We report the metal-assisted synthesis of oligomeric nitrogen-rich DiL1 and PoL1 ⁿ sensitizers from the monomer L1 via acyclic diene metathesis (ADMET) polymerization for the tuning of the photoluminescence properties of europium-containing complexes by ligand design. The binding of [Eu(hfac)₃ ] cargoes (H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) to monomer L1 to give [L1 Eu(hfac)₃ ] during metathesis appeared to be essential (i) for protecting the Grubbs ruthenium catalyst to interact with the nitrogen atoms of L1 and (ii) to drive the Ru-catalyzed ADMET reaction toward dimeric {DiL1 [Eu(hfac)₃ ]₂ } and polymeric {PoL1 ⁿ [Eu(hfac)₃ ] _n } assemblies. Compared with the monomeric tridentate L1 -L4 ligands, significant electronic delocalization occurs through the π-conjugated benzimidazole-CH═CH-benzimidazole skeletons in the dimer DiL1 and polymers PoL1 ⁿ , which results in a stepwise red-shift of the excited levels in these extended polyaromatic ligands. As a consequence, the sensitization of bound [Eu(hfac)₃ ] units reveals a decrease of the photoluminescence quantum yields along the [L4 Eu(hfac)₃ ] > [L3 Eu(hfac)₃ ] > [L2 Eu(hfac)₃ ] ≈ [L1 Eu(hfac)₃ ] > {DiL1 [Eu(hfac)₃ ]₂ } > {PoL1 ¹⁸ [Eu(hfac)₃ ]₁₈ } series due to unfavorable europium-to-ligand back energy transfers.