Synthesis and luminescence properties of terbium complexes based on 4-acyl pyrazolone derivatives

Journal of Luminescence 188 (2017) 223–229

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Synthesis and luminescence properties of terbium complexes based on 4acyl pyrazolone derivatives Ming Daia,b, Haihua Xiaoa, Chunwei Yea, Dehua Shuc, Ling Shia, Dongcai Guoa,b, a b c

MARK

⁎

School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China Hunan Provincial Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions, Changsha 410082, China China Geo Engineering Corporation, Beijing 100082, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Pyrazolone Complex Synthesis Luminescence Electrochemical property

Six new 4-acyl pyrazolone derivatives and their complexes with the terbium nitrates were synthesized and characterized by mass spectra, elemental analysis, EDTA titrimetric analysis, UV spectra, infrared spectra, molar conductivity and thermal analysis. The results showed that pyrazolone derivatives formed good coordination with the terbium ions. The introduction of electron-donating group in the ligands not only increased the luminescence intensity and fluorescence quantum yield, but also enlarged the HOMO and LUMO energy levels of corresponding terbium complexes. All the complexes possessed high luminescence intensity and showed relatively high fluorescence quantum yields. These target complexes may have vast potential to be developed as luminescence materials in the future.

1. Introduction

2. Experimental

Rare earth complexes have a very broad application prospects in the life, scientific research, industry and other fields due to their excellent optical, electrical and magnetic properties [1–6]. Especially, good luminescence properties of rare earth complexes make it used widely for fluorescent anti-counterfeiting materials [7,8], light conversion film and fluorescent tags. Therefore, it has been a diligent direction for the scientific research workers to design and synthesize the new rare earth complexes with excellent luminescent properties [9]. 4-acyl pyrazolone contain aromatic and heterocyclic ring, and possess multiple ligand activity centers that enables it form a single core, dual core and dual different core of complexes with rare earth ions [10]. In order to develop good luminescence complexes, we have designed a series of pyrazolone compounds with β-diketone structure, which can be combined with rare earth ions to form a relatively stable structure, so that the complexes have a variety of properties in optical, electrical, magnetic and biological fields, etc. [11–14]. In this paper, six new 4-acyl pyrazolone derivatives and their complexes with the terbium ions were synthesized and characterized. Meanwhile, the synthetic methods, luminescence properties, fluorescence quantum yields and electrochemical properties of the title ligands and complexes are reported in detail. The synthesis route is shown in Scheme 1.

2.1. Materials and methods All the starting materials and reagents were purchased from commercial suppliers. Terbium nitrate was prepared according to the literature [15]. 1H NMR and 13C NMR spectra were acquired on a Bruker-400 NMR instrument using CDCl3 or DMSO-d6 as solution and TMS as internal standard. Mass spectra were recorded on MAT 95 XP mass spectrometers. Melting points were determined with a TECH XT-4 melting point apparatus. Ultraviolet (UV) spectra were acquired on a LabTech UV-2100 spectrophotometer. IR spectra in the 4000–400 cm−1 region were acquired with KBr disks on a IRAffinity-1 spectrometer. The thermal analysis was performed on a DTG-60 thermal analyzer operating at a heating rate of 20 °C/min. Fluorescence spectra were got on a HIACHI F-2700 spectrophotometer at a scanning rate of 1200 nm/ min. The luminescence spectra of the title complexes were measured with 2.5 nm slit widths for excitation and 5.0 nm slit widths for emission in the solid state at room temperature under a drive voltage of 400 V, and an appropriated filter was placed to remove light scattering for non-absorbed wavelength. Fluorescence quantum yield of the title complexes(Φfx) can be calculated by the formula as follows [16,17].

Abbreviations: UV, Ultraviolet; NMR, nuclear magnetic resonance; EDTA, ethylenediaminetetraacetic acid; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital ⁎ Corresponding author at: School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China. E-mail address: [email protected] (D. Guo). http://dx.doi.org/10.1016/j.jlumin.2017.04.018 Received 13 November 2015; Received in revised form 8 March 2017; Accepted 9 April 2017 Available online 15 April 2017 0022-2313/ © 2017 Published by Elsevier B.V.

Journal of Luminescence 188 (2017) 223–229

M. Dai et al. Table 1 Elementary analysis and molar conductivity data of the title complexes. Complex

TbY1(NO3)3·EtAc 2

TbY (NO3)3·EtAc 3

TbY (NO3)3·EtAc TbY4(NO3)3·EtAc TbY5(NO3)3·EtAc TbY6(NO3)3·EtAc

Measured value (theoretical value) (%)

Λm

C

H

N

Tb

(S cm2 mol−1)

35.64 (35.90) 36.51 (36.79) 35.84 (36.06) 34.16 (34.37) 35.02 (35.22) 33.75 (33.92)

3.57 (3.14) 3.82 (3.35) 3.66 (3.28) 3.31 (2.88) 3.67 (3.21) 3.23 (2.85)

9.01 (9.10) 8.75 (8.94) 8.55 (8.76) 8.56 (8.71) 10.46 (10.71) 10.14 (10.32)

20.32 (20.66) 19.97 (20.29) 19.56 (19.88) 19.53 (19.77) 19.94 (20.26) 19.25 (19.51)

24

fluorescence integrated area Fx and Fstd. The excitation wavelength was range 300–400 nm and the corresponding absorbance value A was less than 0.05, which was to reduce error in estimation of the emission quantum yield. The electrochemical properties of the complexes were got by the cyclic voltammetry data on a CHI 660d electrochemical workstation at a sweep rate of 50 mV/s (external reference: ferrocene, supporting electrolyte: 0.1 mol/L sodium nitrate solution, solvent: DMSO, the sensitivity: 1×e–3 A/V).

31 35

2.2. Synthesis

39

2.2.1. Synthesis of 1-phenyl-3-methyl-5-pyrazolone(A) Phenylhydrazine(2.16 g, 20 mmol) and ethyl acetoacetate(2.60 g, 20 mmol) were dissolved in absolute alcohol (30 mL) in a 100-mL three-neck fask. The reaction solution was heated and refluxed at 80 °C for 6 h, and then the excess solvent was distilled after completion of the reaction. The distillation residue was poured into ice water, a lot of

35 27

Table 2 UV data of the title ligands and complexes. compounds

λ1 (nm)

ε1 (1.0×104 L mol−1 cm−1)

λ2 (nm)

ε2 (1.0×104 L mol−1 cm−1)

Y1 TbY1(NO3)3·EtAc Y2 TbY2(NO3)3·EtAc Y3 TbY3(NO3)3·EtAc Y4 TbY4(NO3)3·EtAc Y5 TbY5(NO3)3·EtAc Y6 TbY6(NO3)3·EtAc

273 280 275 277 276 278 275 278 275 276 – –

0.84 0.93 0.95 1.07 0.98 1.24 0.96 1.13 0.95 0.97 – –

298 – 299 – 301 – 300 – 295 299 294 299

0.61 – 0.68 – 0.71 – 0.64 – 0.85 1.09 1.08 1.19

yellow needle-shaped crystals appeared immediately. The crystals were filtered, washed with cool water, dried, and then recrystallized from absolute ethanol to get target compound A [18]. Yield 76%. 1H NMR (CDCl3) δ/ppm: 7.86 (d, J =7.8 Hz, 2H, ArH), 7.39 (t, J =8.0 Hz, 2H, ArH), 7.18 (t, J =7.4 Hz, 1 H, ArH), 3.43 (s, 2H, CH2), 2.20 (s, 3H, CH3). 2.2.2. Synthesis of 1-phenyl-3-methyl-4-chloracetyl-5-pyrazolone(B) Compound A(3.48 g, 20 mmol) was dissolved in dioxane(30 mL) in a 100-mL three-neck flask, and then calcium hydroxide(3.0 g, 60 mmol) was added with stirring. The mixture was heated to reflux for 1 h at 90 °C after the reaction was tempered about 5 min, and then left to cool down to room temperature. Dilute hydrochloric acid(60 mL, 2 mol L−1) was poured into the resulting mixture to obtain white solid. The white solid was filtered, washed with dilute hydrochloric acid, recrystallized twice from absolute ethanol, and dried in vacuum to get white needleshaped crystals [19]. Yield 53%. 1H NMR (CDCl3) δ/ppm: 7.79 (d, J =7.8 Hz, 2H, ArH), 7.46 (t, J =8.0 Hz, 2H, ArH), 7.32 (t, J =7.4 Hz, 1H, ArH), 4.44 (s, 2H, CH2), 2.51 (s, 3H, CH3); MS (EI) m/z (%): 252 (M+2, 7), 250 (M, 22), 223 (2), 217 (4), 216 (25), 201 (100), 173 (5), 149 (17), 130 (5), 104 (8), 92 (12), 91 (22), 77 (41), 67 (35).

Fig. 1. UV spectra of TbY2(NO3)3·EtAc(a) and Y2(b).

Φfx =

nx2 Fx Astd ⋅ ⋅ ⋅Φfstd 2 Fstd Ax nstd

2.2.3. Synthesis of the pyrazolone derivatives(Y1−6) As the synthesis procedures for pyrazolone derivatives Y1–6 are similar, only the synthesis procedure of the compound Y1 is shown for illustration. Benzoic acid(0.61 g, 5 mmol) and anhydrous potassium carbonate(1.104 g, 8 mmol) were dissolved in DMF in a 100-mL threeneck fask, and the mixture was heated to reflux for 1 h at 90 °C. Afterwards, compound B (1.25 g, 5 mmol) and a little potassium iodide were added into the mixture, and the reaction mixture was refluxed for 8 h. The resultant residue was poured into water(500 mL), and moderate dilute hydrochloric acid was poured to obtain white precipitate. The

–1

The quinine sulfate (1.0 μg mL ) of sulfuric acid solution (0.1 mol L–1) was used as a standard reference in this experiment, and the Фfstd was 0.55. The nx (approximately equal to the refractive index of the solvent (DMSO)) was 1.480, and the nstd (the refractive index of water) was 1.337. A was the absorbance, as the point of intersection of the ultraviolet absorption curves of the standard and the sample was treated as excitation wavelength, Astd was equal to Ax. The luminescence spectra data were measured in DMSO solution (1.0×10−6 mol L−1) at room temperature to get the value of the 224

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Table 3 IR data of the title ligands and complexes(cm−1). compounds

v (O-H)

v (C=O)

v (C=N)

v (C-O-C)

Y1 TbY1(NO3)3·EtAc Y2 TbY2(NO3)3·EtAc Y3 TbY3(NO3)3·EtAc Y4 TbY4(NO3)3·EtAc Y5 TbY5(NO3)3·EtAc Y6 TbY6(NO3)3·EtAc

3416 3415 3405 3402 3413 3411 3422 3421 3414 3413 3419 3415

1725,1672 1712,1656 1721,1681 1710,1665 1719,1676 1704,1658 1723,1662 1707,1645 1722,1667 1710,1642 1717,1678 1708,1657

1527 1526 1532 1531 1535 1532 1531 1530 1523 1522 1540 1537

1167 1164 1164 1165 1171 1170 1174 1173 1182 1181 1192 1190

v (NO3-)

1491,1352,1033,831 1484,1347,1035,825 1482,1347,1037,836 1484,1353,1036,835 1479,1342,1036,833 1491,1342,1035,839

101.77, 65.90, 15.79; IR (KBr) v/cm−1: 3416, 3034, 1725, 1672, 1527, 1456, 1262, 1167, 748; MS (EI) m/z (%):338 (M+2, 1), 337 (M +1, 5), 336 (M, 23), 256 (1), 231 (3), 201 (16), 186 (6), 185 (3), 163 (2), 122 (2), 106 (8), 105 (100), 91 (4), 77 (28), 67 (3), 51 (6); Anal. Calcd. for C19H16N2O4: C, 67.85; H, 4.79; N, 8.33. Found: C, 67.45; H, 5.21; N, 7.98. 2-(1-phenyl-3-methyl-5-pyrazolone-4-yl)−2-oxygen generation of ethyl-4-methyl benzo ate(Y2). Pale yellow powder. Yield 64%. m.p. 81–83 °C. 1H NMR (400 MHz, CDCl3) δ/ppm: 8.04 (d, J =7.7 Hz, 2H, ArH), 7.78 (d, J =7.9 Hz, 2H, ArH), 7.46 (t, J =7.6 Hz, 2H, ArH), 7.30 (dd, J =16.7, 7.8 Hz, 3H, ArH), 5.30 (s, 2H, CH2), 2.53 (s, 3H, CH3), 2.43 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 190.73, 166.12, 159.21, 146.96, 144.41, 136.85, 130.04, 129.30, 129.20, 127.14, 126.29, 121.21, 101.77, 65.78, 21.77, 15.80; IR (KBr) v/cm−1: 3405, 2976, 1712, 1681, 1532, 1461, 1274, 1164, 743; MS (EI) m/z (%): 351 (M+1, 2), 350 (M, 7), 220 (1), 216 (2), 201 (8), 185 (2), 174 (2), 136 (4), 120 (9), 119 (100), 93 (2), 92 (9), 91 (45), 77 (21), 65 (16); Anal. Calcd. for C20H18N2O4: C, 68.56; H, 5.18; N, 8.00. Found: C, 68.17; H, 5.53; N, 7.84. 2-(1-phenyl-3-methyl-5-pyrazolone-4-yl)−2-oxygen generation of ethyl-4-methoxyl benzo ate(Y3). Pale yellow powder. Yield 72%. m.p. 78–80 °C. 1H NMR (400 MHz, CDCl3) δ/ppm: 8.10 (d, J =8.1 Hz, 2H, ArH), 7.78 (d, J =7.9 Hz, 2H, ArH), 7.46 (t, J =7.6 Hz, 2H, ArH), 7.32 (t, J =7.4 Hz, 1H, ArH), 6.96 (d, J =8.1 Hz, 2H, ArH), 5.29 (s, 2H, CH2), 3.88 (s, 3H, CH3), 2.53 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 190.89, 165.77, 163.88, 159.21, 146.98, 136.86, 132.12, 129.19, 127.13, 121.37, 121.20, 113.85, 101.76, 65.69, 55.52, 15.82; IR (KBr) v/cm−1: 3413, 3012, 1719, 1676, 1535, 1462, 1258, 1172, 751; MS (EI) m/z (%): 367 (M+1, 1), 366 (M, 4), 277 (1), 216 (2), 201 (7), 185 (1), 173 (1), 152 (3), 136 (9), 135 (100), 119 (6), 92 (7), 77 (15), 65 (3); Anal. Calcd. for C20H18N2O5: C, 65.57; H, 4.95; N, 7.65. Found: C, 65.23; H, 5.34; N, 7.47. 2-(1-phenyl-3-methyl-5-pyrazolone-4-yl)−2-oxygen generation of ethyl-4-chloro benzoate (Y4). Pale yellow powder. Yield 66%. m.p. 97–99 °C. 1H NMR (400 MHz, CDCl3) δ/ppm: 8.08 (d, J =8.1 Hz, 2H, ArH), 7.77 (d, J =7.8 Hz, 2H, ArH), 7.46 (d, J =7.2 Hz, 4H, ArH), 7.32 (t, J =7.3 Hz, 1H, ArH), 5.31 (s, 2H, CH2), 2.52 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 190.22, 165.23, 159.18, 146.95, 140.12, 136.79, 131.39, 129.22, 128.97, 127.51, 127.19, 121.21, 101.72, 66.01, 15.80; IR (KBr) v/cm−1: 3422, 3026, 1723, 1662, 1531, 1466, 1267, 1174, 747, 724; MS (EI) m/z (%): 372 (M+2, 6), 370 (M, 20), 232 (2), 231 (9), 216 (21), 201 (40), 187 (3), 158 (4), 156 (14), 141 (34), 139 (100), 113 (6), 111 (17), 92 (5), 91 (8), 77 (15), 75 (6); Anal. Calcd. for C19H15ClN2O4: C, 61.55; H, 4.08; N, 7.56. Found: C, 61.24; H, 4.47; N, 7.24. 2-(1-phenyl-3-methyl-5-pyrazolone-4-yl)−2-oxygen generation of ethyl-4-amino benzoate (Y5). Yellow powder. Yield 63%. m.p. 119–121 °C.1H NMR (400 MHz, DMSO) δ/ppm: 7.70 (dd, J =12.7, 8.2 Hz, 4H, ArH), 7.51 (t, J =7.6 Hz, 2H, ArH), 7.32 (t, J =7.3 Hz, 1H, ArH), 6.61 (d, J =8.1 Hz, 2H, ArH), 5.28 (s, 2H, CH2), 2.45 (s, 3H,

Fig. 2. IR spectra of complex TbY4(NO3)3·EtAc(a) and Y4(b).

Fig. 3. Molecular structure of the complex TbY4(NO3)3·EtAc.

precipitate was filtered, washed with cool water, recrystallized from absolute ethanol, and dried in vacuum to form compound Y1. 2-(1-phenyl-3-methyl-5-pyrazolone-4-yl)−2-oxygen generation of ethyl benzoate(Y1). Pale yellow powder. Yield 61%. m.p. 87–89 °C. 1H NMR (400 MHz, CDCl3) δ/ppm: 8.16 (d, J =7.9 Hz, 2H, ArH), 7.79 (d, J =8.1 Hz, 2H, ArH), 7.62 (t, J =7.2 Hz, 1H, ArH), 7.48 (dt, J =11.4, 7.6 Hz, 4H, ArH), 7.32 (t, J =7.4 Hz, 1H, ArH), 5.33 (s, 2H, CH2), 2.54 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 190.52, 166.07, 159.21, 147.00, 136.83, 133.60, 130.00, 129.20, 129.06, 128.59, 127.15, 121.21, 225

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CH3); 13C NMR (DMSO) δ/ppm: 188.11, 165.85, 160.65, 153.81, 150.99, 136.30, 131.80, 129.56, 126.57, 121.25, 116.29, 113.24, 102.98, 67.79, 13.96; IR (KBr) v/cm−1: 3414, 3013, 1722, 1667, 1523, 1457, 1256, 1182, 743; MS (EI) m/z (%): 351 (M, 5), 217 (2), 216 (12), 201 (15), 174 (4), 156 (11), 137 (10), 120 (100), 93 (3), 92(13), 77 (9), 65 (12); Anal. Calcd. for C19H17N3O4: C, 64.95; H, 4.88; N, 11.96. Found: C, 64.72; H, 5.27; N, 11.73. 2-(1-phenyl-3-methyl-5-pyrazolone-4-yl)−2-oxygen generation of ethyl-4-nitro benzoate (Y6). Yellow powder. Yield 62%. m.p. 131–133 °C. 1H NMR (400 MHz, CDCl3) δ/ppm: 8.53–8.15 (m, 4H, ArH), 7.77 (d, J =7.9 Hz, 2H, ArH), 7.46 (t, J =7.5 Hz, 2H, ArH), 7.33 (t, J =7.4 Hz, 1H, ArH), 5.37 (s, 2H, CH2), 2.53 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 189.58, 164.23, 159.10, 150.85, 146.89, 136.72, 134.50, 131.15, 129.24, 127.27, 123.72, 121.20, 101.67, 66.40, 15.79; IR (KBr) v/cm−1: 3419, 3024, 1717, 1678, 1540, 1469, 1259, 1192, 745; MS (EI) m/z (%): 383 (M+2, 2), 382 (M+1, 13), 381 (M, 53), 350 (3), 233 (1), 232 (7), 231 (50), 208 (5), 202 (12), 201 (88), 200 (5), 185 (4), 151 (20), 150 (100), 135 (13), 120 (32), 104 (35), 92 (31), 77 (47), 76 (20); Anal. Calcd. for C19H15N3O6: C, 59.84; H, 3.96; N, 11.02. Found: C, 59.57; H, 4.27; N, 10.85.

Table 4 Thermal analysis data of the title complexes. Complex

Endothermic Peak (°C)

Exothermic Peak (°C)

Residual weigh (theoretical value) (%)

TbY1(NO3)3·EtAc TbY2(NO3)3·EtAc TbY3(NO3)3·EtAc TbY4(NO3)3·EtAc TbY5(NO3)3·EtAc TbY6(NO3)3·EtAc

79, 114 75 80, 145 82 78 75, 152

195, 249, 217, 206, 262, 211,

23.64 24.47 24.33 22.94 24.56 23.78

394, 394, 405, 446, 372, 416,

506, 551 693 632, 667 552 413, 595 613

(24.32) (23.88) (23.41) (23.27) (23.85) (22.97)

2.2.4. Synthesis of the title complexes As the synthesis procedures of the title complexes are similar, only the synthesis procedure of the complex TbY1(NO3)3·EtAc is selected for illustration. The compound Y1(0.0672 g, 0.20 mmol) was dissolved in 20 mL ethyl acetate in a 100 mL three-neck flask. The mixture was heated to 60 °C until completely dissolved, and then terbium nitrate ethyl acetate solution(2 mL, 0.1 mol/L) was added. Meanwhile, the PH value was adjusted to 6–7 by sodium ethoxide. The reaction mixture was refluxed for 4 h, and poured into 50 mL petroleum ether with stirring to get precipitate. The precipitate was filtered, washed with ethyl acetate for several times, and dried in vacuum to obtain the complex TbY1(NO3)3·EtAc.

Fig. 4. TG-DTA curves of TbY5(NO3)3·EtAc.

3. Results and discussions 3.1. Properties of the target complexes The elementary analysis and molar conductivity data of the title complexes are listed in Table 1, it's obvious that the composition of the six novel terbium complexes conformed to TbY1–6(NO3)3·EtAc. The title ligands and terbium complexes were easily soluble in chloroform, dichloromethane, DMSO and DMF as well as methanol, ethanol and ethyl acetate, but hardly soluble in petroleum ether and cyclohexane. The molar conductance data of the complexes in DMF(10−3 mol/L) indicated that all complexes acted as nonelectrolytes [20]. Fig. 5. Excitation spectra(a) and emission spectra (b) of TbY4(NO3)3·EtAc.

3.2. UV spectral analysis Both UV data and the molar absorptivities of the title ligands and complexes were recorded in the DMSO solution (5.0×10−5 mol L−1) are listed in Table 2. As the UV spectra of all the complexes TbY1–6(NO3)3·EtAc are similar, only the UV spectra of TbY2(NO3)3·EtAc and its corresponding ligand Y2 are selected for illustration, as shown in Fig. 1. As shown in Table 2 and Fig. 1, there were two absorption peaks for ligand Y2, one appeared at around 275 nm and the other at 299 nm, which were assigned to the π–π* and n–π* transitions, respectively. Compared with the absorption peak of the ligand Y2, the π–π* transitions absorption peak of TbY2(NO3)3·EtAc was shifted to 277 nm, while n→π* transitions absorption peak appeared obvious deformation. The above results indicated that ligand Y2 had been coordinated to the terbium ion [21]. There was no absorption band in the visible range of the spectra for these complexes, while these complexes are yellow, this was because the absorption spectrum was influenced by many factors, including

Fig. 6. Excitation spectra(a) and emission spectra (b) of Tb(NO3)3Phen.

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Table 5 Luminescence spectral data of the title complexes and Tb(NO3)3Phen. λex (nm)

Complexes

TbY1(NO3)3·EtAc TbY2(NO3)3·EtAc TbY3(NO3)3·EtAc TbY4(NO3)3·EtAc TbY5(NO3)3·EtAc TbY6(NO3)3·EtAc Tb(NO3)3Phen

D4→7F6

D4→7F5

5

I

278 276 276 275 276 274 359

1

TbY (NO3)3·EtAc TbY2(NO3)3·EtAc TbY3(NO3)3·EtAc TbY4(NO3)3·EtAc TbY5(NO3)3·EtAc TbY6(NO3)3·EtAc

781 2250 2852 3789 647 88 2140

I (a.u.)

λem (nm)

I (a.u.)

λem (nm)

I (a.u.)

492 492 492 493 492 493 492

384 974 1098 1533 154 35 1679

547 546 547 547 546 547 545

1088 2436 3175 5538 847 135 2643

587 585 586 586 587 586 585

80 159 191 250 26 8 215

623 624 624 625 624 625 624

24 70 77 92 11 3 64

the central ion, and it was also in agreement with the molar conductivity analysis. Furthermore, there was a weak band at 426 cm−1, which could be assigned to v(Tb-O). All above, ligand Y4 was coordinated to the Tb(III) ion, and stable complex was synthesized. Based on these analysis and elemental analysis, the molecular structure of the complexe TbY4(NO3)3·EtAc may be deduced and shown in Fig. 3.

λ (nm)

I (a.u.)

Фfx

311 307 316 302 318 315

954 1171 1254 1836 778 357

0.49 0.51 0.55 0.65 0.43 0.13

3.4. Thermal analysis The thermal analyses data of the title complexes are listed in Table 4. As the thermal analyses of all the complexes TbY1–6(NO3)3·EtAc are similar, only the thermal analysis of TbY5(NO3)3·EtAc is selected for illustration, as shown in Fig. 4. As shown in Table 4 and Fig. 4, along with a weak endothermic peak in the DTA curve at about 78 °C, there was a obvious mass loss stage between 40 and 160 °C due to decomposition of crystallization solvent molecules EtAc, and the experimental value(89.36%) was corresponding to the theoretical value(88.78%). And then, a obvious mass loss stage ranged from 200 to 540 °C occurred due to the mass loss of the ligand Y5, accompanied by three exothermic peaks in the DSC curve at 262, 372 and 413 °C, and the experimental value(55.47%) was corresponding to the theoretical value(55.23%). At last, a rapid mass loss ranged from 540 to 650 °C was attributed to the mass loss of three nitrate ions, accompanied by a strong exothermic peak in the DSC curve at about 595 °C, and the experimental value (25.69%) was corresponding to the theoretical value (23.98%). The residue finally reached a steady at 650 °C, which indicated that the complexe had been completely decomposed. The rest of residue Tb4O7 of TbY5(NO3)3·EtAc was 24.56%, which matched well with the theoretical value(23.85%). The thermal analyses results were consistent with elemental analysis and IR spectral analysis, which further confirmed the molecular structure of the title complexes.

The IR spectral data of the title ligands and complexes are listed in Table 3. As the IR spectra of all the complexes TbY1–6(NO3)3·EtAc are similar, only the IR spectra of TbY4(NO3)3·EtAc and its corresponding ligand Y4 are selected for illustration, as shown in Fig. 2. As seen from Table 3 and Fig. 2, the C=O on the 4-acyl stretching vibration was shifted, v(C=O) from 1723 cm−1 in Y4 to 1707 cm−1 in TbY4(NO3)3·EtAc. Moreover, the C=O on the ester group stretching vibration was also changed, v(C=O) from 1662 cm−1 to 1645 cm−1. These confirmed that the oxygen of the two carbanyl group were coordinated to the Tb (III) ion. The O-H, C=N and C-O-C absorption had invisible changes, which established that the enol O-H, C=N and the oxygen of C-O-C were not involved in the coordination. In addition, the characteristic frequencies of the coordinating NO3- appeared at around 1484(v1), 1036(v2), 835(v3) and 1329 cm−1(v4), the separation of the two strongest absorption bands |ν1–ν4| was 131 cm−1, which implied that the NO3- ions in the complex were bidentate [23]. Moreover, there was no typical absorption at 1385 cm−1, which made it clear that three NO3- ions were involved in coordination without dissociative,and the oxygen atoms in the nitrate are coordinated to

3.5. Luminescence properties The luminescence spectral data of the title complexes are given in Table 5. As the luminescence spectra of all the title complexes are similar, only the luminescence spectra of TbY4(NO3)3·EtAc is selected for illustration, as shown in Fig. 5. It can be observed Table 5 and Fig. 5 that the maximum excitation wavelength of TbY4(NO3)3·EtAc was at 275 nm from the excitation spectrum. The emission spectrum of the complex displayed four main peaks at 492 nm, 546 nm, 586 nm and 624 nm, which was assigned to the 5D4→7F6, 5D4→7F5, 5D4→7F4, 5D4→7F3 transition, respectively. As for the complex TbY4(NO3)3·EtAc, the emission intensity at 547 nm from the 5D4→7F5 transition was the strongest, which indicated that the complex gave off green fluorescence. at the same time, the width of the half peaks was approximately 10 nm, which made it clear that the complex gave off high purity fluorescence, and ligand Y4 was an excellent organic chelator.

Table 7 The HOMO and LUMO energy levels of the title complexes.

TbY (NO3)3·EtAc TbY2(NO3)3·EtAc TbY3(NO3)3·EtAc TbY4(NO3)3·EtAc TbY5(NO3)3·EtAc TbY6(NO3)3·EtAc

D4→7F3

λem (nm)

3.3. IR spectral analysis

1

5

I (a.u.)

concentration, solution, density, transitions between different energy states and charge transfer between the ligand and the central atom, etc. It is known that nitro group have an absorption band in the visible range of absorption spectrum, while complexe didn’t have absorption bands in the visible range of the spectra, this might because the complexe with electrophilic group(-NO2) exist an internal chargetransfer excited state below the triplet state, which led to the emission quenching [22].

Complex

5

λem (nm)

Table 6 The fluorescence quantum yields of the title complexes. Complex

D4→7F4

5

(a.u.)

λonset (nm)

Eox (v)

EHOMO (ev)

Eg (ev)

ELUMO (ev)

262 261 262 263 262 263

0.684 0.681 0.677 0.693 0.666 0.702

−5.424 −5.421 −5.417 −5.433 −5.406 −5.442

4.733 4.751 4.733 4.715 4.733 4.715

−0.691 −0.670 −0.684 −0.718 −0.673 −0.727

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(NO3)3Phen is widely used as a fluorescent material, which indicates that TbY4(NO3)3·EtAc has vast potential to be developed as luminescence materials in the future. 3.6. Fluorescence quantum efficiency analysis The fluorescence quantum yield of the title complexes are listed in Table 6. As shown in Table 6, the fluorescence quantum yield of Tb (III) complexes increased from 0.13 to 0.65 in the order TbY6(NO3)3·EtAc < TbY5(NO3)3·EtAc < TbY1(NO3)3·EtAc < TbY2 (NO3)3·EtAc < TbY3(NO3)3·EtAc < TbY4(NO3)3·EtAc, which indicated that the introduction of electron-donating group in the ligand increased the fluorescence quantum yield, while the introduction of electrophilic group decreased the fluorescence quantum yield. It's obvious that the triplet level of ligand Y4 was in an appropriate level to center the Tb(III) ion and the fluorescence quantum yield of complex TbY4(NO3)3·EtAc reached up to 0.65, which indicated that TbY4(NO3)3·EtAc was an excellent luminescence material.

Fig. 7. Cyclic voltammetry curve of TbY4(NO3)3·EtAc.

From Table 5, it can be observed that the luminescence intensity of TbY2,3(NO3)3·EtAc was stronger than that of TbY1(NO3)3·EtAc, this was because the electron-donating group(-CH3, -OCH3) was introduced into the target complexes TbY2,3(NO3)3·EtAc respectively, and the introduction of electron-donating group enlarged the π-conjugated systems of complexes TbY2,3(NO3)3·EtAc. The emission intensity of TbY4(NO3)3·EtAc was the strongest of the six complexes, which attributed to the triplet level of ligand Y4 was in an appropriate level to the center Tb(III) ion and the energy transition from ligand Y4 to the Tb(III) ion was easier than that of other ligands [24]. Theoretically, the introduction of electron-donating group would increase the electron cloud density and the luminescence intensity of complexes. However, the luminescence intensity of TbY5(NO3)3·EtAc was slightly weaker than that of TbY1(NO3)3·EtAc, that was because the triplet state energy level of the ligand Y5 wasn’t matched for the lowest excited state of Tb (III) due to the introduction of strong electron-donating group (-NH2), and it reduced energy transfer efficiency and luminescence intensity of TbY5(NO3)3·EtAc [25]. At last, the emission intensities of TbY6(NO3)3·EtAc was weaker than that of TbY1–5(NO3)3·EtAc, on the one hand, the introduction of electrophilic group (-NO2) decreased the π-conjugated systems of ligand; on the other hand, the energy level from the triplet state energy level of the ligand Y6 to the lowest excited state of Tb(III) was too small, which caused the energy back to the triplet state of the ligand. In addition, in order to compare the strength of the luminescence intensity, the complex Tb(NO3)3Phen was synthesized from phenanthroline and terbium nitrate, and measured the luminescence spectral data that are given in Table 4, the luminescence spectra of Tb (NO3)3Phen is shown in Fig. 6. Comparing Fig. 6 with Fig. 5, it can be observed that the luminescence intensity of TbY4(NO3)3·EtAc was much stronger than that of Tb(NO3)3Phen. It is generally known that Tb

3.7. Electrochemical properties The electrochemical properties of the title complexes were studied by cyclic voltammetry(CV) in DMSO solution to get their data of the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO) energy levels, and the HOMO and LUMO energy levels of the title complexes are listed in Table 7. The HOMO and LUMO energy levels of the title complexes were calculated with formulas: EHOMO =– (4.74 ev + EOX) (4.74 was for saturated calomel electrode relative to the vacuum energy, EOX was the onset potential) and ELUMO=EHOMO+Eg [26], the energy gap (Eg) was calculated as Eg=1240/λonset(eV) (λonset was the value of the maximum ultraviolet absorption peak) [27]. As the CV curves of the title complexes are similar, only the CV curves of TbY4(NO3)3·EtAc is selected for illustration, as shown in Fig. 7. As shown in Table 7 and Fig. 7, the HOMO and LUMO energy levels of the complexes TbY2,3,5(NO3)3·EtAc were stronger than that of the complexe TbY1(NO3)3·EtAc, which was attributed to the introduction of the electron-donating group (-CH3, -OCH3, -NH2). The introduction of the electron-donating group increased the ability of lossing electrons, which decreased the oxidation potentials and enlarged the HOMO and LUMO energy levels. However, the HOMO and LUMO energy level of the complexes TbY4(NO3)3·EtAc was lower than that of the complexe TbY1(NO3)3·EtAc, this was because of the p–π conjugation effect and the inductive effect of the halogen atom in ligands Y4. Moreover,the HOMO and LUMO energy levels of the complexe TbY6(NO3)3·EtAc were the lowest among the complexes TbY1–6(NO3)3·EtAc, that was because the electrophilic group(-NO2) was introduced into the target ligands Y6. Meanwhile, The band gaps Eg of complexes TbY1–6(NO3)3·EtAc was ranged from 4.715 to 4.751 eV. 4. Conclusions Six pyrazolone derivatives and their complexes with the terbium ions were synthesized and characterized. The luminescence intensities of terbium complexes are fairly strong, which proves the triplet level of ligands are in an appropriate level to the center Tb(III) ions. Meanwhile, the introduction of electron-donating group in the ligands not only increased the luminescence intensity and fluorescence quantum yield, but also enlarged the HOMO and LUMO energy levels of corresponding terbium complexes. In addition, the luminescence intensity of the complex TbY4(NO3)3·EtAc was the strongest among all title complexes, and the fluorescence quantum yield of the complex TbY4(NO3)3·EtAc even reached up to 0.647. In conclusion, the complexes TbY1–6(NO3)3·EtAc especially TbY4(NO3)3·EtAc have vast potential to be developed as luminescence materials in the future.

Scheme 1. Synthesis route of 4-acyl pyrazolone derivatives.

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Compliance with ethical standards

online version at http://dx.doi.org/10.1016/j.jlumin.2017.04.018.

Funding

References

This study was funded by the National Natural Science Foundation of China (No. J1103312, No. J1210040 and No. 21341010), the Innovative Research Team in University (No. IRT1238), China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE-No. 2011-40) as well as the Science and Technology Project of Hunan provincial Science and Technology Department(No.2016SK2064).

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Conflict of interest statement The authors declare that they have no conflict of interest. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (No. J1103312, No. J1210040 and No. 21341010), the Innovative Research Team in University (No. IRT1238), China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE-No. 2011-40) as well as the Science and Technology Project of Hunan provincial Science and Technology Department No. 2016SK2064. We also thank Dr William Hickey, the U.S. professor of HRM, for the English editing on this paper. Appendix A. Supporting information Supplementary data associated with this article can be found in the

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