What can be learned from the singlet-triplet splitting of the radical pair P+H- [p = primary donor] in the photosynthetic reaction center: conclusions from electric field effects on the P+H- recombination dynamics

Völk, Martin; Haeberle, T.; Feick, R.; Ogrodnik, A.; Michel‐Beyerle, M. E.

doi:10.1021/j100140a047

Cited by 43 publications

(37 citation statements)

References 10 publications

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“…In part, chemically induced dynamic electron polarization depends on the connection between the singlet-triplet (S-T 0 ) electronic energy splitting (exchange coupling: 2 J ϭ E S Ϫ E T in CS state) and V CR . The parameter 2 J is governed by the spin-selective interaction (41) of V CR in accordance with Marcus theory (32,40,(42)(43)(44)(45)(46). Fig.…”

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confidence: 68%

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Primary charge-recombination in an artificial photosynthetic reaction center

Kobori

Yamauchi

Akiyama

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

101

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Photoinduced primary charge-separation and charge-recombination are characterized by a combination of time-resolved optical and EPR measurements of a fullerene-porphyrin-linked triad that undergoes fast, stepwise charge-separation processes. The electronic coupling for the energy-wasting charge recombination is evaluated from the singlet-triplet electronic energy gap in the short-lived, primary charge-separated state. The electronic coupling is found to be smaller by Ϸ40% than that for the primary charge-separation. This inhibition of the electronic interaction for the charge-recombination to excited triplet state largely results from a symmetry-broken electronic structure modulated by configuration interaction between 3 (b1u,b3g) and 3 (au, b3g) electronic states of the free-base porphyrin.electronic coupling ͉ long-range electron transfer ͉ molecular orbital symmetry T o construct nanoscale devices or molecular wires that efficiently convert photon energies to chemical potentials, extensive studies have produced photoinduced, long-distance charge separation (CS) states by using donor (D)-acceptor (A)-linked multiarray systems that mimic natural photosynthesis (1-11). In each intramolecular, sequential electron transfer (ET) step, an electron or a hole tunnels between D and A over substantial distances (Ͼ10 Å) as in the natural photosynthetic reaction centers (12). From Marcus theory (13)(14)(15)(16)(17)(18)(19)(20), the ET rate constant (k ET ) is described (in the weak coupling regime) aswhere -h is Planck's constant divided by 2 and V, the electronic coupling matrix element, represents the electronic tunneling interaction between D and A. FCWDS denotes Franck-Condon weighted density of states parameterized by the reorganization energy ( ) and the driving force (Ϫ⌬G) for the ET reaction. The FCWDS term is given by (4where k B is the Boltzmann constant and T is the temperature. Although for several long-range ET systems, V has been interpreted in terms of a superexchange coupling model (21) that bridges the redox sites (22-28), much remains to be resolved regarding the tunneling mechanism (29-34). In addition, characterizations of V will be useful for designing molecular electronic devices (35). Key to efficient photoinduced CS is prevention of the energywasting charge recombination (CR) characterized in part by V. Although V and have been investigated for various ET steps by experimentally measuring k ET s in systems such as D 2 -D 1 -A triad (7, 9, 36) and protein complexes (24,25,34), primary CR,-A has escaped elucidation, especially in the efficient stepwise CS systems because the CR kinetics is hidden by subsequent CS processes,In the photosynthetic reaction center protein of Rhodobacter sphaeroides, which contains as cofactors a special pair (P), two bacteriopheopheophytins (H A and H B ), and two quinones (Q A and Q B ), the CR kinetics of the primary CS state of P ؉• H A Ϫ• Q A cannot easily be observed, because H A Ϫ• 3 Q A forward ET (Ϸ200 ps) is much faster than the primary CR (10-20 ns, which has b...

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confidence: 68%

“…2). These energy shifts, determined by the V CR s, result in the E S -E T difference (2 J 1 ) for the primary CS state (42,43). The 2 J parameters have been investigated for the P ؉• H A Ϫ• state in the Q A -depleted natural photosynthetic reaction centers through measurements of the magnetic field dependence of the reaction yield (47,48).…”

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confidence: 99%

Primary charge-recombination in an artificial photosynthetic reaction center

Kobori

Yamauchi

Akiyama

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

101

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“…No resonance was observed for compound 1, which most likely results from the magnitude of 2 J being larger than the magnetic field that our apparatus can apply (1.2 T). Measurements of 2 J have been shown to provide a measure of the electronic coupling for charge recombination by superexchange (11)(12)(13) and have been used as a probe of electron transfer mechanism in a number of studies (2,8,9,16,18,50,51). Most importantly, the energetic splitting between the singlet and triplet states within the radical ion pair arises from an indirect exchange between the two spins made possible by their interaction with the bridge.…”

Section: Resultsmentioning

confidence: 99%

“…The indirect electronic coupling between the electron donor and acceptor via the bridge orbitals, V DA , that is a result of superexchange (10) is the same interaction responsible for long-distance communication between the spins within a radical pair (RP), provided that the initial states are paramagnetic (11)(12)(13)(14)(15), which is true for the charge recombination process in the systems studied here. The magnetic field effect (MFE) on the yield of spin-selective RP recombination products directly reveals the magnitude of magnetic coupling between the spins of the RP and is proportional to V DA 2 (2,(16)(17)(18). The mechanistic details of the radical pair intersystem crossing mechanism (RP-ISC) and the theory behind the MFE have been researched extensively (19) and applied to many donor-acceptor systems (2,8,9,(20)(21)(22)(23)(24).…”

Section: -4 Are Only Weakly Distance Dependent After the Initial Phmentioning

confidence: 99%

Wire-like charge transport at near constant bridge energy through fluorene oligomers

Goldsmith

Sinks

Kelley

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

168

187

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The study of photoinitiated electron transfer in donor-bridgeacceptor molecules has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. We report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene (FLn) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FLn bridge. In toluene, the rate constants for both charge separation and recombination as well as the energy levels of the relevant FL n ؉• bridge states for n ‫؍‬ electron transfer ͉ hopping ͉ superexchange E fficient long-distance electron transfer is a prerequisite for molecular materials designed to serve as active components in solar cells and nanoscale devices. Being able to consistently achieve ''wire-like'' charge transport in synthetic systems requires a thorough understanding of the mechanisms involved. Recently progress has been made toward this goal (1, 2), but the complexity of even simple molecular systems poses a formidable challenge. In particular, the distance dependence of electron transfer has been shown to be a complex function of individual parameters, including molecular geometry and energetics (3,4). While the use of rigidly bound electron donor (D), bridge (B), electron acceptor (A) compounds has simplified the investigation of the distance dependence of electron transfer by keeping the donor-acceptor distance (r DA ) well defined, it is difficult to vary r DA by changing the length of the bridge without substantially altering the energy levels of the bridge.In this report we examine electron transfer in a system in which r DA can be changed considerably, whereas the oxidation potential of the intervening bridge changes very little. Approximate matching between the donor and bridge energy levels (2, 5, 6) has been shown to be critical for promoting incoherent, or ''wire-like,'' transport over coherent, or exponentially distance-dependent, superexchange transport (7). These earlier studies have shown how the energetic convergence of the relevant donor and bridge energy levels results in a minimal injection barrier for hole transfer to the bridge, leading to a striking change in mechanism as the bridge is lengthened.The relative importance of incoherent transport at larger values of r DA has been confirmed by independent measurements of the decreasing contribution of superexchange to the overall electron transfer rate with increasing length (8, 9). The indirect electronic coupling between the electron donor and acceptor via the br...

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“…On the radical-ion pairs (RIP), the previous transient EPR investigations [11][12][13] have clarified that J is govemed by the charge-transfer interaction (Jcr) [11][12][13][14][15], which is in accordance with the Marcus electron-transfer (ET) theory [16,17]; J is created by the electronic coupling matrix element (V) permrbation from the charge-recombined electron donor-acceptor (D-A) configurations. The energy difference between the reactant and product equilibrium configuration is defined as the reorganization energy (2.)…”

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confidence: 76%

Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

2003

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The radical pair mechanism (RPM) of chemically induced dynamic electron polarization (CIDEP) is theoretically analyzed to determine what intermolecular separations (reŸ effectively cont¡ to the CIDEP generated from diflhsive, separated radical-ion pairs (RIP) in terms of the chargetransfer interaction in the singlet-triplet energy splitting (J) by taking into aceount the distance-dependent electronic coupling and reorganization energy. The diffusion-model analysis reveals that the hyperŸ237 RPM polarization (PwM) phase is varied with the driving force (-AGc~) for the charge-recombination (CR) process and that the boundary -AGcR between the opposite phases coincides well with the total reorganization energy around the diffusible separation distante, rct r = 1.2 nm, between the ion radicals. For the first time, the roe r is well described by the exponent parameter (13) in the distance-dependent electronic coupling, suggesting that the RPM CIDEP detection can be applied to characterize the electronic coupling in individual solvent-separated RIP systems. It has been concluded that, in contrast to the spin exchange interaction of the neutral radical pairs, the characteristic long-range charge-transfer interaction enables us to utilize the simple diffusion-model analysis to successfully evaluate the rc~ and the P~M in homogeneous liquid polar solvents.

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What can be learned from the singlet-triplet splitting of the radical pair P+H- [p = primary donor] in the photosynthetic reaction center: conclusions from electric field effects on the P+H- recombination dynamics

Cited by 43 publications

References 10 publications

Primary charge-recombination in an artificial photosynthetic reaction center

Primary charge-recombination in an artificial photosynthetic reaction center

Wire-like charge transport at near constant bridge energy through fluorene oligomers

Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

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