Aggregation-Induced Emission of Eu^III Complexes Balanced with Bulky and Amphiphilic Imidazolium Cations in...

A series of Eu^III complexes with thenoyltrifluoroacetone (tta) as the ligands were synthesized, which are balanced with imidazolium cations bearing naphthyl and branched alkyl chains. The complex with the longest alkyl chain (1Eu(tta)₄ ) shows aggregation behavior in ethanol and water mixtures. It exists as individuals at low water percentages (V_H2O ), and form particles with solid interiors at V_H2O ≥50 % the sizes of which decrease with the increase of V_H2O . At V_H2O =90 %, vesicles were observed. The aggregation of 1Eu(tta)₄ in ethanol and water binary mixtures induced changes of the photoluminescence (PL) properties where an abrupt increase in PL intensity, quantum yield, and lifetime were noticed above V_H2O =50 %. This PL enhancement can be explained by the encapsulation of 1Eu(tta)₄ inside the aggregates, preventing it from quenching by the active hydroxyls of water molecules. Contrary to the aggregation-induced emission (AIE) of the Eu complex, the naphthyl group in the imidazolium cation exhibited an aggregation-caused quenching (ACQ) effect. When the length of the alkyl chain in the imidazolium cation is reduced, no PL enhancement can be observed owing to the difficulty in forming the aggregates.