Synthesis, X-ray crystal structures and optical properties of novel substituted pyrazoly 1,3,4-oxadiazole derivatives

Article abstract of DOI:10.1007/s10895-011-0874-7

Novel pyrazoly 1,3,4-oxadiazole derivatives were synthesized and characterized by ¹H NMR, IR, HRMS and X-ray diffraction analysis. UV-vis absorption and fluorescence properties of these compounds in different solutions showed that the maximum a

Full text of DOI:10.1007/s10895-011-0874-7

J Fluoresc (2011) 21:1797–1804
DOI 10.1007/s10895-011-0874-7
ORIGINAL PAPER
Synthesis, X-Ray Crystal Structures and Optical Properties
of Novel Substituted Pyrazoly 1,3,4-Oxadiazole Derivatives
Hong-Shui Lv & Yong-Sheng Xie & Wei-Yong Liu &
Zhong-Liang Gong & Bao-Xiang Zhao
Received: 16 December 2010 /Accepted: 23 February 2011 /Published online: 5 March 2011
#
Springer Science+Business Media, LLC 2011
Abstract Novel pyrazoly 1,3,4-oxadiazole derivatives
were synthesized and characterized by 1H NMR, IR,
HRMS and X-ray diffraction analysis. UV–vis absorption
and fluorescence properties of these compounds in different
solutions showed that the maximum absorption wavelength
was not significantly changed in different solvents; howev-
er, maximal emission wavelength was red-shifted with the
increase of solvent polarity. Absorption 1max and emission
1max was less correlated with substituent groups on aryl
rings.
tron transporting and hole blocking materials in organic
light-emitting diodes (OLEDs) [4–7] and polymer light-
emitting diodes (PLEDs) [8, 9]. In addition, electron-
transporting 1,3,4-oxadiazole moiety has been connected
to many chelating ligands to obtain luminescent complexes
with more new functions [10–18].
The heterocyclic moiety can improve electron injection
and transport properties of the molecules. Thus, many
heterocyclic compounds based 1,3,4-oxadiazole have been
investigated, such as 1,3,4-oxadiazole–pyridine hybrids
[19], 1,3,4-oxadiazole–pyrimidine hybrids [19] and 1,3,4-
oxadiazole-spirobifluorene [20]. Furthermore, 1,3,4-oxadia-
zole derivatives have been a focus of attention in medical
chemistry for a long time because of their wide range of
biological activities including antiinflammatory [21], anti-
cancer [22, 23] and antibacterial [24, 25] activities.
.
.
.
Keywords 1,3,4-Oxadiazole Synthesis X-ray UV–vis
.
absorption Fluorescence
Introduction
We are interested in design, synthesis, structural charac-
terization and fluorescence property of novel compounds
with potential bioactivity in order to investigate the
localization of small molecule in cells. In our previous
work we reported a series of novel 2,5-disubstituted 1,3,4-
oxadiazole derivatives and investigated their optical prop-
erties [26]. Herein, we report synthesis, X-ray crystal
structures and optical properties of novel 1,3,4-oxadiazole
derivatives combined with pyrazole, an electron-rich
heterocycle, in order to investigate their potential applica-
tion in localization of small molecule in cells.
As an important class of five-membered heterocyclic
compounds, 1,3,4-oxadiazoles have been widely studied
as materials in electroluminescent (EL) devices due to their
electron deficiency, high thermal and oxidative stability,
especially high quantum yields [1–3]. In recent years, many
1,3,4-oxadiazole derivatives have been described as elec-
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-011-0874-7) contains supplementary material,
which is available to authorized users.
:
:
:
H.-S. Lv W.-Y. Liu Z.-L. Gong B.-X. Zhao (*)
Institute of Organic Chemistry, School of Chemistry
and Chemical Engineering, Shandong University,
Jinan 250100, People’s Republic of China
e-mail: bxzhao@sdu.edu.cn
Experimental
General
Y.-S. Xie
Department of Chemical and Environment Engineering,
Chongqing Three Gorges University,
Chongqing 404000, People’s Republic of China
Thin-layer chromatography (TLC) was conducted on silica
gel 60 F254 plates (Merck KGaA). H NMR spectra were
1

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J Fluoresc (2011) 21:1797–1804
recorded on a Bruker Avance 400 (400 MHz) spectrometer,
using CDCl3 as solvent and tetramethylsilane (TMS) as
internal standard. Melting points were determined on an
XD-4 digital micro melting point apparatus. IR spectra
were recorded with an IR spectrophotometer VERTEX 70
FT-IR (Bruker Optics). HRMS spectra were recorded on a
Q-TOF6510 spectrograph (Agilent). UV–vis spectra were
recorded on a U-4100 (Hitachi). Fluorescence measure-
ments were recorded on a Perkin–Elmer LS-55 lumines-
cence spectrophotometer.
CH3), 5.29 (s, 2 H, OCH2), 5.89 (s, 2 H, NCH2), 6.93 (d,
2 H, J=8.4 Hz, Ar-H), 7.12 (d, 2 H, J=8.4 Hz, Ar-H), 7.16
(s, 1 H, Pyrazole-H), 7.28 (d, 2 H, J=8.4 Hz, Ar-H), 7.32
(d, 2 H, J=8.4 Hz, Ar-H), 7.39 (d, 2 H, J=8.4 Hz, Ar-H),
7.78 (d, 2 H, J=8.4 Hz, Ar-H); HRMS calcd for [M+H]+
C30H30ClN4O2: 513.2057, found 513.2052.
2-(1-(4-(tert-Butyl)benzyl)-3-(4-chlorophenyl)-1 H-pyrazol-
5-yl)-5-((2-methoxyphenoxy)methyl)-1,3,4-oxadiazole (5b)
White solid, yield 93%; mp 121–122 °C; IR (KBr, cm−1) ν:
3146, 3062, 2961, 2868, 1614, 1594, 1509, 1473, 1257; 1H
NMR (CDCl3): δ 1.26 (s, 9 H, C(CH3)3), 3.85 (s, 3 H,
OCH3), 5.36 (s, 2 H, OCH2), 5.89 (s, 2 H, NCH2), 6.87–
6.95 (m, 2 H, Ar-H), 7.02–7.10 (m, 2 H, Ar-H), 7.16 (s,
1 H, Pyrazole-H), 7.28 (d, 2 H, J=8.4 Hz, Ar-H), 7.32 (d,
2 H, J=8.4 Hz, Ar-H), 7.39 (d, 2 H, J=8.4 Hz, Ar-H), 7.78
(d, 2 H, J=8.4 Hz, Ar-H); HRMS calcd for [M+H]+
C30H30ClN4O3: 529.2006, found 529.1978.
General Procedure for Preparation
of 2-(1-aryl-3-(4-chlorophenyl)-1H-
pyrazol-5-yl)-5-(chloromethyl)-1,3,4-oxadiazole (4)
To a solution of 1-arylmethyl-3-aryl-1H-pyrazole-5-carbo-
hydrazide (1) (3.5 mmol) in dichloromethane (50 ml) was
added 15 drops of Et3N. Subsequently, the solution of 2-
chloroacetyl chloride (2) (4.2 mmol) dissolved in dichloro-
methane (5 ml) was added dropwise in 20 min at room
temperature. The reaction mixture was stirred for 3 h at
room temperature, after which the solvent was removed
under reduced pressure. Water (30 ml) was added to the
residue to remove soluble impurity and the precipitate was
filtrated, washed with water (10 ml×3) and dried to give 3
without further purification.
The mixture of 3 (3.5 mmol) and POCl3 (15 ml) was
refluxed for 5 h. After cooling, it was poured into powder
ice (100 g). The precipitate was filtrated, washed with water
and dried. Product 4 was obtained by column chromatog-
raphy on silica gel using PE/EtOAc (1:1) as an eluent.
2-(1-(4-(tert-Butyl)benzyl)-3-(4-chlorophenyl)-1 H-pyrazol-
5-yl)-5-((4-chlorophenoxy)methyl)-1,3,4-oxadiazole (5c)
White solid, yield 95%; mp 179–181 °C; IR (KBr, cm−1)
1
ν: 3113, 3038, 2955, 2867, 1616, 1596, 1489, 1227; H
NMR (CDCl3): δ 1.26 (s, 9 H, C(CH3)3), 5.29 (s, 2 H,
OCH2), 5.89 (s, 2 H, NCH2), 6.97 (d, 2 H, J=9.2 Hz, Ar-
H), 7.15 (s, 1 H, Pyrazole-H), 7.27–7.34 (m, 6 H, Ar-H),
7.39 (d, 2 H, J=8.4 Hz, Ar-H), 7.79 (d, 2 H, J=8.4 Hz, Ar-
H); HRMS calcd for [M+H]+ C29H27Cl2N4O2: 533.1511,
found 533.1507.
2-(1-(4-(tert-Butyl)benzyl)-3-(4-chlorophenyl)-1 H-pyrazol-
5-yl)-5-((2,4-dichlorophenoxy)methyl)-1,3,4-oxadiazole
(5d) White solid, yield 98%; mp 131–132 °C; IR (KBr,
cm−1) ν: 3144, 3071, 2961, 2869, 1614, 1581, 1478, 1218;
1H NMR (CDCl3): δ 1.26 (s, 9 H, C(CH3)3), 5.36 (s, 2 H,
OCH2), 5.89 (s, 2 H, NCH2), 7.07 (d, 1 H, J=9.2 Hz, Ar-H),
7.17 (s, 1 H, Pyrazole-H), 7.22 (dd, 1 H, J=2.4, 9.2 Hz, Ar-
H), 7.29 (d, 2 H, J=8.8 Hz, Ar-H), 7.32 (d, 2 H, J=8.8 Hz,
Ar-H), 7.39 (d, 2 H, J=8.4 Hz, Ar-H), 7.41 (d, 1 H, J=
2.4 Hz, Ar-H), 7.79 (d, 2 H, J=8.4 Hz, Ar-H); HRMS calcd
for [M+H]+ C29H26Cl3N4O2: 567.1121, found 567.1104.
General Procedure for Preparation
of 2-(1-aryl-3-(4-chlorophenyl)-1
H-pyrazol-5-yl)-5-(aryloxymethyl)-1,3,4-oxadiazole (5a-j)
A mixture of 4 (1 mmol), substituted phenol (1 mmol),
anhydrous potassium carbonate (3 mmol) and dry acetoni-
trile (25 ml) was refluxed for 0.5–2.5 h, after which the
solution was condensed under reduced pressure. The
residue was extracted with dichloromethane (30 ml). The
organic phase was washed with 5% NaOH solution (10 ml),
water (10 ml×3) and dried over MgSO4. After filtered, the
filtrate was concentrated under reduced pressure to afford
title compound 5 in 86–99% (Fig. 1).
2-(1-(4-(tert-Butyl)benzyl)-3-(4-chlorophenyl)-1 H-pyrazol-
5-yl)-5-((4-nitrophenoxy)methyl)-1,3,4-oxadiazole (5e) Yel-
low solid, yield 97%; mp 165–167 °C; IR (KBr, cm−1) ν:
3113, 3083, 2959, 2866, 1614, 1591, 1517, 1496, 1346,
1255; 1H NMR (CDCl3): δ 1.26 (s, 9 H, C(CH3)3), 5.42 (s,
2 H, OCH2), 5.91 (s, 2 H, NCH2), 7.14 (d, 2 H, J=9.2 Hz,
Ar-H), 7.16 (s, 1 H, Pyrazole-H), 7.29 (d, 2 H, J=8.6 Hz,
Ar-H), 7.33 (d, 2 H, J=8.6 Hz, Ar-H), 7.39 (d, 2 H, J=
8.4 Hz, Ar-H), 7.78 (d, 2 H, J=8.4 Hz, Ar-H), 8.25 (d, 2 H,
J=9.2 Hz, Ar-H); HRMS calcd for [M+H]+ C29H27ClN5O4:
544.1752, found 544.1730.
The Spectroscopic Data of Compounds 5
2-(1-(4-(tert-Butyl)benzyl)-3-(4-chlorophenyl)-1 H-pyrazol-
5-yl)-5-((p-tolyloxy)methyl)-1,3,4-oxadiazole (5a) White
solid, yield 99%; mp 159–161 °C; IR (KBr, cm−1) ν:
3114, 3036, 2954, 2866, 1614, 1578, 1511, 1459, 1225; 1H
NMR (CDCl3): δ 1.26 (s, 9 H, C(CH3)3), 2.30 (s, 3 H,

J Fluoresc (2011) 21:1797–1804
1799
Fig. 1 The synthetic route of
unsymmetrical 1,3,4-oxadiazole
derivatives
2-(3-(4-Chlorophenyl)-1-((6-chloropyridin-3-yl)methyl)-
1 H-pyrazol-5-yl)-5-((p-tolyloxy)methyl)-1,3,4-oxadiazole
(5f) White solid, yield 87%; mp 168–171 °C; IR (KBr,
cm−1) ν: 3153, 3055, 2945, 2859, 1619, 1583, 1510, 1473,
1240; 1H NMR (CDCl3): δ 2.30 (s, 3 H, CH3), 5.30 (s, 2 H,
OCH2), 5.93 (s, 2 H, NCH2), 6.92 (d, 2 H, J=8.4 Hz, Ar-
H), 7.12 (d, 2 H, J=8.4 Hz, Ar-H), 7.17 (s, 1 H, Pyrazole-
H), 7.24 (d, 1 H, J=6.6 Hz, Pyridine-H), 7.40 (d, 2 H, J=
8.5 Hz, Ar-H), 7.73 (dd, 1 H, J=2.2, 6.6 Hz, Pyridine-H),
7.76 (d, 2 H, J=8.5 Hz, Ar-H), 8.50 (d, 1 H, J=2.2 Hz,
Pyridine-H); HRMS calcd for [M+H]+ C25H20Cl2N5O2:
492.0994, found 492.0985.
zole (5 h) White solid, yield 86%; mp 179–181 °C; IR
(KBr, cm−1) ν: 3142, 3077, 2951, 2871, 1616, 1584, 1489,
1470, 1238; H NMR (CDCl3): δ 5.31 (s, 2 H, OCH2), 5.94
(s, 2 H, NCH2), 6.97 (d, 2 H, J=8.8 Hz, Ar-H), 7.17 (s, 1 H,
Pyrazole-H), 7.26 (d, 1 H, J=8.0 Hz, Pyridine-H), 7.29 (d,
2 H, J=8.8 Hz, Ar-H), 7.40 (d, 2 H, J=8.4 Hz, Ar-H), 7.74
(dd, 1 H, J=2.6, 8.0 Hz, Pyridine-H), 7.76 (d, 2 H, J=8.4 Hz,
Ar-H), 8.51 (d, 1 H, J=2.6 Hz, Pyridine-H); HRMS calcd for
[M+H]+ C24H17Cl3N5O2: 512.0448, found 512.0436.
1
2-(3-(4-Chlorophenyl)-1-((6-chloropyridin-3-yl)methyl)-
1 H-pyrazol-5-yl)-5-((2,4-dichlorophenoxy)methyl)-1,3,4-
oxadiazole (5i) White solid, yield 93%; mp 211–213 °C;
IR (KBr, cm−1) ν: 3125, 3053, 2955, 1615, 1585, 1477,
2-(3-(4-Chlorophenyl)-1-((6-chloropyridin-3-yl)methyl)-
1 H-pyrazol-5-yl)-5-((2-methoxyphenoxy)methyl)-1,3,4-
oxadiazole (5 g) White solid, yield 90%; mp 149–151 °C;
IR (KBr, cm−1) ν: 3113, 3072, 2956, 2863, 1615, 1586,
1507, 1460, 1259, 1214; 1H NMR (CDCl3): δ 3.87 (s, 3 H,
OCH3), 5.38 (s, 2 H, OCH2), 5.93 (s, 2 H, NCH2), 6.89–
6.98 (m, 2 H, Ar-H), 7.03-7.10 (m, 2 H, Ar-H), 7.18 (s,
1 H, Pyrazole-H), 7.25 (d, 1 H, J=7.2 Hz, Pyridine-H),
7.41 (d, 2 H, J=8.5 Hz, Ar-H), 7.74 (dd, 1 H, J=2.4,
7.2 Hz, Pyridine-H), 7.76 (d, 2 H, J=8.5 Hz, Ar-H), 8.51
(d, 1 H, J=2.4 Hz, Pyridine-H); HRMS calcd for [M+H]+
C25H20Cl2N5O3: 508.0943, found 508.0935.
1
1244; H NMR (CDCl3): δ 5.37 (s, 2 H, OCH2), 5.93 (s,
2 H, NCH2), 7.07 (d, 1 H, J=8.8 Hz, Ar-H), 7.18 (s, 1 H,
Pyrazole-H), 7.22 (d, 1 H, J=1.7 Hz, Ar-H), 7.26 (dd, 1 H,
J=1.7, 8.8 Hz, Ar-H), 7.40 (d, 1 H, J=6.4 Hz, Pyridine-H),
7.41 (d, 2 H, J=8.5 Hz, Ar-H), 7.74 (dd, 1 H, J=2.2,
6.4 Hz, Pyridine-H), 7.77 (d, 2 H, J=8.5 Hz, Ar-H), 8.51
(d, 1 H, J=2.2 Hz, Pyridine-H); HRMS calcd for [M+H]+
C24H16Cl4N5O2: 548.0029, found 548.0019.
2-(3-(4-Chlorophenyl)-1-((6-chloropyridin-3-yl)methyl)-
1 H-pyrazol-5-yl)-5-((4-nitrophenoxy)methyl)-1,3,4-oxadia-
zole (5j) Yellow solid, yield 91%; mp 200–202 °C; IR
(KBr, cm−1) ν: 3112, 3084, 2954, 1615, 1590, 1517, 1491,
1340, 1253; 1H NMR (CDCl3): δ 5.43 (s, 2 H, OCH2), 5.94
2-((4-Chlorophenoxy)methyl)-5-(3-(4-chlorophenyl)-1-((6-
chloropyridin-3-yl)methyl)-1 H-pyrazol-5-yl)-1,3,4-oxadia-

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(s, 2 H, NCH2), 7.13 (d, 2 H, J=9.2 Hz, Ar-H), 7.18 (s, 1 H,
Pyrazole-H), 7.27 (d, 1 H, J=7.0 Hz, Pyridine-H), 7.41 (d,
2 H, J=8.5 Hz, Ar-H), 7.75 (dd, 1 H, J=2.2, 7.0 Hz,
Pyridine-H), 7.77 (d, 2 H, J=8.5 Hz, Ar-H), 8.26 (d, 2 H, J=
9.2 Hz, Ar-H), 8.51 (d, 1 H, J=2.2 Hz, Pyridine-H); HRMS
calcd for [M+H]+ C24H17Cl2N6O4: 523.0688, found
523.0664.
SHELXS-97 program and refinements on F2 were performed
with SHELXL-97 program by full-matrix least-squares
techniques with anisotropic thermal parameters for the non-
hydrogen atoms. All H atoms were initially located in a
difference Fourier map. The methyl H atoms were then
constrained to an ideal geometry, with C–H=0.96Å and
Uiso(H) 1.5Ueq(C). All other H atoms were placed in
geometrically idealized positions and constrained to ride on
their parent atoms, with C–H=0.93Å and Uiso(H)=1.2Ueq(C).
X-Ray Crystallography
Suitable single crystals of 5c and 5e for X-ray structural
analysis were obtained by evaporation of ethyl acetate
solution. The diffraction data for both structures were
collected with a Bruker SMART CCD diffractometer using a
graphite monochromated Mo Kα radiation (1=0.71073Å) at
273(2) K. The structures were solved by direct methods with
Results and Discussion
Synthesis and Characterization
The synthesis of 2-(1-aryl-3-(4-chlorophenyl)-1H-pyrazol-
5-yl)-5-(aryloxymethyl)-1,3,4-oxadiazole (5a-j) was out-
Table 1 Summary of crystallographic data and structure refinement details for 5c and 5e
5c
5e
Empirical formula
Formula weight
Temperature
C29H26Cl2N4O2
533.44
C29H26ClN5O4
544.00
273(2)
298(2)
Wavelength
0.71073Å
0.71073Å
Crystal system
space group
Triclinic
Triclinic
P–1
P–1
Unit cell dimensions
a=9.2259(13) Å,
α=94.166(3)
a=8.931(5) Å,
α=96.435(12)
b=11.302(7) Å,
β=97.049(11)°
c=14.140(8) Å,
γ=108.102(11)°
1329.2(14) A3
2
b=11.2525(16) Å,
β=96.227(2)°
c=13.817(2) Å,
γ=110.292(3)°
1328.1(3) A3
2
Volume
Z
Calculated density
Absorption coefficient
F(000)
1.334 Mg/m3
0.278 mm−1
1.359 Mg/m3
0.189 mm−1
556
568
Crystal size
0.10×0.10×0.10 mm
1.49 to 27.59°
−10≤h≤11, −13≤k≤14, −17≤l≤17
7878/5760 [R(int)=0.0365]
93.7%
0.15×0.10×0.05 mm
1.47 to 23.73°
−10≤h≤10, −10≤k≤12, −15≤l≤15
5835/3934 [R(int)=0.0216]
97.4%
θ range for data collection
Limiting indices
Reflections collected/unique
Completeness to θ
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
Largest diff. peak and hole
CCDC No.
none
Multi–scan
0.9727 and 0.9727
Full-matrix least-squares on F2
5760/0/338
0.9906 and 0.9722
Full-matrix least-squares on F2
3934/1/356
0.931
1.030
R1=0.0723, wR2=0.1969
R1=0.1890, wR2=0.2600
0.612 and −0.271 e Å−3
787253
R1=0.0744, wR2=0.2196
R1=0.1189, wR2=0.2666
0.429 and −0.365 e Å−3
787254

J Fluoresc (2011) 21:1797–1804
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Fig. 4 UV–vis absorption spectra of 5a-5j in dichloromethane
(2×10−5 M)
Fig. 2 The molecular structure of compound 5c, with displacement
ellipsoids drawn at the 30% probability level
methane at room temperature afforded intermediate (3) that
can be used to next reaction step without further purifica-
tion. Then 3 reacted with phosphoryl trichloride to give 2-
(1-aryl-3-(4-chlorophenyl)-1H-pyrazol-5-yl)-5-(chloro-
methyl)-1,3,4-oxadiazole (4) that can be purified by column
chromatography on silica gel using PE/EtOAc (1:1) as an
eluent. The reaction of 4 and substituted phenol in the
presence of anhydrous potassium carbonate in dry acetonitrile
at reflux temperature afforded compound (5) in 86–99%.
The proposed structures were confirmed by IR, 1H NMR
spectra, HRMS, and X-ray single crystal diffraction.
lined in Fig. 1. The starting 1-arylmethyl-3-aryl-1H-
pyrazole-5-carbohydrazide derivatives (1) can be easily
prepared according to the procedure reported in our
previous paper [27]. The reaction of 1 and 2-chloroacetyl
chloride (2) in the presence of triethylamine in dichloro-
X-ray crystallography analysis
The spatial structures of compounds 5c and 5e were
determined by using X-ray diffraction analysis. A summary
of crystallographic data collection parameters and refine-
ment parameters for 5c and 5e are compiled in Table 1.
Fig. 3 The structure of centrosymmetric dimmer of 5c, showing
C–H···N intermolecular hydrogen bonds as dashed lines
Fig. 5 UV–vis absorption spectra of 5a-5j in acetonitrile (2×10−5 M)

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(C12–H12B 0.97Å, H12B···N4 2.42Å, C12···N4 3.377(7)
Å and C12–H12B···N4 168°; Symmetry code: -x, 3-y, 1-z)
2
resulting in a R2 (8) descriptor on a unitary level (Fig. 3).
The dimers are further connected to form a three-
dimensional network by intermolecular C–H···π hydrogen
bonds: C8–H8 0.93Å, H8···Cg 2.89(4) Å, C8···Cg 3.805(6)
Å, C8–H8···Cg 168(4)° and symmetry code: -x, 2-y, 1-z;
C27–H27A 0.96Å, H27A···Cg 2.96Å, C27···Cg 3.880(11)
Å, C8–H8···Cg 161° and symmetry code: -x, 2-y, 2-z (Cg is
the centroid of the C20–C25 ring).
Absorption Spectra
The UV–vis absorption spectra of the compounds 5a-j in
dichloromethane, acetonitrile and ethanol at the concentra-
tion of 2×10−5 mol/L were given in Figs. 4, 5 and 6,
respectively. The results were summarized in Table 2.
These compounds displayed similar absorptions ranging
from 256 to 263 nm that were attributed to π-π transition of
conjugation system. Comparing with 5-aryl-2-(1-aryl-3-(4-
chlorophenyl)-1H-pyrazol-5-yl)- 1,3,4-oxadiazole reported
in previous paper [26], compounds 5a-j have blue shift
about 10 nm. It is easily understood that the aryloxy linked
methylene in C5 cannot extend to the pi-conjugation
system because they are blocked to π-system by methylene.
It is also observed that the 1max values of 5a-5j changes
with the polarity of solutions. Generally in different
solvents, there is a consequence for the 1max values of 5a-
5e: 1max (DCM)>1max (CH3CN)>1max (EtOH). However,
for 5f-5j, 1max (DCM)>1max (EtOH)>1max (CH3CN). In
the same solvent, compounds 5a-5j have the similar
maximum absorption that means substituent R1, R2, R3
and X has no significant impact to the absorption. The
maximum molar extinction coefficients of 5a-5j in
dichloromethane, acetonitrile and ethanol are different,
and although the difference is less, it can be observed that
Fig. 6 UV–vis absorption spectra of 5a-5j in ethanol solution
(2×10−5 M)
The single crystal structure and atomic numbering
chosen for 5c are shown in Fig. 2. The dihedral angles
between the pyrazole ring and the C4 phenyl or 1,3,4-
oxadiazole ring are 7.9(3)° and 10.0(3)°, respectively. All
the small dihedral angles above indicate that the pyrazole,
C4 phenyl and oxadiazole rings in 5c are nearly in the same
plane and conjugated in which the bond lengths of C9–C10
and C4–C7 are 1.453(6) Å and 1.470(6) Å, respectively,
and they have double-bond character. The bond length C9-
C10 (1.453(6) Å) is similar to pyrazoly 1,3,4-oxadiazole
derivatives (C9–C21 1.449Å) we have reported [26]. The
molecular conformation is stabilized by intramolecular C5–
H5···N1 and C19–H19A···N3 hydrogen bonds (Fig. 2): C5–
H5 0.94Å, H5···N1 2.57Å, C5···N1 2.875(7) Å and C5–
H5···N1 100°; C19–H19A 0.97Å, H19A···N3 2.50Å,
C19···N3 3.050(7) Å and C19–H19A···N3 116°. In the
crystal structure, centrosymmetric dimers are formed by
two intermolecular hydrogen bonds of the C12–H12B···N4
Table 2 Maximum absorption
Compound
λmax (nm)
ε (mol−1cm−1L)
wavelength (λmax) and maxi-
mum molar extinction coeffi-
cients (εmax) of 5a-j in
dichloromethane, acetonitrile
and ethanol
DCM
CH3CN
EtOH
DCM
CH3CN
EtOH
5a
5b
5c
5d
5e
5f
260
262
263
262
263
260
263
262
261
263
259
257
260
260
261
256
257
257
257
256
259
257
259
258
260
259
258
259
257
259
35950
41800
39600
41950
43450
36950
37000
43150
41650
43050
39650
46600
43500
45050
45400
40100
40150
45550
45050
46100
40300
47300
45250
46600
46650
41000
40150
46450
37750
44550
5g
5h
5i
5j

J Fluoresc (2011) 21:1797–1804
1803
Fig. 9 Fluorescence spectra of the compounds 5a-5j in acetonitrile
Fig. 7 Fluorescence spectra of the compounds 5a-5j in hexane
(10−7 M)
(10−7 M)
example, in hexane, the maximum emission wavelengths of
5c and 5d are red-shifted 9 nm and 25 nm comparing with
5 h and 5i. These differences might be attributed to
substituted aryl groups in N-1 position. The maximum
emission wavelengths of 5b, 5e and 5 g in DCM have a
little red shift comparing with other compounds. In
CH3CN, all the compounds have almost the same
maximum emission wavelength. Interestingly, the effects
of the solvents on the fluorescence intensities of 5a-5j are
obvious. Compounds 5a-5j have similar fluorescence
intensities in hexane and CH3CN. However, they have
significant differences in DCM. These results indicate
that the molecules have strong coordination effects and
the environment plays an important role in determining
the fluorescence intensity of the compounds. The findings
consist with the report on 1,3,4-oxadiazole derivatives
[3].
the enhancement order is generally ε (DCM) <ε (CH3CN) <ε
(EtOH).
Fluorescence
The emission spectra of these compounds in hexane,
dichloromethane and acetonitrile at the concentration of
10−7 mol/L were investigated as shown in Figs. 7, 8 and 9
respectively. Their excitation wavelengths were all fixed at
260 nm. It can be seen from Table 3 that the maximum
emission wavelengths of 5a-5j are different in three
solvents. Taking 5a as an example, its maximum emission
wavelength is 308, 367 and 408 nm in hexane, dichloro-
methane and acetonitrile, respectively. The results indicate
that the emission wavelengths red-shift with the increase of
the solvent polarity. In addition, substituent groups in π-
conjugation system influence maximal emission bands. For
Table 3 Maximum wavelength (nm) of excitement and emission of
fluorescence of compounds 5a-j
Compound
1ex (nm)
1em (nm)
Hexane
DCM
CH3CN
5a
5b
5c
5d
5e
5f
260
260
260
260
260
260
260
260
260
260
308
307
316
333
308
307
306
307
308
308
367
387
364
357
393
363
392
361
361
360
408
405
406
408
406
405
409
402
405
405
5g
5h
5i
Fig. 8 Fluorescence spectra of the compounds 5a-5j in dichloro-
5j
methane (10−7 M)

1804
J Fluoresc (2011) 21:1797–1804
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A series of novel 1,3,4-oxadiazole-based pyrazole deriva-
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optical properties indicate that the polarity of solvent has no
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properties. Currently, investigations are underway to eluci-
date the bioactivity and localization of small molecule in
cells and the results will be reported in due course.
Acknowledgement This study was supported by National Natural
Science Foundation of China (90813022 and 20972088) and the
Science and Technology Developmental Project of Shandong Prov-
ince (2008GG10002034).
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