Ruthenium, supramolecular complexes

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

The emission spectmm of the H2(TRPyPz) species is characterized by the porphyrazine luminescence at 720 nm, while the excitation profile practically coincides with the absorption spectra, reproducing even the MLCT band at 500 nm, of the peripheral ruthenium complexes. Such coincidence corroborates the occurrence of efficient energy-transfer processes from the ruthenium complexes to the porphyrazine center, in contrast to the related supramolecular tetrapyridyl derivatives. 

According to theoretical calculations (280,281), electronegative substituents on the phenyl ring stabilize the porphyrin ring against oxidation, by diminishing the electronic density on it (259, 282, 283). Similar effects are expected for the supramolecular m o-pyridylporphyrins however, in addition, the catalytic activity also can be modulated by the electronic effects induced by the peripheral ruthenium complexes, particularly in the case of Mn(4-TRPyP). The electronic influence from the peripheral complexes should be smaller for the Mn(3-TRPyP)... 

Analogous to the supramolecular TRPyP systems, the tetramthenated porphyr-azine species also form homogeneous molecular films on solid substrates, by the slow evaporation of the solvent. Such films are suitable for the preparation of modified electrodes displaying versatile electrochemical and photoelectrochem-ical properties. The spectrum of a TRPyPz film on ITO is analogous to the solution spectrum exhibiting the Soret band at 390 and a broad Q band at 705 nm, respectively and the peripheral ruthenium complexes n-n and CT bands at 300 and 500 nm (125, 126). 

There has also been interest in restricted rotations in adenosine Pt complexes in which lower temperatures reveal the effects of rotation in complexes such as (51). Marziltf has been particularly active in the analysis of dynamics of N-donor bioligands, but they have also emphasized studies of octahedral ruthenium complexes. More complicated N-donor hgands can produce multiple processes which can be observed by DNMR and in some cases hindered rotation in pendant rings can be observed. " There has been a recent review of DNMR methods applied to supramolecular systems. ... 

Supramolecular photocatalysts connecting several rhenium complexes to a ruthenium complex and connecting several ruthenium complexes to one rhenimn complex have also been S5mthesized. The photocatalytic abilities toward CO2 reduction of each were RuRe2 (TNco = 190)> Ru2Re (TNco = H0) (99,100), RuRe3 ( co = 0.093, TNco = 240) (96). 

Structure 5.18. Structural representation of [M(II) — TPyP Ru(bipy)2Cl 4] " complex, TRP. Reprinted from Figure 3 H.E. Toma and K. Araki, Supramolecular assemblies of ruthenium complexes and porphyrins. Coordination Chemical Reviews, 196 (2000) 307-329. Cop5 right 2000, with permission of Elsevier. 

Toma, H. and K. Araki (2000). Supramolecular assemblies of ruthenium complexes and porphyrins. Coord. Chem. Rev. 196(1), 307-329. 

Nogueira, A.F., A.L.B. Formiga, H. Winnischofer, M. Nakamura, F.M. Engelmann, K. Araki, and H.E. Toma (2004). Photoelechochemical properties of supramolecular species containing porphyrin and ruthenium complexes on T1O2 films. Photochem. Photobiol. Sci. 3, 56-62. 

Figure 9 Supramolecular dyads containing a ruthenium complex linked to a pyrene moiety by a flexible poly(ethylene glycol) chain of variable length.

A significant research effort has been devoted to the design and application of various supramolecular self-assembled systems in photoelectrochemical solar cells. Fullerenes, fullerene derivatives, and carbon nanotubes are typically used as electron acceptor components of such systems. Porphyrins, phthalocyanines, ruthenium complexes, conjugated oligomers, and polymers are applied as electron donor counterparts. 

Fig. 5 Scheme showing the supramolecular Zn-MCTPyPRus dye anchored on a T1O2 surface. Electron injection or energy transfer (ET) takes place from the peripheral ruthenium complexes after photoexcitation of the zinc(II) porphyrin tetrad enhancing the electron injection to the mesoporous semiconductor and the charge separation efficiency at the inbaface... 

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