Microwave Synthesis and Luminescence Efficiencies in Mixed-Ligand Europium Complexes

The microwave-assisted methodology is now extended and fine-tuned for the synthesis of mixed-ligand europium complexes with an average reaction time of 12 min. Overall, 14 different complexes were synthesized to improve luminescence using our previously proposed strategy to boost luminescence through ligand diversification, specifically by applying it to quaternary europium complexes with at least one DBM (1,3-diphenylpropane-1,3-dionate) ligand. DBM is a strong absorbant of UV radiation that can dissipate energy through nonradiative channels; thus, it is a useful molecular scaffold for sunblockers and cosmetics. Accordingly, the following luminescent tetrakis and quaternary complexes were prepared: K[Eu(DBM)₄], K[Eu(β)₄], K[Eu(DBM)₃(β)], K[Eu(DBM)₂(β)₂], K[Eu(DBM)₂(β)(β')], and the fully mixed complex K[Eu(DBM)(BTFA)(TTA)(HFAC)], where β can be either BTFA (4,4,4-trifluoro-1-phenylbutane-1,3-dionate), TTA (4,4,4-trifluoro-1-(2-thienyl)butane-1,3-dionate), or HFAC (1,1,1,5,5,5-hexafluoropentane-2,4-dionate). For all the complexes, luminescence experiments were performed in chloroform and acetone solutions. Our findings confirm that mixed-ligand complexes exhibit superior quantum efficiencies compared to the average of their homoleptic counterparts. The presence of DBM in the complexes tends to dramatically increase the nonradiative decay rates of the solutions. Finally, we present formulae that provide a detailed understanding of the distinctive roles of each ligand and their relevant interactions in luminescence.