Synthesis and luminescence properties of salicylaldehyde isonicotinoyl hydrazone derivatives and their europium complexes

Article abstract of DOI:10.1016/j.jinorgbio.2015.04.007

Four novel salicylaldehyde isonicotinoyl hydrazone derivatives and their corresponding europium ion complexes were synthesized and characterized, while the luminescence properties and the fluorescence quantum yields of the target complexes were investigated. The results indicated that the ligands favored energy transfers to the emitting energy level of europium ion, and four target europium complexes showed the characteristic luminescence of central europium ion. Besides the luminescence intensity of the complex with methoxy group, which possessed the highest fluorescence quantum yield (0.522), was stronger than that of other complexes. Furthermore, the electrochemical properties of the target complexes were further investigated by cyclic voltammetry, the results indicated that the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels and the oxidation potential of the complexes with electron donating group increased, however, that of the complexes with accepting electron group decreased.

Full text of DOI:10.1016/j.jinorgbio.2015.04.007

Synthesis and luminescence properties of salicylaldehyde isonicotinoyl
hydrazone derivatives and their europium complexes
Wenfei Shan, Fen Liu, Jiang Liu, Yanwen Chen, Zehui Yang, Dongcai
Guo
PII:
DOI:
Reference:
S0162-0134(15)00101-4
doi: 10.1016/j.jinorgbio.2015.04.007
JIB 9707
To appear in:
Journal of Inorganic Biochemistry
Received date:
Revised date:
Accepted date:
8 January 2015
9 April 2015
9 April 2015
Please cite this article as: Wenfei Shan, Fen Liu, Jiang Liu, Yanwen Chen, Zehui Yang,
Dongcai Guo, Synthesis and luminescence properties of salicylaldehyde isonicotinoyl
hydrazone derivatives and their europium complexes, Journal of Inorganic Biochemistry
(2015), doi: 10.1016/j.jinorgbio.2015.04.007
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ACCEPTED MANUSCRIPT
Synthesis and luminescence properties of salicylaldehyde
isonicotinoyl hydrazone derivatives and their europium complexes
Wenfei Shan a, Fen Liu a, Jiang Liu a, Yanwen Chen b, Zehui Yang c, Dongcai Guo a, 
aSchool of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
bHunan Labour Protection Institute of Nonferrous Metals. Changsha 410014, China
cSchool of Chemical Engineering, Ningbo University of Technology, Ningbo 315016, China
 Corresponding author. Tel./fax: +86 0731 88821449
E-mail address: dcguo2001@hnu.edu.cn (D. Guo)
Abstract: Four novel salicylaldehyde isonicotinoyl hydrazone derivatives and their
corresponding europium ion complexes were synthesized and characterized. Whilst
the luminescence properties and the fluorescence quantum yields of the target
complexes were investigated. The results indicated that the ligands favored energy
transfers to the emitting energy level of europium ion, and four target europium
complexes showed the characteristic luminescence of central europium ion. Besides
the luminescence intensity of the complex with methoxy group, which possessed the
highest fluorescence quantum yield (0.522), was stronger than that of other complexes.
Furthermore, the electrochemical properties of the target complexes were further
investigated by cyclic voltammetry, the results indicated that the highest occupied
molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy
levels and the oxidation potential of the complexes with electron donating group
increased, however, that of the complexes with accepting electron group reduced.
Keywords: Acylhydrazone; Complex; Synthesis; Luminescence properties;
Electrochemical properties
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1. Introduction
Schiff base compounds, the aldehyde or ketone analogs in which the carbonyl
group is replaced by an imine or azomethine group, are considered privileged ligands,
because of their simple preparation in the condensation of amines with carbonyl
compounds .Therefore, Schiff base ligands have gained paramount importance due to
their versatility, such as catalytic property, biological activity, antibacterial, dyes, etc.
[1-10].
The Eu3+ ion possesses several unique luminescence properties. However, it has a
very small absorption coefficient. This shortfall is overcome by introducing an
organic ligand to sensitize Eu3+ ion. The sensitizeng ligand transfers energy to the
Eu3+ ion effectively, which can enhance luminescence of Eu3+ ion [11]. Therefore, it is
significant to design and synthesis the ligand, whose triplet energy level matches
better to the emitting level of the rare earth ions. This results are attributed to their
excellent coordination nature to the Eu3+ ion.
Schiff bases ligands and their Eu3+ ion complexes have a variety of applications
in biological, clinical, analytical and industrial fields [12]. Among these, heterocyclic
Schiff base ligands and their Eu3+ ion complexes do have drawed significant interest.
To our konwledge, systsalicylaldehyde−acylhydrazones act as tetradentate ligand
were reported only in the discrete trinuclear metal complexes of special
compartmental ligands[13]. However, systsalicylaldehyde−acylhydrazones have been
seldom reported in literature with their corresponding rare earth ions complexes due
to their poor solubility, not to mention the luminescence properties of their rare earth
complexes. On the other hand, the salicylaldehyde isonicotinoyl hydrazone (SIH)
compounds possess good conjugated plane, rigid stucture and various coordination
sites, and good biological activity[14-18], are theoretically suitably used as the
organic ligand of the rare earth luminescent complexes. Aiming to enrich in the other
application filed, SIH has been focused notably on a salicylaldehyde-derived chelator,
which has been recently shown to possess very strong and concentration-dependent
properties on the optics, sensors, medicine [19-22].
As described herein, we used isonicotinic acid, salicylaldehyde and aniline
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derivatives as raw materials to synthesize four SIH derivatives with acetanilide as the
end group. Their corresponding complexes with europium ions had also been
prepared and characterized. Meanwhile, the luminescence and electrochemical
properties of the title complexes were investigated, and the relationship between the
structure of the ligand and the properties of complexes was also explored in detail.
The synthetic route to salicylaldehyde isonicotinic acid hydrazon (La–d) derivatives
schiff bases as outlined was shown in Scheme 1.
2. Experimental
2.1 Materials and methods
The purity of Eu2O3 was 99.99%. Europium nitrate was prepared according to the
literature [23]. Isonicotinic acid and aniline derivatives were of chemical pure (CP)
grade, dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) and other
reagents were of AR grade and used without further purification.
1H NMR spectra were registered in DMSO−d6/CDCl3 at Brucker
spectrophotometer (400 MHz) with tetramethylsilane (TMS) as an internal standard.
Mass spectra were evaluated with the MAT95XP Mass Spectrometer. Infrared
radiation (IR) spectra (400 cm–1–4000 cm–1) were achieved in KBr discs by a
PERKIN-ELMER Spectrum One. UV-visible (UV-vis) spectra (190–50 nm) were
recorded by LabTech UV-2100 spectrophotometer, with DMSO as solvent and
reference. Elemental analysis of the complexes was carried out on a Flash EA1112
elemental analyzer. Melting points of all compounds were established on a XT-4
binocular microscope apparatus and were uncorrected. The molar conductivity was
recorded with DDS-12A digital display conductivity instrument. Thermal gravimetric
analysis was carried out on a NETZSCH STA 409PC thermal gravimetric analyzer,
with the empty crucible for comparison in air atmosphere, and the rate of temperature
increase was 10/min. Cyclic voltammetry curve testing using three electrodes were
glassy electrode, a platinum electrode and a saturated calomel electrode, ferrocene as
external standard, nitrite solution was used as the supporting electrolyte and DMSO as
the solvent, the test scanning speed was 100 mV·s–1 and the sensitivity was 1 mA.
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The fluorescence spectra was measured by using powder samples on a Hitachi F-2700
fluorescence spectrophotometer at room temperature, the scanning speed was 1200
nm/min, and the voltage was set to 700 V. The europium ion was established by
EDTA titration using xylenol orange as an indicator.
2.2 General procedure for the synthesis of intermediates
2.2.1 General procedure for synthesis of N'−(2−hydroxybenzylidene)
isonicotinohydrazide (2)
To the solution of isonicotinic acid (50 mmol, 6.15 g) in methanol (50 mL), acetyl
chloride (5 mL) was gradually added and the reaction mixture was stirred in ice water
bath. Then the reaction mixture was heated to the required temperature and refluxed
for 24 h with stirring in the oil bath. Next, the mixture was suction filtrated, and
filtrate was distilled under reduced pressure to obtain methyl isonicotinate.
Subsequently, the obtained product was added into a 150 mL three-neck flask,
absolute alcohol (60 mL) was gradually added dropwise into it, and the mixture was
heated to 85 ºC. To this solution, the hydrazine hydrate (10 mL 80%) was added and
refluxed for 6 h, and then the excess solvent was removed under reduced pressure, the
residue was recrystallized from absolute alcohol to get white needle
isonicotinoylhydrazide crystal (compound 1).
To the solution of compound 1 (0.04 mol 5.88 g) in absolute alcohol (60 mL 95%),
salicylaldehyde (0.04 mol 4.88 g) was added in a 150 mL three-neck flask, the
mixture solution was refluxed for 8 h with stiring, the resulting mixture was separated
by suction filtration, the residue was recrystallized with absolute alcohol three times
to get yellow crystal and dried under vacuum for 24
h
to get
N'−(2−hydroxybenzylidene) isonicotinohydrazide (compound 2).
N'−(2−hydroxybenzylidene) isonicotinohydrazide (7.72 g) Yield: 80%. 1H NMR
(400 MHz, DMSO-d6) δ (ppm): 12.31 (s, 1H), 11.08 (s, 1H), 8.81 (dd, J = 4.4, 1.6 Hz,
2H), 8.69 (s, 1H), 7.85 (dd, J = 4.4, 1.6 Hz, 2H), 7.62 (dd, J = 7.7, 1.5 Hz, 1H),
7.44–7.14 (m, 1H), 7.07–6.89 (m, 2H). MS (EI) [MS = mass spectrometry; EI =
electron ioniozation] m/z (%): 241 (M, 27), 123 (100), 106 (97), 91 (10),78 (74), 51
(55).
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2.2.2 Synthesis of 2-chloro-N-phenylacetamide derivatives (3a–3d)
The synthesis procedures of compound 3b−3d were similar to that of compound 3a,
thus we take the synthesis method of compound 3a for example. Aniline (0.066 mol,
6.15 g) was dissolved into acetic acid (40–50 mL) in a 100 mL single-neck flask, then
the chloroacetyl chloride (0.074 mol, 8.36 g) was added dropwise with stirring for 30
min in the ice bath, the resulting mixture was reacted for another one hour at room
temperture. Subsequently, the mixture was poured into saturated sodium acetate (150
mL) solution to form milky white precipitate. Finally, this precipitate was suction
filtrated, washed three times with distilled water and recrystallized from the mixed
solution of absolute alcohol and water (1/2, v/v). The product was dried under
vacuum for 24 h to obtained compound 3a.
1
2-chloro-N-phenylacetamide (3a) A white needle solid. yield: 68%. H NMR
(400 MHz, DMSO-d6) δ/ppm: 10.29 (s, 1H, NH), 7.77–7.47 (m, 2H, ArH), 7.40–7.22
(m, 2H, ArH), 7.18–6.95 (m, 1H, ArH), 4.25 (s, 2H, CH2); MS (EI) m/z (%): 172
(M+3, 3), 171 (M+2, 25), 169 (M, 80), 121 (3), 120 (40), 106 (7), 94 (10), 93 (100),
77 (22), 65 (28).
1
2-chloro-N-(4-chlorophenyl)acetamide (3b) A white needle solid. yield: 94%. H
NMR (400 MHz, DMSO-d6) δ/ppm: 7.67–7.57 (m, 2H, ArH), 7.46–7.34 (m, 2H,
ArH), 4.26 (s, 2H, CH2); MS (EI) m/z (%): 207 (M+3, 5),205 (M+1, 33), 203 (M−1,
52), 156 (4), 154 (13), 129 (32), 127 (100), 126 (15), 111 (5), 99 (14), 77 (5), 63 (7).
1
2-chloro-N-p-tolyacetamide (3c) A white flake solid yield: 78%. H NMR (400
MHz, DMSO-d6) δ/ppm: 10.21 (s, 1H, NH), 7.42–7.46 (m, 2H, ArH), 7.16–7.18 (m,
2H, ArH), 4.22 (s, 2H, CH2), 2.21 (s, 3H, CH3); MS (EI) m/z (%): 186 (M+3, 3), 185
(M+2, 25), 183 (M, 75), 148 (4), 134 (27), 107 (100), 106 (76), 91 (16),77 (26), 51
(10).
2-chloro-N-(4-methoxyphenyl)acetamide (3d) A faint purple solid. yield: 53%.
1H NMR (400 MHz, DMSO-d6) δ/ppm: 10.18 (s, 1H, NH), 7.54–7.48 (m, 2H, ArH),
6.95–6.87 (m, 2H, ArH), 4.22 (s, 2H, CH2), 3.73 (s, 3H, CH3); MS (EI) m/z (%): 202
(M+3, 3), 201 (M+2,32), 199 (M, 100), 124 (29), 123 (72), 108 (72), 95 (13), 80 (6).
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2.2.3 Synthesis of salicylaldehyde isonicotinoyl hydrazone derivatives (La–Ld)
As the synthesis methods of compounds La–d were similar with each others, only
the synthesis procedures of compound La was described as example. To the solution
of compound 2 (3 mmol, 0.72 g) in 30 mL DMF, potassium carbonate (7.5 mmol,
1.04 g) and potassium iodide (3.3 mmol, 0.54 g) were added into a 150 mL three-neck
flask, the reaction solution was heated to 80 ºC and stirred for 1.5 h. Subsequently,
cmpound 3a (3.3 mmol, 0.60 g) dissolved in 30 mL DMF was slowly added into the
flask. The resulting solution was stirred for 20 h at 80 ºC, and then cooled down at
room temperature after the solvent DMF was removed under vacuum. The residue
was gradually poured into 300 mL distilled water and stirred for 30 min. A fine brown
precipitate was formed, filtrated, washed with water, and recrystalled from the mixed
solution of absolute alcohol and chloroform (1/3, v/v) to obtain the compound La
(E)-N'-(2-(2-oxo-2-(phenylamino)ethoxy)benzylidene)isonicotinohydrazide (La) A
brown powder. Yield: 74%. m.p. 196 ºC; 1H NMR (400 MHz, CDCl3) δ/ppm: 11.07 (s,
1H, N=CH), 9.07 (d, J = 7.6 Hz, 1H, pyridine protons), 8.78 (d, J = 6.0 Hz, 1H,
pyridine protons), 8.01 (s, 1H, NH), 7.71 (d, J = 7.7 Hz, 2H, pyridine protons),
7.69–7.32 (m, 6H, ArH), 7.29 (d, J = 8.2 Hz, 2H, ArH), 7.02(s, 1H, NH), 6.96 (d, J =
8.6 Hz, 1H, ArH), 4.66 (s, 2H, CH2); IR (KBr) v/cm–1: 3615, 3430, 3048, 2922, 2860,
1644, 1619, 1547,1492,1456, 1242; MS (EI) m/z (%): 392 (M+18, 2), 374 (M, 6), 328
(M–46, 8), 296 (25), 252 (100), 238 (12), 195 (25), 166 (77), 133 (52), 119 (31), 106
(87), 93 (74), 77 (93), 65 (51); Anal. Calcd. for C21H18N4O3: C, 67.37; H, 4.85; N,
14.96; O, 12.82. Found: C, 67.39; H, 4.82; N, 14.91.
(E)-N'-(2-(2-(4-chlorophenylamino)-2-oxoethoxy)benzylidene)isonicotinohydraz
1
ide (Lb) A brown powder. Yield: 89%. m.p. 215 ºC; H NMR (400 MHz, CDCl3)
δ/ppm: 11.10 (s, 1H, N=CH), 8.81 (s, 1H, pyridine protons), 8.70 (dd, J = 30.1, 9.5 Hz,
1H, pyridine protons), 8.19 (s, 1H, NH), 7.83 (t, J = 12.4 Hz, 2H, pyridine protons),
7.69–7.54 (m, 4H, ArH), 7.44 (d, J = 7.3 Hz, 1H, ArH), 7.22 (dd, J = 18.9, 8.5 Hz, 2H,
ArH), 6.95 (dd, J = 15.5, 7.4 Hz, 1H, ArH), 6.80 (s, 1H, NH), 5.06 (d, J = 6.9 Hz, 2H,
CH2); IR (KBr) v/cm–1: 3602, 3430, 3273, 3041, 2922, 2876, 1646, 1614, 1539, 1492,
1466, 1248; MS (EI) m/z (%): 409 (M+1, 2), 408 (M, 4), 374 (M-35, 6), 328
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(M-35-46, 3), 296 (6), 254 (19), 240 (13), 226 (6), 195 (17), 164 (12), 149 (10), 133
(24), 120 (28), 106 (100), 93 (45), 77 (48), 65 (28); Anal. Calcd. for C21H17ClN4O3: C,
61.69; H, 4.19; Cl, 8.67; N, 13.70; O, 11.74. Found: C, 61.67; H, 4.22; Cl, 8.64; N,
13.73.
(E)-N'-(2-(2-(p-toluidino)-2-oxoethoxy)benzylidene)isonicotinohydrazide (Lc) A
1
dark brown powder. Yield: 76%. m.p. 218 ºC; H NMR (400 MHz, CDCl3) δ/ppm:
11.16 (s, 1H, N=CH), 9.28 (s, 1H, pyridine protons), 8.39 (s, 1H, pyridine protons),
8.22 (s, 1H, NH), 7.56–7.51 (m, 2H, pyridine protons), 7.48–7.17 (m, 4H, ArH),
7.00–6.86 (m, 4H, ArH), 6.81 (s, 1H, NH), 4.88 (s, 2H, CH2), 2.29 (s, 3H, CH3); IR
(KBr) v/cm–1: 3600, 3290, 3422, 3048, 2922, 2868, 1652, 1608, 1539, 1464, 1438,
1243; MS (EI) m/z (%): 389 (M+1, 2), 388 (M, 7), 374 (2), 328 (2), 296 (4), 266 (9),
254 (9), 240 (7), 226 (4), 195 (3), 162 (4), 149 (8), 133 (23), 120 (25), 106 (100), 91
(34), 78 (53), 65 (15); Anal. Calcd. for C22H20N4O3: C, 68.03; H, 5.19; N, 14.42; O,
12.36. Found: C, 68.05; H, 5.21; N, 14.40.
(E)-N'-(2-(2-(4-methoxyphenylamino)-2-oxoethoxy)benzylidene)isonicotinohydr
1
azide (Ld) A dark brown powder. Yield: 56%. m.p. 229 ºC; H NMR (400 MHz,
CDCl3) δ/ppm: 11.06 (s, 1H, N=CH), 8.96 (m, 1H, pyridine protons), 8.77 (d, J = 5.2
Hz, 1H, pyridine protons), 8.33 (s, 1H, NH), 7.59–7.54 (m, 2H, pyridine protons),
7.45 (dd, J = 17.8, 8.3 Hz, 2H, ArH), 6.97–6.81 (m, 5H, ArH), 6.78 (d, J = 8.7 Hz, 1H,
NH), 4.70 (d, J = 28.4 Hz, 2H, CH2), 2.91 (s, 3H, CH3); IR (KBr) v/cm–1: 3598, 3435,
3307, 3065, 2932, 2876, 1641, 1608, 1562, 1511, 1463, 1245; MS (EI) m/z (%): 422
(M+18, 2), 404 (M, 4), 374 (4), 328 (5), 296 (4), 266 (11), 254 (9), 240 (8), 226 (4),
195 (4), 162 (4), 149 (8), 133 (23), 120 (26), 106 (100), 91 (34), 78 (53), 65 (15);
Anal. Calcd. for C22H20N4O4: C, 65.34; H, 4.98; N, 13.85; O, 15.82. Found: C, 65.37;
H, 4.50; N, 13.89.
2.2.4 Preparation of the target europium complexes
To the solution of compound La (0.5 mmol, 0.19 g) dissolved in chloroform (30
mL), the solution of europium nitrate (5 mL, 0.1 M) in absolute alcohol was added
dropwise into a 100 mL three-neck flask, the reaction solution was heated to 80 ºC,
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and stirred and refluxed for 4 h. Then a heavy white precipitate was formed, and
filtrated, purified by washing thoroughly with chloroform several times, and dried for
6 h under vacuum to get the europium complex with ligand La. Other target
complexes were prepared by the same method.
3. Results and discussion
3.1 Elemental analysis and molar conductivity of europium complexes
Analytical data for the newly synthesized europium complexes were listed in Table
1. Results of elemental analysis indicate that the composition of the four novel
europium complexes are conformed to [EuLa–d(NO3)2](NO3)·H2O, ligands La–Ld were
easily dissolved in DMSO, DMF, and slightly dissolved in chloroform, absolute
alcohol, but hardly dissolved in ethylether, benzene and deionized water, while the
synthesized europium complexes were only soluble in DMSO and DMF. The molar
conductance values of the target complexes in acetone solution are in the range of
106–112 S·cm2/mol, which indicates that the synthesised europium complexes are
conformed to a kind of 1:1 electrolytes [24].
3.2 UV-vis spectral analysis
The UV-vis absorption spectra data of the free ligands La–Ld and their europium
complexes were listed in Table 2, which were recorded in DMSO solution. Since the
UV-vis spectra of four synthesized europium complexes are similar, only the UV-vis
spectra of [EuLa,b(NO3)2](NO3)·H2O) as well as their corresponding ligands La,b are
selected, as shown in Figs 1 and 2, respectively.
It is apparently seen from Figs. 1 and 2 that the UV-vis spectral shapes of the
[EuLa(NO3)2](NO3)·H2O and [EuLb(NO3)2](NO3)·H2O are similar to that of the free
ligand La and Lb, which suggests that the coorrdination of the europium ion does not
have a significant influence on the π→π* state energy, and also reveals that the
absorption of the complex is mainly attributed to the ligand. It can be seen from Table
2 that absorb peaks appeared at 285 nm, 319 nm and 286 nm, 323 nm are assigned to
the π→π* and n→π* transitions of La and Lb, respectively. Moreover, the transition
absorption peaks of [EuLa(NO3)2](NO3)·H2O and [EuLb(NO3)2](NO3)·H2O compared
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to the corresponding ligands La and Lb are red shifted, the reasons are that the
introduction of Eu3+ ions enlarged the ligands’conjugated system, which prove that
nitrogen atom of C=N group and the oxygen atom of C=O group of La and Lb are
coordinated to the europium ion [25].
3.3 Infrared radiation (IR) spectra analysis
The IR spectral data of the ligands La–Ld and their europium complexes were listed
in Table 3. Since the IR spectra of four synthesized europium complexes display
similar manners,only the IR spectra of [EuLb(NO3)2](NO3)·H2O and
[EuLd(NO3)2](NO3)·H2O as well as their corresponding ligands Lb and Ld are selected,
as shown in Figs. 3 and 4, respectively.
It is apparently seen from Table 3 and Figs. 3 and 4 that the free ligands Lb and Ld
exhibit absorption bands at 1614 cm–1 and 1608 cm–1, respectively, which are
assigned to the ν(C=N) stretching vibration of the benzene ring. While the absorption
bands of their corresponding europium complexes are shifted subtlely by about 8 cm–1
and 14 cm–1 to a higher frequency compared to that of the free ligands Lb and Ld,
respectively, which confirms that the nitrogen atom of the C=N group is coordinated
to the europium ion successfully [25]. The absorption bands of ligands Lb and Ld
located at 1646 cm–1 and 1641 cm–1, respectively, which are attributed to the
stretching vibration of the ν(N-C=O) group. As for their corresponding europium
complexes, the ν(N-C=O) group stretching frequence are shifted to 1641 cm–1 and
1635 cm–1, respectively. All these results suggest that the oxygen atom of N-C=O
group is coordinated to the europium ion. The absorption bands of Lb and Ld
appearing at 1240 cm–1 and 1050 cm–1, respectively, attributing to the stretching
vibration of the Ar-O-C group, but it does not shift obviously in the target europium
complexes, indicating that the oxygen atom of Ar-O-C group is not coordinate to the
europium ion. The above results reveal that the position of the characteristic IR
absorption bands provides significant indications regarding bonding sites of the
ligands when coordinating to the europium ion [26].
According to the data in Table 3, the characteristic frequencies of the
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coordinating nitrate groups in [EuLb(NO3)2](NO3)H2O are appeared approximately at
1479 cm–1 (υ1), 1388 cm–1 (υ4), 1094 cm–1 (υ2) and 835 cm–1 (υ3). In addition, the
difference between the two strongest absorption bands of the nitrate groups (|υ1–υ4|)
can be defined as Δν. It is generally believed that the Δν value is below 200 for the
bidentate nitrate moiety, but above 200 for the monodentate nitrate moiety. While the
Δν values of [EuLb(NO3)2](NO3)H2O and [EuLd(NO3)2](NO3)H2O are about 91 cm–1
and 104 cm–1, respectively, which indicates that the nitrate groups are coordinated to
the europium ion as bidentate ligands [27]. Meanwhile, there are free nitrate groups in
the target complexes, which are agreement with the results of the conductivity
experiment, and also confirmed to the following thermal gravimetric analyses
[27−30].
3.4 Thermal analysis
To investigate thermal stability and details of thermal decomposition of the target
europium complexes, thermal analysis was carried out and the thermogravimetric
analysis data were summarized in Table 4. The four synthesized target complexes
present similar thermal decomposition behavior, herein, only the TG-DSC curve of
[EuLc(NO3)2](NO3)·H2O were given as a representative example for illustration in
detail, which are shown in Fig. 5.
The TG-DSC curve of [EuLc(NO3)2](NO3)·H2O were measured in the range of
20–800 ºC. It can be observed from Table 4 and Fig. 5 that there is a little mass loss
(observed value 2.40%) from 20 ºC to 91 °C, that is due to the desorption of one
crystal water molecular (calculated value 2.42%). Meanwhile, the weight loss at 287.1
ºC and 630.7 ºC was 8.35% and 16.72%, corresponding to the loss process of the
external a molecules nitrate and internally two molecule nitrate of the target europium
complexes, respectively, which is well corresponding with the calculated
values(8.33% and 16.67%). The target europium complex presentes an exothermic
peak at 475.9 ºC and weight loss 51.79%, attributing to the decomposition of the free
organic ligand. Further heated to 800 ºC, the europium complex was completely
decomposed, and the mass loss percentage is 24.58%, which is close to the calculated
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values of 23.66%, corresponding to the formation of Eu2O3. The thermal analysis
results demonstrate that the target europium complexes have high thermal stability. At
the same time, the structures of the target europium complexes were further
confirmed.
3.5 Fluorescence properties analysis
The luminescence characteristics of the target europium complexes in the solid
state were measured at room temperature under a drive voltage of 700 V, and
excitation and emission slit widths were 5.0 nm. Under identical experimental
conditions, the fluorescence spectra data of the synthesized europium complexes were
summarized in Table 5. Since the observed fluorescence spectra of four target
europium complexes are very similar, the fluorescence spectrum of
[EuLd(NO3)2](NO3)·H2O is selected for illustration, and its excitation and emission
spectrum are presented in Figs. 6 and 7, respectively.
It is shown in Fig. 6 that the complex [EuLd(NO3)2](NO3)H2O exhibits a broad
band in the 320−390 nm region (λmax=362). Therefore, the wavelength at 362 nm was
used to measure the luminescence intensities of the four europium complexes. The
characteristic emission spectrum (Fig. 7) of the target europium complex is consisted
of three main bands at approximately 594 nm (5D0→7F1), 620 nm (5D0→7F2), 621 nm
5
(5D0→7F2). Additionally, we can see the intensity of the D0→7F2 transition (electric
dipole) is greater than that of the 5D0→7F1 transition (magnetic dipole) and the former
is about 3.41 times stronger than that of the latter, which suggests that europium does
not lie in a centro-symmetric coordination site [25]. It can be noted that there is a
narrow and sharp emission peak appearing at approximately 620 nm, which shows
that the target europium complexes have good monochromaticity and the energy is
efficiently transferred from the excited triplet state of the ligand to the vibrational
state of the europium.
The relative luminescence intensity of europium complexes is linked to the
efficiency of the intramolecular energy transfer between the triple levels of the ligand
and the emitting level of the europium ion, which depends on the energy gap between
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the two levels. Thus, it is easily seen from Table 5 that four synthesised europium
complexes present the characteristic luminescence of europium, which indicates that
the four ligands La–Ld are comparatively mild organic chelators for the absorption and
transfer of energy to the europium ion. Meanwhile, the luminescence intensity of
complex [EuLb(NO3)2](NO3)H2O is lower than that of other complexes, which is due
to ligand Lb having an electrophilic group (−Cl) causes the electron density of the
benzene ring to decrease. At the same time, the introduction of the accepting
electronic group easily results in fluorescence quenching. However, the luminescence
intensity of [EuLd(NO3)2](NO3)H2O is better than that of other complexes. It
appeares that the triplet levels of the ligand Ld are in an appropriate level of the
central europium ion and cause an energy transfer from ligand to europium ion more
easily. This stems from the fact that the ligand Ld has an activating group (−OCH3)
that donates electrons to the benzene ring and enlarges the π-conjugated system of
ligand Ld. The above results further highlight that the nature of the substituted group
has significant impact upon the fluorescence intensity of the target europium
complexes.
3.6 Fluorescence quantum yields analysis
The fluorescence quantum yields (Φfx) of the target europium complexes were
determined by the reference method [28] using sulfuric acid solution (0.1 M) of quinine
sulfate (1.0 μg/L) with a known quantum yield (Φfstd=0.55) as standard reference at
room temperature. The fluorescence quantum yields(Φfx) were calculated using the
following formula and summarized in Table 6.
2
F
x
A
std
nX
Φfx = n2std
×
×
×Φfstd
F
AX
std
Where, Φfx is the fluorescence quantum yield of sample, subscripts std and X refer to
the standard and the unkonwn, respectively. n represents the refractive indices
(nX=1.48, nstd=1.337). F is the area of fluorescence integral, and A is the absorbance at
the excitation wavelength. The fluorescence spectral date of the target europium
complexes were measured at room temperature in DMSO solution.
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It can be seen from Table 6 that [EuLd(NO3)2](NO3)H2O exhibits the highest
quantum yield (0.522), resulting from the good p–π conjugation, a rigid plane, and the
triplet energy level of ligand Ld methoxy−substituted group matches better to excite
the state level of europium ion, thus the corresponding europium complex shows good
light–emitting ability. By contrast, [EuLb(NO3)2](NO3)H2O shows the lowest
quantum yield (0.308), which is attributed to the π→π* transition of the
electron−accepting group (−Cl) substituent belongs to forbidden transition, the
excited state molecules are seldom obtained. According to intramolecular
charge–transfer theory [31], introduction of the para–chlorine substituent can prohibit
intramolecular charge–transfer and reduce both benzene ring electron density and the
conjugation of the whole molecular system; this is the reason for the lower
fluorescence quantum yield of [EuLb(NO3)2](NO3)H2O. The above results reveal that
the para–position substituent on the benzene ring has significant effect on the
fluorescence quantum yields of the target europium complexes. In other words, the
electron−donating group can increase the fluorescence quantum yield of the target
europium complexes and electron−accepting group can decrease the fluorescence
quantum yield, which is consistent with fluorescence data of the target europium
complexes.
3.7 Electrochemical properties analysis
Cyclic voltammetry (CV) experiments were conducted to detect the
electrochemical properties of the synthesized target europium complexes. The highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels of the target europium complexes were estimated according to
the electrochemical performance and the UV-vis absorption spectra [32]. The HOMO
and LUMO data for the target europium complexes were obtained using equation
EHOMO=4.74+eEOX, ELUMO= EHOMO-Eg, Eg=1240/λonset(eV) [24] (EOX is the oxidation
potential of the europium complexes; λonset is the maximum UV-vis absorption spectra
peak starting value). Four cyclic voltammetrics were depicted in Fig. 8 and their
electrochemical data were presented in Table 7.
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It can be seen from Table 7 that the oxidation potential of four target europium
complexes occurs in the potential range +0.656 V to +0.764 V. The Eg of four target
europium complexes is between 4.715 eV and 4.751 eV. Compared with that of the
complex [EuLa(NO3)2](NO3)H2O, the oxidation potential and HOMO energy levels
of the complexes [EuLc,d(NO3)2](NO3)H2O are raise, in contrast, that of the complex
[EuLb(NO3)2](NO3)H2O drop; these phenomena are ascribed to the fact of the ligands
Lc,d owning electron–donating groups (−CH3, −OCH3, respectively) and ligand Lb
owning electron–accepting groups (−Cl). The results demonstrate that the introduction
of electron–donating groups can increase both benzene ring electron density and the
oxidation potential of the synthesised europium complexes, while the introduction of
electron–accepting groups can reduce it. In other words, the nature of the substituent
group can affect the HOMO energy levels of the target europium complexes.
4. Conclusions
Four novel salicylaldehyde isonicotinoyl hydrazone derivatives and their
corresponding europium complexes were prepared successfully. Fluorescence
properties and fluorescence quantum yields of the target europium complexes were
investigated, the results showed that the ligands La–Ld were an efficient sensitizer for
europium luminescence, four target complexes exhibited characteristic fluorescence
emissions of europium ion. The fluorescence intensity of the synthesized europium
complex methoxy−substituted group was stronger than that of other complexes and
presented the highest fluorescence quantum yield (0.522). The substituents' nature had
pronounced effect upon the electrochemical properties of the target europium
complexes. Moreover, the test results of cyclic voltammograms of the target europium
complexes showed that the introduction of the electron–accepting groups tended to
decrease the oxidation potential and HOMO energy levels of the target europium
complexes, however, introduction of the electron–donating groups could increase the
corresponding complexes oxidation potential and HOMO energy levels. In addition,
the thermal analysis provided information about the high thermal stability for the
target europium complexes. These results demonstrated that the synthesized target
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europium complexes possessed good application prospects and theoretical research
value.
5. Abbreviations
UV-vis
IR
Ultraviolet visible;
infrared spectra
CV
cyclic voltammetric
HOMO
highest occupied molecular orbital;
LUMO
DMF
lowest unoccupied molecular orbital.
dimethyl formamide
Acknowledgements
The authors are grateful for the financial support of the National Natural Science
Foundation of China (No.J1103312; No.J1210040; No.21341010), the Innovative
Research Team in University (No.IRT1238) and chemical excellent engineer training
program of Hunan university. We also thank Dr. William Hickey, the U.S. professor of
HRM, for the English editing on this paper.
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Reference:
[1] G. Grivani, G. Bruno, H.A. Rudbari, A.D. Khalaji, P. Pourteimouri, Inorg. Chem.
Commun. 18 (2012) 15−20.
[2] J.Y. Mao, N. Li, H.R. Li, X.B. Hu, J. Mol. Catal. A: Chem. 258 (2006) 178−184.
[3] M.S. Nair, D. Arish, R.S. Joseyphus, J. Saudi. Chem. Soc.12 (2012) 83−88.
[4] T.F. Abbs Fen Reji, A. Jeena Pearl, B.A. Rosy, J. Rare Earth. 31 (2013)
1009−1016.
[5] D.F. Xu, S.Z. Ma, G.Y. Du, Q.Z. He, D.Z. Sun, J. Rare Earth. 26 (2008) 643−647.
[6] G. Grivani, A.D. Khalaji, V. Tahmasebi, K. Gotoh, H. Ishida, J. Polyhedron. 31
(2012) 265−271.
[7] X.B. Sun, X.Z. Jin, W. Pan, J.P. Wang, J. Carbohydr. Polym. 113 (2014) 194−199.
[8] K.Kanmani Raja, N.Indra Gandhi, L. Lekha, D. Easwaramoorthy, G. Rajagopal, J.
Mol. Struct. 1060 (2014) 49−57.
[9] L. Lekha, K. Kanmani Raja, G. Rajagopal, D. Easwaramoorthy, J. Mol. Struct.
1056-1057 (2014) 307−313.
[10] L. Lekha, K. Kanmani Raja, G. Rajagopal, D. Easwaramoorthy, J. Organomet.
Chem. 753 (2014) 72−80.
[11] J.C. Lee, Y.K. Jeong, J.M. Kim, J.G. Kang, Spectrochim. Acta, part A. 124 (2014)
256−264.
[12] M.S. Nair, D. Arish, R.S. Joseyphus, J. Saudi. Chem. Soc. 12 (2012) 83−88.
[13] L.M. Wu, G.F. Qiu, H.B. Teng, Q.F. Zhu, S.C. Liang, X.M. Hu, Inorg. Chim.
Acta. 360 (2007) 3069-3074.
16

ACCEPTED MANUSCRIPT
[14] Z.H.Xu, F.J. Chen, P.X. Xi, X.H. Liu, Z.Z. Zeng, J. Photochem. Photobiol, A.
196 (2008) 77−83.
[15] Z.H.Xu, X.W. Zhang, W.Q. Zhang, Y.H. Gao, Z.Z. Zeng, Inorg. Chem. Commun.
14 (2011) 1569−1573.
[16] Y.T. Yang, C.X. Yin, F.J. Huo, Y.Y. Chu, H.B. Tong, J.B. Chao, Sens. Actuator, B.
186 (2013) 212−218.
[17] C. Mouralian, J.L. Buss, B. Stranix, J. Chin, P. Ponka, Biochem. Pharmacol. 71
(2005) 214−222.
[18] J.X. Yu, Vikram D. Kodibagkar, Rami R. Hallac, L. Liu, Ralph P. Mason,
Bioconjugate. Chem. 23 (2012) 596−603.
[19] X.T. Chen, Z.F. Li, Y. Xiang, A.J. Tong, Tetrahedron. Lett. 49 (2008) 4697−4700.
[20] N. Li, Y. Xiang, X.T. Chen, A.J. Tong, J. Tlanta. 79 (2009) 327−332.
[21] K. Li, Y. Xiang, X.Y. Wang, J. Li, R.R. Hu, A.J. Tong, B.Z. Tang, JACS. 136
(2014) 1643−1649.
[22] C.D. Fan, J. Zhao, B.X. Zhao, S.L. Zhang, J.Y. Miao, Chem. Res. Toxicol. 22
(2009) 1517-1525.
[23] K. Nakagawa, K. Amita, H. Mizuno, Y. Inone, T. Hakushi, Bull. Chem. Soc. Jpn.
60 (1987) 2037−2040.
[24] D.Q Wang, X.M Wang, Q.G He, M.Y Zhou, W.W Rui, X.T Tao, Tetrahedron.
Lett. 49 (2008) 5871−5876.
[25] D.C. Guo, W. He, B. Liu, L.N. Gou, R.X. Li, J. Lumin. 28 (2013) 280−286.
[26] M.R. Anoop, P.S. Binil, S. Suma, M.R. Sudarsanakumar, J. Rare Earth. 30 (2012)
17

ACCEPTED MANUSCRIPT
709−715.
[27] Wu. Zhang, Y.C. Chai, K.Y. Li, Y.W. Chen, D. Yan, D.C. Guo, J. Lumin. 29
(2014) 1113−1122.
[28] D. Yan, Y. Xiang, K.Y. Li, Y.W. Chen, Z.H. Yang, D.C. Guo, J. Mol. Struct. 1074
(2014) 487−496.
[29] Y.H. Liu, W. He, Z.H. Yang, Y.W. Chen, X.W. Wang, D.C. Guo, J. Lumin. 160
(2014) 35−42.
[30] D.C. Guo, W. He, B. Liu, L.N. Gou, R.X. Li, Lumin. 28 (2013) 280−286.
[31] D.C Guo, P.L Wu, H. Tan, L. Xia, W.H. Zhou, J. Lumin. 131 (2011) 1272−-1276.
[32] Ge. G, He. J, Guo. H, Wang. F, Zou. D, J. Organomet. Chem. 694 (2009)
3050−3057.
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Tables Captions
Table 1 Elemental analysis and molar conductance data of the target europium
complexes
Table 2 The UV-vis data of the target europium complexes and corresponding ligands
Table 3 The IR data of the target europium complexes and corresponding ligands
Table 4 The thermogravimetric data of the target europium complexes
Table 5 The fluorescence spectral data of the target europium complexes
Table 6 The fluorescence quantum yields of the target europium complexes
Table 7 The EHOMO, ELUMO and Eg of the target europium complexes
Figures/Scheme Captions
Scheme 1. Synthesis route for salicylaldehyde isonicotinoyl hydrazon derivatives
Fig. 1 UV-vis spectra of [EuLa(NO3)2](NO3)·H2O (a) and La (b)
Fig. 2 UV-vis spectra of [EuLb(NO3)2](NO3)·H2O (a) and Lb (b)
Fig. 3 The IR spectra of [EuLb(NO3)2](NO3)·H2O (a) and Lb (b)
Fig. 4 The IR spectra of [EuLd(NO3)2](NO3)·H2O (a) and Ld (b)
Fig. 5 The TG-DSC curve of [EuLc(NO3)2](NO3) H2O
Fig. 6 The excitation spectrum of [EuLd(NO3)2](NO3)H2O
Fig. 7 The emission spectrum of [EuLd(NO3)2](NO3)H2O
Fig. 8. The CV curves of the target europium complexes
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Figures
Scheme 1. Synthesis route for salicylaldehyde isonicotinoyl hydrazon derivatives
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0.4
b
0.3
a
0.2
0.1
b
a
0.0
280
300
320
340
360
380
400
Wavelength/nm
Fig. 1 UV-vis spectra of [EuLa(NO3)2](NO3)·H2O (a) and La (b)
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0.4
0.3
b
0.2
a
0.1
0.0
280
300
320
340
360
380
400
Wavelength/nm
Fig. 2 UV-vis spectra of [EuLb(NO3)2](NO3)·H2O (a) and Lb (b)
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a
b
4000 3500 3000 2500 2000 1500 1000
Wavenumber/cm-1
500
Fig. 3 The IR spectra of [EuLb(NO3)2](NO3)·H2O (a) and Lb (b)
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a
b
4000 3500 3000 2500 2000 1500 1000
Wavenumber/cm-1
500
Fig. 4 The IR spectra of [EuLd(NO3)2](NO3)·H2O (a) and Ld (b)
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100
90
TG
80
70
)
g
/ m
%
60
W
M
T G /
50
40
30
20
C
(
D S
DSC
100 200 300 400 500 600 700 800
Temperature/0C
Fig. 5 The TG-DSC curve of [EuLc(NO3)2](NO3) H2O
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4500
4000
3500
3000
2500
2000
1500
1000
500
300
320
340
360
380
400
420
wavelength/nm
Fig. 6 The excitation spectrum of [EuLd(NO3)2](NO3)H2O
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3000
2500
2000
1500
1000
500
0
550
575
600
625
650
wavelength/nm
Fig. 7 The emission spectrum of [EuLd(NO3)2](NO3)H2O
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a
b
c
d
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
E/V
Fig. 8. The CV curves of the target europium complexes
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Tables
Table 1 Elemental analysis and molar conductance data of the target europium complexes
measured value (theoretical value) (%)
Λm(acetone)
Complex
C
H
N
Eu
(S.cm2.mol–1)
[EuLa(NO3)2](NO3)H2O
[EuLb(NO3)2](NO3)H2O
[EuLc(NO3)2](NO3)H2O
[EuLd(NO3)2](NO3)H2O
34.48 (34.53) 2.79 (2.76) 14.39 (13.42) 20.85 (20.81)
33.01 (32.98) 2.54 (2.50) 12.76 (12.82) 19.89 (19.87)
35.52 (35.50) 2.96 (2.98) 13.14 (13.17) 20.45 (20.41)
34.72 (34.75) 2.94 (2.92) 12.86 (12.89) 19.94 (19.98)
106
119
114
122
29