Charge recombination between

The supramolecular structure of the photosystems ensures the membrane location of the charged species is such that charge recombination between EA and P680+ and between EA2 and P700+ is prevented. 

Phenothiazine (PTZ) and ferrocene (Fc), both of which have a small reorganization energy associated to the electron transfer, are the first comediators that have been considered. Each has a potential that falls between 0.22 and 0.75 V versus SCE, respectively, the potential of the Co(DTB)33+/2+ and of the dye [Ru(H2DCB)-(dnbpy)(NCS)2]+/0. Because of the facile electron transfer, the photooxidized dye would be predominantly reduced by the comediator. Its oxidized form (PTZ+ and Fc +) can then be rapidly intercepted by Co(II), preventing the direct charge recombination between the oxidized comediator and the electrons in the 2. Nanosecond... 

This reaction rate is much faster than that of charge recombination between injected electrons and Ru dye cations. Thus, fast regeneration of the oxidized Ru dye photosensitizer also contributes to the accomplishment of effective charge separation. 

The second process that can reduce the yield of useful ions is charge recombination between any reduced electron acceptor and the original positive hole left in the initially excited molecule (shown as and in Figure 5.8). This can be prevented only by the spatial separation of the positive and negative charges. The reduction potentials of the successive electron acceptors decrease as their spatial separation increases, so that both the reverse electron transfer and the charge recombination become very slow. 

Polivka had investigated the co-adsorption of carotenoid and pheophytin (111) on the surface of TiC>2 electrode and the photophysical properties of pheophytin in this film. The results demonstrated that the fluorescence of 111 was efficiently reductive quenched by carotenoid in this co-assembled film, suggesting similar mechanisms to that in the natural photosynthetic systems. The radical anion of 111 formed during the electron transfer recovered to the neutral state quickly before the charge recombination between carotenoid cation and pheophytin anion took place. It is suspected that the electron injection from the pheophytin anion to the conduction band of Ti02 was responsible for this quick recovery. This result indicated that such a self-assembling strategy may be also considered for novel DSSC constructions [108]. 

Early in 1961, Libby [30] suggested the proton transfer from the radical cation and subsequent charge recombination between the protonated site of polyethylene chain and an electron for the alkyl radical formation ... 

Weller and Zachariasse thoroughly investigated exciplex formation and luminescence for donor acceptor systems in THF [18]. A particularly interesting result from their work came from an examination of the temperature dependence of radiative charge recombination between 9,10-dimethylanthracene anion (DMA") and TPTA+ in THF [19]. They found that both exciplex emission and fluorescence from DMA were observed in solution at low temperature (ca. —50°C). As the solution temperature is raised, the excimer emission decreases in relative intensity, and at room temperature the emission is nearly completely DMA fluorescence. The monomer-to-exciplex emission intensity ratio as a function of temperature follows Arrhenius kinetic behavior and yields an activation barrier that is nearly the same as the energy gap between the exciplex and the DMA states. Thus, their model consisted of reaction of the solvent-separated ions to form an intimate emissive ion pair which could dissociate to yield the singlet anthracene derivative. 

It is noteworthy that the rate of reduction of the Zn porphyrin j-radical cation by the copper(I) complex is much faster than direct charge recombination between the porphyrin radicals, although the reaction exoergonicity for Eq. 19 is very much less than that for Eq. 18. The faster rate for Eq. 19 may arise because of the closer proximity of the reactants and/or because electron transfer occurs via a different route. In addition, /ci9 shows some dependence on the porphyrin donor. This effect could be due to a different spin multiplicity of the precursor excited state. 

Fig. 1. Three difference spectra for P680 photooxidation. See text for discussion. (A) from DOring, Renger, Vater and Witt (1969) Properties of the photoactive Chlorophyll-a f in photosynthesis. Z Naturforsch 24B 1139 (B) from van Gorkom, Pulles and Wessels (1975) Light-induced changes of absorbance and electron spin resonance in small photosystem II particles. Biochim Biophys Acta 408 336 (C) from Gerken, Dekker, Schlodder and Witt (1989) Studies on the multiphasic charge recombination between chlorophyll a/ (P680) and plastoquinone in photosystem II complexes. Ultraviolet difference spectrum of Chl-a / Chl-a. Biochim Biophys Acta 977 57.

S Gerken, JP Dekker, E Schlodder and HT Witt (1989) Studies on the multiphasic charge recombination between chlorophyl a, (P-680 ) and plastoquinone QfP in photosystem II complexes. Ultraviolet difference spectrum ofChl a 7Ch/ a,. Biochim Biophys Acta 977 52-61... 

Ki Satoh, Hansson and P Mathis (1990) Charge recombination between stabilized P680 and reduced cytochrome b559 in quinone-reconstituted PS II reaction center. Biochim Biophys Acta 1016 121-126... 

D. Charge Recombination between P700 and Ar in the CPI Complex [P700-Ao-Ai].588... 

PV Warren, JH Golbeck and JT Warden (1993) Charge recombination between P70CT andAf occurs directly to the ground state of P700 in a photosystem I core devoid ofF,, Fq, andF. Bbchemistry 32 849-857... 

Electron transfer from T to the oxidized dye takes place in 100 ns, which is faster than the charge recombination between the injected electron and the oxidized dye (ps to ms) by three orders of magnitude. 

Heterodimer-containing His - 73 Leu RCs are photochemical active. These RCs exhibit reversible bleaching of their long wavelength absorption band by actinic light. Both the oxidized-minus-reduced difference spectrum and the rate of charge recombination between the oxidized heterodimer and the primary quinone anion are similar to that observed in His OO Leu RCs (data not shown). 

---