Chlorophylls semiconductor

Photoelectrochemlcal conversion from visible light to electric and/or chemical energy has been investigated with chlorophyll thin membranes deposted on semiconductor or metal electrodes (71). Chlorophyll-coated metal (platinum) electrodes derived cathodic photocurrent in acidic electrolyte solutions, although the photocurrent efficiencies tend to be low compared to those of chlorophyll-semiconductor electrodes. The cathodic photoresponse may result from the p-type photoconductive nature of a solid chlorophyll layer and/or the formation of a contact barrier at the metal-chlorophyll interface, which contributes to light-induced carrier separation and leads to photocurrent generation. 

In this chapter, we develop a model of bonding that can be applied to molecules as simple as H2 or as complex as chlorophyll. We begin with a description of bonding based on the idea of overlapping atomic orbitals. We then extend the model to include the molecular shapes described in Chapter 9. Next we apply the model to molecules with double and triple bonds. Then we present variations on the orbital overlap model that encompass electrons distributed across three, four, or more atoms, including the extended systems of molecules such as chlorophyll. Finally, we show how to generalize the model to describe the electronic structures of metals and semiconductors. 

In contrast to a conventional p-n-junction-type solar cell, the mechanism of the DSSC does not involve a charge-recombination process between electrons and holes because electrons are injected from the dye photosensitizers into the semiconductor, and holes are not formed in the valence band of the semiconductor. In addition, electron transport takes place in the Ti02 film, which is separated from the photon absorption sites (i.e., the photosensitizers) thus, effective charge separation is expected. This photon-to-current conversion mechanism of the DSSC is similar to that for photosynthesis in nature, where chlorophyll functions as the photosensitizer and electron transport occurs in the membrane. 

Photoelectrochemical conversion from visible light to electric and/or chemical energy using dye-sensitized semiconductor or metal electrodes is a promising system for the in vitro simulation of the plant photosynthetic conversion process, which is considered one of the fundamental subjects of modern and future photoelectrochemistry. Use of chlorophylls(Chls) and related compounds such as porphyrins in photoelectric and photoelectrochemical devices also has been of growing interest because of its close relevance to the photoacts of reaction center Chls in photosynthesis. 

Photoelectrochemical Systems Involving Chlorophyll-Coated Semiconductor and Metal Electrodes... 

Chlorophyll-Coated Semiconductor Electrodes. Chi has first been employed by Tributsch and Calvin (55,56) in dye sensitization studies of semiconductor electrodes. Solvent-evaporated films of Chi a, Chi b, and bacteriochlorophyll on n-type semiconductor ZnO electrodes (single crystal) gave anodic sensitized photocurrents under potentiostatic conditions in aqueous electrolytes. The photocurrent action spectrum obtained for Chi a showed the red band peak at 673 nm corresponding closely to the amorphous and monomeric state of Chi a. The addition of supersensitizers (reducing agents) increased the anodic photocurrents, and a maximum quantum efficiency of 12.5% was obtained for the photocurrent in the presence of phenylhydrazine. 

Dyes such as erythrosin B [172], eosin [173-177], rose bengal [178,179], rhodamines [180-185], cresyl violet [186-191], thionine [192], chlorophyll a and b [193-198], chlorophyllin [197,199], anthracene-9-carboxylate [200,201], perylene [202,203] 8-hydroxyquinoline [204], porphyrins [205], phthalocyanines [206,207], transition metal cyanides [208,209], Ru(bpy)32+ and its analogs [83,170,210-218], cyanines [169,219-226], squaraines [55,227-230], and phe-nylfluorone [231] which have high extinction coefficients in the visible, are often employed to extend the photoresponse of the semiconductor in photoelectro-chemical systems. Visible light sensitization of platinized Ti02 photocatalyst by surface-coated polymers derivatized with ruthenium tris(bipyridyl) complex has also been attempted [232,233]. Because the singlet excited state of these dyes is short lived it becomes essential to adsorb them on the semiconductor surface with... 

Hotchandani, S. Kamat, P. V. Modification of electrode surface with semiconductor colloids and its sensitization with chlorophyll a, Chem. Phys. Lett. 1992, 191, 320. 

Bedja, I. Hotchandani, S. Carpentier, R. Fessenden, R. W. Kamat, P. V. Chlorophyll b modified nanocrystalline Sn02 semiconductor thin film as a photosensitive electrode, J. Appl. Phy. 1994, 75, 5444. 

Kamat, P. V. Chauvet, J. P. Fessenden, R. W. Photoelectrochemistry in particulate systems. 4. Photosensitization of a Ti02 semiconductor with a chlorophyll analogue, J. Phys. Chem. 1986, 90, 1389. 

Chlorophyll-containing Membrane on an Electrode Surface. Photoelectrochemical and photovoltaic effects have been noted with a variety of chlorophyll assemblies deposited on metal or semiconductor electrode surfaces (Figure 6). 

Figure 6. Molecular organizations of chlorophyll membranes deposited on metal or semiconductor electrodes.

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