Spectro-electrochemical study of iron and ruthenium bis-terpyridine complexes with methyl viologen-like subunits as models for supramolecular polymers.

Metallo-supramolecular polyelectrolytes (MEPE) have a variety of attractive properties concerning electrochromism, spin-crossover, rheology, and cell differentiation. Previous studies suggest that these polynuclear structures can be regarded as an assembly of individual subunits and mononuclear complexes can act as models. In this study, we synthesize a monotopic and a ditopic terpyridine ligand with pyridinium units as well as the corresponding iron and ruthenium MEPEs and their mononuclear counterparts. UV-vis studies show that the mononuclear complexes have similar absorption properties to MEPEs. Furthermore, all complexes and MEPEs exhibit electrochromic behavior. Yet only the MEPEs can be deposited on different substrates using a layer-by-layer approach which makes them attractive for applications as electrochromic devices. However, the low solubility particularly of the ruthenium MEPE, renders characterization in solution impractical. Hence, the use of mononuclear complexes with similar monotopic ligands as presented herein can act as a first instance to evaluate the properties of corresponding MEPEs, facilitating the development of metallo-supramolecular materials.

Dinuclear 2,4-di(pyridin-2-yl)-pyrimidine based ruthenium photosensitizers for hydrogen photo-evolution under red light.

In this study, we report two dinuclear Ru(II) complexes C1 and C2 and compare them to their mononuclear analogues Ref1 and Ref2. The dinuclear species exhibit a much stronger absorption, longer excited-state lifetimes and higher luminescence quantum yields than the mononuclear complexes. In addition, C1 and C2 are easier to reduce. An estimation of the driving forces for the electron transfer processes relevant to photocatalytic hydrogen evolution suggests that C1 and Ref2 possess similar activity as photosensitizer (PS). Yet, the improved photophysical properties of C1 make it a more promising candidate for hydrogen evolution. In hydrogen evolution experiments, C1 indeed exhibits increased activity as PS, however, the catalytic system loses its activity after only a few hours. C2 is less active than the mononuclear complexes despite its superior photophysical properties. This observation is attributed to a lack of driving force for the electron transfer towards the catalyst. Further studies of the dinuclear complex C1 show that it is indeed the PS, which decomposes under the catalytic conditions, presumably due to the electron transfer towards the catalyst being the rate-limiting step.

Substituted 2,4-Di(pyridin-2-yl)pyrimidine-Based Ruthenium Photosensitizers for Hydrogen Photoevolution under Red Light.

The photocatalytic reduction of water to form hydrogen gas (H2) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2-yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue- and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.

The crystal structure of the triclinic polymorph of 1,4-bis-([2,2':6',2''-terpyridin]-4'-yl)benzene.

The title triclinic polymorph (Form I) of 1,4-bis-([2,2':6',2''-terpyridin]-4'-yl)benzene, C36H24N6, was formed in the presence of the Lewis acid yttrium trichloride in an attempt to obtain a coordination compound. The crystal structure of the ortho-rhom-bic polymorph (Form II), has been described previously [Fernandes et al. (2010 ▸). Acta Cryst. E66, o3241-o3242]. The asymmetric unit of Form I consists of half a mol-ecule, the whole mol-ecule being generated by inversion symmetry with the central benzene ring being located about a crystallographic centre of symmetry. The side pyridine rings of the 2,2':6',2''-terpyridine (terpy) unit are rotated slightly with respect to the central pyridine ring, with dihedral angles of 8.91 (8) and 10.41 (8)°. Opposite central pyridine rings are coplanar by symmetry, and the angle between them and the central benzene ring is 49.98 (8)°. The N atoms of the pyridine rings inside the terpy entities, N⋯N⋯N, lie in trans-trans positions. In the crystal, mol-ecules are linked by C-H⋯π and offset π-π inter-actions [inter-centroid distances are 3.6421 (16) and 3.7813 (16) Å], forming a three-dimensional structure.

Growth on Metallo-Supramolecular Coordination Polyelectrolyte (MEPE) Stimulates Osteogenic Differentiation of Human Osteosarcoma Cells (MG63) and Human Bone Marrow Derived Mesenchymal Stem Cells.

BACKGROUND: Culturing of cells is typically performed on standard tissue culture plates generating growth conditions, which in general do not reflect the native three-dimensional cellular environment. Recent investigations provide insights in parameters, which strongly affect the general cellular behavior triggering essential processes such as cell differentiation. The physical properties of the used material, such as stiffness, roughness, or topology, as well as the chemical composition of the cell-surface interface are shown to play a key role in the initiation of particular cellular responses. METHODS: We extended our previous research, which identified thin films of metallo-supramolecular coordination polyelectrolytes (MEPEs) as substrate to trigger the differentiation of muscular precursor cells. RESULTS: Here, we show that the same MEPEs similarly stimulate the osteogenic differentiation of pre-osteoblasts. Remarkably, MEPE modified surfaces also trigger the differentiation of primary bone derived mesenchymal stem cells (BMSCs) towards the osteogenic lineage. CONCLUSION: This result leads to the conclusion that these surfaces individually support the specification of cell differentiation toward lineages that correspond to the natural commitment of the particular cell types. We, therefore, propose that Fe-MEPEs may be used as scaffold for the treatment of defects at least in muscular or bone tissue.

Photocatalytic Hydrogen Evolution Driven by a Heteroleptic Ruthenium(II) Bis(terpyridine) Complex.

Since the initial report by Lehn et al. in 1979, ruthenium tris(bipyridine) ([Ru(bpy)3]2+) and its numerous derivatives were applied as photosensitizers (PSs) in a large panel of photocatalytic conditions while the bis(terpyridine) analogues were disregarded because of their low quantum yields and short excited-state lifetimes. In this study, we prepared a new terpyridine ligand, 4'-(4-bromophenyl)-4,4‴:4″,4‴'-dipyridinyl- 2,2':6',2″-terpyridine (Bipytpy) and used it to prepare the heteroleptic complex [Ru(Tolyltpy)(Bipytpy)](PF6)2 (1; Tolyltpy = 4'-tolyl-2,2':6',2'-terpyridine). Complex 1 exhibits enhanced photophysical properties with a higher quantum yield (7.4 × 10-4) and a longer excited-state lifetime (3.8 ns) compared to those of [Ru(Tolyltpy)2](PF6)2 (3 × 10-5 and 0.74 ns, respectively). These enhanced photophysical characteristics and the potential for PS-catalyst interaction through the peripheral pyridines led us to apply the complex for visible-light-driven hydrogen evolution. The photocatalytic system based on 1 as the PS, triethanolamine as a sacrificial donor, and cobaloxime as a catalyst exhibits sustained activity over more than 10 days under blue-light irradiation (light-emitting diode centered at 450 nm). A maximum turnover number of 764 was obtained after 12 days.

The Kinetics of Growth of Metallo-supramolecular Polyelectrolytes in Solution.

Several transition metal ions, like Fe2+ , Co2+ , Ni2+ , and Zn2+ complex to the ditopic ligand 1,4-bis(2,2':6',2''-terpyridin-4'-yl)benzene (L). Due to the high association constant, metal-ion induced self-assembly of Fe2+ , Co2+ , and Ni2+ leads to extended, rigid-rod like metallo-supramolecular coordination polyelectrolytes (MEPEs) even in aqueous solution. Here, we present the kinetics of growth of MEPEs. The species in solutions are analyzed by light scattering, viscometry and cryogenic transmission electron microscopy (cryo-TEM). At near-stoichiometric amounts of the reactants, we obtained high molar masses, which follow the order Ni-MEPE≈Co-MEPE

Green-to-Red Electrochromic Fe(II) Metallo-Supramolecular Polyelectrolytes Self-Assembled from Fluorescent 2,6-Bis(2-pyridyl)pyrimidine Bithiophene.

The structure and properties of metallo-supramolecular polyelectrolytes (MEPEs) self-assembled from rigid 2,6-bis(2-pyridyl)pyrimidine and the metal ions FeII and CoII are presented. While FeL1-MEPE (L1 = 1,4-bis[2,6-bis(2-pyridyl)pyrimidin-4-yl]benzene) is deep blue, FeL2- and CoL2-MEPE (L2 = 5,5'-bis[2,6-bis(2-pyridyl)pyrimidin-4-yl]-2,2'-bithiophene) are intense green and red in color, respectively. These novel MEPEs display a high extinction coefficient and solvatochromism. Ligand L2 shows a high absolute fluorescence quantum yield (Φf = 82%). Viscosity and static light-scattering measurements reveal that the molar masses of these MEPEs are in the range of 1 × 108 g/mol under the current experimental conditions. In water, FeL1-MEPE forms a viscous gel at 20 °C (c = 8 mM). Thin films of high optical quality are fabricated by dip coating on transparent conducting indium tin oxide (ITO) glass substrate. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. Green to red and blue to colorless electrochromism is observed for FeL2-MEPE and FeL1-MEPE, respectively. The results show that the electrochromic properties are affected by the ligand topology. The Fe-MEPEs show a reversible redox behavior of the FeII/FeIII couple at 0.86 and 0.82 V versus Fc+/Fc and display an excellent redox cycle stability under switching conditions. FeL2-MEPE in its oxidized state exhibits a broad absorption band covering the near-IR region (ca. 1500 nm) due to the ligand-to-metal charge transfer transition originating due to charge delocalization in the bithiophene spacer.

Kinetic Studies of the Coordination of Mono- and Ditopic Ligands with First Row Transition Metal Ions.

The reactions of the ditopic ligand 1,4-bis(2,2':6',2″-terpyridin-4'-yl)benzene (1) as well as the monotopic ligands 4'-phenyl-2,2':6',2″-terpyridine (2) and 2,2':6',2″-terpyridine (3) with Fe(2+), Co(2+), and Ni(2+) in solution are studied. While the reaction of 1 with Fe(2+), Co(2+), and Ni(2+) results in metallo-supramolecular coordination polyelectrolytes (MEPEs), ligands 2 and 3 give mononuclear complexes. All compounds are analyzed by UV/vis and fluorescence spectroscopy. Fluorescence spectroscopy indicates that protonation as well as coordination to Zn(2+) leads to an enhanced fluorescence of the terpyridine ligands. In contrast, Fe(2+), Co(2+), or Ni(2+) quench the fluorescence of the ligands. The kinetics of the reactions are studied by stopped-flow fluorescence spectroscopy. Analysis of the measured data is presented and the full kinetic rate laws for the coordination of the terpyridine ligands 1, 2, and 3 to Fe(2+), Co(2+), and Ni(2+) are presented. The coordination occurs within a few seconds, and the rate constant increases in the order Ni(2+) < Co(2+) < Fe(2+). With the rate constants at hand, the polymer growth of Ni-MEPE is computed.

Thermally induced structural rearrangement of the Fe(II) coordination geometry in metallo-supramolecular polyelectrolytes.

Rigid rod-type metallo-supramolecular coordination polyelectrolytes with Fe(II) centres (Fe-MEPEs) are produced via the self-assembly of the ditopic ligand 1,4-bis(2,2':6',2''-terpyridine-4'-yl)benzene (tpy-ph-tpy) and Fe(II) acetate. Fe-MEPEs exhibit remarkable electrochromic properties; they change colour from blue to transparent when an electric potential is applied. This electrochemical process is generally reversible. The blue colour in the ground state is a result of a metal-to-ligand charge transfer at the Fe(II) centre ion in a quasi-octahedral geometry. When annealed at temperatures above 100 °C, the blue colour turns into green and the formerly reversible electrochromic properties are lost, even after cooling down to room temperature. The thermally induced changes in the Fe(II) coordination sphere are investigated in situ during annealing of a solid Fe-MEPE using X-ray absorption fine structure (XAFS) spectroscopy. The study reveals that the thermally induced transition is not accompanied by a redox process at the Fe(II) centre. From the detailed analysis of the XAFS spectra, the changes are attributed to structural changes in the coordination sphere of the Fe(II) site. In the low temperature state, the Fe(II) ion rests in a quasi-octahedral coordination environment surrounded by six nitrogen atoms of the pyridine rings. The axial Fe-N bond length is 1.94 Å, while the equatorial bond length amounts to 1.98 Å. In the high temperature state, the FeN6-site exhibits a distortion with the axial Fe-N bonds being shortened to 1.88 Å and the equatorial Fe-N bonds being elongated to 2.01 Å.

Detailed study of layer-by-layer self-assembled and dip-coated electrochromic thin films based on metallo-supramolecular polymers.

Electrochromic thin films of metallo-supramolecular polyelectrolytes based on Fe(OAc)2 and 1,4-bis(2,2':6',2″-terpyridin-4'-yl)benzene are readily prepared by layer-by-layer (LbL) deposition or dip-coating on transparent conducting electrode surfaces. By applying a potential, we can switch the color of the films from blue to colorless. Because of the strong absorption and the fast switching speed, the color change can be observed with the eye. The devices show reversible switching and cycle stability.

From terpyridine-based assemblies to metallo-supramolecular polyelectrolytes (MEPEs).

Introducing metal ion coordination as bonding motive into polymer architectures provides new structures and properties for polymeric materials. The metal ions can be part of the backbone or of the side-chains. In the case of linear metallo-polymers the repeat unit bears at least two metal ion receptors in order to facilitate metal-ion induced self-assembly. If the binding constants are sufficiently high, macromolecular assemblies will form in a solution. Likewise, polymeric networks can be formed by metal ion induced crosslinking. The metal ion coordination sites introduce dynamic features, e.g. for self-healing or responsive materials, as well as additional functional properties including spin-crossover, electro-chromism, and reactivity. Terpyridines have attracted attention as receptors in metallo-polymers due to their favorable properties. It is well suited to assemble linear rigid-rod like metallo-polymers in case of rigid ditopic ligands. Terpyridine binds a large number of metal ions and are readily functionalized giving rise to a plethora of available ligands as components in metallo-polymers. By the judicious choice of the metal ions, the design of the ligands, the counter ions and the boundary conditions of self-assembly, the final structure and properties of the resulting metallo-polymers can be tailored at all length scales. Here, we review recent activities in the area of metallo-polymers based on terpyridines as central metal ion receptors.

Electrorheological fluids based on metallo-supramolecular polyelectrolyte-silicate composites.

We present an innovative concept for the design of electrorheological fluids (ERF) based on a dispersed phase of rigid-rod-shaped metallo-supramolecular polyelectrolytes (MEPE) intercalated in mesoporous SBA-15 silica. While applying an electric field to this composite dispersed in silicone oil, rheological measurements reveal a strong increase in the storage modulus, indicating the solidification of the fluid. Besides the strong electrorheological effect and the low current densities, we note that the required amount of MEPE is five times lower than in comparable host-guest ERFs. Composites based on mononuclear complexes do not show a comparable electrorheological effect.

Nanocomposites derived from montmorillonite and metallosupramolecular polyelectrolytes: modular compounds for electrorheological fluids.

Nanocomposites made from Na-montmorillonite and metallo-supramolecular polyelectrolytes (MEPE) based on nickel and ditopic bis-terpyridine ligands are prepared by an aqueous synthesis. Intercalation is confirmed by IR-spectroscopy, thermogravimetric analysis, and X-ray powder diffraction. The rheological response in the presence of an electric field of the dispersed nanocomposites in silicone oil is measured with a rheometer. The nanocomposites show a distinct electrorheological effect depending on the concentration and the kind of intercalated species. The effect occurs with a low content of active material while only very small currents are observed.

X-ray near-edge absorption study of temperature-induced low-spin-to-high-spin change in metallo-supramolecular assemblies.

X-ray absorption near the iron K edge (XANES) was used to investigate the characteristics of temperature-induced low-spin-to-high-spin change (SC) in metallo-supramolecular polyelectrolyte amphiphile complexes (PAC) containing FeN(6) octahedra attached to two or six amphiphilic molecules. Compared to the typical spin-crossover material Fe(phen)(2) (NCS)(2) XANES spectra of PAC show fingerprint features restricted to the near-edge region which mainly measures multiple scattering (MS) events. The changes of the XANES profiles during SC are thus attributed to the structure changes due to different MS path lengths. Our results can be interpreted by a uniaxial deformation of FeN(6) octahedra in PAC. This is in agreement with the prediction that SC is originated by a structural phase transition in the amphiphilic matrix of PAC, but in contrast to Fe(phen)(2) (NCS)(2), showing the typical spin crossover being associated with shortening of all the metal-ligand distances.

Tuning the structure and the magnetic properties of metallo-supramolecular polyelectrolyte-amphiphile complexes.

Self-assembly of Fe(2+) ions and the rigid ditopic ligand 1,4-bis(2,2':6',2''-terpyridin-4'-yl)benzene results in metallo-supramolecular coordination polyelectrolytes (MEPE). Sequential self-assembly of MEPE and dialkyl phosphoric acid esters of varying chain length via electrostatic interactions leads to the corresponding polyelectrolyte-amphiphile complexes (PAC), which have liquid-crystalline properties. The PACs have a stratified architecture where the MEPE is embedded in between the amphiphile layers. Upon heating above room temperature, the PACs show either a reversible or an irreversible spin-crossover (SCO) in a temperature range from 360 to 460 K depending on the architecture of the amphiphilic matrix. As the number of amphiphiles per metal ion is increased in the sequence 1:2, 1:4, and 1:6, the temperature of the SCO is shifted to higher values whereas the amphiphile chain length does not have a significant impact on the SCO temperature. In summary, we describe in this article how the structure and the magnetic response function of PACs can be tailored through the design of the ligand and the composition. To investigate the structure and the magnetic behavior, we use X-ray scattering, X-ray absorption spectroscopy, differential scanning calorimetry, faraday-balance, and superconducting quantum interference measurements in combination with molecular modeling.

Organization of spin- and redox-labile metal centers into Langmuir and Langmuir-Blodgett films.

New sal2(trien) ligands that contain alkoxy substituents of various length in meta position of the phenolate entities were coordinated to electronically and magnetically active iron(III) and cobalt(III) centers. The electrochemical and spectroscopic properties of these amphiphilic complexes are virtually unaffected upon alteration of the alkoxy substituents, thus providing a system in which the physical behavior and the metal-centered chemical activity can be tailored independently. The amphiphilic character has been exploited for preparing Langmuir monolayers at the air-water interface and for constructing Langmuir-Blodgett films, hence allowing for hierarchical assembling of electronically and magnetically active systems. While Langmuir films were stable, transfer onto solid supports was limited, which restricted the magnetic analysis of the Langmuir-Blodgett assemblies.

Optically active metallo-supramolecular polymers derived from chiral bis-terpyridines.

The optically active metallo-supramolecular polymers were successfully synthesized via complexation of Fe(II) ions with new bis-terpyridines containing chiral tetra-ethylene glycol units at the ortho-position of the peripheral pyridine rings. In addition, we also revealed solvent and temperature effects toward assembly and dis-assembly behavior of polymers.

From thiophene [2]rotaxane to polythiophene polyrotaxane.

The polythiophene polyrotaxane was synthesized through electrochemical polymerization of the [2]rotaxane consisting of the electron-rich dumbbell-shaped sexithiophene and the electron-deficient cyclophane of cyclobis(paraquat-p-phenylene). The optical and electrochemical property of the polythiophene polyrotaxane film was characterized. The material reported herein is attractive not only as a component for constructing the macromolecular machine but also as a new type of insulated molecular wire having donor-acceptor interaction between the macrocycle and the conductive polymer.

Supramolecular shape shifter: polymorphs of self-organized fullerene assemblies.

Studies of hierarchical supramolecular assemblies of a fullerene derivative bearing three hexadecyloxy chains (1) have been carried out in various conditions, such as different organic solvents, temperatures, and with ultrasonication. The dimensional control of the hierarchical supramolecular architectures from the fullerene derivative (1), such as spherical vesicles, fibers, nano-disks, and uncommon conical and flower-shaped assemblies, was previously achieved. Here, further morphologies such as microspheres, windmill-like sheets, baton-, maracas-, and jellyfish-like assemblies were discovered. Shape shifting of supramolecular assemblies was seen only in the case of the fullerene derivative (1). Reference derivatives bearing three eicosyloxy (2) and three dodecyloxy chains (3) never showed such polymorphs, however, these derivatives self-organized into microparticles with nano-flake outer surfaces and ultra-thin disks, respectively. The fullerene derivatives studied here formed interdigitated bilayer structures with different interspacing length. There are two essential intermolecular forces present due to fullerene or aliphatic chains. Fullerene moieties exhibit strong pi-pi interactions, while van der Waals interactions between aliphatic chains can be altered by variation of their length. Three hexadecyloxy chains at the fullerene moiety (1) were the most effective substitution pattern for stimulating supramolecular shape shifting.