Design, Synthesis, Crystal Structure and Photoluminescence Properties of Four New Europium (III) Complexes with Fluorinated β-Diketone Ligand

The strong photoluminescence properties of europium complexes with organic ligands attracted the attention of many researchers and found a wide range of uses in medical, industrial and biological fields. In this article, four new Tetrakis europium complexes 3a, 3b, 3c and 3d have been prepared using 1-phenyl-4,4,4-trifluoro-1,3-butenedionato ligand and pyridinium, bipyridinium, piperazinium and piperidinium counter cations. These complexes have been characterized by negative FAB-mass. The crystal structures of 3a, 3b, 3c and 3d were determined by single crystal X-ray diffraction analysis. The complex 3a crystallized in monoclinic form, space group P21/n with four molecules in the unit cell. The complex 3b crystallized in monoclinic form, space group P2/n with two complex molecules in the unit cell. The complex 3c crystallized in monoclinic form, space group C2/c with sixteen molecules in the unit cell. The complex 3d crystallized in monoclinic form, space group P21/n with four complex molecules in the unit cell. The complex 3a has 1,2-alternative structure, 3b has 1,3-alternative structure, 3c has cone like structure and 3d has partial cone like structure. The photoluminescence properties of these complexes have been evaluated. Strong red emissions were observed in all four complexes due to D0 → F2 transition of Europium (III) ions under UV excitation. Four β-diketone ligands acted as strong antenna ligands and transferred the absorbed energy to europium (III) ion effectively; consequently strong red luminescence was observed.


Introduction
The photoluminescence properties of Lanthanide complexes with organic ligands have been greatly enhanced, and led to the development of strong luminescent Lanthanide complexes with important applications in medical, industrial and biological fields [1]- [7]. Europium (III) complex with organic ligands is an example of strong luminescent Lanthanide complex and Europium (III) complexes that have great importance in materials engineering chemistry due to significant improvement in photophysical parameters such as high luminescence emission efficiency, long fluorescence life time, large stokes shift, sharp emission bands [8] [9] [10] [11]. In the past decade various high luminescent europium complexes have been engineered and evaluated for their photoelectronic properties such as OLEDs, electroluminescent displays, bioimaging, sensing and targeting specific DNA structures, melamine detection in milk protein. Europium (III) complexes have also found applications as sensor materials to detect pesticides, temperature, HCl, NO 2 gas, HOCl, pH, phosphate, mitochondria and, 8-oxo-dGTP [12]- [20]. Albumin proteins in human serum have also been detected by Europium complexes, which act as sensor materials [21].
Search for novel europium complexes that uses less energy and exhibits desired application such as sensors, OLEDS, etc. is of great importance in photoelectronic materials. Therefore, new europium complexes should have enhanced degree of change in luminescence to be a good sensor material. On the other hand, understanding the relationship between molecular structures and photoelectronic properties of europium (III) complexes gives valuable information in designing future photoelectronic materials with improved properties. It is stated that luminescence of Europium (III) ion originates from forbidden f-f transitions that totally hinder the Europium (III) ion interaction with light. The ligand that forms complexes with Europium (III) ion acts as antenna. This absorbs energy and transfers to Europium (III) ion through intersystem crossing to triplet excited states. In this context, europium complexes with substituted aromatic β-diketones as organic ligands were explored due to efficiency in generating triplet excited states in close contact with europium (III) ion. Thus, various Europium (III) complexes with β-diketones were synthesized and evaluated for their photoluminescent properties [22] [23] [24] [25] [26].
In our previous studies, we synthesized and investigated the molecular structures and photoelectronic properties of octa-coordinate europate (III) complexes using substituted β-diketone ligands [27] [28]. In this study, we want to investigate the molecular structures and photoluminescence properties of four new octa-coordinate Europium (III) complexes 3a, 3b, 3c and 3d, possessing pyridinium, bipyridinium, piperazinium and bipiperidinium as counter cations.

General Procedure for the Synthesis of Complexes 3a, 3b, 3c and 3d
In a RB flask, a solution of europium (III) chloride (0.650 g, 0.41 mmol) and 1-phenyl-4,4,4-trifluoromethyl-1,3-butanedione 1 (0.370 g, 1.65 mmol) in absolute ethanol (30 mL) was prepared at room temperature. Under protection from air, slightly excess of pyridine, bipyridine, piperazine, bipiperidine were added to the solution to get complexes 3a, 3b, 3c and 3d respectively. Ethanol was removed by rotary evaporator under reduced pressure. Under protection from air, the residue was repeatedly washed with small portions (5 mL) of warm, dry ethanol. The residual powders were dissolved in ethanol for crystallization.
Without protection from air, the crystallized product was filtered off, washed with two portions of cold ethanol, and dried under reduced pressure, affording the complexes 3a, 3b, 3c and 3d as a powder. All four complexes were obtained in moderate to good yields (68% for 3a, 65% for 3b, 75% for 3c and 60% for 3d, respectively).

Single-Crystal X-Ray Analysis and Structure Determination
Crystals of four compounds 3a, 3b, 3c and 3d were obtained at room temperature by crystallization in DCM-ethanol mixed solvent.
The crystal data were recorded on a Bruker APEX II KY CCD diffractometer equipped with graphite monochromatized Mo-Kα radiation of wavelength 0.71073 Å from a sealed micro focus tube, and a nominal crystal to area detector distance of 58 mm. X-ray generator settings were 50 kV and 30 mA. The data were collected at −153˚C (120 K) for 3b-3c and at −123˚C (150 K) for 3a. The crystallographic data of these complexes were summarized in Table 1.
APEX2 software was used for preliminary determination of the unit cell [29].
Determination of integrated intensities and unit cell refinement were performed using SAINT program [30]. The structures were solved with SHELXS-2014/7 [31] and subsequent structure refinements were performed with SHELXL-2014/7.

Results and Discussion
Complexes 3a, 3b, 3c and 3d were synthesized from the corresponding ligand 1,3-diphenyl-1,3-propanedione by complexation reaction with europium (III) chloride in the presence of pyridine, bipyridine, piperazine and bipiperidine as counter cations (Scheme 1). This reaction is a standard preparation procedure for lanthanide (III) complex [32]. Structures of complexes were determined by mass spectrometry and X-ray single crystal structure analyses.
We have measured the UV-Vis and Fluorescence spectra of 3a, 3b, 3c and 3d.
The fluorescence spectrum was measured in solution and solid state as well.
The solution state fluorescence measurements were carried out in dichloromethane solution (1 × 10 −3 mol/L). The corresponding absorption and emission spectrum of 3a, 3b, 3c and 3d were shown below (Figures 1-8, respectively). Suitable single crystals for X-ray structure analysis were easily obtained for all complexes. Since, europium (III) complexes are air stable, preparation of crystals is has been easy. All four complexes were dissolved in suitable solvents and left to slow evaporation at room temperature that resulted in crystals of complex 3a, 3b, 3c and 3d.
The complex 3a has 1,2-alternative structure, 3b has 1,3-alternative structure, 3c has cone like structure and 3d has partial cone like structure. In the crystal, complex 3a crystallized in monoclinic form with P2 1 /n space group and it has four molecules in unit cell with pyridinium cation (Figure 9). The complex 3b also crystallize monoclinic form with P2/n space group and it has two molecules Crystal Structure Theory and Applications

Conclusion
In conclusion, four new europium complexes have been synthesized and cha-