Piling up excited states to reach upconversion (UC) is severely restricted by vibrational quenching mechanisms, especially when one looks at discrete molecular entities in solution. By carefully controlling the supramolecular assembly processes resulting from the strong electrostatic interactions between negatively charged Yb complexes and Tb 3+ cations in aqueous solutions, we engineered the formation of heteropolynuclear complexes of [(YbL) 2 Tb x ] compositions (x = 1 and 2). These edifices display a phenomenon of cooperative photosensitization UC with green emission of the Tb cations upon NIR excitation at 980 nm in the Yb absorption band. The photophysical properties of the complexes were carefully investigated by steady-state and time-resolved luminescence experiments in D 2 O, allowing to quantify the impact of the composition and pD of the solution on the emission intensity, as well as clarifying the exact cooperative photosensitization upconversion mechanism. Using optimized conditions, the energy transfer UC process could be observed for the first time in non-deuterated water with discrete molecular compounds.

A series of polynuclear assemblies based on ligand L (1,4,7-tris[hydrogen (6-methylpyridin-2-yl)phosphonate]-1,4,7-triazacyclononane) has been developed. The coordination properties of ligand L with Ln (Ln = La, Eu, Tb, Yb, Lu) have been studied in water (pH = 7.0) and in DO (pD = 7.0) by UV-absorption spectrometry, spectrofluorimetry, H andP NMR, DOSY, ESI-mass spectrometry, and X-ray diffraction. This nonadentate ligand forms highly stable mononuclear complexes in water and provides a very efficient shielding of the Ln cations, as emphasized by the very good luminescence properties of the Yb complex in DO, especially regarding its lifetime (τ = 10.2 μs) and quantum yield (ϕ = 0.42%). In the presence of excess Ln cation, polynuclar complexes of [(LnL)Ln ] stoichiometry (x = 1 and x = 2) are observed in solution. In the solid state, a dinuclear complex of La could be isolated and structurally characterized by X-ray diffraction, unraveling the presence of strong hydrogen bonding interactions between a La(HO) cation and the [LaL] complex.