Outer-sphere quenching

It is also necessary to take account of outer sphere contributions to the quenching of the metal-centered excited state. The nature of the outer sphere will vary from one complex to another, depending on the size, charge, and shape of the complex. However, it is possible to correct for a generalized outer sphere contribution for each lanthanide giving a new equation for q which can be expressed in several ways 74... 

The Mn04, Mn04 electron transfer has been studied using radioisotopes and quenched-flow as well as by nmr, with good agreement between the results (Tables 1.3 and 3.3). The rate of outer-sphere electron transfer is given by... 

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

Binding of the components in a neutral aqueous solution was confirmed by potentiometric titrations. The feasibility of electron transfer between the components was predicted by cyclic voltammetry and an efficient outer-sphere fast electron transfer was foreseen. Fluorescence spectroscopy measurements showed that the formation of a defined donor-acceptor complex worked even in water at neutral pH. Electron transfer as the quenching mechanism was proved by laser flash photolysis. 

Although chromate(VI) is photochemically inactive in all of its forms in neat aqueous solution, the photochemical oxidation of alcohols by chromate(VI) has been known for more than 80 years and interpreted in terms of photochemical reactivity of the chromate(VI) esters [94], Recent studies have shown, however, that LMCT excitation of CrVI species is quenched not only by inner-sphere but also by outer-sphere electron transfer [23, 87,92,94,95], Moreover, inner-sphere electron transfer in chromate(VI) esters was found to involve two electrons, yielding a CrIV species and appropriate aldehyde or ketone ... 

Kinetic data have been reported for reduction of //-superoxo complexes by Fe2+,7 1 Mov,702 Co11703 and Ru11 complexes,704 and V2+, Cr2+ and Eu2+.705 These processes involve outer-sphere electron transfer and in some cases703,706 the Marcus theory has been applied to the rate constants obtained. Electron transfer quenching of the excited state of [Ru(bipy)3]2+ by various -superoxo cobalt(III) complexes leads to production of [Ru(bipy)3]3+ and the corresponding /z-peroxo species.706... 

Several studies of bimetallic complexes in which the donor and acceptor are linked across aliphatic chains have demonstrated that these are generally weakly coupled systems. " Studies of complexes linked by l,2-bis(2,2 bipyridyl-4-yl)ethane (bb see Figure 5), indicate that these are good models of the precursor complexes for outer-sphere electron-transfer reactions of tris-bipyridyl complexes. A careful comparison of kinetic and spectroscopic data with computational studies has led to an estimate of //rp = 20cm for the [Fe(bb)3pe] + self-exchange electron transfer. In a related cross-reaction, the Ru/bpy MLCT excited state of [(bpy)2Ru(bb)Co(bpy)2] + is efficiently quenched by electron transfer to the cobalt center in several resolved steps, equations (57) and (58). ... 

All lifetimes are in milliseconds, and the Al constant is a proportionality factor specific to a given lanthanide that takes into accoimt the energy gap between the groimd and excited state manifolds. This equation was later modified by Parker to include quenching effects from coordinated N—H oscillators (Eq. 2), where x is the number of N—H oscillators) and outer-sphere water molecules (97). 

Both AP and Ga have a tightly bound hydrate shell in aqueous solution and both are prone to hydrolysis. In terms of the Hertz electrostatic model for quadrupolar relaxation of ionic nuclei in electrolyte solution (see Section III.C) one therefore expects effective quenching of the electric field gradient caused by the surrounding water dipoles, due to a nearly perfect coordination symmetry. Any contribution to the e.f.g. should therefore arise from outer-sphere solvent dipoles. In terms of the fully orientated solvation (FOS) model this would correspond to a distribution width parameter approaching zero (/. -> 0) with the first term in equation (4) vanishing. This is indeed what Hertz (24) found for both AF" and Ga ", and the experimental infinite dilution relaxation rates ( AP" 7-5 s Ga 350 s ) are remarkably well matched by the computed ones... 

Quenching of excited-state [Ru(bipy)3] by reduced blue proteins involves electron transfer from the Cu with rate constants close to the diffusion limit for electron-transfer reactions in aqueous solution. It is suggested that the excited Ru complex binds close to the copper-histidine centre, and that outer-sphere electron transfer occurs from Cu through the imidazole groups to Ru. Estimated electron-transfer distances are about 3.3 A for plastocyanin and 3.8 A for azurin, suggesting that the hydrophobic bipy ligands of Ru " penetrate the residues that isolate the Cu-His unit from the solvent. ... 

At more favorable free energy for the redox reaction, kq increases (the turning point being AG°j 0.2 V) consistent with the increasing importance of outer sphere electron transfer as a component of this quenching. Thus for Cr(tfac)3, (tfac is trifloroacetylacetonate), AG°[ = 0.23 V and kq = 4.2 x 10 s in dichloro-... 

Chemical reaction is an important quenching mechanism of electronically excited states. Because of the short lifetime (generally less than 1 /xsec) of excited states in fiuid solution at room temperature, quenching by chemical reaction must be very fast if it is to occur. We shall consider here only outer-sphere electron transfer reactions of excited states since these reactions are certainly fast enough to compete with the other deactivation modes. 

RuCNHj) ] ", like [Ru(NH3)6 , participates in outer-sphere electron transfer reactions, a range of such reactions involving the reduction of [RufNHj) ] having been reported. [RuCNHj) ] quenches the excited state [Ru(bipy)3] " and has been suggested as a possible oxidant in related... 

The electronic effects in energy and electron transfer reactions, including excited state systems, have been discussed in a review by Endicott. The trends observed in the rate constants for the quenching of the doublet E) excited state of [Cr(bpy)3] by a series of organochromium complexes, [Cr(H20)5R], indicate an outer-sphere electron transfer mechanism. The different reactivity patterns found for the oxidations of [(H20)Co([14]aneN4)R] complexes by [Ru(Bpy)3] and [ Cr(bpy)3] point to electron and energy transfer mechanisms, respectively. The reductive quenching of [ Cr(bpy)3] by Fe produces [Cr(bpy)3], which also quenches the excited state in the absence of added... 

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