Energy transfer pathways in the carbazole functionalized β-diketonate europium complexes

Abstract

Two novel Eu³⁺ complexes Eu(CDBM)₃·2H₂O and Eu(CCDBM)₃·2H₂O have been synthesized (CDBM = 1-(4-(9-carbazol)phenyl)-3-phenyl-1,3-propanedione, CCDBM = 1-((4-(9-carbazol)methyl)phenyl)-3-phenyl-1,3-propanedione). Eu(CDBM)₃·2H₂O showed strong internal ligand charge transfer (ILCT) fluorescence and sensitized emission of Eu³⁺. Due to the charge transfer character, the fluorescence band of CDBM shifted from 403 nm in cyclohexane to 559 nm in acetonitrile. Photophysical studies demonstrated that no energy was migrated from the ILCT excited state of the ligands to Eu³⁺, and that Eu³⁺ was sensitized by the triplet state which was localized in the 1,3-diphenyl-1,3-propanedione (DBM) part. The quantum efficiencies of Eu³⁺ in the three complexes are in the order Eu(DBM)₃·2H₂O > Eu(CCDBM)₃·2H₂O > Eu(CDBM)₃·2H₂O in both solution and the solid state. The energy transfer pathways in the three Eu³⁺ complexes were discussed in detail. Based on the systematic photophysical studies, a new guideline for the organo-lanthanide light emitting materials has been proposed: ILCT should be avoided during molecular modification.