Redox-active dyes

Worked Example 7.3. A redox-active dye is eluting from an HPLC column. Since the analyte is redox-active, the HPLC detector is unusual in that it consists of a small annulus of silver, mounted within a short Teflon tube. Eluent from the column contains analyte, trickling at a constant rate, V, through the cell and over the electrode while the current is monitored. It is assumed that the silver ring only sees the redox-active dye, i.e. the current is wholly faradaic. 

The understanding of the molecular mechanism leads to tailor-made catalytic systems. These do not necessarily have to be of pure native origin. Recent work indicates the potential of engineering sites for surface binding and redox active dyes, de novo designed redox proteins and genetic chimeras [45,48,50]. Thus tailor-made systems are foreseen. 

Up until 1977, the non-covalent polymeric assemblies found in biological membranes rarely attracted any interest in supramolecular organic chemistry. Pure phospholipids and glycolipids were only synthesized for biophysical chemists who required pure preparations of uniform vesicles, in order to investigate phase transitions, membrane stability and leakiness, and some other physical properties. Only very few attempts were made to deviate from natural membrane lipids and to develop defined artificial membrane systems. In 1977, T. Kunitake published a paper on A Totally Synthetic Bilayer Membrane in which didodecyl dimethylammonium bromide was shown to form stable vesicles. This opened the way to simple and modifiable membrane structures. Since then, organic chemists have prepared numerous monolayer and bilayer membrane structures with hitherto unknown properties and coupled them with redox-active dyes, porous domains and chiral surfaces. Recently, fluid bilayers found in spherical vesicles have also been complemented by crystalline mono-... 

The most important fluorescing natural compounds are porphyrins, flavins, and reduced nicotinamide. The most important quenchers are triplet oxygen, redox active dyes, radicals, and transition metal ions. 

The main objective of this chapter is to illustrate how fundamental aspects behind catalytic two-phase processes can be studied at polarizable interfaces between two immiscible electrolyte solutions (ITIES). The impact of electrochemistry at the ITIES is twofold first, electrochemical control over the Galvani potential difference allows fine-tuning of the organization and reactivity of catalysts and substrates at the liquid liquid junction. Second, electrochemical, spectroscopic, and photoelectrochemical techniques provide fundamental insights into the mechanistic aspects of catalytic and photocatalytic processes in liquid liquid systems. We shall describe some fundamental concepts in connection with charge transfer at polarizable ITIES and their relevance to two-phase catalysis. In subsequent sections, we shall review catalytic processes involving phase transfer catalysts, redox mediators, redox-active dyes, and nanoparticles from the optic provided by electrochemical and spectroscopic techniques. This chapter also features a brief overview of the properties of nanoparticles and microheterogeneous systems and their impact in the fields of catalysis and photocatalysis. 

A way to reduce interferences by cooxi-dizable sample constituents is by keeping the applied electrode potential as low as possible. Therefore, a reaction partner is chosen to be electrochemically indicated that is converted at low potential. For this purpose, the natural electron acceptors of many oxidoreductases have been replaced by redox-active dyes or other reversible electron mediators. Among them are the ferricyanide/ferrocyanide couple, V-methylphenazinium sulfate, fer-rocenes, and benzoquinone. With these mediators an electrode potential around -1-200 mV can be applied, which decreases... 

Selective reduction of biomolecules by dihydrogen using soluble platinum carbonyl clusters and redox active dyes as catalysts are known (see S. Bhaduri et al, J. Am. Chem. Soc., 1998, 120, 12127-12128). Based on this work, suggest a plausible catalytic scheme for the reduction of dinitrogen to ammonia by dihydrogen. 

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

A related problem associated with efforts to characterize redox conditions of environmental materials is the lack of equilibrium among the chemical constituents of an environmental system (138-141) or between the environmental constituents and a sensor material (142). Thus, even techniques that are based on specific redox active species—such as H2 (143-146), Hg (147), indicator dyes (148, 149), or other mediators (137)— cannot provide a general characterization of redox conditions. However, we do recommend techniques that quantify the activity of specific oxidants or reductants, because they are necessary for the rigorous application of the approach Section 5.1 describes. Similar considerations apply to the characterization of redox kinetics. 

The mixture of 85 and methylene blue 86 [66] at pH 3 showed the characteristic blue color ( =665 nm) of the dye. Upon addition of F", the oxidation potential of the redox-active ferrocene unit decreased. Under these conditions, the ferrocene group was able to reduce the dye as detected by the disappearance of the band at 665 nm. Other anions had no effect on the ferrocene and dye units. Therefore, this system can be used as a colorimetric detector for fluoride. 

Alternatively, charge injection into the semiconductor can involve the reductive or oxidative quenching of the dye excited state by a redox-active species (a supersensitizer (S)), followed by thermal interfacial electron transfer ... 

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