Synthesis and properties of coumarin derivatives and their terbium complexes

Article abstract of DOI:10.1007/s11164-015-2357-y

A series of coumarin derivatives obtained from salicylaldehyde and phenol were synthesized. Their corresponding terbium complexes were prepared and characterized by elemental analysis, EDTA titrations, molar conductivity, UVvis spectra, IR spectra, and thermal analysis. The luminescent properties and electrochemical properties of the terbium complexes were also investigated. The results showed that all the terbium complexes exhibited characteristic emissions of terbium ions. The introduction of electron-donating groups can improve the luminescent properties, decrease the HOMO and LUMO energy levels of the terbium complex, while electron-withdrawing groups can weaken the luminescent properties, and increase the HOMO and LUMO energy levels of terbium complex.

Full text of DOI:10.1007/s11164-015-2357-y

Res Chem Intermed
DOI 10.1007/s11164-015-2357-y
Synthesis and properties of coumarin derivatives
and their terbium complexes
Defen Meng1 De Xu1 Dong Li1 Ming Dai1
•
•
•
•
Guizhi Li1 Dongcai Guo1
•
Received: 10 August 2015 / Accepted: 8 November 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A series of coumarin derivatives obtained from salicylaldehyde and
phenol were synthesized. Their corresponding terbium complexes were prepared
and characterized by elemental analysis, EDTA titrations, molar conductivity,
UV–vis spectra, IR spectra, and thermal analysis. The luminescent properties and
electrochemical properties of the terbium complexes were also investigated. The
results showed that all the terbium complexes exhibited characteristic emissions of
terbium ions. The introduction of electron-donating groups can improve the lumi-
nescent properties, decrease the HOMO and LUMO energy levels of the terbium
complex, while electron-withdrawing groups can weaken the luminescent proper-
ties, and increase the HOMO and LUMO energy levels of terbium complex.
Keywords Coumarin derivatives Á Terbium complexes Á Luminescent properties
Introduction
Rare earth organic complexes are currently attracting considerable attention due to
their special physical and chemical properties [1–3]. Specially, the luminescent rare
earth complexes used as labels and sensors mainly show narrow emission bands, large
Stokes’ shift, and hypersensitivity to coordinative environments. The luminescent
intensity of rare earth complexes is strongly dependent on the efficiency of ligand-to-
metal energy transfer [4], which is related to the central ion and the structure of the
organic ligand. Therefore, it attracts more chemists to make an effort to synthesize
excellent ligands.
&
Dongcai Guo
dcguo2001@hnu.edu.cn
1
School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082,
People’s Republic of China
123

D. Meng et al.
Coumarin derivatives, a kind of lactone compounds, have been reported to
possess significant biological activities and pharmacological actions, widely used as
laser dyes, organic nonlinear optical material, fluorescent probes, medicine, and
pesticides [5–9].
Based on the above considerations, a series of coumarin derivatives ligands
(L127, Scheme 1) were synthesized. Their terbium complexes were also prepared
and characterized. The fluorescent properties and electrochemical properties of the
terbium complexes were also investigated in detail.
Experimental section
Materials and methods
Terbium nitrate was prepared according to the literature [10]. Chloroacetic acid was
of CP grade, phenol derivatives, salicylaldehyde, and other reagents were of AR
grade, and all were used without further purification.
Elemental analyses were carried out with a Flash EA1112 elemental analyzer.
Molar conductivity measurements were carried out in N,N–dimethylformamide
O
H
H
C
O
O
O
H
C
3
+
H H
C 2C
3
H
3C
O
O
A
O
O
H
H
OC 2COO
H
O
R
R
-
B1 7
H
NHNH
O
OC 2CO
2
H
H5
OC 2COOC2
R
R
-
D1 7
-
C1 7
O
O
N
-
D1 7
NH
A
+
O
H
C
3
R
L1-7
B1-7
D1-7 L1-7 R=4-H 4-F 4-Cl 4-Br 4-NO2 4-OCH3 4-CH3
:
;
C1-7
;
;
(
(
)
)
.
CH CH OH
N2H4 H2O
,
H
N
NaOH
CH CH OH
C
(
)
ClCH COOH
(
)
CH COCl CH CH OH
(
)
,
;
,
;
,
;
3
2
5
11
2
2
3
2
3
2
CH CH OH reflux.
,
3
2
Scheme 1 Synthetic route of coumarin derivatives L127
123

Synthesis and properties of coumarin derivatives and their…
(DMF) using a DDS-12 A digital direct reading conductivity meter. UV–vis
absorption spectra were recorded on a LabTech UV-2100 spectrophotometer, with
DMSO as reference and solvent. IR spectra were recorded on a Shimadzu
IRAffinity-1 FT-IR spectrophotometer by dispersing samples in KBr pellets.
Luminescent spectra were monitored by an HIACHI F-2700 spectrophotometer at
room temperature, with excitation and emission slit widths at 5.0 nm and voltage of
700 V. Thermal gravimetric (TG) and differential thermal analyses (DTA) were
carried out with a heating speed of 20 °C min-1 in the static air atmosphere on an a
SHIM ADZU DTG-60 thermogravimetric analyzer. Cyclic voltammetry (CV)
measurements were performed using a CHI 660d electrochemical workstation on at
a sweep rate of 50 mV s-1 (external reference: ferrocene, supporting electrolyte:
0.1 mol L-1 sodium nitrate solution, solvent: DMSO, sensitivity: 1 mA)
Synthesis of terbium complexes
Ligands L1-7 were synthesized and purified according to the literature [11].The
mixture of L1 (0.50 mmol, 0.18 g) and chloroform (30 mL) was heated at 60 °C
until completely dissolved. Then terbium nitrate ethanol solution (5 mL,
0.1 mol L-1) was added, and the pH was adjusted to 6–7 by sodium ethoxide.
The mixture continued to reflux for 2 h. Then, the resulting mixture was filtered,
washed with chloroform several times, and dried in vacuum for 12 h to get the
complex with L1 as white powder. The synthesis and purification of other
complexes were similar to that of the complex with L1.
Results and discussion
Properties of terbium complexes
Analytical data of terbium complexes, presented in Table 1, indicated that the
composition of terbium complexes conform to TbL1-7(NO3)3Á2H2O. Ligands L127
were soluble in dichloromethane, chloroform, acetone, DMF, and DMSO, but
Table 1 Elemental analysis and molar conductance data of terbium complexes
Complexes
Found (calculated) (%)
Km
C
H
N
Tb
(S cm2 mol-1
)
TbL1(NO3)3Á2H2O
TbL2(NO3)3Á2H2O
TbL3(NO3)3Á2H2O
TbL4(NO3)3Á2H2O
TbL5(NO3)3Á2H2O
TbL6(NO3)3Á2H2O
TbL7(NO3)3Á2H2O
31.18 (31.80)
30.62 (31.02)
29.89 (30.32)
28.30 (28.64)
29.47 (29.92)
31.88 (32.13)
32.35 (32.83)
3.02 (2.79)
2.79 (2.56)
2.63 (2.53)
2.30 (2.39)
2.88 (2.49)
3.20 (2.95)
3.35 (3.00)
10.04 (9.76)
9.73 (9.52)
9.52 (9.31)
8.81 (8.79)
11.13 (11.02)
9.43 (9.37)
11.67 (9.56)
21.93 (22.17)
21.56 (21.63)
21.20 (21.14)
20.06 (19.97)
20.68 (20.87)
21.33 (21.28)
21.59 (21.75)
8
13
10
9
16
12
11
123

D. Meng et al.
insoluble in cyclohexane, petroleum ether, and benzene. Terbium complexes were
freely soluble in DMF and DMSO, but insoluble in methanol, alcohol, ether, benzene,
and cyclohexane. Molar conductivity data of terbium complexes in DMF
(10-3 mol L-1) at room temperature indicated that all complexes were nonelectrolyte
[12].
UV–vis spectra analyses
UV–vis absorption bands of L127, terbium complexes and molar absorptivities (e)
[13] determined in DMSO solution (10-4 mol L-1) are listed in Table 2. Since the
UV–vis spectra of all the complexes are similar, only UV–vis spectra of
TbL3(NO3)3Á2H2O and its corresponding ligand L3 are selected for illustration, as
shown in Fig. 1.
As shown in Fig. 1, UV–vis spectra of TbL3(NO3)3Á2H2O was very similar to that
of L3 and the absorption intensity of the complex TbL3(NO3)3Á2H2O was higher than
ligand L3. There were two absorption peaks for L3, one appeared at around 297 nm
and another 336 nm, which were assigned to the p–p* and n–p* transitions,
respectively. However, these two absorption peaks of the complex TbL3(NO3)3Á2H2O
were redshifted to 300 and 339 nm, respectively. These phenomena were due to the
exlarged conjugation system followed by the coordination of L3 with terbium ion [14].
IR spectra analyses
IR spectra data of L127 and their corresponding terbium complexes are presented in
Table 3. Since the IR spectra of all the complexes are similar, only the spectra of
TbL4(NO3)3Á2H2O and L4 are selected for illustration, as shown in Fig. 2.
Table 2 UV–vis data of L127and their terbium complexes
Ligands and
complexes
k1 (nm)
e1 (1.0 9 104
k2 (nm)
e2 (1.0 9 104
L mol-1 cm-1
)
L mol-1 cm-1
)
L1
296
299
296
299
297
300
296
298
296
299
296
298
297
299
0.74
0.78
0.67
0.79
0.80
0.89
0.78
0.92
0.71
0.87
0.88
0.98
0.78
0.94
335
338
336
339
336
339
335
339
335
338
336
338
336
338
1.08
1.13
1.00
1.11
1.07
1.18
1.10
1.25
1.10
1.18
1.01
1.16
0.69
1.25
TbL1(NO3)3Á2H2O
L2
TbL2(NO3)3Á2H2O
L3
TbL3(NO3)3Á2H2O
L4
TbL4(NO3)3Á2H2O
L5
TbL5(NO3)3Á2H2O
L6
TbL6(NO3)3Á2H2O
L7
TbL7(NO3)3Á2H2O
123

Synthesis and properties of coumarin derivatives and their…
1.2
a
1.0
b
0.8
0.6
0.4
0.2
0.0
300
350
400
450
Wavelength (nm)
Fig. 1 UV–vis spectra of a TbL3(NO3)3Á2H2O and b L3
For ligand L4, the absorption peaks at 1713, 1651, and 1613 cm-1 were assigned
to the stretching vibration of C=O of the lactone group of coumarin, N–C=O and
C=N of the hydrazide group, respectively. However, for TbL4(NO3)3Á2H2O, the
absorption peak for m(C=O) was shifted to 1669 cm-1 (Dm = 44 cm-1), indicating
that the oxygen atom of the C=O group of lactone was coordinated to the terbium
ion.
Table 3 IR data of ligands L127 and their terbium complexes
Ligands and complexes Lactone Hydrazide
mAr–O–C mN–H
mO–H
mNO
3
mC=O
mC=O
mC=N
L1
1708
1665
1711
1666
1710
1671
1713
1669
1708
1668
1710
1661
1711
1670
1651 1611 1246
1625 1590 1245
1653 1613 1247
1626 1587 1245
1650 1613 1249
1623 1591 1244
1651 1613 1246
1627 1587 1245
1651 1611 1247
1633 1590 1245
1649 1613 1245
1624 1587 1245
1650 1612 1245
1626 1589 1243
3243
TbL1(NO3)3Á2H2O
3241 3411 1486,1326,1035,827
L2
3241
TbL2(NO3)3Á2H2O
3240 3415 1490,1308,1030,831
L3
3244
TbL3(NO3)3Á2H2O
3243 3411 1487,1315,1033,835
L4
3243
TbL4(NO3)3Á2H2O
3241 3419 1495,1319,1041,827
L5
3245
TbL5(NO3)3Á2H2O
3245 3417 1498,1335,1037,830
L6
3246
TbL6(NO3)3Á2H2O
L7
TbL7(NO3)3Á2H2O
3243 3405 1496,1325,1023,836
3248
3245 3409 1492,1307,1038,832
123

D. Meng et al.
In addition, compared to L4, the absorption peaks of m(N–C=O) and m(C=N) of
TbL4(NO3)3Á2H2O were redshifted by 24 and 26 cm-1, respectively, suggesting that
the oxygen atom of N–C=O and the nitrogen atom of C=N of hydrazide were
coordinated to the terbium ion. However, the typical absorption peak of Ar–O–C
had invisible changes, which established that the oxygen of Ar–O–C was not
involved in the coordination. This may due to the poor coordination ability of
Ar–O–C and the large sterically hindered effect of L4.
The typical NO3 absorptions of TbL4(NO3)3Á2H2O were observed in the
-
complexes at 1495 cm-1 (m1), 1041 cm-1 (m2), 827 cm-1 (m3), and 1319 cm-1 (m4).
The absorption peaks lying at 1495 and 1319 cm-1 were attributed to the
asymmetric vibration absorption (mas) and symmetric vibration absorption (ms) of
-
NO3 in complex TbL4(NO3)3Á2H2O. The difference between the two strongest
–
absorption bands |m1 - m4| was approximately 176 cm-1, implying that NO3 was
coordinated to the terbium ion in a bidentate manner [15]. The characteristic
absorption of free NO3- at 1385 cm-1 disappeared, indicating that three NO3- ions
were involved in coordination, and this was in accord with molar conductivity
analyses. Furthermore, there were two new weak absorption bands at 489 and
455 cm-1, which could be assigned to m(Tb–O) and m(Tb–N), respectively.
IR spectrum of TbL4(NO3)3Á2H2O exhibited a broad band at 3400–3444 cm-1
,
indicating that there was crystal water existingd in the complex, which is in
agreement with elemental analyses and thermogravimetric analyses.
Thermal analyses
Thermal properties of TbL1-7(NO3)3Á2H2O were evaluated by means of TG and
differential thermal analyses (DTA) under air atmosphere from ambient temperature
up to 800 °C, and the data are listed in Table 4. Since the thermal behaviors of all
a
b
4000
3500
3000
2500
2000
1500
1000
500
Wavelength (cm-1)
Fig. 2 IR spectra of a TbL4(NO3)3Á2H2O and b L4
123

Synthesis and properties of coumarin derivatives and their…
the complexes are similar, only the TG–DTA curves of TbL3(NO3)3Á2H2O are
depicted, as shown in Fig. 3.
As shown in Fig. 3, the DG-DTA curves gave us the decomposed process of the
complex TbL3(NO3)3Á2H2O. The first mass loss occurred between 40 and 200 °C
was attributed to loss of crystal water, and the found value 4.75 % was
corresponding to the calculated value 4.79 %. Then, the DTA curve shows an
exothermic peak at 217 °C, and the mass loss is 49.40 %, which is consistent with
the calculated value 49.34 % of L3 decomposition. In addition, there are two
distinct exothermic peaks at 395 and 510 °C, and the weight loss is 21.53 %,
corresponding to the loss of three nitrate molecules. Further heating to 800 °C, the
complex was completely decomposed into Tb2O3, and the residue weight was
24.67 %, which matched well with the calculated value 24.34 %. The thermal
analyses results showed that the complexes TbL1-7(NO3)3Á2H2O did not decompose
until the temperature reached 200 °C, which demonstrated that the complexes had
satisfactory thermal stability.
Fluorescent properties
Fluorescent spectra of terbium complexes were measured in the solid state at room
temperature, and the fluorescent spectra data are given in Table 5. Since the
fluorescent spectra of TbL1-7(NO3)3Á2H2O are very similar, only the fluorescent
spectrum of the complex TbL6(NO3)3Á2H2O is selected for illustration, and its
excitation and emission spectra are shown in Fig. 4.
All the complexes presented characteristic fluorescence of the terbium ion,
indicating that ligands L1-7 are efficient to sensitize the luminescence of terbium
ion. The emission spectrum of TbL6(NO3)3Á2H2O displayed four main peaks at
7
7
7
approximately 491 nm (5D4 ? F6), 545 nm (5D4 ? F5), 586 nm (5D4 ? F4),
and 625 nm (5D4 ? F3), and the strongest emission peak at 545 nm was
responsible for the green fluorescence of the terbium complex [16].
7
The emission at 491 nm was assigned to magnetic dipole transition, while
emission at 545 nm was assigned to electric dipole transition. The forbidden electric
dipole transition was naturally sensitive to the coordination environment, so the
asymmetric microenvironment around the ligand caused the probability of electric
Table 4 TG–DTA data of terbium complexes
Complexes
Endothermic peak (°C)
Exothermic peak (°C)
Residue (%) found (calcd.)
TbL1(NO3)3Á2H2O
TbL2(NO3)3Á2H2O
TbL3(NO3)3Á2H2O
TbL4(NO3)3Á2H2O
TbL5(NO3)3Á2H2O
TbL6(NO3)3Á2H2O
TbL7(NO3)3Á2H2O
71, 193
70, 191
68, 195
67, 200, 637
65, 161
66, 197
69, 190
211, 393, 519
213, 391, 553
217, 395, 510
220, 421
24.58 (25.52)
23.75 (24.90)
24.67 (24.34)
22.95 (23.01)
23.64 (24.02)
24.16 (24.50)
24.68 (25.03)
255, 383, 504
276, 378, 501
223, 303, 447, 557
123

D. Meng et al.
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
TG
DTA
-10
-20
-30
100
200
300
400
500
600
700
800
Temperature (°C)
Fig. 3 TG–DTA curves of the complex TbL3(NO3)3Á2H2O
Table 5 Fluorescent spectra of terbium complexes
Complexes
kex
(nm)
5D4
?
7F6
5D4
?
7F5
5D4
?
7F4
5D4
?
7F3
I (a.u.)
kem
(nm)
I (a.u.) kem
(nm)
I (a.u.) kem
(nm)
I (a.u.) kem
(nm)
TbL1(NO3)3Á2H2O 361
TbL2(NO3)3Á2H2O 361
TbL3(NO3)3Á2H2O 362
TbL4(NO3)3Á2H2O 360
TbL5(NO3)3Á2H2O 356
TbL6(NO3)3Á2H2O 360
TbL7(NO3)3Á2H2O 361
491
490
491
492
491
491
491
290.3
205.5
173.3
111.4
226.1
914.2
580.7
546
763.6 586
401.3 586
316.5 584
237.1 585
470.5 585
1859.8 586
1227.5 586
45.2
41.5
47.4
13.6
63.8
149.7
72.1
624
14.0
16.1
21.6
545
546
545
546
545
545
626
626
625
625
625
626
6.23
31.9
76.3
38.7
dipole transition. Meanwhile, the width of the half peaks at 545 nm was
approximately several nanometers, which indicated that TbL6(NO3)3Á2H2O had
high color purity, and ligand L6 was a comparatively good organic chelator [17].
As shown in Table 5, compared to the complex without substituent group TbL1
(NO3)3Á2H2O, complexes substituted by electron-donating groups (OCH3 and CH3)
TbL6(NO3)3Á2H2O and TbL7(NO3)3Á2H2O had stronger luminescent intensity;
complexes substituted by electron–accepting groups (F, Cl, Br, NO2) TbL2-5
(NO3)3Á2H2O had weaker luminescent intensity. From the point of view of the
energy level, the efficiency of energy transferred from ligand to terbium ion is one
of the key factors that influence the characteristic fluorescent properties of the
terbium ion. While the efficiency of the intramolecular energy transfer between the
triple level of ligand and the emitting level of terbium ion, depends on the energy
123

Synthesis and properties of coumarin derivatives and their…
2000
1800
1600
1400
1200
1000
800
b
600
a
400
200
0
325 350 375 400 425 450 475 500 525 550 575 600 625 650
Wavelength (nm)
Fig. 4 a Excitation spectra of TbL6(NO3)3Á2H2O; b emission of TbL6(NO3)3Á2H2O
gap between the two levels. The energy difference between the triplet state energy
level of the ligand and the lowest excited state of the terbium ion cannot be too large
or too small. If the energy difference is too large, the energy-transfer efficiency will
decrease due to diminution of the overlap between the energy donor (ligand) and
acceptor (terbium ion). By contrast, if the energy difference is too small, the energy
can back-transfer from the terbium ion to the triplet state energy of the ligand. The
luminescent intensity of TbL6(NO3)3Á2H2O was the highest, indicating that the
triplet level of L6 was close to the lowest excited state of the terbium ion and the
energy transited from L6 to the terbium ion was easier than other complexes. The
electron density of the phenyl ring was increased due to introduction of electron-
donating groups (OCH3 and CH3) and the energy transited from ligand L6 and L7 to
the terbium ion was more efficient. The energy difference between the triplet state
energy level of ligands L225 and the lowest excited state of terbium ion was
enlarged due to decreased electron density of phenyl ring because of the
introduction of electron-withdrawing groups (F, Cl, Br, NO2), which made the
energy transited from L225 to the terbium ion to be hindered.
Fluorescence quantum yield studies
Fluorescence quantum yield of terbium complexes (Afx) can be calculated
according to a reference method with the following formula [18].
n2x Fx Astd
Ufx
¼
Á
Á
Á Ufstd
:
n2std
Fstd Ax
Quinine sulfate (1.0 lg mL-1) of sulfuric acid solution (0.1 mol L-1) was used
as a standard reference. Subscripts std and x refer to the standard and the unknown,
respectively. The nx [the refractive index of the solvent (DMSO)] was 1.480, and the
123

D. Meng et al.
nstd (the refractive index of water) was 1.337. F is the area of the fluorescent
integral, and A is the absorbance of UV–vis absorption spectra. Since the sample
and standard are excited at the same wavelength, Astd is equivalent to Ax. The
fluorescent spectra data of TbL127(NO3)3Á2H2O were measured at room temper-
ature in DMSO (10-6 M) with excitation and emission slit widths 5.0 nm, and the
results of Afx of the complexes are listed in Table 6.
As shown in Table 6, the fluorescence quantum yield of terbium complexes was
in the order TbL6(NO3)3Á2H2O [ TbL7(NO3)3Á2H2O [ TbL1(NO3)3Á2H2O [ TbL5
(NO3)3Á2H2O [ TbL2(NO3)3Á2H2O [ TbL3(NO3)3Á2H2O [ TbL4(NO3)3Á2H2O,
which was consistent with fluorescent spectra in the solid state.
L6, 7 had electron-donating groups (OCH3 and CH3, respectively) that enlarged
the p-conjugated system of ligands L6, 7 and energy transited from L6, 7 to the
terbium ion was more efficient. In addition, fluorescence quantum yield of
TbL6(NO3)3Á2H2O was higher than TbL7(NO3)3Á2H2O, which may be because the
p–p conjugation effect between lone pair electrons of the oxygen atom and the p-
electron of the phenyl ring was stronger than the r–p hyperconjugation effect
between the r-electron of the methylic C–H bond and the p-electron of the phenyl
ring. However, fluorescence quantum yield of TbL225(NO3)3Á2H2O was lower than
that of TbL1(NO3)3Á2H2O. This was because electron-withdrawing groups (F, Cl, Br
and NO2) in L225 respectively caused the electron density on the phenyl ring to
decrease and easily resulted in fluorescence quenching. Meanwhile, the fluorescence
quantum yield of TbL6(NO3)3Á2H2O was the highest among all the complexes,
which meant that the triplet state energy level of ligand L6 best matched the excited
state energy level of the terbium ion. From the above discussions, we concluded that
all the complexes possessed relatively good fluorescence quantum yields.
Electrochemical properties
The electrochemical properties of terbium complexes were analyzed by the CV in
dimethyl sulfoxide solution. The CV curves of the complexes were obtained at room
temperature, and the data are given in Table 7. The hightest occupied molecular
orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) were obtained
ꢀ
ꢁ
according to the equations EHOMO ¼ À 4:74 þ EOX À Efþc=fc eV (4.74 was for
saturated calomel electrode relative to the vacuum energy, EOX was the onset
Table 6 Fluorescence quantum
yield of TbL127(NO3)3Á2H2O
Complexes
kex (nm)
I (a.u.)
Afx
TbL1(NO3)3Á2H2O
TbL2(NO3)3Á2H2O
TbL3(NO3)3Á2H2O
TbL4(NO3)3Á2H2O
TbL5(NO3)3Á2H2O
TbL6(NO3)3Á2H2O
TbL7(NO3)3Á2H2O
305
304
305
303
305
304
305
188
106
95
0.063
0.045
0.036
0.028
0.057
0.127
0.119
73
137
274
262
123

Synthesis and properties of coumarin derivatives and their…
Table 7 HOMO and LUMO energy levels of TbL1-7(NO3)3Á2H2O
Complexes
konset (nm)
EOX (V)
EHOMO (eV)
Eg (eV)
ELUMO (eV)
TbL1(NO3)3Á2H2O
TbL2(NO3)3Á2H2O
TbL3(NO3)3Á2H2O
TbL4(NO3)3Á2H2O
TbL5(NO3)3Á2H2O
TbL6(NO3)3Á2H2O
TbL7(NO3)3Á2H2O
298
297
298
297
296
297
298
0.653
0.614
0.637
0.646
0.602
0.667
0.689
-5.393
-5.354
-5.377
-5.386
-5.342
-5.407
-5.429
4.161
4.175
4.161
4.175
4.189
4.175
4.161
-1.232
-1.179
-1.216
-1.211
-1.153
-1.232
-1.268
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-2.5 -2.0 -1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
Potential (V)
Fig. 5 CV curve of TbL1(NO3)3Á2H2O
potential, Efþc=fc ¼ 0 V) and ELUMO = EHOMO ? Eg. The Eg was obtained by
ultraviolet absorption spectrometry, the corresponding energy of the peak wave-
length (konset, the wavelength of the maximum ultraviolet absorption peak) was
treated as Eg, Eg = hc/konset = 1240/konset eV [19]. The reduction potentials were
obtained according to the electrochemical equation Ered = EOX - Eg. Since the CV
curves of all the complexes are similar, only the CV curve of TbL1(NO3)3Á2H2O is
selected for illustration, as shown in Fig. 5.
As shown in Table 7, the HOMO and LUMO energy levels of TbL6, 7(NO3)3Á2H2O
were higher than TbL1(NO3)3Á2H2O. However, the HOMO and LUMO energy levels
of the complexes TbL225(NO3)3Á2H2O were less than TbL1(NO3)3Á2H2O. These
phenomena confirmed that the HOMO and LUMO energy levels were closely related
to the substituent on the benzene ring of ligand. The introduction of electron-donating
groups can increase the HOMO and LUMO energy levels of the terbium complex,
however the introduction of electron-withdrawing groups can decrease the HOMO and
LUMO energy level of the terbium complex. Electron-donating groups enhanced the
ability of lossing electrons, so the oxidation potentials were decreased and the HOMO
123

D. Meng et al.
energy level was increased. While electron accepting groups weakened the ability of
losing electrons, so the oxidation potentials were increased and the HOMO energy
level was decreased.
Conclusions
A series of coumarin derivatives L127 and their corresponding terbium complexes
were prepared and characterized. All the terbium complexes had relatively good
thermal stability. The improvement of luminescent intensities of terbium complexes
was attributed to the efficience of intramolecular energy transfer from the triplet
state level of ligands (antenna effect) to the lowest excited state level of the terbium
ion. The fluorescent intensity of the complex TbL6(NO3)3Á2H2O was the strongest
among all terbium complexes. Therefore, TbL6(NO3)3Á2H2O could be used as
promising luminescent materials. Moreover, electrochemical property analyses
showed that the introduction of electron-donating groups enhanced the HOMO
energy level and the LUMO energy level. These complexes may be of great
significance in researching electron transport material.
Acknowledgments The authors are grateful for the financial support of the National Natural Science
Foundation of China (Nos. J1103312, 21341010), the Natural Science Foundation of Hunan Province
(No. 11JJ5005), the Innovative Research Team in University (No. IRT1238) and China Outstanding
Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE-
No. 2011-40). We also thank Dr. William Hickey, the U.S. professor of HRM, for the English editing of
this paper.
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