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Spectroscopic studies of an europium(III) tris-β-diketonate complex bearing a pyrazolylpyridine ligand
Journal of Alloys and Compounds 451 (2008) 344–346
Spectroscopic studies of an europium(III) tris--diketonate complex bearing a pyrazolylpyridine ligand Ant´onio Moreira dos Santos a,b , Ana C. Coelho b , Filipe A. Almeida Paz b , Jo˜ao Rocha b , Isabel S. Gonc¸alves b , Lu´ıs D. Carlos a,∗ a b
Department of Physics, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal Department of Chemistry, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal Available online 19 April 2007
Abstract A new europium(III) complex, Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (where BTA: 1-benzoyl-3,3,3-trifluoroacetonate), was synthesised by simple ligand exchange with the solvent adduct Eu(BTA)3 (H2 O)2 . The compound was characterised by elemental analysis, thermogravimetry, FTIR, FT Raman and photoluminescence spectroscopies. A significant increase of the 5 D0 lifetime of the complex, relative to the value found for the water-coordinated adduct (from 0.657 ± 0.001 to 0.835 ± 0.002 ms), and a larger contribution of the ligand levels to the excitation spectrum indicate a better photoluminescence performance for the former complex. © 2007 Elsevier B.V. All rights reserved. Keywords: Europium -diketonate complex; Pyrazolylpyridine ligands; Photoluminescence
1. Introduction There has been a growing research interest in systems containing lanthanide centres which may find direct applications in devices such as fibre amplifiers and solid-state lasers [1,2]. However, since the lanthanide absorption bands are narrow, direct excitation is usually not efficient. It is known that optical absorption can be enhanced by coordinating organic ligands to the lanthanide centre, thus acting as light collectors (antennae) which transfer the absorbed energy to the lanthanide ion (emitter), yielding highly luminescent complexes [3]. For this purpose, tris--diketonates bearing aromatic and fluorine substituents show excellent photoluminescence efficiency. For example, the quantum yield measured for Eu(NTA)3 (DMSO)2 [where NTA: 1-(2-naphthoyl)-3,3,3-trifluoroacetonate; DMSO: dimethyl sulfoxide] is in the order of 75% and is one of the highest reported for europium complexes isolated in the solid state [4]. The photoluminescence properties of lanthanide diketonate complexes can be further optimised by varying the nature of the first coordination sphere. For instance, the presence of water molecules coordinated to lanthanide ions reduces efficiency by providing non-radiative decay pathways from the ∗
Corresponding author. Tel.: +351 234370946; fax: +351 234424965. E-mail address: [email protected] (L.D. Carlos).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.141
excited states [4,5]. Efficiency can thus be improved by replacing these water molecules by Lewis base nitrogen organic ligands, such as 1,10-phenanthroline [5] and 2,2 -bipyridine [6–8]. Herein, the preparation of Eu(BTA)3 (ethyl[3-(2pyridyl)-1-pyrazolyl]acetate) complex bearing a substituted pyrazolylpyridine ligand and its photoluminescence properties are reported. 2. Experimental 2.1. Materials and methods Elemental analysis was performed at the University of Aveiro. Thermogravimetric analysis (TGA) were carried out using a Shimadzu TGA-50 system with a heating rate of 5 ◦ C min−1 under a static air atmosphere. IR spectra were measured with a Unican Mattson Mod 7000 FTIR spectrophotometer using KBr pellets. Raman spectra were collected on a Bruker RFS100/S FT instrument using a 2 cm−1 resolution (Nd:YAG laser, 1064 nm excitation, InGaAs detector). 1 H and 13 C NMR spectra were measured in solution (CDCl3 ) using a Bruker CXP 300 spectrometer. Chemical shifts are quoted in parts per million from TMS. Photoluminescence measurements were recorded on a Fluorolog-3 Model FL3-2T with a double excitation spectrometer (Triax 320), fitted with a 1200 grooves/mm grating blazed at 330 nm, and a single emission spectrometer (Triax 320), fitted with a 1200 grooves/mm grating blazed at 500 nm, coupled to a R928P photomultiplier. The excitation source was a 450 W Xe lamp. Excitation spectra were corrected from 240 to 600 nm for the spectral distribution of the lamp intensity using a photodiode reference detector. Emission and excitation spectra were corrected for the spectral response of the monochromators
A.M. dos Santos et al. / Journal of Alloys and Compounds 451 (2008) 344–346
345
and the detector using typical correction spectra provided by the manufacturer. Time-resolved measurements were carried out using a 1934D3 phosphorimeter coupled to the Fluorolog-3. A Xe flash lamp (6 s/pulse half width and 20–30 s tail) was used as excitation source. 1-Benzoyl-3,3,3-trifluoroacetone (HBTA) and europium trichloride hexahydrate (EuCl3 6H2 O) were purchased from Aldrich and used as received. Reported methods were used to prepare the -diketonate precursor complex Eu(BTA)3 (H2 O)2 [9] and ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate [10]. Elemental analysis, 1 H and 13 C NMR data for this organic ligand were found to be in good agreement with the reported data [10].
2.2. Preparation of Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (1) A solution of ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (0.14 g, 0.60 mmol) in CHCl3 (10 mL) was added to a solution of Eu(BTA)3 (H2 O)2 (0.50 g, 0.60 mmol) in CHCl3 (15 mL) at room temperature. The resulting solution was stirred for 4 h. Solvent was removed in vacuo leading to the isolation of a yellow solid which was recrystallised from a mixture containing diethyl ether and n-hexane (0.47 g, 75%). Anal. Calcd. for C42 H31 N3 O8 F9 Eu (1028.7): C, 49.04; H 3.04; N, 4.08%. Found: C, 49.12; H, 3.25; N, 4.07%. Selected IR (KBr): 3147w, 3075w, 2998m, 1756s, 1637vs, 1610vs, 1597vs, 1577vs, 1540s, 1530s, 1488s, 1477s, 1438m, 1428m, 1372m, 1320vs, 1289vs, 1244s, 1199sh, 1185vs, 1145sh, 1132vs, 1098s, 1075s, 1054m, 1025m, 1014m, 943m, 764s, 716m, 700s, 630m, 579m, 508m, 461m, 408w, 362w, cm−1 . Selected Raman: 3072m, 2946m, 1638m, 1607sh, 1598vs, 1572m, 1532s, 1489s, 1439s, 1320s, 1288s, 1249s, 1160m, 1147m, 1027m, 1012m, 1001vs, 945s, 718m, 630m, 618m, 248m, 191m, 152m, 107s, cm−1 .
3. Results and discussion 3.1. Synthesis and characterisation The complex Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl] acetate) (1) was prepared in good yield and isolated as a microcrystalline powder by simple ligand exchange with the solvent adduct Eu(BTA)3 (H2 O)2 at room temperature (Scheme 1 and Section 2). Infrared spectroscopy was particularly informative regarding the substitution of the coordinated water molecules of Eu(BTA)3 (H2 O)2 by the pyrazolylpyridine ligand (pzpy). Bands arising from the C N stretching ring vibrations and the ligand N C C N fragment are, as expected, shifted to higher wavenumbers upon coordination to the Eu centre. The Raman bands of ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate at 1567 cm−1 (pzpy C C inter-ring stretching mode) and 1594 cm−1 (py C C
Fig. 1. TGA of pyrazolylpyridine ligand, Eu(BTA)3 (H2 O)2 Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (1).
and
stretching mode) are shifted to 1572 and 1607 cm−1 , respectively. Another effect of complexation involves the IR band assigned to the stretching of the carbonyl group, which shifts from 1736 cm−1 for ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate to 1756 cm−1 for 1. Also the ν(O H) vibrations, found in the 3000–3500 cm−1 range, completely disappeared upon coordination of the bidentate ligand. Fig. 1 shows the thermograms for the pyrazolylpyridine ligand, the precursor Eu(BTA)3 (H2 O)2 and Eu(BTA)3 (ethyl[3(2-pyridyl)-1-pyrazolyl]acetate) (1). The pyrazolylpyridine ligand decomposes in a one-step process in the temperature range 130–250 ◦ C, leaving a residual mass of ca. 3.0%. The precursor complex undergoes a gradual weight loss of ca. 4.5% from room temperature up to 165 ◦ C, attributed to the removal of coordinated water. Additional thermal decomposition occurs in two steps in the 165–380 ◦ C and 380–550 ◦ C temperature ranges, with weight losses of ca. 56.5% and 13.0%, respectively, leaving a residual mass of ca. 26.0%. The thermogram of compound 1 does not show any step characteristic of the “free” pyrazolylpyridine ligand. The first weight loss occurs at ca. 180 ◦ C, indicating that the ligand is coordinated to the metal center. After 180 ◦ C, the thermal decomposition proceeds in two steps, very similar to the behaviour registered for the precursor complex: weight losses of ca. 63.0% and 16.0% for the 180–370 and 370–500 ◦ C temperature ranges, respectively, leaving a residue of about 21.0%. Suitable crystalline material was studied using singlecrystal X-ray diffraction at the low temperature of ˚ b = 21.690(4) A, ˚ c = 17.780(4) A, ˚ 100(2) K [a = 11.076(2) A, ˚ 3 , monoclinic, P21 /n], β = 106.83(3)◦ , volume = 4088.5(14) A thus confirming the formulation of the novel compound 1 [11]. 3.2. Photoluminescence studies
Scheme 1.
The room temperature excitation spectra (PLE) of compounds 1 and Eu(BTA)3 (H2 O)2 monitored on the more intense 5 D → 7 F Stark component (613 nm) are shown in Fig. 2. The 0 2
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suggesting that the Eu3+ ions occupy only one chemical environment with low site symmetry. Spectroscopic data further indicates the absence of an inversion centre on the Eu3+ local site since the emission is dominated by the 5 D0 → 7 F2 transition, relative to the magnetic-dipole 5 D0 → 7 F1 transition. The 5 D0 decay times for compounds Eu(BTA)3 (H2 O)2 and 1 were measured monitoring the main 7 F2 Stark component (excitation wavelength of 395 nm). Single exponential functions were used to fit the experimental data yielding lifetime values of 0.657 ± 0.001 and 0.835 ± 0.002 ms, respectively. The observed increase of the 5 D0 lifetime for 1 is another indication of the improvement of the emission features when the coordinated water molecules are replaced by the pyrazolylpyridine ligand. Fig. 2. Excitation spectra, collected at ambient temperature of (a) Eu(BTA)3 (H2 O)2 and (b) Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (1) normalised to the 7 F0 → 5 D2 transition.
4. Concluding remarks The coordinated water molecules of the tris--diketonate complex Eu(BTA)3 (H2 O)2 were readily replaced by the mixed pyridine–pyrazole ligand in chloroform media, yielding the new complex Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (1). The photoluminescence properties indicate the presence of a single Eu3+ site exhibiting a low-symmetry coordination environment. Studies based on time-resolved spectra further illustrate a considerable increase in the lifetime of the Eu3+ centre when the coordinated water molecules of the precursor complex are replaced by the heteroaromatic rings. Acknowledgement The authors are grateful to FCT, OE and FEDER for funding (Project POCI/CTM/58863/2004). References
Fig. 3. Emission spectra, collected at ambient temperature of Eu(BTA)3 (ethyl[3(2-pyridyl)-1-pyrazolyl]acetate) (1) (—) and Eu(BTA)3 (H2 O)2 (- - -). For both compounds no emission band from the ligands is detected. The inset shows the low temperature (12 K) emission spectra of (1) in the region corresponding to the 5 D0 → 7 F0 and 5 D0 → 7 F1 transitions, excited at 393 nm.
spectra consist of a broad absorption band between 240 and 475 nm attributed to the organic ligands, and sharp 4f6 transitions attributed to Eu3+ . The higher relative intensity of the ligand band compared with the sharp Eu3+ transitions, particularly for compound 1, indicates that the Eu3+ ions are essentially excited via an efficient sensitised process, rather than by direct metal excitation. Fig. 3 shows the emission spectra of compounds 1 and Eu(BTA)3 (H2 O)2 excited at 377 nm. The spectra consist of the typical Eu3+ emission lines ascribed to the intra-4f6 5D → 7F 0 0–4 transitions. For both compounds only one line, and three lines are observed for the non-degenerate 5 D0 → 7 F0 and the 5 D0 → 7 F1 transitions respectively (as shown clearly in the low temperature emission of compound 1, inset of Fig. 3),
[1] K. Binnemans, in: K.A. Gschneidner Jr., J.-C.G. B¨unzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, 35, Elsevier, 2005 (chapter 225). [2] G.F. de S´a, O.L. Malta, C. de Mello Doneg´a, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. Da Silva Jr., Coord. Chem. Rev. 196 (2000) 165–195. [3] T. J¨ustel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084–3103. [4] L.D. Carlos, C. de Mello Doneg´a, R.Q. Albuquerque, S. Alves Jr., J.F.S. Menezes, O.L. Malta, Mol. Phys. 101 (2003) 1037–1045. [5] J.A. Fernandes, R.A. S´a Ferreira, M. Pillinger, L.D. Carlos, J. Jepsen, A. Hazell, P. Ribeiro-Claro, I.S. Gonc¸alves, J. Lumin. 113 (2005) 50–63. [6] J.A. Fernandes, S.S. Braga, M. Pillinger, R.A. S´a Ferreira, L.D. Carlos, A. Hazell, P. Ribeiro-Claro, I.S. Gonc¸alves, Polyhedron 25 (2006) 1471–1476. [7] L.C. Thompson, F.W. Atchison, V.G. Young, J. Alloys Compd. 275–277 (1998) 765–768. [8] S.P. Vila Nova, H.J. Batista, S. Alves Jr., C. de Mello Doneg´a, R.L. Longo, G.F. de S´a, L.C. Thompson, J. Lumin. 118 (2006) 83–90. [9] M. Ismail, S.J. Lyle, J.E. Newbery, J. Inorg. Nucl. Chem. 31 (1969) 1715–1724. [10] W.R. Thiel, M. Angstl, T. Priermeier, Chem. Ber. 127 (1994) 2373– 2379. [11] A.C. Coelho, A.M. dos Santos, F.A.A. Paz, J. Rocha, I.S. Gonc¸alves, L.D. Carlos, manuscript in preparation.
Spectroscopic studies of an europium(III) tris--diketonate complex bearing a pyrazolylpyridine ligand Ant´onio Moreira dos Santos a,b , Ana C. Coelho b , Filipe A. Almeida Paz b , Jo˜ao Rocha b , Isabel S. Gonc¸alves b , Lu´ıs D. Carlos a,∗ a b
Department of Physics, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal Department of Chemistry, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal Available online 19 April 2007
Abstract A new europium(III) complex, Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (where BTA: 1-benzoyl-3,3,3-trifluoroacetonate), was synthesised by simple ligand exchange with the solvent adduct Eu(BTA)3 (H2 O)2 . The compound was characterised by elemental analysis, thermogravimetry, FTIR, FT Raman and photoluminescence spectroscopies. A significant increase of the 5 D0 lifetime of the complex, relative to the value found for the water-coordinated adduct (from 0.657 ± 0.001 to 0.835 ± 0.002 ms), and a larger contribution of the ligand levels to the excitation spectrum indicate a better photoluminescence performance for the former complex. © 2007 Elsevier B.V. All rights reserved. Keywords: Europium -diketonate complex; Pyrazolylpyridine ligands; Photoluminescence
1. Introduction There has been a growing research interest in systems containing lanthanide centres which may find direct applications in devices such as fibre amplifiers and solid-state lasers [1,2]. However, since the lanthanide absorption bands are narrow, direct excitation is usually not efficient. It is known that optical absorption can be enhanced by coordinating organic ligands to the lanthanide centre, thus acting as light collectors (antennae) which transfer the absorbed energy to the lanthanide ion (emitter), yielding highly luminescent complexes [3]. For this purpose, tris--diketonates bearing aromatic and fluorine substituents show excellent photoluminescence efficiency. For example, the quantum yield measured for Eu(NTA)3 (DMSO)2 [where NTA: 1-(2-naphthoyl)-3,3,3-trifluoroacetonate; DMSO: dimethyl sulfoxide] is in the order of 75% and is one of the highest reported for europium complexes isolated in the solid state [4]. The photoluminescence properties of lanthanide diketonate complexes can be further optimised by varying the nature of the first coordination sphere. For instance, the presence of water molecules coordinated to lanthanide ions reduces efficiency by providing non-radiative decay pathways from the ∗
Corresponding author. Tel.: +351 234370946; fax: +351 234424965. E-mail address: [email protected] (L.D. Carlos).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.141
excited states [4,5]. Efficiency can thus be improved by replacing these water molecules by Lewis base nitrogen organic ligands, such as 1,10-phenanthroline [5] and 2,2 -bipyridine [6–8]. Herein, the preparation of Eu(BTA)3 (ethyl[3-(2pyridyl)-1-pyrazolyl]acetate) complex bearing a substituted pyrazolylpyridine ligand and its photoluminescence properties are reported. 2. Experimental 2.1. Materials and methods Elemental analysis was performed at the University of Aveiro. Thermogravimetric analysis (TGA) were carried out using a Shimadzu TGA-50 system with a heating rate of 5 ◦ C min−1 under a static air atmosphere. IR spectra were measured with a Unican Mattson Mod 7000 FTIR spectrophotometer using KBr pellets. Raman spectra were collected on a Bruker RFS100/S FT instrument using a 2 cm−1 resolution (Nd:YAG laser, 1064 nm excitation, InGaAs detector). 1 H and 13 C NMR spectra were measured in solution (CDCl3 ) using a Bruker CXP 300 spectrometer. Chemical shifts are quoted in parts per million from TMS. Photoluminescence measurements were recorded on a Fluorolog-3 Model FL3-2T with a double excitation spectrometer (Triax 320), fitted with a 1200 grooves/mm grating blazed at 330 nm, and a single emission spectrometer (Triax 320), fitted with a 1200 grooves/mm grating blazed at 500 nm, coupled to a R928P photomultiplier. The excitation source was a 450 W Xe lamp. Excitation spectra were corrected from 240 to 600 nm for the spectral distribution of the lamp intensity using a photodiode reference detector. Emission and excitation spectra were corrected for the spectral response of the monochromators
A.M. dos Santos et al. / Journal of Alloys and Compounds 451 (2008) 344–346
345
and the detector using typical correction spectra provided by the manufacturer. Time-resolved measurements were carried out using a 1934D3 phosphorimeter coupled to the Fluorolog-3. A Xe flash lamp (6 s/pulse half width and 20–30 s tail) was used as excitation source. 1-Benzoyl-3,3,3-trifluoroacetone (HBTA) and europium trichloride hexahydrate (EuCl3 6H2 O) were purchased from Aldrich and used as received. Reported methods were used to prepare the -diketonate precursor complex Eu(BTA)3 (H2 O)2 [9] and ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate [10]. Elemental analysis, 1 H and 13 C NMR data for this organic ligand were found to be in good agreement with the reported data [10].
2.2. Preparation of Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (1) A solution of ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (0.14 g, 0.60 mmol) in CHCl3 (10 mL) was added to a solution of Eu(BTA)3 (H2 O)2 (0.50 g, 0.60 mmol) in CHCl3 (15 mL) at room temperature. The resulting solution was stirred for 4 h. Solvent was removed in vacuo leading to the isolation of a yellow solid which was recrystallised from a mixture containing diethyl ether and n-hexane (0.47 g, 75%). Anal. Calcd. for C42 H31 N3 O8 F9 Eu (1028.7): C, 49.04; H 3.04; N, 4.08%. Found: C, 49.12; H, 3.25; N, 4.07%. Selected IR (KBr): 3147w, 3075w, 2998m, 1756s, 1637vs, 1610vs, 1597vs, 1577vs, 1540s, 1530s, 1488s, 1477s, 1438m, 1428m, 1372m, 1320vs, 1289vs, 1244s, 1199sh, 1185vs, 1145sh, 1132vs, 1098s, 1075s, 1054m, 1025m, 1014m, 943m, 764s, 716m, 700s, 630m, 579m, 508m, 461m, 408w, 362w, cm−1 . Selected Raman: 3072m, 2946m, 1638m, 1607sh, 1598vs, 1572m, 1532s, 1489s, 1439s, 1320s, 1288s, 1249s, 1160m, 1147m, 1027m, 1012m, 1001vs, 945s, 718m, 630m, 618m, 248m, 191m, 152m, 107s, cm−1 .
3. Results and discussion 3.1. Synthesis and characterisation The complex Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl] acetate) (1) was prepared in good yield and isolated as a microcrystalline powder by simple ligand exchange with the solvent adduct Eu(BTA)3 (H2 O)2 at room temperature (Scheme 1 and Section 2). Infrared spectroscopy was particularly informative regarding the substitution of the coordinated water molecules of Eu(BTA)3 (H2 O)2 by the pyrazolylpyridine ligand (pzpy). Bands arising from the C N stretching ring vibrations and the ligand N C C N fragment are, as expected, shifted to higher wavenumbers upon coordination to the Eu centre. The Raman bands of ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate at 1567 cm−1 (pzpy C C inter-ring stretching mode) and 1594 cm−1 (py C C
Fig. 1. TGA of pyrazolylpyridine ligand, Eu(BTA)3 (H2 O)2 Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (1).
and
stretching mode) are shifted to 1572 and 1607 cm−1 , respectively. Another effect of complexation involves the IR band assigned to the stretching of the carbonyl group, which shifts from 1736 cm−1 for ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate to 1756 cm−1 for 1. Also the ν(O H) vibrations, found in the 3000–3500 cm−1 range, completely disappeared upon coordination of the bidentate ligand. Fig. 1 shows the thermograms for the pyrazolylpyridine ligand, the precursor Eu(BTA)3 (H2 O)2 and Eu(BTA)3 (ethyl[3(2-pyridyl)-1-pyrazolyl]acetate) (1). The pyrazolylpyridine ligand decomposes in a one-step process in the temperature range 130–250 ◦ C, leaving a residual mass of ca. 3.0%. The precursor complex undergoes a gradual weight loss of ca. 4.5% from room temperature up to 165 ◦ C, attributed to the removal of coordinated water. Additional thermal decomposition occurs in two steps in the 165–380 ◦ C and 380–550 ◦ C temperature ranges, with weight losses of ca. 56.5% and 13.0%, respectively, leaving a residual mass of ca. 26.0%. The thermogram of compound 1 does not show any step characteristic of the “free” pyrazolylpyridine ligand. The first weight loss occurs at ca. 180 ◦ C, indicating that the ligand is coordinated to the metal center. After 180 ◦ C, the thermal decomposition proceeds in two steps, very similar to the behaviour registered for the precursor complex: weight losses of ca. 63.0% and 16.0% for the 180–370 and 370–500 ◦ C temperature ranges, respectively, leaving a residue of about 21.0%. Suitable crystalline material was studied using singlecrystal X-ray diffraction at the low temperature of ˚ b = 21.690(4) A, ˚ c = 17.780(4) A, ˚ 100(2) K [a = 11.076(2) A, ˚ 3 , monoclinic, P21 /n], β = 106.83(3)◦ , volume = 4088.5(14) A thus confirming the formulation of the novel compound 1 [11]. 3.2. Photoluminescence studies
Scheme 1.
The room temperature excitation spectra (PLE) of compounds 1 and Eu(BTA)3 (H2 O)2 monitored on the more intense 5 D → 7 F Stark component (613 nm) are shown in Fig. 2. The 0 2
346
A.M. dos Santos et al. / Journal of Alloys and Compounds 451 (2008) 344–346
suggesting that the Eu3+ ions occupy only one chemical environment with low site symmetry. Spectroscopic data further indicates the absence of an inversion centre on the Eu3+ local site since the emission is dominated by the 5 D0 → 7 F2 transition, relative to the magnetic-dipole 5 D0 → 7 F1 transition. The 5 D0 decay times for compounds Eu(BTA)3 (H2 O)2 and 1 were measured monitoring the main 7 F2 Stark component (excitation wavelength of 395 nm). Single exponential functions were used to fit the experimental data yielding lifetime values of 0.657 ± 0.001 and 0.835 ± 0.002 ms, respectively. The observed increase of the 5 D0 lifetime for 1 is another indication of the improvement of the emission features when the coordinated water molecules are replaced by the pyrazolylpyridine ligand. Fig. 2. Excitation spectra, collected at ambient temperature of (a) Eu(BTA)3 (H2 O)2 and (b) Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (1) normalised to the 7 F0 → 5 D2 transition.
4. Concluding remarks The coordinated water molecules of the tris--diketonate complex Eu(BTA)3 (H2 O)2 were readily replaced by the mixed pyridine–pyrazole ligand in chloroform media, yielding the new complex Eu(BTA)3 (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) (1). The photoluminescence properties indicate the presence of a single Eu3+ site exhibiting a low-symmetry coordination environment. Studies based on time-resolved spectra further illustrate a considerable increase in the lifetime of the Eu3+ centre when the coordinated water molecules of the precursor complex are replaced by the heteroaromatic rings. Acknowledgement The authors are grateful to FCT, OE and FEDER for funding (Project POCI/CTM/58863/2004). References
Fig. 3. Emission spectra, collected at ambient temperature of Eu(BTA)3 (ethyl[3(2-pyridyl)-1-pyrazolyl]acetate) (1) (—) and Eu(BTA)3 (H2 O)2 (- - -). For both compounds no emission band from the ligands is detected. The inset shows the low temperature (12 K) emission spectra of (1) in the region corresponding to the 5 D0 → 7 F0 and 5 D0 → 7 F1 transitions, excited at 393 nm.
spectra consist of a broad absorption band between 240 and 475 nm attributed to the organic ligands, and sharp 4f6 transitions attributed to Eu3+ . The higher relative intensity of the ligand band compared with the sharp Eu3+ transitions, particularly for compound 1, indicates that the Eu3+ ions are essentially excited via an efficient sensitised process, rather than by direct metal excitation. Fig. 3 shows the emission spectra of compounds 1 and Eu(BTA)3 (H2 O)2 excited at 377 nm. The spectra consist of the typical Eu3+ emission lines ascribed to the intra-4f6 5D → 7F 0 0–4 transitions. For both compounds only one line, and three lines are observed for the non-degenerate 5 D0 → 7 F0 and the 5 D0 → 7 F1 transitions respectively (as shown clearly in the low temperature emission of compound 1, inset of Fig. 3),
[1] K. Binnemans, in: K.A. Gschneidner Jr., J.-C.G. B¨unzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, 35, Elsevier, 2005 (chapter 225). [2] G.F. de S´a, O.L. Malta, C. de Mello Doneg´a, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. Da Silva Jr., Coord. Chem. Rev. 196 (2000) 165–195. [3] T. J¨ustel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084–3103. [4] L.D. Carlos, C. de Mello Doneg´a, R.Q. Albuquerque, S. Alves Jr., J.F.S. Menezes, O.L. Malta, Mol. Phys. 101 (2003) 1037–1045. [5] J.A. Fernandes, R.A. S´a Ferreira, M. Pillinger, L.D. Carlos, J. Jepsen, A. Hazell, P. Ribeiro-Claro, I.S. Gonc¸alves, J. Lumin. 113 (2005) 50–63. [6] J.A. Fernandes, S.S. Braga, M. Pillinger, R.A. S´a Ferreira, L.D. Carlos, A. Hazell, P. Ribeiro-Claro, I.S. Gonc¸alves, Polyhedron 25 (2006) 1471–1476. [7] L.C. Thompson, F.W. Atchison, V.G. Young, J. Alloys Compd. 275–277 (1998) 765–768. [8] S.P. Vila Nova, H.J. Batista, S. Alves Jr., C. de Mello Doneg´a, R.L. Longo, G.F. de S´a, L.C. Thompson, J. Lumin. 118 (2006) 83–90. [9] M. Ismail, S.J. Lyle, J.E. Newbery, J. Inorg. Nucl. Chem. 31 (1969) 1715–1724. [10] W.R. Thiel, M. Angstl, T. Priermeier, Chem. Ber. 127 (1994) 2373– 2379. [11] A.C. Coelho, A.M. dos Santos, F.A.A. Paz, J. Rocha, I.S. Gonc¸alves, L.D. Carlos, manuscript in preparation.