Metal-to-ligand Charge-transfer Luminescence Research Articles

Cyrene™ as a green alternative to N,N'-dimethylformamide (DMF) in the synthesis of MLCT-emissive ruthenium(II) polypyridyl complexes for biological applications.

Ruthenium(II) polypyridyl complexes (RPCs) that emit from triplet metal-to-ligand charge transfer (MLCT) states find a wide variety of uses ranging from luminophores to potential anti-cancer or anti-bacterial therapeutics. Herein we describe a greener, microwave-assisted synthetic pathway for the preparation of homoleptic [Ru(N^N)3]2+ and bis-heteroleptic [Ru(N^N)2(N'^N')]2+ type complexes. This employs the bio-renewable solvent Cyrene™, dihydrolevoglucosenone, as a green alternative to N,N'-dimethylformamide (DMF) in the synthesis of Ru(N^N)2Cl2 intermediate complexes, obtaining comparable yields for N^N = 2,2'-bipyridine, 1,10-phenanthroline and methylated derivatives. Employing these intermediates, a range of RPCs were prepared and we verify that the ubiquitous luminophore [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) can be prepared by this two-step green pathway where it is virtually indistinguishable from a commercial reference. Furthermore, the novel complexes [Ru(bpy)2(10,11-dmdppz)]2+ (10,11-dmdppz = 10,11-dimethyl-dipyridophenazine) and [Ru(5,5'-dmbpy)2(10,11-dmdppz)]2+ (5,5'-dmbpy = 5,5'-dimethyl-bpy) intercalate duplex DNA with high affinity (DNA binding constants, Kb = 5.7 × 107 and 1.0 × 107 M-1, respectively) and function as plasma membrane and nuclear DNA dyes for confocal and STED microscopies courtesy of their long-lived MLCT luminescence.

Manganese(I) complexes with metal-to-ligand charge transfer luminescence and photoreactivity.

Precious metal complexes with the d6 valence electron configuration often exhibit luminescent metal-to-ligand charge transfer (MLCT) excited states, which form the basis for many applications in lighting, sensing, solar cells and synthetic photochemistry. Iron(II) has received much attention as a possible Earth-abundant alternative, but to date no iron(II) complex has been reported to show MLCT emission upon continuous-wave excitation. Manganese(I) has the same electron configuration as that of iron(II), but until now has typically been overlooked in the search for cheap MLCT luminophores. Here we report that isocyanide chelate ligands give access to air-stable manganese(I) complexes that exhibit MLCT luminescence in solution at room temperature. These compounds were successfully used as photosensitizers for energy- and electron-transfer reactions and were shown to promote the photoisomerization of trans-stilbene. The observable electron transfer photoreactivity occurred from the emissive MLCT state, whereas the triplet energy transfer photoreactivity originated from a ligand-centred 3π-π* state.

An 111In-labelled bis-ruthenium(ii) dipyridophenazine theranostic complex: mismatch DNA binding and selective radiotoxicity towards MMR-deficient cancer cells.

Theranostic radionuclides that emit Auger electrons (AE) can generate highly localised DNA damage and the accompanying gamma ray emission can be used for single-photon emission computed tomography (SPECT) imaging. Mismatched DNA base pairs (mismatches) are DNA lesions that are abundant in cells deficient in MMR (mismatch mediated repair) proteins. This form of genetic instability is prevalent in the MMR-deficient subset of colorectal cancers and is a potential target for AE radiotherapeutics. Herein we report the synthesis of a mismatch DNA binding bis-ruthenium(ii) dipyridophenazine (dppz) complex that can be radiolabelled with the Auger electron emitting radionuclide indium-111 (111In). Greater stabilisation accompanied by enhanced MLCT (metal to ligand charge-transfer) luminescence of both the bis-Ru(dppz) chelator and non-radioactive indium-loaded complex was observed in the presence of a TT mismatch-containing duplex compared to matched DNA. The radioactive construct [111In]In-bisRu(dppz) ([111In][In-2]4+) targets cell nuclei and is radiotoxic towards MMR-deficient human colorectal cancer cells showing substantially less detrimental effects in a paired cell line with restored MMR function. Additional cell line studies revealed that [111In][In-2]4+ is preferentially radiotoxic towards MMR-deficient colorectal cancer cells accompanied by increased DNA damage due to 111In decay. The biodistribution of [111In][In-2]4+ in live mice was demonstrated using SPECT. These results illustrate how a Ru(ii) polypyridyl complex can incorporate mismatch DNA binding and radiometal chelation in a single molecule, generating a DNA-targeting AE radiopharmaceutical that displays selective radiotoxicity towards MMR-deficient cancer cells and is compatible with whole organism SPECT imaging.

Synergetic effect between spin crossover and luminescence in the [Fe(bpp)2][BF4]2 (bpp = 2,6-bis(pyrazol-1-yl)pyridine) complex

A simple spin crossover (SCO) complex, [Fe(bpp)2][BF4]2 (bpp = 2,6-bis(pyrazol-1-yl)pyridine), was found to exhibit luminesce with a broad band from 425 to 565 nm in the solid state at ambient temperature, although it was non-luminescent at <250 K. In addition, an abrupt enhancement in luminescence was found at ∼260 K, which is exactly the temperature for a transition from the low spin (LS) to the high spin (HS) state of the complex. This implied that the luminescence was triggered by the thermal spin transition from the LS to HS state, which was different from the emission quenching effect of a normal paramagnetic metal center. A theoretical study via DFT calculations suggested that the excitation efficiency of the complex from the ground state to the lowest excited state for the HS state was higher than that for the LS state, which was proposed to be the reason for the HS-triggered luminescence. A hole/electron excitation analysis and orbital component analysis demonstrated that metal to ligand charge transfer (MLCT) upon excitation was responsible for the luminescence of this complex. This is the first report about the MLCT luminescence of SCO Fe(II) complex tuned by the thermal spin transition. Although the HS-triggered luminescence of solid [Fe(bpp)2][BF4]2 was very weak and not suitable for practical applications, molecular materials for both the multiple readout application and magnetism-regulated optical signaling could be anticipated by improving the luminescence efficiency of this type of SCO complexes in the future.

Targeting the endoplasmic reticulum with a membrane-interactive luminescent ruthenium(ii) polypyridyl complex†Electronic supplementary information (ESI) available: Experimental details, characterization of 2 and Fig. S1-S6. See DOI: 10.1039/c3sc51725jClick here for additional data file.

The characterization and bioactivity of the dinuclear ruthenium(ii) complex [(Ru(DIP)2)2(tpphz)]4+ (DIP = 4,7-diphenyl-1,10-phenanthroline and tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine) is reported. This new complex is found to be luminescent in acetonitrile, where excitation into MLCT (metal-to-ligand charge-transfer) bands in the visible area of the spectrum (λex = 450 nm, ε = 45 000 M-1 cm-1) result in red emission (λem,max = 620 nm, ΦMLCT = 0.017). Aqueous in vitro binding studies indicate that this complex binds to duplex DNA with an affinity of 1.8 × 106 M-1 through a non-classical groove-binding interaction, however, unlike the parent complex [(Ru(phen)2)2(tpphz)]4+ (phen = 1,10-phenanthroline), it also displays an increase in MLCT luminescence on addition of liposomes. Confocal microscopy and TEM studies show that this lipophilic complex targets the endoplasmic reticulum of eukaryotic cells, where it functions as an imaging agent for this organelle, and cytotoxicity studies in human cancer cell lines indicate a comparable potency to the anti-cancer drug cisplatin.

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.

Synthesis and properties of electrochemiluminescent dinuclear Ru(II) complexes assembled with ester-bridged bis(bipyridine) ligands

New bridging ligands, such as bpy-(COOCH 2)-bpy (BL1), mbpy-(CH 2) 3COOCH 2-bpy (BL2), bpy-COO(CH 2) 6OOC-bpy (BL3), and bpy-COOCH 2PhPhCH 2OOC-bpy (BL4), have been synthesized and coordinated to [RuL 2(acetone) 2](PF 6) 2 for various [Ru(L) 2(BL)Ru(L) 2](PF 6) 4-type dinuclear ruthenium complexes (where BL = BL1, BL2, BL3, BL4, and L = bpy, o-phen, DTDP). Their electrochemical redox potentials, spectroscopic properties and relative electrochemiluminescence were investigated in detail. All dinuclear Ru complexes exhibit MLCT (metal-to-ligand charge transfer) absorption and luminescence bands in the visible region. ECL intensities of dinuclear ruthenium(II) complexes were affected not only by the kind of the ligand, but also by the nature of the bridging ligand. Among the synthesized dinuclear Ru complexes, [(DTDP) 2Ru(mbpy)-(CH 2) 3COOCH 2-(bpy)Ru(DTDP) 2](PF 6) 4 exhibited enhanced ECL intensities as high as 2.9 times greater than that of the reference, [Ru( o-phen) 3](PF 6) 2.

A tris(2,2′-bipyridine)ruthenium(ii) derivative tethered to a cis-PtCl2(amine)2moiety: syntheses, spectroscopic properties, and visible-light-induced scission of DNA

A tris(2,2'-bipyridine)ruthenium(II) derivative having two N-(3-ammoniopropyl)carbamoyl pendant units has been prepared and reacted with cis-PtCl2(DMSO)2 (DMSO = dimethyl sulfoxide) to give a heteronuclear Ru(II)Pt(II) dimer having a cis-Pt(II)Cl2(aliphatic amine)2 unit, [Ru(bpy)2(mu-bridge)PtCl2](PF6)2 (bpy = 2,2'-bipyridine, bridge = 4,4'-bis(N-(3-aminopropyl)carbamoyl)-2,2'-bipyridine). The ESI-TOF mass spectrum of the Ru(II)Pt(II) dimer shows a set of signals corresponding to {[Ru(bpy)2(mu-bridge)PtCl2](PF6)}(+) (m/z 1181.1). The MLCT (metal-to-ligand charge transfer) luminescence intensity is enhanced upon the platination of two amine units, presumably due to the formation of a relatively rigid metallocycle. More interestingly, the luminescence intensity is further enhanced by the complexation of the Ru(II)Pt(II) dimer with either 5'-GMP (guanosine 5'-monophosphate disodium salt) or calf thymus DNA. Visible-light-induced scission of supercoiled pBR322 DNA is found to be efficiently enhanced in the presence of the title Ru(II)Pt(II) dimer.

On the Quenching of MLCTRe→bpy Luminescence by Cu(II) Species in Re(I) Polymer Micelles

Transmission electron microscopy (TEM) and dynamic light scattering (DLS) studies on acetonitrile solutions of the polymer {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 demonstrated that the Re(I) polymer molecules aggregate to form spherical micelles of radius R = 156 nm. Coordination of Cu(II) species to the Re (I) polymer causes a decrease in the micelle radius and a distortion from the spherical shape. Besides, the coordination of Cu(II) species to the {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 polymer produces the quenching of the metal to ligand charge transfer (MLCT) excited state by energy transfer processes that are more efficient than those in the quenching of the monomer pyRe(CO)3bpy+ luminescence by Cu(II). Moreover, the kinetics of the quenching by Cu(II) do not follow a Stern-Volmer behavior. Conversely, the quenching of the MLCT luminescence of the Re(I) polymer by the sacrificial electron donor 2,2',2' '-nitrilotriethanol, TEOA, follows a Stern-Volmer kinetics. A comparison is made between the quenching by CuX2 (X = Cl or CF3SO3) and TEOA.

Syntheses and Crystal Structures of Dinuclear Complexes Containing d-Block and f-Block Luminophores. Sensitization of NIR Luminescence from Yb(III), Nd(III), and Er(III) Centers by Energy Transfer from Re(I)− and Pt(II)−Bipyrimidine Metal Centers

Mononuclear complexes [Re(bpym)(CO)(3)Cl] and [Pt(bpym)(CC-C(6)H(4)CF(3))(2)] (bpym = 2,2'-bipyrimidine), in which one of the bipyrimidine sites is vacant, have been used as "complex ligands" to prepare heterodinuclear d-f complexes in which a lanthanide tris(1,3-diketonate) unit is attached to the secondary bipyrimidine site to evaluate the ability of d-block chromophores to act as antennae for causing sensitized near-infrared (NIR) luminescence from adjacent lanthanide(III) centers. The two sets of complexes so prepared are [Re(CO)(3)Cl(mu-bpym)Ln(fod)(3)] (abbreviated as Re-Ln; where Ln = Yb, Nd, Er) and [(F(3)C-C(6)H(4)-CC)(2)Pt(mu-bpym)Ln(hfac)(3)] (abbreviated as Pt-Ln; where Ln = Nd, Gd). Members of both series have been structurally characterized; the metal-metal separation across the bipyrimidine bridge is approximately 6.3 A in each case. In these complexes, the (3)MLCT (MLCT = metal to ligand charge-transfer) luminescences of the mononuclear [Re(bpym)(CO)(3)Cl] and [Pt(bpym)(CC-C(6)H(4)CF(3))(2)] complexes are quenched by energy transfer to those lanthanides (Ln = Yb, Nd, Er) that have low-lying f-f states capable of NIR luminescence; as a result, sensitized NIR luminescence is seen from the lanthanide center following excitation of the d-block unit. In the solid state, quenching of the luminescence from the d-block chromophore is complete, indicating efficient d --> f energy transfer, as a result of the short metal-metal separation across the bipyrimidine bridge. In a CH(2)Cl(2) solution, partial dissociation of the dinuclear complexes into the mononuclear units occurs, with the result that some (3)MLCT luminescence is observed from mononuclear [Re(bpym)(CO)(3)Cl] or [Pt(bpym)(CC-C(6)H(4)CF(3))(2)] present in the equilibrium mixture. Solution UV-vis and luminescence titrations, carried out by the addition of portions of Ln(fod)(3)(H(2)O)(2) or Ln(hfac)(3)(H(2)O)(2) to the d-block complex ligands, indicate that binding of the lanthanide tris(1,3-diketonate) unit at the secondary bipyrimidine site to give the d-f dinuclear complexes occurs with an association constant of ca. 10(5) M(-)(1).

Photochemically Reversible Luminescence Lifetime Switching in Metal−Organic Systems

We use a diarylethene with solution-stable open and closed forms (1,2-bis(2-methylbenzo[b]thiophen-3-yl) hexafluorocyclopentene), BTF6, as a photochromic energy transfer quencher of the metal-to-ligand charge-transfer (MLCT) based luminescence in [Ru(dpp)3]2+, dpp = 4,7-diphenyl-1,10-phenanthroline. As only the closed form of BTF6 serves as a quencher for the MLCT luminescence, and the read (390 nm), write (λ < 360 nm), and erase (λ > 500 nm) wavelengths are well-separated, this system potentially represents an economic alternative to covalent systems. For the first time, we demonstrate the concept of using luminescence lifetimes to transduce the photochromic binary state of the quencher as opposed to the intensity-based readout schemes that are traditionally employed. The luminescence quenching process is described in terms of Förster-type resonance energy transfer (RET) enhanced by diffusion.

X-ray structures, photophysical characterization, and computational analysis of geometrically constrained copper(I)--phenanthroline complexes.

A series of three geometrically constrained C(2)-symmetric Cu(I) mono-phenanthroline complexes were characterized by X-ray structural analysis, and their photophysical properties were investigated by absorption and emission spectroscopy. Visible light excitation yielded metal-to-ligand charge-transfer (MLCT) excited states with luminescence lifetimes up to 155 ns. Ultrafast transient absorption spectroscopy provided further insights into the excited-state dynamics and suggests for all three complexes the formation of a phenanthroline radical anion. In agreement with electrochemical measurements, the data further indicate that coordinative rearrangements are involved in nonradiative deactivation of the excited states. According to time-dependent density functional theory calculations (B3LYP/6-31G), the major MLCT transitions are polarized along the C(2) axis of the complex and originate predominantly from the copper d(xz) orbital. The computational analysis identifies an excited-state manifold with a number of close-lying, potentially emissive triplet states and is in agreement with the multiexponential decay kinetics of the MLCT luminescence. The relationship between structural and photophysical data of the studied Cu(I) mono-phenanthroline complexes agrees well with current models describing the photophysics of the related Cu(I) bis-diimine complexes.

Di- and tetranuclear ruthenium(II) and/or osmium(II) complexes of polypyridyl ligands bridged by a fully conjugated aromatic spacer: synthesis, characterization, and electrochemical and photophysical studies.

A series of mono-, di-, and tetranuclear homo/heterometallic complexes of Ru(II) and Os(II) based on the bridging ligand dppz(11-11')dppz (where dppz = dipyrido[3,2-a:2',3'-c]phenazine) (BL) have been synthesized and characterized. This bridging ligand is a long rigid rod with only one rotational degree of freedom and provides complete conjugation between the chromophores. The complexes synthesized are of general formula [(bpy)(2)Ru-BL](2+), [(phen)(2)/(bpy)(2)M-BL-M(bpy)(2)/(phen)(2)](4+) (M = Ru(II) and Os(II)), [(bpy)(2)Ru-BL-Os(bpy)(2)](4+), and [((bpy)(2)Ru-BL)(3)M](8+). Detailed (1)H NMR studies of these complexes revealed that each chiral center does not influence its neighbor because of the long distance between the metal centers and the superimposed resonances of the diastereoisomers, which allowed the unambiguous assignment of the signals, particularly for homonuclear complexes. Concentration-dependent (1)H NMR studies show molecular aggregation of the mono- and dinuclear complexes in solution by pi-pi stacking. Electrospray mass spectrometry data are consistent with dimerization of mono- and dinuclear complexes in solution. Electrochemical studies show oxidations of Ru(II) and Os(II) in the potential ranges +1.38 to +1.40 and +0.92 to +1.01 V, respectively. The bridging ligand exhibits two one-electron reductions, and it appears that the added electrons are localized on the phenazene moieties of the spacer. All of these complexes show strong metal-to-ligand charge-transfer (MLCT) absorption and (3)MLCT luminescence at room temperature. Quantum yields have been calculated, and the emission lifetimes of all complexes have been measured by laser flash photolysis experiments. The luminescence intensity and lifetime data suggest that the emission due to the Ru center of the heteronuclear complexes is strongly quenched (>90%) compared to that of the corresponding model complexes. This quenching is attributed to intramolecular energy transfer from the Ru(II) center to the Os(II) center (k = (3-5) x 10(7) s(-1)) across the bridging ligand.

A new polytopic bis-diazacrown-ether-polypyridine ligand and its complexes with Zn(II) salts and mononuclear and dendritic Ru(II) precursors. Synthesis, absorption spectra, redox behavior, and luminescence properties.

The new polytopic receptor 1 containing two terpyridine, one phenanthroline, and two diazacrown-ether sites has been prepared using a modular approach. Such a new species contains several pieces of information in its structure which can be processed by different metal ions to give different supramolecular inorganic architectures. Actually, reaction of 1 with Zn(CH(3)COO)(2) in methanol, and subsequent anion exchange, afforded the intramolecular ring-type [Zn(1)](2+) complex, which appears to be formed by a self-assembling reaction. A different synthetic approach, stepwise synthesis, allowed us to synthesize the two multicomponent compounds [(bpy)(2)Ru(mu-1)Ru(bpy)(2)](4+) (Ru2; bpy = 2,2'-bipyridine) and [[(bpy)(2)Ru(mu-2,3-dpp)](2)Ru(mu-1)Ru[(mu-2,3-dpp)Ru(bpy)(2)](2)](12+) (Ru6; 2,3-dpp = 2,3-bis(2-pyridyl)pyrazine). The absorption spectra and luminescence properties of 1 and [Zn(1)](2+) are dominated by pi --> pi transitions and excited states. The absorption spectra of the ruthenium compounds are dominated by ligand-centered (LC) bands in the UV region and metal-to-ligand charge-transfer (MLCT) bands in the visible. The latter compounds undergo several reversible metal-centered oxidations and ligand-centered reductions in the potential window investigated (-2.0/+2.0 V versus SCE) and exhibit MLCT luminescence in both acetonitrile fluid solution at room temperature and in butyronitrile rigid matrix at 77 K. Both the redox and photophysical properties of Ru2 and Ru6 can be assigned to specific subunits of the multicomponent structures. The data indicate that the [Ru(bpy)(2)](2+) and the dendritic [Ru[(mu-2,3-dpp)Ru(bpy)(2)](2)](6+) fragments appended to the polytopic 1ligands behave as independent components of the multicomponent arrays.

New ruthenium(II) and osmium(II) trinuclear dendrons. Synthesis, redox behavior, absorption spectra, and luminescence properties

The new trinuclear dendrons [Cl2Os{(μ-2,3-dpp)Ru(bpy)2}2]4+3, Cl2OsRu2 (bpy = 2,2′-bipyridine; 2,3-dpp = 2,3-bis(2-pyridyl)pyrazine), [Cl2Os{(μ-2,3-dpp)Os(bpy)2}2]4+4, Cl2OsOs2, and [(bpy)Ru{(μ-2,3-dpp)Os(bpy)2}2}6+6, bpyRuOs2 have been synthesized, and their redox properties, absorption spectra, and luminescence properties studied, together with the same properties of the species [Cl2Ru{(μ-2,3-dpp)Ru(bpy)2}2]4+1, Cl2RuRu2, [Cl2Ru{(μ-2,3-dpp)Os(bpy)2}2]4+2, Cl2RuOs2, and [(bpy)Ru{(μ-2,3-dpp)Ru(bpy)2}2}6+5, bpyRuRu2. All the compounds undergo several reversible metal-centered oxidation and ligand-centered reduction processes which are assigned to specific subunits of the trinuclear structure. Analysis of the reduction patterns of the various compounds allows one to obtain information on the extent of ligand–ligand interactions mediated by the central metals. The compounds show very intense ligand-centered absorption bands in the UV region and intense metal-to-ligand charge-transfer (MLCT) bands in the visible region, which can be assigned to transitions involving specific subunits of the dendron structure. All except 3 and 4 exhibit MLCT luminescence both at room temperature in acetonitrile fluid solution and in a MeOH–EtOH 4 ∶ 1 (v/v) rigid matrix at 77 K. The luminescence properties are dominated by the photophysical properties of the subunit(s) in which the lowest-lying excited state of any structure is localized, suggesting that fast intercomponent energy transfer takes place in all the compounds.

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.