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)₃ LnL4 Ln(hfac)₃ ] ≡ [L4 Ln₂ (hfac)₆ ] adducts. Their crystal structures confirm the formation of dimeric molecular scaffolds made of two nine-coordinated trivalent [LnN₃ O₆ ] 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, [L4 Er₂ (hfac)₆ ] exhibits ESA light upconversion with blueish-green emissions at 525 and 542 nm corresponding to Er(² H_11/2 ,⁴ S_3/2 → ⁴ I_15/2 ) transitions. Thanks to the pertinent speciation in dichloromethane, we could extract a reliable upconversion quantum yield and brightness for the targeted dinuclear [L4 Er₂ (hfac)₆ ] adduct in solution. They largely overpass by 2 orders of magnitude those of the unsaturated mononuclear [L4 Er(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.