The connection of two Cr^III sensitizers around a central Er^III acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of Cr^III-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green Er^III-centered emission.

Triangular luminescent box: Self-assembly of a new multidentate receptor with europium cations results in the formation of trinuclear discrete complexes. X-ray crystallography shows that nine-coordinate cations are linked by ligands to provide a triangular complex in the solid state and in solution. Despite the coordinated solvent molecules, this topologically unusual complex exhibits remarkable luminescent properties.

Unsymmetrical substituted bidentate benzimidazol-2-ylpyridine ligands L2 and L3 react with [Ru(dmso)₄Cl₂] in ethanol to give statistical 1:3 mixtures of fac-[Ru(Li)₃]²⁺ and mer-[Ru(Li)₃]²⁺ (i=2, 3; ΔGΘ_{isomerisation}=-2.7 kJ mol^-1). In more polar solvents (acetonitrile, methanol), the free energy of the facial ↔ meridional isomerisation process favours mer-[Ru(Li)₃]²⁺, which is the only isomer observed in solution at the equilibrium (ΔGΘ_{isomerisation}≤-11.4 kJ mol^-1). Since the latter process takes several days for [Ru(L2)₃]²⁺, fac-[Ru(L2)₃]²⁺ and mer-[Ru(L2)₃]²⁺ have been separated by chromatography, but the 28-fold increase in velocity observed for [Ru(L3)₃]²⁺ provides only mer-[Ru(L3)₃](ClO₄)₂ after chromatography (RuC₆₀H₅₁N₉O₈Cl₂, monoclinic, P2₁/n, Z=4). The facial isomer can be stabilised when an appended tridentate binding unit, connected at the 5-position of the benzimidazol-2-ylpyridine unit in ligand L1, interacts with nine-coordinate lanthanides(III). The free energy of the facial↔meridional isomerisation is reversed (ΔGΘ_{isomerisation}≥11.4 kJ mol^-1), and the Ru — N bonds are labile enough to allow the quantitative thermodynamic self-assembly of HHH-[RuLu(L1)₃]⁵⁺ within hours ([RuLu(L1)₃](CF₃SO₃)_4.5Cl_0.5(CH₃OH)_2.5: RuLuC₁₀₆H₁₀₉Cl_0.5N₂₁O₁₉S_4.5F_13.5, triclinic, P, Z=2). Electrochemical and photophysical studies show that the benzimidazol-2-ylpyridine units in L1-L3 display similar π-acceptor properties to, but stronger π-donor properties than, those found in 2,2'-bipyridine. This shifts the intraligand π→π^* and the MLCT transitions toward lower energies in the pseudo-octahedral [Ru(Li)₃]²⁺ (i=2, 3) chromophores. The concomitant short lifetime of the ³MLCT excited state points to efficient, thermally activated quenching via low-energy Ru-centred d-d states, a limitation which is partially overcome by mechanical coupling in HHH-[RuLu(L1)₃]⁵⁺.