Molecular Upconversion in Water in Heteropolynuclear Supramolecular Tb/Yb Assemblies

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 Tb3+ cations in aqueous solutions, we engineered the formation of heteropolynuclear
complexes of [(YbL)2Tbx] 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 D2O, allowing one 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 nondeuterated water with discrete molecular
compounds.