Influence of polystyrenesulfonate on electron transfer quenching of ruthenium trisbipyridine luminescence by viologens: non-covalent assembly and covalent tethering of the ruthenium complex

A new copolymer (RuB-PSS) of ruthenium(II)bis-(2,2'-bipyridine)(4-vinyl 2,2'-bipyridine) and styrene sulfonate was prepared which tethers the ruthenium chromophore directly to the polymer backbone. The photophysical properties of the copolymer, and its luminescence quenching by viologens, were compared with those of ruthenium(II)tris-bipyridine, [Ru(bpy)(3)](2+), bound non-covalently to polystyrenesulfonate (PSS) via hydrophobic and electrostatic interactions. Enhancement of ruthenium polypyridyl complex luminescence in both systems is due to decreased rates of non-radiative decay when removed from bulk water as well as reduced oxygen quenching. Molecular dynamics simulations show an open PSS chain conformation with induction of local curvature around the ruthenium centres. Hence, the complexes remain exposed to water, albeit less so than in bulk solution, as evidenced by low enhancement of bound [Ru(phen)(2)dppz](2+) emission. Quenching by O(2) is hindered for both systems due to combined polarity, ionic strength, and viscosimetric effects that influence local concentrations and diffusion of reactants. Electron transfer quenching of the Ru centre by zwitterionic propyl viologen sulfonate (PVS(0)) and cationic methyl viologen (MV(2+)) is enhanced for [Ru(bpy)(3)](2+)/PSS, but retarded for RuB-PSS, despite the attraction of the quenchers for PSS. PSS binding hinders separation of the electron transfer products relative to aqueous solution, excepting an increase for RuB-PSS/PVS(0). We conclude that anionic hydrophobic polymers such as PSS can differentially influence forward- and reverse- electron transfer reactions depending on the charge and hydrophobicity of the reactants. In the context of small molecule binding, we find that PSS provides a tenable model for DNA.     

Photoinduced electron transfer catalysis of titania particles modified with a Ru(2,2′-bipyridyl)3 2+-grafted polymer by visible light

 The preparation and photocatalysis of colloidal titania modified with Ru(2,2′-bipyridyl)₃ ²⁺-grafted polyacrylate were investigated. Visible light irradiation of the Ru(2,2′-bipyridyl)₃ ²⁺-tethered titania in pH 7.0 buffer solution gave electron transfer to methyl viologen to form the cation radical via electron migration from the Ru(II) complex to the titania surface. Received: 16 September 1999 Accepted: 24 November 1999     

Photodriven electron and energy transfer from a light-harvesting metallodendrimer

Intermolecular electron and energy transfer from a light-harvesting metallodendrimer [Ru[bpy(C-450)(4)](3)](2+), where bpy(C-450)(4) is a 2,2'-bipyridine derivative containing 4 coumarin-450 units connected together through aryl ether linkages, is observed in acetonitrile solutions at room temperature. The model complex [Ru(dmb)(3)](2+), where dmb is 4,4'-dimethyl-2,2'-bipyridine, is included for quantitative comparison. The excited states of both compounds are metal-to-ligand charge transfer in nature and participate in excited-state electron and triplet energy transfer processes. Quenching constants were determined from luminescence and time-resolved absorption experiments at constant ionic strength. [Ru[bpy(C-450)(4)](3)](2+) displays significantly slower quenching rates to molecular oxygen and methyl viologen relative to the other processes investigated. Triplet energy transfer from [Ru[bpy(C-450)(4)](3)](2+) to 9-methylanthracene is quantitatively indistinguishable from [Ru(dmb)(3)](2+) while reductive electron transfer from phenothiazine was slightly faster in the former. With the exception of dioxygen quenching, our results indicate that the current dendritic structure is ineffective in shielding the core from bimolecular electron and triplet energy transfer reactions. Electrochemical measurements of [Ru[bpy(C-450)(4)](3)](2+) reveal irreversible oxidative processes at potentials slightly negative to the Ru(III/II) potential that are assigned to oxidations in the dendritic structure. Excited-state oxidative electron-transfer reactions facilitate this process resulting in the reduction of ground-state Ru(III) to Ru(II) and the trapping of the methyl viologen radical cation (MV(*+)) when methyl viologen serves as the quencher. This process generates a minimum of 9 MV(*+)'s for every [Ru[bpy(C-450)(4)](3)](2+) molecule and disassembles the metallodendrimer, resulting in the production of a [Ru(dmb)(3)](2+)-like species and "free" C-450-like dyes.     

p-Carborane: a new cage spacer for photoactive metal polypyridine dyads

The first example of a binuclear ruthenium complex involving the p-carborane framework in the bridging ligand is reported. The bridging ligand is a symmetric linear array comprising a central p-carborane unit, two p-phenylene spacers, and two 5-yl-2,2'-bipyridine coordinating units. A homobinuclear Ru(II) complex, with 2,2'-bipyridine as peripheral ligands, was synthesized and characterized. The Ru(II)-Ru(III) mixed-valence species, obtained by partial oxidation, has been investigated with steady-state and time-resolved techniques in CH3CN. The rate of photoinduced electron transfer is 2.3 x 10(8) s(-1).     

Visible-light-driven photoreactions of [(bpy)2Ru(II)L]Cl2 in aqueous solutions (bpy = bipyridine, l = 1,2-bis(4-(4'-methyl)-2,2'-bipyridyl) ethene)

Visible-light-induced photoreactions of [(bpy)2Ru(II)L]Cl2 (bpy = bipyridine, L = trans-1,2-bis(4-(4'-methyl)-2,2'-bipyridyl) ethene) in aqueous solution are examined. From pH titrations, it is found that the Ru complex is a stronger base (pKa* = 6) in the excited state than in the ground state (pKa = 4). Photolysis of the [(bpy)2Ru(II)L] complex in solutions at pH 7 and 12 led to formation of species with increased emission quantum yields, approximately 55 nm blue-shift of the emission maximum to 625 nm, and disappearance of the absorption band at 330 nm, the latter arising from the olefinic bond of the L ligand. No spectral changes are observed in solutions at pH < or = 4. With the help of chromatography, mass spectroscopy, Raman spectroscopy, and NMR, photoproducts formed at neutral pH have been analyzed. It is found that the major product is a dimer of [(bpy)2Ru(II)L], dimerizing around the double bond. Photoreactions do not occur in the dark or in the aprotic solvent acetonitrile. We propose that a Ru(III) radical intermediate is formed by photoinduced excited-state electron and proton transfer, which initiates the dimerization. The radical intermediate can also undergo photochemical degradative reductions. Below pH 4, the emission quenching is proposed to arise via protonation of the monoprotonated [(bpy)2Ru(II)LH] followed by electron transfer to the viologen-type moiety created by protonation. The products of photodegradation at pH > 12 are different from those of pH 7, but the mechanism of the degradation at pH > 12 was not elucidated.     

Long-lived charge-separation by retarding reverse flow of charge-balancing cation and zeolite-encapsulated Ru(bpy)(3)(2+) as photosensitized electron pump from zeolite framework to externally placed viologen

K(+)-exchanged, Ru(bpy)(3)(2+)-encapsulating zeolite-Y [K(+)-Ru(bpy)(3)(2+)Y] and N-[3-(dicyclohexylmethyl)oxypropyl-N'-methyl-4,4'-bipyridinium [DCH-MV(2+)] were prepared, and visible light-induced electron transfer from the zeolite-encapsulated Ru(II) complex to the size-excluded viologen was studied in acetonitrile. Addition of a series of crown ethers (CEs) into the heterogeneous solution leads to over a 10-fold increase in the yield of DCH-MV(*)(+), where the yield linearly increases as the formation constant of CE with K(+) [K(f)(K(+))(CE)] increases. The following two sequential events are attributed to be responsible for the above novel phenomenon. First, K(+) ions are liberated from the zeolite to solution during interfacial electron transfer from the photoexcited Ru(II) complexes to DCH-MV(2+). Second, the liberated K(+) ions form strong host-guest complexes with the added CE molecules, which leads to retardation of the reverse flow of the cations, hence the charge-balancing electrons, from the solution to the zeolite. Surprisingly, the yield of DCH-MV(*)(+) reaches more than approximately 50 times the amount of Ru(bpy)(3)(2+) situated in the outermost supercages, despite the absence of electron relay in the zeolite. This is attributed to photosensitized electron pumping from the zeolite framework to viologen by the outermost Ru(bpy)(3)(2+) ions. In support of the above conclusion, Ru(bpy)(3)(3+) does not accumulate in the zeolite host while DCH-MV(*)(+) accumulates in the supernatant solution. Consistent with the above, the independently prepared hexafluorophosphate salt of Ru(bpy)(3)(3+) is reduced to Ru(bpy)(3)(2+) in acetonitrile upon contact with Ru(bpy)(3)(2+)-free M(+)Y (M(+) = Li(+), Na(+), K(+), Rb(+), and Cs(+)), where the yield increases as the donor strength of the framework oxygen increases. Although small, thermal electron transfer also takes place from the zeolite framework to DCH-MV(2+), where the yield increases upon increasing the donor strength of the framework, concentration of DCH-MV(2+), temperature, and K(f)(K(+))(CE) (when K(+)Y is the zeolite host). The photoyield is always higher than the thermal yield by 4-30 times, confirming that the zeolite-encapsulated Ru(bpy)(3)(2+) serves as the photosensitized electron pump.     

Photoinduced energy- and electron-transfer processes in dinuclear Ru(II)-Os(II), Ru(II)-Os(III), and Ru(III)-Os(II) trisbipyridine complexes containing a shape-persistent macrocyclic spacer.

The PF6- salt of the dinuclear [(bpy)2Ru(1)Os(bpy)2]4+ complex, where 1 is a phenylacetylene macrocycle which incorporates two 2,2'-bipyridine (bpy) chelating units in opposite sites of its shape-persistent structure, was prepared. In acetonitrile solution, the Ru- and Os-based units display their characteristic absorption spectra and electrochemical properties as in the parent homodinuclear compounds. The luminescence spectrum, however, shows that the emission band of the Ru(II) unit is almost completely quenched with concomitant sensitization of the emission of the Os(II) unit. Electronic energy transfer from the Ru(II) to the Os(II) unit takes place by two distinct processes (k(en) = 2.0x10(8) and 2.2x10(7) s(-1) at 298 K). Oxidation of the Os(II) unit of [(bpy)2Ru(1)Os(bpy)2]4+ by Ce(IV) or nitric acid leads quantitatively to the [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ complex which exhibits a bpy-to-Os(III) charge-transfer band at 720 nm (epsilon(max) = 250 M(-1) cm(-1)). Light excitation of the Ru(II) unit of [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ is followed by electron transfer from the Ru(II) to the Os(III) unit (k(el,f) = 1.6x10(8) and 2.7x10(7) s(-1)), resulting in the transient formation of the [(bpy)2Ru(III)(1)Os(II)(bpy)2]5+ complex. The latter species relaxes to the [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ one by back electron transfer (k(el,b) = 9.1x10(7) and 1.2x10(7) s(-1)). The biexponential decays of the [(bpy)2*Ru(II)(1)Os(II)(bpy)2]4+, [(bpy)2*Ru(II)(1)Os(III)(bpy)2]5+, and [(bpy)2Ru(III)(1)Os(II)(bpy)2]5+ species are related to the presence of two conformers, as expected because of the steric hindrance between hydrogen atoms of the pyridine and phenyl rings. Comparison of the results obtained with those previously reported for other Ru-Os polypyridine complexes shows that the macrocyclic ligand 1 is a relatively poor conducting bridge.     

Photoinduced Electron and Energy Transfer in Rigidly Bridged Ru(II)-Rh(III) Binuclear Complexes

A series of binuclear Ru(II)-Rh(III) complexes of general formula (ttpy)Ru-tpy-(ph)(n)-tpy-Rh(ttpy)(5+) (n = 0-2) have been synthesized, where ttpy = 4'-p-tolyl-2,2':6,2"-terpyridine and tpy-(ph)(n)-tpy represents a bridging ligand where two 2,2':6',2"-terpyridine units are either directly linked together (n = 0) or connected through one (n = 1) or two (n = 2) phenyl spacers in the 4'-position. This series of complexes is characterized by (i) rigid bridge structures and (ii) variable metal-metal distances (11 ? for n = 0, 15.5 ? for n = 1, 20 ? for n = 2). The photophysics of these binuclear complexes has been investigated in 4:1 methanol/ethanol at 77 K (rigid glass) and 150 K (fluid solution) and compared with that of mononuclear [Ru(ttpy)(2)(2+) and Rh(ttpy)(2)(3+)] or binuclear [(ttpy)Ru-tpy-tpy-Ru(ttpy)(4+)] model compounds. At 77 K, no quenching of the Ru(II)-based excited state is observed, whereas energy transfer from excited Rh(III) to Ru(II) is observed for all complexes. At 150 K, energy transfer from excited Rh(III) to Ru(II) is again observed for all complexes, while quenching of excited Ru(II) by electron transfer to Rh(III) is observed, but only in the complex with n = 0. The reasons for the observed behavior can be qualitatively understood in terms of standard electron and energy transfer theory. The different behavior between n = 0 and n = 1, 2 can be rationalized in terms of better electronic factors and smaller reorganizational energies for the former species. The freezing of electron transfer quenching but not of energy transfer, in rigid glasses reflects the different reorganizational energies involved in the two processes. Unusual results arising from multiphotonic and conformational effects have also been observed with these systems.     

Light-driven tyrosine radical formation in a ruthenium-tyrosine complex attached to nanoparticle TiO2

We demonstrate a possibility of multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystalline TiO(2). The complex mimics the function of the tyrosine(Z)() and chlorophyll unit P(680) in natural photosystem II (PSII). A ruthenium(II) tris(bipyridyl) complex covalently linked to a L-tyrosine ethyl ester through an amide bond was attached to the surface of nanocrystalline TiO(2) via carboxylic acid groups linked to the bpy ligands. Synthesis and characterization of this complex are described. Excitation (450 nm) of the complex promotes an electron to a metal-to-ligand charge-transfer (MLCT) excited state, from which the electron is injected into TiO(2). The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 5 micros with a yield of 15%. This rather low yield is a result of a fast back electron transfer reaction from the nanocrystalline TiO(2) to the photogenerated Ru(III).     

Ultrafast processes in bimetallic dyads with extended aromatic bridges. Energy and electron transfer pathways in tetrapyridophenazine-bridged complexes

The energy and electron transfer processes taking place in binuclear polypyridine complexes of ruthenium and osmium based on the tetrapyrido[3,2-a:2',3'-c:3' ',2' '-h:2' "-3' "-j]phenazine bridging ligand (tpphz) have been investigated by ultrafast absorption spectroscopy. In the binuclear complexes, each chromophore is characterized by two spectrally distinguishable metal-to-ligand charge transfer (MLCT) excited states: MLCT1 (with promoted electron mainly localized on the bpy-like portion of tpphz, higher energy) and MLCT0 (with promoted electron mainly localized on the pyrazine-like portion of tpphz, lower energy). In the homodinuclear complexes Ru(II)-Ru(II) and Os(II)-Os(II), MLCT1 --> MLCT0 relaxation (intraligand electron transfer) is observed, with strongly solvent-dependent kinetics (ca. 10(-10) s in CH2Cl2, ca. 10(-12) s in CH3CN). In the heterodinuclear Ru(II)-Os(II) complex, *Ru(II)-Os(II) --> Ru(II)-Os(II) energy transfer takes place by two different sequences of time-resolved processes, depending on the solvent: (a) in CH2Cl2, ruthenium-to-osmium energy transfer at the MLCT1 level followed by MLCT1 --> MLCT0 relaxation in the osmium chromophore, (b) in CH3CN, MLCT1 --> MLCT0 relaxation in the ruthenium chromophore followed by osmium-to-ruthenium metal-to-metal electron transfer. In the mixed-valence Ru(II)-Os(III) species, the *Ru(II)-Os(III) --> Ru(III)-Os(II) electron transfer quenching is found to proceed by two consecutive steps in CH3CN: intraligand electron transfer followed by ligand-to-metal electron transfer. On a longer time scale, charge recombination leads back to the ground state. Altogether, the results show that the tpphz bridge plays an active mechanistic role in these systems, efficiently mediating the transfer processes with its electronic levels.     

Switchable antenna: a star-shaped ruthenium/osmium tetranuclear complex with azobis(bipyridine) bridging ligands

A star-shaped Ru/Os tetranuclear complex, in which a central Os unit is linked to three peripheral Ru units by 4,4'-azobis(2,2'-bipyridine) (azobpy) bridging ligands, was prepared to examine the unique photodynamics regulated by its redox state. The Ru/Os tetranuclear complex exhibits Ru-based luminescence at 77 K, whereas the three-electron reduction (one for each azobpy) of the Ru/Os complex results in luminescence from the Os unit. The photoexcited state of the Ru/Os complex rapidly decays into low energy metal-to-ligand charge-transfer states, in which the excited electron is localized in the azobpy ligand in the form of azobpy(.-). Upon the one-electron reduction of the azobpy ligands, the above-mentioned low-energy states become unavailable to the photoexcited complex. As a result, an energy transfer from the Ru-based excited state to the Os-based excited state becomes possible. Ultrafast transient absorption measurements revealed that the energy transfer process consists of two steps; intramolecular electron transfer from the terminal bipyridine ligand (bpy(.-)) to form azobpy(2-) followed by a metal-to-metal electron transfer. Thus, the Ru/Os tetranuclear complex collects light energy into the central Os unit depending on the redox state of the bridging ligands, qualifying as a switchable antenna.     

Ligand Substitution Reaction of Bis(ethylenediamine)glycinatocobalt(III) Complex with Ethylenediamine Catalyzed by the Photo-excited Tris(2,2′-bipyridine)ruthenium(II) Complex

Substitution reaction with ethylenediamine of coordinated glycinate ligand in bis(ethylenediamine)-glycinatocobalt(III) complex has been studied in the presence of photo-excited tris(2,2′-bipyridine)ruthenium(II) complex in alkaline aqueous solution (buffered around pH 12) containing 1.0M chloride ion at 25°C. VIS absorption and CD spectra were used for the racemate and the optically active isomers of the Co(III) complexes, respectively. The reaction was catalyzed by the excited Ru(II) complex to give tris(ethylenediamine)cobalt(III) complex. Mechanism of the ligand-substitution reaction and role of the excited Ru(II) complex were discussed.     

Characterization of a stable ruthenium complex with an oxyl radical

The ruthenium oxyl radical complex, [Ru(II)(trpy)(Bu(2)SQ)O(.-)] (trpy = 2,2':6',2"-terpyridine, Bu(2)SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone) was prepared for the first time by the double deprotonation of the aqua ligand of [Ru(III)(trpy)(Bu(2)SQ)(OH(2))](ClO(4))(2). [Ru(III)(trpy)(Bu(2)SQ)(OH(2))](ClO(4))(2) is reversibly converted to [Ru(III)(trpy)(Bu(2)SQ)(OH-)](+) upon dissociation of the aqua proton (pK(a) 5.5). Deprotonation of the hydroxo proton gave rise to intramolecular electron transfer from the resultant O(2-) to Ru-dioxolene. The resultant [Ru(II)(trpy)(Bu(2)SQ)O(.-)] showed antiferromagnetic behavior with a Ru(II)-semiquinone moiety and oxyl radical, the latter of which was characterized by a spin trapping technique. The most characteristic structural feature of [Ru(II)(trpy)(Bu(2)SQ)O(.-)] is a long Ru-O bond length (2.042(6) A) as the first terminal metal-O bond with a single bond length. To elucidate the substituent effect of a quinone ligand, [Ru(III)(trpy)(4ClSQ)(OH(2))](ClO(4))(2) (4ClSQ = 4-chloro-1,2-benzosemiquinone) was prepared and we compared the deprotonation behavior of the aqua ligand with that of [Ru(III)(trpy)(Bu(2)SQ)(OH(2))](ClO(4))(2). Deprotonation of the aqua ligand of [Ru(III)(trpy)(4ClSQ)(OH(2))](ClO(4))(2) induced intramolecular electron transfer from OH- to the [Ru(III)(4ClSQ)] moiety affording [Ru(II)(trpy)(4ClSQ)(OH.)]+, which then probably changed to [Ru(II)(trpy)(4ClSQ)O(.-)]. The antiferromagnetic interactions (J values) between Ru(II)-semiquinone and the oxyl radical for [Ru(II)(trpy)(Bu(2)SQ)O(.-)] and for [Ru(II)(trpy)(4ClSQ)O(.-)] were 2J = -0.67 cm(-1) and -1.97 cm(-1), respectively.     

Photophysical and electrochemical properties of platinum(II) complexes bearing a chromophore-acceptor dyad and their photocatalytic hydrogen evolution

A series of platinum(II) complexes bearing a chromophore-acceptor dyad obtained by reacting 4-(p-bromomethylphenyl)-6-phenyl-2,2'-bipyridine or 4'-(p-bromomethylphenyl)-2,2':6',2'-terpyridine with pyridine, 4-phenylpyridine, 4,4'-bipyridine, 1-methyl-4-(pyridin-4'-yl)pyridinium hexafluorophosphate respectively, were synthesized. Their photophysical properties, emission quenching studies by Pt nanoparticles and methyl viologen, electrochemical properties and photoinduced electron-transfer reactions in a photocatalytic hydrogen-generating system containing triethanolamine and colloidal Pt without an extra electron relay, were investigated. A comparison of the rates of hydrogen production for the two photocatalytic systems, one containing a metal-organic dyad and the other comprising a 1:1 mixture of the parental platinum(II) complexes and the corresponding electron relay, showed that intramolecular electron transfer improves the photocatalytic efficiency. Compared with cyclometalated platinum(II) complexes, the related platinum(II) terpyridyl complexes exhibited poor performance for photocatalytic hydrogen evolution. An investigation into the amount of hydrogen generated by three platinum(II) complexes containing cyclometalated ligands with methyl groups located on different phenyl rings revealed that the efficiency of hydrogen evolution was affected by a subtle change of functional group on ligand, and the hydrogen-generating efficiency in the presence or absence of methyl viologen is comparable, indicating electron transfer from the excited [Pt(C^N^N)] chromophore to colloidal Pt. (1)H NMR spectroscopy of the metal-organic dyads in an aqueous solution in the presence of excess triethanolamine revealed that the dyad with a viologen unit was unstable, and a chemical reaction in the compound occurred prior to irradiation by visible light under basic conditions.     

Synthesis, structure, and electronic properties of RuN6 dinuclear Ru-Hbpp complexes

A series of RuN(6) dinuclear Ru-Hbpp complexes (Hbpp is the dinucleating tetraaza ligand 3,5-bis(pyridyl)pyrazole) of general formula {[Ru(II)(R(2)-trpy)(MeCN)](2)(μ-R(1)-bpp)}(3+), 10(3+)-14(3+), (R(1) = H, Me, or NO(2). and R(2) = H, Me, MeO; see Scheme 1) has been prepared from their Cl(-) or AcO(-) bridged precursors. The complexes have been characterized by UV-vis, NMR, CV, and some by X-ray. Complexes 10(3+)-14(3+), Ru(2)(II,II), were oxidized by 1 equiv in solution, leading to the mixed valence Ru(2)(II,III) complexes 10(4+)-14(4+) containing one unpaired electron and were characterized by EPR and UV-vis-near-IR, which showed metal-centered spin and the presence of low-energy IVCT bands. The H(ab) parameter indicates a relatively strong electronic coupling between the two ruthenium centers (class II). Further two electron oxidation in solution of the 10(3+)-14(3+) led to the formation of EPR silent Ru(2)(III,III) complexes 10(5+)-14(5+), that were further characterized by UV-vis-NIR. TD-DFT calculations are employed to assign the nature of the UV-vis transitions for the complexes in the various oxidation states, which are of metal to ligand charge transfer (MLCT) type for Ru(2)(II,II) and ligand to metal charge transfer (LMCT) type for Ru(2)(III,II) and Ru(2)(III,III).     

Electro- and photochemical properties of a (mu-alkoxo)bis(mu-carboxylato)diruthenium complex having two tetraphenylporphinato zinc(II) moieties

The novel (mu-alkoxo)bis(mu-carboxylato)diruthenium complex K[Ru(2)(dhpta)(mu-O(2)C-p-ZnTPP)(2)] 3 was prepared by simple ligand substitution reaction. Strong antiferromagnetic interaction between two Ru(III) ions of 3 was observed with a coupling constant of -425 approximately -404 cm(-1). The cyclic voltammogram of 3 can be explained in terms of superposition of those of ZnTPP-p-CO(2)H and K[Ru(2)(dhpta)(mu-O(2)CPh)(2)] 2, indicating no significant electrochemical interaction. The large conproportionation constant estimated from the reduction potentials for Ru(III)Ru(III) and Ru(II)Ru(III) indicates great stability of the mixed-valence state. The mixed-valence species [Ru(II)Ru(III)(dhpta)(mu-O(2)C-p-ZnTPP)(2)](2-) 4 was prepared by controlled potential electrolysis. The electronic absorption spectrum of 4 was quite similar to that of [Ru(II)Ru(III)(dhpta)(mu-O(2)CCH(3))(2)](2-) which is a typical Class II complex. The fluorescence from the S(2) state of the ZnTPP unit of 3 was significantly (78%) quenched. The electron transfer from the ZnTPP unit to Ru(III) ions in 3 is a plausible mechanism, even though energy transfer could not be ruled out completely. The free energy change for electron transfer, Delta G(CS), was estimated to be ca.-1.1 eV, which is similar to typical values for the reorganization energy lambda in polar solvents. Hence, the electron transfer scheme is situated almost at the top of the Marcus parabola, enabling ultrafast electron transfer.     

Electron transfer through clay monolayer films fabricated by the Langmuir-Blodgett technique

Hybrid films composed of amphiphilic molecules and clay particles were constructed by the modified Langmuir-Blodgett (LB) method. Clays used were sodium montmorillonite (denoted as mont) and synthetic smectite containing Co(II) ions in the octahedral sites (denoted as Co). Two kinds of amphiphilic molecules were used-[Ru(dC(18)bpy)(phen)2](ClO4)2 (dC(18)bpy = 4,4'-dioctadecyl-2,2'-bipyridyl and phen = 1,10-phenanthroline) (denoted as Ru) and octadecylammonium choloride (ODAH+Cl- or denoted as ODAH). Three kinds of hybrid films (denoted as Ru-mont, Ru-Co, and ODAH-Co films) were prepared by spreading an amphiphilic molecule onto an aqueous suspension of a clay. Atomic force microscopy (AFM) analyses of the films deposited on silicon wafers indicated that closely packed films were obtained at 20 ppm for all the above three cases. Cyclic voltammetry (CV) was measured on an ITO electrode modified with a hybrid film or a monolayer film of pure Ru(II) complex salt (denoted as Ru film). The Ru(II) complexes incorporated in the Ru-mont film lost their redox activity, indicating that montmorillonite layers acted as a barrier against electron transfer. In contrast, the same complexes in the Ru-Co film were electrochemically active with the simultaneous appearance of the redox peaks due to the Co(II)/Co(III) (or Co(II)/Co(IV)) couple. The results implied that electron transfer through cobalt clay layers was possible via mediation by Co(II) ions in a clay sheet. For an aqueous solution containing nitrite ions (NO2-) at pH 3.0, a large catalytic oxidation current was observed for both the electrodes modified with the Ru-mont and Ru-Co films. The results were interpreted in terms of the mechanisms that the charge separation of an incorporated Ru(II) complex took place to produce a pair of a Ru(III) complex and an electron and that the generated Ru(III) complex was reduced by a nitrite ion before it recombined with the electron.     

Zeolite membrane-based artificial photosynthetic assembly for long-lived charge separation

Photochemically generated long-lived charge separation is the key step in processes that aim for conversion of solar energy into chemical energy. In this study, we focus on a Ru polypyridyl complex [(bpy)(2)Ru(II)L, bpy = bipyridine, L = 1,2-bis[4-(4(')-2,2(')-bipyridyl) ethene] encapsulated on the surface of a pinhole-free zeolite membrane by quaternization of L and surrounded with intrazeolitic bipyridinium ions (N,N'-trimethyl-2,2'-bipyridinium ion, 3DQ(2+)). Visible-light irradiation of the Ru complex side of the membrane in the presence of a sacrificial electron donor led to formation of PVS(-*) on the other side. Pore-blocking disilazane-based chemistry allows for Na(+) to migrate through the membrane to maintain charge balance, while keeping the 3DQ(2+) entrapped in the zeolite. These results provide encouragement that the zeolite membrane based architecture has the necessary features for not only incorporating molecular assemblies with long-lived charge separation but also for ready exploitation of the spatially separated charges to store visible light energy in chemical species.     

Appraisal of the redox behaviour of the antimetastatic ruthenium(III) complex [ImH][RuCl(4)(DMSO)(Im)], NAMI-A

The imidazolium trans-tetrachloro(dimethylsulfoxide)imidazoleruthenate(III) complex [ImH][Ru(III)Cl(4)(DMSO)(Im)], NAMI-A, has shown an interesting antimetastatic activity. Since Ru(III) complexes are coordinatively more inert than the corresponding Ru(II) derivatives, an "activation by reduction" mechanism has been proposed to explain the biological activity of NAMI-A, thus acting as a pro-drug. We report here an electrochemical study on NAMI-A in aqueous solutions which emphasizes the structural and chemical consequences accompanying the easy Ru(III)/Ru(II) electron transfer (e.g., axial imidazole/water exchange in acidic solution in the short timescale of cyclic voltammetry followed by equatorial chloride/water exchange in the longer timescale of macroelectrolysis).     

Synthesis and study of Ru,Rh,Ru triads: modulation of orbital energies in a supramolecular architecture

Supramolecular trimetallic complexes [((tpy)RuCl(BL))(2)RhCl(2)](3+) where tpy = 2,2':6',2' '-terpyridine and BL = dpp or bpm [dpp = 2,3-bis(2-pyridyl)pyrazine and bpm = 2,2'-bipyrimidine] have been synthesized and characterized. The mixed-metal complexes couple a reactive rhodium(III) center to two ruthenium(II) light absorbers to form a light absorber-electron collector-light absorber triad. The variation of the bridging (dpp and bpm) and terminal (tpy in lieu of bpy) ligands has some profound effects on the properties of these complexes, and they are remarkably different from the previously reported [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+) system. The electrochemical data for both title trimetallics consist of overlapping Ru(III/II) couples for both terminal metals at 1.12 V versus the Ag/AgCl reference electrode. Cathodically an irreversible Rh(III/I) reduction followed by bridging ligand reductions is seen. This is indicative of highest occupied molecular orbitals (HOMO) localized on the terminal ruthenium metal centers and a lowest unoccupied molecular orbital (LUMO) residing on the rhodium. This rhodium-based LUMO is in contrast to the bpy analogue [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+), which has a bpm(pi) localized LUMO. This orbital inversion by terminal ligand variation illustrates the similar energy of these Rh(dsigma) and bpm(pi) orbitals within this structural motif. Both title trimetallics possess broad, low-energy Ru --> BL charge transfer absorbances at 540 nm (dpp) and 656 nm (bpm). A comparison of the spectroscopic, electrochemical, and spectroelectrochemical properties of these trimetallic complexes is presented.