Secondary Electron Transfer Research Articles

Photochemical carbon–sulfur bond cleavage in some alkyl and benzyl sulfides

Irradiation (254 nm) of five alkyl and benzyl ethyl sulfides causes efficient ( Φ r 0.27–0.90) homolytic cleavage of the C–S bond. Of the resulting fragments, thiyl radicals mainly couple, while alkyl radicals abstract hydrogen, disproportionate or couple when stabilized (benzyl). Selective trapping of either of the two types of radicals occurs in the presence of nucleophilic (methyl vinyl ether and 1-hexene) and, respectively, electrophilic (acrylonitrile) alkenes. When an easily oxidized radical is formed, e.g. cumyl, secondary electron transfer leads to the corresponding cation.

Kinetics and mechanism of electron transfer in intact photosystem II and in the isolated reaction center: Pheophytin is the primary electron acceptor

The mechanism and kinetics of electron transfer in isolated D1/D2-cyt(b559) photosystem (PS) II reaction centers (RCs) and in intact PSII cores have been studied by femtosecond transient absorption and kinetic compartment modeling. For intact PSII, a component of approximately 1.5 ps reflects the dominant energy-trapping kinetics from the antenna by the RC. A 5.5-ps component reflects the apparent lifetime of primary charge separation, which is faster by a factor of 8-12 than assumed so far. The 35-ps component represents the apparent lifetime of formation of a secondary radical pair, and the approximately 200-ps component represents the electron transfer to the Q(A) acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are approximately 3 and 11 ps, respectively. It is shown (i) that pheophytin is reduced in the first step, and (ii) that the rate constants of electron transfer in the RC are identical for PSII cores and for isolated RCs. We interpret the first electron transfer step as electron donation from the primary electron donor Chl(acc D1). Thus, this mechanism, suggested earlier for isolated RCs at cryogenic temperatures, is also operative in intact PSII cores and in isolated RCs at ambient temperature. The effective rate constant of primary electron transfer from the equilibrated RC* excited state is 170-180 ns(-1), and the rate constant of secondary electron transfer is 120-130 ns(-1).

Charge Separation Kinetics in Intact Photosystem II Core Particles Is Trap-Limited. A Picosecond Fluorescence Study

The fluorescence kinetics in intact photosystem II core particles from the cyanobacterium Thermosynechococcus elongatus have been measured with picosecond resolution at room temperature in open reaction centers. At least two new lifetime components of approximately 2 and 9 ps have been resolved in the kinetics by global analysis in addition to several known longer-lived components (from 42 ps to approximately 2 ns). Kinetic compartment modeling yields a kinetic description in full agreement with the one found recently by femtosecond transient absorption spectroscopy [Holzwarth et al. (2005) submitted to Proc. Natl. Acad. Sci. U.S.A.]. We have for the first time resolved directly the fluorescence spectrum and the kinetics of the equilibrated excited reaction center in intact photosystem II and have found two early radical pairs before the electron is transferred to the quinone Q(A). The apparent lifetime for primary charge separation is 7 ps, that is, by a factor of 8-12 faster than assumed on the basis of earlier analyses. The main component of excited-state decay is 42 ps. The effective primary charge separation rate constant is 170 ns(-)(1), and the secondary electron-transfer rate constant is 112 ns(-)(1). Both electron-transfer steps are reversible. Electron transfer from pheophytin to Q(A) occurs with an apparent overall lifetime of 350 ps. The energy equilibration between the CP43/CP47 antenna and the reaction center occurs with a main apparent lifetime of approximately 1.5 ps and a minor 10 ps lifetime component. Analysis of the overall trapping kinetics based on the theory of energy migration and trapping on lattices shows that the charge separation kinetics in photosystem II is extremely trap-limited and not diffusion-to-the-trap-limited as claimed in several recent papers. These findings support the validity of the assumptions made in deriving the earlier exciton radical pair equilibrium model [Schatz, G. H., Brock, H., and Holzwarth, A. R. (1988) Biophys. J. 54, 397-405].

Ultrafast Transient Absorption Studies on Photosystem I Reaction Centers from Chlamydomonas reinhardtii. 2: Mutations near the P700 Reaction Center Chlorophylls Provide New Insight into the Nature of the Primary Electron Donor

The energy transfer and charge separation kinetics in several core Photosystem I particles of Chlamydomonas reinhardtii with point mutations around the P A and P B reaction center chlorophylls (Chls) have been studied using ultrafast transient absorption spectroscopy in the femtosecond to nanosecond time range to characterize the influence on the early electron transfer processes. The data have been analyzed in terms of kinetic compartment models. The adequate description of the transient absorption kinetics requires three different radical pairs in the time range up to ∼100 ps. Also a charge recombination process from the first radical pair back to the excited state is present in all the mutants, as already shown previously for the wild-type (Müller, M. G., J. Niklas, W. Lubitz, and A. R. Holzwarth. 2003. Biophys. J. 85:3899–3922; and Holzwarth, A. R., M. G. Müller, J. Niklas, and W. Lubitz. 2005. J. Phys. Chem. B. 109:5903–59115). In all mutants, the primary charge separation occurs with the same effective rate constant within the error limits as in the wild-type (»350 ns −1), which implies an intrinsic rate constant of charge separation of <1 ps −1. The rate constant of the secondary electron transfer process is slowed down by a factor of ∼2 in the mutant B-H656C, which lacks the ligand to the central metal of Chl P B. For the mutant A-T739V, which breaks the hydrogen bond to the keto carbonyl of Chl P A, only a slight slowing down of the secondary electron transfer is observed. Finally for mutant A-W679A, which has the Trp near the P A Chl replaced, either no pronounced effect or, at best, a slight increase on the secondary electron transfer rate constants is observed. The effective charge recombination rate constant is modified in all mutants to some extent, with the strongest effect observed in mutant B-H656C. Our data strongly suggest that the Chls of the P A and P B pair, constituting what is traditionally called the “primary electron donor P700”, are not oxidized in the first electron transfer process, but rather only in the secondary electron transfer step. We thus propose a new electron transfer mechanism for Photosystem I where the accessory Chl(s) function as the primary electron donor(s) and the A 0 Chl(s) are the primary electron acceptor(s). This new mechanism also resolves in a straightforward manner the difficulty with the previous mechanism, where an electron would have to overcome a distance of ∼14 Å in <1 ps in a single step. If interpreted within a scheme of single-sided electron transfer, our data suggest that the B-branch is the active branch, although parallel A-branch activity cannot be excluded. All the mutations do affect to a varying extent the energy difference between the reaction center excited state RC* and the first radical pair and thus affect the rate constant of charge recombination. It is interesting to note that the new mechanism proposed is in fact analogous to the electron transfer mechanism in Photosystem II, where the accessory Chl also plays the role of the primary electron donor, rather than the special Chl pair P680 (Prokhorenko, V. and A. R. Holzwarth. 2000. J. Phys. Chem. B. 104:11563–11578).

Photoinduced interlayer electron transfer in alternating porphyrin–fullerene dyad and regioregular poly(3-hexylthiophene) Langmuir–Blodgett films

Alternating Langmuir–Blodgett (LB) bi-layer structures containing a donor–acceptor (DA) dyad layer and a layer of conducting polymer were used to study interlayer vectorial photoinduced electron transfer (VPET). The used porphyrin–fullerene dyads were known to be capable of VPET, from the porphyrin to fullerene moiety, as a LB monolayer structure. The DA dyad and the poly(3-hexylthiophene), PHT, layers were deposited as adjacent LB monolayers in order to promote the secondary electron transfer from the polymer to the porphyrin cation, the product of the primary electron transfer. The VPET reaction was studied with the time-resolved Maxwell displacement charge (TRMDC) method both in the photovoltage (PV) and photocurrent (PC) modes. The PHT monolayer at the side of the porphyrin moieties lengthened the charge separation (CS) distance. Because of PHT the photovoltage amplitudes were increased and the recombination kinetics of the charge-separated state was delayed. The electron transfer direction in the bi-layer system was determined by the orientation of the DA dyad. The charge separation in the system took place with yield close to unity.

Construction and Characterization of Genetically Modified Synechocystis sp. PCC 6803 Photosystem II Core Complexes Containing Carotenoids with Shorter π-Conjugation than β-Carotene

Beta-carotene has been identified as an intermediate in a secondary electron transfer pathway that oxidizes Chl(Z) and cytochrome b(559) in Photosystem II (PS II) when normal tyrosine oxidation is blocked. To test the redox function of carotenoids in this pathway, we replaced the zeta-carotene desaturase gene (zds) or both the zds and phytoene desaturase (pds) genes of Synechocystis sp. PCC 6803 with the phytoene desaturase gene (crtI) of Rhodobacter capsulatus, producing carotenoids with shorter conjugated pi-electron systems and higher reduction potentials than beta-carotene. The PS II core complexes of both mutant strains contain approximately the same number of chlorophylls and carotenoids as the wild type but have replaced beta-carotene (11 double bonds), with neurosporene (9 conjugated double bonds) and beta-zeacarotene (9 conjugated double bonds and 1 beta-ionylidene ring). The presence of the ring appears necessary for PS II assembly. Visible and near-infrared spectroscopy were used to examine the light-induced formation of chlorophyll and carotenoid radical cations in the mutant PS II core complexes at temperatures from 20 to 160 K. At 20 K, a carotenoid cation radical is formed having an absorption maximum at 898 nm, an 85 nm blue shift relative to the beta-carotene radical cation peak in the WT, and consistent with the formation of the cation radical of a carotenoid with 9 conjugated double bonds. The ratio of Chl(+)/Car(+) is higher in the mutant core complexes, consistent with the higher reduction potential for Car(+). As the temperature increases, other carotenoids become accessible to oxidation by P(680)(+).

Rewiring photosynthesis: engineering wrong-way electron transfer in the purple bacterial reaction centre

The purple bacterial reaction centre uses light energy to separate charge across the cytoplasmic membrane, reducing ubiquinone and oxidizing a c-type cytochrome. The protein possesses a macroscopic structural two-fold symmetry but displays a strong functional asymmetry, with only one of two available membrane-spanning branches of cofactors (the so-called A-branch) being used to catalyse photochemical charge separation. The factors underlying this functional asymmetry have been the subject of study for many years but are still not fully understood. Site-directed mutagenesis has been partially successful in rerouting electron transfer along the normally inactive B-branch, allowing comparison of the kinetics of equivalent electron transfer reactions on the two branches. Both the primary and secondary electron transfer steps on the B-branch appear to be considerably slower than their A-branch counterparts. The effectiveness of different mutations in rerouting electron transfer along the B-branch of cofactors is discussed.

Energetics of primary and secondary electron transfer in Photosystem II membrane particles of spinach revisited on basis of recombination-fluorescence measurements

Photon absorption by one of the roughly 200 chlorophylls of the plant Photosystem II (PSII) results in formation of an equilibrated excited state (Chl200*) and is followed by chlorophyll oxidation (formation of P680+) coupled to reduction of a specific pheophytin (Phe), then electron transfer from Phe− to a firmly bound quinone (QA), and subsequently reduction of P680+ by a redox-active tyrosine residue denoted as Z. The involved free-energy differences (Δ G) and redox potentials are of prime interest. Oxygen-evolving PSII membrane particles of spinach were studied at 5 °C. By analyzing the delayed and prompt Chl fluorescence, we determined the equilibrium constant and thus free-energy difference between Chl200* and the [Z+,QA−] radical pair to be −0.43 ± 0.025 eV, at 10 μs after the photon absorption event for PSII in its S 3-state. On basis of this value and previously published results, the free-energy difference between P680* and [P680+,QA−] is calculated to be −0.50 ± 0.04 eV; the free-energy loss associated with electron transfer from Phe to QA is found to be 0.34 ± 0.04 eV. The given uncertainty ranges do not represent a standard deviation or likely error, but an estimate of the maximal error. Assuming a QA−/QA redox potential of −0.08 V [Krieger et al., 1995, Biochim. Biophys. Acta 1229, 193], the following redox-potential estimates are obtained: +1.25 V for P680/P680+; +1.21 V for Z/Z+ (at 10 μs); −0.42 V for Phe−/Phe; −0.58 V for P680*/P680+.

Asymmetric Electron Transfer in Cyanobacterial Photosystem I: Charge Separation and Secondary Electron Transfer Dynamics of Mutations Near the Primary Electron Acceptor A0

Point mutations were introduced near the primary electron acceptor sites assigned to A0 in both the PsaA and PsaB branches of Photosystem I in the cyanobacterium Synechocystis sp. PCC 6803. The residues Met688PsaA and Met668PsaB, which provide the axial ligands to the Mg2+ of the eC-A3 and eC-B3 chlorophylls, were changed to leucine and asparagine (chlorophyll notation follows Jordan et al., 2001). The removal of the ligand is expected to alter the midpoint potential of the A0/A0− redox pair and result in a change in the intrinsic charge separation rate and secondary electron transfer kinetics from A0− to A1. The dynamics of primary charge separation and secondary electron transfer were studied at 690nm and 390nm in these mutants by ultrafast optical pump-probe spectroscopy. The data reveal that mutations in the PsaB branch do not alter electron transfer dynamics, whereas mutations in the PsaA branch have a distinct effect on electron transfer, slowing down both the primary charge separation and the secondary electron transfer step (the latter by a factor of 3–10). These results suggest that electron transfer in cyanobacterial Photosystem I is asymmetric and occurs primarily along the PsaA branch of cofactors.

Resonance Raman spectroscopy of carotenoids in Photosystem II core complexes

Resonance Raman (RR) spectroscopy has been used to examine the configuration of the carotenoids bound to Synechocystis PCC 6803 Photosystem II (PS II) core complexes. The excitation wavelengths used (514.5, 488.0, 476.5 and 457.9 nm) span the absorption bands of all of the approximately 12-17 neutral carotenoids in the PS II core complex. The RR spectra of the two carotenoids associated with the D1-D2 polypeptides (Car507 and Car489) of the reaction center are extracted via light versus dark difference experiments measured at 20 K. The RR results are consistent with all-trans configurations for both Car507 and Car489 and indicate that majority of the other carotenoids in the PS II core complex must also be in the all-trans configuration. The configuration of beta-carotene is relevant to its proposed function as a molecular wire in the secondary electron-transfer reactions of PS II.

Electron transfer reactions induced by the triplet state of thiacarbocyanine dimers

Abstract The photoinduced electron transfer between either cationic 5,5 ′ -dichloro-3,3 ′ ,9-triethylthiacarbocyanine ( 1 ) or a structurally similar anionic dye ( 2 ) and appropriate donors, e.g. ascorbic acid, and acceptors, e.g. methyl viologen, was studied by ns-laser photolysis. In aqueous solution the dyes in the ground state are present as an equilibrated mixture of dimers and monomers, whereas the triplet state is mainly populated from dimers. The triplet states of both dimers and monomers are quenched by electron donors or acceptors and the rate constant for quenching is generally 2–4 times higher for dimers than for monomers. The kinetics of triplet decay and radical formation and decay as a result of primary and secondary electron transfer were analyzed. While the one-electron reduced dimer decays due to back reactions, the one-electron oxidized dimer rapidly dissociates into the monomer and the monomeric dye radical. For the dimeric dye/donor/acceptor systems the primary photoinduced electron transfer occurs either from the donor or to the acceptor yielding the dimeric dye radicals. The one-electron reduced dimer can be efficiently oxidized by acceptors, e.g. the rate constant for reaction of the dimeric dye radical of 1 with methyl viologen (photoreductive pathway of sensitization) is 1.6×10 9 M −1 s −1 . The photooxidative pathway of sensitization is more complicated; after dissociation of the dimeric dye radical, the monomeric dye radical is reduced in a secondary electron transfer from ascorbic acid, e.g. with a rate constant of 1×10 9 M −1 s −1 for 2 , yielding the monomer. On increasing the donor concentration the photooxidative pathway of sensitization is switched to a photoreductive one.

2P253 光化学系IIにおけるカロテノイド及びクロロフィルZを経由する副次的電子移動経路(光生物 B) 光合成)

2P253 光化学系IIにおけるカロテノイド及びクロロフィルZを経由する副次的電子移動経路(光生物 B) 光合成)

Photoinduced Electron Transfer for Pyrromethene Dyes

The photophysics and electrochemistry of pyrromethene dyes (PM), an important class of laser dyes, have been investigated. Selected dyes were found to be moderately good electron donors. Diffusion-limited fluorescence quenching (ca. 6 × 109 M-1 s-1) is observed for the dyes having half-wave oxidation potentials <1.07 V (vs SCE) in the presence of an electron acceptor, such as methyl viologen or pyromellitic dianhydride. For pyrromethene 567, the dye radical cation and triplet state, along with the pyromellitic dianhydride radical anion and the methyl viologen radical cation, were identified using transient absorption spectroscopy. The dye radical cation, which displays a sharp absorption band at 410 nm, is found to be insensitive toward oxygen. Secondary electron transfer involving the reduction of oxygen and producing superoxide radical anion is suggested. Oxygen plays a critical role in PM dye photochemistry as it induces intersystem crossing to generate the dye triplet state and gives rise to products ...

Two redox-active beta-carotene molecules in photosystem II.

Photosystem II (PS II) contains secondary electron-transfer paths involving cytochrome b(559) (Cyt b(559)), chlorophyll (Chl), and beta-carotene (Car) that are active under conditions when oxygen evolution is blocked such as in inhibited samples or at low temperature. Intermediates of the secondary electron-transfer pathways of PS II core complexes from Synechocystis PCC 6803 and Synechococcus sp. and spinach PS II membranes have been investigated using low temperature near-IR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. We present evidence that two spectroscopically distinct redox-active carotenoids are formed upon low-temperature illumination. The Car(+) near-IR absorption peak varies in wavelength and width as a function of illumination temperature. Also, the rate of decay during dark incubation of the Car(+) peak varies as a function of wavelength. Factor analysis indicates that there are two spectral forms of Car(+) (Car(A)(+) has an absorbance maximum of 982 nm, and Car(B)(+) has an absorbance maximum of 1027 nm) that decay at different rates. In Synechocystis PS II, we observe a shift of the Car(+) peak to shorter wavelength when oxidized tyrosine D (Y(D)*) is present in the sample that is explained by an electrostatic interaction between Y(D)* and a nearby beta-carotene that disfavors oxidation of Car(B). The sequence of electron-transfer reactions in the secondary electron-transfer pathways of PS II is discussed in terms of a hole-hopping mechanism to attain the equilibrated state of the charge separation at low temperatures.

Quinone reduction via secondary B-branch electron transfer in mutant bacterial reaction centers.

Symmetry-related branches of electron-transfer cofactors-initiating with a primary electron donor (P) and terminating in quinone acceptors (Q)-are common features of photosynthetic reaction centers (RC). Experimental observations show activity of only one of them-the A branch-in wild-type bacterial RCs. In a mutant RC, we now demonstrate that electron transfer can occur along the entire, normally inactive B-branch pathway to reduce the terminal acceptor Q(B) on the time scale of nanoseconds. The transmembrane charge-separated state P(+)Q(B)(-) is created in this manner in a Rhodobacter capsulatus RC containing the F(L181)Y-Y(M208)F-L(M212)H-W(M250)V mutations (YFHV). The W(M250)V mutation quantitatively blocks binding of Q(A), thereby eliminating Q(B) reduction via the normal A-branch pathway. Full occupancy of the Q(B) site by the native UQ(10) is ensured (without the necessity of reconstitution by exogenous quinone) by purification of RCs with the mild detergent, Deriphat 160-C. The lifetime of P(+)Q(B)(-) in the YFHV mutant RC is >6 s (at pH 8.0, 298 K). This charge-separated state is not formed upon addition of competitive inhibitors of Q(B) binding (terbutryn or stigmatellin). Furthermore, this lifetime is much longer than the value of approximately 1-1.5 s found when P(+)Q(B)(-) is produced in the wild-type RC by A-side activity alone. Collectively, these results demonstrate that P(+)Q(B)(-) is formed solely by activity of the B-branch carriers in the YFHV RC. In comparison, P(+)Q(B)(-) can form by either the A or B branches in the YFH RC, as indicated by the biexponential lifetimes of approximately 1 and approximately 6-10 s. These findings suggest that P(+)Q(B)(-) states formed via the two branches are distinct and that P(+)Q(B)(-) formed by the B side does not decay via the normal (indirect) pathway that utilizes the A-side cofactors when present. These differences may report on structural and energetic factors that further distinguish the functional asymmetry of the two cofactor branches.

Crystal structure of oxygen-evolving photosystem II from Thermosynechococcus vulcanus at 3.7-A resolution.

Photosystem II (PSII) is a multisubunit membrane protein complex performing light-induced electron transfer and water-splitting reactions, leading to the formation of molecular oxygen. The first crystal structure of PSII from a thermophilic cyanobacterium Thermosynechococcus elongatus was reported recently [Zouni, A., Witt, H. T., Kern, J., Fromme, P., Krauss, N., Saenger, W. & Orth, P. (2001) Nature 409, 739-743)] at 3.8-A resolution. To analyze the PSII structure in more detail, we have obtained the crystal structure of PSII from another thermophilic cyanobacterium, Thermosynechococcus vulcanus, at 3.7-A resolution. The present structure was built on the basis of the sequences of PSII large subunits D1, D2, CP47, and CP43; extrinsic 33- and 12-kDa proteins and cytochrome c550; and several low molecular mass subunits, among which the structure of the 12-kDa protein was not reported previously. This yielded much information concerning the molecular interactions within this large protein complex. We also show the arrangement of chlorophylls and cofactors, including two beta-carotenes recently identified in a region close to the reaction center, which provided important clues to the secondary electron transfer pathways around the reaction center. Furthermore, possible ligands for the Mn-cluster were determined. In particular, the C terminus of D1 polypeptide was shown to be connected to the Mn cluster directly. The structural information obtained here provides important insights into the mechanism of PSII reactions.

Selective perturbation of the second electron transfer step in mutant bacterial reaction centers

In order to specifically perturb the primary electron acceptor B A — a monomeric bacteriochlorophyll (BChl) a — involved in bacterial photosynthetic charge separation (CS), the protein environment of B A in the reaction center (RC) of Rhodobacter sphaeroides was modified by site-directed mutagenesis. Isolated RCs were characterized by redox titrations, low temperature optical spectroscopy, ENDOR/TRIPLE resonance spectroscopy and femtosecond time-resolved spectroscopy. Two mutations were studied: In the GS(M203) mutant a serine is introduced near the ring E keto group of B A, while in FY(L146) a phenylalanine near the ring A acetyl group of B A is replaced by tyrosine. In all mutations the oxidation potential of the primary electron donor P as well as the electronic structure of both the P + radical cation and the radical anion of the secondary electron acceptor, H A − , are not significantly altered compared to the wild type (WT), while changes of the optical absorption spectra at 77 K in the BChl Q X and Q Y regions are observed. The GS(M203) mutation only leads to a minor retardation of the CS reactions at room temperature, whereas for FY(L146) significant deviations from the native electron transfer (ET) rates could be detected: In addition to a faster first (2.9 ps) and a slower second (1 ps) ET step, a new 8-ps time constant was found in the FY(L146) mutant, which can be ascribed to a fraction of RCs with slowed down secondary ET. The results allow us to address the functional role of the acetyl group of B A and question the role of the free energy changes as the main determining factor of ET rates in RCs. It is concluded that structural rearrangements alter the electronic coupling between the pigments and thereby influence the rate of fast CS.

Hystons, new quasi-particles, and electron transfer in bacterial photosynthesis

For photosynthetic purple bacteria the collective vibrations of the linked polar fragments and polar elements both in the LH2 antenna complex and reaction center (RC) with their protein environment are described as a motion of hystons, new quasi-particles. Taking into account the fact that hystons modulate the frequency of exciton transitions, the half-width of absorption spectra had been defined as a function of the difference in permanent dipole moment between excited and ground states. Obtained equations for the temperature dependence of full half-width provide an excellent fit to known experimental data for B800 spectra of Rb. sphaeroides and Rps. acidophila as well as for P870 primary electron donor band of Rb. sphaeroides with 63, 52 and 142 cm −1 hyston modes, respectively. The theory has been extended and applied to the calculation of temperature dependence at the rate of electron transfer (ET). After fitting to known data, it has been shown that the 304(±10) cm −1 hyston mode, localized mainly on the bacterioheophytin, governs the secondary ET (from the bacterioheophytin to ubiquinone) in the Rps. sphaeroides RC. It is also shown that the primary ET in the RCs of Rps. viridis and Rb. sphaeroides are controlled by special pair pseudo-hyston modes with frequencies 139(±11) and 166(±28) cm −1 , respectively.

Porphyrinic dyads and triads assembled around iridium(III) bis-terpyridine: photoinduced electron transfer processes.

Multicomponent arrays based on a central iridium(III) bis-terpyridine complex (Ir) used as assembling metal and free-base, zinc(II) or gold(III) tetraaryl-porphyrins (PH(2), PZn, PAu) have been designed to generate intramolecular photoinduced charge separation. The rigid dyads PH(2)-Ir, PZn-Ir, PAu-Ir, and the rigid and linear triads PH(2)-Ir-PAu, PZn-Ir-PAu, as well as the individual components Ir, PH(2), PZn, PAu have been synthesized and characterized by various techniques including electrochemistry. Their photophysical properties either in acetonitrile or in dichloromethane and toluene have been determined by steady-state and time-resolved methods. In acetonitrile, excitation of the triad PH(2)-Ir-PAu leads to a charge separation with an efficiency of 0.5 and a resulting charge-separated (CS) state with a lifetime of 3.5 ns. A low-lying triplet localized on PH(2) and the presence of the heavy Ir(III) ion offer the CS state an alternative deactivation path through the triplet state. The behavior of the triad PZn-Ir-PAu in dichloromethane is rather different from that of PH(2)-Ir-PAu in acetonitrile since the primary electron transfer to yield PZn(+)()-Ir(-)-PAu is not followed by a secondary electron transfer. In this solvent, both unfavorable thermodynamic and electronic parameters contribute to the inefficiency of the second electron-transfer reaction. In contrast, in toluene solutions, the triad PZn-Ir-PAu attains a CS state with a unitary yield and a lifetime of 450 ns. These differences can be understood in terms of ground-state charge-transfer interactions as well as different stabilization of the intermediate and final CS states by solvent.

Energy transfer and trapping in the CP47RC complex of photosystem II

The CP47RC complex of Photosystem II (PS II) differs from the isolated reaction center (RC) by the presence of a single antenna unit (CP47). Both complexes can be prepared in such a way that the primary quinone acceptor (QA) is absent, implying that secondary electron transfer can not occur in these complexes. The excited state kinetics of RC has previously been investigated by several groups, but such kinetics of CP47RC have hardly been measured. The only additional process upon excitation of the CP47RC complex compared to the RC complex is energy migration from the antenna to RC and back. Our goal was to determine the fluorescence kinetics of CP47RC complex and to analyze the influence of the CP47 antenna on the kinetics. We have measured time-resolved fluorescence kinetics of CP47RC complexes at room temperature using a synchroscan streak camera with a time resolution of about 3 ps. The intensity of the excitation laser pulses was low, which prevented significant annihilation contributions. Global analysis of the data revealed that the excited state kinetics of CP47RC has a 4-exponential behavior. We consider a model in which at room temperature excitations reach equilibrium in CP47 within less than 2 ps (see the abstract of F.L.de Weerd et.al.) and in which the energy transfer from CP47 to RC is represented by single rate constant. The possible ways of excitation energy migration and trapping in the CP47RC complex are discussed.