Claude Piguet

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.).

Mono- and polymetallic lanthanide-containing functional assemblies: a field between tradition and novelty

The variable and versatile co-ordination behaviour of lanthanide metal ions, LnIII, limits their selective introduction into organised molecular or supramolecular architectures. The design of lanthanide-based devices is thus a special challenge since their specific electronic, magnetic or spectroscopic properties result from a precise control of the co-ordination sphere around the metal ions. The lock-and-key principle associated with the preorganisation of rigid macropolycylic multidentate ligands tailored for one particular LnIII only partially fulfils these structural requirements. The development of less constrained macrocyclic ligands or macrocycles bearing pendant arms allows a smooth transition toward flexible (predisposed) receptors leading to the application of the induced fit principle in lanthanide co-ordination chemistry. According to this concept, programmed secondary non-covalent interstrand interactions (π-stacking, hydrogen bonds, electrostatic repulsion) assist the complexation process leading to an ultra-fine tuning of the metallic co-ordination sites. These two complementary approaches are discussed and evaluated for the design of organised mono-, di- and polymetallic lanthanide complexes together with the consideration of new semi-rigid multidentate podands which combine both aspects.

Near-Infrared→Visible Light Upconversion in a Molecular Trinuclear d-f-d Complex

Giving the green light: The connection of two CrIII sensitizers around a central ErIII acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of CrIII-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green ErIII-centered emission. Copyright

Optimizing Millisecond Time Scale Near-Infrared Emission in Polynuclear Chrome(III)–Lanthanide(III) Complexes

This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF(3)SO(3))(2) (M = Cr, Zn) and Ln(CF(3)SO(3))(3) (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D(3)-symmetrical trinuclear [MLnM(L2)(3)](CF(3)SO(3))(n) complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)(3)](9+) induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)(3)](6+), the connection of a second strong-field [CrN(6)] sensitizer in [CrLnCr(L2)(3)](9+) significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)(3)](9+) under reasonable pumping powers.

Design of luminescent building blocks for supramolecular triple-helical lanthanide complexes

The ligand 2,6-bis(1′-ethyl-5′-methylbenzimidazol-2′-yl)pyridine (L5) reacts with lanthanide perchlorate in acetonitrile to give the mononuclear triple-helical complexes [Ln(L5)3]3+(Ln = Eu, Gd or Tb). The crystal structure of [Eu(L5)3][ClO4]3·4MeCN has been determined, which shows three unco-ordinated perchlorate anions and an [Eu(L5)3]3+ cation where the three tridentate ligands are wrapped around a pseudo-C3 axis. The co-ordination sphere around EuIII may be best described as a slightly distorted trigonal-tricapped prism where the six benzimidazole nitrogen atoms occupy the vertices of the prism and the three pyridine nitrogen atoms occupy the capping positions. A detailed geometrical analysis showed that the ethyl groups in L5 produce a slide of the strands which is responsible for the distortion of the triple-helical structure as exemplified by the low symmetry for the EuIII site in the luminescence spectra of [Eu(L5)3]3+. Proton NMR spectra in acetonitrile indicate that the triple-helical structure is maintained for [Ln(Li)3]3+{Ln = Eu or Tb; L = 2,6-bis(1′-R-benzimidazol-2′-yl)pyridine [R = Me L1, Et L2, Pr L3 or CH2C6H3(OMe)2-3,5 L4] or L5} on the NMR time-scale, but the stability of the complexes together with the structural arrangement of the ligands depend on the size of the substituents bound to the benzimidazole nitrogen atoms. Photophysical studies of [Eu(Li)3]3+ show that these steric effects affect the quantum yield in solution and that methyl groups bound to the 5 positions of the benzimidazole rings in L5 shift the π→π* transitions centred on the ligand, but do not strongly modify the emission properties of [Eu(L5)3]3+. Extended Huckel calculations give a qualitative insight into the factor controlling the π→π* transitions of the ligands and complexes.

Optimizing sensitization processes in dinuclear luminescent lanthanide oligomers: selection of rigid aromatic spacers.

This work illustrates a simple approach for optimizing the lanthanide luminescence in molecular dinuclear lanthanide complexes and identifies a particular multidentate europium complex as the best candidate for further incorporation into polymeric materials. The central phenyl ring in the bis-tridentate model ligands L3–L5, which are substituted with neutral (X = H, L3), electron-withdrawing (X = F, L4), or electron-donating (X = OCH3, L5) groups, separates the 2,6-bis(benzimidazol-2-yl)pyridine binding units of linear oligomeric multi-tridentate ligand strands that are designed for the complexation of luminescent trivalent lanthanides, Ln(III). Reactions of L3–L5 with [Ln(hfac)3(diglyme)] (hfac– is the hexafluoroacetylacetonate anion) produce saturated single-stranded dumbbell-shaped complexes [Ln2(Lk)(hfac)6] (k = 3–5), in which the lanthanide ions of the two nine-coordinate neutral [N3Ln(hfac)3] units are separated by 12–14 Å. The thermodynamic affinities of [Ln(hfac)3] for the tridentate binding sites in L3–L5 are average (6.6 ≤ log(β(2,1)(Y,Lk)) ≤ 8.4) but still result in 15–30% dissociation at millimolar concentrations in acetonitrile. In addition to the empirical solubility trend found in organic solvents (L4 > L3 >> L5), which suggests that the 1,4-difluorophenyl spacer in L4 is preferable, we have developed a novel tool for deciphering the photophysical sensitization processes operating in [Eu2(Lk)(hfac)6]. A simple interpretation of the complete set of rate constants characterizing the energy migration mechanisms provides straightforward objective criteria for the selection of [Eu2(L4)(hfac)6] as the most promising building block.

Ion spray-tandem mass spectrometry of supramolecular coordination complexes

Double-helical [M2L2]n+, triple-helical [M2L3]n+, and toroidal [M3L3]n+ (M = Cu, Co, Fe, Ni, La, Eu, Gd, Tb, or Lu) supramolecular complexes have been fully characterized by ion spray mass spectrometry (IS-MS). The IS-MS spectra from pure acetonitrile solutions reflect the nature of the cations present in solution with conservation of the charge state and allow an efficient qualitative speciation of the compounds. The mass spectrometry results can be correlated with other powerful techniques (nuclear magnetic resonance and electronic spectroscopy) for the characterization of supramolecular complexes in solution, Structural information is obtained by collision-induced dissociation, which strongly depends on the metal ions used in the supramolecular complexes and on the various connectivities and topologies of the ligands. When the ligand contains 3,5dimethoxybenzyl groups bound to the benzimidazole rings, the partial fragmentation of the complexes is associated with a decrease of the total charge of the complexes and the appearance of the characteristic fragment at m/z 151 that corresponds to the 3,5-dimethoxybenzyl cation. A detailed analysis of the fragmentation pathways of these supramolecular complexes suggests that the metal-nitrogen coordination bonds are very strong in the gas phase.

Trivalent lanthanide ions: versatile coordination centers with unique spectroscopic and magnetic properties

At first sight, trivalent lanthanide ions LnIII are not very attractive to the chemist: the spherical entities with “inner” 4f valence electrons interact electrostatically with their surroundings, display little stereochemical preferences, and have very similar chemical behavior. On the other hand, these ions exhibit rich and unique spectroscopic and magnetic properties that can be taken advantage of either for spectroscopic and magnetic probes, or to construct materials with specific physico-chemical properties. Moreover, the intrinsic chemical drawbacks of the LnIII ions can be turned into a benefit since the ions adapt easily to almost any chemical environment and can therefore be readily introduced into a variety of ionic, molecular, and supramolecular edifices where they act as functional centers. We will first outline the historical aspects of LnIII coordination chemistry. Fundamental properties of the LnIII ions, including coordination numbers and geometries, solvation, hydrolysis and thermodynamic aspects of complexation, are then briefly reviewed. We finally focus on the several methods developed by inorganic chemists to trap the elusive lanthanide ions into environments preserving or even enhancing their physical properties, or increasing the differences in their chemical characteristics.

Helicates and Related Metallosupramolecular Assemblies: Toward Structurally Controlled and Functional Devices

According to the first use of the word ‘helicate’ in 1987 [1] and its accepted definition [2]: a helicate is a discrete helical supramolecular complex constituted by one or more covalent organic strands wrapped about and coordinated to a series of ions defining the helical axis. The ligand strands are made of several domains corresponding to binding units coordinated to the central ions (in general cations) separated by spacers exhibiting specific structural requirements (Figure 1). Three peculiar characteristics are associated with idealized helicates: (i) several metal ions are located on a straight line according to a mono-dimensional arrangement, (ii) the wrapped ligand strands induce helicity, a special case of chirality [3] and (iii) the final helicate is generated by a strict self-assembly process [4], i.e. it corresponds to the thermodynamically most stable complex. This latter point requires that the interactions between the strands and the metal ions are reversible in order to allow a complete exploration of the energy hypersurface of the assembly process [2, 5]. The coordinate bonds involved between the components in the helicates (i.e. the ligand strands and 3dor 4f-block ions) are particularly well-suited for this purpose since they are labile enough to ensure reversibility, but strong enough to provide thermodynamically stable complexes. The generation of one particular supramolecular helicate thus results from the combination of two crucial factors: (1) a judicious match between the intrinsic information borne by the metal

Understanding, Controlling and Programming Cooperativity in Self-Assembled Polynuclear Complexes in Solution

Deviations from statistical binding, that is cooperativity, in self-assembled polynuclear complexes partly result from intermetallic interactions DeltaE(M,M), whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point-charge model, which suggests severe and counter-intuitive deviations from predictions based solely on the Coulomb law when considering the variation of DeltaE(M,M) with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty-nine homometallic and heterometallic polynuclear triple-stranded helicates obtained from the coordination of the segmental ligands L1-L11 with Zn(2+) (a spherical d-block cation) and Lu(3+) (a spherical 4f-block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance (DeltaE(1-4)(Lu,Lu)>DeltaE(1-2)(Lu,Lu)), ii) of larger magnitude when Zn2+ replaces Lu3+ (DeltaE(1-2)(Zn,Lu)>DeltaE(1-2)(Lu,Lu) and iii) attractive between two triply charged cations held at some specific distance (DeltaE(1-3)(Lu,Lu)<0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.