Electron transfer quenching kinetics

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. 

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 ... 

Excited state potentials can also be estimated from kinetic studies of electron transfer quenching reactions involving a series of acceptors and/or donors with varying potentials. By applying electron transfer theory to the quenching step, in conjunction with the predicted dependence of the quenching rate constant on AG° for the electron transfer reaction, estimates for the redox potentials may be obtained (2 ). These approaches have been used successfully in the evaluation of the redox properties of several metal complexes,... 

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... 

FIGURE 12. Kinetic scheme proposed for electron transfer quenching of Ru(bipy)32+ by some nitrobenzene derivatives (245). 

Kinetics of Electron-Transfer Quenching Figure 8.6 shows that the excited-state complex [Cr(phen)3]3+ can have either two or three reaction pathways, depending whether or not a quencher Q is present in solution. As the concentration of Q increases, the rate of the bimolecular electron-transfer pathway increases. We can take advantage of this to determine the rate constant of the electron-transfer pathway. 

Many varied laser systems have been used for ns-flash photolysis experiments. For example, the kinetics of the electron-transfer quenching of triplet methylene... 

To study the mechanism by which HCIO4 catalyzes electron transfer, the kinetic order in acid was measured by performing a series of Stem-Volmer luminescence quenching experiments at various concentrations of HCIO4. These experiments revealed a linear relationship between k t and [HCIO4] (Fig. 42) showing electron transfer from [Ru (bpy)3] " is first order in acid. In this work, the authors proposed a stepwise ET/PT mechanism in which protonation of the ketyl radical anion provides additional thermodynamic driving force which causes an increase in et- This work predated the wide-spread acceptance of concerted proton-coupled electron transfer as an elementary step, however these seminal observations provided the conceptual framework for PCET to be applied further in contemporary synthetic chemistry. 

The kinetics of electron transfer quenching of excited states can be described by a reaction cycle that involves a sequence of forward and equilibrium steps. Such a cycle for an oxidative quencher O reacting with the excited state Ru(bpy)3 is shown in Fig. 5.9. The rates and equilibria that must be considered are the formation of an encounter complex (exciplex) between the excited state and the quencher (A i2), followed by electron transfer within this complex ( 32). This Ru(III)/0 product can either dissociate to give Ru(bpy) and 0 ( 34), or undergo back electron transfer to reform ground state Ru(bpy)3 and the quencher O. These steps... 

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