Synthesis and fluorescence properties of 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazol derivatives and their terbium complexes

Article abstract of DOI:10.1002/bio.2668

Eight novel 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazol derivatives have been designed and synthesized, and their corresponding Tb³⁺ complexes were also prepared successfully. The fluorescence properties and fluorescence quantum yields of the target complexes were investigated, the results showed that the ligands were an efficient sensitizer for Tb³⁺ luminescence, and the target complexes exhibited characteristic fluorescence emissions of Tb³⁺ ion. The fluorescence intensity of the complex substituted by chlorine was stronger than that of other complexes. The substituents' nature has a great effect upon the electrochemical properties of the target complexes. The results showed that the introduction of the electron-withdrawing groups tended to decrease the oxidation potential and highest occupied molecular orbital energy levels of the target Tb³⁺ complexes; however, introduction of the electron-donating groups can increase the corresponding complexes' oxidation potential and highest occupied molecular orbital energy levels.

Full text of DOI:10.1002/bio.2668

Research article
Received: 09 November 2013,
Accepted: 19 February 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2668
Synthesis and fluorescence properties of 1,2,4-
triazolo[3,4-b]-1,3,4-thiadiazol derivatives and
their terbium complexes
Wu Zhang,a Yuchao Chai,a Kangyun Li,a Yanwen Chen,b Dong Yana
and Dongcai Guoa*
ABSTRACT: Eight novel 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazol derivatives have been designed and synthesized, and their
corresponding Tb3+ complexes were also prepared successfully. The fluorescence properties and fluorescence quantum yields
of the target complexes were investigated, the results showed that the ligands were an efficient sensitizer for Tb3+
luminescence, and the target complexes exhibited characteristic fluorescence emissions of Tb3+ ion. The fluorescence
intensity of the complex substituted by chlorine was stronger than that of other complexes. The substituents’ nature has a great
effect upon the electrochemical properties of the target complexes. The results showed that the introduction of the electron-
withdrawing groups tended to decrease the oxidation potential and highest occupied molecular orbital energy levels of the
target Tb3+ complexes; however, introduction of the electron-donating groups can increase the corresponding complexes’
oxidation potential and highest occupied molecular orbital energy levels. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: 1,2,4-Triazoles; synthesis; terbium complex; fluorescence properties; electrochemical properties
The synthesis route for the 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazol
Introduction
Rare earth complexes are the subject of increasing interest in
derivatives (L1–8) is shown in Scheme 1.
bioinorganic and coordination chemistry because of their
successful application as diagnostic tools in biomedical analysis,
Experimental
laser systems, optical amplification, etc. (1–4). Luminescence of
Materials and methods
rare earth complexes results from the unique 4f orbitals, which
are well shielded by the outer 5 s2 and 5p6 electrons (5–7).
Pyridine-2,6-dicarboxylic acid (purity 99.99%, Entai Science and
Technology Co., Ltd. Wuhan, China) and Tb2O3 (purity 99.99%)
were purchased from commercial suppliers. Tb(NO3)3 (0.2mol/L)
solution was prepared by dissolving Tb2O3(99.99%) in HNO3
according to the literature (10). Monochloroacetic acid (Sinopharm
Group Co., Ltd. China) and ethyl acetate were of C.P. grade, phenol
derivatives and other chemicals were of A.R. grade and used
without purification.
Therefore, the luminescence excited directly from the rare earth
ions is unfavorable and so, it is very important to design and
synthesize novel ligands possessing high absorption efficiency.
Sujith et al. (8) has synthesized a series of 1,2,4-triazolo[3,4-b]-
1,3,4-thiadiazol derivatives, which represent one of the most
biologically active classes of compounds, possessing a wide
spectrum of pharmacological activities, but, few studies exist
of 1,2,4-triazolo[3 ,4-b]-1,3,4-thiadiazol derivatives as ligands,
synthesis and applications in rare earth complexes. This
motivated our research interest in Tb3+ complexes based
on 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazol. Because the 1,2,4-
triazolo[3,4-b]-1,3,4-thiadiazol derivatives present appropriate
π-conjugated system, good planarity and a variety of coordi-
nation sites, it is possible in theory that they would be good
organic ligands. However, studies are needed to prove
whether these kinds of ligands possess highly efficient
energy transfer (9).
Proton nuclear magnetic resonance (1H NMR) spectra was
recorded in dimethyl sulfoxide (DMSO)-d6/CDCl3 on Brucker
spectrophotometer (America) (400 MHz) with TMS as an internal
standard. Infrared (IR) spectra (400–4000/cm) were obtained in
KBr discs by a Perkin-Elmer Spectrum One (PERKIN-ELMER).
Ultraviolet (UV) spectra (190–450 nm) were recorded by LabTech
UV-2100 (LabTech) spectrophotometer, with DMSO as solvent
and reference. Elemental analysis of the complexes was carried
out on a VarioEL 111 CHNS (Germany) analyzer. Melting points
In this work, compounds containing the 1,2,4-triazolo
[3,4-b]-1,3,4-thiadiazol were designed and synthesized, and their
corresponding Tb3+ complexes were also prepared. The
fluorescence properties of target complexes were studied in
detail and the fluorescence quantum yields of the Tb3+
complexes was also investigated. Meanwhile, the relationship
between the structure of ligands and the electrochemical
properties of the target complexes were discussed in detail.
* Correspondence to: Dongcai Guo, Dongcai Guo School of Chemistry
and Chemical Engineering, Hunan University, Changsha 410082, China.
E-mail: dcguo2001@hnu.edu.cn
a
School of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, China
b
Institute of Nonferrous Metallurgy Labor Protection, Hunan, Changsha
410014, China
Luminescence 2014
Copyright © 2014 John Wiley & Sons, Ltd.

W. Zhang et al.
(30 mL) was added and stirring was continued for 30 min. The
precipitated potassium dithiocarbazinate was collected by
filtration, washed with anhydrous ether two times and dried in
a vacuum for 24 h. Thus, the potassium salt was obtained and
used in the next step without further purification (12).
A suspension of potassium dithiocarbazinate in water (2 mL)
and hydrazine hydrate (85%, 2 mL) were added into a 100 mL
single-necked flask and heated for 6 h at 110 °C. The resulting mix-
ture was cooled to room temperature and gradually poured on to
crushed ice (30 mL) with stirring. The color of the reaction mixture
changed to reddish brown. The pH value of the solution was
adjusted to 6.0 by dropwise addition of aqueous HCl. The white
precipitate was obtained during the reaction process and allowed
to stand overnight. The resulting solid was dissolved in a solution
of a mixture (30 mL) of water and ethanol (1 : 1). The reaction mix-
ture was heated to boiling while stirring; a white solid was obtained
by hot filtration and dried in vacuum for 24 h. Yield was 20%.
Scheme 1. The synthetic route for the ligand L1~8
.
Synthesis of phenoxyacetic acid derivatives 2′a–h. The procedures
for synthesis of compounds 2′a–h were carried out according to the
literature (10) and partly revised by ourselves. As the synthesis
methods of compounds 2′a–h were similar, only the synthesis pro-
cess of the phenoxyacetic acid compound 2′a was described as an
example. The solution of sodium monochloroacetate was obtanied
that a solution of monochloroacetic acid (0.05 mmol, 4.72 g) in
water (20 mL) was cooled in an ice bath and then the sodium
hydroxide (25%) added with stirring. Thus, a solution of sodium
monochloroacetate was obtained when the pH value was 9–10.
The mixture of sodium hydroxide (0.04 mol, 1.60 g), water (20 mL),
ethanol (5 mL) and phenol (0.04 mol, 3.76 g) were added
portionwise into a 150 mL three-neck flask and then the above
solution of sodium monochloroacetate was slowly added. The reac-
tion mixture was refluxed at 105 ºC for 5 h. The resulting solution
was cooled to room temperature and acidified with dilute
hydrochloric acid until the pH value reached 1–2. At that moment,
a large number of white precipitate was formed, and then filtered
off, washed several times with HCl and dried in vacuum. Thus, the
white solid of phenoxyacetic acid was obtained.
of all compounds were determined on an X-4 binocular micro-
scope. The thermal gravimetric analysis was carried out on a
NETZSCH STA 409PC thermal gravimetric analyzer (Germany).
In the cyclic voltammetry curve testing three electrodes were
used (glassy electrode, platinum electrode and a saturated
calomel electrode), ferrocene was used as an external standard,
nitrite solution as the supporting electrolyte and DMSO as the
solvent; the test scanning speed was 100 mV/s and the sensi-
tivity was 1 mA. The fluorescence spectra were measured by
using powder samples on a Hitachi F-2700 Fluorescence
Spectrophotometer (Japan) at room temperature. The width of
excitation and emission slits were both 2.5 nm.
General procedure for synthesis of the intermediates
Synthesis of compound 2. A solution of 2,6-pyridine dicarboxylic
acid (60 mmol, 10.02 g) in anhydrous methanol (80 mL) was
added to a 150 mL three-neck flask and then the acetyl chloride
was gradually added dropwise while stirring in the ice water
bath (11). The reaction mixture was heated and refluxed for
24 h with stirring in the oil bath. The formed precipitate was
collected by filtration, washed several times with ethyl acetate
and dried at room temperature. The white powder of compound
2 was obtained. Yield was 91%.
Phenoxyacetic acid (2′a). White crystals, yield 67%. 1H NMR
(400 MHz, CDCl3), δ/ppm: 4.70 (s, 2H, CH2), 6.94 (d, 2H,
J = 8.0 Hz, ArH), 7.02–7.05 (t, 1H, J = 7.2 Hz, ArH), 7.30–7.35
(m, 2H, ArH); MS(ESI) m/z (%): 304 (2 M, 13), 303 (2 M – 1, 100),
151 (M – 1, 28).
p-Tolyloxyacetic acid (2′b). White crystals, yield 69%. 1H NMR
(400 MHz, CDCl3), δ/ppm: 2.30 (s, 3H, CH3), 4.66 (s, 2H, CH2),
6.83 (d, 2H, J = 8.8 Hz, ArH), 7.12 (d, 2H, J = 8.8 Hz, ArH); MS (EI)
m/z (%): 167 (M + 1, 10), 166 (M, 100), 121 (60), 107 (50), 91
(64), 77 (26), 65 (17).
Synthesis of compound 3. A mixture of compound 2 (53 mmol)
in absolute ethanol (200 mL) was added to a 500 mL three-neck
flask and refluxed for some time, until the temperature was
maintained to 85 ºC. The hydrazine hydrate (30 mL, 85%) was
added to the refluxing mixture. The resulting mixture was further
allowed to reflux for 6 h. Excess ethanol was distilled out under
reduced pressure and diluted with water. The precipitate were
formed and stood for some time, and then the reaction mixture
was filtered, thoroughly washed with cold water and recrystallized
from water. Yield was 92%.
(4-Methoxy-phenoxy)-acetic acid (2′c). White crystals, yield 68%.
1H NMR (400 MHz, CDCl3), δ/ppm:3.78 (s, 3H, CH3), 4.63 (s, 2H,
CH2), 6.84–6.86 (m, 2H, ArH), 6.87–6.90 (m, 2H, ArH); MS (EI)
m/z (%): 183 (M + 1, 6), 182 (M, 64), 123 (100), 109 (16), 95 (26),
92 (9), 77 (8).
Synthesis of compound 4. Potassium hydroxide (20 mmol) was
dissolved in absolute ethanol (30 mL). The resulting solution
was cooled in ice bath, and compound 3 (5 mmol, 0.98 g) was
added with stirring. A solution of carbon disulfide (20 mmol,
1.52 g) in anhydrous ethanol was added while stirring. Most of
the precipitate appeared after dropped and then diluted with
10 mL of anhydrous ethanol. The reaction mixture was agitated
continuously for 14 h at room temperature, and then the ether
(4-Nitro-phenoxy)-acetic acid (2′d). White powder, yield 72%. 1H
NMR (400 MHz, DMSO-d6), δ/ppm: 4.88 (s, 2H, CH2), 7.13–7.15
(t, 2H, ArH), 8.20–8.22 (t, 2H, ArH); MS (EI) m/z (%): 198 (M + 1,
9), 197 (M, 100), 181 (3), 167 (17), 152 (85), 139 (11), 122 (22),
109 (38), 92 (29), 76 (18).
(4-Fluoro-phenoxy)-acetic acid (2′e). White crystals, yield 52%. 1H
NMR (400 MHz, CDCl3), δ/ppm: 4.67 (s, 2H, CH2), 6.87–6.90 (m, 2H,
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Synthesis and fluorescence properties of the terbium complexes
ArH), 6.99–7.03 (m, 2H, ArH); MS (EI) m/z (%): 171 (M + 1, 9), 170
(M, 100), 125 (79), 112 (41), 95 (68), 83 (30), 75 (17).
54.08; H, 3.53; N, 21.02; S, 10.69. Found: C, 54.50; H, 3.58; N,
20.53; S, 10.67.
2,6-bis(6-((4-nitrophenoxy)methyl)-[1,2,4]triazolo[3,4-b][1,3,4]
thiadiazol-3-yl)pyridine (L4). White solid, yield 60%. m.p.
310 ºC; 1H NMR (400 MHz, CDCl3) δ/ppm: 5.47 (s, 4H, CH2),
7.21 (t, 4H, ArH), 8.33 (t, 4H, ArH), 8.13–8.18 (t, 1H, J = 6.0 Hz,
pyridine proton), 8.48 (d, 2H, J = 6.0 Hz, pyridine protons); IR
(KBr) v/cm: 3119, 1611, 1544, 1509, 1466, 1400, 1259, 1054,
687; MS (ESI) m/z (%): 653 (M + 23, 36), 652 (M + 22, 100), 630
(M, 22); Anal. Calcd. for C25H15N11O6S2: C, 47.69; H, 2.40; N,
24.47; S, 10.19. Found: C, 47.88; H, 2.41; N, 24.68; S, 10.14.
2,6-bis(6-((4-fluorophenoxy)methyl)-[1,2,4]triazolo[3,4-b][1,3,4]
thiadiazol-3-yl)pyridine (L5). Gray solid, yield 92%. m.p. 228 ºC;
1H NMR (400 MHz, CDCl3), δ/ppm: 5.40 (s, 4H, CH2), 6.93–6.98
(m, 4H, ArH), 7.00–7.05 (t, 4H, ArH), 8.11–8.15 (t, 1H, J = 8.0 Hz,
pyridine proton), 8.48 (d, 2H, J = 8.0 Hz, pyridine protons); IR
(KBr) v/cm: 3118, 1593, 1574, 1542, 1464, 1402, 1251, 1098,
1081, 687; MS (ESI) m/z (%): 599 (M + 23, 25), 598 (M + 22,
100), 576 (M, 30); Anal. Calcd. for C25H15F2N9O2S2: C, 52.17;
H, 2.63; N, 21.90; S, 11.14. Found: C, 52.14; H, 2.53; N, 21.92;
S, 11.11.
(4-Chloro-phenoxy)-acetic acid (2′f). White crystals, yield 85%. 1H
NMR (400 MHz, CDCl3), δ/ppm: 4.67 (s, 2H, CH2), 6.87 (d, 2H,
J = 8.8 Hz, ArH), 7.27 (d, 2H, J = 8.4 Hz, ArH); MS (EI) m/z (%):
188 (M + 2, 32), 186 (M, 100), 141 (79), 128 (56), 111 (59), 99
(32), 75 (33).
(4-Bromo-phenoxy)-acetic acid (2′g). White powder, yield (78%).
1H NMR (400 MHz, CDCl3), δ/ppm: 4.76 (s, 2H, CH2), 6.82 (d, 2H,
J = 9.2 Hz, ArH), 7.42 (d, 2H, J = 9.2 Hz, ArH); MS (EI) m/z (%): 232
(M + 1, 97), 230 (M – 1, 100), 187 (47), 185 (49), 174 (34), 172
(37), 157 (40), 155 (35), 143 (17), 76 (20).
1
(4-Iodo-phenoxy)-acetic acid (2′h). White powder, yield 76%. H
NMR (400 MHz, CDCl3), δ/ppm: 4.67 (s, 2H, CH2), 6.71 (d, 2H,
J = 9.2 Hz, ArH), 7.60 (d, 2H, J = 8.4 Hz, ArH); MS (EI) m/z (%): 279
(M + 1, 9), 278 (M, 100), 233 (23), 220 (21), 203 (15), 191 (7), 106
(6), 92 (6), 76 (11).
Synthesis of 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazol derivatives L1–8
.
As
the synthesis methods of the compounds L1–8 were similar, the
synthesis of compound L1 was described as an example. In the
100 mL single-necked flask, both compound 4 (2 mmol, 0.61 g)
and phenoxyacetic acid (6 mmol, 0.91 g) were dissolved in
15 mL of dry phosphorous oxychloride. The resulting solution
was refluxed at 106 ºC for 7 h. Excess of POCl3 was removed by
vacuum distillation. The reaction mixture was cooled to room
temperature and then gradually poured into 400 mL of crushed
ice while stirring. The potassium hydroxide solution (2 mol/L)
was added until the pH value of the mixture raised to 8. The
resulting mixture was allowed to stand overnight and precipitate
completely, then filtered, washed thoroughly with cold water,
dried and recrystallized from anhydrous ethanol to give the
target compound L1.
2,6-bis(6-((4-chlorophenoxy)methyl)-[1,2,4]triazolo[3,4-b][1,3,4]
thiadiazol-3-yl)pyridine (L6). White solid, yield 67%. m.p.
1
258 ºC; H NMR (400 MHz, CDCl3), δ/ppm: 5.41 (s, 4H, CH2), 6.93
(d, 4H, J = 8.8 Hz, ArH), 7.29 (d, 4H, J = 9.2 Hz, ArH), 8.11–8.15
(t, H, J = 7.8 Hz, pyridine proton), 8.48 (d, 2H, J = 8.0 Hz, pyridine
protons); IR (KBr) v/cm: 3138, 1595, 1572, 1546, 1464, 1400,
1243, 1040, 709, 687; MS (ESI) m/z (%): 632 (M + 24, 83),
630 (M + 22, 100), 608 (M, 16); Anal. Calcd. for C25H15Cl2N9O2S2:
C, 49.35; H, 2.48; N, 20.72; S, 10.54. Found: C, 49.60; H, 2.49;
N, 20.47; S, 10.60.
2,6-bis(6-((4-bromophenoxy)methyl)-[1,2,4]triazolo[3,4-b][1,3,4]
thiadiazol-3-yl)pyridine (L7). Gray solid, yield 83%. m.p. 245 ºC;
1H NMR (400 MHz, CDCl3), δ/ppm: 5.40 (s, 4H, CH2), 6.88 (d, 4H,
J = 9.2 Hz, ArH), 7.43 (d, 4H, J = 9.2 Hz, ArH), 8.11–8.15 (t, 1H,
J = 7.8 Hz, pyridine proton), 8.48 (d, 2H, J = 8.0, pyridine protons);
IR (KBr) v/cm: 3140, 1594, 1581, 1546, 1464, 1403, 1246, 1052,
680, 599; MS (ESI) m/z (%): 720 (M + 23,100), 718 (M + 21, 49),
698 (M + 1, 38), 696 (M – 1, 15); Anal. Calcd. for C25H15Br2N9O2S2:
C, 43.06; H, 2.17; N, 18.08; S, 9.20. Found: C, 43.05; H, 2.14; N,
18.27; S, 9.34.
2,6-bis(6-((4-iodophenoxy)methyl)-[1,2,4]triazolo[3,4-b][1,3,4]
thiadiazol-3-yl)pyridine (L8). Black solid, yield 88%. m.p. 155 ºC;
1H NMR (400 MHz, CDCl3), δ/ppm: 5.40 (s, 4H, CH2), 6.76 (d, 4H,
J = 9.2, ArH), 7.62 (d, 4H, J = 9.2 Hz, ArH), 8.11–8.15 (t, 1H,
J = 8.0 Hz, pyridine proton), 8.48 (d, 2H, J = 8.4 Hz, pyridine
protons); IR (KBr) v/cm: 3152, 1634, 1578, 1547, 1467, 1400,
1244, 1050, 680, 419; MS (ESI) m/z (%): 814 (M + 23, 100), 792
(M + 1, 14); Anal. Calcd. for C25H15I2N9O2S2: C, 37.94; H, 1.91; N,
15.93; S, 8.10. Found: C, 37.82; H, 1.93; N, 16.04; S, 8.18.
2,6-bis(6-(phenoxymethyl)-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazol-3-
yl)pyridine (L1). Brown solid, yield 81%. m.p. 192 ºC; 1H NMR
(400 MHz, CDCl3), δ/ppm: 5.45 (s, 4H, CH2), 6.98–7.00 (dd, 4H,
J1 = 8.8 Hz, J2 = 0.8 Hz, ArH), 7.06–7.10 (m, 2H, ArH), 7.31–7.35 (m,
4H, ArH), 8.11–8.15 (t, 1H, J = 7.8Hz, pyridine proton), 8.49 (d, 2H,
J = 8 Hz, pyridine protons); IR (KBr) v/cm: 3138, 1598, 1573, 1540,
1454, 1401, 1246, 1080, 690; MS (ESI) m/z (%): 1079 (2M-1, 47),
563 (M + 23, 15), 562 (M + 22, 53), 540 (M, 51); Anal. Calcd. for
C25H17N9O2S2: C, 55.65; H, 3.18; N, 23.36; S, 11.88. Found: C,
55.25; H, 3.62; N, 23.75; S, 11.46.
2,6-bis(6-(p-tolyloxymethyl)-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazol-
1
3-yl)pyridine (L2). Brown solid, yield 92%. m.p. 205 ºC; H NMR
(400 MHz, CDCl3), δ/ppm: 2.31 (s, 6H, CH3), 5.41 (s, 4H, CH2),
6.88 (d, 4H, J = 8.8 Hz, ArH), 7.06–7.15 (m, 4H, ArH), 8.10–8.14
(t, 1H, J = 7.8Hz, pyridine proton), 8.48 (d, 2H, J = 8.0 Hz, pyridine
protons); IR (KBr) v/cm: 3122, 2860, 1590, 1574, 1542, 1458, 1401,
1241, 1083, 685; MS (ESI) m/z (%): 1157 (2 M + 21, 100), 1135
(2 M – 1, 40), 590 (M + 22, 58), 568 (M, 47); Anal. Calcd. for
C27H21N9O2S2: C, 57.13; H, 3.73; N, 22.21; S, 11.30. Found: C,
57.35; H, 3.57; N, 22.15; S, 11.12.
2,6-bis(6-((4-methoxyphenoxy)methyl)-[1,2,4]triazolo[3,4-b][1,3,4]
thiadiazol-3-yl)pyridine (L3). Brown solid, yield 63%. m.p. 191 ºC;
1H NMR (400 MHz, CDCl3), δ/ppm: 3.77 (s, 6H, OCH3), 5.38
(s, 4H, CH2), 6.84 (d, 4H, 8.8 Hz, ArH), 6.92 (d, 4H, J = 9.2 Hz,
ArH), 8.10–8.14 (t, 1H, J = 7.8 Hz, pyridine proton), 8.48 (d, 2H,
J = 8.0 Hz, pyridine protons); IR (KBr) v/cm: 3121, 1594, 1574,
1460, 1401, 1232, 1109, 1032, 688; MS (ESI) m/z (%): 623
(M + 23, 38), 622 (M + 22, 100); Anal. Calcd. for C27H21N9O4S2: C,
Synthesis of the target terbium complexes
The synthesis methods of target Tb3+ complexes were similar, so
the synthesis of the Tb3+ complexes of compound L1 is
described as an example. The compound L1 was dissolved in
30 mL chloroform at 80 ºC, and 1.25 mL Tb(NO3)3 (0.2 mol/L)
solution was added, then a large number of white precipitate
was formed immediately. The reaction mixture was maintained
at reflux for 4 h, the resulting solid complexes were collected
by the hot filtration, and washed several times with chloroform.
Finally, the target Tb3+ complex was dried under vacuum for 6 h.
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W. Zhang et al.
Results and discussion
Elemental analysis and solubility of the target complexes
The compositions of eight Tb3+ complexes were confirmed on
the basis of the elemental analysis and molar conductivity, and
their data are summarized in Table 1, which are in accordance
with the theoretical values calculated, indicating that the
composition of the target complexes are (TbL1–8(NO3))(NO3)
2ꢀH2O. The molar conductivity values of the target
complexes in dimethylformamide (DMF) solution are in the
range of 155–204 S · cm2/mol, which indicated that the target
complexes are 2 : 1 electrolytes (13). The results revealed that
the possible chemical structure of the target complexes was
shown in Fig. 1. The ligands L1–8 are very soluble in DMF and
DMSO, and only partially soluble in chloroform, but insoluble in
water, ethanol, benzene, etc., while the target complexes are
only soluble in DMF and DMSO.
Figure 1. The possible chemical structure of the target complexes.
Table 2. Ultraviolet spectra data of complexes
Complexes
λmax (nm)
Ligand
λ
max (nm)
[TbL1(NO3)](NO3)2ꢀH2O
[TbL2(NO3)](NO3)2ꢀH2O
[TbL3(NO3)](NO3)2ꢀH2O
[TbL4(NO3)](NO3)2ꢀH2O
[TbL5(NO3)](NO3)2ꢀH2O
[TbL6(NO3)](NO3)2ꢀH2O
[TbL7(NO3)](NO3)2ꢀH2O
[TbL8(NO3)](NO3)2ꢀH2O
276,308
278,308
289
267,308
278,308
279,308
278,308
277,308
L1
L2
L3
L4
L5
L6
L7
L8
276,306
277,307
286
267,307
277,306
277,306
277,305
275,305
Ultraviolet spectra analysis
The UV absorption spectra of the ligands L1–8 and their corre-
sponding Tb3+ complexes were recorded in DMSO solution,
and the data are summarized in Table 2. As the UV absorption
spectra of all target complexes are similar, so the UV spectra of
ligand L8 as well as its corresponding Tb3+ complex is selected
for illustration and shown in Fig. 2.
It can be seen from Table 2 and Fig. 2 that the transition
absorption peaks of the target complexes compared to the
corresponding ligands are all red-shifted; in particular, the n→ π*
transitions absorption peak are all shifted to 308 nm, which are
attributed to the impact of the central Tb3+ ion.
1.0
b
0.8
Figure 2 shows that there are two absorption peaks at 275 nm
and 305 nm, which are ascribed to the π–π* and n–π* transitions
in ligand L8, respectively. The UV spectrum of the complex
[TbL8(NO3)](NO3)2ꢀH2O is very similar to that of the free ligand
L8. The only difference is that the absorption intensity of the
complex is decreased, which is attributed to Tb3+ ion that
enlarged the ligand’s conjugated system. This evidence shows
that ligand L8 is coordinated successfully with the center Tb3+
ion (14).
a
0.6
0.4
0.2
0.0
260
280
300
320
340
360
380
400
Infrared spectra analysis
The IR data of all ligands and their corresponding Tb3+
complexes are presented in Table 3. The results show that the
Wavelength (nm)
Figure 2. Ultraviolet spectra of the [TbL8(NO3)](NO3)2ꢀH2O (a) and the ligand L8 (b).
Table 1. Elemental analysis and molar conductance data for target complexes
Found (calculated) (%)
Complexes
C
H
N
Tb
Λm (S · cm2/mol)
[TbL1(NO3)](NO3)2ꢀH2O
[TbL2(NO3)](NO3)2ꢀH2O
[TbL3(NO3)](NO3)2ꢀH2O
[TbL4(NO3)](NO3)2ꢀH2O
[TbL5(NO3)](NO3)2ꢀH2O
[TbL6(NO3)](NO3)2ꢀH2O
[TbL7(NO3)](NO3)2ꢀH2O
[TbL8(NO3)](NO3)2ꢀH2O
33.24 (33.22)
34.78 (34.80)
33.67 (33.64)
30.17 (30.21)
30.87 (31.95)
30.78 (30.90)
28.35 (28.30)
26.25 (26.00)
2.11 (2.10)
2.49 (2.47)
2.30 (2.39)
1.71 (1.71)
1.85 (1.81)
1.72 (1.75)
1.54 (1.60)
1.38 (1.47)
18.54 (18.60)
18.01 (18.05)
17.32 (17.45)
19.58 (19.74)
17.78 (17.89)
17.23 (17.30)
15.85 (15.85)
14.58 (14.56)
17.48 (17.61)
17.26 (17.08)
16.55 (16.51)
16.21 (16.01)
16.72 (16.93)
16.18 (16.37)
15.04 (15.00)
13.72 (13.78)
161
174
158
192
185
155
197
204
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Synthesis and fluorescence properties of the terbium complexes
Table 3. Infrared spectra data of ligands and their corresponding terbium complexes
νNO-31
Compound
νC=N
νC=N-N
νAr-O-C
νC-S-C
νAr-H
νOH
ν1
ν4
ν2
ν3
L1
1598
1598
1590
1614
1594
1615
1611
1612
1593
1615
1595
1614
1594
1615
1634
1616
1246
1248
1241
1238
1232
1230
1259
1261
1251
1261
1243
1246
1246
1242
1244
1241
1080
1081
1083
1090
1032
1030
1053
1055
1081
1097
1040
1044
1052
1070
1050
1057
690
690
685
686
688
688
687
687
687
688
687
687
680
680
680
680
3138
3163
3122
3130
3121
3156
3119
3138
3118
3153
3138
3140
3140
3129
3152
3194
3414
3397
3413
3414
3444
3411
3413
3398
3422
3400
3414
3424
3409
3409
3429
3401
[TbL1(NO3)](NO3)2ꢀH2O
1479
1484
1484
1487
1489
1480
1484
1480
1388
1385
1385
1384
1388
1385
1385
1385
1025
1028
1030
1022
1026
1027
1021
1029
815
815
816
816
816
818
817
817
L2
[TbL2(NO3)](NO3)2ꢀH2O
L3
[TbL3(NO3)](NO3)2ꢀH2O
L4
[TbL4(NO3)](NO3)2ꢀH2O
L5
[TbL5(NO3)](NO3)2ꢀH2O
L6
[TbL6(NO3)](NO3)2ꢀH2O
L7
[TbL7(NO3)](NO3)2ꢀH2O
L8
[TbL8(NO3)](NO3)2ꢀH2O
IR spectra of ligands and corresponding complexes present
similar features to each other, so the IR spectra of the free
ligands L4 and L6 as well as their corresponding complexes
[TbL4(NO3)](NO3)2ꢀH2O and [TbL6(NO3)](NO3)2ꢀH2O are selected
for illustration and shown in Figs 3 and 4, respectively.
ion (15). In the free ligands L4 and L6, the C = N-N group
stretching modes is appeared at 1259/ cm-1 and 1243/ cm-1, but
the shifting of the C = N-N stretching peak in the corresponding
Tb3+ complexes to a longer wavelength indicates involvement
of the nitrogen atom of the C = N-N group in coordination (16).
While, the C-S-C group stretching vibration locates at around
685/cm in all ligands, but it is not obviously shifted in the
target complexes. This evidence shows that the sulfur atom of
the C-S-C group does not participate in coordination, which is
ascribed to sterically hindered occurring, and the central ion is dif-
ficult to coordinate with the sulfur atom of the fused heterocyclic
and nitrogen atom of the pyridine ring simultaneously. The above
results reveal that the position of the characteristic IR absorption
bands provides significant indications regarding bonding sites
of the ligands when coordinating with the Tb3+ ion (17).
Figures 3 and 4 show that the free ligands L4 and L6 exhibit
absorption bands at 1611/cm and 1595/cm respectively, which
are assigned to the ν(C = N) stretching vibration of the pyridine
ring, while, in their corresponding Tb3+ complexes, the
absorption band shifts by about 2/cm and 19/cm to a higher
frequency as compared to that of their counterpart for the free
ligands L4 and L6, respectively, which indicate that the nitrogen
atom of the C = N group coordinates with the rare earth ion. The
IR absorption spectra of ligands L4 and L6 are at 1053/cm and
1040/cm, which are attributed to the stretching vibration of the
Ar-O-C group, but in their corresponding Tb3+ complexes, the
Ar-O-C group stretching frequency are red-shifted to 1055/cm
and 1044/cm, respectively. All these results indicate that the
oxygen atom of Ar-O-C group coordinates with the rare earth
It is seen from Table 3 that the characteristic frequencies of
the coordinating nitrate groups appeared approximately at
1481 (υ1), 1324 (υ4), 1031 (υ2) and 815 cm (υ3). In addition, the
difference between the two strongest absorption bands of the
a
a
b
b
4000
3500
3000
2500
2000
1500
1000
500
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm)
Wavenumber (cm)
Figure 3. Infrared spectra of [TbL4(NO3)](NO3)2ꢀH2O (a) and L4 (b).
Figure 4. Infrared spectra of [TbL6(NO3)](NO3)2ꢀH2O (a) and L6 (b).
Luminescence 2014
Copyright © 2014 John Wiley & Sons, Ltd.
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W. Zhang et al.
nitrate groups (|υ1–υ4|) can be defined as Δν. It is generally
believed that the Δν value is below 200 for the bidentate nitrate
moiety, but above 200 for the monodentate nitrate moiety (18).
While, the Δν values of [TbL4(NO3)](NO3)2ꢀH2O and [TbL6(NO3)]
(NO3)2ꢀH2O are about 103/cm and 95/cm, respectively, which
indicate that the nitrate groups coordinate to the Tb3+ ion as
bidentate ligands in the target complexes. Meanwhile, there
are free nitrate groups in the target complexes, which are
consistent with the results of the conductivity experiment, and
confirmed in thermal gravimetric analyses. The broad
absorption band of the –OH stretching vibration in the range
3398–3444/cm can be seen in free ligands and target
complexes, which indicate that the presence of the water
molecules adsorbed on to ligands, the complexes contained
crystallization water, these results will be further confirmed in
thermal gravimetric analyses.
105
100
95
90
85
80
75
70
65
60
55
50
45
40
4
3
2
1
0
Thermal analysis
To investigate thermal stability and details of thermal decompo-
sition of the target complexes, thermal analysis was carried out.
The eight target complexes present similar thermal behavior,
thus, in this paper, we only give the thermogravimetric and
differential thermal analysis curves of [TbL5(NO3)](NO3)2ꢀH2O
and [TbL6(NO3)](NO3)2ꢀH2O as examples for illustration, which
are shown in Figs 5 and 6, respectively. The thermogravimetric
analysis data were obtained at a heating rate of 10 ºC/min and
summarized in Table 4.
100
200
300
400
T (
500
600
700
800
)
Figure 5. TG–DSC curves of [TbL5(NO3)](NO3)2ꢀH2O. DSC, ?????; TG, thermogravimetric.
The thermogravimetric and differential thermal analysis
analyses for complexes [TbL5(NO3)](NO3)2ꢀH2O and [TbL6(NO3)]
(NO3)2ꢀH2O were studied in the range of 20–800 ºC. Figures 5
and 6 show that the weight losses during the first step for these
complexes are 1.73% and 1.66% and between 70 and 150 ºC.
These values have good agreement with the loss of one lattice
of water molecules depending on the complexes (19), and the
calculated values are 1.92% and 1.85%, respectively. In addition,
the target complexes are without endothermic peak and
weightlessness at 150–230 ºC, which illustrate that the water
does not participate in coordination with the target complexes,
because the coordination water of the analogous complexes is
usually released over 200 ºC. Although there are endothermic
peaks of all complexes appearing in the vicinity 265 ºC, but no
apparent weightlessness, which is ascribed to the melt of target
complexes. Meanwhile, there are weight loss steps at tempera-
tures 284 ºC and 348 ºC with 13.30% and 6.65% for [TbL5(NO3)]
(NO3)2ꢀH2O, 12.73% and 6.35% for [TbL6(NO3)](NO3)2ꢀH2O,
corresponding to the process that loss of the outside two
molecules nitrate and internally a molecule nitrate of the
target complexes, respectively, those values correspond with
105
100
3
2
1
0
95
90
85
80
75
70
65
60
55
50
45
40
100
200
300
400
T (
500
600
700
800
)
Figure 6. TG–DSC curves of [TbL6(NO3)](NO3)2ꢀH2O. DSC, ?????; TG, thermogravimetric.
Table 4. Thermal analysis data for target complexes
Complexes
Endothermic peak (ºC)
Temp.
H2O (lost) (%)
TG (Calcd.)
NO-3(lost) (%)
TG (Calcd.)
Ligand (lost) (%)
TG (Calcd.)
Metal residue (%)
TG (Calcd.)
[TbL1(NO3)](NO3)2ꢀH2O
[TbL2(NO3)](NO3)2ꢀH2O
[TbL3(NO3)](NO3)2ꢀH2O
[TbL4(NO3)](NO3)2ꢀH2O
[TbL5(NO3)](NO3)2ꢀH2O
[TbL6(NO3)](NO3)2ꢀH2O
[TbL7(NO3)](NO3)2ꢀH2O
[TbL8(NO3)](NO3)2ꢀH2O
90, 265, 460
265, 372
2.15 (1.99)
1.62 (1.93)
2.20 (1.87)
1.97 (1.81)
1.73 (1.92)
1.66 (1.85)
1.59 (1.70)
1.96 (1.56)
20.16 (20.60)
19.83 (19.98)
19.08 (19.31)
18.82 (18.73)
19.95 (19.81)
19.08 (19.16)
17.26 (17.55)
16.15 (16.12)
35.18 (36.88)
36.14 (38.78)
39.59 (40.81)
38.32 (42.60)
36.87 (39.29)
38.16 (41.30)
43.66 (46.22)
50.72 (50.60)
42.58 (40.53)
43.02 (39.31)
40.51 (38.01)
42.16 (36.86)
42.56 (38.98)
42.04 (37.69)
38.52 (34.53)
31.60 (31.72)
264, 337, 395
241, 291, 408
257, 284, 414
263, 284, 415
86, 266, 404
96, 266, 435
TG, thermogravimetric analysis.
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Synthesis and fluorescence properties of the terbium complexes
100
50
0
the calculated values. The last weight loss step is at 414 ºC for
[TbL5(NO3)](NO3)2ꢀH2O and 415 ºC for [TbL6(NO3)](NO3)2ꢀH2O,
and the weightlessness is 36.87% and 38.16%; these values are
consistent with the calculated values of organic ligands
decomposition, respectively. The target complexes are
decomposed completely at 800 ºC, and the remaining solid
composition is Tb2O3 and its quality percentage is 42.56% for
[TbL5(NO3)](NO3)2ꢀH2O and 42.04% for [TbL6(NO3)](NO3)2ꢀH2O,
which is identical to the calculated values 38.98% and 37.69%,
respectively. The thermal analysis results show that the target
complexes have high thermal stability, and are consistent with
the results of other analyses.
Fluorescence properties analysis
The fluorescence properties of target complexes were
determined on solid-state samples at room temperature, and
both the excitation and emission light slits were 2.5 nm. The
fluorescence spectra data of the target complexes are
summarized in Table 5. The observed fluorescence spectra of
all target complexes are very similar; the fluorescence spectrum
of [TbL6(NO3)](NO3)2ꢀH2O was selected for illustration, and its
excitation and emission spectra are shown in Figs 7 and 8,
respectively.
250
300
35
40
450
500
Wavelength (nm)
Figure 7. Excitation spectra of [TbL6(NO3)](NO3)2ꢀH2O.
800
It is seen from Fig. 7 that the excitation spectrum of
[TbL6(NO3)](NO3)2ꢀH2O, which was measured under an emission
wavelength of 540 nm, possesses a broad excitation band with
the strongest excitation peak at 364 nm. Meanwhile, Fig. 8 shows
that the characteristic emission spectra of the target Tb3+
complex consisted of four main bands at approximately
491.5 nm (5D4 → 7 F6), 545.5 nm (5D4 → 7 F5), 585 nm (5D4 → 7 F4)
and 623 nm (5D4 → 7 F3). It can be noted that there are narrow
and sharp emission peaks appearing at approximately
491.5 nm, which indicates that the complexes have good
monochromaticity and the ligand L6 is a comparatively good
organic chelator for the absorption and transfer of energy to
the center Tb3+ ion (20).
600
400
200
0
450
500
550
600
650
Wavelength (nm)
Figure 8. Emission spectra of [TbL6(NO3)](NO3)2ꢀH2O.
Table 5 shows that the fluorescence intensity of complex
[TbL4(NO3)](NO3)2ꢀH2O is the lower than that of other
complexes, which is due to ligand L4 having an electrophilic
group (-NO2). In addition, the fluorescence intensity of the
complex [TbL2,3(NO3)](NO3)2ꢀH2O is better than that of the
complex [TbL1(NO3)](NO3)2ꢀH2O, which is attributed to ligands
L2 and L3 having an activating group (-CH3 and -OCH3, respec-
tively). The above results show that the electron-withdrawing
groups on the ligands can decrease the fluorescence intensity
of the corresponding target complexes, and this is due to
(i) the electron-withdrawing groups can reduce electron
density of the phenyl ring in the target complexes, and (ii),
introduction of the electron-withdrawing groups can easily
result in fluorescence quenching. On the contrary, the
electron-donating groups on the ligands resulted in an increase
in the fluorescence intensity of target complexes, which is due
to the electron-donating groups donating electrons to the
Table 5. Fluorescence spectra data of the target terbium complexes
5D4 → 7 F6
5D4 → 7 F5
5D4 → 7 F4
5D4 → 7 F3
em (nm) I (a.u.)
1.067
Complexes
λex (nm)
λ
em (nm)
I (a.u.)
λem (nm)
I (a.u.)
λem (nm) I (a.u.)
λ
[TbL1(NO3)](NO3)2ꢀH2O
[TbL2(NO3)](NO3)2ꢀH2O
[TbL3(NO3)](NO3)2ꢀH2O
[TbL4(NO3)](NO3)2ꢀH2O
[TbL5(NO3)](NO3)2ꢀH2O
[TbL6(NO3)](NO3)2ꢀH2O
[TbL7(NO3)](NO3)2ꢀH2O
[TbL8(NO3)](NO3)2ꢀH2O
364
364
364
364
364
364
364
364
492
12.85
492.8
9.169
1.219
340.9
789.3
161.3
28.69
543.5
545.5
545.5
545.5
545.5
545.5
545.5
545.5
22.24
441.6
8.997
1.547
293.9
685.3
164.2
26.69
585
585
585
585
585
585
585
585
3.92
622.5
623
623
623
623
623
623
623
491.5
491.5
491.5
491.5
491.5
491.5
491.5
103.9
2.178
0.434
68.25
165.1
42.86
6.684
22.02
0.539
0.117
14.07
34.73
9.722
1.565
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W. Zhang et al.
aromatic ring and enlarging the π-conjugated system of the li-
gands. Whereas, the fluorescence intensity of complex
[TbL6(NO3)](NO3)2ꢀH2O is the strongest in all target complexes,
which is attributed to the following two factors. First, the chlo-
rine atom has an inductive effect, resulting in the triplet level
of ligand L6 corresponding with the lowest excited state level
of Tb3+ ions. Secondly, electrostatic factors in ligand–metal
bonding, which may be affected by the counter anion in com-
plexes, influenced the triplet state energy level (T1) of the ligand
L6 and lowered this T1 energy level in the target complexes.
Thus, the triplet state energy level of L6 in the Tb3+ complexes
corresponds with the lowest excited state level of Tb3+. In
comparison with the complex [TbL6(NO3)](NO3)2ꢀH2O, the
fluorescence intensity of the complex [TbL7-8(NO3)](NO3)2ꢀH2O
is relatively decreased, which is attributed to the heavy atom
effect. Based on the theory of the antenna effect (21), the
fluorescence intensity of the target complexes is related to
the efficiency of the intramolecular energy transfer between
the triple level of the ligands and the vibrational level of the rare
earth ions, which depends on the energy gap (Eg) between the
two levels. It is shown in Table 5 that the fluorescence intensity
of complex [TbL6(NO3)](NO3)2ꢀH2O is the strongest in all target
complexes, which demonstrate that the triplet level of the
ligand L6 correspond with the vibrational level of the rare earth
ions in all target complexes. At the same time, all target
complexes show characteristic lines for the sensitization of
Tb3+ ions, which shows that the energy will be absorbed and
transferred to central Tb3+ ions effectively. The above results
further highlight that the nature of the substituted group has
a significant effect upon the fluorescence intensity of the Tb3+
complexes.
areas under the fluorescence curves of the sample and the used
standard. Ax and Astd are the absorbances of the sample and
standard at the excitation wavelength, respectively, both the
sample and standard are excited at the same relevant
wavelength, so that the Astd is equal to the Ax. The fluorescence
quantum yields data of all target complexes are summarized in
Table 6.
As shown in Table 6, [TbL2(NO3)](NO3)2ꢀH2O exhibits the
highest quantum yield (0.607), resulting from the larger donor
character of the methyl moiety and larger π-conjugated system
of the phenyl moiety. By contrast, [TbL4(NO3)](NO3)2ꢀH2O shows
the lowest quantum yield (0.490), which is attributed to the n →
π* transition of the electron-withdrawing group (-NO2) substitu-
ent belongs to forbidden transition, the excited state molecules
are seldom obtained. For the halogen-substituted target com-
plexes, the fluorescence quantum yield of [TbL6(NO3)](NO3)
2ꢀH2O is the highest among [TbL5-8(NO3)](NO3)2ꢀH2O. Different
from L5and L7,8, the ligand L6 can induce an electronic
rearrangement of the complexes, which may improve the
energy level matching between the ligands and the central
Tb3+ ions. The results reveal that the 4-position substituent on
the benzene ring has a significant effect on the fluorescence
quantum yields of the target complexes, the electron-donating
substituents can increase the fluorescence quantum yields of
the target complexes, on the contrary, electron-withdrawing
groups can decrease the fluorescence quantum yields of the
target complexes.
12
10
a
b
c
d
e
8
6
4
2
0
Fluorescence quantum yields analysis
The fluorescence quantum yields (Φfx) of the target complexes
were determined using a sulfuric acid solution (0.1 mol/L)
of quinine sulfate (1.0 μg/mL) with a known quantum yield
(ϕfstd = 0.55) as standard reference at room temperature because
of the similarly between its absorption and fluorescent spectra
with those of the target complexes. The fluorescence quantum
yields (Φfx) are calculated by a comparative method (22) using
the following equation.
f
-2
g
-4
h
-6
-8
n2x Fx Astd
-10
-2.5
Φfx
¼
ꢀ
ꢀ
ꢀΦfstd
n2std Fstd Ax
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
E/V
where nx (1.48) and nstd (1.34) are the refractive indices of
solvents used for the sample and standard. Fx and Fstd are the
Figure 9. Oxidation potential of [TbL1~8(NO3)](NO3)2ꢀH2O (a–h).
Table 6. Fluorescence quantum yield data of the target complexes in dimethyl sulfoxide (1.0 × 10–6 mol/L)
Complexes
Excitation wavelength, λ (nm)
Fluorescence intensity, I (a.u.)
Fluorescence quantum yield (Ф)
[TbL1(NO3)](NO3)2ꢀH2O
[TbL2(NO3)](NO3)2ꢀH2O
[TbL3(NO3)](NO3)2ꢀH2O
[TbL4(NO3)](NO3)2ꢀH2O
[TbL5(NO3)](NO3)2ꢀH2O
[TbL6(NO3)](NO3)2ꢀH2O
[TbL7(NO3)](NO3)2ꢀH2O
[TbL8(NO3)](NO3)2ꢀH2O
318
316
320
314
317
314
315
318
2048
2606
1731
1327
2095
2193
2062
1886
0.585
0.607
0.562
0.490
0.594
0.598
0.588
0.571
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Synthesis and fluorescence properties of the terbium complexes
Table 7. Electrochemical data for terbium complexes
Complexes
λonset (nm)
Eox (v)
EHOMO (ev)
Eg (ev)
ELUMO (ev)
[TbL1(NO3)](NO3)2ꢀH2O
[TbL2(NO3)](NO3)2ꢀH2O
[TbL3(NO3)](NO3)2ꢀH2O
[TbL4(NO3)](NO3)2ꢀH2O
[TbL5(NO3)](NO3)2ꢀH2O
[TbL6(NO3)](NO3)2ꢀH2O
[TbL7(NO3)](NO3)2ꢀH2O
[TbL8(NO3)](NO3)2ꢀH2O
254
254
251
249
251
252
250
251
0.677
0.742
0.705
0.646
0.658
0.664
0.670
0.675
5.417
5.482
5.445
5.386
5.398
5.404
5.410
5.415
4.882
4.882
4.940
4.980
4.940
4.921
4.960
4.940
0.535
0.600
0.505
0.406
0.458
0.483
0.450
0.475
Electrochemical properties analysis
substituted by chlorine is the strongest in all complexes. The
electrochemical analysis results showed that the introduction of
electron-donating groups can increase the oxidation potential
and HOMO energy levels of the target complexes, and, introduc-
tion of electron-withdrawing groups can reduce the oxidation
potential and HOMO energy levels of the target complexes. In
addition, the thermal analysis provided information about the
high thermal stability for the target complexes. In summary,
the above results show that the target Tb3+ complexes are
promising candidates for luminescent materials.
The electrochemical properties of the target complexes were
investigated by means of a cyclic voltammetric technique in
DMSO solution (23). The highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels of the target complexes are estimated according
to the electrochemical performance and the UV absorption
spectra (24). The HOMO and LUMO data for target complexes
are obtained using equation EHOMO = 4.74 + eEOX, ELUMO = Eg +
E
HOMO, Eg = 1240/λonset(eV) (13) (λonset is the largest UV
absorption spectra peak starting value). All cyclic voltammetrics
are depicted in Fig. 9 and their electrochemical data are
summarized in Table 7.
Acknowledgments
The authors are grateful for the financial support of the National
Natural Science Foundation of China (no. J1103312), the Natural
Science Foundation of Hunan Province (no. 11JJ5005) and the
Innovative Research Team in University (no. IRT1238) as well as
the Science and Technology Project of Hunan Provincial Science
and Technology Department (no. 2012GK3156). We also thank
Dr. William Hickey, the US Professor of HRM, for the English
editing on this paper.
It is seen from Table 7 that the oxidation potential of all com-
plexes occurs in the potential range +0.646 to +0.742 V. The Eg
of all target complexes is between 4.882 eV and 4.980 eV.
Compared with the complex [TbL1(NO3)](NO3)2ꢀH2O, the oxida-
tion potential and HOMO energy levels of complexes [TbL2-
3(NO3)](NO3)2ꢀH2O are higher, and in contrast, that of the
complexes [TbL4–8(NO3)](NO3)2ꢀH2O are lower; these phenom-
ena are ascribed to the ligands L2–3 having electron-donating
groups and L4-8 having electron-withdrawing groups. The
oxidation potentials and HOMO energy levels of the Tb3+
complexes with halogen substituents are increased in the order
of F < Cl < Br < I, the results show that introducing of electron-
donating groups can increase the oxidation potential of the
Tb3+ complexes, while introduction of an electron-withdrawing
groups can reduce it. In other words, the nature of the
substituent group can affect the HOMO energy levels of the
target complexes. The frontier orbital theory (25) may provide
an explanation for the above phenomena; the HOMO possesses
the priority to provide electrons. For electron donor compounds,
the oxidation process corresponds to the process of losing
electrons in HOMO.
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