Luminescence from Ru

PBE dendrons bearing a focal bipyridine moiety have been demonstrated to coordinate to Ru + cations, exhibiting luminescence from the metal cation core by the excitation of the dendron subunits [28-30]. The terminal peripheral unit was examined (e.g., phenyl, naphthyl, 4-f-butylphenyl) to control the luminescence. The Ru +-cored dendrimer complexes are thought to be photo/redox-active, and photophysical properties, electrochemical behavior, and excited-state electron-transfer reactions are reported. 

The first report of luminescence from a ruthenium(II) complex of this type did not involve an aromatic ligand. Krug and Demas used [Ru(bt)3]2+3I) to demonstrate that an aromatic system is not essential for CT luminescence. However, there was no emission at room temperature and even at 77 K emission was weak. Efficiency improves in complexes of the related aromatic heterocycle (ligand 38), which we have observed to be... 

Beer and coworkers have modified their approach by derivatizing the upper rim of calix[4]arene with two (48, Tos = tosylate) and four (49) Ru(II) tris-bipyridyl reporter sites. The amide linkers of 48 and 49 form a hydrogen-bonding cleft for anion occupation. A preliminary report [385] indicates that MLCT luminescence from the Ru(II) centers is sensitive to anion association. 

The 1,2-dithienylethene unit has also been used to link together metal tris(bipyridyl) moieties including an unsymmetrical Ru(II)/Os(II) complex. In the open form luminescence from the MLCT state is observed with efficient energy transfer from ruthenium to osmium. However, the emission is quenched upon conversion to the closed form because of energy transfer to the photo-chromic 1,2-dithienylethene orbitals.55... 

As Ru(bpy) + is known to exhibit luminescence from the lowest excited state irrespective of the excitation wavelength (Section 9.2), chemiluminescence should be observed in the one-electron reduction of Ru(bpy)3+ if (spectroscopic) excited states are formed. This was found to be the case in the reduction of Ru(bpy)3+ by the hydroxide ion273 and by hydrazine273,274 in aqueous solution and by Ru(bpy)3 11 27S in acetonitrile solution. Chemiluminescence was also observed... 

What is actually observed, however, is that [Rh(phi)2(phen)]3+ intercalated into DNA quenches the intensity of [Ru(phen)2(dppz)]2+ much more effectively than it quenches the two lifetimes, as summarized in Table II. This effect is most pronounced when the DNA helix is a short oligonucleotide. The direct comparison of quenching in the absence of DNA cannot be accomplished because the ruthenium(II) complex does not luminesce in aqueous solution however, electron transfer from [Ru(phen)3]2+ to [Rh(phi)2(phen)]3+ in buffered solution provides a control with the same thermodynamic driving force (40). 

It is also useful to consider the luminescence from metallated oligonucleotides in the presence of noncovalent metallointercalator. Adding one equivalent of free [Ru(phen)2(dppz)]2+ to the ruthenium-modified duplex doubles the intensity in luminescence, consistent with independent intercalation by the two species. As described earlier, steady-state luminescence reaches saturation at approximately three times the luminescence of the ruthenium-modified duplex when two equivalents of [Ru(phen)2(dppz)]2+ have been added. It is not surprising, then, that addition of a stoichiometric amount of [Rh(phi)2(phen)]3+ to the ruthenium-modified duplex leads to substantial but not complete quenching of the ruthenium emission. Statistically, some duplexes will accommodate two rhodium(III) complexes, leaving a few ruthenium-modified duplexes unoccupied and therefore unquenched. Thus, complete quenching is observed only when the acceptor is covalently bound to the same duplex as the donor. 

Veggel and coworkers [137, 138] first reported the use of ruthenium(II) tris(2,2 -bipyridine) complexes ([Ru(bpy)3] +) and ferrocene as light-harvesting chromophores for sensitization of NIR luminescence from Nd(III) and Yb(III) ions. The Ru-Ln complexes (Ln = Nd 104, Yb 105) resulted from incorporating [Ru(bpy)3] + with m-terphenyl-based lanthanide complexes. Upon excitation of the Ru(bpy)s chromophore absorption with visible light up to 500 nm, both Ru-Nd and Ru-Yb complexes exhibited typical NIR luminescence because of effective Ru Ln energy transfer with the rates of 1.1 x 10 s for Ru-Nd complex and <1.0 X 10 s for Ru—Yb species. 

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