Heteropolynuclear Lanthanide(III) Complexes for Cooperative Sensitization Upconversion in Water

We report the synthesis of a tritopic ligand, L², composed of two strongly binding lanthanide (Ln) sites using tris-functionalized triazacyclononane (tacn) scaffolds bridged by a weaker Ln binding triethylene glycol chain. Coordination chemistry of Ln³⁺ (Ln = Eu, Tb, Yb, Lu) was investigated by using NMR and photoluminescent spectroscopies. The first two Ln³⁺ ions are coordinated by the tacn scaffolds to form [LnL²] and [Ln₂L²] species, followed by tri- and tetranuclear complexes, [Ln(Ln₂L²)] and [Ln₂(Ln₂L²)]. The third and fourth exomacrocyclic binding events occur at the polyethylene glycol binding site, buttressed by a synergistic interaction of the phosphonate arms, confirmed by DFT modeling. [Ln₂L²] (Ln = Tb, Eu, Yb, and Lu) homobimetallic complexes were prepared, and characterized and their spectroscopic properties determined in H₂O and D₂O. Titration of the [Yb₂L²] dinuclear complex by Tb salts in D₂O confirmed the formation of the tri- and tetranuclear species. Upon excitation into the ²F_5/2 ← ²F_7/2 absorption band of Yb at 980 nm, a cooperative sensitization upconversion process is evidenced, displaying visible Tb emission bands. Heating resulted in Ln scrambling in the tacn coordination sites, increasing the UC efficiency by ca. 10³. The most efficient emitter for UC is the tetranuclear [TbYb(TbYbL²)], with one of each Ln³⁺ species in the tacn scaffolds and one of each Ln³⁺ species coordinated by the polyethylene glycol chain. Optimization on the pD led to an overall 9.0 × 10^-7 UC quantum yield (λ_exc = 980 nm, P = 10.8 W·cm^-2). The same experiment was repeated in water, affording UC at the molecular level.