Molecular Fe(II)-Ln(III) dyads for luminescence reading of spin-state equilibria at the molecular level.

Due to the primogenic effect, the valence shells of divalent iron Fe(II) ([Ar]3d) and trivalent lanthanides Ln(III) ([Xe]4f) are compact enough to induce spin-state equilibrium for the 3d-block metal and atom-like luminescence for the 4f-block partner in Fe(II)-Ln(III) dyads. In the specific case of homoleptic pseudo-octahedral [Fe(II)N] units, programming spin crossover (SCO) around room temperature at normal pressure requires the design of unsymmetrical didentate five-membered ring chelating NN' ligands, in which a five-membered (benz)imidazole heterocycle (N) is connected to a six-membered pyrimidine heterocycle (N'). Benefiting from the influence, the facial isomer -[Fe(II)(NN')] is suitable for inducing SCO properties at room temperature in solution.

Symmetry and Rigidity for Boosting Erbium-Based Molecular Light-Upconversion in Solution.

Previously limited to highly symmetrical homoleptic triple-helical complexes [Er(Lk) ] , where Lk are polyaromatic tridentate ligands, single-center molecular-based upconversion using linear optics and exploiting the excited-state absorption mechanism (ESA) greatly benefits from the design of stable and low-symmetrical [LkEr(hfa) ] heteroleptic adducts (hfa =hexafluoroacetylacetonate anion). Depending on (i) the extended π-electron delocalization, (ii) the flexibility and (iii) the heavy atom effect brought by the bound ligand Lk, the near-infrared (801 nm) to visible green (542 nm) upconversion quantum yield measured for [LkEr(hfa) ] in solution at room temperature can be boosted by up to three orders of magnitude.

Rational Loading of Linear Multi-Site Receptors with Functional Lanthanide Containers: The Missing Link between Oligomers and Polymers.

Although metal-containing organic polymers are becoming essential for modern applications in lighting, catalysis, and electronic devices, very little is known about their controlled metallic loading, which mainly limits their design to empirical mixing followed by characterization and often hampers rational developments. Focusing on the appealing optical and magnetic properties of 4f-block cations, the host-guest reactions leading to linear lanthanidopolymers already display some unexpected dependence of the binding-site affinities on the length of the organic polymer backbone: a drift usually, and erroneously, assigned to intersite cooperativity. Taking advantage of the parameters obtained for the stepwise thermodynamic loading of a series of rigid linear multi-tridentate organic receptors with increasing length, N = 1 (monomer L1), N = 2 (dimer L2), and N = 3 (trimer L3), with [Ln(hfa)3] containers in solution (Ln = trivalent lanthanide cations, hfa = 1,1,1,5,5,5-hexafluoro-pentane-2,4-dione anion), it is demonstrated here that the site-binding model, based on the Potts-Ising approach, successfully predicts the binding properties of the novel soluble polymer P2 made up of nine successive binding units .

Preorganized Polyaromatic Soft Terdentate Hosts for the Capture of [Ln(β-diketonate) ] Guests in Solution.

The concept of preorganization is famous in coordination chemistry for having transformed flexible bidentate 2,2'-bipyridine scaffolds into rigid 1,10-phenanthroline platforms. The resulting boosted affinities for d-block cations has successfully paved the way for the design of a wealth of functional complexes, devices and materials for analysis and optics. Its extension toward terdentate homologues adapted for the selective complexation of f-block cations with larger coordination numbers remains more overlooked.

Metal-Based Linear Light Upconversion Implemented in Molecular Complexes: Challenges and Perspectives.

The piling up of low-energy photons to produce light beams of higher energies while exploiting the nonlinear optical response of matter was conceived theoretically around 1930 and demonstrated 30 years later with the help of the first coherent ruby lasers. The vanishingly small efficacy of the associated light-upconversion process was rapidly overcome by the implementation of powerful successive absorptions of two photons using linear optics in materials that possess real intermediate excited states working as relays. In these systems, the key point requires a favorable competition between the rate constant of the excited-state absorption (ESA) and the relaxation rate of the intermediate excited state, the lifetime of which should be thus maximized.

Bottom-Up Approach for the Rational Loading of Linear Oligomers and Polymers with Lanthanides.

The adducts between luminescent lanthanide tris(β-diketonate)s and diimine or triimine ligands have been explored exhaustively for their exceptional photophysical properties. Their formation, stability, and structures in solution, together with the design of extended metallopolymers exploiting these building blocks, remain, however, elusive. The systematic peripheral substitution of tridentate 2,6-bis(benzimidazol-2-yl)pyridine binding units ( = -), taken as building blocks for linear oligomers and polymers, allows a fine-tuning of their affinity toward neutral [Ln(hfa)] (hfa = hexafluoroacetylacetonate) lanthanide containers in the [Ln(hfa)] adducts.

Ligand-Sensitized Near-Infrared to Visible Linear Light Upconversion in a Discrete Molecular Erbium Complex.

While the low-absorption cross section of lanthanide-based upconversion systems, in which the trivalent lanthanides (Ln) are responsible for converting low- to high-energy photons, has restricted their application to intense incident light, the emergence of a cascade sensitization through an organic dye antenna capable of broadly harvesting near-infrared (NIR) light in upconversion nanoparticles opened new horizons in the field. With the aim of pushing molecular upconversion within the range of practical applications, we show herein how the incorporation of an NIR organic dye antenna into the ligand scaffold of a mononuclear erbium coordination complex boosts the upconversion brightness of the molecule to such an extent that a low-power (0.7 W·cm) NIR laser excitation of [Er(hfa)] (hfa = hexafluoroacetylacetonate) at 801 nm results in a measurable visible upconverted signal in a dilute solution (5 × 10 M) at room temperature.

Molecular light-upconversion: we have had a problem! When excited state absorption (ESA) overcomes energy transfer upconversion (ETU) in Cr(III)/Er(III) complexes.

Nine-coordinate [ErN9] or [ErN3O6] chromophores found in triple helical [Er(L)3]3+ complexes (L corresponds to 2,2',6',2''-terpyridine (tpy), 2,6-(bisbenzimidazol-2-yl)pyridine (bzimpy), 2,6-diethylcarboxypyridine (dpa-ester) or 2,6-diethylcarboxamidopyridine (dpa-diamide) derivatives), [Er(dpa)3]3- (dpa is the 2,6-dipicolinate dianion) and [GaErGa(bpb-bzimpy)3]9+ (bpb-bzimpy is 2,6-bis((pyridin-2-benzimidazol-5-yl)methyl-(benzimidazol-2-yl))pyridine) exhibit NIR (excitation at 801 nm) into visible (emission at 542 nm) linear light upconversion processes in acetonitrile at room temperature. The associated quantum yields 5.5(6) × 10-11 ≤ φuptot(ESA) ≤ 1.

Deciphering and quantifying linear light upconversion in molecular erbium complexes.

Single-center light upconversion corresponds to the piling up of low-energy photons successive linear absorptions: a phenomenon commonly observed in lanthanide-doped low-phonon ionic solids or nanoparticles. Its ultimate miniaturization in molecular complexes opens challenging perspectives in terms of improved reproducibility, chemical control and optical programming. However, high-energy vibrations inherent in coordination complexes severely limit the efficiency of successive excited-state absorptions (ESAs) responsible for the gain in photon energy.

Neutral Heteroleptic Lanthanide Complexes for Unravelling Host-Guest Assemblies in Organic Solvents: The Law of Mass Action Revisited.

The binding of lanthanide containers [Ln(β-diketonate)dig] [dig = 1-methoxy-2-(2-methoxyethoxy)ethane] to aromatic tridentate N-donor ligands () in dichloromethane produces neutral nine-coordinate heteroleptic [Ln(β-diketonate)] complexes, the equilibrium reaction quotients of which vary with the total concentrations of the reacting partners. This problematic drift prevents the determination of both reliable thermodynamic stability constants and intrinsic host-guest affinities. The classical solution theory assigns this behavior to changes in the activity coefficients of the various partners in nonideal solutions, and a phenomenological approach attempts to quantitatively attribute this effect to some partition of the solvent molecules between bulk-innocent and contact-noninnocent contributors to the chemical potential.

Room-Temperature Linear Light Upconversion in a Mononuclear Erbium Molecular Complex.

To date, the piling up of successive photons of low energies (near infrared; NIR) using a single lanthanide center and linear optics to ultimately produce upconverted visible emission was restricted to low-phonon solid materials and nanoparticles. Now we show that the tight helical wrapping of three terdentate N-donor ligands around a single nine-coordinate trivalent erbium cation provides favorable conditions for a mononuclear molecular complex to exhibit unprecedented related upconverted emission. Low power NIR laser excitations into the metal-centered transitions Er( I ← I ) at 801 nm or Er( I ← I ) at 966 nm result in upconverted blue-green emissions, where two or three photons respectively are successively absorbed by a molecular lanthanide complex possessing high-energy vibrations.

Thermodynamic Programming of Erbium(III) Coordination Complexes for Dual Visible/Near-Infrared Luminescence.

Intrigued by the unexpected room-temperature dual visible/near-infrared (NIR) luminescence observed for fast-relaxing erbium complexes embedded in triple-stranded helicates, in this contribution, we explore a series of six tridentate N-donor receptors L4-L9 with variable aromaticities and alkyl substituents to extricate the stereoelectronic features responsible for such scarce optical signatures. Detailed solid-state (X-ray diffraction, differential scanning calorimetry, optical spectroscopy) and solution (speciations and thermodynamic stabilities, spectrophotometry, NMR and optical spectroscopy) studies of mononuclear unsaturated [Er(Lk) ] and saturated triple-helical [Er(Lk) ] model complexes reveal that the stereoelectronic changes induced by the organic ligands affect inter- and intramolecular interactions to such an extent that 1) melting temperatures in solids, 2) the affinity for trivalent erbium in solution, and 3) optical properties in luminescent complexes can be rationally varied and controlled. With this toolkit in hand, mononuclear erbium complexes with low stabilities displaying only NIR emission can be transformed into molecular-based dual Er-centered visible/NIR emitters operating at room temperature in both solid and solution states.

A Rational Approach to Metal Loading of Organic Multi-Site Polymers: Illusion or Reality?

Since its identification as an independent topic after the first world war, the chemistry of (bio)polymers and macromolecules rapidly benefited from intense synthetic activities driven by contributors focusing on formulation and structural aspects. Satisfying rationalization and predictions concerning polymer organization, stability, and reactivity were, however, delayed until the late fifties, when physical chemists set the basis of an adapted thermodynamic modeling. The recent emergence of metal-containing (bio)organic polymers (i.

Controlling Lanthanide Exchange in Triple-Stranded Helicates: A Way to Optimize Molecular Light-Upconversion.

The kinetic lability of hexadentate gallium-based tripods is sufficient to ensure thermodynamic self-assembly of luminescent heterodimetallic [GaLn(L3) ] helicates on the hour time scale, where Ln is a trivalent 4f-block cation. The inertness is, however, large enough for preserving the triple-helical structure when [GaLn(L3) ] is exposed to lanthanide exchange. The connection of a second gallium-based tripod further slows down the exchange processes to such an extent that spectroscopically active [CrErCr(L4) ] can be diluted into closed-shell [GaYGa(L4) ] matrices without metal scrambling.

Cr as an alternative to Ru in metallo-supramolecular chemistry.

Compared to divalent ruthenium coordination complexes, which are widely exploited as parts of multi-component photonic devices, optically active trivalent chromium complexes are under-represented in multi-metallic supramolecular architectures performing energy conversion mainly because of the tricky preparation of stable heteroleptic Cr building blocks. We herein propose some improvements with the synthesis of a novel family of kinetically inert heteroleptic bis-terdentate mononuclear complexes, which can be incorporated into dinuclear rod-like dyads as a proof-of-concept. The mechanism and magnitude of intermetallic CrCr communication have been unraveled by a combination of magnetic, photophysical and thermodynamic investigations.

Taming Lanthanide-Centered Upconversion at the Molecular Level.

Considered at the beginning of the 21th century as being incompatible with the presence of closely bound high-energy oscillators, lanthanide-centered superexcitation, which is the raising of an already excited electron to an even higher level by excited-state energy absorption, is therefore a very active topic strictly limited to the statistical doping of low-phonon bulk solids and nanoparticles. We show here that molecular lanthanide-containing coordination complexes may be judiciously tuned to overcome these limitations and to induce near-infrared (NIR)-to-visible (VIS)-light upconversion via the successive absorption of two low-energy photons using linear-optical responses. Whereas single-ion-centered excited-state absorption mechanisms remain difficult to implement in lanthanide complexes, the skillful design of intramolecular intermetallic energy-transfer processes operating in multimetallic architectures is at the origin of the recent programming of erbium-centered molecular upconversion.

Metal loading of lanthanidopolymers driven by positive cooperativity.

This work demonstrates how the thermodynamic loading of monodisperse polymeric single-stranded multi-tridentate receptors of variable lengths is controlled by the nature of the metallic carrier Ln(hfac)3 (Ln is La, Eu or Y, and hfac is hexafluoroacetylacetonate). Whereas the intrinsic affinity of the tridentate binding site is maximum for medium-sized Eu(3+) and decreases for Y(3+), the contraction of the hydrodynamic radius of the polymer upon metal loading induces positive allosteric cooperativity for the smaller cations. The origin of this behaviour is rationalized within the frame of intermetallic dipole-dipole interactions modulated by the solvation potential of dipolar solutes in dielectric materials.

Smaller than a nanoparticle with the design of discrete polynuclear molecular complexes displaying near-infrared to visible upconversion.

This work shows that the operation of near-infrared to visible light-upconversion in a discrete molecule is not limited to non-linear optical processes, but may result from superexcitation processes using linear optics. The design of nine-coordinate metallic sites made up of neutral N-heterocyclic donor atoms in kinetically inert dinuclear [GaEr(L1)(3)](6+) and trinuclear [GaErGa(L2)(3)](9+) helicates leads to [ErN(9)] chromophores displaying unprecedented dual visible nanosecond Er((4)S(3/2)→(4)I15/2) and near-infrared microsecond Er((4)I(13/2)→(4)I1(5/2)) emissive components. Attempts to induce one ion excited-state absorption (ESA) upconversion upon near-infrared excitation of these complexes failed because of the too-faint Er-centred absorption cross sections.

Lanthanide-to-lanthanide energy-transfer processes operating in discrete polynuclear complexes: can trivalent europium be used as a local structural probe?

This work, based on the synthesis and analysis of chemical compounds, describes a kinetic approach for identifying intramolecular intermetallic energy-transfer processes operating in discrete polynuclear lanthanide complexes, with a special emphasis on europium-containing entities. When all coordination sites are identical in a (supra)molecular complex, only heterometallic communications are experimentally accessible and a Tb → Eu energy transfer could be evidenced in [TbEu(L5)(hfac)6] (hfac = hexafluoroacetylacetonate), in which the intermetallic separation amounts to 12.6 Å.

Lanthanide loading of luminescent multi-tridentate polymers under thermodynamic control.

This work illustrates the use of basic statistical mechanics for rationalizing the loading of linear multitridentate polymers with trivalent lanthanides, Ln(III), and identifies the specific ionic sizes of europium and yttrium as promising candidates for the further design of organized heterometallic f–f′ materials. Using [Ln(hfac)3] (hfac = hexafluoroacetylacetonate) as lanthanide carriers, the thermodynamically controlled formation of Wolf type-II lanthanidopolymers [{Ln(hfac)3}m(L4)] is modeled with the help of two simple microscopic descriptors: (i) the intrinsic affinity of Ln(III) for the tridentate binding sites fN3(Ln) and (ii) the intermetallic interactions ΔE1–2(Ln,Ln) operating between two occupied adjacent sites. Selective complexation (fN3La << fN3Eu > fN3(Y)) modulated by anticooperative interactions (ΔE1–2(La,La) ≃ ΔE1–2(Eu,Eu) > ΔE1–2(Y,Y) ≈ 0) favors the fixation of Eu(III) in semiorganized lanthanidopolymers [{Eu(hfac)3}m(L4)] displaying exploitable light-downshifting.

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.

N-Heterocyclic tridentate aromatic ligands bound to [Ln(hexafluoroacetylacetonate)3] units: thermodynamic, structural, and luminescent properties.

Herein, we discuss how, why, and when cascade complexation reactions produce stable, mononuclear, luminescent ternary complexes, by considering the binding of hexafluoroacetylacetonate anions (hfac(-)) and neutral, semi-rigid, tridentate 2,6-bis(benzimidazol-2-yl)pyridine ligands (Lk) to trivalent lanthanide atoms (Ln(III)). The solid-state structures of [Ln(Lk)(hfac)(3)] (Ln=La, Eu, Lu) showed that [Ln(hfac)(3)] behaved as a neutral six-coordinate lanthanide carrier with remarkable properties: 1) the strong cohesion between the trivalent cation and the didentate hfac anions prevented salt dissociation; 2) the electron-withdrawing trifluoromethyl substituents limited charge-neutralization and favored cascade complexation with Lk; 3) nine-coordination was preserved for [Ln(Lk)(hfac)(3)] for the complete lanthanide series, whilst a counterintuitive trend showed that the complexes formed with the smaller lanthanide elements were destabilized. Thermodynamic and NMR spectroscopic studies in solution confirmed that these characteristics were retained for solvated molecules, but the operation of concerted anion/ligand transfers with the larger cations induced subtle structural variations.

Dimerization of dendrimeric lanthanide complexes: thermodynamic, thermal, and liquid-crystalline properties.

A series of 10 different mesomorphic semidendrimeric tridentate ligands L5-L14 grafted with terminal cyanobiphenyl groups have been synthesized. Upon reaction with Ln(NO(3))(3) (Ln = trivalent lanthanide), the central 2,6-bis(N-ethylbenzimidazol-2-yl)pyridine unit is meridionally tricoordinated to the metal to give rodlike monomeric [Ln(Lk)(NO(3))(3)] and H-shaped dimeric [Ln(2)(Lk)(2)(NO(3))(6)] complexes. For the small Lu(III) cation, the monomeric complexes are quantitatively formed in a noncoordinating CD(2)Cl(2) solution.

Rational tuning of melting entropies for designing luminescent lanthanide-containing thermotropic liquid crystals at room temperature.

The connection of twelve peripheral and divergent dodecyloxy chains to a central tridentate aromatic binding unit provides the dodecacatenar ligand L11, for which room-temperature mesomorphism is detected. An enthalpically unbalanced large melting entropy (DeltaSmL11=226 J mol(-1) K(-1)) results from the programmed microsegregation induced in the crystalline phase, a phenomenon which is maintained in the associated lanthanide complexes [Ln(L11)(NO3)3] and [Ln(L11)(CF3CO2)3]2. Low-temperature melting processes (-43 View Article and Find Full Text PDF

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.