Redox relay

The photoelectrochemical properties of 283 colloids prepared by chemical solution growth [193] have been demonstrated by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nation were found to provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. 

Carbides were first proposed as anodes for H2 ionization in electrochemical power sources [422], The higher activity of WC with respect to W for H2 evolution was discovered about forty years ago [423], but the first practical proposals for the use of carbides as cathodes are found only recently under the influence of research aimed at the development of more efficient water electrolyzers [424]. More recently, aqueous suspensions of WC have been proposed to catalyze H2 formation in the presence of the reduced form of a redox relay that is continuously generated through a photochemical reaction [425]. 

The recombination behavior of this immobilized dyad is illustrated in Figure 6.30, which shows the time-resolved transient absorption spectra both without and in the presence of I—/13. This figure shows clearly that recombination in the absence of the redox relay is slow and takes place on the microsecond time-scale. 

Considering that the injection is faster than 10 ns, i.e. faster than the excited-state lifetime, a considerable degree of charge separation is obtained in these interfacial supramolecular triads. As shown for other systems, the incorporation of a solid substrate is very beneficial in this respect. Particularly surprising at first instance is the slow recombination reaction in the presence of the redox relay. In the presence of the I /13 redox couple, the following reaction is expected ... 

Fig. 16. Structures of different types of ligands acting as two-electron redox relays in natural and artificial systems Flavins such as (11) are the essential constituents of flavodoxines and flavoproteins 137). The a-ketoglutarate anion (a-KG, 12) is a typical example of a sacrificial redox mediator which decomposes during catalysis (72). Synthetic chelates such as bis-arylimino-acenaphthene (BIAN, 13) have been proposed for the development of bio-inspired multielectron transfer photosensitizers 138).

For example, C-Cl bonds can be easily cleaved by reduction because of their low-lying cr orbitals, whereas C-N bonds are much more difficult to cleave. Hence, it is advisable to incorporate the cr (C-N) into a (cr 4-ti ) system by overlap of cr (C-N) with a low-lying n (e.g. from nitrobenzene). If overlap of the a with a n is not possible, we still can set up a low-lying n redox relay to shuffle the electron to the a by intramolecular ET. 

Bond cleavage after intramolecular ET via n redox relays... 

Common n acceptors as redox relays for reductive bond cleavage. Combine... 

Scheme 16. Frequently used n acceptors that function as redox relays to shuttle electrons to the...

Frequently, bond cleavage is used for R-Hal —> R-H dehalogenations [74], and the formation of carbanions [75]. More seldom encountered are reactions of n acceptors that have the a bond spatially well separated and that depend on long-range ET. Such a situation occurs in phenyl-substituted alkyl chlorides [76], and bridgehead halides [77] with various redox relay functions (e.g. a nitroaryl group) [78]. 

The elegant synthesis of the harringtonine alkaloid skeleton might involve the photoinduced oxidation of an electron-rich phenyl group that acts as a redox relay by accepting an electron from the o-(C-Si) donor [188]. 

Electrochemistry of Enzymes at Electrodes Functionalized with Monolayers of Redox Relays... 

Electrochemical Activation of Enzymes by the Attachment of Redox Relays to the Protein Backbone... 

Enzyme entrapment in polymers functionalized with redox relay units... 

A major advance in the construction of electrically contacted enzyme electrodes involves the structural alignment of the enzyme redox center with respect to the electrode interface in conjunction with the site-specific positioning of a redox relay component between the enzyme redox center and the electrode. The design of such electrodes promotes a new level of molecular architecture of biomolecules on surfaces, enabling us to optimize the electrical contact of the resulting enzyme elec-... 

Photoswitchable electrical communication between enzymes and electrodes has also been achieved by the application of photoisomerizable electron-transfer mediators [195, 199]. DilTusional electron mediators (viologen or ferrocene derivatives) were functionalized with photoisomerizable spiropyran/merocyanine units. These mediators can be reversibly photoisomerized from the spiropyran state to the merocyanine state (360 < A < 380 nm) and back (A > 475 nm). An enzyme multilayer array composed of glutathione reductase or glucose oxidase was electrically contacted only when the photoactive group linked to the redox relay (viologen or ferrocene derivative, respectively) was in the spiropyran state. 

Subsequently, Schuhmann reported GOx entrapment by ferrocene derivatives attached to the enzyme outer surfrce via long flexible polyethylene spacer chains within polyp5Trole films [126]. However, this system still did not significantly improve sensor sensitivity and the author pointed out that modifications are needed to increase the concentration of redox relays to enhance the sensor efficiency. 

However, ET efficiency is not only dependent on the distance of the involved redox relays but also on the properties of the electrode material, the nature of the enzyme, the properties of the immobilization matrix, and the redox mediator (if any) in a complex manner. 

---