Synthesis and Fluorescence Properties of Eu3+, Tb3+ Complexes with Schiff Base Derivatives

Article abstract of DOI:10.1007/s10895-015-1741-8

Novel Schiff base ligands derived from N′-benzylidene-benzohydrazide (substituted by-H,-CH₃,-OCH₃,-Cl) and 2-chloro-N-phenylacetamide were synthesized. The solid complexes of rare earth (Eu, Tb) nitrate with these Schiff base ligands

Full text of DOI:10.1007/s10895-015-1741-8

J Fluoresc
DOI 10.1007/s10895-015-1741-8
ORIGINAL ARTICLE
Synthesis and Fluorescence Properties of Eu3+, Tb3+ Complexes
with Schiff Base Derivatives
Yanhong Liu1,2 & Weihua Kong1 & Zehui Yang3 & Ming Dai1 & Ling Shi1 & Dongcai Guo1,2
Received: 30 June 2015 /Accepted: 7 December 2015
#
Springer Science+Business Media New York 2015
Abstract Novel Schiff base ligands derived from N′-
benzylidene-benzohydrazide (substituted by –H, −CH3,
−OCH3, −Cl) and 2-chloro-N-phenylacetamide were synthe-
sized. The solid complexes of rare earth (Eu, Tb) nitrate with
these Schiff base ligands were synthesized and characterized
by elemental analysis, EDTA titrimetric analysis, ther-
mal analysis, infrared spectra and UV–Vis spectra anal-
ysis. The fluorescence properties of rare earth (Eu, Tb)
complexes in solid were studied. Under the excitation of
ultraviolet light, these complexes exhibited characteristic
emission of europium and terbium ions. The results
showed that the ligand favored energy transfer to the
emitting energy of Eu and Tb ions. Effects of different
ligands on the fluorescence intensity of rare earth (Eu,
Tb) complexes had been discussed. The electrochemical
properties of rare earth (Eu, Tb) complexes were also
investigated.
Abbreviations
UV–Vis Ultraviolet visible
EDTA
Ethylenediaminetetraacetic acid
HOMO Highest occupied molecular orbital
LUMO Lowest unoccupied molecular orbital
Introduction
The study of the rare earth complex continues to be an
active research area, which is attributed to its applica-
tion as optical telecommunication devices, as well as
biological imaging probes and sensors [1–4], due to
characteristic spectroscopic and magnetic properties of
rare earth ions. The fluorescence rare earth complexes
are of special interest due to their narrow emission
bands, a large stokes’ shift. The emission.
mechanism of rare earth ion is based on f–f transition,
which results in emission bands with extremely narrow band-
width and no theoretical cap on the quantum efficiency.
The f–f transition forbidden by parity (Laporte) selec-
tion rules lead to weak absorbance and low quantum
yields, so a suitable chromophoric moiety (antenna)
[5–7] is helpful to trigger their emission through effi-
ciently energy transfer from the ligand to the rare earth
ion. Our work stems from our interest in the study of
coordination behaviors of rare earth ion with Schiff base
ligands. Schiff base derivative containing various donor
atoms such as nitrogen, oxygen, which enhances the
coordination of these ligands with rare earth ions and
even yields stable metal complex [8–13].
.
.
Keywords Schiff base Rare earth complexes Fluorescence
properties
* Dongcai Guo
dcguo2001@hnu.edu.cn
1
School of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, China
2
Hunan Provincial Key Laboratory for Cost-effective Utilization of
In this paper, novel Schiff bases ligands have been synthe-
sized, as shown in Scheme 1. The Schiff bases ligands and
their complexes have been characterized by FT–IR, 1H NMR,
UV–Vis spectroscopy, elemental analysis. The contribution of
Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions,
Changsha 410082, China
3
School of Chemical Engineering, Ningbo University of Technology,
Ningbo 315016, China

J Fluoresc
CHO
R
Scheme 1 The synthetic routes
of phenylacetamide derivatives
L1–4 and rare earth (Eu,
Tb)complexes
O
COOCH2CH3
NH2NH2.H2O
CONHNH2
N
R
N
H
a
c
b1-4
NH2
NHCOCH2Cl
ClCOCH2Cl
R
O
O
R
NHCOCH2Cl
N
N
H
NH
N
N
O
c
b1-4
L1-4
RE Complexes
RE3+
R1= H, R2= CH3, R3= OCH3, R4=Cl
RE= Eu; Tb
different ligands to the fluorescence properties can be prelim-
inarily understood.
General Procedure for Synthesis of Benzohydrazide (a)
Ethyl benzoate (100 mmol, 15.0 g) was dissolved in absolute
ethyl alcohol (30 mL) in round bottom flask. Hydrazine hy-
drate 80 % (20 mL) was added drop wise to the solution and
refluxed for 6 h at 80 °C. After completion of reaction, the
mixture was cooled to rt. for 12 h. The precipitate was filtered,
dried and recrystallized in ethanol.
Experimental Section
Materials and Methods
All the chemicals used were of the analytical grade (AR) and
of highest purity.
General Procedure for Synthesis of Hydrazones (b1–4
)
The rare earth ions were determined by EDTA titration
using xylenol oranges as an indicator. Elemental analyses
(C, N, and H) was measured on a Vario–EL–III elemental
analyzer. The molar conductivity measurements were carried
out in acetone using DDS–12A digital direct reading conduc-
tivity meter. 1H NMR and spectra were recorded on a Bruker–
400 spectrometer (solvent: DMSO–d6/CDCl3, internal refer-
ence: TMS). Mass spectra were recorded on a MAT95XP
apparatus. UV–Vis absorption spectra were carried out on a
LabTech UV–2100 spectrophotometer. IR spectra were re-
corded on a PERKIN–ELMER Spectrum One spectrometer
by dispersing samples in KBr pellets. Melting points were
determined with XT–4 binocular microscopy melting point
apparatus. The thermal analysis were performed on a DTG–
60 thermal analyzer operating at a heating rate of 20 °C/min in
the range of 40 °C up to 800 °C under in air. Fluorescence
spectra were obtained on a HIACHI F–2700 spectrophotom-
eter at a scanning rate of 1500 nm/min, at room temperature,
and the voltage was 700 V. The electrochemical properties of
the complexes were got by the cyclic voltammetry data on a
CHI 660d electrochemical workstation.
A mixture of (50 mmol, 6.80 g) benzohydrazine (a) and ap-
propriate substituted aromatic aldehyde (50 mmol) was
refluxed in ethanol (30 mL) in the presence of a catalytic
amount of glacial acetic acid for about 4 h. The progress of
the reaction was monitored through TLC. The resulting mix-
ture was cooled to rt. The precipitates obtained was filtered
off, washed with ethanol, dried and recrystallized in ethanol to
afford the corresponding hydrazone derivatives. Structures of
the synthesized compounds were confirmed on the basis of
spectral data.
N′-benzylidene-benzohydrazide (b1) White solid, Yield
90 %, 1 H NMR (400 MHz, DMSO–d6) δ/ppm: 11.82 (s,
1 H, CH = N), 8.47 (s, 1 H, NH), 7.92 (d, J = 7.1 Hz, 2 H,
ArH), 7.73 (s, 2 H, ArH), 7.63–7.56 (m, 1 H, ArH), 7.52 (d,
J = 14.1 Hz, 2 H, ArH), 7.46 (d, J = 6.3 Hz, 3 H, ArH); MS
(EI) m/z (%):225 (m + 1, 1), 224 (m, 5), 223 (m-1, 1), 123 (1),
122 (3), 121 (36), 105 (100), 103 (4), 77 (32), 65 (4), 51 (9).
N′-(4-methylbenzylidene)benzohydrazide (b2) White sol-
1
id, Yield 87 %, H NMR (400 MHz, DMSO–d6) δ/ppm:
11.77 (s, 1 H, CH = N), 8.42 (s, 1 H, NH), 7.91 (d,

J Fluoresc
J = 7.4 Hz, 2 H, ArH), 7.63 (d, J = 7.9 Hz, 2 H, ArH), 7.58 (d,
J = 7.0 Hz, 1 H, ArH), 7.52 (t, J = 7.4 Hz, 2 H, ArH), 7.28 (d,
J = 7.7 Hz, 2 H, ArH), 2.35 (s, 3 H, CH3); MS (EI) m/z (%):
239 (m + 1, 2), 238 (m,9), 237 (m-1, 1), 121 (2), 105 (100), 78
(6), 77 (34), 76 (2), 51 (5).
(400 MHz, CDCl3) δ/ppm: 8.61 (s, 1 H, CH = N), 8.19 (s,
1 H, NH), 7.88 (d, J = 7.3 Hz, 2 H, ArH), 7.59–7.45 (m, 7 H,
ArH), 7.38–7.30 (m, 5 H, ArH), 7.12 (t, J = 7.4 Hz, 1 H, ArH),
4.94 (s, 2 H, CH2); IR (KBr) v/cm−1: 3283, 2361, 1691, 1632,
1612, 1556, 1447, 1423, 1348, 1136, 1078, 754, 690; MS (EI)
m/z (%): 358 (m + 1,3), 357 (m, 8) 267 (1), 266 (5), 265 (28),
135 (1), 134 (5), 106 (9), 105 (100), 93 (24), 89 (6), 77 (35),
76 (3), 51 (6); Anal. Calcd for C22H19N3O2: C, 73.93; H, 5.36;
N, 11.76. Found: C, 73.78; H, 5.26; N, 11.89.
N′-(4-methoxybenzylidene)benzohydrazide (b3) White
1
solid, Yield 89 %, H NMR (400 MHz, DMSO–d6) δ/ppm:
11.70 (s, 1 H, CH = N), 8.40 (s, 1 H, NH), 7.90 (d, J = 7.5 Hz,
2 H, ArH), 7.68 (d, J = 8.5 Hz, 2 H, ArH), 7.61–7.55 (m, 1 H,
ArH), 7.52 (t, J = 7.3 Hz, 2 H, ArH), 7.03 (d, J = 8.5 Hz, 2 H,
ArH), 3.81 (s, 3 H, OCH3); MS (EI) m/z (%): 256 (m + 2, 1),
255 (m + 1, 3), 254 (m, 16), 135 (3), 134 (14), 133 (100), 106
(9), 105 (92), 78 (5), 77 (38), 76 (5), 51 (7).
2-(N-Benzoyl-N’-benzylidene-hydrazino)-N-p-tolyl-acet-
1
amide (L2) Yield 54 %, m.p. 151–153 °C; H NMR
(400 MHz, CDCl3) δ/ppm: 8.65 (s, 1 H, CH = N), 8.16 (s,
1 H, NH), 7.88 (d, J = 7.4 Hz, 2 H, ArH), 7.50 (ddd, J = 26.7,
16.6, 7.7 Hz, 7 H, ArH), 7.32 (t, J = 7.6 Hz, 2 H, ArH), 7.13
(dd, J = 17.0, 7.6 Hz, 3 H, ArH), 4.93 (s, 2 H, CH2), 2.35 (s,
3 H, CH3); IR (KBr) v/cm−1: 3283, 2359, 1691, 1632, 1614,
1556, 1446, 1427, 1346, 1138, 1078, 752, 692; MS (EI) m/z
(%): 372 (m + 1, 3), 371 (m, 12), 281 (1), 280 (5), 279 (16),
119 (8), 118 (12), 105 (100), 93 (32), 91 (7), 77 (31), 65 (4);
Anal. Calcd for C23H21N3O2: C, 74.37; H, 5.70; N, 11.31.
Found: C, 74.21; H, 5.64; N, 11.50.
N′-(4-chlorobenzylidene)benzohydrazide (b4) White solid,
Yield 82 %, 1H NMR (400 MHz, DMSO–d6) δ/ppm: 11.90 (s,
1 H, CH = N), 8.45 (s, 1 H, NH), 7.91 (d, J = 7.3 Hz, 2 H,
ArH), 7.76 (d, J = 8.0 Hz, 2 H, ArH), 7.59 (d, J = 7.1 Hz, 1 H,
ArH), 7.52 (d, J = 6.0 Hz, 4 H, ArH); MS (EI) m/z (%): 261
(m + 3, 1), 260 (m + 2, 6), 258 (m, 14), 257 (m-1, 2), 121 (37),
105 (100), 90 (2), 89 (5), 77 (25), 63 (3), 51 (4).
General Procedure for Synthesis
of 2-Chloro-N-Phenylacetamide (c)
2-(N-Benzoyl-N’-benzylidene-hydrazino)-N-(4-methoxy-
1
phenyl)-acetamide (L3) Yield 62 %, m.p. 143–144 °C;
H
NMR (400 MHz, CDCl3) δ/ppm: 8.67 (s, 1 H, CH = N),
8.15 (s, 1 H, NH), 7.87 (d, J = 7.3 Hz, 2 H, ArH), 7.58–7.44
(m, 7 H, ArH), 7.32 (t, J = 7.9 Hz, 2 H, ArH), 7.12 (t,
J = 7.4 Hz, 1 H, ArH), 6.87 (d, J = 8.7 Hz, 2 H, ArH), 4.92
(s, 2 H, CH2), 3.81 (s, 3 H, CH3); IR (KBr) v/cm−1: 3277,
2363, 1687, 1636, 1609, 1556, 1446, 1429, 1350, 1136, 1080,
758, 694; MS (EI) m/z (%): 388 (m + 1, 2), 387 (m, 7), 296
(1), 295 (6), 135 (15), 134 (14), 133 (17), 120 (18), 105 (100),
93 (37), 91 (12), 65 (6), 51 (10); Anal. Calcd for C23H21N3O3:
C, 71.30; H, 5.46; N, 10.85. Found: C, 71.14; H, 5.36; N,
10.89.
Aniline (50 mmol, 4.65 g) was dissolved in glacial acetic acid
(40 ml), the solution was cooled in ice bath with stir-
ring. Chloracetylchloride (50 mmol, 5.60 g) was added
dropwise to avoid the vigorous reaction. After 0.5 h, it
was moved to react at room temperature for 1 h. Then
it was poured to 200 mL saturated solution of sodium
acetate. The precipitate was filtered, washed with water,
recrystallized from alcohol-water solution to afford
white product. Yield 87 %.
General Procedure for Synthesis of Ligands L1-4
2-(N-Benzoyl-N’-benzylidene-hydrazino)-N-(4-chloro-
1
phenyl)-acetamide (L4) Yield 72 %, m.p. 151–152 °C;
H
Hydrazones (b1–4) (20 mmol), anhydrous K2CO3 (50 mmol,
6.90 g) and dimethyl formamide (20 mL) were warmed to
85 °C, then 2-chloro-N-phenylacetamide (c) (20 mmol,
3.38 g) in dimethyl formamide (10 mL) and a little KI were
added slowly. The mixture was refluxed for 3 h. After cooling
down, it was poured into distilled water (300 mL), and
then diluted hydrochloric acid (2 mol/L) was added un-
til the pH value was 3–4 for a further 10 min. The
crude final product was filtered with suction, washed
several times with cold water, and dried. The crude product
was dissolved in appropriate dichloromethane, filtered,
washed with a little dichloromethane and recrystallized from
suitable solvents to obtain white flocculent solids [15]. The
yield, melting point, and spectral data of each compound are
given below.
NMR (400 MHz, CDCl3) δ/ppm: 8.62 (s, 1 H, CH = N),
8.16 (s, 1 H, NH), 7.90–7.77 (m, 2 H, ArH), 7.52 (ddd,
J = 19.9, 16.6, 8.1 Hz, 7 H, ArH), 7.38–7.27 (m, 4 H, ArH),
7.12 (t, J = 7.4 Hz, 1 H, ArH), 4.92 (s, 2 H, CH2); IR (KBr)
v/cm−1: 3287, 2372, 1699, 1638, 1612, 1556, 1447, 1427,
1342, 1140, 1089, 754, 696; MS (EI) m/z (%): 393(m + 2,
2), 391 (m, 4), 302 (1), 301 (6), 299 (19), 298 (1), 106 (9), 105
(100), 93 (16), 89 (3), 77 (27), 51 (4); Anal. Calcd for
C22H18ClN3O2: C, 67.43; H, 4.63; N, 10.72. Found: C,
67.31; H, 4.57; N, 10.86.
Preparation of the Complexes
An ethanol solution (5 mL) of RE(NO3)3·6H2O (RE = Eu, Tb)
(1 mmol) was added dropwise to a solution of 1 mmol
ligand L1–4 in the chloroform (30 mL). The mixture
was refluxed at 80 °C for 4 h. And then the pH value
2-(N-Benzoyl-N’-benzylidene-hydrazino)-N-phenyl-acet-
1
amide (L1) Yield 64 %, m.p. 147–148 °C; H NMR

J Fluoresc
Table 1 Elementary analysis and
conductivity data of Eu(III),
Tb(III) complexes
Complex
Measured value (Theoretical value %)
Λm
(S·cm2 mol−1
)
C
H
N
RE
[Eu(NO3)2L1]NO3·2H2O
[Eu(NO3)2L2]NO3·2H2O
[Eu(NO3)2L3]NO3·2H2O
[Eu(NO3)2L4]NO3·2H2O
[Tb(NO3)2L1]NO3·2H2O
[Tb(NO3)2L2]NO3·2H2O
[Tb(NO3)2L3]NO3·2H2O
[Tb(NO3)2L4]NO3·2H2O
36.01 (36.11)
36.87 (37.05)
36.12 (36.27)
34.46 (34.51)
35.66 (35.77)
36.54 (36.70)
35.82 (35.93)
34.16 (34.20)
3.06 (3.15)
3.19 (3.35)
3.05 (3.28)
2.75 (2.87)
3.09 (3.12)
3.19 (3.32)
3.15 (3.25)
2.85 (2.85)
11.23 (11.49)
11.05 (11.27)
10.94 (11.04)
10.84 (10.98)
11.23 (11.38)
11.05 (11.17)
10.84 (10.94)
10.84 (10.88)
20.85 (20.79)
20.52 (20.40)
20.23 (19.97)
19.98 (19.87)
21.75 (21.54)
21.32 (21.14)
20.83 (20.70)
20.51 (20.59)
70
72
75
81
77
72
75
75
was adjusted to 6.0–6.5 by a solution of sodium
ethoxide in ethanol (1 mol/L). The precipitated solid
complex was filtered, washed with hot chloroform, dried
in vacuum for 48 h.
are 1:1 ionic compounds [20]. From the analyses above, compo-
sition of the complexes can be inferred: [RE(NO3)2 L1–4]NO3·
2H2O (RE = Eu, Tb).
IR Spectral Analyses
Results and Discussion
The main IR peaks of the ligands and their Eu(III), Tb(III)
complexes are presented in Table 2. From Figs. 1 and 2, the
IR spectrum of the free ligand L4 showed strong absorption
band at 1699 cm−1, which was assigned to the stretch vibration
of the carbonyl group [ν(C = O)]. In the nitrate complexes, the
red shifts to about 1653 cm−1(Δν = 46 cm−1) and
1647 cm−1(Δν = 52 cm−1) in [Eu(NO3)2L4]NO3·2H2O,
1647 cm−1(Δν = 52 cm−1) and 1636 cm−1(Δν = 63 cm−1)
in [Tb(NO3)2L4]NO3·2H2O, indicating that the oxygen atom
of C = O and N–C = O took part in coordination to RE (Eu3+,
Tb3+) ions, respectively. It resulted in the equalization of elec-
tron cloud on the C = O group, then the absorption frequencies
moved to the low wave number due to the decrease of the
Properties of Target Complexes
Analytical data for the complexes, presented in Table 1, indi-
cated that the nitrate complexes conformed to a 1:1 metal–to–
ligand stoichiometry [RE(NO3)3 L1–4]·2H2O. The compounds
L1–4 were white powers and soluble in dichloromethane, all
the nitrate complexes were white powers and slightly soluble
in DMSO, DMF, insoluble in benzene and diethyl ether. The
molar conductance data of these complexes in DMF
solution (1 × 10−3 mol·L−1) was in the range of 70–
81 S·cm2 mol−1, which indicated that all complexes
Table 2 IR data (cm−1) of ligands L1−4 and their Eu(III), Tb(III) complexes
−
Compounds
ν(O-H)
ν(N-C = O)
ν(C = O)
ν(NO3
)
ν1
ν2
ν3
ν4
ν
|ν1-ν4|
L1
2361
2359
2357
2359
2360
2355
2363
2359
2355
2372
2360
2359
1691
[Eu(NO3)2L1]NO3·2H2O
[Tb(NO3)2L1]NO3·2H2O
L2
[Eu(NO3)2L2]NO3·2H2O
[Tb(NO3)2L2]NO3·2H2O
L3
[Eu(NO3)2L3]NO3·2H2O
[Tb(NO3)2L3]NO3·2H2O
L4
[Eu(NO3)2L4]NO3·2H2O
[Tb(NO3)2L4]NO3·2H2O
3524
3524
1653,1647
1651,1645
1691
1456
1456
1041
1045
835
835
1362
1362
1385
1385
94
94
3524
3524
1653,1647
1651,1645
1687
1458
1456
1041
1043
837
835
1362
1362
1385
1385
96
94
3524
3500
1653,1647
1651,1645
1699
1456
1454
1041
1041
837
835
1362
1362
1385
1385
94
92
3503
3524
1653,1647
1647,1636
1456
1456
1041
1041
835
835
1362
1362
1385
1385
94
94

J Fluoresc
Table 3 UV-Vis spectra data of ligands L1−4 and their Eu(III), Tb(III)
complexes
b
compounds
λ1 (nm)
λ2 (nm)
L1
267
265
266
268
266
264
266
265
267
268
265
264
298
[Eu(NO3)2 L1]NO3·2H2O
[Tb(NO3)2 L1]NO3·2H2O
L 2
[Eu(NO3)2 L2]NO3·2H2O
[Tb(NO3) L2 ]NO3·2H2O
—
a
a
—
302
—
—
2
L3
305
—
[Eu(NO3)2 L3]NO3·2H2O
[Tb(NO3)2 L3]NO3·2H2O
L 4
[Eu(NO3)2 L4]NO3·2H2O
[Tb(NO3)2 L4]NO3·2H2O
—
4000
3500
3000
2500
2000
1500
1000
500
305
—
Wavenumber/cm-1
Fig. 1 IR spectra of the ligand L4 (a) and the complex [EuL4(NO3)2]
NO3·2H2O (b)
—
a —, No absorption peak
force constant. Meanwhile, the stretch vibration of triple bond
[ν(N–C = O)] shifted from 2372 cm−1 in L4 to 2360 cm−1 in
[Eu(NO3)2L4]NO3·2H2O (Δν = 8 cm−1), and to 2359 cm−1 in
[Tb(NO3)2L4]NO3·2H2O (Δν = 13 cm−1), respectively. This
was also confirmed that the oxygen of N–C = O was involved
in the coordination.
The characteristic frequencies of the coordinating nitrate
groups (C2v) in [Eu(NO3)2L4]NO3·2H2O and [Tb(NO3)2L4]
NO3·2H2O both appeared at about 1456 cm−1 (ν1),
1041 cm−1 (ν2), 835 cm−1(ν3), and 1362 cm−1 (ν4) and the
difference between the two highest frequency bands (|ν1-ν4|)
was 94 cm−1, indicating that the coordinated nitrate groups in
the complexes were bidentate state [14, 19, 21]. The free ni-
trate groups (D3h) appeared at ca. 1385 cm−1 in the spectra of
the complexes, which explained that one nitrate group can not
participate in the coordination. Additionally, broad bands at
3503 cm−1 in [Eu(NO3)2L4]NO3·2H2O and 3524 cm−1 in [Tb
(NO3)2L4]NO3·2H2O indicated that H2O were existent in Eu
(III) and Tb(III) complexes, confirming the elemental analy-
sis. These results confirmed that the coordination bond was
formed between the ligand L4 and the Eu(III), Tb(III) ions,
respectively.
UV–Vis Spectral Analyses
UV–vis spectra of rare earth complexes (Eu, Tb) and their
corresponding Schiff base ligands were recorded in DMSO
solution (1 × 10−5 mol·L−1), the data are listed in Table 3.
Two absorption peaks of the free ligand L4 appeared at 268
and 305 nm, which were assigned to the π → π* and n → π*
transitions of the ligand, as shown in Fig. 3. However, the first
strong absorption wavelength of [Eu(NO3)2L4]NO3·2H2O
was 265 nm, while the second was disappeared. This was
b
0.4
a
0.3
0.2
a
0.1
b
0.0
-0.1
-0.2
-0.3
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber/cm-1
260
280
300
320
340
360
380
400
Fig. 2 IR spectra of the ligand L4 (a) and the complex [TbL4(NO3)2]
NO3·2H2O (b)
Wavelength/nm
Fig. 3 UV–Vis spectra of L4 (a) and [Eu(NO3)2L4]NO3·2H2O (b)

J Fluoresc
Table 4 TG–DTA data of Eu(III), Tb(III)complexes
Complexes Endothermic Exothermic
R
Residual
Weight
Peak(°C)
Peak(°C)
(Theoretical
Value)(%)
N
O
[Eu(NO3)2L1]NO3·2H2O 109, 566
[Eu(NO3)2L2]NO3·2H2O 110, 567
[Eu(NO3)2L3]NO3·2H2O 106, 571
[Eu(NO3)2L4]NO3·2H2O 117, 574
[Tb(NO3)2L1]NO3·2H2O 103, 569
[Tb(NO3)2L2]NO3·2H2O 100, 571
[Tb(NO3)2L3]NO3·2H2O 106, 576
[Tb(NO3)2L4]NO3·2H2O 107, 573
415, 607
418, 610
420, 617
418, 620
411, 618
417, 619
420, 621
416, 611
24.09 (24.08)
23.11 (23.62)
29.84 (23.13)
29.79 (23.01)
25.01 (25.34)
24.51 (24.87)
24.48 (24.35)
24.17 (24.22)
N
N
O
O
O
NO3 · 2H2O
RE
O
O
N
O
NH
O
because that the steady coordinate bond changed the distribu-
tion of electron cloud.
R = H; CH3; OCH3; Cl
RE = Eu;Tb
Fig. 5 The molecular structure of rare earth (Eu, Tb) complexes
Thermal Analyses
Thermal properties were evaluated by means of DTA and TG
in air. The results are presented in Table 4. Figure 4 showed
that the [Eu(NO3)2L1]NO3·2H2O started to decompose at
100–180 °C, and the mass loss (4.83 %) was match well with
the theoretical value (4.92 %), indicating that there were two
H2O molecules in the complex. The TG graph showed that the
ligand L1 was lost at 200–480 °C, and the mass loss (42.71 %)
was in according to the theoretical value (48.84 %). Three
Fluorescence Properties
The fluorescence properties of the ligand and rare earth (Eu,
Tb) complexes are investigated. The excitation and emission
spectra of all the samples were measured at room temperature
in the solid state. The results are presented in Tables 5, 6, 7 and
Figs. 6, 7 and 8.
Under the excitation of UV light, the L1–4 exhibited blue
light. From Fig. 6, it was obvious that peaks shape were sim-
ilar to each other, the excitation wavelength of the ligand was
338.0 nm, and emission intensity was decreased in the order
L1 > L4 > L2 > L3. The fluorescence intensity of the ligands
L2–4 were decreased, mainly due to the effects of substituent
(−CH3, −OCH3, −Cl) on the fluorescence quantum efficiency.
The substituent changed the conjugated π–system, and the
smaller the π–system, the weaker the fluorescence intensity.
Importantly, the substituent decreased molecular rigidity
and increased the molecular vibration, thus it enhanced
the probability of deactivation due to the collisions with
−
NO3 were lost at 480–661 °C accompanying with exother-
mic peaks at 607 °C. The experimental mass loss (26.64 %)
was close to the calculated value (25.44 %). When the com-
plex was completely decomposed into rare earth oxide, the
residual weigh was 24.08 %, which was very close to the
experimental result (24.09 %).
From the analyses (IR spectra, elemental analysis, and ther-
modynamic analysis) above we can deduce the composition
and molecular structure of the complexes, as shown in Fig. 5.
110
100
90
80
70
60
50
40
30
20
50
40
30
20
10
0
Table 5 Fluorescence spectral data of ligands L1−4
Compound Ex Slit (nm) Em Slit (nm) λexa(nm) λemb(nm) RLIC
L1
L2
L3
L4
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
338.0
338.0
338.0
338.0
435.0
443.5
439.5
430.0
6483
2058
1792
2389
-10
-20
a The maximum fluorescence excitation wavelength
b The fluorescence characteristic emission wavelength
C Relative fluorescence intensity
100
200
300
400
T/
500
600
700
800
Fig. 4 TG-DTA of [Eu(NO3)2L1]NO3·2H2O

J Fluoresc
Table 6 Fluorescence spectral data of Eu (III) complexes
Compound
Ex Slit (nm)
5.0
Em Slit (nm)
2.5
λ
exa(nm)
λemb(nm)
RLIC
Assignment
[Eu(NO3)2L1]NO3·2H2O
396.0
396.0
396.0
396.0
595.0
618.5
595.0
618.5
595.0
618.5
595.0
618.5
1412
1184
1733
1462
1316
1143
2287
1859
5D0 → F1
7
7
5D0 → F2
[Eu(NO3)2L2]NO3·2H2O
[Eu(NO3)2L3]NO3·2H2O
[Eu(NO3)2L4]NO3·2H2O
5.0
5.0
5.0
2.5
2.5
2.5
5D0 → F1
7
7
5D0 → F2
7
5D0 → F1
7
5D0 → F2
7
5D0 → F1
7
5D0 → F2
a The maximum fluorescence excitation wavelength
b The fluorescence characteristic emission wavelength
C Relative fluorescence intensity
other molecules, and decreased the fluorescence quan-
tum efficiency.
of the 5D0 → 7F1, and the ratio (5D0 → 7F2/5D0 → 7F1) of the
complexes [Eu(NO3)2 L1–4]NO3·2H2O were 0.77, 0.85, 0.87,
0.81. Thus, the central Eu3+ ion was located in a symmetric
coordination site [18].
From Table 6 and Fig. 7, peaks shape of all Eu(III) com-
plexes were similar to each other. The excitation wavelength
of all complexes were at 396.0 nm, the emission wavelength at
595.0 and 618.5 nm, which were assigned to the magnetic
As shown in Table 7 and Fig. 8, the excitation wavelength
of all complexes were at 370.0 nm, the characteristic emission
7
5
7
dipole transition (5D0 → F1) and electric dipole transition
peaks were assigned to the D4 → Fj (j = 3, 4, 5, 6) of the
central Tb3+ ion. As for [Tb(NO3)2L4]NO3·2H2O, the main
band of Tb (III) ion at 491.0 nm, 546.5 nm, 586.5 nm, and
(5D0 → 7F2) of the Eu3+ ion. The magnetic dipole transition
7
(5D0 → F1) was independent of the ligand environment,
while the electric dipole transition (5D0 → 7F2) was sensitive
to the surrounding electric field. As shown in Fig. 5, the emis-
sion intensity of the 5D0 → 7F2 transition was weaker than that
623.5 nm, was assigned to the D4 → F6, D4 → F5,
5D4 → 7F4, 5D4 → 7F3 transition, respectively. The emission
intensity at 546.5 nm from the electric dipole transition
5
7
5
7
Table 7 Fluorescence spectral
data of Tb (III) complexes
Compound
Ex Slit (nm)
Em Slit (nm)
2.5
λexa(nm)
λemb(nm)
RLIc
Assignment
[Tb(NO3)2L1]NO3·2H2O
5.0
5.0
5.0
5.0
370.0
491.0
547.5
586.0
623.5
491.0
547.5
586.5
623.5
491.0
546.0
586.5
622.5
491.0
546.5
586.5
623.5
1231
1099
657.0
587.0
3621
4883
1170
658.3
1449
1427
703.0
570.9
6712
5D4 → F6
7
7
5D4 → F5
7
5D4 → F4
7
5D4 → F3
[Tb(NO3)2L2]NO3·2H2O
[Tb(NO3)2L3]NO3·2H2O
[Tb(NO3)2L4]NO3·2H2O
2.5
2.5
2.5
370.0
370.0
370.0
5D4 → F6
7
7
5D4 → F5
7
5D4 → F4
7
5D4 → F3
7
5D4 → F6
7
5D4 → F5
7
5D4 → F4
7
5D4 → F3
7
5D4 → F6
d
7
…
5D4 → F5
7
1997
5D4 → F4
7
803.2
5D4 → F3
a The maximum fluorescence excitation wavelength
b The fluorescence characteristic emission wavelength
c Relative fluorescence intensity
d …, the numberal of relative intensity is larger than 10,000

J Fluoresc
8000
7000
6000
5000
4000
3000
2000
1000
0
12000
10000
8000
6000
4000
2000
0
b
a
H
b
a
H
Cl
CH3
OCH
3
CH
3
OCH3
Cl
340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640
Wavelength/nm
320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640
Wavelength/nm
Fig. 6 Excitation (a) and emission (b) spectra of ligand L1-4
Fig. 8 Excitation (a) and emission (b) spectra of the complexes [Tb
(NO3)2 L1–4]NO3·2H2O
(5D4 → 7F5) was strongest, which deduced that the Tb (III) ion
was located in an asymmetric coordination site. The strongest
emission peaks of all complexes were different, which indi-
cated that the ligand had different influence on the coordina-
tion environment. Among all Tb(III) complexes, the ratio of
of the central ions (Antenna effect). The sensitization involves
placing lanthanide cations in proximity to chromophoric mol-
ecules having high absorptivity, and the closer the triplet state
levels of the ligands to the emitting energy level of the central
ions, the more efficient the intra-molecular energy transfer
[16]. Among all ligands (L1–4), L4 substituted by –Cl,
achieved energy transfer best, eventually the complex
(RE = Eu, Tb) had the strongest intrinsic fluorescence than
that of [RE(NO3)2 L1–3]NO3·2H2O. Meanwhile, the fluores-
cence intensity of [RE(NO3)2 L2–4]NO3·2H2O substituted by
–CH3, −OCH3, −Cl was stronger than that of [RE(NO3)2L1]
NO3·2H2O, respectively, this was because that the substituent
changed the elecron cloud on the benzene ring, the energy
level difference between the triplet state level of the ligands
and the lowest excited state level of the central ions was
changed, thus the emission intensity of the fluorescence was
influenced.
7
7
emission intensity (5D4 → F5/5D4 → F6) were 0.89, 1.39,
0.98, 1.49, which explained that the Tb3+ ions in [Tb(NO3)
2L1,3]NO3·2H2O were in the center of symmetry, but [Tb
(NO3)2L2,4]NO3·2H2O conversely.
Figures 7, and 8 showed that the emission peak of the
ligand L1–4 at 430–443 nm were disappeared, and the emis-
sion intensity of RE (Eu, Tb) complexes were decreased in the
order [RE(NO3)2L4]NO3·2H2O > [RE(NO3)2L2]NO3·
2H2O > [RE(NO3)2L3]NO3·2H2O > [RE(NO3)2L1]NO3·
2H2O. Obviously, the ligand transferred energy to the central
ion, the fluorescence of rare earth complexes were attributed
to the efficient intra-molecular energy transfer from the triplet
state level of the ligands (L1–4) to the lowest excited state level
By comparison between Eu(III) and Tb(III) complexes, the
emission intensity of [Tb(NO3)2L2,4]NO3·2H2O were obvi-
ously stronger than that of [Eu(NO3)2L2,4]NO3·2H2O, which
deduced that the ligands L2,4 had better energy level matching
degree with lowest excited state of Tb3+ ion.
10000
H
OCH3
9000
a
b
8000
CH3
Cl
7000
6000
5000
4000
3000
2000
1000
Electrochemical Properties
The electrochemical properties of rare earth (Eu, Tb) complexes
were analyzed by the cyclic voltammetry (CV) in dimethyl sulf-
oxide solution. The hightest occupied molecular orbit (HOMO,
E
HOMO = − (4.74 + EOX – Efc/fc+)) and the lowest unoccupied
molecular orbit (LUMO, ELUMO = EHOMO + Eg, Eg = hc/
onset = 1240/λonset) [17, 22] were obtained, as shown in Table
8. Since the cyclic voltammetry curves of all the complexes are
similar, only the cyclic voltammetry curves of [Eu(NO3)2 L1–4
NO3·2H2O are seleted for illustration, as shown in Fig. 9.
0
λ
360 380 400 420 440 460 480 500 520 540 560 580 600 620 640
Wavelength/nm
]
Fig. 7 Excitation (a) and emission (b) spectra of the complexes [Eu
(NO3)2 L1–4]NO3·2H2O

J Fluoresc
Table 8 The HOMO and LUMO
energy levels of the Eu3+, Tb3+
complexes
Complex
λ
onset (nm)
Eoxa (v)
Egb (ev)
EHOMO c(ev)
ELUMOd (ev)
[Eu(NO3)2L1]NO3·2H2O
[Eu(NO3)2L2]NO3·2H2O
[Eu(NO3)2L3]NO3·2H2O
[Eu(NO3)2L4]NO3·2H2O
[Tb(NO3)2L1]NO3·2H2O
[Tb(NO3)2L2]NO3·2H2O
[Tb(NO3)2L3]NO3·2H2O
[Tb(NO3)2L4]NO3·2H2O
256
256
256
256
255
256
257
257
0.537
0.404
0.466
0.667
0.568
0.518
0.508
0.667
4.844
4.844
4.844
4.844
4.863
4.844
4.825
4.825
−5.277
−5.144
−5.206
−5.407
−5.308
−5.258
−5.248
−5.407
−0.433
−0.300
−0.362
−0.563
−0.445
−0.414
−0.423
−0.582
a Oxidation potentials measured by cyclic voltammetry with ferrocene as the standard (the oxidation potential of
ferrocene set as zero)
b Band gap estimated from the UV–vis absorption spectrum
c Calculated from the oxidation potentials
d Deduced from the HOMO and Eg
From Table 8, it was obvious that the oxidation potential of
rare earth [RE(NO3)2L4]NO3·2H2O complexes was the
highest than that of [RE(NO3)2 L1–3]NO3·2H2O, while [RE
(NO3)2L1]NO3·2H2O was stronger than that of [RE(NO3)2L2,
3]NO3·2H2O, this mainly due to the influence of the substitu-
ent on the benzene ring. The introduction of electron–donating
group (−CH3, −OCH3) to the ligand enhanced the ability of
lossing electrons, so the oxidation potentials were de-
creased. As for the L4, the inductive effect and
conjugative effect of the substituent (−Cl) contributed to
stablizing the electron, so the oxidation potential was in-
creased. Thus, the hightest occupied molecular orbit were in-
Conclusions
Solid complexes of rare earth (Eu, Tb) nitrate with Schiff base
ligands were synthesized and characterized. The thermody-
namic analysis showed that the starting decomposition tem-
peratures of rare earth complexes were approximately 100 °C.
The fluorescence analysis showed that the ligands L1–4 pos-
sessed excellent fluorescence properties and the emission in-
tensity of L4 was the strongest. The fluorescence intensity of
rare earth (Eu, Tb) complexes were decreased in the order [RE
(NO3)2L4]NO3·2H2O > [RE(NO3)2L2]NO3·2H2O > [RE
(NO3)2L3]NO3·2H2O > [RE(NO3)2L1]NO3·2H2O (RE = Eu,
Tb). This tendency was attributed to the efficient
intra-molecular energy transfer from the triplet state level of
the ligands to the lowest excited state level of the central ions
(Eu3+, Tb3+). The synthesized compounds showed three–pri-
mary colours, which may be significant for studying organic
light–emitting device and fluorescence probe. The electro-
chemical properties of rare earth (Eu, Tb) complexes were
also investigated. The results showed that the electron–donat-
ing group (−CH3, −OCH3) in ligands L2,3 increased the
hightest occupied molecular orbit, while electron–withdraw-
ing group (−Cl) in L4 decreased.
creased in the order [Eu(NO3)2L2]NO3·2H2O <
[Eu
(NO3)2L3]NO3·2H2O < [Eu(NO3)2L1]NO3·2H2O < [Eu(NO3)
2L4]NO3·2H2O and [Tb(NO3)2L3]NO3·2H2O < [Tb(NO3)2L2]
NO3·2H2O < [Tb(NO3)2L1]NO3·2H2O < [Tb(NO3)2L4]NO3·
2H2O.
14
12
d
10
8
6
Acknowledgments We are grateful to 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.
c
4
2
0
b
-2
-4
-6
a
-8
-10
-12
-14
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