Coupling of metal-based light-harvesting antennas and electron-donor subunits: trinuclear ruthenium(II) complexes containing tetrathiafulvalene-substituted polypyridine ligands

Three new tetrathiafulvalene-substituted 2,2'-bipyridine ligands, cis-bpy-TTF(1), trans-bpy-TTF(1), and cis-bpy-TTF(2) have been prepared and characterized. X-ray analysis of trans-bpy-TTF(1) is also reported. Such ligands have been used to prepare two new trinuclear Ru(II) complexes, namely, [[(bpy)(2)Ru(micro-2,3-dpp)](2)Ru(bpy-TTF(1))](PF(6))(6) (9; bpy=2,2'-bipyridine; 2,3-dpp=2,3-bis(2'-pyridyl)pyrazine) and [[(bpy)(2)Ru(micro-2,3-dpp)](2)Ru(bpy-TTF(2))](PF(6))(6) (10). These compounds can be viewed as coupled antennas and charge-separation systems, in which the multichromophoric trinuclear metal subunits act as light-harvesting antennas and the tetrathiafulvalene electron donors can induce charge separation. The absorption spectra, redox behavior, and luminescence properties (both at room temperature in acetonitrile and at 77 K in a rigid matrix of butyronitrile) of the trinuclear metal complexes have been studied. For the sake of completeness, the mononuclear compounds [(bpy)(2)Ru(bpy-TTF(1))](PF(6))(2) (7) and [(bpy)(2)Ru(bpy-TTF(2))](PF(6))(2) (8) were also synthesized and studied. The properties of the tetrathiafulvalene-containing species were compared to those of the model compounds [Ru(bpy)(2)(4,4'-Mebpy)](2+) (4,4'-Mebpy=4,4'-dimethyl-2,2'-bipyridine) and [[(bpy)(2)Ru(micro-2,3-dpp)](2)Ru(bpy)](6+). The absorption spectra and redox behavior of all the new metal compounds can be interpreted by a multicomponent approach, in which specific absorption features and redox processes can be assigned to specific subunits of the structures. The luminescence properties of the complexes in rigid matrices at 77 K are very similar to those of the corresponding model compounds without TTF moieties, whereas the new species are nonluminescent, or exhibit very weak emissions relative to those of the model compounds in fluid solution at room temperature. Time-resolved transient absorption spectroscopy confirmed that the potentially luminescent MLCT states of 7-10 are significantly shorter lived than the corresponding states of the model species. Photoinduced electron-transfer processes from the TTF moieties to the (excited) MLCT chromophore(s) are held responsible for the quenching processes.     

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.     

Eilatin complexes of ruthenium and osmium. synthesis, electrochemical behavior, and near-IR luminescence

The synthesis and characterization of new Ru(II) and Os(II) complexes of the ligand eilatin (1) are described. The new complexes [Ru(bpy)(eil)(2)](2+) (2), [Ru(eil)(3)](2+) (3), and [Os(eil)(3)](2+) (4) (bpy = 2,2'-bipyridine; eil = eilatin) were synthesized and characterized by NMR, fast atom bombardment mass spectrometry, and elemental analysis. In the series of complexes [Ru(bpy)(x)(eil)(y)()](2+) (x + y = 3), the effect of sequential substitution of eil for bpy on the electrochemical and photophysical properties was examined. The absorption spectra of the complexes exhibit several bpy- and eil-associated pi-pi and metal-to-ligand charge-transfer (MLCT) transitions in the visible region (400-600 nm), whose energy and relative intensity depend on the number of ligands bound to the metal center (x and y). On going from [Ru(bpy)(2)(eil)](2+) (5) to 2 to 3, the d(pi)(Ru) --> pi(eil) MLCT transition undergoes a red shift from 583 to 591 to 599 nm, respectively. Electrochemical measurements performed in dimethyl sulfoxide reveal several ligand-based reduction processes, where each eil ligand can accept up to two electrons at potentials that are significantly anodically shifted (by ca. 1 V) with respect to the bpy ligands. The complexes exhibit near-IR emission (900-1100 nm) of typical (3)MLCT character, both at room temperature and at 77 K. Along the series 5, 2, and 3, upon substitution of eil for bpy, the emission maxima undergo a blue shift and the quantum yields and lifetimes increase. The radiative and nonradiative processes that contribute to deactivation of the excited level are discussed in detail.     

Fused donor-acceptor ligands in RuII chemistry: synthesis, electrochemistry and spectroscopy of [Ru(bpy)3-n(TTF-dppz)n](PF6)2.

Three ruthenium(II) polypyridine complexes of general formula [Ru(bpy)(3-n)(TTF-dppz)n](PF6)2 (n=1-3, bpy=2,2'-bipyridine), with one, two or three redox-active TTF-dppz (4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine) ligands, were synthesised and fully characterised. Their electrochemical and photophysical properties are reported together with those of the reference compounds [Ru(bpy)3](PF6)2, [Ru(dppz)3](PF6)2 and [Ru(bpy)2(dppz)](PF6)2 and the free TTF-dppz ligand. All three complexes show intraligand charge-transfer (ILCT) fluorescence of the TTF-dppz ligand. Remarkably, the complex with n=1 exhibits luminescence from the Ru(2+)-->dppz metal-to-ligand charge-transfer ((3)MLCT) state, whereas for the other two complexes, a radiationless pathway via electron transfer from a second TTF-dppz ligand quenches the (3)MLCT luminescence. The TTF fragments as electron donors thus induce a ligand-to-ligand charge-separated (LLCS) state of the form TTF-dppz- -Ru(2+)-dppz-TTF(+). The lifetime of this LLCS state is approximately 2.3 micros, which is four orders of magnitude longer than that of 0.4 ns for the ILCT state, because recombination of charges on two different ligands is substantially slower.     

Ruthenium(II) coordination chemistry of a fused donor-acceptor ligand: synthesis, characterization, and photoinduced electron-transfer reactions of [{Ru(bpy)2}(n)(TTF-ppb)](PF6)(2n) (n = 1, 2)

A pi-extended, redox-active bridging ligand 4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[2,3-a:3',2'-c]phenazine (L) was prepared via direct Schiff-base condensation of the corresponding diamine-tetrathiafulvalene (TTF) precursor with 4,7-phenanthroline-5,6-dione. Reactions of L with [Ru(bpy)(2)Cl(2)] afforded its stable mono- and dinuclear ruthenium(II) complexes 1 and 2. They have been fully characterized, and their photophysical and electrochemical properties are reported together with those of [Ru(bpy)(2)(ppb)](2+) and [Ru(bpy)(2)(mu-ppb)Ru(bpy)(2)](4+) (ppb = dipyrido[2,3-a:3',2'-c]phenazine) for comparison. In all cases, the first excited state corresponds to an intramolecular TTF --> ppb charge-transfer state. Both ruthenium(II) complexes show two strong and well-separated metal-to-ligand charge-transfer (MLCT) absorption bands, whereas the (3)MLCT luminescence is strongly quenched via electron transfer from the TTF subunit. Clearly, the transient absorption spectra illustrate the role of the TTF fragment as an electron donor, which induces a triplet intraligand charge-transfer state ((3)ILCT) with lifetimes of approximately 200 and 50 ns for mono- and dinuclear ruthenium(II) complexes, respectively.     

DNA mismatch detection by resonance energy transfer between ruthenium(II) and osmium(II) tris(2,2'-bipyridyl) chromophores

Octahedral tris-chelate complexes [M(II)(bpy)(3)](2+) (M = Ru or Os, bpy = 2,2'-bipyridyl), covalently attached to the 3'- and 5'-phosphates of two oligonucleotides, are juxtaposed when hybridized contiguously to a fully complementary DNA target. Visible metal-to-ligand charge-transfer (MLCT) excitation of the [Ru(II)(bpy)(3)](2+) unit leads to resonance energy transfer to the MLCT state of the [Os(II)(bpy)(3)](2+) moiety, with the energy transfer efficiency depending on the degree of hybridization. The extent of attenuation of the intense red luminescence from the Ru(II) chromophore hence allows highly sensitive structural probing of the assembly and constitutes a novel approach to DNA sensing which is capable of detecting mutations.     

Luminescent ruthenium(II) polypyridine biotin complexes: synthesis, characterization, photophysical and electrochemical properties, and avidin-binding studies

Two luminescent ruthenium(II) polypyridine complexes containing a biotin moiety [Ru(bpy)(2)(L1)](PF(6))(2) (1) and [Ru(bpy)(2)(L2)](PF(6))(2) (2) (bpy = 2,2'-bipyridine; L1 = 4-(N-((2-biotinamido)ethyl)amido)-4'-methyl-2,2'-bipyridine; L2 = 4-(N-((6-biotinamido)hexyl)amido)-4'-methyl-2,2'-bipyridine) have been synthesized and characterized, and their photophysical and electrochemical properties have been studied. Upon photoexcitation, complexes 1 and 2 display intense and long-lived triplet metal-to-ligand charge-transfer ((3)MLCT) (dpi(Ru) --> pi*(L1 or L2)) emission in fluid solutions at 298 K and in low-temperature glass. We have studied the binding of these ruthenium(II) biotin complexes to avidin by 4'-hydroxyazobenzene-2-carboxylic acid (HABA) assays, luminescence titrations, competitive assays using native biotin, and quenching experiments using methyl viologen. On the basis of the results of these experiments, a homogeneous competitive assay for biotin has been investigated.     

Synthesis,characterization, and photophysical studies of new bichromophoric ruthenium(II) complexes

The photophysical properties of a series of prepared ruthenium tris(bipyridine) complexes, covalently linked to aromatic species, of type [Ru(bpy)(2)-(4-methyl-4'-(arylaminocarbonyl)-2,2'-bipyridine)](2+) ([Ru(bpy)(2)(mbpy-L)](2+), where bpy = 2,2'-bipyridine; mbpy = 4-methyl-4'-carbonyl-2,2'-bipyridine; and L = 2-aminonaphthyl (naph), 9-aminoanthryl (anth), 1-aminopyrenyl (pyr), or 9-aminoacridinyl (acrd)) were studied by electronic absorption spectroscopy and steady state and time resolved luminescence spectroscopies. The absorption spectra of the MLCT electronic transition of the complexes are similar, which is in agreement with a practically constant redox potential of Ru(III/II) close to 1.28 V versus Ag/AgCl. However, the luminescence spectra of the new complexes are red shifted compared to Ru(bpy)(3)(2+), and this effect is ascribed to solvation and inductive effects of the amide group which enhance the symmetry breakdown among the three bipyridyl ligands. The energy stabilization of the (3)MLCT state is in the range 2.1-8.4 kJ/mol. The triplet-triplet energy transfer between the Ru complex and the aromatic species linked by an amide spacer is a slow process with rate constants of 2.6 x 10(4), 3.6 x 10(4), and 4.9 x 10(4) s(-)(1) for anthracene, acridine, and pyrene as acceptors in methanol, respectively. The energy transfer rate constant increases with decreasing polarity of the solvent. In dichloromethane, the rate constants for anthracene, acridine, and pyrene acceptors are 2.6 x 10(5), 1.5 x 10(5), and 2.9 x 10(5) s(-)(1), respectively. The low efficiency of energy transfer is due to the small difference in triplet energy between donor and acceptor species, weak electronic coupling, and unfavorable Franck-Condon factors, despite the short separation distance between donor and acceptor species in an amide bridge.     

Changing the role of 2,2'-bipyridine from secondary ligand to protagonist in [Ru(bpy)2(N-N)]2+ complexes: low-energy, red emission from a ruthenium(II)-to-2,2'-bipyridine 3MLCT state

Two new bidentate ligands (1 and 2) with bicyclic guanidine moieties were synthesized and attached to a Ru(II)(bpy)(2) core (bpy = 2,2'-bipyridine) to afford complexes 3 and 4, which were characterized by spectroscopic and electrochemical methods. Complex 4 was further characterized by X-ray crystallography. In cyclic voltammetric studies, both complexes show a Ru(II/III) couple, which is 500 mV less positive than the Ru(II/III) couple of Ru(bpy)(3)(2+). The (1)MLCT and (3)MLCT states of 3 (560 nm/745 nm) and 4 (550 nm/740 nm) are significantly red-shifted with respect to Ru(bpy)(3)(2+) (440 nm/620 nm). Compounds 3 and 4 exhibit emission from a Ru(II)-to-bpy (3)MLCT state, which is rarely the emitting state at λ > 700 nm in [Ru(bpy)(2)(N-N)](2+) complexes.     

Characteristics and properties of metal-to-ligand charge-transfer excited states in 2,3-bis(2-pyridyl)pyrazine and 2,2'-bypyridine ruthenium complexes. Perturbation-theory-based correlations of optical absorption and emission parameters with electrochemistry and thermal kinetics and related Ab initio calculations

The absorption, emission, and infrared spectra, metal (Ru) and ligand (PP) half-wave potentials, and ab initio calculations on the ligands (PP) are compared for several [L(n)()Ru(PP)](2+) and [[L(n)Ru]dpp[RuL'(n)]](4+) complexes, where L(n) and L'(n) = (bpy)(2) or (NH(3))(4) and PP = 2,2'-bipyridine (bpy), 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), or 2,3-bis(2pyridyl)benzoquinoxaline (dpb). The energy of the metal-to-ligand charge-transfer (MLCT) absorption maximum (hnu(max)) varies in nearly direct proportion to the difference between Ru(III)/Ru(II) and (PP)/(PP)(-) half-wave potentials, DeltaE(1/2), for the monometallic complexes but not for the bimetallic complexes. The MLCT spectra of [(NH(3))(4)Ru(dpp)](2+) exhibit three prominent visible-near-UV absorptions, compared to two for [(NH(3))(4)Ru(bpy)](2+), and are not easily reconciled with the MLCT spectra of [[(NH(3))(4)Ru]dpp[RuL(n)]](4+). The ab initio calculations indicate that the two lowest energy pi orbitals are not much different in energy in the PP ligands (they correlate with the degenerate pi orbitals of benzene) and that both contribute to the observed MLCT transitions. The LUMO energies calculated for the monometallic complexes correlate strongly with the observed hnu(max) (corrected for variations in metal contribution). The LUMO computed for dpp correlates with LUMO + 1 of pyrazine. This inversion of the order of the two lowest energy pi orbitals is unique to dpp in this series of ligands. Configurational mixing of the ground and MLCT excited states is treated as a small perturbation of the overall energies of the metal complexes, resulting in a contribution epsilon(s) to the ground-state energy. The fraction of charge delocalized, alpha(DA)(2), is expected to attenuate the reorganizational energy, chi(reorg), by a factor of approximately (1 - 4alpha(DA)(2) + alpha(DA)(4)), relative to the limit where there is no charge delocalization. This appears to be a substantial effect for these complexes (alpha(DA)(2) congruent with 0.1 for Ru(II)/bpy), and it leads to smaller reorganizational energies for emission than for absorption. Reorganizational energies are inferred from the bandwidths found in Gaussian analyses of the emission and/or absorption spectra. Exchange energies are estimated from the Stokes shifts combined with perturbation--theory-based relationship between the reorganizational energies for absorption and emission values. The results indicate that epsilon(s) is dominated by terms that contribute to electron delocalization between metal and PP ligand. This inference is supported by the large shifts in the N-H stretching frequency of coordinated NH(3) as the number of PP ligands is increased. The measured properties of the bpy and dpp ligands seem to be very similar, but electron delocalization appears to be slightly larger (10-40%) and the exchange energy contributions appear to be comparable (e.g., approximately 1.7 x 10(3) cm(-1) in [Ru(bpy)(2)dpp](2+) compared to approximately 1.3 x 10(3) cm(-1) in the bpy analogue).     

Mononuclear and dinuclear complexes of dibenzoeilatin: synthesis, structure, and electrochemical and photophysical properties

This work describes a study of Ru(II) and Os(II) polypyridyl complexes of the symmetrical, fused-aromatic bridging ligand dibenzoeilatin (1). The synthesis, purification, and structural characterization by NMR of the mononuclear complexes [Ru(bpy)(2)(dbneil)](2+) (2), [Ru(tmbpy)(2)(dbneil)](2+) (3), and [Os(bpy)(2)(dbneil)](2+) (4), the homodinuclear complexes [[Ru(bpy)(2)](2)[micro-dbneil]](4+) (5), [[Ru(tmbpy)(2)](2)[micro-dbneil]](4+) (6), and [[Os(bpy)(2)](2)[micro-dbneil]](4+) (7), and the heterodinuclear complex [[Ru(bpy)(2)][micro-dbneil][Os(bpy)(2)]](4+) (8) are described, along with the crystal structures of 4, 6, and 7. Absorption spectra of the mononuclear complexes feature a low-lying MLCT band around 600 nm. The coordination of a second metal fragment results in a dramatic red shift of the MLCT band to beyond 700 nm. Cyclic and square wave voltammograms of the mononuclear complexes exhibit one reversible metal-based oxidation, as well as several ligand-based reduction waves. The first two reductions, attributed to reduction of the dibenzoeilatin ligand, are substantially anodically shifted compared to [M(bpy)(3)](2+) (M = Ru, Os), consistent with the low-lying pi orbital of dibenzoeilatin. 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 conjugated dibenzoeilatin ligand. Luminescence spectra, quantum yield, and lifetime measurements at room temperature in argon-purged acetonitrile have shown that the complexes exhibit (3)MLCT emission, which occurs in the IR-region between 950 and 1300 nm. The heterodinuclear complex 8 exhibits luminescence only from the Ru-based fragment, the intensity of which is less than 1% of that observed in the corresponding homodinuclear complex 5; no emission from the Os-based unit is observed, and an intramolecular quenching constant of k(q) > or = 3 x10(9) s(-)(1) is evaluated. The nature of the quenching process is briefly discussed.     

Synthesis and structural,electrochemical, and optical properties of Ru(II) complexes with azobis(2,2'-bipyridine)s

Two new ditopic ligands, 5,5"-azobis(2,2'-bipyridine) (5,5"-azo) and 5,5"-azoxybis(2,2'-bipyridine) (5,5"-azoxy), were prepared by the reduction of nitro precursors. Mononuclear and dinuclear Ru(II) complexes having one of these bridging ligands and 2,2'-bipyridine terminal ligands were also prepared, and their properties were compared with previously reported Ru(II) complexes having 4,4"-azobis(2,2'-bipyridine) (4,4"-azo). The X-ray crystal structure showed that 5,5"-azo adopts the trans conformation and a planar rodlike shape. The X-ray crystal structure of [(bpy)(2)Ru(5,5"-azo)Ru(bpy)(2)](PF(6))(4) (Ru(5,5"-azo)Ru) showed that the bridging ligand is in the trans conformation and nearly planar also in the complex and the metal-to-metal distance is 10.0 A. The azo or azoxy ligand in these complexes exhibits reduction processes at less negative potentials than the terminal bpy's due to the low-lying pi level. The electronic absorption spectra for the complexes having 5,5"-azo or 5,5"-azoxy exhibit an extended low-energy metal-to-ligand charge-transfer absorption. The ligands, 5,5"-azo and 5,5"-azoxy, and the mononuclear complex, [(bpy)(2)Ru(5,5"-azo)](2+), isomerize reversibly upon light irradiation. The low-energy MLCT state sensitizes the isomerization of the azo moiety in this complex. While [(bpy)(2)Ru(4,4"-azo)Ru(bpy)(2)](PF(6))(4) exhibits light switch properties, namely, significant electrochromism and a large luminescence enhancement, upon reduction, Ru(5,5"-azo)Ru does not show these properties. The radical anion formation upon reduction of these complexes has been confirmed by ESR spectroscopy.     

Synthesis, structure and spectral and redox properties of new mixed ligand monomeric and dimeric Ru(II) complexes: predominant formation of the "cis-alpha" diastereoisomer and unusual 3MC emission by dimeric complexes

The tetradentate ligands 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane (pdto) and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (bbdo) form the complexes [Ru(pdto)(mu-Cl)](2)(ClO(4))(2) 1 and [Ru(bbdo)(mu-Cl)](2)(ClO(4))(2) 2 respectively. The new di-mu-chloro dimers 1 and 2 undergo facile symmetrical bridge cleavage reactions with the diimine ligands 2,2'-bipyridine (bpy) and dipyridylamine (dpa) to form the six-coordinate complexes [Ru(pdto)(bpy)](ClO(4))(2) 3, [Ru(bbdo)(bpy)](ClO(4))(2) 4, [Ru(pdto)(dpa)](ClO(4))(2) 5 and [Ru(bbdo)(dpa)](ClO(4))(2) 6 and with the triimine ligand 2,2':6,2'-terpyridine (terpy) to form the unusual seven-coordinate complexes [Ru(pdto)(terpy)](ClO(4))(2) 7 and [Ru(bbdo)(terpy)](ClO(4))(2) 8. In 1 the dimeric cation [Ru(pdto)(mu-Cl)](2)(2+) is made up of two approximately octahedrally coordinated Ru(II) centers bridged by two chloride ions, which constitute a common edge between the two Ru(II) octahedra. Each ruthenium is coordinated also to two pyridine nitrogen and two thioether sulfur atoms of the tetradentate ligand. The ligand pdto is folded around Ru(II) as a result of the cis-dichloro coordination, which corresponds to a "cis-alpha" configuration [DeltaDelta/LambdaLambda(rac) diastereoisomer] supporting the possibility of some attractive pi-stacking interactions between the parallel py rings at each ruthenium atom. The ruthenium atom in the complex cations 3a and 4 exhibit a distorted octahedral coordination geometry composed of two nitrogen atoms of the bpy and the two thioether sulfur and two py/bzim nitrogen atoms of the pdto/bbdo ligand, which is actually folded around Ru(II) to give a "cis-alpha" isomer. The molecule of complex 5 contains a six-coordinated ruthenium atom chelated by pdto and dpa ligands in the expected distorted octahedral fashion. The (1)H and (13)C NMR spectral data of the complexes throw light on the nature of metal-ligand bonding and the conformations of the chelate rings, which indicates that the dithioether ligands maintain their tendency to fold themselves even in solution. The bis-mu-chloro dimers 1 and 2 show a spin-allowed but Laporte-forbidden t(2g)(6)((1)A(1g))--> t(2g)(5) e(g)(1)((1)T(1g), (1)T(2g)) d-d transition. They also display an intense Ru(II) dpi--> py/bzim (pi*) metal-to-ligand charge transfer (MLCT) transition. The mononuclear complexes 3-8 exhibit dpi-->pi* MLCT transitions in the range 340-450 nm. The binuclear complexes 1 and 2 exhibit a ligand field ((3)MC) luminescence even at room temperature, whereas the mononuclear complexes 3 and 4 show a ligand based radical anion ((3)MLCT) luminescence. The binuclear complexes 1 and 2 undergo two successive oxidation processes corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)Ru(III) state (K(c): 1, 3.97 x 10(6); 2, 1.10 x 10(6)). The mononuclear complexes 3-7 exhibit only one while 8 shows two quasi-reversible metal-based oxidative processes. The coordinated 'soft' thioether raises the redox potentials significantly by stabilising the 'soft' Ru(II) oxidation state. One or two ligand-based reduction processes were also observed for the mononuclear complexes.     

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.     

Marked differences in light-switch behavior of Ru(II) complexes possessing a tridentate DNA intercalating ligand

The tridentate ligand 3-(pyrid-2'-yl)dipyrido[3,2-a:2',3'-c]phenazine (pydppz) has been prepared in two steps by elaboration of 2-(pyrid-2'-yl)-1,10-phenanthroline. Both homoleptic [Ru(pydppz)(2)](2+) and heteroleptic [Ru(tpy)(pydppz)](2+) (tpy = 2,2';6',2' '-terpyridine) complexes have been prepared and characterized by (1)H NMR. The absorption and emission spectra are consistent with low-lying MLCT excited states, which are typical of Ru(II) complexes. Femtosecond transient absorption measurements show that that the (3)MLCT excited state of the heteroleptic complex [Ru(tpy)(pydppz)](2+) (tau approximately 5 ns) is longer-lived than that of the homoleptic complex [Ru(pydppz)(2)](2+) (tau = 2.4 ns) and that these lifetimes are significantly longer than that of the (3)MLCT state of the parent complex [Ru(tpy)(2)](2+) (tau = 120 ps). These differences are explained by the lower-energy (3)MLCT excited state present in [Ru(tpy)(pydppz)](2+) and [Ru(pydppz)(2)](2+) compared to [Ru(tpy)(2)](2+), resulting in less deactivation of the former through the ligand-field state(s). DFT and TDDFT calculations are consistent with this explanation. [Ru(tpy)(pydppz)](2+) and [Ru(pydppz)(2)](2+) bind to DNA through the intercalation of the pydppz ligand; however, only the heteroleptic complex exhibits luminescence enhancement in the presence of DNA. The difference in the photophysical behavior of the complexes is explained by the inability of [Ru(pydppz)(2)](2+) to intercalate both pydppz ligands, such that one pydppz always remains exposed to the solvent. DNA photocleavage is observed for [Ru(tpy)(pydppz)](2+) in air, but not for [Ru(pydppz)(2)](2+). The DNA damage likely proceeds through the production of small amounts of (1)O(2) by the longer-lived complex. Although both complexes possess the intercalating pydppz ligand, they exhibit different photophysical properties in the presence of DNA.     

Binuclear wirelike dimers based on ruthenium(II)-bipyridine units linked by ethynylene-oligothiophene-ethynylene bridges

The syntheses, structural characteristics, electrochemical behavior, ground-state spectra, photophysical properties, and transient absorption (TA) spectra in CH(3)CN solvent are reported for binuclear [(bpy)(2)Ru(bpy-E(T)(n)()E-bpy)Ru(bpy)(2)](4+) complexes, Ru(bpyT(n)bpy)Ru, where the Ru-based units are connected by alternating 3,4-dibutylthiophene (DBT')/thiophene (T') fragments linked via ethynyl groups (E) to bpy ligands at the 5-position (bpy is 2,2'-bipyridine). The ligand bpyT(3)bpy represents a module containing DBT'/T'/DBT' subunits, and bpyT(5)bpy accounts for a DBT'/T'/DBT'/T'/DBT' pattern. The syntheses and electrochemical and spectroscopic (emission and TA) properties in CH(2)Cl(2) solvent of the bpyT(n)()bpy ligands are likewise reported. The behavior of the Ru(bpyT(n)bpy)Ru dimers has been compared to that of the bpyT(n)bpy ligands and to that of a related mononuclear complex, [(bpy)(2)Ru(bpy-E-DBT')](2+), Ru(bpyDBT'). For the dimeric complexes, the electrochemical results show that the first reduction step takes place at the bpy ligand(s) bearing an ethynylene group, the first oxidation step is thiophene-centered, and further oxidation involves the metal centers, which are only weakly interacting. The photophysical and TA results for the Ru(bpyT(n)bpy)Ru dimers account for the presence of low-lying oligothiophene-centered (3)pi,pi levels, while higher-lying metal-ligand charge transfer ((3)MLCT) levels are thermally accessible only for the case of Ru(bpyT(3)bpy)Ru; the possible role of charge separation (CS) levels (from oxidation at the thiophene bridge and reduction at one of the coordinated bpy's) is also discussed.     

Ruthenium(II) 2,2'-Bipyridyl tetrakis acetonitrile undergoes selective axial photocleavage

The complex [Ru(bpy)(AN)4]2+ (bpy = 2,2'-bipyridyl, AN = acetonitrile) has a Ru(II) --> pi(*)(bpy) MLCT band at 388 nm. Upon irradiation on this absorption band, the compound undergoes total regioselective photocleavage yielding complexes fac-[Ru(bpy)(AN)(3)(H(2)O)](2+) and trans-[Ru(bpy)(AN)(2)(H(2)O)(2)](2+) in two consecutive steps with quantum yields of 0.43 and 0.09, respectively. This behavior is a consequence of the stronger sigma-donor ability of the bpy nitrogens that determines the orbital ordering and therefore the nature of the lowest lying 3d-d state responsible for the photochemistry. The two-step photoreaction, which can be followed by UV-vis and NMR spectra, provides a quantitative path to the preparation of trans-polypyridine species with potentially interesting photochemical properties.     

Computational exploration of heterolytic halogen-carbon bond scission photoreactions in ruthenium polypyridyl complexes

Energy wasting charge recombination is an efficiency limiting process in efforts to achieve solar energy storage. Here, density functional theory is used to explore the thermodynamics of photochemical energy storage reactions in several ruthenium polypyridyl complexes where heterolytic halogen-carbon bond scission occurs after light-induced formation of the triplet metal to ligand charge transfer ((3)MLCT) state, as seen in the following reaction: [Ru(II)(A)(n)(L-X)](2+) + hν → [Ru(III)(A)(n)(L-X)(?-)](2+)* → [Ru(III)(A)(n)(L·)](3+) + X(-) (L = polypyridine ligand; X = Cl, Br, and I; A = ancillary ligand). A thermochemical cycle is employed to determine structural and electronic factors influencing ΔE(rxn). Significant energetic penalties in the oxidation of the metal center are mitigated through methylation of ancillary ligands or introduction of amine ancillary ligands. Methylation of the halogenated ligand maintains energy stored in the (3)MLCT state. Reduction in ΔE(rxn) is obtained by exploiting strain in the coordination geometry or in sterically encumbered ligands that is released upon bond breaking. Formation of a contact ion pair is significantly more favorable than complete separation of charged products, and shows negative ΔE with respect to the (3)MLCT state in certain cases. Future tunability in stored energy may be achieved through careful manipulation of ligand structure and charge on ancillary ligands.     

Electrochemiluminescent peptide nucleic acid-like monomers containing Ru(II)-dipyridoquinoxaline and Ru(II)-dipyridophenazine complexes

A series of Ru(II)-peptide nucleic acid (PNA)-like monomers, [Ru(bpy)(2)(dpq-L-PNA-OH)](2+) (M1), [Ru(phen)(2)(dpq-L-PNA-OH)](2+) (M2), [Ru(bpy)(2)(dppz-L-PNA-OH)](2+) (M3), and [Ru(phen)(2)(dppz-L-PNA-OH)](2+) (M4) (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, dpq-L-PNA-OH = 2-(N-(2-(((9H-fluoren-9-yl)methoxy)carbonylamino)ethyl)-6-(dipyrido[3,2-a:2',3'-c]phenazine-11-carboxamido)hexanamido)acetic acid, dppz-L-PNA-OH = 2-(N-(2-(((9H-fluoren-9-yl) methoxy)carbonylamino)ethyl)-6-(dipyrido[3,2-f:2',3'-h]quinoxaline-2-carboxamido)acetic acid) have been synthesized and characterized by IR and (1)H NMR spectroscopy, mass spectrometry, and elemental analysis. As is typical for Ru(II)-tris(diimine) complexes, acetonitrile solutions of these complexes (M1-M4) show MLCT transitions in the 443-455 nm region and emission maxima at 618, 613, 658, and 660 nm, respectively, upon photoexcitation at 450 nm. Changes in the ligand environment around the Ru(II) center are reflected in the luminescence and electrochemical response obtained from these monomers. The emission intensity and quantum yield for M1 and M2 were found to be higher than for M3 and M4. Electrochemical studies in acetonitrile show the Ru(II)-PNA monomers to undergo a one-electron redox process associated with Ru(II) to Ru(III) oxidation. A positive shift was observed in the reversible redox potentials for M1-M4 (962, 951, 936, and 938 mV, respectively, vs Fc(0/+) (Fc = ferrocene)) in comparison with [Ru(bpy)(3)](2+) (888 mV vs Fc(0/+)). The ability of the Ru(II)-PNA monomers to generate electrochemiluminescence (ECL) was assessed in acetonitrile solutions containing tripropylamine (TPA) as a coreactant. Intense ECL signals were observed with emission maxima for M1-M4 at 622, 616, 673, and 675 nm, respectively. At an applied potential sufficiently positive to oxidize the ruthenium center, the integrated intensity for ECL from the PNA monomers was found to vary in the order M1 (62%) > M3 (60%) > M4 (46%) > M2 (44%) with respect to [Ru(bpy)(3)](2+) (100%). These findings indicate that such Ru(II)-PNA bioconjugates could be investigated as multimodal labels for biosensing applications.     

Characterization of low energy charge transfer transitions in (terpyridine)(bipyridine)ruthenium(II) complexes and their cyanide-bridged bi- and tri-metallic analogues

The lowest energy metal-to-ligand charge transfer (MLCT) absorption bands found in ambient solutions of a series of [Ru(tpy)(bpy)X](m+) complexes (tpy = 2,2':3',2'-terpyridine; bpy = 2,2'-bipyridine; and X = a monodentate ancillary ligand) feature one or two partly resolved weak absorptions (bands I and/or II) on the low energy side of their absorption envelopes. Similar features are found for the related cyanide-bridged bi- and trimetallic complexes. However, the weak absorption band I of [(bpy)(2)Ru{CNRu(tpy)(bpy)}(2)](4+) is missing in its [(bpy)(2)Ru{NCRu(tpy)(bpy)}(2)](4+) linkage isomer demonstrating that this feature arises from a Ru(II)/tpy MLCT absorption. The energies of the MLCT band I components of the [Ru(tpy)(bpy)X](m+) complexes are proportional to the differences between the potentials for the first oxidation and the first reduction waves of the complexes. Time-dependent density functional theory (TD-DFT) computational modeling indicates that these band I components correspond to the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition, with the HOMO being largely ruthenium-centered and the LUMO largely tpy-centered. The most intense contribution to a lowest energy MLCT absorption envelope (band III) of these complexes corresponds to the convolution of several orbitally different components, and its absorption maximum has an energy that is about 5000 cm(-1) higher than that of band I. The multimetallic complexes that contain Ru(II) centers linked by cyanide have mixed valence excited states in which more than 10% of electronic density is delocalized between the nearest neighbor ruthenium centers, and the corresponding stabilization energy contributions in the excited states are indistinguishable from those of the corresponding ground states. Single crystal X-ray structures and computational modeling indicate that the Ru-(C≡N)-Ru linkage is quite flexible and that there is not an appreciable variation in electronic structure or energy among the conformational isomers.