Photo-initiated electron transfer

Brown, A. R., L. J. Yellowlees, and H. H. Girault, Photo-initiated electron transfer reactions across the interface between two immiscible electrolyte solutions, J Chem Soc Faraday T, Vol. 89, (1993) p. 207. 

Vision involves cis-trans photoisomerization of a chromophore and many studies have been done using different models/ For example, a CASSCF/AMBER procedure has been used to study the nonadiabatic dynamics of retinal in rhodopsin proteins/ In another study, a simple model of a photosynthetic center was examined by Worth and Cederbaum. They proposed that the presence of conical intersections facilitated the long-range intermolecular photo-initiated electron transfer between the protein s porphyrin and a nearby quinone. Semiempirical methods and QM/MM methods have been developed by Martinez and coworkers " to study the cis-trans isomerization dynamics of the Green Fluorescent Protein chromophore in solution, which occurs through conical intersections.The chromophore in this protein consists of two rings connected with a double bond and has been studied in vacuo as well. ... 

Oxidations initiated by thermally induced electron transfer in an oxygen-CT complex represent the thermal analog of the Frei photo-oxidation and are properly classified as hybrid type IlAOi-type IIaRH oxidations (Fig, 2), Such reactions require either zeolites with high electrostatic fields or substrates with low oxidation potentials. In addition, elevated temperatures are known to promote the thermally initiated electron-transfer step, although the possible intrusion of a classical free-radical initiation chain oxidation at higher temperatures must be considered. 

Intermolecular photo-oxidation-reduction reactions involve a light initiated electron transfer between a complex and any other suitable molecule available in the medium. An oxidized or a reduced form of the complex may be obtained. 

Electron impact mass spectrometry of the cyclobutanedione (24) gives rise to dimethylcarbene radical cation.35 Appearance energy measurements and ah initio calculations indicated that the radical cation lies 84 kJ mol 1 above the propene radical cation and is separated from it by a barrier of 35 kJ mol-1. Diarylcarbene radical cations have been generated by double flash photolysis of diaryldiazomethanes in the presence of a quinolinium salt (by photo-induced electron transfer followed by photo-initiated loss of N2).36 Absolute rate constants for reactions with alkenes showed the radicals to be highly electrophilic. In contrast to many other cation radicals, they also showed significant radicophihc properties. 

A transfer of the excitation energy from the donor to the acceptor will occur when an energy acceptor molecule is placed at the proximity of an excited energy donor molecule. After energy transfer, the donor relaxes to its ground state and the acceptor is promoted to one of its excited states. A photo-induced electron transfer can be initiated after photoexcitation when an excited single electron in the LUMO of the electron donor is transferred to a vacant molecular orbital (LUMO) of the acceptor. 

In 1983 Bryant and Coyle described the cyclization of N-(co-morpholino-alkyl)-maleimides 107, which is initiated by a photo-induced electron transfer (Sch. 35) [77]. To the best of our knowledge,... 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium(II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polyproline chain to a [Co(NH3)5] + or [(-NH-py)Ru (NH3)5] acceptor. Chains composed of from zero to six cis-prolines have been examined. The apparent distance dependence of the electron-transfer rate constant, corrected for variations in the solvent reorganizational energy, seems to exhibit two types of distance dependence, 0.7-1A for short chains and /3 a0.3 A for long chains. A very detailed theoretical analysis of electron transfer in the complexes with four proline linkers has indicated that the electronic coupling is sensitive to conformational variations within the proline chain. ... 

Thus, the band positions of the irradiated semiconductor are key thermodynamic variables for in the control of the observed redox chemistry resulting from photo-catalyzed single-electron transfer (charge trapping) across the semiconductor-electrolyte interface. Whether oxygenation, rearrangement, isomerization, or other consequences follow this initial electron transfer seems to be controlled by surface effects on the relative rates of reaction. 

In the initial description of the cationic dye-borate system [24, 76], it was postulated that electron transfer was possible because, in nonpolar solvents, dye/borate salts exist predominantly as ion pairs. Since the lifetime of the cyanine singlet excited state is quite short [24, 25], this prerequisite is crucial for eflfective photo-induced electron transfer. Recently initiator systems in which neutral dyes are paired with triarylalkylborate anions have appeared in the literature [77]. In the latter case, the borate ion acts as the electron donor while neutral merocyanine, coumarin, xanthene, and thioxanthene dyes act as the electron acceptors. It is obvious that these initiating systems are not organized for effective electron transfer processes. The formation of an encounter complex (EC) between excited dye and electron donor is required. 

The excited-state redox reaction, equation (8.12), is thermodynamically favorable (E° > 0) while ground-state reaction, equation (8.13) is not (E° < 0). Therefore, a mixture of [Cr(phen)3]3+ and such a substrate will only undergo a redox reaction after the chromium complex has been excited. This is the process of photo-induced electron transfer light initiates an electron-transfer reaction. This experiment will explore how substrates such as DNA may be oxidized by the excited-state [Cr(phen)3]3+ complex. Because the electron-transfer reaction competes kinetically with luminescence, the presence of such a suitable substrate leads to a decrease in the intensity of luminescence. For this reason, the substrate is termed a quencher. 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium (II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polypro-... 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

The discussion so far has focused on reactions in the Marcus-normal region , for which lAGoA I/Xr < 1. The regime in which A(jDA /Xr> 1 is most readily accessed by means of photochemical or pulsed radiolysis techniques. Photochemical techniques are very powerful, but they usually involve an initial photo-induced electron transfer process, and this requires pre-assembly of the reactants. In the context of outer-sphere electron transfer processes, such pre-assembly involves ground state ion pairs. 

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