ON THE NATURE OF THE INTERMEDIATE ELECTRON ACCEPTOR (A1) IN THE PHOTOSYSTEM I REACTION CENTER

Abstract— A sodium dodecyl sulfate-Photosystem I (PSI) complex has been prepared and characterized with respect to its electron acceptors. Component X and iron-sulfur centers A and B are absent from this preparation but the intermediate electron acceptor (A₁) is present. Flash-induced absorbance changes at 25°C show charge separation, followed by a back-reaction with a half-time of 5 µs. The spectrum of the flash-induced change from 350 to 550 nm indicates a contribution from the intermediate electron acceptor, A₁, as well as from P700⁺. EPR studies show that A₁ is associated with a free-radical signal having a g-value of 2.0025 and a linewidth of 12 gauss. A, would appear to be associated with a monomeric form of either Chi α or pheophytin a.     

THE USE OF LIPID HEADGROUP CHARGE TO ENHANCE PHOTOINDUCED ELECTRON TRANSFER IN VESICLES: REACTIONS OF FUNCTIONALIZED PYRENE WITH A TWO COMPONENT VIOLOGEN SYSTEM

Abstract—
Kinetics of photoinduced electron transfer from a lipid functionalized pyrene, 1-(10-(6(8)-octadecylpyrenyl)decanoyl)-2-hexanoyl-.sn-glycero-3-phosphorylcholine (OPyPC), to a two component viologen acceptor system have been measured by laser flash photolysis. N, N'-tetramethylene-2,2'-bi-pyridinium ion (DQ²⁺) and N, N' -dipropyl-4,4'-bipyridinium sulfonate (PVS), have been utilized as the primary and secondary acceptors. It has been shown that utilization of a lipid with a net negatively charged phosphatidylglycerol headgroup provides a driving force for localizing high concentrations of primary acceptor (DQ²⁺) in the region of donor. Subsequently, the charged interface can act to maintain long-term separation between the oxidized pyrene donor (OPyPC⁺) and the reduced secondary acceptor, PVS^-. When a dioleoyl lipid is used, reaction of (OPyPC⁺) with the double bond competes significantly with back reaction. However, substitution of diphytylphosphatidylglycerol for the dioleoyl analog results in a cation lifetime of about 0.5 ms and a continued very long-lived reduced species (˜4 h). Quantum yields of ˜0.15 may be obtained in this system.     

An Electron‐Deficient Building Block Based on the B←N Unit: An Electron Acceptor for All‐Polymer Solar Cells

A double B←N bridged bipyridyl (BNBP) is a novel electron‐deficient building block for polymer electron acceptors in all‐polymer solar cells. The B←N bridging units endow BNBP with fixed planar configuration and low‐lying LUMO/HOMO energy levels. As a result, the polymer based on BNBP units (P‐BNBP‐T) exhibits high electron mobility, low‐lying LUMO/HOMO energy levels, and strong absorbance in the visible region, which is desirable for polymer electron acceptors. Preliminary all‐polymer solar cell (all‐PSC) devices with P‐BNBP‐T as the electron acceptor and PTB7 as the electron donor exhibit a power conversion efficiency (PCE) of 3.38 %, which is among the highest values of all‐PSCs with PTB7 as the electron donor.     

Conjugated polymers containing B←N unit as electron acceptors for all-polymer solar cells

Polymer electron acceptors are the key materials in all-polymer solar cells(all-PSCs).In this review,we focused on introducing the principle of boron-nitrogen coordination bond(B←N),and summarizing our recent research on polymer electron acceptors containing B←N unit for efficient all-PSC devices.Two approaches have been reported to design polymer electron acceptors using B←N unit.One is to replace a C-C unit by a B←N unit in conjugated polymers to transform a polymer electron donor to a polymer electron acceptor.The other approach is to construct novel electron-deficient building block based on B←N unit for polymer electron acceptors.The polymer electron acceptors containing B←N unit showed tunable lowest unoccupied molecular orbital(LUMO) energy levels and exhibited excellent all-PSC device performance with power conversion efficiency of exceeding6%.These results indicate that organic boron chemistry is a new toolbox to develop functional polymer materials for optoelectronic device applications.     

PHOTOOXIDATION AND ELECTRON-TRANSFER PROCESSES IN FREE-BASE TETRAPHENYLPORPHIN PROBED BY RESONANCE RAMAN SPECTROSCOPY

Abstract
We report here the resonance Raman studies of photooxidation of free base tetraphenylporphin (H₂TPP) in the presence of external electron acceptors such as CCl₄ and chloranil under selective laser irradiation. From the dependence of photooxidation on the concentration of electron acceptors, polarity of solvents, excitation lines and temperatures, we have inferred that a weak triplet exciplex formed between the excited H₂TPP and electron acceptor in non-polar solvents serves as transient species and the light-induced intermolecular charge transfer from H₂TPP to the electron acceptor is the primary process involved in photooxidation. Observation of partial photooxidation in the rigid matrix at low temperatures has been interpreted to be due to long-range quantum mechanical electron tunneling process. Almost complete photooxidation is observed in a soft matrix as the donor and acceptor molecules can attain favorable relative orientation and separation for electron transfer during the excited state lifetime of the exciplex.     

PHOTOOXIDATION AND ELECTRON-TRANSFER PROCESSES IN FREE-BASE TETRAPHENYLPORPHIN PROBED BY RESONANCE RAMAN SPECTROSCOPY

Abstract —We report here the resonance Raman studies of photooxidation of free base tetraphenylporphin (H₂TPP) in the presence of external electron acceptors such as CCl₄ and chloranil under selective laser irradiation. From the dependence of photooxidation on the concentration of electron acceptors, polarity of solvents, excitation lines and temperatures, we have inferred that a weak triplet exciplex formed between the excited H₂TPP and electron acceptor in non-polar solvents serves as transient species and the light-induced intermolecular charge transfer from H₂TPP to the electron acceptor is the primary process involved in photooxidation. Observation of partial photooxidation in the rigid matrix at low temperatures has been interpreted to be due to long-range quantum mechanical electron tunneling process. Almost complete photooxidation is observed in a soft matrix as the donor and acceptor molecules can attain favorable relative orientation and separation for electron transfer during the excited state lifetime of the exciplex.     

SYNTHESIS OF AMPHIPATHIC PORPHYRINS AND THEIR PHOTOINDUCED ELECTRON TRANSFER REACTIONS AT THE LIPID BILAYER-WATER INTERFACE

A one flask synthesis of cis -substituted amphipathic porphyrins is reported. These porphyrins were used to study electrostatic effects on photoinduced electron transfer across the lipid bilayer-water interface. A neutral porphyrin undergoes only dynamic interfacial electron transfer reactions irrespective of charge of the acceptor, although ionic strength effects indicate a negative charge on the porphyrin donor species. A dianionic porphyrin forms an interfacial static complex with a dicationic electron acceptor, methyl viologen, at low ionic strength. The electron transfer rate within the complex is slow, 10⁵∼ 10⁶ s^-1, which is attributed to a near orthogonal orientation between the donor and the acceptor ∼ orbitals.     

PHOTOPHYSICS AND IONIC PHOTODISSOCIATION OF POLYESTERS WITH PENDANT 1-PYRENYL GROUPS IN SOLUTION

Primary photoprocesses of polyesters with pendant pyrenyl groups in solution were studied by means of nanosecond laser flash photolysis method. The intrapolymer process of the S_l S_l annihilation was observed. Intense laser excitation brings about dense population of several fluorescent states in a polymer chain, and their mutual interaction leads to efficient deactivation. This process is confirmed by excitation intensity dependence of fluorescence spectra and fluorescence intensity. It was also confirmed that the T-T absorption intensity decreases as the degree of polymerization increases. This low formation yield of the triplet state in the polyesters is ascribed mainly to the above S_l S_l interaction. In addition the polymer effect on the ionic photodissociation yield of the polyester-electron acceptor was investigated. The yield decreases as the degree of polymerization increases. This effect is due to slow solvation processes, which may be due to low micropolarity and high microviscosily around the polymer chain.     

INTERACTION OP POLYVINYL ALCOHOL WITH LECITHIN AT THE AIR/WATER AND OIL/WATER INTERFACES AND ITS EFFECT ON EMULSION STABILITY

The interaction of polyvinyl alcohol (PVA) with lecithin films at the air/water and xylene/water interfaces was studied at pH 3, the isoelectric point of lecithin. Surface and interfacial tension measurements and electro-phoretie mobility data showed that PVA either replaces the lecithin molecules or becomes coadsorbed with it at the interface. This mixed film has no elasticity and very low interfacial viscosity. However, coalescence rate measurements showed that the emulsion formed by the polymer* lecithin is more stable than that formed with either polymer or lecithin alone. The enhanced stability by the PVA-lecithin mixed film was attributed to steric interaction produced by the adsorbed polymer.     

Polymer Acceptor Based on B←N Units with Enhanced Electron Mobility for Efficient All‐Polymer Solar Cells

We demonstrate that polymer electron acceptors with excellent all‐polymer solar‐cell (all‐PSC) device performance can be developed from polymer electron donors by using B←N units. By alleviating the steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain B←N units, the π–π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all‐PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all‐PSCs with state‐of‐the‐art polymer acceptors.     

COPPER QUENCHING OF THE VARIABLE FLUORESCENCE IN Dunaliella tertiolecta. NEW EVIDENCE FOR A COPPER INHIBITION EFFECT ON PSII PHOTOCHEMISTRY

Abstract— The copper quenching effect on fluorescence in Dunaliella tertiolecta was studied. 30% of variable fluorescence was quenched in the presence of 70 μ,M CuS0₄. We confirmed that the copper inhibitory effect on photosystem II (PSII) activity is located on its oxidizing side. Further, we indicate that the complementary area is decreased by copper. Since the quantum yield of PSII photochemistry was lowered and the rate of PSII primary electron acceptor Q_A remained unaffected, we can conclude that some PSII reaction centers were inactivated by copper.     

Enhanced Charge Transfer,Transport and Photovoltaic Efficiency in All‐Polymer Organic Solar Cells by Polymer Backbone Fluorination

We successfully designed and synthesized two BDT‐BT‐T (BDT=benzo[1,2‐b:4,5‐b']dithiophene, BT‐T=4,7‐dithien‐2‐yl‐2,1,3‐benzothiadiazole) based polymers as the electron donor for application in all‐polymer solar cells (all‐PSCs). By adopting N2200 as the electron acceptor, we systematically investigated the impact of fluorination on the charge transfer, transport, blend morphology and photovoltaic properties of the relevant all‐PSCs. A best power conversion efficiency (PCE) of 3.4% was obtained for fluorinated PT‐BT2F/N2200 (BT2F=difluorobenzo[c][1,2,5]thiadiazole) all‐PSCs in comparison with that of 2.7% in non‐fluorinated PT‐BT/N2200 (BT=benzothiadiazole) based device. Herein, all‐polymers blends adopting either non‐fluorinated PT‐BT or fluorinated PT‐BT2F exhibit similar morphology features. In depth optical spectrum measurements demonstrate that molecular fluorination can further enhance charge transfer between donor and acceptor polymer. Moreover, all‐polymer blends exhibit improved hole mobilities and more balanced carriers transport when adopting fluorinated donor polymer PT‐BT2F. Therefore, although the PCE is relatively low, our findings may become important in understanding how subtle changes in molecular structure impact relevant optoelectronic properties and further improve the performance of all‐PSCSs.     

靛蓝衍生物的电子转移研究

合成了两个N,N′-取代的靛蓝染料,反式-N,N′-双对硝基苯甲酰基靛蓝(1)和反式N,N′-双间硝基苯甲酰基靛蓝(Ⅱ)。研究了这两个染料与电子给体(胺类化合物)或与电子受体(紫精化合物)之间的电子转移。结果表明,这两个染料在激发单重态经荧光猝灭,被紫精猝灭的速度常数(k_q)比被胺猝灭的速度常数大。     

PHOTOSYNTHETIC NITRITE REDUCTION BY DITHIOERYTHRITOL AND THE EFFECT OF NITRITE ON ELECTRON TRANSPORT IN ISOLATED CHLOROPLASTS

Abstract. Photosynthetic reduction of nitrite to ammonia with type C chloroplasts from the heterocont alga Bumilleriopsis filiformis was investigated using 3,6-diaminodurene/ascorbate and 3,6-diaminodurene/dithioerythritol (DAD/DTE) as electron donor couple. Rates approach 6–10 μmol NO^-₂ reduced/mg chlorophyll/h and are steady for up to 30 min. The presence of oxygen or NADP⁺ only slightly diminished the rates of nitrite reduction obtained with DAD/DTE. Illuminated chloroplasts reduce oxygen in the presence of DAD/DTE at 135 μmol/mg chlorophyll/h without acceptor supplied. Photosynthetic oxygen uptake by this system in the presence of ferredoxin and NO^-₂, however, is inhibited to 42% by nitrite reductase with concurrent nitrite reduction. NO^-₃ and NO^-₂ have no effect on photosystem I-mediated NADP⁺ reduction, NO^-₂ (10 m M ) inhibits ferricyanide-mediated oxygen evolution to 72%. Also photosystem II reactions assayed e.g. with silicomolybdate are inhibited significantly by NO^-₂ (1 m M ), but only slightly by NO^-₃. Nitrite reductase is inhibited by p -chloromercuribenzoate ( p CMB), and this inhibition is prevented by DTE. Results suggest that photosynthetic nitrite reduction can cope with low concentrations of either compound, provided relevant thiol groups are protected.     

AN FMN-PHOTOSYSTEM I PHOTOVOLTAIC CELL

Abstract— We have made a photovoltaic cell using Photosystem I subchloroplast particles isolated according to the method of Shiozawa et al. (1974). The particles were placed on a filter between two compartments one of which contained the electron donor, K₄Fe(CN)₆ and the other the electron acceptor, FMN. Upon illumination with white light ( I = 80 W/m²) a potential of 300 mV was observed across a 3000 Ω load resistance. Both Photosystem I photochemistry and direct photoreactions of FMN contribute to the process. A power output of 20 μW was observed for a 2 cm² filter containing 60 μg chlorophyll. This corresponds to 0.1 W/m². The power efficiency was 0.13%. The short circuit current was 108 μA.     

SPECTRAL FEATURES OF THE BOUND ELECTRON ACCEPTOR A2 OF PHOTOSYSTEM I

Abstract— Difference spectrum for the reduction of A₂, a bound secondary electron acceptor of photo-system I, in the thylakoid membranes of a thermophilic blue-green alga, Synechococcus sp., was determined by subtracting the difference spectrum of P700 photooxidation from the difference spectrum for flash-induced absorption changes due to oxidation of P700 and reduction of A₂, or by measuring light-induced absorption changes under reducing conditions where reduced A₂ accumulates. The spectrum showing a broad bleaching with two maxima at 420 and 440 nm indicates that A₂ is an iron-sulfur center different from P430.     

The mechanism of perylene photo-oxidation in a water-soluble polymeric photocatalyst

The photo-oxidation of perylene in aqueous solutions of a polymeric photocatalyst was investigated to probe the mechanism of polycyclic aromatic hydrocarbon degradation. Perylene and other hydrophobic molecules are efficiently solubilized in aqueous polymer solutions with distribution coefficients as high as 4 x 10⁶. The rate of perylene photo-oxidation was much more rapid in aqueous polymer solutions than in organic solvents. In organic solvents, ¹0₂ sensitizers (rose bengal) had little effect on the reaction, but electron acceptors, such as dicyanobenzene, caused an acceleration in rate. Naphthoquinone was suggested as a potential electron acceptor in the naphthalene-containing polymer, and it was shown to be formed in small concentrations by polymer oxidation. It was concluded that the polymer plays several key functions in perylene photo-oxidation: (1) solubilization of the hydrophobic molecule; (2) energy migration through the polymer coil and energy transfer, providing additional photochemical energy to the reactants; (3) the enhancement of oxidation by photoinduced electron transfer via provision of an electron acceptor and facilitation of charge separation.     

ELECTRON AND ENERGY TRANSFER IN ILLUMINATED POROUS VYCOR GLASS

Abstract— Porous Vycor glass samples containing adsorbed molecules were illuminated at 77 K by a mercury lamp jacketed by a filter cutting off wavelengths below 250 nm. Oxygen or carbon dioxide on Vycor produces an asymmetric electron paramagnetic resonance (EPR) signal best described as holes trapped in the glass. Methyl bromide produces an identical EPR signal plus four other lines due to methyl radicals. Evidence is presented that the products result from excitonic energy transfer from the Vycor to the adsorbed materials. Triphenylamine (TPA) adsorbed on Vycor can also be photoionized by similar illumination, and the cation radical TPA⁺ can be stabilized at 77 K if an electron acceptor is also adsorbed. Attachment of the photoejected electron by carbon dioxide forms CO₂^-, and that by methyl bromide leads to methyl radicals. The CH₃ radical yield is dependent on the surface separation between the electron donor (TPA) and the acceptor (CH₃Br). By monitoring the relative quantum yield of the methyl radicals as a function of distance separating the TPA and CH₃Br, it is shown that the photoelectron is capable of migrating on the Vycor glass surface.     

2H NMR studies of phase behaviour and molecular motions of doped discotic liquid-crystalline systems

A discotic triphenylene monomer as well as a dimer and a main chain polymer, all substituted with heptyloxy side groups, were doped with an electron acceptor (2,4,7-trinitrofluorenone (TNF)) to give charge transfer complexes. These doped systems were aligned in a magnetic field, thus proving their liquid crystallinity. ²H NMR measurements show that the electron acceptor molecules are incorporated into the columns built of triphenylene cores. In the charge transfer complex with the triphenylene monomer almost all the electron acceptor molecules stack in the columns even close to the clearing temperature T₁, while for the dimer and especially for the polymer a significant fraction of the TNF molecules exhibits isotropic motion, which is attributed to their location in the region between the columns, already way below T₁. This isotropically distributed part increases on approaching T₁. Fast rotation of the discs around their column axes takes place in the monomer and is quenched in the dimer and the polymer, due to the interlinking of the columns by the spacer. The electron acceptor molecules, on the other hand, exhibit free rotation in all samples, even in the charge transfer complexes with the triphenylene dimer and polymer. In the side group labelled triphenylenes increased ²H NMR spectral narrowing is detected in the charge transfer complex samples compared with the corresponding pure triphenylenes. This indicates higher side group mobility in the doped systems, since the incorporation of TNF molecules increases the spacing between the discotic units.     

AFFINITY FOR OXYGEN IN PHOTOREDUCTION OF MOLECULAR OXYGEN AND SCAVENGING OF HYDROGEN PEROXIDE IN SPINACH CHLOROPLASTS

Abstract— The apparent K _m for O₂ in the photoreduction of molecular oxygen by spinach class II chloroplasts and photosystem I subchloroplast fragments was determined. In both cases, a value of 2 ∼ 3 μ M O₂ was obtained. The reaction rate constant between O₂ and P-430, the primary electron acceptor of PS I, is estimated to be ∼ 1.5 × 10⁷ M ^-1 s^-1 and the factors affecting the production of superoxide by the photoreduction of O₂ in chloroplasts are discussed. Preliminary evidence is presented indicating the occurrence of an azide-insensitive scavenging system for H₂O₂ in chloroplast stroma.