Complexes Eu(III) and Gd(III) with 1-([1,1'-Biphenyl]-4-yl)-4,4,4-trifluorobutane-1,3-dione: π-Stacking and Luminescence Properties

Two new coordination compounds of europium(III) and gadolinium(III) with biphenyl-substituted β-diketone and 1,10-phenanthroline (Phen) ligands, [Eu(BPhTFB)3(Phen)] and [Gd(BPhTFB)(Phen)] (HBPhTFB = 1-([1,1′-biphenyl]-4-yl)-4,4,4-trifluorobutane-1,3-dione), have been synthesized and comprehensively studied. The introduction of a biphenyl fragment into the β-diketone ligand extends the π-conjugation system and enhances light absorption in the near-visible region, while the use of Phen as an ancillary ligand eliminates inner-sphere water or alcohol molecules that typically cause luminescence quenching. Structural studies revealed that the biphenyl moiety adopts a twisted conformation, enabling π–π stacking interactions with the phenanthroline ligand. This stacking leads to the formation of intermolecular charge-transfer states, as evidenced by features observed in the excitation spectra. The europium complex [Eu(BPhTFB)(Phen)] exhibits intense red emission characteristic of the 5D0 → 7FJ transitions, with an internal quantum yield of 57% and an overall luminescence quantum yield of 32%. The gadolinium analogue serves as a non-luminescent reference, confirming the ligand-centered origin of the sensitization process. The combined structural and photophysical data demonstrate that introducing extended aromatic substituents and appropriate ancillary ligands effectively enhances the luminescence performance of europium β-diketonate complexes, making them promising candidates for use in photonic and sensing applications.