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 2F5/2 ← 2F7/2 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 (λexc = 980 nm, P = 10.8 W·cm-2). The same experiment was repeated in water, affording UC at the molecular level.