Ruthenium complexes luminescent quenching

Ruthenium(II) bipyridyl and Cr(III) aquo complexes luminesce strongly when photostimulated. The emission of light can be quenched effectively by such species as oxygen, paraquat, Fe(II) aquo complexes, Ru(II) complexes and Cr(NCS)i (Sutin [15]). Pfeil [16] found that the quenching rate coefficients are typically a third to a half of the value which might be predicted from the Smoluchowski theory [3]. 

Noted that it is essential that the polymers used to immobilize the emitter be amenable to the application (44, 47, 50-52). An additional factor that plagues both luminescent polypyridyl ruthenium complexes and the 2- or 4-pyridine substituted platinum 1,2-enedithiolate in this application is the oxygen-induced triplet quenching. 

When [Rh(phi)2(phen)]3+ is titrated into a solution containing [Ru(phen)2(dppz)]2+ and B-form DNA, the photoinduced luminescence of the ruthenium(II) complex is quenched dramatically (53). In these experiments, luminescence is monitored by laser flash as quencher is added. Data are then plotted in Stern-Volmer format, where the ratio of initial intensity/intensity (I0/I) is given as a function of quencher concentration [Q. The degree of lifetime quenching can also be described by plotting the inverse of the lifetime (r0/r) versus [Q. Normally, when chromophore and quencher interact bimolecularly, Stern-Volmer graphs are linear with [Q] and the slope for r0/r is the same as that for I0/I. 

Quenching of the ( CT)[Ru(bipy)3] by [Cr(bipy)]3 has been studied. This is via electron transfer to the Cr complex and a rapid back reaction. The ruthenium complex will also quench the 727 nm emission of the metal-centred doublet excited state of the chromium species, by a similar mechanism. Evidently both ligand- and metal-centred excited states can be quenched by bimolecular redox processes. A number of Ru complexes, e.g. [Ru(bipy)3] and [Ru(phen)3] also have their luminescence quenched by electron transfer to Fe or paraquat. Both the initial quenching reactions and back reactions are close to the diffusion-controlled limit. These mechanisms involve initial oxidation of Ru to Ru [equation (1)]. However, the triplet excited state is more active than the ground state towards reductants as well as... 

There is an impressive battery of spectroscopic techniques available for probing interactions between metal complexes and DNA. The oldest of these, UV/vis spectroscopy, is still one of the most sensitive ways to analyze dye-DNA interactions. For chiral metal complexes, circular dichroism is an invaluable tool. Fluorescence spectroscopy has in particular made great strides in recent years with respect to these applications, and aside from the measurement of simple emission from an excited metal complex, one can utilize emission polarization, luminescence lifetimes, and differential fluorescence quenching to obtain still more information about the environment about a metal complex. The application of ruthenium complexes, in particular, to developing probes for DNA, has been initiated in our laboratory and we focus here on some of its applications. 

The excited state reactions of tris(polypyridine)ruthenium complexes have been reviewed. The rate constants for the oxidative quenching of the luminescence of [ Ru(bpy)3] " and [ Ru(bpy)2(4,4 -Cl2bpy)] by the heteropolytungstate [Mn(0H)PWii039] and [Co(H20)SiW,039] ions have been measured as a... 

Diaxler S, Lippitsch ME, Klimant 1 et al (1995) Effects of polymer matrices on the time-resolved luminescence of a ruthenium complex quenched by oxygen. J Phys Chem 99 3162-3167... 

Morris KJ, Roach MS, Xu WY, Demas JN, DeGraff BA (2007) Luminescence lifetime standards for the nanosecond to microsecond range and oxygen quenching of ruthenium(II) complexes. Anal Chem 79 9310-9314... 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

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