Achieving visible light excitation in carbazole-based Eu-β-diketonate complexes via molecular engineering

Herein, we present the synthesis, characterization and photophysical properties of a series of Eu complexes prepared with novel carbazole-based fluorinated β-diketones, namely, 4,4,5,5,5-pentafluoro-3-hydroxy-1-(9-phenyl-9H-carbazol-2-yl)pent-2-en-1-one (L1) and 4,4,5,5,5-pentafluoro-3-hydroxy-1-(9-(4-methoxyphenyl)-9H-carbazol-2-yl)pent-2-en-1-one (L2) as primary ligands and a bidentate phosphine oxide molecule, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene oxide (DDXPO) as ancillary ligand. Using the Sparkle/PM3 model the molecular geometries of the designed complexes are optimized and the luminescent parameters are calculated by the LUMPAC software. The results demonstrated that suitably expanded π-conjugation in the developed Eu-carbazole based β-diketonate complexes dramatically red-shifted the excitation maximum to the visible region (λ = 420 nm) with an impressive quantum yield (34-42%). The triplet state energy levels of L1 and L2 in the complexes are higher than that of the lowest excited level of Eu ion, D, so the photoluminescence mechanism of the Eu complexes was proposed as a ligand-sensitized luminescence process. The predicted luminescent parameters from the Sparkle/PM3 structures are in agreement with the experimental data, which shows the efficacy of the theoretical models adopted in the present study.