Primary donor triplet states

The primary donor triplet state 3P The triplet state of the primary donor is formed by recombination of the primary radical pair P,+4>X of prereduced bRCs in which the ET to the quinones is blocked ... 

The Primary Donor Triplet State iP7a0 If in the charge separation process electron-transfer in PS I beyond the first acceptor A0 is blocked by treatment with sodium dithionite at high pH and illumination, which reduces the iron-sulfur centres (F) and the quinone (A, the triplet state of the donor, 3P7ao, is obtained via radical-pair recombination from the triplet RP according to ... 

The main result of the early EPR and ODMR work was a reduction of the ZFS parameters D( and when comparing monomeric 3BChl and the 3P state of the primary donor in bRCs (for a collection of data see reference 22). This has been interpreted as resulting from a delocalization of the triplet exciton in the BChl-dimer. However, due to the complexity of the electronic system, that could also involve charge transfer states, a final quantitative conclusion has been difficult. From a determination of the triplet axes in bRC single crystals104 107 it was concluded that in R. sphaeroides the triplet is indeed delocalized whereas in B. viridis it seems to be located on one monomeric BChl b half. 

Further work using time-resolved EPR and magnetophotoselection (MPS), using plane-polarized light to excite the triplet state, gave information on the orientation of the optical transition dipole axes relative to the principal axes of the triplet state. By this technique the transition moments of the primary donor"6, the carotenoid in the bRC"7 and the bacteriopheophytin in the inactive B branch 4>0"8 were determined. 

The obtained data clearly show that the g-anisotropy of the triplet states is larger than that of the respective cation-radical. A similar effect has been observed for the triplet states of the primary donors in PS II231 and in the bacterial RC.111112114 This can be explained by the fact that the triplet electrons probe the spin distribution in two different orbitals (HOMO and LUMO), and the latter has a rather large spin density at the nitrogens and the central magnesium (cf. references 216, 218), by which the spin-orbit coupling and the g-anisotropy is increased. 

The rate of another important process, the recombination of the primary product of the charge separation, i.e. the reduced primary acceptor (QA ) and oxidised primary donor, bacteriochlorophyl dimer (P+), falls from 103 to 102 s 1 when dynamic processes with uc = 103 s 1 monitored by the triplet labelling method occur. Very fast electron transfer from P+ to bacteriochlorophyl (Bchl) and from (Bchl) to QA does not depend on media dynamics and occurs via conformationally non-equilibrium states (Fig. 3.16). 

In purple bacteria a number of different lines of evidence led to the conclusion that bacteriopheophytin (BPh) acts as an electron carrier between the primary donor and Qa (Chapter 3). When is reduced illumination results in the photoaccumulation of reduced bacteriopheophytin, detected by its characteristic absorption changes and by an EPR signal split due to its interaction with Q Fe. At temperatures too low for rapid photoaccumulation of BPh to take place, illumination results in formation of a triplet state of the primary donor P-870 which has a polarization pattern characteristic of its formation by recombination of a radical pair. When BPh is reduced this triplet state cannot be formed. The most direct proof that BPh acts as a primary acceptor comes from the direct observation by absorption spectroscopy of BPh reduction within a few picoseconds after the flash. The BPh is reoxidized in 200 ps by electron transfer to or, if is already reduced, by recombination in 14 ns (see Chapter 3). 

The orientation of the transition moment of the long-wavelength absorbance of the primary donor with respect to the dipolar axes of the triplet state can be found from magnetophotoselection EPR or from EPR on oriented or crystalline material [80-85], From EPR on single crystals of Rps. viridis it was found that the triplet X- and y-axes are very close to the pyrrole N-N axes of one of the BChls b making up the primary donor (Refs. 78,79 and J.R. Norris, personal communication). 

Figure 1. JabJonski-type diagram of the lowest energy levels of electron donor-acceptor molecules formally linked by a single bond which show dual fluorescence phenomenon. D-A, (D A), (D+-A ), (D -A ) and (D-A) denote the ground state, the primary excited and charge-transfer (CT) singlet states, and CT and locally excited triplet states, respectively. The arrows correspond to the radiative (absorption, A, fluorescence, F, and phosphorescence, Ph) and the radiationless (internal conversion, IC, and intersystem crossing, ISC) processes.

Primary step I The relative efficiencies of biacetyl and butene-2 in the quenching of reaction I were found to be not far from unity in the case of the ketones studied consequently, primary process I occurred from the high vibrational levels of the triplet state, possessing sufficient energy to induce efficiently the isomerization of butene-2. The efficiency of energy transfer to cw-butene-2 compared with that to biacetyl, measured in terms of reaction I, was found to be in the following order 2-pentanone > 2-butanone > acetone. It is possible that this order reflects the facility with which the vibrational energy of the donors may be made available to... 

Photochemical generation of the radical cations derived from A -vinylcar-bazole/acceptor charge-transfer complexes and subsequent polymerization is well known. Perhaps somewhat more interesting are the cationic photopolymerizations of styrene and a-methylstyrene. With these monomers of relatively weak electron donor character photolysis of the charge-transfer complexes formed with tetracyanobenzene and pyromellitic dianhydride produces monomer radical cation species from both singlet and triplet states, and the photophysics of the primary processes have been elucidated in some detail. ... 

An assessment of experimental observations concerning the influence of butyl methacrylate on the primary photochemical processes occurring in the reduction of benzophenone by triethylamine suggests that they may point to the participation of ternary exciplexes. Such species may be of the form [donor + acceptor + alkene] and may play an important role in the electron transfer processes. Studies have been reported which characterise the triplet states of m-, and p-hydroxybenzophenones. In non-hydrogen bond... 

The reaction center contains one carotenoid molecule, except in the carotenoidless strain R-26 of Rb. sphaeroides. Both the spheroidene in Rb. sphaeroides and the 1,2-dihydroneurosporene in Rp. viridis assume the 15,15 -cA configuration and are located near the Bb molecule (see Figs. 9 and 10). The protein environment around the carotenoid consists of a large number of aromatic residues, which probably impose strong steric constraints on the carotenoid, and may account for the red shift in the absorption spectrum of the carotenoid relative to that of the free carotenoid. The proximity of the carotenoid to Bb suggests that the latter could serve as a conduit for the transfer of triplet-state energy from the primary donor to the carotenoid. 

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