Molecular electron transport

Section 5 is on one particular molecule, p-benzene dithiol. This is one of the most commonly studied molecules in molecular electronic transport junctions [7] (although it is also one of the most problematic). Section 6 discusses a separate measurement, inelastic electron tunneling spectroscopy [8, 9] (IETS). This can be quite accurate because it can be done on single molecules at low temperatures. It occurs because of small perturbations on the coherent transport, but it can be very indicative of such issues as the geometrical arrangement in the molecular transport junction, and pathways for electron transport through the molecular structure. 

Electron-transporting layers have been used in conjunction with conjugated polymers by several other groups, with a similar effect.64,65 Unfortunately, the stability of LEDs using molecular electron-transporting materials dispersed in polymeric matrices tends to be poor, probably due to slow recrystallization of the electron-transporting molecules. This problem has led to the development of various polymeric electron-transporting materials, which show improved stability.66... 

For the attainment of marvelous electron transfer processes in the natural sequential potential fields, many noncovalentaly-bound donor-acceptor (DA) systems and covalently-bound DA systems " " have been previously reported. Most of them are artificial models of the photosynthesis comprising simple assemblies of the dyad (DA) or triad [donor-spacer-acceptor (DSA)] functional molecules with a chromophore such as a porphyrin. The quantum efficiency of such systems is lower (<25%) compared with the biological systems (=100%), and thus more efforts for constructing more efficient systems are necessary. Some of the covalen-taly-bound DA systems have been designed for the fabrication of molecule-scale devices based on a molecular electron-transport wire and/or highly ordered molecular arrays on the surface. " Most of such studies employed the DA nonconju-gated molecules. 

Microporous inorganic solids, such as zeolites, clays, and layered oxide semiconductors offer several advantages as organizing media for molecular electron transport assemblies. Because these materials are microcrystalline, their internal pore spaces have well-defined size and shape. This property can be exploited to cause self-assembly, by virtue of size exclusion effects, ion exchange equilibria, and specific adsorption, of photosensitizers, electron donors, and electron acceptors at the solid/solution interface. 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

The electron transport protein, cytochrome c, found in the mitochondria of all eukaryotic organisms, provides the best-studied example of homology. The polypeptide chain of cytochrome c from most species contains slightly more than 100 amino acids and has a molecular weight of about 12.5 kD. Amino acid sequencing of cytochrome c from more than 40 different species has revealed that there are 28 positions in the polypeptide chain where the same amino acid residues are always found (Figure 5.27). These invariant residues apparently serve roles crucial to the biological function of this protein, and thus substitutions of other amino acids at these positions cannot be tolerated. 

Wall Piece IV (1985), a kinetic sculpture by George Rhoads. This complex meehanieal art form can be viewed as a metaphor for the molecular apparatus underlying electron transport and ATP synthesis by oxidative phosphorylation. (1985 ty George Rhoaeh)... 

As we have seen, the metabolic energy from oxidation of food materials—sugars, fats, and amino acids—is funneled into formation of reduced coenzymes (NADH) and reduced flavoproteins ([FADHg]). The electron transport chain reoxidizes the coenzymes, and channels the free energy obtained from these reactions into the synthesis of ATP. This reoxidation process involves the removal of both protons and electrons from the coenzymes. Electrons move from NADH and [FADHg] to molecular oxygen, Og, which is the terminal acceptor of electrons in the chain. The reoxidation of NADH,... 

The electron transport chain involves several different molecular species, including ... 

Consider the oxidation of succinate by molecular oxygen as carried out via the electron transport pathway... 

Write a balanced equation for the reduction of molecular oxygen by reduced cytochrome e as carried out by complex IV (cytochrome oxidase) of the electron transport pathway. 

Another approach to molecular assembly involves siloxane chemistry [61]. In this method, the electrically or optically active oligomers are terminated with tii-chlorosilane. Layers are built up by successive cycles of dip, rinse, and cure to form hole transport, emissive, and electron transport layers of the desired thicknesses. Similar methods have also been used to deposit just a molecular monolayer on the electrode surface, in order to modify its injection properties. 

Fig. 3-4 Electron transport process schematic, showing coupled series of oxidation-reduction reactions that terminate with the reduction of molecular oxygen to water. The three molecules of ATP shown are generated by an enzyme called ATPase which is located in the cell membrane and forms ATP from a proton gradient created across the membrane.

Interfaces between two different media provide a place for conversion of energy and materials. Heterogeneous catalysts and photocatalysts act in vapor or liquid environments. Selective conversion and transport of materials occurs at membranes of biological tissues in water. Electron transport across solid/solid interfaces determines the efficiency of dye-sensitized solar cells or organic electroluminescence devices. There is hence an increasing need to apply molecular science to buried interfaces. 

McLendon, G. Control of Biological Electron Transport via Molecular Recognition and Binding The Velcro Model. Vol. 75, pp. 159-174. 

---