Charge-separated Radical Pair Research Articles

Femtosecond optical studies of the primary charge separation reactions in far-red photosystem II from Synechococcus sp. PCC 7335

Primary processes of light energy conversion by Photosystem II (PSII) were studied using femtosecond broadband pump-probe absorption difference spectroscopy. Transient absorption changes of core complexes isolated from the cyanobacterium Synechococcus sp. PCC 7335 grown under far-red light (FRL-PSII) were compared with the canonical Chl a containing spinach PSII core complexes upon excitation into the red edge of the Qy band. Absorption changes of FRL-PSII were monitored at 278 K in the 400–800 nm spectral range on a timescale of 0.1–500 ps upon selective excitation at 740 nm of four chlorophyll (Chl) f molecules in the light harvesting antenna, or of one Chl d molecule at the ChlD1 position in the reaction center (RC) upon pumping at 710 nm. Numerical analysis of absorption changes and assessment of the energy levels of the presumed ion-radical states made it possible to identify PD1+ChlD1− as the predominant primary charge-separated radical pair, the formation of which upon selective excitation of Chl d has an apparent time of ∼1.6 ps. Electron transfer to the secondary acceptor pheophytin PheoD1 has an apparent time of ∼7 ps with a variety of excitation wavelengths. The energy redistribution between Chl a and Chl f in the antenna occurs within 1 ps, whereas the energy migration from Chl f to the RC occurs mostly with lifetimes of 60 and 400 ps. Potentiometric analysis suggests that in canonical PSII, PD1+ChlD1− can be partially formed from the excited (PD1ChlD1)* state.

Triplet states in the reaction center of Photosystem II.

In oxygenic photosynthesis sunlight is harvested and funneled as excitation energy into the reaction center (RC) of Photosystem II (PSII), the site of primary charge separation that initiates the photosynthetic electron transfer chain. The chlorophyll ChlD1 pigment of the RC is the primary electron donor, forming a charge-separated radical pair with the vicinal pheophytin PheoD1 (ChlD1+PheoD1-). To avert charge recombination, the electron is further transferred to plastoquinone QA, whereas the hole relaxes to a central pair of chlorophylls (PD1PD2), subsequently driving water oxidation. Spin-triplet states can form within the RC when forward electron transfer is inhibited or back reactions are favored. This can lead to formation of singlet dioxygen, with potential deleterious effects. Here we investigate the nature and properties of triplet states within the PSII RC using a multiscale quantum-mechanics/molecular-mechanics (QM/MM) approach. The low-energy spectrum of excited singlet and triplet states, of both local and charge-transfer nature, is compared using range-separated time-dependent density functional theory (TD-DFT). We further compute electron paramagnetic resonance properties (zero-field splitting parameters and hyperfine coupling constants) of relaxed triplet states and compare them with available experimental data. Moreover, the electrostatic modulation of excited state energetics and redox properties of RC pigments by the semiquinone QA- is described. The results provide a detailed electronic-level understanding of triplet states within the PSII RC and form a refined basis for discussing primary and secondary electron transfer, charge recombination pathways, and possible photoprotection mechanisms in PSII.

Readout of spin quantum beats in a charge-separated radical pair by pump-push spectroscopy.

Spin quantum beats prove the quantum nature of reactions involving radical pairs, the key species of spin chemistry. However, such quantum beats remain hidden to transient absorption–based optical observation because the spin hardly affects the absorption properties of the radical pairs. We succeed in demonstrating such quantum beats in the photoinduced charge-separated state (CSS) of an electron donor–acceptor dyad by using two laser pulses—one for pumping the sample and another one, with variable delay, for further exciting the CSS to a higher electronic state, wherein ultrafast recombination to distinct, optically detectable products of singlet or triplet multiplicity occurs. This represents a spin quantum measurement of the spin state of the CSS at the time instant of the second (push) pulse.

Indications of 5' to 3' Interbase Electron Transfer as the First Step of Pyrimidine Dimer Formation Probed by a Dinucleotide Analog.

Pyrimidine dimers are the most common DNA lesions generated under UV radiation. To reveal the molecular mechanisms behind their formation, it is of significance to reveal the roles of each pyrimidine residue. We thus replaced the 5'-pyrimidine residue with a photochemically inert xylene moiety (X). The electron-rich X can be readily oxidized but not reduced, defining the direction of interbase electron transfer (ET). Irradiation of the XpT dinucleotide under 254 nm UV light generates two major photoproducts: a pyrimidine (6-4) pyrimidone analog (6-4PP) and an analog of the so-called spore photoproduct (SP). Both products are formed by reaction at C4=O of the photo-excited 3'-thymidine (T), which indicates that excitation of a single "driver" residue is sufficient to trigger pyrimidine dimerization. Our quantum-chemical calculations demonstrated that photo-excited 3'-T accepts an electron from 5'-X. The resulting charge-separated radical pair lowers its energy upon formation of interbase covalent bonds, eventually yielding 6-4PP and SP.

The Protein Environment of the Bacteriopheophytin Anion Modulates Charge Separation and Charge Recombination in Bacterial Reaction Centers

The kinetics and pathway of electron transfer has been explored in a series of reaction center mutants from Rhodobacter sphaeroides, in which the leucine residue at M214 near the bacteriopheophytin cofactor in the A-branch has been replaced with methionine, cysteine, alanine, and glycine. These amino acids have substantially different volumes, both from each other and, except for methionine, from the native leucine. Though the mutation site of M214 is close to the bacteriopheophytin cofactor, which is involved in the electron transfer, none of the mutations alter the cofactor composition of the reaction center and the primary charge separation reaction is essentially undisturbed. However, the kinetics of electron transfer from HA(-) → QA becomes both slower and substantially heterogeneous in three of the four mutants. The decreased HA(-) → QA electron transfer rate allows charge recombination between P(+) and HA(-) to compete with the forward reaction, resulting in a drop in the overall yield of charge separation. Both the yield change and the variation in kinetics correlate well with the volume of the mutant amino acid side chains. Analysis of the kinetics suggests that the introduction of a smaller side chain at M214 results in greater protein structural heterogeneity and dynamics on multiple time scales, resulting in perturbation of the electronic environment and its evolution in the vicinity of the early charge-separated radical pair, P(+)HA(-), and the subsequent acceptor QA, affecting both the extent and time scale of dielectric relaxation. It appears that the reaction center has been optimized not only in terms of its static structure-function relationships, but also finely tuned to favor particular reaction pathways on particular time scales by adjusting protein dynamics.

ChemInform Abstract: Fullerenes: Three Dimensional Electron Acceptor Materials

This feature article highlights the advantages of employing [60]fullerene as a viable electron accepting building block in novel donor acceptor systems. Different strategies that aim towards improving charge separation in fullerene containing systems are presented. This is accomplished, for example, via utilisation of additional stabilisation forces of the radical pair or, alternatively, diffusional splitting of the last mentioned. Fine-tuning the topology of the electron donor moiety has been shown to be a powerful means of influencing the relative energies of the two states involved (e.g. the charge-separated state vs. the singlet ground state). Remarkable effects concerning the lifetime of the charge-separated radical pair were observed, in particular, in systems that upon charge separation led to a gain of aromaticity and planarity of the oxidised fragment.

2]Catenanes decorated with porphyrin and [60]fullerene groups: design, convergent synthesis, and photoinduced processes.

A new class of [2]catenanes containing zinc(II)-porphyrin (ZnP) and/or [60]fullerene (C(60)) as appended groups has been prepared. A complete description of the convergent synthetic approach based on Cu(I) template methodology and "click" 1,3-dipolar cycloaddition chemistry is described. This new electron donor-acceptor catenane family has been subjected to extensive spectroscopic, computational, electrochemical and photophysical studies. (1)H NMR spectroscopy and computational analysis have revealed that the ZnP-C(60)-[2]catenane adopts an extended conformation with the chromophores as far as possible from each other. A detailed photophysical investigation has revealed that upon irradiation the ZnP singlet excited state initially transfers energy to the (phenanthroline)(2)-Cu(I) complex core, producing a metal-to-ligand charge transfer (MLCT) excited state, which in turn transfers an electron to the C(60) group, generating the ZnP-[Cu(phen)(2)](2+)-C(60)(*-) charge-separated state. A further charge shift from the [Cu(phen)(2)](2+) complex to the ZnP subunit, competitive with decay to the ground state, leads to the isoenergetic long distance ZnP(*+)-[Cu(phen)(2)](+)-C(60)(*-) charge-separated radical pair state, which slowly decays back to the ground state on the microsecond time scale. The slow rate of back-electron transfer indicates that in this interlocked system, as in previously studied covalently linked ZnP-C(60) hybrid materials, this process occurs in the Marcus-inverted region.

Azobenzene-Linked Porphyrin−Fullerene Dyads

A new group of porphyrin-fullerene dyads with an azobenzene linker was synthesized, and the photochemical and photophysical properties of these materials were investigated using steady-state and time-resolved spectroscopic methods. The electrochemical properties of these compounds were also studied in detail. The synthesis involved oxidative heterocoupling of free base tris-aryl-p-aminophenyl porphyrins with a p-aminophenylacetal, followed by deprotection to give the aldehyde, and finally Prato 1,3-dipolar azomethineylide cycloaddition to C60. The corresponding Zn(II)-porphyrin (ZnP) dyads were made by treating the free base dyads with zinc acetate. The final dyads were characterized by their 1H NMR, mass, and UV-vis spectra. 3He NMR was used to determine if the products are a mixture of cis and trans stereoisomers, or a single isomer. The data are most consistent with the isolation of only a single configurational isomer, assigned to the trans (E) configuration. The ground-state UV-vis spectra are virtually a superimposition of the spectral features of the individual components, indicating there is no interaction of the fullerene (F) and porphyrin (H2P/ZnP) moieties in the ground state. This conclusion is supported by the electrochemical data. The steady-state and time-resolved fluorescence spectra indicate that the porphyrin fluorescence in the dyads is very strongly quenched at room temperature in the three solvents studied: toluene, tetrahydrofuran (THF), and benzonitrile (BzCN). The fluorescence lifetimes of the dyads in all solvents are sharply reduced compared to those of H2P and ZnP standards. In toluene, the lifetimes of the free base dyads are 600-790 ps compared to 10.1 ns for the standard, while in THF and BzCN the dyad lifetimes are less than 100 ps. For the ZnP dyads, the fluorescence lifetimes were 10-170 ps vs 2.1-2.2 ns for the ZnP references. The mechanism of the fluorescence quenching was established using time-resolved transient absorption spectroscopy. In toluene, the quenching process is singlet-singlet energy transfer (k approximately 10(11) s-1) to give C60 singlet excited states which decay with a lifetime of 1.2 ns to give very long-lived C60 triplet states. In THF and BzCN, quenching of porphyrin singlet states occurs at a similar rate, but now by electron transfer, to give charge-separated radical pair (CSRP) states, which show transient absorption spectra very similar to those reported for other H2P-C60 and ZnP-C60 dyad systems. The lifetimes of the CSRP states are in the range 145-435 ns in THF, much shorter than for related systems with amide, alkyne, silyl, and hydrogen-bonded linkers. Thus, both forward and back electron transfer is facilitated by the azobenzene linker. Nonetheless, the charge recombination is 3-4 orders of magnitude slower than charge separation, demonstrating that for these types of donor-acceptor systems back electron transfer is occurring in the Marcus inverted region.

Porphyrin–fullerene photosynthetic model systems with rotaxane and catenane architectures

Abstract Routes for the synthesis of a series of [2]rotaxanes and [2]catenanes incorporating porphyrin, ferrocene and fullerene moieties by the Sauvage metal template technique are described. Photophysical studies on these materials show that excitation of the porphyrin component causes a series of events along an energy gradient leading to long-lived long distance charge-separated radical pair states with lifetimes as long as 32 μs in tetrahydrofuran solution at ambient temperatures. To cite this article: D.I. Schuster et al., C. R. Chimie 9 (2006) .

Synthesis and photophysics of porphyrin–fullerene donor–acceptor dyads with conformationally flexible linkers

The synthesis and photophysics of a series of porphyrin–fullerene (P–C 60) dyads in which the two chromophores are linked by conformationally flexible polyether chains is reported. Molecular modeling indicates the two moieties adopt a stacked conformation in which the two chromophores are in close proximity. Photoexcitation of the free base dyads in polar solvents such as tetrahydrofuran and benzonitrile, causes electron transfer (ET) to generate charge-separated radical pair (CSRP) states, which were directly detected using transient absorption (TA) techniques. In nonpolar solvents such as toluene, where CSRP states were not directly detected, fullerene triplet state states were formed, according to TA studies as well as singlet oxygen sensitization measurements. The low value of the quantum efficiency for sensitized formation of singlet molecular oxygen [O 2( 1Δ g)] in toluene and chloroform indicates that singlet energy transduction to give H 2P– 1C 60*, followed by intersystem crossing to H 2P– 3C 60* and energy transfer to 3O 2, is not the operative mechanism. Rather, a mechanism is proposed involving ET to give CSRP states followed by exergonic charge recombination to eventually generate fullerene triplets. Such a mechanism has been demonstrated experimentally for structurally related P–C 60 dyads. For the corresponding ZnP–C 60 dyads with flexible linkers, only photoinduced ET to generate long-lived CSRP states is observed. Photoinduced charge separation in these dyad systems is extremely rapid, consistent with a through space rather than through-bond mechanism. Charge recombination is up to three orders of magnitude slower, indicating this process occurs in the inverted region of the Marcus curve that relates ET rates to the thermodynamic driving force. These observations once again demonstrate the advantages of incorporating fullerenes as electron acceptor components in photosynthetic model systems.

Zwitterionic Acceptor Moieties: Small Reorganization Energy and Unique Stabilization of Charge Transfer Products

A series of fulleropyrrolidine- and fulleropyrrolidinium-based donor−acceptor ensembles were tested in light of intrinsic reorganization energies for photoinduced electron transfer events. Among these complexes, the fulleropyrrolidinium ions are particularly well-suited for the role of an acceptor moiety. In fact, once reduced, fulleropyrrolidinium ions exhibit a zwitterionic character, due to the contemporary presence of a positive pyrrolidinium ion together with the fullerene radical ion. We demonstrate that this zwitterionic character of the acceptor plays a central role in improving acceleration of charge separation and deceleration of charge recombination. While the charge separation occurs with dynamics close to the top region of the Marcus parabola, the charge recombination rates are pushed deeply into the inverted region of the parabolic dependence. Overall, we determined reorganization energies (λ) for the fulleropyrrolidinium-based ensemble that are appreciably smaller than those seen for the fulleropyrrolidine-based analogue, while keeping the electronic coupling element constant (!). Remarkable is the oxygen effect on the zwitterionic fulleropyrrolidinium acceptornot seen on the fulleropyrrolidine acceptorin the charge-separated radical pair.

Rigidified tetrathiafulvalene–[60]fullerene assemblies: towards the control of through-space orientation between both electroactive units

Our recent works on fused TTF–C60 dyads, (TTF)n–C60 polyads and C60–TTF–C60 dumbbell triads in which the acceptor C60 is doubly tethered to the donor tetrathiafulvalene through a rigidified cyclohexene ring are presented. This approach was developed in order to control the relative orientation as well as the distance between both donor and acceptor entities. Thereby, through-space interactions which are of great importance for photoinduced electron- and/or energy-transfer processes are expected to dominate because of the special topology of the molecules. The two linked C60 and TTF chromophores in such adducts are not only in close proximity but also have optimal orbital orientations, thus facilitating these through-space electronic interactions. These new C60-based assemblies were synthesized by [4 + 2] Diels–Alder cycloaddition reactions. The different methodologies considered for their synthesis are discussed, their analytical, spectroscopic characterizations and electrochemical properties are also described. The selective electro-oxidation or reduction afforded the corresponding radical cation and radical anion which were characterized by EPR. These C60-based assemblies were studied for their nonlinear optical and optical limiting applications. Moreover, intramolecular photoinduced charge-separation and charge-recombination processes in a fused C60–TTF–C60 dumbbell triad which was designed to be soluble in organic solvents were investigated by time-resolved absorption and fluorescence techniques. Appreciable interaction between the C60 moiety and TTF moiety in the ground state was suggested by steady-state absorption spectra and the fluorescence spectra showed considerable interaction in the singlet excited state. The nanosecond transient absorption spectra displayed the formation of the charge-separated radical pair C60–TTF˙+–C60˙−, characterized by a lifetime of ca. 20 ns in benzonitrile.

Synthesis, cyclic voltammetry, and photophysical properties of a bridged o-phenylenediamine-C(60) dyad.

The synthesis, complete spectroscopic characterization, cyclic voltammetry, and photophysical measurements of a new o-phenylenediamine-C(60) dyad are described. By using a tether strategy, only a single regioisomer was obtained. Cyclic voltammetry measurements indicate that the two electroactive groups do not interact in their singlet ground states. Photophysical investigations reveal a rapid photoinduced electron transfer between the singlet excited state of the fullerene acceptor and the o-phenylenediamine donor, yielding a charge-separated radical pair.

Evidence for Two Separate One-Electron Transfer Events in Excited Fulleropyrrolidine Dyads Containing Tetrathiafulvalene (TTF)

1,3-Dipolar cycloadditions of TTF-azomethine ylides (TTF = tetrathiafulvalene) to C60 have been used to synthesize a series of novel donor−bridge−acceptor dyads. In these dyads the pyrrolidine[3‘,4‘:1,2][60]fullerene is covalently attached to the electron donor TTF either directly (5) or alternatively through one (2a) or two (7) vinyl groups. In the ground state, dyads 2a, 5, and 7 undergo four quasireversible one-electron reductions and two reversible oxidation steps. The former are associated with the reduction of the C60 core, whereas the latter correspond to the formation of the radical cation and dication of the TTF moiety, respectively. Semiempirical PM3 calculations reveal donor−acceptor distances of 4.8 Å (5), 7.6 Å (2a), and 10.5 Å (7), and a deviation from planarity between the TTF fragment and the vinylogous spacer. In relation to an N-methylfulleropyrrolidine, the emission of the fullerene singlet excited state in dyads 2a, 5, and 7 is substantially reduced. Furthermore, the fluorescence quantum yield correlates well with the solvent dielectric constant and also with the spatial separation of the donor and acceptor moieties in the dyads. These correlation suggest that intramolecular electron-transfer processes evolving from the fullerene singlet excited state generate the (C60•-)−(TTF•+) pair. Pico- and nanosecond-resolved transient spectroscopy further substantiate a rapid transformation of the initially formed singlet excited state into the charge-separated radical pair with intramolecular rates ranging between 1.17 × 1010 s-1 and 1.47 × 109 s-1. In all cases, the product of back electron transfer is the triplet excited state, which is generated with markedly high quantum yields (0.61−0.97). The latter is, in addition to the rapid primary intramolecular electron transfer, subject to a slower, secondary intermolecular electron transfer with rate constants of 7 × 108 M-1s-1 (5) in benzonitrile and 1.6 × 109 M-1 s-1 (5) in toluene.

Fullerenes: three dimensional electron acceptor materials

This feature article highlights the advantages of employing [60]fullerene as a viable electron accepting building block in novel donor acceptor systems. Different strategies that aim towards improving charge separation in fullerene containing systems are presented. This is accomplished, for example, via utilisation of additional stabilisation forces of the radical pair or, alternatively, diffusional splitting of the last mentioned. Fine-tuning the topology of the electron donor moiety has been shown to be a powerful means of influencing the relative energies of the two states involved (e.g. the charge-separated state vs. the singlet ground state). Remarkable effects concerning the lifetime of the charge-separated radical pair were observed, in particular, in systems that upon charge separation led to a gain of aromaticity and planarity of the oxidised fragment.

Time-resolved EPR studies of photoinduced electron transfer reactions in photosynthetic model porphyrin quinone triads

Time-resolved electron paramagnetic resonance (EPR) investigations, performed at microwave frequencies of 9.5 GHz (X band) and 95 GHz (W band), on porphyrin quinone quinone and porphyrin porphyrin quinone triads are presented. Time-resolved EPR at different microwave frequencies makes it possible to elucidate the electron transfer (ET) reactions after pulsed laser excitation and to determine their intermediates, the charge-separated radical pair (RP) states. After photoexcitation in the nematic and soft glass phase of a liquid crystal (LC), spin-polarized EPR spectra are observed for the RPs and for the triplet states of the porphyrins created by spin-selective intersystem crossing (ISC) from the excited porphyrin singlet state. The EPR polarization patterns of the RPs are discussed in terms of the favoured decay channels of the photoexcited singlet state of the porphyrin donor. The decay pathway can be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character, or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene groups, significantly influences the polarization patterns of the RP spectra. Saturation (inversion) recovery measurements reveal that the polarization pattern of the radical pairs is governed mainly by a combination of spin relaxation and charge recombination of the radical pairs. In addition, information about the orientation of the guest molecules in the LC matrix with respect to the long axes of the LC molecules can be obtained. The EPR spectrum of the excited triplet state of free-base porphyrin quinone systems, in which no ET occurs, is used as an additional probe for investigating the orientation distributions. In the porphyrin porphyrin quinone triads the energy and ET processes depend strongly on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal, or both porphyrins bear a zinc atom in their macrocycles.

Time-resolved EPR studies of photoinduced electron transfer reactions in photosynthetic model porphyrin quinone triads

Time-resolved electron paramagnetic resonance (EPR) investigations, performed at microwave frequencies of 9.5 GHz (X band) and 95 GHz (W band), on porphyrin quinone quinone and porphyrin porphyrin quinone triads are presented. Time-resolved EPR at different microwave frequencies makes it possible to elucidate the electron transfer (ET) reactions after pulsed laser excitation and to determine their intermediates, the charge-separated radical pair (RP) states. After photoexcitation in the nematic and soft glass phase of a liquid crystal (LC), spin-polarized EPR spectra are observed for the RPs and for the triplet states of the porphyrins created by spin-selective intersystem crossing (ISC) from the excited porphyrin singlet state. The EPR polarization patterns of the RPs are discussed in terms of the favoured decay channels of the photoexcited singlet state of the porphyrin donor. The decay pathway can be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character, or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene groups, significantly influences the polarization patterns of the RP spectra. Saturation (inversion) recovery measurements reveal that the polarization pattern of the radical pairs is governed mainly by a combination of spin relaxation and charge recombination of the radical pairs. In addition, information about the orientation of the guest molecules in the LC matrix with respect to the long axes of the LC molecules can be obtained. The EPR spectrum of the excited triplet state of free-base porphyrin quinone systems, in which no ET occurs, is used as an additional probe for investigating the orientation distributions. In the porphyrin porphyrin quinone triads the energy and ET processes depend strongly on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal, or both porphyrins bear a zinc atom in their macrocycles.

EPR studies of photoinduced electron transfer in triad model compounds of photosynthesis

Time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads obtained after photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin created by spin-selective intersystem crossing (ISC) from the excited singlet state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RP are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanism and thus the polarization patterns of the RP spectra. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. In the porphyrin-porphyrin-quinone triads the energy and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal or both porphyrins bear a zinc atom.

High-field/high-frequency EPR/ENDOR — A powerful new tool in photosynthesis research

In photosynthesis research the elucidation of the spatial and electronic structures of the electron donor and acceptor ion radicals is very important for an understanding of the light-induced electron transfer process. Recent 3 mm (W-band, 95 GHz) high-field EPR and ENDOR studies on the primary donor cation radicals P+. (bacteriochlorophyll dimer), the acceptor anion radicals Q+. (quinones), and the charge-separated radical pair (P+.Q−.) in photosynthetic bacteria and biomimetic model systems are presented. From single crystals of, for instance,Rb. sphaeroides R-26 reaction centers both the hyperfine tensors of various protons and theg-tensor of P865 +. have been determined and compared with calculated tensor values based on recent X-ray structure data. The results consistently reveal a breaking of the local C2 symmetry of the electronic structure at the primary donor side of the reaction center. This is of particular interest since it might be relevant for the vectorial electron transfer along the protein complex. Among the quinone radical anions studied are frozen solutions of the electron acceptors of bacterial and plant reaction centers (ubiquinone and plastoquinone, respectively). The increased electron Zeeman interaction in high-field EPR leads to almost completely resolvedg-tensor components even in disordered samples. Theg-tensors and component linewidths are sensitive probes for specific anisotropic interactions with the environment. In the case of the transient correlated coupled radical pair P865 +.-QA −. ofRb. sphaeroides (Fe replaced by Zn) the spin-polarized high-field EPR spectra allow an unambiguous determination of the relative orientation of theg-tensors of the donor and acceptor parts. Thereby high-precision structure information is obtained on the electron transfer pigments after light-induced charge separation.

On the viability of the superexchange mechanism in the primary charge separation step of bacterial photosynthesis

We investigate the feasibility of participation of the intermediate bacteriochlorophyll of the reaction center via a superexchange mechanism in the primary charge separation process of bacterial photosynthesis. When an internal charge transfer state of the special pair is incorporated into the theoretical model, an adequate rate of electron transfer can be attained without creating a discrepancy in the calculated singlet-triplet splitting of the charge-separated radical pair state, in contrast to results obtained by R.A. Marcus (Chem. Phys. Lett. 133 (1988) 471–477) without the internal charge transfer state.