Upconversion in a d–f [RuYb₃] Supramolecular Assembly

We have prepared a hetero-tetrametallic assembly consisting of three ytterbium ions coordinated to a central [Ru­(bpm)₃]²⁺ (bpm = 2,2′-bipyrimidine) motif. Irradiation into the absorption band of the peripheral ytterbium ions at 980 nm engenders emission of the ³MLCT state of the central [Ru­(bpm)₃]²⁺ core at 636 nm, which represents the first example of f → d molecular upconversion (UC). Time-resolved measurements reveal a slow rise of the UC emission, which was modeled with a mathematical treatment of the observed kinetics according to a cooperative photosensitization mechanism using a virtual Yb centered doubly excited state followed by energy transfer to the Ru centered ¹MLCT state.