Excited-State Coordination Chemistry:  A New Quenching Mechanism

Dougherty, Thomas J.; Hicks, Charles H.; Maletta, Anthony; Fan, Jianwei; Rutenberg, and Irina; Gafney, Harry D.

doi:10.1021/ja9724396

Cited by 9 publications

(7 citation statements)

References 17 publications

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“…Taking the average of the interior (N20, C21, C22, and N1) and exterior (C12, C21, C22, and C9) torsion angles, we find a dihedral angle of 11°. For the tetramethylene-bridged free ligand 3c, we estimate a torsion angle of 58°1 9 and thus would expect twisting of N3 away from the coordinated water, weakening this hydrogen bond. However, due to the constraints of the five-membered chelate ring, the ligand flattens considerably in the complex so that the average of the interior (N1, C24, C23, and N22) and exterior (C9, C24, C23, and C14) angles is 35°.…”

Section: Resultsmentioning

confidence: 95%

“…8 In the excited state, this ligand is effectively reduced and, therefore, its proton affinity should be enhanced. 9 We would like to use this more basic, photoreduced ligand to assist with the deprotonation of a metal-bound water molecule 10 and, thus, promote effective photooxidation of the system. This objective requires that either the bpy or the tpy ligand be redesigned to make it more electronegative and, thus, a good charge acceptor in the excited state.…”

Section: Introductionmentioning

confidence: 99%

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Design and Study of Bi[1,8]naphthyridine Ligands as Potential Photooxidation Mediators in Ru(II) Polypyridyl Aquo Complexes

et al. 2004

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A series of 3,3'-polymethylene-bridged bi[1,8]naphthyridine (binap) ligands, 3a-c, are complexed with Ru(II) to afford [Ru(tpy)(3a-c)(H(2)O)](2+) where an uncomplexed nitrogen on 3a-c is situated so it can form a H-bond with the coordinated water. An additional complex involving [Ru(4'-NMe(2)tpy)(3b)(H(2)O)](2+) is also prepared. X-ray analyses of the [Ru(tpy)(3a,c)(H(2)O)](2+) complexes indicate well-organized H-bonds even when the binap is nonplanar. In an attempt to realize photooxidation, the effects of light, varying potential, and pH were examined. A Pourbaix diagram indicated that the oxidation potential decreased by approximately 0.5 V in the pH range of 1.9-11.6. The lowest-energy electronic absorption for the binap complexes involves the metal-to-ligand charge transfer to the binap ligand and is sensitive to ligand planarity. The absorbance shifted to a lower energy as the auxiliary ligand became a better donor (4'-NMe(2)tpy) or as the water was deprotonated. Acetonitrile was found to displace water most easily for the complex of 3c, where the ligand is the least planar. Despite promising features, photooxidation of the bound water was not observed.

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Section: Resultsmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Design and Study of Bi[1,8]naphthyridine Ligands as Potential Photooxidation Mediators in Ru(II) Polypyridyl Aquo Complexes

et al. 2004

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“…5 orders of magnitude. 5 The [H + ] dependence of the quantum yield of dissociation of the bimetallic [bpy) 2 Ru(CN)(µ-CN)Rh(NH 3 ) 4 Br] 2+ and the appearance of a (bpy) 2 Ru(CN) 2 emission from a bimetallic composed of two components that, as individual molecules quench at a diffusion controlled rate, led to the suggestion that optical excitation of the (bpy) 2 Ru(CN) 2 ligand in [bpy) 2 Ru(CN)(µ-CN)Rh(NH 3 ) 4 Br] 2+ leads to a dissociative excited state. 6 Dissociation is attributed to the reduction in electron density at the cyano groups as evidenced by the decrease in their basicity on the population of the (bpy) 2 Ru(CN) 2 ligand's MLCT state.…”

Section: Introductionmentioning

confidence: 99%

“…Supporting Information Available: Proton NMR of the titration of Ru(bpy) 2 (dpp) 2+ with D 2 SO 4 showing the downfield shifts of specific dpp protons with increasing acid concentration; plot of the relative quantum yields (Φ/Φ o ) and relative emission lifetimes (τ/τ o ) of the 705 nm emission of Ru(bpy) 2 (dpp) 2+ vs pH; plots and sigmoidal fits of contributions of the 705 nm ()) and 735 nm (0) emissions to the composite emission spectra in the acid titration of Ru(bpy) 2 (dpp) 2+ . The fits inflect and cross at pH 3.1.…”

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confidence: 99%

Excited-State Coordination Chemistry: Excited-State Basicity of Bis(2,2′-bipyridyl)(2,3-dipyridylpyrazine)ruthenium(II)

2009

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The proton dependencies of the absorption and emission spectra of bis(2,2′-bipyridyl)(2,3-bis(2-pyridyl)pyrazine)ruthenium(II), (bpy) 2 Ru(dpp) 2+ indicate that population of the dpp-localized MLCT state increases the basicity of dpp peripheral nitrogens. NMR spectra reveal the protonation of the peripheral dpp pyridine in the ground state, pK a of 1.12 ( 0.03, occurs intermediate between the changes evident in the absorption and emission spectra. As a result, the emissivity of aqueous solutions of (bpy) 2 Ru(dpp) 2+ as a function of [H + ] derives from two emissive species: the unprotonated complex and the monoprotonated complex [(bpy) 2 Ru(dppH py )] 3+ with the proton attached to the peripheral dpp pyridine. Although protonation in the MLCT state generally quenches the emission, the emissivity of the monoprotonated complex, albeit weak, is attributed to the asymmetric distribution of the charge in the MLCT state. The majority of the transferred charge resides at the peripheral pyrazinyl nitrogen, and excited-state acid-base chemistry occurs predominantly at this site. Nonetheless, ground-state protonation of the peripheral dpp pyridine dramatically increases the nonradiative decay rate and significantly influences subsequent excited-state protonation processes. Protonation of the excited state changes from a bimolecular process to a combination of inter-and intramolecular processes where the proton transfers from the dpp pyridyl nitrogen to the dpp pyrazinyl nitrogen and from the surrounding aqueous solvent shell. Energetically, changes in the absorption spectra originally attributed to the first protonation of the complex and from which the ∆pK a of the excited state have been estimated, in fact, correspond to the second protonation of the complex. IntroductionThe hydrogen ion dependencies of the absorption and emission spectra of Ru(II) diimines reveal substantial differences in the acid-base properties of the ground and emissive MLCT states of the complexes. 1 Depending on the direction of the charge transfer relative to the acid-base site, and the location of the acceptor orbital in the MLCT state, excitation changes electron distribution, which in turn increases or decreases acid-base properties by as much as 5 to 6 orders of magnitude. 2 Brønsted basicity usually does not correlate with the coordinating ability of a ligand, but, with diimine ligands, the equilibrium constant for coordination increases linearly with the Brønsted basicity of the ligand's coordinating nitrogens. 3 Work in this laboratory focuses on whether these photoinduced changes in acid-base properties translate into an excited-state coordination chemistry where a Ru(II) diimine possessing one or more acid-base sites on the ligand periphery functions as a ligand and whether excitation enhances or reduces its ability to coordinate to another metal.Excited-state coordination chemistry arises from the observation that excitation of bis(2,2′-bipyridyl)(2,3-dipyridylpyrazine)ruthenium(II), (bpy) 2 Ru(dpp) 2+ , in the presence of PtCl 6

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“…Optical excitation of certain Ru(II) diimines produces immense changes in Bronsted acid-base properties. [1][2][3] In cis-(bpy) 2 Ru(CN) 2 , excitation transfers electron density to the diimine reducing the basicity of the cyano ligand by g10 5 in the excited state. 4 Evidence presented here indicates that population of the MLCT state of the "(bpy) 2 Ru(CN) 2 ligand" 5 in the bimetallic [(bpy) 2 Ru(CN)(µ-CN)Rh(NH 3 ) 4 Br] 2+ leads to a dissociative excited state.…”

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Excited-State Acid−Base Chemistry: Evidence for a Dissociative Excited State

et al. 2003

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Excitation (410 nm) of the bimetallic [(bpy)(2)Ru(CN)(mu-CN)Rh(NH(3))(4)Br](2+) produces the MLCT state localized on the (bpy)(2)Ru(CN)(2) ligand. Photoinduced cleavage of the bimetallic occurs in the presence of [H(+)], and the dependence yields a K(a) equivalent to that for ground-state cis-(bpy)(2)Ru(CN)(2) implying separation of the bimetallic prior to relaxation. The pH dependence and the emissivity of a bimetallic composed of components that individually quench at a diffusion controlled rate suggest that rupture of the RuCN-Rh bond is due to the reduction in electron density at the cyano ligand that occurs on population of the MLCT state. Unlike known photoinduced metal ligand dissociations, where the excitation energy is consumed in the dissociation, the dissociated "(bpy)(2)Ru(CN)(2) ligand" remains excited.

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Excited-State Coordination Chemistry: A New Quenching Mechanism

Cited by 9 publications

References 17 publications

Design and Study of Bi[1,8]naphthyridine Ligands as Potential Photooxidation Mediators in Ru(II) Polypyridyl Aquo Complexes

Design and Study of Bi[1,8]naphthyridine Ligands as Potential Photooxidation Mediators in Ru(II) Polypyridyl Aquo Complexes

Excited-State Coordination Chemistry: Excited-State Basicity of Bis(2,2′-bipyridyl)(2,3-dipyridylpyrazine)ruthenium(II)

Excited-State Acid−Base Chemistry: Evidence for a Dissociative Excited State

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