Multisite effects on intervalence charge transfer in a clusterlike trinuclear assembly containing ruthenium and osmium

The intervalence charge transfer (IVCT) properties of the mixed-valence forms of the diastereoisomers of the dinuclear [[Ru(bpy)2](mu-HAT)[M(bpy)2]]5+ (M = Ru or Os) complexes and the trinuclear heterochiral [[Ru(bpy)2]2[Os(bpy)2](mu-HAT)]n+ (n = 7, 8; HAT = 1,4,5,8,9,12-hexaazatriphenylene; bpy = 2,2'-bipyridine) species display a marked dependence on the nuclearity and extent of oxidation of the assemblies, while small differences are also observed for the diastereoisomers of the same complex in the dinuclear cases. The mixed-valence heterochiral [[Ru(bpy)2]2[Os(bpy)2](mu-HAT)]n+ (n = 7, 8) forms exhibit IVCT properties that are intermediate between those of the diastereoisomeric forms of the localized hetero-dinuclear complex [[Ru(bpy)2](mu-HAT)[Os(bpy)2]]5+ and the borderline localized-to-delocalized homo-trinuclear complex [[Ru(bpy)2]3(mu-HAT)]n+ (n = 7, 8). The near-infrared (NIR) spectrum of the +7 mixed-valence species exhibits both interconfigurational (IC) and IVCT transitions which are quantitatively similar to those in [[Ru(bpy)2](mu-HAT)[Os(bpy)2]]5+ and are indicative of the localized mixed-valence formulation [[Ru(II)(bpy)2]2[Os(III)(bpy)2](mu-HAT)]7+. The +8 state exhibits a new band attributable to an IVCT transition in the near-infrared region.     

Intervalence charge transfer in the stereoisomers of a dinuclear ruthenium complex containing the bridging ligand dibenzoeilatin

The dinuclear species [{Ru(bpy)2}2(micro-dbneil)]4+(bpy = 2,2'-bipyridine; dbneil = dibenzoeilatin) was separated into its three stereoisomeric forms by cation-exchange chromatographic techniques. The NMR characteristics of the two diastereoisomers (meso and rac) are very similar, consistent with the decreased anisotropic interactions between the terminal bpy ligands located on different ruthenium centres because of their large spatial separation. Spectroelectrochemical measurements of the IVCT transitions in the mixed valence (5+) species show the system to be in the class II-III regime with substantial electronic communication between the metal centres. There is no significant difference in IVCT behaviour between the two diastereoisomers.     

Influence of anions on intervalence charge transfer (IVCT) in mixed-valence dinuclear complexes

Spectroelectrochemical studies of the intervalence charge transfer (IVCT) characteristics of both diastereoisomeric forms of the dinuclear complex [{Ru(bpy)2}2(mu-dpi-)]n+ [bpy=2,2'-bipyridine; dpi-=4,5-di(2-pyridyl)imidazolate] showed that the degree of inter-metal electronic coupling (or valence delocalization) is dependent on stereochemical identity. Increasing the relative concentration of the strongly associating anion toluene-4-sulfonate in acetonitrile/[(n-C4H9)4N]{B(C6F5)4} solution differentially decreased the level of delocalization for the two diastereoisomers. In a comparative investigation of electrochemical and spectroelectrochemical techniques of the anion-induced electron localization in [{Ru(bpy)2}2(mu-dpo)]5+ [dpo=3,4-di(2-pyridyl)-1,2,5-oxadiazole], differences were observed between the two methods in the order and extent of effects induced by a number of inorganic anions (PF6-, BF4-, ClO4-). It was determined that the measure of coupling derived from electrochemical methods was less reliable than that obtained from spectral methods. Comparative electrochemical studies were undertaken on [{M(bpy)2}2(mu-BL)]n+ {M=Ru, Os; BL=dpo, dpi-), which revealed substantial differences in DeltaEox (the separation between the redox potentials for the MII-MII/MIII-MII and MII-MIII/MIII-MIII couples) for the two metal centers and therefore the comproportionation constant Kc, dependent on the neutral or anionic nature of the bridging ligand.     

Variable coordination mode of chloranilic acid. Synthesis, structure, and electrochemical properties of some osmium complexes

Reaction of chloranilic acid (H2ca) with [Os(bpy)2 Br2] (bpy = 2,2'-bipyridine) affords a dinuclear complex of type [{Os(bpy)2}2 (ca)]2+, isolated as the perchlorate salt. A similar reaction of H2ca with [Os(PPh3)2 (pap)Br2] (pap = 2-(phenylazo)pyridine) affords a dinuclear complex of type [{Os(PPh3)2 (pap)}2 (ca)]2+ (isolated as the perchlorate salt) and a mononuclear complex of type [Os(PPh3)2 (pap)(ca)]. Reaction of H2ca with [Os(PPh3)2(CO)2(HCOO)2] gives a dinuclear complex of type [{Os(PPh3)2(CO)2}2 (r-ca)], where r-ca is the two electron reduced form of the chloranilate ligand. The structures of the [{Os(PPh3)2 (pap)}2 (ca)](ClO4)2, [Os(PPh3)2 (pap)(ca)], and [{Os(PPh3)2(CO)2}2 (r-ca)] complexes have been determined by X-ray crystallography. In the [{Os(bpy)2}2 (ca)]2+ and [{Os(PPh3)2 (pap)}2 (ca)]2+ complexes, the chloranilate dianion is serving as a tetradentate bridging ligand. In the [Os(PPh3)2 (pap)(ca)] complex, the chloranilate dianion is serving as a bidentate chelating ligand. In the [{Os(PPh3)2(CO)2}2 (r-ca)] complex, the reduced form of the chloranilate ligand (r-ca(4-)) is serving as a tetradentate bridging ligand. All the four complexes are diamagnetic and show intense metal-to-ligand charge-transfer transitions in the visible region. The [Os(PPh3)2 (pap)(ca)] complex shows an Os(II)-Os(III) oxidation, followed by an Os(III)-Os(IV) oxidation on the positive side of a standard calomel electrode. The three dinuclear complexes show two successive oxidations on the positive side of SCE. The mixed-valent Os(II)-Os(III) species have been generated in the case of the two chloranilate-bridged complexes by coulometric oxidation of the homovalent Os(II)-Os(II) species. The mixed-valent Os(II)-Os(III) species show intense intervalence charge-transfer transitions in the near-IR region.     

Thiolato-bridged RuIIAgIRuII trinuclear complex composed of bis(bipyridine)ruthenium(II) units with chelating 2-aminoethanethiolate: conversion to a disulfide-bridged RuIIRuII dinuclear complex

The reaction of [Ru(solvent)2(bpy)2]2+ (bpy = 2,2'-bipyridine) with Haet (2-aminoethanethiol) in ethanol/water in the presence of Ag+ gave a thiolato-bridged RuIIAgIRuII trinuclear complex, [Ag{Ru(aet)(bpy)2}2]3+, in which two [RuII(aet)(bpy)2]+ units are linked by an AgI atom. When this complex was treated with HCl in acetonitrile/water, a disulfide-bridged RuIIRuII dinuclear complex, [Ru2(cysta)(bpy)4]4+ (cysta = cystamine), was produced as a result of the removal of an AgI atom and the autoxidation of thiolato groups. It was found that the dinuclear structure in [Ru2(cysta)(bpy)4]4+ is reverted back to [Ag{Ru(aet)(bpy)2}2]3+ by treatment with Ag+ assisted by Zn reduction.     

Mononuclear and dinuclear complexes of isoeilatin

This work describes the synthesis and characterization of mononuclear and dinuclear Ru(II) and Os(II) complexes based on the symmetrical bridging ligand isoeilatin (1). The crystal structure of 1.[HCl]2 consists of layers of tightly pi-stacked molecules of the biprotonated isoeilatin. The mononuclear complexes [Ru(bpy)2(ieil)]2+ (2(2+)) and [Os(bpy)2(ieil)]2+ (3(2+)) form discrete dimers in solution held together by face-selective pi-stacking interactions via the isoeilatin ligand. Coordination of a second metal fragment does not hinder the pi-stacking completely, as demonstrated by the concentration dependence of the 1H NMR spectra of the dinuclear complexes [{Ru(bpy)2}2{mu-ieil}]4+ (4(4+)), [{Os(bpy)2}2{mu-ieil}]4+ (5(4+)), and [{Ru(bpy)2}{mu-ieil}{Os(bpy)2}]4+ (6(4+)) and supported by the solid-state structure of meso-4.[Cl]4. The bridging isoeilatin ligand conserves its planarity even upon coordination of a second metal fragment, as demonstrated in the solid-state structures of meso-4.[Cl]4, meso-4.[PF6]4, and meso-5.[PF6]4. All of the dinuclear complexes exhibit a preference (3/2-3/1) for the formation of the heterochiral as opposed to the homochiral diastereoisomer. Absorption spectra of the mononuclear complexes feature a low-lying dpi(M) --> pi*iel MLCT band around 600 nm that shifts to beyond 700 nm upon coordination of a second metal fragment. Cyclic and square-wave voltammetry measurements of the complexes exhibit two isoeilatin-based reduction waves that are substantially anodically shifted compared to [M(bpy)3]2+ (M = Ru, Os). Luminescence spectra, quantum yields, and lifetime measurements at room temperature and at 77 K demonstrate that the complexes exhibit 3MLCT emission that occurs in the IR region between 950 and 1300 nm. Both the electrochemical and photophysical data are consistent with the low-lying pi orbital of the isoeilatin ligand. The dinuclear complexes exhibit two reversible, well-resolved, metal-centered oxidation waves, despite the chemical equivalence of the two metal centers, indicating a significant metal-metal interaction mediated by the bridging isoeilatin ligand.     

Underlying spin-orbit coupling structure of intervalence charge transfer bands in dinuclear polypyridyl complexes of ruthenium and osmium

The mixed-valence systems meso- and rac-[{M(bpy)2}2(mu-BL)]5+ {M = Ru, Os; BL = a series of polypyridyl bridging ligands such as 2,3-bis(2-pyridyl)benzoquinoxaline (dpb)} are characterized by multiple intervalence charge transfer (IVCT) and interconfigurational (IC) bands in the mid-infrared and near-infrared (NIR) regions. Differences in the relative energies of the IC transitions for the fully oxidized (+6) states of the osmium systems demonstrate that stereochemical effects lead to fundamental changes in the energy levels of the metal-based dpi orbitals, which are split by spin-orbit coupling and ligand-field asymmetry. An increase in the separation between the IC bands as BL is varied reflects the increase in the degree of electronic coupling through the series of ruthenium and osmium complexes. The increase in the IVCT bandwidths for the former is therefore attributed to the increase in the separation of the three underlying components of the bands. Stark effect measurements reveal small dipole moment changes accompanying IVCT excitation in support of the localized-to-delocalized or delocalized classification for the dinuclear ruthenium and osmium systems.     

Probing mixed valence in a new tppz-bridged diruthenium(III,II) complex {(mu-tppz)[Ru(bik)Cl]2}3+ (tppz=2,3,5,6-tetrakis(2-pyridyl)pyrazine, bik=2,2'-bis(1-methylimidazolyl)ketone): EPR silence, intervalence absorption, and nu(CO) line broadening

The complex dication of the diruthenium(II) compound {(mu-tppz)[Ru(bik)Cl]2}(ClO4)2 can be oxidized and reduced in two one-electron steps each. In CH3CN/0.1 M Bu4NPF6, the odd-electron intermediates{(mu-tppz)[Ru(bik)Cl]2}n+, n=1 and 3, have comproportionation constants of 7x10(8) and 1x10(5), respectively. Both exhibit near-infrared absorptions, in the case of n=3 the 1640 nm band (epsilon=1200 M-1 cm-1, Deltanu1/2=1560 cm-1) is attributed to an intervalence charge-transfer transition. While the mixed-valent intermediate (n=3) is EPR silent even at 4 K, the n=1 form shows g(parallel) 2.005 and g( perpendicular) 1.994 at that temperature, signifying a diruthenium(II) complex of the tppz*- radical anion. The variation of energy and intensity of nuCO and of the ring vibration band around 1590 cm-1 has been monitored not only for {(mu-tppz)[Ru(bik)Cl]2}n+, n=0-4, but also for the mononuclear {(tppz)Ru(bik)Cl}n+, n=0-2. In the dinuclear complex the carbonyl stretching bands of the spectator ligand bik are shifted by about 15 cm-1 on each one-electron-transfer step, increasing with the positive charge. The mixed-valent {(mu-tppz)[Ru(bik)Cl]2}3+ shows a perceptibly broader nuCO band, suggesting incomplete valence averaging (partial localization).     

meso-[{Ru(phen)2}2(mu-HAT)]4+: a high-affinity DNA hairpin probe {HAT = 1,4,5,8,9,12-hexaazatriphenylene; phen = 1,10-phenanthroline}

1H NMR spectroscopy and fluorescent intercalator displacement (FID) assays have been used to investigate the DNA-binding abilities of two series of dinuclear polypyridyl ruthenium(II) complexes of the form [{Ru(L)2}2(mu-BL)]4+ {L = 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (Me2bpy), 1,10-phenanthroline (phen), or 4,7-dimethyl-1,10-phenanthroline (Me2phen); BL = 2,2'-bipyrimidine (bpm) or 1,4,5,8,9,12-hexaazatriphenylene (HAT)}. Preliminary FID surveys of these metal complexes against a variety of different oligonucleotides revealed that those complexes based upon the HAT bridging ligand induced greater fluorescence decreases in dye-bound DNA than did their bpm-bridged counterparts, suggesting a higher binding affinity by the HAT-bridged species. Furthermore, the greatest fluorescence decreases were typically observed in an oligonucleotide featuring a six-base hairpin loop. The apparent binding affinity of the metal complexes was also found to be a function of the stereochemistry and identity of the terminal ligands of the complex. The meso (DeltaLambda) stereoisomer generally induced greater fluorescence decreases than did either enantiomer (DeltaDelta or LambdaLambda), phen-based terminal ligands performed better than bpy-based terminal ligands, and those terminal ligands with methyl substituents demonstrated stronger apparent binding than did their non-methylated analogues. NMR experiments on meso-[{Ru(phen)2}2(mu-HAT)]4+ and meso-[{Ru(Me2phen)2}2(mu-HAT)]4+ demonstrated that both complexes bound with high affinity to the six-base hairpin oligonucleotide at the stem-loop interface and provided evidence to support stronger binding by the methylated species. meso-[{Ru(phen)2}2(mu-HAT)]4+ was found to bind poorly to duplex DNA and smaller four-base hairpin loops in FID and NMR experiments, whereas FID data suggest that the methylated analogue binds relatively strongly to most oligonucleotide sequences (the four- and six-base hairpins in particular). These results demonstrate that binding affinity can come at the expense of selectivity, with meso-[{Ru(phen)2}2(mu-HAT)]4+ proving to be an efficient compromise between the two as a high-affinity DNA hairpin probe.     

Synthesis and characterization of dinuclear ruthenium complexes covalently linked to Ru(II) tris-bipyridine: an approach to mimics of the donor side of photosystem II

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

Synthesis, structure, spectroscopic properties, and photochemistry of homo- and heteropolynuclear copper(I) complexes bridged by the 2,5-bis(2-pyridyl)pyrazine ligand

Cu(I)-Cu(I) and Cu(I)-Ru(II) dinuclear complexes bridged by the 2,5-bppz (2,5-bis(2-pyridyl)pyrazine) ligand have been prepared and characterized including the X-ray crystallographic study of the dinuclear [{CuI(PPh3)2}2(mu-2,5-bppz)](PF6)2)2CH3Cl complex: a = 13.974(2), b = 13.993(2), c = 13.537(2) A; alpha = 101.98(1), beta = 103.22(1), gamma = 113.90(1) degrees ; triclinic, P, Z = 1. The trinuclear [{(bpy)2RuII(mu-2,5-bppz)}2CuI](PF6)5 complex was also prepared, and the structure of the complex in solution was studied by spectrometric titration. The dinuclear Cu(I) complex and [(bpy)2RuII(mu-2,5-bppz)CuI(PPh3)2](PF6)5 show photoluminescence in the solid state, which should arise from MLCT states. Photochemical oxidation of the trinuclear RuII2CuI complex occurs in the presence of oxygen to give a RuII2CuII complex. The MLCT states and the redox reaction in the excited state are discussed.     

Synthesis, nucleic acid binding and cytotoxicity of oligonuclear ruthenium complexes containing labile ligands

We report the synthesis, nucleic acid binding and cytotoxicity of the complexes [Ru(terpy)(Me₂bpy)Cl]⁺, [Ru(terpy)(phen)Cl]⁺ and dinuclear [{Ru(terpy)Cl}₂(??-bb_n)]²⁺ {where Me₂bpy = 4,4??-dimethyl-2,2??-bipyridine; phen = 1,10-phenanthroline; and bb_n = bis[4(4??-methyl-2,2??-bipyridyl)]-1,n-alkane, with n = 7, 10, 12, 14}. The complexes were isolated from the reaction of the [Ru(terpy)Cl₃] precursor with the respective bidentate and di-bidentate bridging ligands. The time-course UV?CVisible spectroscopy of the reaction of the mono- and dinuclear complexes with guanosine 5-monophosphate (GMP) showed the movement of the metal-to-ligand charge transfer (MLCT) band to lower wavelengths, accompanied by a hypochromism effect. The formation of the aqua complex and phosphate-bound intermediates in the reaction were detected by the time-course ¹H NMR and ³¹P NMR experiments, which also demonstrated that the complex bound to the N7 guanine was the major product. The UV?CVisible and ¹H NMR studies showed no evidence of the interaction of the complexes with both adenosine 5-monophosphate (AMP) and cytidine 5-monophosphate (CMP). Cytotoxicity studies of these complexes against a murine leukemia L1210 cell line revealed that the dinuclear [{Ru(terpy)Cl}₂(??-bb_n)]²⁺ complexes were significantly more cytotoxic than mononuclear [Ru(terpy)(Me₂bpy)Cl]⁺. The [{Ru(terpy)Cl}₂(??-bb₁₄)]²⁺ complex appeared to be the most active (IC₅₀ = 4.2 ??M).     

Oligonuclear polypyridylruthenium(II) complexes incorporating flexible polar and non-polar bridges: synthesis, DNA-binding and cytotoxicity

The paper reports the synthesis and characterisation of a series of flexible di-bidentate bridging ligands in which two 4-methyl-2,2'-bipyridine groups are linked at the 4'-position by polymethylene (bb(n)), linear polyether (bbO(n)) or linear alkylamine (bbN(n)) chains of varying length (n). The enantiomers (ΔΔ/ΛΛ) of the rac forms of the ruthenium(ii) dinuclear complexes incorporating these ligands -i.e. [{Ru(phen)(2)}(2)(μ-BL)](4+) (phen = 1,10-phenanthroline; BL = bb(n), bbO(n) or bbN(n)) - have been isolated by reaction of Δ- or Λ-[Ru(phen)(2)(py)(2)](2+) (py = pyridine) with the respective bridging ligands. Mononuclear species - in which only one of the bidentate moieties of the bridging ligand is coordinated - have also been isolated, as well as trinuclear and tetranuclear species involving the bb(7) bridge. Fluorescence displacement studies of the DNA-binding of the dinuclear complexes containing the bbO(n) and bbN(n) bridges generally revealed a lower affinity than their bb(n) analogues for an oligonucleotide containing a single bulge site; the mononuclear complexes showed a lower affinity - and the trinuclear and tetranuclear complexes a higher affinity - than the dinuclear species, revealing an interesting interplay of lipophilicity, electrostatics and size in the complex/nucleic acid interaction. Cytotoxicity studies of these complexes against a murine leukaemia cell line revealed that the presence of the polyether or polyamine links in the chain lowered the cytotoxicity compared with their polymethylene analogues, and that the bb(7)-bridged trinuclear and tetranuclear complexes showed considerably enhanced cytotoxicity compared with the dinuclear Rubb(7) analogue.     

Rational Control of Conformational Distributions and Mixed‐Valence Characteristics in Diruthenium Complexes

The electronic characteristics of mixed‐valence complexes are often inferred from the shape of the inter‐valence charge transfer (IVCT) band, which usually falls in the near infrared (NIR) region, and relationships derived from Marcus‐Hush theory. These analyses typically assume one single, dominant molecular conformation. The NIR spectra of the prototypical delocalised (Class III Robin–Day mixed‐valence) complexes [{Ru(pp)Cp’}₂(μ‐C≡C?C≡C)]⁺ ([ 1 ]⁺: Cp’=Cp, pp=(PPh₃)₂; [ 2 ]⁺: Cp’=Cp, pp=dppe; [ 3 ]⁺: Cp’=Cp*, pp=dppe) feature a ‘two‐band’ pattern, which complicates band‐shape analysis using these traditional methods. In the past, the appearance of sub‐bands within or near the IVCT transition has been attributed to vibronic effects or localised d‐d transitions. Quantum‐chemical modelling of a series of rotational conformers of [ 1 ]⁺–[ 3 ]⁺ reveals the two components that contribute to the NIR absorption band envelope to be a π‐π* transition and an MLCT transition. The MLCT components only gain appreciable intensity when the orientation of the half‐sandwich ruthenium ligand spheres deviates from idealised cis (Ω P?Ru?Ru?P=0°) or trans (Ω P?Ru?Ru?P=180°) conformations. The increased steric demand of the supporting ligands, together with some underlying inter‐phosphine ligand T‐shaped CH???π stacking interactions across the series [ 1 ]⁺ to [ 2 ]⁺ to [ 3 ]⁺ results in local minima biased towards such non‐idealised conformations of the metal‐ligand fragments (Ω P?Ru?Ru?P=33–153°). Experimentally, this is indicated by appearance of multiple bands within the IR (C≡C) band envelopes and increasing intensity of the higher‐energy MLCT transition(s) relative to the π‐π* transition across the series, and the appearance of a pronounced ‘two‐band’ pattern in the experimental NIR absorption envelopes. These conformational effects and the methods of analysis presented here, which combine analysis of IR and NIR spectra with quantum‐chemical calculations on a range of energetically similar conformational minima, are expected to be quite general for mixed‐valence systems.     

Probing the transition between the localised (class II) and localised-to-delocalised (class II-III) regimes by using intervalence charge-transfer solvatochromism in a series of mixed-valence dinuclear ruthenium complexes

Intervalence charge-transfer (IVCT) solvatochromism studies on the diastereoisomeric forms of [{Ru(bpy)(2)}(2)(mu-BL)](5+) (bpy=2,2'-bipyridine; BL=a series of di-bidentate polypyridyl bridging ligands) reveal that the solvent dependencies of the IVCT transitions decrease as the "tail" of the bridging ligand is extended, and the extent of delocalisation increases. Utilising a classical theoretical approach for the analysis of the intervalence charge-transfer (IVCT) solvatochromism data, the subtle and systematic variation in the electronic properties of the bridging ligands can be correlated with the shift between the localised (class II) and localised-to-delocalised (class II-III) regimes. The investigation of the diastereoisomeric forms of two series of complexes incorporating analogous structurally rigid (fused) and nonrigid (unfused) bridging ligands demonstrates that the differences in the IVCT characteristics of the diastereoisomers of a given complex are accentuated in the latter case, due to a stereochemically induced redox asymmetry contribution. The marked dependence of the IVCT transitions on the stereochemical identity of the complexes provides a quantitative measure of the fundamental contributions of the reorganisational energy and redox asymmetry to the intramolecular electron-transfer barrier at the molecular level.     

Comparison of Physical and Photophysical Properties of Monometallic and Bimetallic Ruthenium(II) Complexes Containing Structurally Altered Diimine Ligands

The physical and photophysical properties of a series of monometallic, [Ru(bpy)(2)(dmb)](2+), [Ru(bpy)(2)(BPY)](2+), [Ru(bpy)(Obpy)](2+) and [Ru(bpy)(2)(Obpy)](2+), and bimetallic, [{Ru(bpy)(2)}(2)(BPY)](4+) and [{Ru(bpy)(2)}(2)(Obpy)](4+), complexes are examined, where bpy is 2,2'-bipyridine, dmb is 4,4'-dimethyl-2,2'-bipyridine, BPY is 1,2-bis(4-methyl-2,2'-bipyridin-4'-yl)ethane, and Obpy is 1,2-bis(2,2'-bipyridin-6-yl)ethane. The complexes display metal-to-ligand charge transfer transitions in the 450 nm region, intraligand pi --> pi transitions at energies greater than 300 nm, a reversible oxidation of the ruthenium(II) center in the 1.25-1.40 V vs SSCE region, a series of three reductions associated with each coordinated ligand commencing at -1.3 V and ending at approximately -1.9 V, and emission from a (3)MLCT state having energy maxima between 598 and 610 nm. The Ru(III)/Ru(II) oxidation of the two bimetallic complexes is a single, two one-electron process. Relative to [Ru(bpy)(2)(BPY)](2+), the Ru(III)/Ru(II) potential for [Ru(bpy)(2)(Obpy)](2+) increases from 1.24 to 1.35 V, the room temperature emission lifetime decreases from 740 to 3 ns, and the emission quantum yield decreases from 0.078 to 0.000 23. Similarly, relative to [{Ru(bpy)(2)}(2)(BPY)](4+), the Ru(III)/Ru(II) potential for [{Ru(bpy)(2)}(2)(Obpy)](4+) increases from 1.28 to 1.32 V, the room temperature emission lifetime decreases from 770 to 3 ns, and the room temperature emission quantum yield decreases from 0.079 to 0.000 26. Emission lifetimes measured in 4:1 ethanol:methanol were temperature dependent over 90-360 K. In the fluid environment, emission lifetimes display a biexponential energy dependence ranging from 100 to 241 cm(-)(1) for the first energy of activation and 2300-4300 cm(-)(1) for the second one. The smaller energy is attributed to changes in the local matrix of the chromophores and the larger energy of activation to population of a higher energy dd state. Explanations for the variations in physical properties are based on molecular mechanics calculations which reveal that the Ru-N bond distance increases from 2.05 ? (from Ru(II) to bpy and BPY) to 2.08 ? (from Ru(II) to Obpy) and that the metal-to-metal distance increases from approximately 7.5 ? for [{Ru(bpy)(2)}(2)(Obpy)](4+) to approximately 14 ? for [{Ru(bpy)(2)}(2)(BPY)](4+).     

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.     

Mixed-valence electron delocalized thiocyanato bridged ruthenium dinuclear complex [{Ru(NH3)5}2SCN]4+

The thiocyanato bridged mixed-valence ruthenium dinuclear species [{Ru(NH₃)₅}2SCN]⁴⁺ has been prepared and characterized. A solvent independent, low intensity intervalence transfer band was observed in the near IR absorption spectrum suggesting a delocalized limit in the [Ru(II)-SCN-Ru(III)] unit.     

A new heptanuclear dendritic ruthenium(II) complex featuring photoinduced energy transfer across high-energy subunits.

The new heptanuclear ruthenium(II) dendron, [Cl(2)Ru{(micro-2,3dpp)Ru[(micro-2,3-dpp)Ru(bpy)2]2}2](PF6)12 (1; 2,3-dpp=2,3-bis(2'-pyridyl)pyrazine; bpy = 2,2'-bipyridine), was prepared by means of the "complexes as ligands/complexes as metals" synthetic strategy, and its absorption spectrum, redox behavior, and luminescence properties were investigated. Compound 1 is a multicomponent species, which contains three different types of chromophores (namely, the {Cl(2)Ru(micro-2,3-dpp)2} core, the {Ru(micro-2,3dpp)3}2+ intermediate, and the {(bpy)2Ru(micro-2,3-dpp)}2+ peripheral subunits) and several redox-active sites. The new species exhibits very intense absorption bands in the UV region (epsilon value in the 10(5)-10(6) M(-1) cm(-1) range) as a result of spin-allowed ligand-centered (LC) transitions, and intense bands in the visible region (epsilon value in the 10(4)-10(5) M(-1) cm(-1) range) as a result of the various spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. The redox investigation (accomplished by cyclic and differential pulse voltammetry) indicates that 1 undergoes a series of reversible metal-centered oxidation and ligand-centered reduction processes within the potential window investigated (+1.90 / -1.40 V vs. the standard calomel electrode, SCE). The assignment of each absorption bond and redox process to specific subunits of 1 was achieved by comparison with the properties of smaller multinuclear species of the same family, namely [Cl(2)Ru{(micro-2,3-dpp)Ru(bpy)2}2]4+ (2), [(bpy)2Ru(u-2,3-dpp)Ru(bpy)2]4+ (4), and [Ru{(micro-2,3-dpp)Ru(bpy)2}3]4+ (5). The title compound exhibits luminescence both at room temperature in acetonitrile fluid solution and at 77 K in butyronitrile rigid matrix. The emission is attributed to the triplet MLCT (3MLCT) state involving the core {Cl(2)Ru(micro-2,3-dpp)2} subunit. Interestingly, the 3MLCT levels involving the peripheral {(bpy)2Ru(micro-2,3-dpp)}2+ subunits are deactivated by energy transfer to the emitting level, in spite of the presence of interposed high-energy (Ru(micro-2,3-dpp)3}2+ components, which, in other dendrimers, acted as "isolating" subunits toward energy-transfer processes. Ultrafast experiments on 1 and on the parent species 2 and 5 allowed us to rationalize this behavior and highlight that a sequential two-step electron-transfer process can be held responsible for the efficient overall energy transfer, which offers a way to overcome a limitation in antenna metal dendrimers.