Surface confinement and its effects on the luminescence quenching of a ruthenium-containing metallopolymer

Luminescence quenching of the metallopolymers [Ru(bpy)(2)(PVP)(10)](2+) and [Ru(bpy)(2)(PVP)(10)Os(bpy)(2)](4+), both in solution and as thin films, is reported, where bpy is 2,2'-bipyridyl and PVP is poly(4-vinylpyridine). When the metallopolymer is dissolved in ethanol, quenching of the ruthenium excited state, Ru(2+*), within [Ru(bpy)(2)(PVP)(10)](2+) by [Os(bpy)(3)](2+) proceeds by a dynamic quenching mechanism and the rate constant is (1.1 +/- 0.1) x 10(11) M(-1) s(-1). This quenching rate is nearly two orders of magnitude larger than that found for quenching of monomeric [Ru(bpy)(3)](2+) under the same conditions. This observation is interpreted in terms of an energy transfer quenching mechanism in which the high local concentration of ruthenium luminophores leads to a single [Os(bpy)(3)](2+) centre quenching the emission of several ruthenium luminophores. Amplifications of this kind will lead to the development of more sensitive sensors based on emission quenching. Quenching by both [Os(bpy)(3)](2+) and molecular oxygen is significantly reduced within a thin film of the metallopolymer. Significantly, in both optically driven emission and electrogenerated chemiluminescence, emission is observed from both ruthenium and osmium centres within [Ru(bpy)(2)(PVP)(10)Os(bpy)(2)](4+) films, i.e. the ruthenium emission is not quenched by the coordinated [Os(bpy)(2)](2+) units. This observation opens up new possibilities in multi-analyte sensing since each luminophore can be used to detect separate analytes, e.g. guanine and oxoguanine.     

Photoredox vs. energy transfer in a Ru(II)-Fe(II) supramolecular complex built with an heteroditopic bipyridine-terpyridine ligand

A trinuclear [[Ru(II)(bpy)(2)(bpy-terpy)](2)Fe(II)](6+) complex (I) in which a Fe(II)-bis-terpyridine-like centre is covalently linked to two Ru(II)-tris-bipyridine-like moieties by a bridging bipyridine-terpyridine ligand has been synthesised and characterised. Its electrochemical, photophysical and photochemical properties have been investigated in CH(3)CN and compared with those of mononuclear model complexes. The cyclic voltammetry of (I) exhibits, in the positive region, two successive reversible oxidation processes, corresponding to the Fe(III)/Fe(II) and Ru(III)/Ru(II) redox couples. These systems are clearly separated (DeltaE(1/2) = 160 mV), demonstrating the lack of an electronic connection between the two subunits. The two oxidized forms of the complex, [[Ru(II)(bpy)(2)(bpy-terpy)](2)Fe(III)](7+) and [[Ru(III)(bpy)(2)(terpy-bpy)](2)Fe(III)](9+), obtained after two successive exhaustive electrolyses, are stable. (I) is poorly luminescent, indicating that the covalent linkage of the Ru(II)-tris-bipyridine to the Fe(II)-bis-terpyridine subunit leads to a strong quenching of the Ru(II)* excited state by energy transfer to the Fe(II) centre. Luminescence lifetime experiments show that the process occurs within 6 ns. The nature of the energy transfer process is discussed and an intramolecular energy exchange is proposed as a preferable deactivation pathway. Nevertheless this energy transfer can be efficiently quenched by an electron transfer process in the presence of a large excess of the 4-bromophenyl diazonium cation, playing the role of a sacrificial oxidant. Finally complete photoinduced oxidation of (I) has been performed by continuous photolysis experiments in the presence of a large excess of this sacrificial oxidant. The comparison with a mixture of the corresponding mononuclear model complexes has been made.     

Ultrafast fluorescence detection in tris(2,2'-bipyridine)ruthenium(II) complex in solution: relaxation dynamics involving higher excited states

The excited-state dynamics of a transition metal complex, tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)(3)](2+), has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck-Condon singlet excited state in this molecule was missing. In this study, emission from the Franck-Condon excited singlet state of [Ru(bpy)(3)](2+) has been observed for the first time, and its lifetime has been estimated to be 40 +/- 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited (1)(MLCT) states to the vibrationally hot triplet manifold occurs with an intersystem crossing time constant of 40 +/- 15 fs. Mixing of the higher levels in the triplet state with the singlet state due to strong spin-orbit coupling is proposed. This enhances the radiative rate constant, k(r), of the vibrationally hot states within the triplet manifold, facilitating the upconversion of the emitted photons. The vibrationally excited triplet, which is emissive, undergoes vibrational cooling with a decay time in the range of 0.56-1.3 ps and relaxes to the long-lived triplet state. The results on the relaxation dynamics of the higher excited states in [Ru(bpy)(3)](2+) are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.     

Syntheses, characterization, and photochemical properties of amidate-bridged Pt(bpy) dimers tethered to Ru(bpy)3(2+) derivatives

Amidate-bridged diplatinum(II) entities [Pt(2)(bpy)(2)(μ-amidato)(2)](2+) (amidate = pivalamidate and/or benzamidate; bpy = 2,2'-bipyridine) were covalently linked to one or two Ru(bpy)(3)(2+)-type derivatives. An amide group was introduced at the periphery of Ru(bpy)(3)(2+) derivatives to give metalloamide precursors [Ru(bpy)(2)(BnH)](2+) (abbreviated as RuBnH, n = 1 and 2), where deprotonation of amide BnH affords the corresponding amidate Bn, B1H = 4-(4-carbamoylphenyl)-2,2'-bipyridine, and B2H = ethyl 4'-[N-(4-carbamoylphenyl)carbamoyl]-2,2'-bipyridine-4-carboxylate. From a 1:1:1 reaction of [Pt(2)(bpy)(2)(μ-OH)(2)](NO(3))(2), RuBnH, and pivalamide, trinuclear complexes [Pt(2)(bpy)(2)(μ-RuBn)(μ-pivalamidato)](4+) (abbreviated as RuBn-Pt(2)) were isolated and characterized. Tetranuclear complexes [Pt(2)(bpy)(2)(μ-RuBn)(2)](6+) (abbreviated as (RuBn)(2)-Pt(2)) were separately prepared and characterized in detail. The quenching of the triplet excited state of the Ru(bpy)(3)(2+) derivative (i.e., Ru*(bpy)(3)(2+)) upon tethering the Pt(2)(bpy)(2)(μ-amidato)(2)(2+) moiety is strongly enhanced in RuB1-Pt(2) and (RuB1)(2)-Pt(2), while it is only slightly enhanced in RuB2-Pt(2) and (RuB2)(2)-Pt(2). These are partly explained by the driving forces for the electron transfer from the Ru*(bpy)(3)(2+) moiety to the Pt(2)(bpy)(2)(μ-amidato)(2)(2+) moiety (ΔG°(ET)); the ΔG°(ET) values for RuB1-Pt(2), (RuB1)(2)-Pt(2), RuB2-Pt(2), and (RuB2)(2)-Pt(2) are estimated as -0.01, 0.00, +0.22, and +0.28 eV, respectively. The considerable difference in the photochemical properties of the B1- and B2-bridged systems were further examined based on the emission decay and transient absorption measurements, which gave results consistent with the above conclusions.     

Photophysical and novel charge-transfer properties of adducts between [RuII(bpy)3]2+ and [S2Mo18O62]4-

The photophysical properties of acetonitrile solutions of [Ru(bpy)(3)](2+) and [S(2)Mo(18)O(62)](4-) are described. We discuss evidence for ion cluster formation in solution and the observation that despite the strong donor ability of the excited state of [Ru(bpy)(3)](2+) and its inherent photolability, adducts with [S(2)Mo(18)O(62)](4-) were photostable. Photophysical studies suggest that the quenching of the [Ru(bpy)(3)](2+) excited state by [S(2)Mo(18)O(62)](4-) occurs via a static mechanism and that binding is largely electrostatic in nature. Evidence is provided from difference spectroscopy and luminescence excitation spectroscopy for good electronic communication between [Ru(bpy)(3)](2+) and [S(2)Mo(18)O(62)](4-) with the presence of a novel, luminescent, inter-ion charge-transfer transition. The identity of the transition is confirmed by resonance Raman spectroscopy.     

Photophysical properties of ligand localized excited state in ruthenium(II) polypyridyl complexes: a combined effect of electron donor-acceptor ligand

We have synthesized ruthenium(II) polypyridyl complexes (1) Ru(II)(bpy)(2)(L(1)), (2) Ru(II)(bpy)(2)(L(2)) and (3) Ru(II)(bpy)(L(1))(L(2)), where bpy = 2,2'-bipyridyl, L(1) = 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L(2) = 4-(N,N-dimethylamino-phenyl)-(2,2'-bipyridine) and investigated the intra-ligand charge transfer (ILCT) and ligand-ligand charge transfer (LLCT) states by optical absorption and emission studies. Our studies show that the presence of electron donating -NMe(2) functionality in L(2) and electron withdrawing catechol fragment in L(1) ligands of complex 3 introduces low energy LLCT excited states to aboriginal MLCT states. The superimposed LLCT and MLCT state produces redshift and broadening in the optical absorption spectra of complex 3 in comparison to complexes 1 and 2. The emission quantum yield of complex 3 is observed to be extremely low in comparison to that of complex 1 and 2 at room temperature. This is attributed to quenching of the (3)MLCT state by the low-emissive (3)LLCT state. The emission due to ligand localized CT state (ILCT and LLCT) of complexes 2 and 3 is revealed at 77 K in the form of a new luminescence band which appeared in the 670-760 nm region. The LLCT excited state of complex 3 is populated either via direct photoexcitation in the LLCT absorption band (350-700 nm) or through internal conversion from the photoexcited (3)MLCT (400-600 nm) states. The internal conversion rate is determined by quenching of the (3)MLCT state in a time resolved emission study. The internal conversion to LLCT and ILCT excited states are observed to be as fast as ～200 ps and ～700 ps for complexes 3 and 2, respectively. The present study illustrates the photophysical property of the ligand localized excited state of newly synthesized heteroleptic ruthenium(II) polypyridyl complexes.     

Redox-switchable direction of photoinduced electron transfer in an Ru(bpy)3(2+)-viologen dyad

Quenching of the 3MLCT excited state of [Ru(bpy)3]2+ (bpy=bipyridine) by the reduction products (MV*+ and MV0) of methyl viologen (MV2+) was studied by a combination of electrochemistry with laser flash photolysis or femtosecond pump-probe spectroscopy. Both for the bimolecular reactions and for the reactions in an Ru(bpy)3(2+)-MVn+ dyad, quenching by MV*+ and MV0 is reductive and gives the reduced ruthenium complex [Ru(bpy)3]+, in contrast to the oxidative quenching by MV2+. Rate constants of quenching (kq), and thermal charge recombination (krec) and cage escape yields (phi(ce)) were determined for the bimolecular reactions, and rates of forward (kf) and backward (kb) electron transfer in the dyad were measured for quenching by MV2+, MV*+, and MV0. The reactions in the dyad are very rapid, with values up to kf = 1.3 x 10(12) s(-1) for *Ru(bpy)3(2+)-MV*+. In addition, a long-lived (tau = 15 ps) vibrationally excited state of MV*+ with a characteristically structured absorption spectrum was detected; this was generated by direct excitation of the MV*+ moiety both at 460 and 600 nm. The results show that the direction of photoinduced electron transfer in a Ru(bpy)3-MV molecule can be switched by an externally applied bias.     

Preparation and multicolor electrochromic performance of a WO3/tris(2,2'-bipyridine)ruthenium(II)/polymer hybrid film

A tungsten trioxide (WO(3))/tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)(3)](2+); bpy=2,2'-bipyridine)/poly(sodium 4-styrenesulfonate) (PSS) hybrid film was prepared by electrodeposition from a colloidal triad solution containing peroxotungstic acid (PTA), [Ru(bpy)(3)](2+), and PSS. A binary solution of [Ru(bpy)(3)](2+) and PTA (30 vol % ethanol in water) gradually gave an orange precipitate, possibly caused by the electrostatic interaction between the cationic [Ru(bpy)(3)](2+) and the anionic PTA. The addition of PSS to the binary PTA/[Ru(bpy)(3)](2+) solution remarkably suppressed this precipitation and caused a stable, colloidal triad solution to form. The spectrophotometric measurements and lifetime analyses of the photoluminescence from the excited [Ru(bpy)(3)](2+) ion in the colloidal triad solution suggested that the [Ru(bpy)(3)](2+) ion is partially shielded from electrostatic interaction with anionic PTA by the anionic PSS polymer chain. The formation of the colloidal triad made the ternary [Ru(bpy)(3)](2+)/PTA/PSS solution much more redox active. Consequently, the rate of electrodeposition of WO(3) from PTA increased appreciably by the formation of the colloidal triad, and fast electrodeposition is required for the unique preparation of this hybrid film. The absorption spectrum of the [Ru(bpy)(3)](2+) ion in the film was close to its spectrum in water, but the photoexcited state of the [Ru(bpy)(3)](2+) ion was found to be quenched completely by the presence of WO(3) in the hybrid film. The cyclic voltammogram (CV) of the hybrid film suggested that the [Ru(bpy)(3)](2+) ion performs as it is adsorbed onto WO(3) during the electrochemical oxidation. An ohmic contact between the [Ru(bpy)(3)](2+) ion and the WO(3) surface could allow the electrochemical reaction of adsorbed [Ru(bpy)(3)](2+). The composition of the hybrid film, analyzed by electron probe microanalysis (EPMA), suggested that the positive charge of the [Ru(bpy)(3)](2+) ion could be neutralized by partially reduced WO(3)(-) ions, in addition to Cl(-) and PSS units, based on the charge balance in the film. The electrostatic interaction between the WO(3)(-) ion and the [Ru(bpy)(3)](2+) ion might be responsible for forming the electron transfer channel that causes the complete quenching of the photoexcited [Ru(bpy)(3)](2+) ion, as well as the formation of the ohmic contact between the [Ru(bpy)(3)](2+) ion and WO(3). A multicolor electrochromic performance of the WO(3)/[Ru(bpy)(3)](2+)/PSS hybrid film was observed, in which transmittances at 459 and 800 nm could be changed, either individually or at once, by the selection of a potential switch. Fast responses, of within a few seconds, to these potential switches were exhibited by the electrochromic hybrid film.     

Tetranuclear polybipyridyl complexes of Ru(II) and Mn(II), their electro- and photo-induced transformation into di-mu-oxo Mn(III)Mn(IV) hexanuclear 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.     

Ultrafast photochemical dissociation of an equatorial CO ligand from trans(X,X)-[Ru(X)2(CO)2(bpy)] (X = Cl, Br, I): a picosecond time-resolved infrared spectroscopic and DFT computational study

Ultrafast photochemistry of the complexes trans(X,X)-[Ru(X)(2)(CO)(2)(bpy)] (X = Cl, Br, I) was studied in order to understand excited-state reactivity of equatorial CO ligands, coordinated trans to the 2,2'-bipyridine ligand (bpy). TD-DFT calculations have identified the lowest electronic transitions and singlet excited states as mixed X -->bpy/Ru --> bpy ligand to ligand/metal to ligand charge transfer (LLCT/MLCT). Picosecond time-resolved IR spectroscopy in the region of nu(CO) vibrations has revealed that, for X = Cl and Br, subpicosecond CO dissociation is accompanied by bending of the X-Ru-X moiety, producing a pentacoordinated intermediate trans(X,X)-[Ru(X)(2)(CO)(bpy)]. Final movement of an axial halide ligand to the vacant equatorial position and solvent (CH(3)CN) coordination follows with a time constant of 13-15 ps, forming the photoproduct cis(X,X)-[Ru(X)(2)(CO)(CH(3)CN)(bpy)]. For X = I, the optically populated (1)LLCT/MLCT excited state undergoes a simultaneous subpicosecond CO dissociation and relaxation to a triplet IRuI-localized excited state which involves population of an orbital that is sigma-antibonding with respect to the axial I-Ru-I bonds. Vibrationally relaxed photoproduct cis(I,I)-[Ru(I)(2)(CO)(CH(3)CN)(bpy)] is formed with a time constant of ca. 55 ps. The triplet excited state is unreactive, decaying to the ground state with a 155 ps lifetime. The experimentally observed photochemical intermediates and excited states were assigned by comparing calculated (DFT) and experimental IR spectra. The different behavior of the chloro and bromo complexes from that of the iodo complex is caused by different characters of the lowest triplet excited states.     

Photolysis of (Diamine)bis(2,2'-bipyridine)ruthenium(II) Complexes Using On-Line Electrospray Mass Spectrometry

Photolysis of Ru(bpy)(2)(en)(2+) and Ru(bpy)(2)(tn)(2+), where bpy = 2,2'-bipyridine, en = ethylenediamine, and tn = 1,3-propylenediamine, was studied in acetonitrile using on-line electrospray mass spectrometry (ES-MS). These complexes are known to undergo a four-electron oxidation photochemically, giving the alpha,alpha'-diimine complexes. The monoimine complexes involved in this stepwise process were detectable after photoirradiation (lambda >420 nm). Also, new ligand-oxidized complexes Ru(bpy)(2)(en+14)(2+) and Ru(bpy)(2)(tn+14)(2+) were observed together with photosubstitution products such as Ru(bpy)(2)(AN)(2)(2+) and Ru(bpy)(2)(AN)(2)X(+) (AN = acetonitrile). The notation (en+14) and (tn+14) represents loss of two hydrogen atoms and addition of an oxygen atom to the en and tn ligands. Photosubstitution intermediates with the monodentate diamine, Ru(bpy)(2)(tn)(AN)(2+) and Ru(bpy)(2)(tn)(AN)X(+), were detected in the ES mass spectrum of the tn complex but not in that of the en complex. Other photosubstituted intermediates with the monodentate (en+14) and (tn+14) ligands were detected by on-line mass analysis. The electrospray technique combined with use of a flow-through photoreaction cell was shown to be a useful tool for studying photolysis of inorganic metal complexes.     

A Tetranuclear Ruthenium(II) Complex Containing both Electron-Rich and Electron-Poor Bridging Ligands. Absorption Spectrum, Luminescence, Redox Behavior, and Intercomponent Energy Transfer

The first luminescent and redox active multinuclear Ru(II) compound containing both electron-poor (2,3-bis(2-pyridyl)pyrazine, 2,3-dpp) and electron-rich (3,5-bis(pyridyn-2-yl)-1,2,4-triazole, Hbpt) polypyridine bridging ligands has been synthesized. The novel compound is [(bpy)(2)Ru(&mgr;-bpt)Ru{(&mgr;-2,3-dpp)Ru(bpy)(2)}(2)](7+) (1; bpy = 2,2'-bipyridine). Its absorption spectrum, luminescence properties, and redox behavior have been studied and are compared with the properties of the parent complexes [Ru{(&mgr;-2,3-dpp)Ru(bpy)(2)}(3)](8+) (2) and [(bpy)(2)Ru(&mgr;-bpt)Ru(bpy)(2)](3+) (3). The absorption spectrum of 1 is dominated by ligand-centered bands in the UV region and by metal-to-ligand charge transfer bands in the visible region. Excited states and oxidation and reduction processes are localized in specific sites of the multicomponent structure. However, perturbations of each component on the redox and excited states of the others, as well as electronic interactions between the chromophores, can be observed. Intercomponent energy transfer from the upper-lying (&mgr;-bpt)(bpy)Ru-->bpy CT excited state of the Ru(bpy)(2)(&mgr;-bpt)(+) component to the lower-lying (bpy)(2)Ru-->&mgr;-2,3-dpp CT excited state of the Ru(bpy)(2)(&mgr;-2,3-dpp)(2+) subunit(s) is efficient in 1 in fluid solution at room temperature, whereas this process is not observed in a rigid matrix at 77 K. A two-step energy transfer mechanism is proposed to explain the photophysical properties of the new compound.     

Static and dynamic quenching of Ru(II) polypyridyl excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(bpy)(2)(deeb)(PF(6))(2), where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, in dichloromethane were found to be efficiently quenched by iodide at room temperature. The ionic strength dependence of the UV-visible absorption spectra gave evidence for ion pairing. Iodide was found to quench the excited states by static and dynamic mechanisms. Stern-Volmer and Benesi-Hildebrand analysis of the spectral data provided a self-consistent estimate of the iodide-Ru(bpy)(2)(deeb)(2+) adduct in dichloromethane, K = 59 700 M(-1). Transient absorption studies clearly demonstrated an electron-transfer quenching mechanism with transient formation of I(2)(*)(-) in high yield, phi = 0.25 for 355 or 532 nm excitation. For Ru(bpy)(2)(deeb)(PF(6))(2) in acetonitrile, similar behavior could be observed at higher iodide concentrations than that required in dichloromethane. The parent Ru(bpy)(3)(2+) compound also ion pairs with iodide in CH(2)Cl(2), and light excitation gave a higher I(2)(*)(-) yield, phi = 0.50. X-ray crystallographic, IR, and Raman data gave evidence for interactions between iodide and the coordinated deeb ligand in the solid state.     

Light-induced charge separation and photocatalytic hydrogen evolution from water using Ru(II)Pt(II)-based molecular devices: effects of introducing additional donor and/or acceptor sites

In our hopes to improve the photocatalytic efficiency of photo-hydrogen-evolving molecular devices, several new dyads and triads possessing a photosensitizing Ru(bpy)(phen)(2)(2+) (or Ru(phen)(3)(2+)) chromophore (abbreviated as Ru(II)) attached to both/either a phenothiazine moiety (abbreviated as Phz) and/or H(2)-evolving PtCl(2)(bpy) units (abbreviated as Pt), such as Phz-Ru(II)-Pt2 (triad), Ru(II)-Pt2 (dyad), and Ru(II)-Pt3 (dyad), were synthesized and their basic properties together with the photo-hydrogen-evolving characteristics were investigated in detail. The (3)MLCT phosphorescence from the Ru(II) moiety in these systems is substantially quenched due to the highly efficient photoinduced electron transfer (PET). Based on the electrochemical studies, the driving forces for the PET were estimated as -0.07 eV for Phz-Ru(II)-Pt2, -0.24 eV for Ru(II)-Pt2, and -0.22 eV for Ru(II)-Pt3, revealing the exergonic character of the PET in these systems. Luminescence lifetime studies revealed the existence of more than two decay components, indicative of a contribution of multiple PET processes arising from the presence of at least two different conformers in solution. The major luminescence decay components of the hybrid systems [τ(1) = 6.5 ns (Ru(II)-Pt2) and τ(1) = 1.04 ns (Phz-Ru(II)-Pt2) in acetonitrile] are much shorter than those of Phz-free/Pt-free Ru(bpy)(phen)(2)(2+) derivatives. An important finding is that the triad Phz-Ru(II)-Pt2 affords a quite long-lived charge separated (CS) state (τ(CS) = 43 ns), denoted as Phz(+)˙-Ru(Red)-Pt2, as a result of reductive quenching of the triplet excited state of Ru(bpy)(phen)(2)(2+) by the tethering Phz moiety, where Ru(Red) denotes Ru(bpy)(phen)(2)(+). Moreover, the lifetime of Phz(+)˙-Ru(Red)-Pt2 was observed to be much longer than that of Phz(+)˙-Ru(Red). The photocatalytic H(2) evolution from water driven by these systems was examined in an aqueous acetate buffer solution (pH 5.0) containing 4-19% dimethylsulfoxide (solubilising reagent) in the presence of EDTA as a sacrificial electron donor. Dyads Ru(II)-Pt2 and Ru(II)-Pt3 were found to exhibit improved photo-hydrogen-evolving activity compared to the heterodinuclear Ru-Pt dyads developed so far in our group. On the other hand, almost no catalytic activity was observed for Phz-Ru(II)-Pt2 in spite of the formation of a strongly reducing Ru(Red) site (Phz(+)˙-Ru(Red)-Pt2), indicating that the electron transfer from the photogenerated Ru(Red) unit to the PtCl(2)(bpy) unit is not favoured presumably due to the slow electron transfer rate in the Marcus inverted region.     

Fullerene polypyridine ligands: synthesis, ruthenium complexes, and electrochemical and photophysical properties

Fullerene coordination ligands bearing one bipyridine or terpyridine unit were synthesized, and their coordination to ruthenium(II) formed linear rod-like donor-acceptor systems. Steady-state fluorescence of [Ru(bpy)(2)(bpy-C(60))](2+) showed a rapid solvent-dependent, intramolecular quenching of the ruthenium(II) MLCT excited state. Time-resolved flash photolysis in CH(3)CN revealed characteristic transient absorption changes that have been ascribed to the formation of the C(60) triplet state, suggesting that photoexcitation of [Ru(bpy)(2)(bpy-C(60))](2+) results in a rapid intramolecular transduction of triplet excited state energy. The electrochemical studies on both [Ru(bpy)(2)(bpy-C(60))](2+) and [Ru(tpy)(tpy-C(60))](2+) indicated electronic coupling between the metal center and the fullerene core.     

Picosecond isomerization in photochromic ruthenium-dimethyl sulfoxide complexes

The complexes [Ru(tpy)(bpy)(dmso)](OSO(2)CF(3))(2) and trans-[Ru(tpy)(pic)(dmso)](PF(6)) (tpy is 2,2':6',2' '-terpyridine, bpy is 2,2'-bipyridine, pic is 2-pyridinecarboxylate, and dmso is dimethyl sulfoxide) were investigated by picosecond transient absorption spectroscopy in order to monitor excited-state intramolecular S-->O isomerization of the bound dmso ligand. For [Ru(tpy)(bpy)(dmso)](2+), global analysis of the spectra reveals changes that are fit by a biexponential decay with time constants of 2.4 +/- 0.2 and 36 +/- 0.2 ps. The first time constant is assigned to relaxation of the S-bonded (3)MLCT excited state. The second time constant represents both excited-state relaxation to ground state and excited-state isomerization to form O-[Ru(tpy)(bpy)(dmso)](2+). In conjunction with the S-->O isomerization quantum yield (Phi(S)(-->)(O) = 0.024), isomerization of [Ru(tpy)(bpy)(dmso)](2+) occurs with a time constant of 1.5 ns. For trans-[Ru(tpy)(pic)(dmso)](+), global analysis of the transient spectra reveals time constants of 3.6 +/- 0.2 and 118 +/- 2 ps associated with these two processes. In conjunction with the S-->O isomerization quantum yield (Phi(S)(-->)(O) = 0.25), isomerization of trans-[Ru(tpy)(pic)(dmso)](+) occurs with a time constant of 480 ps. In both cases, the thermally relaxed excited states are assigned as terpyridine-localized (3)MLCT states. Electronic state diagrams are compiled employing these data as well as electrochemical, absorption, and emission data to describe the reactivity of these complexes. The data illustrate that rapid bond-breaking and bond-making reactions can occur from (3)MLCT excited states formed from visible light irradiation.     

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.     

Nanoscale-enhanced Ru(bpy)3(2+) electrochemiluminescence labels and related aptamer-based biosensing system

A unique multilabeling at a single-site protocol of the Ru(bpy)(3)(2+) electrochemiluminescence (ECL) system is proposed. Nanoparticles (NPs) were used as assembly substrates to enrich ECL co-reactants of Ru(bpy)(3)(2+) to construct nanoscale-enhanced ECL labels. Two different kinds of NP substrates [including semiconductor NPs (CdTe) and noble metal NPs (gold)] capped with 2-(dimethylamino)ethanethiol (DMAET) [a tertiary amine derivative which is believed to be one of the most efficient of co-reactants of the Ru(bpy)(3)(2+) system] were synthesized through a simple one-pot synthesis method in aqueous media. Although both CdTe and gold NPs realized the enrichment of ECL co-reactants, they presented entirely different ECL performances as nanoscale ECL co-reactants of Ru(bpy)(3)(2+). The different effects of these two NPs on the ECL of Ru(bpy)(3)(2+) were studied. DMAET-capped CdTe NPs showed enormous signal amplification of Ru(bpy)(3)(2+) ECL, whereas DMAET-capped gold NPs showed a slight quenching effect of the ECL signal. DMAET-capped CdTe NPs can be considered to be excellent nanoscale ECL labels of the Ru(bpy)(3)(2+) system, as even a NP solution sample of 10(-18) M was still detectable after an electrostatic self-assembly concentration process. DMAET-capped CdTe NPs were further applied in the construction of aptamer-based biosensing system for proteins and encouraging results were obtained.     

Long-range electron transfer across molecule-nanocrystalline semiconductor interfaces using tripodal sensitizers

Four tripodal sensitizers, Ru(bpy)(2)(Ad-tripod-phen)(2+) (1), Ru(bpy)(2)(Ad-tripod-bpy)(2+) (2), Ru(bpy)(2)(C-tripod-phen)(2+) (3), and Ru(bpy)(2)(C-tripod-bpy)(2+) (4) (where bpy is 2,2'-bipyridine, phen is 1,10-phenanthroline, and Ad-tripod-bpy (phen) and C-tripod-bpy (phen) are tripod-shaped bpy (phen) ligands based on 1,3,5,7-tetraphenyladamantane and tetraphenylmethane, respectively), have been synthesized and characterized. The tripodal sensitizers consist of a rigid-rod arm linked to a Ru(II)-polypyridine complex at one end and three COOR groups on the other end that bind to metal oxide nanoparticle surfaces. The excited-state and redox properties of solvated and surface-bound 1-4 have been studied at room temperature. The absorption spectra, emission spectra, and electrochemical properties of 1-4 in acetonitrile solution are preserved when 1-4 are bound to nanocrystalline (anatase) TiO(2) or colloidal ZrO(2) mesoporous films. This behavior is indicative of weak electronic coupling between TiO(2) and the sensitizer. The kinetics for excited-state decay are exponential for 1-4 in solution and are nonexponential when 1-4 are bound to ZrO(2) or TiO(2). Efficient and rapid (k(cs) > 10(8) s(-)(1)) excited-state electron injection is observed for 1-4/TiO(2). The recombination of the injected electron with the oxidized Ru(III) center is well described by a second-order kinetic model with rate constants that are independent of the sensitizer. The sensitizers bound to TiO(2) were reversibly oxidized electrochemically with an apparent diffusion coefficient approximately 1 x 10(-11) cm(2) s(-)(1).     

Photocatalytic generation of a non-heme oxoiron(IV) complex with water as an oxygen source

The photocatalytic formation of a non-heme oxoiron(IV) complex, [(N4Py)Fe(IV)(O)](2+) [N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine], efficiently proceeds via electron transfer from the excited state of a ruthenium complex, [Ru(II)(bpy)(3)](2+)* (bpy = 2,2'-bipyridine) to [Co(III)(NH(3))(5)Cl](2+) and stepwise electron-transfer oxidation of [(N4Py)Fe(II)](2+) with 2 equiv of [Ru(III)(bpy)(3)](3+) and H(2)O as an oxygen source. The oxoiron(IV) complex was independently generated by both chemical oxidation of [(N4Py)Fe(II)](2+) with [Ru(III)(bpy)(3)](3+) and electrochemical oxidation of [(N4Py)Fe(II)](2+).