Electron donor-acceptor complexes redox reactions

Electron donor-acceptor complexes, electron transfer in the thermal and photochemical activation of, in organic and organometallic reactions, 29, 185 Electron spin resonance, identification of organic free radicals, 1, 284 Electron spin resonance, studies of short-lived organic radicals, 5, 23 Electron storage and transfer in organic redox systems with multiple electrophores, 28, 1... 

The fundamental theories behind electron transfer were discussed above in Section 2.1. Indeed, some of the most important empirical proofs for these theories have originated from photoinduced electron transfer in supramolecular donor-acceptor complexes. The difference between thermally and photochemi-cally induced electron transfer lies in both the orbitals participating in the reaction and in the additional thermodynamic driving force provided by the excited state. It is therefore important to consider the redox properties of excited-state species. 

Very little is known about the nature of the weak interactions of CAs in solutions where a vast majority of their chemical reactions has been studied. Particularly, the study of donor-acceptor complexes of CAs by modern physical-chemical methods is still of great interest. Besides, complexation of CAs with donors or acceptors of electron density is a useful tool for modifying the stability, reactivity and spectral properties of CAs. Systematic investigations of the redox properties of CAs are needed in order to elucidate the role of electron transfer in the transformations of CAs. 

The thionine function, possessing a relatively high redox potential, acts as an electron acceptor upon irradiation with visible light, which is considered to first form the electron-donor-acceptor (EDA) complex (exciplex) with the electron donor (D reductant). The electron in the exciplex further added to the thionine function to produce the colorless leucothionine and the electron acceptor (A). The A may be an oxidized form of D. The dark reaction may take place either by back electron transfer to the thionine function via the EDA complex or by air oxidation of the leucothionine when atmospheric oxygen is present. 

Table II summarizes the sources and key properties of isolated HiPIPs, almost all of which have been isolated from photosynthetic organisms, and there has been extensive speculation on their involvement in respiratory electron transport chains (18, 21, 91-93, 95, 96, 102-105). Evidence in support of such a hypothesis has recently emerged from studies of a partially reconstructed reaction center (RC) complex from Rhodoferax fermentans (93, 95). The kinetics of photo-induced electron transfer from HiPIP to the reaction center suggested the formation of a HiPIP-RC complex with a dissociation constant of 2.5 fx,M. In vivo and in vitro studies by Schoepp et al. (94) similarly have demonstrated that the only high-redox-potential electron transfer component in the soluble fraction of Rhodocyclus gelatinosus TG-9 that could serve as the immediate electron transfer donor to the reaction-center-bound C3d ochrome was a HiPIP. In vitro experiments have shown HiPIP to be an electron donor to the Chromatium reaction center (106). Fukumori and Yamanaka (107) also reported that Chromatium vinosum HiPIP is an efficient electron acceptor for a thiosulfate-oxidizing enzyme isolated from that organism.

A number of rate constants for reactions of transients derived from the reduction of metal ions and metal complexes were determined by pulse radiolysis [58]. Because of the shortlived character of atoms and oligomers, the determination of their redox potential is possible only by kinetic methods using pulse radiolysis. In the couple Mj/M , the reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. These reactions obviously occur in competition with the cascade of coalescence processes. The unknown potential °(M /M ) is derived by comparing the action of several reference systems of different potentials. 

For each of the three reactions catalyzed by the NADH dehydrogenase complex, identify (a) the electron donor, (b) the electron acceptor, (c) the conjugate redox pair, (d) the reducing agent, and (e) the oxidizing agent. 

Poly(pyridyl)ruthenium complexes, typically, [Ru(bpy)3]2+ have frequently been used as photocatalysts in the redox reactions between electron donors (Dred) and acceptors (Aox) to yield the oxidized (Dox) and reduced (Ared) forms (Eq. 20) [34-37] ... 

---