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

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.