Definition of an Intramolecular Eu-to-Eu Energy Transfer within a Discrete [Eu2L] Complex in Solution

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer

The chromium(III) polypyridyl complexes are appealing for their long-lived near-infrared (NIR) emission reaching the millisecond range and for the strong circularly polarized luminescence of their isolated enantiomers. However, harnessing those properties in functional polynuclear Cr^III devices remains mainly inaccessible because of the lack of synthetic methods for their design and functionalization. Even the preparation and investigation of most basic nonsymmetrical Cr^III dyads exhibiting directional intramolecular intermetallic energy transfer remain unexplored. Taking advantage of the inertness of heteroleptic chromium(III) polypyridyl building blocks, we herein adapt the "complex-as-ligand" strategy, largely used with precious 4d and 5d metals, for the preparation of a binuclear nonsymmetrical Cr^III complex (3d metal). The resulting [(phen)₂Cr(L)Cr(tpy)]⁶⁺ dyad shows dual long-lived NIR emission and a directional intermetallic energy transfer that is controlled by the specific arrangements of the different coordination spheres. This strategy opens a route for building predetermined polynuclear assemblies with this earth-abundant metal.

The special case of the spectral emission of a Tb3+ mono metal complex

The fine structure in the spectral lines of the visible fluorescence of Tb 3+  complexes are replaced by a single peak in the case of a singular molecular complex Tb(H 3 PTC) 3  , where H 4 PTC represents perylene-3,4,9,10-tetracarboxylic acid, and its emission wavelength depends on the film thickness. This single peak challenges the old creed that the f-orbital electrons of Tb 3+  are always protected from the influence of the surrounding atoms. We perform density functional theory calculations to show that the wavefunction of the ground state is localized and in addition, spin-polarized, and this facilitates fluorescent transitions under UV to the first excited state instead of the fundamental state. We discuss the possibility of making a spintronic device with the molecule, Tb(H 3 PTC) 3  .

Accessing Lanthanide-to-Lanthanide Energy Transfer in a Family of Site-Resolved [LnIII LnIII '] Heterodimetallic Complexes

The ligand H₃ L (6-[3-oxo-3-(2-hydroxyphenyl)propionyl]pyridine-2-carboxylic acid), which exhibits two different coordination pockets, has been exploited to engender and study energy transfer (ET) in two dinuclear [Ln^III Ln^III '] analogues of interest, [EuYb] and [NdYb]. Their structural and physical properties have been compared with newly synthesised analogues featuring no possible ET ([EuLu], [NdLu], and [GdYb]) and with the corresponding homometallic [EuEu] and [NdNd] analogues, which have been previously reported. Photophysical data suggest that ET between Eu^III and Yb^III does not occur to a significant extent, whereas emission from Yb^III originates from sensitisation of the ligand. In contrast, energy migration seems to be occurring between the two Nd^III centres in [NdNd], as well as in [NdYb], in which Yb^III luminescence is thus, in part, sensitised by ET from Nd. This study shows the versatility of this molecular platform to further the investigation of lanthanide-to-lanthanide ET phenomena in defined molecular systems.

Formation of Heteropolynuclear Lanthanide Complexes Using Macrocyclic Phosphonated Cyclam-Based Ligands

Ligands L₁ and L₂, respectively based on a cyclam and a cross-bridged cyclam scaffold functionalized at N1 and N8 by 6-phosphonic-2-methylene pyridyl groups, are described. While complexation of lanthanide (Ln) cations with L₂ was not possible, a family of complexes has been prepared with L₁, of the general formulae [LnL₁H₂]Cl (Ln³⁺ = Lu, Tb, Yb) or [LnL₁H] (Ln³⁺ = Eu). The solution, structural, potentiometric, and photophysical data for these novel ligands and their complexes have been investigated, including a solid-state study by X-ray diffraction (L₁, L₂, and [EuL₁H]), ¹H NMR complexation investigations (Lu³⁺), as well as UV-vis absorption and luminescence spectroscopy in water and D₂O (pH ≈ 7). L₁ forms 1:1 metal-ligand stoichiometric octadentate complexes in solution. Importantly, the pyridyl phosphonate functions are capable of simultaneous chelation to the metal center and of interaction with a second metal center. ¹H NMR (Lu³⁺) and spectrophotometric titrations of the isolated [TbL₁]^- complex by EuCl₃ salts demonstrated the formation of high-order (hetero)polymetallic species in aqueous solution (H₂O, pH = 7). Global analysis of the luminescence titration experiment points to the formation of 4:1, 3:1, and 3:2 [TbL₁]/Eu heteropolynuclear assemblies, exhibiting a strong preference to forming [TbL₁]₃Eu₂ at increased europium concentrations, with energy transfer occurring between the kinetically inert terbium complex and added europium cations.

The role of ligand to metal charge-transfer states on the luminescence of Europium complexes with 18-membered macrocyclic ligands

The stabilization of a divalent Europium provides an efficient pathway for the quenching of the luminescence in ten-coordinate macrocyclic complexes.

Lanthanoid complexes supported by retro-Claisen condensation products of β-triketonates

β-Triketonates have been recently used as chelating ligands for lanthanoid ions, presenting unique structures varying from polynuclear assemblies to polymers. In an effort to overcome low solubility of the complexes of tribenzoylmethane, four β-triketones with higher lipophilicity were synthesised. Complexation reactions were performed for each of these molecules using different alkaline bases in alcoholic media. X-ray diffraction studies suggested that the ligands were undergoing decomposition under the reaction conditions. This is proposed to be caused by in situ retro-Claisen condensation reactions, consistent with two examples that have been reported previously. The lability of the lanthanoid cations in the presence of a varying set of potential ligands gave rise to structures where one, two, or three of the molecules involved in the retro-Claisen condensation reaction were linked to the lanthanoid centres. These results, along with measurements of ligand decomposition in the presence of base alone, suggest the solvent used will modulate the impact of the retro-Claisen condensation in these complexes.

Coordination Properties of GdDO3A-Based Model Compounds of Bioresponsive MRI Contrast Agents

We report a detailed characterization of the thermodynamic stability and dissociation kinetics of Gd³⁺ complexes with DO3A derivatives containing a (methylethylcarbamoylmethylamino)acetic acid (L¹), (methylpropylcarbamoylmethylamino)acetic acid (L²), 2-dimethylamino- N-ethylacetamide (L³), or 2-dimethylamino- N-propylacetamide (L⁴) group attached to the fourth nitrogen atom of the macrocyclic unit. These ligands are model systems of Ca²⁺- and Zn²⁺-responsive contrast agents (CA) for application in magnetic resonance imaging (MRI). The results of the potentiometric studies ( I = 0.15 M NaCl) provide stability constants with log K_GdL values in the range 13.9-14.8. The complex speciation in solution was found to be quite complicated due to the formation of protonated species at low pH, hydroxido complexes at high pH, and stable dinuclear complexes in the case of L^1,2. At neutral pH significant fractions of the complexes are protonated at the amine group of the amide side chain (log K_GdL×H = 7.2-8.1). These ligands form rather weak complexes with Mg²⁺ and Ca²⁺ but very stable complexes with Cu²⁺ (log K_CuL = 20.4-22.3) and Zn²⁺ (log K_ZnL = 15.5-17.6). Structural studies using a combination of ¹H NMR and luminescence spectroscopy show that the amide group of the ligand is coordinated to the metal ion at pH ∼8.5, while protonation of the amine group provokes the decoordination of the amide O atom and a concomitant increase in the hydration number and proton relaxivity. The dissociation of the complexes occurs mainly through a rather efficient proton-assisted pathway, which results in kinetic inertness comparable to that of nonmacrocyclic ligands such as DTPA rather than DOTA-like complexes.

Dinuclear Lanthanide(III)-m-ODO2A-dimer Macrocyclic Complexes: Solution Speciation, DFT Calculations, Luminescence Properties, and Promoted Nitrophenyl-Phosphate Hydrolysis Rates

Potentiometric speciation studies, mass spectrometry, and DFT calculations helped to predict the various structural possibilities of the dinuclear trivalent lanthanide ion (Ln^III , Ln=La, Eu, Tb, Yb, Y) complexes of a novel macrocyclic ligand, m-ODO2A-dimer (H₄ L), to correlate with their luminescence properties and the promoted BNPP and HPNP phosphodiester bond hydrolysis reaction rates. The stability constants of the dinuclear Ln₂ (m-ODO2A-dimer) complexes and various hydrolytic species confirmed by mass spectrometry were determined. DFT calculations revealed that the Y₂ LH_-1 and the Y₂ LH_-2 species tended to form structures with the respective closed- and open-form conformations. Luminescence lifetime data for the heterodimetallic TbEuL system confirmed the fluorescence resonance energy transfer from the Tb^III to Eu^III ion. The internuclear distance R_TbEu values were estimated to be in the range of 9.4-11.3 Å (pH 6.7-10.6), which were comparable to those of the DFT calculated open-form conformations. Multiple linear regression analysis of the k_obs data was performed using the equation: k_obs,corr. =k_obs -k_obs,OH =kLn2LHM->1 [Ln₂ LH_-1 ]+kLn2LH-2 [Ln₂ LH_-2 ] for the observed Ln₂ L-promoted BNPP/HPNP hydrolysis reactions in solution pH from 7 to 10.5 (Ln=Eu, Yb). The results showed that the second-order rate constants for the Eu₂ LH_-2 and Yb₂ LH_-2 species were about 50-400 times more reactive than the structural analogous Zn₂ (m-12 N₃ O-dimer) system.

On the consequences of the stereochemical activity of the Bi(iii) 6s2 lone pair in cyclen-based complexes. The [Bi(DO3A)] case

The spatial arrangement of donor atoms in Bi(iii) cyclen derivatives modulates the orientation and activity of the 6s² lone pair.

Cooperative loading of multisite receptors with lanthanide containers: an approach for organized luminescent metallopolymers

Metal loading of multi-terdentate receptors with [Eu(pbta)₃] provides the first anti-cooperative factors large enough for programming metal alternation in lanthanidopolymers at room temperature.

Metal-containing (bio)organic polymers are materials of continuously increasing importance for applications in energy storage and conversion, drug delivery, shape-memory items, supported catalysts, organic conductors and smart photonic devices. The embodiment of luminescent components provides a revolution in lighting and signaling with the ever-increasing development of polymeric light-emitting devices. Despite the unique properties expected from the introduction of optically and magnetically active lanthanides into organic polymers, the deficient control of the metal loading currently limits their design to empirical and poorly reproducible materials. We show here that the synthetic efforts required for producing soluble multi-site host systems Lk are largely overcome by the virtue of reversible thermodynamics for mastering the metal loading with the help of only two parameters: (1) the affinity of the luminescent lanthanide container for a single binding site and (2) the cooperative effect which modulates the successive fixation of metallic units to adjacent sites. When unsymmetrical perfluorobenzene-trifluoroacetylacetonate co-ligands (pbta^–) are selected for balancing the charge of the trivalent lanthanide cations, Ln³⁺, in six-coordinate [Ln(pbta)₃] containers, the explored anti-cooperative complexation processes induce nearest-neighbor intermetallic interactions twice as large as thermal energy at room temperature (RT = 2.5 kJ mol^–1). These values have no precedent when using standard symmetrical containers and they pave the way for programming metal alternation in luminescent lanthanidopolymers.

Metal loading of lanthanidopolymers driven by positive cooperativity

The contraction of the lanthanidopolymers [L3^N(Ln(hfac)₃)_m] (Ln is La, Eu or Y) observed upon metal loading with small lanthanides favours solvation in solution, a trend which induces positive cooperativity in the thermodynamic complexation process.