Flash-quench technique employed to study the one-electron reduction of triiodide in acetonitrile: evidence for a diiodide reaction product

The one-electron reduction of triiodide (I(3)(-)) by a reduced ruthenium polypyridyl compound was studied in an acetonitrile solution with the flash-quench technique. Reductive quenching of the metal-to-ligand charge-transfer excited state of [Ru(II)(deeb)(3)](2+) by iodide generated the reduced ruthenium compound [Ru(II)(deeb(-))(deeb)(2)](+) and diiodide (I(2)(?-)). The subsequent reaction of [Ru(II)(deeb(-))(deeb)(2)](+) with I(3)(-) indicated that I(2)(?-) was a product that appeared with a second-order rate constant of (5.1 ± 0.2) × 10(9) M(-1) s(-1). After correction for diffusion and some assumptions, Marcus theory predicted a formal potential of -0.58 V (vs SCE) for the one-electron reduction of I(3)(-). The relevance of this reaction to solar energy conversion is discussed.     

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.     

Evidence for static quenching of MLCT excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(deeb)(bpy)(2)(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 acetonitrile or dichloromethane were found to be quenched by iodide at room temperature. The ionic strength dependence of the optical spectra gave evidence for ion pairing. Iodide is found to quench the photoluminescence (PL) intensity and influence the spectral distribution of the emitted light. A static component to the time-resolved PL quenching provided further evidence for ground-state adduct. Stern-Volmer analysis of the static component provided an estimate of the iodide-Ru(deeb)(bpy)(2)(2+) adduct equilibrium constant in dichloromethane, K(sv) = 40,000 M(-)(1). Transient absorption studies clearly demonstrate that an electron-transfer quenching mechanism is operative and that I(2)(-)(*) can be photoproduced in high yield, phi = 0.25. For Ru(bpy)(3)(PF(6))(2) in acetonitrile, similar behavior could be observed at iodide concentrations >100 times that required for dichloromethane.     

Triiodide quenching of ruthenium MLCT excited state in solution and on TiO2 surfaces: an alternate pathway for charge recombination

The excited states of [Ru(bpy)2(deeb)](PF6)2, where bpy is 2,2-bipyridine and deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, were found to be efficiently quenched by triiodide (I3-) in acetonitrile and dichloromethane. In dichloromethane, I3- was found to quench the excited states by static and dynamic mechanisms; Stern-Volmer analysis of the time-resolved and steady-state photoluminescence data produced self-consistent estimates for the I3- + Ru(bpy)2(deeb)2+ <==> [Ru(II)(bpy)2(deeb)2+,(I3-)]+ equilibrium, K = 51,000 M(-1), and the bimolecular quenching rate constant, kq = 4.0 x 10(10) M(-1) s(-1). In acetonitrile, there was no evidence for ion pairing and a dynamic quenching rate constant of k(q) = 4.7 x 10(10) M(-1) s(-1) was calculated. Comparative studies with Ru(bpy)2(deeb)2+ anchored to mesoporous nanocrystalline TiO2 thin films also showed efficient excited-state dynamic quenching by I3- in both acetonitrile and dichloromethane, kq = 1.8 x 10(9) and 3.6 x 10(10) M(-1) s(-1), respectively. No reaction products for the excited-state quenching processes were observed by nanosecond transient absorption measurements from 350 to 800 nm under any experimental conditions. X-ray crystallographic, IR, and Raman data gave evidence for interactions between I3- and the bpy and deeb ligands in the solid state.     

Electronic and photophysical properties of adducts of [Ru(bpy)3]2+ and Dawson-type sulfite polyoxomolybdates α/β-[Mo18O54(SO3)2]4-

The spectroscopic and photophysical properties of [Ru(bpy)(3)](2)[[Mo(18)O(54)(SO(3))(2)], where bpy is 2,2'-bipyridyl and [Mo(18)O(54)(SO(3))(2)](4-) is either the α or β-sulfite containing polyoxomolybdate isomer, have been measured and compared with those for the well known but structurally distinct sulfate analogue, α-[Mo(18)O(54)(SO(4))(2)](4-). Electronic difference spectroscopy revealed the presence of new spectral features around 480 nm, although they are weak in comparison with the [Ru(bpy)(3)](2)[Mo(18)O(54)(SO(4))(2)] analogue. Surprisingly, Stern-Volmer plots of [Ru(bpy)(3)](2+) luminescence quenching by the polyoxometallate revealed the presence of both static and dynamic quenching for both α and β-[Mo(18)O(54)(SO(3))(2)](4-). The association constant inferred for the ion cluster [Ru(bpy)(3)](2)α-[Mo(18)O(54)(SO(4))(2)] is K = 5.9 ± 0.56 × 10(6) and that for [Ru(bpy)(3)](2)β-[Mo(18)O(54)(SO(4))(2)] is K = 1.0 ± 0.09 × 10(7). Unlike the sulfate polyoxometalates, both sulfite polyoxometalate-ruthenium adducts are non-luminescent. Despite the strong electrostatic association in the adducts resonance Raman and photoelectrochemical studies suggests that unlike the sulfato polyoxometalate analogue there is no sensitization of the polyoxometalate photochemistry by the ruthenium centre for the sulfite anions. In addition, the adducts exhibit photochemical lability in acetonitrile, attributable to decomposition of the ruthenium complex, which has not been observed for other [Ru(bpy)(3)](2+) -polyoxometalate adducts. These observations suggest that less electronic communication exists between the [Ru(bpy)(3)](2+) and the sulfite polyoxoanions relative to their sulfate polyoxoanion counterparts, despite their structural and electronic analogy. The main distinction between sulfate and sulfite polyoxometalates lies in their reversible reduction potentials, which are more positive by approximately 100 mV for the sulfite anions. This suggests that the capacity for [Ru(bpy)(3)](2+) or analogues to sensitize photoreduction in the adducts of polyoxometalates requires very sensitive redox tuning.     

Self assembled composites of luminescent Ru(II) metallopolymers and the Dawson polyoxometalate α-[Mo18O54(SO4)2]4-

The interaction of two luminescent metallopolymers; [Ru(bpy)(2)(PVP)(10)](2+) and [Ru(bpy)(2)(CAIP)co-poly(7)](+), where bpy is 2,2'-bipyridyl, PVP is polyvinylpyridine, and (CAIP)co-poly(7) is poly(styrene(6)-co-p-(aminomethyl)styrene) amide linked to 2-(4-carboxyphenyl)imidazo[4,5-f] [1,10]phenanthroline, with the Dawson polyoxomolybdate α-[Mo(18)O(54)(SO(4))(2)](4-) is described. Both metallopolymers undergo electrostatic association with the polyoxometalate. From both electronic and luminescence spectroscopy the thermodynamic products were determined to be {[Ru(bpy)(2)(PVP)(10)](4.5)[Mo(18)O(54)(SO(4))(2)]}(5+) and {[Ru(bpy)(2)(CAIP)co-poly(7)](5)[Mo(18)O(54)(SO(4))(2)]}(+), i.e. in both instances, the number of ruthenium centres in the cluster exceeds the number required for charge neutralization of the molybdate centre. Association quenches the luminescence of the metallopolymer although, consistent with the excess of Ru(ii) present in the associated composites, emission is not completely extinguished even when a large excess of [Mo(18)O(54)(SO(4))(2)](4-) is present. The observed emission lifetime was not affected by [Mo(18)O(54)(SO(4))(2)](4-) therefore quenching was deemed static. The luminescent intensity data was found to fit best to a (sphere of action) Perrin model from which the radii of the quenching were calculated as 4.6 ? and 5.8 ? for [Ru(bpy)(2)(PVP)(10)](2+) and [Ru(bpy)(2)(CAIP co-poly)(7)](+) respectively. Both UV/Vis and resonance Raman data indicate the presence of a new optical transition centered around 490 nm for the composite, {[Ru(bpy)(2)(PVP)(10)](4.5)[Mo(18)O(54)(SO(4))(2)]}(5+) but not for {[Ru(bpy)(2)(CAIP)co-poly(7)](5)[Mo(18)O(54)(SO(4))(2)]}(+). This indicates strong electronic interaction between the metal centres in the former composite, which despite good thermodynamic analogy, is not observed for {[Ru(bpy)(2)(CAIP)co-poly(7)](5)[Mo(18)O(54)(SO(4))(2)]}(+). These results are consistent with photoelectrochemical studies of layer by layer assemblies of these films which indicate that the ruthenium centre sensitizes polyoxometalate photo-oxidation of benzyl alcohol in {[Ru(bpy)(2)(PVP)(10)](4.5)[Mo(18)O(54)(SO(4))(2)]}(5+) but not in {[Ru(bpy)(2)(CAIP)co-poly(7)](5)[Mo(18)O(54)(SO(4))(2)]}(+).     

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.     

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.     

New dicarboxylic acid bipyridine ligand for ruthenium polypyridyl sensitization of TiO2

An ambidentate dicarboxylic acid bipyridine ligand, (4,5-diazafluoren-9-ylidene) malonic acid (dfm), was synthesized for coordination to Ru(II) and mesoporous nanocrystalline (anatase) TiO(2) thin films. The dfm ligand provides a conjugated pathway from the pyridyl rings to the carbonyl carbons of the carboxylic acid groups. X-ray crystal structures of [Ru(bpy)(2)(dfm)]Cl(2) and the corresponding diethyl ester compound, [Ru(bpy)(2)(defm)](PF(6))(2), were obtained. The compounds displayed intense metal-to-ligand charge transfer (MLCT) absorption bands in the visible region (ε > 11,000 M(-1) cm(-1) for [Ru(bpy)(2)(dfm)](PF(6))(2) in acetonitrile). Significant room temperature photoluminescence, PL, was absent in CH(3)CN but was observed at 77 K in a 4:1 EtOH:MeOH (v:v) glass. Cyclic voltammetry measurements revealed quasi-reversible Ru(III/II) electrochemistry. Ligand reductions were quasi-reversible for the diethyl ester compound [Ru(bpy)(2)(defm)](2+), but were irreversible for [Ru(bpy)(2)(dfm)](2+). Both compounds were anchored to TiO(2) thin films by overnight reactions in CH(3)CN to yield saturation surface coverages of 3 × 10(-8) mol/cm(2). Attenuated total reflection infrared measurements revealed that the [Ru(bpy)(2)(dfm)](2+) compound was present in the deprotonated carboxylate form when anchored to the TiO(2) surface. The MLCT excited states of both compounds injected electrons into TiO(2) with quantum yields of 0.70 in 0.1 M LiClO(4) CH(3)CN. Micro- to milli-second charge recombination yielded ground state products. In regenerative solar cells with 0.5 M LiI/0.05 M I(2) in CH(3)CN, the Ru(bpy)(2)(dfm)/TiO(2) displayed incident photon-to-current efficiencies of 0.7 at the absorption maximum. Under the same conditions, the diethylester compound was found to rapidly desorb from the TiO(2) surface.     

Fe(II), Ru(II) and Re(I) complexes of endotopic, sterically non-hindering, U-shaped 8,8'-disubstituted-3,3'-biisoquinoline ligands: syntheses and spectroscopic properties

The redox behaviour, optical-absorption spectra and emission properties of U-shaped and elongated disubstituted biisoquinoline ligands and of derived octahedral Fe(ii), Ru(ii), and Re(i) complexes are reported. The ligands are 8,8'-dichloro-3,3'-biisoquinoline (1), 8,8'-dianisyl-3,3'-biisoquinoline (2), and 8,8'-di(phenylanisyl)-3,3'-biisoquinoline (3), and the complexes are [Fe(3)(3)](2+), [Fe(2)(3)](2+), [Ru(1)(phen)(2)](2+), [Ru(2)(3)](2+), [Ru(3)(3)](2+), [Re(2)(py)(CO)(3)](+), and [Re()(py)(CO)(3)](+). For the ligands, the optical properties as observed in dichloromethane are in line with expectations based on the predominant (1)pipi* nature of the involved excited states, with contributions at lower energies from (1)npi* and (1)ILCT (intraligand charge transfer) transitions. For all of the Fe(ii), Ru(ii), and Re(i) complexes, studied in acetonitrile, the transitions associated with the lowest-energy absorption band are of (1)MLCT (metal-to-ligand charge transfer) nature. The emission properties, as observed at room temperature and at 77 K, can be described as follows: (i) the Fe(ii) complexes do not emit, either at room temperature or at 77 K; (ii) the room-temperature emission of the Ru(ii) complexes (phi(em) > 10(-3), tau in the micros range) is of mixed (3)MLCT/(3)LC character (and similarly at 77 K); and (iii) the room-temperature emission of the Re(i) complexes (phi(em) approximately 3 x 10(-3), tau < 1 ns) is of (3)MLCT character and becomes of (3)LC (ligand-centered) character (tau in the ms time scale) at 77 K. The interplay of the involved excited states in determining the luminescence output is examined.     

Ruthenium nitrosyl complexes with 1,4,7-trithiacyclononane and 2,2'-bipyridine (bpy) or 2-phenylazopyridine (pap) coligands. Electronic structure and reactivity aspects

The present article describes ruthenium nitrosyl complexes with the {RuNO}(6) and {RuNO}(7) notations in the selective molecular frameworks of [Ru(II)([9]aneS(3))(bpy)(NO(+))](3+) (4(3+)), [Ru(II)([9]aneS(3))(pap) (NO(+))](3+) (8(3+)) and [Ru(II)([9]aneS(3))(bpy)(NO˙)](2+) (4(2+)), [Ru(II)([9]aneS(3))(pap)(NO˙)](2+) (8(2+)) ([9]aneS(3) = 1,4,7-trithiacyclononane, bpy = 2,2'-bipyridine, pap = 2-phenylazopyridine), respectively. The nitrosyl complexes have been synthesized by following a stepwise synthetic procedure: {Ru(II)-Cl} → {Ru(II)-CH(3)CN} → {Ru(II)-NO(2)} → {Ru(II)-NO(+)} → {Ru(II)-NO˙}. The single-crystal X-ray structure of 4(3+) and DFT optimised structures of 4(3+), 8(3+) and 4(2+), 8(2+) establish the localised linear and bent geometries for {Ru-NO(+)} and {Ru-NO˙} complexes, respectively. The crystal structures and (1)H/(13)C NMR suggest the [333] conformation of the coordinated macrocyclic ligand ([9]aneS(3)) in the complexes. The difference in π-accepting strength of the co-ligands, bpy in 4(3+) and pap in 8(3+) (bpy < pap) has been reflected in the ν(NO) frequencies of 1945 cm(-1) (DFT: 1943 cm(-1)) and 1964 cm(-1) (DFT: 1966 cm(-1)) and E°({Ru(II)-NO(+)}/{Ru(II)-NO˙}) of 0.49 and 0.67 V versus SCE, respectively. The ν(NO) frequency of the reduced {Ru-NO˙} state in 4(2+) or 8(2+) however decreases to 1632 cm(-1) (DFT: 1637 cm(-1)) or 1634 cm(-1) (DFT: 1632 cm(-1)), respectively, with the change of the linear {Ru(II)-NO(+)} geometry in 4(3+), 8(3+) to bent {Ru(II)-NO˙} geometry in 4(2+), 8(2+). The preferential stabilisation of the eclipsed conformation of the bent NO in 4(2+) and 8(2+) has been supported by the DFT calculations. The reduced {Ru(II)-NO˙} exhibits free-radical EPR with partial metal contribution revealing the resonance formulation of {Ru(II)-NO˙}(major)?{Ru(I)-NO(+)}(minor). The electronic transitions of the complexes have been assigned based on the TD-DFT calculations on their DFT optimised structures. The estimated second-order rate constant (k, M(-1) s(-1)) of the reaction of the nucleophile, OH(-) with the electrophilic {Ru(II)-NO(+)} for the bpy derivative (4(3+)) of 1.39 × 10(-1) is half of that determined for the pap derivative (8(3+)), 2.84 × 10(-1) in CH(3)CN at 298 K. The Ru-NO bond in 4(3+) or 8(3+) undergoes facile photolytic cleavage to form the corresponding solvent species {Ru(II)-CH(3)CN}, 2(2+) or 6(2+) with widely varying rate constant values, (k(NO), s(-1)) of 1.12 × 10(-1) (t(1/2) = 6.2 s) and 7.67 × 10(-3) (t(1/2) = 90.3 s), respectively. The photo-released NO can bind to the reduced myoglobin to yield the Mb-NO adduct.     

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.     

Coordination and Redox Chemistry of Substituted-Polypyridyl Complexes of Ruthenium

The complexes [Ru(tpy)(acac)(Cl)], [Ru(tpy)(acac)(H(2)O)](PF(6)) (tpy = 2,2',2"-terpyridine, acacH = 2,4 pentanedione) [Ru(tpy)(C(2)O(4))(H(2)O)] (C(2)O(4)(2)(-) = oxalato dianion), [Ru(tpy)(dppene)(Cl)](PF(6)) (dppene = cis-1,2-bis(diphenylphosphino)ethylene), [Ru(tpy)(dppene)(H(2)O)](PF(6))(2), [Ru(tpy)(C(2)O(4))(py)], [Ru(tpy)(acac)(py)](ClO(4)), [Ru(tpy)(acac)(NO(2))], [Ru(tpy)(acac)(NO)](PF(6))(2), and [Ru(tpy)(PSCS)Cl] (PSCS = 1-pyrrolidinedithiocarbamate anion) have been prepared and characterized by cyclic voltammetry and UV-visible and FTIR spectroscopy. [Ru(tpy)(acac)(NO(2))](+) is stable with respect to oxidation of coordinated NO(2)(-) on the cyclic voltammetric time scale. The nitrosyl [Ru(tpy)(acac)(NO)](2+) falls on an earlier correlation between nu(NO) (1914 cm(-)(1) in KBr) and E(1/2) for the first nitrosyl-based reduction 0.02 V vs SSCE. Oxalate ligand is lost from [Ru(II)(tpy)(C(2)O(4))(H(2)O)] to give [Ru(tpy)(H(2)O)(3)](2+). The Ru(III/II) and Ru(IV/III) couples of the aqua complexes are pH dependent. At pH 7.0, E(1/2) values are 0.43 V vs NHE for [Ru(III)(tpy)(acac)(OH)](+)/[Ru(II)(tpy)(acac)(H(2)O)](+), 0.80 V for [Ru(IV)(tpy)(acac)(O)](+)/[Ru(III)(tpy)(acac)(OH)](+), 0.16 V for [Ru(III)(tpy)(C(2)O(4))(OH)]/[Ru(II)(tpy)(C(2)O(4))(H(2)O)], and 0.45 V for [Ru(IV)(tpy)(C(2)O(4))(O)]/[Ru(III)(tpy)(C(2)O(4))(OH)]. Plots of E(1/2) vs pH define regions of stability for the various oxidation states and the pK(a) values of aqua and hydroxo forms. These measurements reveal that C(2)O(4)(2)(-) and acac(-) are electron donating to Ru(III) relative to bpy. Comparisons with redox potentials for 21 related polypyridyl couples reveal the influence of ligand changes on the potentials of the Ru(IV/III) and Ru(III/II) couples and the difference between them, DeltaE(1/2). The majority of the effect appears in the Ru(III/II) couple. ()A linear correlation exists between DeltaE(1/2) and the sum of a set of ligand parameters defined by Lever et al., SigmaE(i)(L(i)), for the series of complexes, but there is a dramatic change in slope at DeltaE(1/2) approximately -0.11 V and SigmaE(i)(L(i)) = 1.06 V. Extrapolation of the plot of DeltaE(1/2) vs SigmaE(i)(L(i)) suggests that there may be ligand environments in which Ru(III) is unstable with respect to disproportionation into Ru(IV) and Ru(II). This would make the two-electron Ru(IV)O/Ru(II)OH(2) couple more strongly oxidizing than the one-electron Ru(IV)O/Ru(III)OH couple.     

Sensitization and stabilization of TiO(2) photoanodes with electropolymerized overlayer films of ruthenium and zinc polypyridyl complexes: a stable aqueous photoelectrochemical cell

Overlayer thin films of vinylbipyridine (vbpy)-containing Ru and Zn complexes have been formed on top of ruthenium dye complexes adsorbed to TiO(2) by reductive electropolymerization. The goal was to create an efficient, water-stable photoelectrode or electrodes. An adsorbed-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Ru(vbpy)(3)](PF(6))(2) surface composite displays excellent stability toward dissolution in water, but the added overlayer film greatly decreases incident photon-to-current conversion efficiencies (IPCE) in propylene carbonate with I(3)(-)/I(-) as the carrier couple. An ads-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Zn(vbpy)(3)](PF(6))(2) composite displays no loss in IPCE compared to ads-[Ru(vbpy)(2)(dcb)](PF(6))(2) but is susceptible to film breakdown in the presence of water by solvolysis and loss of the cross-linking Zn(2+) ions. Success was attained with an ads-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Ru(vbpy)(2)(dppe)](PF(6))(2) composite. In this case the electropolymerized layer is transparent in the visible. The composite electrode is stable in water, the IPCE in propylene carbonate with I(3)(-)/I(-) is comparable to the adsorbed complex, and a significant IPCE is observed in water with the quinone/hydroquinone carrier couple. The assembly [(bpy)(2)(CN)Ru(CN)Ru(vbpy)(2)(NC)Ru(CN)(bpy)(2)](PF(6))(2) ([Ru(CN)Ru(NC)Ru](PF(6))(2)) adsorbs spontaneously on TiO(2), and electropolymerization of thin layers of the assembly to give ads-[Ru(CN)Ru(NC)Ru](PF(6))(2)/poly-[Ru(CN)Ru(NC)Ru](PF(6))(2) enhances IPCE and has no deleterious effect on the IPCE/Ru.     

Toward exceeding the Shockley-Queisser limit: photoinduced interfacial charge transfer processes that store energy in excess of the equilibrated excited state

Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine, bq is 2,2'-biquinoline, and deeb and deebq are 4,4'-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (+/- 2) x 10(-8) mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were phi(417 nm) = 0.18 +/- 0.02, phi(532.5 nm) = 0.08 +/- 0.02, and phi(683 nm) = 0.05 +/- 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.     

Excited state interfacial electron transfer from a compound with a single pyridine ligand

The coordination compound Ru(NH(3))(5)(eina)(PF(6))(2), where eina is ethyl isonicotinate, was synthesized and attached to optically transparent nanocrystalline (anatase) TiO(2) films, abbreviated Ru(NH(3))(5)(eina)/TiO(2). The metal-to-ligand-charge-transfer (MLCT) absorption was found to shift in wavelength with solvent. The absorption maximum of the low energy MLCT band was observed at 486 nm in acetonitrile and 528 nm in dimethylformamide for Ru(NH(3))(5)(eina)(PF(6))(2) and at 512 and 555 nm for Ru(NH(3))(5)(eina)/TiO(2), respectively. The compound was found to be nonemissive with an excited state lifetime <10 ns under all conditions studied. Light excitation in fluid solution and when attached to insulating ZrO(2) films resulted in a loss of the MLCT absorption, consistent with ligand field photochemistry. Pulsed light excitation of Ru(NH(3))(5)(eina)/TiO(2) yields an absorption difference spectrum consistent with an interfacial charge separated state, Ru(III)(NH(3))(5)(eina)/TiO(2)(e(-)). This state forms within 10 ns and returns cleanly to ground state product within milliseconds. The injection quantum yields were determined by comparative actinometry and were found to be excitation wavelength dependent: phi(inj)(417 nm) = 0.30 +/- 0.05 and phi(inj)(532.5 nm) = 0.15 +/- 0.03. Regenerative solar cells based on Ru(NH(3))(5)(eina)/TiO(2) with 0.5 M TBAI, where TBA is tetrabutylammonium, and 0.05 M I(2) in acetonitrile were very inefficient. Sluggish iodide oxidation is expected, on the basis of the negative E degrees (Ru(III/II)) = +0.17 (V vs Ag/AgCl) reduction potential, and this presumably allows a greater fraction of the injected electrons to recombine with the oxidized compound thereby lowering the solar cell efficiency.     

Synthesis of ruthenium(II) complexes of tetradentate bis(N-pyridylimidazolylidenyl)methane and their reactivities towards N- donors

A family of hexa-coordinated ruthenium(II) complexes of bis(N-pyridylimidazolylidenyl)methane (L) were prepared and structurally characterized. Carbene transfer reactions of [Ru(p-cymene)Cl(2)](2), [Ru(CO)(2)Cl(2)](n) and RuHCl(CO)(PPh(3))(3) with silver-NHC complexes in situ generated from [H(2)L](PF(6))(2) and Ag(2)O afforded [RuL(CH(3)CN)(2)](PF(6))(2) (1), [Ru(2)L(p-cymene)(2)Cl(2)](PF(6))(2) (2), [RuL(CO)(2)](PF(6))(2) (3) and [RuL(PPh(3))(2)](PF(6))(2) (4), respectively. The reactions of 1 towards several N- and P-donors were studied. The treatment of 1 with 1,10-phenanthroline resulted in the substitution of one pyridine and one acetonitrile molecule affording [RuL(phen)(CH(3)CN)](PF(6))(2) (5) as a mixture of two isomers. Reaction of 1,2-bis(diphenylphosphino)ethane (dppe) and 1 gave [RuL(dppe)(CH(3)CN)(2)](PF(6))(2) (7), in which two pyridines were substituted by a dppe ligand trans to two NHC groups. In contrast, reactions of 1 with ethane-1,2-diamine, propane-1,3-diamine and 3,5-dimethyl-1H-pyrazole led to the substitution of acetonitrile and subsequent N-H addition of the C≡N bond of the coordinated acetonitrile yielding [RuL(ethane-1,2-diamine)(N-(2-aminoethyl)acetimidamide)](PF(6))(2) (8), [RuL(propane-1,3-diamine)(N-(3-aminopropyl)acetimidamide)](PF(6))(2) (9) and RuL(1-(3,5-dimethyl-1H-pyrazol-1-yl)ethanimine)(CH(3)CN)](PF(6))(2) (10), respectively.     

High-extinction ruthenium compounds for sunlight harvesting and hole transport

The compounds Ru(bpy) 2(BTL)(PF 6) 2 and Ru(deeb) 2(BTL)(PF 6) 2, where bpy is 2,2'-bipyridine, deeb is 4,4'-(C 2H 5CO 2) 2-bpy, and BTL is 9'-[4,5-bis(cyanoethylthio)]-1,3-dithiol-2-ylidene]-4',5'-diazafluorene, were found to have very high extinction coefficients in the visible region. In an acetonitrile solution, the extinction of Ru(deeb) 2(BTL)(PF 6) 2 was = 44 000 +/- 1000 M (-1) cm (-1) at lambda = 470 nm. Two quasi-reversible oxidation waves, E 1/2 = +0.88 and +1.16 V, and an irreversible reduction, E pr = -1.6 V, were observed versus ferrocene (Fc (+/0)). At -40 degrees C, a state was observed with spectroscopic properties characteristic of a metal-to-ligand charge-transfer excited state, tau = 25 ns. This same compound was found to photoinject electrons into TiO 2 with a quantum yield Phi = 0.3 +/- 0.2 for 532.5 or 417 nm light excitation in a 0.1 M LiClO 4/acetonitrile electrolyte. In regenerative solar cells, a sustained photocurrent was observed with a maximum incident photon-to-current efficiency of 0.4. The photocurrent action and absorptance spectra were in good agreement, consistent with injection from a single excited state.     

Enhancement of metal-metal coupling at a considerable distance by using 4-pyridinealdazine as a bridging ligand in polynuclear complexes of rhenium and ruthenium

Novel polynuclear complexes of rhenium and ruthenium containing PCA (PCA = 4-pyridinecarboxaldehyde azine or 4-pyridinealdazine or 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a bridging ligand have been synthesized as PF(6-) salts and characterized by spectroscopic, electrochemical, and photophysical techniques. The precursor mononuclear complex, of formula [Re(Me(2)bpy)(CO)(3)(PCA)](+) (Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine), does not emit at room temperature in CH(3)CN, and the transient spectrum found by flash photolysis at lambda(exc) = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me(2)bpy)(CO)(3)Re(II)(PCA(-))](+), with lambda(max) = 460 nm and tau < 10 ns. The spectral properties of the related complexes [[Re(Me(2)bpy)(CO)(3)}(2)(PCA)](2+), [Re(CO)(3)(PCA)(2)Cl], and [Re(CO)(3)Cl](3)(PCA)(4) confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(II)(NH(3))(5)](3+), presents an intense absorption in the visible spectrum that can be assigned to a MLCT d(pi)(Ru) --> pi(PCA); in CH(3)CN, the value of lambda (max) = 560 nm is intermediate between those determined for [Ru(NH(3))(5)(PCA)](2+) (lambda(max) = 536 nm) and [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](4+) (lambda(max) = 574 nm), indicating a significant decrease in the energy of the pi-orbital of PCA. The mixed-valent species, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(III)(NH(3))(5)](4+), was obtained in CH(3)CN solution, by bromine oxidation or by controlled-potential electrolysis at 0.8 V in a OTTLE cell of the [Re(I),Ru(II)] precursor; the band at lambda(max) = 560 nm disappears completely, and a new band appears at lambda(max) = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) Re(I) --> Ru(III). By using the Marcus-Hush formalism, both the electronic coupling (H(AB)) and the reorganization energy (lambda) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (approximately 15.0 Angstroms), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](5+) (K(c) approximately 10(2), in CH(3)CN), puts into evidence an unusual enhancement of the metal-metal coupling in the bridged PCA complexes. This effect can be accounted for by the large extent of "metal-ligand interface", as shown by DFT calculations on free PCA. Moreover, lambda is lower than the driving force -DeltaG degrees for the recombination charge reaction [Re(II),Ru(II)] --> [Re(I),Ru(III)] that follows light excitation of the mixed-valent species. It is then predicted that this reverse reaction falls in the Marcus inverted region, making the heterodinuclear [Re(I),Ru(III)] complex a promising model for controlling the efficiency of charge-separation processes.     

Reactivity of radicals generated on irradiation of trans-[Ru(NH3)4(NO2)P(OEt)3](PF6)

The electronic absorption spectrum of trans-[Ru(NH(3))(4)(NO(2))(P(OEt)(3)](+) in aqueous solution is characterized by a strong absorption band at 334 nm (lambda(max) = 1800 mol(-1) L cm(-1)). On the basis of quantum mechanics calculations, this band has been assigned to a MLCT transition from the metal to the nitro ligand. Molecular orbital calculations also predict an LF transition at 406 nm, which is obscured by the intense MLCT transition. When trans-[Ru(NH(3))(4)(NO(2))(P(OEt)(3)](+) in acetonitrile is irradiated with a 355 nm pulsed laser light, the absorption features are gradually shifted to represent those of the solventocomplex trans-[Ru(NH(3))(4)(solv)(P(OEt)(3)](2+) (lambda(max) = 316 nm, epsilon = 650 mol(-1) L cm(-1)), which was also detected by (31)P NMR spectroscopy. The net photoreaction under these conditions is a photoaquation of trans-[Ru(NH(3))(4)(NO(2))(P(OEt)(3)](+), although, after photolysis, the presence of the nitric oxide was detected by differential pulse polarography. In phosphate buffer pH 9.0, after 15 min of photolysis, a thermal reaction resulted in the formation of a hydroxyl radical and a small amount of a paramagnetic species as detected by EPR spectroscopy. In the presence of trans-[Ru(NH(3))(4)(solv)P(OEt)(3)](2+), the hydroxyl radical initiated a chain reaction. On the basis of spectroscopic and electrochemical data, the role of the radicals produced is analyzed and a reaction sequence consistent with the experimental results is proposed. The 355 nm laser photolysis of trans-[Ru(NH(3))(4)(NO(2))(P(OEt)(3)](+) in phosphate buffer pH 7.4 also gives nitric oxide, which is readily trapped by ferrihemeproteins (myoglobin, hemoglobin, and cytochrome C), giving rise to the formation of their nitrosylhemeproteins(II), (NO)Fe(II)hem.