Excited Ru Research Articles

The effects of Pt cocatalyst particle size on charge transfer kinetics in dye-sensitized SrTiO3 photocatalysts for hydrogen evolution studied by time-resolved emission/absorption spectroscopy

Dye-sensitized H2 evolution systems were constructed from Pt-loaded SrTiO3 and Ru(II) complex photosensitizers as building blocks, and the impact of the Pt cocatalyst was investigated. In the presence of a stronger electron donor (sodium ascorbate), SrTiO3 loaded with larger Pt particles showed higher activity. On the other hand, the sample having smaller Pt particles gave higher activity in the presence of a weaker electron donor (ethylenediaminetetraacetic acid disodium salt dihydrate). Time-resolved emission/absorption measurements revealed that the larger Pt particles facilitated electron injection from the excited Ru sensitizer to Pt/SrTiO3, while the smaller one improved charge carrier separation by suppressing the back electron transfer reaction from Pt/SrTiO3 to the oxidized sensitizer. These results show that the cocatalyst particle size can affect the kinetics of the electron injection and carrier separation processes. Judicious choice of the Pt particle size can therefore improve the H2 evolution activity in accordance with the reaction system.

Synthesis of Coumarin 3‐aldehyde Derivatives via Photocatalytic Cascade Radical Cyclization‐Hydrolysis

AbstractNew synthetic method of coumarins 3‐aldehyde via visible light stimulation and oxidation promotion cascade radical cyclization/hydrolysis process is reported. this reaction involves a two‐step procedure, the 1,3‐dioxolane radical is initiated in situ from 1,3‐dioxolane via the excited Ru(bpy)3Cl2 and oxidant TBHP, subsequently an addition to triple bond of various alkynes produces vinyl radicals, the radical intermediates can smoothly convert to coumarin 3‐aldehyde via tandem cyclization/acidic hydrolysis process in good to excellent yields. Furthermore, the synthetic application valuable of coumarin 3‐aldehyde is also evaluated in this work.

Nitrogen Doping Excited Ru and Ti3c2-Xnx Support for Hydrogen Generation from Ammonia Borane

Nitrogen Doping Excited Ru and Ti3c2-Xnx Support for Hydrogen Generation from Ammonia Borane

Enhanced photocatalytic CO2 reduction with suppressing H2 evolution via Pt cocatalyst and surface SiO2 coating

The photocatalytic CO2 reduction to chemical feedstocks represents a promising route for the utilization of stranded renewable resources, but faces poor activity and selectivity resulting from the fast recombination of photogenerated charge carriers and competing hydrogen (H2) evolution reaction. Herein, we report the synthesis of core-shell structured Pt/SiO2@SiO2 composites via electrostatic self-assembly and surface SiO2 coating for photocatalytic CO2 reduction with Ru(bpy)32+ (abbreviated as Ru, bpy = 2′2-bipyridine) as photosensitizer and Pt as cocatalyst under visible light irradiation (λ > 420 nm). Photoluminescence results illustrate that Pt cocatalyst enhances the transfer of photogenerated electrons from the excited Ru, thereby improving photocatalytic performance. Further surface coating SiO2 prevents Pt from directly contacting with H2O and ameliorates CO2 adsorption ability of catalyst, thus suppressing H2 evolution and increasing CO selectivity, which also endows the catalyst with enhanced stability. This work would provide an instructive guideline in regulating the selectivity and stability of using transition metal complexes as photosensitizers for photocatalytic CO2 reduction through the cocatalyst loading and surface coating strategy.

A photoredox catalysed Heck reaction via hole transfer from a Ru(ii)-bis(terpyridine) complex to graphene oxide.

The attachment of homoleptic Ru bis-terpy complexes on graphene oxide significantly improved the photocatalytic activity of the complexes. These straightforward complexes were applied as photocatalysts in a Heck reaction. Due to covalent functionalization on graphene oxide, which functions as an electron reservoir, excellent yields were obtained. DFT investigations of the charge redistribution revealed efficient hole transfer from the excited Ru unit towards the graphene oxide.

Visible Light-Driven Gas-Phase Artificial Photosynthesis Reactions over Ruthenium Metal Nanoparticles Modified with Anatase TiO2

Ruthenium metal nanoparticles with a narrow size distribution have been synthesised via a solvothermal method. The solids were characterised using a range of analytical techniques (XRD, TEM, TPD, and XPS) and tested in the CO2+H2O reaction under simulated solar radiation, showing photocatalytic activity towards the production of CH4 and CO. The photocatalysis was promoted through a plasmonic excitation of the Ru. The addition of Ti to the preparation resulted in the formation of anatase TiO2. Notwithstanding the fact that the energy of the light used during the photocatalysis was insufficient to excite TiO2, its presence affects the catalysts’ optical and chemical properties and the product (CH4/CO) ratios, favouring the evolution of CO over that of CH4 (suggesting exciton transfer to TiO2 from plasmonically excited Ru).

Metallic Cobalt-Carbon Composite as Recyclable and Robust Magnetic Photocatalyst for Efficient CO2 Reduction.

CO2 conversion into value-added chemical fuels driven by solar energy is an intriguing approach to address the current and future demand of energy supply. Currently, most reported surface-sensitized heterogeneous photocatalysts present poor activity and selectivity under visible light irradiation. Here, photosensitized porous metallic and magnetic 1200 CoC composites (PMMCoCC-1200) are coupled with a [Ru(bpy)3 ]Cl2 photosensitizer to efficiently reduce CO2 under visible-light irradiation in a selective and sustainable way. As a result, the CO production reaches a high yield of 1258.30 µL with selectivity of 64.21% in 6 h, superior to most reported heterogeneous photocatalysts. Systematic investigation demonstrates that the central metal cobalt is the active site for activating the adsorbed CO2 molecules and the surficial graphite carbon coating on cobalt metal is crucial for transferring the electrons from the triplet metal-to-ligand charge transfer of the photosensitizer Ru(bpy)32+ , which gives rise to significant enhancement for CO2 reduction efficiency. The fast electron injection from the excited Ru(bpy)32+ to PMMCoCC-1200 and the slow backward charge recombination result in a long-lived, charge-separated state for CO2 reduction. More impressively, the long-time stability and easy magnetic recycling ability of this metallic photocatalyst offer more benefits to the photocatalytic field.

Investigation of excited state, reductive quenching, and intramolecular electron transfer of Ru(ii)-Re(i) supramolecular photocatalysts for CO2 reduction using time-resolved IR measurements.

Supramolecular photocatalysts in which Ru(ii) photosensitizer and Re(i) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO2 reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the 3MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.0-1.1) × 104 s-1 as a major component and (3.5-4.3) × 106 s-1 as a minor component, in acetonitrile. The produced charge-separated state has a long lifetime, with charge recombination rate constants of only (6.5-8.4) × 104 s-1. Thus, although it has a large driving force (-ΔG0CR ∼ 2.6 eV), this process is in the Marcus inverted region. On the other hand, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH), reductive quenching of the excited Ru unit proceeds much faster (kq[BNAH (0.2 M)] = (3.5-3.8) × 106 s-1) than the abovementioned intramolecular oxidative quenching, producing the one-electron-reduced species (OERS) of the Ru unit. Nanosecond TRIR data clearly show that intramolecular electron transfer from the OERS of the Ru unit to the Re unit (kET > 2 × 107 s-1) is much faster than from the excited state of the Ru unit, and that it is also faster than the reductive quenching process of the excited Ru unit by BNAH. To measure the exact value of kET, picosecond TRIR spectroscopy and a stronger reductant were used. Thus, in the case of the binuclear complex with tri(p-fluorophenyl)phosphine ligands (RuRe(FPh)), for which intramolecular electron transfer is expected to be the fastest among the three binuclear complexes, in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), kET was measured as kET = (1.4 ± 0.1) × 109 s-1. This clearly shows that intramolecular electron transfer in these RuRe binuclear supramolecular photocatalysts is not the rate-determining process in the photocatalytic reduction of CO2, which is one of the main reasons why they work so efficiently.

Electrochemiluminescence Microscopy Imaging Using Ru(bpy)3 2+ Doped in Silica Microspheres

Electrochemiluminescence (ECL) is a controllable form of chemiluminescence where emission intensity is affected by a voltage imposed at an electrode surface. ECL microscopy imaging is a imaging technique aimed at light-emitting close to electrode surface with highly temporal and spatial resolution. Because ECL emitters are generated in situ at electrode surfaces, ECL imaging reflects the local surface reactivity and has a near-zero background. These features make ECL very useful in imaging applications, such as multiplexed ECL sandwich immunoassays, parallel single-cell analysis and latent fingerprints ECL imaging. Here we synthesized Ru(2,2’-bipyridine)3Cl2-doped silica (RuDS) microspheres via Stober method for single microsphere ECL imaging. The RuDS microsphere consisted of a silica core and a Ru(2,2’-bipyridine)3Cl2-doped silica shell via sol−gel process to immobilize Ru(bpy)3Cl2 into silica shell. Rougher sufaces and some defects indicated imperfect growth of Ru(2,2’-bipyridine)3Cl2-doped silica shell. We also noticed that RuDS microspheres tended to aggregate in the growth of Ru(2,2’-bipyridine)3Cl2-doped silica shell, since electrostatic interaction between Ru(bpy)3Cl2 and silica matrices decreased negatice charges on the surfaces of silica microspheres. Then an electrode modified with RuDS microspheres was placed in a cell under the microscope. The bright field image showed the positions of the RuDS microspheres. The bright ECL image of RuDS microspheres suggested that intermediates generated by electrooxidation of tripropylamine (TPA) can move up to several micron to cause the excitation of Ru(bpy)3Cl2. Interestingly, the ECL intensity from different single microspheres exhibited obvious distinctions, which may derive from local heterogeneity of electrode surface and RuDS microspheres. Moreover, the image sequences showed the areas without any microspheres also produced a weak ECL background signal, as a result of the electrooxidation of TPA itself. Simultaneously, the oxidation current of TPA in cyclic voltammetry and the ECL emission intensity of RuDS microspheres dramatically decreased along with increase of scan times, indicating the passivating of the surface of electrode was occured when the potential was applied. These results demonstrated that ECL microscopy imaging of single RuDS microsphere appearing high temporal and spatial resolution, is a powerful tool in terms of further understanding of detailed ECL mechanisms, both emiters and coreactants, and potential application in single molecule detection, etc.

A Long-Lived Mononuclear Cyclopentadienyl Ruthenium Complex Grafted onto Anatase TiO2 for Efficient CO2 Photoreduction.

This work shows a novel artificial donor-catalyst-acceptor triad photosystem based on a mononuclear C5 H5 -RuH complex oxo-bridged TiO2 hybrid for efficient CO2 photoreduction. An impressive quantum efficiency of 0.56 % for CH4 under visible-light irradiation was achieved over the triad photocatalyst, in which TiO2 and C5 H5 -RuH serve as the electron collector and CO2 -reduction site and the photon-harvester and water-oxidation site, respectively. The fast electron injection from the excited Ru(2+) cation to TiO2 in ca. 0.5 ps and the slow backward charge recombination in half-life of ca. 9.8 μs result in a long-lived D(+) -C-A(-) charge-separated state responsible for the solar-fuel production.

Disentangling the Physical Processes Responsible for the Kinetic Complexity in Interfacial Electron Transfer of Excited Ru(II) Polypyridyl Dyes on TiO2.

Interfacial electron transfer at titanium dioxide (TiO2) is investigated for a series of surface bound ruthenium-polypyridyl dyes whose metal-to-ligand charge-transfer state (MLCT) energetics are tuned through chemical modification. The 12 complexes are of the form Ru(II)(bpy-A)(L)2(2+), where bpy-A is a bipyridine ligand functionalized with phosphonate groups for surface attachment to TiO2. Functionalization of ancillary bipyridine ligands (L) enables the potential of the excited state Ru(III/)* couple, E(+/)*, in 0.1 M perchloric acid (HClO4(aq)) to be tuned from -0.69 to -1.03 V vs NHE. Each dye is excited by a 200 fs pulse of light in the visible region of the spectrum and probed with a time-delayed supercontiuum pulse (350-800 nm). Decay of the MLCT excited-state absorption at 376 nm is observed without loss of the ground-state bleach, which is a clear signature of electron injection and formation of the oxidized dye. The dye-dependent decays are biphasic with time constants in the 3-30 and 30-500 ps range. The slower injection rate constant for each dye is exponentially distributed relative to E(+/)*. The correlation between the exponentially diminishing density of TiO2 sub-band acceptor levels and injection rate is well described using Marcus-Gerischer theory, with the slower decay components being assigned to injection from the thermally equilibrated state and the faster components corresponding to injection from higher energy states within the (3)MLCT manifold. These results and detailed analyses incorporating molecular photophysics and semiconductor density of states measurements indicate that the multiexponential behavior that is often observed in interfacial injection studies is not due to sample heterogeneity. Rather, this work shows that the kinetic heterogeneity results from competition between excited-state relaxation and injection as the photoexcited dye relaxes through the (3)MLCT manifold to the thermally equilibrated state, underscoring the potential for a simple kinetic model to reproduce the complex kinetic behavior often observed at the interface of mesoporous metal oxide materials.

Photochemical Hydrogen Generation Initiated by Oxidative Quenching of the Excited Ru(bpy)3 (2+) * by a Bio-Inspired [2Fe2S] Complex.

A diiron dithiolate complex 1 containing 1,8-naphthalic anhydride bridge was prepared, which possessed the lowest reduction potential for the synthetic diiron complexes modeled on the active site of [FeFe] hydrogenase reported so far. For the first time, oxidative quenching of the excited Ru(bpy)3 (2+) * through electron transfer to a bio-inspired [2Fe2S] complex was corroborated. Hydrogen evolution, driven by visible light, was successfully observed for a three-component system, consisting of Ru(bpy)3 (2+) , complex 1, and EDTA as electron donor in aqueous/organic media. These results provide a basis and also opportunity to develop a photo water splitting system employing Fe-based catalysts without sacrificial electron donors.

Photocatalyses of Ru(II)–Re(I) binuclear complexes connected through two ethylene chains for CO2 reduction

We synthesized two novel supramolecular photocatalysts for CO2 reduction [Ru(BL1)Re(CO)3Cl]2+ (12+) and [Ru(BL1)Re(CO)2{P(p-F-C6H4)3}2]3+ (33+), where BL1 is two 2,2′-bipyridine moieties connected through two ethylene chains. Compared to the corresponding supramolecular photocatalysts (22+, 43+) with only one ethylene chain in the bridging ligand (BL2), these complexes had stronger electronic interactions between the Ru and Re units through the two ethylene chains, which strengthened the oxidation power of the excited Ru unit. Although BL1 compounds functioned as photocatalysts for CO2 reduction by selectively providing CO, their photocatalyses were very different, especially, when compared with BL2 compounds. In the case of 12+, the quantum yield of CO formation (ΦCO) was larger than that of the corresponding BL2 compound. Conversely, ΦCO of the system using 33+ was approximately 40% less than the corresponding BL2 compound. The reasons for these differences were successfully clarified through mechanistic studies of these photocatalytic CO2 reductions.

Ruthenium(II)Bis(2,2′-Bipyridine) Interactions with Nucleaobases- Models for DNA Binding

The cis diammine dichloroplatinum (II) (cisplatin), FDA approved for treatment of testicular cancer in 1978, represents the current paradigm in transition metal based chemotherapy agents. Cisplatin and various analogs have now emerged as significant antineoplastic chemotherapy agents towards a wide range of solid tumors cancers. The (NH3)2(H2O)2 Pt(II) complex binds to the major groove of gDNA targeting the N7 atoms of guanine bases resulting in both 1,2 intrastrand (65%) and interstrand (25%) crosslinks. In the case of intrastrand crosslinking, the toxicity has been suggested to arise from bending of the DNA. Despite the success of cisplatin type drugs in treating many forms of cancer, the drug suffers from the propensity of cancer cells to relapse after the initial exposure. In addition, cisplatin type complexes also exhibit general cellular toxicity and are not specific to cancer cells. Ruthenium based metal complexes may offer an alternative to Pt based chemotherapy agents. Octahedral Ru(II)tris(2,2′-bipyridine) (RuBpy) complexes containing bidentate ligands exhibit a rich photochemistry that has been exploited in applications ranging from solar cells to drug development. Of specific interest is the excitation of Ru(II)bis(2,2′-bipyridine)(L)2 (RuBpy2L2) type complexes where L = sterically hindered bpy, primary amine, nitrile, etc which leads to the photoexchange of L with solvent molecules. Photolysis of RuBpy(L)2 complexes in aqueous solutions and in the presence of DNA results in the formation of an RuBpy2-DNA adduct providing for a potential photo-active cisPlatin analog. However, little is known on the exact mechanism through which RuBpy2 binds to DNA bases. Here we have examined the binding of nucleobases, nucleosides and nucleotides to the RuBpy2(H2O)2 complex using optical spectroscopy and computational chemistry methods. Our results demonstrate a strong affinity of the RuBpy2 toward guanine with binding taking place at the N7 position, similar to cisPlatin.

Photoinduced energy and electron transfer in rubrene–benzoquinone and rubrene–porphyrin systems

Excited-state electron and energy transfer from singlet excited rubrene (Ru) to benzoquinone (BQ) and tetra-(4-aminophenyl) porphyrin (TAPP) were investigated by steady-state absorption and emission, time-resolved transient absorption, and femtosecond (fs)–nanosecond (ns) fluorescence spectroscopy. The low reduction potential of BQ provides the high probability of electron transfer from the excited Ru to BQ. Steady-state and time-resolved results confirm such an excited electron transfer scenario. On the other hand, strong spectral overlap between the emission of Ru and absorption of TAPP suggests that energy transfer is a possible deactivation pathway of the Ru excited state.

Photon Upconversion by Triplet–Triplet Annihilation in Ru(bpy)3- and DPA-Functionalized Polymers

Two photophysically active polymers, each containing one absorbing Ru(bpy)3 triplet sensitizing core and multiple 9,10-diphenylanthracene (DPA) triplet energy acceptors, are shown to exhibit noncoherent photon upconversion by triplet–triplet annihilation (TTA) in both fluid solution and in thin solid films. An analysis of the kinetics and absorbed power dependencies of the upconverted fluorescence demonstrates that, in fluid solution, TTA is occurring in the low-power, pseudo-first order limit, and that the energy transfer mechanism involves fast, efficient intramolecular triplet energy transfer from the excited Ru(bpy)3 core to the DPA acceptors in the macromolecule’s pendant arms, followed by intermolecular TTA. Intermolecular triplet energy transfer, diffusion coefficient, and quantum yield measurements using Ru(bpy)3 and DPA controls suggest that, although a minority of the intermolecular encounters result in TTA in this one absorbing core system, efficient intramolecular TTA could be realized in a po...

Trifluoromethylation of Allylsilanes under Photoredox Catalysis

A new catalytic method to access allylic secondary CF3 products is described. These reactions use the visible light excited Ru(bpy)3Cl2·6H2O catalyst and the Togni or Umemoto reagent as the CF3 source. The photoredox catalytic manifold delivers enantioenriched allylic trifluoromethylated products not accessible under Cu(I) catalysis.

Syntheses and Excitation Transfer Studies of Near-Orthogonal Free-Base Porphyrin–Ruthenium Phthalocyanine Dyads and Pentad

A new series of molecular dyads and pentad featuring free-base porphyrin and ruthenium phthalocyanine have been synthesized and characterized. The synthetic strategy involved reacting free-base porphyrin functionalized with one or four entities of phenylimidazole at the meso position of the porphyrin ring with ruthenium carbonyl phthalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor-acceptor polyads (dyad and pentad) is investigated in nonpolar toluene and polar benzonitrile solvents using both steady-state and time-resolved emission techniques. Electrochemical and computational studies suggested that the photoinduced electron transfer is a thermodynamically unfavorable process in nonpolar media but may take place in a polar environment. Selective excitation of the donor, free-base porphyrin entity, resulted in efficient excitation transfer to the acceptor, ruthenium phthalocyanine, and the position of imidazole linkage on the free-base porphyrin could be used to tune the rates of excitation transfer. The singlet excited Ru phthalocyanine thus formed instantly relaxed to the triplet state via intersystem crossing prior to returning to the ground state. Kinetics of energy transfer (k(ENT)) was monitored by performing transient absorption and emission measurements using pump-probe and up-conversion techniques in toluene, respectively, and modeled using a Förster-type energy transfer mechanism. Such studies revealed the experimental k(ENT) values on the order of 10(10)-10(11) s(-1), which readily agreed with the theoretically estimated values. Interestingly, in polar benzonitrile solvent, additional charge transfer interactions in the case of dyads but not in the case of pentad, presumably due to the geometry/orientation consideration, were observed.

A novel ruthenium(II)-cobaloxime supramolecular complex for photocatalytic H2 evolution: synthesis, characterisation and mechanistic studies.

We report the synthesis and characterization of novel mixed-metal binuclear ruthenium(II)-cobalt(II) photocatalysts for hydrogen evolution in acidic acetonitrile. First, 2-(2'-pyridyl)benzothiazole (pbt), 1, was reacted with RuCl(3)·xH(2)O to produce [Ru(pbt)(2)Cl(2)]·0.25CH(3)COCH(3), 2, which was then reacted with 1,10-phenanthroline-5,6-dione (phendione), 3, in order to produce [Ru(pbt)(2)(phendione)](PF(6))(2)·4H(2)O, 4. Compound 4 was then reacted with 4-pyridinecarboxaldehyde in order to produce [Ru(pbt)(2)(L-pyr)](PF(6))(2)·9.5H(2)O, 5 (where L-pyr = (4-pyridine)oxazolo[4,5-f]phenanthroline). Compound 5 was then reacted with [Co(dmgBF(2))(2)(H(2)O)(2)] (where dmgBF(2) = difluoroboryldimethylglyoximato) in order to produce the mixed-metal binuclear complex, [Ru(pbt)(2)(L-pyr)Co(dmgBF(2))(2)(H(2)O)](PF(6))(2)·11H(2)O·1.5CH(3)COCH(3), 6. [Ru(Me(2)bpy)(2)(L-pyr)Co(dmgBF(2))(2)(OH(2))](PF(6))(2), 7 (where Me(2)bpy = 1,10-phenanthroline, 4,4'-dimethyl-2,2'-bipyridine) and [Ru(phen)(2)(L-pyr)Co(dmgBF(2))(2)(OH(2))](PF(6))(2), 8 were also synthesised. All complexes were characterized by elemental analysis, ESI MS, HRMS, UV-visible absorption, (11)B, (19)F, and (59)Co NMR, ESR spectroscopy, and cyclic voltammetry, where appropriate. Photocatalytic studies carried out in acidified acetonitrile demonstrated constant hydrogen generation longer than a 42 hour period as detected by gas chromatography. Time resolved spectroscopic measurements were performed on compound 6, which proved an intramolecular electron transfer from an excited Ru(II) metal centre to the Co(II) metal centre via the bridging L-pyr ligand. This resulted in the formation of a cobalt(I)-containing species that is essential for the production of H(2) gas in the presence of H(+) ions. A proposed mechanism for the generation of hydrogen is presented.

Development of highly efficient supramolecular CO2reduction photocatalysts with high turnover frequency and durability

New Ru(II)-Re(I) supramolecular photocatalysts with a rhenium(I) biscarbonyl complex as a catalyst unit were synthesized. They photocatalyzed CO2 reduction to CO using a wide-range of visible light, and their photocatalytic abilities were strongly affected by the phosphorus ligands on the Re site. Especially, Ru-Re(FPh), with two P(p-FPh)3 ligands, exhibited tremendous photocatalytic properties, i.e. TN(CO) = 207 and phi(CO) = 0.15, and, in addition, this is one of the fastest-operating photocatalysts for CO2 reduction to CO, with TF(CO) = 281 h(-1). We also clarified a balance of transferred electrons in this photocatalytic reaction and found that the two electrons necessary for CO formation were provided by two sequential reductive quenching processes of the excited Ru photosensitizer unit by the reductant BNAH.