Quantum Yield Of Charge Separation Research Articles

Symmetry breaking in photosystem I: ultrafast optical studies of variants near the accessory chlorophylls in the A- and B-branches of electron transfer cofactors.

Femtosecond absorption spectroscopy of Photosystem I (PSI) complexes from the cyanobacterium Synechocystis sp. PCC 6803 was carried out on three pairs of complementary amino acid substitutions located near the second pair of chlorophyll molecules Chl2A and Chl2B (also termed A-1A and A-1B). The absorption dynamics at delays of 0.1-500ps were analyzed by decomposition into discrete decay-associated spectra and continuously distributed exponential components. The multi-exponential deconvolution of the absorption changes revealed that the electron transfer reactions in the PsaA-N600M, PsaA-N600H, and PsaA-N600L variants near the B-branch of cofactors are similar to those of the wild type, while the PsaB-N582M, PsaB-N582H, and PsaB-N582L variants near the A-branch of cofactors cause significant alterations of the photochemical processes, making them heterogeneous and poorly described by a discrete exponential kinetic model. A redistribution of the unpaired electron between the second and the third monomers Chl2A/Chl2B and Chl3A/Chl3B was identified in the time range of 9-20ps, and the subsequent reduction of A1 was identified in the time range of 24-70ps. In the PsaA-N600L and PsaB-N582H/L variants, the reduction of A1occurred with a decreased quantum yield of charge separation. The decreased quantum yield correlates with a slowing of the phylloquinone A0 → A1 reduction, but not with the initial transient spectra measured at the shortest time delay. The results support a branch competition model, where the electron is sheared between Chl2A-Chl3A and Chl2B-Chl3B cofactors before its transfer to phylloquinone in either A1A or A1B sites.

Insight into Electron Transfer from a Redox Polymer to a Photoactive Protein.

Biohybrid photoelectrochemical systems in photovoltaic or biosensor applications have gained considerable attention in recent years. While the photoactive proteins engaged in such systems usually maintain an internal charge separation quantum yield of nearly 100%, the subsequent steps of electron and hole transfer beyond the protein often limit the overall system efficiency and their kinetics remain largely uncharacterized. To reveal the dynamics of one of such charge-transfer reactions, we report on the reduction of Rhodobacter sphaeroides reaction centers (RCs) by Os-complex-modified redox polymers (P-Os) characterized using transient absorption spectroscopy. RCs and P-Os were mixed in buffered solution in different molar ratios in the presence of a water-soluble quinone as an electron acceptor. Electron transfer from P-Os to the photoexcited RCs could be described by a three-exponential function, the fastest lifetime of which was on the order of a few microseconds, which is a few orders of magnitude faster than the internal charge recombination of RCs with fully separated charge. This was similar to the lifetime for the reduction of RCs by their natural electron donor, cytochrome c2. The rate of electron donation increased with increasing ratio of polymer to protein concentrations. It is proposed that P-Os and RCs engage in electrostatic interactions to form complexes, the sizes of which depend on the polymer-to-protein ratio. Our findings throw light on the processes within hydrogel-based biophotovoltaic devices and will inform the future design of materials optimally suited for this application.

Quantum yield and energy efficiency of photoinduced intramolecular charge separation.

Kinetics of photoinduced intramolecular charge separation (CS) and the ensuing ultrafast charge recombination (CR) in electron-donor-acceptor dyads are studied numerically, taking into account the excitation of charge-transfer active intramolecular vibrations and multiple relaxation time scales of the surrounding polar solvent. Both energetic and dynamic properties of intramolecular and solvent reorganization are considered, and their influence on the CS/CR kinetics and quantum yield of ultrafast CS is explored. Particular attention is paid to the energy efficiency of CS, as one of the most important parameters indicating the promise of using a molecular compound as a basis for emerging optoelectronic devices. The CS quantum yield and the energy efficiency of CS are shown to depend differently on the key model parameters. Necessary conditions for the highly efficient CS are evaluated using analytic formulae for the electron transfer rates and derived from numerical simulation data. The reasons why low-exergonic CS taking place in the Marcus normal region can be much slower than CR in the deep inverted region are discussed.

Photophysical insights on a new supramolecular recognition element comprising PyC60 and a bisporphyrin studied in solution

The present work reports detailed photophysical insights on supramolecular interaction between C60 pyrrolidine tris-acid ethyl ester (PyC60) and a designed diporphyrin (1) in toluene. Both UV–vis and steady state fluorescence studies indicate effective complexation between PyC60 and 1 as revealed from the value of binding constant (K) of PyC60–1 complex, i.e., KPyC60–1 (UV–vis) = 7625 dm3·mol−1 and KPyC60–1 (fluorescence) = 9550 dm3·mol−1. Time-resolved fluorescence measurements evoke that the decay of photoexcited 1 in presence of PyC60 occurs only via static quenching mechanism and the magnitude of bimolecular quenching constant value for such system is enumerated to be 9.033 × 1011 dm3·mol−1·sec−1. In 1,2-dichlorobenzene (DCB), PyC60–1 system exhibits appreciable value of rate-constant of charge-separation (kcs) and quantum yield of charge-separation (ϕCS) than that observed in toluene. However, magnitude and order of both kcs (=5.35 × 107) and ϕCS (= 0.085) value for the PyC60–1 system estimated in DCB indicate that like toluene, there is no possibility of electron transfer in DCB. Both 1H and 13C NMR spectra of PyC60 molecule show appreciable shift of functional protons and carbon moiety of parent molecule in presence of 1, respectively. Quantum chemical calculations by ab initio method in vacuo establish the geometric structure of PyC60–1 complex and predict that dispersive forces associated with π-π interaction play significant role behind non-covalent interaction between PyC60 and 1.

Redox Exciton Quenching in Cadmium Selenide Quantum Dots by Methyl Viologen: Quantum Yield of Charge Separation

Using femtosecond laser spectroscopy with a time resolution of 25 fs, it has been shown that methyl viologen (MV2+) quenches the lower exciton state X1 in CdSe quantum dots in toluene. The quenching kinetics is approximated by a three-exponential function with time constants of τ1 = 350 fs (79%), τ2 = 1.3 ps (15%), and τ3 = 4 ps (6%). The quantum yield of charge separation to produce the h+//MV+ pair possessing a lifetime of more than 1 ns does not exceed 24%. The low quantum yield of the radical ion MV+ is explained in terms of fast recombination of MV+ and the hole. The methyl viologen adsorbed on the quantum dot surface mainly acts as the center of nonradiative electron–hole recombination.

Molecular complexation between PCBM and porphyrazine in solution: A case study of non-covalent interaction

The present investigations report the non-covalent interaction of a fullerene derivative, namely, [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) with 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraaza-21H,23H-porphine (1) and zinc-2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraaza-21H,23H-porphine (2) in toluene and also in mixed solvent comprising toluene and 1,2-dichlorobenzene (volume:volume=3:2). Complex formation by steady state fluorescence studies reveal high value of binding constant (K) for both PC70BM-1 and PC70BM-2 systems in toluene, i.e., KPCBM-1=10,820dm3·mol−1 and KPCBM-2=9655dm3·mol−1, and very good selectivity of binding for 1 as a result of change in solvent polarity, viz., KPC70BM-1 (Toluene)/KPC70BM-1 (Mixed Solvent) ~3.0. Due to solvophobic effect, the selectivity in binding for 2 in two different solvents, i.e., KPC70BM-2 (Toluene)/KPC70BM-2 (Mixed Solvent) estimated to be ~1.0. Lifetime measurements establish that static quenching mechanism is operative behind the photoexcited decay of 1⁎ (and/2⁎) in presence of PC70BM both in toluene and in mixed solvent. However, in mixed solvent, PC70BM-1 system is found to exhibit much higher value of magnitude of rate constant of charge-separation (i.e., kCSs(PC70BM-1)=2.97×107s−1) and kCSs(PC70BM-2)=5.60×106s−1) and quantum yield of charge-separation (ϕCS(PC70BM-1)=0.070) compared to PC70BM-2 system (ϕCS(PC70BM-1)=0.015). Ab initio calculations in vacuo predict the geometric structures of PC70BM-1 and PC70BM-2 systems and provide very good support in favour of charge-separation in former system in terms of multipole moment calculations.

Artificial Photosynthetic Reaction Center Exhibiting Acid-Responsive Regulation of Photoinduced Charge Separation.

Nonphotochemical quenching (NPQ) is a photoprotective regulatory mechanism employed by many photosynthetic organisms to dynamically modulate energy flow within the photosynthetic apparatus in response to fluctuating light conditions. Activated by decreases in lumen pH produced during periods of high photon flux, NPQ induces rapid thermal dissipation of excess excitation energy. As a result, the rate of charge separation (CS) decreases, thereby limiting the accumulation of potentially deleterious reactive intermediates and byproducts. Herein, a molecular triad that functionally mimics the effects of NPQ associated with an artificial photosynthetic reaction center is described. Steady-state absorption and emission, time-resolved fluorescence, and transient absorption spectroscopies have been used to demonstrate a 1 order of magnitude reduction in the CS quantum yield via reversible protonation of an excited-state-quenching molecular switch moiety. As in the natural system, the populations of unquenched and quenched states and therefore the overall yields of CS were found to be dependent on acid concentration.

Computational Visual Stress Level Analysis of Calcareous Algae Exposed to Sedimentation.

This paper presents a machine learning based approach for analyses of photos collected from laboratory experiments conducted to assess the potential impact of water-based drill cuttings on deep-water rhodolith-forming calcareous algae. This pilot study uses imaging technology to quantify and monitor the stress levels of the calcareous algae Mesophyllum engelhartii (Foslie) Adey caused by various degrees of light exposure, flow intensity and amount of sediment. A machine learning based algorithm was applied to assess the temporal variation of the calcareous algae size (∼ mass) and color automatically. Measured size and color were correlated to the photosynthetic efficiency (maximum quantum yield of charge separation in photosystem II, ) and degree of sediment coverage using multivariate regression. The multivariate regression showed correlations between time and calcareous algae sizes, as well as correlations between fluorescence and calcareous algae colors.

Photoinduced Charge Separation on the Microsecond Timescale in Graphene Oxide and Reduced Graphene Oxide Suspensions.

Transient absorption spectroscopy of graphene oxide (GO) and reduced graphene oxide (rGO) suspensions provides evidence for the photochemical generation of a charge-separated state on the microsecond timescale upon laser excitation. The lifetime and quantum yield of charge separation in suspended rGO were found to be higher than for GO. This could be advantageous for optoelectronic and photocatalytic applications, where graphene-based materials act as charge (electron) carriers. The electron-transfer quenching of the rGO charge-separated state by different amines is more efficient when the amine is a better electron donor and more easily oxidized.

Natural isoforms of the Photosystem II D1 subunit differ in photoassembly efficiency of the water-oxidizing complex.

Oxygenic photosynthesis efficiency at increasing solar flux is limited by light-induced damage (photoinhibition) of Photosystem II (PSII), primarily targeting the D1 reaction center subunit. Some cyanobacteria contain two natural isoforms of D1 that function better under low light (D1:1) or high light (D1:2). Herein, rates and yields of photoassembly of the Mn4CaO5 water-oxidizing complex (WOC) from the free inorganic cofactors (Mn(2+), Ca(2+), water, electron acceptor) and apo-WOC-PSII are shown to differ significantly: D1:1 apo-WOC-PSII exhibits a 2.3-fold faster rate-limiting step of photoassembly and up to seven-fold faster rate to the first light-stable Mn(3+) intermediate, IM1*, but with a much higher rate of photoinhibition than D1:2. Conversely, D1:2 apo-WOC-PSII assembles slower but has up to seven-fold higher yield, achieved by a higher quantum yield of charge separation and slower photoinhibition rate. These results confirm and extend previous observations of the two holoenzymes: D1:2-PSII has a greater quantum yield of primary charge separation, faster [P680 (+) Q A (-) ] charge recombination and less photoinhibition that results in a slower rate and higher yield of photoassembly of its apo-WOC-PSII complex. In contrast, D1:1-PSII has a lower quantum yield of primary charge separation, a slower [P680 (+) Q A (-) ] charge recombination rate, and faster photoinhibition that together result in higher rate but lower yield of photoassembly at higher light intensities. Cyanobacterial PSII reaction centers that contain the high- and low-light D1 isoforms can tailor performance to optimize photosynthesis at varying light conditions, with similar consequences on their photoassembly kinetics and yield. These different efficiencies of photoassembly versus photoinhibition impose differential costs for biosynthesis as a function of light intensity.

Chemical physics behind phthalocyanine–gold nanoparticle interaction and its effect over supramolecular interaction between PC70BM and phthalocyanine in solution

Zinc phthalocyanine (ZnPc) is shown to form charge transfer complex with gold nanoparticles (AuNps) in toluene. Significant enhancement in quantum yield of charge-separation is observed for ZnPc-AuNp conjugate. Role of AuNp over the non-covalent interaction between ZnPc and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) is studied by various spectroscopic tools. The strongest binding between PC70BM and ZnPc is confirmed from the evaluation of binding constant (K) as well as estimation of heat of formation value for this complex obtained from theoretical calculations. Both dynamic light scattering and transmission electron microscope measurements reveal formation of particles ranging in smaller dimension.

New photophysical insights on effect of gold nanoparticles on the interaction between phthalocyanine and PC70BM in solution

Photoinduced processes in phthalocyanine/functionalized gold nanoparticles (Pc/AuNps) have been investigated by spectroscopic measurements. It is observed while AuNps reduce the magnitude of binding constant for the non-covalent complexation between [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) and H2-Pc by 15.8 times in comparison to PC70BM/ZnPc system in toluene, fluorescence of ZnPc is strongly quenched in presence of AuNp. Fluorescence lifetime determined by time-correlated single photon counting is also strongly reduced for ZnPc in presence of AuNp compared to H2-Pc. Symbolic enhancement in quantum yield of charge-separation associated with well-defined electrostatic interaction has been confirmed for PC70BM/ZnPc supramolecule in presence of AuNp. Transient absorption measurements establish that energy transfer mechanism is operative for both PC70BM/H2-Pc and PC70BM/ZnPc supramolecules in absence and presence of AuNp in toluene.

The effect of sediment mimicking drill cuttings on deep water rhodoliths in a flow-through system: Experimental work and modeling

The impact of sediment coverage on two rhodolith-forming calcareous algae species collected at 100m water depth off the coast of Brazil was studied in an experimental flow-through system. Natural sediment mimicking drill cuttings with respect to size distribution was used. Sediment coverage and photosynthetic efficiency (maximum quantum yield of charge separation in photosystem II, ϕPSIImax) were measured as functions of light intensity, flow rate and added amount of sediment once a week for nine weeks. Statistical experimental design and multivariate data analysis provided statistically significant regression models which subsequently were used to establish exposure–response relationship for photosynthetic efficiency as function of sediment coverage. For example, at 70% sediment coverage the photosynthetic efficiency was reduced 50% after 1–2weeks of exposure, most likely due to reduced gas exchange. The exposure–response relationship can be used to establish threshold levels and impact categories for environmental monitoring.

Action spectra of photosystems II and I and quantum yield of photosynthesis in leaves in State 1

The spectral global quantum yield (YII, electrons/photons absorbed) of photosystem II (PSII) was measured in sunflower leaves in State 1 using monochromatic light. The global quantum yield of PSI (YI) was measured using low-intensity monochromatic light flashes and the associated transmittance change at 810nm. The 810-nm signal change was calibrated based on the number of electrons generated by PSII during the flash (4·O2 evolution) which arrived at the PSI donor side after a delay of 2ms. The intrinsic quantum yield of PSI (yI, electrons per photon absorbed by PSI) was measured at 712nm, where photon absorption by PSII was small. The results were used to resolve the individual spectra of the excitation partitioning coefficients between PSI (aI) and PSII (aII) in leaves. For comparison, pigment–protein complexes for PSII and PSI were isolated, separated by sucrose density ultracentrifugation, and their optical density was measured. A good correlation was obtained for the spectral excitation partitioning coefficients measured by these different methods. The intrinsic yield of PSI was high (yI=0.88), but it absorbed only about 1/3 of quanta; consequently, about 2/3 of quanta were absorbed by PSII, but processed with the low intrinsic yield yII=0.63. In PSII, the quantum yield of charge separation was 0.89 as detected by variable fluorescence Fv/Fm, but 29% of separated charges recombined (Laisk A, Eichelmann H and Oja V, Photosynth. Res. 113, 145–155). At wavelengths less than 580nm about 30% of excitation is absorbed by pigments poorly connected to either photosystem, most likely carotenoids bound in pigment–protein complexes.

Electronic structure of the Mn-cofactor of modified bacterial reaction centers measured by electron paramagnetic resonance and electron spin echo envelope modulation spectroscopies

The electronic structure of a Mn(II) ion bound to highly oxidizing reaction centers of Rhodobacter sphaeroides was studied in a mutant modified to possess a metal binding site at a location comparable to the Mn4Ca cluster of photosystem II. The Mn-binding site of the previously described mutant, M2, contains three carboxylates and one His at the binding site (Thielges et al., Biochemistry 44:389-7394, 2005). The redox-active Mn-cofactor was characterized using electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) spectroscopies. In the light without bound metal, the Mn-binding mutants showed an EPR spectrum characteristic of the oxidized bacteriochlorophyll dimer and reduced quinone whose intensity was significantly reduced due to the diminished quantum yield of charge separation in the mutant compared to wild type. In the presence of the metal and in the dark, the EPR spectrum measured at the X-band frequency of 9.4 GHz showed a distinctive spin 5/2 Mn(II) signal consisting of 16 lines associated with both allowed and forbidden transitions. Upon illumination, the amplitude of the spectrum is decreased by over 80 % due to oxidation of the metal upon electron transfer to the oxidized bacteriochlorophyll dimer. The EPR spectrum of the Mn-cofactor was also measured at the Q-band frequency of 34 GHz and was better resolved as the signal was composed of the six allowed electronic transitions with only minor contributions from other transitions. A fit of the Q-band EPR spectrum shows that the Mn-cofactor is a high spin Mn(II) species (S = 5/2) that is six-coordinated with an isotropic g-value of 2.0006, a weak zero-field splitting and E/D ratio of approximately 1/3. The ESEEM experiments showed the presence of one (14)N coordinating the Mn-cofactor. The nitrogen atom is assigned to a His by comparing our ESEEM results to those previously reported for Mn(II) ions bound to other proteins and on the basis of the X-ray structure of the M2 mutant that shows the presence of only one His, residue M193, that can coordinate the Mn-cofactor. Together, the data allow the electronic structure and coordination environment of the designed Mn-cofactor in the modified reaction centers to be characterized in detail and compared to those observed in other proteins with Mn-cofactors.

Quantum Yield of Charge Separation in Photosystem II: Functional Effect of Changes in the Antenna Size upon Light Acclimation

We have studied thylakoid membranes of Arabidopsis thaliana acclimated to different light conditions and have related protein composition to excitation energy transfer and trapping kinetics in Photosystem II (PSII). In high light: the plants have reduced amounts of the antenna complexes LHCII and CP24, the overall trapping time of PSII is only ∼180 ps, and the quantum efficiency reaches a value of 91%. In low light: LHCII is upregulated, the PSII lifetime becomes ∼310 ps, and the efficiency decreases to 84%. This difference is largely caused by slower excitation energy migration to the reaction centers in low-light plants due to the LHCII trimers that are not part of the C2S2M2 supercomplex. This pool of "extra" LHCII normally transfers energy to both photosystems, whereas it transfers only to PSII upon far-red light treatment (state 1). It is shown that in high light the reduction of LHCII mainly concerns the LHCII-M trimers, while the pool of "extra" LHCII remains intact and state transitions continue to occur. The obtained values for the efficiency of PSII are compared with the values of Fv/Fm, a parameter that is widely used to indicate the PSII quantum efficiency, and the observed differences are discussed.

Photonic Modulation of Electron Transfer with Switchable Phase Inversion

Photochromes may be reversibly photoisomerized between two metastable states and their properties can influence, and be influenced by, other chromophores in the same molecule through energy or electron transfer. In the photochemically active molecular tetrad described here, a porphyrin has been covalently linked to a fullerene electron acceptor, a quinoline-derived dihydroindolizine photochrome, and a dithienylethene photochrome. The porphyrin first excited singlet state undergoes photoinduced electron transfer to the fullerene to generate a charge-separated state. The quantum yield of charge separation is modulated by the two photochromes: one isomer of each quenches the porphyrin excited state, reducing the quantum yield of electron transfer to near zero. Interestingly, when the molecule is illuminated with white light, the quantum yield decreases as the white light intensity is increased, generating an out-of-phase response of the quantum yield to white light. However, when the same experiment is performed in the presence of additional, steady-state UV illumination, a phase inversion occurs. The quantum yield of electron transfer now increases with increasing white light intensity. Such effects illustrate emergent complexity in a relatively simple system and could find applications in molecular logic, photochemical labeling and drug delivery, and photoprotection for artificial photosynthetic molecules. The photochemistry leading to this behavior is discussed.

Photoinduced energy and charge transfer in a p-phenylene-linked dyad of boron dipyrromethene and monostyryl boron dipyrromethene

Boron dipyrromethenes (BDPs) are excellent building blocks for design of artificial light harvesting and charge separation systems. In the present work, we report the results of photophysical studies of a novel dyad, in which a BDP and a mono-styryl BDP (MSBDP) are covalently linked to each other at the meso-position via a p-phenylene unit. It was found that the photophysical properties of the dyad dissolved in polar as well as nonpolar solvents are strongly affected by two different types of interactions between the BDP and MSBDP parts, namely excitation energy transfer and photoinduced electron transfer. The first process delivers the excitation energy to the first excited singlet state of the MSBDP-part upon excitation of the BDP unit. The direct or indirect (via excitation energy transfer) population of the first excited singlet state of the MSBDP moiety is followed by hole transfer to generate the charge-separated state. In non-polar toluene, the probability of charge separation is low, whereas in polar acetonitrile the charge separation quantum yield is close to unity, resulting in strong quenching of the MSBDP fluorescence.

Dynamics and Efficiency of Hole Transport in LNA:DNA Hybrid Diblock Oligomers

We report here the effect of replacing one or both of the purine or pyrimidine blocks of a diblock stilbene donor-acceptor capped hairpin with locked nucleic acid (LNA) bases on the dynamics and efficiency of hole transport. The structures of the DNA and LNA:DNA hybrids are tentatively assigned to B- or A-type structures on the basis of their circular dichroism spectra. Replacing the bases in either the A-block or the G-block of the diblock DNA hairpin with LNA bases results in a modest decrease in the base-to-base hopping rate constant and quantum yield for charge separation. Somewhat larger decreases are observed when all of the purine or pyrimidine bases are replaced by LNA bases.

Optimization of variable fluorescence measurements of phytoplankton communities with cyanobacteria

Excitation–emission fluorescence matrices of phytoplankton communities were simulated from laboratory-grown algae and cyanobacteria cultures, to define the optical configurations of theoretical fluorometers that either minimize or maximize the representation of these phytoplankton groups in community variable fluorescence measurements. Excitation sources that match the photosystem II (PSII) action spectrum of cyanobacteria do not necessarily lead to equal representation of cyanobacteria in community fluorescence. In communities with an equal share of algae and cyanobacteria, inducible PSII fluorescence in algae can be retrieved from community fluorescence under blue excitation (450–470 nm) with high accuracy (R 2 = 1.00). The highest correlation between community and cyanobacterial variable fluorescence is obtained under orange-red excitation in the 590–650 nm range (R 2 = 0.54). Gaussian band decomposition reveals that in the presence of cyanobacteria, the emission detection slit must be narrow (up to 10 nm) and centred on PSII chlorophyll-a emission (~683 nm) to avoid severe dampening of the signal by weakly variable phycobilisomal fluorescence and non-variable photosystem I fluorescence. When these optimizations of the optical configuration of the fluorometer are followed, both cyanobacterial and algal cultures in nutrient replete exponential growth exhibit values of the maximum quantum yield of charge separation in PSII in the range of 0.65–0.7.