PHOTOVOLTATC EFFICIENCIES OF MICROCRYSTALLINE AND ANHYDROUS CHLOROPHYLL a

Abstract— A thin layer of chlorophyll a (around 2000 Å). a p -type organic semiconductor. was sandwiched between two different metals. aluminum and silver. We used the photovoltaic effect in order to study the efficiency of light conversion by the crystalline and anhydrous forms of Chl a . When the photovoltaic cell is illuminated through the semi-transparent aluminum electrode. an action spectrum similar to the visible absorption spectrum of Chl a is obtained. The anhydrous form. always shows a maximum, in the red, at 672 nm and the crystalline one at 746 nm to 738 nm depending on the amount of water vapor present in the measuring area. The light conversion efficiency has been measured at the maximum absorption in the red for both forms of Chl a . For the anhydrous form, we found values as high as 0.036%, which is very significant even compared to 0.21%, the highest value found for the crystalline form. In both cases. the incident light power was approximately 10 μW on the cell. The light conversion efficiency of both forms of Chl a cells was found to depend on a combine effect of the nature of the gas (O₁. N₂. air. Ar. N₂O. SF ₆ , H₂) and the amount of water vapor present in the measuring area. The best conversion efficiency was obtained with O ₂ , or air saturated with water. This combined effect was very large for the crystalline form whereas the anhydrous form was only slightly affected.     

PHOTOVOLTAIC ACTION SPECTRA AND EFFICIENCIES OF CHLOROPHYLL a SPECIES ABSORBING NEAR 700 nm

Abstract— Alcohol vapors affect the photovoltaic properties of anhydrous chlorophyll a (Chi a). At 23°C, a photovoltaic cell of the type Allanhydrous Chi a|Ag has been successively submitted to non saturating vapors of methanol, ethanol, 1-propanol, 2-propanol, cyclopropyl carbinol, cyclopentanol, methyl cyclopropyl carbinol, 1-butanol, 1-pentanol and chloroethanol. The action spectrum of anhydrous Chi a has a maximum in the red at 672 nm. This maximum is shifted towards 700 nm under the influence of alcohol vapors. The most important changes occur for ethanol, 1-propanol and 2-propanol. In the same way, the conversion efficiencies of light energy into electrical energy, measured at the maximum in the red are, for the same alcohols, higher than the initial value 1.7 times 10^-2% obtained on the average for anhydrous Chi a. A maximum value of 6.3 times 10^-2% has been obtained after rehydration of Chi a solvated with 2-propanol. The influence of alcohol vapors has been interpreted in terms of microcapillarity of anhydrous Chi a and a mean microcavity radius of 11 ± 6 Å has been deduced from the photocurrent variation with the amount of alcohol vapor present in the measuring area. Furthermore, the action spectrum shift towards 700 nm has been interpreted by the formation, at 23°C, of Chi a special aggregates whose action spectrum has been obtained by difference. Infrared spectroscopy demonstrated that anhydrous Chi a obtained from electrodeposition is an assembly of Chi a dimers and that alcohol vapors, producing the special aggregates formation at 23°C, induce a decrease of the free C=O ketone band and an increase of the associated C=O ketone. The C=O esters band is not affected by the Chi a reorganization.     

ELECTRON TRANSFER FROM CHLOROPHYLL a TO QUINONE IN MONO- AND MULTILAYER ARRAYS

Abstract— Mono- and multilayers of chlorophyll a (Chl a )– lecithin have been prepared on quartz slides, by means of the Blodgett-Langmuir technique, for fluorescence studies. Self-quenching of the Chl a fluorescence has been observed in Chl a -lecithin single layer excited with a laser light at 632.8 nm. The fluorescence yield is reduced by 50% at a concentration of 7 ± 10¹² Chl a molecules cm⁻². Chl a fluorescence quenching, by adding N,N -distearoyl-1,4-diaminoanthraquinone (SAQ), has been studied. in a single layer, in pure Chl a and also at various dilutions of Chl a in lecithin. The results are explained in terms of a dynamic quenching rather than in terms of a permanent complex formation, at the ground state, between Chl a and SAQ. The fluorescence quenching has been interpreted as the result of an electron transfer from excited Chl a to SAQ, and rate constants of 8.3 ± 10⁻⁵ cm² molecule⁻¹ S⁻¹ and 2.4 ± 10⁻⁴ cm² molecule⁻¹ s⁻¹ have been found for pure diluted Chl a , respectively. Ten per cent of the diluted Chl a fluorescence always remains unquenchable and independent of the quinone concentration. In multilayers, where SAQ and Chl a are in different layers, there is no fluorescence quenching for pure or diluted Chl a even when the chromophores are in two adjacent layers. This happens only if SAQ is not able to diffuse from one layer to another. A minimum value of 22.4 nm has been found for the singlet exciton diffusion length in pure Chl a multilayers.     

Heat-treated Fe/N/C catalysts for O2 electroreduction: are active sites hosted in micropores?

Limited availability of platinum is a potential threat to fuel cell commercialization. Since the 1970s, alternative catalysts to the electrochemical reduction of oxygen have been obtained from heat treatment at T > 600 degrees C of carbon with a non-noble metal and a source of nitrogen atoms. However, the process by which the heat treatment activates these materials remains an open question. Here, we report that the activation process of carbon black and iron acetate heat-treated in NH(3) comprises three consecutive steps: (i) incorporation of nitrogen atoms in the carbon, (ii) micropore formation through reaction between carbon and ammonia, and (iii) completion of active sites in the micropores by reaction of iron with ammonia. Step (ii) is the slowest. Moreover, the microporous surface per mass of catalyst controls the macroscopic activity when enough nitrogen atoms are incorporated in the structure of the carbon support. These facts should help in determining the structure of the active sites and in identifying methods to increase the site density of such catalysts.     

Synthesis of poly(arylene ether)s containing hole‐transport moieties from an isocyanate masked bisphenol

The design and synthesis of novel charge (hole‐ or electron‐) transport materials have been the focus of much research in recent years because of their wide variety of applications. In this study, three high molecular weight poly(arylene ether)s, 6a–c, containing naphthyl‐substituted benzidine moieties have been synthesized from carbamates derived from bisphenols. After masking with n‐propyl isocyanate, the carbamate is stable, can be readily purified by recrystallization from toluene, and can be polymerized directly with difluoro compounds under mild conditions. The resulting polymers possess high glass‐transition temperatures, excellent thermal stability, and good film‐forming properties. In comparison, the poly(arylene ether)s 6a′–c′, synthesized from unprotected bisphenol, have lower molecular weights and wider polydispersity and contain some brown impurities. Preliminary experiments show that both 6a and 6a′ can function well as hole‐transport materials in light‐emitting diodes. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2740–2748, 2000     

Does CO poison Fe-based catalysts for ORR?

Catalysts for the oxygen reduction reaction (ORR) were prepared on carbon black (C) using Fe^IIphthalocyanine (FePc) and Cl–Fe^IIItetramethoxyphenylporphyrin (ClFeTMPP), as Fe precursors with and without a pyrolysis step at 800 °C. CO poisoning of the ORR catalytic sites for all these Fe/N/C electrocatalysts was attempted at pH 1 and 13, but to no avail, even if an iron ion is known to occupy the center of the active sites in at least the unpyrolyzed FePc/C or ClFeTMPP/C. The exact nature of the active center of these Fe-based heat-treated catalysts may still be a subject of debate but, in light of the absence of CO poisoning for unpyrolyzed FePc/C and ClFeTMPP/C, resistance to CO poisoning by the heat-treated catalysts cannot be used as evidence that the active center of their catalytic site is devoid of iron.     

Molecular oxygen reduction in PEM fuel cell conditions: ToF-SIMS analysis of co-based electrocatalysts

A series of Co-based electrocatalysts for oxygen reduction in acid media has been prepared using two different Co precursors: cobalt acetate (CoAc) and a cobalt porphyrin (CoTMPP). These catalysts have been analyzed by ToF-SIMS to obtain information on the number and the structure of catalytic active sites in these materials. The results are compared with the results of a similar analysis already performed on a series of Fe-based electrocatalysts (J. Phys. Chem. B 2002, 106, 8705) also prepared with two different Fe precursors: iron acetate (FeAc) and an iron porphyrin (ClFeTMPP). The interpretation of ToF-SIMS data for Fe-based catalysts allowed us to conclude that whatever the Fe precursor was, the same catalytic sites (FeN2/C and FeN4/C, with their respective dominant ToF-SIMS signatures: FeN2C4+ and FeN4C8+ ions) were found. The comparison of the ToF-SIMS data with the activity of those catalysts led to the conclusion that the FeN2C catalytic site was more active than FeN4/C. When the same procedure is applied to ToF-SIMS data measured for Co-based catalysts, the following conclusions are drawn: (i) as for Fe precursors, both Co precursors also give similar results; (ii) as for Fe-based catalysts, the same four families of MetalNxCy+ ions, with 1, 2, 3, and 4 nitrogen atoms, are also found in the spectra of Co-based catalysts, but there is no dominant CoNxCy+ ion signature; (iii) only CoN4/C can be ascertained on the basis of ToF-SIMS measurements. There is no strong support from ToF-SIMS measurements for (or against) the existence of CoN2/C in Co-based catalysts as there is for FeN2/C in Fe-based catalysts; (iv) contrary to Fe-based catalysts, all catalytic sites (if there are any besides CoN4/C) are about equally active in Co-based electrocatalysts.