Low Energy Bioimaging through Molecular Up-Conversion Emission with Imidazolium- and Benzenetriol-Based Er(III) Complexes

New water-soluble imidazolium-based and benzenetriol-based ligands and their 3:2 ligand-to-lanthanide sandwich-type complexes, [Ln₂(L₁)₃H₂O], [Ln₂(L₂)₃H₂O], and [Ln₂(L₃)₃H₂O] (Ln = Er(III), Gd(III), and Lu(III)), were isolated. Upon 980 nm excitation, the Er(III) complexes exhibited metal-centered up-conversion emission in the solid state and in aqueous solution. When incubated in HeLa cells, the complexes appear to localize in the mitochondria, and red and green Er(III)-centered up-conversion emission was observed.