Dinuclear Heteroleptic Erbium Complexes for Improving Molecular-Based Light Upconversion in Solution?

In an attempt to boost molecular-based excited-state absorption (ESA) via cross-relaxation (CR), the back-to-back ditridentate polyaromatic 2,2',6,6'-tetrakis(1-methyl-1H-benzo[d]imidazole-2-yl)-4,4'-bipyridine ligand (L4) was reacted with neutral [Ln(hfac)3] lanthanide cargoes (Ln = Y, Eu, and Er and H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) to give dinuclear erbium [(hfac)3LnL4Ln(hfac)3] ≡ [L4Ln2(hfac)6] adducts. Their crystal structures confirm the formation of dimeric molecular scaffolds made of two nine-coordinated trivalent [LnN3O6] chromophores separated by a 4,4'-bipyridine bridge with Ln···Ln distances within the nanometric range (Eu···Eu = 11.69 Å, Y···Y = 11.65 Å, and Er···Er = 12.12 Å). Thermodynamic studies in dichloromethane provide critical insights into the formation and stability of these adducts. Under near-infrared (NIR) excitation at 801 nm in solution, [L4Er2(hfac)6] exhibits ESA light upconversion with blueish-green emissions at 525 and 542 nm corresponding to Er(2H11/2,4S3/2 → 4I15/2) transitions. Thanks to the pertinent speciation in dichloromethane, we could extract a reliable upconversion quantum yield and brightness for the targeted dinuclear [L4Er2(hfac)6] adduct in solution. They largely overpass by 2 orders of magnitude those of the unsaturated mononuclear [L4Er(hfac)3] intermediate but remain comparable to data reported for related saturated monomeric adducts in the same conditions. No global beneficial cross-relaxation effect could thus be unambiguously identified.