Synthesis, characterization and fluorescence properties of novel pyridine dicarboxylic acid derivatives and corresponding Tb(III) complexes

Article abstract of DOI:10.1016/j.saa.2007.12.006

Two novel ligands containing two pyridine-2,6-dicarboxylic acid conjugative units, 4-(2-(2,6-dicarbox-ypyridin-4-yl)vinyl)pyridine-2,6-dicarboxylic acid (L₁) and 4-(4-(2-(2,6-dicarboxypyridin-4-yl)vinyl)styryl)pyridine-2,6-dicarboxylic acid (L₂) and their complexes with Tb(III) have been synthesized and characterized by elemental analysis, IR spectra and NMR. The ligand synthetic route was optimized and the yield of ligands reached over 78% as a result of the Wittig-Horner reaction used. The fluorescent intensities of these complexes with corresponding complexes with single pyridine-2,6-dicarboxylic acid unit was compared. The result has shown that the ligands with two pyridine-2,6-dicarboxylic acid units are the excellent sensitizers to lanthanide fluorescence. Also, we investigated the fluorescence properties of these complexes in different solution and in different pH value. Due to their excellent green-emmiter, they would be a potential candidate material for applications in organic light-emitting devices and medical diagnosis.

Full text of DOI:10.1016/j.saa.2007.12.006

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 71 (2008) 209–214
Synthesis, characterization and fluorescence properties
of novel pyridine dicarboxylic acid derivatives
and corresponding Tb(III) complexes
Guo-liang Gu, Rui-ren Tang∗, You-hu Zheng, Xiao-ming Shi
School of Chemistry and Chemical Engineering, Central South University,
Changsha, Hunan 410083, PR China
Received 20 July 2007; received in revised form 3 December 2007; accepted 9 December 2007
Abstract
Two novel ligands containing two pyridine-2,6-dicarboxylic acid conjugative units, 4-(2-(2,6-dicarbox-ypyridin-4-yl)vinyl)pyridine-2,6-
dicarboxylic acid (L1) and 4-(4-(2-(2,6-dicarboxypyridin-4-yl)vinyl)styryl)pyridine-2,6-dicarboxylic acid (L2) and their complexes with Tb(III)
have been synthesized and characterized by elemental analysis, IR spectra and NMR. The ligand synthetic route was optimized and the yield
of ligands reached over 78% as a result of the Wittig–Horner reaction used. The fluorescent intensities of these complexes with corresponding
complexes with single pyridine-2,6-dicarboxylic acid unit was compared. The result has shown that the ligands with two pyridine-2,6-dicarboxylic
acid units are the excellent sensitizers to lanthanide fluorescence. Also, we investigated the fluorescence properties of these complexes in different
solution and in different pH value. Due to their excellent green-emmiter, they would be a potential candidate material for applications in organic
light-emitting devices and medical diagnosis.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Pyridine dicarboxylic acid derivatives; Tb(III) complexes; IR spectra; Fluorescence property
1. Introduction
benzoyltrifluoroacetone (BFA) [8], ␣-thenoyltrifluoroacetone
(TTA) [9] and pyridine-2,6-dicarboxylic acid (DPA) [10].Since
the lanthanide complexes with DPA derivatives are chiral, they
can get more information about structure of biologic molecule
through measurement Circularly Polarized Luminescence Spec-
tra [11,12], and can be used to tag proteins in time-resolved
fluoroimmunoassay (TR-FIA) successfully.
With an aim to develop novel fluorescence materials, our
group has synthesized compound 1 containing single pyridine-
2,6-dicarboxylic acid unit and found that their Tb(III) complex
display relatively weak fluorescence intensity [13]. In an
attempt to increase the intensity of the lanthanide fluores-
cence exploiting the antenna effect, we synthesized two novel
ligands named 4-(2-(2,6-dicarboxypyridin-4-yl)vinyl)pyridine-
2,6-dicarboxylic acid (L1) and 4-(4-(2-(2,6-dicar-boxypyridin-
4-yl)vinyl)styryl)pyridine-2,6-dicarboxylic acid (L2) and their
Tb(III) complexes. Both of ligands have rigid conjugated planar
structure, which would be beneficial to the electronic negotiabil-
ity [14]. In the Tb(III) complexes, two pyridine-2,6-dicarboxylic
conjugative units of each ligand can coordinate with two lan-
thanide ions respectively to form a ␲-extended conjugated netted
The desire for tailored complexes for use as fluorescent
probes in biological media and optical amplification in fiber-
optic telecommunications systems is a driving interest in
research on luminescent lanthanide complexes [1–4]. In partic-
ular, Eu(III) and Tb(III) complexes have attracted attention due
to their well-defined luminescence properties, including hyper-
sensitivity to the coordination environment, narrow bandwidth
and millisecond luminescence decay times [5,6]. The absorption
coefficients of the optical transitions for these ions are, however,
very low which limits their practical application considerably.
This drawback can be overcome through the use of highly
absorbent chelating ligands, which serve as efficient sensitizers.
In recent years, some rare earth complexes with ␤-diketon, aro-
matic carboxylic acid and heterocyclic carboxylic acid group
were under development, such as benzoyl acetone (BA) [7],
∗
Corresponding author. Tel. +86 731 8836961; fax: +86 731 8879616.
E-mail address: trr@mail.csu.edu.cn (R.-r. Tang).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.12.006

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G.-l. Gu et al. / Spectrochimica Acta Part A 71 (2008) 209–214
Fig. 1. Synthesis route of ligands (L1 and L2).
structure, which would increase the fluorescence efficiency. The
synthetic route of ligands was expressed in Fig. 1.
2.1.1. Preparation of 4-(chloromethyl)pyridine-2,
6-dicarboxylate (2)
To a solution of compound 1 (3.0 g, 13.3 mmol) in anhy-
drous CHCl3 (25 mL), sulfuryl dichloride (2.4 g, 20.0 mmol)
was added dropwise under nitrogen atmosphere at −5 ◦C with
continuousstirringforupto40 min. Excesssolventwasremoved
under reduced pressure and the crude product was purified by
recrystallization from ethanol to give the flaxen solid 2 (2.8 g)
after drying in vacuum. Yield: 86.0%; m.p. 168–170 ◦C.
IR (KBr), ν/cm−1: 3079, 2959, 2836, 1725, 1710, 1380,
1257, 1125, 798. MS, m/z: 243 (M+), 213 (M–OCH3, 62%), 184
(M–OCH3–CO, 100%,). 1H NMR (400 MHz, CDCl3), δ: 8.27
(s, 2H, Py–H ), 4.45 (s, 2H, CH2Cl), 4.06 (s, 6H, OCH3). EA
(calculated for C9H10O5N): % C 49.65 (49.18), H 4.15 (4.23),
N 5.52 (5.37).
2. Experimental
2.1. Materials and apparatus
The purity of terbium oxide exceeds 99.99%. 4-
(Hydroxylme-thyl)pridine-2,6-dicarboxylate 1 and p-xylylene
bis-(triphenyl-phosphonium bromide) 7 were synthesized as
described in the literatures [15,16]. Other chemicals were of
A.R. grade and used without further purification.
Melting points were determined on a XR-4 apparatus (ther-
mometer uncorrected); elemental analysis was carried out by
a PerkinElmer 2400 elemental analyzer; all nuclear magnetic
resonance (NMR) spectra were obtained on a Vario-ET-III appa-
ratus (400 MHz, Vanian company in America). Fluorescence
properties were recorded on RF-2000 apparatus. The infrared
spectra were obtained in the 4000–400 cm−1 region by using a
Avator-360-FT spectrophotometer with KBr plates.
2.1.2. Preparation of 4-formylpyridine-2,6-dicarboxylate
(4)
A solution of pyridinium chlorochromate (7.5 g, 37.5 mmol)
and compound 1 (5.6 g, 25.0 mmol) were refluxed for 5 h in

G.-l. Gu et al. / Spectrochimica Acta Part A 71 (2008) 209–214
211
dry CH2Cl2 (50 mL) at room temperature. CH2Cl2 (25 mL) was
removed under reduced pressure. To the above solution, ethyl
acetate (100 mL) was added and washed with distilled water
(50 mL), 5% Na2CO3 (60 mL × 2) and saturated brine (70 mL)
sequentially. The solution was dried on anhydrous Na2SO4 and
then concentrated under reduced pressure. The rude product was
recrystallized from ethyl acetate and chloroform (3:1, v/v) when
the acicular whitish solid 4 (4.2 g) was obtained. Yield: 75.0%;
m.p. 152–154 ◦C.
was obtained by 732 cation resin column. Yield: 98.4%; m.p.
286–288 ◦C. IR (KBr), ν/cm−1: 3 450, 2918, 1734, 1636, 1593,
1
1399,1259, 981, 696. H NMR (DMSO-d6), δ: 12.50 (s, H,
COOH), 8.05 (s, 2H, Py–H), 6.96 (d, H, J = 17.0 Hz, Py–CH ).
The ligand 9 was prepared by reaction of 8 with the solution
of NaOH by a method similar to the ligand 6. Yield: 96.1%.
1H NMR (400 MHz, DMSO-D6), δ: 7.83 (ds, 2H, J = 17.02 Hz,
Ar–CH ), 7.32 (d, 2H, J = 16.352 Hz, Py–CH ), 8.37 (s, 4H,
Py–H), 7.68 (s, 4H, Ar–H).
IR(KBr), ν/cm−1:3037, 2960, 2861, 1734, 1706, 1355, 1213,
978, 781. MS, m/z: 193 (M–OCH3, 58%), 165 (M–OCH3–CO,
2.2. Preparation of complexes of Tb(III)
1
100%). H NMR (400 MHz, CDCl3 ), δ: 10.02 (s, 1H, CHO),
6.64 (s, 2H, Py–H), 4.06 (s, 6H, OCH3). EA (calculated for
C9H10O5N): % C 53.65 (53.82), H 4.10 (4.06), N 6.30 (6.28).
The solution of TbCl3 (0.4610 mol/L) was prepared accord-
ing to the literature method [13].
To a hot solution of L (L = L1, L2) in distilled water and
ethanol, a two-third molar amount of TbCl3 was added and
stirred for 5 h at 50 ◦C. The pH value of the solution was adjusted
to 7.0 by dropwise addition of aqueous NaOH (0.01 mol/L).
The precipitate obtained on concentration was filtered, washed
thoroughly with alcohol and chloroform mixture (1:1, v/v) and
air-dried.
2.1.3. Preparation of dimethyl
4-(2-(2,6-bis(methoxycarbonyl)
pyridin-4-yl)vinyl)pyridine-2,6-dicarboxylate (5) and
dimethyl
4-(4-(2-(2,6-bis(methoxycarbonyl)pyridine-4-yl)vinyl)
styryl)pyridine-2,6-dicarboxylate (8)
A solution of triphenylphosphine (3.4 g, 12.5 mmol) and
compound 2 (2.5 g, 10.0 mmol) in DMF (5 mL) was refluxed
for 5 h, then cooled to the room temperature. To the above solu-
tion, the anhydrous benzene (15 mL) was added and refluxed
for 1 h. The reaction mixture was filtered to obtain the colorless
solid 3 (3.5 g). Yield: 86.4%.
3. Results and discussion
3.1. Properties of the complexes
The results of elemental analysis (Table 1) indicated that the
composition of the complexes conforms to Na6Tb2(L1)3·18H2O
and Na6Tb2(L2)3·23H2O. The complexes are faint yellow col-
ored and stable. They are soluble in H2O, DMF, DMSO, slightly
soluble in ethanol and acetone, and insoluble in benzene, diethyl
ether and tetrahydrofuran. Because of the insolubility of the
complexes in suitable solvents we were unsuccessful in growing
crystals for single crystal X-ray structural studies.
To the absolute methanol (50 mL) solution of compound 3
(2.6 g, 5.0 mmol) and compound 4 (1.2 g, 5.0 mmol), the solu-
tion of sodium methanolate (0.4 g, 6.0 mmol) in 20 mL absolute
methanol was added dropwise and stirred up to 1 h under nitro-
gen atmosphere at −20 ◦C, and kept it overnight with stirring at
room temperature. The solution was concentrated under reduced
pressure and a whitish solid 5 was obtained by flash chromatog-
raphyelutingfromethylacetate, petroleumetherandchloroform
(1:1:1, v/v). Yield: 81.4%; m.p. 191–193 ◦C. IR (KBr), ν/cm−1
:
3.2. IR spectra
3412, 2954, 1712, 1675, 1592, 1439, 1399, 1253, 994, 721.
1H NMR (CDCl3 ), δ: 6.97 (d, 2H, J = 16.0 Hz, Py–CH ),
8.05 (s, 4H, Py–H), 3.98 (s, 6H, OCH3). EA (calculated for
C20H18O8N2): % C 57.21 (57.97), H 4.79 (4.35), N 6.30 (6.76).
Compound 8 was obtained following the same procedure
described as compound 5. Wheat solid, yield: 78.3%, m.p.
223–225 ◦C. IR (KBr ), ν/cm−1: 3448, 2981, 1718, 1654, 1594,
1396, 1288, 964. 1H NMR (400 MHz, CDCl3 ), δ: 7.18 (d, 2H,
J = 16.4 Hz, Ar–CH ), 7.51 (d, 2H, J = 16.0 Hz, Py–CH ), 8.37
(s, 4H, Py–H), 7.51 (s, 4H, Ar–H), 3.92 (s, 6H, OCH3). EA (cal-
culated for C24H28O8N2): % C 65.21 (65.11), H 4.79 (4.86), N
5.30 (5.42).
In the IR spectra of compounds 5 and 8, the strong and
sharp bands at 994 and 964 cm−1 are assigned to δ ( C H), and
the medium intensity and wide bands at 1675 and 1654 cm−1
attributed to ν (C C), which suggests compounds 5 and 8 are
trans-configuration with double bonds.
The bands useful for suggesting the mode of coordination of
ligands are listed in Table 2. The uncoordinated ligands show
the bands of middle intensity at 2918 cm−1 (L1) and 2915 cm−1
(L2) attributed to free υOH of COOH, and strong and sharp
bands at 1734 cm−1 (L1) and 1718 cm−1 (L2) assignable to
C O. The bands at 1592 cm−1 (L1) and 1594 cm−1 (L2) were
assigned to pyridine ring stretching [υ (C N)] [17]. In the com-
plexes, the band for υ (C N) of pyridine ring shows a shift to a
lower frequency about 180 cm−1 as a result of the coordination
through metal-nitrogen bond. The bands (C O) in free ligands
disappear and the new bands appear at 1617, 1444 cm−1 and
1620, 1443 cm−1 assignable to [νas (COO−) + νS (COO−)]. The
asymmetric stretching vibration frequencies (υas) of COO− in
2.1.4. Preparation of 4-(2-(2,6-dicarboxypyridin-4-
yl)vinyl)pyridine-2,6-dicarboxylic acid (6, L1) and
4-(4-(2-(2,6-dicarboxypyridin-4-yl)vinyl)styryl)
pyridine-2,6-dicarboxylic acid (9, L2)
A solution of NaOH (30%, 1.2 mL) was added into 5 (2.3 g,
5.5 mmol) in methanol (50 mL) and the mixture was stirred at
room temperature for 24 h. Then the solvent was completely
removed under reduced pressure. The whitish solid 6 (1.6 g)
complexesshowashifttoalowerfrequencyabout98–128 cm−1
meanwhile the symmetric stretching vibration frequencies (υs)
;

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G.-l. Gu et al. / Spectrochimica Acta Part A 71 (2008) 209–214
Table 1
Elemental analytical data for the complexes
Complex
M (%) found (calc.)
C (%) found (calc.)
H (%) found (calc.)
N (%) found (calc.)
Na6Tb2(L1)3·18H2O
Na6Tb2(L2)3·23H2O
17.12 (17.26)
14.33 (14.21)
31.02 (31.27)
38.34 (38.61)
2.87 (2.93)
3.31 (3.66)
4.73 (4.56)
3.81 (3.75)
of COO− show a shift to a higher frequency about 40–45 cm−1
,
which indicates the ligand coordinates the Tb(III) ions through
their carboxyl.
These above suggest that each lanthanide ion coordinates
with three double-chelated pyridine rings which have three
nitrogen atoms and six oxygen atoms forming a contorted
three coronary triangle prism polyhedral structure [18,19]. So
the coordination numbers of Tb(III) is nine. A broad band at
3350–3420 cm−1 and a peak between 920 and 935 cm−1 in
the complexes indicates the presence of lattice water molecules
[20]. The non-ligand bands observed in the region about 430
and 525 cm−1 are assigned to the υ (Tb–O) and (Tb–N) bands
[17,21].
3.3. Fluorescence properties of complexes
Fig. 2. Excitation (left) and emission (right) spectrum of Na6Tb2(L1)3·18H2O.
The fluorescence spectra of the ligands and their Tb(III) com-
plexes was carried out in solid and different solutions at room
temperature.
level. The most intense transition is observed at 543 and 546 nm
7
due to the 5D4 → F5, which can exhibit some structuring under
7
high resolution. The 5D4 → F6 translation at 491 and 492 nm
is moderately strong and is known to be sensitive to ligand
environment. It has a strong magnetic dipole character. The
3.3.1. Fluorescence properties of Tb(III) complexes in solid
Fluorescence properties of Tb(III)complexes in solid was
shown in Table 3, Figs. 2 and 3.
5D4 → F4 transition at 582 and 585 nm is of moderate intensity.
7
7
The component at 620 and 623 nm is assigned to the 5D4 → F3
The Tb(III) complexes show a strong emission when excited
transition.
5
at 279 and 280 nm with the emission emanating from the D4
The typical narrow emission bands of Tb(III) ions can be
detected upon excitation of the ligand-centered absorption band,
and the ligand-luminescence is completely suppressed, indi-
cating that the ligand is a comparative good organic chelator
to absorb energy and transfer them to Tb(III) ion. It can be
explained the giving of energy (ligand) is much more than the
accepted (Tb(III)) and the intersystem crossing (S1 → T1) from
the first singlet excited state of ligand to the excited triplet state
Table 2
Diagnostic IR bands (cm−1) of the ligand and its Ln(III) complexes
Compound
ν
ν
Py
δ
νas
νS
(C O) (OH) ‘N’
(
C
H) (COO−) (COO−)
L1
L2
1734
1718
2918 1592 994
2915 1594 964
1413 1023
Na6Tb2(L1)3·18H2O
1617
1620
1444
1443
Na6Tb2(L2)3·23H2O
1415 985
Table 3
Fluorescence properties of Tb(III) complex in solid
Complex
λex (nm)
λem (nm)
RFIa
Assignment
7
Na6Tb2(L1)3·18H2O
280
492
546
585
623
4207
8986
960
5D4 → F6
5D4 → F5
7
7
5D4 → F4
7
312
5D4 → F3
7
Na6Tb2(L2)3·23H2O
279
491
543
582
620
4682
8455
748
5D4 → F6
7
5D4 → F5
7
5D4 → F4
7
266
5D4 → F3
The width of emission slit and excitation slit were 2.5 nm, the voltage of photo-
multiplier tube was 400 V.
a
RFI: Relative fluorescence intensity.
Fig. 3. Excitation (left) and emission (right) spectrum of Na6Tb2(L2)3·23H2O.

G.-l. Gu et al. / Spectrochimica Acta Part A 71 (2008) 209–214
213
of the organic solvent [26]. At the same concentration, the fluo-
rescence intensity order of the complexes in different solvents is
H2O > DMF > DMSO, but the order of solvent dipole moment
is contrary. Samy [27] had brought forward a theory to explain
the phenomenon: the Franck–Condon excites state exists in the
system of fluorescent molecules. The solvent which has lower
dipole moment can stabilize the ground state that corresponds to
the Franck–Condon excited state. This effect results in that the
triplet states of T2 (␲–␲*) exchanges with those of T1 (␲–␲*)
in ligand, thus, the transition S (n–␲*) → T2 (␲–␲*) becomes
possible. And the probability of intersystem crossing (Kiso)
from the singlet state to the triplet state of ligand increases. The
fluorescent mechanism of the ligand-Tb(III) (λem = 543 nm) is
5
7
S0 → Sh → S1 → T → D4 → F5, and the increasing of Kiso
5
7
causes the enhancement of emissive probability D4 → F5 in
Tb(III), thus, the less the dipole moment of solvent molecule is,
the stronger the fluorescence intensity is [28].
Fig. 4. Changes of fluorescence intensities of Tb(III) complexes in different pH
environment.
of ligand is increased. The emission spectra in Fig. 2 or Fig. 3
4. Conclusions
7
consists of two main lines at 491–492 nm (5D4 → F6) and
543–546 nm (5D4 → F5); the intensity ratio of the two lines
7
The preparation and fluorescence properties of Tb(III) com-
plexes of the novel ligands (L1 and L2) are reported. Each ligand
comprises two pyridine-2,6-dicarboxyl acid units. The study of
fluorescence shows the Tb(III) complexes display characteristic
metal-centered fluorescence, indicating that efficient ligand-to-
metal energy transfer (antenna effect) occurred; the ligand with
two pyridine-2,6-dicarboxylic acid units is the excellent sen-
sitizer to lanthanide fluorescence over the ligand with single
pyridine-2,6-dicarboxylic acid unit. The fluorescence intensity
of the complexes in solid is stronger than that in H2O, DMF
or DMSO solvents, and effect of pH value shows the fluores-
cence intensity of complexes in neutral solution is stronger
than that in acidic or basic solution. Due to their excellent
green-emmiter, they would be a potential candidate material
for applications in organic light-emitting devices and medical
diagnosis.
(5D4 → F5/5D4 → F6) is about 2.1, which indicates that the
7
7
Tb(III) ion is at the center of an asymmetric coordination field
[22,23]. The 5D4 → F5 peak of Tb(III) complexes is sharp and
7
strong, showing higher color purity and emission intensity [24].
3.3.2. The effect of pH on fluorescence properties
The effect of pH value on the fluorescence intensity of com-
plexes is shown in Fig. 4.
When the pHwas at7, thefluorescenceintensityof theTb(III)
complexes would get to the maximum, which can be explained
that carboxylate anions is easy to be protonized in the acidic
aqueous solution; while the lanthanide ions can coordinate with
hydroxyl in the alkaline aqueous solution, which lead precipita-
tion to be produced as the pH increases gradually. The result is
similar to the literature reported by Xian-Hong Yin [25] about
the relationships between the pH and the fluorescence intensity
of lanthanide complexes of other DPA derivatives.
Acknowledgement
3.3.3. The effect of different solvents on fluorescence
properties
This work is financially supported by National Natural Sci-
ence Foundation of China (No. 50573019).
The fluorescence intensity of Tb(III) complexes in H2O,
DMF and DMSO are listed in Table 4. Each solution was diluted
to 1.0 × 10−5 mol/L.
References
The spectra shows that the fluorescence intensity of the com-
plexes in solution are weaker than that in solid, which can be
due to the excited state of the lanthanide ions is efficiently
quenched by interactions with high-energy vibrations groups
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