Synthesis, characterization, optical properties and theoretical studies of novel substituted imidazo[1,5-a]pyridinyl 1,3,4-oxadiazole derivatives

Article abstract of DOI:10.1007/s10895-012-1091-8

Novel imidazo[1,5-a]pyridinyl 1,3,4-Oxadiazole derivatives were synthesized and characterised by IR, ¹H NMR and HRMS.UV-vis absorption and fluorescence properties of these compounds in different solutions showed that the maximal emission wavelength was not significantly changed in different solvents; however, maximum absorption wavelength was blue-shifted with the increase of solvent polarity. Absorption λ_max and emission λ_max was less correlated with substituent groups on benzene rings. The calculated molecular orbital correlates well with their absorption. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10895-012-1091-8

J Fluoresc
DOI 10.1007/s10895-012-1091-8
ORIGINAL PAPER
Synthesis, Characterization, Optical Properties
and Theoretical Studies of Novel Substituted
Imidazo[1,5-a]pyridinyl 1,3,4-Oxadiazole Derivatives
Yan-Qing Ge & Teng Wang & Gui Yun Duan &
Li Hua Dong & Xiao Qun Cao & Jian Wu Wang
Received: 26 March 2012 /Accepted: 20 June 2012
#
Springer Science+Business Media, LLC 2012
Abstract Novel imidazo[1,5-a]pyridinyl 1,3,4-Oxadiazole
derivatives were synthesized and characterised by IR, H
effect transistors (FETs) [8] and as precursors of N-
heterocyclic carbenes [9].
1
NMR and HRMS.UV-vis absorption and fluorescence prop-
erties of these compounds in different solutions showed that
the maximal emission wavelength was not significantly
changed in different solvents; however, maximum absorp-
tion wavelength was blue-shifted with the increase of sol-
vent polarity. Absorption λmax and emission λmax was less
correlated with substituent groups on benzene rings. The
calculated molecular orbital correlates well with their
absorption.
Aromatic 1, 3, 4-oxadiazole-based compounds have
attracted intense attention for their potential use in organic
electronics because either polymers or small molecules of
this kind have been widely used as electron-transporting
materials or electroluminescence (EL) materials in organic
light emitting diodes (OLEDs) [10–20]. In addition, electro-
ntransporting 1, 3, 4-oxadiazole moiety has been connected
to many chelating ligands to obtain luminescent complexes
with more new functions [21–24].
We are interested in design, synthesis, structural charac-
terization and fluorescence properties of novel compounds
with potential bioactivity. In our previous work we reported
a series of novel 2, 5-disubstituted 1, 3, 4-oxadiazole deriv-
atives and investigated their optical properties [25]. In con-
tinuation of our efforts in synthesizing various bioactive
molecules [26–30] and fluorescent small molecules
[31–33], herein, we report the synthesis, characterization
and optical properties of novel 1, 3, 4-oxadiazole derivatives
combined with imidazo[1,5-a]pyridine in order to investi-
gate their potential application in localization of small mol-
ecule in cells.
.
.
Keywords Synthesis Oxadiazole Imidazo[1,5-a]
.
.
.
pyridine Nitrogen-bridgehead UV-vis absorption
Fluorescence
Introduction
Imidazo[1,5-a]pyridine derivatives have been of particular
interest for their pharmacological and biological activities
such as cardiotonic agents [1], aromatase inhibitors in
estrogen-dependent diseases [2], thromboxane A2 synthe-
tase inhibitors [3] and angiotensin II receptor antagonists
[4]. They also have potential applications in organic light-
emitting diodes (OLEDs) [5–7], in organic thin-layer field
Experimental
General
:
:
:
:
Y.-Q. Ge T. Wang G. Y. Duan L. H. Dong X. Q. Cao (*)
Taishan Medical University,
Thin-layer chromatography (TLC) was conducted on silica
gel 60 F254 plates (Merck KGaA). H NMR spectra were
1
Taian, Shandong 271016, People’s Republic of China
e-mail: xqcao@yahoo.cn
recorded on a Bruker Avance 400 (400 MHz) spectrometer,
using DMSO-d6 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). The High Resolution Mass Spectrometry
J. W. Wang (*)
School of Chemistry and Chemical Engineering,
Shandong University,
Jinan, Shandong 250100, People’s Republic of China
e-mail: jwwang@sdu.edu.cn

J Fluoresc
1
(HRMS) spectra were recorded on a Q-TOF6510 spectro-
graph (Agilent). UV-vis spectra were recorded on a U-4100
(Hitachi). Fluorescent measurements were recorded on a
Perkin-Elmer LS-55 luminescence spectrophotometer.
mp 130.6–130.9 °C; H NMR (400 MHz, DMSO-d6): δ
8.40 (d, J07.2 Hz, 1H), 7.98 (s, 1H), 7.35 (m, 2H), 7.18
(dd, J07.6, 1.6 Hz, 1H), 7.12 (d, J08.0 Hz, 2H), 7.03 (m,
1H), 5.49 (s, 2H), 3.02 (t, J07.2 Hz, 2H), 1.73 (m, 2 H),
1.38 (m, 2H), 0.92 (t, J07.2 Hz, 3H); IR (KBr) ν02945,
2872, 1638, 1601, 1494, 1403, 1220, 1078, 749 cm−1;
HRMS: m/z calcd for C20H20ClN4O2 [M+H]+ 383.1275,
found 383.1288.
Computation Details
The hybrid density function B3LYP (Becke-Lee-Young-Parr
composite of exchange-correction functional) method [34, 35]
and the standard 6-31G (d, p) basis set [36] were used for both
structure optimization and the property calculations. All the
calculations were performed using the Gaussian 03 program
in the IBM P690 system at the Shandong Province High
Performance Computing Centre.
2-(3-butyl-1-chloroimidazo[1,5-a]pyridin-7-yl)-5-(o-toly-
loxymethyl)-1,3,4-oxadiazole (5b) Yellow solid (86 %
yield): mp 127.7–128 °C; H NMR (400 MHz, DMSO-
d6): δ 8.41 (dd, J07.6, 0.8 Hz, 1H), 7.98 (s, 1H), 7.22-
7.16 (m, 4H), 6.94 (m, 1H), 5.50 (s, 2H), 3.02 (t, J07.2 Hz,
2H), 2.21 (s, 3H), 1.73 (m, 2 H), 1.38 (m, 2H), 0.92 (t, J0
7.2 Hz, 3H); IR (KBr) ν02954, 2868, 1637, 1591, 1494,
1289, 1251, 1128, 1078, 748 cm−1; HRMS: m/z calcd for
C21H22ClN4O2 [M+H]+ 397.1431, found 397.1426.
1
General Procedure for Preparation of 2-(3-butyl-1-
chloroimidazo[1,5-a]pyridin-7-yl)- 5-(chloromethyl)-1,3,4-
oxadiazole 3
To a solution of 3-butyl-1-chloroimidazo[1,5-a]pyridine-7-
carbohydrazide (1) (3.5 mmol) in dichloromethane (50 ml)
was added 15 drops of Et3N. Subsequently, the solution of 2-
chloroacetyl chloride (4.2 mmol) dissolved in dichlorome-
thane (5 ml) was added dropwise in 20 min at room temper-
ature. The reaction mixture was stirred for 6 h at room
temperature, after which the solvent was removed under re-
duced 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 2 without
further purification. The mixture of 2 (3.5 mmol) and POCl3
(15 ml) was refluxed for 8 h. After cooling, it was poured into
powder ice (100 g). The precipitate was filtrated, washed with
water and dried. Product 3 was obtained by column chroma-
tography on silica gel using PE/EtOAc (4:1) as an eluent.
2-(3-butyl-1-chloroimidazo[1,5-a]pyridin-7-yl)-5-(m-toly-
loxymethyl)-1,3,4-oxadiazole (5c) Yellow solid (92 %
1
yield): mp 126.1–126.9 °C; H NMR (400 MHz, DMSO-
d6): δ 8.39 (d, J07.2 Hz, 1H), 7.96 (s, 1H), 7.24–7.16 (m,
2H), 6.90–6.95 (m, 2H), 6.84 (d, J07.2 Hz, 1H), 5.46 (s,
2H), 3.02 (t, J07.2 Hz, 2H), 2.30 (s, 3H), 1.73 (m, 2H), 1.38
(m, 2H), 0.92 (t, J07.2 Hz, 3H); IR (KBr) ν02953, 2929,
2868, 1637, 1588, 1491, 1450, 1246, 1159, 1078, 769 cm−1;
HRMS: m/z calcd for C21H22ClN4O2 [M+H]+ 397.1431,
found 397.1424.
2-(3-butyl-1-chloroimidazo[1,5-a]pyridin-7-yl)-5-(p-toly-
loxymethyl)-1,3,4-oxadiazole (5d) Yellow solid (88 %
1
yield): mp 154.4–154.9 °C; H NMR (400 MHz, DMSO-
d6): δ 8.39 (dd, J07.6, 0.8 Hz, 1H), 7.97 (d, J00.8 Hz, 1H),
7.18-7.13(m, 3H), 7.00 (d, J08.8 Hz, 2H), 5.43 (s, 2H), 3.02
(t, J07.2 Hz, 2H), 2.25 (s, 3H), 1.73 (m, 2H), 1.38 (m, 2H),
0.92 (t, J07.2 Hz, 3H); IR (KBr) ν02966, 2869, 1711, 1638,
1589, 1481, 1456, 1277, 1031, 752 cm−1; HRMS: m/z calcd
for C21H22ClN4O2 [M+H]+ 397.1431, found 397.1431.
General Procedure for Preparation of 2-(3-butyl-1-
chloroimidazo[1,5-a]pyridin-7-yl)- 5-(aryloxymethyl)-
1,3,4-oxadiazole 5a-i
A mixture of 3 (1 mmol), substituted phenol 4 (1.05 mmol),
anhydrous potassium carbonate (1.2 mmol) and dry aceto-
nitrile (25 ml) was refluxed for 1–3 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 82–92 % (Fig. 1).
2-(3-butyl-1-chloroimidazo[1,5-a]pyridin-7-yl)-5-((4-
methoxyphenoxy)methyl)-1,3,4-oxadiazole (5e) Yellow sol-
1
id (82 % yield): mp 128.8–129.8 °C; H NMR (400 MHz,
DMSO-d6): δ 8.39 (d, J07.6 Hz, 1H), 7.97 (s, 1H), 7.17(d,
J07.6 Hz, 1H), 7.06 (d, J08.8 Hz, 2H), 6.90 (d, J08.8 Hz,
2H), 5.41 (s, 2H), 3.71 (s, 3H), 3.02 (t, J07.2 Hz, 2H), 1.73
(m, 2H), 1.38 (m, 2H), 0.92 (t, J07.2 Hz, 3H); IR (KBr) ν0
2958, 2933, 2872, 1635, 1580, 1564, 1508, 1494, 1403,
1231, 1044, 813, 727 cm−1; HRMS: m/z calcd for
C21H22ClN4O3 [M+H]+ 413.1830, found 413.1842.
The Spectroscopic Data of Compounds 5
2-(3-butyl-1-chloroimidazo[1,5-a]pyridin-7-yl)-5-(phenoxy-
2-(3-butyl-1-chloroimidazo[1,5-a]pyridin-7-yl)-5-((4-chlor-
methyl)-1,3,4-oxadiazole (5a) Yellow solid (90 % yield):
ophenoxy)methyl)-1,3,4-oxadiazole (5f) Yellow solid (87 %

J Fluoresc
Fig. 1 The synthetic route of
unsymmetrical 1,3,4-
oxadiazole derivatives
1
yield): mp 155.9–156.7 °C; H NMR (400 MHz, DMSO-
d6): δ 8.39 (d, J07.6 Hz, 1H), 7.97 (s, 1H), 7.39 (d, J0
8.8 Hz, 2H), 7.18–7.15 (m, 3H), 5.51 (s, 2H), 3.02 (t, J0
7.2 Hz, 2H), 1.73 (m, 2H), 1.38 (m, 2H), 0.92 (t, J07.2 Hz,
3H); IR (KBr) ν02958, 2929, 2864, 1638, 1585, 1560,
1493, 1226, 1078, 819, 722 cm−1; HRMS: m/z calcd for
C20H19Cl2N4O2 [M+H]+ 417.0885, found 417.0891.
7.38–7.34 (m, 2H), 7.17 (dd, J07.6, 1.6 Hz, 1H), 5.70 (s,
2H), 3.02 (t, J07.2 Hz, 2H), 1.73 (m, 2H), 1.38 (m, 2H),
0.92 (t, J07.2 Hz, 3H); IR (KBr) ν02962, 2929, 2864,
1637, 1593, 1513, 1494, 1343, 1299, 1074, 747 cm−1;
HRMS: m/z calcd for C20H19ClN5O4 [M+H]+ 428.1126,
found 428.1133.
Methyl 4-((5-(3-butyl-1-chloroimidazo[1,5-a]pyridin-7-yl)-
1,3,4-oxadiazol-2-yl)methoxy)benzoate (5h) Yellow solid
(90 % yield): mp 155.9–157 °C; 1H NMR (400 MHz,
DMSO-d6): δ 8.39 (d, J07.6 Hz, 1H), 7.97 (s, 1H), 7.95
2-(3-butyl-1-chloroimidazo[1,5-a]pyridin-7-yl)-5-((4-nitro-
phenoxy)methyl)-1,3,4-oxadiazole (5g) Yellow solid (85 %
yield): mp 166–167.3 °C; 1H NMR (400 MHz, DMSO-d6):
δ 8.39 (d, J07.6 Hz, 1H), 8.29–8.25 (m, 2H), 7.98 (s, 1H),
0.09
5a
5b
5c
5d
Table 1 Maximum absorption wavelength (λmax) and maximum mo-
lar extinction coefficients (εmax) of 5a-i in dichloromethane and
acetonitrile
0.08
0.07
5e
5f
Ε (mol−1 cm−1 L)
0.06
Compound
λmax
5g
0.05
CH3CN
DCM
CH3CN
DCM
5h
5i
0.04
0.03
0.02
0.01
0.00
5a
5b
5c
5d
5e
5f
377
377
378
361
375
369
368
386
367
390
392
394
390
393
393
393
394
395
32000
37000
33000
29000
32000
35000
27000
67000
53000
50000
58000
54000
44000
48000
47000
48000
80000
61000
300
320
340
360
380
400
420
440
460
5g
5h
5i
Wavelength/nm
Fig. 2 UV-vis absorption spectra of 5a-i in dichloromethane solution
(10−6 M)

J Fluoresc
700
600
500
400
300
200
100
0
0.06
0.04
0.02
0.00
5a
5b
5c
5d
5e
5f
5g
5h
5i
5a
5b
5c
5d
5e
5f
5g
5h
5i
320
340
360
380
400
420
440
460
400
450
500
550
600
650
wavelength/nm
Wavelength/nm
Fig. 3 UV-vis absorption spectra of 5a-i in acetonitrile solution (10−6 M)
Fig. 5 Fluorescence spectra of the compounds 5a-5i in acetonitrile
(10−6 M)
(d, J09.2 Hz, 2H), 7.24 (d, J08.8Hz, 2H), 7.17 (dd, J07.6,
1.6 Hz, 1H), 5.61 (s, 2H), 3.83 (s, 3H), 3.02 (t, J07.2 Hz,
2H), 1.73 (m, 2H), 1.38 (m, 2H), 0.92 (t, J07.2 Hz, 3H); IR
(KBr) ν02956, 2875, 1720, 1636, 1607, 1510, 1493, 1282,
1170, 1040, 767 cm−1; HRMS: m/z calcd for C22H22ClN4O4
[M+H]+ 441.1330, found 441.1319.
Results and Discussion
Synthesis and Characterization
The synthetic procedures of compound 5a-i are shown in
Fig. 1. The starting 3-butyl-1-chloroimidazo[1,5-a]pyridine-
7-carbohydrazide 1 can be easily prepared according to the
procedure reported in our previous paper [25]. The reaction
of 1 and 2-chloroacetyl chloride in the presence of triethyl-
amine in dichloromethane at room temperature afforded
intermediate 2 that can be used to the next reaction step
without further purification. Then 2 reacted with phosphoryl
trichloride to give 2-(3-butyl-1-chloroimidazo[1,5-a]pyri-
din-7-yl)-5-(chloromethyl)-1,3,4-oxadiazole 3 that can be
purified by column chromatography on silica gel using
PE/EtOAc (4:1) as an eluent. The reaction of 3 and substi-
tuted phenol 4 in the presence of anhydrous potassium
2-((biphenyl-4-yloxy)methyl)-5-(3-butyl-1-chloroimi-
dazo[1,5-a]pyridin-7-yl)-1,3,4-oxadiazole (5i) Yellow solid
1
(88 % yield): mp 145.9–146.5 °C; H NMR (400 MHz,
DMSO-d6): δ 8.39 (d, J07.6 Hz, 1H), 7.84 (s, 1H), 7.54
(d, J07.6 Hz, 1H), 7.43–7.29 (m, 6H), 7.16–7.11 (m, 2H),
5.50 (s, 2H), 3.02 (t, J07.2 Hz, 2H), 1.73 (m, 2H), 1.38 (m,
2H), 0.92 (t, J07.2 Hz, 3H); IR (KBr) ν02933, 2870, 1633,
1584, 1559, 1508, 1492, 1206, 1073, 810, 723 cm−1;
HRMS: m/z calcd for C26H24ClN4O2 [M+H]+ 459.1588,
found 459.1586.
900
800
800
700
700
5a
5a
5b
5c
5d
5e
5f
5g
5h
5i
600
500
400
300
200
100
0
5b
5c
5d
5e
5f
5g
5h
5i
600
500
400
300
200
100
0
-100
400
450
500
550
600
650
-100
400
450
500
550
600
650
Wavelength/nm
Wavelength/nm
Fig. 4 Fluorescence spectra of compounds 5a-i in dichloromethane
(10−6 M)
Fig. 6 Fluorescence spectra of the compounds 5a-i in DMF (10−6 M)

J Fluoresc
-0.03284
-0.07124
-0.03290
-0.03251
-0.07068
-0.03378
-0.03347
-0.03384
-0.07225
Table 2 Maximum wavelength of excitement and emission of fluo-
rescence of compounds 5a-i
-0.04167
-0.07455
-0.05
-0.10
-0.15
-0.20
-0.25
-0.07190 -0.07219
-0.07130
-0.07925
Compound
λex (nm)
λem (nm)
-0.08687
DCM
CH3CN
DMF
HOMO-1
HOMO
LUMO
5a
5b
5c
5d
5e
5f
390
390
390
390
390
390
390
390
390
477
475
477
476
477
475
478
476
476
477
479
477
476
479
475
474
476
474
476
477
476
476
478
478
477
476
476
LUMO+1
-0.20291
-0.20319
-0.20238
-0.22112
-0.20216
-0.20243
-0.22558
-0.20319
-0.20505
-0.23946
-0.20891
-0.21641
-0.21500
-0.22387
-0.22906
-0.25939
5g
5h
5i
5a
5b
5c
5h
5i
5d
5e
5j
Fig. 8 Molecular orbital energy levels of compounds 5a-i
values of 5a-i: λmax (DCM) > λmax (CH3CN). In the same
solvent compounds 5a-i have the similar maximum absorp-
tion, which means substituent R1, R2 and R3 have no sig-
nificant impact on the absorption. The maximum molar
extinction coefficients of 5a-i in dichloromethane and ace-
tonitrile are different, and it can be observed that the en-
hancement order is generally ε (CH3CN) < ε (DCM).
carbonate in dry acetonitrile at reflux temperature afforded
compound 5 in 82–92 %.
1
The proposed structures were confirmed by H NMR
spectra, IR, and HRMS.
Absorption Spectra of the Compounds 5a-i
For UV-visible absorption measurements, the concentration
was 5×10−6 mol L−1, and the absorption data are summa-
rized in Table 1. The UV-visible absorption spectra of com-
pounds 5a-i measured in dichloromethane and acetonitrile
solutions are given in Figs. 2 and 3. These compounds
displayed similar absorptions ranging from 367 to 394 nm
that were attributed to π-π transition of conjugation system.
Comparing with 2-(3-butyl-1-chloroimidazo[1,5-a]pyridin-
7-yl)-5-aryl-1,3,4-oxadiazole reported in previous paper
[25], compounds 5a-i have blue shifted about 20 nm, the
reason being that the aryloxy which are linked to methylene
in C5 cannot extend to the pi-conjugation system due to the
block effect of methylene. It is also observed that the λmax
values of 5a-i change with the polarity of solutions. Gener-
ally in different solvents, there is a consequence for the λmax
Fluorescence Spectra
The emission spectra of compounds 5a-i in dichlorome-
thane, acetonitrile and DMF solution (10−6 mol L−1) are
presented in Figs. 4, 5 and 6 respectively and their excitation
wavelengths are shown in Table 2. It can be found that their
intensity of fluorescence differed from each other. In com-
parison with the effect on the absorption spectrum, the
group in benzene ring had weak influence on the emission
intensity of 1,3,4-oxadiazole compounds except compound
5h with a methoxycarbonyl group. It can be seen from
Table 2 that the maximum emission wavelengths of 5a-i
are almost the same in three solvents. These results indicate
that the environment plays no significant role in determining
Fig. 7 The minimized structures of compounds 5

J Fluoresc
Fig. 9 Molecular orbital maps
of compounds 5
5a
5b
5c
5d
5e
LUMO + 1
LUMO
HOMO
HOMO – 1
5g
5h
5i
LUMO + 1
LUMO
HOMO
HOMO – 1

J Fluoresc
the emission wavelengths of the compounds. However, the
fluorescence intensity decreased with the increase in solvent
polarity. It is believed that the potential energy surface of the
emitting state is different from that of the ground state and a
photo-induced intramolecular charge transfer (ICT) takes
place in the fluorescence states with increasing solvent
polarity. That is to say, the molecule is solvated significantly
in the S1 excited state, resulting in a difference in dipolemo-
ment between the S1 excited state and the ground state.
absorption and fluorescence emission. Quantum calculation
correlates well with their absorption and fluorescence emis-
sion. However, the maximum absorption wavelength
changes significantly with the increase of solvent polarity.
In contrast with the previous report, present work provides
small molecules with more interesting structure diversity
and similar optical properties. Currently, investigations are
underway to elucidate the bioactivity and localization of
small molecule in cells and the results will be reported in
due course.
Theoretical Calculations
Acknowledgement This study was supported by the Science and Tech-
nology Development Project of Shandong Province (2011GGH22112 and
2012GSF11812).
To enhance our understanding of the relationship between
molecular structures and the electronic spectra of these new
1,3,4-oxadiazoles derivatives, we carried out structure opti-
mization and molecular orbital (MO) calculations on com-
pound 5a-i based on a simplified model with density
functional theory (DFT) on the level of B3LYP. The mini-
mized structures are shown in Fig. 7 and the calculated
molecular orbital (HOMO and LUMO) energies are shown
in Fig. 8.
The minimized structures of compound 5 revealed that
imidazo[1,5-a]pyridine ring presents a coplanar conforma-
tion with the 1,3,4-oxadiazole ring while the benzene ring
does not. This indicates that imidazo[1,5-a]pyridine group
conjugates with the 1,3,4-oxadiazole ring efficiently
through the single bond yet the benzene does not. This result
is in accordance with the similar absorptions spectra of
compound 5 with different substituents on the benzene ring.
The molecular energy levels of the frontier molecular
orbital shown in Fig. 8 revealed that the introduction of
electron-withdrawing group to benzene ring raised the en-
ergy levels of HOMO and also raised the energy levels of
LUMO for the compound; therefore, the energy gap be-
tween HOMO and LUMO changed little. This corresponds
well to the experimentally recorded similar absorption spec-
tra of compound 5.
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