Abstract The N,N’-coordinated mononuclear complex Eu(TTA)3(pdn), prepared by treating 1,10-phenanthroline- 5,6-dione (pdn) with Eu(TTA)3(H2O)2 (HTTA = 2-thenoyltrifluoroacetone), may be used as a benzoquinonelike ligand through the oxygen donor atoms in reaction with an additional Eu(TTA)3(H2O)2 precursor to give the dinuclear complex Eu2(TTA)6(pdn). Both the mono- and dinuclear complexes were characterized by elemental analysis, IR, TG, MS, and 1H NMR spectra. The results indicated that they both exhibited a characteristic red emission peak at 616 nm, corresponding to the 5D0 →7F2 transition of Eu(III) ions under UV excitation. Compared with the mononuclear complex, the dinuclear complex presented better thermal stability and stronger luminescence, which makes it a promising light-conversion molecular device.48145

Keywords   Metal complexes  Fluorescencespectroscopy  Lanthanoids  Absorption  Synthesis

Catalogue

1 Abstract 2

2  Introduction 4

3 Results and discussion 6

3.1 Synthesis and characterization 6

3.2 Thermogravimetric analysis (TGA) 8

3.3 UV spectra 9

3.4 uminescence properties 10

3.5 Experimental 12

3.5.1 1,10-Phenanthroline-5,6-dione 13

3.5.2 Tris(2-thenoyltrifluoroacetonato)europium(III) dehydrate 14

3.5.3(1,10-Phenanthroline-5,6-dione)tris(2-thenoyltrifluoroacetonato)europium(III) 14

3.5.4(1,10-Phenanthroline-5,6-dione)hexa(2-thenoyltrifluoroacetonato)dieuropium(III) 14

4 Acknowledgments 16

5 References 17

2  Introduction

  The lanthanide coordination chemistry has become increasingly important in recent years for a wide range of promising applications of lanthanide complexes in many areas, such as inorganic chemistry [1], biology [2], and medicine [3]. As well known, lanthanide complexes can emit luminescence in the visible and near-IR wavelength regions under ultraviolet excitation [4], and have found extensive applications as fluorescence biomarkers [5]. Nowadays, the design and synthesis of dinuclear lanthanide complexes have become an important theme in bioinorganic and coordination chemistry, especially their uses in metalloproteins as spectroscopic probes for active sites [6–8].

On the other hand, 1,10-phenanthroline-5,6-dione (pdn) bearing two kinds of coordinating functionalities (the quinonoid and the diimine) is often used as a bridging ligand in the construction of multinuclear complexes. In the recent years, the synthesis and characterization of transition-metal dinuclear complexes using the pdn ligand as a bridging agent has received much interest [9]. However, the photophysical properties of dinuclear lanthanide complexes with the pdn ligand still rarely addressed [10]. Here, we present a novel dinuclear europium(III) complex 2 (Scheme 1) by complexing 2-thenoyltrifluoroacetone (HTTA), bifunctional pdn and two Eu(III) ions in a sequent way. Note that the HTTA ligand is known to have the high energy-transfer efficiency from the ligand to Eu(III) ion owing to its high absorption coefficient [11]. We will demonstrate that the synergism of the pdn and HTTA ligands and two Eu cores leads to the remarkable photophysical properties of the resultant dinuclear Eu complex.

3 Results and discussion

3.1 Synthesis and characterization

The mono- and dinuclear Eu(III) complexes Eu(TTA)3(pdn) (1) and Eu2(TTA)6(pdn) (2) have been prepared according to Scheme 1. In the solid state, both complexes are air- and moisture-stable. The mononuclear complex was obtained using the diimine properties of the pdn ligand by substitution of the labile water ligands of the Eu(III) precursor compound Eu(TTA)3(H2O)2. The high resolution infrared spectrum of complex 1 shows that ν(C=O) and ν(C=C) shift from 1,659 and 1,632 cm-1 for free HTTA to 1,601 and 1,541 cm-1 for the complex, respectively (Fig. 1). A strong absorption is observed at about 1,700 cm-1 for 1, only a small shift to higher energy of ν(C=O) which occurs at 1686 cm-1 in the free pdn ligand. It should be noted that [Hg(pdn)2Cl2] [12] and [Pd(pdn)2(PPh3)2] [13], formulated as N,N’-adducts of the dione, have absorptions at about 1,700 cm-1, and that the TiCl4 adduct of 9,10-phenanthrenequinone [14] shows the ν(C=O) vibrational band at 1,575 cm-1 (9,10-phenanthrenequinone absorbs at 1,675 cm-1, △ν(C=O) =-100 cm-1). Thus, it can be clearly said that the mononuclear complex 1 contains the N,N’-coordinated metal center because of the higher wavenumber absorption (△ν(C=O) = +14 cm-1). In addition, the ν(C=N) and ν(C–H) absorption around 1,560, 808, and 739 cm-1 for the free pdn shift to 1,508, 788, and 718 cm-1 for complex 1,respectively, which further supports the formation of the coordination bonds between Eu(III) ion and nitrogen atoms of pdn ligand [15]. The bands at 463 and 494 cm-1 can be attributed to bands of ν(Eu–O) and ν(Eu–N), respectively.