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

We report the metal-assisted synthesis of oligomeric nitrogen-rich DiL1 and PoL1n 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 [L1Eu(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 {PoL1n[Eu(hfac)3]n} assemblies. Compared with the monomeric tridentate L1−L4 ligands,
significant electronic delocalization occurs through the π-conjugated benzimidazole−CHCH−benzimidazole
skeletons in the dimer DiL1 and polymers PoL1n, 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 [L4Eu(hfac)3] > [L3Eu(hfac)3] > [L2Eu(hfac)3]
≈ [L1Eu(hfac)3] > {DiL1[Eu(hfac)3]2} > {PoL118[Eu(hfac)3]18} series due to unfavorable europium-to-ligand
back energy transfers.