Intrigued by the unexpected room-temperature dual visible-NIR luminescence observed for fastrelaxing erbium complexes embedded in triple-stranded helicates, this contribution explores a series of six tridentate N-donor receptors L4-L9 with variable aromaticities and alkyl substituents for extricating the stereo-electronic requirements 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 stereo-electronic changes induced by the organic ligands affect inter- and intra-molecular interactions to such an extent that (i) melting temperatures in solids, (ii) the affinity for trivalent erbium in solution, and (iii) optical properties in luminescent complexes can be rationally varied and controlled. With this toolkit in hand, mononuclear erbium complexes with low stabilities displaying only near-infrared (NIR) emission can be transformed into molecular-based dual Ercentered visible/NIR emitters operating at room temperature in solids and in solutions.