ESR studies of the redox orbitals in diimine complexes of iron(II) and ruthenium(II)

Morris, David E.; Hanck, Kenneth W.; DeArmond, M. Keith

doi:10.1021/ja00348a017

Cited by 93 publications

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“…The formality of the Ru III oxidation state is most meaningful for the initially formed FranckCondon state, and it should be regarded with caution in the thermally equilibrated excited state because significant mixing of metal-t 2g and ligand-π Ã orbitals occurs (15,16,24,25). In fluid solution, the excited electron was best described as being localized on a single diimine ligand as opposed to being delocalized over all three (16,26,27). For heteroleptic Ru-tris(diimine) compounds, where the diimine ligands are inequivalent, it has been shown that the excited state is localized on the ligand that is most easily reduced.…”

Section: Discussionmentioning

confidence: 99%

Visible light generation of I–I bonds by Ru-tris(diimine) excited states

Farnum

Jou

Meyer

2012

Proc. Natl. Acad. Sci. U.S.A.

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Seven Ru-tris(diimine) compounds were prepared to study the photooxidation of iodide. Iodide oxidation results in the formation of I-I bonds, and it is therefore relevant to the conversion and storage of solar energy. Iodide oxidation is also a key step for electrical power generation in dye-sensitized solar cells. The mechanistic details of iodide oxidation and I-I bond formation were elucidated through time-resolved spectroscopic measurements. Bimolecular electron-transfer reactions between Ru-tris(diimine) excited states and iodide first yielded the iodine atom that subsequently reacted with excess I − to yield the I-I bond of diiodide (I 2•− ). An important finding was that excited-state iodide oxidation was rapid (k > 10 9 M −1 s −1 ) even for thermodynamically uphill reactions. These results indicated that iodide oxidation to the iodine atom may account for a significant fraction of sensitizer regeneration within dye-sensitized solar cells. •− ) and triiodide (I 3 − ) (1-6). Photoinitiation of iodide oxidation with visible light thus provides a fundamental means by which solar photons can be converted to chemical energy in the form of I-I bonds. This bond formation chemistry is also of major importance in dye-sensitized solar cells where I − /I 3 − is the most widely used redox mediator (7-11). We have previously reported the photooxidation of iodide using Ru-tris(diimine) compounds in fluid solution (1,2,(12)(13)(14). These compounds are known for their exceptional absorption properties in the visible region, and their long-lived metal-toligand charge transfer (MLCT) excited states (15, 16). Visible light generation of Ru-tris(diimine) excited states in the presence of I − resulted in electron-transfer reactions that generated I-I bonds. In acetonitrile solutions, iodide oxidation proceeded through an iodine atom intermediate, and I 2•− appeared as a secondary reaction product (1, 2, 13). In dichloromethane solutions, iodide oxidation was greatly enhanced due to strong ion-pair interactions between the Ru-tris(diimine) compounds and iodide (1,12,14).A series of Ru-tris(diimine) photooxidants have been prepared to characterize photo-initiated I-I bond formation in acetonitrile. The diimine ligands employed in this study are shown in Fig. 1, where a vertical arrow is drawn to indicate increased electron donation of the chelating nitrogens to the ruthenium metal center. The compounds prepared allowed the free-energy change for electron transfer (ΔG°) to be systematically varied over a 400-meV range. A strong dependence of the observed rate constants for iodide oxidation on ΔG°was apparent and rate constants on the order of 10 9 M −1 s −1 were determined for ΔG°≥ 0 eV. ResultsThe series of Ru-tris(diimine) compounds was synthesized following literature procedures and are hereby referred to as Ru 2þ . To our knowledge, ½RuðdmbÞ 2 ðbpzÞðPF 6 Þ 2 and ½RuðbpmÞ 2 ðdeebÞðPF 6 Þ 2 are compounds that have not been previously reported. The photophysical properties for all other compounds have been well characterized i...

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Section: Discussionmentioning

confidence: 99%

Visible light generation of I–I bonds by Ru-tris(diimine) excited states

Farnum

Jou

Meyer

2012

Proc. Natl. Acad. Sci. U.S.A.

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“…4 ) In related ligand reduced cxxnplexes like (( u(b,y) ] generated e1ectrochnica11y, there is clear spectral evidence that the added electron is localized on a single ligand and frau epr line broadening that the rate of electron Ixpping between ligands is rapid (ref . 32,33).…”

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confidence: 99%

Photochemistry of metal coordination complexes: metal to ligand charge transfer excited states

Meyer¹

1986

Pure and Applied Chemistry

740

478

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Systanatic variations appear in the othphysical and photochnical properties of t4LC excited states whicth n be accounted for qualitatively or even quantitatively based on the properties of the nxlecules and of the surrounding rreditin.In 1959, Paris and Brandt first reported anission fran the cxinplex (RuUpy) (bpy is 2,2'-bipyridine) in solution (ref. 1). The excited state involved is sufficienty long-lived (-8OO ns in water at roan tanperature) that the possibility existed for using it as a thernical reagent and based on quanching studies using ccmplexes of Co(III), 1dainson and cazorkers suggested that the excited state could act as a reducing agent (ref. 2). Their suggestion was soon verified by additional quanching and flash çkiothlysis studies (ref.3,4). It was also shawn that the excited state could act as an oxidizing agent (ref. 5), that its redox potentials could be estimated experisontally by a kinetic qunnching techniqun (ref. 6,7), and that it could undergo facile, bisolecular electron transfer with a varie of oxidants or reductants (ref. 8-10). For example, oxidative qcnching by paraquat (PQ ), Scheme 1 Ru(Lpy)32 _Y) Ru(bpy)32 (1) 2+* 2+ kq 3+ + Ru(bpy) + PQ -p Ru(bpy). + PQ (2) k 2 2 -*Ru(bpy)3+PQ (3) provides a basis for the conversion of visible light energy into a transiently stored redox pair. Similar electron transfer schanes had been established previously for organic excited states (ref. II), bit the high c±ianical stability of the Ru ccznplex in sore than one oxidation state and its high light absorptivity in the visible made it an attractive candidate for exploiting the energy conversion possibilities associated with E. 1-3. The successful utilization of RU(bpy) 2+ and related cxiiplexes in energy conversion schanes relies on establishing a basis for un&rstanding the light absorptivity and otophysical and *iotochanical properties of the cthrcrtophores involved (ref. 9a ,l2). The goal of this account is to describe those properties and to provide a basis for understanding than at the microscopic level. ELECTRONIC STRUCTURE. LIGHT ABSORPTIVITY (Ru(1:py) ]2+ and (Os(phen) ]2+ (phan is l,l0-enantroline) tare with oanplexes like (Mo(bçy)CO) 1, (Re(phen)(o)3Cl], and (Ir(bpy) Cl)] the ground state electronic configuratio (dii)6 (t in O syninetry) and vaant, 1CM lying ir*(bpy or men) orbitals on the polypyridyl ligand.of apircpriate syrrmetry to mix with the dir orbitals. Visible light absorption in these carlexes is usually cinated by intense absorption bands arising f ran drr .-ir* metal to ligand charge transfer (MLCT) transitions, e.g., b(kpy)(CO) + hvr4o'(&y (CD) . For the lox oxidation state oarlexes of tb(O) and Re(I) the crbonyl groups are secessai4 to mix with and stabilize the dir levels thus decreasing sensitivity taGards oxidation and bringing the dir_ir* (kçq or p&en) energy gap into the visible region of the spectrun. In the higher oxidation state axilexes of Rh(III) or Ir(III) the dir levels are so stabilized by the higher effective nuclear charge at the metal that the MLCT transitio...

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“…The signals with g = 2.0004 are assigned to RuC + . [23] Spin-trapping of guanine radicals: Samples contained a ruthenium complex (10…”

Section: Methodsmentioning

confidence: 99%

Ligand Selection in Ru^II Complexes for Direct One‐Electron Photooxidation of Guanine: A Combined Computational and Experimental Study

Boggio‐Pasqua

Vicendo

Oubal

et al. 2009

Chemistry A European J

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Making the right ligand selection: DFT calculations of Gibbs energies for the one-electron photooxidation of guanine by six Ru(II)-polypyridine complexes are reported. The theoretical predictions are compared with new EPR spectra. Our theoretical calculations confirm the experimental observations that the direct photooxidation of guanine by [Ru(bpz)(3)](2+), [Ru(tap)(3)](2+), and [Ru(bpz)(2)(dppz)](2+) is thermodynamically favorable, but is unfavorable with [Ru(bpy)(3)](2+), [Ru(phen)(3)](2+), and [Ru(bpy)(2)(dppz)](2+) (see figure).

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ESR studies of the redox orbitals in diimine complexes of iron(II) and ruthenium(II)

Cited by 93 publications

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Visible light generation of I–I bonds by Ru-tris(diimine) excited states

Visible light generation of I–I bonds by Ru-tris(diimine) excited states

Photochemistry of metal coordination complexes: metal to ligand charge transfer excited states

Ligand Selection in Ru^II Complexes for Direct One‐Electron Photooxidation of Guanine: A Combined Computational and Experimental Study

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ESR studies of the redox orbitals in diimine complexes of iron(II) and ruthenium(II)

Cited by 93 publications

References 0 publications

Visible light generation of I–I bonds by Ru-tris(diimine) excited states

Visible light generation of I–I bonds by Ru-tris(diimine) excited states

Photochemistry of metal coordination complexes: metal to ligand charge transfer excited states

Ligand Selection in RuII Complexes for Direct One‐Electron Photooxidation of Guanine: A Combined Computational and Experimental Study

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Ligand Selection in Ru^II Complexes for Direct One‐Electron Photooxidation of Guanine: A Combined Computational and Experimental Study