CHLOROPHYLL-PHOTOSENSITIZED OXIDATION OF HYDROQUINONE AND p-PHENYLENEDIAMINE DERIVATIVES

Abstract— ESR and photovoltaic studies on light-induced one-electron transfer between chlorophyll a and electron donors in the absence of oxygen show (1) the possible conversion of photo-reduced chlorophyll a and p-benzosemiquinone ion radicals to their non-ionic radicals in methanol solutions of low pH, (2) the production of ESR absorption of tetrachloro-p-benzosemi-quinone even in benzene, enhanced by the addition of triethylamine or methanol, and (3) the transfer of one electron from tetramethyl-p-phenylenediamine to either excited chlorophyll a or pheophytin a in methanol at pH above 3.6 but not to pheophytin a at pH below 1 0 where its radical cation appears to accept an electron from excited pheophytin a . Bacteteriochloro-phyll is also shown to be capable of photooxidizing hydroquinones and tetramethyl-p-phenyl-enediamine.
The presence of oxygen enhances chlorophyll a -photosensitized oxidation of hydroquinone and tetrachloro-hydroquinone by one-electron transfer to oxygen and of trimethylhydro-quinone probably by two-electron trnasfer to oxygen. A free radical from excited chlorophyll a-oxygen interaction is formed in these reactions, but rapidly quenched in the case of trimethyl-hydroquinone. This, kind of free radical is not formed in pheophytin a . Tetramethyl- p -phenyl-enediamine readily undergoes chlorophyll a-photosensitized oxidation by oxygen in any pH region.     

Charge transport in photofunctional nanoparticles self-assembled from zinc 5,10,15,20-tetrakis(perylenediimide)porphyrin building blocks

Molecules designed to carry out photochemical energy conversion typically employ several sequential electron transfers, as do photosynthetic proteins. Yet, these molecules typically do not achieve the extensive charge transport characteristic of semiconductor devices. We have prepared a large molecule in which four perylene-3,4:9,10-tetracarboxydiimide (PDI) molecules that both collect photons and accept electrons are attached to a central zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP) electron donor. This molecule self-assembles into ordered nanoparticles both in solution and in the solid-state, driven by van der Waals stacking of the PDI molecules. Photoexcitation of the nanoparticles results in quantitative charge separation in 3.2 ps to form ZnTPP(+)PDI(-) radical ion pairs, in which the radical anion rapidly migrates to PDI molecules that are, on average, 21 A away, as evidenced by magnetic field effects on the yield of the PDI triplet state that results from radical ion pair recombination. These nanoparticles exhibit charge transport properties that combine important features from both photosynthetic and semiconductor photoconversion systems.     

Photoinduced electron transfer studies of Nile red in the presence of TiO2 colloidal nanoparticles

Photoinduced electron transfers between Nile red (NR) with TiO2 colloidal nanoparticles are studied using picosecond transient absorption and time resolved fluorescence spectroscopy. The dynamics of electron transfer from the dye molecule to the semiconductor were understood from the transient, and also the formation of conduction band electron and Nile red cation radical were detected.     

QUINONES AS MEDIATORS OF ELECTRON TRANSFER FROM MEMBRANE-BOUND CHLOROPHYLL TO CYTOCHROME c IN THE AQUEOUS PHASE IN LIPID BILAYER VESICLES: SYNERGISTIC ENHANCEMENT OF CYTOCHROME c REDUCTION BY LIPOPHILIC/ HYDROPHILIC QUINONE PAIRS

Laser flash photolysis has been used to determine the kinetics of cytochrome c reduction by chlorophyll triplet state in negatively-charged lipid bilayer vesicles, as mediated by quinones. Large synergistic enhancements in the yield of reduced cytochrome were obtained using a pair of quinones, one of which was lipophilic (e.g. benzoquinone, 2,6-di-f-butylbenzoquinone) and the other of which was hydrophilic (e.g. l,2-naphthoquinone-4-sulfonate). The mechanism was shown to involve initial quenching of the triplet by the membrane-associated quinone to form chlorophyll cation radical and quinone anion radical. An interquinone electron transfer process followed this reaction, which occurred at the membrane-water interface, and greatly facilitated electron transport from within the bilayer to the aqueous phase. This process formed the basis of the synergistic effect. Cytochrome c reduction occurred in the water phase by reaction with the anion radical of the hydrophilic quinone. Finally, the reduced cytochrome was reoxidized by a slow reaction with chlorophyll cation radical. Under the most favorable conditions, we estimate that the quantum yield of conversion of triplet quenching events to reduction of cytochrome approached unity. The lifetime of the reduced protein and oxidized chlorophyll could be as long as 140 ms, under the best conditions. This system has properties which are thus quite favorable for solar energy conversion in a biomimetic process.     

The effect of axial Mg ligation on the geometry and spin density distribution of chlorophyll and bacteriochlorophyll cation free radical models: a density functional study.

Density functional calculations are performed on models of chlorophyll and bacteriochlorophyll to examine the effect of Mg ligation on the geometry and spin density distribution of the cation free radicals formed. It is shown that, whereas the properties of the bacteriochlorophyll model can be explained on the basis of the electron density distribution of the highest occupied molecular orbital (HOMO), for the chlorophyll model the geometry and spin density properties of the ligated species do not follow this trend. For the ligated chlorophyll models it is shown that, due to the closeness in energy of the HOMO and HOMO-1 orbitals, a Jahn-Teller distortion occurs on one-electron oxidation, leading to an admixed hybrid orbital for the cation radical form. Orbital mixing is shown to lead to significant changes in the geometry and spin density distribution of the cation free radical formed. It is also shown that orbital mixing does not lead to an increase in the magnitude of the (14)N hyperfine couplings thereby invalidating reports in the literature which have dismissed mixed orbital states for the primary donor cation radicals of photosynthetic reaction centers based on this criterion.     

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

Dibenzo[a,e]pentalene (DBP) is a non-alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP-phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift-calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP-phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.     

Functional in situ evaluation of photosynthesis-protecting carotenoids in mutants of the cyanobacterium Synechocystis PCC6803

Cyanobacteria possess different carotenoids as scavengers of reactive oxygen species. In Synechocystis PCC6803, zeaxanthin, echinenone, beta-carotene and myxoxanthophyll are synthesized. By disruption of the ketolase and hydroxylase genes, it was possible to obtain mutants devoid of either zeaxanthin, echinenone, or a combination of both carotenoids. With these mutants, their function in protecting photosynthetic electron transport under high light stress as well as chlorophyll and carotenoid degradation after initiation of singlet oxygen or radical formation was analyzed. Wild type Synechocystis is very well protected against high light-mediated photooxidation. Absence of echinenone affects photosynthetic electron transport to only a small extent. However, complete depletion of zeaxanthin together with a modification of myxoxanthophyll resulted in strong photoinhibition of overall photosynthetic electron transport as well as the photosystem II reaction. In the double mutant lacking both carotenoids the effects were additive. The light saturation curves of photosynthetic electron transport of the high light-treated mutants exhibited not only a lower saturation value but also smaller slopes. Using methylviologen or methylene blue as a radical or singlet oxygen generators, respectively, massive degradation of chlorophyll and carotenoids, indicative of photooxidative destruction of the photosynthetic apparatus, was observed, especially in the mutants devoid of zeaxanthin.     

CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID VESICLE SYSTEMS: COMPARISON OF INSIDE-OUTSIDE ASYMMETRY BETWEEN LARGE AND SMALL UNILAMELLAR VESICLES*

Abstract— We have determined the chlorophyll triplet quenching efficiencies, the chlorophyll cation radical yields and the conversion efficiencies of chlorophyll triplet to radical in large and small unilamellar phosphatidylcholine vesicles (LUV and SUV, respectively) in the presence of electrically-charged electron acceptors (ferricyanide and oxidized cytochrome c) located in either the inner or outer aqueous compartments of the vesicles. Both types of vesicles displayed inside-outside asymmetry, although the properties were reversed. Triplet quenching in SUV was more efficient when ferricyanide was located within the vesicle interior, whereas the reverse was true in LUV. When ferricyanide was located on the outside of the vesicles, the extent of triplet quenching in LUV was about two times that in SUV and the amount of cation radical formed in LUV was about two times that in SUV. Under these conditions, the conversion efficiencies of chlorophyll triplet to radical were 12.2% for LUV and 8.5% for SUV. With cytochrome c as an electron acceptor in negatively charged vesicles (25 mol per cent dixhexadecylphosphate incorporated) similar results were obtained. Again, the triplet quenching and radical yield inside-outside asymmetry properties were reversed between the two types of vesicles, and radical formation efficiencies when cyt c was located outside the vesicles were higher in LUV (11.7%) than in SUV (4.2%). We conclude that the inside-outside asymmetric photochemical behavior of unilamellar phosphatidylcholine vesicles is influenced by factors in addition to the difference in radius of curvature between the inside and outside surfaces. It is suggested that transmembrane electrostatic potentials may be involved. Furthermore, in the present system the properties of LUV were more favorable to photochemical electron transfer product formation than those of SUV.     

Modern Electrochemical Aspects for the Synthesis of Value‐Added Organic Products

The use of electricity instead of stoichiometric amounts of oxidizers or reducing agents in synthesis is very appealing for economic and ecological reasons, and represents a major driving force for research efforts in this area. To use electron transfer at the electrode for a successful transformation in organic synthesis, the intermediate radical (cation/anion) has to be stabilized. Its combination with other approaches in organic chemistry or concepts of contemporary synthesis allows the establishment of powerful synthetic methods. The aim in the 21st Century will be to use as little fossil carbon as possible and, for this reason, the use of renewable sources is becoming increasingly important. The direct conversion of renewables, which have previously mainly been incinerated, is of increasing interest. This Review surveys many of the recent seminal important developments which will determine the future of this dynamic emerging field.     

Photoinduced Electron‐Transfer Oxidation of Olefins with Molecular Oxygen Sensitized by Tetrasubstituted Dimethoxybenzenes: A Non‐Singlet‐Oxygen Mechanism

α‐Methylstyrene ( 1 ) was photo‐oxidized in the presence of a series of alkylated dimethoxybenzenes as sensitizers in an oxygen‐saturated MeCN solution to afford the cleaved ketone 2 , epoxide 3 , as well as a small amount of the ene product 4 in ca. 1 : 1 : 0.04 ratio. The relative rate of conversion was well‐correlated with the fluorescence quantum yield of sensitizers. Thus, a non‐singlet‐oxygen mechanism is proposed, in which an excited sensitizer is quenched by (ground‐state) molecular oxygen to produce a sensitizer radical cation and a superoxide ion (O), the former of which oxidizes the substrate, while the latter reacts with the resulting olefin radical cation ( 1 ^{+ .}) to give the major oxidation products. Photodurability of such electron‐donating sensitizers is dramatically improved by substituting four aromatic H‐atoms in 1,4‐dimethoxybenzene with Me or fused alkyl groups, which provides us with an environmentally friendly, clean method of photochemical functionalization with molecular oxygen, alternative to the ene reaction via singlet oxygenation.     

TEMPERATURE DEPENDENCE OF PHOTOSYNTHETIC EXCITATION TRANSFER—ACTIVATIONLESS TRANSFER

Abstract— We have investigated the temperature dependence of photosynthetic excitation transfer by quantum-mechanical treatment. Our theoretical analyses predict that the temperature dependence of most excitation transfers is quite weak in the relatively high temperature range (kT 100cm^-1), whereas some excitation transfers exhibit complex temperature dependence in the low temperature range due to the environmental effect. The temperature-independency in the high temperature range results from the fact that most excitation transfers among various Chi forms characterized by different absorption peaks almost satisfy the activationless condition where the energy gap for the excitation transfer matches with the nuclear rearrangement energy. This activationless condition, which has already been noticed in the electron transfers in the reaction center, is suggested to play central roles in the efficient energy conversion in the primary stage of photosynthesis.     

Multifunctional Conjugated Polymers with Main‐Chain Donors and Side‐Chain Acceptors for Dye Sensitized Solar Cells (DSSCs) and Organic Photovoltaic Cells (OPVs)

A novel multifunctional conjugated polymer (RCP‐1) composed of an electron‐donating backbone (carbazole) and an electron‐accepting side chain (cyanoacetic acid) connected through conjugated vinylene and terthiophene has been synthesized and tested as a photosensitizer in two major molecule‐based solar cells, namely dye sensitized solar cells (DSSCs) and organic photovoltaic cells (OPVs). Promising initial results on overall power conversion efficiencies of 4.11% and 1.04% are obtained from the basic structure of DSSCs and OPVs based on RCP‐1, respectively. The well‐defined donor (D)‐acceptor (A) structure of RCP‐1 has made it possible, for the first time, to reach over 4% of power conversion efficiency in DSSCs with an organic polymer sensitizer and good operation stability.     

Aromatic Radical Anions in Neat Aromatic Hydrocarbons as Solvents. Direct evidence of through space spin-density transfer to the ligand of the counter ion

Contact ion pairs of aromatic radical anions, with a crown ether complex of potassium as cation in a neat aromatic hydrocarbon, can be obtained by reducing the aromatic hydrocarbon in which a small amount of crown ether is dissolved. The unpaired electron stays attached to one aromatic molecule during a time interval which is long on the ESR. time scale. The radicals are stabilized by ion-pair formation in the low polarity solvent. As a consequence of this stabilization, radicals of compounds with low electron affinities, e.g. mesitylene, can be prepared. Mesitylene, m-xylene, and toluene show additional hyperfine splitting in the ESR. spectra of their anion radical pairs of the order of 18 μT. The proton ENDOR. spectra have signals at the corresponding frequencies, indicating a hyperfine coupling with protons of the crown ether ligand. Using mixtures of two aromatic compounds, their relative electron affinities can be determined by studying the temperature dependence of the radical concentrations.     

PHOTO-CIDNP STUDY OF PYRIMIDINE DIMER SPLITTING I: REACTIONS INVOLVING PYRIMIDINE RADICAL CATION INTERMEDIATES

The light-induced splitting of pyrimidine dimers was studied using the electron acceptor anthraquinone-2-sulfonate (AQS) as a photosensitizer. To this end, photochemically induced dynamic nuclear polarization (photo-CIDNP) experiments were performed on a series of pyrimidine monomers and dimers. The CIDNP spectra demonstrate the existence of both the dimer radical cation, which is formed by electron transfer from the dimer to the photoexcited sensitizer AQS*, and its dissociation product, the monomer radical cation. In spectra of 1,1′-trimethylene bridged cis,syn pyrimidine dimers, polarization is observed that originates from a spin-sorting process in the dimer radical pair. This points to a relatively long lifetime of the dimer radical cation involved, which is presumably due to stabilization by the trimethylene bridge. Polarization originating from a dimer radical pair is detected in the spectrum of trans,anti (1,3-dimethyluracil) dimer as well. The spectra of the bridged pyrimidines also demonstrate the reversibility of the dissociation of dimer radical cation into monomer radical cation, which is concluded from the observation of polarization in the dimer as a result of spin sorting in the monomer radical pair.     

From a localized H3O radical to a delocalized H3O+···e- solvent-separated pair by sequential hydration

The impact of microhydration on the electronic structure and reactivity of the H(3)O moiety is investigated by ab initio calculations. In the gas phase, H(3)O is a radical with spin density localized on its hydrogen end, which is only kinetically stable and readily decomposes into a water molecule and a hydrogen atom. When solvated by a single water molecule, H(3)O preserves to a large extent its radical character, however, two water molecules are already capable to shift most of the spin density to the solvent. With three solvating water molecules this shift is practically completed and the system is best described as a solvent-separated pair of a hydronium cation and a hydrated electron. The electronic structure of this system and its proton transfer reactivity leading to formation of a hydrogen atom already resemble those of a proton-electron pair in bulk water.     

PHOTOCHEMISTRY, PHOTOPHYSICS, AND MECHANISM OF PYRIMIDINE DIMER REPAIR BY DNA PHOTOLYASE

Abstract— DNA photolyases photorepair pyrimidine dimers (PyroPyr) in DNA as well as RNA and thus reverse the harmful effects of UV-A (320–400 nm) and UV-B (280–320 nm) radiations. Photolyases from various organisms have been found to contain two noncovalently bound cofactors; one is a fully reduced flavin adenine dinucleotide (FADH^-) and the other, commonly known as second chromophore, is either methenyltetrahydrofolate (MTHF) or 8-hydroxydeazaflavin (8-HDF). The second chromophore in photolyase is a light-harvesting molecule that absorbs mostly in the near-UV and visible wavelengths (300–500 nm) with its high extinction coefficient. The second chromophore then transfers its excitation energy to the FADH^-. Subsequently, the photoexcited FADH^- transfers an electron to the Pyr<>Pyr generating a dimer radical anion (Pyr<>Pyr^-) and a neutral flavin radical (FADH^-). The Pyr<>Pyr^- is very unstable and undergoes spontaneous splitting followed by a back electron transfer to the FADH^-. In addition to the main catalytic cofactor FADH^-, a Trp (Trp277 in Escherichia coli ) in apophotolyase, independent of other chromophores, also functions as a sensitizer to repair Pyr <> Pyr by direct electron transfer.     

Methionine radical cation: structural studies as a function of pH using X- and Q-band time-resolved electron paramagnetic resonance spectroscopy

A comprehensive high resolution electron paramagnetic resonance (EPR) characterization of the l-methionine radical cation and its N-acetyl derivative in liquid solution at room temperature is presented. The cations were generated photochemically in high yield by excimer laser excitation of a water soluble dye, anthraquinone sulfonate sodium salt, the excited triplet state of which is quenched by electron transfer from the side chain sulfur atom of methionine or N-acetylmethionine. The radicals were detected by continuous wave (CW) time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band (9.5 GHz) and Q-band (35 GHz) microwave frequencies. At pH values well below the pK(a) of the protonated amine nitrogen, the cation forms a dimer with another ground-state methionine molecule through a S-S three-electron bond. In basic solution, the lone pair on the nitrogen of the amino acid is available to make an intramolecular S-N three-electron bond with the side chain sulfur atom, leading to a five-membered ring structure for the cation. When the amino acid nitrogen is unsubstituted (methionine itself), rapid deprotonation to an aminyl radical takes place at high pH values. If the nitrogen is substituted (N-acetylmethionine), the cyclic structure is observed within its electron spin relaxation time at about 1 micros. Spectral simulation provides chemical shifts (g-factors) and hyperfine coupling constants for all structures, and isotopic labeling experiments strongly support the assignments.     

Self-aggregates of natural chlorophylls and their synthetic analogues in aqueous media for making light-harvesting systems

Chlorophyll molecules are well organized for efficient energy or electron transfer in a light-harvesting antenna or a reaction center of photosynthetic organisms. In order to make effective photosynthetic mimics, self-aggregates of natural chlorophylls and their synthetic analogues have been prepared with the specific intermolecular interactions. Many studies have been carried out to prepare aqueous chlorophyll aggregates by use of surfactants or chemical modifications of the natural pigments, because chlorophylls basically are poorly soluble in water. This review article focuses on the preparation and function of aqueous chlorophyll aggregates used in making artificial photosynthetic systems.     

New perspective of electron transfer chemistry

A new perspective of electron transfer chemistry is described for fine control of electron transfer reactions including back electron transfer in the charge separated state of artificial photosynthetic compounds and its synthetic application. Fundamental electron transfer properties of suitable components of efficient electron transfer systems are described in light of the Marcus theory of electron transfer, in particular focusing on the Marcus inverted region, and they are applied to design multi-step electron transfer systems which can well mimic the function of a photosynthetic reaction center. Both intermolecular and intramolecular electron transfer processes are finely controlled by complexation of radical anions, produced in the electron transfer, with metal ions which act as Lewis acids. Quantitative measures to determine the Lewis acidity of a variety of metal ions are given in relation to the promoting effects of metal ions on the electron transfer reactions. The mechanistic viability of metal ion catalysis in electron transfer reactions is demonstrated by a variety of examples of chemical transformations involving metal ion-promoted electron transfer processes as the rate-determining steps, which are made possible by complexation of radical anions with metal ions.     

A REACTION BETWEEN PHEOPHYTIN and CHLOROPHYLL ADSORBED TO POLYETHYLENE-TETRADECANE PARTICLES*

Abstract— When chlorophyll is adsorbed to polyethylene-tetradecane particles along with ligating amphiphiles, and pheophytin is present also, a slow but irreversible photobleaching of the chlorophyll is observed in suspensions of the particles in aqueous media. It is suggested that the reaction starts by transfer of an electron from singlet excited chlorophyll to pheophytin and concludes by hydration and reduction of the chlorophyll radical cation.