Cage escape yields

Cage escape yield, 202 Catalysis, 90-94 acid-base, 232-238 dual substrates in, 94 ucleophilic, 237-238 Catalytic reactions, circle diagrams for, 189... 

Photo-induced electron transfer between [Ru(bpy)3]2+-like centres covalently bound to positively-charged polymers (N-ethylated copolymers of vinylpyridine and [Ru(bpy)2(MVbpy)]2+) and viologens or Fe (III) has been studied using laser flash photolysis techniques. It is found that the backbone affects the rates of excited state quenching, the cage escape yield, and the back electron transfer rate because of both electrostatic and hydrophobic interactions. The effect of ionic strength on the reactions has been studied. Data on the electron transfer reactions of [Ru(bpy)3]2+ bound electrostatically or covalently to polystyrenesulphonate are also presented. 

Table 1. Quenching (k ) and Back Reaction (kp Rate Constants and Cage Escape Yields Reactions of [ Ru(bipy)j] ...

Kinetic analysis of the oxidative quenching of Ru(bipy)2+ at different MV2+ concentrations by laser flash photolysis yields the rate constants for the forward and reverse electron transfer as 5.0 x 108M-1s-1 and 2.4 x 109M-1s- respectively. The cage escape yield of the redox products in the quenching process in water is about 30%. The back reaction in Eq. (7.6) can be prevented by adding a third component such as TEOA, which is capable of reducing Ru(bipy)3+ to the 2 + state ... 

Cage escape yield (< >cage) to the yield of separated electron-transfer products (corrected for fraction of excited states unquenched) that can be intercepted and/or utilized in other secondary reactions. It is a measure of the escape probability from the geminate pair and is given by ... 

The cage escape yield will be maximal when k3o 1 34. Back reactions to reform ground state reactants are typically highly exergonic leading to low barriers and large values of k3Q. However, even when the back reaction is barrierless, it is still possible to have k3o k34 provided the back reaction is strongly non-adiabatic (Kgi 1, i.e., reaction is controlled by electronic rather than by nuclear factors). 

For excited states of metal complexes that are well characterized, the procedure can be reversed and the free energy dependence of the quenching can, in turn, be used to determine the quencher exchange rates and/or quencher potentials that may be difficult to obtain by other means. For practical applications, the cage escape yield must be high. Laser flash photolysis techniques allow measurement of cage escape yields of redox products and there have been a number of studies reporting such yields for various quenchers. 

Table 9 Quenching rate constants and cage escape yields for the electron transfer quenching of [Ru(bpy)q] by various Co(lll) complexes [33]. ...

In all four cases, the assignment of the electron/energy transfer mechanisms have further experimental support from continuous and flash photolysis studies. Formation of Ru(III) and reduced acceptor ions can be observed in the first three cases and cage escape yields determined. For Eu(III) and Cr(III), in spite of the similarity in the redox potentials (E == - 0.43V and -0.41V respectively) and self-exchange rate constants, marked difference exists in the quenching behaviour. 

In the excited state quenching of a number of Ru- and Os-polypyridyl complexes by methyl viologen, the cage escape yield for MV+- radical varies only slightly, ranging from 0.14 to 0.27 ... 

The anthracene triplet, in turn, undergoes oxidative quenching with methyl viologen (reaction 57) and yields the redox products AnC02- and MV -with cage escape yields of nearly unity [84] ... 

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