Photo-induced electron transfer reactions

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

The stereoselectivity of anti-Markovnikov adducts (161) and (162) produced through photo-induced electron-transfer reaction of (160) with MeOH in MeCN depends on the optimum structures and stabilities of the corresponding radical and carbanion intermediates (163) and (164). In PhH, steric hindrance in an exciplex, comprising an excited singlet sensitizer and (160), forced cis addition of MeOH to (160) to give trans-isomer (161) as the major addition product. 

The catalytic effect of acid or metal ions on the thermal or photo-induced electron-transfer reactions has been reviewed.60... 

Excited complex formation and photo-induced electron-transfer reactions in organised systems. 94... 

Nakadaira, Y., Sekiguchi, A., Funada, Y., and Sakurai, H. (1991) Photo-induced electron transfer reaction of polysilanes, intramolecular trapping of a transient radical cation with a nucleophile. Chemistry Letters, 327-330. 

PHOTO-INDUCED ELECTRON TRANSFER REACTIONS IN POLYMER-BOUND RUTHENIUM BIPYRIDYL COMPLEXES... 

Ru(bpy)3]2+ in these syntheses is worthy of note because this complex is a typical photosensitizer. Thus we can expect to construct the stereoselective photo-induced electron transfer reaction with optically pure [Ru(bpy)3]2+. 

In the absence of nucleophilic species, the radical cations generated in the photo-induced electron-transfer reactions may undergo other reactions. Thus, 1-phenoxypropene undergoes cis-trans isomerisation on irradiation in the presence of electron acceptors such as dicyanobenzene (Majima et al., 1979). Some alkenes undergo dimerisation giving cyclobutanes on irradiation in the... 

Perylene and tetracene both undergo photo-induced electron-transfer reactions with pyromellitic anhydride (Levin, 1976). If a mixture of perylene and tetracene is used, and the light absorbed by the perylene, the perylene radical cation will be formed which because of the relative oxidation potentials will react with tetracene to give the tetracene radical ion. Thus the photogenerated perylene radical cation has undergone a redox reaction with tetracene. In effect, the perylene has acted as a sensitiser for the production of the tetracene radical cation. This type of sensitisation has been used to effect a number of reactions. 

Photoreduction of benzophenone by primary and secondary amines leads to the formation of benzpinacol and imines [145]. Quantum yields greater than unity for reduction of benzophenone indicated that the a-aminoalkyl radical could further reduce the ground state of benzophenone. Bhattacharyya and Das confirmed this in a laser-flash photolysis study of the benzophenone-triethylamine system, which showed that ketyl radical anion formation occurs by a fast and a slow process wherein the slow process corresponds to the reaction of a-aminoalkyl radical in the ground state of benzophenone [148]. Direct evidence for similar secondary reduction of benzil [149] and naphthalimides [150] by the a-aminoalkyl radical have also been reported. The secondary dark reaction of a-aminoalkyl radicals in photo-induced electron-transfer reactions with a variety of quinones, dyes, and metal complexes has been studied by Whitten and coworkers [151]. 

Nucleobases can serve as either electron donors or electron acceptors in photo-induced electron transfer reactions with electronically excited acceptors (Ac) or donors (D), respectively. The free energy of photoinduced electron transfer reaction in which the nucleobase (N) serves as a donor or acceptor can be estimated using the Rehm-Weller equations (Eqs. 5 and 6), respectively. 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into solution. The most interesting question here is whether an electron transfer from higher quantized levels to the oxidized species of the redox system can occur, as illustrated in Fig. 9.31. In order to accomplish such a hot electron transfer, the rate of electron transfer must be competitive with the rate of electron relaxation. It has been shown that quantization can slow down the carrier cooling dynamics and make hot carrier transfer competitive with carrier cooling. 

For electron transfer reaction involving neutral species such as BP and LCV alone, ionic field effects are confined to reactions after primary electron transfer. When reacting species bears charge before redox reaction, situation is more complicated particularly in the case of photo-induced electron transfer reactions. Both formation and back electron transfer processes will be influenced by ionic field in different manners. For example, the well-known positive effect of polyelectrolyte bearing cation on reactions between anions(or vice versa) will not be applicable if the overall reaction including back electron transfer process is under consideration. 

Fluorescence characteristics of the products were studied in relationship to complexing activity, and the 2- and the 6-derivatives can be used as fluorescence sensors. Treatment of p-cyclodextrin with the disulfonyl chloride 24 gave capped derivatives linked through 0-6 of the AD rings or AC rings in the ratio 76 24. Yields were low, but the products are potential hosts for photo-induced electron transfer reactions. In related work cycloinulohexaose was capped by use of diphenyl-4,4 -disulfonyl chloride through 0-6 of the A-C related units. This allowed displacement reactions and, for example, phenylthio-derivatives could be made in the AC relationship. ... 

Tran -nitroso dimers have been prepared by the reaction of thionitrate esters with (9-acyl derivatives of A-hydroxyl-2-thiopyridines (Scheme 28)., The potential-energy surface for the reaction of the fluoroformyl radical [FC(0)] with NO has been calculated using ab initio studies at the B3LYP level. The reaction was calculated to proceed via addition to NO to form an FC(0)N0 adduct which upon fragmentation yielded FNO and CO. Photo-induced electron-transfer reactions between allylsilane-allylstannane bifunctional reagents and 1,2-diketones have been shown to give a-keto homoallylic alcohols having an allylsilane moiety. 

The photo-induced electron-transfer reactions of the type delineated above are completely reversible, and result in no net chemical transformation of the molecular complex or radical pair. This chemical unreactivity is due to the facility of the back-electron transfer reaction — the application of light serving merely to generate a photostationary state, i.e. 

This efficient reaction can be contrasted with the wholly reversible photo-induced electron-transfer reactions of anthracenes (ANTH) with tetracyano-ethylene or TCNE [207], e.g. 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into... 

Hasegawa, E. Kato, T. Kitazume, T. Yanagi, K. Hasegawa, K. Horaguchi, T. (1996) Photo induced electron transfer reactions of a,P-epoxy ketones with 2-phenyl-A,.Y-dimethylbenz-imidazoline (PDMBI) significant water effect on the reaction pathway. Tetrahedron Lett., 37, 7079-82. 

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