Previously limited to highly symmetrical homoleptic triple-helical complexes [Er(Lk)₃]³⁺, where Lk are polyaromatic tridentate ligands, single-center molecular-based upconversion using linear optics and exploiting the excited-state absorption mechanism (ESA) greatly benefits from the design of stable and low-symmetrical [LkEr(hfa)₃] heteroleptic adducts (hfa⁻=hexafluoroacetylacetonate anion). Depending on (i) the extended π-electron delocalization, (ii) the flexibility and (iii) the heavy atom effect brought by the bound ligand Lk, the near-infrared (801 nm) to visible green (542 nm) upconversion quantum yield measured for [LkEr(hfa)₃] in solution at room temperature can be boosted by up to three orders of magnitude.