Light-induced transfer reaction, reversible

Electron-transfer Reactions - Light-induced electron transfer from a donor to a suitable acceptor has been described for numerous bimolecular systems. The reagents have been dispersed in a polar solvent,at microscopic or macroscopic interfaces, in latex dispersions, in nematic liquid crystals, in reverse micelles, in vesicles, and in lipid bilayer membranes. Additional studies have been concerned with electron transfer... 

A striking result is the complete reversibility of the light-induced iron-sulphur signal at 5 K. In current thinking the iron-sulphur clusters are the "last" acceptors of the electron transfer chain, and therefore it is remarkable that these clusters are reversibly reduced instead of the g= 2.00 acceptors. It is unlikely that the iron-sulphur is placed before the other acceptors, because of its relatively hi redox potential. Therefore it has to be assumed that under the conditions of the experiment in part of the reaction centre population the iron-sulphur cluster is reduced instead of the Aq/Aj acceptor complex. It is clear that further experiments are needed to shed more light on this puzzling effect. 

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. 

In summary, the present work demonstrates for the first time that some of the light-induced electron transfer reactions in photos)mthetic reaction centers can be induced to occur in the reverse direction in the dark by application of external electric fields. 

Sometimes, thermal agitation (i.e., Brownian motion) induces a nonradiative transfer (i.e., reverse of the internal conversion) from a nonexcited triplet state to an excited triplet state. Afterward, intercombination between the excited triplet and excited singulet state occurs. Finally, by internal conversion down to the nonexcited singulet state, light is emitted by fluorescence, according to the following successive reaction schemes ... 

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