Nd-to-Yb energy transfer (ET) at the molecular level has been investigated in the novel heterobimetallic NdYb₂Q₉ (Q = 8-quinolinolate) complex where the Q ligand works as a high cross-sectional optical antenna. The remarkable ion size-driven templating effect of Nd³⁺ allows for good control of molecular speciation through a simple one-pot synthetic procedure. Short intermetallic distances and enhanced donor (Nd)/acceptor (Yb) spectral overlap in the molecular assembly strongly favor Nd-to-Yb energy transfer, which reaches nearly unitary efficiency, while detrimental processes such as concentration quenching and energy back transfer are ruled out. The devised approach allows us to obtain a controlled assembly of heterolanthanide compounds whose features hold potential for the development of highly performing Yb³⁺-based optical materials.