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. In the presence of different coordination sites, as found in the trinuclear complex [Eu3(L2)(hfac)9], homometallic communication can be induced by selective laser excitation and monitored with the help of high-resolution emission spectroscopy. The narrow and non-degenerated character of the Eu((5)D0 ↔ (7)F0) transition excludes significant spectral overlap between donor and acceptor europium cations. Intramolecular energy-transfer processes in discrete polynuclear europium complexes are therefore limited to short distances, in agreement with the Fermi golden rule and with the kinetic data collected for [Eu3(L2)(hfac)9] in the solid state and in solution. Consequently, trivalent europium can be considered as a valuable local structural probe in discrete polynuclear complexes displaying intermetallic separation in the sub-nanometric domain, a useful property for probing lanthanido-polymers.

Invited for the cover of this issue is the group of Emmanuel Terazzi at the University of Geneva, Switzerland. The cover image shows how single-stranded dinuclear lanthanide helicates (green) survive immersion into a drop of less polar solvent (e.g. CH 2 Cl 2 ) and their eventual self-organization into a liquid-crystalline material.

This work demonstrates how minor structural and electronic changes between Ln(NO3)3 and Ln(hfac)3 lanthanide carriers (Ln = trivalent lanthanide, hfac = hexafluoroacetylacetonate) lead to opposite thermodynamic protocols for the metal loading of luminescent polynuclear single-stranded oligomers. Whereas metal clustering is relevant for Ln(hfac)3, the successive fixation of Ln(NO3)3 provides stable microspecies with an alternated occupancy of the binding sites. Partial anion dissociation and anion/ligand bi-exchange processes occur in polar aprotic solvents, which contribute to delay the unambiguous choice of a well-behaved neutral lanthanide carrier for the selective complexation of different trivalent lanthanides along a single ligand strand. Clues for further improvement along this stepwise strategy are discussed.

The connection of flexible protodendritic wedges to the bis-tridentate rigid polyaromatic ligand L1 provides amphiphilic receptors L5 and L6; their reduced affinities for complexing trivalent lanthanides (Ln = La, Y, Lu) in organic solvent (by fifteen orders of magnitude!) prevent the formation of the expected dinuclear triple-stranded helicates [Ln2(Lk)3]6+. This limitation could be turned into an advantage because L1 or L6 can be treated with [Ln(hfac)3] (Hhfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione) to give neutral single-stranded [Ln2(Lk)(hfac)6] complexes with no trace of higher-order helicates. Whereas ligands L1 and L5 are not liquid crystals, L6 can be melted above room temperature (41 degrees C) to give a nematic mesophase, and its associated dinuclear helical complex [Y2(L6)(hfac)6] self-organises at the same temperature into a fluidic smectic mesophase.

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. Combined with the strong red photoluminescence of [Eu(Lk)(hfac)(3)], the ternary system Ln(III)/hfac(-)/Lk is a promising candidate for the planned metal-loading of preformed multi-tridentate polymers.