Design, synthesis and characterization of π-conjugated 2,5-diphenylsubstituted-1,3,4-oxadiazole-based D-π-A-π’-D′ form of efficient deep blue functional materials: Photophysical properties and fluorescence “Turn-off” chemsensors approach

Article abstract of DOI:10.1016/j.molliq.2021.115443

This paper primarily deals with the design and synthesis of three novel conjugated organic molecules containing 1,3,4-oxadiazole unit (abbreviated as CHEM-6(a-c)) with Donor-π-Acceptor-π’-Donor’ structures by utilizing palladium catalyzed Suzuki cross coupling reaction. Molecular structures of these compounds were characterized by using various spectroscopic and analytical techniques namely ¹H NMR, ¹³C NMR, FT-IR, GC–MS and elemental analysis. Optical properties of these compounds have been investigated by using UV–visible absorption and fluorescence spectroscopy. Also, the PL investigations on solid state samples (as thin films) were carried out. Furthermore, solvatochromism and Density Functional Theory (DFT) computations are studied in detail. The experimentally measured optical band gap (E_g^opt) values of compounds CHEM-6a, CHEM-6b and CHEM-6c were determined to be 3.11, 3.03 and 2.70 eV, respectively. The DSC and TGA results revealed that the compounds exhibit good thermal stabilities. Current results indicate that the compounds CHEM-6(a-c) are promising candidates and could play an important role in the field of optoelectronic devices and possibly find potential applications in development of OLEDs. In addition, the compounds CHEM-6(a-c) showed an interesting property of chemical sensing for Hg(II) ion which can be clearly observed by naked eyes from the changes in solution color and fluorescence (under UV excitation).

Full text of DOI:10.1016/j.molliq.2021.115443

Journal of Molecular Liquids 328 (2021) 115443
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Design, synthesis and characterization of π-conjugated
2
,5-diphenylsubstituted-1,3,4-oxadiazole-based D-π-A-π’-D′ form of
efficient deep blue functional materials: Photophysical properties and
fluorescence “Turn-off” chemsensors approach
a,
b
a
a
Mahesh S. Najare ⁎, Mallikarjun K. Patil , Manjunatha Garbhagudi , Mohammed Yaseen ,
b
a
Sanjeev R. Inamdar , Imtiyaz Ahmed M. Khazi
Department of Chemistry, Karnatak University, Dharwad 580003, Karnataka, India
a
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:
This paper primarily deals with the design and synthesis of three novel conjugated organic molecules containing
Received 25 September 2020
Received in revised form 12 January 2021
Accepted 18 January 2021
1,3,4-oxadiazole unit (abbreviated as CHEM-6(a-c)) with Donor-π-Acceptor-π’-Donor’ structures by utilizing
palladium catalyzed Suzuki cross coupling reaction. Molecular structures of these compounds were characterized
1
13
by using various spectroscopic and analytical techniques namely H NMR, C NMR, FT-IR, GC–MS and elemental
analysis. Optical properties of these compounds have been investigated by using UV–visible absorption and fluo-
rescence spectroscopy. Also, the PL investigations on solid state samples (as thin films) were carried out. Further-
Available online 24 January 2021
Keywords:
more, solvatochromism and Density Functional Theory (DFT) computations are studied in detail. The
1,3,4-Oxadiazole
opt
Photophysical properties
Fluorescence
Chemsensors
DFT computations
Thermal stability
g
experimentally measured optical band gap (E ) values of compounds CHEM-6a, CHEM-6b and CHEM-6c
were determined to be 3.11, 3.03 and 2.70 eV, respectively. The DSC and TGA results revealed that the com-
pounds exhibit good thermal stabilities. Current results indicate that the compounds CHEM-6(a-c) are promising
candidates and could play an important role in the field of optoelectronic devices and possibly find potential ap-
plications in development of OLEDs. In addition, the compounds CHEM-6(a-c) showed an interesting property of
chemical sensing for Hg(II) ion which can be clearly observed by naked eyes from the changes in solution color
and fluorescence (under UV excitation).
©
2021 Elsevier B.V. All rights reserved.
1. Introduction
encouraged for the successful utilization in our day to day life applica-
tions [10–13].
In recent years enormous research interest has been devoted in the
However, for the development of more efficient OLEDs and their
practical use into large scale commercial application, present OLEDs
still have some issues including thermal/device stability, cost, balanced
charge carriers and color purity. The performance of OLEDs further
needs improvement in terms of efficiency and lifetime [14–17]. Further-
more, organic luminescent materials with different emitting colors can
directly affect the quality of displays. To date, among the primary
color organic luminescent materials, red and green emitting materials
with excellent performance in OLEDs have been effectively commercial-
ized. However, the performance of blue-emitting luminescent mate-
rials, especially deep-blue ones still remains a challenge due to their
wide energy band gap, color purity, lower efficiencies and lifespan
[18–20]. Some of these issues can be solved by investigating the new
class of π-conjugated organic molecules with excellent luminescent
properties or by the modification of organic framework, substituents,
linking positions etc., of the organic materials to modulate or to control
the properties of the organic materials [21,22]. Wide range of organic
field of organic electronics, including various organic optoelectronic
devices such as organic field-effect transistors (OFETs), organic light-
emitting diodes (OLEDs), organic solar cells (OSCs), sensors and photo-
voltaics [1]. Especially, organic light-emitting diodes (OLEDs) have
attracted significant attention of both research and industry due to
their outstanding applications in full-color flat panel displays and
solid-state lighting [2–8]. In addition, OLEDs are considered as one of
the most promising competitor for next-generation lighting resources
compared to traditional incandescent bulbs and compact fluorescent
lamps (CFL) in terms of device efficiency and non-ecofriendly nature
of the fluorescence tubes [9]. Wide range of advantages of OLEDs, such
as low power consumption, easy processing, high efficiency, light
weight, flexibility, fast response time and wide viewing angle have
⁎
Corresponding author.
E-mail address: mahesh.s.najare@gmail.com (M.S. Najare).
https://doi.org/10.1016/j.molliq.2021.115443
167-7322/© 2021 Elsevier B.V. All rights reserved.
0

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
materials used in optoelectronic devices are π-conjugated materials
with delocalized electrons. Small organic molecules have attracted
more potential applications as light emitting or charge carrier layers in
OLEDs due to their unique advantages of electrochemical, thermal sta-
bility, balanced charge carriers, exact molecular structure and high pu-
rity, which induce a significant increase in the work efficiency of the
optoelectronic devices [23–25].
Of late, the inclusion of electron-donating groups as donors (D) and
electron-deficient groups as acceptors (A) by organic π-conjugated
linkers in the organic dyes with donor-π-acceptor (D–π–A) geometry
appear to be the most appealing strategy for their various applications
in optoelectronic devices such as organic photovoltaic devices, solar
cells and organic light emitting diodes [26–28]. In order to reduce
power consumption, enhance ambipolar electrical charge carriers and
electroluminescence (EL) efficiency of optoelectronic devices/OLEDs,
conjugated donor-π-acceptor type compounds are considered promis-
ing [29–31]. The general architecture of OLEDs is composed of an
anode, hole transport layer (HTL), emissive layer (EML), electron trans-
port layer (ETL) and a cathode. The basic mechanism in OLED devices is
charge transport and recombination of charges which leads to the emis-
sion of light [32–34].
At present 1,3,4-oxadiazole derivatives have attracted significant in-
terest in electroactive and optoactive materials for the development of
optoelectronic devices as these compounds possess advantages such
as high thermal stabilities, good film forming capabilities, mechanical
properties, chemical stabilities, stability towards hydrolytic and oxida-
tive degradation, wide energy gap, high electron mobility and exhibit
strong fluorescence with high quantum yields [35,36]. The combination
of these wide range properties of 1,3,4-oxadiazole containing molecules
leads to extensive use in variety of optoelectronic applications such as:
electron-transporting layer and hole-blocking materials in OLEDs, in
thin-film electroluminescent devices, due to their electron deficient na-
ture and as photosensitive materials in photovoltaic cells. Also, the or-
ganic materials containing 1,3,4-oxadiazole ring have a significant
impact in the field of sensitive materials for reliable sensors with poten-
tial application in environmental monitoring [37–39].
Currently, the fluorogenic materials have become a hot research
topic since the debut of aggregation induced emission (AIE) concept
and they have been extensively used as chemosensors, bioprobes,
stimuli-responsive nanomaterials and active layers in the production
of efficient OLEDs. However, synthesis of efficient fluorogenic molecules
have synthetic difficulties, so in this view Schiff bases are the appropri-
ate candidates having simple synthetic path ways and also applied to
many fluorescent sensors. In order to develop Schiff base sensors the
presence of strong fluorophores is required. In this regards
orthohydroxyphenyl Schiff base act as one of the attracting group
with core molecule containing 2,5-diphenylsubstituted-1,3,4-
oxadiazole motif, which are used for the synthesis of the transition
metal complexes of 1,3,4-oxadiazole dyes, photosensitive materials,
liquid crystals and mesogens. They were evidenced as excellent
fluorophores and widely used in the developments of fluorescence
solvents. Effect of concentration on the absorption/emission properties
of the compounds were studied in ethanol solvent at room tempera-
ture. DFT computations were performed in gas-phase by using the
Gaussian 09 software and the ground state geometries were optimized
at CAM-B3LYP and 6-311G+ (d,p) level of study. The thermal stability
of the synthesized compounds were examined by using DSC and TGA
measurements. The obtained results indicate that the synthesized or-
ganic compounds CHEM-6(a-c) are better promising candidates and
could play an important role in the field of optoelectronic devices/
OLEDs. Furthermore, the metal ion sensing properties of the com-
pounds CHEM-6(a-c) will be preliminarily observed in order to
study the feasibility for using as fluorescent sensing materials in an
application.
2. Results and discussion
2.1. Chemistry
By employing a relatively simple and efficient synthetic pathway,
the required key intermediates and proposed target compounds
CHEM-6(a-c) were synthesized as shown in Scheme 1. The essential
precursors ethyl 4-bromobenzoate (1), 4-bromobenzohydrazide (2) and
2-(4-bromophenyl)-5-(4-nitrophenyl)-1,3,4-oxadiazole (CHEM-3) were
prepared according to the procedures reported previously [36,43]. A
mixture of 4-bromobenzoic acid and thionyl chloride in ethanol is
refluxed to yield the precursor ethyl 4-bromobenzoate (1), followed
by the reaction with hydrazine hydrate in ethanol at reflux condition
giving the intermediate 4-bromobenzohydrazide (2). The mixture of
4-bromobenzohydrazide (2) and 4-nitrobenzoic acid in phosphoryl
3
chloride (POCl ) was then refluxed to acquire the key intermediate 2-
(4-bromophenyl)-5-(4-nitrophenyl)-1,3,4-oxadiazole (CHEM-3). The
reduction of the nitro group of intermediate CHEM-3 with iron powder
follows the method reported [44] to get the essential intermediate 4-(5-
(4-bromophenyl)-1,3,4-oxadiazol-2-yl)benzenamine (CHEM-4). The
subsequent Schiff's base condensation of CHEM-4 and salicylaldehyde
in absolute ethanol in the presence of catalytic amount of glacial acetic
acid gives the building block intermediate (E)-2-((4-(5-(4-bromophenyl)-
1,3,4-oxadiazol-2-yl)phenylimino)methyl)phenol (CHEM-5). Palladium
catalyzed Suzuki–Miyaura cross coupling reactions are performed between
intermediate CHEM-5 and boronic acids (viz., naphthalen-2-ylboronic acid,
4-(trifluoromethoxy)phenylboronic acid and 5’-Hexyl-2, 2′-bithiophene-
5‑boronic acid pinacol ester) to yield the desired products (E)-2-((4-(5-
(4-(naphthalen-2-yl)phenyl)-1,3,4-oxadiazol-2-yl)phenylimino)methyl)
phenol(CHEM-6a),(E)-2-((4-(5-(4-(4-(trifluoromethoxy)benzene)phe-
nyl)-1,3,4-oxadiazol-2-yl)phenylimino) methyl) phenol (CHEM-6b) and
(E)-2-((4-(5-(4-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)phenyl)-1,3,4-
oxadiazol-2-yl)phenylimino) methyl)phenol (CHEM-6c) in 72–77% yields.
The synthesized compounds CHEM-6(a-c) are easily soluble in general
organic solvents like methanol, ethanol, acetone, tetrahydrofuran, acetoni-
3
trile, toluene, chloroform (CHCl ), dimethyleformamide (DMF), di-
chloromethane and dimethylesulfoxide (DMSO). The purification of
conjugated Schiff's base consist of 1,3,4-oxadiazole derivatives
CHEM-6(a-c) were done by using column chromatography on silica
gel. The molecular structural integrity of all the products was charac-
terized by using various spectroscopic and analytical techniques
such as TGA, DSC, 1H NMR, C NMR, FT-IR, GCMS and elemental
analysis. The obtained results were in good agreement with the pro-
posed molecular structures confirming the purity of the compounds.
(
FL) sensors and found to have applications in OLEDs because of their
excellent photoluminescence properties, high electron affinities and
chemical stabilities [35,40–42].
In the present investigation we have designed and synthesized three
novel unsymmetrical small π-conjugated organic molecules containing
13
1
,3,4-oxadiazole moiety CHEM-6(a-c) (Scheme 1) with Donor-π-Ac-
ceptor-π’-Donor’ geometry. Molecular structures of the synthesized
compounds were characterized by using various spectroscopic
methods. Optical properties of the compounds CHEM-6(a-c) have
been thoroughly investigated by using UV–visible and fluorescence
2.2. Photophysical properties
emission spectra. Fluorescence quantum yields (Ф
f
) and optical band
2.2.1. UV–visible absorption and photoluminescence (PL) measurements
For a better understanding of the efficiency of organic materials for
the possible applications in optoelectronic devices/OLEDs applications,
optical properties of synthesized derivatives CHEM-6(a-c) were investi-
gated by using UV–visible absorption and photoluminescence (PL)
opt
gaps (E
g
) were calculated. In order to explore more about intramolec-
ular charge transfer (ICT) properties of the synthesized molecular
probes a detailed solvatochromism study is carried out in various sol-
vents with variations in solvent polarity from polar to non polar
2

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
Scheme 1. Synthetic route for the preparation of designed compounds and chemical structures of the target compounds CHEM-6(a-c)
measurements. They were carried out in ethanol solution and are
displayed in Fig. 1(A and B), respectively. The corresponding optical
characteristics (absorption maxima, emission maxima and Stokes
shift) of compounds CHEM-6(a-c) are summarized in Table 1. From
the Fig. 1(A), absorption spectra of the compounds CHEM-6(a-c) in eth-
yl) thiophen-2-yl result in extending the π-conjugation in the synthe-
sized organic molecules.
Photoluminescence spectra of compounds CHEM-6(a-c) exhibiting a
single emission peaks in the range of 468–485 nm in ethanol solution
with deep blue emission at room temperature have been depicted in
Fig. 1(B) and the relevant data (emission maxima and Stokes shift) are
shown in Table 1. In comparison with the UV–visible absorption spec-
trum of the compounds CHEM-6(a-c), a significant bathochromic shift
abs
anol showed absorption maxima (λmax) at 340, 336 and 379 nm, corre-
sponding to the compounds CHEM-6a, CHEM-6b and CHEM-6c,
respectively. It is observed that all the compounds exhibited two ab-
sorption bands, one in the lower wavelength region (weak band)
emi
(red shift) was noticed in the emission maxima (λmax) of compounds
emi
2
3
50–300 nm and in higher wavelength region (strong band)
00–440 nm. The relatively weak bands appearing at lower wavelength
CHEM-6(a-c). Theλmaxof the synthesized compounds follows the
trend, CHEM-6c (468 nm) < CHEM-6a (484 nm) < CHEM-6b
(485 nm) which is opposite to the trend shown by the absorption max-
imum. This suggests that the possible structural reorganization in the
region (˂300 nm) is assigned to the localized π → π* transitions originat-
ing from the 1,3,4-oxadiazole system, while the intense bands appearing
at longer wavelength absorption (>300 nm) is attributed to the
delocalized π → π* electronic transition of the entire organic molecule
excited state of the synthesized molecules. Notably, the Stokes shifts
of the compounds CHEM-6(a-c) are in the range of 5018–9143 cm−
1
,
[
45]. The above assessment is supported by the fact that the longer wave-
which are comparative to the oxadiazole derivatives reported in the
literatures [46]. Considering the Stokes shift as the factor used to
determine the structural reorganization, the extent of structural defor-
length absorption shifts gradually to the red region (red shift) reflecting
variation in the π conjugation length of the organic molecules. Thus
CHEM-6c exhibited the most red-shifted absorption owing to the pres-
ence of elongated π-conjugation. The absorption red-shift from CHEM-
1
mation in the excited state (S ) of the organic compounds may be in
−
1
−1
the order: CHEM-6c (5018 cm ) < CHEM-6a (8751 cm ) < CHEM-
−
1
6
b, CHEM-6a to CHEM-6c, suggests that the effect of addition of extra
6b (9143 cm ). From this we can observe that all the synthesized
fluorophores exhibited high Stokes shift values. The high Stokes shift
may be due to the π-extended conjugation of the synthesized com-
pounds and ICT property. Such fluorophores with a high Stokes shift
conjugation fragments in compounds CHEM-6b and CHEM-6a which
are 4-(trifluoromethoxy) benzene and naphthalen-2-yl, respectively,
whereas in the compound CHEM-6c it is 5-(5-hexylthiophen-2-
3

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
solid state are shown in Fig. 1(C) and resultant emission maxima
emi
(
(
λmax) are listed in Table 1. Interestingly it is noticed from the Fig. 1
C), that the compounds CHEM-6(a-c) solid state show dual fluores-
cence at distinct regions. The emission spectra of the compounds are
broader and cover a wide-range (violet to orange color). Compounds
emi
CHEM-6a, CHEM-6b and CHEM-6c showed emission maxima (λmax) at
4
69/544, 468/546 and 473/542 nm, respectively. The phenomena of
dual fluorescence is a photophysical aspect, have been extensively in-
vestigated and observed recently in some of the dyes like eosine-B,
fluorescein-Na, Exalite (E404, E417, E428) and frequently found in
benzazole, benzothiazole, benzoxazole and benzimidazole derivatives
[45,48,49]. The dual fluorescence, where a two-color radiative process
occurs relating two distinct excited electronic states, is attributed to:
[
[
1] normal and dimers, [2] the aggregation-induced emission process,
3] tautomerism, [4] the excimer/exciplex formation, [5] excited state
photoisomers, [6] protonated species and [7] the excited-state intramo-
lecular proton transfer (ESIPT). Generally, the materials emitting mate-
rials dual fluorescence have a broad range of applications in technology,
such as in photonic and optoelectronic devices due to their sensitivity to
different external stimuli and to multicolor light. Also, the high solid
state emissive nature of the organic materials makes them suitable can-
didates for application in chemical sensors where change in their lumi-
nescence property can be utilized to detect the presence of an analyte
[40–52].
opt
2
.2.2. Optical band gap (E
g
)
Investigation of optical properties like optical band gap of the or-
ganic materials has been of fundamental importance to design, optimize
and development of optoelectronic devices/OLEDs. Optical band gap of
the organic materials provides information about the estimation of en-
ergy levels in the organic materials. When they absorb light energy/
photons correspond to the energy carried by UV–visible radiation in
electromagnetic radiations (EMR), which in turn equals to the optical
band gap of the organic materials, then an electron in the low-energy
level molecular orbital is promoted to the high-energy level molecular
opt
orbital [53,54]. Optical band gap (E
g
) was evaluated from the absorp-
tion edge (long wavelength edge) of optical absorption spectra [55].
opt
g
E
were determined from the absorption onset wavelength of the cor-
responding absorption spectra (λonset, solution) to the baseline utilizing
the Eq. (1) [56] and the corresponding values are listed in Table 1.
1
240
opt
g
E
¼
ð1Þ
λonset
opt
g
The optical band gap (E
) values of the synthesized compounds
CHEM-6a, CHEM-6b and CHEM-6c are 3.11 eV, 3.03 eV and 2.70 eV, re-
spectively. All the three synthesized compounds have low optical band
gap values indicating the increase in the extended π-conjugation.
Among them, CHEM-6c have relatively low optical band gap about
2
.70 eV suggesting that owned a better conjugation backbone. Low op-
tical band gap values containing organic materials have attracted and
can find possible applications in the area of optoelectronic devices/
OLEDs applications.
Fig. 1. (A) UV–visible absorption spectra and (B) Emission spectra of the synthesized
compounds in ethanol solution at room temperature (C) Emission spectra of
compounds CHEM-6(a-c) in solid state (thin film). Ref - Coumarin 440 dye and
Coumarin 480 dye are used for comparison [1].
2.2.3. Solvatochromism investigations
Further, to check the suitability of organic probes in the field of opto-
electronic devices, a detailed study of solvent polarity dependence
(
solvatochromism) of CHEM-6(a-c) has been carried out in various sol-
value ensure that there is no overlap between the absorption and emis-
sion spectra and results in the negligible re-absorption of the emitted
light. Also, compounds having such large Stokes shifts may find unique
applications in the detection of emitted fluorescence light in biological
applications and in organic electronic fields [35,47].
In order to explore the potential of the synthesized organic materials
in optoelectronic devices, study of their emission spectra in solid state
form (thin films) will be helpful. Emission spectra recorded in the
vents such as, polar/less polar protic solvents like methanol, pentanol,
hexanol, octanol, nonanol and in polar aprotic/non-polar solvents such
as DMSO, DMF, DMA, toluene and octane. Corresponding photophysical
data are summarized in Table 2 and the normalized UV–visible absorp-
tion, emission spectra in various solvents with varying polarities are
shown in Fig. 2(A and B). Solvatochromism is very important property
as it gives information about interaction of the solvent molecules with
the excited state (S ) of the organic molecules, also gives an information
1
4

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
Table 1
opt
f g
Optical characteristics, fluorescence quantum yields (Φ ), optical bandgap energies(E ) and thermal characteristics of CHEM-6(a-c).
abs
max
nm)a
emi
max
(nm)b
emi
max
opt d
Compounds
λ
(
λ
λ
Stokes shift Δλ
E
g
Quantum Yield
(Φ
T
[°C]
g
T
[°C]
m
T
[°C]
c
T
[°C]
5d
Optimized fluorescence
concentration (μM)
c
−1
e
e
e
e
(nm)
(cm
)
(eV)
f
)
CHEM-6a
CHEM-6b
CHEM-6c
Ref. C440
Ref. C480
340
336
379
355
391
484
485
468
434
469
469/544
468/546
473/542
–
8751
9143
5018
–
3.11
3.03
2.70
–
0.20
0.37
0.80
0.98
0.95
65
–
132
–
177
180
179
–
–
–
–
–
–
301
302
322
–
8
10
6
–
–
–
–
–
–
–
–
a
abs
Absorption maxima (λmax) in ethanol at 10 μM concentration.
b
c
emi
Emission maxima (λmax) in ethanol at 10 μM concentration.
Emission maxima measured in solid state film.
Optical bandgap energies (E
Results from DSC and TGA measurements; T
d
e
opt
g
opt
g
) were calculated from the long edge of the absorption spectrum using E
= hc/λ = 1240/λ (eV).
g
-glass transition temperature, T -melting temperature, T -crystallization temperature and T5d-5% weight loss temperature. Ref-Coumarin
m
c
440 and Coumarin 480 [1].
Table 2
intramolecular charge transfer (ICT) between different electron donat-
ing (D) aryl/hetero-aryl groups and electron acceptor (A) 1,3,4-
oxadiazole group in π-extended conjugation of the form D-π-A-π’-D′
leads to the delocalization of the electron cloud from donor to acceptor
groups.
Solvatochromism studies: Absorption and emission parameters of organic compounds
CHEM-6(a-c) in solvents of varying polarity.
Solvents
CHEM-6a
CHEM-6b
CHEM-6c
λ
a
λ
f
Δλ
λ
a
λ
f
Δλ
λ
a
λ
f
Δλ
1
The polar excited states (S ) are likely to be stabilized by polar sol-
vents than that of non-polar solvents [59]. This is further confirmed by
Methanol
Pentanol
Hexanol
Octanol
Nonanol
336 496 9601 332 498 10,040 381 468 4879
339 460 7759 337 464 8122
340 466 7953 337 466 8214
341 462 7680 338 462 7941
341 459 7539 337 461 7982
346 520 9671 342 517 9897
386 463 4308
387 462 4195
388 460 4034
388 463 4175
391 488 5085
387 480 5006
388 477 4809
381 458 4413
388 449 3501
the large Stokes shifts of the compounds CHEM-6(a-c) found to be in
−1
−1
the range 4879–10,040 cm in methanol and 3501–6165 cm in oc-
tane solution (Table 2), which further support that the intramolecular
charge transfer character of these D-π-A-π’-D′ chromophores between
the donor units and the electron acceptor oxadiazole moiety. The com-
pounds showing a strong fluorescence solvatochromic effect and photo-
induced ICT are found to be advantageous for the performance in OLED,
also to achieve high efficiency of devices [60,61]. Hence, the synthesized
compounds CHEM-6(a-c) are promising candidates for the use in
OLEDs/optoelectronic applications.
Dimethyl sulfoxide
Dimethylformamide 342 509 9593 342 508 9555
Dimethylacetamide 345 507 9262 341 504 9484
Toluene
Octane
332 408 5611 328 411 6157
326 408 6165 319 384 5306
a f
λ : Absorption maxima in nm, λ
: Emission maxima in nm and Δλ-Stokes shift in cm−1.
1 0
about the polarized nature of the excited state (S ) or ground state (S )
of organic materials [57]. From solvatochromism studies we can explore
the intramolecular charge transfer (ICT) properties of the synthesized
compounds.
As it is witnessed from the absorption spectra of the organic com-
pounds CHEM-6(a-c) (Table 2 and Fig. 2(A)) a small spectral shift in ab-
sorption maxima is observed as the polarity of the solvent decreased
gradually from high-polarity dimethyl sulfoxide to low-polarity octane,
2.2.4. Quenching studies
In order to evaluate the effect of concentration on UV–visible ab-
sorption and emission properties of all the synthesized compounds
CHEM-6(a-c), we have investigated the absorption and emission spec-
tra of the organic compounds in ethanol solvent with different concen-
trations as shown in Figs. 3(A and B), 4(A and B) and 5(A and B),
respectively. The corresponding data are represented in Table 1. In the
UV–visible absorption spectra, the intensity of the absorption gradually
increases with increase in the concentration of the molecules CHEM-6
(a-c) from 0.5 μM to 40 μM which is an ideal condition for good
OLEDs. The representative UV–visible absorption spectra of the com-
pounds CHEM-6(a-c) in ethanol which shows the effect of concentra-
tion is represented in Figs. 3A, 4A and 5A, respectively. In the emission
spectra of the compounds, initially the intensity of the emission gradu-
ally increases with increase in the concentration and reaches a maxi-
mum intensity (optimized concentration) and then intensity of
emission gradually begins to decrease due to self quenching. The effect
of concentration on emission spectra of compounds CHEM-6(a-c) is
shown in Fig. 3B, 4B and 5B, respectively. The optimized fluorescence
concentrations of all the compounds are tabulated in Table 1. The com-
pound CHEM-6b has shown the highest optimized fluorescence concen-
tration of 10 μM and the compound CHEM-6c has shown the lowest
optimized fluorescence concentration of 6 μM. The compound CHEM-
6a has shown moderate optimized fluorescence concentration of 8 μM.
0
which indicates a small change in polarity of the ground state (S ) of or-
ganic compounds CHEM-6(a-c). The compounds CHEM-6(a-c) exhib-
ited a very small hypsochromic shift (blue-shift about 7 nm) as the
polarity of the solvents (in case of polar/less polar protic) increased
gradually from nonanol to ethanol, which indicates a small stabilization
0
in the ground state (S ). The bathochromic shift (red-shift about 23 nm)
of absorption maxima of the compounds CHEM-6(a-c) is observed as
the polarity of the solvents (in case of polar aprotic/non-polar) in-
creased gradually from octane to dimethyl sulfoxide. This clearly reveals
the stabilization of the exited state (S ) of the organic compounds. This
1
may be attributed to the interactions of the synthesized molecule with
the solvent molecules (solute-solvent interactions).
In relationship with the absorption spectra, the emission spectra of
the compounds CHEM-6(a-c) displayed a strong positive solvatochromic
response to solvents with varying polarities. All the compounds CHEM-6
(
a-c) exhibited a large bathochromic shift (red-shift) as the polarity of
the solvent gradually increased from less-polar octane to high-polar
DMSO. The compounds CHEM-6(a-c) showed red shifts of 112, 113
and 39 nm from octane to DMSO, respectively. This large solvatochromic
shift in the fluorescence spectra compared to the absorption spectra sug-
2.2.5. Theoretical computations
The optoelectronic properties of the organic materials depend
gest dipolar stabilization of the highly polar excited state (S
1
) than
0
mainly on the changes in the ground state (S ) electronic structures,
ground state (S ) of the compounds CHEM-6(a-c), exhibiting marked
0
so it is imperative to study the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) of the
photoinduced intramolecular charge transfer (ICT) properties. The
solvatochromic extent may be determined by molecular conjugative ef-
fect [58]. These results clearly confirm that the existence of strong
0
organic materials in its ground state (S ) [62]. To gain further informa-
tion about the molecular geometries, electronic properties and provide
5

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
Fig. 2. (A) Normalized absorption and emission spectra of organic compounds CHEM-6(a-c) in polar/less polar protic solvents such as methanol, pentanol, hexanol, octanol and nonanol.
B) Normalized absorption and emission spectra of organic compounds CHEM-6(a-c) in polar aprotic/non-polar solvents such as DMSO, DMF, DMA, toluene and octane.
(
6

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
Fig. 3. (A) Effect of concentration on the UV–visible absorption spectra of the compounds CHEM-6a in ethanol solution: (a) 0.5 μM (b) 1 μM (c) 2 μM (d) 4 μM (e) 6 μM (f) 8 μM (g) 10 μM
abs
(
(
h) 20 μM (i) 40 μM. λmax= 340 nm and (B) Emission spectra of the compounds CHEM-6a in ethanol solution: (a) 0.5 μM (b) 1 μM (c) 2 μM (d) 4 μM (e) 6 μM (f) 8 μM (g) 10 μM (h) 20 μM
emi
i) 40 μM (j) 60 μM (k) 80 μM. λmax= 484 nm.
Fig. 4. (A) Effect of concentration on the UV–visible absorption spectra of the compounds CHEM-6b in ethanol solution: (a) 0.5 μM (b) 1 μM (c) 2 μM (d) 4 μM (e) 6 μM (f) 8 μM (g) 10 μM
abs
(
(
h) 20 μM (i) 40 μM. λmax= 335 nm and (B) Emission spectra of the compounds CHEM-6b in ethanol solution: (a) 0.5 μM (b) 1 μM (c) 2 μM (d) 4 μM (e) 6 μM (f) 8 μM (g) 10 μM (h) 20 μM
emi
i) 40 μM (j) 60 μM (k) 80 μM. λmax= 485 nm.
Fig. 5. (A) Effect of concentration on the UV–visible absorption spectra of the compounds CHEM-6c in ethanol solution: (a) 0.5 μM (b) 1 μM (c) 2 μM (d) 4 μM (e) 6 μM, (f) 8 μM (g) 10 μM,
abs
(
(
h) 20 μM. λmax= 385 nm and (B) Emission spectra of the compounds CHEM-6c in ethanol solution: (a) 0.5 μM (b) 1 μM (c) 2 μM (d) 4 μM (e) 6 μM (f) 8 μM (g) 10 μM (h) 20 μM (i) 40 μM
emi
j) 60 μM (k) 80 μM. λmax= 468 nm
additional information about the process of intramolecular charge
transfer (ICT) properties of the synthesized compounds CHEM-6(a-c)
for the possible applications in the field of optoelectronics, density func-
tional theory (DFT) computations were performed in gas-phase by
functional and using 6-311G+ (d,p) basis set. In addition frontier mo-
lecular orbitals (FMO) amplitude plots of HOMOs and LUMOs level of
compounds CHEM-6(a-c) are illustrated in Fig. 6. DFT model was ex-
tended to investigate the dihedral angles of optimized ground states ge-
ometries of compounds (Table 3) and molecular electrostatic potential
energy surfaces (MEP) as depicted in Fig. 7.
using the Gaussian 09 software [63]. The ground state (S
0
) geometries
of compounds CHEM-6(a-c) were optimized using hybrid CAM-B3LYP
7

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
Fig. 6. Presentation of the frontier molecular orbitals of HOMOs and LUMOs level of compounds CHEM-6(a–c) at the CAM-B3LYP/6-311G+ (d,p) level of theory in the gas phase.
Table 3
Dihedral angles (DA) [°] are calculated around different aryl/hetero-aryl groups, phenyl and 1,3,4-oxadiazole group from ground state optimized molecular structures of CHEM-6(a-c).
The electron densities of HOMO energy levels largely located on the
electron-donor (D) groups such as naphthalen-2-yl, 4-(trifluoromethoxy)
benzene, 5-(5-hexylthiophen-2-yl) thiophen-2-yl and phenol units,
spreading slightly over phenyl π-linker, whereas in LUMOs, the electron
clouds are mainly situated on the electron-transport 1,3,4-oxadiazole
unit. As it is noticed from Fig. 6 electron density on the donor groups de-
creases, while the electron density on 1,3,4-oxadiazole unit increases
from HOMO to LUMO indicating electron deficient nature of oxadiazole
group. This clearly indicates that 1,3,4-oxadiazole is the better electron-
acceptor unit (A). In the photoexcitation process from the ground state
of the organic molecules. The presence of a high degree of planarity and
rigidity in the organic materials with extended π overlap results in over-
all improvement in hole or electron mobility [64,65]. The optimized
ground states geometries of compounds CHEM-6(a-c) are illustrated
in Fig. 7. By using the optimized molecular structures, the dihedral an-
gles (DA) are calculated around different electron donating aryl/
hetero-aryl groups, phenyl and 1,3,4-oxadiazole group and given in
Table 3, are in the range 0.45° - 40.79°. These results reveal that the syn-
thesized organic compounds CHEM-6(a-c) are comparatively planar in
nature. In the compounds donor units (D) connected to an acceptor
unit (A) through a phenyl π-linkage shows extended π-π conjugation.
Such novel class of π-conjugated compounds have drawn considerable
attention as they exhibit promising optoelectronic properties.
0 1
S to the singlet S electronic state of the compounds CHEM-6(a-c),
there is a substantial effect on the shift in electron cloud density from
the electron donor groups (D) to electron acceptor oxadiazole group
through phenyl π-linker and this clearly confirms an intramolecular
charge transfer property of the synthesized compounds, thereby reduc-
ing to the D-π-A-π’-D′ form.
Molecular electrostatic potential (MEP) surfaces of organic materials
are important factors in the field of optoelectronics, as they are used to
study the interpretation of molecular electronic structures. Electron
density around organic molecules is considered a very important factor
for understanding the reactive behavior of organic molecular system
Charge carrier mobility is material property which depends on the
extent of π-conjugation, which in turn depends chiefly on the planarity
8

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
Fig. 7. Density functional theory (DFT) results. Ground state optimized geometries and molecular electrostatic potential (MEP) surfaces of CHEM-6(a-c) molecules at CAM-B3LYP and 6-
11G+ (d,p) level of study.
3
[
66,67]. In the MEP plots of compounds CHEM-6(a-c), the most electro-
visible spectroscopy. Of these, fluorescence spectroscopy as a technique
for the detect Hg2 ion is of current research interest due to various ad-
vantages like high sensitivity, low-cost, direct visual detection, getting
the response quickly and relatively easy compared to other techniques.
Towards this aim, many mercury sensors based on small fluorescent or-
+
negative potential is represented by red regions, which are mainly local-
ized above the 1,3,4-oxadiazole unit and electronegative oxygen atoms.
The most positive electrostatic potential is indicated by blue regions lo-
calized around different electron donating aryl/hetero-aryl groups,
which gives additional evidence for the presence of strong intramolecu-
lar charge transfer properties from donors to acceptor groups of the π-
conjugated molecules CHEM-6(a-c) in the form D-π-A-π’-D′.
2+
ganic molecules have been reported. As the Hg metal ion having an
10
0
electronic configuration of 5d 6s , oxygen atoms or nitrogen atoms
are good binding sites for it [68,71–74].
Motivated by the above point of view, based on molecular structure
and photophysical properties, the synthesized organic compounds
CHEM-6(a-c) were interesting candidates as a functional material for
metal ions sensing applications. From preliminary investigations, we
found that compounds CHEM-6(a-c) exhibited the metal ion responsive
property towards Hg(II) which could be observed by the changing of its
solution color and fluorescence (under UV excitetion). In the experi-
2
.2.6. Metal ion sensing investigation
Detection of heavy metal ions is very essential since they cause seri-
ous damage to the environment and result in human health issues. For
example, mercury is considered one of the most toxic and highly dan-
gerous heavy metal used in several industrial products such as batteries,
paints and electrical equipment [68–70]. Also, the sources of mercury
are various geological activities, mine tailings, burning coal, waste
from chlorine-alkali industries, etc. Mercury in the form of Hg2 ions
is a carcinogenic material with a high degree of cellular toxicity causes
many serious diseases such as damage to the central nervous system,
−5
ment, aqueous solution containing of Hg(II) (1 × 10 M) was tested
for the presence of Hg(II) by adding with the one equivalent of com-
+
−5
2
pounds CHEM-6(a-c) in acetonitrile-H O (9:1) (1 × 10 M) solution.
The corresponding optical characteristics (absorption maxima, emis-
sion maxima and excitation wavelength of fluorescence) of compounds
CHEM-6(a-c) are summarized in Table 4. Absorption and fluorescence
2
+
kidney failure, chronic Hg poisoning, heart, liver and thyroid diseases.
2+
Therefore, selective and sensitive detection studies for Hg is highly
demanded. Numerous techniques are employed to detect Hg2 ions,
such as electrochemistry, high performance liquid chromatography
+
2
spectra of the compounds CHEM-6(a-c) (acetonitrile-H O (9:1)
−
5
2+
2+
(
1 × 10
M)) and CHEM-6(a-c) + Hg
1 equivalent of Hg
ion
were depicted in Fig. 8 and Fig. 9, respectively. From the results of
naked eye observation, the solution color changed from colorless Hg
(
HPLC), X-ray fluorescence spectroscopy, inductively coupled plasma
mass spectrometry, atomic absorption/emission spectroscopy and UV/
Table 4
2
+
Absorption and fluorescence spectral data of compounds CHEM-6(a-c) and their mixtures with Hg (1 eq) in acetonitrile-H
2
O (9:1).
System
λ
max (nm)a
Absorbance
Excitation wavelength of Fluorescence (nm)
λ
max (nm)b
Florescence counts
CHEM-6a
CHEM-6a + Hg 1 eq
CHEM-6b
CHEM-6b + Hg 1 eq
CHEM-6c
CHEM-6c + Hg 1 eq
332
332
328
328
380
380
0.71114
0.35018
0.49062
0.25052
0.22259
0.09461
332
328
380
500
498
507
–
471
481
2169
512
2230
–
3836
884
2
+
2
+
2
+
a
abs
−5
Absorption maxima (λmax) in acetonitrile-H
2
O (9:1) at 1 × 10 M concentration.
b
emi
−5
Emission maxima (λmax) in acetonitrile-H
2
O (9:1) at 1 × 10 M concentration.
9

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
2
+
2 2
Fig. 8. Absorption spectra and generality under ambient light (inside) of Hg (H O (9:1)
O, (1 × 10−5 M)), CHEM-6(a-c) and CHEM-6(a-c) + Hg2+ 1 equivalent in acetonitrile-H
1 × 10− M) solution.
5
(
Fig. 9. Fluorescence spectra and generality under UV irradiation (inside) of compounds CHEM-6(a-c) and CHEM-6(a-c) + Hg2+ 1 equivalent in acetonitrile-H
O (9:1) (1 × 10−5 M)
2
solution.
(
II) aqueous solution into the dark yellow of Fe(II)/CHEM-6(a-c) com-
obtain amorphous morphology, prevent light scattering and crystalliza-
tion induced device degradation of OLED/optoelectronic devices. The
thermal instability or low Tg of the organic materials may result in the
degradation of optoelectronic devices due to morphological changes
[75–77]. The key solutions for such challenges is by overcoming the syn-
thesis of thermally stable organic materials with high glass transition
temperatures (Tg) and 5% weight loss temperature (T5d) can produce
stable optoelectronic devices. The thermal properties of the synthesized
molecules CHEM-6(a-c) were examined by using thermal gravimetric
analysis (TGA) and differential scanning calorimetry (DSC) under nitro-
plex or excimer which corresponded to the absorption maxima at 332,
3
28 and 380 nm for the compounds CHEM-6(a-c) + Hg2 , respectively
+
in UV–Vis spectrum as seen in Fig. 8 and Table 4. Moreover, from the
Fig. 9, CHEM-6(a-c) + Hg2 1 equivalent solutions displayed fluores-
cent turn-off mechanism from deep blue color fluorescence of com-
pounds CHEM-6(a-c) to light yellow/colorless under UV irradiation
corresponding to absorption wavelength of the compounds. CHEM-6
+
(
a-c) exhibited emission wavelength of 500, 507 and 471 nm,
respectively (Table 4). The slight shift in the emission maxima of
CHEM-6(a-c) + Hg from that of sole CHEM-6(a-c) compounds, which
2
+
−1
gen atmosphere at scan rates of 10 °C min in the temperature range
2+
showed fluorescent turn-off upon addition of Hg ions 1 equivalent (as
seen from the Table 4, Figs. 8 and 9 decrease in the absorbance in the ab-
sorption spectra and decrease in the fluorescence counts in fluorescence
spectra), also indicated the presence of Hg(II) parallel with the results
from UV–Vis spectroscopy. From the three synthesized compounds,
CHEM-6c shown excellent fluorescence turn-off property from deep
blue color to colorless (under UV).
from 25 to 450 °C, respectively. The corresponding thermal characteris-
tics data are summarized in Table 1 and thermograms represented in
Fig. 10(A and B). In the DSC thermograms, endothermic peaks are ob-
served which correspond to melting point temperatures (T ) of the tar-
m
get compounds CHEM-6a, CHEM-6b and CHEM-6c as 177 °C, 180 °C and
179 °C, respectively. In addition, there was no observed exothermic
c
peak corresponding to crystallization temperature (T ) of the com-
From both results, it clearly concluded that the compounds CHEM-6
a-c) could exhibit dual sensing properties from two simultaneous phe-
nomena, the changes in solution color and fluorescence turn-off (under
UV) and therefore, they could be used as functional material for Hg(II)
ion sensing application.
pounds CHEM-6(a-c). In DSC thermograms, the prepared compounds
reveal endothermic baseline shift due to the glass transition tempera-
tures (Tg) for the compounds CHEM-6a and CHEM-6c at 65 °C and
132 °C, respectively. Importantly, the glass transition temperatures
(Tg) of the present compounds CHEM-6a and CHEM-6c are significantly
higher than that of commonly used popular 1,3,4-oxadiazole's including
non-bipolar host-emitters such as BOBP (Tg = 45 °C). Also, they are
higher than the well known 1,3,4-oxadiazole based electron-transporting
materials present in the literature, namely 2-(4-biphenylyl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole (PBD, Tg = 60 °C) [43,78]. The relatively
high Tg of the compounds CHEM-6a and CHEM-6c indicate the excellent
amorphous glass state stability of the compounds. The TGA measure-
ments reveal that the T5d temperature (defined as the temperature at
(
3
. Thermal stability
From the point of view to generate the stable and long lifetime
OLED/optoelectronic devices the thermal stability of organic materials
is highly desired. Predominantly, glass transition temperature (Tg) of
organic molecules is one of the most significant factor that influences
the device stability and operation lifetime. Also, high Tg is necessary to
10

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
Fig. 10. (A) DSC and (B) TGA thermograms of CHEM-6(a-c) recorded under nitrogen atmosphere at scan rates of 10 °C min−1 in the temperature range from 25 to 450 °C.
which organic sample loses its 5% weight) of compounds CHEM-6a,
CHEM-6b and CHEM-6c under N atmosphere are at 301 °C, 302 °C
production of high efficient OLEDs/optoelectronic devices. Ongoing re-
search draws attention towards developing new light-emitting organic
materials with very high fluorescence quantum yield. The fluorescence
quantum yield of fluorophores is defined as the ratio of the number of
photons emitted divided by the number of photons absorbed. As more
photons are re-emitted in post-excitation, signifying a higher fluores-
cence quantum yield, the less probable organic materials will lose en-
ergy in a non-useful way (nonradiative transitions) [79–81]. The
fluorescence quantum yields of the final compounds CHEM-6(a-c)
were determined in ethanol solvent by using a comparison method rel-
ative to a standard dye Coumarin 440 (7-amino-4-methyl-2H-1-
benzopyran-2-one) (Фref = 0.98) and Coumarin 480 (2,3,5,6-1H, 4H-
tetrahydro-8-methylquinolazino-[9, 9a,l-gh] coumarin) (Фref = 0.95)
[1] according to the following Eq. (2) [82].
2
and 322 °C, respectively. Among the three synthesized compounds
CHEM-6c have shown highest T5d temperature value 322 °C. From the
DSC and TGA results, a relatively high Tg and T5d of the present com-
pounds suggested that all of them were thermally stable and highly de-
sirable for applications in OLED/optoelectronic devices, as stability and
lifetime of such devices depend critically on the thermal stability of or-
ganic materials.
4. Conclusion
In the present work, we have designed and synthesized three π-
conjugated organic molecules CHEM-6(a-c) containing 2,5-
diphenylsubstituted-1,3,4-oxadiazole motif with good to excellent
yields by using simple and efficient synthetic methodology. The struc-
2
Iunk Astd unk
η
Φfunk ¼ Φstd
ð2Þ
1
13
2
tures were characterized by using H, C NMR, FT-IR, GCMS and ele-
Istd Aunk η
std
mental analysis. The optical properties of the synthesized compounds
opt
were studied in detail. Optical band gap (E
g
) values of compounds
where Φfunk is the fluorescence quantum yield of the sample, I is the in-
tegrated fluorescence intensity, A is the absorbance of the solution, η is
the refractive index and the subscripts of “unk” and “std” refer to the so-
lutions of the sample and reference standard dye of known quantum
yield, respectively. The fluorescence quantum yields of final compounds
CHEM-6a, CHEM-6b and CHEM-6c are 0.20, 0.37 and 0.80, respectively
in ethanol solvent as shown in Table 1. Interestingly fluorescence quan-
CHEM-6(a-c) are in the range 2.70–3.11 eV and have high Stokes shifts
−1
values in the range of 5018–9143 cm . DFT computations are investi-
gated in detail. Fluorescence quantum yields of compounds CHEM-6(a-
c) are in the range 0.20–0.80. Meanwhile, DSC and TGA measurements
indicate that these organic materials acquired good thermal stability
with glass transition temperatures (Tg) of compound CHEM-6a and
CHEM-6c observed at 65 °C and 132 °C, respectively and 5% decomposi-
tion temperature (T5d) above 300 °C. The current results demonstrate
that organic compounds CHEM-6(a-c) are very promising candidates
for possible applications for the development of displays, OLEDs and
could play an important role in the field of optoelectronics. In addition,
the compounds CHEM-6(a-c) showed the sensing property to Hg(II) ion
which could be observed with visual detection by the naked eyes as
changes in solution color and by the changing of its optical properties
like rapid fluorescence quenching by Hg ion (under UV). This metal
ion responsive property and its interaction could be feasible for further
study as a fluorescent turn-off chemosensors probes for Hg(II) in the fu-
ture and should be an interesting topic, not only for the chemistry and
physics but also for functional materials towards sensing applications
in medical research and detection of metal ion in biological/environ-
mental samples.
tum yield of CHEM-6c is high about 0.80 as compared to Φ of com-
f
pounds CHEM-6a and CHEM-6b which have moderate values. The
moderate Φ of compounds CHEM-6a and CHEM-6b is attributed to
f
the structural modification on organic molecules leads to the variation
of fluorescence intensity which directly impacts on the quantum yield.
In the compound CHEM-6c the presence of alkyl group and thiophene
rings (hexylthiophen-2-yl) results in better solubility as well as ex-
−1
tended π-π conjugation and low Stokes shift 5018 cm result in high
fluorescence quantum yield. Thus, the synthesized compounds are
2+
found to have moderate to high fluorescence quantum yields (Φ ) and
f
it makes them excellent candidates for the use in further applications
like chem/fluorescent sensors, optoelectronic devices/OLEDs and so on.
5.2. Materials and synthesis
All required starting materials and reagents were obtained from
commercially available sources unless otherwise stated and used with-
out further purification. Solvents used for analytical measurements and
synthesis were pre-dried and freshly distilled from appropriate drying
agents prior to use. Naphthalen-2-ylboronic acid, 4-(trifluoromethoxy)
phenylboronic acid, 5’-Hexyl-2, 2′-bithiophene-5‑boronic acid pinacol
5
. Experimental section
.1. Fluorescence quantum yield (Ф
In the field of organic electronics fluorescence quantum yield (Ф
5
f
)
f
) is
the most important property which can be used to measure the possible
ester and catalyst Pd(PPh
3 4
) were purchased from Sigma-Aldrich.
usefulness of an organic material and their optical quality for the
The essential key intermediates ethyl 4-bromobenzoate (1), 4-
11

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
bromobenzohydrazide (2) and 2-(4-bromophenyl)-5-(4-nitrophenyl)-
over anhydrous sodium sulfate. It was concentrated by rotary evapora-
tion under reduced pressure to give solid residue. The crude solid was
dried and recrystallized from ethanol to give intermediate compound
1
,3,4-oxadiazole (CHEM-3) were prepared according to the literature
method [36,43].
4
-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)benzenamine (CHEM-4)
5
4
.3. General procedure for the synthesis of intermediates ethyl
-bromobenzoate (1) and 4-bromobenzohydrazide (2)
in 86% yield.
5
.5.1. 4-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)benzenamine
(CHEM-4)
It was obtained as pale yellow color solid in 86% yield having m. p.
4
-bromobenzoic acid (10.00 g, 49.75 mmol) was dissolved in anhy-
drous ethanol (80 mL) in a 250 mL round-bottomed flask fitted with
dropping funnel and reflux condenser. SOCl (3.97 mL, 54.72 mmol)
−1
2
174–175 °C; FT-IR (KBr, cm ): 3349, 3225 (-N-H asym and -N-H sym-
was added slowly with the help of dropping funnel at r.t. under contin-
uous stirring. After refluxing for 8 h at 80 °C, the mixture was cooled and
quenched with ice water. The resulting mixture was then extracted
twice with ethyl acetate and the combined organic layer was washed
three times with sodium carbonate solution, dried over anhydrous so-
dium sulfate and concentrated by rotary evaporation to give intermedi-
ate ethyl 4-bromobenzoate (1) in 89% yield.
The mixture of ethyl 4-bromobenzoate (1) (5.00 g, 21.73 mmol) and
hydrazine hydrate (1.16 mL, 23.91 mmol) were taken in (30 mL) etha-
nol. The reaction mixture was refluxed for 10–12 h at 80 °C. Progress of
the reaction was monitored by TLC. The reaction mixture was then
allowed to cool and quenched with ice water. The solid that separated
out was filtered and re-crystallized from ethanol to give required inter-
mediate 4-bromobenzohydrazide (2) with 87% yield.
metric stretching), 3090 (-C-H aromatic), 1611, 1560 (-C=C- and -C=
N- aromatic), 1072, 1010 (=C-O-C = asym and = C-O-C = symmetric,
1
oxadiazole ring), 518 (-C-Br stretching); H NMR (400 MHz, CDCl
3
):
δ = 7.90 (d, J = 8.5 Hz, 2H), 7.84 (d, J = 8.4 Hz, 2H), 7.58 (d, J =
8.4 Hz, 2H), 6.69 (d, J = 8.4 Hz, 2H), 4.04 (s, 2H); 13C NMR (100 MHz,
3
CDCl ): δ = 165.17, 162.98, 149.88, 132.33, 128.71, 128.13, 125.94,
123.14, 114.70, 113.29; EI-MS: m/z calculated for C14
found 315 (M ).
3
H10BrN O 315.00,
+
5.6. General procedure for synthesis of intermediate (E)-2-((4-(5-(4-
bromophenyl)-1,3,4-oxadiazol-2-yl)phenylimino)methyl)phenol
(CHEM-5)
4-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)benzenamine (CHEM-
4
) (3.00 g, 9.52 mmol) and salicylaldehyde (1.00 mL, 9.55 mmol)
5
.4. General procedure for synthesis of intermediate 2-(4-bromophenyl)-5-
were taken in 20 mL of absolute ethanol in a 100 mL RBF fitted with re-
flux condenser. To the reaction mixture was added 2–3 drops of glacial
(
4-nitrophenyl)-1,3,4-oxadiazole (CHEM-3)
3
CH COOH (catalytic amount). The reaction mixture was heated under
Mixture of 4-bromobenzohydrazide (2) (4.00 g, 18.60 mmol) and
-nitrobenzoic acid (3.10 g, 18.60 mmol) in round-bottomed flask fitted
reflux for 3 h. The resulting mixture was left over night to get an orange
precipitate. The precipitate obtained was filtered and crystallized from
ethanol to give the respective key intermediate compound (E)-2-((4-
(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)phenylimino)methyl)phe-
nol (CHEM-5) in 84% yield.
4
with reflux condenser containing 30 mL of POCl was refluxed at 100 °C
3
for 12 h. After the completion of the reaction as confirmed by the TLC.
The reaction mixture was allowed to cool to room temperature. The
mixture was added to crushed ice under efficient fume hood. The ob-
tained solid residue was filtered and washed with excess of water. Fur-
ther, in order to remove the any unreacted carboxylic acid left, the
collected solid was washed with excess of saturated sodium bicarbonate
solution. The solid residue was dried and recrystallized from ethanol to
obtain the required intermediate to give 2-(4-bromophenyl)-5-(4-ni-
trophenyl)-1,3,4-oxadiazole (CHEM-3) as white solid in 85.7% yield.
5.6.1. (E)-2-((4-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)phenylimino)
methyl)phenol (CHEM-5)
It was obtained as yellow color solid in 84% yield having m. p.
−1
231–233 °C; FT-IR (KBr, cm ): 3458 (-O-H stretching, phenolic), 3063
(-C-H aromatic), 1620 (-CH=N- stretching, imine), 1599, 1569 (-C=C-
and -C=N- aromatic), 1280 (-C-O stretching, phenolic), 1075, 1008 (=
C-O-C = asym, and = C-O-C = symmetric, oxadiazole ring), 535 (-C-Br
1
5.4.1. 2-(4-bromophenyl)-5-(4-nitrophenyl)-1,3,4-oxadiazole (CHEM-3)
stretching); H NMR (400 MHz, CDCl
3
): δ = 12.90 (s, 1H), 8.68 (s, 1H),
It was obtained as white color solid in 85.7% yield having m. p.
8.44–8.27 (m, 1H), 8.19 (d, J = 8.1 Hz, 2H), 8.02 (d, J = 8.5 Hz, 2H),
7.69 (d, J = 8.5 Hz, 2H), 7.55 (d, J = 5.8 Hz, 1H), 7.43 (d, J = 5.3 Hz,
2
(
16–218 °C; FT-IR (KBr, cm−1): 3106 (-C-H aromatic), 1600, 1556
13
-C=C- and -C=N- aromatic), 1534, 1347 (-N-O asym and -N-O sym-
2H), 7.05 (d, J = 8.3 Hz, 1H), 7.01–6.93 (m, 1H); C NMR (100 MHz,
CDCl ): δ = 165.09, 163.68, 161.33, 149.87, 143.62, 139.22, 137.49,
136.29, 128.77, 127.48, 127.32, 127.29, 126.90, 123.03, 114.81, 114.58,
metric stretching), 1073, 1009 (=C-O-C = asym and = C-O-C = sym-
3
metric, oxadiazole ring), 527 (-C-Br stretching); 1H NMR (400 MHz,
+
CDCl
J = 8.6 Hz, 2H), 7.65 (d, J = 8.6 Hz, 2H); C NMR (100 MHz, CDCl
δ = 164.96, 163.10, 132.74, 129.27, 128.60, 128.43, 127.95, 127.28,
3
): δ = 8.35 (d, J = 9.0 Hz, 2H), 8.27 (d, J = 9.1 Hz, 2H), 7.97 (d,
113.60; EI-MS: m/z calculated for C21
5.7. General procedure for the synthesis of target compounds CHEM-6(a-c)
To a mixture of 1,4-dioxane (10 mL) and aqueous 2 M K CO (5 mL)
3 2
H14BrN O 419.02, found 419 (M ).
13
3
):
1
3
24.57, 122.27; EI-MS: m/z calculated for C14
45 (M ).
H
8
3
BrN O
3
344.97, found
+
2
3
in a pressure tube, a mixture of (E)-2-((4-(5-(4-bromophenyl)-
1,3,4-oxadiazol-2-yl)phenylimino)methyl)phenol (CHEM-5) (0.20 g,
5
.5. General procedure for synthesis of intermediate 4-(5-(4-
bromophenyl)-1,3,4-oxadiazol-2-yl)benzenamine (CHEM-4)
0.476 mmol), boronic acid (0.081 g, 0.476 mmol) and Pd(PPh
3 4
)
(
0.11 g, 0.2 equiv), as a catalyst were added. The resulting reaction mix-
The compound 4-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)
benzenamine (CHEM-4) was synthesized according to the reported
procedure [44], the compound CHEM-3 (3.50 g, 10.14 mmol, 1.0
ture was heated with stirring for 8 h at 100 °C under a nitrogen atmo-
sphere. After completion of the reaction (as indicated by thin layer
chromatographic analysis), the reaction mixture was allowed to cool
to room temperature. Then the compound was extracted by DCM
(20 mL × 3) and the combined organic phase was washed with water
and brine solution. The DCM solution was dried over anhydrous
equiv) was suspended in EtOH/H
2
O (v/v: 3:1) system. The iron powder
Cl (1.62 g, 30.43 mmol, 3.0
(
5.66 g, 101.44 mmol, 10.0 equiv) and NH
4
equiv) were added to the reaction mixture. Then the reaction mixture
was heated to reflux for 2 h at 80 °C with vigorous stirring. Once com-
pletion of the reaction as confirmed by thin layer chromatographic anal-
ysis (TLC), the precipitate was separated from the reaction mixture by
filtration and washed with excess of ethanol. The filtrate was dried
2 4
Na SO and concentrated under reduced pressure by rotary evaporation
to give the crude solid, which was purified by column chromatography
on silica gel using ethyl acetate: hexane as eluent, to offer desired final
compounds CHEM-6(a-c) in 72–77% yield.
12

M.S. Najare, M.K. Patil, M. Garbhagudi et al.
Journal of Molecular Liquids 328 (2021) 115443
5
.7.1. (E)-2-((4-(5-(4-(naphthalen-2-yl)phenyl)-1,3,4-oxadiazol-2- yl)
Acknowledgements
phenylimino)methyl)phenol (CHEM-6a)
It was obtained as pale yellow color solid in 75% yield having m. p.
We are thankful to the University Scientific Instrument Centre
(USIC) and SAIF, Karnatak University, Dharwad for providing the spec-
tral data. Also to the University Grants Commission (UGC), New Delhi,
India, for providing financial support under UGC-SRF (Sr. No.
2121510180 Ref. No. 20/12/2015) and DST-Purse Phase-II program for
providing necessary facilities. One of the authors Mallikarjun K. Patil
would like to thank KSTePS Govt. of Karnataka for providing DST-Ph.D
fellowship.
1
3
(
79–180 °C; FT-IR (KBr, cm−1): 3342 (-O-H stretching, phenolic),
223 (-C-H aromatic), 1608 (-CH=N- stretching, imine), 1587, 1553
-C=C- and -C=N- aromatic), 1271 (-C-O stretching, phenolic), 1073,
012 (=C-O-C = asym and = C-O-C = symmetric, oxadiazole ring);
1
1
H NMR (400 MHz, CDCl
.04–7.99 (m, 6H), 7.94 (d, J = 3.4 Hz, 1H), 7.92–7.89 (m, 2H), 7.77
d, J = 8.6 Hz, 2H), 7.74 (d, J = 8.6 Hz, 1H), 7.58–7.55 (m, 1H),
.54–7.51 (m, 2H), 6.68 (d, J = 8.6 Hz, 2H), 5.98 (s, 1H); 13C NMR
100 MHz, CDCl ): δ = 165.38, 163.03, 153.02, 152.99, 143.22, 136.64,
3
): δ = 8.32 (s, 1H), 8.15 (d, J = 8.4 Hz, 2H),
8
(
7
(
3
Appendix A. Supplementary data
1
1
1
33.78, 133.11, 129.90, 129.23, 128.92, 128.83, 128.78, 128.76, 128.30,
28.08, 127.97, 127.50, 127.15, 127.11, 126.83, 126.31, 125.36, 123.19,
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2021.115443.
14.11, 114.08, 110.07; EI-MS m/z calculated for C31
H
21
N
3
O
2
467.16,
+
found 467 (M ); Elem. Anal. Calcd (%) for C31
H
21
N
3
O
2
: C 79.64, H
4.53, N 8.99 Found: C 79.53, H 4.51, N 8.96.
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