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Claude Piguet

Publications and source records attributed to Claude Piguet.

At least 19 recordsLinked to original sources
PubMed2007

Tuning the polarization along linear polyaromatic strands for rationally inducing mesomorphism in lanthanide nitrate complexes.

The opposite orientation of the ester spacers in the rodlike ligands L 4C12 (benzimidazole-OOC-phenyl) and L 5C12 (benzimidazole-COO-phenyl) drastically changes the electronic structure of the aromatic systems, without affecting their meridional tricoordination to trivalent lanthanides, Ln(III), and their thermotropic liquid crystalline (i.e., mesomorphic) behaviors. However, the rich mesomorphism exhibited by the complexes [Ln(L 4C12)(NO3)3] (Ln=La-Lu) vanishes in [Ln(L 5C12)(NO3)3], despite superimposable molecular structures and comparable photophysical properties. Density functional theory (DFT) and time-dependant DFT calculations performed in the gas phase show that the inversion of the ester spacers has considerable effects on the electronic structure and polarization of the aromatic groups along the strands, which control residual intermolecular interactions responsible for the formation of thermotropic liquid-crystalline phases. As a rule of thumb, an alternation of electron-poor and electron-rich aromatic rings favors intermolecular interactions between the rigid cores and consequently mesomorphism, a situation encountered for L 4C12, L 5C12, [Ln(L 4C12)(NO3)3], but not for [Ln(L 5C12)(NO3)3]. The intercalation of an additional electron-rich diphenol ring on going from [Ln(L 5C12)(NO3)3] to [Ln(L 6C12)(NO3)3] restores mesomorphism despite an unfavorable orientation of the ester spacers, in agreement with our simple predictive model.

Journal Article↗
PubMed2006

Thermotropic lanthanidomesogens.

Due to their high and variable coordination numbers leading to poorly predictable three-dimensional coordination spheres, the trivalent lanthanide metal ions are challenging molecular objects for introduction into thermotropic liquid crystals. Conversely, their predictive electronic, optical and magnetic metal-centred properties make them particularly attractive for being incorporated into switchable macroscopic materials responding to external electric and magnetic stimuli. We briefly describe here some of the important concepts and strategies leading to the recent successful preparation of luminescent thermotropic lanthanide-containing mesophases, for which the generic term lanthanidomesogens is proposed.

Journal Article↗
PubMed2006

How to adapt Scatchard plot for graphically addressing cooperativity in multicomponent self-assemblies.

A graphical method has been developed for the reliable detection of cooperativity in polymetallic complexes involving intra- and intermolecular complexation processes. The method relies on the determination of the partial occupancy r(AL)n, which represents the average number of metals bound per preassembled receptor AL(n) made up of n ligands bound to a linker A. We observe nonlinear, i.e., nonstatistical, Scatchard-like plots (r(AL)n/[M] vs r(AL)n) for metal-binding in double-stranded helicates. The present concept is extended to a virtual, pre-organized receptor L(n), in which no specific linker is involved. Applications to several polymetallic helicates reveal the presence of negatively cooperative processes attributed mainly to intermetallic repulsions, in agreement with recent thermodynamic models.

Journal Article↗
PubMed2006

Encapsulation of labile trivalent lanthanides into a homobimetallic chromium(III)-containing triple-stranded helicate. Synthesis, characterization, and divergent intramolecular energy transfers.

The segmental bidentate-tridentate-bidentate ligand L2 reacts with M(II) (M = Cr, Zn) and Ln(III) (Ln = La, Eu, Gd, Tb, Lu) to give the heterotrimetallic triple-stranded helicates [MLnM(L2)3]7+. For M = Zn(II), the isolated complexes [ZnLnZn(L2)3](CF3SO3)7 (Ln = Eu, Tb) display only lanthanide-centred luminescence arising from the pseudo-tricapped trigonal prismatic LnN9 coordination site. For M = Cr(II), rapid air oxidation provides Cr(III) and leads to the isolation of inert [CrLnCr(L2)3](CF3SO3)9 (Ln = Eu, Tb) complexes, in which divergent intramolecular Ln --> Cr energy transfers can be evidenced. Taking [ZnEuZn(L2)3]7+ as a luminescent standard for Eu-centred emission, a quantitative treatment of the energy migration processes indicates that the rate constant characterizing the Eu --> Cr energy transfer is more efficient in the trimetallic system, than in the analogous simple bimetallic edifice. Particular attention is focused on potential control of directional energy transfer processes in Cr-Ln pairs.

Journal Article↗
PubMed2006

Structural, thermodynamic, and mesomorphic consequences of replacing nitrates with trifluoroacetate counteranions in ternary lanthanide complexes with hexacatenar tridentate ligands.

The promesogenic hexacatenar tridentate ligands L3(Cn) (I shape) and L4(Cn) (V shape) react with trivalent lanthanide trifluoroacetates, Ln((CF3CO2)3, to give either monometallic [Ln(Li(Cn))(CF3CO2)3] or trifluoroacetato-bridged bimetallic [Ln(Li(Cn))(CF3CO2)3]2 complexes in the solid state, as exemplified by the crystal structures of [Lu(L4(CO))(CF3CO2)3(H2O)], [Lu(L4(CO))(CF3CO2)3]2, and [La(L3(C4))(CF3CO2)3]2. Although the dimerization process is influenced by the competiting complexation of anions or solvent molecules, the coordination of CF3CO2- instead of NO3- to Ln(III) produces a significant lengthening of the Ln-N(ligand) bond distances. This translates into a considerable decrease of the affinity of the Li(C12) (i = 3, 4) ligands for Ln(CF3CO2)3 in solution, thus leading to significant dissociation of the [Ln(Li(C12))(CF3CO2)3] complexes at millimolar concentrations. The thermal properties of these complexes also suffer from their limited thermodynamic stability, and the thermotropic liquid crystalline phases produced at high temperatures reflect mixtures of different species. However, a hexagonal columnar organization characterizes the main component in the mesophases obtained with [Ln(L3(C12))(CF3CO2)3] at high temperature. A tentative interpretation of the small-angle X-ray scattering (SAXS) profiles suggests that disklike dimers of [Ln(L3(C12))(CF3CO2)3]2 are packed along the columnar axes. For [Ln(L4(C12))(CF3CO2)3], SAXS profiles are compatible with a lamellar organization in the mesophases originating from the existence of rodlike dimers of [Ln(L4(C12))(CF3CO2)3]2 as the major component in the liquid-crystal state.

Journal Article↗
PubMed2006

Simple thermodynamics for unravelling sophisticated self-assembly processes.

During the past 15 years, coordination chemistry has rapidly developed toward multicomponent assemblies involving several ligands and metal ions, which are connected via intra- or intermolecular processes. The fascinating structural aspect of these complexation reactions has been early recognized for the design of sophisticated (supra)molecular architectures with novel topologies and functions, while the concomitant energetic part only recently emerged as a potential tools for controlling and programming self-assemblies. In this Perspective, we focus on the modelling of the free energy changes accompanying self-assembly processes. Starting with the original protein-ligand model borrowed from biology, which describes complicated multicomponent assemblies, we present (i) its adaptation to coordination chemistry and (ii) its significance for addressing cooperativity as an extra energy cost resulting from intercomponent interactions. An additional entropic concept arising from the separation of intra- and intermolecular complexation processes is then discussed, together with its explicit consideration for modeling multicomponent complexation reactions. Finally, both aspects (i.e. cooperativity and intra-/intermolecular connections) are combined in the extended site binding model, which is able to dissect free energy changes occurring in sophisticated metal-ligand assemblies with a minimum set of microscopic parameters. Applications to experimental complexation reactions of increasing complexity are systematically discussed, and illustrate the potential and limitations of each model.

Kinetics↗
PubMed2006

A justification for using NMR model-free methods when investigating the solution structures of rhombic paramagnetic lanthanide complexes.

The detailed analysis of the 1H NMR hyperfine shifts according to the model-free methods shows that the semi-rigid monometallic complexes [Ln(L)(NO3)3] (Ln = Eu-Yb) are isostructural in solution. The associated separation of contact and pseudo-contact contributions to the hyperfine NMR shifts in each rhombic lanthanide complex at room temperature provides paramagnetic susceptibility tensors whose principal magnetic axes match the expected symmetry requirements. Moreover, both axial (Delta chi(ax)) and rhombic (Delta chi(rh)) paramagnetic anisotropies display satisfactory linear dependence on Bleaney's factors, a correlation predicted by the approximate high-temperature expansion of the magnetic susceptibility limited to T(-2). Consequently, the simple, and chemically attracting NMR model-free methods are not limited to axial systems, and can be safely used for the investigation of the solution structures of any lanthanide complexes. Molecular-based structural criteria for the reliable estimation of paramagnetic susceptibility tensors by NMR are discussed, together with the assignment of the labels of the crystal-field and magnetic axes within Bleaney's approach.

Journal Article↗
PubMed2005

Taking advantage of luminescent lanthanide ions.

Lanthanide ions possess fascinating optical properties and their discovery, first industrial uses and present high technological applications are largely governed by their interaction with light. Lighting devices (economical luminescent lamps, light emitting diodes), television and computer displays, optical fibres, optical amplifiers, lasers, as well as responsive luminescent stains for biomedical analysis, medical diagnosis, and cell imaging rely heavily on lanthanide ions. This critical review has been tailored for a broad audience of chemists, biochemists and materials scientists; the basics of lanthanide photophysics are highlighted together with the synthetic strategies used to insert these ions into mono- and polymetallic molecular edifices. Recent advances in NIR-emitting materials, including liquid crystals, and in the control of luminescent properties in polymetallic assemblies are also presented. (210 references.).

Cations↗
PubMed2005

A simple thermodynamic model for quantitatively addressing cooperativity in multicomponent self-assembly processes--Part 2: Extension to multimetallic helicates possessing different binding sites.

The extended site-binding model, which explicitly separates intramolecular interactions (i.e., intermetallic and interligand) from the successive binding of metal ions to polytopic receptors, is used for unravelling the self-assembly of trimetallic double-stranded Cu(I) and triple-stranded Eu(III) helicates. A thorough analysis of the available stability constants systematically shows that negatively cooperative processes operate, in strong contrast with previous reports invoking either statistical behaviours or positive cooperativity. Our results also highlight the need for combining successive generations of complexes with common binding units, but with increasing metallic nuclearities, for rationalizing and programming multicomponent supramolecular assemblies.

Journal Article↗
PubMed2005

A simple thermodynamic model for quantitatively addressing cooperativity in multicomponent self-assembly processes--part 1: Theoretical concepts and application to monometallic coordination complexes and bimetallic helicates possessing identical binding sites.

A thermodynamic model has been developed for quantitatively estimating cooperativity in supramolecular polymetallic [M(m)L(n)] assemblies, as the combination of two simple indexes measuring intermetallic (I(c)MM) and interligand (I(c)LL) interactions. The usual microscopic intermolecular metal-ligand affinities (f(i)(M,L)) and intermetallic interaction parameters (uMM), adapted to the description of successive intermolecular binding of metal ions to a preorganized receptor, are completed with interligand interactions (uLL) and effective concentrations (c(eff)), accounting for the explicit free energy associated with the aggregation of the ligands forming the receptor. Application to standard monometallic pseudo-octahedral complexes [M(L)(n)(H2O)(6 - n)] (M = Co, Ni, Hf, L = ammonia, fluoride, imidazole, n = 1-6) systematically shows negative cooperativity (uLL < 1), which can be modulated by the electronic structures, charges, and sizes of the entering ligands and of the metal ions. Extension to the self-assembly of more sophisticated bimetallic helicates possessing identical binding sites is discussed, together with the origin of the positively cooperative formation of [Eu2(L3)3].

Journal Article↗
PubMed2005

Tuning the decay time of lanthanide-based near infrared luminescence from micro- to milliseconds through d-->f energy transfer in discrete heterobimetallic complexes.

Inert and optically active pseudo-octahedral Cr(III)N6 and Ru(II)N6 chromophores have been incorporated by self-assembly into heterobimetallic triple-stranded helicates HHH-[CrLnL3]6+ and HHH-[RuLnL3]5+. The crystal structures of [CrLnL(3)](CF(3)SO(3))(6) (Ln=Nd, Eu, Yb, Lu) and [RuLnL3](CF3SO3)5 (Ln=Eu, Lu) demonstrate that the helical structure can accommodate metal ions of different sizes, without sizeable change in the intermetallic MLn distances. These systems are ideally suited for unravelling the molecular factors affecting the intermetallic nd-->4f communication. Visible irradiation of the Cr(III)N6 and Ru(II)N6 chromophores in HHH-[MLnL3]5/6+ (Ln=Nd, Yb, Er; M=Cr, Ru) eventually produces lanthanide-based near infrared (NIR) emission, after directional energy migration within the complexes. Depending on the kinetic regime associated with each specific d-f pair, the NIR luminescence decay times can be tuned from micro- to milliseconds. The origin of this effect, together with its rational control for programming optical functions in discrete heterobimetallic entities, are discussed.

Amides↗
PubMed2005

Isolation and characterization of the first circular single-stranded polymetallic lanthanide-containing helicate.

A thorough examination of the disassembly of bimetallic triple-stranded lanthanide helicates [Ln2(Li)3]6+ (stoichiometry S = m/n = 2/3 = 0.67, global complexity GC = m + n = 2 + 3 = 5) in excess of metals shows the competitive formation of standard linear bimetallic complexes [Ln2(Li)2]6+ (S= 1.0, GC = 4), and circular trimetallic single-stranded helicates [Ln3(Li)3]9+ (S= 1.0, GC = 6).

Crystallography, X-Ray↗
PubMed2005

Molecular control of macroscopic cubic, columnar, and lamellar organizations in luminescent lanthanide-containing thermotropic liquid crystals.

The connection of lipophilic gallic acid derivatives at the 5,5'- or 6,6'-positions of the rigid 2,6-bis(1-ethyl-benzimidazol-2-yl)pyridine core provides two pro-mesogenic tridentate ligands L10 and L12, whose molecular shapes, anisometries, and directional intermolecular pi-stacking can be tuned. X-ray diffraction data in the crystalline state, combined with solution 1H NMR measurements, show that complexation with trivalent lanthanides, Ln(III), produces the neutral hemi-disklike complexes [Ln(Li)(NO3)3] (i = 10, 12), which dimerize to give the rodlike bimetallic complexes [Ln2(Li)2(NO3)6] at lower temperature. The relevant thermodynamic parameters for the latter process depend on the nature of the ligand, the size of the metal ion, and the strength of the intermolecular interactions involved in the condensed phase. These three-dimensional models obtained for the complexes in the crystals and in solution are eventually confronted with small-angle XRD profiles recorded in the intermediate thermotropic liquid crystalline phase, in which the rigidity of the packed polyaromatic cores is maintained, while the alkyl chains are molten. According to the specific geometries and nuclearities of the molecular complexes, three types of mesophases (lamellar, columnar, and cubic) can be induced, which provides a direct correlation between the microscopic arrangements and the macroscopic ordering in lanthanide-containing metallomesogens.

Journal Article↗
PubMed2004

Statistical mechanical approach to competitive binding of metal ions to multi-center receptors.

A microscopic site binding model to treat binding of several metal ions to multi-center receptors is proposed. The model introduces the appropriate parameterization in terms of microscopic complexation constants and metal-metal pair interaction energies. The model is solved with statistical mechanical techniques, including direct enumeration or transfer matrices. We obtain microscopic and macroscopic complexation constants, microstate probabilities, and binding isotherms for chain-like receptors, including the long-chain limit. Various examples to illustrate the usefulness of the model are given.

Binding Sites↗
PubMed2004

A simple thermodynamic model for rationalizing the formation of self-assembled multimetallic edifices: application to triple-stranded helicates.

Reaction of the bis-tridentate ligand bis[1-ethyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl]methane (L2) with Ln(CF(3)SO(3))(3).xH(2)O in acetonitrile (Ln = La-Lu) demonstrates the successive formation of three stable complexes [Ln(L2)(3)](3+), [Ln(2)(L2)(3)](6+), and [Ln(2)(L2)(2)](6+). Crystal-field independent NMR methods establish that the crystal structure of [Tb(2)(L2)(3)](6+) is a satisfying model for the helical structure observed in solution. This allows the qualitative and quantitative beta23 (bi,Ln1,Ln2)characterization of the heterobimetallic helicates [(Ln(1))(Ln(2))(L2)(3)](6+). A simple free energy thermodynamic model based on (i) an absolute affinity for each nine-coordinate lanthanide occupying a terminal N(6)O(3) site and (ii) a single intermetallic interaction between two adjacent metal ions in the complexes (DeltaE) successfully models the experimental macroscopic constants and allows the rational molecular programming of the extended trimetallic homologues [Ln(3)(L5)(3)](9+).

Journal Article↗
PubMed2004

CD spectra of d-f heterobimetallic helicates with segmental di-imine ligands.

We have investigated the CD spectra of a series of enantiomerically pure heterobimetallic helicates, Lambda,Lambda-[LnCr(1)(3)](6+) (Ln = Eu, Gd, Tb), which contain segmental di-imine ligands. For the mononuclear precursor of these helicates, Lambda-[Cr(1)(3)](3+), a positive exciton couplet was observed around 330 nm, as expected for a tris(di-imine) complex with this absolute configuration. The titration of Ln(III) ions into a solution of this complex leads to the formation of Lambda,Lambda-[LnCr(1)(3)](6+). During this process, the CD signal was observed to invert to give a signal which was negative at lower energies. We investigated the observed changes in the CD spectra using a ZINDO-based computational method which we have previously developed. We were able to show that the exciton coupling of the chromophores coordinated to the Cr and Ln ions give rise to CD signals of opposite phase, despite having the same nominal absolute configuration. Exciton coupling between chromophores located on different metal centers ("internuclear" exciton coupling) is also predicted to have a significant impact on the observed spectrum. We were able to "deconstruct" the observed CD spectra into a set of competing exciton coupling effects and show that the sign of these spectra does not correlate with the absolute configuration of the individual metal centers.

Journal Article↗