Marcus electron transfer rate quenching

The inverted region was initially predicted by Marcus and the decrease in the electron transfer rate constant with —AG° has been observed experimentally many times.18 This is an important and remarkable result both for natural and artificial photosynthesis and energy conversion it predicts that, following electron transfer quenching of the excited A -B, the back electron transfer in the inverted region for the charge-separated state A + -B becomes slower as the energy stored increases. 

However, in the usual fluorescence quenching, the Rehm-Weller treatment (the curve b ) well accounts for the forward electron transfer rate constants from D (or D ) to A (or A), and the inverted region is not usually observed in the rate constants for the emission quenching. More sophisticated theories than the Marcus theory have been advanced in recent years [20-21]. However, we will not discuss this subject further. 

If one plots tile degree of luminescence quenching and the ratio of the charged photoexcitation bands to the neutral photoexcitation bands versus the electrochemical reduction potential (which measures qualitatively the electron affinity of the molecules), a maximum is observed [95] (Figure 8.37). This maximum in the photoinduced electron transfer rate on increasing the electron affinity suggests the existence of the inverted regime predicted by the Marcus theory (see [82,83] and references therein). Further experiments for quantitative determination of the electron transfer rate are needed. 

The anionic and cationic ends of the NC s TICT state were found to be quenched by electron donors and acceptors respectively. The quenching rate constants were analyzed with the help of the Marcus model and found consistent with the expected value for electron transfer mechanism. This was confirmed by photochemical reactions running through a radical cation or a radical anion intermediate. The isomerization of quadricyclane to norborna-diene was used as a check for the reactivity of the radical-cation end of the NC s... 

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... 

Fig. 2. Plot of the fluorescence quenching rate constants k, vs the free energy changes AG for the electron transfer process (a) Marcus relationship, (b) Rehm-Weller relationship...

Structure on hydrogel properties of 2-hydroxyethyl acrylate determined. " Polymers bearing tertiary amino groups have been synthesised and their fluorescence spectra found to be significantly quenched while maleic anhydride " and cyclododecanones have been found to be effective initiators of the photopolymerisation of styrene. Poly(methylphenylsilane) is also an effective photoinitiator for styrenes and acrylates via a photolytic process to give silyl radicals. Iron oxalate is also an effective photo initiator for acrylate monomers while a theoretical description of the kinetics of free radical dye-initiated polymerisation via an electron transfer process has been proposed. Using the Marcus theory it has been shown that the rate of electron transfer can affect the rate of initiation. 

Photolysis of the naphthylmethyl esters (365) gives naphthylmethyl radicals and phenylacetoxyl radical. The radical pair may transfer an electron to yield phenyl acetate and naphthylmethyl cation which is quenched by nucleophilic solvent. Alternatively, the radical pair may escape their solvent cage and so yield free radical derived products. Pincock has estimated the rate constant for the electron transfer in the radical pair for different X-substituents on the naphthalene by monitoring the ratio of free radical products to ionic products produced in the photolysis reaction, and has correlated this rate constant with the free energy of the electron transfer reaction in the radical pair. The results are discussed in terms of the Marcus theoretical relationship between reaction rate constant and equilibrium constant. 

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