dy](2)(mu(3)-eta(2):eta(2):eta(2)-N(2))K(THF), 3. The 1.396(7) and 1.402(7) A N-N bond distances in 2 and 3, respectively, are consistent with an (N(2))(3-) ligand, but the high magnetic moment of 4f(9) Dy(3+) precluded definitive identification. The Y[N(SiMe(3))(2)](3)/K reduction system was used to synthesize yttrium analogues of 2 and 3, {[(Me(3)Si)(2)N](2)(THF)Y}(2)(mu-eta(2):eta(2)-N(2))[K(THF)(6)] and {[(Me(3)Si)(2)N](2)(THF)Y}(2)(mu(3)-eta(2):eta(2):eta(2)-N(2))K, that had similar N-N distances and allowed full characterization. EPR, Raman, and DFT studies are all consistent with the presence of (N(2))(3-) in these complexes. (15)N analogues were also prepared to confirm the spectroscopic assignments. The DFT studies suggest that the unpaired electron is localized primarily in a dinitrogen pi orbital isolated spatially, energetically, and by symmetry from the metal orbitals.

Three-chromophore excited-state mixed valence.

The lowest energy optical electronic absorption band of the three-chromophore system tris(4-bromophenyl)amine radical cation is analyzed. The lowest energy electronic transition corresponds to a p-bromophenyl orbital to nitrogen p orbital transition that places the positive charge on three equivalent p-bromophenyl chromophores. The excited electronic state is an example of excited-state mixed valence (ESMV), and the spectrum is interpreted using two ESMV models. The simplest model invokes the concept of an "effective coupling" between the three identical chromophores with an excited-state energy splitting equal to three times the coupling. A more accurate model, the "neighboring orbital model", utilizes the coupling between the bridge's and charge-bearing unit's orbitals closest in energy. The three-chromophore system provides a striking illustration of the failure of an effective coupling term to account for ESMV splitting. The calculated relative energies of the diabatic and adiabatic states are different, but the calculated absorption spectra of the two models show nearly identical vibrational fine structure. Resonance Raman data and the time-dependent theory of electronic and resonance Raman spectroscopies are used to calculate the spectra.