Marye Anne Fox

PHOTOINDUCED ELECTRON TRANSFER

Abstract— Given the pre‐eminent roles of photoinduced electron transfer and energy transfer as primary events in photobiology, it is incumbent on practitioners of the science to understand those principles which govern these elementary events. Recent developments in both theory and experiment on photoinduced electron transfer have allowed for important insights into understanding the factors governing such steps. For example, shown in Fig. 1 is a representation of the positions of the chromophores in the photosynthetic reaction center of Rhodopseudomonas sphaeroides, as determined from crystallographic measurements (Chang et al., 1986), showing that its three dimensional arrangement within an intact membrane is analogous to that observed earlier in Rhodopseudomonas viridis (Deisenhofer et al., 1984). In both systems, the critical event of photosynthesis is the transfer of an electron from the photoexcited special pair of bacteriochlorophylls located at the top via the pheophytin at the far right to the quinone at the bottom which acts as an ultimate repository for the separated charge. Of great importance is a determination of those factors which govern the efficiency and the rate of electron transfer through this photoinduced cascade and a better understanding of how this fixed, prearranged structure maximizes the efficiency of photochemical energy storage.

Size quantization effects in cadmium sulfide layers formed by a Langmuir-Blodgett technique

Abstract Layers of Q-state particles ( 2 S to yield cadmium sulfide particles in a lattice of arachidic acid. The onset of absorption of this layer is blue-shifted by 0.38 eV with respect to bulk CdS. The adherence to Beers law and insensitivity of the absorbance edge to the number of LB layers deposited (up to seven) suggest no strong electronic interactions between successive layers. Ellipsometric measurements show that exposure of the LB films of cadmium arachidic acid to H 2 S gas results in an increase in thickness of about 3 A per layer.

Photophysical characterization of dilute solutions and ordered thin films of alkyl-substituted polyfluorenes

Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1-3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the effect of polymer chain interactions on excited state relaxation. Film morphology is controlled by annealing of 250 nm thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in turn, a function of the length of the attached alkyl substituents. Alkyl substituents also affect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band. In the annealed films, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields.

Photosensitization of TiO2 colloids by Erythrosin B in acetonitrile

Abstract Photosensitization by Erythrosin B of a TiO2 colloidal dispersion in acetonitrile has been studied by fast kinetic spectroscopy. The dye molecules adsorbed on the TiO2 surface had a significantly shorter lifetime (2 250 ps) compared to those in homogeneous solution (≈ 1.6 ns) in acetonitrile. The photosensitization of TiO2 occurred more efficiently from the singlet than from the triplet state of the dye.

Photocatalytic reduction of CO2 with H2O on TiO2 and Cu/TiO2 catalysts

Photoinduced reduction of CO2 by H2O to produce CH4 and CH3OH has been investigated on wellcharacterized standard TiO2 catalysts and on a Cu2+ loaded TiO2 catalyst. The efficiency of this photoreaction depends strongly on the kind of catalyst and the ratio of H2O to CO2. Anatase TiO2, which has a large band gap and numerous surface OH groups, shows high efficiency for photocatalytic CH4 formation. Photogenerated Ti3+ ions, H and CH3 radicals are observed as reactive intermediates, by ESR at 77 K. Cu-loading of the small, powdered TiO2 catalyst (Cu/TiO2) brings about additional formation of CH3OH. XPS studies suggest that Cu+ plays a significant role in CH3OH formation.

Hydrogen photoproduction by Nafion/cadmium sulfide/platinum films in water/sulfide ion solutions

The construction, characterization, and evaluation of a novel system for photocatalytic hydrogen generation that comprises semiconductor (CdS) crystallites embedded in a polymer (Nafion) matrix that also contains a hydrogen-evolution catalyst (Pt) are reported. Hydrogen-production efficiencies from water containing a sacrificial electron donor (sulfide ion) were greater than those commonly obtained with unsupported colloidal or powdered semiconductors under similar conditions. Several factors dramatically influence performance: the crystalline form and surface composition of the semiconductor and the extent of catalyst dispersion throughout the film.

Photooxidation of Anthracene on Atmospheric Particulate Matter

Anthracene that has been dispersed into atmospheric particulate matter is photooxidized to an array of products reminiscent of oxidation by singlet oxygen in the solution phase. Photooxidation is a more important degradative pathway under ambient conditions than ozonation. Significant disappearance of anthracene also occurs by pathways not involving oxygenation.

Photoelectrochemical Doping of TiO2 Particles and the Effect of Charge Carrier Density on the Photocatalytic Activity of Microporous Semiconductor Electrode Films

Irradiation with a very high intensity laser pulse (355 nm) induces blue color in TiO 2 powders and films. The blue color, which is assigned to the formation of Ti 3+ in the particle, persists in air for several months. With an increase of the concentration of Ti 3+ in a given particle, photocatalytic activity is decreased because of a decrease in the thickness of the space-charge layer as deep Ti 3+ traps are populated.

Transients generated upon photolysis of colloidal titanium dioxide in acetonitrile containing organic redox couples

This report investigates the identity of transients formed upon irradiating colloidal TiO/sub 2/ particles in nonaqueous solvents in the presence of organic redox systems, a medium which simultaneously incorporates the heterogeneity of the powder-catalyzed reactions and the possibility for spectroscopic analysis of mechanistically relevant transients. Although semiconductor band theory predicts that photocatalyzed oxidations or reductions could occur with donors or acceptors whose redox potentials lie within the range between the valence and conduction bands of the metal oxide, little evidence for organic radical ion intermediates generated in this way from neutral precursors has yet been reported. This investigation describes transient spectra that affirm the occurrence of semiconductor-mediated electron transfer and demonstrate the involvement of radical ions, the first such data reported for organic redox couples in nonaqueous solvents.

Selectivity in the TiO2- mediated photocatalytic oxidation of thioethers

The relative rates of photooxygenation of substituted diphenyl sulfides on irradiated TiO2 powders suspended in oxygenated acetonitrile correlate linearly with σ+, as is consistent with formation of a surface bound cation radical in the primary photoprocess. Substituted dibenzyl sulfides, in contrast, suffer C-S cleavage upon photocatalyzed oxidation. Factors disposing these reactions toward preferential sulfoxide formation or bond cleavage are discussed.