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The synthesis and characterization of neutral heteroleptic molecular iridium(III) complexes with (2,4-difluoro)phenylpyridine and different pyridine-1,2,4-triazole ligands are described. The studies have shown high external quantum efficiencies and color points better than the standard for blue iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,[C.sup.2]]picolinate.

A comparison of rate constants measured for an intervalence (IV) compounds with electron-transfer parameters obtained from its optical absorption spectrum is done using resonance Raman analysis. It is suggested that the Zhu-Nakamura preexponential factor provides good consistency for the ESR rate constants using a value, averaged single-mode which closer to those predicted from the resonance Raman spectroscopic data but need slightly higher electronic coupling than [V.sub.Hush].

Photochemical charge separation within aromatic hydrazines and the effect of exited-state intervalence in dihydrazines are studied. The results suggest that the most noticeable consequence of the exited-state intervalence is, under 60% as large for Hy2(super 14)NA compared to Hy(super 1)NA and for a significantly smaller Strokes shift for the distributed compared to the monosubstituted compounds Hy2(super 26)NA compared to Hy(super 2)NA.

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 from a localized class II compound to a delocalized class II compound by changing solvents.

A model for the quantitative treatment of dihydrazine diradical dication, which possesses mixed valence excited states is introduced and utilized for explaining the observed spectroscopic consequences. Results indicate that the excited-state mixed valence could play an important role in the electronic spectra of many organic and inorganic compounds.

The electron transfer (ET) in four symmetrically substituted naphthalene-bridged bis-hydrazine radical cations is compared within the Marcus-Hush framework. The results show that ET rates estimated for four naphthalene bridged bis-hydrazine radical cations vary significantly and show no correlation with the distance between the charge-bearing units.

The optical spectrum of 2,7-dinitronaphthalene radical anion generated by Na(Hg) reduction in acetonitrile containing a large excess of cryptand[2.2.2] exhibits a Hush-type intervalence charge-transfer band at 1070nm, estimated to correspond to an off-diagonal matrix coupling element. Intramolecular electron transfer rate constants are predicted within an order of magnitude by a Marcus-Hush-type two-state classical model analysis of the optical spectrum.

Research has been conducted on intervalence radical anions which demonstrate charge-transfer bands in their near-IR spectra. The authors discuss the absorption bands of delocalized dinitroaromatic anions with aromatic bridges, and compare their optically derived electron transfer parameters with those of radical anions.

The optical spectrum of 2,7-dinitronaphthalene radical anion generated by Na(Hg) reduction in acetonitrile containing a large excess of cryptand[2.2.2] exhibits a Hush-type intervalence charge-transfer band at 1070 nm, estimated to correspond to an off-diagonal matrix coupling element of 310 cm-1. The interpolated rate constant for intramolecular electron transfer at 293 K measured by ESR between 225 and 320 K for this solution is 3.1(±.2) x 109 s-1. Rate constants estimated in two ways from the optical parameters using the Marcus-Hush assumption that the diabatic surfaces should be parabolae are 1.0 and 0.11 x 109 s-1, and those using diabatic surfaces that fit the observed charge-transfer band are 9.6 and 3.4 x 109 s-1, when used with an electron-transfer distance on the adiabatic surfaces of 6.42 Å. Similar measurements and comparisons were also carried out using dimethylformamide and butyronitrile as solvents. The success of simple, classical two-state Marcus-Hush theory precludes an electron-hopping mechanism. UHF calculations predict a planar unsymmetrical gas-phase structure for 1 ,3-dinitrobenzene radical anion but give serious spin contamination. Semiempirical AM1 calculations using singles excitation configuration interaction with an active space of 70 orbitals and the COSMO solvent model also give a planar unsymmetrical structure. These calculations make the internal vibrational component of the reorganization energy nearly constant, and much smaller than the solvent reorganizational component, and predict the transition energy to lie between that observed in acetonitrile (9360 cm-1) and those observed in dimethylformamide (8100 cm-1) and butyronitrile (8040 cm-1). [ABSTRACT FROM AUTHOR]

Excited-state mixed valence (ESMV) occurs in the 1,2-diphenyl-1,2-diisopropyl hydrazine radical cation, a molecule in which the ground state has a symmetrical charge distribution localized primarily on the hydrazine, but the phenyl to hydrazine charge-transfer excited state has two interchangeably equivalent phenyl groups that have different formal oxidation states. Electronic absorption and resonance Raman spectra are presented. The neighboring orbital model is employed to interpret the absorption spectrum and coupling. Resonance Raman spectroscopy is used to determine the excited-state distortions. The frequencies of the enhanced modes from the resonance Raman spectra are used together with the time-dependent theory of spectroscopy to fit the two observed absorption bands that have resolved vibronic structure. The origins of the vibronic structure and relationships with the neighboring orbital model are discussed. [ABSTRACT FROM AUTHOR]

A quantitative model of mixed-valence excited-state spectroscopy is developed and applied to 2,3-diphenyl-2,3-diazabicyclo[2.2.2]octane. The lowest-energy excited state of this molecule arises from a transition from the ground state, where the charge is located on the hydrazine bridge, to an excited state where the charge is associated with one phenyl group or the other. Coupling splits the absorption band into two components with the lower-energy component being the most intense. The sign of the coupling, derived by using a neighboring orbital model, is positive. The transition dipole moments consist of parallel and antiparallel vector components, and selection rules for each are derived. Bandwidths are caused by progressions in totally symmetric modes determined from resonance Raman spectroscopic analysis. The absorption, emission, and Raman spectra are fit simultaneously with one parameter set.

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, H(ab), 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 E(op) = 2H(ab) for class III compounds is not true in this case.

[structure: see text] The optical diffuse reflectance and solution spectra of two bis-hydrazine radical cationic intervalence compounds have been compared. The results are consistent with an ion-pairing increase and an "effective polarity" in these crystals that is not far from that of acetonitrile or other polar solvents.