Quenching rate constants, free energy dependence

Eqs. 9 and 10 make clear predictions about the dependence of quenching rate constants on the free energy change in the quenching step. One way of testing the theory is to observe the quenching of the excited state by a series of related quenchers where the parameters kq(0), K, and k j) should remain sensibly constant and yet where the potentials of the quenchers as oxidants or re-ductants can be varied systematically. Such experiments have been carried out, most notably with the MLCT excited state, Ru(bpy)3 + (1). The experiments have utilized both a series of oxidative nitroaromatic and alkyl pyridinium quenchers, and a series of reductive quenchers based on aniline derivatives. From the data and known redox potentials for the quenchers, plots of RTlnk q vs. 

Regular dependence (Fig. 28) of quenching rate constants, feq, on the free-energy driving force AGq(AGq = — AGe) strongly suggests another reason for the observed ECL behavior. 

The driving-force dependence of the quenching rate constant and the redox potential of the ES couple can be obtained from Eq. (d) or (0 if the free-energy dependences of AGj3 and AG are known. In terms of the classical formalism, AG23 is given by... 

Progress in the understanding of ES reactions now allows ESs to be so well characterized that quencher exchange rates and potentials are being determined from the free-energy dependence of [ RuL3] + quenching rate constants . Study of ES electron transfer permits a detailed examination of very exothermic reactions and provides a probe of the intimate details of electron transfer. 

In experimental studies of photoinduced electron transfer reactions, the free energy dependence of the quenching of a particular metal complex (e.g, Ru(bpy)3 + ) by a series of structurally related quenchers of known properties is used to confirm the calculated potential of the excited state couple and to estimate its exchange rate. For a given quencher a graded series of polypyridine complexes with different substituted bpy/phen ligands can also be used equivalently. These provide a set of experimentally measured data to check the observation of normal and inverted behaviour predicted by Marcus theory under certain conditions. Rate constants for back electron transfer of photoredox products have been measured in a similar manner in some cases and these were also subjected to analysis. 

For QDs with tCl-l,2-BQ at x=5, the observed variation of PL quenching efficiency on is explained by the dependence of the photoinduced electron transfer rate constant on the Gibbs free energy ACf for PET in the normal region ... 

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