Polypyridyl ruthenium complexes reactions

Light-Induced Electron Transfer Reactions of Metalloporphyrins and Polypyridyl Ruthenium Complexes in Organized Assemblies... 

Cyclic voltammetry is an excellent tool to explore electrochemical reactions and to extract thermodynamic as well as kinetic information. Cyclic voltammetric data of complexes in solution show waves corresponding to successive oxidation and reduction processes. In the localized orbital approximation of ruthenium(II) polypyridyl complexes, these processes are viewed as MC and LC, respectively. Electrochemical and luminescence data are useful for calculating excited state redox potentials of sensitizers, an important piece of information from the point of view of determining whether charge injection into Ti02 is favorable. 

With the aim of mimicking, on a basic level, the photoinduced electron-transfer process from WOC to P680+ in the reaction center of PSII, ruthenium polypyridyl complexes were used (182-187) as photosensitizers as shown in Fig. 19. These compounds are particularly suitable since their photophysical and photochemical properties are well known. For example, the reduction potential [Rum(bpy)3]3+/-[Run(bpy)3]2+ (bpy = 2,2 -bipyridine) of 1.26 V vs NHE is sufficiently positive to affect the oxidation of phenols (tyrosine). As traps for the photochemically mobilized electron, viologens or [Co(NH3)5C1]2+ were used. 

The analogous osmium polymers have also been studied in great detail. The synthetic procedures required for these metallopolymers are the same as those described above for ruthenium however, the reaction times are longer. The similarity between the analogous mononuclear and polymeric species is further illustrated by the fact that the corresponding osmium polymers have considerably lower redox potentials and are also photostable, as expected on the basis of the behavior observed for osmium polypyridyl complexes. 

Chirality effects are also reported in the energy transfer from the ruthen-ium(II) polypyridyl complex to the osmium(II) complex in Langumuir-Blodgett (LB) films [76]. In this experiment, the LB film was prepared with [Ru(dp-phen)3]2+ (dp-phen = 4,7-dipheny 1-1,10-phenanthroline) and stearic acid, where the molar ratio was 1 1 to 1 4. The quenching reaction of the ruthenium(II) complex was carried out with optically pure osmium(II) complex, [Os(dp-phen)3]2+. This reaction consists of photoexcitation of the ruthenium(II) and osmium(II) complexes [Eqs. (29) and (30)], spontaneous decays of the excited ruthenium(H) and osmium(II) complexes [Eqs. (32) and (33)], and the energy transfer between the exited ruthenium(II) complex and the osmium(II) complex [Eq. (31)]. 

We have applied this theoretical formulation [26-28] to a series of PCET reactions. The systems were chosen based on the availability of experimental data that had not yet been fully explained. The systems that will be discussed in this section are iron bi-imidazoline complexes, ruthenium polypyridyl complexes, amidinium-car-boxylate interfaces, DNA-acrylamide complexes, tyrosine oxidation, and the enzyme lipoxygenase. In all cases, the solvent was treated as a dielectric continuum [58, 59]. 

Figure 16.4 PCETcomproportionation reactions in ruthenium polypyridyl complexes [46].

The ability of the MLCT ES in Ru(ll) polypyridyl compounds to undergo facile excited state ET reactions, combined with the stability of the complexes and the tunability of their ES properties, makes this class of compounds attractive candidates for a number of practical applications. Ruthenium(ll) polypyridyl compounds have already found applications in the field of photovoltaics as sensitizers for wide-bandgap semiconductors (such as Ti02) and as photocatalysts for the photochemical splitting of water. Their photophysical and photoredox properties... 

The challenge in this field is to control both the physical architecture and chemical reactivity of the film so as to promote selected electron transfer reactions while inhibiting others. With polymer-modified electrodes (PMEs), the electrode is conferred with the molecular selectivity and specificity that is lacking at a conventional pristine electrode. For example, poly(4-vinyl)pyridine and poly(N-vinyl)imi-dazole can be functionalized with osmium and ruthenium polypyridyl complexes. These synthetic macromolecules act as useful model systems for... 

It has been remarked that in many respects, the chemistry of the luminescent excited state of tris-(2,2 -bipyridyl)ruthenium(ii)... is as well characterized as that of many ground-state metal complexes . Certainly kinetic data on the electron-transfer reactions of [Ru(bipy)3] +, both as oxidant and as reductant, are becoming plentiful year by year. These, and data for other excited metal ions, are listed in Table 6 (p. 58). Emission quenching of a series of polypyridyl-type ruthenium(ii) complexes with Cu + proceeds by electron transfer,... 

Ruthenium (II) polypyridyl complexes promote the photoredox reactions such as Diels-Alder reaction (see formulae 29 31) and azomethine ylide formation followed by [3 + 2] cycloaddition (32-34) by use of visible light. Yoon found 2,2 -bipyrazine ligand is better than 2,2 -bipyridine. Bach reported enantioselective intramolecular [2 + 2] and intermolecular [3 + 2] photocycloaddition by use of chiral hydrogen bond templates (37) and (41). ... 

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