Research article Received: 20 June 2014,

Revised: 10 August 2014,

Accepted: 24 September 2014

Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/bio.2804

Synthesis and luminescence properties of pyrazolone derivatives and their terbium complexes Haihua Xiao,a,b Xi Jiang,a Dong Li,a Limin Wu,a Wu Zhanga and Dongcai Guoa* ABSTRACT: Seven novel pyrazolone derivatives were synthesized and characterized by 1H NMR and 13C NMR spectra, mass spectra, infrared spectra and elemental analysis. Their terbium complexes were prepared and characterized by elemental analysis, EDTA titrimetric analysis, UV/vis spectra, infrared spectra and molar conductivity, as well as thermal analysis. The fluorescence properties and fluorescence quantum yields of the complexes were investigated at room temperature. The results indicated that pyrazolone derivatives had good energy-transfer efficiency for the terbium ion. All the terbium complexes emitted green fluorescence characteristic of terbium ions, possessed strong fluorescence intensity, and showed relatively high fluorescence quantum yields. Cyclic voltammograms of the terbium complexes were studied and the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) energy levels of these complexes were estimated. Copyright © 2014 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: pyrazolone; synthesis; terbium complex; fluorescence quantum yield; cyclic voltammetry

Introduction

Experimental

Rare earth complexes have attracted considerable attention due to their special physical and chemical properties. To date, rare earth complexes have been widely used in medicinal chemistry (1), material chemistry (2) and biochemistry (3). In recent years, the design and synthesis of rare earth functional coordination compounds have been hot topics in coordination chemistry (4,5). Some complexes bearing rare earth ions show very intense luminescence, which is characterized by a very narrow half-peak width, a long decay time and fine monochromic light (6). The luminescence intensity of rare earth complexes is dependent on the absorption efficiency of ligands in the ultraviolet region and the efficiency of energy transfer from the ligand to the rare earth ion (7). Therefore, it is very important to design and synthesize novel chelating ligands possessing high absorption efficiency. Pyrazolone and its derivatives are important heterocyclic compounds containing N and O atoms, and they have been widely applied in pharmaceutical and biological reagents, metallurgy, dyes and luminescent materials (8–10). Meanwhile, pyrazolone derivatives is a kind of excellent chelator, and they can form corresponding rare earth complexes which possess good stability, fine luminescent monochromaticity and strong fluorescence intensity (11,12). Based on the above considerations, seven novel pyrazolone derivatives were synthesized via the condensation reaction between 1-phenyl-3-methyl-4-acetyl-5-pyrazolone and p-substituted 2-phenoxyacetohydrazide, and characterized. Their terbium complexes were also prepared and characterized. The fluorescence spectral properties, fluorescence quantum yields and electrochemical properties of the terbium complexes were investigated at room temperature. The synthesis route for the pyrazolone derivatives (L1–7) is shown in Scheme 1.

Materials and methods

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All the starting materials and reagents were purchased from commercial suppliers. Terbium nitrate was prepared according to the literature (13). Melting points were determined using TECH XT-4 melting point apparatus and uncorrected. Infrared spectra (4000–400 cm
1) were recorded with KBr discs on a PerkinElmer Spectrum One spectrometer. 1H NMR and 13C NMR spectra were registered on a Bruker 400 MHz spectrometer using CDCl3 or DMSO-d6 as solvent and tetramethylsilane (TMS) as an internal standard, chemical shifts were reported as δ values in units of ppm. Mass spectra were recorded on LC-MS-Agilent 1100 series. Elemental analyses were performed on Vario EL(III) elemental analyzer. The terbium ion was determined by EDTA titration using xylenol orange as an indicator. UV/vis spectra were detected on a LabTech UV-2100 spectrophotometer using dimethylsulfoxide (DMSO) as the solvent. Fluorescence spectra were obtained on a Hitachi F-2700 spectrophotometer at room temperature, the width of the excitation and emission slits was 2.5 nm and the voltage of the photomultiplier tube (PMT) was 700 V. Molar conductivities were measured on a DDS-12A digital

* Correspondence to: D. Guo, School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China. Tel: +86-0731-88821449; Fax: +86-0731-88821449. E-mail: [email protected] a

School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China

b

Hunan Xiangjiang Kansai Paint Co. Ltd, Changsha 410000, China

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H. Xiao et al.

Scheme 1. Synthesis route for pyrazolone derivatives.

direct reading conductivity meter using N,N′-dimethylformamide (DMF) as the solvent. Thermal gravimetric (TG) and differential thermal analyses (DTA) were performed in static air atmosphere on a Shimadzu DTG-60 thermogravimetric analyzer at a heating rate of 20°C/min from 40 to 800°C. The electrochemical properties of the terbium complexes were investigated by cyclic voltammetry on a CHI 660d electrochemical workstation; sodium acetate solution (0.1 mol/L) was used as supporting electrolyte and the terbium complexes were dissolved in DMSO solution (1.0 × 10
3 mol/L); the test sweep rate was 100 mV/s and the sensitivity was 1 mA.

General procedure for the synthesis of the intermediates Synthesis of 1-phenyl-3-methyl-5-pyrazolone (1). Phenyl hydrazine (0.02 mol, 2.16 g) and ethyl acetoacetate (0.02 mol, 2.60 g) were dissolved in absolute alcohol (30 mL) in a 100-mL three-neck flask, and the reaction mixture was refluxed for 6 h with stirring. After completion of the reaction, the excess solvent was distilled, and the resultant residue was poured into ice water to obtain yellow needle-shaped crystals. These crystals were filtered, and then recrystallized from absolute ethanol to give compound 1 (14). 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, 1H, ArH), 3.43 (s, 2H, CH2), 2.20 (s, 3H, CH3).

1-Phenyl3-methyl-4-acetyl-5-pyrazolone (2) was prepared from 1-phenyl-3methyl-5- pyrazolone 1 and acetyl chloride using a modified Jensen procedure (15). In a 100-mL three-neck flask, compound 1 (0.02 mol, 3.48 g) was dissolved in 20 mL dioxane with stirring, then 3.0 g of calcium hydroxide was added with strong stirring and 2.0 mL acetyl chloride was added dropwise within 2 min. The reaction mixture became a thick paste and the temperature increased during the first few minutes. The mixture was heated to reflux for 1 h at 90°C, and then the calcium complexes were decomposed by pouring the mixture into dilute hydrochloric acid (50 mL, 2 mol/L) which caused the yellowish crystals to separate. The crystals were filtered and recrystallized from ethanol–water to form white crystals. Yield 72%. 1H NMR (CDCl3) δ/ppm: 7.83 (d, J = 8.6 Hz, 2H, ArH), 7.45 (t, J = 7.8 Hz, 2H, ArH), 7.27-7.32 (m, 1H, ArH), 2.48 (s, 3H, CH3), 2.47 (s, 3H, CH3); MS (EI) m/z (%): 218 (M + 2, 2), 217 (M + 1, 15), 216(M, 100), 201 (90), 173 (8), 135 (4), 130 (6), 106 (4), 105 (6), 92 (10), 91 (14), 77 (20). Synthesis of 1-phenyl-3-methyl-4-acetyl-5-pyrazolone (2).

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Synthesis of phenoxyacetic acid derivatives (1′a–g). The synthesis procedures for compounds 1′a–g are similar, so only the synthesis of phenoxyacetic acid 1′a is described for illustration. In a 50-mL beaker, chloroacetic acid (0.04 mol, 3.78 g) was dissolved in 12 mL of distilled water, then 10 mL of sodium hydroxide solution (25%) was added dropwise to form sodium chloroacetate solution. Meanwhile, sodium hydroxide (0.04 mol, 1.60 g), 12 mL of distilled water and 5 mL of absolute ethanol were added to a 100-mL three-neck flask with stirring, then phenol (0.04 mol, 3.76 g) was added and the mixture was stirred for 20 min. The above-prepared sodium chloroacetate solution was added to the three-neck flask using a constant pressure drop funnel. The reaction mixture was heated to reflux for 5 h, then cooled to room temperature, and acidified using dilute hydrochloric acid to form white solid. The white solid was filtered and then recrystallized from absolute ethanol to obtain white crystals, which were dried under vacuum for 24 h to obtain phenoxyacetic acid 1′a. 1 Phenoxyacetic acid (1′a). A white crystal. Yield 74%. H NMR (CDCl3) δ/ppm: 7.32 (dd, J = 8.5, 7.6 Hz, 2H, ArH), 7.04 (t, J = 7.4 Hz, 1H, ArH), 6.94 (d, J = 8.6 Hz, 2H, ArH), 4.69 (s, 2H, CH2); MS (ESI) m/z (%): 304 (2M, 12), 303 (2M – 1, 100), 151 (M – 1, 28).

A white crystal. Yield 71%. 1H NMR (CDCl3) δ/ppm: 7.11 (d, J = 8.4 Hz, 2H, ArH), 6.83 (d, J = 8.4 Hz, 2H, ArH), 4.66 (s, 2H, CH2), 2.30 (s, 3H, CH3); MS (EI) m/z (%): 167 (M + 1, 10), 166 (M, 100), 121 (62), 107 (50), 91 (64), 77 (26), 65 (17). p-Methylphenoxyacetic acid (1′b).

A white crystal. Yield 72%. 1H NMR (CDCl3) δ/ppm: 6.84-6.90 (m, 4H, ArH), 4.64 (s, 2H, CH2), 3.78 (s, 3H, OCH3); MS (EI) m/z (%): 183 (M + 1, 6), 182 (M, 64), 123 (100), 109 (18), 95 (26), 77 (10). p-Methoxyphenoxyacetic acid (1′c).

A white solid. Yield 69%. 1H NMR (CDCl3) δ/ppm: 8.24 (d, J = 9.2 Hz, 2H, ArH), 7.00 (d, J = 9.2 Hz, 2H, ArH), 4.79 (s, 2H, CH2); MS (EI) m/z (%): 198 (M + 1, 10), 197 (M, 100), 181 (4), 167 (16), 152 (85), 139 (10), 122 (22), 109 (38), 92 (28), 76 (18). p-Nitrophenoxyacetic acid (1′d).

A white crystal. Yield 71%. 1H NMR (CDCl3) δ/ppm: 7.01 (d, J = 8.0 Hz, 2H, ArH), 6.88 (d, J = 8.0 Hz, 2H, ArH), 4.66 (s, 2H, CH2); MS (EI) m/z (%): 171 (M + 1, 10), 170 (M, 100), 125 (78), 112 (42), 95 (68), 75 (17). p-Fluorophenoxyacetic acid (1′e).

A white crystal. Yield 78%. 1H NMR (CDCl3) δ/ppm: 7.28 (d, J = 8.4 Hz, 2H, ArH), 6.86 (d, J = 8.4 Hz, 2H, ArH), 4.67 (s, 2H, CH2); MS (EI) m/z (%): 188 (M + 2, 33), 186 (M, 100), 141 (78), 128 (56), 111 (58), 99 (32), 75 (34). p-Chlorophenoxyacetic acid (1′f).

A white solid. Yield 75%. 1H NMR (CDCl3) δ/ppm: 7.41 (d, J = 9.0 Hz, 2H, ArH), 6.82 (d, J = 9.0 Hz, 2H, ArH), 4.66 (s, 2H, CH2); MS (EI) m/z (%): 232 (M + 1, 98), 230 (M – 1, 100), 187 (47), 185 (48), 174 (34), 172 (36), 157 (40), 155 (38), 143 (17), 76 (20). p-Bromophenoxyacetic acid (1′g).

Synthesis of ethyl phenoxyacetate derivatives (2′a–g). In a 100-mL three-neck flask, p-substituted phenoxyacetic acid (0.02 mol) was dissolved in absolute ethanol (40 mL) with stirring, then the acetyl chloride (1.0 mL) was added dropwise. The reaction mixture was refluxed for 20 h at 80°C, and an excess of ethanol was distilled off using a rotary evaporator to obtain the ethyl phenoxyacetate derivatives 2′a–g. Compounds 2′a–g were used in the next reaction directly without purification. Synthesis of 2-phenoxyacetohydrazide derivatives (3′a–g). The synthesis procedures of compounds 3′a–g are similar, so only

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Luminescence properties of terbium complexes the synthesis of 2-phenoxyacetohydrazide 3′a is given for illustration. Ethyl phenoxyacetate (0.01 mol, 1.80 g), 5 mL of hydrazine hydrate (80%) and 30 mL of absolute ethanol were added to a 100-mL three-neck flask with stirring; the reaction mixture was then refluxed for 6 h at 85°C. After cooling to room temperature, white needle-shaped crystals were obtained, filtered and washed thoroughly with ethanol, and then dried under vacuum to give compound 3′a. 2-Phenoxyacetohydrazide (3′a). A white needle-shaped crystal. Yield 88%. 1H NMR (CDCl3) δ/ppm: 7.80 (s, 1H, NH), 7.35 (t, J = 7.9 Hz, 2H, ArH), 7.06 (t, J = 7.4 Hz, 1H, ArH), 6.93 (d, J = 8.5 Hz, 2H, ArH), 4.61 (s, 2H, CH2), 3.93 (s, 2H, NH2); MS (EI) m/z (%): 167 (M + 1, 4), 166 (M, 34), 135 (2), 134 (8), 108 (4), 107 (23), 94 (100), 77 (62), 65 (6). p-Methyl-2-phenoxyacetohydrazide (3′b). A white needle-shaped crystal. Yield 85%. 1H NMR (CDCl3) δ/ppm: 7.75 (s, 1H, NH), 7.12 (d, J = 8.4 Hz, 2H, ArH), 6.80 (d, J = 8.4 Hz, 2H, ArH), 4.55 (s, 2H, CH2), 3.92 (s, 2H, NH2), 2.30 (s, 3H, CH3); MS (EI) m/z (%): 181 (M + 1, 2), 180 (M, 14), 122 (2), 121 (12), 108 (100), 107 (17), 91 (54), 77 (10), 65 (14).

A white needle-shaped crystal. Yield 86%. 1H NMR (CDCl3) δ/ppm: 7.72 (s, 1H, NH), 6.82–6.90 (m, 4H, ArH), 4.53 (s, 2H, CH2), 3.91 (s, 2H, NH2), 3.77 (s, 3H, OCH3); MS (EI) m/z (%): 197 (M + 1, 4), 196 (M, 30), 138 (1), 137 (9), 124 (100), 109 (28), 107 (16), 92 (10), 77 (16), 64 (6). p-Methoxy-2-phenoxyacetohydrazide (3′c).

p-Nitro-2-phenoxyacetohydrazide (3′d). A pale yellow crystal. Yield 82%. 1H NMR (CDCl3) δ/ppm: 8.25 (d, J = 8.0 Hz, 2H, ArH), 7.62 (s, 1H, NH), 7.01 (d, J = 8.0 Hz, 2H, ArH), 4.67 (s, 2H, CH2), 3.95 (s, 2H, NH2); MS (EI) m/z (%): 212 (M + 1, 4), 211 (M, 22), 153 (4), 152 (24), 123 (11), 122 (22), 106 (8), 92 (12), 76 (14), 73 (100), 65 (5). p-Fluoro-2-phenoxyacetohydrazide (3′e). A white needle-shaped crystal. Yield 87%. 1H NMR (CDCl3) δ/ppm: 7.70 (s, 1H, NH), 7.02 (d, J = 8.0 Hz, 2H, ArH), 6.89 (d, J = 8.0 Hz, 2H, ArH), 4.54 (s, 2H, CH2), 3.93 (s, 2H, NH2); MS (EI) m/z (%): 185 (M + 1, 2), 184 (20), 169 (2), 126 (4), 125 (24), 112 (100), 97(28), 95 (76), 83 (22), 75 (24), 73 (28). p-Chloro-2-phenoxyacetohydrazide (3′f). A white needle-shaped crystal. Yield 90%. 1H NMR (CDCl3) δ/ppm: 7.72 (s, 1H, NH), 7.30 (d, J = 9.0 Hz, 2H, ArH), 6.87 (d, J = 9.0 Hz, 2H, ArH), 4.57 (s, 2H, CH2), 3.95 (s, 2H, NH2); MS (EI) m/z (%): 202 (M + 2, 7), 200 (M, 20), 143 (6), 141 (17), 130 (33), 128 (100), 111 (34), 99 (4), 77 (7), 65 (4). p-Bromo-2-phenoxyacetohydrazide (3′g). A white needle-shaped crystal. Yield 87%. 1H NMR (CDCl3) δ/ppm: 7.69 (s, 1H, NH), 7.42 (d, J = 9.0 Hz, 2H, ArH), 6.80 (d, J = 9.0 Hz, 2H, ArH), 4.55 (s, 2H, CH2), 3.93 (s, 2H, NH2); MS (EI) m/z (%): 246 (M + 1, 16), 244 (M – 1, 16),187 (16), 185 (18), 174 (97), 172 (100), 157 (49), 155 (46), 145 (8), 143 (8), 106 (7), 93 (11), 77 (18), 65 (24). General procedure for the synthesis of the pyrazolone derivatives 1–7 1–7 (L ). The synthesis procedures for pyrazolone derivatives L

are similar, so only the synthesis procedure of compound L1 is shown for illustration. Compound 2 (4 mmol, 0.864 g) was dissolved in 20 mL of absolute ethanol in a 100-mL three-neck flask. Meanwhile, 2-phenoxyacetohydrazide (4 mmol, 0.664 g) was dissolved in hot absolute ethanol (20 mL) in a 50 mL beaker, then added to the above solution; 1 mL of glacial acetic acid was added to the flask as a catalyst. The reaction mixture was heated to reflux for 4 h at 105°C, then cooled to room temperature to form pale yellow needle-shaped crystals. The crystals were filtered and

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washed with absolute ethanol, then dried under vacuum to give compound L1. (E)-N′-(1-(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl)ethyli1 dene)-2-phenoxyacetohydrazide (L ). A pale yellow needle-shaped

crystal. Yield 71%. m.p. 160–162°C. 1H NMR (CDCl3) δ/ppm: 12.36 (s, 1H, OH of enol-isomer), 8.84 (s, 1H, NH), 7.99 (d, J = 8.0 Hz, 2H, ArH), 7.37 (q, J = 8.1 Hz, 4H, ArH), 7.16 (t, J = 7.4 Hz, 1H, ArH), 7.11 (t, J = 7.4 Hz, 1H, ArH), 6.99 (d, J = 8.0 Hz, 2H, ArH), 4.69 (s, 2H, CH2), 2.39 (s, 3H, CH3), 2.27 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 167.67 (C=O), 166.60 (C5 of pyrazolone ring), 165.25 (C3 of pyrazolone ring), 156.84 (C1 of phenoxyl), 147.50 (C=N), 138.66 (C1 of 1-substituted aromatic ring), 129.90, 128.79, 124.70, 122.49, 119.22, 114.58 (aromatic ring carbons), 99.92 (CH2), 66.79 (C4 of pyrazolone ring), 17.16 (CH3), 14.25 (CH3); IR (KBr) ν/cm
1: 3426, 3211, 2981, 1711, 1617, 1589, 1487, 1374, 1214, 756; MS (EI) m/z (%): 366 (M + 2, 4), 365 (M + 1, 36), 364 (M, 100), 257 (46), 243 (18), 215 (26), 199 (59), 185 (12), 123 (17), 107 (14), 91 (10), 77 (46), 67 (11); Anal. calcd for C20H20N4O3: C, 65.92; H, 5.53; N, 15.38. Found: C, 65.78; H, 5.47; N, 15.44. (E)-N′-(1-(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl)ethyli2 dene)-2-(p-tolyloxy)acetohydrazide (L ). A yellow needle-shaped

crystal. Yield 72%. m.p. 181–182°C. 1H NMR (CDCl3) δ/ppm: 12.35 (s, 1H, OH of enol-isomer), 8.59 (s, 1H, NH), 7.96 (d, J = 7.6 Hz, 2H, ArH), 7.36 (t, J = 8.0 Hz, 2H, ArH), 7.13–7.16 (m, 3H, ArH), 6.85 (d, J = 8.6 Hz, 2H, ArH), 4.66 (s, 2H, CH2), 2.39 (s, 3H, CH3), 2.33 (s, 3H, CH3), 2.30 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 167.78 (C=O), 166.53 (C5 of pyrazolone ring), 165.24 (C3 of pyrazolone ring), 154.79 (C1 of phenoxyl), 147.45 (C=N), 138.67 (C1 of 1-substituted aromatic ring), 131.91, 130.31, 128.79, 124.66, 119.18, 114.42 (aromatic ring carbons), 99.95 (CH2), 67.02 (C4 of pyrazolone ring), 20.53 (CH3), 17.17 (CH3), 14.27 (CH3); IR (KBr) ν/cm
1: 3415, 3214, 2979, 1712, 1618, 1586, 1482, 1374, 1212, 752; MS (EI) m/z (%): 380 (M + 2, 4), 379 (M + 1, 26), 378 (M, 100), 257 (20), 243 (10), 199 (6), 126 (4), 72 (18), 59 (26); Anal. calcd for C21H22N4O3: C, 66.65; H, 5.86; N, 14.81. Found: C, 66.78; H, 5.77; N, 14.64. (E)-N′-(1-(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl)ethyli3 dene)-2-(p-tolyloxy)acetohydrazide (L ). A yellow crystal. Yield 70%.

m.p. 135–137°C. 1H NMR (CDCl3) δ/ppm: 12.38 (s, 1H, OH of enolisomer), 8.67 (s, 1H, NH), 7.97 (d, J = 8.0 Hz, 2H, ArH), 7.36 (d, J = 7.7 Hz, 2H, ArH), 7.14 (t, J = 7.4 Hz, 1H, ArH), 6.85–6.89 (m, 4H, ArH), 4.63 (s, 2H, CH2), 3.79 (S, 3H, OCH3), 2.38 (s, 3H, CH3), 2.28 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 168.05 (C=O), 166.71 (C5 of pyrazolone ring), 165.26 (C3 of pyrazolone ring), 154.89 (C1 of phenoxyl), 151.01 (C4 of phenoxyl), 147.55 (C=N), 138.69 (C1 of 1substituted aromatic ring), 128.80, 124.66, 119.15, 115.62, 114.85 (aromatic ring carbons), 99.75 (CH2), 67.54 (C4 of pyrazolone ring), 55.68 (OCH3), 17.14 (CH3), 14.26 (CH3); IR (KBr) ν/cm
1: 3422, 3216, 2984, 1709, 1615, 1582, 1486, 1372, 1208, 749; MS (EI) m/z (%): 396 (M + 2, 4), 395 (M + 1, 26), 394 (M, 100), 257 (20), 243 (39), 215 (20), 199 (24), 123 (21), 109 (8), 77 (12); Anal. calcd for C21H22N4O4: C, 63.95; H, 5.62; N, 14.20. Found: C, 63.78; H, 5.74; N, 14.04. (E)-N′-(1-(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl)ethyli4 dene)-2-(4-nitrophenoxy)acetohydrazide (L ). A pale yellow crystal.

Yield 68%. m.p. 241–242°C. 1H NMR (DMSO-d6) δ/ppm: 12.27 (s, 1H, OH of enol-isomer), 11.09 (S, 1H, NH), 8.26 (d, J = 9.2 Hz, 2H, ArH), 7.97 (d, J = 8.1 Hz, 2H, ArH), 7.39 (t, J = 7.9 Hz, 2H, ArH), 7.24 (d, J = 9.2 Hz, 2H, ArH), 7.13 (t, J = 7.4 Hz, 1H, ArH), 4.96 (s, 2H, CH2), 2.35 (s, 3H, CH3), 2.35 (s, 3H, CH3); 13C NMR (DMSO-

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H. Xiao et al. d6) δ/ppm: 167.01 (C=O), 166.69 (C5 of pyrazolone ring), 165.30 (C3 of pyrazolone ring), 163.21 (C1 of phenoxyl), 147.92 (C=N), 141.95 (C4 of phenoxyl), 139.45 (C1 of 1-substituted aromatic ring), 129.16, 126.28, 124.31, 118.46, 115.87 (aromatic ring carbons), 98.45 (CH2), 66.79 (C4 of pyrazolone ring), 17.30 (CH3), 14.88 (CH3); IR (KBr) ν/cm
1: 3438, 3308, 2984, 1718, 1625, 1589, 1486, 1377, 1219, 746; MS (EI) m/z (%): 411 (M + 2, 5), 410 (M + 1, 26), 409 (M, 100), 258 (10), 257 (59), 243 (12), 215 (24), 199 (55), 174 (9), 123 (16), 109 (12), 91 (12), 77 (20), 65 (6); Anal. calcd for C20H19N5O5: C, 58.68; H, 4.68; N, 17.11. Found: C, 58.49; H, 4.79; N, 17.04. (E)-2-(4-fluorophenoxy)-N′-(1-(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H5 pyrazol-4-yl)ethylidene)acetohydrazide (L ). A yellow needle-

shaped crystal. Yield 72%. m.p. 174–176°C. 1 H NMR (CDCl 3 ) δ/ppm: 12.23 (s, 1H, OH of enol-isomer), 9.00 (S, 1H, NH), 7.95 (d, J = 7.6 Hz, 2H, ArH), 7.33 (t, J = 8.0 Hz, 2H, ArH), 7.13 (t, J = 7.4 Hz, 1H, ArH), 7.02 (dd, J = 11.4, 5.7 Hz, 2H, ArH), 6.87-6.91 (m, 2H, ArH), 4.59 (s, 2H, CH 2 ), 2.34 (s, 3H, CH 3 ), 2.21 (s, 3H, CH 3 ); 13 C NMR (CDCl 3 ) δ/ppm: 167.72 (C=O), 166.79 (C5 of pyrazolone ring), 165.24 (C3 of pyrazolone ring), 156.83 (C1 of phenoxyl), 152.98 (C4 of phenoxyl), 147.59 (C=N), 138.60 (C1 of 1-substituted aromatic ring), 128.81, 124.81, 119.21, 116.10 (aromatic ring carbons), 99.70 (CH2 ), 67.35 (C4 of pyrazolone ring), 17.10 (CH 3 ), 14.26 (CH 3 ); IR (KBr) ν/cm
1 : 3412, 3215, 2984, 1709, 1613, 1586, 1484, 1375, 1208, 753; MS (EI) m/z (%): 384 (M + 2, 3), 383 (M + 1, 18), 382 (M, 76), 258 (10), 257 (64), 243 (42), 215 (45), 199 (100), 185 (18), 174 (14), 125 (31), 123 (43), 95 (62), 77 (78), 67 (30); Anal. Calcd for C 20 H 19 FN 4 O 3 : C, 62.82; H, 5.01; N, 14.65. Found: C, 62.56; H, 5.27; N, 14.51. (E)-2-(4-chlorophenoxy)-N′-(1-(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H6 pyrazol-4-yl)ethylidene)acetohydrazide (L ). A pale yellow needle-

(12), 77 (18), 67 (5); Anal. calcd for C20H19ClN4O3: C, 60.23; H, 4.80; N, 14.05. Found: C, 60.51; H, 4.67; N, 14.21. (E)-2-(4-bromophenoxy)-N’-(1-(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H7 pyrazol-4-yl)ethylidene)acetohydrazide (L ). A pale yellow solid.

Yield 74%. m.p. 206–208°C. 1H NMR (CDCl3) δ/ppm: 12.33 (s, 1H, OH of enol-isomer), 8.71 (S, 1H, NH), 7.95 (d, J = 7.8 Hz, 2H, ArH), 7.45 (d, J = 9.0 Hz, 2H, ArH), 7.35 (t, J = 7.6 Hz, 2H, ArH), 7.14 (t, J = 7.4 Hz, 1H, ArH), 6.84 (d, J = 9.0 Hz, 2H, ArH), 4.63 (s, 2H, CH2), 2.37 (s, 3H, CH3), 2.27 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 167.44 (C=O), 166.64 (C5 of pyrazolone ring), 165.19 (C3 of pyrazolone ring), 155.95 (C1 of phenoxyl), 147.56 (C=N), 138.57 (C1 of 1-substituted aromatic ring), 132.64, 128.83, 124.87, 119.27, 116.38, 114.75 (aromatic ring carbons), 99.79 (CH2), 66.93 (C4 of pyrazolone ring), 17.15 (CH3), 14.30 (CH3); IR (KBr) ν/cm
1: 3407, 3218, 2985, 1708, 1613, 1587 1486, 1379, 1207, 754; MS (EI) m/z (%): 444 (M + 1, 86), 442 (M – 1, 86), 364 (6), 258 (10), 257 (68), 243 (45), 215 (59), 199 (100), 174 (30), 123 (28), 77 (54), 65 (22); Anal. Calcd for C20H19BrN4O3: C, 54.19; H, 4.32; N, 12.64. Found: C, 54.43; H, 4.57; N, 12.37. General procedure for the synthesis of the terbium complexes. Because the synthesis procedures of the terbium

complexes are very similar, only the synthesis procedure of the complex TbL1(NO3)3·2H2O is selected for illustration. The compound L1 (0.5 mmol, 0.182 g) was dissolved in 20 mL absolute ethanol in a 100 mL three-neck flask. The pH of the mixture was adjusted to 6–7 with an aqueous solution of NaOH (1 mol/L), and then 5 mL terbium nitrate ethanol solution (0.1 mol/L) was added. The mixture was stirred for 4 h at 60°C to form white precipitated solids, which were filtered and washed several times with ethanol, and then dried under vacuum for 24 h to get the complex TbL1(NO3)3·2H2O.

Results and discussions

1

shaped crystal. Yield 74%. m.p. 179–180°C. H NMR (CDCl3) δ/ppm: 12.34 (s, 1H, OH of enol-isomer), 8.80 (S, 1H, NH), 7.97 (d, J = 7.8 Hz, 2H, ArH), 7.35 (dd, J = 18.0, 8.3 Hz, 4H, ArH), 7.16 (t, J = 7.4 Hz, 1H, ArH), 6.91 (d, J = 8.9 Hz, 2H, ArH), 4.65 (s, 2H, CH2), 2.39 (s, 3H, CH3), 2.28 (s, 3H, CH3); 13C NMR (CDCl3) δ/ppm: 167.41 (C=O), 166.62 (C5 of pyrazolone ring), 165.20 (C3 of pyrazolone ring), 155.42 (C1 of phenoxyl), 147.54 (C=N), 138.57 (C1 of 1-substituted aromatic ring), 129.72, 128.82, 127.45, 124.86, 119.26, 115.90 (aromatic ring carbons), 99.83 (CH2), 67.02 (C4 of pyrazolone ring), 17.14 (CH3), 14.29 (CH3); IR (KBr) ν/cm
1: 3409, 3218, 2984, 1706, 1611, 1594, 1486, 1376, 1204, 748; MS (EI) m/z (%): 400 (M + 2, 35), 398 (M, 100), 258 (8), 257 (50), 243 (22), 215 (26), 199 (56), 185 (10), 174 (8), 123

Composition and properties of the terbium complexes The results of elemental analysis of the terbium complexes are expressed in Table 1, which are in accordance with the theoretical values calculated, and indicated that the composition of the seven novel terbium complexes conformed to TbL1–7(NO3) 1–7 were soluble in DMF, DMSO, THF, chloro3·2H2O. Ligands L form and acetone as well as dichloromethane, methanol, ethanol and ethyl acetate, except that ligand L4 was only soluble in DMF and DMSO; but they hardly dissolve in cyclohexane, petroleum ether and water. All the terbium complexes were soluble in DMF and DMSO, but only slightly soluble in THF, chloroform and acetone. The molar conductivity data of the complexes

Table 1. Elemental analytical and molar conductivity data of the terbium complexes Complex C TbL1(NO3)3·2H2O TbL2(NO3)3·2H2O TbL3(NO3)3·2H2O TbL4(NO3)3·2H2O TbL5(NO3)3·2H2O TbL6(NO3)3·2H2O TbL7(NO3)3·2H2O

Λm (S/m2/mol)

Found (calcd) (%)

32.01 33.12 32.37 30.24 31.31 30.67 29.01

(32.23) (33.21) (32.53) (30.39) (31.47) (30.80) (29.14)

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H 3.14 3.31 3.26 2.74 2.87 2.75 2.75

(3.25) (3.45) (3.38) (2.93) (3.04) (2.97) (2.81)

N 13.06 (13.15) 12.77 (12.91) 12.57 (12.64) 14.04 (14.18) 12.61 (12.84) 12.39 (12.57) 11.69 (11.90)

Copyright © 2014 John Wiley & Sons, Ltd.

Tb 21.54 21.12 20.74 20.43 21.05 20.57 19.62

(21.32) (20.93) (20.50) (20.11) (20.82) (20.38) (19.28)

11 12 10 7 15 18 17

Luminescence 2014

Luminescence properties of terbium complexes

UV/vis spectral studies The UV/vis spectra data of the ligands L1–7 and their terbium complexes were determined in the DMSO solution (10
4 mol/L) as well as the molar absorptivities (ε) (17) are listed in Table 2. Because the UV/vis spectra of all the complexes TbL1–7(NO3)3·2H2O are similar, only the UV/vis spectra of TbL6(NO3)3·2H2O and its corresponding ligand L6 are selected for illustration, as shown in Fig. 1. There were two absorption peaks for ligand L6, one appeared at around 277 nm and another 335 nm, which were assigned to the π–π* and n–π* transitions, respectively, however, these two absorption peaks of complex TbL6(NO3)3·2H2O appeared at 282 and 338 nm, respectively. This was due to the introduction of terbium ion that enlarged the conjugation system of ligand L6. The above results indicated that ligand L6 was coordinated to the terbium ion (18). IR spectral studies The characteristic infrared absorption bands of the pyrazolone derivatives L1–7 and their corresponding terbium complexes are listed in Table 3. Because the IR spectra of all the complexes are similar, only the IR spectra of TbL6(NO3)3·2H2O and its corresponding ligand L6 are selected for illustration, as shown in Fig. 2. For ligand L6, the medium intensity band at 3409 cm
1 can be attributed to the v(O–H) of the pyrazolone ring, this was in agreement with the fact that there was a weak broad peak at 12.34 ppm in the 1H NMR spectrum. These results indicated that ligand L6 existed in the form of an enol-isomer. The weak band at 3218 cm
1 corresponds to the v(N–H) stretching vibration (19), the strong bands at 1706 and 1611 cm
1 were allocated to the v(C=O) and v(C=N) group of the hydrazide, respectively, and the band at 1594 cm
1 was assigned to v(C=N) of the pyrazolone ring. For the infrared spectrum of complex TbL6(NO3)3·2H2O, a new band appeared at ~3374 cm
1, which was due to the presence of crystal water molecules. Meanwhile, the enol-isomer v(OH) of the pyrazolone ring disappeared, implying that ligand L6

was chelated with the Tb(III) ion in an enol form. The absorption bands of C=O and C=N groups in the hydrazide were shifted to the lower wavenumber, which demonstrated that ligand L6 was coordinated to the Tb(III) ion by the oxygen atom and nitrogen atom of the hydrazide group. The wavenumber of cyclic C=N did not change in the complex, which indicated that ligand L6 was not coordinated to the Tb(III) ion via cyclic C=N. In addition, the characteristic frequencies of the coordinating nitrate groups appeared at around 1478 (v1), 1329 (v4), 1026 (v2) and 819 cm
1 (v3) (20). The difference between the two strongest absorption bands of the nitrate groups (|v1 – v4|) was ~149 cm
1, which indicated that the coordinated nitrate groups in the complex were bidentate (21). Furthermore, there were two weak bands at 457 and 419 cm
1, which could be assigned to v(Tb-O) and v (Tb-N), respectively.

Fluorescence spectral studies The fluorescence spectra of the terbium complexes were measured with 2.5 nm slit widths for excitation and emission in the solid state at room temperature, and the fluorescence spectral data of the complexes are given in Table 4.

1.2 1.0

b

0.8

Abs

in DMF (10
3 mol/L) indicated that all complexes acted as nonelectrolytes (16).

a

0.6 0.4 0.2 0.0 250

300

350

400

450

wavelength/nm 6

6

Figure 1. UV/vis spectra of complex TbL (NO3)3·2H2O (a) and ligand L (b).

Table 2. UV/vis data of the complexes and ligands L1–7 in DMSO solution (10
4 mol/L) Compound L1 TbL1(NO3)3·2H2O L2 TbL2(NO3)3·2H2O L3 TbL3(NO3)3·2H2O L4 TbL4(NO3)3·2H2O L5 TbL5(NO3)3·2H2O L6 TbL6(NO3)3·2H2O L7 TbL7(NO3)3·2H2O

Luminescence 2014

λ1 (nm)

ε1 (1.0 × 104 L/mol/cm)

λ2 (nm)

ε2 (1.0 × 104 L/mol/cm)

276 279 277 280 284 288 280 282 277 282 277 282 278 281

1.03 1.15 1.03 1.14 1.01 1.16 1.01 1.12 1.06 1.20 1.09 1.22 1.07 1.19

332 336 333 337 334 337 330 337 335 338 335 338 330 336

0.71 0.77 0.73 0.79 0.67 0.70 0.60 0.67 0.70 0.74 0.84 0.89 0.69 0.81

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H. Xiao et al. Table 3. IR spectral data of the ligands L1–7 and their terbium complexes (cm
1) Compound

1

L TbL1(NO3)3·2H2O L2 TbL2(NO3)3·2H2O L3 TbL3(NO3)3·2H2O L4 TbL4(NO3)3·2H2O L5 TbL5(NO3)3·2H2O L6 TbL6(NO3)3·2H2O L7 TbL7(NO3)3·2H2O

v(O–H)

v(N–H)

3426 3373 3415 3372 3422 3377 3438 3375 3412 3368 3409 3374 3407 3373

Hydrazide

3211 3210 3214 3214 3216 3216 3308 3309 3215 3216 3218 3217 3218 3218

v(NO3
)

v(Ar–O–C)

v(C=O)

v(C=N)

1711 1668 1712 1665 1709 1665 1718 1675 1709 1662 1706 1657 1708 1661

1617 1598 1618 1599 1615 1601 1625 1607 1613 1601 1611 1597 1613 1598

1214 1214 1212 1211 1208 1208 1219 1219 1208 1207 1204 1204 1207 1206

v(Tb–O)

v(Tb–N)

v1

v4

v2

v3

1481

1325

1024

822

465

429

1480

1324

1026

822

463

427

1480

1329

1022

821

460

426

1482

1321

1018

824

469

432

1479

1326

1025

819

460

424

1478

1329

1026

819

457

419

1480

1328

1027

820

458

422

360

380

4000

3000

T/%

I/a.u.

a

2000

1000

b

0 300 4000

3500

3000

2500

2000

1500

1000

320

340

400

420

440

wavelength/nm

500

wavenumber/cm-1 6

6

Figure 3. Excitation spectrum of complex TbL (NO3)3·2H2O.

6

Figure 2. IR spectra of complex TbL (NO3)3·2H2O (a) and ligand L (b).

It can be observed from the excitation spectrum that the maximum excitation wavelength was at 379 nm. The emission spectrum of the complex TbL6(NO3)3·2H2O displayed four main peaks at approximately 491 nm (5D4→7F6), 545 nm (5D4→7F5), 586 nm (5D4→7F4) and 625 nm (5D4→7F3), which was typical for terbium derivative, and we can see that the strongest emission peak at

Because the fluorescence spectra of the complexes are very similar, only the fluorescence spectrum of the complex TbL6(NO3)3·2H2O is selected for illustration. The excitation and emission spectra of TbL6(NO3)3·2H2O are shown in Figs. 3 and 4, respectively.

Table 4. Fluorescence spectral data of the complexes TbL1
7(NO3)3·2H2O Complex

TbL1(NO3)3·2H2O TbL2(NO3)3·2H2O TbL3(NO3)3·2H2O TbL4(NO3)3·2H2O TbL5(NO3)3·2H2O TbL6(NO3)3·2H2O TbL7(NO3)3·2H2O

λex (nm)

377 376 389 381 383 380 378

5

D4→7F6

5

D4→7F5

D4→7F4

5

5

D4→7F3

λem (nm)

I (a.u.)

λem (nm)

I (a.u.)

λem (nm)

I (a.u.)

λem (nm)

I (a.u.)

491 491 491 491 491 491 491

1376 1949 2436 614 2842 3611 3242

546 545 546 545 546 545 545

3456 4831 5780 1551 7445 9091 8334

586 586 584 585 585 586 586

278 296 383 102 441 543 507

624 626 626 625 625 625 626

139 91 138 33 144 167 154

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Luminescence 2014

Luminescence properties of terbium complexes 10000 9000 8000 7000

I/a.u.

6000 5000 4000 3000 2000 1000 0 -1000 460

480

500

520

540

560

580

600

620

640

wavelength/nm 6

Figure 4. Emission spectrum of complex TbL (NO3)3·2H2O.

545 nm was responsible for the green fluorescence of the Tb(III) complex (22). Meanwhile, the width of the half peaks was approximately several nanometers, which indicated that the complex TbL6(NO3)3·2H2O had high color purity, and ligand L6 was a comparatively good organic chelator (23,24). The energy-transfer efficiency from the ligand to the center ion is one of the key factors that influence the characteristic fluorescence properties of the rare earth ions. From Table 4, it can be observed that all the complexes emit fluorescence characteristic of the Tb(III) ion, which indicates that ligands L1–7 are all comparatively good organic chelators and have an excellent ‘antenna’ effect (25). The relative fluorescence intensity of rare earth complexes is related to the efficiency of the intramolecular energy transfer between the triple level of the ligand and the emitting level of the Tb(III) ion, which depends on the energy gap between the two levels. The energy difference between the triplet state energy level of the ligand and the lowest excited state of Tb(III) can not be too large or too small. If the energy difference is too large, the energy-transfer efficiency will decrease due to diminution of the overlap between the energy donor (ligands) and acceptor (rare earth ions). By contrast, if the energy difference is too small, the energy can back-transfer from the rare earth ions to the triplet state energy of the ligands (26). It can be seen in Table 4 that the fluorescence intensity of TbL6(NO3)3·2H2O was stronger than that of the other six complexes, which indicated that the triplet level of ligand L6 was in an appropriate level to center the Tb(III) ion and the energy transition from ligand L6 to the Tb(III) ion was easier than with the other ligands (18). The reason for this was that the p–π conjugation effect between the lone pair electrons of the Cl atom and the π-electron of the phenyl ring was stronger than the inductive effect of the Cl atom in the phenyl ring, therefore the electron density of ligand L6 was increased and the energy transition from ligand L6 to the Tb(III) ion was more efficient. Meanwhile, the fluorescence intensities of complexes TbL2,3(NO3)3·2H2O were stronger than that of TbL1(NO3)3·2H2O, because ligand L2 and ligand L3 had an donative electron group (
CH3 and OCH3, respectively) which enlarged the π-conjugated systems of ligand L2 and ligand L3. In addition, it can be seen that the fluorescence intensity of complex TbL3(NO3)3·2H2O was better than that of TbL2(NO3)3·2H2O, which may be because the p–π conjugation effect between the lone pair electrons of the O atom and the π-electron of the phenyl ring was stronger than

Luminescence 2014

the σ–π hyperconjugation effect between the σ-electron of the methylic C–H bond and the π-electron of the phenyl ring. Otherwise, the fluorescence intensity of complex TbL4(NO3)3·2H2O was worse than that of TbL1(NO3)3·2H2O, because ligand L4 had an electrophilic group (
NO2) that caused the electron density of the phenyl ring to decrease; however, the introduction of the accepting electron group easily resulted in fluorescence quenching. The above results indicated that the fluorescence intensity of the terbium complex substituted by the donative electron group of the corresponding ligand was increased, whereas the fluorescence intensity of the terbium complex substituted by the electrophilic group of the corresponding ligand was decreased. Moreover, with regard to the fluorescence intensities of TbL5–7(NO3)3·2H2O substituted by the halogen atom of the corresponding ligand L5–7, the fluorescence intensity of TbL6(NO3)3·2H2O was better than that of TbL5(NO3)3·2H2O. This was because the p–π conjugation effect between the lone pair electrons of the chlorine atom and the π-electron of the phenyl ring was stronger than the p–π conjugation effect between the lone pair electrons of the fluorine atom and the π-electron of the phenyl ring, and the triplet state energy of ligand L6 matched most closely with the excited state energy of the Tb (III) ion. However, the fluorescence intensity of complex TbL7(NO3)3·2H2O was worse than that of TbL6(NO3)3·2H2O, which was due to the heavy atom effect of the bromine atom in ligand L7 (27,28). Fluorescence quantum yield studies The fluorescence quantum yield of the unknown terbium complexes (Фfx) can be calculated according to the following formula (29,30): Φfx ¼

n2x Fx Astd   Φfstd n2std Fstd Ax

The quinine sulfate (1.0 μg/mL) of a sulfuric acid solution (0.1 mol/L) was used as a standard reference. Where Фfstd was the fluorescence quantum yield of the standard and Фfstd = 0.55. The fluorescence spectral data of complexes TbL1–7(NO3)3·2H2O were measured at room temperature in DMSO solution with excitation and emission slit widths of 5.0 nm. The fluorescence quantum yields of complexes TbL1–7(NO3)3·2H2O are listed in Table 5. As shown in Table 5, the fluorescence quantum yields of complexes TbL2(NO3)3·2H2O and TbL3(NO3)3·2H2O were both greater than that of TbL1(NO3)3·2H2O because ligands L2 and L3 had an Table 5. The fluorescence quantum yields of the complexes TbL1
7(NO3)3·2H2O Complex TbL1(NO3)3·2H2O TbL2(NO3)3·2H2O TbL3(NO3)3·2H2O TbL4(NO3)3·2H2O TbL5(NO3)3·2H2O TbL6(NO3)3·2H2O TbL7(NO3)3·2H2O

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λex (nm)

I (a.u.)

Фfx

313 311 315 308 315 316 314

823 896 1015 524 1104 1357 1232

0.372 0.393 0.405 0.276 0.414 0.482 0.457

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H. Xiao et al. Table 6. The HOMO and LUMO energy levels of the complexes TbL1
7(NO3)3·2H2O Complex

λonset (nm)

EOX (V)

EHOMO (eV)

Eg (eV)

ELUMO (eV)

1

257 256 257 257 258 258 257

0.677 0.658 0.651 0.692 0.696 0.688 0.691


5.417
5.398
5.391
5.432
5.436
5.428
5.431

4.825 4.844 4.825 4.825 4.806 4.806 4.825


0.592
0.554
0.566
0.607
0.630
0.622
0.606

TbL (NO3)3·2H2O TbL2(NO3)3·2H2O TbL3(NO3)3·2H2O TbL4(NO3)3·2H2O TbL5(NO3)3·2H2O TbL6(NO3)3·2H2O TbL7(NO3)3·2H2O

8

Current/uA

4

0

-4

-8 -2.4

-1.6

-0.8

0.0

0.8

1.6

Potential/V 6

Figure 5. Cyclic voltammogram of complex TbL (NO3)3·2H2O.

donating electron group (
CH3 and -OCH3, respectively) that enlarged the π-conjugated system of ligands L2,3. However, the fluorescence quantum yield of complex TbL4(NO3)3·2H2O was lower than that of TbL1(NO3)3·2H2O because ligand L4 had an electrophilic group (
NO2) that caused the electron density on the phenyl ring to decrease and easily resulted in fluorescence quenching. Meanwhile, the fluorescence quantum yield of complex TbL6(NO3)3·2H2O was the greatest among complexes TbL5–7(NO3)3·2H2O. This was because the p–π conjugation effect of ligand L6 was stronger than that of L5, and also because of the heavy atom effect of the bromine atom in ligand L7. Meanwhile, the triplet state energy level of ligand L6 best matched the excited state energy level of the Tb(III) ion, so the fluorescence quantum yield of complex TbL6(NO3)3·2H2O reached 0.482. From the above discussions, we concluded that all the complexes TbL1–7(NO3)3·2H2O possessed relatively good fluorescence quantum yields.

The HOMO and LUMO energy levels of title terbium complexes were obtained according to the equations EHOMO =
(4.74 + eEOX), where EOX is the starting value of the oxidation potential peak (32), and ELUMO = EHOMO + Eg. The energy gap (Eg) was calculated as Eg = 1240/λonset (eV) (33), and λonset was the value of the largest UV/vis absorption peak. Table 6 shows that the HOMO energy levels of the complexes TbL2,3(NO3)3·2H2O were both higher than that of complex TbL1(NO3)3·2H2O, however, the HOMO energy level of complex TbL4(NO3)3·2H2O was less than that of complex TbL1(NO3)3·2H2O. This may be because in terbium complexes with different substituted groups in the relevant ligands, the introduction of a donative electron group in the ligand can increase the HOMO energy levels of the corresponding complexes, whereas the introduction of an electrophilic group in the ligand can decrease the HOMO energy level of the corresponding complexes. Moreover, it can be seen that the HOMO energy levels of complexes TbL5–7(NO3)3·2H2O were both less than that of complex TbL1(NO3)3·2H2O, this was because of the p–π conjugation effect and the inductive effect of the halogen atom in ligands L5–7. Meanwhile, the value of Eg for complexes TbL1–7(NO3)3·2H2O was between 4.806 and 4.844 eV. Also, we can see that the change rule about the LUMO energy levels was in keeping with the that for the HOMO energy levels of complexes TbL1–7(NO3)3·2H2O.

Conclusions The pyrazolone derivatives L1–7 existed in the form of an enolisomer in the solid state, and their corresponding terbium complexes were also prepared and characterized. All the terbium complexes had relatively high thermal stability. The fluorescence intensity of the complex TbL6(NO3)3·2H2O was the strongest among all terbium complexes. Moreover, the fluorescence quantum yield of complex TbL6(NO3)3·2H2O reached 0.482. In summary, complex TbL6(NO3)3·2H2O could be used as promising candidate luminescent materials.

Electrochemical properties studies The electrochemical properties of the terbium complexes were studied using cyclic voltammetry (CV) in SMSO solution (31). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the complexes were estimated and the corresponding data are listed in Table 6. Because the CVs of all the complexes TbL1–7(NO3) 6 3·2H2O are similar, only the CV of TbL (NO3)3·2H2O is selected for illustration, as shown in Fig. 5.

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Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (No.J1103312, No.J1210040 and No.21341010), and the Innovative Research Team in University (No.IRT1238) as well as the Science and Technology Project of Hunan provincial Science and Technology Department (No.2012GK3156). We also thank Dr William Hickey, the U.S. professor of HRM, for the English editing on this paper.

Copyright © 2014 John Wiley & Sons, Ltd.

Luminescence 2014

Luminescence properties of terbium complexes

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