Proton Ejection Accompanying Light Induced Electron Transfer Between Chlorophyll and Hydroquinone

Abstract— The photovoltaic effect in ethanolic solutions of chlorophyll a with benzoquinone or hydroquinone has been measured as a function of pH. In acidic media, stable, fast‐rising photovoltaic signals of positive sign are obtained. In neutral and basic media, less stable, slower rising photovoltaic signals of negative sign are found. The results in acid media are explained in terms of a photochemical interaction between chlorophyll and either the benzoquinone or hydroquinone producing protons which can then act as the electrode active agent. In basic media, negatively charged radical species, such as the benzoquinone radical‐anion, are considered to be the most likely electrode‐active species.

Section: Discussionsupporting

confidence: 39%

“…Chlorophyll and porphyrin type compounds have also been found to undergo protonejection accompanying light-induced electron transfer with hydroquinone [4] or benzoquinone[S, 61. The reaction was reversible, slightly quenched by air, and inhibited by large concentrations of quinone.…”

Section: Introductionmentioning

confidence: 99%

PHOTOELECTRONIC EFFECTS IN ORGANIC MATERIALS–II. VARIATION OF PHOTOVOLTAGE WITH pH IN CHLOROPHYLL‐QUINONE SOLUTIONS

Simpson

Freeman

Reucroft

1970

“…ESR studies. Since a degassed solution of chlorophyll a and QH, in methanol, upon illumination with red light, produces a p-benzosemiquinone radical ion [ 19,201 and ejects protons [21], the effect of pH was investigated by adding *mall amounts of hydrochloric acid in methanol. At an apparent pH of 3.7, where no appreciable pheophytinization took place, an ESR spectrum exhibited a five-line structure ( Fig.…”

Section: Hydroquinonementioning

confidence: 99%

CHLOROPHYLL‐PHOTOSENSITIZED OXIDATION OF HYDROQUINONE AND p‐PHENYLENEDIAMINE DERIVATIVES

Fujimori

Tavla

1968

Self Cite

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.

“…According to this interpretation, the pH change is a result 'of the initial formation of the semiquinone anion radical which then acts as a base. This seems unlikely in view of the ESR evidence for the semiquinone anion radical in the steady-state and observations of pH decreases in such systems [39]. Thus one could instead postulate a stabilization resulting from coordination of quinone by the magnesium (see below).…”

Section: )mentioning

confidence: 97%

Chlorophyll One‐electron Photochemistry: Light‐induced Absorbance Changes and Esr Signals for Various Porphyrin‐quinone and Hydroquinone Systems in Alcohol Solvents*

White

Tollin

1971

Light-dark optical difference spectra of degassed ethanol or pyridine solutions of chlorophyll and benzoquinone or hydroquinone at temperatures above -50°C show only the semiquinone absorbance band. Decay of the signals is second order, with a rate constant in agreement with earlier ESR results.Light-induced optical changes due to chlorophyll can be elicited by lowering the temperature of ethanol solutions of chlorophyll and benzoquinone to a region of high viscosity. Hydroquinone is not effective in producing these optical changes.Similar results are achieved at room temperature by using as solvent a degassed mixture of the alcohols: cyclohexanol, rert-butanol, and ethanol (CBE). Difference spectra show bleaching of the chlorophyll bands and increased absorbance in the intermediate wavelength region (460-580 nm). Decay kinetics are first order, while the rise is complicated (probably biphasic).ESR signals have no hyperfine structure and also decay by first order kinetics, at a rate which is faster than that of the optical changes. The ESR signals reach a steady state more rapidly than the optical signals, without biphasic kinetics. These results demonstrate that at least two species are generated.Addition of acid increases the amount of bleaching in CBE, while small amounts of base decrease it. Larger amounts of base cause chlorophyll bleaching to completely disappear and only the semiquinone anion is observed. Activation energies for the chlorophyll a-benzoquinone photoreaction in CBE are 10-14 kcal/mole. Lower potential quinones give lower activation energies. The rate constant for quenching of the triplet state of chlorophyll a by p-carotene in CBE is 7.5 &0.5 X 108 (M set)-'. p-carotene also quenches photoproduct formation. The bimolecular rate constant for formation of the photoproduct with benzoquinone was calculated to be 7 X 108 (M sec)-I.The redox potential of the quinone affects both the magnitude of the shlorophyll absorbance changes and the rate of decay. The higher the potential, the larger the changes and the slower the decay. Other porphyrin systems show similar photoreactions only if they are chelated with a group I1 metal, such as MgZ+, Cd+*, or Zn+z.The results are interpreted in terms of the formation, by a triplet-sensitized one-electron transfer from solvent to quinone, of a chlorophyll-semiquinone complex which is stabilized via coordination with the chelated metal.