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 pi-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.|Moving from homoleptic triple helical erbium complexes to heteroleptic adducts boosts the quantum yield of near-infrared to visible molecular-based light-upconversion by three orders of magnitude in solution at room temperature.image