Ruthenium excited state

A novel Os and Ru bis(bipyridyl) containing an amide macrocyclic receptor has been shown to detect the presence of anions by both electrochemical and optical methodologies. Photophysical studies have clearly shown that the rate constants of the energy transfer process responsible for the quenching of the luminescent ruthenium excited state significantly decreases in the presence of chloride ions. ... 

Similarities between (Riitbpy) ]" (discussed in Chapter 13) and [Pu(pop)4.1 are apparent. Reactive excited states are produced in each when it is subjected to visible light. The excited state ruthenium cation, [Rulbpyjj] ", can catalytically convert water to hydrogen and oxygen. The excited state platinum anion, [Pt2(pop)4J , can catalytically convert secondary alcohols to hydrogen and ketones. An important difference, however, is that the ruthenium excited state species results from the transfer of an electron from the metal to a bpy ligand, while in the platinum excited state species the two unpaired electrons are metal centered. As a consequence, platinum reactions can occur by inner sphere mechanisms (an axial coordination site is available), a mode of reaction not readily available to the 18-electron ruthenium complex.203... 

A good example is the excited state of the tris(bipyridine)ruthenium(2+) ion, Ru(bpy)5+. This species results from the transfer of an electron from the metal to a ligand. In the language of localized valences, it is a ruthenium(3+) ion, coordinated to two bipyridines and to one bipyridyl radical anion in other words, [Ru3+(bpy)2(bpy )]2+. This excited state is a powerful electron donor and acceptor.17 The following equations show an example of each quenching mode ... 

The X-ray structure of zinc naphthalocyanate has been determined with Zn—N bond lengths of 1.983(4) A.829 Pentanuclear complexes with a zinc phthalocyanine core and four ruthenium subunits linked via a terpyridyl ligand demonstrate interaction between the photoactive and the redox active components of the molecule. The absorbance and fluorescence spectra showed considerable variation with the ruthenium subunits in place.830 Tetra-t-butylphthalocyaninato zinc coordinated by nitroxide radicals form excited-state phthalocyanine complexes and have been studied by time-resolved electron paramagnetic resonance.831... 

The intense colors in 2,2/-bipyridyl complexes of iron(II), ruthenium(II), and osmium(II) are due to excitation of an electron from metal t2g orbitals to an empty, low-lying ir orbital of a conjugated 2,2 bipyridyl ligand. The photoexcitation of this MLCT excited state can lead to emission as the excited state collapses back to the ground state. However, not all complexes are... 

Based on extensive screening of hundreds of ruthenium complexes, it was discovered that the sensitizer s excited state oxidation potential should be negative of at least —0.9 V vs. SCE, in order to inject electrons efficiently into the Ti02 conduction band. The ground state oxidation potential should be about 0.5 V vs. SCE, in order to be regenerated rapidly via electron donation from the electrolyte (iodide/triiodide redox system) or a hole conductor. A significant decrease in electron injection efficiencies will occur if the excited and ground state redox potentials are lower than these values. 

Cyclic voltammetry is an excellent tool to explore electrochemical reactions and to extract thermodynamic as well as kinetic information. Cyclic voltammetric data of complexes in solution show waves corresponding to successive oxidation and reduction processes. In the localized orbital approximation of ruthenium(II) polypyridyl complexes, these processes are viewed as MC and LC, respectively. Electrochemical and luminescence data are useful for calculating excited state redox potentials of sensitizers, an important piece of information from the point of view of determining whether charge injection into Ti02 is favorable. 

Studies like those mentioned here on the osmium complexes are more difficult for related complexes of ruthenium because of the intervention of a lowlying, thermally populable d-d excited state. However, it is possible to separate the two contributions to excited state decay by temperature dependent measurements. In the case of Ru(bpy>32+, temperature dependent lifetime studies have been carried out in a series of solvent, and the results obtained for the variation of knr with Eem are in agreement with those obtained for the Os complexes (19). 

Case Study Time-Resolved Resonance Raman Studies of the Excited States of Tris(Bipyridine)Ruthenium(II) ... 

In the ruthenium frA-bipyridine system, an orange emission at 610 nm arises when the excited stated [Ru(bpy)32+] decays to the ground state. Ru(bpy)32+ is the stable species in the solution and the reactive species—Ru(bpy)33+—can be generated from Ru(bpy)32+ on the electrode surface by oxidation at about +1.3 V. Adding Ru(bpy)32+ to the electrolyte and using an end-column electrode to convert the Ru(bpy)32+ into the active Ru(bpy)33+ form allow a simple and sensitive ECL detection mode. The reaction lends itself to electrochemical control due to the electrochemically induced interconversion of the key oxidation states ... 

Co(terpy)2 +/Co(terpy)2 + value is much smaller than that for either the ground- or excited-state ruthenium polypyridine couples because substantial rearrangement of the metal... 

So we conclude that the excited state ruthenium(II)-oxygen reaction produces singlet oxygen, as does the one involving... 

G. Orellana, M. C. Moreno-Bondi, E. Segovia, M. D. Marazuela, Fiber-optic sensing of carbon dioxide based on excited-state proton transfer to a luminescent ruthenium(II) complex, Anal. Chem. 64, 2210-2215(1992). 

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