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The absorption, emission, and infrared spectra, metal (Ru) and ligand (PP) half-wave potentials, and ab initio calculations on the ligands (PP) are compared for several [L(n)()Ru(PP)](2+) and [[L(n)Ru]dpp[RuL'(n)]](4+) complexes, where L(n) and L'(n) = (bpy)(2) or (NH(3))(4) and PP = 2,2'-bipyridine (bpy), 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), or 2,3-bis(2pyridyl)benzoquinoxaline (dpb). The energy of the metal-to-ligand charge-transfer (MLCT) absorption maximum (hnu(max)) varies in nearly direct proportion to the difference between Ru(III)/Ru(II) and (PP)/(PP)(-) half-wave potentials, DeltaE(1/2), for the monometallic complexes but not for the bimetallic complexes. The MLCT spectra of [(NH(3))(4)Ru(dpp)](2+) exhibit three prominent visible-near-UV absorptions, compared to two for [(NH(3))(4)Ru(bpy)](2+), and are not easily reconciled with the MLCT spectra of [[(NH(3))(4)Ru]dpp[RuL(n)]](4+). The ab initio calculations indicate that the two lowest energy pi orbitals are not much different in energy in the PP ligands (they correlate with the degenerate pi orbitals of benzene) and that both contribute to the observed MLCT transitions. The LUMO energies calculated for the monometallic complexes correlate strongly with the observed hnu(max) (corrected for variations in metal contribution). The LUMO computed for dpp correlates with LUMO + 1 of pyrazine. This inversion of the order of the two lowest energy pi orbitals is unique to dpp in this series of ligands. Configurational mixing of the ground and MLCT excited states is treated as a small perturbation of the overall energies of the metal complexes, resulting in a contribution epsilon(s) to the ground-state energy. The fraction of charge delocalized, alpha(DA)(2), is expected to attenuate the reorganizational energy, chi(reorg), by a factor of approximately (1 - 4alpha(DA)(2) + alpha(DA)(4)), relative to the limit where there is no charge delocalization. This appears to be a substantial effect for these complexes (alpha(DA)(2) congruent with 0.1 for Ru(II)/bpy), and it leads to smaller reorganizational energies for emission than for absorption. Reorganizational energies are inferred from the bandwidths found in Gaussian analyses of the emission and/or absorption spectra. Exchange energies are estimated from the Stokes shifts combined with perturbation--theory-based relationship between the reorganizational energies for absorption and emission values. The results indicate that epsilon(s) is dominated by terms that contribute to electron delocalization between metal and PP ligand. This inference is supported by the large shifts in the N-H stretching frequency of coordinated NH(3) as the number of PP ligands is increased. The measured properties of the bpy and dpp ligands seem to be very similar, but electron delocalization appears to be slightly larger (10-40%) and the exchange energy contributions appear to be comparable (e.g., approximately 1.7 x 10(3) cm(-1) in [Ru(bpy)(2)dpp](2+) compared to approximately 1.3 x 10(3) cm(-1) in the bp...

The radiolytic production of CdS particles from solutions containing cadmium ions and a thiol (3-mercapto-1,2-propanediol, RSH) is described. The production of the colloids is initiated by the reaction of solvated electron with the thiol to release HS" ions. The polynuclear complexes of cadmium with the thiolate form of RSH act as moderators to the growth of the particles and allow reproducible production of practically any predetermined size particles. The complexes at the surface of the particles also stabilize the particles for long periods of time. The particles are strongly fluorescent from size of ca. 8 Á in radius. Excess electrons in these particles lead to bleaching of the exciton band due to Coulomb screening effects. At higher doses, formation of cadmium atoms leads to increased absorption in the visible range.

differing greatly in doping density. Electrodes with the largest doping densities (>1019 cm"3) yield anomalous impedance, kinetic current, and photopotential behavior. Kinetic current vs potential curves for redox couples with similar structure and reorganization energy but different reduction potentials indicate that different mechanisms are responsible for electron transfer across the WSe2/acetonitrile interface. Exchange current densities are not in close agreement with the predictions of commonly used theoretical relationships. Acknowledgment. This research was supported by the U.S. Department of Energy (Division of Chemical Sciences) Grant No. DE-FG02-84ER13247. Valuable discussions with B. B. Smith (University of Colorado, Boulder) are gratefully acknowledged.

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Explosive-type emulsions with internal-phase fractions from 0.74 to 0.92 were studied to quantify the effects of average droplet size, continuous-phase viscosity, volume fraction, and temperature on emulsion viscosity measured at low rates of shear. Plots of emulsion viscosity vs volume fraction of the dispersed phase (0) were obtained at 80 °C for two different emulsions varying in continuous-phase composition, but having constant droplet size profiles. Emulsion viscosity increased steeply as the internal phase increased, but varied inversely with average droplet radius. The effect of droplet size became more prominent at higher values of . The viscosity of the emulsion (when corrected for the viscosity of the continuous phase) increased approximately linearly with increased internal phase when drop size distribution was held constant. A mathematical analysis is given that ascribes the increase to droplet distortion. At low shear, the studied emulsions exhibited Boltzmann temperature dependence to flow (2Ja = 2-6 kcal/mol) with breaks at 22 and 72 °C. The continuous phase also exhibited a break at 22 °C.

The previously observed facile photooxidation of Ru(bpy)3 2+ to Ru(bpy)3 3+ in oxygenated solutions of 9 M H2SO4 (Kotkar, D; Joshi, V.; Ghosh, P. K. Chem. Commun. 1987, 4; Indian Patent No. 164358 (1989)) is further studied. A similar phenomenon was observed with Ru(phen)3 2+ but not with Ru(bpy)2[bpy-(CO2H)2]2+. The reaction is strongly dependent on acid concentration, with a sharp change in the region of 2−7 M H2SO4. The quantum yield of Ru(bpy)3 3+ formation in 9 M H2SO4 is close to the quantum yield of steady-state luminescence quenching by O2. Photooxidation is accompanied by near-stoichiometric formation of H2O2 as reduced product. Chromatographic, spectroscopic, electrochemical and optical rotation studies reveal that Ru(bpy)3 2+ survives the strongly acidic environment with little evidence of either any change in coordination sphere or ligand degradation, even after repeated cycles of photolytic oxidation followed by electrolytic reduction. The high quantum yield and selectivity of the reaction is ascribed to (i) predominance of the electron transfer quenching pathway over all others and (ii) highly efficient trapping of O2 •- by H+ followed by rapid disproportionation to H2O2 and O2. These are likely on account of the high ionic strength of the medium which favors the required shifts in the potentials of the O2/O2 •- and O2/H2O2 couples. Upon storage of the photooxidized Ru(III) solution in dark, partial recovery of Ru(bpy)3 2+ occurs gradually. Studies with the electrooxidized complex over a range of acid concentrations indicate that Ru(bpy)3 2+ is regenerated by reaction of Ru(bpy)3 3+ with H2O2. The reaction is promoted by increasing concentrations of [H2O2] and inhibited by [O2] and [H+]. The fraction of Ru(III) remaining after the reverse reaction is allowed to plateau in solutions of varying acid concentrations follows a similar trend to that found after attainment of steady state in the photooxidation reaction, although in all cases the forward reaction produces more Ru(III) than what remains in the reverse reaction. These observations are consistent with the following equation 2Ru(bpy)3 2+ + O2 + 2H+ →(hν)/←(dark) 2Ru(bpy)3 3+ + H2O2 for which the equilibrium constant has been computed. Light helps overcome the activation barrier of the forward reaction by driving it via *Ru(bpy)3 2+, and to the extent that the photooxidation is driven past the equilibrium, there is conversion of light energy in the form of long-lived chemical products. Spectroscopic evidence rules out any significant shift in the redox potential of Ru(bpy)3 3+/2+, suggesting thereby that H2O2 is much more stable in the more strongly acidic medium and less capable of reducing Ru(bpy)3 3+ unlike at higher pH.

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