| 1. |
- Karnahl, Michael, et al.
(författare)
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Synthesis and photophysics of a novel photocatalyst for hydrogen production based on a tetrapyridoacridine bridging ligand
- 2012
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Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104 .- 1873-4421. ; 393:1, s. 65-73
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Tidskriftsartikel (refereegranskat)abstract
- Molecular photocatalysts allow for selectively tuning their function on a molecular level based on an in-depth understanding of their chemical and photophysical properties. This contribution reports the synthesis and photophysical characterization of the novel molecular photocatalyst [(tbbpy)(2)Ru(tpac)PdCl(2)](2+) RutpacPd (with tpac = tetrapyrido[3,2-a:2',3'-c:3 '',2 ''-h:2''',3'''-j]acridine) and its mononuclear building block. Furthermore, detailed photocatalytic activity measurements of RutpacPd are presented. The introduction of the tpac-ligand into the molecular framework offers a potential route to reduce the impact of water as compared to the well-studied class of RutpphzPd (with tpphz = tetrapyrido[3,2-a:2',3'-c:300,2 ''-h: 2''',3'''-j] phenazine) complexes. The distinct impact of water on the electron-transfer processes in tpphz-ligands stems from the possibility of water to form hydrogen bonds to the phenazine nitrogen atoms and will potentially reduced when replacing the phenazine by the acridine unit. The effect of this structural variation on the catalytic properties and the underlying ultrafast intramolecular charge transfer behavior will be discussed in detail.
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| 2. |
- Karnahl, Michael, et al.
(författare)
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Tuning of Photocatalytic Hydrogen Production and Photoinduced Intramolecular Electron Transfer Rates by Regioselective Bridging Ligand Substitution
- 2011
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Ingår i: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 12:11, s. 2101-2109
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Tidskriftsartikel (refereegranskat)abstract
- Artificial photosynthesis based on supramolecular photocatalysts offers the unique possibility to study the molecular processes underlying catalytic conversion of photons into chemical fuels in great detail and to tune the properties of the photocatalyst by alterations of the molecular framework. Herein we focus on both possibilities in studying the photocatalytic reduction of protons by derivatives of the well-known photocatalyst [(tbbpy)(2)Ru(tpphz)PdCl(2)](PF(6))(2) [4,4'-di-tert-butyl-2,2'-bipyridine (tbbpy), tetrapyrido[3,2-a:2',3'-c:3 '',2 ''-h:2'", 3'"-j]phenazine (tpphz)]. We report on a modified photocatalyst where the crucial bridging ligand tpphz is substituted by bromine and investigate the effect of the structural variation on the catalytic properties of the complex and its ultrafast intramolecular charge transfer behavior. It is found that structural modification stabilizes the phenanthroline-centered metal-to-ligand charge-transfer state on the tpphz moiety, thereby reducing the electron transfer gradient across the entire electron-relaying bridging ligand and at the same time accelerating nanosecond ground-state recovery. The same structural modifications cause an overall reduction of the catalytic activity of the complex. Thus, the results highlight the potential of small structural variations in the molecular framework of supramolecular catalysts in understanding the photoinduced charge-transfer processes and optimizing their catalytic performance.
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| 3. |
- Kositzki, Ramona, et al.
(författare)
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Electronic and molecular structure relations in diiron compounds mimicking the [FeFe]-hydrogenase active site studied by X-ray spectroscopy and quantum chemistry
- 2017
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Ingår i: Dalton Transactions. - : ROYAL SOC CHEMISTRY. - 1477-9226 .- 1477-9234. ; 46:37, s. 12544-12557
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Tidskriftsartikel (refereegranskat)abstract
- Synthetic diiron compounds of the general formula Fe-2(mu-S2R)(CO)(n)(L)(6-n) (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and K beta main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(I) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe-Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds.
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| 4. |
- Kuhnt, Christian, et al.
(författare)
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The impact of bromine substitution on the photophysical properties of a homodinuclear Ru-tpphz-Ru complex
- 2011
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 516:1-3, s. 45-50
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Tidskriftsartikel (refereegranskat)abstract
- Ruthenium-polypyridine complexes play an important role as photosensitizers in supramolecular photocatalysis. Using multiple Ru-centers within a single supramolecular catalyst might be a promising path for improving its efficiency. The connection of several chromophores may, however, lead to direct interaction amongst individual photoactive centers, which is at the core of the work at hand. The work focuses on the photophysics of [(tbbpy)(2)Ru(3,16-Br(2)-tpphz)Ru(tbbpy)(2)](PF(6))(4) (1, tpphz = tetrapyrido[3,2-a: 2',3'-c: 3 '',2 ''-h:2''',3'''-j] phenazine, tbbpy = 4,4'-di-tert.-butyl-2,2'-bipyridine) and aims at detailing the impact of the bromine substituents on bridging ligand photoinduced intramolecular charge-transfer dynamics. It is shown that the introduction of the bromine reduces the driving force for intra-ligand charge-transfer steps and impacts exciton-exciton annihilation at high pump intensities.
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| 5. |
- Tschierlei, Stefanie, et al.
(författare)
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Photophysics of an Intramolecular Hydrogen-Evolving Ru-Pd Photocatalyst
- 2009
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Ingår i: Chemistry: A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 15:31, s. 7678-7688
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Tidskriftsartikel (refereegranskat)abstract
- Photoinduced electron-transfer processes within a precatalyst for intramolecular hydrogen evolution [(tbbpy)(2)Ru(tpphz)PdCl2](2+) (RuPd; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'c:3 '',2 '',-h:2"',3'''-j]phenazine) have been studied by resonance Raman and ultrafast time-resolved absorption spectroscopy. By comparing the photophysics of the [(tbbpy)(2)Ru(tpphz)](2+) subunit Ru with that of the supramolecular catalyst RuPd, the individual electron-transfer steps are assigned to kinetic components, and their dependence on solvent is discussed. The resonance Raman data reveal that the initial excitation of the molecular ensemble is spread over the terminal tbbpy and the tpphz ligands. The subsequent excited-state relaxation of both Ru and RuPd on the picosecond timescale involves formation of the phenazine-centered intraligand charge-transfer state, which in RuPd precedes formation of the Pd-reduced state. The photoreaction in the heterodinuclear supramolecular complex is completed on a subnanosecond timescale. Taken together, the data indicate that mechanistic investigations must focus on potential rate-determining steps other than electron transfer between the photoactive center and the Pd unit. Furthermore, structural variations should be directed towards increasing the directionality of electron transfer and the stability of the charge-separated states.
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| 6. |
- Beyler, Maryline, et al.
(författare)
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Pentacoordinate iron complexes as functional models of the distal iron in [FeFe] hydrogenases
- 2011
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Ingår i: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 47:42, s. 11662-11664
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Tidskriftsartikel (refereegranskat)abstract
- Mononuclear pentacoordinate iron complexes with a free coordination site were prepared as mimics of the distal Fe (Fe(d)) in the active site of [FeFe] hydrogenases. The complexes catalyze the electrochemical reduction of protons at mild overpotential.
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| 7. |
- Gardner, James M, 1982-, et al.
(författare)
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Light-Driven Electron Transfer between a Photosensitizer and a Proton-Reducing Catalyst Co-adsorbed to NiO
- 2012
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:47, s. 19322-19325
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Tidskriftsartikel (refereegranskat)abstract
- While intermolecular hole-hopping along the surface of semiconductors is known, there are no previous examples of electron-hopping between molecules on a surface. Herein, we present the first evidence of electron transfer from the photoreduced sensitizer Coumarin-343 (C343) to complex 1, both bound on the surface of NiO. In solution, 1 has been shown to be a mononuclear Fe-based proton-reducing catalyst. The reduction of 1 is reversible and occurs within 50 ns after excitation of C343. Interfacial recombination between the reduced 1(-) and NiO hole occurs on a 100 µs time scale by non-exponential kinetics. The observed process is the first essential step in the photosensitized generation of H2 from a molecular catalyst in the absence of a sacrificial donor reagent.
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| 8. |
- Karnahl, Michael, et al.
(författare)
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Mixed-valence [(FeFeII)-Fe-I] hydrogenase active site model complexes stabilized by a bidentate carborane bis-phosphine ligand
- 2012
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 41:40, s. 12468-12477
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Tidskriftsartikel (refereegranskat)abstract
- A series of [FeFe]-hydrogenase active site analogues, with the general formula [Fe-2(dt)(CO)(4)(BC)] 1-3 (dt = dithiolate, pdt = propyl-1,3-dt (1), bdt = benzene-1,2-dt (2), edt = ethyl-1,2-dt (3); BC = 1,2-bisdiphenylphosphine-1,2-o-carborane), has been prepared and structurally characterized. While the electrochemical reductions of 1-3 are largely invariant to the different nature of their dt bridges, the oxidations differ by more than 120 mV in between the series. Remarkably, all three compounds are reversibly oxidized, with complex 1 that contains the most electron-donating pdt ligand at the mildest potential of -0.09 V vs. Fc/Fc(+). The one-electron oxidized state 1(ox) is stable for several minutes and was spectroscopically characterized by FTIR and EPR. EPR spectroscopy provided evidence that in the mixed-valence [(FeFeII)-Fe-I] state most of the spin density is located on the iron with the BC-ligand. This is monitored through the strong P-31 hyperfine coupling of the phenyl groups of the BC ligand, while further delocalization into the o-carborane unit is negligible.
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| 9. |
- Karnahl, Michael, et al.
(författare)
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Structural and spectroscopic characterization of tetranuclear iron complexes containing a (P2N2Ph)-N-R bridge
- 2012
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Ingår i: Journal of coordination chemistry (Print). - : Informa UK Limited. - 0095-8972 .- 1029-0389 .- 1026-7441. ; 65:15, s. 2713-2723
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Tidskriftsartikel (refereegranskat)abstract
- A pair of tetranuclear iron complexes consisting of two Fe-2(Cl(2)bdt)(CO)(5) subunits (Cl(2)bdt = 3,6-dicholorobenzene-1,2-dithiolate) bridged by different cyclic 1,5-diaza-3,7-diphosphacyclooctane (P2N2) ligands were prepared and structurally characterized. In the solid state, the P2N2 ligands adopt a boat conformation, which results in rather short distances between the two Fe-2(Cl(2)bdt)(CO)(5) clusters that promotes electronic communication across the diphosphine ligand.
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| 10. |
- Orthaber, Andreas, et al.
(författare)
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Coordination and conformational isomers in mononuclear iron complexes with pertinence to the [FeFe] hydrogenase active site
- 2014
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 43:11, s. 4537-4549
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Tidskriftsartikel (refereegranskat)abstract
- A series of six mononuclear iron complexes of the type [Fe(X-bdt)((P2N2Ph)-N-R)(CO)] ((P2N2Ph)-N-R = 1,5-diaza-3,7-diphosphaoctane, bdt = benzenedithiolate with X = H, Cl-2 or Me and R = Ph, Bn, Cyc or tert-Bu) was prepared. This new class of penta-coordinate iron complexes contains a free coordination site and a pendant base as essential structural features of the [FeFe]-hydrogenase active site. The bidentate nature of the (P2N2Ph)-N-R ligands was found to be crucial for the preferential formation of coordinatively unsaturated penta-coordinate complexes, which is supported by first principle calculations. IR-spectroscopic data suggest the presence of coordination isomers around the metal center, as well as multiple possible conformers of the (P2N2Ph)-N-R ligand. This finding is further corroborated by X-ray crystallographic and computational studies. P-31{H-1}-NMR- and IR-spectroscopic as well as electrochemical measurements show that the electronic properties of the complexes are strongly, and independently, influenced by the P-substituents at the (P2N2Ph)-N-R ligand as well as by modifications of the bdt bridge. These results illustrate the advantages of this modular platform, which allows independent and selective tuning through site specific modifications. Potential catalytic intermediates, namely singly reduced and protonated complexes, have been further investigated by spectroscopic methods and exhibit remarkable stability. Finally, their general capacity for electro-catalytic reduction of protons to molecular hydrogen was verified.
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| 11. |
- Schulz, Martin, et al.
(författare)
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The role of the bridging ligand in photocatalytic supramolecular assemblies for the reduction of protons and carbon dioxide
- 2012
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Ingår i: Coordination chemistry reviews. - : Elsevier BV. - 0010-8545 .- 1873-3840. ; 256:15-16, s. 1682-1705
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Tidskriftsartikel (refereegranskat)abstract
- Molecular photocatalysts, combining a light absorber and a catalytic active center via a bridging ligand, have received much attention during the last decade. Therefore, many new types of photocatalysts containing novel bridging ligands are reported. The bridging ligand is expected to play an important role and does not only represent the chemical connection between a photosensitizing unit and a catalytically active center, but can also greatly affect the overall catalytic activity and stability of the photocatalysts. Moreover, a well-chosen combination of the three parts photosensitizer-bridge-catalyst is expected to yield intramolecular photocatalysts superior over intermolecular systems. This review will discuss the coordination, photophysical, electrochemical and photocatalytic properties of multinuclear metal complexes based on a selection of heterocyclic bridging ligands that show significant photocatalytic activity for hydrogen generation or CO2 reduction. Of particular interest in the discussion will be the effect of the bridging ligand on the photocatalytic properties of the compounds. Based on the results obtained, possible design principles for the optimization of the photocatalytic properties of this type of compounds will be suggested.
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| 12. |
- Streich, Daniel, et al.
(författare)
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Comparing the Reactivity of Benzenedithiolate- versus Alkyldithiolate-Bridged Fe2(CO)6 Complexes with Competing Ligands
- 2011
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Ingår i: European Journal of Inorganic Chemistry. - : Wiley. - 1434-1948 .- 1099-1948 .- 1099-0682. ; :7, s. 1106-1111
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Tidskriftsartikel (refereegranskat)abstract
- The reactivity of [(mu-X(2)bdt)Fe-2(CO)(6)] [(bdt)1, X(2)bdt = 3,6-di-substituted bezenedithiolate; X = H, Cl] with ligands of different donor strengths is investigated and compared to that of [(mu-pdt)Fe-2(CO)(6)] [(pdt) 1, pdt = propyldithiolate] and [(mu-edt)Fe-2(CO)(6)] [(edt)1, edt = ethyldithiolate]. Strong donor ligands (L = CN-, PMe3) when added to (bdt) 1 lead to mononuclear [(bdt)Fe(L)(2)(CO)(2)], (bdt)6(L), in a disproportionation and fragmentation reaction, while simple ligand-substitution reactions occur on (edt)1 and (pdt)1 under identical conditions. In the presence of weaker ligands such as secondary amines or dmf, the alkyldithiolate-bridged complexes are unreactive, while (bdt)1 transforms to an O-2-sensitive, magnetically uncoupled species, potentially a mononuclear Fe-I complex coordinated by bdt and at least 2 CO ligands.
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