Metal chlorophyll-coated

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

Chlorophyll-Coated Metal Electrodes. Photoelectrochemical reactions at Chl-coated metal electrodes have been investigated respecting the various configurational modes of Chi. Platinum electrodes have widely been employed as substrates of Chi films. 

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. 

LB chlorophyll monolayers (25 mN/m 1.4 nm2/molecule) on Pt electrodes showed low photoactivity, possibly caused by a quenching of excited states by the metal electrode or by total reversibility of electron exchange. Addition of electron acceptors, e.g., quinones, had no effect. The optically transparent tin oxide semiconductor electrode proves to be a much better subphase for the generation of photocurrents. Chlorophyll-coated Sn02 combined with a platinum electrode gave approximately 100 nA/cm. Similar results were obtained with photovoltaic systems of the form mercury droplet/buffer solution/chlorophyll a monolayer/electron acceptor monolayer/aluminum (Fig. 6.9.3). The quantum yield of such monolayer arrangements never exceeded 10" in any of these systems and is thus far away from competitive inorganic semiconductor cells (Norris and Meisel, 1989). 

Copper phthalocyanine (1) was developed in the 1930s and is the most commonly used blue organic pigment in the coatings, paint, and printing inks industry. Phthalocyanine forms complexes with numerous metals. Various complexes with 66 chemical elements are known. Phthalocyanines are structurally related to naturally occurring dyes such as hemoglobin and chlorophyll A. 

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... 

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