Synthesis and rare earth metal ion-sensing properties of aza-crown derivative incorporating with diaryl-1,3,4-oxadiazole

Article abstract of DOI:10.1016/j.saa.2010.10.027

A new fluorescent chemosensor (A18C6-Ox) in which a monoaza-18-crown-6 is linked to a diaryl-1,3,4-oxadiazole fluorophore by a methylene spacer has been synthesized to evaluate binding interaction with the rare earth ions by means of absorption and emission spectrophotometry. Absorption spectra of A18C6-Ox showed a broad band at 289 nm and there was no significant change in the presence of Sc³⁺, La³⁺, Pr³⁺, Sm³⁺, Gd³⁺, Tb³⁺, Yb³⁺ and Lu³⁺ except for Ce³⁺ and Eu³⁺. From the emission spectral change of A18C6-Ox, interaction of the rare earth ions with A18C6-Ox is very strong. The formation of A18C6-Ox complexing with Sc³⁺, La³⁺, Pr ³⁺, Sm³⁺, Gd³⁺, Tb³⁺, Yb ³⁺ and Lu³⁺ leads to an increase in fluorescence intensity of A18C6-Ox, while Ce³⁺ and Eu³⁺ ions interact strongly causing fluorescence quenching of A18C6-Ox. In addition, the optimal complexation stoichiometry of the rare earth ions with A18C6-Ox was investigated by the fluorescent titration.

Full text of DOI:10.1016/j.saa.2010.10.027

Spectrochimica Acta Part A 78 (2011) 396–400
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and rare earth metal ion-sensing properties of aza-crown derivative
incorporating with diaryl-1,3,4-oxadiazole
Tianzhi Yua,∗, Jing Menga,b, Yuling Zhaob, Hui Zhanga, Xiaoqian Hanb, Duowang Fana
a Key Laboratory of Opto-Electronic Technology and Intelligent Control (Lanzhou Jiaotong University), Ministry of Education, Lanzhou 730070, PR China
b School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 April 2010
Received in revised form 8 October 2010
Accepted 25 October 2010
A new fluorescent chemosensor (A18C6-Ox) in which a monoaza-18-crown-6 is linked to a diaryl-1,3,4-
oxadiazole fluorophore by a methylene spacer has been synthesized to evaluate binding interaction with
the rare earth ions by means ofabsorption and emission spectrophotometry. Absorption spectra of A18C6-
Ox showed a broad band at 289 nm and there was no significant change in the presence of Sc3+, La3+, Pr3+
,
Sm3+, Gd3+, Tb3+, Yb3+ and Lu3+ except for Ce3+ and Eu3+. From the emission spectral change of A18C6-Ox,
interaction of the rare earth ions with A18C6-Ox is very strong. The formation of A18C6-Ox complexing
with Sc3+, La3+, Pr3+, Sm3+, Gd3+, Tb3+, Yb3+ and Lu3+ leads to an increase in fluorescence intensity of
A18C6-Ox, while Ce3+ and Eu3+ ions interact strongly causing fluorescence quenching of A18C6-Ox. In
addition, the optimal complexation stoichiometry of the rare earth ions with A18C6-Ox was investigated
by the fluorescent titration.
Keywords:
Synthesis
Diaryl oxadiazole
Azacrown ether
Rare earth ions
UV–Visible spectrum
Fluorescence
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
was expected that the oxadiazole moiety could be used to con-
struct new PET chemosensors. In addition, the radius of RE (III) ion
˚
˚
The design and synthesis of functional luminescent probes for
metal cations are of growing interest because of their poten-
tial applications in biomedical and environmental chemosensors
[1] or molecular logic devices [2]. Chemosensors which applied
the principle of photoinduced electron transfer are called PET
chemosensors. Fluorescent aromatic/heteroaromatic derivatives
such as anthracene, pyrene, coumarin and benzothiazole have been
widely employed for the construction of PET based chemosen-
sors [3–10]. The sensing abilities of the molecular probes to metal
cations strongly relied on the photophysical and electrochemi-
cal properties of the fluorophores. Therefore, research for new
fluorophores is closely related to further development of PET
chemosensors.
(1.1–1.2 A) is comparable with the radius of cavity size (1.3 A) of
18-crown-6 (18-C-6), the complex of RE (III) ion with 18-C-6 by
1:1 molar ration was easily formed, i.e. RE (III) entered in the cavity
of the crown ether. In this paper we report the synthesis and fluo-
rescence property of monoaza-18-crown-6 derivative (A18C6-Ox)
comprising a diaryl-1,3,4-oxadiazole substituent linked directly to
macrocyclic nitrogen atom viaamethylene spacer, andinvestigated
the spectral properties of diaryl-1,3,4-oxadiazole chromophore in
the presence of RE (III) ions by means of UV–Vis absorption and
emission spectrophotometry. The results of our studies concern-
ing the interaction of RE (III) ions (Sc3+, La3+, Ce3+, Pr3+, Sm3+, Eu3+
Gd3+, Tb3+, Yb3+ and Lu3+) with fluoroionophore are reported.
,
Aza-crown ethers can bind with various metal cations because
of the nitrogen atoms in the ring, such as alkali metal ions, alkaline-
earth metal ions, transition metal ions, rare-earth metal ions and
so on [11–13]. Diaryl-1,3,4-oxadiazole derivatives are well known
to exhibit strong fluorescence and often used as electron-transport
materials in electroluminescence devices [14–19], and their appli-
cations as the signaling component in molecular sensory systems
have been investigated for metal cation recognitions [20–23]. As
the chromophoric oxadiazole is an effective electron acceptor, it
2. Experimental
2.1. Materials and methods
Rare earth oxides were purchased from the Shanghai Yao-
long Chemical Factory. Rare earth chlorides were prepared by
the method described in the literature [24]. Monoaza-18-crown-
6 was purchased from ABCR GmbH & Co. KG. 4-tert-Butylbenzoyl
chloride was obtained from Alfa Aesar. P-Toluic acid and N-
bromosuccinimide (NBS) were obtained from Shanghai Zhongqin
chemical reagent Co. Ltd. and Tianjin Guangfu Fine Chemical
Research Institute, respectively. All of the solvents used were ana-
lytical grade reagents.
∗
Corresponding author. Tel.: +86 931 4956935; fax: +86 931 4938756.
E-mail address: ytz823@hotmail.com (T. Yu).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.10.027

T. Yu et al. / Spectrochimica Acta Part A 78 (2011) 396–400
397
CONHNH2
COOH
COCH2CH3
H2O
NH2NH2
CH3CH2OH
H2SO4
H3C
H3C
H3C
O
O
C
N
ClOC
Pyridine
N
POCl3
H3C
C
NHNH
H3C
NBS
O
CCl4 / BPO
N
N
BrH2C
O
O
O
O
CH3CN / K2CO3
O
HN
O
O
N
O
O
O
N
N
O
O
A18C6-Ox
Scheme 1. Synthetic routes to A18C6-Ox.
IR spectra (400–4000 cm−1) were measured on a Shimadzu
IRPrestige-21 FT-IR spectrophotometer. 1H NMR spectra were
obtained on Unity Varian-500 MHz. C, H, and N analyses were
obtained using an Elemental Vario-EL automatic elemental anal-
ysis instrument. UV–Vis absorption and photoluminescent spectra
were recorded on a Shimadzu UV-2550 spectrometer and Perkin
Elmer LS-55 spectrometer, respectively. Melting points were mea-
sured by using an X-4 microscopic melting point apparatus made in
Beijing Taike Instrument Limited Company, and the thermometer
was uncorrected.
2-(4-methylphenyl)-5-(4-tert-Butylphenyl)-1,3,4-oxadiazole:
(12.4 g, 33.9 mmol) of
1-(4ꢀ-Methylbenzoyl)-2-(4ꢀ-tert-
butylbenzoyl)-hydrazine was dispersed in 80 mL of POCl3 at
room temperature. The mixture was refluxed for 10 h at 80 ◦C.
After cooling, the reaction mixture was poured carefully into
500 mL of ice water under stirring. The precipitate was collected by
filtration, washed with water and dried under vacuum. The crude
product was purified by recrystallization from methanol to give
a white solid. Yield: 81%. m.p. 110–111 ◦C. IR (KBr pellet, cm−1):
2951, 2864, 1615, 1551, 1494, 1416, 1273, 1181, 1118, 1069, 1015,
844, 822 cm−1 1H NMR (CDCl3): ı 8.0 (d, J = 6.7 Hz, 2H, –ArH), 7.85
;
(d, J = 7.1 Hz, 2H, –ArH), 7.3 (d, J = 6.7 Hz, 2H, –ArH), 7.15 (d, J = 7.1,
2H, –ArH), 2.3 (s, 3H, –CH3), 1.3 (s, 9H, –(CH3)3).
2.2. Synthesis and characterization of A18C6-Ox
2-[4-(bromomethyl)phenyl]-5-(4-tert-butylphenyl)-
1,3,4-oxadiazole:
2-(4-methylphenyl)-5-(4-tert-Butylphenyl)-1,3,4-oxadiazole,
(5.0 g,
17.0 mmol)
of
The synthetic routes of A18C6-Ox are described in Scheme 1.
p-Toluic hydrazide: P-Toluic acid (10 g, 73.5 mmol) was dissolved
in anhydrous ethanol (200 mL), and H2SO4 (5 mL) was added drop-
wise to the stirred solution and the mixture was refluxed for 20 h.
The excess ethanol was removed by distillation. The crude product
was then stirred vigorously at 80 ◦C in excessive hydrazine hydrate
(20 mL) for 15 h, and the reaction was standed at room temperature
overnight. The white precipitate was obtained by filtration, washed
with water and ethanol. The product was dried under reduced pres-
sure, and purified with recrystallization from methanol. Yield: 84%.
m.p. 115–117 ◦C. IR (KBr pellet, cm−1): 3306, 3225, 3022, 1621,
1351, 830, 722.
(3.6 g, 1.2 equiv) of N-bromosuccinimide (NBS) and a catalytic
amount of benzoyl peroxide (BPO) were dissolved in 100 mL
of carbon tetrachloride. The reaction mixture was refluxed for
about 8 h. The warm reaction mixture was filtered under suction
and the precipitated succinimide washed with hot CCl4. After
the solvent was evaporated, the crude product was purified by
recrystallization from methanol to afford a white solid. Yield: 85%.
m.p. 150–152 ◦C (lit. m.p. 150–154 ◦C [21]). IR (KBr pellet, cm−1):
3090, 3051, 3011, 2965, 2867, 1615, 1552, 1499, 1422, 1230, 1197,
1100, 1072, 1015, 965, 850, 711.
1-(4ꢀ-Methylbenzoyl)-2-(4ꢀ-tert-butylbenzoyl)-hydrazine: 4-tert-
butylbenzoyl chloride (17.3 g, 88.3 mmol) was added dropwise to a
solution of p-toluic hydrazide (13.2 g, 88.3 mmol) in 100 mL of dry
pyridine at 0 ◦C under vigorous stirring. The mixture was heated
to reflux for 5 h. After cooled, the mixture was poured carefully
into 200 mL of ice water. The white precipitate was collected by
filtration, washed with water and dried under vacuum. The crude
product was purified by recrystallization from methanol to afford
a white solid. Yield: 78%. m.p. 237–238 ◦C. IR (KBr pellet, cm−1):
3225, 2968, 1669, 1635, 1540, 1506, 1277, 1107, 850, 749.
N-[2-(4-methylphenyl)-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole]aza-18-crown-6 (A18C6-Ox): Under the protection
of argon,
(4-tert-butylphenyl)-1,3,4-oxadiazole
a
mixture of 2-[4-(bromomethyl)phenyl]-5-
(0.85 g, 2.28 mmol),
1-aza-18-crown-6 (0.50 g, 1.90 mmol) and anhydr. K2CO3 (0.60 g,
4.35 mmol) were dissolved in 30 mL of dry acetonitrile. The reaction
mixture was stirred and refluxed for about 5 h. Then the mixture
was filtered and the filtrate was concentrated in vacuum. The
brown oily mass obtained was extracted with dichloromethane
(3 × 50 mL). The combined organic layers were washed with water

398
T. Yu et al. / Spectrochimica Acta Part A 78 (2011) 396–400
0.6
UV absorption
Emission
A18C6-Ox
Ce3+
A18C6-Ox + Ce
1.0
0.5
0.0
1.0
0.5
0.0
3+
0.4
0.2
0.0
250
300
350
400
450
500
Wavelength (nm)
250
300
350
400
Wavelength (nm)
Fig. 1. UV–Vis absorption and fluorescence spectra of A18C6-Ox in MeCN solution
at room temperature (C = 1.0 × 10−5 mol/L).
Fig. 3. Absorption spectra of free Ce3+ (1.0 × 10−3 mol/L) A18C6-Ox
(1.0 × 10−5 mol/L) in MeCN and A18C6-Ox containing Ce3+ (CCe3+ = 1.0 × 10−3 mol/L).
(2 × 50 mL), and dried over anhydrous MgSO4. The solvent was
removed in vacuum, and the residue was purified by silica-gel
column chromatography using CHCl3/CH3OH (99:1, v/v) as the
eluent to afford pale yellow oil. Yield: 56% (0.58 g). IR (KBr pellet,
cm−1): 2967, 2874, 1616, 1490, 1343, 1263, 1110, 957, 837, 717.
1H NMR (500 MHz, CDCl3, ı, ppm): 8.08 (d, 2H, J = 6.8 Hz, –Ar–H),
8.06 (d, 2H, J = 6.6 Hz, –Ar–H), 7.56 (d, 2H, J = 6.6 Hz, –Ar–H), 7.54
(d, 2H, J = 6.8 Hz, –Ar–H), 3.78 (s, 2H, –CH2–), 3.72–3.63 (m, 20H,
–OCH2CH2–), 2.83 (t, 4H, J = 7.4 Hz, >NCH2CH2–), 1.38 (s, 9H, –CH3).
only a slight increase or decrease in the band intensity. These results
show that no significant ground state perturbation of absorption
characteristics of the fluorophore was not produced in the pres-
ence of the above ions chlorides because of the absence of direct
electronic link between the aza-crown and oxadiazole fluorophore
[21]. However, the differences appeared in the addition of Ce3+ and
Eu3+ chlorides to the MeCN solution of A18C6-Ox (Figs. 3 and 4).
Compared with the absorption spectra of the free Ce3+ solution and
the free A18C6-Ox solution, the absorption spectra of A18C6-Ox
with the addition of Ce3+ showed two bands at 266 nm and 316 nm,
respectively, which were similar to that of the free Ce3+ in MeCN
solution, and the absorption peak at 289 nm did not appear (Fig. 3).
In other words, upon adding chloride of Ce3+ to the MeCN solution
of A18C6-Ox, the UV spectrum of the mixture mainly exhibited the
characteristic absorption band of Ce3+, which is much stronger than
that of the free Ce3+ at the same concentration, and the absorption
band of A18C6-Ox was covered. From Fig. 4, after adding chlo-
ride of Eu3+, the absorption spectrum of the A18C6-Ox solution
also showed the characteristic absorption band of Eu3+, the maxi-
mal absorption peak was at 282 nm and was stronger than that of
the free Eu3+ in MeCN solution. Similarly, the absorption band of
A18C6-Ox was covered. The absorption of free metal ions Ce3+ and
Eu3+ were stronger, which is the reason why did the absorption
band of A18C6-Ox be covered.
3. Result and discussion
3.1. UV absorption spectra
UV absorption spectrum of A18C6-Ox measured in diluted
MeCN is shown in Fig. 1. From Fig. 1, A18C6-Ox exhibited a broad
absorption band at 289 nm which is similar to MOX because of the
same fluorophore diaryl-1,3,4-oxadiazole in the structures of them
[21]. The effects of the addition of the selected rare earth ions (Sc3+
,
La3+, Pr3+, Sm3+, Gd3+, Tb3+, Yb3+ and Lu3+) chlorides on the UV
absorption spectra of A18C6-Ox in MeCN solution are shown in
Fig. 2.
Addition of MeCN solution of Sc3+, La3+, Pr3+, Sm3+, Gd3+, Tb3+
,
Yb3+, Lu3+ (1.0 × 10−3 mol/L) to the MeCN solution of A18C6-Ox
(1.0 × 10−5 mol/L) did not result in any obvious changes either in
the shape or the position of absorption spectra of A18C6-Ox, but
0.4
Sm3+>Pr3+>La3+>Tb3+>Yb3+>Lu3+>Gd3+>Sc3+
A18C6-Ox
3+
0.6
0.4
0.2
0.0
A18C6-Ox + Eu
0.3
0.2
0.1
0.0
Eu3+
A18C6-Ox
250
300
350
400
250
300
350
400
Wavelength (nm)
Wavelength (nm)
Fig. 2. Absorption spectra of A18C6-Ox (1.0 × 10−5 mol/L) in MeCN and A18C6-Ox
containing Sc3+, La3+, Pr3+, Sm3+, Gd3+, Tb3+, Yb3+ and Lu3+ (CM3+ = 1.0 × 10−3 mol/L).
Fig. 4. Absorption spectra of free Eu3+ (1.0 × 10−3 mol/L) A18C6-Ox
(1.0 × 10−5 mol/L) in MeCN and A18C6-Ox containing Eu3+ (CEu3+ = 1.0 × 10−3 mol/L).

T. Yu et al. / Spectrochimica Acta Part A 78 (2011) 396–400
399
3+
A18C6-Ox + La
3+
800
600
400
200
0
A18C6-Ox + Gd
3+
A18C6-Ox + Sc
800
3+
A18C6-Ox + Yb
A18C6-Ox + Pr
3+
3+
A18C6-Ox + Tb
3+
A18C6-Ox + Lu
3+
600
A18C6-Ox + Sm
A18C6-Ox
3+
A18C6-Ox + Eu
3+
A18C6-Ox + Ce
400
300
350
400
450
500
0.0
0.5
1.0
1.5
2.0
Eq. of Tb3+ ions
Wavelength (nm)
Fig. 5. Fluorescence spectra of A18C6-Ox (1.0 × 10−5 mol/L) in MeCN before and
after the addition of Sc3+, La3+, Ce3+, Pr3+, Sm3+, Eu3+, Gd3+, Tb3+, Yb3+ and Lu3+
(CM3+ = 1.0 × 10−4 mol/L).
Fig. 6. Variation in fluorescence intensity of a solution of A18C6-Ox in MeCN
(1.0 × 10−5 mol/L) at 358 nm as a function of equivalents of Tb3+ ions.
800
600
400
3.2. Emission spectra
Excitation of A18C6-Ox at its absorption maximum at 289 nm
produced emission band at 358 nm which presumably originates
from the locally excited stated (Fig. 1). Addition of the selected rare-
earth chlorides (Sc3+, La3+, Ce3+, Pr3+, Sm3+, Eu3+, Gd3+, Tb3+, Yb3+
,
Lu3+) upto a concentration of 1.0 × 10−4 mol/L to a MeCN solution
of A18C6-Ox (1.0 × 10−5 mol/L) led to no noticeable shift either in
the shape or position of the emission spectra (Fig. 5). However, the
luminescence intensities were increased or decreased in various
degrees depending upon the nature of the cations.
From Fig. 5, addition of Sc3+, La3+, Pr3+, Sm3+, Gd3+, Tb3+, Yb3+
and Lu3+ chlorides produced several folds enhancements of fluores-
cence, the order of emission enhancement being La3+ > Gd3+ > Sc3+
≈ Yb3+ > Pr3+ ≈ Tb3+ > Lu3+ ꢁ Sm3+. However, with the addition of
Eu3+ and Ce3+, the fluorescence intensity of A18C6-Ox was induced
quenching, and the highest quenching efficiency is observed for
Ce3+. The results were suggested that the singlet (S1) and the triplet
0.0
0.5
1.0
1.5
2.0
Eq. of Lu3+ ions
Fig. 7. Variation in fluorescence intensity of a solution of A18C6-Ox in MeCN
(1.0 × 10−5 mol/L) at 358 nm as a function of equivalents of Lu3+ ions.
(T1) of A18C6-Ox are lower than the lowest 4f levels of Sc3+, La3+
,
Pr3+, Sm3+, Gd3+, Tb3+, Yb3+ and Lu3+, but are higher than the low-
est 4f levels of Eu3+ and Ce3+. So diaryl-1,3,4-oxadiazole-to-Eu3+
or Ce3+ energy transfer mechanism can be invoked to explain the
observed fluorescence quenching by Eu3+ or Ce3+. In the case of
Ce3+ ion, the radiationless energy transfer process from diaryl-
1,3,4-oxadiazole (energy donor) to Ce3+ ion (energy acceptor) is
highly probable because of a strong overlap of the emission spec-
trum of diaryl-1,3,4-oxadiazole and absorption spectrum of Ce3+
ion (Figs. 1 and 3). In the case of Eu3+ ion a lower overlap of the emis-
sion spectrum of diaryl-1,3,4-oxadiazole and absorption spectrum
of Eu3+ ion causes a lower fluorescence quenching of A18C6-Ox by
this ion (Figs. 1 and 4). The special phenomenon observed from Eu3+
and Ce3+ indicates that A18C6-Ox has selected effect on Eu3+ and
Ce3+ at certain extent which can let us distinguish Eu3+ and Ce3+
from the other rare earth metal ions.
Figs. 6 and 7 show the plots of changes in fluorescent intensity
of A18C6-Ox as a function of increasing Tb3+ and Lu3+ ions concen-
tration in MeCN solvent, respectively. At the beginning, with the
increase of the ions concentration, the fluorescent intensity also
increased. As expected, addition of ca. l equiv of Tb3+ or Lu3+ to a
solution of A18C6-Ox in MeCN led to an approximate plateau indi-
cating 1:1 complexation stoichiometry. The fact indicates that the
cavity of the aza-18-crown-6 could accommodate one rare earth
metal ion.
4. Conclusions
A new PET based fluorescent chemosensor (A18C6-Ox) has
been synthesized and its absorption and emission spectra investi-
gated in the presence of the selected rare-earth metal ions. While,
the absorption spectra of A18C6-Ox had no obvious shifts when
exposed to the various selected metal ions except for Ce3+ and Eu3+
,
significant emission enhancements of A18C6-Ox were detected
in the presence of Sc3+, La3+, Pr3+, Sm3+, Gd3+, Tb3+, Yb3+ and
Lu3+ chlorides, the highest emission enhancements were caused
by La3+ and Gd3+ ions. The quenching of fluorescence of A18C6-
Ox was observed in the presence of Ce3+ and Eu3+ chlorides. The
result showed that the 1:1 complexes between A18C6-Ox and the
selected rare-earth metal ions were formed in MeCN solutions.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (Grant 60776006) and also supported by ‘Qing Lan’
Talent Engineering Funds (QL-05-23A) by Lanzhou Jiaotong Uni-
versity.

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T. Yu et al. / Spectrochimica Acta Part A 78 (2011) 396–400
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