Tuning luminescent property of trivalent europium complex nanocrystals with fluorinated anionic ligands

Although the f-f transitions of lanthanide ions have many unique luminescent properties such as high color purity and lifetime ranging from micro- to milliseconds, their luminescence can be quenched by the high-frequency vibration of X-H (X = C, N, O) of solvents. Herein, we present synthesis and luminescent property of the f-f transition trivalent europium complex nanocrystals, which are successfully prepared with a reverse-micelle strategy with oleylamine (OAm) as the surfactant and n-hexane as the nonpolar solvent. Four Eu(III) complexes are synthesized with 1,3-Diphenylpropane-1,3-dione (DBM), 1-(2-Thenoyl)-3,3,3-trifluoroacetone (TTA), 4,4,4-trifluoro-1-phenylbutane-1,3-dione (BTFA), and 4,4,4-trifluoro-1-(naphthalen-2-yl)butane-1,3-dione (NTA), respectively, as the anionic ligands and 4,4'-di-tert-butyl-2,2'-bipyridine as the neutral ligand. The complex nanocrystals show characteristic red emission of Eu(III) complexes. Their photoluminescence quantum yields are increased from 12.5 % to 28.0 %, 30.7 % and 35.2 % as the anionic ligand is changed from DBM to TTA, BTFA, and NTA, which induces compacted packing of the Eu (III) complex molecules by the noncovalent interactions deduced from the substituted fluorine and π-chromophores. The emission enhancement is consistent with the increase of the ratios of the radiative-to-nonradiative rate constants, in which the radiative rate constants are independent on the structure of the anionic ligands, while the nonradiative rate constants are significantly suppressed with anionic ligands changing from DBM to TTA, BTFA, and finally NTA. In all, our results demonstrate that the f-f transition trivalent europium complexes can be formed into nanocrystals via the reverse-micellar approach and fluorinated ligands provide an effective strategy for tuning packing-induced luminescence enhancement.