Synthesis, X-ray crystal structure and optical properties of novel 2,5-diaryl-1,3,4-oxadiazole derivatives containing substituted pyrazolo[1,5-a]pyridine units

Article abstract of DOI:10.1016/j.dyepig.2011.03.035

A series of pyrazolo[1,5-a]pyridine-containing 2,5-diaryl-1,3,4-oxadiazole derivatives were synthesized and their structures were characterized by IR, ¹H NMR and HRMS spectra. The crystal structure of 3a was determined using single crystal X-ray crystallography. Its spatial structure was found to be monoclinic, and all aromatic rings were approximately coplanar, which allowed conjugation. The absorption results showed that compounds 1a-f presented their absorption peaks ranging from 264 nm to 290 nm, while compounds 3a-f with a larger conjugation system exhibited red-shifted absorption character (absorption maxima between 283 nm and 303 nm) compared to the corresponding absorption of 1a-f. Fluorescence spectra revealed that these compounds exhibited blue fluorescence (421-444 nm) in dilute solutions and showed quantum yields of fluorescence between 0.32 and 0.83 in dichloromethane.

Full text of DOI:10.1016/j.dyepig.2011.03.035

Dyes and Pigments 91 (2011) 446e453
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, X-ray crystal structure and optical properties of novel 2,5-diaryl-1,
3,4-oxadiazole derivatives containing substituted pyrazolo[1,5-a]pyridine units
,
a,
He Yang a, Jinglin Mu a, Xia Chen a, Lei Feng a, Jiong Jia b , Jianwu Wang
*
*
a School of Chemistry and Chemical Engineering, Shandong University, 27 Shanda Nanlu, Jinan 250100, Shandong, PR China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of pyrazolo[1,5-a]pyridine-containing 2,5-diaryl-1,3,4-oxadiazole derivatives were synthesized
and their structures were characterized by IR, 1H NMR and HRMS spectra. The crystal structure of 3a was
determined using single crystal X-ray crystallography. Its spatial structure was found to be monoclinic,
and all aromatic rings were approximately coplanar, which allowed conjugation. The absorption results
showed that compounds 1aef presented their absorption peaks ranging from 264 nm to 290 nm, while
compounds 3aef with a larger conjugation system exhibited red-shifted absorption character (absorp-
tion maxima between 283 nm and 303 nm) compared to the corresponding absorption of 1aef. Fluo-
rescence spectra revealed that these compounds exhibited blue fluorescence (421e444 nm) in dilute
solutions and showed quantum yields of fluorescence between 0.32 and 0.83 in dichloromethane.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 21 December 2010
Received in revised form
27 March 2011
Accepted 30 March 2011
Available online 13 April 2011
Keywords:
Pyrazolo[1,5-a]pyridine
1,3,4-Oxadiazole
Fluorescence
Absorption
Crystal structure
Heterocycle
1. Introduction
[22,23]. The azaindole-like pyrazolo[1,5-a]pyridine ring has a large
dipole moment due to the non-uniform charge distribution in the
2,5-Diaryl-1,3,4-oxadiazoles have enjoyed widespread use in
organic electronics. Due to the electron-deficient property of 1,3,4-
oxadiazole derivatives, as well as good thermal stabilities and high
photoluminescence quantum yields [1], 2,5-diaryl-1,3,4-oxadia-
zoles are the famous electron-transporting and emitting materials
for organic light emitting diodes (OLEDs) [2e6]. In recent years,
many two-photon absorption (TPA) molecules containing oxadia-
zole segment as the electron-accepting group have been synthe-
sized and investigated [7e10]. In addition, oxadiazole-based
materials have been also applied in organic field-effect transistors
(OFETs) [11,12] and molecular level devices and machines [13,14].
Pyrazolo[1,5-a]pyridine derivatives are nitrogen-containing
heterocycles and have been of particular interest for their phar-
macological and biological activities. Of these heterocycles, 2-
substituted pyrazolo[1,5-a]pyridine derivatives have been shown to
be effective dopamine D3 and D4 receptor antagonists [15e18],
adenosine A1 receptor antagonists [19], p38 kinase inhibitors [20]
and EphB3 receptor tyrosine kinase inhibitors [21]. They have
been also reported to exhibit potent activity against herpesviruses
five-membered and six-membered rings (Fig. 1). Within the pyr-
azolo[1,5-a]pyridine ring, electronic charge builds up significantly
in the five-membered ring while low electron density appears in
the six-membered ring. Conjugated organic compounds with large
polarizabilities are of particular use in the field of optical elec-
tronics, such as non-linear optics [24]. To date little attention has
been paid to the photophysical properties of these pyrazolo[1,5-a]
pyridine derivatives. Thus, as a continuation of our efforts in
synthesizing nitrogen-bridgehead heterocycles and exploring
novel fluorescent nitrogen-containing compounds [25,26], we
would like to synthesize a series of pyrazolo[1,5-a]pyridine-con-
taining 2,5-diaryl-1,3,4-oxadiazole derivatives with both potential
bioactivities and optical characters and investigate their structural
and spectroscopic properties.
2. Materials and methods
2.1. General
All reagents were commercially available and used without
further purification. Melting points were recorded on an XD-4
digital micro melting point apparatus and uncorrected. 1H NMR
* Corresponding authors. Tel.: þ86 531 88362708; fax: þ86 531 88564464.
E-mail addresses: jiongjia@sdu.edu.cn (J. Jia), jwwang@sdu.edu.cn (J. Wang).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.035

H. Yang et al. / Dyes and Pigments 91 (2011) 446e453
447
2.2.2.2. 2-(4-Biphenyl)-pyrazolo [1,5-a]pyridine-5-carbohydrazide
(2b). White solid, yield 92%, mp 219e220 ꢀC. 1H NMR (300 MHz,
d6-DMSO):
d 10.01 (s, 1H, NH), 8.77e8.83 (m, 1H, ArH), 8.38 (s, 1H,
ArH), 8.09e8.20 (m, 2H, ArH), 7.74e7.83 (m, 4H, ArH), 7.29e7.52 (m,
5H, ArH), 4.61 (s, 2H, NH2); HRMS calcd for [M þ H]þ C20H17N4O:
329.1402; found: 329.1406.
Fig. 1. Charge distribution within pyrazolo[1,5-a]pyridine moiety.
spectra were recorded on a Bruker Avance 300 MHz spectrometer,
using CDCl3 or d6-DMSO as solvent and TMS as internal standard. IR
spectra were recorded with an IR spectrophotometer VERTEX
70 FT-IR (Bruker Optics). HRMS spectra were recorded on
a Q-TOF6510 spectrograph (Agilent). UVevis spectra were recorded
on a U-4100 (Hitachi) spectrophotometer. Fluorescent measure-
ments were recorded on a PerkineElmer LS-55 luminescence
spectrophotometer.
2.2.2.3. 2-(4-Fluorophenyl)-pyrazolo [1,5-a]pyridine-5-carbohydrazide
(2c). White solid, yield 84%, mp 247e248 ꢀC. 1H NMR (300 MHz,
d6-DMSO):
d
10.00 (s, 1H, NH), 8.75e8.78 (d, J ¼ 7.2 Hz, 1H, ArH), 8.18
(s, 1H, ArH), 8.01e8.04 (m, 2H, ArH), 7.50e7.56 (m, 2H, ArH),
7.27e7.30 (m, 2H, ArH), 4.60 (s, 2H, NH2); HRMS calcd for [M þ H]þ
C14H12FN4O: 271.0995; found: 271.0992.
2.2.2.4. 2-(4-Chlorophenyl)-pyrazolo [1,5-a]pyridine-5-carbohydrazide
(2d). White solid, yield 80%, mp 233e234 ꢀC. 1H NMR (300 MHz,
2.2. Synthesis
d6-DMSO):
d
10.00 (s, 1H, NH), 8.75e8.78 (d, J ¼ 7.2 Hz, 1H, ArH), 8.18
(s, 1H, ArH), 8.02e8.05 (m, 2H, ArH), 7.53e7.56 (m, 2H, ArH),
7.27e7.30 (m, 2H, ArH), 4.61 (s, 2H, NH2); HRMS calcd for [M þ H]þ
C14H12ClN4O: 287.0700; found: 287.0707.
2.2.1. General procedure for the synthesis of 1aef
Compounds 1a, 1b and 1f were synthesized using the same
method for 1c, 1d and 1e we reported before [25].
2.2.2.5. 2-Phenyl-pyrazolo
(2e). White solid, yield 89%, mp 242e243 ꢀC. 1H NMR
(300 MHz, d6-DMSO):
1H, ArH), 8.18 (s, 1H, ArH), 8.00e8.03 (m, 2H, ArH), 7.38e7.51 (m,
3H, ArH), 7.25e7.29 (m, 2H, ArH), 4.59 (s, 2H, NH2); HRMS calcd
for [M þ H]þ C14H13N4O: 253.1089; found: 253.1093.
[1,5-a]pyridine-5-carbohydrazide
2.2.1.1. Ethyl 2-(3,4-dimethoxyphenyl)-pyrazolo [1,5-a]pyridine-5-
carboxylate (1a). Yellow solid, yield 61%, mp 190e191 ꢀC. IR (KBr)
d
10.00 (s, 1H, NH), 8.76e8.78 (d, J ¼ 7.2 Hz,
n
/cmꢁ1: 3056 (ArH), 1709 (C]O), 1485 (Ar), 1269 (CeOeC), 760
(ArH); 1H NMR (300 MHz, CDCl3):
d
8.45e8.48 (d, J ¼ 7.2 Hz, 1H,
ArH), 8.26 (s, 1H, ArH), 7.56 (d, J ¼ 1.8 Hz, 1H, ArH), 7.48e7.51 (m,
1H, ArH), 7.29e7.32 (dd, J1 ¼7.2 Hz, J2 ¼ 1.8 Hz, 1H, ArH), 6.93e6.97
(m, 2H, ArH), 4.38e4.45 (q, J ¼ 7.2 Hz, 2H, CH2), 4.01 (s, 3H), 3.94 (s,
3H), 1.40e1.45 (t, J ¼ 7.2 Hz, 3H, CH3); HRMS calcd for [M þ H]þ
C18H19N2O4: 327.1345; found: 327.1346.
2.2.2.6. 2-(Naphthalen-2-yl)-pyrazolo
carbohydrazide (2f). White solid, yield 93%, mp 230e231 ꢀC. 1H
NMR (300 MHz, d6-DMSO): 10.03 (s, 1H, NH), 8.79e8.82 (d,
[1,5-a]pyridine-5-
d
J ¼ 7.5 Hz, 1H, ArH), 8.59 (s, 1H, ArH), 8.16e8.23 (m, 2H, ArH),
7.94e8.04 (m, 3H, ArH), 7.52e7.59 (m, 2H, ArH), 7.42 (s, 1H, ArH),
7.28e7.31 (m, 1H, ArH), 4.64 (s, 2H, NH2); HRMS calcd for
[M þ H]þ C18H15N4O: 303.1246; found: 303.1248.
2.2.1.2. Ethyl 2-(4-biphenyl)-pyrazolo [1,5-a]pyridine-5-carboxylate
(1b). White solid, yield 68%, mp 201e202 ꢀC. IR (KBr) /cmꢁ1
n :
3052 (ArH), 1713 (C]O), 1474 (Ar), 1269 (CeO), 760 (ArH); 1H NMR
(300 MHz, CDCl3):
d
8.48e8.50 (d, J ¼ 7.2 Hz, 1H, ArH), 8.29 (s, 1H,
ArH), 8.03e8.05 (m, 2H, ArH), 7.64e7.71 (m, 4H, ArH), 7.43e7.48 (m,
2H, ArH), 7.31e7.39 (m, 2H, ArH), 7.02 (s, 1H, ArH), 4.38e4.45 (q,
J ¼ 7.2 Hz, 2H, CH2), 1.41e1.45 (t, J ¼ 7.2 Hz, 3H, CH3); HRMS calcd
for [M þ H]þ C22H19N2O2: 343.1447; found: 343.1443.
Table 1
Crystal data and structure refinement for 3a.
3a
Empirical formula
Formula weight
Temperature
C23H18N4O3
398.41
298(2) K
2.2.1.3. Ethyl
carboxylate (1f). White solid, yield 70%, mp 178e179 ꢀC. IR (KBr)
/cmꢁ1: 3051 (ArH), 1709 (C]O), 1528 (Ar), 1256 (CeO), 750 (ArH);
1H NMR (300 MHz, CDCl3):
8.48e8.50 (m, 1H, ArH), 8.43 (s, 1H,
2-(naphthalen-2-yl)-pyrazolo
[1,5-a]pyridine-5-
Wavelength
Crystal system
Space group
0.71073 Å
Monoclinic
P 1 21/c 1
n
d
¼ 90ꢀ
ArH), 8.28 (s, 1H, ArH), 8.06e8.09 (m, 1H, ArH), 7.90e7.93 (m, 2H,
ArH), 7.83e7.86 (m, 1H, ArH), 7.47e7.58 (m, 2H, ArH), 7.30e7.33 (m,
1H, ArH), 7.08 (s, 1H, ArH), 4.37e4.46 (m, 2H, CH2), 1.40e1.45 (m,
3H, CH3); HRMS calcd for [M þ H]þ C20H17N2O2: 317.1290; found:
317.1291.
Unit cell dimensions
a ¼ 19.962(8) Å,
b ¼ 11.030(4) Å,
a
b
¼ 91.273(7)ꢀ
c ¼ 8.856(4) Å,
g
¼ 90ꢀ
Volume
1949.44(140) Å3
Z
4
Calculated density
Absorption coefficient
F(000)
1.3574 Mg/m3
0.093 mmꢁ1
832
2.2.2. General procedure for the synthesis of 2aef
A mixture of compound 1 (3 mmol) and 80% hydrazine mono-
hydrate (10 mL) was maintained under reflux for 6 h. After the
solution was cooled, white solid was precipitated. The crude
product was filtered and washed with ethanol to afford compound
2 as white crystals.
Crystal size
0.15 ꢂ 0.15 ꢂ 0.10 mm
q
range for data collection
2.04e27.38ꢀ
Limiting indices
ꢁ13 ꢃ h ꢃ 25
ꢁ14 ꢃ k ꢃ 12
ꢁ10 ꢃ l ꢃ 10
Reflections collected/unique
Completeness to
Absorption correction
Max. and min. transmission
Refinement method
10,970/4311 [R(int) ¼ 0.0214]
q
¼ 27.56ꢀ
97.4%
None
2.2.2.1. 2-(3,4-Dimethoxyphenyl)-pyrazolo
carbohydrazide (2a). White solid, yield 89%, mp 215e216 ꢀC. 1H
NMR (300 MHz, d6-DMSO): 10.00 (s, 1H, NH), 8.73e8.76 (d,
[1,5-a]pyridine-5-
0.9908 and 0.9862
Full-matrix least-squares on F2
4311/0/320
d
Data/restraints/parameters
Goodness-of-fit on F2
0.998
J ¼ 7.2 Hz, 1H, ArH), 8.14 (s, 1H, ArH), 7.55e7.57 (d, J ¼ 7.2 Hz, 2H,
ArH), 7.21e7.24 (m, 2H, ArH), 7.04e7.07 (m, 1H, ArH), 4.61 (s, 2H,
NH2), 3.86 (s, 3H, OCH3), 3.81 (s, 3H, OCH3); HRMS calcd for
[M þ H]þ C16H17N4O3: 313.1301; found: 313.1308.
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
s
(I)]
R1 ¼ 0.0413, wR2 ¼ 0.1147
R1 ¼ 0.0616, wR2 ¼ 0.1294
0.191 and ꢁ0.163 eÅꢁ3

448
H. Yang et al. / Dyes and Pigments 91 (2011) 446e453
Fig. 2. Synthetic routes of 3aef.
2.2.3. General procedure for the synthesis of 3aef
2.2.3.3. 2-(4-Fluorophenyl)-5-(5-phenyl-1,3,4-oxadiazol-2-yl)-pyr-
A solution of benzoyl chloride (2 mmol) in CHCl3 (5 mL) was
added dropwise into a CHCl3 (10 mL) solution of compound 2
(2 mmol) and triethylamine (2 mmol) at room temperature. The
resulting mixture was stirred for 5 h. The solvent was removed
under vacuum and the crude product was washed with petroleum
ether and water. The solid was dried and added to phosphorus
oxychloride (caution: reacts violently with water; incompatible
with many metals, alcohols, amines, phenol, DMSO, and strong
bases) (10 mL). The mixture was refluxed for 12 h and cooled to
room temperature. The solution was added dropwise into ice water
(50 mL), and yellow solid was precipitated. The crude product was
collected by filtration and washed with water. The solid was puri-
fied by chromatography (silica gel, hexane/ethyl acetate, 3/1, v/v).
azolo [1,5-a]pyridine (3c). White solid, yield 82%, mp 355e356 ꢀC.
IR (KBr)
n
/cmꢁ1: 3050 (ArH), 1441 (Ar), 1227 (CeF), 791, 726 (ArH);
1H NMR (300 MHz, CDCl3):
d
8.57e8.59 (d, J ¼ 7.2 Hz, 1H, ArH), 8.31
(s, 1H, ArH), 8.16e8.19 (m, 2H, ArH), 7.95e7.99 (m, 2H, ArH),
7.50e7.59 (m, 4H, ArH), þ7.14e7.20 (m, 2H, ArH), 6.97 (s, 1H, ArH);
HRMS calcd for [M þ H] C21H14FN4O: 357.1152; found: 357.1137.
2.2.3.4. 2-(4-Chlorophenyl)-5-(5-phenyl-1,3,4-oxadiazol-2-yl)-pyr-
azolo [1,5-a]pyridine (3d). White solid, yield 80%, mp 371e372 ꢀC.
IR (KBr)
NMR (300 MHz, CDCl3):
ArH), 8.19 (m, 2H, ArH), 7.92e7.95 (m, 2H, ArH), 7.44e7.59 (m, 6H,
ArH), 7.00 (s, 1H, ArH); HRMS calcd for [M þ H]þ C21H14ClN4O:
373.0856; found: 373.0847.
n
/cmꢁ1: 3049 (ArH), 1436 (Ar), 1089 (CeCl), 794 (ArH); 1H
d
8.59 (d, J ¼ 7.2 Hz, 1H, ArH), 8.31 (s, 1H,
2.2.3.1. 2-(3,4-Dimethoxyphenyl)-5-(5-phenyl-1,3,4-oxadiazol-2-yl)-
pyrazolo [1,5-a]pyridine (3a). White solid, yield 73%, mp
2.2.3.5. 2-Phenyl-5-(5-phenyl-1,3,4-oxadiazol-2-yl)-pyrazolo [1,5-a]
397e398 ꢀC. IR (KBr)
n
/cmꢁ1: 3037 (ArH), 2995, 2836 (CeH in CH3),
pyridine (3e). White solid, yield 88%, mp 337e338 ꢀC. IR (KBr)
n/
1253 (CeOeC), 730 (ArH); 1H NMR (300 MHz, CDCl3):
d
8.56e8.58
cmꢁ1: 3049 (ArH), 1563 (Ar), 763 (ArH), 682 (Ar); 1H NMR
(300 MHz, CDCl3):
(d, J ¼ 7.2 Hz, 1H, ArH), 8.27 (s, 1H, ArH), 8.15e8.18 (m, 2H, ArH),
7.47e7.58 (m, 6H, ArH), 6.95e6.98 (m, 2H, ArH), 4.02 (s, 3H), 3.95
(s, 3H); HRMS calcd for [M þ H]þ C23H19N4O3: 399.1457; found:
399.1449.
d
8.58e8.60 (d, J ¼ 7.2 Hz, 1H, ArH), 8.30 (s, 1H,
ArH), 8.15e8.19 (m, 2H, ArH), 7.98e8.01 (m, 2H, ArH), 7.53e7.60
(m, 3H, ArHþ), 7.38e7.52 (m, 4H, ArH), 7.02 (s, 1H, ArH); HRMS calcd
for [M þ H] C21H15N4O: 339.1246; found: 339.1238.
2.2.3.2. 2-(4-Biphenyl)-5-(5-phenyl-1,3,4-oxadiazol-2-yl)-pyrazolo
2.2.3.6. 2-(Naphthalen-2-yl)-5-(5-phenyl-1,3,4-oxadiazol-2-yl)-pyr-
[1,5-a]pyridine (3b). White solid, yield 77%, mp 413e414 ꢀC. IR
azolo [1,5-a]pyridine (3f). White solid, yield 70%, mp 264e265 ꢀC.
(KBr)
n
/cmꢁ1: 3051 (ArH), 1447 (Ar), 760, 728 (ArH); 1H NMR
IR (KBr)
(300 MHz, CDCl3):
n
/cmꢁ1: 3048 (ArH),1563 (Ar), 724 (ArH), 681 (Ar); 1H NMR
(300 MHz, CDCl3):
d
8.60e8.62 (d, J ¼ 7.2 Hz, 1H, ArH), 8.33 (s, 1H,
d
8.63e8.65 (d, J ¼ 7.5 Hz, 1H, ArH), 8.48 (s, 1H,
ArH), 8.06e8.09 (m, 2H, ArH), 7.63e7.74 (m, 5H, ArH), 7.45e7.59
(m, 6H, ArH), 7.38e7.40 (m, 2H, ArH), 7.06 (s, 1H, ArH); HRMS calcd
for [M þ H]þ C27H19N4O: 415.1559; found: 415.1585.
ArH), 8.34 (s, 1H, ArH), 8.11e8.20 (m, 3H, ArH), 7.87e7.97 (m, 3H,
ArH), 7.51e7.60 (m, 6H, ArH), 7.16 (s, 1H); HRMS calcd for [M þ H]þ
C25H17N4O: 389.1402; found: 389.1407.
Fig. 3. The molecular structure of 3a with displacement ellipsoids drawn at the 50% probability level.

H. Yang et al. / Dyes and Pigments 91 (2011) 446e453
449
Table 2
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 CeH ¼ 0.96 Å and
Uiso(H) ¼ 1.5 Ueq(C). All other H atoms were placed in geometrically
idealized positions and constrained to ride on their parent atoms,
with CeH ¼ 0.93 Å and Uiso(H) ¼ 1.2 Ueq(C). A summary of the
crystallographic data and structure refinement details is given in
Table 1.
The selected bond lengths (Å) and bond angles (ꢀ) for compound 3a.
O(1)eC(8)
O(1)eC(7)
N(3)eN(4)
N(3)eC(11)
N(2)eC(8)
N(2)eN(1)
1.3684(17)
1.3727(17)
1.3570(17)
1.373(2)
C(9)eC(8)
1.462(2)
1.479(2)
1.3667(19)
1.456(2)
1.3533(19)
1.422(2)
C(15)eC(16)
O(2)eC(18)
C(6)eC(7)
1.2892(19)
1.4102(18)
N(4)eC(15)
O(2)eC(22)
C(8)eO(1)eC(7)
N(4)eN(3)eC(12)
C(14)eC(12)eC(13)
N(4)eC(15)eC(14)
N(1)eC(7)eO(1)
C(7)eN(1)eN(2)
N(2)eC(8)eC(9)
102.90(11)
112.54(12)
137.01(14)
111.89(14)
111.88(13)
106.54(12)
128.31(13)
N(4)eN(3)eC(11)
C(11)eN(3)eC(12)
C(8)eN(2)eN(1)
N(4)eC(15)eC(16)
O(1)eC(7)eC(6)
N(2)eC(8)eO(1)
O(1)eC(8)eC(9)
124.78(13)
122.66(13)
106.48(12)
119.61(13)
118.90(12)
112.19(13)
119.47(12)
3. Results and discussion
3.1. Synthesis and structure characterization
The synthetic route to the target heterocycles is shown in Fig. 2.
The starting materials 1aef can be easily prepared from 3-aryl-
pyrazole-5-carbaldehyde and ethyl 4-bromobut-2-enoate in
76e87% yields by a tandem reaction we previously reported [25].
Compounds 1 reacted with hydrazine hydrate to afford compounds
2 in good yield. The compounds 2 then reacted with aryl chlorides
in the presence of triethylamine, followed by dehydration in
phosphorus oxychloride to give desired compounds 3.
The assumed structures of target products were characterized
by IR, 1H NMR and HRMS spectra. For example compound 3a,
obtained as white solid, gave an [M þ H]-ion peak at m/z 399.1449
2.3. X-ray crystallography
Suitable single crystals of 3a for X-ray structure analysis were
obtained by slow evaporation of a solution of the solid in ethyl
acetate. The diffraction data of crystals with approximate dimen-
sions of 0.15 ꢂ 0.15 ꢂ 0.10 mm were collected with a Bruker-Nonius
SMART APEX ⅡCCD diffractometer using
chromated Mo K radiation (
a graphite mono-
¼ 0.71073 Å) at 298(2) K. The
a
l
structures were solved by direct methods with SHELXS-97 program
and refinements on F2 were performed with SHELXL-97 program
Fig. 4. Crystal packing diagram of 3a along the b-axis (A) and c-axis (B).

450
H. Yang et al. / Dyes and Pigments 91 (2011) 446e453
Table 3
The absorption characteristics of compounds 1aef and 3aef.
Compounds lmax (nm)
3max (L molꢁ1 cmꢁ1
CH2Cl2
)
CH2Cl2
Cyclohexane
296
Cyclohexane
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
279 297 277
44,790 32,944 39,519 26,256
63,995 58,125
290
e
289
e
e
e
264 274 264
270 279 269
267 275 264
267 298 266
301 335 298
299 348 297
283 338 383
286 339 285
283 340 284
303 349 290
273
280
275
296
340
351
328
342
342
351
59,906 48,060 58,918 46,853
57,664 49,299 55,952 46,060
56,635 47,850 58,532 48,438
71,775 33,131 70,008 31,212
50,878 17,539 18,880
49,914 10,442 18,270
53,375 14,648 21,670
61,983 14,845 11,236
7116
3232
6145
2261
57,580 15,415 42,547 11,237
47,811 13,312 5660 1401
in the HRMS, in accord with the molecular formula C23H19N4O3. The
IR spectra of compound 3a showed the characteristic absorption
bands at 3037 cmꢁ1 (ArH) and 1253 cmꢁ1 (CeOeC). The 1H NMR
spectra (CDCl3) of compound 3a revealed two singlet peaks at
Fig. 6. UVevis absorption spectrum of compounds 3aef in dichloromethane solution
(1 ꢂ 10ꢁ5 mol Lꢁ1).
d
3.95 (3H, CH3) and 4.02 (3H, CH3) which were readily assigned to
the hydrogen atoms of two methoxy moieties. The peaks between
6.98e8.58 for compound 3a were also consistent with protons on
d
basically intermediate between typical CeC single (1.54 Å) and C]
C double (1.34 Å) bonds. The carbonenitrogen bond lengths are
also intermediate between typical CeN single (1.47 Å) and C]N
aromatic rings. Moreover, the structure of compound 3a was
confirmed by X-ray diffraction analysis.
double (1.27 Å) bonds. This indicates that the
molecule are delocalized.
p-electrons in the
3.2. Crystal structure
The stacking structure of 3a along the b and c axis of the unit cell
is shown in Fig. 4. Compound 3a packs in a herringbone arrange-
ment with a dihedral angle between non-parallel molecules of
56.88ꢀ. The distance between two neighboring molecules, which
have face to face parallel orientation to each other, is 6.4946 Å. The
crystal packing structure also demonstrates the existence of inter-
molecular hydrogen bond between the C13eH13 on the pyrazolo
[1,5-a]pyridine ring and N1 atom on the 1,3,4-oxadiazole ring with
bond length of 2.4829 Å.
The single crystal structure and atomic numbering chosen for
compound 3a are shown in Fig. 3. The structure of 3a is in
a monoclinic crystal system with a P1 21/c 1 space group. The
pyrazolo[1,5-a]pyridine subunit is substantially planar with
a dihedral angles of 2.70ꢀ between pyrazole and pyridine segments.
The dihedral angles between the pyrazolo[1,5-a]pyridine ring and
1,3,4-oxadiazole or C16 phenyl ring are 11.25ꢀ and 7.50ꢀ, respec-
tively. Furthermore, the dihedral angle between 1,3,4-oxadiazole
ring and the C6 phenyl ring is 2.31ꢀ. All these dihedral angles are so
small that the four rings in 3a nearly construct a plane, providing
a large conjugated system within the molecule. Selected bond
lengths and bond angles are listed in Table 2. The results reveal that
the carbonecarbon bond lengths on the molecular skeleton are
3.3. Absorption spectral characteristics of the compounds 1ae1f
and 3ae3f
The UVevis spectral data of the compounds 1aef and 3aef are
summarized in Table 3. Fig. 5 shows the absorption spectrum of
Fig. 5. UVevis absorption spectrum of compounds 1aef in dilute (1 ꢂ 10ꢁ5 mol Lꢁ1
)
dichloromethane solution.
Fig. 7. UVevis absorption spectrum of compound 1a in different solvents.

H. Yang et al. / Dyes and Pigments 91 (2011) 446e453
451
Fig. 8. Emission spectrum of compounds 1aef in dilute (1 ꢂ 10ꢁ5 mol Lꢁ1) dichloro-
Fig. 10. The fluorescence spectra of compound 3d in different solvents.
methane solution.
compounds 1aef in dichloromethane. The results show that 1aef
display absorptions ranging from 235 to 340 nm, which are
attributed to the
pep transition of conjugated backbone.
*
Compounds 1c,1d,1e and 1f have similar absorption characteristics
and present sharp absorption peaks at 264 nm, 270 nm, 267 nm and
267 nm, respectively, with shoulders at longer wavelengths of
274 nm for 1c, 279 nm for 1d, 275 nm for 1e and 298 nm for 1f. In
the case of 1a, it exhibits one sharp absorption peak at 279 nm,
about 12 nm red-shifted compared with the corresponding peaks
of 1e, due to the strong electron-donating effect of two methoxy
groups on 2-phenyl ring. Compound 1a shows its shoulder
absorption towards longer wavelength at 297 nm. It is worth noting
that compound 1b presents a broad absorption ranging from
250 nm to 340 nm, with absorption maxima red-shifted distinctly
to 290 nm, resulting from the electron-donating effect of phenyl
Fig. 9. Emission spectrum of compounds 3aef in dichloromethane solution
(1 ꢂ 10ꢁ5 mol Lꢁ1).
Table 4
Fluorescence spectral data for 1aef and 3aef in dichloromethane.
lmaxex (nm)a
lmaxem (nm)a
Stoke’s shift (nm)a
VPL
b
Compounds
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
283
291
267
272
268
264
312
298
340
332
338
339
442
426
422
421
421
423
444
428
425
422
427
430
159
135
155
149
153
159
132
130
85
0.32
0.60
0.52
0.57
0.42
0.48
0.73
0.83
0.62
0.46
0.59
0.58
90
89
91
a
Solution in 1 ꢂ 10ꢁ5 mol Lꢁ1 CH2Cl2.
b
VPL is relative PL quantum efficiency of target compound solution in
1 ꢂ 10ꢁ6 mol Lꢁ1 CH2Cl2 which is determined by using 1 ꢂ 10ꢁ6 mol Lꢁ1 H2SO4
(V ¼ 0.55) as the standard reference.
Fig. 11. The simplified molecular structures during the DFT calculation and the mini-
mized structures of compounds 3a (A) and 3b (B).

452
H. Yang et al. / Dyes and Pigments 91 (2011) 446e453
Fig. 12. Molecular orbital maps of compounds 3a (A) and 3b (B).
group on the 2-phenyl ring and the formation of a more delocalized
and extended -conjugated system.
caused by the incorporation of oxadiazole moiety into the mole-
cules. The Stoke’s shifts of compounds 1aef and 3aef range from
85 to 159 nm, indicating the existence of molecular conformation
deviation between the ground state and the exited state.
The solvent effects on the fluorescence characteristics of these
compounds were investigated in cyclohexane, CH2Cl2 and THF at
the concentration of 1 ꢂ 10ꢁ5 mol Lꢁ1. For compound 3d in Fig. 10,
the emission wavelengths are red shifted with the increase of
solvent polarity from 411 nm in cyclohexane to 420 nm in CH2Cl2
and 426 nm in THF. Due to the interaction between fluorescent
molecules and solvent, the fluorescence is also enhanced with the
increase of solvent polarity.
p
For compounds 3aef (Fig. 6), they show absorption peaks at
longer wavelengths compared with the corresponding ethyl 2-aryl-
pyrazolo[1,5-a]pyridine-5-carboxylates (1aef), which may be due
to the extension of conjugation from 5-phenyl-1,3,4-oxadiazole
segment. Compounds 3c, 3d and 3e demonstrate similar absorption
properties, with absorption peaks at 283 nm, 286 nm and 283 nm,
respectively. The result reveals that halogen atoms (F or Cl) on
2-phenyl rings only slightly affect the absorption character of these
1,3,4-oxadiazole compounds. However, for compounds 3a, 3b and
3f with strong electron-donating groups or a high degree of
conjugation within their molecules, the absorption maxima are
red-shifted to 301 nm, 299 nm and 303 nm, respectively. It should
be noted that 3aef also present weak charge transfer absorption
bands between 335 nm and 349 nm, attributed to charge transfer
from pyrazolo[1,5-a]pyridine moiety to electron-accepting oxa-
diazole units.
Influence of solvent on the absorption behavior was investi-
gated. The absorption spectra of 1a, as an example, in three
different solvents (cyclohexane, dichloromethane and acetonitrile)
at a concentration of 1 ꢂ 10ꢁ5 mol Lꢁ1 are shown in Fig. 7. It is
observed that the absorption spectra change slightly with the
increase of solvent polarity although there is a tendency of a shorter
lmax in acetonitrile, indicating that there is no charge transfer in the
ground state.
3.5. Theoretical calculation
Density functional theory (DFT) calculations (B3LYP; 6-31G*) set
in the Gaussian 03 program package [27] were carried out to obtain
information about the HOMO and LUMO distributions of these
compounds. As examples, the minimized structures of 3a and 3b
are shown in Fig. 11 and their calculated molecular orbital (HOMO
and LUMO) energies are shown in Fig. 12. The minimized structure
of compound 3a reveals that all aromatic rings are strictly coplanar,
which is different from the molecular conformation in the crystal
structure, with an energy difference of 0.79 kJ molꢁ1. The confor-
mation deviation may results from hydrogen bond existing in the
crystal structure, as demonstrated in Fig. 4. For compound 3b, the
theoretical model indicates a dihedral angle of 37.3ꢀ within its
biphenyl unit while other aromatic rings present in the same plane.
The molecular energy levels of the frontier molecular orbital
3.4. Fluorescence spectral characteristics
shown in Fig.12 illustrates that HOMOs are
on the 2-aryl-pyrazolo[1,5-a]pyridine moieties and LUMOs are of
* character distributing in the electron-accepting 5-(5-phenyl-
1,3,4-oxadiazol-2-yl)-pyrazolo[1,5-a]pyridine segments. It should
be noted that the HOMO distributes in 3,4-dimethoxyphenyl ring at
a larger degree for compound 3a than that in biphenyl group for
compound 3b, due to the strong electron-donating property of
methoxyl group.
p orbitals concentrating
The emission spectra of compounds 1aef in CH2Cl2 are shown in
Fig. 8. They present efficient blue emissions in dilute solution, with
maximum emission spectra ranging from 421 to 442 nm. It can be
found that their intensity of fluorescence differed from each other.
The fluorescence intensity of compound 1a is extremely weak
compared with other compounds, which may be due to the inter-
action of molecules leading to the quenching of fluorescence at the
measured concentration of 1 ꢂ10ꢁ5 mol Lꢁ1. For compound 1a, the
DonoreAcceptor structure character resulting from the electron-
donating methoxy groups and electron-withdrawing ester group
is in favor of dipoleedipole interactions between molecules.
The maximum emissions of compounds 3aef are similar to
1aef, as demonstrated in Fig. 9. But the fluorescence intensity is
stronger than the corresponding intensity of 1aef. The Stoke’s shift
and fluorescence quantum yields of these compounds are
summarized in Table 4. Compounds 1aef and 3aef show moderate
fluorescence quantum yields ranging from 0.32 to 0.83. The result
reveals that 3aef exhibit higher VPL than 1aef, which may be
p
4. Conclusions
A series of novel 2,5-diaryl-1,3,4-oxadiazole derivatives con-
taining pyrazolo[1,5-a]pyridine moiety have been synthesized and
their structures were characterized by IR, 1H NMR and HRMS
spectra. Representatively, the spatial structure of compound 3a was
determined by X-ray diffraction analysis. The absorption maxima of
compounds 1aef vary from 264 nm to 290 nm depending on
the groups bonded to benzene rings. Compounds 3aef show

H. Yang et al. / Dyes and Pigments 91 (2011) 446e453
453
[4] Lee DW, Kwon KY, Jin JI, Park Y, Kin YR, Hwang IW. Chem Mater 2001;13:
565e74.
[5] Hughes G, Bryce MR. J Mater Chem 2005;15:94e107.
[6] Kulkarni AP, Tonzola CJ, Babel A, Jenekhe SA. Chem Mater 2004;16:4556e73.
[7] Kim OK, Lee KS, Woo HY, Kim KS, He GS, Swiatkiewicz J, et al. Chem Mater
2000;12:284e6.
[8] Huang PH, Shen JY, Pu SC, Wen YS, Lin JT, Chou PT, et al. J Mater Chem 2006;
16:850e7.
[9] Qian Y, Meng K, Lu CG, Lin BP, Huang W, Cui YP. Dyes Pigm 2009;80:174e80.
[10] Goudreault T, He Z, Guo Y, Ho CL, Zhan H, Wang Q, et al. Macromolecules
2010;43:7936e49.
red-shifted absorption peaks ranging from 283 to 303 nm
compared to the corresponding absorption of 1aef, due to their
extended conjugation. The maximum emission spectra of the target
compounds range from 421 to 444 nm and compounds 3 have
higher fluorescence quantum yields (VPL ¼ 0.46e0.83) than cor-
responding those of compounds 1 (VPL ¼ 0.32e0.60). These small
molecules have potential bioactivities and fluorescent properties,
and their use as fluorescent pigments and electroluminescent
materials are currently under investigation.
[11] Landis CA, Dhar BM, Lee T, Sarjeant A, Katz HE. J Phys Chem C 2008;112:
7939e45.
[12] Lee T, Landis CA, Dhar BM, Jung BJ, Sun J, Sarjeant A, et al. J Am Chem Soc
2009;131:1692e705.
Acknowledgements
[13] Li AF, Ruan YB, Jiang QQ, He WB, Jiang YB. Chem Eur J 2010;16:5794e802.
[14] Fang YK, Liu CL, Li C, Lin CJ, Mezzenga R, Chen WC. Adv Funct Mater 2010;20:
3012e24.
[15] Bettinetti L, Schlotter K, Hubner H, Gmeiner P. J Med Chem 2002;45:4594e7.
[16] Prante O, Tietze R, Hocke C, Lober S, Hubner H, Kuwert T, et al. J Med Chem
2008;51:1800e10.
[17] Lober S, Aboul-Fadl T, Hubner H, Gmeiner P. Bioorg Med Chem Lett 2002;12:
633e6.
[18] Ehrlich K, Gotz A, Bollinger S, Tschammer N, Bettinetti L, Harterich S, et al.
J Med Chem 2009;52:4923e35.
The authors thank the Shandong Natural Science Foundation
(No. Y2008B40) and Shandong Excellent Young and Mid-aged
Scientist Promotive Foundation (No. 2008BS04024) for financial
support of this work.
Appendix. Supplementary materials
[19] Akahane A, Katayama H, Mitsunaga T, Kato T, Kinoshita T, Kita Y, et al. J Med
Chem 1999;42:779e83.
[20] Cheung M, Harris PA, Badiang JG, Peckham GE, Chamberlain SD, Alberti MJ,
et al. Bioorg Med Chem Lett 2008;18:5428e30.
[21] Qiao L, Choi S, Case A, Gainer TG, Seyb K, Glicksman MA, et al. Bioorg Med
Chem Lett 2009;19:6122e6.
[22] Gudmundsson KS, Johns BA, Allen SH. Bioorg Med Chem Lett 2008;18:
1157e61.
[23] Allen SH, Johns BA, Gudmundsson KS, Freeman GA, Boyd Jr FL, Sexton CH,
et al. Bioorg Med Chem 2006;14:944e54.
[24] Marder SR, Stucky GD, Sohn JE. ACS symposium series, vol. 455. Washington,
DC: ACS; 1991.
CCDC 787181 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/data_request/cif, by emailing data_request@
ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: þ44 1223
336033.
References
[25] Ge YQ, Jia J, Li Y, Yin L, Wang JW. Heterocycles 2009;78:197e206.
[26] Ge YQ, Jia J, Yang H, Tang XT, Wang JW. Dyes Pigm 2011;88:344e9.
[27] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 03, revision A. 1. Pittsburgh, PA: Gaussian, Inc.; 2004.
[1] Schulz B, Bruma M, Brehmer L. Adv Mater 1997;9:601e13.
[2] Jin SH, Kim MY, Kim JY, Lee K, Gal YS. J Am Chem Soc 2004;126:2474e80.
[3] Leung MK, Yang CC, Lee JH, Tsai HH, Lin CF, Huang CY, et al. Org Lett 2007;9:
235e8.