Charge Transfer Processes in the Dark

Rate-limiting Steps in Charge-transfer Processes in the Dark... 

Photorechargeable Cell, Fig.1 Two-electrode sandwich-type structure of photocapacitor and charge-transfer mechanism in the processes of photo-charge and dark discharge... 

Stoicescu and Dimonie103 studied the polymerization of 2-vinylfuran with iodine in methylene chloride between 20 and 50 °C. The time-conversion curves were not analysed for internal orders but external orders with respect to catalyst and monomer were both unity. Together with an overall activation energy of 2.5 kcal/mole for the polymerization process, these were the only data obtained. Observations about the low DP s of the products, their dark colour, their lack of bound iodine and the presence of furan rings in the oligomers, inferred by infrared spectra (not reported), completed the experimental evidence. The authors proposed a linear, vinylic structure for the polymer, and a true cationic mechanism for its formation and discussed the occurrence of an initial charge-transfer complex on the... 

The charge-transfer nitrations of the aromatic donors are generally carried out to rather low actinic conversions to avoid complications from light absorption by the nitroarene products, and in duplicate sets (with a dark control) to monitor simultaneously any competition from thermal processes. For example, the yellow solution of anisole and Me2PyN02 in acetonitrile at — 40°C is irradiated with the aid of the cut-off filter that effectively removes all excitation light with Aexc<400nm. After reasonable photochemical conversions are attained, the H NMR spectrum is found to be virtually identical to that of the reaction mixture obtained by electrophilic (thermal) nitration (60). 

More detailed investigations [38,39] have shown the kinetics of low-temperature electron transfer reaction (1) in bacteria to have a biphase character, i.e. to consist of two sections, one with a faster and the other with a slower decay of the Pf centres. Also, the type of kinetics of reaction (1) in bacteria at low temperatures has been found to depend on the conditions of sample preparation. The region of fast (t 50 ms) charge recombination at T < 230 K was observed only for the samples frozen in the dark. The extent of P decay was observed to decrease upon freezing the samples in the light. These results were explained by the presence of two channels for the decay of P+ centres by reactions with particles A and Q". The faster decay of P+ was assumed to be due to its reaction with A and the slower decay of P4 to its reaction with Q . The relative amounts of A and Q particles (i.e. the extent of electron transfer from the reduced form of the primary acceptor A to the secondary acceptor Q) was assumed to depend on temperature. This assumption explains why the character of P decay depends on whether P+ species are formed after or in the process of freezing the sample. 

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