A strategy for improving the room-temperature luminescence properties of Ru(II) complexes with tridentate ligands

Several ruthenium(II) complexes with new tridentate polypyridine ligands have been prepared, and their photophysical properties have been studied. The new tridentate ligands are tpy-modified systems (tpy = 2,2':6',2' '-terpyridine) in which aromatic substituents designed to be coplanar with the tpy moiety are introduced, with the aim of enhancing delocalization in the acceptor ligand of the potentially luminescent metal-to-ligand charge-transfer (MLCT) state and increasing the MLCT-MC energy gap (MC = metal-centered excited state). Indeed, the Ru(II) complexes obtained with this new family of tridentate ligands exhibit long-lived luminescence at room temperature (up to 200 ns). The enhanced luminescence properties of these complexes support this design strategy and are superior to those of the model Ru(tpy)22+ compound and compare favorably with those of the best Ru(II) complexes with tridentate ligands reported so far.     

Synthesis, luminescence, and electrochemical studies of tris(homoleptic) ruthenium(II) and osmium(II) complexes of 6'-tolyl-2,2':4',2'-terpyridine

The synthesis and photophysical and electrochemical properties of tris(homoleptic) complexes [Ru(tpbpy)3](PF6)2 (1) and [Os(tpbpy)3](PF6)2 (2) (tpbpy = 6'-tolyl-2,2':4',2' '-terpyridine) are reported. The ligand tpbpy is formed as the side product during the synthesis of 4'-tolyl-2,2':6',2' '-terpyridine (ttpy) and characterized by single-crystal X-ray diffraction: monoclinic, P21/c. The tridentate tpbpy coordinates as a bidentate ligand. The complexes 1 and 2 exhibit two intense absorption bands in the UV region (200-350 nm) assignable to the ligand-centered (1LC) pi-pi* transitions. The ruthenium(II) complex exhibits a broad absorption band at 470 nm while the osmium(II) complex exhibits an intense absorption band at 485 nm and a weak band at 659 nm assignable to the MLCT (dpi-pi*) transitions. A red shifting of the dpi-pi* MLCT transition is observed on going from the Ru(II) to the Os(II) complex as expected from the high-lying dpi Os orbitals. These complexes exhibit ligand-sensitized emission at 732 and 736 nm, respectively, upon light excitation onto their MLCT band through excitation of higher energy LC bands at room temperature. The MLCT transitions and the emission maxima of 1 and 2 are substantially red-shifted compared to that of [Ru(bpy)3](PF6)2 and [Os(bpy)3](PF6)2. The emission of both the complexes in the presence of acid is completely quenched indicating that the emission is not due to the protonation of the coordinated ligands. Our results indicate the occurrence of intramolecular energy transfer from the ligand to the metal center. Both the complexes undergo quasi-reversible metal-centered oxidation, and the E1/2 values for the M(II)/M(III) redox couples (0.94 and 0.50 V versus Ag/Ag+ for 1 and 2, respectively) are cathodically shifted with respect to that of [Ru(bpy)3](PF6)2 and [Os(bpy)3](PF6)2 (E1/2 = 1.28 and 1.09 V versus Ag/Ag+, respectively). The tris(homoleptic) Ru(II) and Os(II) complexes 1 and 2 could be used to construct polynuclear complexes by using the modular synthetic approach in coordination compounds by exploiting the coordinating ability of the pyridine substituent. Furthermore, these complexes offer the possibility of studying the influence of electron-withdrawing and electron-donating substituents on the photophysical properties of Ru(II) and Os(II) polypyridine complexes.     

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.     

Metal-directed synthesis and photophysical studies of trinuclear v-shaped and pentanuclear x-shaped ruthenium and osmium metallorods and metallostars based upon 4'-(3,5-dihydroxyphenyl)-2,2':6',2''-terpyridine divergent units

A new series of V-shaped trinuclear metallorods and X-shaped pentanuclear metallostars has been prepared by the reaction of metal complexes bearing pendant phenolic functionalities with complexes containing electrophilic ligands. Specifically, {M(tpy)2} motifs (M=Ru or Os; tpy=2,2':6',2'-terpyridine) bearing one or two pendant 3,5-dihydroxyphenyl substituents at the 4-position of the central ring of the tpy have been reacted with the complexes [Ru(tpy)(Xtpy)]2+ (X=Cl or Br) to form new ether-linked species. The energy transfer from ruthenium to osmium in these complexes has been investigated in detail and the efficiency of transfer shown to be highly temperature dependent; the energy transfer is highly efficient at low temperature, whereas at room temperature nonradiative and nontransfer deactivation of the excited {Ru(tpy)2}* domains is most significant.     

Synthesis and properties of the elusive ruthenium(II) complexes of 4'-cyano-2,2':6',2' '-terpyridine

The heteroleptic and homoleptic ruthenium(II) complexes of 4'-cyano-2,2':6',2' '-terpyridine are synthesized by palladium catalyzed cyanation of the corresponding Ru(II) complexes of 4'-chloro-2,2':6',2' '-terpyridine. The introduction of the strongly electron-withdrawing cyano group into the Ru(tpy)(2)(2+) moiety dramatically changes its photophysical and redox properties as well as prolongs its room temperature excited-state lifetime.     

Near-infrared absorbing and emitting Ru(II)-Pt(II) heterodimetallic complexes of dpdpz (dpdpz = 2,3-di(2-pyridyl)-5,6-diphenylpyrazine)

The reaction of 2,3-di(2-pyridyl)-5,6-diphenylpyrazine (dpdpz) with K(2)PtCl(4) in a mixture of acetonitrile and water afforded mono-Pt complex (dpdpz)PtCl(2)4 in good yield, with two lateral pyridine nitrogen atoms binding to the metal center. Two types of Ru(II)-Pt(II) heterodimetallic complexes bridged by dpdpz, namely, [(bpy)(2)Ru(dpdpz)Pt(C≡CC(6)H(4)R)](2+) (7-9, R = H, NMe(2), or Cl, respectively) and [(tpy)Ru(dpdpz)Pt(C≡CPh)] (+) (12), were then designed and prepared, where bpy = 2,2'-bipyridine and tpy = 2,2';6',2'-terpyridine. In both cases, the platinum atom binds to dpdpz with a C(∧)N(∧)N tridentate mode. However, the coordination of the ruthenium atom with dpdpz could either be noncyclometalated (N(∧)N bidentate) or cyclometalated (C(∧)N(∧)N tridentate). The electronic properties of these complexes were subsequently studied and compared by spectroscopic and electrochemical analyses and theoretical calculations. These complexes exhibit substantial absorption in the visible to NIR (near-infrared) region because of mixed MLCT (metal-to-ligand-charge-tranfer) transitions from both the ruthenium and the platinum centers. Complexes 7 and 9 were found to emit NIR light with higher quantum yields than those of the mono-Ru complex [(bpy)(2)Ru(dpdpz)](2+) (5) and bis-Ru complex [(bpy)(2)Ru(dpdpz)Ru(bpy)(2)](4+) (13). However, no emission was detected from complex 8 or 12 at room temperature in acetonitrile.     

Direct measurement of excited-state intervalence transfer in [(tpy)Ru(III)(tppz(*-))Ru(II)(tpy)](4+) by time-resolved near-infrared spectroscopy

Extension of time-resolved infrared (TRIR) measurements into the near-infrared region has allowed the first direct measurement of a mixed-valence band in the metal-to-ligand charge transfer (MLCT) excited state of a symmetrical ligand-bridged complex. Visible laser flash excitation of [(tpy)Ru(tppz)Ru(tpy)]4+ (tppz is 2,3,5,6-tetrakis(2-pyridyl)pyrazine; tpy is 2,2':6',6' '-terpyridine) produces the mixed-valence, MLCT excited state [(tpy)RuIII(tppz*-)RuII(tpy)]4+* with the excited electron localized on the bridging tppz ligand. A mixed-valence band appears at numax = 6300 cm-1 with a bandwidth-at-half- maximum, Deltanu1/2 = 1070 cm-1. In the analogous ground-state complex, [(tpy)Ru(tppz)Ru(tpy)]5+, a mixed-valence band appears at numax = 6550 cm-1 with Deltanu1/2 = 970 cm-1 which allows a comparison to be made of electronic coupling across tppz0 and tppz*- as bridging ligands.     

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.     

Self-assembled light-harvesting systems: Ru(II) complexes assembled about Rh-Rh cores

Ru(II) polypyridine species have been assembled about dirhodium(II, II) tetracarboxylate cores. The complexes prepared have general formulas [{(terpy)Ru(La)}n{Rh2(CH3COO)4-n(CH3CN)2}]2n+ (a-type compounds: terpy = 2,2':6',2' '-terpyridine; La = 4'-(p-carboxyphenyl)-2,2':6',2' '-terpyridine; n = 1, 1a; n = 2, cis-2a and trans-2a-cis and trans refer to the arrangement of the Ru(II) species around the dirhodium core; n = 3, 3a), [{(Lb)Ru(La)}n{Rh2(CH3COO)4-n(CH3CN)2}]2n+ (b-type compounds: Lb = 6-phenyl-2,4-di(2-pyridyl)-s-triazine; n = 1, 1b; n = 2, an inseparable mixture of cis-2b and trans-2b; n = 3, 3b; n = 4, 4b), and [{(terpy)Ru(Lc)}{Rh2(CH3COO)3(CH3CN)2}]2+ (1c; Lc = 6-(p-carboxyphenyl)-2,4-di(2-pyridyl)-s-triazine). As model species, also the mononuclear [(terpy)Ru(La)]2+ (5a), [(La)Ru(Lb)]2+ (5b), and [(terpy)Ru(Lc)]2+ (5c) have been prepared. All of the complexes have been characterized by several techniques, including NMR and mass spectra, and the stability of the various species is discussed. The absorption spectra of all of the compounds are dominated by the Ru(II) polypyridine moieties, showing intense ligand-centered (LC) bands in the UV region and intense metal-to-ligand charge-transfer (MLCT) bands in the visible. The compounds exhibit several metal-centered oxidation and ligand-centered reduction processes, which have been assigned to specific subunits. Both absorption and redox data indicate a supramolecular nature of the assembled systems. Efficient energy transfer from the MLCT triplet state of the Ru-based components to the lowest-energy excited state of the dirhodium core takes place for the a-type compounds at 298 K in acetonitrile solution, whereas such a process is inefficient for the b-type and c-type species, which exhibit the typical MLCT emission. At 77 K in butyronitrile matrix, Ru-to-Rh2 energy transfer is partly efficient for both the a-type and the b-type compounds and is inefficient for 1c. The reasons for such behavior are discussed by taking into account arguments concerning the driving force and reorganization energy of the complexes.     

A novel heteroditopic terpyridine-pincer ligand as building block for mono- and heterometallic Pd(II) and Ru(II) complexes

A palladium-catalyzed Stille coupling reaction was employed as a versatile method for the synthesis of a novel terpyridine-pincer (3, TPBr) bridging ligand, 4'-{4-BrC6H2(CH2NMe2)2-3,5}-2,2':6',2' '-terpyridine. Mononuclear species [PdX(TP)] (X = Br, Cl), [Ru(TPBr)(tpy)](PF6)2, and [Ru(TPBr)2](PF6)2, synthesized by selective metalation of the NCNBr-pincer moiety or complexation of the terpyridine of the bifunctional ligand TPBr, were used as building blocks for the preparation of heterodi- and trimetallic complexes [Ru(TPPdCl)(tpy)](PF6)2 (7) and [Ru(TPPdCl)2](PF6)2 (8). The molecular structures in the solid state of [PdBr(TP)] (4a) and [Ru(TPBr)2](PF6)2 (6) have been determined by single-crystal X-ray analysis. Electrochemical behavior and photophysical properties of the mono- and heterometallic complexes are described. All the above di- and trimetallic Ru complexes exhibit absorption bands attributable to (1)MLCT (Ru --> tpy) transitions. For the heteroleptic complexes, the transitions involving the unsubstituted tpy ligand are at a lower energy than the tpy moiety of the TPBr ligand. The absorption bands observed in the electronic spectra for TPBr and [PdCl(TP)] have been assigned with the aid of TD-DFT calculations. All complexes display weak emission both at room temperature and in a butyronitrile glass at 77 K. The considerable red shift of the emission maxima relative to the signal of the reference compound [Ru(tpy)2]2+ indicates stabilization of the luminescent 3MLCT state. For the mono- and heterometallic complexes, electrochemical and spectroscopic studies (electronic absorption and emission spectra and luminescence lifetimes recorded at room temperature and 77 K in nitrile solvents), together with the information gained from IR spectroelectrochemical studies of the dimetallic complex [Ru(TPPdSCN)(tpy)](PF6)2, are indicative of charge redistribution through the bridging ligand TPBr. The results are in line with a weak coupling between the {Ru(tpy)2} chromophoric unit and the (non)metalated NCN-pincer moiety.     

Synthesis and study of the spectroscopic and redox properties of Ru(II),Pt(II) mixed-metal complexes bridged by 2,3,5,6-tetrakis(2-pyridyl)pyrazine

The mixed-metal supramolecular complexes [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 (tpy = 2,2':6',2'-terpyridine and tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were synthesized and characterized. These complexes contain ruthenium bridged by tppz to platinum centers to form stereochemically defined linear assemblies. X-ray crystallographic determinations of the two complexes confirm the identity of the metal complexes and reveal intermolecular interactions of the Pt sites in the solid state for [(tpy)Ru(tppz)PtCl](PF6)3 with a Pt...Pt distance of 3.3218(5) A. The (1)H NMR spectra show the expected splitting patterns characteristic of stereochemically defined mixed-metal systems and are assigned with the use of (1)H-(1)H COSY and NOESY. Electronic absorption spectroscopy displays intense ligand-based pi --> pi* transitions in the UV and MLCT transitions in the visible. Electrochemically [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 display reversible Ru (II/III) couples at 1.63 and 1.83 V versus Ag/AgCl, respectively. The complexes display very low potential tppz (0/-) and tppz(-/2-) couples, relative to their monometallic synthons, [(tpy)Ru(tppz)](PF6)2 and [Ru(tppz)2](PF6)2, consistent with the bridging coordination of the tppz ligand. The Ru(dpi) --> tppz(pi*) MLCT transitions are also red-shifted relative to the monometallic synthons occurring in the visible centered at 530 and 538 nm in CH3CN for [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4, respectively. The complex [(tpy)Ru(tppz)PtCl](PF6)3 displays a barely detectable emission from the Ru(dpi) --> tppz(pi*) (3)MLCT in CH 3CN solution at RT. In contrast, [ClPt(tppz)Ru(tppz)PtCl](PF6)4 displays an intense emission from the Ru(dpi) --> tppz(pi*) (3)MLCT state at RT with lambda max(em) = 754 nm and tau = 80 ns.     

Diorganoruthenium complexes incorporating noninnocent [C(6)H(2)(CH(2)ER(2))(2)-3,5](2)(2-) (E = N, P) bis-pincer bridging ligands: synthesis, spectroelectrochemistry, and DFT studies

The dinuclear complex [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 (bridging PCP-PCP = 3,3',5,5'-tetrakis(diphenylphosphinomethyl)biphenyl, [C6H2(CH2PPh2)2-3,5]22-) was prepared via a transcyclometalation reaction of the bis-pincer ligand [PC(H)P-PC(H)P] and the Ru(II) precursor [Ru(NCN)(tpy)]Cl (NCN = [C6H3(CH2NMe2)2-2,6]-) followed by a reaction with 2,2':6',2' '-terpyridine (tpy). Electrochemical and spectroscopic properties of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 are compared with those of the closely related [(tpy)RuII(NCN-NCN)RuII(tpy)](PF6)2 (NCN-NCN = [C6H2(CH2NMe2)2-3,5]22-) obtained by two-electron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4. The molecular structure of the latter complex has been determined by single-crystal X-ray structure determination. One-electron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4 and one-electron oxidation of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 yielded the mixed-valence species [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ and [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+, respectively. The comproportionation equilibrium constants Kc (900 and 748 for [(tpy)RuIII(NCN-NCN)RuIII(tpy)]4+ and [(tpy)RuII(PCP-PCP)RuII(tpy)]2+, respectively) determined from cyclic voltammetric data reveal comparable stability of the [RuIII-RuII] state of both complexes. Spectroelectrochemical measurements and near-infrared (NIR) spectroscopy were employed to further characterize the different redox states with special focus on the mixed-valence species and their NIR bands. Analysis of these bands in the framework of Hush theory indicates that the mixed-valence complexes [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+ and [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ belong to strongly coupled borderline Class II/Class III and intrinsically coupled Class III systems, respectively. Preliminary DFT calculations suggest that extensive delocalization of the spin density over the metal centers and the bridging ligand exists. TD-DFT calculations then suggested a substantial MLCT character of the NIR electronic transitions. The results obtained in this study point to a decreased metal-metal electronic interaction accommodated by the double-cyclometalated bis-pincer bridge when strong sigma-donor NMe2 groups are replaced by weak sigma-donor, pi-acceptor PPh2 groups.     

Mixed-metal supramolecular complexes coupling phosphine-containing Ru(II) light absorbers to a reactive Pt(II) through polyazine bridging ligands

Supramolecular bimetallic Ru(II)/Pt(II) complexes [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) and their synthons [(tpy)Ru(L)(BL)](n)()(+) (where L = Cl(-), CH(3)CN, or PEt(2)Ph; tpy = 2,2':6',2'-terpyridine; and BL = 2,2'-bipyrimidine (bpm) or 2,3-bis(2-pyridyl)pyrazine (dpp)) have been synthesized and studied by cyclic voltammetry, electronic absorption spectroscopy, mass spectral analysis, and (31)P NMR. The mixed-metal bimetallic complexes couple phosphine-containing Ru chromophores to a reactive Pt site. These complexes show how substitution of the monodentate ligand on the [(tpy)RuCl(BL)](+) synthons can tune the properties of these light absorbers (LA) and incorporate a (31)P NMR tag by addition of the PEt(2)Ph ligand. The redox potentials for the Ru(III/II) couples occur at values greater than 1.00 V versus the Ag/AgCl reference electrode and can be tuned to more positive potentials on going from Cl(-) to CH(3)CN or PEt(2)Ph (E(1/2) = 1.01, 1.55, and 1.56 V, respectively, for BL = bpm). The BL(0/-) couple at -1.03 (bpm) and -1.05 V (dpp) for [(tpy)Ru(PEt(2)Ph)(BL)](2+) shifts dramatically to more positive potentials upon the addition of the PtCl(2) moiety to -0.34 (bpm) and -0.50 V (dpp) for the [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) bridged complex. The lowest energy electronic absorption for these complexes is assigned as the Ru(d pi) --> BL(pi*) metal-to-ligand charge transfer (MLCT) transition. These MLCT transitions are tuned to higher energy in the monometallic synthons when Cl(-) is replaced by CH(3)CN or PEt(2)Ph (516, 452, and 450 nm, for BL = bpm, respectively) and to lower energy when Pt(II)Cl(2) is coordinated to the bridging ligand (560 and 506 nm for BL = bpm or dpp). This MLCT state displays a broad emission at room temperature for all the dpp systems with the [(tpy)Ru(PEt(2)Ph)(dpp)PtCl(2)](2+) system exhibiting an emission centered at 750 nm with a lifetime of 56 ns. These supramolecular complexes [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) represent the covalent linkage of TAG-LA-BL-RM assembly (TAG = NMR active tag, RM = Pt(II) reactive metal).     

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.     

Design and spectroscopic study of new ruthenium(II) complexes containing ligands derived from terpyridine and dipyrido[3,2‐a:2′,3′‐c]phenazine: {Ru(4′‐Rph‐tpy) [dppz(COOH)]Cl}PF6 with R = NO2, Br,Cl

A series of polypyridine ruthenium complexes of the general formula {Ru(Rph‐tpy)[dppz(COOH)]Cl} PF₆ with R = Br ( 1 ), Cl ( 2 ), NO₂ ( 3 ) where Rph‐tpy is 4′‐(4‐Rphenyl‐2,2′:6′,2″‐terpyridine and dppz(COOH) is dipyrido[3,2‐a:2′,3′‐c]phenazine‐2‐carboxylic acid were prepared and characterized. These complexes display intense metal‐to‐ligand charge‐transfer (MLCT) bands centered about 500 nm. The effect of pH on the absorption spectra of these complexes consisting of protonatable ligands has been investigated in water solution by spectrophotometric titration. The electrochemistry shows oxidation potentials for the Ru(II)–Ru(III) couple at +0.881 ( 1 ), +0.907 ( 2 ) and +0.447 V ( 3 ), respectively. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis, characterization and electrochemistry of 4'-functionalized 2,2':6',2'-terpyridine ruthenium(II) complexes and their biological activity

The synthesis and characterization of Ru(II) terpyridine complexes derived from 4'-functionalized 2,2':6',2'-terpyridine ligands by a multi step procedure have been described. The complexes are redox-active, showing both metal-centred (oxidation) and ligand-centred (reduction) processes. The antibacterial and antifungal activity of the synthesized ruthenium(II) complexes [Ru(attpy)2](PF6)2 (attpy = 4'-(4-acryloyloxymethylphenyl)-2,2':6',2'-terpyridine); [Ru(mttpy)2](PF6)2 (mttpy = 4'-(4-methacryloyloxymethylphenyl)-2,2':6',2'- terpyridine); [Ru(mttpy)(MeOPhttpy)](PF6)2 (MeOPhttpy = 4'-(4-methoxyphenyl)-2,2':6',2'-terpyridine); and [Ru(mttpy)(ttpy)](PF6)2 (ttpy = 4'-(4-methylphenyl)-2,2':6',2'-terpyridine) were tested against four human pathogens (Proteus vulgaris, Proteus mirabilis, Pseudomonas aeruginosa and Escherichia coli) and five plant pathogens (Curvularia lunata, Fusarium oxysporum, Fusarium udum, Macrophomina phaseolina and Rhizoctonia solani) by the well diffusion method and MIC values of the complexes are reported. A biological study of the complexes indicated that the complexes [Ru(mttpy)2](PF6)2 and [Ru(mttpy)(MeOPhttpy)](PF6)2 exhibit very good activity against most of the test pathogens and their activity is better than those of some of the commercially available antibiotics like tetracycline and the fungicide carbendazim.     

The multichromophore approach: prolonged room-temperature luminescence lifetimes in Ru(II) complexes based on tridentate polypyridine ligands

A new family of ruthenium(II) complexes with multichromophoric properties was prepared based on a "chemistry-on-the-complex" synthetic approach. The new compounds are based on tridentate chelating sites (tpy-type ligands, tpy=2,2':6',2'-terpyridine) and most of them carry appended anthryl chromophores. Complexes 2 a and 2 b were synthesized through the Pd-catalyzed Suzuki coupling reaction between 9-anthrylboronic acid and the chloro ligands on the presursor species 1 a and 1 b, respectively. The monocoupling product 2 c was also synthesized as the starting complex for a dimetallic complex under optimized Suzuki coupling conditions. The palladium(0)-catalyzed homocoupling reaction on complexes 1 a and 2 c led to dimetallic Ru(II) species 2 d and 2 e, respectively. The solid structures of complexes 2 a and 2 b were characterized by X-ray diffraction. The absorption spectra, redox behavior, luminescence properties (both at room temperature and at 77 K), and transient absorption spectra and decays of 2 a-e were investigated. The absorption spectra of all new species are dominated by ligand-centered (LC) bands in the UV region and metal-to-ligand charge-transfer (MLCT) bands in the visible region. The new compounds undergo reversible metal-centered oxidation processes and several ligand-centered reduction processes, which have been assigned to specific sites. The complexes exhibit luminescence both at room temperature in fluid solution and at 77 K in rigid matrices; the emission was attributed to (3)MLCT states at room temperature and to the lowest-lying anthracene triplet ((3)An) at low temperature, except for 2 c, which does not contain any anthryl chromophore and whose low temperature emission is also of MLCT origin. The luminescence lifetimes of complexes 2 a-d showed that multichromophoric behavior occurs in these species, allowing the luminescence lifetime of the Ru(II)-based chromophores to be prolonged to the microsecond timescale, with the anthryl groups behaving as energy-storage elements for the repopulation of the (3)MLCT state. Nanosecond transient-absorption spectroscopy confirmed the equilibration process between the triplet MLCT and An levels at room temperature. Thermodynamic and kinetic factors governing the equilibration time and the lifetime of the equilibrated excited state are discussed.     

trans-[Ru(II)(dpp)Cl2]: a convenient reagent for the preparation of heteroleptic Ru(dpp) complexes, where dpp is 2,9-di(pyrid-2'-yl)-1,10-phenanthroline

The reaction of 2,9-di(pyrid-2'-yl)-1,10-phenanthroline (dpp) with [RuCl(3)·3H(2)O] or [Ru(DMSO)(4)Cl(2)] provides the reagent trans-[Ru(II)(dpp)Cl(2)] in yields of 98 and 89%, respectively. This reagent reacts with monodentate ligands L to replace the two axial chlorides, affording reasonable yields of a ruthenium(II) complex with dpp bound tetradentate in the equatorial plane. The photophysical and electrochemical properties of the tetradentate complexes are strongly influenced by the axial ligands with electron-donating character to stabilize the ruthenium(III) state, shifting the metal-to-ligand charge-transfer absorption to lower energy and decreasing the oxidation potential. When the precursor trans-[Ru(II)(dpp)Cl(2)] reacts with a bidentate (2,2'-bipyridine), tridentate (2,2';6,2'-terpyridine), or tetradentate (itself) ligand, a peripheral pyridine on dpp is displaced such that dpp binds as a tridentate. This situation is illustrated by an X-ray analysis of [Ru(dpp)(bpy)Cl](PF(6)).     

Expanded ligands: bis(2,2':6',2'-terpyridine carboxylic acid)ruthenium(ii) complexes as metallosupramolecular analogues of dicarboxylic acids

Ligands in which multiple metal-binding domains are linked by a metal-containing moiety rather than a conventional organic group are described as "expanded ligands". The use of 4,4'-difunctionalised {Ru(tpy)(2)} units provides a linear spacer between metal-binding domains and we have extended this motif to expanded ligands containing two carboxylic acid metal-binding domains. In this paper, we describe the synthesis and structural characterisation of ruthenium(ii) complexes of 2,2':6',2'-terpyridine-4'-carboxylic acid and 4'-carboxyphenyl-2,2':6',2'-terpyridine. The ability of the ruthenium(ii) centre to charge compensate deprotonation of the carboxylic acid leads to Zwitterionic complexes and three representative compounds have been structurally characterised.     

Enhancement of luminescence lifetimes of mononuclear ruthenium(II)-terpyridine complexes by manipulation of the sigma-donor strength of ligands

The synthesis and characterization of mixed ligand 2,2';6',2' '-terpyridine (tpy) ruthenium complexes with 2,6-bis([1,2,4]triazol-3-yl)pyridine, 2,6-bis(5-phenyl-[1,2,4]triazol-3-yl)pyridine, and 2,6-bis([1,2,3,4]tetrazol-5-yl)pyridine are reported. The species are characterized by HPLC, 1H NMR, UV/vis, and emission spectroscopy. The photophysical properties of the complexes are investigated as a function of temperature over the range 80-320 K. The emission lifetime observed for the fully deprotonated compounds at room temperature is about 80 ns. This increase by 2 orders of magnitude with respect to the parent "[Ru(tpy)2](2+)" complex is rationalized by an increase in the energy of the metal based dsigma orbital, rather than by manipulation of the pi* orbitals on the ligands. The acid-base and electrochemical properties of the compounds are reported also.