Metal-Assisted Synthesis of Extended PolyaromaticNitrogen-Rich Ligands for Tunable Sensitization of Eu(III) Complexes

We report the metal-assisted synthesis of oligomeric nitrogen-rich <b>DiL1</b> and <b>PoL1</b>^<i>n</i> sensitizers from the monomer <b>L1</b> 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 <b>L1</b> to give [<b>L1</b>Eu(hfac)₃] during metathesis appeared to be essential (i) for protecting the Grubbs ruthenium catalyst to interact with the nitrogen atoms of <b>L1</b> and (ii) to drive the Ru-catalyzed ADMET reaction toward dimeric {<b>DiL1</b>[Eu(hfac)₃]₂} and polymeric {<b>PoL1</b>^<i>n</i>[Eu(hfac)₃]_<i>n</i>} assemblies. Compared with the monomeric tridentate <b>L1</b>–<b>L4</b> ligands, significant electronic delocalization occurs through the π-conjugated benzimidazole–CHCH–benzimidazole skeletons in the dimer <b>DiL1</b> and polymers <b>PoL1</b>^<i>n</i>, 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 [<b>L4</b>Eu(hfac)₃] <b>></b> [<b>L3</b>Eu(hfac)₃] <b>></b> [<b>L2</b>Eu(hfac)₃] ≈ [<b>L1</b>Eu(hfac)₃] <b>></b> {<b>DiL1</b>[Eu(hfac)₃]₂} <b>></b> {<b>PoL1</b>^<b>18</b>[Eu(hfac)₃]₁₈} series due to unfavorable europium-to-ligand back energy transfers.