Kinetic analysis and yield of redox products

The quenching rate constants measured in fluid solutions need to be corrected for diffusional effects and for this reason it is useful to define a diffusion-controlled quenching rate constant kq. as follows  

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  

As per the above sequence of reactions, the back reaction to reform the ground state reactants is given by a second order rate constant 

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). 

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