John R. Harbour

THE INVOLVEMENT OF THE HYDROXYL RADICAL IN THE DESTRUCTIVE PHOTOOXIDATION OF CHLOROPHYLLS IN VIVO AND IN VITRO

Abstract— The photooxidation of chlorophylls in vivo and in aqueous micellar solutions has been investigated. Through spin trapping experiments, it was found that hydroxyl radicals are photo‐produced in these systems if oxygen is present. Optical studies indicate that the rate of photo‐destruction of chlorophyll is spin trap dependent. This suggests that the hydroxyl radical may play a key role in the destruction of chlorophyll.

Transient radicals in heterogeneous systems: detection by spin trapping.

The application of spin trapping to the detection of transient radicals generated in heterogeneous systems is discussed. The major part of this review focuses on the interaction of light with particles in dispersion and the resultant radicals which are produced. This includes photoactive pigments such as CdS, TiO2, ZnO and phthalocyanine in a variety of solvents as well as micelles, vesicles and microemulsions containing molecules such as phthalocyanine or chlorophyll. The utility of spin trapping in heterogeneous systems which are not a function of illumination (i.e. electrochemical reactions, catalysis and biological systems) is also discussed. It is demonstrated that reaction mechanisms can be better understood by the indirect detection of the radical intermediates which are involved in the various processes. This impacts studies in solar energy utilization, electrophotography, catalysis, electrochemistry, photosynthesis and the disease process.

Localized paramagnetism in carbon blacks: Inherent vs electrochemically reduced sites

Abstract The results of an electron paramagnetic resonance investigation of four different carbon blacks are presented. These include an unoxidized, an oxidized, a devolatilized and a surface-modified black. All of these blacks manifest a narrow EPR signal at low temperatures. This signal, referred to as the inherent signal, has an intensity which correlates with the number of functional groups on the surface. Electrochemical reduction of these four blacks also yields narrow EPR signals but with intensities approximately 1 order of magnitude greater than their corresponding inherent signals. The magnitude of these electrochemically generated signals also correlates with the number of surface functionalities. A temperature study of both the inherent and electrochemical EPR signals reveals that these paramagnetic sites are most likely localized. This is deduced from the fact that below 185 K, the Curie law is followed. However, at higher temperatures (> 185 K) the inherent signal is exponentially dependent on the temperature, suggesting a Boltzmann-type equilibrium between charge carriers and surface states.

PHOTOINDUCED ELECTRON TRANSFER REACTIONS IN PHTHALOCYANINE DISPERSIONS

Abstract— The photochemistry of phthalocyanine particles suspended in a fluid has been investigated. In alcoholic media, these particles were shown to be capable of reducing benzoquinone and oxidizing hydroquinone. In aqueous media the light induced formation of superoxide was detected. This reaction could be substantially enhanced by addition of EDTA. The role of surfactant in the photoproduction of superoxide is associated with the charge acquired by the particle as a result of the surfactant used. These results can be explained in terms of the band model for semiconductors where band bending by surfactant molecules is invoked. Such studies have relevance to photoevents occurring in photosynthesis, photocatalysis and to the types of solar energy conversion systems where a photoactive semiconductor interfaces with an aqueous medium.

Generation of electron accepting surface sites in carbon blacks by peracetic acid oxidation

Abstract Surface oxidation of three different carbon blacks was carried out using peracetic acid. An electron spin resonance investigation of these oxidized blacks revealed that a significant number of surface sites were generated. These surface sites were in equilibrium with the bulk conduction electrons as evidenced by the large deviation from the Curie Law. Electrochemical reduction of these blacks also resulted in electron transfer to these surface sites as evidenced by ESR. The ESR results on the reduced and oxidized carbon blacks correlated with the measured contact potentials. This is consistent with a mechanism of electron acceptance by surface sites during contact electrification in a xerographic developer system.

Effect of electroreduction on the EPR spectra due to charge carriers in carbon blacks

Abstract The electrochemical reduction of carbon black particles dispersed in methylene chloride was monitored in situ with electron paramagnetic resonance (EPR) spectroscopy. The broad EPR signal is converted to a signal with intermediate linewidth (ΔHpp ~ 23 G) in both CSX-99 and BP-1300 carbon blacks. This narrowing was coincident with the manifestation of the narrow EPR signal (ΔHpp ~ 5 G, which has been previously shown to arise during electroreduction. In addition, changes in the cavity Q were also observed under electro-reduction and subsequent electrooxidation. This decrease in the dielectric loss and the narrowing of the EPR linewidth observed upon electroreduction are discussed.

Electrochemical generation of narrow conduction electron spin resonance signals in carbon blacks

Abstract Electrochemical reduction of either of two different carbon blacks dispersed in methylene chloride gave rise to a temperature independent narrow ESR signal superimposed upon the characteristic broad signal. The observation of this conduction ESR (CESR) signal implies that there are two distinct environments available for charge carriers within the carbon black particles. With the oxidized black, an additional ESR spectrum (g = 2.0034) revealing hyperfine interaction was observed. This was associated with surface oxidized molecules which had been electrochemically reduced and reside in solution as radical anions.

ESR detection of charge exchange in carbon black dispersions

Etude des signaux de RSE provoques electrochimiquement dans des dispersions de 2 voies de carbone dans differents electrolytes

An electron spin resonance investigation into dark and light-induced paramagnetism in metal-free phthalocyanines

Abstract The origin of the g = 2.003 ESR signal observed in diamagnetic metal-free phthalocyanines (H2Pc) is discussed. A model is proposed which suggests that holes and electrons delocalized over one or more phthalocyanine molecules gives rise to the signal. Results on the variation of polymorphic state, chemical purity, oxygen content and surface area of phthalocyanine samples give support to these conclusions. Visible light absorbed by H2Pc was found to increase this signal which reversibly returns to the dark level upon blocking the light. These results suggest that structural defects and chemical impurities can act as deep and shallow traps for holes and electrons and that ESR is therefore an exceptional method for probing trap densities.

Endor studies of carbon black and carbon black-polymer composites

Abstract Electron-nuclear double resonance (ENDOR) spectra have been obtained at the free proton frequency for a carbon black and a carbon black-polymer composite, at temperatures ranging from 130 K to 350 K. The ENDOR spectra show an unusual dependence on the saturation of the associated homogeneously broadened ESR resonance, as “non-saturating” microwave power gives rise to largest ENDOR enhancements. This behaviour is attributed to a “lineshift” ENDOR mechanism. In the composite, the polymer 1 H nuclei contribute very strongly to the ENDOR signal, and this signal shows a marked dependence on molecular motion within the polymer.