Electron monolayer assembly

Figure 29. For the latter case, it is considered that the semiconducting re-electron systems are separated by insulating hydrocarbon spacers, resulting in alternate thin layers of organic semiconductor and insulator in these monolayer assemblies. The direct current - voltage (I - V) characteristics were mea- sured for the multilayers H2Pc(R)8 and Cu-Pc(R)8 in directions perpendicular and parallel to the film plane. In both cases, the linear I - V relationships of Ohm s law were observed at low electric field and obtained DC conductivities are summarized in Table 3. The normal conductivity (ajJ were ca. 10 13 S cm-1, while the lateral ones p//) were 3.4 x 10-7 and 9.9 x 10 7 S cm 1 for films of the metal-free and copper Pc derivatives, respectively. The former (ojJ tended to decrease slightly with increase of Figure 29. Schematical illustration of the substituent alkyl chain length,...

Figure 1. Schematic representation of the artificial photosynthetic reaction center by a monolayer assembly by A-S-D triad and antenna molecules for light harvesting (H), lateral energy migration and energy transfer, and charge separation across the membrane via multistep electron transfer (a) Side view of mono-layer assembly, (b) top view of a triad surrounded by H molecules, and (c) energy diagram for photo-electric conversion in a monolayer assembly.

Keywords Metal vapor deposition Metal-organic interface Molecular devices Molecular electronics Self-Assembled Monolayers... 

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

When amphiphilic dyes are incorporated into such monolayers, their fluorescence properties can be used to characterize the organization of the array [84-87]. An amphiphilic cyanine dye used in this way showed a much lower distance damping factor (P value) between layers containing the donor cyanine and the acceptor viologen (P = 0.3) than is usually observed in fixed donor-acceptor systems [88]. An even smaller p value was obtained from photocurrent measurements in monolayer assemblies containing amphiphilic porphyrins (p = 0.005) [89], The former value was attributed to electron tunnelling occurring in parallel... 

Willner et al. [52] have created some elegant interfacial supramolecular assemblies to address this issue by removing the non-covalently bound flavin adenine dinucleotide (FAD) redox center from glucose oxidase and immobilizing the enzyme on a tether consisting of cystamine chemisorbed on a gold surface, a pyrroloquinoline quinone (PQQ) link and FAD. The mediator potential and electron transfer distances of this assembly were carefully chosen so that transfer of electrons from the FAD to the PQQ and to the electrode is very fast. A maximum rate of 900 150 s-1 for the enzymatic reaction within this monolayer assembly was obtained, which is indistinguishable from the value of about 1000 s-1 obtained for the enzyme in solution. While monolayers can offer molecular-level control of the interfacial structure, the... 

H. Kuhn, Electron-Transfer in Monolayer Assemblies , Pure Appl. Chem., 51, 341 (1979)... 

Mediation of electron transfer between a dissolved redox probe and the electrode by surface-confined redox groups has also been used to assess the integrity of electroactive monolayer assemblies. As an example, the reduction of Ru (NH3)e, normally occurring at about 0.1 V vs. Ag/AgCl, is fully blocked by an assembly containing tethered A-ethylviologen redox probes. However, at the observed reduction potential of the viologen probes (ca. —0.4 V), these probes mediate the reduction of Ru (NH3)g [87]. Similarly, the delayed oxidation of Fe (CN)6 at... 

Cysteamine was used to couple redox-active carboxylalkyl-4,4 -bipyridinium salts to the gold surface . The nonordered monolayer assembly was then transformed into a densely packed monolayer with 1-hexadecanethiol and cyclic voltammetry of the surface bound viologen was performed. The electron transfer rate constants to the bipyridinium sites depended on the alkyl chain length Abridging the redox site to the electrode. Electron transfer rate constants followed the Marcus theory. Cysteic-acid-active ester monolayers chemisorbed on gold were used to electrode-immobilize the protein glutathione reductase, then a bipyridinium carboxylic acid was condensed onto the enzyme in the presence of urea to wire the protein towards electrochemical reduction (Figure 6.26). 

Figure 3. Monolayer assembly on a hydrophobic support for the investigation of photoinduced electron transfer. The N,N -dioctadecyIoxacyanine dye is excited with UV radiation and transfers an electron to the electron acceptor, N,N -diocta-decyl-4,4 -bipyridinium, in the adjacent monolayer.

Supersensitization of Photoinduced Electron Transfer in Monolayer Assemblies... 

Photoinduced electron-transfer reactions have also been observed in monolayer assemblies (23, 24). Such systems can be made essentially free of molecular diffusion and thus most closely resemble the solid state. They can be fabricated with precise geometry and therefore provide well-defined structures often lacking in amorphous solids or adsorbed layers. However, until now no evidence for enhancement of the yield of reaction (1) by added supersensitizers had been obtained, although the concept had been discussed (25). 

This work explored whether supersensitization of electron transfer could be demonstrated in a monolayer assembly. We also studied the properties of excited cyanine dyes, the class of compound used almost exclusively as photographic spectral sensitizers (26). Kuhn, Mtfbius, and co-workers have demonstrated the detailed information that can be gained on energy and charge-transfer properties through the use of monolayer assemblies of such dyes (23, 27). We emphasized investigations of cyanine dye... 

Figure 2. Arrangement of layers in monolayer assembly of cyanine sensitizer (C) and viologen electron acceptor (A) on glass coated with an arachidic acid spacer...

Although the fluorescence quenching by viologen electron acceptor measures the extent of the primary electron transfer from excited dye [reaction (1)], it cannot be used to determine the supersensitizing influence of an added third component that may act by a mechanism such as reaction (3) that does not influence the quenching reaction. It is thus necessary to measure A directly. This is relatively easy in solution but, because of the low concentrations involved, rather difficult in monolayers. However, the electron-adduct radical is relatively stable and in fact has been detected optically in a monolayer assembly (33,... 

We have constructed a monolayer assembly in which electron transfer occurs from excited cyanine dye J aggregates to a violo-gen electron acceptor. This process is manifested by efficient, rapid quenching of the dye fluorescence accompanied by slow, inefficient growth of relatively stable radicals, detected by absorption. If electron donors are added to either acceptor or dye layer, this radical yield can be enhanced. This enhancement or supersensitization can be as large as a factor of 3.5, depending on donor and conditions. The supersensitization is due to electron transfer from donor to either excited dye or the dye radical cation. The net effect is photocatalyzed electron transfer from donor to acceptor. Electron transport through the dye aggregate appears. to be necessary for supersensitization. 

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. 

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