4,4,5,5,5-Pentafluoro-1-(9H-fluoren-2-yl)-1,3-pentanedione complex of Eu3+ with 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene oxide as a promising light-conversion molecular device

Abstract

A novel β-diketone ligand, 4,4,5,5,5-pentafluoro-1-(9H-fluoren-2-yl)-1,3-pentanedione (Hpffpd), which contains a polyfluorinated alkyl group, as well as a long conjugated fluorene unit, and a chelate phosphine oxide ligand, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene oxide (DDXPO) were synthesized and utilized for the synthesis of two new europium complexes [Eu(pffpd)₃(C₂H₅OH)(H₂O)] 1 and [Eu(pffpd)₃(DDXPO)] 2. For comparison, the [Eu(pffpd)₃(DPEPO)] 3 complex was also synthesized involving a known chelate phosphine oxide, bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO). The synthesized complexes have been characterized by various spectroscopic techniques and their solid-state photophysical properties were investigated. The single-crystal X-ray diffraction analyses of 2 and 3 revealed that these complexes are mononuclear, and that the central Eu³⁺ ion is surrounded by eight oxygen atoms, six of which are from the three bidentate fluorinated β-diketonates, and the other two oxygen atoms from the chelate phosphine oxide. The coordination polyhedra can be described as distorted square antiprism. Compound 2 has a solid-state photoluminescence quantum yield of 48%, which is about two times higher than that of compound 3 (28%). This may be due to the fact that DDXPO in 1 has the mezzo first triplet excited state energy level (T₁) between the first singlet excited energy level (S₁) and T₁ of Hpffpd, which may support one more additional energy transfer from the T₁ energy level of DDXPO to that of Hpffpd, and consequently improves the energy transfer in the Eu³⁺ complex. Furthermore, DDXPO (average Eu–O = 2.34 Å) in complex 2 coordinates more strongly with the central Eu³⁺ as compared to DPEPO in complex 3 (average Eu–O = 2.38 Å) which can improve the energy transfer between the ligands and central metal ion, and consequently enhances the photoluminescence efficiency of the corresponding Eu³⁺ complex.