Synthesis, Characterization and Properties of Novel Coumarin Derivatives and Their Europium Complexes

Article abstract of DOI:10.1007/s10895-015-1563-8

Seven novel coumarin derivatives derived from salicylaldehyde and phenol were synthesized and characterized by ¹H NMR and ¹³C NMR spectra, mass spectra, infrared spectra and elemental analysis. Their corresponding Eu(III) complexes having general formula EuL^1-7(NO₃)₃·2H₂O were successfully prepared and characterized by elemental analysis, EDTA titrimetric, molar conductivity, UV-Vis, FT-IR and thermal performance studies. The luminescence properties, fluorescence quantum yields and the electrochemical properties of the title complexes were investigated. The results showed that the title complexes exhibited characteristic emissions of europium ions and possessed relatively good fluorescence quantum yields. The luminescence intensity of the complex with bromine-substituted group is the strongest among all the title complexes. The introduction of electron-withdrawing groups can increase the luminescence properties and fluorescence quantum yields, decrease the HOMO and LUMO energy levels of the title europium complexes, but electron-withdrawing group conversely. And these title complexes may possibly be useful for studying in luminescent materials field.

Full text of DOI:10.1007/s10895-015-1563-8

J Fluoresc
DOI 10.1007/s10895-015-1563-8
ORIGINAL PAPER
Synthesis, Characterization and Properties of Novel Coumarin
Derivatives and Their Europium Complexes
Dong Yan1 & Dong Li1 & Guang Cheng1 & Zehui Yang2 & Ling Shi1 & Dongcai Guo1
Received: 23 January 2015 /Accepted: 23 March 2015
#
Springer Science+Business Media New York 2015
Abstract Seven novel coumarin derivatives derived from
salicylaldehyde and phenol were synthesized and character-
ized by 1H NMR and 13C NMR spectra, mass spectra, infrared
spectra and elemental analysis. Their corresponding Eu(III)
complexes having general formula EuL1−7(NO3)3⋅2H2O were
successfully prepared and characterized by elemental analysis,
EDTA titrimetric, molar conductivity, UV–Vis, FT-IR and
thermal performance studies. The luminescence properties,
fluorescence quantum yields and the electrochemical proper-
ties of the title complexes were investigated. The results
showed that the title complexes exhibited characteristic emis-
sions of europium ions and possessed relatively good fluores-
cence quantum yields. The luminescence intensity of the com-
plex with bromine-substituted group is the strongest among all
the title complexes. The introduction of electron-withdrawing
groups can increase the luminescence properties and fluores-
cence quantum yields, decrease the HOMO and LUMO ener-
gy levels of the title europium complexes, but electron-
withdrawing group conversely. And these title complexes
may possibly be useful for studying in luminescent materials
field.
Introduction
Rare earth organic complexes have attracted significant atten-
tion due to their functional optical, electronic and magnetic
properties. Their fascinating optical properties make the com-
plexes attractive for multiple applications, such as MRI con-
trast agents [1, 2], optical amplifiers [3, 4], organic light emit-
ting diodes (OLEDs) [5, 6], probes [7, 8], lasers [9], etc. The
luminescence intensity of rare earth complexes is strongly
dependent on the efficiency of ligand absorption in the UV
region, the efficiency of ligand-to-metal energy transfer, and
the efficiency of rare earth luminescence [10], thus the lumi-
nescence intensity of rare earth complexes is relationship to
central ion and the structure of organic ligand. Therefore, the
luminescence excited directly from the rare earth ions is un-
favorable and so, it is very important to design and synthesize
novel chelating ligands possessing high absorption efficiency
and high efficient of ligand-to-RE(III) ions energy transfers. It
is well known that coumarin derivatives are one kind of lac-
tone compounds and widely present in nature. They have been
reported to possess significant biological activities and phar-
macological actions, such as anti-tumor, anti-HIV, anti-oxida-
tion, anti-coagulation, anti-arrhythmic, anti-osteoporosis, an-
algesic, antibacterial and photosensitivity [11, 12], and widely
used in the fields of medicine and pesticides, fluorescent probe
[13]. Coumarin derivatives have been extensively studied due
to their good planarity and high energy transfer efficiency. The
lactone coumarin skeleton of 3-acetyl coumarin possessing
good conjugated plane, rigid structure and various coordina-
tion sites, so this kind of compounds is usually used as the
organic ligand of the rare earth luminescent complexes.
In this paper, seven novel coumarin derivatives and their
corresponding europium complexes were synthesized, the lu-
minescent and electrochemical properties of the title com-
plexes were investigated, and the relationship between the
.
.
.
Keywords Europium complexes Coumarin Synthesis
.
Electrochemical properties Luminescence
* Dongcai Guo
dcguo2001@163.com
1
School of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, China
2
School of Chemical Engineering, Ningbo University of Technology,
Ningbo 315016, China

J Fluoresc
structure of the ligand and the properties of complexes was
also explored in detail. The synthesis route for the coumarin
derivatives is shown in Scheme 1.
the same temperature. Then, the reaction mixture was cooled to
room temperature, the resulting precipitate was collected by
filtration and washed several times with hot ethanol [15]. After
drying, the product was recrystallized from ethanol and dried
under vacuum to give yellow needle crystals (A). Yield: 82 %.
m.p. 118 °C; 1H NMR (400 MHz, CDCl3) δ/ppm 8.51 (s, 1H,
CH), 7.68−7.63 (m, 2H, ArH), 7.39−7.34 (m, 2H, ArH), 2.73
(s, 1H, CH3). MS (EI) m/z (%): 190.1 (M+2, 1), 189.1 (M+1,
7), 188.1 (M, 53), 173.1 (100), 145.1 (15), 89.1(23).
Experimental
Materials
The purity of Eu2O3 exceeded 99.99 %, Eu(NO3)3 were pre-
pared according to literature methods [14], chloroacetic acid
was of CP grade, phenol derivatives, salicylaldehyde and oth-
er reagents were of AR grade, and all used without further
purification.
Synthesis of Phenoxyacetic Acid Derivatives (B1−7
)
Compounds B1−7 were prepared by similar procedures. In a
typical synthesis of B1, monochloroacetic acid (0.04 mol,
3.78 g) was dissolved in deionized water (15 mL) under the
condition of stirring and an ice bath. Then NaOH (25 %) was
added dropwise until the pH value was adjusted to 9−10, then
a solution of sodium chloroacetate was obtained. To a solution
of NaOH (0.03 mol, 1.20 g), deionized water (15 mL) and
ethanol (5 mL), phenol (0.04 mol, 3.76 g) was slowly added
under stirring. After addition, the mixture was stirred for
20 min, then the above sodium chloroacetate was added
dropwise, and heated to 105 °C and refluxed for 5 h. The
reaction mixture was cooled to room temperature. The pH
value of the mixture was acidified to 1−2 with diluted hydro-
chloric acid. The precipitate was filtered, washed with diluted
hydrochloric acid many times, and recrystallized and dried in
vacuum, resulting in a white solid product of the
Physical Measurements
The melting points of the synthesized compounds were deter-
mined on a TECH XT-4 melting point apparatus and were
uncorrected. 1H NMR and 13C NMR spectra were measured
with a Bruker-400 MHz nuclear magnetic resonance spec-
trometer with deuterated chloroform (CDCl3) or dimethyl
sulfoxide (DMSO)-d6 as a solvent and TMS as internal refer-
ence. The electron impact mass spectra (EI-MS) were mea-
sured using a MAT95XP analyzer. Elemental analysis were
carried out with a Flash EA1112 elemental analyzer. The UV–
Vis spectra were recorded on a LabTech UV-2100 UV-Visible
Spectrophotometer and dimethyl sulfoxide (DMSO) was used
as a reference and a solvent. The infrared spectra (KBr pellets)
were recorded in the region 4000–400 cm−1 on a Shimadzu
IRAffinity-1 FT-IR spectrophotometer. Luminescence spectra
were monitored by HIACHI F-2700 spectrophotometer at
room temperature, the widths of both the excitation and emis-
sion slit were 2.5 nm and the voltage of photomultiplier tube
was 700 V. Thermal gravimetric (TG) and differential thermal
analyses (DTA) were carried out up to 800 °C with a heating
speed of 20 °C/min in the static air atmosphere on an a SHIM
ADZU DTG-60 thermogravimetric analyzer. Cyclic volt-
ammetry (CV) measurements were performed using a CHI
660d electrochemical workstation, the sodium nitrite solution
(0.1 M) used as electrolyte, DMSO as solvent. The cyclic
voltammogram was recorded at a scan speed of 50 mV·s−1
at a sensitivity of 1 mA.
phenoxyacetic acid (B1). White crystals. Yield: 76 %. H
1
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 (2 M, 14), 303
(2 M-1, 100), 151 (M-1, 29).
p-fluorophenoxyacetic acid (B2). White solid. Yield: 72 %.
1H NMR (CDCl3) δ/ppm: 7.00 (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 (77), 112 (42), 95 (69), 75 (16).
p-chlorophenoxyacetic acid (B3). White solid. Yield: 82 %.
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, 31), 186 (M, 100), 141 (80), 128 (56), 111 (57), 99
(33), 75 (34).
p-bromophenoxyacetic acid (B4). White solid. Yield: 77 %.
1H NMR (CDCl3) δ/ppm: 7.42 (d, J=9.0 Hz, 2H, ArH), 6.81
(d, J=9.0 Hz, 2H, ArH), 4.72 (s, 2H, CH2); MS (EI) m/z (%):
232 (M+1, 96), 230 (M-1, 100), 187 (46), 185 (48), 174 (35),
172 (36), 157 (41), 155 (38), 143 (18), 76 (21).
General Procedure for Sythesis of the Intermediates
Synthesis of 3-acetyl Coumarin (A)
p-nitrophenoxyacetic acid (B5). White crystals. Yield:
1
1
A mixture of salicylaldehyde (0.02 mmol, 2.10 mL), ethyl
acetoacetate (0.02 mol, 2.50 mL), absolute ethyl alcohol
(50 mL) was placed in a 150 mL three-neck flask, which
was heated to reflux under stirring, then several drops of pi-
peridine was added. The mixture was stirred for a further 5 h at
68 %. H NMR (400 MHz, DMSO-d6) δ/ppm: H NMR
(CDCl3) δ/ppm: 8.21 (d, J=9.2 Hz, 2H, ArH), 7.14 (d, J=
9.2 Hz, 2H, ArH), 4.85 (s, 2H, CH2); MS (EI) m/z (%): 198
(M+1, 11), 197 (M, 100), 181 (5), 167 (18), 152 (86), 139
(12), 122 (23), 109 (37), 92 (30), 76 (19).

J Fluoresc
p-methoxyphenoxyacetic acid (B6). White crystals. Yield:
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).
76 %. 1H NMR (CDCl3) δ/ppm: 6.90-6.83 (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 (19), 95 (27), 77 (9).
p-methylphenoxyacetic acid (B7). White crystals. Yield:
1
71 %. H NMR (CDCl3) δ/ppm: 7.11 (d, J=8.4 Hz, 2H,
p-nitro-2-phenoxyacetohydrazide (D5). Light yellow crys-
1
ArH), 6.83 (d, J=8.4 Hz, 2H, ArH), 4.66 (s, 2H, CH2), 2.31
(s, 3H, CH3); MS (EI) m/z (%): 167 (M+1, 10), 166 (M, 100),
121 (63), 107 (49), 91 (65), 77 (25), 65 (16).
tals. Yield: 78 %. H NMR (CDCl3) δ/ppm: 8.25 (d, J=
8.0 Hz, 2H, ArH), 7.63 (s, 1H, NH), 7.02 (d, J=8.0 Hz, 2H,
ArH), 4.66 (s, 2H, CH2), 3.94 (s, 2H, NH2); MS (EI) m/z (%):
212 (M+1, 4), 211 (M, 22), 153 (5), 152 (24), 123 (11), 122
(22), 106 (8), 92 (12), 76 (14), 73 (100), 65 (5).
Synthesis of Ethyl Phenoxyacetate Derivatives (C1−7
)
p-methoxy-2-phenoxyacetohydrazide (D6). White needle
crystals. Yield: 85 %. 1H NMR (CDCl3) δ/ppm: 7.72 (s, 1H,
NH), 6.90−6.83 (m, 4H, ArH), 4.53 (s, 2H, CH2), 3.92 (s, 2H,
NH2), 3.77 (s, 3H, OCH3); MS (EI) m/z (%): 197 (M+1, 4),
196 (M, 30), 138 (2), 137 (9), 124 (100), 109 (28), 107 (16),
92 (11), 77 (17), 64 (6).
Compounds (C1−7) were prepared by similar procedures. In
a typical synthesis of 4a, a mixture of phenoxyacetic acid
(0.02 mol, 3.04 g) and absolute ethanol (40 mL) was added
to a 150 mL three-neck flask in an ice bath, then acetyl chlo-
ride (1.0 mL) was added dropwise with stirring. The reaction
mixture was heated to 80 °C and refluxed for 24 h. The excess
solvent was completely removed under reduced pressure on a
rotary evaporator. The residue product was used for the next
reaction directly without purification.
p-methyl-2-phenoxyacetohydrazide (D7). White needle
crystals. Yield: 79 %. 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).
Synthesis of Phenoxyacetohydrazide Derivatives (D1−7
)
Compounds D1-7 were prepared by similar procedures. In a
typical synthesis of D1, a mixture of phenoxyacetate
(10 mmol, 1.80 g), hydrazine hydrate (80 %, 5 mL) and ab-
solute ethanol (30 mL) was added to a 150 mL three-neck
flask, then the reaction mixture was refluxed for 5 h. The
mixture was cooled to room temperature. The crude product
was collected by filtration and washed several times with eth-
anol. After drying, white needle crystal was recrystallized
from ethanol and dried in vacuum.
Synthesis of Coumarin Derivatives (L1−7
)
Compounds L1−7 were prepared by similar procedures. In a
typical synthesis of L1, 3-acetyl coumarin (4 mmol, 0.75 g)
was dissolved in 30 mL absolute ethanol in a 150 mL three-
neck flask, then phenoxyacetohydrazide (4 mmol, 0.66 g) in
20 mL absolute ethanol was added dropwise with constant stir-
ring and finally heated under reflux for 6 h. A light yellow solid
was precipitated, filtered and dried in vacuum. The light yellow
crystal was purified by recrystallization from ethyl acetate.
(E)-N’-(1-(2-oxo-2H-chromen-3-yl)ethylidene)-2-
phenoxyacetohydrazide (L1). Pale yellow needle crystals.
Yield: 71 %. m.p. 132−136 °C; 1H NMR (400 MHz, CDCl3)
δ/ppm: 9.58 (s, 1H, NH), 8.21 (s, 1H, CH), 7.57 (d, J=7.6 Hz,
2H, ArH), 7.32−7.38 (m, 4H, ArH, ArH), 7.08 (m, J=7.4 Hz,
1H, ArH), 6.98 (d, J=7.9 Hz, 2H, ArH), 4.74 (s, 2H, CH2),
2.31 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3) δ/ppm:
164.43, 159.89, 156.79, 154.14, 151.82, 142.67, 132.56,
130.04, 129.52, 129.03, 124.85, 122.71, 118.86, 116.52,
114.68, 67.30, 14.85; IR (KBr) ν/cm−1: 3243, 3053, 1708,
1651, 1611, 1246 736; MS (EI) m/z (%): 338 (M+2, 1), 337
(M+1, 10), 336 (M, 40), 229 (100), 187 (23), 115 (14), 77 (28);
Anal. Calcd. for C19H16N2O4: C, 67.85; H, 4.79 N, 8.33; O,
19.03. Found: C, 67.83; H, 4.51; N, 8.34.
Phenoxyacetohydrazide (D1). White needle crystals. Yield:
83 %. 1H NMR (CDCl3) δ/ppm: 7.80 (s, 1H, NH), 7.36 (t, J=
7.9 Hz, 2H, ArH), 7.06 (t, J=7.4 Hz, 1H, ArH), 6.92 (d, J=
8.5 Hz, 2H, ArH), 4.60 (s, 2H, CH2), 3.93 (s, 2H, NH2); MS
(EI) m/z (%): 167 (M+1, 5), 166 (M, 33), 135 (3), 134 (8),
108 (4), 107 (23), 94 (100), 77 (62), 65 (6).
p-fluoro-2-phenoxyacetohydrazide (D2). White needle
crystals. Yield: 65 %. 1H NMR (CDCl3) δ/ppm: 7.72 (s, 1H,
NH), 7.02 (d, J=8.0 Hz, 2H, ArH), 6.90 (d, J=8.0 Hz, 2H,
ArH), 4.54 (s, 2H, CH2), 3.94 (s, 2H, NH2); MS (EI) m/z (%):
185 (M+1, 3), 184 (21), 169 (2), 126 (5), 125 (24), 112 (100),
97(29), 95 (75), 83 (23), 75 (25), 73 (28).
p-chloro-2-phenoxyacetohydrazide (D3). White needle
crystals. Yield: 87 %. 1H NMR (CDCl3) δ/ppm: 7.72 (s, 1H,
NH), 7.31 (d, J=9.0 Hz, 2H, ArH), 6.88 (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, 21), 143 (6), 141 (17), 130 (33), 128
(100), 111 (35), 99 (4), 77 (7), 65 (5).
(E)-2-(4-fluorophenoxy)-N’-(1-(2-oxo-2H-chromen-3-
yl)ethylidene)acetohydrazide (L2). Yellow crystals. Yield:
72 %. m.p. 143−145 °C; 1H NMR (400 MHz, CDCl3)
δ/ppm: 9.53 (s, 1H, NH), 8.22 (s, 1H, CH), 7.58 (m, J=
7.6 Hz, 2H), 7.32−7.38 (m, 2H, ArH), 7.06 (m, J=8.3 Hz,
p-bromo-2-phenoxyacetohydrazide (D4). White needle
crystals. Yield: 80 %. 1H NMR (CDCl3) δ/ppm: 7.69 (s, 1H,

J Fluoresc
2H, ArH), 6.94 (dd, J=7.7, 3.9 Hz, 2H, ArH), 4.71 (s, 2H,
CH2), 2.33 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3) δ/
ppm 164.23, 159.91, 154.15, 152.69, 151.98, 147.57, 142.75,
134.45, 132.62, 129.05, 124.88, 118.85, 116.54, 116.42,
116.01, 68.05, 14.88; IR (KBr) ν/cm−1: 3241, 3053, 1711,
1653, 1613, 1247, 736; MS (EI) m/z (%): 356 (M+2, 1), 355
(M+1, 4), 354 (M, 23), 229 (100), 187 (32), 115 (35), 89 (17),
75 (13), 63 (12); Anal. Calcd. for C19H15FN2O4: C, 64.40; H,
4.27, N, 7.91; O, 18.06. Found: C, 67.45; H, 4.32, N, 7.87.
(E)-2-(4-chlorophenoxy)-N’-(1-(2-oxo-2H-chromen-3-
yl)ethylidene)acetohydrazide (L3). Yellow crystals. Yield:
(E)-2-(4-methoxyphenoxy)-N’-(1-(2-oxo-2H-chromen-3-
yl)ethylidene)acetohydrazide (L6). Pale yellow needle crys-
tals. Yield: 76 %. m.p. 151−153 °C; H NMR (400 MHz,
1
CDCl3) δ/ppm: 9.58 (s, 1H, NH), 8.22 (s, 1H, CH), 7.57−
7.59 (d, J=7.7 Hz, 2H, ArH), 7.39−7.31 (m, 2H, ArH), 6.86−
6.93 (m, 4H, ArH), 4.69 (s, 2H, CH2), 3.80 (s, 3H, OCH3),
2.32 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3) δ/ppm:
164.72, 159.90, 155.14, 151.79, 151.07, 150.94, 142.73,
132.58, 129.04, 124.87, 118.86, 116.53, 116.03, 115.81,
115.05, 68.18, 55.72, 14.89; IR (KBr) ν/cm−1: 3246, 3055,
1710, 1649, 1613, 1245, 738; MS (EI) m/z (%): 368 (M+2,
1), 367 (M+1, 7), 366 (M, 28). 200 (100), 171 (13), 115 (37),
89 (15), 63 (16); Anal. Calcd. for C20H18N2O5: C, 65.57; H,
4.95 N, 7.65; O, 21.84. Found: C, 65.56; H, 4.91; N, 7.67.
(E)-N’-(1-(2-oxo-2H-chromen-3-yl)ethylidene)-2-(p-
tolyloxy)acetohydrazide (L7). Pale yellow solid. Yield: 74 %.
m.p. 147−150 °C; 1H NMR (400 MHz, CDCl3) δ/ppm: 9.57 (s,
1H, NH), 8.22 (s, 1H, CH), 7.57−7.59 (m, J=7.7 Hz, 2H, ArH),
7.31−7.38 (m, 2H, ArH), 7.15 (d, J=8.0 Hz, 2H, ArH), 6.88 (d,
J=7.9 Hz, 2H, ArH), 4.71 (s, 2H, CH2), 2.32 (d, J=4.9 Hz, 6H,
CH3); 13C NMR (100 MHz, CDCl3) δ/ppm: 164.65, 159.89,
154.76, 154.13, 151.72, 142.65, 132.54, 130.44, 129.03,
128.76, 126.00, 124.85, 118.86, 116.51, 114.53, 67.52, 20.54,
14.85; IR (KBr) ν/cm−1: 3248, 3058, 1711, 1650, 1612, 1245,
746; MS (EI) m/z (%): 352 (M+2, 2), 351 (M+1, 13), 350 (M,
45), 229 (85), 215 (M, 43), 187 (43), 115 (47), 91 (100), 65 (26);
Anal. Calcd. for C20H18N2O4: C, 68.56; H, 5.18 N, 8.00; O,
18.27. Found: C, 68.55; H, 5.20; N, 7.96.
1
70 %. m.p. 157−159 °C; H NMR (400 MHz, CDCl3)
δ/ppm: 9.50 (s, 1H, NH), 8.22 (s, 1H, CH), 7.57−7.59 (m,
J=8.0 Hz, 2H), 7.36−7.39 (d, J=11.2 Hz, 2H, ArH), 7.32−
7.34 (m, J=9.4 Hz, 2H, ArH), 6.92 (d, J=7.7 Hz, 2H,ArH),
4.71 (s, 2H, CH2), 2.33 (s, 3H, CH3); 13C NMR (100 MHz,
CDCl3) δ/ppm: 163.93, 159.92, 155.38, 154.17, 147.54,
142.72, 132.62, 129.98, 129.42, 129.04, 124.88, 118.84,
116.56, 116.17, 116.02, 67.62, 14.89; IR (KBr) ν/cm−1:
3244, 3065, 1710, 1650, 1613, 1249, 736; MS (EI) m/z (%):
373.1 (M+3, 2), 372.1 (M+2, 7), 370.1 (M, 25), 229.1 (100),
187.1 (27), 141.1 (14) 115.1 (45), 89.1 (21), 75.1 (17), 63.1
(15); Anal. Calcd. for C19H15ClN2O4: C, 61.55; H, 4.08 N,
7.56; O, 17.26. Found: C, 61.63; H, 4.06; N,7.58.
(E)-2-(4-bromophenoxy)-N’-(1-(2-oxo-2H-chromen-3-
yl)ethylidene)acetohydrazide (L4). Pale yellow crystals. Yield:
68 %. m.p. 165−168 °C; 1H NMR (400 MHz, CDCl3) δ/ppm:
9.49 (s, 1H, NH), 8.22 (s, 1H, CH), 7.58−7.60 (m, J=8.0 Hz,
2H, ArH), 7.46−7.48 (d, J=8.2 Hz, 2H), 7.36−7.38 (m, J=
4.5 Hz, 2H, ArH), 6.88 (d, J=8.4 Hz, 2H), 4.71 (s, 2H, CH2),
2.33 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3) δ/ppm: 163.89,
159.87, 155.90, 154.16, 152.03, 142.72, 132.92, 132.62, 132.35,
129.04, 124.88, 118.83, 116.68, 116.49, 115.13, 67.53, 14.90. IR
(KBr) ν/cm−1: 3243, 3055, 1713, 1651, 1613, 1246, 744; MS
(EI) m/z (%): 418 (M+3, 1), 417 (M+2, 4), 415 (M, 13), 414
(M-1, 15), 229 (100), 215 (23), 187 (33), 115 (38), 89 (17), 76
(12), 63 (13); Anal. Calcd. for C19H15BrN2O4: C, 54.96; H, 3.64,
N, 6.75; O, 15.41. Found: C, 54.97; H, 3.66; N, 6.73.
(E)-2-(4-nitrophenoxy)-N’-(1-(2-oxo-2H-chromen-3-
yl)ethylidene)acetohydrazide (L5). Yellow needle crystals.
Yield: 72 %. m.p. 160−163 °C; 1H NMR (400 MHz, DMSO)
δ/ppm 11.13 (s, 1H, NH), 8.36 (s, 1H, CH), 8.21 (d, J=8.5 Hz,
2H, ArH), 7.81 (d, J=7.5 Hz, 1H, ArH), 7.65 (t, J=7.7 Hz,
1H, ArH), 7.41−7.44 (dd, J=15.1, 7.7 Hz, 2H, ArH), 7.15 (d,
J=8.3 Hz, 2H, ArH), 5.38 (s, 2H, CH2), 2.23 (s, 3H, CH3);
13C NMR (101 MHz, DMSO) δ/ppm: 169.65, 164.11, 159.57,
153.87, 147.42, 142.23, 141.40, 132.95, 129.60, 126.84,
126.19, 125.28, 119.32, 116.51, 115.66, 66.35, 16.04. IR
(KBr) ν/cm−1: 3245, 3067, 1708, 1651, 1611, 1247, 745;
MS (EI) m/z (%): 382 (M+1, 4), 381 (M, 16), 229 (100),
200 (62), 186 (22), 115 (45), 89 (20); Anal. Calcd. for
C19H15N3O6: C, 59.84; H, 3.96 N, 11.02; O, 25.17. Found:
C, 59.87; H, 3.99; N, 10.98.
Synthesis of the Title Europium Complexes
Synthetic methods of the title europium complexes were sim-
ilar, so the synthetic process of the complex of compound L1
was expressed as example. The compound L1 (0.50 mmol,
0.18 g) was dissolved in 30 mL chloroform in a 100 mL
three-neck flask, then 5 mL europium nitrate ethanol solution
(0.1 mol·L−1) was added. The pH value of the mixture was
adjusted to 6−7 by an aqueous solution of NaOH (1 mol·L−1).
The reaction mixture was heated to reflux for 2 h at 60 °C. The
precipitated was collected by filtration, and washed several
times with chloroform, and then dried in drying oven for 8 h
to give complex with ligand L1. The general synthesis proce-
dures of other complexes with ligands L2−7 are similar to that
of complex with ligand L1.
Results and Discussion
Composition and Physical Properties of the Complexes
Analytical data for the newly synthesized title complexes are
given in Table 1. Compounds L1−7 were easily dissolved in
chloroform, DMSO, DMF, acetone and dichloromethane, but

J Fluoresc
Table 1 The elemental analysis
and molar conductance data of the
europium complexes
Complexes
Found (calculated) (%)
Λm
C
H
N
Eu
(S·m2·mol−1
)
EuL1(NO3)3·2H2O
EuL2(NO3)3·2H2O
EuL3(NO3)3·2H2O
EuL4(NO3)3·2H2O
EuL5(NO3)3·2H2O
EuL6(NO3)3·2H2O
EuL7(NO3)3·2H2O
31.70 (32.11)
30.84 (31.32)
30.12 (30.60)
28.37 (28.90)
29.78 (30.20)
32.01 (32.43)
32.85 (33.15)
2.93 (2.82)
2.78 (2.61)
2.72 (2.55)
2.53 (2.41)
2.66 (2.52)
3.15 (2.97)
3.21 (3.04)
9.80 (9.86)
9.48 (9.61)
9.25 (9.40)
8.81 (8.87)
11.02 (11.12)
9.28 (9.46)
9.55 (9.67)
21.15 (21.41)
20.48 (20.88)
20.05 (20.40)
19.00 (19.26)
19.75 (20.13)
20.11 (20.54)
20.58 (20.99)
16
20
18
12
25
17
22
hardly dissolved in ethanol, methanol and cyclohexane, while
the title Eu(III) complexes were only dissolved in DMSO and
DMF.
The results of elemental analyses are in good agreement
with the theoretical values calculated, indicating that the com-
position of the seven novel europium complexes conformed to
EuL1−7(NO3)3·2H2O. The molar conductivity values of the
title complexes in DMF are lower than that of 1:1 electrolytes
(65–90 S·cm2·mol−1), indicating that all the europium nitrate
complexes conformed to a kind of nonelectrolyte [16].
EuL2(NO3)3·2H2O and EuL4(NO3)3·2H2O as well as their
corresponding ligands are shown in Figs. 1 and 2,
respectively.
In Figs. 1 and 2, the absorption of L2 and L4 were charac-
terized by two main absorption bands in the regions 280−
350 nm. The ultraviolet absorption spectra of L2 and L4 ex-
hibited two peaks at 296 nm, 336 nm and 296 nm, 335 nm,
respectively, which were assigned to the π→π* transition of
aromatic ring and n→π* transition of conjugation between
the lone pair of electrons of p-orbital of N-atom in C = N [18].
The UV spectra of the complexes EuL2(NO3)3·2H2O and
EuL4(NO3)3·2H2O show the characteristic absorption of their
free ligands, which suggests that the coordination of europium
ion does not have a significant influence on the π→π* and
n→π* state energy, and also reveals that the absorption of the
europium complexes is mainly attributed to the ligands, this
maybe due to the f-f transitions are Laporte-forbidden and
very weak in nature [19]. But there are certain changes both
in frequencies and intensities. The peaks of EuL2(NO3)3·
2H2O and EuL4(NO3)3·2H2O appear at 299 nm, 338 nm
and 298 nm, 338 nm are slightly shifted to lower frequencies
compared to that of free ligands. The modifications of the
shifts confirmed the coordination of the ligands to the europi-
um ions.
UV Spectra
The UV spectra of the ligands and their corresponding euro-
pium complexes were carried out in the DMSO solution
(10−4 mol·L−1), the numerical values of the maximum absorp-
tion wavelength (λ) and the molar absorptivities (ε) [17] are
shown in Table 2. Since the UV spectra of all the complexes
exhibit the similar features, only the UV spectra of
Table 2 The UV data of the europium complexes and ligands in
DMSO solution
(10−4 mol·L−1
)
Compounds
λ1
(nm)
ε1 (1.0×104
L·mol−1·cm−1) (nm)
λ2
ε2 (1.0×104
L·mol−1·cm−1
)
L1
296
0.74
0.81
0.67
0.74
0.80
0.86
0.78
0.89
0.71
0.76
0.88
0.99
0.78
1.01
335
337
336
338
336
339
335
338
335
338
336
338
336
338
1.08
1.16
1.00
1.19
1.07
1.13
1.10
1.19
1.10
1.21
1.01
1.10
1.07
1.21
1.2
a
EuL1(NO3)3·2H2O 299
L2
296
EuL2(NO3)3·2H2O 299
L3
297
EuL3(NO3)3·2H2O 299
L4
296
EuL4(NO3)3·2H2O 298
L5
296
EuL5(NO3)3·2H2O 298
L6
296
EuL6(NO3)3·2H2O 298
L7
297
EuL7(NO3)3·2H2O 299
b
0.8
0.4
0.0
270
300
330
360
390
420
450
Wavelength/nm
Fig. 1 The UV-Visible spectra of EuL2(NO3)3·2H2O (a) and L2 (b)

J Fluoresc
TG
DTA
100
80
60
40
20
1.2
0.8
0.4
0.0
90
a
b
60
30
0
-30
100
200
300
400
500
600
700
800
Temperature/oC
270
300
330
360
390
420
450
Wavelength/nm
Fig. 3 TG-DTA curves of the complex EuL6(NO3)3·2H2O
Fig. 2 The UV-Visible spectra of EuL4(NO3)3·2H2O (a) and L4 (b)
FT-IR Spectra
Thermogravimetric Analysis
The IR spectral data for the title europium complexes and their
corresponding ligands L1−7 are listed in Table 4. IR spectra of
all the Eu(III) complexes display the same manner, only the IR
spectra of EuL3(NO3)3·2H2O as well as its corresponding
ligand are selected for illustration in Fig. 4.
The thermal behaviour of the title europium complexes were
investigated by thermal analysis. The thermogravimetric (TG)
and differential thermal analysis (DTA) was carried out in air
atmosphere from ambient temperature up to 800 °C. All the
TG-DTA data of the title complexes are listed in Table 3. The
TG-DTA curves give the similar thermal decomposition be-
havior, only the TG-DTA curve of the complex EuL6(NO3)3·
2H2O is selected for illustration as an example (Fig. 3).
From Fig. 3, it shows that the first mass loss occurrs be-
tween 40 and 200 °C (observed 4.82 %, calc. 4.87 %), and the
DTA curve shows a small endothermic peak at 197 °C. The
result was coincided with the release of two crystal water
molecules. Then, the mass loss between 250 and 280 °C is
49.30 %, and this value is consistent with the calculated value
49.46 % of organic ligand decomposition. In addition, there
are two successive exothermic peaks at 358 and 485 °C, and
the weight loss ratio of residue is 21.83 %, corresponding to
the loss of internally three nitrate molecules. Further heating
two complexes to 800 °C, the weight ratio of residues was
24.35 %, the values basically agree with calculated 23.78 %,
corresponding to the formation of Eu2O3. The thermal analy-
sis results demonstrate that the title complexes have a satisfac-
tory thermal stability.
Figure 4 shows that the IR spectrum of the Eu(III) complex
is different from its corresponding ligand. The IR spectrum of
the free ligand L3 shows a band at 1710 cm−1, which can be
assigned to v(C = O) of the lactone group of coumarin. The
bands at 1654 cm−1 and 1613 cm−1 are attributed to the v(N-C
= O) and v(C = N) of hydrazide group. In the IR spectra of the
complex EuL3(NO3)3·2H2O, the bands for v(C = O) shifted to
1668 cm−1 (Δν=42 cm−1), which indicates that the oxygen
atom of C = O group in lactone takes part in coordination to
the Eu(III) ions. Further, the absorption peaks of v(N-C = O)
and v(C = N) red shift upfield by 28 cm−1 and 29 cm−1, re-
spectively, it suggest that Eu(III) ions bonded with the oxygen
atom of N-C = O and nitrogen atom of C = N. However, the
v(Ar-O-C) absorption band appears at around 1249 cm−1 and
remains unchanged. This is due to the poor coordination abil-
ity of Ar-O-C and the free ligand’s large sterically hindered
effect, which prevents oxygen atom of Ar-O-C group from
coordinating to Eu(III) ions. The absorption bands lying at
1480 cm−1 and 1317 cm−1 are observed in the complex,
Table 3 The thermogravimetric
data of the europium complexes
Complexes
Endothermic peak (°C)
Exothermic peak (°C)
Residue (%) Found (Calcd.)
EuL1(NO3)3·2H2O
EuL2(NO3)3·2H2O
EuL3(NO3)3·2H2O
EuL4(NO3)3·2H2O
EuL5(NO3)3·2H2O
EuL6(NO3)3·2H2O
EuL7(NO3)3·2H2O
68, 196
67, 187
73, 195
67, 200
63, 158
67, 197
65, 183
211, 387, 521
211, 382, 551
219, 385, 532
223, 390,510
250, 405
24.27 (24.78)
24.36 (24.17)
23.91 (23.62)
24.32 (22.30)
25.26 (23.31)
24.35 (23.78)
25.14 (24.31)
271, 358, 485
249, 361, 437,482

J Fluoresc
Table 4 IR spectral data of the
ligands and their europium
complexes
Compounds
lactone
hydrazide
νAr-O-C
νN-H
νO-H
νNO3-
νC=O
νC=O
νC=N
L1
1708
1662
1711
1663
1710
1668
1713
1666
1708
1664
1710
1658
1711
1666
1651
1623
1653
1622
1650
1622
1651
1624
1651
1633
1649
1623
1650
1623
1611
1587
1613
1585
1613
1584
1613
1583
1611
1584
1613
1587
1612
1587
1246
1245
1247
1244
1249
1247
1246
1246
1247
1245
1245
1243
1245
1243
3243
3240
3241
3238
3244
3243
3243
3241
3245
3241
3246
3246
3248
3243
–
3411
–
EuL1(NO3)3·2H2O
L2
EuL2(NO3)3·2H2O
L3
EuL3(NO3)3·2H2O
L4
EuL4(NO3)3·2H2O
L5
EuL5(NO3)3·2H2O
L6
EuL6(NO3)3·2H2O
L7
EuL7(NO3)3·2H2O
1480,1325,1026,819
1481,1322,1028,823
1480,1317,1026,818
1483,1320,1024,815
1479,1305,1028,830
1486,1321,1026,819
1490,1306,1031,816
3415
–
3411
–
3419
–
3417
–
3405
–
3409
which is attributed the asymmetric vibration absorption
(vas) and symmetric vibration absorption (vs). The fre-
quency separation [Δν=|υ1-υ4|] between the asymmetric
and symmetric stretching of this group can be made to
distinction between these binding states. The difference
between the two strongest absorption bands of the ni-
trate groups (|υ1–υ4|) can be defined as Δν. It is gen-
erally believed that the Δν value is below 200 for the
bidentate nitrate moiety, but above 200 for the
monodentate nitrate moiety [20]. The difference between
υ1 and υ4 of EuL3(NO3)3 ·2H2O is approximately
around 526 cm−1 and 430 cm−1 assigned to Eu-O and
Eu-N stretching vibrations [21].
The IR spectrum of the complex exhibits a broad band at
3411 cm−1, indicating that lattice water molecules are existent
in the molecular unit of complexes, which is in agreement
with the elemental analysis and thermogravimetric analysis
discussed in the previous section. So we propose the most
probable coordination structure of the europium complexes
of the ligands shown in Fig. 5.
Luminescence Properties
163 cm−1 which can be suggested that the coordinated
,
NO3− ions in the europium complexes are bidentate co-
ordination. The coordination is further confirmed
through the appearance of two medium intensity bands
The luminescence spectra for the solid complexes EuL1
−7(NO3)3·2H2O were measured at room temperature under a
drive voltage of 700 V, and excitation and emission slit widths
were both 2.5 nm. The luminescence spectral data of the title
complexes are listed in Table 5. Since the luminescence spec-
tra of all the complexes are similar, only the luminescence
spectrum of EuL4(NO3)3·2H2O is selected for illustration,
and the excitation spectra and emission spectra are shown in
Fig. 6.
a
As shown in Table 5, all complexes present the character-
istic luminescence of europium ions, and there is no trace
emission from ligands in the complexes, indicating that the
ligands are comparatively organic chelators to sensitize lumi-
nescence of Eu(III) ions. It is shown in Fig. 6 that the maxi-
mum excitation wavelength of the complex EuL4(NO3)3·
2H2O is observed at 370 nm, due to the π→π* transition
centered at the ligand L4. The complex appears emission
bands at 595, 620 nm with the characteristic properties of
europium ions, which are assigned to 5D0→7F1 and 5D0→7
F2 transitions, respectively. The 5D0→7F1 transition is a mag-
netic transition, which is less affected by the free ligand, and is
b
4000 3500 3000 2500 2000 1500 1000
500
Wavelength/cm-1
Fig. 4 The IR spectra of EuL3(NO3)3·2H2O (a) and L3(b)

J Fluoresc
8000
7000
6000
5000
4000
3000
2000
1000
0
H
N
N
O
Eu
O
2H2O
O
O
O
O
N
O
3
a
b
R
R= H, F, Cl, Br, NO2, OCH3, CH3
350
400
450
500
550
600
650
700
Fig. 5 The most probable coordination structure of the complexes
Wavelength/nm
irrelevant to the symmetry of rare earth ions. In the process of
coordination, ligand field environment has a significant im-
Fig. 6 The excitation spectrum (a) and emission spectrum (b) of
EuL4(NO3)3·2H2O
5
pact on the D0→7F2 transition (electric dipole transition),
will be affected, resulting in that is not completely consistent
with the form of ultraviolet absorption spectra.
which is closely related to the symmetry of the ligand field.
It can be observed that the intensity of the hypersensitive
5D0→7F2 transition is greater than that of the 5D0→7F1 tran-
sition and the former is about 3.78 times higher than the latter,
which suggests that europium ion do not lie in a centro-
symmetric coordination site [22]. The asymmetric microenvi-
ronment causes the polarization of the Eu(III) ions under the
influence of electric field of the surrounding ligands, which
increases the probability for the electric dipole transition [23].
Additionally, this typical narrow and sharp emission peak of
Eu(III) ions can be detected at 620 nm, indicating that the
complex has good monochromaticity and the energy trans-
ferred from the excited triplet state of the ligand to the vibra-
tional state of the europium ion is efficient [24].
By comparison of the luminescence excitation spectra and
the UV absorption spectroscopy of the title europium com-
plexes, their absorption peak position and shape change a
lot. The reason may be that some electrons of complexes mo-
lecular of triplet triplet state (T1) crossing to the triplet state
and then back to the ground state, thus the number of electrons
directly from the excited state back to the ground state is
decreased. The shape of the luminescence excitation spectra
The intensity of the intra-molecular energy transfer be-
tween the triplet levels of the ligand and the emitting level of
europium ions is one of the key factors influencing the lumi-
nescence properties of lanthanide complexes(Antenna effect).
From Table 5, compared with unsubstituted complex
EuL1(NO3)3·2H2O, the luminescence intensities of the com-
plexes with electron-withdrawing groups (F, Cl, Br, NO2) are
much more stronger, and the luminescence intensities of the
complexes with electron-donating groups (OCH3, CH3,) are
decreased. Among the title complexes EuL1−7(NO3)3·2H2O,
the luminescence intensity of the complex EuL4(NO3)3·2H2O
was the highest, which indicated that the triplet level of ligand
L4 was in an appropriate level to center the Eu(III) ion and the
energy transition from ligand L4 to the Eu(III) ion was easier
than with the other ligands. However, the luminescence inten-
sities of EuL6−7(NO3)3·2H2O were rather weaker, this was
because too small the energy levels difference that caused
the energy back to the triplet state of the ligands. All the above
analyses confirmed that the ligand L4 was more suitable than
another ligands to transfer energy to the lowest excited state
level of the Eu(III) ion.
Table 5 The fluorescence spectra data of the europium complexes
Fluorescence Quantum Yield Studies
Complexes
λex/nm 5D0—7F1
5D0—7F2
em / nm I/a.u.
λem / nm I/a.u.
λ
The fluorescence quantum yields (Фfx) of the title europium
complexes were determined by the reference method and cal-
culated by the following equation [25]:
EuL1(NO3)3·2H2O 370
EuL2(NO3)3·2H2O 368
EuL3(NO3)3·2H2O 370
EuL4(NO3)3·2H2O 371
EuL5(NO3)3·2H2O 370
EuL6(NO3)3·2H2O 371
EuL7(NO3)3·2H2O 369
595
595
594
595
595
595
595
525.5 6202
2176.3
2195.7
4875.2
7944.3
3041.8
1607.6
1143.5
631.5
1316.1
2100.3
794.5
619
619
620
620
620
619
n2x Fx
A
Φfx
¼
⋅
⋅
std⋅Φfstd
n2std
Fstd Ax
In the formula, subscripts std and x refer to the standard and
the unknown, respectively, Where, nx and the nstd are the refrac-
tive indices of sample and standard (nx=1.480, nstd =1.337),
456.5
313.7

J Fluoresc
Table 7 The EHOMO, ELUMO and Eg of the europium complexes
respectively. F is the area of fluorescence integral, and A is the
absorbance at the exciting wavelength of UV absorption. Both
the sample and standard are excited at the same relevant wave-
length, so that the Astd is equivalent to Ax. The fluorescence
spectral data of EuL1−7(NO3)3·2H2O were measured at room
temperature in DMSO (10−6 M) solution with excitation and
emission slit widths were 5.0 nm, and the calculated results of
Фfx of the title complexes are listed in Table 6.
Complexes
λonset
(nm)
EOX
(V)
EHOMO
(eV)
Eg
(eV)
ELUMO
(eV)
EuL1(NO3)3·2H2O 297
EuL2(NO3)3·2H2O 296
EuL3(NO3)3·2H2O 298
EuL4(NO3)3·2H2O 296
EuL5(NO3)3·2H2O 298
EuL6(NO3)3·2H2O 297
EuL7(NO3)3·2H2O 298
0.701 5.441
0.679 5.419
0.685 5.425
0.693 5.433
0.667 5.407
0.709 5.449
0.712 5.452
4.175 1.266
4.189 1.230
4.161 1.264
4.189 1.244
4.161 1.246
4.175 1.274
4.161 1.291
As shown in Table 6, the change rule of the luminescence
intensities of different complexes recorded in solid is consistent
with that in liquid, and the same as the data of fluorescence
quantum yields. The fluorescence quantum yields of EuL2
−5(NO3)3·2H2O with electron-withdrawing groups (F, Cl, Br,
NO2) are higher than that of EuL1(NO3)3·2H2O, and halogen-
containing complexes show higher fluorescence quantum
yields, EuL4(NO3)3·2H2O shows the highest quantum yield
(0.562), which is due to the fact that L4 possesses the stronger
p-π conjugation effect compared with L3 and L2, and the triplet
energy level of bromine-substituted ligand matches better to the
excited state level of Eu(III). On the contrary, the introduction
of the electron-donating group (CH3, OCH3) in the ligands can
decrease the fluorescence quantum yields of the complexes
EuL6−7(NO3)3·2H2O, and the fluorescence quantum yield of
the complex with methyl substituent is the lowest 0.259, The
reason is that the electron density of the phenyl rings increases
with improvement of the triplet energy level of free ligands, so
the energy gap between the triplet energy level and the excited
state level of Eu(III) became larger. From the above discus-
sions, we conclude that all the title complexes EuL1−7(NO3)3·
2H2O possess relatively good fluorescence quantum yields.
summarized in Table 7 and the cyclic voltammetry curve of
EuL1(NO3)3·2H2O is depicted in the Fig. 7.
As shown in Table 7, the oxidation potential and HOMO
and LOMO energy levels of EuL2−5(NO3)3·2H2O are lower
than that of EuL1(NO3)3·2H2O. The reason is that the intro-
duction of electron-withdrawing groups can reduce the elec-
tron cloud density on the benzene ring, weaken the betatopic
ability of the title complexes, thus the oxidation potential re-
duced. The HOMO energy levels and the oxidation potential
of halogen atom-substituted europium complexes are deter-
mined in the following order: F<Cl<Br. The oxidation poten-
tial and HOMO and LOMO energy levels of EuL6−7(NO3)3·
2H2O are higher than that of EuL1(NO3)3·2H2O, this stems
from the fact that the introduction of electron-donating group
will increase oxidation potential. The energy gap of europium
complexes range from 4.161 to 4.189 eV, which indicates that
the electron-donating groups and electron-withdrawing
groups have no apparent effect on energy gap. However, the
substituent type of the title complexes has a certain degree of
influence on Eox, HOMO and LOMO energy levels. Accord-
ing to the discussion above, the introduction of electron-
Electrochemical Properties
To study the electrochemical properties of the title europium
complexes, cyclic voltammetry (CV) experiments were con-
ducted in DMSO solution and the oxidation potential EOX of
the complexes were measured. The HOMO and LUMO ener-
12
8
gy levels are calculated using equations EHOMO=4.74+eEOX
,
ELUMO=EHOMO−Eg, Eg=1240/λonset (eV), where λonset was
the starting value of the largest UV–Vis absorption spectra
peak [26]. The electrochemical data of the complexes are
4
Table 6 The fluorescence quantum yields of the europium complexes
0
Complexes
λex (nm)
I (a.u.)
Фfx
EuL1(NO3)3·2H2O
EuL2(NO3)3·2H2O
EuL3(NO3)3·2H2O
EuL4(NO3)3·2H2O
EuL5(NO3)3·2H2O
EuL6(NO3)3·2H2O
EuL7(NO3)3·2H2O
310
311
309
312
312
311
310
851
868
0.466
0.471
0.536
0.562
0.493
0.318
0.259
-4
1322
1638
933
-8
-2.4
-1.6
-0.8
0.0
0.8
1.6
535
Potential/V
511
Fig. 7 The CV curve of EuL2(NO3)3·2H2O

J Fluoresc
using a tridentate N, N, O-ligand for near-infrared organic light-
emitting diodes. Dalton Trans 42:8951–8960
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Conclusions
Seven new coumarin derivatives have been designed and syn-
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series of stable solid complexes with europium ions. The co-
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oxygen atom of the coumarin, one nitrogen atom and one
oxygen atom of acyl hydrazone. Thermal study indicates that
title europium complexes are thermally stable. The lumines-
cent test results revealed that all the title Eu(III) complexes
emitted characteristic luminescence of Eu(III) ions and the
complex of the ligand with bromo-substituted group possesses
the strongest luminescence intensity and the highest fluores-
cence quantum yield among all the title complexes. The intro-
duction of electron-donating groups to the ligands can in-
crease the HOMO and LOMO energy levels, the oxidation
potential of the title Eu(III) complexes. However, the result
of introduction of electron-withdrawing groups was just op-
posite. These results demonstrate that the title complexes are
expected to be used as luminescent materials in various fields.
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Acknowledgments 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 Univer-
sity (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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