Chlorophyll, monolayers

Photodegradation as well as fluorescence quenching has been observed in chlorophyll monolayers [302,316]. Whitten [317] observed a substantial decrease in the area of mixed films of tripalmitin and a ci5-thioindigo dye as isomerization to the trti 5-thioindigo dye occurred on irradiation with UV light. 

Optical reflection from a monomolecular film must be measured from the interface with a very small amount of material present. Therefore, in these methods, repeated interaction of the light beam with one or more identical films is generally employed. The simplest way to observe a light beam that has passed through several identical monolayers is to transfer portions of the layer to suitable transfer end plates, which are then stacked and examined in a conventional spectrophotometer. This method was used where monolayers of chlorophyll were deposited on glass slides by the LB method. Ferrodoxin and chlorophyll monolayers were investigated by measuring the spectra (550-750 nm) of these films at the interface. 

A typical time response for a short-circuited photocurrent in the presence of hydroquinone ( Q) as an added solution redox species is shown in Figure 9. These photocurrents were stable for several hours. In the absence of in the electrolyte, the photocurrent also increased rapidly upon the onset of illumination, but subsequently decayed exponentially to 70% of its initial value in a half-decay time of ca. 25 s. This behavior is similar to that observed for chlorophyll monolayers deposited on SnC (12). Photocurrents under potentially-controlled conditions were also stable upon illumination, but exhibited slower decay characteristics when the light was turned off. This effect is unusual and is currently under further investigation. 

In contrast to cyanine dyes and Ru complexes, fairly concentrated chlorophyl monolayers could be deposited on Sn02- In the latter case, the quantum yield of the photocurrent was determined as a function of the molar ration of chlorophyl and stearic acid (concerning the definition of quantum yield see Section 10.2.5). An optimum was obtained for a 1 1 ratio, whereas the quantum yield dropped by a factor of nearly 3 for a pure chlorophyl layer [35]. This effect was interpreted as concentration quenching which cannot be further discussed here. The same authors have also investigated chlorophyl multilayers, all of which were deposited using the Langmuir-... 

Fig. 10.15 Dependence of quantum efficiency (solid curve) and photocurrenl (dashed curve) at 675 nm on the number of chlorophyl monolayers on Sn02 at 0.1 V (SCE). Electrolyte. pH 6.9 + 0.05 hydroquinone. (After ref. [38])...

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). 

In contrast to cyanine dyes and Ru complexes, fairly concentrated chlorophyll monolayers could be deposited on SnOj. In the latter case, the quantum yield of the photocurrent was determined as a function of the molar ration of chlorophyll... 

T. Watanabe, T. Miyasaka, A. Fujishima, and K. Honda, Photoelectrochemical study on chlorophyll monolayer electrodes, Chemistry Letters, vol. 7, no. 4,... 

Photoexcited fluorescence from spread monolayers may be studied [158,159] if the substance has both a strong absorption band and a high emission yield as in the case for chlorophyll [159]. Gaines and co-workers [160] have reported on the emission from monolayers of Ru(bipyridine)3, one of the pyridine ligands having attached C g aliphatic chains. Ruorescence depolarization provides information about the restriction of rotational diffusion of molecules in a monolayer [161], Combining pressure-area... 

General Technique for Chlorophyll Utilization. In photoelectrochemical measurements, the methods commonly employed for preparing Chi interfacial layers on electrode substrates are solvent evaporation, electrodeposition (for crystalline Chi), and monolayer deposition techniques, as outlined previously. 

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

Hydrophobic and/or van der Waals interaction has not been well studied. Only as late as 1988 were Yamamura et al. [143] venturesome enough to choose octadecylsilyl monolayers as a polymeric material for hydrophobic interactions. Templates such as adamantane, cholesterol, vitamin Kj, pyrene derivatives, and chlorophyll, amongst others, formed ester bonds to a long alkyl- or oligooxy-ethylene chain and these were present when octadecylsilylchloride was covalently bound to a SnOj electrode surface. The electrode surface was freed from the... 

Pigmented Monolayer Membrane on the Solid Surface. Monolayers and mixed monolayers of pigments can be prepared on a neutral aqueous buffer solution surface and deposited, at a controlled surface concentration, onto a solid substrate by the Langmuir-Blodgett technique (63,64). As a typical feature, a monolayer membrane of chlorophyll a possesses absorption peaks at 675-680 nm and 435-440 nm with corresponding optical sensltles of 0.008-0.01 and 0.01-0.013 in the red and blue bands, respectively (65). 

The phthalocyanines are still of some interest, and Ayers has interpreted the photoelectrochemistry of copper phthalocyanine in terms of the band-model for semiconductors. Menzel et alf have studied the photoconductivity and electrical field-induced fluorescence quenching in metal-free phthalocyanine films. Doping of the film increases charge-carrier photogeneration, and it seems likely that the mechanism is extrinsic, involving field-assisted exciplex dissociation. These are complicated materials, which are difficult to purify, and the mechanisms of carrier generation in polycrystalline and amorphous films are still obscure. The porphyrins and monolayer assemblies of chlorophyll on SnOj electrodes 2 72,273 ijggjj studied. 

Janzen, A. F., Bolton, J. R., Stillman, M. J. (1979). Photochemical electron transfer in monolayer assemblies. 1. Spectroscopic study of radicals produced in chlorophyll a/acceptor systems, J. Am. Chem. Soc., 101 6337. 

Moncelli MR, Becucci L, Dolfi A et al. Monolayers and multilayers of chlorophyl a on a mercury electrode. Bioelectrochem 2002 56 159-162. 

Volkov AG, Gugeshashvili MI, Zelent B et al. Light energy conversion with chlorophyll a and pheophytin a monolayers at the optically transparent Sn02 electrode Artificial photosynthesis. 

Attention should be drawn at this point to a distinctive feature important for phospholipid vesicles. It was found that the electron transport reaction goes with different efficiency on the outside and inside of the bilayer, which is due to different lateral mobility of chlorophyll in the external and internal monolayers and different possibilities of recombination of the charges arising under illumination.101 On the opposite side, inside the liposome the reducer is... 

Saga, Y. and H. Tamiaki (2004). Facile synthesis of chlorophyll analog possessing a disulfide bond and formation of self-assembled monolayer on gold surface. J. Pho-tochem. Photobiol. B 73(1-2), 29-34. 

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