Redox process light-induced

Flavins — Riboflavin is first of all essential as a vitamin for humans and animals. FAD and FMN are coenzymes for more than 150 enzymes. Most of them catalyze redox processes involving transfers of one or two electrons. In addition to these well known and documented functions, FAD is a co-factor of photolyases, enzymes that repair UV-induced lesions of DNA, acting as photoreactivating enzymes that use the blue light as an energy source to initiate the reaction. The active form of FAD in photolyases is their two-electron reduced form, and it is essential for binding to DNA and for catalysis. Photolyases contain a second co-factor, either 8-hydroxy-7,8-didemethyl-5-deazariboflavin or methenyltetrahydrofolate. ... 

The present technique enables light-induced redox reaction UV light-induced oxidative dissolution and visible light-induced reductive deposition of silver nanoparticles. Reversible control of the particle size is therefore possible in principle. The reversible redox process can be applied to surface patterning and a photoelectrochemical actuator, besides the multicolor photochromism. 

Light-Induced Redox Processes at the Semiconductor-Electrolyte Interface... 

Of major interest in geochemistry and in natural water systems are semiconducting minerals for which the absorption of light occurs in the near UV or visible spectral region and as a result of which redox processes at the mineral-water interface are induced or enhanced. Table 10.1 gives band gap energies of a variety of semiconductors. 

Of more apparent significance in the aquatic environment are redox processes induced or enhanced on absorbance of light by chromophores at metal oxide surfaces in which the metal of the oxide lattice constitutes the cationic partner. Light induced electron transfer within such a chromophore often results in disruption of the oxide lattice. The photoredox-induced dissolution of iron and manganese oxides by such a mechanism has been proposed as a possible means of supply of essential trace-metal nutrients to plants and aquatic organisms (29-31). ... 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

When interaction between the metal-based components is weak, polynuclear transition metal complexes belong to the field of supramolecular chemistry. At the roots of supramolecular chemistry is the concept that supramolecular species have the potential to achieve much more elaborated tasks than simple molecular components while a molecular component can be involved in simple acts, supramolecular species can performIn other words, supramolecular species have the potentiality to behave as molecular devices. Particularly interesting molecular devices are those which use light to achieve their functions. Molecular devices which perform light-induced functions are called photochemical molecular devices (PMD). Luminescent and redox-active polynuclear complexes as those described in this chapter can play a role as PMDs operating by photoinduced energy and electron transfer processes. ... 

Single Electron Transfer (SET) has an important place in many bimolecular reactions.277 Several types of initiation have been employed. When this transfer is induced by light, which provides sufficient redox potential difference between the two interacting molecules to initiate it, the process is known as the Photoinduced Electron Transfer (PET) process.278,279 Light-induced electron transfer provides an excellent synthetic means to alkylate MSMA. 

Many metal chalcogenides and oxides are known to be semiconductors. These materials can act as sensitizers for light-induced redox processes due to their electronic structure consisting of a valence band with filled molecular orbitals (MO s) and a conduction band with empty MO s. Absorption of a photon with an energy above the bandgap energy Eg generally leads to the formation of an electron/hole pair in the semiconductor particle (18). ... 

As far as the external action is concerned, several external stimuli can be conveniently used to achieve rotation of one ring into the other some success has been achieved by chemical means like addition of protons [60, 89, 90], demetalation of catenates [90, 91] or change of solvent [85], but by far the most convenient system for its ease of control is a redox action which can be either chemically, electro-chemically, or photochemically induced. The latter method, based on light-induced electron transfer processes, could take advantage of the development of laser technology and grant a complete control both in the space and in the time domain. 

Upon irradiation, redox dye photosensitizers adsorbed on the surface of wide-bandgap metal oxide semiconductors readily inject an electron in the conduction band of the solid. While charge injection has been found for numerous efficient systems to occur in the femtosecond time frame, the electron back transfer takes place much more slowly, typically in the microsecond-millisecond domain. This charge recombination process can be intercepted by reaction of a reducing mediator M with the oxidized dye (Eq. (43)). The overall efficiency of the light-induced charge separation then depends upon the kinetic competition between back electron transfer and dye regeneration processes. 

Chromophore absorbing light to induce heterogeneous redox processes (including reductive dissolution of higher-valent oxides) (semiconductors)... 

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