Exciplex formation encounter theories

There are two parallel channels of energy quenching by either contact formation of exciplex with subsequent dissociation to RIP [Eq. (3.604), scheme I] or by remote formation of RIP with subsequent association (transformation) to exciplex [Eq. (3.604), scheme II]. The last one was considered first by means of unified theory [29], which was extended later to account for both schemes together [30]. Since the results were comprehensively reviewed in Chapter IX of Ref. 32, there is no need to do the same here. It should only be noted that the theory of scheme I has been generalized to account exciplex formation, not only by encounters of excited reactants but also by a straightforward light excitation of existing complexes of the same particles [31]. 

Thus, electron transfers from a series of unhindered, partially hindered, and heavily hindered aromatic electron donors (with matched oxidation potentials) to photoactivated quinone acceptors are kinetically examined by laser flash photolysis, and the free-energy correlations of the ET rate constants are scrutinized [31]. The second-order rate constants of electron transfers from hindered donors such as hexaethylbenzene or tri-icrt-butylbenzene strongly depend on the temperature, the solvent polarity and salt effects, and they follow the free-energy correlation predicted by Marcus theory (see Figure 20A). Moreover, no spectroscopic or kinetic evidence for the formation of encounter complexes (exciplexes) with the photo-activated quinones prior to electron transfer is observed. 

In contrast, electron transfers from unhindered (or partially hindered) donors such as hexamethylbenzene, mesitylene, di-ferr-butyltoluene, etc. to photoactivated quinones exhibit temperature-independent rate constants that are up to 100 times faster than predicted by Marcus theory, poorly correlated with the accompanying free-energy changes (see Figure 20A), and only weakly affected by solvent polarity and salt effects. Most importantly, there is unambiguous (NIR) spectroscopic and kinetic evidence for the pre-equilibrium formation K c) of long-lived encounter complexes (exciplexes) between arene donor (ArH) and photoexcited quinone acceptor (Q ) prior to electron transfer (A et) [20] (Eq. 95). 

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