Mono- and dinuclear ruthenium(II) and osmium(II) polypyridine complexes built around spiro-bridged bis(phenanthroline) ligands: synthesis, electrochemistry, and photophysics

Two new dyads have been synthesized in which terminal Ru(II) and Os(II) polypyridine complexes are separated by sterically constrained spiro bridges. The photophysical properties of the corresponding mononuclear complexes indicate the importance of the decay of the lowest-energy triplet states localized on the metallo fragments through the higher-energy metal-centered excited states. This effect is minimized at 77 K, where triplet lifetimes are relatively long, and for the Os(II)-based systems relative to their Ru(II)-based counterparts. Intramolecular triplet energy transfer takes place from the Ru(II)-based fragment to the appended Os(II)-based unit, the rate constant being dependent on the molecular structure and on temperature. In all cases, the experimental rate constant matches surprisingly well with the rate constant calculated for F?rster-type dipole-dipole energy transfer. As such, the disparate rates shown by the two compounds can be attributed to stereochemical factors. It is further concluded that the spiro bridging unit does not favor through-bond electron exchange interactions, a situation confirmed by cyclic voltammetry.     

Electrochemical, spectroscopic, and photophysical properties of structurally diverse polyazine-bridged Ru(II),Pt(II) and Os(II),Ru(II),Pt(II) supramolecular motifs

Five new tetrametallic supramolecules of the motif [{(TL)(2)M(dpp)}(2)Ru(BL)PtCl(2)](6+) and three new trimetallic light absorbers [{(TL)(2)M(dpp)}(2)Ru(BL)](6+) (TL = bpy = 2,2'-bipyridine or phen = 1,10-phenanthroline; M = Ru(II) or Os(II); BL = dpp = 2,3-bis(2-pyridyl)pyrazine, dpq = 2,3-bis(2-pyridyl)quinoxaline, or bpm = 2,2'-bipyrimidine) were synthesized and their redox, spectroscopic, and photophysical properties investigated. The tetrametallic complexes couple a Pt(II)-based reactive metal center to Ru and/or Os light absorbers through two different polyazine BL to provide structural diversity and interesting resultant properties. The redox potential of the M(II/III) couple is modulated by M variation, with the terminal Ru(II/III) occurring at 1.58-1.61 V and terminal Os(II/III) couples at 1.07-1.18 V versus Ag/AgCl. [{(TL)(2)M(dpp)}(2)Ru(BL)](PF(6))(6) display terminal M(dπ)-based highest occupied molecular orbitals (HOMOs) with the dpp(π*)-based lowest unoccupied molecular orbital (LUMO) energy relatively unaffected by the nature of BL. The coupling of Pt to the BL results in orbital inversion with localization of the LUMO on the remote BL in the tetrametallic complexes, providing a lowest energy charge separated (CS) state with an oxidized terminal Ru or Os and spatially separated reduced BL. The complexes [{(TL)(2)M(dpp)}(2)Ru(BL)](6+) and [{(TL)(2)M(dpp)}(2)Ru(BL)PtCl(2)](6+) efficiently absorb light throughout the UV and visible regions with intense metal-to-ligand charge transfer (MLCT) transitions in the visible at about 540 nm (M = Ru) and 560 nm (M = Os) (ε ≈ 33,000-42,000 M(-1) cm(-1)) and direct excitation to the spin-forbidden (3)MLCT excited state in the Os complexes about 720 nm. All the trimetallic and tetrametallic Ru-based supramolecular systems emit from the terminal Ru(dπ)→dpp(π*) (3)MLCT state, λ(max)(em) ≈ 750 nm. The tetrametallic systems display complex excited state dynamics with quenching of the (3)MLCT emission at room temperature to populate the lowest-lying (3)CS state population of the emissive (3)MLCT state.     

A trimetallic mixed Ru(II)/Fe(II) terpyridyl complex with a long-lived excited state in solution at room temperature

The photophysical behavior of a series of mono- and trimetallic Ru(II) and mixed Ru(II)/Fe(II) bis-terpyridyl complexes was examined. The complexes have bridging terpyridyl ligands linked by phenylene-vinylene substituents on the terpyridyl. For the complexes bridged by a single phenylene-vinylene, the lowest-energy excited state is metal-to-ligand charge transfer (MLCT), and excited-state decay is on the 1-10 ns time scale. The complexes with two phenylene-vinylene groups have thermally equilibrated excited states that are localized on the phenylene-vinylene bridge and have much longer lifetimes (>200 ns). Remarkably, the trimetallic complex having an Fe(II) terpyridyl center also has a long-lived excited state, despite the fact that low-energy iron-localized excited states exist on the complex.     

Photophysical behavior of Ru(II) and Os(II) terpyridyl phenylene vinylene complexes: perturbation of MLCT state by intra-ligand charge-transfer state

The photophysical behavior of novel bimetallic Ru(II) and Os(II) complexes having a bridging ligand consisting of two terpyridyl moieties covalently linked in the 4′ position through a distyrylbenzene bridge (tp vp vpt) is reported. The Ru(II) complex has a unique red emission with an excited state lifetime nearly 2000-times longer than the parent complex, [Ru(mpt)₂](PF6)₂ (mpt=4′-(methylphenyl)-2,2′,6′,2″-terpyridine). Combined spectral data suggest the presence of an emissive intra-ligand charge-transfer (ILCT) state lower in energy than the metal-to-ligand charge transfer (MLCT) state. The Os(II) complex exhibits red emission that is similar to that of the parent complex [Os(mpt)₂](PF₆)₂. However, the excited state absorption spectrum reveals a unique transient absorption in the far red that suggests perturbation of the MLCT state by the ILCT state.     

Probing the excited states of d6 metal complexes containing the 2,2'-bipyrimidine ligand using time-resolved infrared spectroscopy. 1. Mononuclear and homodinuclear systems

This paper reports time-resolved infrared (TRIR) spectroscopic studies on a series of weakly luminescent or nonluminescent 2,2'-bipyrimidine-based complexes to probe their electronic structure and the dynamic behavior of their excited states on the picosecond and nanosecond time scales. The complexes are mononuclear [Re(CO)3Cl(bpm)] (1), [Ru(CN)4(bpm)]2- (2), and [Ru(bpyam)2(bpm)]2+ (3) [bpm=2,2'-bipyrimidine; bpyam=2,2'-bipyridine-4,4'-(CONEt2)2] and their homodinuclear analogues [{Re(CO)3Cl}2(mu-bpm)] (4), [{Ru(CN)4}2(mu-bpm)]2- (5), and [{Ru(bpyam)2}2(mu-bpm)]4+ (6). Complex 1 shows the characteristic shift of the three nu(CO) bands to higher energy in the Re-->bpm triplet metal-to-ligand charge-transfer (3MLCT) state, which has a lifetime of 1.2 ns. In contrast, the dinuclear complex 4 shows nu(CO) transient bands to both higher and lower energy than the ground state indicative of, on the IR time scale, an asymmetric excited state [(OC)3ClReI(bpm*-)ReII(CO)3Cl] whose lifetime is 46 ps. The cyanoruthenate complexes 2 and 5 show comparable behavior, with a shift of the nu(CN) bands to higher energy in the excited state for mononuclear 2 but two sets of transient bands-one to higher energy and one to lower energy-in dinuclear 5, consistent with an asymmetric charge distribution [(NC)4RuII(bpm*-)RuIII(CN)4]4- in the 3MLCT state. These cyanoruthenate complexes have much longer lifetimes in D2O compared with CH3CN, viz., 250 ps and 3.4 ns for 2 and 65 ps and 1.2 ns for 5 in CH3CN and D2O, respectively. In complex 3, both higher-energy Ru-->bpyam and lower-energy Ru-->bpm 3MLCT states are formed following 400 nm excitation; the former decays rapidly (tau=6-7 ps) to the latter, and the subsequent decay of the Ru-->bpm 3MLCT state occurs with a lifetime of 60 or 97 ns in D2O or CH3CN, respectively. Similar behavior is shown by dinuclear 6 in both D2O and CH3CN, with initial interconversion from the Ru-->bpyam to the Ru-->bpm 3MLCT state occurring with tau approximately 7 ps and the resultant Ru-->bpm 3MLCT state decaying on the nanosecond time scale.     

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.     

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.     

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.     

Electronic energy transfer and collection in luminescent molecular rods containing ruthenium(II) and osmium(II) 2,2':6',2"-terpyridine complexes linked by thiophene-2,5-diyl spacers

The electronic absorption spectra, luminescence spectra and lifetimes (in MeCN at room temperature and in frozen n-C3H7CN at 77 K), and electrochemical potentials (in MeCN) of the novel dinuclear [(tpy)Ru(3)Os(tpy)]4+ and trinuclear [(tpy)Ru(3)Os(3)Ru(tpy)]6- complexes (3 = 2,5-bis(2,2':6',2'-terpyridin-4-yl)thiophene) have been obtained and are compared with those of model mononuclear complexes and homometallic [(tpy)Ru(3)Ru(tpy)]4+, [(tpy)Os(3)Os(tpy)]4+ and [(tpy)Ru(3)Ru(3)Ru(tpy)]6+ Complexes. The bridging ligand 3 is nearly planar in the complexes, as seen from a preliminary X-ray determination of [(tpy)Ru(3)Ru(tpy)][PF6]4, and confers a high degree of rigidity upon the polynuclear species. The trinuclear species are rod-shaped with a distance of about 3 nm between the terminal metal centres. For the polynuclear complexes, the spectroscopic and electrochemical data are in accord with a significant intermetal interaction. All of the complexes are luminescent (phi in the range 10(-4)-10(-2) and tau in the range 6-340 ns, at room temperature), and ruthenium- or osmium-based luminescence properties can be identified. Due to the excited state properties of the various components and to the geometric and electronic properties of the bridge, Ru --> Os directional transfer of excitation energy takes place in the complexes [(tpy)Ru(3)Os(tpy)]4+ (end-to-end) and [(tpy)Ru(3)Os(3)Ru(tpy)]6+ (periphery-to-centre). With respect to the homometallic case, for [(tpy)Ru(3)Os(3)Ru(tpy)]6+ excitation trapping at the central position is accompanied by a fivefold enhancement of luminescence intensity.     

Building Block Approach to the Construction of Long-Lived Osmium(II) and Ruthenium(II) Multimetallic Complexes Incorporating the Tridentate Bridging Ligand 2,3,5,6-Tetrakis(2-pyridyl)pyrazine

Two classes of synthetically useful bimetallic complexes of the form [(tpy)M(tpp)RuCl(3)](PF(6)) and [(tpy)M(tpp)Ru(tpp)](PF(6))(4) have been prepared and their spectroscopic and electrochemical properties investigated (tpy = 2,2':6',2"-terpyridine, tpp = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, and M = Ru(II) or Os(II)). Synthetic methods have been developed for the stepwise construction of tpp-bridged systems using a building block approach. In all four complexes, the tpp that serves as the bridging ligand is the site of localization of the lowest unoccupied molecular orbital (LUMO). The nature of the HOMO (highest occupied molecular orbital) varies depending upon the components present. In the systems of the type [(tpy)M(tpp)RuCl(3)](PF(6)), the ruthenium metal coordinated to tpp and three chlorides is the easiest to oxidize and is the site of localization of the HOMO. In contrast, for the [(tpy)M(tpp)Ru(tpp)](PF(6))(4) systems, the HOMO is based on the metal, M, that is varied, either Ru or Os. This gives rise to systems which possess a lowest lying excited state that is always a metal-to-ligand charge transfer state involving tpp but can be tuned to involve Os or Ru metal centers in a variety of coordination environments. The synthetic variation of the components within this framework has allowed for understanding the spectroscopic and electrochemical properties. Bimetallic systems incorporating this tpp ligand have long-lived excited states at room temperature (lifetimes of ca. 100 ns). The bimetallic system [(tpy)Ru(tpp)Ru(tpp)](PF(6))(4) has a longer excited state lifetime than the monometallic system from which it was constructed, [(tpy)Ru(tpp)](PF(6))(2). Details of the spectroscopic and electrochemical studies are reported herein.     

Complexes of substituted derivatives of 2-(2-pyridyl)benzimidazole with Re(I), Ru(II) and Pt(II): structures, redox and luminescence properties

N,N'-Chelating ligands based on the 2-(2-pyridyl)benzimidazole (PB) core have been prepared with a range of substituents (phenyl, pentafluorophenyl, naphthyl, anthracenyl, pyrenyl) connected to the periphery via alkylation of the benzimidazolyl unit at one of the N atoms. These PB ligands have been used to prepare a series of complexes of the type [Re(PB)(CO)(3)Cl], [Pt(PB)(CCR)(2)](where -CCR is an acetylide ligand) and [Ru(bpy)(2)(PB)][PF(6)](2)(bpy = 2,2'-bipyridine). Six of the complexes have been structurally characterised. Electrochemical and luminescence studies show that all three series of complexes behave in a similar manner to the analogous complexes with 2,2'-bipyridine in place of PB. In particular, all three series of complexes show luminescence in the range 553-605 nm (Pt series), 620-640 nm (Re series) and 626-645 nm (Ru series) arising from the (3)MLCT state, with members of the Pt(II) series being the most strongly emissive with lifetimes of up to 500 ns and quantum yields of up to 6% in air-saturated CH(2)Cl(2) at room temperature. In the Re and Ru series there was clear evidence for inter-component energy-transfer processes in both directions between the (3)MLCT state of the metal centre and the singlet and triplet states of the pendant organic luminophores (naphthalene, pyrene, anthracene). For example the pyrene singlet is almost completely quenched by energy transfer to a Re-based MLCT excited state, which in turn is completely quenched by energy transfer to the lower-lying pyrene triplet state. For the analogous Ru(II) complexes the inter-component energy transfer is less effective, with (1)anthracene --> Ru((3)MLCT) energy transfer being absent, and Ru((3)MLCT)-->(3)anthracene energy transfer being incomplete. This is rationalised on the basis of a greater effective distance for energy transfer in the Ru(II) series, because the MLCT excited states are localised on the bpy ligands which are remote from the pendant aromatic group; in the Re series in contrast, the MLCT excited states involve the PB ligand to which the pendant aromatic group is directly attached, giving more efficient energy transfer.     

Photophysics and electron transfer in poly(3-octylthiophene) alternating with Ru(II)- and Os(II)-bipyridine complexes

A series of soluble metal-organic polymers that contain Ru(II)- and Os(II)-polypyridine complexes interspersed within a pi-conjugated poly(3-octylthiophene) backbone are prepared. Detailed electrochemical and photophysical studies are carried out on the polymers and two model complexes to determine the extent that the metal-polypyridine units interact with the pi-conjugated system. The results indicate that there is a strong electronic interaction between the metal-based chromophores and the pi-conjugated organic segments, and consequently the photophysical properties are not simply based on the sum of the properties of the individual components. In the Ru(II) polymers, the metal-to-ligand charge-transfer (MLCT) excited state is slightly higher in energy than the 3 pi,pi* state of the poly(3-octylthiophene) backbone. This state ordering results in a material that displays only a weak MLCT luminescence and a long-lived transient absorption spectrum that is dominated by the 3 pi,pi* state. In the Os(II) polymer the MLCT state is lower in energy than the polythiophene-based 3 pi,pi* state and the "unperturbed" MLCT emission is observed. Finally, all of the metal-organic polymers undergo photoinduced bimolecular electron-transfer (ET) reactions with the oxidative quencher dimethyl viologen. Transient absorption spectroscopy reveals that photoinduced ET to dimethyl viologen produces the oxidized polymers, and in most cases, the transient spectra are dominated by features characteristic of a poly(3-octylthiophene) polaron.     

Primary photoinduced processes in bimetallic dyads with extended aromatic bridges. tetraazatetrapyridopentacene complexes of ruthenium(II) and osmium(II)

The photophysics of the binuclear complexes [(phen)2M(tatpp)M(phen)2]4+, where M = Ru or Os, phen = 1,10-phenanthroline, and tatpp = 9,11,20,22-tetraazatetrapyrido[3,2-a:2'3'-c:3',2'-l:2',3']pentacene, has been studied in acetonitrile and dichloromethane by femtosecond and nanosecond time-resolved techniques. The results demonstrate that complexes of different metals have different types of lowest excited state: a tatpp ligand-centered (LC) triplet in the case of Ru(II); a metal-to-ligand charge-transfer (MLCT) triplet state in the case of Os(II). The excited-state kinetics is strongly solvent-dependent. In the Ru(II) system, the formation and decay of the LC state take place, respectively, in 25 ps and ca. 5 ns in CH3CN and in 0.5 ps and 1.3 micros in CH2Cl2. These solvent effects can be rationalized on the basis of a thermally activated decay of the LC state through the upper MLCT state. In the Os(II) system, the formation and decay of the MLCT state take place, respectively, in 3.8 and 60 ps in CH3CN and in 0.5 and 4 ps in CH2Cl2. These effects are consistent with the solvent sensitivity of the MLCT energy, in terms of driving force and energy-gap law arguments. The relevance of these results for the use of ladder-type aromatic bridges as potential molecular wires is discussed.     

Intermolecular energy transfer across nanocrystalline semiconductor surfaces

The yields and dynamics for energy transfer from the metal-to-ligand charge-transfer excited states of Ru(deeb)(bpy)(2)(PF(6))(2), Ru(2+), and Os(deeb)(bpy)(2)(PF(6))(2), Os(2+), where deeb is 4,4'-(CH(3)CH(2)CO(2))(2)-2,2'-bipyridine, anchored to mesoporous nanocrystalline (anatase) TiO(2) thin films were quantified. Lateral energy transfer from Ru(2+)* to Os(2+) was observed, and the yields were measured as a function of the relative surface coverage and the external solvent environment (CH(3)CN, THF, CCl(4), and hexanes). Excited-state decay of Ru(2+)*/TiO(2) was well described by a parallel first- and second-order kinetic model, whereas Os(2+)*/TiO(2) decayed with first-order kinetics within experimental error. The first-order component was assigned to the radiative and nonradiative decay pathways (tau = 1 micros for Ru(2+)*/TiO(2) and tau = 50 ns for Os(2+)*/TiO(2)). The second-order component was attributed to intermolecular energy transfer followed by triplet-triplet annihilation. An analytical model was derived that allowed determination of the fraction of excited-states that follow the two pathways. The fraction of Ru(2+)*/TiO(2) that decayed through the second-order pathway increased with surface coverage and excitation intensity. Monte Carlo simulations were performed to estimate the Ru(2+)* --> Ru(2+) intermolecular energy transfer rate constant of (30 ns)(-1).     

Electropolymerization of vinylbipyridine complexes of ruthenium(II) and osmium(II) in SiO2 sol-gel films

PF(6)(-) salts of the complexes [Ru(vbpy)(3)](2+) and [Os(vbpy)(3)](2+) (vbpy = 4-methyl-4'-vinyl-2,2'-bipyridine) have been electropolymerized into the pores of SiO(2) sol-gel films deposited on conductive Tin(IV)-doped indium oxide-coated glass slides (ITO, In(2)O(3):Sn). The resulting transparent composites represent a new class of materials of general formulas ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) and ITO/SG-poly-[Os(vbpy)(3)](PF(6))(2). The composites are stable with respect to loss of complexes to the external solution and demonstrate several interesting phenomena: (1) Sol-gel pores, serving as diffusion channels for the vbpy complexes and counterions, play a key role in the formation of the polymer and dictate the electrochemical properties of the resulting composite. (2) Dynamic polymer growth occurs within individual diffusion channels creating parallel structures of filled and unfilled channels. (3) Unidirectional charge transfer and a "bilayer" effect have been shown to operate in ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) films exposed to [Os(vbpy)(3)](PF(6))(2) in the external solution. (4) Photophysical properties of the metal-to-ligand charge transfer (MLCT) excited states in ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) composites are significantly modified compared to electropolymerized films on ITO or model monomeric complexes in solution.     

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

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

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

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

Contemporary progresses on neutral, highly emissive Os(II) and Ru(II) complexes

This tutorial review highlights recent and current advances in Os(II) and Ru(II) based luminescent complexes in view of their potential in providing models for photophysical properties and in serving as active materials in optoelectronic devices. It starts with a discussion of the fundamentals of pyridyl azolate chromophores and presents several prototypical designs that allow subtle variation of their basic properties. The third section of this article concerns the preparation of Os(II) and Ru(II) metal complexes and discusses the key factors that control their phosphorescence efficiencies and peak wavelengths. Attention is focused on the properties of their lowest lying excited states. In the last section, we present a series of related Os(II) complexes possessing pyridyl azolate, cyclometalated benzo[h]quinoline, beta-diketonates and quinolinates to demonstrate the power of fundamental basis to chemistry and theoretical approaches in rationalizing the corresponding photophysical behavior and hence to discuss the implications regarding their possible routes for future research.     

Dendrimers made of Ru(II) and Os(II) polypyridine subunits as artificial light-harvesting antennae

Dendrimers based on Ru(II) and Os(II) polypyridine complexes as building blocks and 2,3–dpp (2,3–dpp = 2,3–bis(2′–pyridyl)pyrazine) as bridging ligands are presented and their properties as light-harvesting antenna systems are illustrated. The dendrimers exhibit a huge absorption in the visible region and energy migration patterns whose direction and efficiency depend on the synthetically determined topography of the systems. New recent developments are also discussed, with particular regard towards ultrafast energy transfer processes and long-range electron transfer within the dendritic arrays. To cite this article: S. Serroni et al., C. R. Chimie 6 (2003).     

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.