Photoinduced electron transport

Hwang K C and Mauzerall D C 1993 Photoinduced electron transport across a lipid bilayer mediated by Nature 361 138-40... 

Many inhibitors of photoinduced electron transport, including a large number of herbicides, have been shown to bind reversibly and competitively to the protein. The studies volve displacement of a radiolabeled inhibitor, such as [ C]-atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)- -trlazine], by an uy abeled inhibitor (29). Only marginal displacement of [ Cj-atrazine was observed by high concentrations of umbelliferone and naringenin. The other allelochemicals produced no measurable effects. 

In a broad sense photosynthesis in plants is a photoinduced electron transport reaction. Chlorophyll molecules in the green plants are the main light harvesting molecules. They are assisted by carotenoids and phycocyanins in this act. These molecules have absorption in the visible region covering the whole spectrum from blue to red. The energy absorbed by all these molecules is transferred to chlorophyll a (Chi a), which is the main light sensitive molecule, by mechanisms discussed in Section 6.6.4. 

Taken together with the results from the photoinduced electron-transport experiments in which diffusive thermal electron hopping can occur over a few... 

Photoinduced electron transport and the coupled phosphorylation reactions as they are postulated to occur in chloroplasts are presented schematically in Figure 2. Not all investigators agree on the details of this scheme, and some even question the sequence of the intermediates. The numbers and locations of the phosphorylation sites also remain to be identified precisely. However, the scheme is a reasonable approximation based on available information. Reactions that occur in the light are represented by the open arrows and the solid arrows represent electron transfers that occur in the dark. 

Figure 2. Schematic of photoinduced electron transport and phosphorylation reactions considered to occur in chloroplast lamellae [from Moreland and Hilton (2)]. Open arrows indicate light reactions solid arrows indicate dark reactions and the narrow dashed line represents the cyclic pathway. Abbreviations used PS I, photosystem I PS II, photosystem II Y, postulated electron donor for photosystem II Q, unknown primary electron acceptor for photosystem II PQ, plastoquinones cyt b, b-type cytochromes cyt f, cytochrome f PC, plastocyanin P700, reaction center chlorophyll of photosystem I FRS, ferredoxin-reducing substance Fd, ferredoxin Fp, ferredoxin-NADP oxidoreductase FeCy, ferricyanide asc, ascorbate and DPIP, 2,6-dichloropheno-lindophenol. The numbers la, lb, 2, 3, and 4 indicate postulated sites of action by...

Photoinduced electron transport from EDTA to viologen through the excited Zn porphyrin and Ti02 particle entrapped within lithium aluminum-layered double hydroxide was attempted [387] (Fig. 19). Since the fluorescence of Zn porphyrin was quenched by TiO, the excited singlet state of porphyrin was supposed to transfer an electron to Xi02 particle. 

Figure 19 Photoinduced electron transport from EDTA to viologen through the excited Zn porphyrin and TiOj particle entrapped within lithium aluminum layered double hydroxide [387].

Another example of photoinduced charge-transfer phenomena with the participation of C o was presented by Kamat by utilizing ZnO suspension composites with buckminsterfullerene [49]. Recent reports on photoinduced polymerization of C60 [50] as well and photoinduced electron transport across lipid bilayers mediated by Ceo [51] have continued to broaden the importance of buckminsterfullerene as a photoexcited electron acceptor in photophysics and photochemistry. 

M. Avron. Photoinduced electron transport in chloroplasts. Current Topics in Bioenergetics, 2 1—22, 1967. 

Let us now return to MMCT effects in semiconductors. In this class of compounds MMCT may be followed by charge separation, i.e. the excited MMCT state may be stabilized. This is the case if the M species involved act as traps. A beautiful example is the color change of SrTiOj Fe,Mo upon irradiation [111]. In the dark, iron and molybdenum are present as Fe(III) and Mo(VI). The material is eolorless. After irradiation with 400 nm radiation Fe(IV) and Mo(V) are created. These ions have optical absorption in the visible. The Mo(VI) species plays the role of a deep electron trap. The thermal decay time of the color at room temperature is several minutes. Note that the MMCT transition Fe(III) + Mo(VI) -> Fe(IV) -I- Mo(V) belongs to the type which was treated above. In the semiconductor the iron and molybdenum species are far apart and the conduction band takes the role of electron transporter. A similar phenomenon has been reported for ZnS Eu, Cr [112]. There is a photoinduced charge separation Eu(II) -I- Cr(II) -> Eu(III) - - Cr(I) via the conduction band (see Fig. 18). 

Fig. 4 Coupling of the redox participants to the DNA w-stack is requisite to DNA-mediated charge transport. Rapid (>109 s 1) photoinduced electron transfer occurs between the metallointercalators, [Ru(phen)2dppz]2+ and [Rh(phi) 2phen]3+, when they are tethered to opposite ends of a DNA duplex over 40 A apart. Conversely, electron transfer does not occur between non-intercalated Ru(II) and Rh(III) complexes tethered to DNA...

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Under the advisement of PhD mentor Professor Joseph T. Hupp, the PI successfully used spectroelectrochemical quartz crystal microgravimetry to elucidate the mechanism of charge transport... for both aqueous and nonaqueous sytems. This was the first demonstration of proton-coupled electron transfer at oxide semiconductor interfaces. These findings were then successfully applied to a new interpretation of photoinduced electron transfer at similar interfaces, which are of importance in the field of solar energy conversion. ... 

Figure 3.32. Energy level scheme of the device in Figure 3.31. Photoinduced electron transfer takes place from the photoexcited ruthenium dye into the Ti02 conduction band. The recombination directly back to the dye has to be suppressed. Instead, the current is directed through the circuit to the counterelectrode and the hole conductor that brings the electrons back via hopping transport. HTM hole transport material.

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

Solar cells, or photovoltaic devices, have been studied for many years [3], Most of the current work is focused on dye-sensitized nanocrystalline solar cells. These provide a technical and economically viable alternative to present-day photovoltaic devices. In contrast to conventional systems, in which the semiconductor assumes both the task of light absorption and charge carrier transport, the two functions are separated in dye-sensitized nanocrystalline solar cells [54] (cf. OPCs). Light is absorbed by the dye sensitizer, which is anchored to the surface of a wide-band-gap semiconductor. Charge separation takes place at the interface via photoinduced electron injection from the dye into the conduction band of the... 

Energy (ATP) Availability. All of the herbicides that interfere with the photoinduced reactions limit the availability of ATP. Action is expressed at different sites on the electron transport and energy generation pathways, but the net result is... 

Wasielewski s research interests comprise photoinduced electron transfer and charge transport in organic molecules and materials, artificial and natural photosynthesis, self-assembly of nanoscale materials, spin dynamics of multispin organic molecules, materials for molecule-based optoelectronics and spintronics, and time-resolved optical and magnetic resonance spectroscopy. His research has resulted in over 300 publications. Dr. Wasielewski was elected a fellow of the American Association for the Advancement of Science in 1995, and has held numerous distinguished lectureships and fellowships. Among Wasielewski s recent awards are the 2004 Photochemistry Research Award of the Inter-American Photochemical Society and the 2006 James Flack Norris Award in Physical Organic Chemistry of the American Chemical Society. 

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