Osmium intramolecular electron transfer

FIGURE 12.22 As an extension of previous work with inorganic-organic dyads (Figure 12.21), the bimetallic Ru-Os compound shown was designed to promote rapid intramolecular electron transfer (2), after electron injection (1). This process was demonstrated as well as a novel remote injection from the osmium MLCT excited state. 

When a metal complex like osmium(II) fris(bipyridyl) or a metallocene, as in compound 3, is appended to the ligand, the MLCT excited state is quenched by energy transfer to the adjacent metal center. The rate of this quenching decreases as an anionic substrate binds between the two metal eomplexes. A similar switching mechanism is seen in the calix[4]diquinone-appended complex, 4. In this case, the fluorescence emission is quenched via an intramolecular electron transfer to the quinone. Anion binding between the Ru complex and the quinone blocks the quenching process, and the emission intensity is significantly recovered. 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

Molecular dyads of ruthenium(ii)- or osmium(ii)-bis(terpyridine) chromophores and expanded pyridinium acceptors have been used to demonstrate the effect of the bridge and the metal ions to the photophysical properties of linear systems. In particular, via ultrafast transient absorption spectroscopy, an equilibration between MLCT and photo-induced charge-separated excited states has been observed demonstrating that intramolecular photoinduced electron transfers can occur within multicomponent systems in spite of driving forces virtually approaching zero. ... 

Holmberg RC, Tierney MT, Ropp PA, Berg EE, Grinstaff MW, Thorp HH (2003) Intramolecular electrocatalysis of 8-oxo-guanine oxidation secondary structure control of electron transfer in osmium-labeled oligonucleotides. Inorg Chem 42 6379-6387... 

The electrochemical and photophysical properties of a variety of mixed-metal supramolecular complexes incorporating Ru(II)/Os(II)-polyazine LAs to reactive Rh(III) systems have been investigated. The coupling of Rh(III) to ruthenium and osmium chromophores has been explored in some detail due to the known energy and electron transfer quenching of MLCT states of ruthenium and osmium by Rh(III) complexes in bimolecular processes. The systems studied to date most frequently included tris(bidentate) or bis(tridentate) coordination on Rh(III). While these studies provide considerable insight into the intramolecular excited state dynamics, these coordination environments typically prevent reactivity at the rhodium site. 

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