Synthesis and photoluminescent properties of Eu (III) complexes with fluorinated β-diketone and nitrogen heterocyclic ligands

Accepted Manuscript Synthesis and photoluminescent properties of Eu (III) complexes with fluorinated βdiketone and nitrogen heterocyclic ligands Dan Wang, Zheng Luo, Zhao Liu, Dunjia Wang, Ling Fan, Guodong Yin PII:

S0143-7208(16)30216-9

DOI:

10.1016/j.dyepig.2016.05.026

Reference:

DYPI 5258

To appear in:

Dyes and Pigments

Received Date: 14 March 2016 Revised Date:

14 May 2016

Accepted Date: 17 May 2016

Please cite this article as: Wang D, Luo Z, Liu Z, Wang D, Fan L, Yin G, Synthesis and photoluminescent properties of Eu (III) complexes with fluorinated β-diketone and nitrogen heterocyclic ligands, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.05.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Graphical Abstract

400

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C1: Eu(TMPD)3Bipy C2: Eu(TFPD)3Bipy C3: Eu(TMPD)3Phen C4: Eu(TFPD)3Phen C5: Eu(TMPD)3Phterpy C6: Eu(TFPD)3Phterpy O

350 300 250 200

O

150

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F

F F

R

100

560

580

600

620

640

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(TMPD : R = -OCH3, TFPD : R = -F)

660

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Wavelength / nm

680

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0 C1 C2 C3 C4

C5 C6

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Relative Intensity / a.u.

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Synthesis and photoluminescent properties of Eu (III) complexes with fluorinated β-diketone and nitrogen heterocyclic ligands

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Dan Wang, Zheng Luo, Zhao Liu, Dunjia Wang*, Ling Fan, Guodong Yin

College of Chemistry and Chemical Engineering, Hubei Collaborative Innovation Center for Rare Metal Chemistry, Hubei Key Laboratory of Pollutant Analysis and

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Reuse Technology, Hubei Normal University, Huangshi 435002, China

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Received…

Abstract: Two fluorinated β-diketones and their six europium (III) complexes using 2,2-dipyridine, 1,10-phenanthroline or 4′-phenyl-terpyridine as the secondary ligand were synthesized and characterized. The photoluminescence behavior for these

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complexes was investigated in solid state in detail. Based on the emission spectra and luminescence decay curves of these complexes in solid state, the Judd−Ofelt intensity parameters (Ωt), lifetime (τ) and luminescence quantum efficiency (η)

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were determined. The Ω2 values indicate that the europium (III) ion is in a highly

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polarizable chemical environment in these complexes. Europium (III) complexes with the secondary ligands 2,2-dipyridine or 1,10-phenanthroline exhibited much better photoluminescence properties than complexes with the secondary ligand 4′-phenyl-terpyridine.

Especially,

europium

4,4,4-trifluoro-1-(4-fluorophenyl)-butane-1,3-dione

(III) containing

complexes the

of

secondary

ligands 2,2-dipyridine or 1,10-phenanthroline showed a longer lifetime (τ = 0.799

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and 0.826 ms, respectively) and a higher luminescence quantum efficiency (η = 56.5 and 56.1, respectively) among these complexes.

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Keywords: Europium (III) complex, fluorinated β-diketone, heterocyclic nitrogen donors, photoluminescence property, quantum efficiency, Judd−Ofelt intensity parameters.

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* Corresponding author. Tel.: +86 714 6515602; fax: +86 714 6573832

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E-mail address: [email protected] (Dunjia Wang)

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1. Introduction β-Diketones have a high complexing ability due to their O,O-donor ligands,

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which are widely used in coordination with transition metal ions [1]. Especially, their europium (III) complexes have been of great attention to the chemists because of their high fluorescence emission efficiency, long fluorescence lifetime and

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extremely sharp emission bands [2−4]. Many applications have been found in laser

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materials, chemical sensor, fluorescent probes, photoluminescent materials, magnetic molecular materials and organic light-emitting diodes [5−7]. Recently, the design, synthesis and optical properties of many novel europium (III) complexes with β-diketones have been reported [8–10]. In addition, some reports demonstrated

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that the β-diketones containing high-energy oscillators, such as C−H and O−H bonds, could quench the metal excited states nonradiatively, thereby resulting in

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lower luminescence intensities and shorter excited-state lifetimes [11,12]. Thus, it is very important to replace C−H bonds with C−F bonds in the synthesis and design of

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new β-diketone ligands in order to improve the luminescent properties of europium (III) complexes.

Therefore, with further investigations into the relationship between the structure of organic ligands and the luminescent behavior of europium (III) complexes, we designed and synthesized the fluorinated β-diketones with 4-methoxyphenyl and 4-fluorophenyl moieties to investigate the spectroscopic and 3

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photoluminescence properties of their europium complexes. Herein, the fluorinated β-diketone ligands, 4,4,4-trifluoro-1-(4-methoxyphenyl)-butane-1,3-dione (TMPD)

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and 4,4,4-trifluoro-1-(4-fluorophenyl)-butane-1,3-dione (TFPD), were synthesized by the Claisen condensation. With these two fluorinated β-diketones as the first ligand and 2,2-dipyridine (Bipy), 1,10-phenanthroline (Phen) or 4′-phenyl-

Eu(TMPD)3⋅Bipy,

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terpyridine (Phterpy) as the second ligand, six new europium (III) ternary complexes, Eu(TFPD)3⋅Bipy,

Eu(TMPD)3⋅Phen,

Eu(TFPD)3⋅Phen,

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Eu(TMPD)3⋅Phterpy and Eu(TFPD)3⋅Phterpy, were prepared, characterized, and their photoluminescence behavior were studied as well. Meanwhile, the Judd–Ofelt intensity parameters (Ωλ), radiative (Arad), nonradiative (Anrad), and luminescent yield

(η)

were

calculated

and

analyzed

according

to

their

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quantum

photoluminescence spectra and luminescence decay curves in the solid state.

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2. Experimental

2.1. Apparatus and Chemicals

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FT-IR spectra were recorded in KBr pellets at 1 cm−1 resolution on a Nicolet FTIR 5700 spectrophotometer. Melting points were measured using X-4 digital melting-point apparatus and uncorrected. Elemental analysis (C, H, N) was performed on a Perkin−Elmer 2400 elemental analyzer. The contents of europium (III) were determined by the complexometric titration with EDTA. 1H NMR spectra

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were measured on an Avance IIITM 300 MHz NB Digital NMR spectrometer in CDCl3 or DMSO-d6 solution with TMS as internal standard. Electrospray ionization

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mass spectra (ESI−MS) were carried out on a Finnigan LCQ Advantage Max spectrometer. The UV-vis spectra were measured on a Hitachi U-3010 spectrometer. The fluorescence excitation, emission spectra and fluorescence lifetimes were

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obtained on a Varian Cary Eclipse fluorescence spectrometer.

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4-Methoxyacetophenone, 4-fluoroacetophenone, ethyl trifluoroacetate, sodium methoxide, europium(III) oxide, 2,2-dipyridine and 1,10-phenanthroline were purchased from Sun Chemical Technology (Shanghai) Co., Ltd. 4′-Phenylterpyridine (Phterpy) was synthesized by our group in the literature [13]. Europium

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trichloride was obtained by dissolving Eu2O3 in concentrated hydrochloric acid. Other Reagents used were of analytical grade, and were used without further

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purification.

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2.2. Synthesis of fluorinated β-diketone ligands 4,4,4-Trifluoro-1-(4-methoxyphenyl)-butane-1,3-dione (TMPD): 4-Methoxyacetophenone (3.0g, 20 mmol) and sodium methoxide (2.16 g, 40 mmol) were added into 80 ml dry benzene, and the mixture was stirred for 15min. To this mixture ethyl trifluoroacetate (5ml, 42 mmol) was added dropwise, and then stirred at 50 °C for 8h. The resulting mixture was cooled to the room temperature and acidified with dilute

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hydrochloric acid. The organic layer was collected, washed with a saturated NaHCO3 solution and dried over anhydrous MgSO4. The organic solvent was

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removed by rotary evaporation. The residue was recrystallized with ethanol to give the light yellow crystal TMPD in yield 70%. Mp 56−57 oC; IR (KBr): ν 3452 (b, m), 3059 (m), 2972(w), 2850 (m), 1602 (s), 1509 (s), 1450 (m), 1313 (m), 1271 (s), 1256

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(s), 1195 (s), 1170 (s), 1140 (s), 1110 (s), 1069 (m), 1020 (m), 844 (s), 794 (s), 698

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(m) cm−1; 1H NMR (300 MHz, CDCl3): δ 3.88 (s, 3H, OCH3), 6.51 (s, 1H, enol CH), 6.99 (d, 2H, Ar−H, J = 8.1 Hz), 7.94 (d, 2H, Ar−H, J = 8.4 Hz), 15.43 (brs, 1H, enol OH); ppm; ESI−MS: m/z 246.30 [M]+. Anal. Calcd. for C11H9O3F3: C, 53.67; H, 3.68; Found C, 53.85; H, 3.66.

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4,4,4-Trifluoro-1-(4-fluorophenyl)-butane-1,3-dione (TFPD): The synthesis was performed as for the previous compound TMPD and the colorless crystal

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TFPD was obtained in 67% yield. Mp 42−43 oC; IR (KBr): ν 3442 (b, m), 3077 (m), 1600 (s), 1510 (s), 1432 (m), 1275 (s), 1159 (s), 1122 (s), 1103 (s), 1063 (s), 1013

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(m), 917 (m), 859 (s), 799 (s), 695 (s) cm−1; 1H NMR (300 MHz, CDCl3): δ 6.54 (s, 1H, enol CH), 7.21 (t, 2H, Ar−H, J = 8.2 Hz), 7.99 (dd, 2H, Ar−H, J =5.6, 8.0 Hz), 15.12 (brs, 1H, enol OH); ppm; ESI−MS: m/z 234.02 [M]+; Anal. Calcd. for C10H6O2F4: C, 51.30; H, 2.58; Found C, 51.53; H, 2.57. 2.3. Preparation of europium (III) ternary complexes (C1−C6) The procedure for preparation of all these europium (III) complexes is below 6

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described: The fluorinated β-diketone ligand (3 mmol), nitrogen heterocyclic ligand (1 mmol) and NaOH (0.12 g, 3 mmol) were dissolved in 30 ml ethanol and stirred at

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50 °C for 15min. To this an ethanolic solution containing 1 mmol EuCl3 was added dropwise and the mixture was stirred at 60 ºC for 5h. The resulting mixture was cooled to the room temperature and the light yellow solid was precipitated. The

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precipitate was purified by washing for several times with deionized water and ethanol to remove the free ligands and salt to give europium (III) ternary complexes

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(C1− −C6).

Eu(TMPD)3⋅Bipy (C1): Light yellow power, yield 79%, mp 179−181 oC; IR ν (KBr): 3046(w), 2966(w), 2842(w), 1599(s), 1548(s), 1505(s), 1466(s), 1439(s),

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1293(s), 1263(s), 1246(s), 1175(s), 1139(s), 1029 (s), 942 (m), 846 (m), 791 (s), 762 (m), 690 (m), 576(m), 472(m) cm–1; 1H NMR (300 MHz, CDCl3): δ 3.03 (s, 3H,

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C=CH), 3.93 (s, 9H, OCH3), 6.82 (d, 6H, Ar−H, J = 7.7Hz), 7.46 (d, 6H, Ar−H, J = 7.8Hz), 8.54 (d, 2H, Bipy−H, J = 6.0Hz), 9.83 (br, 2H, Bipy−H), 10.76 (d, 2H,

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Bipy−H, J = 6.4Hz), 12.44 (br, 2H, Bipy−H) ppm. Anal. Calcd. for EuC43H32N2O9F9: C, 49.49; H, 3.09; N, 2.68; Eu, 14.56; Found C, 49.23; H, 3.07; N, 2.69; Eu, 14.61. Eu(TFPD)3⋅Bipy (C2): Light yellow power, yield 81%, mp 192−194 oC; IR ν (KBr): 3051(m), 1592(s), 1544(s), 1502(s), 1476(m), 1308(s), 1286(s), 1238(s), 1186(s), 1161(s), 1133(s), 1064(m), 1014(m), 851(m), 790(s), 758(m), 693 (m), 576(m), 474(m) cm–1; 1H NMR (300 MHz, CDCl3): δ 3.26 (s, 3H, C=CH), 7.00 (t, 7

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6H, Ar−H, J = 8.3Hz), 7.40 (t, 6H, Ar−H, J = 6.7Hz), 8.52 (d, 2H, Bipy−H, J = 6.7Hz), 9.77 (t, 2H, Bipy−H, J = 7.4Hz), 10.47 (d, 2H, Bipy−H, J = 7.7Hz), 12.78

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(br, 2H, Bipy−H) ppm. Anal. Calcd. for EuC40H23N2O6F12: C, 47.68; H, 2.30; N, 2.78; Eu, 15.08; Found C, 47.92; H, 2.29; N, 2.75; Eu, 14.96.

Eu(TMPD)3⋅Phen (C3): Light yellow power, yield 80%, mp 204−206 oC; IR ν

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(KBr): 3072(w), 2998(w), 2842(w), 1599(s), 1543(s), 1504(s), 1466(m), 1295(s),

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1263(s), 1178(s), 1142(s), 1028(m), 943(m), 844(m), 789(s), 729(m), 692 (m), 577(m), 473(m) cm–1; 1H NMR (300 MHz, CDCl3): δ 3.06 (s, 3H, C=CH), 3.91 (s, 9H, OCH3), 6.79 (d, 6H, Ar−H, J = 8.0Hz), 7.37 (d, 6H, Ar−H, J = 8.1Hz), 8.53 (d, 2H, Phen−H, J = 7.6Hz), 8.90 (br, 2H, Phen−H), 10.1 (s, 2H, Phen−H), 10.59 (d, 2H,

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Phen−H, J = 7.8Hz) ppm. Anal. Calcd. for EuC45H32N2O9F9: C, 50.62; H, 3.02; N, 2.62; Eu, 14.23; Found C, 50.84; H, 3.04; N, 2.65; Eu, 14.17.

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Eu(TFPD)3⋅Phen (C4): Light yellow power, yield 82%, mp 190−192 oC; IR ν (KBr): 3036(w), 1595(s), 1545(s), 1502(s), 1312(s), 1289(s), 1239(m), 1189(m),

1

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1160(s), 1143(s), 1036(m), 845(m), 791(s), 726(m), 690 (m), 577(m), 474(m) cm–1; H NMR (300 MHz, CDCl3): δ 3.27 (s, 3H, C=CH), 6.96 (br, 6H, Ar−H), 7.28 (br,

6H, Ar−H), 8.49 (br, 2H, Phen−H), 9.10 (br, 2H, Phen−H), 9.92 (br, 2H, Phen−H), 10.49 (br, 2H, Phen−H) ppm. Anal. Calcd. for EuC42H23N2O6F12: C, 48.90; H, 2.25; N, 2.72; Eu, 14.73; Found C, 49.15; H, 2.23; N, 2.75; Eu, 14.79.

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Eu(TMPD)3⋅Phterpy (C5): Light yellow power, yield 73%, mp 202−204 oC; IR ν (KBr): 3078(w), 2948 (w), 2841 (w), 1598 (s), 1537 (s), 1496 (s), 1471(s), 1406

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(m), 1296 (s), 1253 (s), 1178 (s), 1134 (s), 1024(s), 943 (m), 843 (m), 789 (s), 690(m), 574(m), 471(m) cm–1; 1H NMR (300 MHz, CDCl3): δ 3.02 (s, 3H, C=CH), 3.80 (s, 9H, OCH3), 6.56 (br, 6H, Ar−H), 6.90 (br, 6H, Ar−H), 7.56 (br, 3H,

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Terpy−H), 7.72 (br, 2H, Terpy−H), 7.88 (br, 2H, Terpy−H), 8.07 (br, 2H, Terpy−H),

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8.82 (br, 2H, Terpy−H), 10.37 (br, 2H, Terpy−H), 12.09 (br, 2H, Terpy−H) ppm. Anal. Calcd. for EuC54H39N3O9F9: C, 54.19; H, 3.28; N, 3.51; Eu, 12.70; Found C, 54.49; H, 3.24; N, 3.49; Eu, 12.76.

Eu(TFPD)3⋅Phterpy (C6): Light yellow power, yield 72%, mp 214−216 oC; IR

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ν (KBr): 3073(m), 1596 (s), 1538 (s), 1492 (s), 1409(m), 1299 (s), 1237 (s), 1146 (s), 1011(m), 944 (m), 845 (m), 789 (s), 573(m), 468(m) cm–1; 1H NMR (300 MHz,

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CDCl3): δ 3.28 (s, 3H, C=CH), 6.73 (br, 6H, Ar−H), 6.91 (br, 6H, Ar−H), 7.53 (br, 3H, Terpy−H), 7.69 (br, 4H, Terpy−H), 7.83 (d, 2H, Terpy−H, J = 7.5Hz), 8.67 (d,

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2H, Terpy−H, J = 7.6Hz), 10.39 (d, 2H, Terpy−H, J = 7.0Hz), 12.03 (br, 2H, Terpy−H) ppm. Anal. Calcd. for EuC51H30N3O6F12: C, 52.77; H, 2.61; N, 3.62; Eu, 13.09; Found C, 52.97; H, 2.56; N, 3.65; Eu, 12.98. 3. Results and discussion 3.1. Synthesis

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The synthetic routes for fluorinated β-diketone ligands and their europium (III) complexes are outlined in Scheme 1. The fluorinated β-diketones (TMPD and TFPD)

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were synthesized by Claisen condensation between 4-Methoxyacetophenone or 4-fluoroacetophenone and ethyl trifluoroacetate using sodium methoxide as the condensing agent in benzene. Their europium (III) ternary complexes C1−C6 were

and

2,2-dipyridine

(Bipy),

1,10-phenanthroline

(Phen)

or

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chloride

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prepared by the reaction of the fluorinated β-diketone ligands with Europium

4’-phenyl-terpyridine (Phterpy) in ethanol solution according to the method of the literature [14].

3.2. IR spectra

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The characteristic infrared absorption bands of the free fluorinated β-diketones

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and their europium (III) ternary complexes are listed in Table 1. In the fluorinated β-diketones, the IR spectra of the uncoordinated ligands TMPD and TFPD showed a broad absorption at 3452 and 3442 cm−1, respectively, which can be attributed to their enolic O−H stretching vibration. The strong bands at 1602−1600 cm−1 and 1510−1509 cm−1 were due to their C=O and enolic C=C stretching vibrations. But in the complexes C1−C6, their enolic O−H absorption of the IR spectra was absence. 10

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The strong absorption bands appeared at 1599–1592 cm−1 and 1505–1492 cm−1 assigned to their C=O and enolic C=C stretching vibrations, and red-shifted 3–18

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cm−1 with respect to those of the corresponding β-diketone ligands. The results suggested that the coordination bands were formed between the fluorinated β-diketones and europium (III) ion. Meanwhile, the strong bands at 1537−1548 cm−1

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in complexes were attributed to C=N stretching vibrations of the nitrogen

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heterocyclic ligands (Bipy, Phen or Phterpy). The new absorption bands at 577−573 cm−1 and 468−474 cm−1 were assigned to the stretching vibrations of the coordinated Eu−N and Eu−O, respectively [15,16]. These evidences further confirmed the conformation of europium (III) ternary complexes with the fluorinated β-diketones

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and nitrogen heterocyclic ligands.

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3.3. 1H NMR spectra

The 1H NMR spectra of europium (III) ternary complexes exhibited three most obvious changes in comparison with those of the β-diketone and nitrogen heterocyclic ligands. The first one is that the enolic proton signals at δ = 15.43−15.12 ppm in the β-diketones were not observed in those of their europium

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complexes. The second is that the methine proton signals at δ = 6.54−6.51 ppm in the keto-enol tautomerism of β-diketones were shifted to high-field at δ = 3.28−3.02

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ppm in their europium complexes, which was due to the closeness for these protons binding site of the β-diketones and the paramagnetic europium ion [4,17]. The third is that the aryl proton signals of the nitrogen heterocyclic ligands in complexes

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exhibited a larger downfield shift compared with those of the corresponding free

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ligands [18,19], which was attributed to the electron-withdrawing effect of complexation. Meanwhile, the chemical shifts for aryl protons of the β-diketones had some shifts toward high-field in the europium (III) complexes, but those of methoxyl group had few changes. Here, we chose the complex C1 as an example to

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discuss their chemical shifts. The 1H NMR spectrum of the complex C1 was presented in Fig. 1. The Fig. 1 showed two broad two-proton singlet peaks at δ 12.44 and 9.83 ppm, which were due to H1 H1’ and H3 H3’, respectively. The two

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two-proton doublet (J = 6.4Hz, 6.0Hz) peaks at δ 10.76 and 8.54 ppm was assigned

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to H4 H4’ and H2 H2’, respectively. Proton H6 H6’ and H5 H5’ both exhibited a six-proton doublet (J = 7.8Hz, 7.7Hz) peaks at δ 7.46 and 6.82 ppm, respectively. The nine-proton singlet peak at δ 3.93 ppm was attributable to the H8 of the methoxyl group and the three-proton singlet peak at δ 3.03 ppm was corresponding to the methine protons H7.

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3.4. UV-visible spectra The UV−vis absorption spectra for the ligands and their europium corresponding complexes in chloroform solution (1 × 10−5 mol/L) was shown in Fig.

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2. For complexes C1−C6, two main absorption bands at 332−340 nm and 266−276

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nm were observed, which were due to the characteristic π–π* enol absorption of the β-diketone ligands and the π–π* absorption of the nitrogen donor heterocyclic ligands, respectively. Compared with their corresponding β-diketone ligands, the absorption bands at 332−340 nm in complexes were red-shifted 6−10 nm, which

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was attributed to the formation of the large conjugated chelate rings. The absorption bands at 266−276 nm in complexes were blue shifted 6−14 nm in comparison with

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the corresponding nitrogen heterocyclic ligands, caused by the perturbation induced by the metal coordination. These results suggested that all ligands had coordinated

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to europium (III) ion.

3.5. Photoluminescence properties The excitation spectra of complexes C1−C6 in solid state at room temperature

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were shown in Fig. 3, which was obtained by monitoring the emission at the hypersensitive 5D0→7F2 transition at 612 nm. These excitation spectra exhibited a

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weak band at 263−272 nm and a strong broad band at 366−378 nm, corresponding to the absorber of the nitrogen donor heterocyclic ligands and fluorinated β-diketones, respectively. The results suggested that the nitrogen donor and

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β-diketone ligands were both binding to the europium (III) ion, and their presence

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together afforded a strong sensitization of the emission. In addition, their excitation spectra also present narrow lines at 464nm, assigned to the europium (III) ion transition for 7F0→5D2 [20].

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The photoluminescence spectra of complexes C1−C6 in the solid state under excitation at 368, 370, 369, 366, 378 and 370 nm, respectively, were presented in

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Fig. 4. From the emission spectra, all europium (III) complexes exhibited characteristic sharp peaks in the region of 580–710nm associated with the 5D0→7FJ (J = 0–4) transitions of the europium (III) ion. The five expected peaks of the 5D0→ 7

F0−4 transitions are well resolved, and their emissions have only one sharp peak in

the region of the 5D0→7F0 transition, suggesting that the europium (III) ion occupies

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only a single site and a single chemical environment exists around it [21]. The 5

D0→7F1 transition was split into two or three components in their emission spectra,

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indicating that the environment of the center europium (III) ion was asymmetric. Compared to other transitions, the very high intensity of 5D0→7F2 transition may be observed in these spectra, pointing to a highly polarizable chemical environment

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around the europium ions, which was responsible for the brilliant red emission of

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these complexes. It was also observed that the emission intensities of complexes C1−C4 were much stronger than those of complex C5 and C6, which was attributed to the high affinity of the second ligands, Bipy and Phen, toward various lanthanide

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ions, especially toward europium (III) ion.

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To further understand the photoluminescence behavior of these europium (III)

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complexes in solid state, the luminescence decay curves of the 5D0 excited state in room temperature were determined by monitoring the most intense emission lines (5D0→7F2) at about 612 nm. Fig. 5 presented the luminescence decay curves of 5D0 level for complexes C1–C6 by monitoring the emission of 612, 613, 612, 612, 612 and 611 nm, respectively. The luminescence decay curves of all europium (III) complexes fitted a first-order exponential decay law, indicating the presence of a

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single chemical environment around the europium ion in complexes, which was in accord with the results of only one 5D0→7F0 line in their emission spectra. The

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lifetime values (τ) of the emitting 5D0 level are summarized in Table 2, which were

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obtained from the luminescence decay curves of complexes C1−C6.

In Table 2, the lifetime (τ) of the emitting state 5D0, nonradiative (Anrad),

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radiative (Arad) and tot-radiative rates (Atot) can be related through the following equation [3]:

τ

= Arad + Anrad = Atot

(1)

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1

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The radiative (Arad) rates were obtained by summing over the radiative rates A0J for each 5D0→7FJ transition (ΣJ A0J) of the europium (III) ion and A0J is calculated according to the following relation [3].

A0 J = A01

I 0 J ν 01 I 01 ν 0 J

(2)

where I0J is the integrated intensities of the 5D0→7FJ transitions (J = 1, 2 and 4) and

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ν0J is energy barycenters of these transitions. The magnetic dipole allowed 5D0→7F1 transition was taken as the reference for the whole spectrum and the value of A01 is

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estimated to be ~50 s−1 [22]. The luminescent quantum efficiency (η) for the emitting 5D0 level in complexes C1−C6 can be calculated by the following equation [23].

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Arad Arad + Anrad

(3)

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η=

The calculated photoluminescence data of complexes C1−C6 (Arad, Anrad, Atot and

η ) are also presented in Table 2. It is clear that complexes C1−C4 containing the secondary ligands Bipy and Phen exhibited a longer lifetime (τ) and a higher

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luminescence quantum efficiency (η), which was in agreement with the results from the excitation and emission intensity of complexes C1−C6. This could be due to the

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introduction of the ligands Bipy and Phen, which were useful ligands with high affinity toward europium (III) ion. The asymmetry of complexes C1−C4 was

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increased and prevented the 4f–4f transition attributed to the secondary ligands Bipy or Phen participating in the coordination [24]. This is beneficial to the photoluminescence properties of complexes C1−C4. In addition, complexes C1, C3 and C5 based on the β-diketone TMPD displayed the shorter lifetime (τ) and lower quantum efficiency (η) in comparison with the corresponding complexes C2, C4

17

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and C6 based on the β-diketone TFPD as ligand. This is because the β-diketone TMPD contains high-energy oscillators C−H bonds of methoxyl group in center

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benzene ring [12], which are able to quench the europium (III) ion excited states nonradiatively, thereby leads to shorter lifetime and lower quantum efficiency of complexes C1, C3 and C5. Consequently, complexes C2 and C4 with the

SC

β-diketone TFPD and the secondary ligands Bipy or Phen have longer lifetime (τ =

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0.799 and 0.826 ms, respectively) and higher luminescence quantum efficiency (η = 56.5 and 56.1, respectively).

3.6. Judd−Ofelt intensity parameters

The the Judd−Ofelt intensity parameters, Ω2 and Ω4, are sensitive to the

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changes in the chemical environment around the europium (III) ion. To further investigate the possible structural changes around the emitting center europium ion in complexes C1−C6, the experimental intensity parameters Ωt (t = 2, 4) can be

EP

calculated from the emission spectra based on the 5D0→7F2 and 5D0→7F4 electronic

AC C

transitions of the europium (III) ion according to the following equation [25]:

Ωt =

3hc 3 A0 J

4e 2 ω 3 χ

5

D0 U ( t ) 7 FJ

(4)

2

where ω is the angular frequency of the transition, A0J, where J = 2 and 4, are the coefficient of spontaneous emission for the 5D0→7FJ transition, χ is the Lorentz

18

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local-field correction term given by χ = n02 (n02 + 2) 2 / 9 , n is the refractive index of the medium (in this case n = 1.5), the squared matrix elements are 〈 5D0 U ( 2 ) 7 F2 〉 2 =

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0.0032 and 〈 5D0 U ( 4 ) 7 F4 〉 2 = 0.0023.

The the Judd−Ofelt intensity parameters Ω2 and Ω4 are given in Table 2. It was

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found that complexes C2 and C4 exhibited higher values of the Ω2 parameters, which indicated a more polarizable chemical environment in their complexes and

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suggested a higher covalent character between the europium (III) ion and ligands. Meanwhile, complexes C5 and C6 with the Phterpy ligand both revealed a higher value for the Ω4 parameter, which was due to the perturbation on the coordination

4. Conclusions six

Eu(TFPD)3⋅Bipy

new

europium

(C2),

(III)

complexes,

Eu(TMPD)3⋅Phen

EP

The

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effect of the bidentate β-diketones by the steric factors from the Phterpy ligand [26].

(C3),

Eu(TMPD)3⋅Bipy

(C1),

Eu(TFPD)3⋅Phen

(C4),

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Eu(TMPD)3⋅Phterpy (C5) and Eu(TFPD)3⋅Phterpy (C6), were synthesized and characterized, and their photophysical properties were investigated in detail. Their characteristics emission lines of europium (III) ion in emission spectra evidently exhibited that europium (III) ion was presented in a polarizable chemical environment acting as only one luminescent centre. The Judd−Ofelt intensity parameters, Ω2 and Ω4 were determined from their emission spectra in solid state.

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Complexes C2 and C4 displayed higher values of the Ω2 parameters, suggested a higher covalent character between the europium (III) ion and ligands. The higher Ω4

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values for Complexes C5 and C6 indicated a perturbation on the coordination effect by the steric factors from the Phterpy ligand. Meanwhile, complexes C1−C4 with the secondary ligands Bipy or Phen exhibited much better photoluminescence

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properties than complexes C5 and C6 with the secondary ligand Phterpy, which was

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assigned to the steric effect preventing the ligand Phterpy from getting closer to the europium (III) ion. In particular, complexes C2 and C4 showed a longer lifetime (τ = 0.799 and 0.826 ms, respectively) and a higher luminescence quantum efficiency (η = 56.5 and 56.1, respectively). These results indicated that the europium (III)

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complexes with the fluorinated β-diketone could effectively improve their

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photoluminescence properties.

Acknowledgment

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The authors would like to acknowledge the support from National Natural Science Foundation of China (No. 21542009) and the Educational Commission of Hubei Province, PR China (No. B2015132).

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TE D

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EP

containing lanthanide complexes. Chem Rev 2002;102:2347−2356. [6] Mahajan RK, Kaur I, Kaur R, Uchida S, Onimaru A, Shinoda S, Tsukube H.

AC C

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pyridyltriazolylmethane and 1,3-diketonate ligands. Inorg Chim Acta 2014;414: 71−77.

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tris-β-diketone ligand. J Alloys Comp 2014;613:13−17.

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EP

[12] Beeby A, Clarkson IM, Dickins RS, Faulkner S, Parker D, Royle L, de Sousa

AC C

AS, Williams JAG, Woods M. Non-radiative deactivation of the excited states of europium, terbium and ytterbium complexes by proximate energy-matched OH, NH and CH oscillators: an improved luminescence method for establishing solution hydration states. J Chem Soc Perkin Trans 1999;2:493−504. [13] Wang D, Liu H, Fan L, Yin G, Hu Y, Zheng J. Synthesis and photoluminescent behavior of Eu(III) complexes with 4,4,4-trifluoro-1-(6-methoxy-naphthalen-

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2-yl)-butane-1,3-dione. Synth Met 2015;209:267−272. [14] Melby LR, Rose NJ, Abramson E, Caris JC. Synthesis and fluorescence of

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some trivalent lanthanide lomplexes. J Am Chem Soc 1964;86:5117−5125. [15] Yang C, Xu J, Ma J, Zhu D, Zhang Y, Liang L, Lu M. An efficient long fluorescence lifetime polymer-based sensor based on europium complex as

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chromophore for the specific detection of F−, CH3COO−, and H2PO4−. Polym

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Chem 2012;3:2640−2648.

[16] Nakamoto K. Infrared and Raman Spectra of Inoganic and Coordination Compounds, New Jersey: John Wiley & Sons Inc; 2009.

[17] Fu L, Ferreira RAS, Silva NJO, Fernandes AJ, Ribeiro-Claro P, Goncalves IS,

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Bermudez V de Z, Carlos LD. Structure–photoluminescence relationship in Eu(III) β-diketonate-based organic–inorganic hybrids. Influence of the synthesis method:

3117−3125.

EP

carboxylic acid solvolysis versus conventional hydrolysis. J Mater Chem 2005;15:

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[18] Bhacca NS, Selbin J, Wander JD. Nuclear magnetic resonance spectra of 1:1 adducts of 1,10-phenanthroline and α,α'-bipyridyl with tris[2,2,6,6-tetramethylheptane-3,5-dionato] complexes of the lanthanides. J Am Chem Soc 1972;94:8719−8722. [19] Iftikhar K. Mixed-ligand lanthanide complexes-IX NMR spectral study of the adducts of Ln(fod)3 with 2,2’-bipyridyl and 1,10-phenanthroline. Polyhedron

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and luminescent properties of supramolecules of β-diketonate of Eu(III) and crown ethers as ligands. J Solid State Chem 2003;171:189−194.

[21] Malta OL, Brito HF, Menezes JFS, e Silva FRG, Alves Jr S, Farias Jr FS, de

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Andrade AVM. Spectroscopic properties of a new light-converting device

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Eu(thenoyltrifluoroacetonate)3 2(dibenzyl sulfoxide). A theoretical analysis based on structural data obtained from a sparkle model. J Lumin 1997;75: 255−268.

[22] Ferreira R, Pires P, de Castro B, Ferreira RAS, Carlos LD, Pischel U.

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radiative lifetime of Eu3+ in luminescent lanthanide complexes. Phys Chem Chem Phys 2002;4:1542−1548. [24] Bekiari V, Lianos P. Strongly Luminescent poly(ethylene glycol)-2,2’bipyridine Lanthanide Ion Complexes. Adv Mater 1998;10:1455−1458. [25] Rodrigues MO, da Costa Júnior NB, de Simone CA, Araújo AAS, Brito-Silva AM, Almeida Paz FA, de Mesquita ME, Júnior SA, Freire RO. Theoretical and

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experimental studies of the photoluminescent properties of the coordination polymer

[Eu(DPA)(HDPA)(H2O)2]·4H2O.

J

Phys

Chem

B

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2008;112:4204−4212. [26] Araújo AAS, Brito HF, Malta OL, Matos JR, Teotonio EES, Storpirtis S, Izumi CMS. Synthesis and photophysical study of highly luminescent coordination

SC

compounds of rare earth ions with thenoyltrifluoroacetonate and AZT. J Inorg

AC C

EP

TE D

M AN U

Biochem 2002;88:87−93

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Tables:

ν (O−H)

ν (C=O)

ν (C=N)

ν (C=C) ν (Eu−N) ν (Eu−O)

TMPD

3452 (m)

1602 (s)



1509 (s)





TFPD

3442 (m)

1600 (s)



1510 (s)





C1



1599 (s)

1548 (s)

1505 (s)

576 (m)

472 (m)

C2



1592 (s)

1544 (s)

1502 (s)

576 (m)

474 (m)

C3



1599 (s)

1543 (s)

1504 (s)

577 (m)

473 (m)

C4



1595 (s)

1545 (s)

1502 (s)

577 (m)

474 (m)

C5



1598 (s)

1537 (s)

1496(s)

574 (m)

471 (m)

C6



1596 (s)

1538 (s)

1492 (s)

573 (m)

468 (m)

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SC

RI PT

Compound

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Table 1 The characteristic IR peaks (cm−1) of β-diketones and their complexes

Table 2 The photoluminescence data of europium (III) complexes in solid state

τ (ms)

Atot(s−1)

C1

0.702

1424

AC C

EP

Complex

Arad(s−1) Anrad(s−1) Ω2(10−20 Ω4(10−20 cm2) cm2) 609 815 14.53 2.06

η (%) 42.8

C2

0.799

1252

708

544

17.50

2.18

56.5

C3

0.741

1350

650

700

15.84

1.96

48.1

C4

0.826

1211

679

532

16.62

2.02

56.1

C5

0.246

4065

530

3535

12.11

2.44

13.0

C6

0.346

2890

634

2256

14.94

2.59

21.9

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Schemes: N

N Eu

Bipy

O

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O

F

O

F F

R

+

CF3COOEt

C1 : R = -OCH3, C2 : R = -F

SC

R

O F

R

F F

+ EuCl3

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Benzene, NaOCH3 O

Phen

NaOH

N

N Eu

O

O

Ethanol

(TMPD : R = -OCH3, TFPD : R = -F)

3

F F F

R

3

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C3 : R = -OCH3, C4 : R = -F

EP

N N

AC C

Phenylterpy

N

Eu O

O F

R

F F

3

C5 : R = -OCH3, C6 : R = -F

Scheme 1 The synthetic routes of fluorinated β-diketones and their complexes

27

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Figure Captions: Fig. 1. 1H NMR (300 MHz) spectrum of the complex C1 in CDCl3. Fig. 2. UV−vis absorption spectra of ligands TMPD, TFPD, Bipy, Phen, Phterpy

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and complexes C1−C6 in chloroform solution (1 × 10−5 mol/L).

Fig. 3. Excitation spectra of complexes C1−C6 in solid state at room

SC

temperature monitoring at 612 nm.

Fig. 4. Emission spectra of complexes C1−C6 in solid state at room temperature,

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under excitation at 368, 370, 369, 366, 378 and 370 nm, respectively. Fig. 5. Luminescence decay curves for complexes C1−C6 in the powder at room

AC C

EP

TE D

temperature monitoring at 612, 613, 612, 612, 612 and 611 nm, respectively.

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Figures:

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SC

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Fig. 1

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Fig. 2

0.7 0.6

0.3

EP

0.4

AC C

Absorbance

0.5

C1 C2 C3 C4 C5 C6 TMPD TFPD Bipy Phen Phterpy

0.2 0.1 0.0

225

250

275

300

325

350

Wavelength / nm

Fig. 3

375

400

425

ACCEPTED MANUSCRIPT

400

C1 C2 C3 C4 C5 C6

300 250

RI PT

Relative Intensity / a.u.

350

200 150

SC

100 50 0 275

300

325

350

375

400

425

M AN U

250

450

475

500

Wavelength / nm

400 350 300 250

EP

200 150 100

AC C

50

560

580

600

0 C1 C2 C3 C4 C5 C6 620

640

660

Wavelength / nm

Fig. 5

680

700

720

Relative Intensity / a.u.

450

TE D

Fig. 4

ACCEPTED MANUSCRIPT

800 700

C1 C2 C3 C4 C5 C6

RI PT

500 400 300 200 100 0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

EP

TE D

Time / ms

AC C

0.8

0.9

M AN U

0.0

SC

Relative Intensity / a.u.

600

1.0

1.1

1.2

ACCEPTED MANUSCRIPT

Highlights ► Six europium (III) complexes with the fluorinated β-diketones were

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synthesized. ► Photoluminescence behavior of europium (III) complexes were investigated.

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► The Judd−Ofelt intensity parameter, lifetime and quantum yield were determined. Eu(TFPD)3⋅Bipy

and

AC C

EP

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photoluminescence behavior.

Eu(TFPD)3⋅Phen

exhibited

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►

a

good

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