Tris-Polypyridyl Complexes

The absorption spectra of tris-polypyridyl Rhodium(III) complexes are characterised by several intense Ligand Centered (LC) absorption bands in the UV. Neither MC absorption bands, nor CT bands are observed in the visible region of the spectrum in contrast to their Ruthenium analogues. This makes tris(polypyridyl)Rh(III) complexes formed with bpy and phen practically colorless [1]. 

At low temperature, the emission spectra of the complexes are well-structured and assigned to ligand-localised nn phosphorescences , responsible for multiexponential luminescence decays observed with mixed-ligand compounds [125,126], 

Emissions from both a MC and LC excited state were observed at low temperature with sterically hindered ligands such as 3,3 -Me2-bpy [127] and 2,2 6, 2 -terpyridine [128], The MC emission is the dominant feature at 77 K, but the LC emission is enhanced relative to the metal centred one in fluid solution [127], 

The behaviour in room-temperature fluid solutions of excited Rhodium(III)-polypyridyl complexes remains unclear. These compounds are weak emitters, and perhaps because of this, contradictory reports on the room temperature emissions of Rh(bpy)3 and Rh(phen)3 have been published. Indelli et al. [129] detected the emission at 588 nm (dd ) and 455 nm nn ) for Rh(phen)3 while Nishizawa et al. [127] observed only the nn emission at 455 nm. The tris-polypyridyl Rhodium(III) complexes photodissociate, giving rise to the loss of a ligand [130], as is expected when the MC state can be populated. 

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On the basis of the reduction potential of Rh(phen) (Eo = — 0.75 V/SCE) and of its nn energy (2.75 eV), Rh(phen)3 in the nn state is expected to be a very powerful oxidising agent (with a reduction potential of 2.0 V/SCE [133]), making it a stronger oxidant than the MLCT states of the Ru(II) complexes discussed above. Electron transfer from aromatic amines [134] or di-and tri-methoxybenzenes [135] to excited Rh(III) polypyridyl complexes have indeed been observed. 

Bis- and tris polypyridyl Rh(III) complexes which have been studied with DNA can be found in Table 2. 

It has been shown that polypyridyl Rh(III) complexes induce photo-cleavages of the sugar phosphate backbone of double-stranded DNA with a higher relative quantum yield than Ru(II) complexes of phen or DIP. Thus replacement of Ru(II) ions by Rh(III) in Tris(phen) complexes, increases the efficiency of DNA photo-cleavages. However, in contrast to the Ru(II) complexes, Rh(III) samples have to be illuminated in the UV because of the absence of absorption bands in the visible region. 

In the transition metal polypyridyl complex group, tris(4,7-diphenylphen-antroline) ruthenium(II) (Ru(dpp)2+) is widely used as a probe for a PSP. The luminescence lifetime of Ru(dpp)2+ is long compared with the other ruthe-nium(II) polypyridyl complexes [17]. The absorption and emission maxima of Ru(dpp)2+ are 457 and 610 nm, respectively. The luminescence lifetimes under nitrogen- and air-saturated conditions are ca. 4.0 and 2.0 ps, respec-... 

The lowest energy MLCT transition of Ru polypyridyl complexes of the type tris-[Ru(4,4/-dicarboxy-2,2/-bipyridine)3] (1), can be lowered so that it absorbs more in the red region of the visible spectrum by replacing one 4,4/-dicarboxy-2,2/-bipyridine (dcbpy) with two thiocyanate donor ligands [Ru(dcbpy)2(NCS)2] (2). In complex 2, the two 4,4/-dicarboxylic acid 2,2 -bipyridine ligands pull while the two thiocyanate donor ligands push electrons. The oxidation potential of the complex 2 is 0.85 V vs. SCE, which is cathodically shifted significantly (0.65 V vs. SCE) compared to the homoleptic type of complex 1, which shows Ru(III/II) couple at 1.5 V vs. SCE. Thus, the... 

In late 1996, a new family of DNA-mediated ET experiments began to be reported. In these the ET donor is comprised of one or more DNA nucleotides (either G, GG, GGG, or a covalent thymine dimer) while the other is a covalently attached, photoactivated electron acceptor (either anthraquinone, pyrene, or an tris-polypyridyl M(III) complex, where M = Ru or Rh) [74-80]. These experiments have much in common with the kind of experiments outlined immediately above and with the new hairpin studies recently reported. They will be discussed toward the end of this chapter. They differ from the pre-1997 ET kinetics experiments that will be discussed immediately below in that none of them has reported ET rate measurements. Rather, yields of net photochemistry (either DNA strand cleavage or thymine dimer scission) are presented as evidence that photoinduced ET events occurred. For comparison of experimental results with ET theory it is clear that at a minimum ET rates must be measured and at a maximum several rates should be measured for a given D/A pair at a variety of separation distances. 

There is much interest in transition metal polypyridyl complexes, largely due to their numerous applications in a variety of fields (247-250). In particular, ruthenium(II) tris(2,2 -bipyridyl) has been one of the most extensively studied complexes of the last decade due to its chemical stability, redox properties, excited-state reactivity, and luminescent emission (251, 252). 

The alumina/dodecylsulfonate system has also been probed by excited state resonance Raman spectroscopy using tris(2,2 -bipyridyl) ruthenium(II) chloride, RuCbpy) " as a probe molecule (Somasundaran et al., 1989). It has been shown that ruthenium polypyridyl complexes serve as excellent photophysical probes for biopolymers like nucleic acids. The excited state of RuCbpy) shows strong resonance enhanced Raman transitions when probed at 355 nm Furthermore, it has been shown that binding of these ions to clay... 

In order to avoid these limitations, there is a need for alternative electrolytes which work in both n-type and p-type systems. One such class of species is the cobalt polypyridyl complexes such as Co tris(4,4"-di-ferf-butyl-2,2"-dipyridyl) perchlorate.Gibson et al. employed this redox couple in a NiO-based p-type device in conjunction with two perylene-based dyes, PI and PINDI (Figure 3.74). ... 

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