Thermodynamic Programming of Erbium(III) Coordination Complexes for Dual Visible/Near-Infrared Luminescence

Intrigued by the unexpected room-temperature dual visible/near-infrared (NIR) luminescence observed for
fast-relaxing erbium complexes embedded in triple-stranded helicates, in this contribution, we explore a series of six tridentate
N-donor receptors L4–L9 with variable aromaticities and alkyl substituents to extricate the stereoelectronic features
responsible for such scarce optical signatures. Detailed solid-state (X-ray diffraction, differential scanning calorimetry,
optical spectroscopy) and solution (speciations and thermodynamic
stabilities, spectrophotometry, NMR and optical spectroscopy) studies of mononuclear unsaturated
[Er(Lk)2]3+ and saturated triple-helical [Er(Lk)3]3+ model complexes reveal that the stereoelectronic changes induced by
the organic ligands affect inter- and intramolecular interactions to such an extent that 1) melting temperatures in
solids, 2) the affinity for trivalent erbium in solution, and 3) optical properties in luminescent complexes can be rationally
varied and controlled. With this toolkit in hand, mononuclear erbium complexes with low stabilities displaying
only NIR emission can be transformed into molecularbased dual Er-centered visible/NIR emitters operating at
room temperature in both solid and solution states.