Redox molecular orientation

The molecular orientation and interactions of redox chromophores are very important in controlling photoresponses at the molecular level. Absorption and fluorescence spectra will give important information on them. We have studied, photoresponses, specific interactions, in-plane and out-of-plane orientation of various chomophores in LB films composed of amphiphiles shown in Figure 1 [3-12]. 

The LB deposition is one of the best methods to prepare highly organized molecular systems, in which various molecular parameters such as distance, orientation, extent of chromophore interaction, or redox potential can be controlled in each monolayer. We have been studying photophysical and photochemical properties of LB films in order to construct molecular electronic and photonic devices. The molecular orientation and interactions of redox chromophores are very important in controlling photoresponses at the molecular level. Absorption and fluorescence spectra give important information on them. We have studied photoresponses, specific interactions, and in-plane and out-of-plane orientation of various chromophores in LB films [3-11], In addition to the change of absorp-... 

Estimation of the Molecular Orientation on the Electrode Surface using the Redox ER Signal... 

Let us consider the ER signal due to the redox reaction of a surface-confined dye molecule. For simplicity, we assume that, for an electrode/adsorption layer incorporating a chromophore/solution interface, the absorption of the oxidized form is negligibly small, i.e. the oxidized form is colorless. We also assume that the electric dipole moment of the reduced form is of a single hnear dipole and that it has a unique director angle f with respect to the surface normal while its azimuthal angle is two-dimensionally isotropic (Fig. 2.15a). The angle

director vector with respect to the surface normal represents the molecular orientation. 

Under potentiostatic conditions, photoinduced heterogeneous electron transfer between specifically adsorbed porphyrins and redox couples confined to the organic phase manifests itself by photocurrent responses. As in the case of dynamic photoelectrochemistry, these photoresponses provide information on the dynamics of heterogeneous electron transfer and recombination processes. In addition, we shall demonstrate that photocurrent measurements can be used to characterise the interfacial coverage of the specifically adsorbed porphyrins as well as their molecular orientation. 

SERS has found application across a broad range of electrochemical investigations, including studies of morphological changes associated with redox states (91). It has been used to dynamically determine molecular orientation, and to characterize the structure of... 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

The Langmuir-Blodgett deposition is one of the best methods to prepare highly organized molecular systems, in which various molecular parameters such as distance, orientation, extent of chromophore interaction, or redox potential can be controlled in each monolayer. We have been studying... 

We have shown that redox chromophores organized in LB films with resped to their orientation, alignment, or electronic interactions make very useful and specific photoresponses such as amplified fluorescence quenching, photocurrents controlled at the molecular level, photoinduced anisotropic eledrochromism, and photochemically modulated second harmonic generation. These results may contribute to facilitate the design and construction of novel photonic devices in the near future. 

Equations 10 and 11 indicate that the redox potential of the HQ/BQ couple is shifted in the negative direction when ti6-chemisorbed but shifted in the positive direction when 2,3-ti2-bonded. This orientation-dependent shift in redox potential is not unexpected by analogy with molecular organometallic compounds. For example, the redox potential for the reversible, one-electron reduction of duroquinone in acetonitrile is shifted from -0.90 V (vs. SCE) to -0.69 V in bis (duroquinone) Ni (0) and to -1.45 V in (1,5 — cyclooctadiene) (duroquinone)Ni (0) (22.) ... 

This interpretation is based on the fact that the variation of the overall charge which occurs when the metal complex undergoes a redox process influences the number of solvent molecules surrounding the complex (which are hence electrostatically orientated), as well as the number of solvent molecules which form hydrogen bonds with the complex. The change in the number of solvent molecules near the metal complex can be translated into an increase or decrease of the molecular order of the entire system and, hence, into an increase or decrease of entropy. 

Not all cytochromes from sulfate-reducing bacteria reduce Fe(III) or other metals. D. vulgaris produces a cyt C553, which has a molecular mass of 9 kDa, midpoint redox potential of OmV, and a single heme and the iron atom is coordinated by histidine methionine. It is unclear at this time if the inability of this cyt C553 to reduce metals is due to lack of a bishistidinyl iron coordination or to some other factor, such as steric hinderance owing to orientation of heme in the protein. 

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