Multipartite quantum entanglement evolution in photosynthetic complexes

We investigate the evolution of entanglement in the Fenna-Matthew-Olson (FMO) complex based on simulations using the scaled hierarchical equations of motion approach. We examine the role of entanglement in the FMO complex by direct computation of the convex roof. We use monogamy to give a lower bound for entanglement and obtain an upper bound from the evaluation of the convex roof. Examination of bipartite measures for all possible bipartitions provides a complete picture of the multipartite entanglement. Our results support the hypothesis that entanglement is maximum primary along the two distinct electronic energy transfer pathways. In addition, we note that the structure of multipartite entanglement is quite simple, suggesting that there are constraints on the mixed state entanglement beyond those due to monogamy.     

Photoinduced ultrafast electron transfer from CdSe quantum dots to Re-bipyridyl complexes

Ultrafast dissociation of excitons in CdSe quantum dots via electron transfer to adsorbed Re-bipyridyl complexes was demonstrated. The dissociation pathway was determined by the observation of reduced adsorbate using femtosecond IR spectroscopy. The rate of electron transfer was shown to increase at smaller QD sizes. Electron transfer time as fast as 2.3 ps was observed, faster than the exciton annihilation time in CdSe. The ultrafast charge separation in this quantum dot-adsorbate donor-acceptor complex provides a potential approach for separating multiple excitons in quantum dots.     

Photoinduced electron transfer in bisporphyrin-diimide complexes

The bisporphyrin host ZnH was synthesized, and its complexation with two aromatic diimide guest molecules, bis(pyridyl)naphthalenediimide NIN and bis(pyridyl)phenyldiimide PIN, was investigated by (1)H NMR and UV/Vis spectroscopy. The diimide guests were complexed simultaneously with both metalloporphyrins of the host, with association constants on the order of 10(8)M(-1). The processes occurring in the complex after excitation of the porphyrinic host were studied by steady-state and time-resolved emission and transient absorption spectroscopy. Complexation alters the photophysical properties of the host ZnH; the luminescence bands shift to the red by 30 nm in the complexed forms, while the emission quantum yield and the lifetime decrease. Comparison of a complex between ZnH and a model guest unable to undergo photoinduced processes allowed us to establish that, in the diimide complexes, quenching of the porphyrinic luminescence occurs with a rate of 1.1 x 10(10)s(-1). The process is identified as an electron transfer from the excited singlet of the porphyrinic host to the imide guest, which yields charge-separated states with a lifetime of 710 ps for ZnH(+)-NIN(-) and 260 ps for ZnH(+)-PIN(-).     

Synthesis of a model compound for the photosynthetic electron transfer

In order to elucidate the role of special pair chlorophyll in photosynthetic electron transfer, a new model compound 1, where two etioporphyrins are covalently tied in a face-to-face orientation and further linked to a quinone with a polymethylene chain, was synthesized.     

Solvent adiabaticity effects on ultrafast electron transfer in viologen charge transfer complexes

Charge recombination (CR) kinetics following photoinduced charge transfer are measured by optical transient absorption for complexes of dimethyl viologen and diheptyl viologen with 4,4'-biphenol (MVBP and HVBP) in methanol. Exponential time constants for MVBP and HVBP are 480 and 790 fs, respectively. Kinetic differences cannot be rationalized with a standard equilibrium nonadiabatic rate formula using parameters obtained from linear absorption and resonance Raman measurements, which give nearly indistinguishable results for the two complexes. Solvent relaxation times and adiabaticities of MVBP are calculated using a full solvation correlation function approach. This analysis suggests that the smaller CR rate of HVBP is due to solvent reorganization differences, and is consistent with a greater adiabatic contribution for HVBP than MVBP. We conjecture that interactions between the diheptyl aliphatic groups of HVBP and the local solvent structure are responsible for the CR differences.     

Ultrafast back electron transfer processes in the photoexcited methylviologen-iodide charge transfer complexes

The ultrafast back electron transfer in the excited charge transfer complexes of the methylviologen with iodide ions has been investigated using femtosecond transient absorption spectroscopy. Methylviologen and iodide form two types of charge transfer complexes each characterized by a charge transfer band in the same spectral region. At low I^- concentrations mainly a 1:1 complex MV²⁺(I^-) is present while at high I^- concentrations both 1:1 and 1:2 complexes MV²⁺(I^-)₂ can be observed. Ultrashort laser pulses at 400 nm are used to excite both complexes in their charge transfer band. The observed transient absorption can be represented by a biexponential function with 1 ps and 20 ps time constants and attributed to the decay of the MV^+./I^. and MV^+./I₂ ^.- radical pair respectively. The excitation of the 1:1 complex leads to the formation of the MV^+./I^. radical pair while the excitation of the 1:2 complex leads to the formation of the MV^+./I^. and MV^+./I₂ ^.- radical pairs.     

Role for bound water and CH-pi aromatic interactions in photosynthetic electron transfer

Photosystem I (PSI) is one of two photosynthetic reaction centers present in plants, algae, and cyanobacteria and catalyzes the reduction of ferredoxin and the oxidation of cytochrome c or plastocyanin. The PSI primary chlorophyll donor, which is oxidized in the primary electron-transfer events, is a heterodimer of chl a and a' called P700. It has been suggested that protein relaxation accompanies light-induced electron transfer in this reaction center (Dashdorj, N.; Xu, W.; Martinsson, P.; Chitnis, P. R.; Savikhin, S. Biophys. J. 2004, 86, 3121. Kim, S.; Sacksteder, C. A.; Bixby, K. A.; Barry, B. A. Biochemistry 2001, 40, 15384). To investigate the details of electron transfer and relaxation events in PSI, we have employed several experimental approaches. First, we report a pH-dependent viscosity effect on P700+ reduction; this result suggests a role for proton transfer in the PSI electron-transfer reactions. Second, we find that changes in hydration alter the rate of P700+ reduction and the interactions of P700 with the protein environment. This result suggests a role for bound water in electron transfer to P700+. Third, we present evidence that deuteration of the tyrosine aromatic side chain perturbs the vibrational spectrum, associated with P700+ reduction. We attribute this result to a linkage between CH-pi interactions and electron transfer to P700+.     

Ab inito study on triplet excitation energy transfer in photosynthetic light-harvesting complexes

We have studied the triplet energy transfer (TET) for photosynthetic light-harvesting complexes, the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The electronic coupling factor was calculated with the recently developed fragment spin difference scheme (You and Hsu, J. Chem. Phys. 2010, 133, 074105), which is a general computational scheme that yields the overall coupling under the Hamiltonian employed. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond triplet energy transfer from the chlorophylls to the carotenoids. This result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π conjugation lengths in the carotenoids and their analogues reported in the literature. We have also explored the possibility of forming triplet excitons in these complexes. In B850 of LH2 or the peridinin cluster in PCP, it is unlikely to have triplet exciton since the energy differences of any two neighboring molecules are likely to be much larger than their TET couplings. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.     

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.     

Unprecedented optically induced long-lived intramolecular electron transfer in cobalt-dioxolene complexes

The observation of very long lifetimes of the metastable states of two cobalt-dioxolene complexes undergoing photoinduced and high T(c) thermally-induced valence tautomer interconversion opens new research perspectives.     

Photoinduced electron transfer in supramolecular assemblies of transition metal complexes

Photoinduced electron transfer in supramolecular assemblies consisting of π-donor dialkoxyarene-functionalized photosensitizers and bipyridinium electron acceptors is examined. The photosensitizers include Ru(II)-tris-bipyridine complexetethered by multi-branch one-shell and two-shell dialkoxybenzene π-donor sites or a Zn(II)-porphyrin capped by a dialkoxybenzene receptor site. The photosensitizer/electron-acceptor supramolecular complexes behave as non-covalent diads and polyads. Effective internal electron transfer quenching within the supramolecular assemblies proceeds. A quantitative model that accounts for the photoinduced electron transfer in the systems is formulated.     

Ultrafast time-resolved carotenoid to-bacteriochlorophyll energy transfer in LH2 complexes from photosynthetic bacteria

Steady-state and ultrafast time-resolved optical spectroscopic investigations have been carried out at 293 and 10 K on LH2 pigment-protein complexes isolated from three different strains of photosynthetic bacteria: Rhodobacter (Rb.) sphaeroides G1C, Rb. sphaeroides 2.4.1 (anaerobically and aerobically grown), and Rps. acidophila 10050. The LH2 complexes obtained from these strains contain the carotenoids, neurosporene, spheroidene, spheroidenone, and rhodopin glucoside, respectively. These molecules have a systematically increasing number of pi-electron conjugated carbon-carbon double bonds. Steady-state absorption and fluorescence excitation experiments have revealed that the total efficiency of energy transfer from the carotenoids to bacteriochlorophyll is independent of temperature and nearly constant at approximately 90% for the LH2 complexes containing neurosporene, spheroidene, spheroidenone, but drops to approximately 53% for the complex containing rhodopin glucoside. Ultrafast transient absorption spectra in the near-infrared (NIR) region of the purified carotenoids in solution have revealed the energies of the S1 (2(1)Ag-)-->S2 (1(1)Bu+) excited-state transitions which, when subtracted from the energies of the S0 (1(1)Ag-)-->S2 (1(1)Bu+) transitions determined by steady-state absorption measurements, give precise values for the positions of the S1 (2(1)Ag-) states of the carotenoids. Global fitting of the ultrafast spectral and temporal data sets have revealed the dynamics of the pathways of de-excitation of the carotenoid excited states. The pathways include energy transfer to bacteriochlorophyll, population of the so-called S* state of the carotenoids, and formation of carotenoid radical cations (Car*+). The investigation has found that excitation energy transfer to bacteriochlorophyll is partitioned through the S1 (1(1)Ag-), S2 (1(1)Bu+), and S* states of the different carotenoids to varying degrees. This is understood through a consideration of the energies of the states and the spectral profiles of the molecules. A significant finding is that, due to the low S1 (2(1)Ag-) energy of rhodopin glucoside, energy transfer from this state to the bacteriochlorophylls is significantly less probable compared to the other complexes. This work resolves a long-standing question regarding the cause of the precipitous drop in energy transfer efficiency when the extent of pi-electron conjugation of the carotenoid is extended from ten to eleven conjugated carbon-carbon double bonds in LH2 complexes from purple photosynthetic bacteria.     

Energy and photoinduced electron transfer in a wheel-shaped artificial photosynthetic antenna-reaction center complex

Functional mimics of a photosynthetic antenna-reaction center complex comprising five bis(phenylethynyl)anthracene antenna moieties and a porphyrin-fullerene dyad organized by a central hexaphenylbenzene core have been prepared and studied spectroscopically. The molecules successfully integrate singlet-singlet energy transfer and photoinduced electron transfer. Energy transfer from the five antennas to the porphyrin occurs on the picosecond time scale with a quantum yield of 1.0. Comparisons with model compounds and theory suggest that the F?rster mechanism plays a major role in the extremely rapid energy transfer, which occurs at rates comparable to those seen in some photosynthetic antenna systems. A through-bond, electron exchange mechanism also contributes. The porphyrin first excited singlet state donates an electron to the attached fullerene to yield a P(*+)-C(60)(*-) charge-separated state, which has a lifetime of several nanoseconds. The quantum yield of charge separation based on light absorbed by the antenna chromophores is 80% for the free base molecule and 96% for the zinc analogue.     

Structural dependence of electron transfer to non-covalent polar complexes

Electron attachment to polar molecules and their non-covalent complexes can lead to different kinds of anions which differ from their excess electron localization. Spectroscopic methods for studying anion structures are reviewed. In many cases, the neutral and anion structures are identical and can be deduced from the electron attachment properties. Examples are given for complexes containing polar solvents or building blocks of biomolecules (nucleobases, amino acid residues...).     

Communication: Exciton-phonon information flow in the energy transfer process of photosynthetic complexes

Non-Markovian and nonequilibrium phonon effects are believed to be key ingredients in the energy transfer in photosynthetic complexes, especially in complexes which exhibit a regime of intermediate exciton-phonon coupling. In this work, we utilize a recently developed measure for non-Markovianity to elucidate the exciton-phonon dynamics in terms of the information flow between electronic and vibrational degrees of freedom. We study the measure in the hierarchical equation of motion approach which captures strong coupling effects and nonequilibrium molecular reorganization. We propose an additional trace distance measure for the information flow that could be extended to other master equations. We find that for a model dimer system and for the Fenna-Matthews-Olson complex the non-Markovianity is significant under physiological conditions.     

Proton coupled electron transfer and redox-active tyrosine Z in the photosynthetic oxygen-evolving complex

Proton coupled electron transfer (PCET) reactions play an essential role in many enzymatic processes. In PCET, redox-active tyrosines may be involved as intermediates when the oxidized phenolic side chain deprotonates. Photosystem II (PSII) is an excellent framework for studying PCET reactions, because it contains two redox-active tyrosines, YD and YZ, with different roles in catalysis. One of the redox-active tyrosines, YZ, is essential for oxygen evolution and is rapidly reduced by the manganese-catalytic site. In this report, we investigate the mechanism of YZ PCET in oxygen-evolving PSII. To isolate YZ(?) reactions, but retain the manganese-calcium cluster, low temperatures were used to block the oxidation of the metal cluster, high microwave powers were used to saturate the YD(?) EPR signal, and YZ(?) decay kinetics were measured with EPR spectroscopy. Analysis of the pH and solvent isotope dependence was performed. The rate of YZ(?) decay exhibited a significant solvent isotope effect, and the rate of recombination and the solvent isotope effect were pH independent from pH 5.0 to 7.5. These results are consistent with a rate-limiting, coupled proton electron transfer (CPET) reaction and are contrasted to results obtained for YD(?) decay kinetics at low pH. This effect may be mediated by an extensive hydrogen-bond network around YZ. These experiments imply that PCET reactions distinguish the two PSII redox-active tyrosines.     

Artificial photosynthetic reaction centers: mimicking sequential electron and triplet-energy transfer.

An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylporphyrin (P), and a bis(heptafluoropropyl)porphyrin (P(F)), C-P-P(F) , and the related triad in which the central porphyrin has been metalated to give C-P(Zn)-P(F) have been synthesized and characterized by transient spectroscopy. These triads are models for amphipathic triads having a carboxylate group attached to the P(F) moiety; they are designed to carry out redox processes across lipid bilayers. Triad C-P-P(F) undergoes rapid singlet-singlet energy transfer between the porphyrin moieties, so that their excited states are in equilibrium. In benzonitrile, photoinduced electron transfer from the first excited singlet state of P and hole transfer from the first excited singlet state of P(F) yield the initial charge-separated state C-P(.) (+)-P(F) (.) (-). Subsequent hole transfer to the carotenoid moiety generates the final charge-separated state C(.) (+)-P-P(F) (.) (-), which has a lifetime of 1.1 mus and is formed with a quantum yield of 0.24. In triad C-P(Zn)-P(F) energy transfer from the P(Zn) excited singlet to the P(F) moiety yields C-P(Zn)-(1)P(F) . A series of electron-transfer reactions analogous to those observed in C-P-P(F) generates C(.) (+)-P(Zn)-P(F) (.) (-), which has a lifetime of 750 ns and is formed with a quantum yield of 0.25. Flash photolysis experiments in liposomes containing an amphipathic version of C-P(Zn)-P(F) demonstrate that the added driving force for photoinduced electron transfer in the metalated triad is useful for promoting electron transfer in the low-dielectric environment of artificial biological membranes. In argon-saturated toluene solutions of C-P-P(F) and C-P(Zn)-P(F) , charge separation is not observed and a considerable yield of triplet species is generated upon excitation of the porphyrin moieties. In both triads triplet energy localized in the P(F) moiety is channeled to the carotenoid chromophore by a triplet energy-transfer relay mechanism. Certain photophysical characteristics of these triads, including the sequential electron transfer and the triplet energy-transfer relay mechanism, are reminiscent of those observed in natural reaction centers of photosynthetic bacteria.     

Redox control of photoinduced electron transfer in axial terpyridoxy porphyrin complexes

The photophysical properties of axial-bonding types (terpyridoxy)aluminum(III) porphyrin (Al(PTP)), bis(terpyridoxy)tin(IV) porphyrin (Sn(PTP) 2), and bis(terpyridoxy)phosphorus(V) porphyrin ([P(PTP) 2] (+)) are reported. Compared with their hydroxy analogues, the fluorescence quantum yields and singlet-state lifetimes were found to be lower for Sn(PTP) 2 and [P(PTP) 2] (+), whereas no difference was observed for Al(PTP). At low temperature, all of the compounds show spin-polarized transient electron paramagnetic resonance (TREPR) spectra that are assigned to the lowest excited triplet state of the porphyrin populated by intersystem crossing. In contrast, at room temperature, a triplet radical-pair spectrum that decays to the porphyrin triplet state with a lifetime of 175 ns is observed for [P(PTP) 2] (+), whereas no spin-polarized TREPR spectrum is found for Sn(PTP) 2 and only the porphyrin triplet populated by intersystem crossing is seen for Al(PTP). These results clarify the role of the internal molecular structure and the reduction potential for electron transfer from the terpyridine ligand to the excited porphyrin. It is argued that the efficiency of this process is dependent on the oxidation state of the metal/metalloid present in the porphyrin and the reorganization energy of the solvent.