Reactions excited state outer sphere electron transfer

Excited State Outer Sphere Electron Transfer Reactions... 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

An ideal photosensitizer must satisfy several stringent requirements (Balzani et. al., 1986) 1) stability towards thermal and photochemical decomposition reactions 2) sufficiently intense absorption bands in a suitable spectral region 3) high efficiency of population of the reactive excited state 4) long lifetime in the reactive excited state 5) suitable ground state and excited state potentials 6) reversible redox behavior 7) good kinetic factors for outer sphere electron transfer reactions. 

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). ... 

Cobalt(II,III) sepulchrates have been used in the chemical education [415] and considerable number of the chemical and physicochemical studies as efficient quencher of the phosphorescence [416] and electronic excited states [417, 418], as a reductant in kinetic studies of redox reactions [419, 420], as a model for study of magnetodynamic [421], solvent [422] and pressure [423] effects on the outer-sphere electron-transfer reactions. Transfer chemical potentials (from solubility measurements) [424], electrochemical reduction potentials [425] and ligand-field parameters [426] for cobalt sepulchrates have been calculated. Solvent effect on Co chemical shift of cobalt(III) ion encapsulated in the sepulchrate cavity [427]... 

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... 

Outer-sphere electron transfer reactions, involving ground or excited states (equations (17), (18), and (19)), and energy transfer via an exchange mechanism (equation (20)) are the most simple bimolecular reactions. In these processes, no bond breaking or bond making occurs, and only electron or electron energy are transferred from a reactant to another. 

The theoretical results obtained for outer-sphere electron transfer based on self-exchange reactions provide the essential background for discussing the interplay between theory and experiment in a variety of electron transfer processes. The next topic considered is outer-sphere electron transfer for net reactions where AG O and application of the Marcus cross reaction equation for correlating experimental data. A consideration of reactions for which AG is highly favorable leads to some peculiar features and the concept of electron transfer in the inverted region and, also, excited state decay. 

For systems that are powerful excited-state reductants, photoreduction of alkyl halides is observed (6.16). This reaction was initially interpreted to be an outer-sphere electron transfer to form the radical anion, which rapidly decomposes to yield R- and X . Subsequent thermal reactions yield the observed products, an SrnI mechanism (Figure 3a). While such a mechanism, SrnI, appears plausible for a metal complex with E°(M2 /3M2 ) < -1.5 V (SSCE), it seems unlikely for complexes with E°(M2 /3M2 ) > -1.0 V (SSCE). Reduction potentials for alkyl halides of interest are generally more negative than -1.5 V (SSCE) (1/7). Alkyl halide photoreduction is observed for binudear d complexes whose excited-state reduction potentials are more positive than -1.0 V (SSCE) in CH3CN. 

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. ... 

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... 

The exciplexes of n,7i -excited states are critical intermediates, which can lead to three follow-up reactions (1) (exergonic) inner-sphere electron transfer (which competes with a direct outer-sphere electron transfer) [169], (2) exciplex-mediated hydrogen atom abstraction (the occurrence of which in n,n exciplexes is still in debate) [70,164,170], and (3), exciplex-induced quenching [62]. The last mechanism prevails when the energetics of electron transfer is endergonic. Again, high-level... 

Halogen atom transfer reactions are relatively uncommon for alkyl halides and Ru(bpj)3. This lack of reactivity is primarily due to the excited state Ru(bpj)3 being an outer-sphere redox reagent, and for alkyl halides both the oxidation and reduction potentials have values that make such electron transfer reactions unfavorable. These considerations are particularly valid for chlorocarbons, but for bromocarbons or iodocarbons it is possible that selective photoreactions with Ru(bpj)3 may be observed. 

---