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

Metal-Assisted Synthesis of Extended Polyaromatic Nitrogen-Rich Ligands for Tunable Sensitization of Eu(III) Complexes

We report the metal-assisted synthesis of oligomeric nitrogen-rich DiL1 and PoL1n sensitizers from the monomer L1 via acyclic diene metathesis (ADMET) polymerization for the tuning of the photoluminescence properties of europium-containing complexes by ligand design. The binding of [Eu(hfac)3] cargoes (H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) to monomer L1 to give [L1Eu(hfac)3] during metathesis appeared to be essential (i) for protecting the Grubbs ruthenium catalyst to interact with the nitrogen atoms of L1 and (ii) to drive the Ru-catalyzed ADMET reaction toward dimeric {DiL1[Eu(hfac)3]2} and polymeric {PoL1n[Eu(hfac)3]n} assemblies. Compared with the monomeric tridentate L1-L4 ligands, significant electronic delocalization occurs through the π-conjugated benzimidazole-CH═CH-benzimidazole skeletons in the dimer DiL1 and polymers PoL1n, which results in a stepwise red-shift of the excited levels in these extended polyaromatic ligands. As a consequence, the sensitization of bound [Eu(hfac)3] units reveals a decrease of the photoluminescence quantum yields along the [L4Eu(hfac)3] > [L3Eu(hfac)3] > [L2Eu(hfac)3] ≈ [L1Eu(hfac)3] > {DiL1[Eu(hfac)3]2} > {PoL118[Eu(hfac)3]18} series due to unfavorable europium-to-ligand back energy transfers.

Tuning Selectivity and Stability in Heteroleptic Lanthanide Adducts by Ligand Design

Terdentate ligands L10-L14 and their heteroleptic [LkLn(hfac)3] complexes (Ln = La, Eu, Gd, Er, or Y; H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) exhibit multifactorial correlations between the ligand's structural frameworks, including their level of preorganization and steric congestion and their affinities and selectivities for catching the trivalent lanthanide containers [Ln(hfac)3]. The polyaromatic ligand scaffolds could be stepwise modulated via lanthanide-template synthetic strategies, using intermolecular rhodium-catalyzed insertion reactions. The increasing level of preorganization along the L10 → L11 → L12 series leads to a duality in which larger thermodynamic formation constants with lanthanides in CD2Cl2 are accompanied by an unexpected decrease in the Ln-N affinities in the solid state, which could be assigned to a limited match between the lanthanide size and the enlarged preorganized cavities. On the contrary, a reduced stability is induced by the connection of additional methyl groups at position 1 of the benzimidazole moieties in L13 and L14, which is accompanied by an optimization of metal-nitrogen bond lengths. This study contributes to the rational design of highly stable neutral heteroleptic lanthanide β-diketonate adducts that resist dissociation in solution, a prerequisite for photophysical applications using these highly luminescent systems at the molecular level.