Charge separation limits

Fig. 1.10 Left. Canonical resonance structures for the l-dimethylamino-l,3,5,7-octatetraen-8-al molecule, DAO. (Top) Neutral polyene limit (middle) cyanine limit (bottom) zwitterionic charge-separated limit. Right Corresponding evolution of bond length alternation BLA (in A) (x) and bond order alternation BOA ( ), as a function of the applied external electric field F.

If we apply our rule (about limiting charge separation to no more than two charges), then we might say that the second resonance structure above has too many charges... 

The total quantum yield [4>cs(total)] for CS is decreased to 0.17 in dimethyl-formamide (DMF) due to the competition of the CSH from Fc-ZnP-H2F+-C6o (1.63 eV) to Fc-ZnP- -HzP-Cso (1.34 eV) versus the decay of Fc-ZnP-Fl2P -C6o to the triplet states of the freebase porphyrin (1.40 eV) and the Ceo (1.50 eV) [47]. In contrast to the case of most donor-acceptor-linked systems, the decay dynamics of the charge-separated radical pair (Fc -ZnP-H2P-C6o ) does not obey first-order kinetics, but, instead, obeys second-order kinetics [47]. This indicates that the mframolecular electron transfer in Fc -ZnP-H2P-C6o" is too slow to compete with the diffusion-limited inter-molecular electron transfer in solution. 

As previously mentioned, a key point in the optimization of the catalysts photodegradation of phenol and its derivates, is the minimization of the electron-hole recombination and the intimate connection of this process with the anion vacancies present in the size-limited, nanometric oxide particles. Minimization of the overall amount of oxide defects has a significant impact on the reaction rate. Traditional methods for improving electron-hole charge separation beyond what can be obtained with bare titania, involves doping mainly with although surface... 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

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