Donors radical cation

Determination of g-tensor components from resolved 327-670 GHz EPR spectra allows differentiation between carotenoid radical cations and other C-H jt-radicals which possess different symmetry. The principal components of the g-tensor for Car"1 differ from those of other photosynthetic RC primary donor radical cations, which are practically identical within experimental error (Table 9.2) (Robinson et al. 1985, Kispert et al. 1987, Burghaus et al. 1991, Klette et al. 1993, Bratt et al. 1997) and exhibit large differences between gxx and gyy values. 

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 3 Special TRIPLE resonance spectra of the primary donor radical-cation P in the bRC of R. sphaeroides wild type and mutant HE(M202) (His - Glu) and of monomeric BChl a " in organic solvents, all spectra in isotropic solution. The isotropic hfcs are directly obtained from the special TRIPLE frequency nSJ = Ais,J2.H The oxidation potential of the primary donor is also given (vs. NHE). Adapted from reference 68.

In the BChl g containing heliobacteria Heliobacillus mobilis and Heliobacterium chlorum symmetric dimers for the primary donor radical cation PgJ5 have been found based on EPR and ENDOR data.85 This symmetric dimer is consistent with the homodimeric structure of the RC. The same reason was invoked to explain the high symmetry of the donor radical-cation Pgg5 in green sulfur bacteria, which is made up from a BChl a dimer.86 For a review see reference 87. Note that these RCs belong to the type I RCs. 

The Primary Donor. - The radical-cation P+ In the bRC of purple bacteria and also in PS I the primary electron donors have been identified as (B)Chl dimers and EPR/ENDOR clearly showed that the unpaired electron and the positive charge - is (asymmetrically) distributed in a supermolecular orbital extending over both dimer halves (see sections 2.1,3.1). Dimer formation has the important consequence of charge delocalization and this stabilization of the primary donor radical-cation leads to a decrease of the oxidation potential. A fine tuning of the potential is possible through interactions with the environment, e.g. via H-bonds. 

This is shown by the formation of transient absorptions of both the donor radical cations and the C6o or C70 radical anion [120,125,127,133,139,141,... 

However, the initial step of the electron transfer reaction strongly depends on the solvent polarity. By changing the solvent to less polar or nonpolar solvents like benzene or nonaromatic hydrocarbons the transient absorptions of 3C 0, G)0 and donor radical cation appear immediately after the laser pulse. The decay of all the absorptions is also completed at the same time. The fast appearance and the fast decay of the Go and donor radical cation absorption suggest that there is an interaction between fullerene and donor in less polar and nonpolar solvents before laser irradiation [120,125,133-139],... 

Time profiles of the formation of fullerene radical anions in polar solvents as well as the decay of 3C o obey pseudo first-order kinetics due to high concentrations of the donor molecule [120,125,127,146,159], By changing to nonpolar solvents the rise kinetics of Go changes to second-order as well as the decay kinetics for 3C o [120,125,133,148], The analysis of the decay kinetics of the fullerene radical anions confirm this suggestion as well. In the case of polar solvents, the decay of the radical ion absorptions obey second-order kinetics, while changing to nonpolar solvents the decay obey first-order kinetics [120,125,127,133,147]. This can be explained by radical ion pairs of the C o and the donor radical cation in less polar and nonpolar solvents, which do not dissociate. The back-electron transfer takes place within the ion pair. This is also the reason for the fast back-electron transfer in comparison to the slower back-electron transfer in polar solvents, where the radical ions are solvated as free ions or solvent-separated ion pairs [120,125,147]. However, back-electron transfer is suppressed when using mixtures of fullerene and borates as donors in o-dichlorobenzene (less polar solvent), since the borate radicals immediately dissociate into Ph3B and Bu /Ph" [Eq. (2)][156],... 

The first-order rate constant can be evaluated from the decay curves of 3C o and the rise curves of Qo and the donor radical cation [125,154], The observed electron transfer rate constants for C6o are usually in the order of 109-1010 dm3 mol-1 s-1 and thus near the diffusion controlled limit which depends on the solvent (e.g., diffusion controlled limit in benzonitrile -5.6 X 109 M-1 s-1) [120,125,127,141,154-156],... 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

One strategy to overcome the regioselectivity problem and to enhance the efficiency for the photocyclization is the use of a suitable leaving group in a-position to the donor (Sch. 31). The intermediate donor radical-cation formed after electron transfer subsequently undergoes... 

Kass, H., Fromme, P. Witt, H.T. and Lubitz, W. (Orientation and electronic structure ofthe primary donor radical cation in Photosystem I a single crystal EPR and ENDOR study, J. Phys. Chem. B 105, 1225-1239. 

Of particular relevance to the present discussion is the observation that the CSS, which is a biradical cation, is formed with essentially pure triplet spin correlation. For energetic reasons, this triplet radical pair cannot recombine to form the MLCT state and can only form the singlet ground state. Therefore, direct recombination is spin forbidden. Moreover, because the radical pair which constitute the CSS product can separate only to a limited distance, essentially every CSS recombination event is between the same geminate radical pair—in other words, every reduced acceptor is ultimately oxidized by the donor radical cation that was formed from the same initial photochemical event. The spin behavior of the DC A triad CSS can be effectively explained by application of the relaxation mechanism of Hayashi and Nagakura. ... 

Figure 10.3 shows the results for the EPR measurements on the three oxidized donor radical cations. The loss of hyperfine resolution across the series (as noted previously by Gilbert et al.) is obvious within this methylated series. The isotropic... 

As mentioned above, triplet Cgo is readily photoreduced by amines and other donors to Cgo radical anion and the donor radical cations [64], We expected this reaction to lead to adducts with covalent bonds. Such adducts are formed with some amines in ground state chemistry [33, 60, 83], but the photochemical process should be more selective and easily controlled, since only one-electron reduction is possible in the photochemical process. C o in the Si state has been suggested to produce an exciplex with triethylamine which seems to react with ground-state Cgo to give a stable product [117]. The reduction potential of the triplet is high enough that electron-transfer from many donors such as electron-rich aromatics and alkenes should be possible. 

At least two points should be especially emphasized, (i) From the solvent part, the parent radical cations exist in a non polar surrounding. Hence, the cations have practically no solvation shell which makes the electron jump easier in respect to more polar solvents. In a rough approximation the kinetic conditions of FET stand between those of gas phase and liquid state reactions, exhibiting critical properties such as collision kinetics, no solvation shell, relaxed species, etc. (ii) The primary species derived from the donor molecules are two types of radical cations with very different spin and charge distribution. One of the donor radical cations is dissociative, i.e. it dissociates within some femtoseconds, before relaxing to a stable species. The other one is metastable and overcomes to the nanosecond time range. This is the typical behavior needed for (macroscopic) identification of FET ... 

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