Charge shift reaction

In a general description of intramolecular electron-transfer (ET) processes one has to differentiate between charge separation in donor/acceptor (D/A) systems via the formation of photoexcited states and a charge-transfer or charge-shift reaction that is thermally activated (Cannon, 1980 Fox and Chanon, 1988 Meyer, 1978). 

Normally, the reaction partners in PET reactions are neutral molecules. That is why a donor radical cation—acceptor radical anion pair is obtained by the PET step. These highly reactive intermediates can be used for triggering interesting reactions. Since the PET is not restricted to neutral molecules PET reactions of donor anions and neutral acceptors or neutral donors and acceptor cations resulting in radical—radical anion (cation) pairs are known as well. These reactions are also called charge shift reactions due to the fact that the overall number of charged species is kept constant throughout the PET step. Finally, a PET process of a donor anion and a acceptor cation is possible as well (Scheme 2). 

Figure 8. Some covalently linked dyads for investigating thermal charge shift reactions in the radical anions.

Figure 10. Charge shift reactions instigated by pulse radiolysis [33].

D and A are represented as neutral species for simplicity, although often one or both is charged (and electrostatic effects can influence ET rates). When both D and Areally are neutral, the forward electron transfer is sometimes referred to as charge separation (CS) and the backreaction as charge recombination (CR). When D or A bears a charge, forward ET is sometimes referred to as a charge-shift reaction. 

Because there are no long-range forces between either the reactants or the products in the charge-shift reaction, D + A D + A , AG° is expected to be independent of distance. Figure 4 demonstrates that this is so. Also plotted in Figure 4 are the calculated estimates of A G° for charge separation-recombination, D + A + A", using the Weller equation. 

For the charge shift reactions studied in pulse radiolysis, the situation is simpler. 

Particularly the first of these two results is feasible only with charge-shift reactions studied by pulse radiolysis. This is one case where pulse radiolysis is superior for truly quantitative examination of ET mechanisms and testing of theory. 

In 1983 Closs and Miller found the bell-shqied curve for charge shift reaction in a linked molecular system to verify the validity of Marcus theory [479,480,488,490,530,540,733-735]. A porphyrin-linked molecular system also exhibited the bell-shaped curve as described in Section IV.C. 

For the general concept of artificial photosynthesis it is vital that the photoinduced charge shift reactions, which are one-electron processes, can be coupled to the multi-electron chemistry that results in stable products (H2, O2) and avoids high-energy intermediates. This requires catalytic donor and acceptor units that can accumulate oxidation and reduction equivalents, stabilize intermediates and catalyze 0-0 and H-H bond formation, respectively. The accumulative oxidation or reduction of such catalysts necessitate... 

ABSTRACT. Intercalation of ediidium bromide (EB ) and N,N -dimethyl-2,7-diazapyienium dichloride (DAP ) into calf-thymus DNA is highly efficient and gives stable supramolecular systems. Selective excitation of intercalated EB results in electron transfer to a nearby intercalated DAP molecule. The transient products formed by this charge-shift reaction are readily observed by laser flash photolysis techniques. The rate of forward electron transfer decreases with increasing number of interspersed nucleic acid bases and, for the closest approach ( 10 A), kf =... 

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