Photophysical, thermal properties, solvatochromism and DFT/TDDFT studies on novel conjugated D-A-π-A-D form of small molecules comprising thiophene substituted 1,3,4-oxadiazole

Article abstract of DOI:10.1016/j.molstruc.2019.127032

In this paper, we report the design and synthesis of a series of novel conjugated thiophene substituted 1,3,4-oxadiazole derivatives BSTO-4(a-f) of Donor-Acceptor-π-Acceptor-Donor form by utilizing palladium catalyzed Suzuki cross coupling reaction. Structures were characterized by using spectroscopic techniques namely FT-IR, ¹H NMR, ¹³C NMR and Mass spectra. Their thermal, photophysical and solvatochromic properties were studied in detail. Also, the PL investigations in the solid state were carried out. Furthermore, theoretical calculations such as density functional theory (DFT) were performed to get a better understanding of the intramolecular charge transfer property and electronic structures of the compounds. Experimentally measured optical bandgap values are in the range 3.02 to 3.54 eV. All compounds exhibit Stokes shifts in the range of 2776–3964 cm^?1 and fluorescence quantum yields (Φ_f) in the range of 0.06–0.70 in chloroform. These derivatives exhibit high thermal stability that is the 5% weight loss temperature of derivatives are in the range 178–356 °C and the compounds BSTO-4(b, e and f) had the glass transition temperatures (Tg) at 107 °C, 147 °C and 68 °C, respectively. These results demonstrate that the novel thiophene substituted 1,3,4-oxadiazole compounds are promising candidates for potential applications in development of OLEDs, organic electronic devices/optoelectronic devices.

Full text of DOI:10.1016/j.molstruc.2019.127032

Journal of Molecular Structure 1199 (2020) 127032
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Photophysical, thermal properties, solvatochromism and DFT/TDDFT
studies on novel conjugated D-A-p-A-D form of small molecules
comprising thiophene substituted 1,3,4-oxadiazole
Mahesh S. Najare a, Mallikarjun K. Patil b, AfraQuasar A. Nadaf a, Shivaraj Mantur a,
Manjunatha Garbhagudi a, Supreet Gaonkar a, Sanjeev R. Inamdar b,
,
*
Imtiyaz Ahmed M. Khazi a
a Department of Chemistry, Karnatak University, Dharwad, 580003, Karnataka, India
b Laser Spectroscopy Programme, Department of Physics and UGC-CPEPA, Karnatak University, Dharwad, 580003, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, we report the design and synthesis of a series of novel conjugated thiophene substituted
1,3,4-oxadiazole derivatives BSTO-4(a-f) of Donor-Acceptor-p-Acceptor-Donor form by utilizing palla-
Received 8 May 2019
Received in revised form
15 July 2019
Accepted 2 September 2019
Available online 5 September 2019
dium catalyzed Suzuki cross coupling reaction. Structures were characterized by using spectroscopic
techniques namely FT-IR, 1H NMR, 13C NMR and Mass spectra. Their thermal, photophysical and sol-
vatochromic properties were studied in detail. Also, the PL investigations in the solid state were carried
out. Furthermore, theoretical calculations such as density functional theory (DFT) were performed to get
a better understanding of the intramolecular charge transfer property and electronic structures of the
compounds. Experimentally measured optical bandgap values are in the range 3.02 to 3.54 eV. All
Keywords:
Thiophene
Oxadiazole
compounds exhibit Stokes shifts in the range of 2776e3964 cmꢀ1 and fluorescence quantum yields (Ff
)
in the range of 0.06e0.70 in chloroform. These derivatives exhibit high thermal stability that is the 5%
weight loss temperature of derivatives are in the range 178e356 ꢁC and the compounds BSTO-4(b, e and
f) had the glass transition temperatures (Tg) at 107 ꢁC, 147 ꢁC and 68 ꢁC, respectively. These results
demonstrate that the novel thiophene substituted 1,3,4-oxadiazole compounds are promising candidates
for potential applications in development of OLEDs, organic electronic devices/optoelectronic devices.
© 2019 Elsevier B.V. All rights reserved.
Photophysical properties
Solvatochromism
DFT/TDDFT
Thermal properties
1. Introduction
application in the field of next-generation general lighting as they
are more efficient than fluorescent tubes and are mercury free
Organic light-emitting diodes (OLEDs) have attracted most of
the scientific, academic and industrial communities owing to their
amazing potential applications in full-colour-flat-panel displays
and solid-state lighting sources [1e6]. In recent years, the huge
achievements in the field of OLEDs can be attributed to their
comparatively low power consumption, high efficiency, thermal
stability and realistic colour tunability by molecular structure
modification; also they have ability to produce thin, lightweight,
transparent, flexible and efficient devices [7e9]. Nowadays, OLED is
extensively used as displays in many electronics devices, such as
smart-phones, digital cameras, smart watches and ultra-high
definition TVs. In addition to this OLEDs have a promising
[10,11].
However, present day OLEDs still have some unresolved prob-
lems such as low quantum efficiency, lifetimes, cost, high driving
voltage and low efficiency of the devices. In addition to the above
issues, the electroluminescence properties of blue-emitting mate-
rials remain difficult in terms of their easy availability, lower cost,
efficiency, colour purity and thermal stability, in order to use them
as efficient blue light emitting materials for organic light-emitting
diodes. Some of these problems can be solved by exploring the
new class of organic molecules or structural modification of organic
molecules that specifically help to enhance the optoelectronic
properties of organic materials for the better applications in opto-
electronic devices/OLEDs [12e17].
In this regard, new organic
attained significant interest due to their potential applications in
p-conjugated small molecules have
* Corresponding author.
E-mail address: drimkorgchem@gmail.com (I.A.M. Khazi).
https://doi.org/10.1016/j.molstruc.2019.127032
0022-2860/© 2019 Elsevier B.V. All rights reserved.

2
M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
optoelectronic devices, such as organic light emitting diodes
(OLEDs), organic field effect transistors (OFETs), organic photovol-
taics (OPVs) and sensors [18e20]. Also, small molecules have
unique optical and electronic properties, viz, (i) well defined mo-
lecular structure; (ii) precise control on functional properties of the
materials leads to easily tunable optical properties and electronic
energy levels; (iii) low cost fabrication by solution-processing and
vacuum deposition methods [21,22]. Organic materials constituted
applications.
2. Results and discussion
2.1. Synthesis and characterization
We have synthesized and characterized thiophene substituted
1,3,4-oxadiazole derivatives BSTO-4(a-f). A relatively simple and
efficient synthetic pathway for the synthesis of proposed com-
pounds BSTO-4(a-f) and their precursors is illustrated in Scheme 1.
Chemical structures of designed final compounds BSTO-4(a-f) are
out-lined in Fig. 1. The required key intermediates 1, 2 and BSTO-3
were synthesized according to the reported procedures from the
literature [34,35]. A reaction between terephthalic acid and SOCl2
in ethanol offered the precursor diethyl terphthalate (1), subse-
quently diethyl terphthalate were converted to the corresponding
hydrazide derivative terephthalohydrazide (2) on treatment with
hydrazine hydrate in ethanol. Then terephthalohydrazide (2) is
treated with 5-chlorothiophene-2-carboxylic acid in POCl3 on
refluxing obtain the key intermediates 2-(5-chlorothiophen-2-yl)-
by donor (D),
p-conjugation and acceptor (A) moieties (D-p-A
form) have gained significant attention from the scientific com-
munity in recent decades due to their flexibility in designing solar
cells and as colour tunable emitting layers in organic light emitting
diodes (OLEDs) [23e25]. A typical OLED is made up of the hole-
transporting layer (HTL) and electron-transporting layer (ETL) of
organic materials sandwiched between two electrodes, the anode
and cathode. When a voltage is applied across the two electrodes,
then charges (holes from the anode and electrons from the cath-
ode) are injected in the organic material. Then, the charges move
inside the material, and recombine to form excitons. A photon is
emitted when the exciton recombines [26,27].
It is essential to examine the selection of building blocks care-
fully during the construction of materials for optoelectronic devices
because they tend to dominate the functional properties of the
materials [21]. Some of the electron transporting groups reported
in the literature are phosphine oxide, phosphine sulfide, quinoxa-
line, phenanthroline, benzimidazolyl, oxadiazole and triazine.
Among them 1,3,4-Oxadiazoles have unique properties such as
electron-deficient nature, high photoluminescence quantum yield,
good thermal and chemical stabilities, these properties encourage
for the extensive uses of 1,3,4-oxadiazoles in electroluminescent
diodes and non-linear optical materials as emitting layers and
electron-transporting/hole blocking materials. More specifically
2,5-diaryl-1,3,4-oxadiazoles are well known electron-transporting
5-(4-(5-(5-chlorothiophen-2-yl)-1,3,4-oxadiazol-2-yl)
phenyl)-
1,3,4-oxadiazole (BSTO-3) derivative. In the final step, key inter-
mediate BSTO-3 is subjected to Pd-catalyzed Suzuki cross coupling
reaction with different aryl/heteroaryl boronic acid to afford the
final compounds BSTO-4(a-f) in moderate yields. The obtained
derivatives were purified using column chromatography. These
synthesized derivatives are easily soluble in common organic sol-
vents such as like methanol, ethanol, acetonitrile, chloroform,
tetrahydrofuran and dichloromethane etc. The structural integrity
and purity of the compounds BSTO-4(a-f) were confirmed and
characterized by various analytical techniques such as 1H NMR, 13
NMR, FT-IR and GC-MS/ESI-MS/APCI-MS.
C
materials and have been reported [28e30]. Thiophene based
p-
2.2. Photophysical properties
conjugated systems are high demand for the developing of high
performance organic solar cells/organic optoelectronics due to
their excellent light-harvesting properties, structural versatility
2.2.1. UVevisible absorption and photoluminescence (PL)
measurements
and intrinsic charge-transport properties. In addition, thiophene
p
-
In order to explore the potentiality of organic materials in op-
toelectronic device applications, examination of their photo-
physical properties is important, and it provides extensive
information about the electronic properties of organic materials
[36,37]. The photophysical properties of BSTO-4(a-f) derivatives
were studied in detail by UVevisible absorption and photo-
luminescence (PL) measurements in chloroform solution illustrated
in the Fig. 2 (a & b). The corresponding photophysical data like
absorption maxima, emission maxima and Stokes shift of each
conjugated backbones are normally combined with selected ac-
ceptors in order to fine-tune their photophysical properties
[31e33].
In view of the above observations, we have designed, synthe-
sized and characterized a series of novel
p-conjugated thiophene
substituted Donor-Acceptor- -Acceptor-Donor form of 1,3,4-
p
oxadiazole derivatives by using Suzuki cross coupling reaction. A
detailed investigation of photophysical properties viz., optical
bandgap, quantum yields, solvatochromism, intramolecular charge
transfer property (ICT) and thermal properties was carried out.
Also, the absorption maxima of compounds BSTO-4(a-f) are ob-
tained computationally by using time dependent density functional
theory (TDDFT) in vacuum. For a better understanding of the mo-
lecular geometries and intramolecular charge transfer character of
the molecules, the frontier molecular orbitals, highest occupied
molecular orbitals and lowest unoccupied molecular orbitals of the
compounds BSTO-4(a-f) were computed using theoretical calcula-
tions density functional theory (DFT). To explore the optical prop-
erties and intramolecular charge transfer property of the
compounds BSTO-4(a-f), the electron-donor (D) and electron-
compound are summarized in Table 1. The absorption spectra for
abs
max
the compounds BSTO-4(a-f) showed absorption maxima (
l
) in
the range of 338e357 nm. Also, computationally obtained values of
absorption maxima of compounds BSTO-4(a-f) are found in the
range 320e396 nm at the TDDFT/COM-B3LYP/6-311G(d,p) level of
theory, in vacuum. The divergence in experimentally and compu-
tationally obtained values may be due to that computational
determination belongs to isolated system of molecule with no
physical solute-solvent interaction. It can be observed that, the
UVevisible absorption spectra of BSTO-4(a-f) in chloroform
showed two absorption bands (Fig. 2a); the absorption bands at
lower wavelength region can be assigned to the
tron transition of the 1,3,4-oxadiazole system and absorption bands
at longer wavelength regions were mainly assigned to the
p/p* local elec-
acceptor (A) moieties are linked via
through a phenyl spacer with para linkages results in extended
conjugation, thereby reducing in the form D-A- -A-D. The de-
p-extending conjugation
p
-p
p/p*
p
electronic transition of the whole bipolar conjugate thiophene
rivatives exhibit enhanced fluorescence quantum yields and high
thermal stability. These results demonstrate that the novel thio-
phene substituted 1,3,4-oxadiazole compounds can be used widely
and can play an important role in OLEDs/optoelectronic
substituted 1,3,4-oxadiazole molecules [38]. However, their ab-
abs
sorption maximum (
l
) are slightly shifted because of changes in
max
their electronic environment by the introduction of the different
electron donating aryl/hetero-aryl groups with varying strength in

M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
3
Scheme 1. Synthetic pathway for the synthesis of proposed compounds BSTO-4(aef).
Fig. 1. Chemical structures of designed final compounds BSTO-4(aef).
their organic backbones (Fig. 2a).
and the relevant data are presented in Table 1. In comparison with
the absorption spectrum, a considerable bathochromic shift (red
shift) was observed in emission maxima of compounds BSTO-4(a-
f), as shown in Fig. 2b and Table 1. This depends upon the nature
Emission spectra of compounds BSTO-4(a-f) exhibited a single
emission peaks in the violet-blue region centred at 373e410 nm in
chloroform solution at room temperature are displayed in Fig. 2b

4
M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
Fig. 2. (a) UVevisible absorption spectra and (b) Emission spectra of the synthesized compounds in CHCl3 solution at room temperature (c) Emission spectra of compounds BSTO-
4(aef) in solid state (thin film). Ref - Coumarin 440 dye used for comparison [39].
Table 1
Photophysical properties, quantum yield (
F), optical bandgap energies and thermal data for BSTO-4(aef).
abs
max
emi
max
emi
max
Eogpte (eV)
Compounds
l
(nm)
l
(nm)c
l
(nm)d
Stokes shift Dl (cmꢀ1
)
Quantum Yield (
F
)
Tg [oC]f
Tm [oC]f
Tc [oC]f
T5d [oC]f
Exp.a
Theo.b
BSTO-4a
BSTO-4b
BSTO-4c
BSTO-4d
BSTO-4e
BSTO-4f
Ref
357
346
345
346
338
346
337
396
324
339
346
320
336
e
410
398
397
399
373
401
404
463
462
421
395
455
429
e
3621
3776
3797
3839
2776
3964
4921
3.02
3.19
3.20
3.22
3.54
3.12
e
0.18
0.62
0.70
0.61
0.06
0.60
0.98
e
228
e
e
e
e
e
e
e
e
300
241
239
242
356
178
e
107
e
222
210
e
e
147
68
e
e
e
a
Absorption maxima in chloroform at 10
m
M concentration.
Theoretical absorption maxima obtained by using TD-DFT/COM-B3LYP/6-311G(d,p) level of theory.
Emission maxima in chloroform at 10 M concentration.
b
c
d
e
f
m
Emission maxima measured in solid state film.
Optical bandgap energies were calculated from the long edge of the absorption spectrum using Eogpt ¼ hc/
l
¼ 1240/
l (eV).
Results from DSC and TGA measurements; Tg-glass transition temperature, Tm-melting temperature, Tc-crystallization temperature and T5d-5% weight loss temperature.
Ref - Coumarin 440 dye used for comparison [39].
of the different electron donating groups such as naphthalen-1-yl,
4-vinylphenyl, 4-ethylbenzoate, benzo[b]thiophen-2-yl, pyren-1-yl
and 4-ethylphenyl substituents on the thiophene moiety, also such
substituents on the thiophene moiety greatly effect the intensity of
absorption and emission of the target compounds and results in a
BSTO-4(a-f) exhibit large Stokes shifts in the range
2776e3964 cmꢀ1, high values of Stokes shifts ensure that there is
no re-absorption of the emitted radiation [36].
In order to bring out efficient optoelectronic devices, the un-
derstanding of properties, interactions of the molecules due to
substitution and packing in the solid state is necessary [45]. Also
most of the organic fluorophores that are highly fluorescent in
solution states show significant decrease in their fluorescence ef-
ficiency in the solid state due to non-radiative decay routes and
plane to plane stacking of organic chromophores [46]. Emission
spectra also recorded in the solid state are shown in Fig. 2c and
better p-extended conjugation of the synthesized compounds [40].
The large Stokes shift provides significant information about
configuration rearrangement of molecules, changes that occur in
the dipole moment during excitation, appropriate electronic tran-
sition energies and structure, properties of organic materials be-
tween the ground and the excited states [41e44]. The compounds

M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
5
resultant emission maxima (lemmaix) are listed in Table 1. The emission
spectra of the compounds BSTO-4(a-f) in solid phase are broader
and show wide-ranging fluorescence (violet to green colour).
Therefore all compounds have larger value of FWHM in solid phase.
Compounds BSTO-4(a-f) showed emission maxima (lemmaix) at 463,
462, 455, 395, 421 and 429 nm, respectively, with a violet-green
emission in solid state.
order to explore the intramolecular charge transfer (ICT) properties
on the synthesized compounds, corresponding results are recorded
in Table 2 and the normalized UVevisible absorption, emission
spectra in various solvents are shown in Fig. 3(a and b). It is noticed
from the absorption spectra of the molecules BSTO-4(a-f) (Table 2)
a small spectral shifts in absorption maxima with respect to solvent
polarity (ethanol-toluene), is representing a small change in po-
larity of the ground state (S0) of solvated molecules BSTO-4(a-f). As
the polarity of the solvents decreased from ethanol-toluene, bath-
ochromic shift of absorption maxima of molecules BSTO-4b and
BSTO-4e is observed that is due to the stabilization of the ground
state (S0). But hypsochromic as well as bathochromic shifts of ab-
sorption maxima are noticed for the compounds BSTO-4a, BSTO-4c,
BSTO-4d and BSTO-4f which reflects on how the molecule interacts
with the solvent molecules, i.e. solute-solvent interactions.
An excited state of the molecule in solution interacts with
varying solvent polarity of the solvent molecules. These excited
state solute-solvent interactions lead to change in the spectral
position and shape of the emission bands. The emission of the
compounds BSTO-4a, BSTO-4b and BSTO-4d are highly sensitive to
solvent polarity. As we move from polar/weakly polar protic sol-
vents to polar aprotic/non-polar solvents a general positive sol-
vatochromism with broad red-shifted emission is observed
(Table 2). It is attributed to the difference in charge distribution of
ground and excited state of compounds BSTO-4a, BSTO-4b and
BSTO-4d, subsequently they interacts with the solvent molecules
and results in the reduction of energy gap between the electronic
ground and excited states leads to the net stabilization of the
excited state than the ground state. However, the emission of the
compounds BSTO-4c, BSTO-4e and BSTO-4f on moving from polar/
weakly polar protic solvents to polar aprotic/non-polar solvents
bathochromic/hypsochromic shifts are observed and they have less
solvent dependence on the emission profile indicating a less-polar
excited state of the compounds and may be affected by environ-
ment of the media also [52]. Interestingly the compound BSTO-4e
show dual fluorescence at distinct regions in all solvents, a pho-
tophysical feature recently observed in some of the dyes like
eosine-B, fluorescein-Na and Exalite (E404, E417, E428) [53,54].
Dual fluorescence may be due to the aggregationeinduced emis-
sion process, the conformational changes and the excimer/exciplex
formation. The dual fluorescence materials have a large number of
applications in sensor and optoelectronic devices [55,56]. The
larger Stokes shift values ranging from 2848 to 6415 cmꢀ1 (Table 2),
suggest significant structural reorganizations or electronic redis-
tribution of the molecules BSTO-4(a-f) in the excited state, which
imply considerably reduced self-absorption and enhanced fluo-
rescence efficiency of the synthesized compounds [21,50,57]. As the
polarity of solvent increases from toluene to ethanol, the values of
Stokes shift also increases. The above solvatochromism studies
2.2.2. Optical band gap (Egopt
)
Optical bandgap is an important property for designing of op-
toelectronic device. It gives better information about estimation of
energy levels in organic compounds and has direct impact on the
efficiency as well as lifetime of OLED device. Materials with large
optical band gap possess partial delocalization and subsequently
obstruct the electron injection characteristics and the conductivity
[36,47]. The optical band (Egopt) correlate with the red edge (long
wavelength edge) of the exciton absorption band [48]. Eogptwere
obtained from the onset of the low energy side of the absorption
spectra (lonset, solution) to the baseline utilizing the Eqn. (1) and
data were tabulated in Table 1 [49].
1240
Eogpt
¼
(1)
l
onset
The optical bandgap ranges from 3.02 to 3.54 eV were recorded
(Table 1). As the -conjugation increases in the molecule, an elec-
p
tron can delocalize over the whole section from the donor side to
the acceptor side resulting in lowering of the bandgap. A low op-
tical band gap value ensures the extended
p-conjugation of the
compounds BSTO-4(a-f) leading to increase in efficiency as well as
lifetime of OLED device. Such molecules can be used with high
efficiency in the area of OLEDs/optoelectronic applications.
2.3. Solvatochromism investigations
The solvatochromism studies on organic materials in optoelec-
tronic fields are very important as it gives the information about
intramolecular charge transfer (ICT), changes in the shape, position
and intensity of the absorption and fluorescence bands. As the
polarity of the solvents changes, a variation in the excitation
maxima and emission maxima wavelengths occurred and is
attributed to whether the excited state or ground state is more
stabilized by the solvent [18,50]. Also it explains interaction of
solvents with the excited state and the polar nature of the excited
state or ground state of organic molecules [51]. The study of solvent
polarity on the molecules BSTO-4(a-f) has been carried out in
various solvents such as polar/weakly polar protic solvents
(ethanol, butanol, hexanol, octanol, decanol) and polar aprotic/non-
polar solvents (acetonitrile, DMA, DMSO, 1,4-dioxane, toluene) in
Table 2
Results of solvatochromism studies on organic compounds BSTO-4(aef).
Solvents
BSTO-4a
BSTO-4b
BSTO-4c
BSTO-4d
BSTO-4e
BSTO-4f
la
lf
Dl
la
lf
Dl
la
lf
Dl
la
lf
Dl
la
lf
Dl
la
lf
Dl
Ethanol
Butanol
Hexanol
Octanol
Decanol
Acetonitrile
DMA
DMSO
1,4-dioxane
Toluene
357
356
360
359
362
354
359
360
357
360
459
451
445
442
440
458
462
468
434
434
6225
5917
5306
5231
4897
6415
6210
6410
4970
4736
341
342
344
344
345
338
343
343
342
346
400
400
399
400
399
400
407
423
397
399
4326
4240
4007
4070
3923
4586
4584
5514
4051
3839
333
337
340
334
336
338
341
350
344
345
400
394
389
399
408
400
417
420
400
399
5030
4293
3705
4877
5252
4586
5345
4762
4070
3923
345
341
343
345
345
340
343
344
343
347
400
399
399
399
389
399
406
423
397
399
3986
4263
4092
3923
3279
4349
4524
5429
3966
3756
333
335
335
335
336
334
337
338
336
337
393
393
394
393
418
398
373
374
373
419
4585
4405
4470
4405
5838
4814
2864
2848
2952
5807
342
344
346
345
347
340
345
346
334
348
399
401
400
400
400
413
420
426
396
402
4177
4132
3902
3986
3818
5199
5176
5428
4688
3860
La: Absorption maxima in nm; lf: Emission maxima in nm, Dl-Stokes shift in cmꢀ1
.

6
M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
Fig. 3. (a) Normalized absorption and emission spectra of compounds BSTO-4(aef) in polar/weakly polar protic solvents such as ethanol, butanol, hexanol, octanol and decanol.
Fig.3 (b) Normalized absorption and emission spectra of compounds BSTO-4(aef) in polar aprotic/non-polar solvents such as acetonitrile, DMA, DMSO, 1,4-dioxane and toluene.
reveal that the presence of strong intramolecular charge transfer
(ICT) between electron donor (D) groups like different electron
donating aryl/hetero-aryl groups, thiophene and electron acceptor
(A) 1,3,4-oxadiazole groups in conjugation of the form D-A-p-A-D
leads to the delocalization of the electron cloud from donor (D) to
acceptor (A) groups.
compounds BSTO-4(a-f), we performed density functional theory
(DFT) and Time-dependent density functional theory (TD-DFT)
computations in gas-phase by using the Gaussian 09 software [58].
Optimized ground state geometries of compounds BSTO-4(a-f)
were investigated by using DFT model employing hybrid func-
tional COM-B3LYP/6-311G(d,P) basis set. Also DFT model is used to
investigate frontier molecular orbitals amplitude plots of HOMOs
and LUMOs levels of compounds BSTO-4(a-f) as shown in Fig. 4. In
addition, dihedral angles (DA) and Molecular electrostatic potential
(MEP) were inspected as shown in Fig. 5.
2.4. Theoretical computations
In order to underscore the extent of planarization of molecules,
geometrical properties, delocalization of the frontier energy levels,
electronic structures and to assist in the analysis of the structure-
property relationships of the optically significant synthesized
For a clearer understanding of the molecular geometries and
intramolecular charge transfer character of the molecules, the
frontier molecular orbitals (FMOs), highest occupied molecular

M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
7
orbitals (HOMOs) and lowest unoccupied molecular orbitals
(LUMOs) of compounds BSTO-4(a-f) were examined and the results
are represented in Fig. 4. The distribution pattern of highest occu-
pied molecular orbitals and lowest unoccupied molecular orbitals
give remarkable information for the charge-transfer character of
the molecules, also the photophysical properties of intramolecular
charge transfer character of compounds are extremely dependent
on the strengths of electron donor (D)/acceptor (A) groups [59,60].
It can be viewed from the HOMOs frontier molecular orbitals
(Fig. 4), the electron density is mainly concentrated on the thio-
phene and different electron donating (D) aryl/hetero-aryl groups
of the compounds BSTO-4(a-f). By observing the LUMOs frontier
molecular orbitals (Fig. 4), the electron density is mainly dispersed
over the 1,3,4-oxadiazole moiety. Now, the analysis of the FMOs of
BSTO-4(a-f) strongly supports that the thiophene and different
electron donating aryl/hetero-aryl groups acts as good electron
donor units (D) and electron deficient nature of 1,3,4-oxadiazole
group, which act as an electron acceptor moiety (A). It is noticed
in the HOMO to LUMO orbitals in Fig. 4, that the electronic density
flow mainly from the thiophene, different electron donating aryl/
hetero-aryl groups to the 1,3,4-oxadiazole group which strongly
supports the strong intramolecular charge transfer (ICT) properties
of the compounds BSTO-4(a-f), thereby reducing in the form D-A-
p-A-D. In addition, the energy level diagram of the frontier orbitals
are depicted in Fig. 4 and HOMO/LUMO energy values are tabulated
in Table 3. The computationally estimated HOMO/LUMO energy
values are in the range of ꢀ6.8820 eV to ꢀ7.3957 eV and ꢀ1.2555 eV
to ꢀ1.6362 eV, respectively, and the energy gaps ð EÞare in the
D
range of 5.2458 eVe5.9070 eV. The divergence between experi-
mentally and computationally measured values HOMO and LUMO
energy gap can be recognized to the fact that computational
calculation belong to isolated (Gas Phase) molecules with no
Fig. 3. (continued).

8
M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
Fig. 4. Frontier molecular orbital amplitude plots of HOMOs and LUMOs levels of compounds BSTO-4(aef) and there HOMO, LUMO energies (eV) in S0 states at the COM-B3LYP/6-
311G(d,P) level of theory.
physical interaction between the solute and solvent. Such a charge-
transfer character of -conjugated molecules BSTO-4(a-f) with
electron-donor and electron-acceptor moieties are can be widely
exploited in optoelectronic and light-emitting diodes (OLEDs)
applications.
The molecular electrostatic potential (MEP) has been exten-
sively used to study the molecular interactions and its physico-
chemical property relationships in the area of research molecular
structures [63,64]. MEP of compounds BSTO-4(a-f) were explored
as shown in Fig. 5. As seen from this figure, in all molecules BSTO-
4(a-f), the red regions exhibiting the negative electrostatic poten-
tial are localized above the 1,3,4-oxadiazole units and the bluish
regions presenting the positive electrostatic potential are localized
on the thiophene and different electron donating aryl/hetero-aryl
groups gives same conclusion about the confirmation of strong
p
The ground state (GS) optimized molecular structures of com-
pounds BSTO-4(a-f) are shown in Fig. 5. The calculated dihedral
angles (DA) from ground state optimized molecular structures
around phenyl, 1,3,4-oxadiazole group, thiophene and different
electron donating aryl/hetero-aryl groups are recorded in Table 4
and they are in the range of 0.01ꢁ-66.05ꢁ. From these results it
confirms that the synthesized compounds BSTO-4(a-f) are quite
planar in nature. Charge carrier mobility of a material depends on
the degree of conjugation, intermolecular arrangement and or-
dered morphologies of the organic moieties in the thin films. The
presence of planarity, rigidity and high steric hindrance in the
molecules leads to good solubility, which helps to produce ther-
mally stable amorphous thin films deposited by using inexpensive
solution processes. Further, fluorescence efficiency in the solid state
is strongly correlated to intramolecular non-radiative decay
[3,61,62]. Thus the planarity and rigidity between donor units (D)
along with an acceptor unit (A) of the molecules BSTO-4(a-f) con-
intramolecular charge transfer properties and the D-A-p-A-D
approach in the compounds BSTO-4(a-f). Thiophene and different
electron donating aryl/hetero-aryl groups acts as an electron donor
(D) unit and 1,3,4-oxadiazole acts as an electron acceptor (A) unit
linked via extending
with para linkages results in extended
D-A- -A-D.
p
conjugation through a phenyl spacer (
p)
p-p
conjugation in the form
p
3. Thermal stability
For OLED/optoelectronic applications, the thermal stability of
the organic materials and formation of morphologically stable thin-
films is a crucial condition for the device stability, lifetime and
successful operation of optoelectronic devices. Thermal instability
or low glass transition temperature (Tg) of the organic layer may
result in faster degradation of organic devices due to morphological
changes in the amorphous organic layer. Synthesis of thermally
taining
p-linkage show the extended p-p conjugation. This results
in overall improvement in electron mobility due to effective
p-p
interactions and decrease the possible pathways for intramolecular
nonradiative decay which leading to enhanced fluorescence emis-
sion and high quantum yields.

M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
9
Fig. 5. Optimized ground state geometries and Molecular electrostatic potentials for BSTO-4(aef), Regions of higher electron density are showing in red and of lower electron
density in blue.

10
M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
Table 3
4. Conclusion
HOMO, LUMO energy and energy gaps ð EÞcalculated from DFT computations in
D
gas-phase.
In this paper, a series of novel conjugated thiophene substituted
1,3,4-oxadiazole derivatives BSTO-4(a-f) of Donor-Acceptor-
Compounds
HOMO (eV)
LUMO (eV)
D
Ea (eV)
p
-
Acceptor-Donor were designed and synthesized by making use of
palladium catalyzed Suzuki cross coupling reaction. Their struc-
tures were determined by spectroscopic techniques such as FT-IR,
1H NMR, 13C NMR and GC-MS/ESI-MS/APCI-MS. We have thor-
oughly investigated the photophysical properties like UVeVis
spectra, fluorescence emission spectra, solvatochromism,
HOMOeLUMO calculations and quantum yields. All the compounds
exhibit violet-blue fluorescence centred at 373e410 nm in chloro-
form solution and has high fluorescence quantum yields. The
photophysical properties were influenced reasonably by sub-
stituents on the thiophene moiety. DFT computations have been
carried out to investigate the intramolecular charge transfer
property of the probes. Differential scanning calorimetry (DSC) and
thermal gravimetric analysis (TGA) results suggested that de-
rivatives have good thermal stability as well as the compounds
BSTO-4(b, e and f) have shown good glass transition temperatures.
The compound BSTO-4b (2-(5-(4-vinylphenyl)thiophen-2-yl)-5-(4-
(5-(5-(4-vinylphenyl)thiophen-2-yl)-1,3,4-oxadiazol-2-yl)phenyl)-
1,3,4-oxadiazole) is the most perspective, as we see from the
manuscript. It has absorption and emission maxima at 346 and
398 nm in chloroform, respectively, and also emission maxima
measured in solid state film is 462 nm. This compound has shown a
BSTO-4a
BSTO-4b
BSTO-4c
BSTO-4d
BSTO-4e
BSTO-4f
ꢀ6.9522
ꢀ6.8820
ꢀ7.3957
ꢀ7.1971
ꢀ6.9522
ꢀ7.1625
ꢀ1.2560
ꢀ1.6362
ꢀ1.5015
ꢀ1.4337
ꢀ1.2560
ꢀ1.2555
5.6962
5.2458
5.8942
5.7334
5.6962
5.9070
a
Band gap obtained using DFT/B3LYP/6-311G(d,p).
stable organic materials with high glass transition temperatures
(Tg) is a key solution for the production of stable optoelectronic
devices [62,65,66]. The thermal properties of the investigated
compounds BSTO-4(a-f) were estimated by using differential
scanning calorimetry (DSC) and thermal gravimetric analysis (TGA)
carried out in the tempeꢀra1ture range from 25 to 450 ꢁC with a
heating rate of 10 ꢁC min under nitrogen atmosphere; the ob-
tained results are tabulated in Table 1 and shown in Fig. 6(a, b). The
DSC thermograms of the compounds BSTO-4a, BSTO-4c and BSTO-
4d are exhibited endothermic peaks corresponding to their melting
point temperatures (Tm) at 228 ꢁC, 222 ꢁC and 210 ꢁC respectively. In
DSC thermograms, there was no observed exothermic peak
resulting from crystallization temperature (Tc) for the compounds
BSTO-4(a-f). In DSC thermograms, endothermic baseline shift
related to the glass transition temperatures (Tg) for the compounds
BSTO-4b, BSTO-4e and BSTO-4f at 107 ꢁC, 147 ꢁC and 68 ꢁC,
respectively, confirm the excellent amorphous glass state stability
exhibited by the compounds. There are no observed endothermic
peaks corresponding to their melting point temperatures (Tm) for
the compounds BSTO-4b, BSTO-4e and BSTO-4f. These results
indicated that the compounds BSTO-4b, BSTO-4e and BSTO-4f were
stable amorphous materials with high Tg values that were higher
than those of commonly used popular oxadiazole including non-
bipolar host-emitters such as, (BOBP, Tg ¼ 45 ꢁC) and they are
higher than the well known oxadiazole-based electron-trans-
porting materials reported in literature, viz, 2-(4-biphenylyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole (PBD, Tg ¼ 60 ꢁC) [35,67]. The
thermal gravimetric analysis results reveal that the 5% weight loss
temperature (T5d) of compounds BSTO-4(a, b, c, d, e and f) under
nitrogen atmosphere are 300, 241, 239, 242, 356 and 178 ꢁC
respectively. DSC and TGA results suggested that all compounds
were thermally stable, encouraging for their applications in OLEDs/
optoelectronic device stability and lifetime.
fluorescence quantum yield
(F) of 0.62 and experimentally
measured optical bandgap value of 3.19 eV. From the DSC and TGA
results the glass transition temperatures (Tg) and 5% weight loss
temperature (T5d) of compound are 107 ꢁC and 241 ꢁC, respectively.
These results suggest that BSTO-4b compound is the most
perspective one for possible applications in organic light-emitting
devices (OLED), organic electronic/optoelectronic devices.
5. Experimental section
5.1. Fluorescence quantum yield (Фf
)
For light-emitting applications in optoelectronic devices, prime
importance is given to the luminescence process which can be
measured by quantum efficiency. To achieve high quantum effi-
ciency, one has to enhance the probability of radiative transition by
suppressing the energy loss due to nonradiative transitions via
vibrational relaxation or internal conversion (IC) and intersystem
crossing (ISC). Fluorescence quantum yield (
Фf) is a significant
parameter in the optoelectronic field for the quantitative
Table 4
Dihedral angles (DA) [o] from ground state (GS) optimized molecular structures of BSTO-4(aef).
Structure
BSTO-4a
GS
BSTO-4b
GS
BSTO-4c
GS
BSTO-4d
GS
BSTO-4e
GS
BSTO-4f
GS
DA1
DA2
DA3
DA4
DA5
DA6
66.05
0.52
0.54
0.54
0.53
66.05
31.56
0.55
0.67
0.56
0.35
31.59
31.12
0.45
0.28
0.10
0.15
30.93
33.31
0.02
0.30
0.30
0.02
33.31
64.71
0.23
0.18
0.18
0.23
64.71
32.05
0.01
0.03
0.20
0.46
32.34

M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
11
Fig. 6. (a) DSC and (b) TGA thermograms of the compounds BSTO-4(aef) with a heating rate of 10 ꢁC minꢀ1 under nitrogen atmosphere in the temperature range from 25 to 450 ꢁC.
characterization of organic materials and provides essential infor-
mation about radiationless transitions [38,68,69]. The fluorescence
quantum yields (Ff) of the organic compounds BSTO-4(a-f) were
calculated in chloroform solvent in comparison with a standard dye
film.
5.3. Materials and synthesis
Coumarin
440
(7-amino-4-methyl-2H-1-benzopyran-2-one)
All required starting chemicals and reagents were purchased
from chemical supplier companies and used without further puri-
fication. Solvents used for synthesis and analytical measurements
were freshly distilled over appropriate drying reagents prior to use.
The catalyst Pd(PPh3)4 and boronic acids were purchased from
Sigma-Aldrich. The required key intermediates 1, 2 and BSTO-3
were synthesized according to the reported procedures from the
literature [34,45 For procedure see ESI].
(Фref ¼ 0.98) [39] according to the Eq. (2) [70].
Is Ar h2r
Ir As
Ff
¼
Fr
(2)
hs2
where Ff is the fluorescence quantum yield of the sample, A is the
absorbance of the solution, I is the integrated fluorescence in-
tensity,
h is the refractive index and the subscripts of “s” and “r”
refer to the solutions of the sample and reference standard dye of
known quantum yield, respectively. The fluorescence quantum
yields (Ff) of compounds BSTO-4(a-f) are found to show potential
and were in the range of 0.06e0.70 in CHCl3 solvent and are
summarized in Table 1. As the structural modification or func-
tionalization on organic molecules is varied, it results in the vari-
ation of fluorescence intensity which is directly proportional to
quantum yield. The variations in the quantum yields of the target
compounds BSTO-4(a-f) depends upon the nature of the different
electron donating groups such as naphthalen-1-yl, 4-vinylphenyl,
4-ethylbenzoate, benzo[b]thiophen-2-yl, pyren-1-yl and 4-
ethylphenyl on the thiophene moiety. The fluorescence quantum
yields are found to be excellent for BSTO-4b, BSTO-4c, BSTO-4d and
BSTO-4f. While the compounds BSTO-4a and BSTO-4e shows a
good quantum yields. This ensures that the thiophene substituted
1,3,4-oxadiazole derivatives are highly efficient and show prom-
ising applications in optoelectronic devices.
5.4. General procedure for the synthesis of target compounds BSTO-
4(aef)
A
mixture
of
2-(5-chlorothiophen-2-yl)-5-(4-(5-(5-
phenyl)-1,3,4-
chlorothiophen-2-yl)-1,3,4-oxadiazol-2-yl)
oxadiazole (BSTO-3) (0.20 g, 0.448 mmol), boronic acid (0.154 g,
0.896 mmol), Pd(PPh3)4 (0.10 g, 0.2 eq), as a catalyst were added to
a mixture of 1,4-dioxane (10 mL) and aqueous 2 M K2CO3(5 mL) in a
pressure tube. The reaction mixture was heated with stirring for 8 h
at 95e100 ꢁC under a N2 atmosphere. After completion of the re-
action as indicated by TLC. The resulting reaction mixture was
allowed to cool to r.t. The compound was extracted with
dichloromethane (3 ꢂ 30 mL) and the organic layer was washed
with water and brine solution. Then organic layer was dried over
Na2SO4 and removed under reduced pressure by rotary evaporation
to dryness. The resultant crude products were purified by column
chromatography on silica gel using ethyl acetate/hexane as eluent,
to afford desired final compounds BSTO-4(a-f) with 75e80% yield.
5.2. Film preparation procedure
5.4.1. 2-(5-(naphthalen-1-yl)thiophen-2-yl)-5-(4-(5-(5-
(naphthalen-1-yl)thiophen-2-yl)-1,3,4-oxadiazol-2-yl)phenyl)-
1,3,4-oxadiazole (BSTO-4a)
The solid state film is prepared by using spin coating technique.
In this process, the substrate spins around an axis which is
perpendicular to the coating area. An excessive amount of fluid
(compounds BSTO-4(a-f) in chloroform solvent) is deposited on
slowly spinning substrate by using a nozzle at the centre of the
substrate. Then the substrate is accelerated to its final spin speed. In
liquid, rotational forces are exerted on the upward direction
causing a wave front to be formed. This wave front flows to the
substrate edge by centrifugal force and hence results in a uniform
layer. After that the excess solvent is removed off from the substrate
surface by rotating it at a speed between 1000 and 1500 rpm. The
fluid is being thinned primarily by centrifugal forces. Then solvent
is evaporated from the substrate surface to get uniform thin solid
Yield: 79%, yellow colour solid, MP ¼ 230e232 ꢁC; FT-IR (cmꢀ1):
3045 (CeH aromatic), 1504, 1485 (C]C and C]N aromatic), 1072,
1034 (CeOeC asym, and CeOeC sym, oxadiazole ring), 1568 (C]C
aromatic, naphthalene ring), 724 (CeSeC stretching, thiophene
ring); 1H NMR (400 MHz, CDCl3)
d: 8.33 (s, 4H), 7.99e7.90 (m,
J ¼ 9.2, 5.3 Hz, 7H), 7.65 (dd, J ¼ 7.1, 1.1 Hz, 2H), 7.57 (d, J ¼ 2.9 Hz,
2H), 7.56e7.52 (m, J ¼ 5.8 Hz, 5H), 7.37 (d, J ¼ 3.8 Hz, 2H); 13C NMR
(100 MHz, CDCl3) d: 163.21, 161.28, 147.72, 133.93, 131.46, 130.82,
130.42, 129.59, 128.60, 128.55, 128.46, 127.59, 127.04, 126.47, 126.41,
125.28, 125.22, 124.57; ESI-MS: m/z [M þ H]þ calculated for
C38H22N4O2S2 631.7369, found 631.7368.

12
M.S. Najare et al. / Journal of Molecular Structure 1199 (2020) 127032
5.4.2. 2-(5-(4-vinylphenyl)thiophen-2-yl)-5-(4-(5-(5-(4-
vinylphenyl)thiophen-2-yl)-1,3,4-oxadiazol-2-yl)phenyl)-1,3,4-
oxadiazole (BSTO-4b)
3098 (CeH aromatic), 2965 (CeH aliphatic), 1586, 1571 (C]C and
C]N aromatic), 1075, 1016 (CeOeC asym, and CeOeC sym, oxa-
diazole ring), 719 (CeSeC stretching, thiophene ring); 1H NMR
Yield: 80%, light green colour solid, MP ¼ 234e235 ꢁC; FT-IR
(cmꢀ1): 3095 (CeH aromatic), 1588 (C]C stretching, olefin) 1571,
1495 (C]C and C]N aromatic), 1078, 1017 (CeOeC asym, and
CeOeC sym, oxadiazole ring), 719 (CeSeC stretching, thiophene
(400 MHz, CDCl3)
d
: 8.24 (s, 4H), 7.80 (d, J ¼ 3.7 Hz, 2H), 7.64 (d,
J ¼ 3.9 Hz, 4H), 7.34 (d, J ¼ 3.8 Hz, 2H), 7.03 (d, J ¼ 4.0 Hz, 4H),
2.72e2.62 (q, J ¼ 18.9, 9.5 Hz, 4H), 1.26 (t, J ¼ 7.6 Hz, 6H); 13C NMR
(100 MHz, CDCl3) d: 164.27, 163.75, 143.52, 140.14, 137.09, 129.25,
ring); 1H NMR (500 MHz, (CD3)2SO)
d: 7.75 (s, 4H), 7.67 (d,
128.75, 128.53, 128.01, 127.57, 127.12, 126.51, 28.29, 15.10; MS
(APCI): m/z [M ꢀ 1]þ calculated C34H26N4O2S2 581.7258, found
585.7142.
J ¼ 8.6 Hz, 2H), 7.55 (d, J ¼ 8.5 Hz, 2H), 7.22 (d, J ¼ 4.1 Hz, 2H), 7.19
(d, J ¼ 8.1 Hz, 4H), 6.71 (d, J ¼ 4.1 Hz, 2H), 6.16 (dd, J ¼ 17.6, 10.9 Hz,
2H), 5.30 (dd, J ¼ 17.7, 1.0 Hz, 2H), 4.71 (dd, J ¼ 10.9, 0.9 Hz, 2H); 13
C
NMR (126 MHz, (CD3)2SO)
d 164.72, 160.53, 139.01, 137.25, 134.90,
Acknowledgements
132.54, 132.51, 132.00, 131.93, 129.29, 129.20, 128.16, 125.62, 115.23;
ESI-MS: m/z [M þ H]þ calculated for C34H22N4O2S2 583.11, found
583.
We are thankful to the University Scientific Instrument Centre
(USIC) and SAIF, Karnatak University, Dharwad for providing the
spectral data. Also to the University Grants Commission (UGC), New
Delhi, India, for providing financial support under UGC-SRF (Sr. No.
2121510180 Ref. No. December 20, 2015), DST-Purse Phase-II pro-
gram for providing necessary facilities. One of the authors Afra-
Quasar A. Nadaf is thankful to DST-Inspire for providing JRF.
5.4.3. 2-(5-(4-ethylbenzoate)thiophen-2-yl)-5-(4-(5-(5-(4-
ethylbenzoate)thiophen-2-yl)-1,3,4-oxadiazol-2-yl)phenyl)-1,3,4-
oxadiazole (BSTO-4c)
Yield: 78%, pale yellow colour solid, MP ¼ 228e230 ꢁC; FT-IR
(cmꢀ1): 3104 (CeH aromatic), 3085 (CeH aliphatic), 1586, 1495
(C]C and C]N aromatic), 1099, 1017 (CeOeC asym, and CeOeC
sym, oxadiazole ring), 1711 (C]O stretching, ester carbonyl), 719
Appendix A. Supplementary data
(CeSeC stretching, thiophene ring); 1H NMR (400 MHz, CDCl3)
d:
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.127032.
8.25 (s, 4H), 8.17 (d, J ¼ 7.4 Hz, 2H), 8.13 (d, J ¼ 7.7 Hz, 2H), 7.65 (d,
J ¼ 3.4 Hz, 4H), 7.04 (d, J ¼ 3.2 Hz, 4H), 4.40 (q, J ¼ 13.4, 6.2 Hz, 4H),
1.40 (t, J ¼ 10.9 Hz, 6H); 13C NMR (100 MHz, CDCl3)
d: 166.99,
References
164.49, 161.78, 140.88, 137.83, 132.71, 129.99, 129.49, 129.27, 128.75,
128.31, 127.86, 127.25, 61.00, 15.84; MS (APCI): m/z [M ꢀ 1]þ
calculated for C36H26N4O6S2 673.1294, found 673.1250.
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stretching, thiophene ring); 1H NMR (400 MHz, CDCl3)
d: 8.25 (s,
4H), 7.80 (d, J ¼ 7.4 Hz, 2H), 7.75 (d, J ¼ 7.2 Hz, 2H), 7.64 (d,
J ¼ 3.2 Hz, 2H), 7.50 (s, 2H), 7.39e7.29 (m, J ¼ 12.7, 6.4 Hz, 4H), 7.04
(d, J ¼ 3.0 Hz, 2H); 13C NMR (100 MHz, CDCl3)
d: 165.25, 164.73,
138.07, 132.49, 130.87, 130.19, 129.72, 128.74, 128.10, 127.91, 127.51,
127.36, 125.63, 124.59, 123.01, 121.57; MS (APCI): m/z [M ꢀ 1]þ
calculated for C34H18N4O2S4 641.7923, found 641.4285.
5.4.5. 2-(5-(pyren-1-yl)thiophen-2-yl)-5-(4-(5-(5-(pyren-1-yl)
thiophen-2-yl)-1,3,4-oxadiazol-2-yl)phenyl)-1,3,4-oxadiazole
(BSTO-4e)
Yield: 75%, pale yellow colour solid, MP ¼ 237e239 ꢁC; FT-IR
(cmꢀ1): 3039 (CeH aromatic), 1597, 1578 (C]C and C]N aro-
matic), 1183, 1075 (CeOeC asym, and CeOeC sym, oxadiazole ring),
725 (CeSeC stretching, thiophene ring); 1H NMR (400 MHz, CDCl3)
d
: 8.27 (d, J ¼ 8.3 Hz, 4H), 8.20 (d, J ¼ 3.6 Hz, 2H), 8.17 (d, J ¼ 4.2 Hz,
2H), 8.08 (s, 4H), 8.05e7.97 (m, 4H), 7.85 (dd, J ¼ 3.7,1.1 Hz, 2H), 7.77
(d, J ¼ 8.2 Hz, 4H), 7.56 (dd, J ¼ 5.0, 1.2 Hz, 2H), 7.19 (dd, J ¼ 5.0,
3.7 Hz, 2H); 13C NMR (100 MHz, CDCl3)
d: 164.09, 163.72, 138.73,
135.85, 134.46, 131.11, 131.02, 130.74, 130.56, 129.74, 128.76, 128.05,
127.68, 127.53, 127.25, 127.04, 126.72, 125.88, 125.13, 124.80, 124.65,
124.48, 124.43, 124.37; ESI-MS: m/z [M þ H]þ calculated for
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