Photoactivated chemical reaction

A fast reaction technique that employs sudden photoactivation or photolysis to initiate or alter a chemical reaction system. This sudden perturbation creates a nonequilibrium situation, allowing one to determine the time course of the relaxation of a chemical reaction system back to equilibrium. 

Most reactions on surfaces are complicated by variations in mass transfer and adsorption equilibrium [70], It is precisely these complexities, however, that afford an additional means of control in electrochemical or photoelectrochemical transformations. Not only does the surface assemble a nonstatistical distribution of reagents compared with the solution composition, but it also generally influences both the rates and course of chemical reactions [71-73]. These effects are particularly evident with photoactivated surfaces the intrinsic lifetimes of both excited states and photogenerated transients and the rates of bimolecular diffusion are particularly sensitive to the special environment afforded by a solid surface. Consequently, the understanding of surface effects is very important for applications that depend on chemical selectivity in photoelectrochemical transformation. 

Phototoxicity is photosensitivity that is independent of immunological responses. Phototoxic responses are dose dependent and will affect almost anyone when sufficient dosage is applied or when taken concurrent with UV exposure. In phototoxic reactions, photoactivated chemicals cause direct cellular damage. UV absorption produces either excited state chemicals or metabolites of these chemicals. These, in turn, can be converted into either free radicals or singlet oxygen, either of which results in biomo-lecular oxidation)10 ... 

If the chemical reactions of the photoactivated intermediate were completely nonspecific, the thermodynamic dissociation constant, Kt, would be the primary consideration (assuming that the reactive intermediate has the same Ki), Since there may be highly reactive entities outside the site, such as other macromolecules or water, the rate of dissociation and the chemical half-life of the intermediate are also important. Further, noncovalently bound reaction products can prevent stoichiometric labeling by their occupancy of the ligand binding site. 

According to the existing notions, unimolecular processes occur at a non-zero rate only if the reacting molecules possess an internal energy exceeding a certain threshold value known as the activation energy. Such molecules are called active. Active molecules are produced in the course of a chemical reaction either by inelastic collisions with the heat bath molecules (thermal activation) or by photoactivation, by light irradiation, electron impact, etc. (non-thermal activation). 

Today, ultrafast pulsed-laser techniques, high-speed computers, and other sophisticated instrumentation make it possible to measure the time evolutions of reactants, intermediates, transition structures, and products following an abrupt photoactivation of a starting material. Detailed theoretical calculations, experienced judgments based on the literature, and newly accessible femtosecond-domain experimental data providing observed intensities of chemical species versus time can provide insights on the atomic-scale events responsible for overall reaction outcomes. 

Chemically modified DNAs can also be used as hybridization probes, provided that the modification does not interfere with the formation of hybrid DNA molecules. A psoralen biotin label has also been developed. Psoralen is a photoactivable agent that can intercalates into single- or double-stranded nucleic acids. On irradiation at 365 nm, it will covalently bind to the probes. This labeling reaction is simple and straightforward. However, the reagents for labeling and detection are only available in a kit format. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Adsorption of dyes onto a semiconductor surface allows for another mode of photoactivation [44-48]. The dye adsorbs a photon, generating an excited state in which a sufficiently higher-energy orbital is populated to allow direct injection of an electron into the conduction band edge [1]. The dye thus becomes oxidized and can either react chemically with nucleophiles by bond formation or can be restored to its original oxidation level by electron transfer. In the latter case, the reaction partner is oxidized, regenerating the ground state of the sensitizer ready to participate in... 

Photons of energies 2—8eV incident on gas/solid interfaces may produce, in addition to the photophysical processes considered above, the rupture and/or formation of chemical bonding within adsorbates or between them on the surface. These photochemical processes at the interface may be further subdivided into (i) photoassisted surface reactions yielding products which remain at the interface and so irreversibly alter its chemical composition and reactivity in the selected reaction conditions and (ii) photocatalytic processes wherein the products from photoinitiated reactions at the interface are continuously removed to the gas phase in the reaction conditions (e.g. by thermally assisted or photo-assisted desorption) with the result that the active surface is continuously regenerated and can become responsible for high turnover accomplished per photoactivated surface site (t.a.p.s. ). 

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