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Multiple arrays of Si wires were sequentially grown and transferred into a flexible polymer film from a single Si(111) wafer. After growth from a patterned, oxide-coated substrate, the wires were embedded in a polymer and then mechanically separated from the substrate, preserving the array structure in the film. The wire stubs that remained were selectively etched from the Si(111) surface to regenerate the patterned substrate. Then the growth catalyst was electrodeposited into the holes in the patterned oxide. Cycling through this set of steps allowed regrowth and polymer film transfer of several wire arrays from a single Si wafer.

One-electron reduction of [Ru(menbpy)3]2+ [menbpy = 4,4′-di{(1R,2S,5R)-(−)-menthoxycarbonyl}-2,2′-bipyridine] by pulse radiolysis produces a Ru(II) complex of a menbpy-centered anion radical, which reduces Δ-[Co(acac)3] more rapidly than Λ-[Co(acac)3] with an enantioselectivity of 2.7 in 85% EtOH/H2O.

This paper reports results for metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) absorption bands of Ru(II) and Ru(III) complexes and shows that the dipole-moment changes can be understood in terms of a two-state model which considers polarization, induced-dipole, and delocalization effects. In summary, the change in the charge of the metal center produced by charge-transfer excitation results in a change in ligand polarization which reduces the expected dipole-moment change. It is shown that including this effect, together with the effect of the permanent dipoles of the NH{sub 3} ligands, results in encouragingly good agreement with the predictions of a two-state model for MLCT and LMCT transitions. 22 refs.

Seven new complex salts trans-[RuII(NCS)(NH3)4(LA)][PF6]2 [LA = N-methyl-4,4′-bipyridinium (MeQ+) 1, N-methyl-2,7-diazapyrenium (Medap+) 2, N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+) 3, N-phenyl-4,4′-bipyridinium (PhQ+) 4, N-phenyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Phbpe+) 5, N-(4-acetylphenyl)-4,4′-bipyridinium (4-AcPhQ+) 6, or N-(2-pyrimidyl)-4,4′-bipyridinium (2-PymQ+) 7] have been prepared. The electronic absorption spectra of 1–7 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, the energies (Emax) of which decrease in the order 2 > 1 > 3 > 4 > 6 > 5 > 7, as the electron-accepting ability of LA increases. The Emax values generally correlate with cyclic voltammetric and 1H NMR data. Comparisons of the MLCT absorption and electrochemical data for 1–7 show that a trans-{RuII(NCS)(NH3)4}+ centre is a stronger electron donor than analogous groups containing only neutral ligands. Molecular first hyperpolarizabilities β have been measured by using the hyper-Rayleigh scattering technique with acetonitrile solutions and a 1064 nm laser, and static first hyperpolarizabilities β0[H] were obtained by application of the two-state model. Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K have afforded dipole moment changes Δμ12 for the MLCT transitions which have been used to calculate β0[S] values according to the two-state equation β0 = 3Δμ12(μ12)2/(Emax)2 (μ12 = transition dipole moment). Both the β0[H] and β0[S] data confirm that the increased electron donor ability of a trans-{RuII(NCS)(NH3)4}+ centre affords enhanced β0 values, when compared with related complexes containing the neutral trans ligands NH3, 4-(dimethylamino)pyridine, N-methylimidazole (mim) or pyridine. However, the magnitude of the observed β0 increase with respect to the analogous mim complexes varies over a range of ca. 25–120%, depending upon LA.

The static molecular first hyperpolarizabilities β0 of a series of V-shaped RuII chromophores with two overlapping visible metal-to-ligand charge-transfer absorptions have been determined by using data obtained from Stark spectroscopy.

When amido-bonded ruthenium(III) complexes of both nicotinamide and isonicotinamide are reduced, the resulting ruthenium(II) complexes, Ru[sup II](NH[sub 3])[sub 5](NHC(O)-3-Py)[sup +] and Ru[sup II](NH[sub 3])[sub 5](NHC(O)-4-Py)[sup +], undergo rapid protonation, followed by intramolecular isomerization (k[sub 4-Py] = 9.6 s[sup [minus]1] at 25[degree]C) to the pyridyl-bonded forms. The authors believe these rearrangements proceed via [eta][sup 2]- arene-bonded intermediates in a walk of the aromatic ring as has been found for related osmium(II) complexes.

Three novel, mixed-ligand complexes of ruthenium and osmium are prepared1; the key example [Ru(bpy)2bpaH](PF6)2[bpy = 2,2′-bipyridine; bpaH = bis(picolyl)amine]1 bears a ‘dangling’ pyridine ligand according to X-ray crystallography and has a luminescence lifetime comparable with that of [Ru(bpy)3]2+.

Expressions for the rate constants for nonadiabatic electron-transfer reactions are presented. These expressions are valid when the electronic coupling between the two redox sites is small and the energy surfaces for the initial and final states are harmonic with identical force constants. High- and low-temperature limiting forms of the rate equations are presented and closed-form expressions which are good approximations to the Franck-Condon sums appearing in these equations are described. Systems in which the electron transfer causes a displacement in one or more vibrational modes are considered. When several vibrational modes are “active,” the higher frequency modes have different effects on the rate constants in the normal and inverted free-energy regions. In the inverted region nuclear tunneling effects are large when hwf > 2kT. Under these conditions the rate has only a weak dependence on temperature and the rate constants can exhibit “quantum beats.” Nuclear tunneling effects are smaller i...

The kinetics of the quenching of the long-lived excited states of CrL_3^(3+) and RuL_3^(2+) complexes (L is 1,10-phenanthroline and 2,2’-bipyridine or substituted derivatives) by the copper proteins plastocyanin, azurin, and stellacyanin have been studied in aqueous solution. The rate constants for quenching by the Cu(I) proteins approach a limiting value of ~10^(6) s^(-1) at high protein concentration. The kinetic behavior for plastocyanin is discussed in terms of a model in which the metal complex binds at a remote site 10-12 A from the copper center. The model allows for electron transfer both from this remote site and by attack of the metal complex adjacent to the copper center. The results show that at low protein concentration the adjacent pathway is about 10 times faster than the remote pathway. The rate constant for the intramolecular electron transfer from the remote site is consistent with the value expected on the basis of theoretical calculations.

The nuclear barrier to electron transfer arises from the need for reorganization of intramolecular and solvent internuclear distances prior to electron t ransfer. For reactions with relatively small driving force ("normal" free-energy region) the nuclear factors and rates increase as intrinsic inner-shell and outer-shell barriers decrease; this is illustrated by data for transition metal complexes in their ground electronic states. By contrast, in the inverted free-energy region, rates and nuclear factors decrease with d ecreasing "intrinsic" barriers; this is illustrated by data for the decay of charge-transfer excited states. Several approaches to the evaluation of the outer-shell barrier are explored in an investigation of the distance dependence of the nuclear factor in intramolecular electron-transfe r processes. One-electron-tra nsfer reactions are at the heart of biological energy transduction and many electrochemical, photochemical, and thermal reaction sequences. For electron transfer

The yields of the metal-to-ligand charge-transfer excited state of poly(pyridine)ruthenium(II) complexes (*RuL/sub 3//sup 2 +/) produced in the reaction of RuL/sub 3//sup 3 +/ with poly(pyridine)cobalt(I) complexes in acetonitrile have been measured by use of a combination of continuous flow methods and chemical actinometry. The *RuL/sub 3//sup 2 +/ yields are high, ranging from 0.31 +/- 0.04 for the most exergonic system (Ru(bpy)/sub 3//sup 3 +/ and Co(4,4'-(CH/sub 3/)/sub 2/bpy)/sub 3//sup +/) to 0.07 +/- 0.02 for the least exergonic system studied (Ru(4,7-(CH/sub 3/)/sub 2/phen)/sub 3//sup 3 +/ and Co(bpy)/sub 3//sup +/. The results are interpreted in terms of competing electron-transfer channels yielding primarily *RuL/sub 3//sup 2 +/ + CoL/sub 3//sup 2 +/ and RuL/sub 3//sup 2 +/ + *CoL/sub 3//sup 2 +/(/sup 2/T), with the formation of entirely ground-state products being negligible. A new actinometer consisting of Os(terpy)/sub 2//sup 2 +/ and Co(NH/sub 3/)/sub 5/Cl/sup 2 +/, for use at wavelengths up to approx.720 nm, is described.

The lifetimes of the excited states produced by 527 nm excitation of tris(1,10-phenanthroline)iron(II) have been measured in a 4:1 (v/v) ethanol/methanol mixture in the 14–295 K temperature range. The results are analyzed in terms of the ⩽10 ps population of ligand-field (LF) states from the initially generated metal-to-ligand charge-transfer (MLCT) states.

The production of ions from the first excited triplet state (T) of magnesium octaethylporphyrin (MgOEP or simply P) has been studied. Triplet-triplet annihilation is shown to be the major route for the production of the ions P/sup +/ and P/sup -/. Evidence is obtained indicating that the reaction of T with ground-state P is not a significant source of ions. Evidence is also presented indicating that the two triplets initially combine to form an excited charge-transfer complex. The relationship between the multiplicity of this charge-transfer complex and triplet quenching, delayed fluorescence, and ion formation is discussed. 13 figures, 3 tables.

The spontaneous reduction of Ru(bpy)/sub 3//sup 3 +/ to Ru(bpy)/sub 3//sup 2 +/ in aqueous solutions yields only trace O/sub 2/ and is accompanied by degradation of approx.10% of the tris(bipyridine)(bpy) complex. Carbon dioxide (5-6mol of CO/sub 2/ per 100 mol of Ru(bpy)/sub 3//sup 3 +/ taken) is produced over the entire pH range, 1 to 12. In addition, modified Ru(bpy)/sub 3//sup 2 +/ -like complexes whose yields (0.2-5.9 mol of Ru per 100 mol of Ru(bpy)/sub 3//sup 3 +/ taken) are a function of pH have been identified as products by high-performance liquid chromatographic analysis of the product solutions. The latter have been characterized by electrochemical and spectroscopic techniques and are not formed when sufficient Co/sup 2 +//sub aq/ (which catalyzes O/sub 2/ formation) is added to the reaction mixture. At pH 7, near stoichiometric O/sub 2/ formation is found for (0.01-1.0) X 10/sup -3/ M initial Ru(bpy)/sub 3//sup 3 +/ when the Co(II) catalyst concentration is approx. =0.1 (Ru(III))/sub 0/ and added (Ru(II) is less than or equal to 10 (Ru(III))/sub 0/. Kinetic studies as a function of pH, (Co(II)), Ru(bpy)/sub 3//sup 3 +/ , and Ru(bpy)/sub 3//sup 7 +/ give the rate law -d(Ru(III))/dt = k(Ru(III))/sup 2/(Co(II))/(Ru(II))(H/supmore » +/)/sup 2/ (k = 5.3 X 10/sup -10/ Ms/sup -1/ at 25/sup 0/C, 0.1 M ionic strength, pH 6.5-7.2) thus implicating a Co(IV) species as a crucial intermediate in O/sub 2/ formation. Irradiation of Ru(bpy)/sub 3//sup 3 +/ solutions with red light (lambda = 660 +/- 30 nm) accelerates the reduction of Ru(bpy)/sub 3//sup 3 +/ ; the quantum yields for Ru(II) and CO/sub 2/ formation are 7 X 10/sup -5/ and 2.8 X 10/sup -6/ mol/einstein, respectively, in 4 M H/sub 2/SO/sub 4/ or CF/sub 3/SO/sub 3/H and 2 X 10/sup -4/ and 1.1 X 10/sup -5/ mol/einstein, respectively, in 1 M H/sub 2/SO/sub 4/ or CF/sub 3/SO/sub 3/H. 47 references, 9 figures, 4 tables.« less

Dielectric continuum models used to calculate the solvent reorganization energy in thermal and photochemical electron-transfer reactions are discussed. Expressions are derived for the reorganization energy for charge redistribution within a single ellipsoidal or spherical solvent cavity. The expressions for the ellipsoidal cavity model differ in important respects from those derived previously. The Marcus two-sphere and the ellipsoidal cavity model differ in important respects from those derived previously. The Marcus two-sphere and the ellipsoidal cavity models can satisfactorily predict the band maximum for the metal-to metal charge-transfer transition in a number of ion-paired and bridged binuclear systems, provided careful consideration is given to the cavity dimensions. In complexes containing 2,2'-bipyridine ligands the solvent can approach close to the charge centers by filling the space between the ligands: this results in a small effective radius and relatively large values for the solvent reorganization energy. On the other hand, in complexes containing ammine ligands the effective radius is increased due to the effects of dielectric saturation resulting from specific ligand interactions with the solvent. Despite the limitations of the two-sphere model the reorganization energies calculated are, in general, not very different from those obtained with the ellipsoidal cavity model.

Mecanisme de reaction relatif au systeme Ru (dimethyl-4,7 phenanthroline-1,10) 3 2+ /Co(bpy) 2+ /triethanolamine. Le rendement en hydrogene augmente de 0,02 dans l'eau a 0,29 dans le melange eau-acetonitrile (50%). Donnees des potentiels d'oxydo reduction de CoL 3 3+ / 2+ et CoL 3 2+ / +

Metal-to-ligand charge transfer (MLCT) excitation of pentacyano(2-methylpyrazine)ferrate(II) ion in solution has been investigated, employing pulsed laser light of wavelengths 440 to 530 nm. Bleaching of the MLCT absorption due to loss of the heterocycle occurs in less than 20 ns. In the presence of excess heterocycle the starting material is regenerated via two intermediates, characterized by their substitution kinetics. One of the intermediates is the ion pentacyanoaquaferrate(II), for which the rates of recombination with 2-methylpyrazine were measured in the concentration range 0.0025 M less than or equal to (2-Mepyr) less than or equal to 9.6 M. The rate law for the process when (2-Mepyr) less than or equal to 1 M is rate = k/sub f/(Fe(CN)/sub 5/OH/sub 2//sup 3 -/)(2-Mepyr) (k/sub f/ = 475 +- 25 M/sup -1/ s/sup -1/, 25/sup 0/C, ..mu.. = 0.1 M LiTFMS, ..delta..H/sup + +/ = 16.0 +- 0.3 kcal/mol, ..delta..S* = 7.7 +- 1 cal mol/sup -1/ deg/sup -1/). The rate of this reaction is independent of pH in the range 6.6 less than or equal to pH less than or equal to 11.4. At (2-Mepyr) > 1 M the apparent order in that reagent's concentration increases markedly. The second intermediate decays via amore » much smaller absorbance change than the first and exhibits a tendency toward rate saturation in its substitution kinetics. This species is identified tentatively as the sterically hindered N-1 isomer of pentacyano(2-methylpyrazine)ferrate(II).« less

The interaction between conformation changes and electron-transfer rates is considered for systems containing two redox centers. The effect of conformation changes on the free-energy barrier to intramolecular electron transfer is examined. Unstable conformers R* of the reactant (or P* of the product) that are either more (low-{lambda} intermediates) or less (high-{lambda} intermediates) redox-active than the stable reactant R (or product P) are considered. The relative energies of the conformers determine whether the observed electron-transfer reaction proceeds by a two-step mechanism involving the intermediate formation of an unstable conformer or by a direct (concerted) reaction involving only the stable reactant and product. The present analysis generates a set of equations that indicate when the two-step or the direct mechanism is energetically favored. Only when the two-step mechanisms are favored is gating or conformational control possible.

Expressions for the shift of the absorption and emission band maxima of a solute with changes of the solvent's dielectric properties are presented for solute molecules with shapes and polarizabilities that can be approximated by those of spherical dielectric-continuum cavity. The derivations use a nonequilibrium thermodynamic approach developed by Marcus. An series of accurate approximations for the solvent shifts are presented. The new expressions are shown to reduce to equations previously derived by McRae, Ooshika, Mataga, and others when point-dipole approximations are made and particular values of the solute polarizability are assigned. The polarizability of the solute can be related to the internal dielectric constant of the cavity, and general expressions for the band shift in the point-dipole limit containing the dielectric constant of the cavity as a parameter are derived.

The kinetics of the oxidation of ferrocytochrome c by tris(1,10-phenanthroline)cobalt(III) has been studied by the stopped-flow technique. The oxidation reaction is first order with respect to each reactant. Measurements in chloride media (..mu.. = 0.13 M, 25/sup 0/C) over the pH range 1-7 revealed a rate maximum at pH 2.9 (k = 6.7 x 10/sup 4/ M/sup -1/ s/sup -1/). By contrast, the rate constants at pH 1 and 5.8 are 3.2 x 10/sup 4/ and 2.1 x 10/sup 3/ M/sup -1/ s/sup -1/, respectively. Below pH 1.7 biphasic kinetics are observed with the slower reaction having a first-order rate constant of approx. 2 s/sup -1/, independent of oxidant concentration. The slow process is ascribed to a conformational rearrangement of the ferricytochrome c which is produced in the mixed-spin form but rearranges to the more stable high-spin form. (auth)

The nuclear barrier to electron transfer arises from the need for reorganization of intramolecular and solvent internuclear distances prior to electron t ransfer. For reactions with relatively small driving force ("normal" free-energy region) the nuclear factors and rates increase as intrinsic inner-shell and outer-shell barriers decrease; this is illustrated by data for transition metal complexes in their ground electronic states. By contrast, in the inverted free-energy region, rates and nuclear factors decrease with d ecreasing "intrinsic" barriers; this is illustrated by data for the decay of charge-transfer excited states. Several approaches to the evaluation of the outer-shell barrier are explored in an investigation of the distance dependence of the nuclear factor in intramolecular electron-transfe r processes. One-electron-tra nsfer reactions are at the heart of biological energy transduction and many electrochemical, photochemical, and thermal reaction sequences. For electron transfer

A recently proposed semiclassical model, in which an electronic transmission coefficient and a nuclear tunneling factor are introduced as corrections to the classical activated-complex expression, is described. The nuclear tunneling corrections are shown to be important only at low temperatures or when the electron transfer is very exothermic. By contrast, corrections for nonadiabaticity may be significant for most outer-sphere reactions of metal complexes. The rate constants for the Fe(H/sub 2/O)/sub 6//sup 2 +/-Fe(H/sub 2/O)/sub 6//sup 3 +/, Ru(NH/sub 3/)/sub 6//sup 2 +/-Ru(NH/sub 3/)/sub 6//sup 3 +/ and Ru(bpy)/sub 3//sup 2 +/-Ru(bpy)/sub 3//sup 3 +/ electron exchange reactions predicted by the semiclassical model are in very good agreement with the observed values. The implications of the model for optically-induced electron transfer in mixed-valence systems are noted. 6 figures, 4 tables, 53 references.

Mecanisme de reaction relatif au systeme Ru (dimethyl-4,7 phenanthroline-1,10) 3 2+ /Co(bpy) 2+ /triethanolamine. Le rendement en hydrogene augmente de 0,02 dans l'eau a 0,29 dans le melange eau-acetonitrile (50%). Donnees des potentiels d'oxydo reduction de CoL 3 3+ / 2+ et CoL 3 2+ / +

In the presence of moderately strong acids in MeCN, Co complexes with BF2-bridged diglyoxime ligands are active catalysts for the redn. of protons to H2 at potentials as pos. as -0.28 V vs. SCE. [on SciFinder (R)]

Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF_2)_2(CH_3CN)_2 and Co(dpgBF_2)_2(CH_3CN)_2 (where dmgBF_2 is difluoroboryl-dimethylglyoxime and dpgBF_2 is difluoroboryl-diphenylglyoxime), as well as cobalt complexes with [14]-tetraene-N_4 (Tim) ligands of the type [Co(Tim^R)X_2]^(n+) (R = methyl or phenyl, X = Br or CH_3CN; n = 1 with X = Br and n = 3 with X = CH_3CN), have been observed to evolve H_2 electrocatalytically at potentials between −0.55 V and −0.20 V vs SCE in CH_3CN. The complexes with more positive Co(II/I) redox potentials exhibited lower activity for H_2 production. For the complexes Co(dmgBF_2)_2(CH_3CN)_2, Co(dpgBF_2)_2(CH_3CN)_2, [Co(Tim^(Me))Br2]Br, and [Co(Tim^(Me))(CH_3CN)_2](BPh_4)_3, bulk electrolysis confirmed the catalytic nature of the process, with turnover numbers in excess of 5 and essentially quantitative faradaic yields for H_2 production. In contrast, the complexes [Co(Tim^(Ph/Me))Br_2]Br and [Co(Tim^(Ph/Me))(CH_3CN)_2](BPh_4)_3 were less stable, and bulk electrolysis only produced faradaic yields for H_2 production of 20−25%. Cyclic voltammetry of Co(dmgBF_2)_2(CH_3CN)_2, [Co(Tim^(Me))Br_2]^+, and [Co(Tim^(Me))(CH_3CN)_2]^(3+) in the presence of acid revealed redox waves consistent with the Co(III)−H/Co(II)−H couple, suggesting the presence of Co(III) hydride intermediates in the catalytic system. The potentials at which these Co complexes catalyzed H_2 evolution were close to the reported thermodynamic potentials for the production of H_2 from protons in CH_3CN, with the smallest overpotential being 40 mV for Co(dmgBF_2)_2(CH_3CN)_2 determined by electrochemistry. Consistent with this small overpotential, Co(dmgBF_2)_2(CH_3CN)_2 was also able to oxidize H_2 in the presence of a suitable conjugate base. Digital simulations of the electrochemical data were used to study the mechanism of H_2 evolution catalysis, and these studies are discussed.

It has been found that the dependences of the electronic coupling element and of the solvent reorganization energy on distance have important implications for intramolecular electron transfers and for the forward and back reaction rates and cage-escape yields in light-induced electron-transfer processes. An equation is presented for the net second order rate constant for bimolecular transfer. A derived difference in the optimum distance for the forward and back reactions is pointed out to have important implications for the formulation of steady-state quenching schemes and for the interpretation of cage-escape yields.