Redox properties excited electronic states

REDOX PROPERTIES OF SPECIES IN EXCITED ELECTRONIC STATES 489... 

Ruthenium, tris(2,2 -bipyridyl)-chemical actinometer, 409 excited electronic states redox properties, 490 structure, 64... 

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. 

Ru(bpy)2(CN)2] to Nd3+ can be assumed to be equal to the efficiency of the quenching of the [Ru(bpy)2(CN)2] emission (ca. 90%) because quenching by electron transfer can be ruled out in view of the Nd3+ redox properties. No evidence of energy transfer in the adduct from the naphthyl-localized 7) excited state of the dendrimer to the lowest 3MLCT state of [Ru(bpy)2(CN)2] has been found since no change in the Tx lifetime at 77 K has been observed. 

Spectroscopic properties of [Ru(bpy)3] " ", and the effects of varying the diimine ligands in [Ru(bpy)3 L ] + (L = diimine) on the electronic spectra and redox properties of these complexes have been reviewed. The properties of the optical emission and excitation spectra of [Ru(bpy)3] +, [Ru(bpy)2(bpy-d )] + and [Ru(bpy-d )3] " " and of related Os, Rh , and Pt and Os species have been analyzed and trends arising from changes in the metal d or MLCT character in the lowest triplet states have been discussed. A study of the interligand electron transfer and transition state dynamics in [Ru(bpy)3] " " has been carried out. The results of X-ray excited optical luminescence and XANES studies on a fine powder film of [Ru(bpy)3][C104]2 show that C and Ru localized excitation enhances the photoluminescence yield, but that of N does not. 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

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. 

Because of this difference in electron-donorand electron-acceptor properties, excited states have very different redox properties from those of related ground states. The effect is so marked that many photochemical processes begin with a complete transfer of an electron from (or to) an excited state (1.2), and the subsequent chemistry is that of radical cations and radical anions, species that are regarded as unusual in ground-state organic reactions. The importance of photochemical electron transfer is underlined by its extensive involvement in photobiological processes such as photosynthesis. 

Whether a complex in an excited state can manifest its enhanced redox properties will depend on whether it can undergo electron transfer faster than it undergoes something else, such as relaxation to the ground slate (luminescence). The emission lifetime of t Ru(bpy)j]2 in aqueous solution at 25 °C is 0.6 /us and it increases... 

Excited state potentials can also be estimated from kinetic studies of electron transfer quenching reactions involving a series of acceptors and/or donors with varying potentials. By applying electron transfer theory to the quenching step, in conjunction with the predicted dependence of the quenching rate constant on AG° for the electron transfer reaction, estimates for the redox potentials may be obtained (2 ). These approaches have been used successfully in the evaluation of the redox properties of several metal complexes,... 

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...

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. 

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