Polarons reaction

The final remark of this section concerns the polaronic transition of m-LPPP around 1.9 eV, where we can observe P2 with its vibronic replica P3 at 2.1 eV. In Figure 9-20 we show this polaronic absorption in m-LPPP as detected by photoin-duced absorption (a), chaige-induced absorption in conventional light-emitting devices (b), and chemical redox-reaction (c). Only under pholoexcilation, which creates both neutral and charged species, the triplet signal at 1.3 eV is also observed. 

Warshel, A. Chu, Z.T. Parson, W.W., Dispersed polaron simulations of electron transfer in photosynthetic reaction centers, Science 1989, 246, 112-116. 

Debierne (1914) was the first to suggest a radical reaction theory for water radiolysis (H and OH). In various forms, the idea has been regenerated by Risse (1929), Weiss (1944), Burton (1947, 1950), Allen (1948), and others. Platzman (1953), however, criticized the radical model on theoretical grounds and proposed the formation of the hydrated electron. Stein (1952a, b) meanwhile had suggested that both electrons and H atoms may coexist in radiolyzed water and proposed a model in which the electron digs its own hole. Later, Weiss (1953, 1960) also favored electron hydration with ideas similar to those of Stein and Platzman. In some respects, the theoretical basis of these ideas is attributable to the polaron (Landau, 1933 Platzman and... 

On the basis of their results the authors concluded that the crossover from the neutral to oxidised form of poly pyrrole, and vice versa, requires the number of spins to pass through a maximum, i.e. the creation and anihilation of bipolarons involves the passage through the polaron state. They expressed this as a two-step redox reaction ... 

Because electron transfer is a rate phenomenon, considerations of timescale are unavoidable in the modeling and understanding of ET reactions. Indeed, even in simple polaron theory (vibronic theory for electron transfer between two sites), there are several timescales including... 

Generally, reactions are considered to be nonadiabatic when the effective splitting, IVyp, is smaller than the thermal energy. This is, however, an inexact prediction—polaron theory provides a more complete set of demarcations between the two limits [86]. 

It is perhaps useful to mention that the use of electronically conducting polymers, such as poly(acetylene), (CH) poly(pyrrole), (C4H5N)X, and poly(aniline), (CeHgNf ), has been proposed for positives for lithium batteries. The electrochemical process of these lithium-polymer positives is somewhat similar to an intercalation reaction. On charging, the polymer (P) is oxidized by acquiring a positive charge to form a polaron, and this is... 

Such a process can be considered to be a catalyzed recombination of the electrons and holes via a suitable solute. On the other hand when there are two solutes (X and Y) capable of reacting with the electrons as well as with the holes, Reactions 38a and 37a can effectively compete with the back Reactions 37b and 38b. From these considerations it may be inferred that in irradiated ice and frozen aqueous solutions the electron is closely coupled to the hole and may be considered to be a polaron bound to a hole polaron (36). This would also account for the first-order nature of the recombination process. 

Recent pulse radiolysis experiments (20, 30) have fully established the presence of the negative polarons (hydrated electrons) in irradiated water, and also have demonstrated the occurrence of Reaction 39 (15). 

Although it has been suggested55 that a similar species, the positive polaron, might have a sufficiently long lifetime in liquid water to undergo chemical reaction, no conclusive evidence for such reactions has yet been put forward. Positive ions formed in organic liquids probably decompose rapidly to free radicals with the elimination of a proton. 

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