Proton Coupled Electron Transfer from the Excited State of a Ruthenium(II) Pyridylimidazole Complex

Proton coupled electron transfer (PCET) from the excited state of [Ru(bpy)2pyimH]2+ (bpy = 2,2′-bipyridine; pyimH = 2-(2′-pyridyl)imidazole) to N-methyl-4,4′-bipyridinium (monoquat, MQ+) was studied. While this complex has been investigated previously, our study is the first to show that the formal bond dissociation free energy (BDFE) of the imidazole-N–H bond decreases from (91 ± 1) kcal mol−1 in the electronic ground state to (43 ± 5) kcal mol−1 in the lowest-energetic 3MLCT excited state. This makes the [Ru(bpy)2pyimH]2+ complex a very strong (formal) hydrogen atom donor even when compared to metal hydride complexes, and this is interesting for light-driven (formal) hydrogen atom transfer (HAT) reactions with a variety of different substrates. Mechanistically, formal HAT between 3MLCT excited [Ru(bpy)2pyimH]2+ and monoquat in buffered 1 : 1 (v : v) CH3CN/H2O was found to occur via a sequence of reaction steps involving electron transfer from Ru(II) to MQ+ coupled to release of the N–H proton to buffer base, followed by protonation of reduced MQ+ by buffer acid. Our study is relevant in the larger contexts of photoredox catalysis and light-to-chemical energy conversion

Photoinduced Electron Transfer Coupled to Donor Deprotonation and Acceptor Protonation in a Molecular Triad Mimicking Photosystem II

The first artificial donor–sensitizer–acceptor compound in which photoinduced long-range electron transfer is coupled to donor deprotonation and acceptor protonation is reported. The long-lived photoproduct stores energy in the form of a radical pair state in which the charges of the donor and the acceptor remain unchanged, much in contrast to previously investigated systems that exhibit charge-separated states comprised of electron–hole pairs. This finding is relevant for light-driven accumulation of redox equivalents, because it exemplifies how the buildup of charge can be avoided yet light energy can be stored. Proton-coupled electron transfer (PCET) reactions at a phenol donor and a monoquat acceptor triggered by excitation of a Ru(II) sensitizer enable this form of photochemical energy storage. Our triad emulates photosystem II more closely than previously investigated systems, because tyrosine Z is oxidized and deprotonated, whereas plastoquinone B is reduced and protonated

Photoinduced electron and proton transfer with ruthenium complexes and organic donors and acceptors

Proton-coupled electron transfer (PCET) plays a crucial role in photosynthesis and catalytic conversions such as water oxidation and carbon dioxide reduction. It can formally be split into proton and electron transfer. In photosynthesis, light is used as principal energy resource, which is also desirable for artificial conversions. Therefore, PCET and individual proton and electron transfers from the long-living triplet excited state of ruthenium polyimine complexes were investigated in this thesis. In the first project, photoinduced PCET from the excited state of [Ru(bpy)2(pyimH)]2+ (bpy = 2,2’-bipyridine, pyimH = (2-pyridyl)imidazole) was investigated with electrochemical and time-resolved spectroscopic techniques. Depending on the pH, simple ET or PCET to a suitable organic substrate is favored. Simple excited state ET is facilitated significantly upon deprotonation of the ruthenium photosensitizer. The reducing power of this type of complex was further tuned by electron donating and electron withdrawing substituents on the bpy-spectator-ligands. Formal hydrogen atom donation is facilitated by approximately 50 kcal mol 1 in the photochemically generated 3MLCT, making these complexes strong formal hydrogen atom donors, even when compared to metal hydride complexes. In the second project, a molecular triad was investigated which is inspired by photosystem II. This triad combines long-range photoinduced charge transfer with two PCETs. The investigated donor-photosensitizer-acceptor assembly is based on a phenol as combined electron and proton donor, a [Ru(bpy)3]2+-type photosensitizer and a 4,4’-bipyridinium proton and electron acceptor. The photochemically generated radicals are separated by 20 Å. They are formed via two PCETs which mimics enzymatic long-range charge transfer more closely than any previously reported molecular model system. In the third project, the influence of a hydrogen bonded carboxylate on the luminescent excited state of acidic [Ru(bpy)2(biimH2)]2+ (biimH2 = 2,2’-biimidazole) type complexes was examined. Luminescence of monocrystalline samples was characterized by DFT calculations and monitored in the solid state at variable temperature and pressure. A pressure-induced red-shift in luminescence was observed in complexes with electron donating tert-butyl substituents on the bpy ligands whereas the more acidic complex with CF3-substituents showed only small pressure dependent luminescence. The origin of the difference in luminescence is either due to pressure induced proton transfer or secondary coordination sphere interactions via the hydrogen bonds

Quadrisecants give new lower bounds for the ropelength of a knot

Using the existence of a special quadrisecant line, we show the ropelength of any nontrivial knot is at least 15.66. This improves the previously known lower bound of 12. Numerical experiments have found a trefoil with ropelength less than 16.372, so our new bounds are quite sharp.Comment: v3 is the version published by Geometry & Topology on 25 February 200

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

The conversion of CO2, H2O, or N2 to energy-rich compounds such as CH3OH, H2 or NH3 requires the properly orchestrated transfer of multiple electrons and protons. Artificial photosynthetic systems therefore must be able to synchronize the rapid primary photoinduced transfer of single electrons to the slower catalytic (multi-electron) turnover of substrates, and this generates a need for temporary accumulation and storage of redox equivalents. This is a very difficult task, particularly in absence of sacrificial reagents. Toward this end, proton-coupled multi-electron transfer (PCMET) driven by light is now receiving increased attention. This invited Feature article considers recent pertinent studies of donor-sensitizer-acceptor compounds and inorganic-organic hybrid systems, as well as some recent photoredox catalysis studies of proton-coupled multi-electron reductions. Key principles for successful light-driven accumulation and storage of redox equivalents are discussed, and the relevance of PCMET for the formation of solar fuels and for photoredox catalysis is emphasized

Prevalence of Gall Bladder Stones among Type 2 Diabetic Patients in Benghazi Libya: A Case-control Study

Background: Diabetes mellitus and gall bladder stones are both common and costly diseases. Increasing age, female gender, overweight, familial history of the disease and type 2 diabetes mellitus is all associated with an increased risk of gallstones. Several studies from around the world reported an increased prevalence of gall bladder stones in patients with diabetes mellitus. Aims and objectives: The aim of this study was to define the frequency of gall bladder stones among Libyan diabetics and to evaluate the possible associated risk factors in these patients. Patients and methods: A case-control study was performed during 2007 at Benghazi Diabetes and endocrinology Center. The study involved 161 randomly selected type-2 diabetic patients under regular follow up at the center, and 166 age and sex matched non-diabetic outpatients at the 7th of October teaching hospital. Real-time abdominal ultrasound was performed by two radiologists to examine the abdomen after an overnight fast. Results: About 40% of the diabetic cohort had gall bladder stones as compared to 17.5% of non-diabetic patients. Females were significantly more affected than males. Patients with gall bladder stones were significantly older and had a significantly higher body mass index than those without stones. Conclusion: The prevalence of gallstones in Libyan diabetic patients is higher than the rates reported in other parts of the world. Libyan diabetic patients with gallstones tend to be older and more obese than those without gallstones. Duration of diabetes mellitus and type of treatment does not seem to influence the frequency of gall bladder stones among Libyan diabetics

Mechanistic insights into the charge transfer dynamics of photocatalytic water oxidation at the lipid bilayer: water interface

Photosystem II, the natural water-oxidizing system, is a large protein complex embedded in a phospholipid membrane. A much simpler system for photocatalytic water oxidation consists of liposomes functionalized with amphiphilic ruthenium(II)-trisbipyridine photosensitizer (PS) and 6,6 '-dicarboxylato-2,2 '-bipyr-idine-ruthenium(II) catalysts (Cat) with a water-soluble sacrificial electron acceptor (Na2S2O8). However, the effect of embedding this photocatalytic system in liposome membranes on the mechanism of photocatalytic water oxidation was not well understood. Here, several phenomena have been identified by spectroscopic tools, which explain the drastically different kinetics of water photo oxidizing liposomes, compared with analogous homogeneous systems. First, the oxidative quenching of photoexcited PS* by S2O82- at the liposome surface occurs solely via static quenching, while dynamic quenching is observed for the homogeneous system. Moreover, the charge separation efficiency after the quenching reaction is much smaller than unity, in contrast to the quantitative generation of PS+ in homogeneous solution. In parallel, the high local concentration of the membrane-bound PS induces self quenching at 10:1-40:1 molar lipid-PS ratios. Finally, while the hole transfer from PS+ to catalyst is rather fast in homogeneous solution (kobs > 1 x 104 s-1 at [catalyst] > 50 mu M), in liposomes at pH = 4, the reaction is rather slow (kobs approximate to 17 s-1 for 5 mu M catalyst in 100 mu M DMPC lipid). Overall, the better understanding of these productive and unproductive pathways explains what limits the rate of photocatalytic water oxidation in liposomal vs homogeneous systems, which is required for future optimization of light-driven catalysis within self-assembled lipid interfaces.Horizon 2020(H2020)828838−SoFiAMetals in Catalysis, Biomimetics & Inorganic Material

Controlling Second Coordination Sphere Effects in Luminescent Ruthenium Complexes by External Pressure

Two luminescent heteroleptic Ru(II) complexes with a 2,2’ - biimidazole (biimH 2) ligand form doubly hyrogen-bonded salt bridges to 4-sulfobenzoate anions in single crystals. The structure of one of these cation-anion adducts shows that the biimH 2 ligand is deprotonated. Its 3MLCT luminescence bands does not shift significantly under the influence of an external hydrostatic pressure, a behavior typical for these electronic transitions. In contrast, hydrostatic pressure on the other crystalline cation-anion adduct induces a shift of proton density from the peripheral N-H groups of biimH 2 towards benzoate, leading to a pronounced red- shift of the 3MLCT luminescence band. Such a significant and pressure-tuneable influence from an interaction in the second coordination sphere is unprecedented in artificial small molecule-based systems

Light-driven electron injection from a biotinylated triarylamine donor to [Ru(diimine)3]2+-labeled streptavidin

Electron transfer from a biotinylated electron donor to photochemically generated Ru(III) complexes covalently anchored to streptavidin is demonstrated by means of time-resolved laser spectroscopy. Through site-selective mutagenesis, a single cysteine residue was engineered at four different positions on streptavidin, and a Ru(II) tris-diimine complex was then bioconjugated to the exposed cysteines. A biotinylated triarylamine electron donor was added to the Ru(II)-modified streptavidins to afford dyads localized within a streptavidin host. The resulting systems were subjected to electron transfer studies. In some of the explored mutants, the phototriggered electron transfer between triarylamine and Ru(III) is complete within 10 ns, thus highlighting the potential of such artificial metalloenzymes to perform photoredox catalysis

Increased Prevalence of Trichinella spp., Northeastern Germany, 2008

Migration of raccoon dogs from Poland may have caused this change