Quenching electron-transfer

For systems such as these, which consist of electron transfer quenching and back electron transfer, it is in general possible to determine the rates both of quenching and of the back reaction. In addition to these aspects of excited state chemistry, one can make another use of such systems. They can be used to synthesize other reactive molecules worthy of study in their own right. The quenching reaction produces new and likely reactive species. They are Ru(bpy)3+ and Ru(bpy)j in the respective cases just shown. One can have a prospective reagent for one of these ions in the solution and thereby develop a lengthy and informative series of kinetic data for the transient. 

State decarbonylation reaction in total synthesis was reported recently in the case of natnral prodnct (+)-herbetenolide, which farther illustrates the exquisite control that the solid state may exert on the chemical behavior of the otherwise highly promiscuous reactive intermediates. As word or caution, it should be mentioned that intramolecular quenching effects known to act in solution can also affect that reaction in the solid state. Recently reported examples include the well-known intramolecular P-phenyl and electron transfer quenching. ... 

Quantitative Quantum Yield/Lifetime Predictions from Electron Transfer Quenching... 

A-Methylphthalimide (288) undergoes photoaddition in acetonitrile to ds-but-2-ene (289) to give d.s-l,6,7-trimethyl-3,4-benzo-6,7-dihydroazepine-2,5-dione (290).238 Evidence supports a concerted [ 2 + J2] pathway to the intermediate 291. Similar additions to other alkenes have been reported.239 Electron transfer quenching has been shown to compete with cycloaddition... 

A.B.P. Lever, York University Most model photocatalysts undergo electron transfer reactions via their spin triplet states. You point out that there are advantages to using the spin singlet state for electron transfer as exemplified by chlorophyll. What kind of structural or electronic features should be built into model photocatalysts to favour use of their spin singlet states for electron transfer quenching ... 

The phosphorescence of Pt2 (I OsKHq in aqueous solution is quenched by l,l-bis(2-sulfoethyl)-4,4f-bipyridinium inner salt (BSEP). Transient absorption attributable to BSEP ( nax 610 nn) is observed in flash kinetic spectroscopic studies of aqueous solutions containing Pt2( Os Hq and BSEP, thereby establishing an electron transfer quenching mechanism ... 

The redox potential diagram in eq. 1 illustrates that the effect of optical excitation is to create an excited state which has enhanced properties both as an oxidant and reductant, compared to the ground state. The results of a number of experiments have illustrated that it is possible for the excited state to undergo either oxidative or reductive electron transfer quenching (2). An example of oxidative electron transfer quenching is shown in eq. 2 where the oxidant is the alkyl pyridinium ion, paraquat (3). 

Application of the energy gap law to the energy conversion mechanism in Scheme 1 leads to a notable conclusion with regard to the efficiency for the appearance of separated redox products following electron transfer quenching. From the scheme, the separation efficiency, sep> is given by eq. 18. Diffusion apart of the... 

Since the excited state may act as an electron donor or as an electron acceptor, two electron-transfer quenching pathways, reductive and oxidative, are possible. 

An example where electron transfer from PhO- is important comes from a related publication on the reaction of phenol with O2 where [Ru(bpy)3]2+ is used as a photosensitizer (14). In acidic media the reaction involves generation of 02 by quenching of excited [Ru(bpy)3]2+ reaction of 2 with phenol leads to the production of benzoquinone. The quantum yields for benzoquinone production are highly pH dependent, showing a sharp peak at pH 8.4. This unusual pH dependence arises from the competition of several pathways, and one of the most important being the electron-transfer quenching of [ Ru(bpy)3]2+ by PhO- ... 

Scheme 6 Electron transfer quenching of the excited state of Ru(L)32+ by differ-ent quenchers...

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

Reversible electron transfer within Mg and Zn-substituted hemoglobin hybrids is initiated by flash photoproduction of the long-lived triplet state ( MP). According to Scheme I, the triplet return to the ground state involves two pathways, intrinsic triplet decay (with rate constant kp) and electron transfer quenching (with rate constant k,). 

Triplet decay in the [Mg, Fe " (H20)] and [Zn, Fe (H20)] hybrids monitored at 415 nm, the Fe " / P isosbestic point, or at 475 nm, where contributions from the charge-separated intermediate are minimal, remains exponential, but the decay rate is increased to kp = 55(5) s for M = Mg and kp = 138(7) s for M = Zn. Two quenching processes in addition to the intrinsic decay process (k ) can contribute to deactivation of MP when the iron containing-chain of the hybrid is oxidized to the Fe P state electron transfer quenching as in Eq. (1) (rate constant kj, and Forster energy transfer (rate constant kj. The triplet decay in oxidized hybrids thus is characterized by kp, the net rate of triplet disappearance (kp = k -I- ki -I- kj. The difference in triplet decay rate constants for the oxidized and reduced hybrids gives the quenching rate constant, k = kp — kj, = k, -I- k , which is thus an upper bound to k(. 

Photoelectron Ejection from t-St and Electron Transfer Quenching of St ... 

Electronic transfer quenching of t-Sf proceeds at ks = kdifr = 7.1 x 10 M sec in DMF. Similar to radical ion pair, (D /A )soiv formed during ET between donor (D) and acceptor (A) molecules in the excited singlet or triplet state, it is suggested that ET quenching initially gives (St/Bp )soiv with competition of the internal conversion of St to Sf . (St/Bp )soiv then undergoes solvent separation into St and Bp at k or returns to Sf and Bp via back ET at k. Therefore, the fraction of free Bp formed is represented by R = k k + k p). 

Selective Electron Transfer Quenching of Radical Anions in the Excited State... 

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