Redox Properties of the Excited State

Excited-state species are better electron donors and electron acceptors than the ground-state species. Thus the redox properties of the ground and excited states are different. 

Photoinduced electron transfer processes involving electron donor (D) and acceptor (A) components can be tuned via redox reactions. Namely, the excited-state properties of fluorophores can be manipulated by either oxidation of electron donors or reduction of electron acceptors. Also, the oxidized and the reduced species show different properties compared to the respective electron donors and acceptors. By making use of these properties of electron donors and acceptors, a number of molecular switches and logic gates have been described in recent years. In the following, we will introduce these redox-controlled molecular switches according to the redox centers. 

The properties which are of interest in this context are those connected with the generation and lifetime of the excited state, [Ru(bipy)3]2+, and the redox potentials of this and the related Ru1 and Rum species. 

Much of the work in this area has centred around efforts to optimize the photochemical and redox properties of the Ru11 complexes which are related to water cleavage reactions, e.g. lifetime of excited state, absorption maxima, etc. A detailed account of these properties is found in Chapter 8.3 and hence it is only intended here to present the results of these studies on hydrogen producing systems. 

Fig. 7. Schematic diagram showing the difference in the redox properties of the ground and excited molecule according to Eqs. (12) and (13) c is the one-electron potential corresponding to the zero-zero spectroscopic energy of the excited state...

Mineralization (meaning a complete oxidation) of numerous organic species cannot be regarded only as an oxidation process. In many cases oxidation must be preceded by reduction steps, eg photocatalyzed transformation of CC14 to C02 and Cl requires first reduction of carbon(IV) to lower oxidation states, followed by its reoxidation to C02 [30], In this context redox properties of an excited semiconductor play a crucial role. Photogenerated holes should support highly oxidative potential, but at the other surface sites an efficient reducer (electron) should also be available [64,65],... 

Because of the high selectivity often observed and the mild reaction conditions required, PET provides a powerful tool for carrying out single electron transfer reactions . By photochemical oxidation excitation of either the electron donor or the acceptor, the redox properties of the respective species change. For example, 9,10-dicyanoanthracene in its singlet state (dca ) is a powerful oxidant = +1.28 V vs. Ag/AgNOs with an... 

Optical measurements have been reported for [Ru(bipy)3], [Ru(bipy)2(biq)f, and [Ru(biq)3] (biq = 2,2 -biquinolyl) together with the temperature dependence (84—330 K) of the luminescence emission. The behaviour of the three complexes is rationalized in terms of states derived from a simple orbital model. A description of the photophysical and redox properties of the luminescent complexes [RuLL L ] (where L = 2,2 -bipyridyl, L = 2,2, 2"-terpyridyl, and L" = phenothiazine, N-methylphenothiazine, thian-threne, or H2O) has appeared, and this suggests that states other than the luminescent state are populated. Coupling between dissimilar ligands in the excited states of [Ru(bipy) (phen)3 f, [Ru(bipy) L3 and... 

The extended set of locally excited conformations will produce the charge-separated (CS) set of states, whereas the folded set will generate the exciplex state. Finally, Coulombic-induced molecular folding (harpooning [57]) in the extended set of CS states will lead to increased production of the exciplex state. The dynamic competition between the various processes outlined in Figure 15 depends not only on the chain length and the redox properties of the donor and acceptor groups, but also on solvent polarity. 

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