Ruthenium Moiety Research Articles

Controlling the Activation of White Phosphorus: Formation of Phosphorous Acid and Ruthenium‐Coordinated 1‐Hydroxytriphosphane by Hydrolysis of Doubly Metalated P4

Successfully tamed: When coordinated to two {CpRu(PPh3)2} moieties, white phosphorus (P4) reacts selectively with a large excess of water to yield phosphorous acid (H3PO3) and the previously unknown 1-hydroxytriphosphane (PH(OH)PHPH2), which is stabilized as a bridging ligand by coordination to the ruthenium moieties (see scheme).

Synthesis and Photophysical Properties of Ruthenium-Based Dendrimers and Their Use in Dye Sensitized Solar Cells

First- and second-generation dendrimers (Ru3 and Ru6) have been synthesized, and their photophysical properties were investigated in solution and when adsorbed on the nanocrystalline TiO2 surface. The performance of Ru3 and Ru6 as charge transfer photosensitizers in nanocrytalline TiO2 based solar cells was also investigated. The best photovoltaic performance was obtained by the Ru3 based solar cell yielding a short circuit current of J sc = 5.52 mA.cm (-2) and an open circuit voltage of V oc = 626 mV, corresponding to an overall conversion efficiency of eta = 1.80% that is approximately double the conversion efficiency of the reference compound Ru1 (eta = 0.91%) and of the second generation dendrimer Ru6 (eta = 0.95%). The particular efficiency of the first generation dendrimer, Ru3, is attributed to the better light-harvesting properties of the doped nanocrystalline TiO2 film when compared to Ru1, whereas the poor performance of the second generation dendrimer, Ru6, is attributed to the uneven adsorption of all of the ruthenium moieties to the nanocrystalline TiO2 surface at the same time.

The synthesis of two novel hybrids containing a zinc(II) porphyrin unit and a polypyridyl ruthenium(II) complex unit and their photoinduced intramolecular electron and energy transfer

Two new hybrids, namely [C 4-ZnTPP-(ip)Ru(phen) 2](ClO 4) 2 and [C 4-ZnTPP-(ip)Ru(pip) 2](ClO 4) 2, containing a zinc(II) porphyrin unit and a polypyridyl ruthenium(II) complex unit were synthesized and characterized. Their photoinduced intramolecular electron and energy transfer properties were investigated using electronic absorption spectra, fluorescence spectra and decayed luminescence spectra. In both hybrid compounds, an electronic interaction and energy transfer between the zinc(II) porphyrin moiety and the polypyridyl ruthenium(II) moiety were observed. The efficiency of the energy transfer from 3MLCT of the polypyridyl ruthenium(II) moiety to the zinc(II) porphyrin moiety and the rate of electron transfer from the singlet excited state of the porphyrin moiety to the polypyridyl ruthenium(II) may be related to the axial coordination of the N atom from pip in polypyridyl ruthenium(II) moiety to the zinc(II) ion.

Dinuclear Nickel–Ruthenium Complexes as Functional Bio‐Inspired Models of [NiFe] Hydrogenases

AbstractThree new dinuclear nickel ruthenium complexes, namely [Ni(xbsms)Ru(p‐cymene)Cl]+, [Ni(emi)Ru(CO)2Cl2]2– and [Ni(emi)Ru(p‐cymene)Cl]– [H2xbsms = 1,2‐bis(4‐mercapto‐3,3‐dimethyl‐2‐thiabutyl)benzene, H2emi = N,N′‐ethylenebis(2‐mercaptoisobutyramide], have been synthesized and fully characterized as models of the active site of[NiFe] hydrogenases. The three‐dimensional structure of [Ni(xbsms)Ru(p‐cymene)Cl]+ reveals a nickel center in a square‐planar dithioether‐dithiolate environment that is connected to a ruthenium moiety via a Ni(μ‐SR)2Ru bridge. These complexes catalyze hydrogen evolution by electroreduction of the weakly acidic Et3NH+ ions in dimethylformamide and are therefore among the first bio‐inspired functional models of [NiFe] hydrogenases. The catalytic activity of these new complexes is explained by the existence of a cooperative effect between the metal centers and is further related to their electronic properties. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

A novel porphyrin–polypyridyl ruthenium(II) hybrid. Synthesis, characterization and photoinduced DNA cleavage activity

A novel porphyrin–polypyridyl ruthenium(II) hybrid compound, [C4-TPP-(ip)Ru(phen)2](ClO4)2 (TPP = 5,10, 15,20-tetraphenylporphyrin, ip = imidazo[4,5-f][1,10]phenanthroline and phen = 1,10-Phenanthroline), in which a polypyridyl ruthenium(II) moiety is linked to a porphyrin moiety by a butyl chain, has been synthesized by the reaction of (C4ip)TPP with cis-Ru(phen)2Cl2 in glacial acetic acid under an Ar atmosphere and characterized by elemental analysis, i.r., 1H-n.m.r., u.v.–vis., ESI-MS and electrochemistry. Its photoinduced DNA cleavage activity was investigated preliminarily. It has been found that the hybrid compound promoted photocleavage of plasmid pBR 322 DNA upon irradiation. Singlet oxygen (1O2) is suggested to be the reactive species responsible for the cleavage.

Photoluminescence and electrogenerated chemiluminescence of a bis(bipyridyl)ruthenium(II)–porphyrin complex

The photoluminescence (PL) and electrogenerated chemiluminescence (ECL) of [H 2(MPy3,4DMPP)Ru(bpy) 2Cl](PF 6), where H 2MPy3,4DMPP = meso-tris-3,4-dimethoxyphenyl-mono-(4-pyridyl)porphyrin and bpy = 2,2′-bipyridine, are reported in acetonitrile. The compound has a complex absorbance spectrum with bands characteristic of both the porphyrin and ruthenium moieties. PL emission maxim are observed at 655 nm when excited at the maximum absorption intensity corresponding to the porphyrin Soret π → π ∗ band, and around 600 nm when excited at wavelengths corresponding to Ru(dπ)–bpy (π ∗) MLCT transition. The photoluminescence efficiency ( ϕ em) of the 655 nm emission is 0.039 and that of the free porphyrin is 0.69 compared to Ru(bpy) 3 2 + at 0.042. [H 2(MPy3,4DMPP)Ru(bpy) 2Cl](PF 6) displays complex electrochemical behavior, with one electrochemically reversible Ru II–Ru III oxidation and two quasi-reversible waves at more cathodic potentials corresponding to the porphyrin moiety. Oxidative ECL was generated using the coreactant tri- n-propylamine (TPrA). ECL efficiencies ( ϕ ecl) were 0.14 for [H 2(MPy3,4DMPP)Ru(bpy) 2Cl] + and 0.099 for H 2MPy3,4DMPP using Ru(bpy) 3 2 + as the standard ( ϕ ecl = 1). ECL intensity was linear with respect to concentration from 1 to 0.001 μM. The ECL intensity peaks at potentials corresponding to oxidation both the ruthenium and porphyrin moieties as well as TPrA, indicating that multiple pathways for formation of the excited state are possible. However, an ECL spectrum shows a band similar in energy and shape to that of the Soret emission (655 nm for the PL and 656 nm for the ECL, respectively), indicating the same excited state is formed in each experiment.

Electronic and fluorescence spectral studies of a novel porphyrin-polypyridyl ruthenium(II) hybrid linked by a butyl chain

The electronic and fluorescence spectroscopic properties of a novel porphyrin-polypyridyl ruthenium(II) hybrid, [C 4-TPP-(ip)Ru(phen) 2](ClO 4) 2 (TPP = 5,10,15,20-tetraphenylporphyrin, ip = imidazo[4,5-f][1,10]phenanthroline and phen = 1,10-Phenanthroline), in which a polypyridyl ruthenium(II) moiety is linked to a porphyrin moiety by a butyl chain have been investigated and compared to its corresponding reference compounds. The studies of electronic absorption spectra have shown that there is an electronic interaction between the porphyrin moiety and the polypyridyl ruthenium(II) moiety in the hybrid. It can be found that intramolecular photoinduced electron and energy transfer processes may occur in the hybrid from the fluorescence spectra. When exciting in Soret band and Q band of porphyrin, the fluorescence quenching of the porphyrin moiety of the hybrid takes place due to electron transfer from the lowest singlet excited state (S 1) to the appended polypyridyl rutherium(II) moiety, while the decay of S 2 (the second-excited singlet state) of the porphyrin moiety is mainly contributed to internal conversion to S 1. When exciting in MLCT band of the polypyridyl ruthenium(II) moiety, fluorescence corresponding to the polypyridyl ruthenium(II) moiety is quenched by intramolecular energy transfer from 3MLCT of the ruthenium moiety to the lowest-energy triplet state localized on the porphyrin moiety.

Ni(xbsms)Ru(CO)2Cl2]: A Bioinspired Nickel−Ruthenium Functional Model of [NiFe] Hydrogenase

As a model of the active site of [NiFe] hydrogenases, a dinuclear nickel-ruthenium complex [Ni(xbsms)Ru(CO)2Cl2] was synthesized and fully characterized. The three-dimensional structure reveals a nickel center in a square-planar dithioether-dithiolate environment connected to a ruthenium moiety via a Ni(mu-SR)2Ru bridge. This complex catalyzes hydrogen evolution by electroreduction of the weakly acidic Et3NH+ ions in N,N-dimethylformamide and is therefore the first functional bioinspired model of [NiFe] hydrogenases.

Structure and Photochemical Properties of (μ-Alkoxo)bis(μ-carboxylato)diruthenium Complexes with Naphthylacetate Ligands

Two new dinuclear Ru(III) complexes containing naphthalene moieties, K[Ru2(dhpta)(mu-O2CCH2-1-naph)2] (1) and K[Ru2(dhpta)(mu-O2CCH2-2-naph)2] (2) (H5dhpta = 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid, naph-1-CH2CO2H = 1-naphthylacetic acid, naph-2-CH2CO2H = 2-naphthylacetic acid), were synthesized. Complex 2 crystallized as an orthorhombic system having a space group of Pbca with unit cell parameters a = 10.6200(5) A, b = 20.270(1) A, c = 35.530(2) A, and Z = 8. EXAFS analysis of 1 and 2 in the solid states and in solution clarified that the dinuclear structures of 1 and 2 were kept in DMSO solutions. Variable-temperature magnetic susceptibility data indicated that the two Ru(III) centers are strongly antiferromagnetically coupled as shown by the large coupling constants, J = -581 cm(-1) (1) and -378 cm(-1) (2). In the cyclic voltammograms of 1 and 2, one oxidation peak and two reduction peaks which were assigned to the redox reaction of the ruthenium moieties were observed in DMF. The large conproportionation constants estimated from the reduction potentials of Ru(III)Ru(III) and Ru(III)Ru(II) indicated the great stability of the mixed-valent state. The mixed-valent species [Ru(III)Ru(II)(dhpta)(mu-O2CCH2-R)2](2-) (R = 1-naph (6) and R = 2-naph (7)) were prepared by controlled potential electrolysis of 1 and 2 in DMF. The electronic absorption spectra of 6 and 7 were similar to that of [Ru(III)Ru(II) (dhpta)(mu-O2CCH3)2](2-) which is a typical Class II type mixed-valent complex. The fluorescence decay of 1 and 2 indicated that there are two quenching processes which come from the excimer and monomer states. The short excimer lifetimes of 1 and 2 were ascribed to the energy transfer from the naphthyl moieties to the Ru centers. The different excimer ratio between 1 and 2 suggested that the excimer formation is affected by the conformation of the naphthyl moieties in the diruthenium(III) complexes.

Tetranuclear polybipyridyl complexes of RuIIand MnII, their electro- and photo-induced transformation into di-µ-oxo MnIIIMnIVhexanuclear complexes

Three heterotetranuclear complexes, [{Ru(II)(bpy)(2)(L(n))}(3)Mn(II)](8+) (bpy = 2,2'-bipyridine, n = 2, 4, 6), in which a Mn(II)-tris-bipyridine-like centre is covalently linked to three Ru(II)-tris-bipyridine-like moieties using bridging bis-bipyridine L(n) ligands, have been synthesised and characterised. The electrochemical, photophysical and photochemical properties of these complexes have been investigated in CH(3)CN. The cyclic voltammograms of the three complexes exhibit two successive very close one-electron metal-centred oxidation processes in the positive potential region. The first, which is irreversible, corresponds to the Mn(II)/Mn(III) redox system (E(pa) approximately 0.82 V vs Ag/Ag(+) 0.01 M in CH(3)CN-0.1 M Bu(4)NClO(4)), whereas the second which is, reversible, is associated with the Ru(II)/Ru(III) redox couple (E(1/2) approximately 0.91 V). In the negative potential region, three successive reversible four electron systems are observed, corresponding to ligand-based reduction processes. The three stable dimeric oxidized forms of the complexes, [Mn(2)(III,IV)O(2){Ru(II)(bpy)(2)(L(n))}(4)](11+), [Mn(2)(IV,IV)O(2){Ru(II)(bpy)(2)(L(n))}(4)](12+) and [Mn(2)(IV,IV)O(2){Ru(III)(bpy)(2)(L(n))}(4)](16+) are obtained in fairly good yields by sequential electrolyses after consumption of respectively 1.5, 0.5 and 3 electrons per molecule of initial tetranuclear complexes. The formation of the di-micro-oxo binuclear complexes are the result of the instability of the {[Ru(II)(bpy)(2)(L(n))](3)Mn(III)}(9+) species, which react with residual water, via a disproportionation reaction and the release of one ligand, [Ru(II)(bpy)(2)(L(n))](2+). A quantitative yield can be obtained for these reactions if the electrochemical oxidations are performed in the presence of an added external base like 2,6-dimethylpyridine. Photophysical properties of these compounds have been investigated showing that the luminescence of the Ru(II)-tris-bipyridine-like moieties is little affected by the presence of manganese within the tetranuclear complexes. A slight quenching of the excited states of the ruthenium moieties, which occurs by an intramolecular process, has been observed. Measurements made at low concentration (<1 x 10(-5) M) indicate that some decoordination of Mn(2+) arises in 1a-c. These measurements allow the calculation of the association constants for these complexes. Finally, photoinduced oxidation of the tetranuclear complexes has been performed by continuous photolysis experiments in the presence of a large excess of a diazonium salt, acting as a sacrificial oxidant. The three successive oxidation processes, Mn(II)--> Mn(III)Mn(IV), Mn(III)Mn(IV)--> Mn(IV)Mn(IV) and Ru(II)--> Ru(III) are thus obtained, the addition of 2,6-dimethylpyridine in the medium giving an essentially quantitative yield for the two first photo-induced oxidation steps as found for electrochemical oxidation.

Synthesis and Characterization of Dinuclear Ruthenium Complexes Covalently Linked to RuII Tris‐bipyridine: An Approach to Mimics of the Donor Side of Photosystem II

To mimic the electron-donor side of photosystem II (PSII), three trinuclear ruthenium complexes (2, 2a, 2b) were synthesized. In these complexes, a mixed-valent dinuclear Ru2(II,III) moiety with one phenoxy and two acetato bridges is covalently linked to a Ru(II) tris-bipyridine photosensitizer. The properties and photoinduced electron/energy transfer of these complexes were studied. The results show that the Ru2(II,III) moieties in the complexes readily undergo reversible one-electron reduction and one-electron oxidation to give the Ru2(II,III) and Ru2(III,III) states, respectively. This could allow for photooxidation of the sensitizer part with an external acceptor and subsequent electron transfer from the dinuclear ruthenium moiety to regenerate the sensitizer. However, all trinuclear ruthenium complexes have a very short excited-state lifetime, in the range of a few nanoseconds to less than 100 ps. Studies by femtosecond time-resolved techniques suggest that a mixture of intramolecular energy and electron transfer between the dinuclear ruthenium moiety and the excited [Ru(bpy)3]2+ photosensitizer is responsible for the short lifetimes. This problem is overcome by anchoring the complexes with ester- or carboxyl-substituted bipyridine ligands (2a, 2b) to nanocrystalline TiO2, and the desired electron transfer from the excited state of the [Ru(bpy)3]2+ moiety to the conduction band of TiO2 followed by intramolecular electron transfer from the dinuclear Ru2(II,III) moiety to photogenerated Ru(III) was observed. The resulting long-lived Ru2(III,III) state decays on the millisecond timescale.

Photochemically Induced Metallization of Surface Silicon Using Dinuclear Metal Carbonyl Compounds. Anchoring of Ruthenium to a Si(111) Surface through Covalent Ru−Si Bond Formation

Near-UV irradiation of hydrogen-terminated Si(111) wafers with the ruthenium−ruthenium bonded dinuclear compounds Cp2Ru2(CO)4 (where Cp = η5-MeC5H4, η5−C5Me5) and [HB(pz)3]2Ru2(CO)4 [where HB(pz)3 = hydrotris(1-pyrazolyl)borate] in benzene solution at room temperature leads to the covalent attachment of CpRu(CO)2 and HB(pz)3Ru(CO)2 moieties to surface silicon sites on the wafer. ATR FTIR, XPS, and RBS data support the proposed Ru−Si bond formation as the dominant mode of wafer metallization. The surface area coverage of these ruthenized wafers is limited by the size the mononuclear ruthenium moiety that is attached to the Si anchor. A plausible reaction sequence involving the photochemically generated ruthenium-centered radicals CpRu(CO)2 and [HB(pz)3]Ru(CO)2 that both activate the hydrogen-terminated Si(111) surface and combine with the resulting silicon radicals is presented and discussed.

Right-Handed Helical Structure of Expanded Oligo(l-leucine) Containing [Ru(terpyridine)2]2+ Moieties

Expanded oligo(l-leucine)s, containing an alternate arrangement of a bis(terpyridine)ruthenium(II) moiety and a l-leucine residue, were synthesized and characterized by 1H NMR, UV, CD, and electrochemical properties. The intensity of CD spectra per ruthenium unit increased with the elongation of the peptide chain. 1H NMR analysis of a tetramer indicated the right-handed helical structure in acetonitrile.

Synthesis and characterization of ruthenium(ii) complexes with the new ligand 2-phenylazopyridine-5-sulfonic acid (Hsazpy): in search for new anticancer agents

Isomers of dichlorobis(2-phenylazopyridine)ruthenium(II) [Ru(azpy)2Cl2], especially the so-called α isomer, display remarkably high cytotoxicities against human tumor cell lines. Unfortunately, the parent [Ru(azpy)2Cl2] compounds are poorly water-soluble. In this paper the synthesis and characterization of the new water-soluble ligand 2-phenylazopyridine-5-sulfonic acid (Hsazpy) is described. Use of this ligand in reaction with RuCl3 gave two isomers, which were isolated as α- and β-[NEt4]2[Ru(sazpy)2Cl2]. The compounds have been fully characterized by (2D) NMR spectroscopy. The molecular structure of the α isomer of [NEt4]2[Ru(sazpy)2Cl2]·2H2O has been determined by single-crystal structure analysis. The packing in the crystal structure of α-[NEt4]2[Ru(sazpy)2Cl2]·2H2O shows an interesting hydrogen-bonding pattern in which two water molecules are involved. One water molecule bridges between a Cl ligand and a SO3− group within one ruthenium moiety, the other water molecule forms a bridge between two SO3− groups from two different ruthenium centers, resulting in a chain-like structure. Preliminary evaluation of the cytotoxicity by means of the IC50 value in A2780 cell line classifies α-[NEt4]2[Ru(sazpy)2Cl2] as non-toxic, but this does not rule out other anticancer activities.

Multimetallic ruthenium(II) complexes as electrochemiluminescent labels.

Two homometallic complexes containing two and three ruthenium polypyridyl units linked by amino acid lysine (Lys) and the related dipeptide (LysLys) were synthesized and their electrochemical, spectroscopic, and electrochemiluminescence (ECL) properties were investigated. The electrochemical and photophysical data indicate that the two metal complexes largely retain the electronic properties of the reference compound for the separate ruthenium moieties in the two bridged complexes, [4-carboxypropyl-4'-methyl-2,2'-bipyridine]bis(2,2'-bipyridine)ruthenium(II) complex. The ECL studies, performed in aqueous media in the presence of tri-n-propylamine as co-reactant, show that the ECL intensity increases by 30% for the dinuclear and trinuclear complexes compared to the reference. Heterogeneous ECL immunoassay studies, performed on larger dendritic complexes containing up to eight ruthenium units, demonstrate that limitations due to the slow diffusion can easily be overcome by means of nanoparticle technology. In this case, the ECL signal is proportional to the number of ruthenium units. Multimetallic systems with several ruthenium centers may, however, undergo nonspecific bonding to streptavidin-coated particles or to antibodies, thereby increasing the background ECL intensity and lowering the sensitivity of the immunoassay.

Synthesis, photophysical and nonlinear optical properties of macromolecular architectures featuring octupolar tris(bipyridine) ruthenium(II) moieties: evidence for a supramolecular self-ordering in a dentritic structure.

The synthesis, photophysical and nonlinear optical properties of several new multi-octupolar tris(bipyridine) ruthenium complexes are reported. The preparation on these complexes is based on the initial construction of multipodal 4,4'-dialkylaminostyryl-2,2'-bipyridine ligands (DAAS-bpy). Thermally stable polyimides featuring octupolar ruthenium trisbipyridyl complexes have been readily obtained by a polycondensation reaction. The controlled coordination strategy of dipodal and tripodal bipyridines to ruthenium(II) has also been successfully used to build bimetallic, trimetallic as well as the first metallodendrimer made of seven metallo-octupoles. These polymetallic species exhibit very intense absorption bands in the visible and long-lived luminescence. The quadratic NLO-susceptibilities beta of these macromolecules have been characterized by harmonic light scattering at 1.91 microm and compared with those of the corresponding monometallic species. The NLO studies clearly demonstrates a quasi-supramolecular ordering in the metallodendrimer.

Electron donor-acceptor dyads and triads based on tris(bipyridine)ruthenium(II) and benzoquinone: synthesis, characterization, and photoinduced electron transfer reactions.

Two electron donor-acceptor triads based on a benzoquinone acceptor linked to a light absorbing [Ru(bpy)(3)](2+) complex have been synthesized. In triad 6 (denoted Ru(II)-BQ-Co(III)), a [Co(bpy)(3)](3+) complex, a potential secondary acceptor, was linked to the quinone. In the other triad, 8 (denoted PTZ-Ru(II)-BQ), a phenothiazine donor was linked to the ruthenium moiety. The corresponding dyads Ru(II)-BQ (4) and PTZ-Ru(II) (9) were prepared for comparison. Upon light excitation in the visible band of the ruthenium moiety, electron transfer to the quinone occurred with a rate constant k(f) = 5 x 10(9) s(-)(1) (tau(f) = 200 ps) in all the quinone containing complexes. Recombination to the ground state followed, with a rate constant k(b) approximately 4.5 x 10(8) s(-)(1) (tau(b) approximately 2.2 ns), for both Ru(II)-BQ and Ru(II)-BQ-Co(III) with no indication of a charge shift to generate the reduced Co(II) moiety. In the PTZ-Ru(II)-BQ triad, however, the initial charge separation was followed by a rapid (k > 5 x 10(9) s(-)(1)) electron transfer from the phenothiazine moiety to give the fairly long-lived PTZ(*)(+)-Ru(II)-BQ(*)(-) state (tau = 80 ns) in unusually high yield for a [Ru(bpy)(3)](2+)-based triad (> 90%), that lies at DeltaG degrees = 1.32 eV relative to the ground state. Unfortunately, this triad turned out to be rather photolabile. Interestingly, coupling between the oxidized PTZ(*)(+) and the BQ(*)(-) moieties seemed to occur. This discouraged further extension to incorporate more redox active units. Finally, in the dyad PTZ-Ru(II) a reversible, near isoergonic electron transfer was observed on excitation. Thus, a quasiequilibrium was established with an observed time constant of 7 ns, with ca. 82% of the population in the PTZ-Ru(II) state and 18% in the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state. These states decayed in parallel with an observed lifetime of 90 ns. The initial electron transfer to form the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state was thus faster than what would have been inferred from the Ru(II) emission decay (tau = 90 ns). This result suggests that reports for related PTZ-Ru(II) and PTZ-Ru(II)-acceptor complexes in the literature might need to be reconsidered.

Photoinduced electron transfer between metal-coordinated cyclodextrin assemblies and viologens.

Two novel tris(bipyridine)ruthenium(II) complexes bearing two and six beta-cyclodextrin binding sites on their ligands have been synthesised and characterised. Complex 1, bearing two cyclodextrins, adopts a conformation in aqueous solution where parts of the aromatic ligands are self-included into the cyclodextrin moieties. This results in a loss of symmetry of the complex and gives rise to a much more complicated 1H NMR spectrum than expected. Photophysical studies indicate that the appended cyclodextrins protect the luminescent ruthenium core from quenching by oxygen, which results in longer excited state lifetimes and higher emission quantum yields compared with the reference compound, the unsubstituted ruthenium tris(bipyridine). Inclusion of suitable guests such as dialkyl-viologens leads to a quenching of the luminescence of the central unit. In these supramolecular donor-acceptor dyads an efficient photoinduced electron transfer from the excited ruthenium moiety (the donor) to the viologen unit (the acceptor) is observed. The alkyl chain length of the acceptor plays an important role on the binding properties; when it exceeds a certain limit the binding becomes strong enough for electron transfer to occur. Interestingly, a viologen with only one long alkyl tail instead of two shows no efficient quenching; this indicates that cooperative interactions between two cyclodextrins binding one viologen are essential to raise the binding constant of the supramolecular dyad.

Ruthenium-manganese complexes for artificial photosynthesis: factors controlling intramolecular electron transfer and excited-state quenching reactions.

Continuing our work toward a system mimicking the electron-transfer steps from manganese to P(680)(+) in photosystem II (PS II), we report a series of ruthenium(II)-manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (k(ET)) values span a large range, 1 x 10(5)-2 x 10(7) s(-1), and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally (lambda = 1.5-2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically lambda approximately 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)-ruthenium(III) electron-transfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.

Heteroditopic ligands containing a 2,2 ′-bipyridine and a 2,2 ′:6 ′,2″-Terpyridine moiety and their ruthenium tris(bipyridine)-like complexes

Two novel heteroditopic ligands in which the bidentate 2,2 ′-bipyridine (bpy) ligand is covalently linked to one or two 2,2 ′:6 ′,2″-terpyridine (terpy) terdentate ligands have been prepared and characterized. The synthesis and the physico-chemical features of their corresponding complexes in which the bpy site is complexed by a ruthenium moiety are also reported.