Photophysical and Electrochemical Properties of Highly π-Conjugated Bipolar Carbazole-1,3,4-Oxadiazole-based D-π-A Type of Efficient Deep Blue Fluorescent Dye

Article abstract of DOI:10.1007/s10895-021-02778-1

In this contribution, we have designed and synthesized a novel carbazole-1,3,4-oxadiazole based bipolar fluorophore (E)-2-(4-(4-(9H-carbazol-9-yl)styryl)phenyl)-5-(4-(tertbutyl) phenyl)-1,3,4-oxadiazole (CBZ-OXA-IV). Wittig reaction is utilised for the synthesis of the designed bipolar target compound CBZ-OXA-IV.¹H NMR, ¹³C NMR, FT-IR and ESI–MS results confirmed the designed chemical structure of the fluorophore CBZ-OXA-IV. The photophysical properties have been investigated in detail using UV–Vis absorption, photoluminescence spectroscopy. Also, the photoluminescence studies on solid state samples (as thin films) were carried out. The CBZ-OXA-IV dye emits intense deep blue fluorescence with observed absorption and emission maxima occurring are at 353?nm and 470?nm, respectively. Fluorophore CBZ-OXA-IV has shown high Stokes shift of 7052?cm^?1. The experimentally measured optical band gap (Egopt) value is found to be 3.01?eV and the fluorescence quantum yields (Φ_f) is 0.40. The intramolecular charge transfer property of CBZ-OXA-IV dye was examined by using photophysical properties such as absorption, emission in different solvents of different varying polarities. In addition, Density Functional Theory computations are studied in detail including the MEP surface plots and natural bond orbital analysis. The electrochemical properties have been investigated in detail by using cyclic voltammetry measurements. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement results display a high thermal stability with decomposition temperature (T_d5%) 387?°C and a large glass transition temperature (Tg) of 98?°C. The obtained results demonstrated that the novel bipolar fluorophore CBZ-OXA-IV could play an important role in organic optoelectronics and possibly can be utilized as bipolar transport materials for electroluminescence applications in optoelectronic devices/OLEDs. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10895-021-02778-1

Journal of Fluorescence
https://doi.org/10.1007/s10895-021-02778-1
ORIGINAL ARTICLE
Photophysical and Electrochemical Properties of Highly π‑Conjugated
Bipolar Carbazole‑1,3,4‑Oxadiazole‑based D‑π‑A Type of Efficient
Deep Blue Fluorescent Dye
Mahesh Sadashivappa Najare1 · Mallikarjun Kalagouda Patil2 · Tarimakki Shankar Tilakraj2 · Mohammed Yaseen1 ·
AfraQuasar A Nadaf1 · Shivaraj Mantur1 · Sanjeev Ramchandra Inamdar2 · Imtiyaz Ahmed M Khazi1
Received: 17 May 2021 / Accepted: 5 July 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
In this contribution, we have designed and synthesized a novel carbazole-1,3,4-oxadiazole based bipolar fuorophore (E)-
2-(4-(4-(9H-carbazol-9-yl)styryl)phenyl)-5-(4-(tertbutyl) phenyl)-1,3,4-oxadiazole (CBZ-OXA-IV). Wittig reaction is utilised
for the synthesis of the designed bipolar target compound CBZ-OXA-IV. 1H NMR, 13C NMR, FT-IR and ESI–MS results
confrmed the designed chemical structure of the fuorophore CBZ-OXA-IV. The photophysical properties have been inves-
tigated in detail using UV–Vis absorption, photoluminescence spectroscopy. Also, the photoluminescence studies on solid
state samples (as thin flms) were carried out. The CBZ-OXA-IV dye emits intense deep blue fuorescence with observed
absorption and emission maxima occurring are at 353 nm and 470 nm, respectively. Fluorophore CBZ-OXA-IV has shown
high Stokes shift of 7052 cm−1. The experimentally measured optical band gap (Egopt) value is found to be 3.01 eV and the
fuorescence quantum yields (Φf) is 0.40. The intramolecular charge transfer property of CBZ-OXA-IV dye was examined
by using photophysical properties such as absorption, emission in diferent solvents of diferent varying polarities. In addi-
tion, Density Functional Theory computations are studied in detail including the MEP surface plots and natural bond orbital
analysis. The electrochemical properties have been investigated in detail by using cyclic voltammetry measurements. Thermal
gravimetric analysis (TGA) and diferential scanning calorimetry (DSC) measurement results display a high thermal stability
with decomposition temperature (Td5%) 387 °C and a large glass transition temperature (Tg) of 98 °C. The obtained results
demonstrated that the novel bipolar fuorophore CBZ-OXA-IV could play an important role in organic optoelectronics and
possibly can be utilized as bipolar transport materials for electroluminescence applications in optoelectronic devices/OLEDs.
Keywords Photophysical properties · Bipolar fluorophore · Carbazole · 1,3,4-Oxadiazole · Electrochemical properties ·
DFT Computations
Introduction
photovoltaics [1]. Since the invention of organic light-emitting
diodes (OLEDs), frst introduced by Tang and VanSlyke
in 1987, OLEDs are considered as one of the most prom-
ising technologies, due to their promising properties and
numerous advantages such as light weight, rapid response,
wide-viewing angles, low driving voltage, high brightness,
color-purity, high optical transparency and the possibility
for fabrication into large and fexible displays [2–6]. There-
fore, OLEDs have attracted immense attention in academic,
scientifc and industrial areas for their potential applications
as ecofriendly candidates for the next-generation full-color
fat panel displays and solid-state lighting sources [7–9].
The simplest architecture of multilayer OLEDs comprise of
an anode, a hole transporting material (HTM) layer, emis-
sive material layer (EML), an electron transporting material
During the last three decades enormous research has been
dedicated to the feld of optoelectronics, including organic
light-emitting diodes (OLEDs), organic feld-efect tran-
sistors (OFETs), sensors, organic solar cells (OSCs) and
* Mahesh Sadashivappa Najare
mahesh.s.najare@gmail.com
1
Department of Chemistry, Karnatak University,
Dharwad 580003, Karnataka, India
2
Laser Spectroscopy Programme, Department of Physics,
UGC-CPEPA, Karnatak University, Dharwad 580003,
Karnataka, India
Vol.:(0123456789)
1 3

Journal of Fluorescence
(ETM) layer and a cathode [4, 10]. The general working
principle of an OLED devices is, when a voltage is applied
across the two electrodes, then charges (holes from the
anode and electrons from the cathode) 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 [11]. Despite the advancements in
the feld of OLEDs, the development of efcient and stable
OLEDs for their practical use into large-scale commercial
applications, still have some unresolved problems such as
high production cost, high power consumption, low ef-
ciency and lifetime [12, 13]. Organic emitting materials
with low thermal decomposition temperature (Td) or low
glass transition temperature (Tg) may result in the faster deg-
radation of organic devices due to morphological changes
leads to the lower lifetime of the devices [14]. Further, the
performance of blue and deep blue OLEDs is in fact not
good enough, in terms of their lower efciency and stability,
which are extensively dependent on the blue luminescent
organic materials [15]. In order to address these drawbacks,
a new class of organic materials should be synthesized as
organic light-emitting materials. Among all the possible
organic materials, small π-conjugated organic molecules
are most extensively used. Due to their various advantages
such as easy, tunable and reproducible synthesis along with
simple purifcation protocols [7]. Over the past two dec-
ades, the organic materials with donor-π-acceptor (D-π-A)
form are accepted as one of the most promising strategies
for improving OLEDs performance [16]. They have received
overwhelming importance as electroluminescent materials
for organic electronics, particularly for energy consumption
and energy harvesting devices, such as organic light-emitting
devices [17]. An extensively used strategy for design
OLED over the past year is to introduction of electron-rich
donors or electron-defcient acceptors to a suitable organic
core structure to get hole transporting or electron transport-
ing materials, respectively [18]. A major drawback in this
device structure is the imbalance of charge-transporting
character of organic materials when only electron-transport
or hole-transport materials are used as it gives rise to low
device efciency and reduced lifetime [18]. Efective control
of charge balance is essential for high performance OLED
devices [19]. This can be achieved by the synthesis of new
organic materials possessing bipolar properties. Which is the
combination of both the electron donating (D) and electron-
accepting (A) moieties within a single molecule, the result-
ing bipolar materials provide the background for a good bal-
ance between electron and hole mobilities (charge balance),
resulting in the broadening of exciton recombination zones
[20–22]. This constitutes one of the mandatory conditions
for the fabrication of single-layer material and consequen-
tially, to give high efciency organic light-emitting diodes
(OLEDs) and low efciency roll-of [21]. The devices with
bipolar fuorophores are easier to fabricate and the manu-
facturing costs are reduced [23]. Organic fuorophores with
comparable hole- and electron-transporting abilities are also
termed ambipolar molecules [23].
In view of the above mentioned points, carbazole is
a known popular heterocyclic moiety, widely used as a
electron donor component for optoelectronic materials
[23] due to their excellent optical property and ease
of functionalization. Carbazole derivatives have been
extensively used as host organic materials for multiple
optoelectronic and photovoltaic applications such as
in OLEDs, organic solar cells and fluorescent probes,
due to their outstanding advantages like rigid structure,
synthesis flexibility, good hole-transporting ability,
photoconductivity, luminescence property, good thermal
and photochemical stability [24–27]. D-A configured
carbazole based dyes used as active photosensitizers are
also reported in the literature [28]. Among them, CBP
(4,4’-N,N’-dicarbazole-biphenyl) and mCP (1,3-di(9H-
carbazol-9-yl)benzene) are the most commonly used
carbazole-based hosts. Unfortunately, the two compounds
exhibit poor thermal stability with low glass transition
temperatures (CBP: 62 °C, mCP: 60 °C) [29]. On the
other hand, symmetrical and unsymmetrical substituted
oxadiazole (OXD) derivatives more particularly
2,5-diaryl-1,3,4-oxadiazoles are extensively used as
multipurpose electron-transporting/hole-blocking
materials in OLEDs/optoelectronic devices, due to their
good electron-transporting/hole-blocking nature and high
thermal stability, thereby increasing the quantum yield of
fluorescence (Φf) [18, 23, 30].
Against this background, in the present work we
have designed, synthesized and characterized a new
D-π-A configured heterocyclic bipolar fluorophore
carbazole-oxadiazole (CBZ-OXA-IV) based organic
dye. Photophysical properties of the compound CBZ-
OXA-IV have been thoroughly examined by using
UV–visible and fluorescence emission spectra. Optical
band gaps (Egopt ) and fluorescence quantum yields (Фf)
were calculated. Intramolecular charge transfer (ICT)
properties of the synthesized bipolar fluorophore CBZ-
OXA-IV have been investigated with the help of solvent
polarity effects (solvatochromism) in various solvents
with variations in solvent polarity from polar to non
polar solvents. Further, with the aim of understanding
their molecular geometry and electronic distribution in
their frontier molecular orbitals, the Density Functional
Theory (DFT) computations including MESP maps and
natural bond orbital analysis were performed using the
Gaussian 09 software and the ground state geometries
were optimized at CAM‐B3LYP and 6‐311G (d,p) level
of study. Additionally, the electrochemical properties of
the bipolar fluorophore CBZ-OXA-IV was carried out
1 3

Journal of Fluorescence
by using the cyclic voltammetry (CV) measurements.
From the DSC and TGA measurements, the thermal
stability of the fluorophore was confirmed. The current
results indicates that the synthesized Donor-π-Acceptor
configured bipolar fluorophore carbazole-oxadiazole
(CBZ-OXA-IV) based organic dye is a promising
candidate and could play an important role in the field
of OLEDs/optoelectronic devices.
Results and Discussion
Design and Synthesis
As shown in Scheme 1, a relatively simple and efcient syn-
thetic pathway is utilised for the synthesis of the designed
bipolar target compound CBZ-OXA-IV. The primary key
intermediates were prepared according to the our previously
Scheme 1 Synthetic pathway
for the synthesis of designed
compound and the chemical
structure of the fuorophore
CBZ-OXA-IV
1 3

Journal of Fluorescence
published and literature procedures [31, 32]. In short, syn-
thesis of the intermediate compounds and target fuoro-
phore were described as follows, 4-methylbenzohydrazide
and 4-(tert-butyl)benzoic acid were taken in POCl3 and
refuxed for 12 h at 100 °C to get the intermediate compound
2-(4-(tert-butyl)phenyl)-5-(p-tolyl)-1,3,4-oxadiazole (I).
Subsequently NBS bromination is carried out at the allylic
position of the compound I by using N-Bromosuccinimide
in the presence of catalytic amount of dibenzoyl peroxide
(DBP) in CCl4 heating at 80 °C for 8 h gave the required
intermediate compound 2-(4-(bromomethyl)phenyl)-5-(4-
(tert-butyl)phenyl)-1,3,4-oxadiazole (II). Further, the nucle-
ophilic substitution reaction of triphenyl phosphine at the
allylic brominated carbon of compound II was carried out
by taking, a mixture of triphenyl phosphine and compound
II in ethyl acetate, refuxing for 8 h at 80 °C gives Wittig
salt (III). The intermediate compound 4-(9H-carbazol-9-yl)
benzaldehyde was prepared according to published proce-
dure [33]. A solution of 4-fuorobenzaldehyde, carbazole
and K2CO3 were taken in DMSO was heated at 150 ºC
overnight to yield the intermediate compound 4-(9H-carbazol-
9-yl)benzaldehyde. In the fnal step, a mixture of phos-
phonium salt (III) and 4-(9H-carbazol-9-yl)benzaldehyde
were taken in a round bottom fask containing DCM:water
(60:40) mixture. To the reaction mixture 50% NaOH was
added dropwise and reaction was stirred for 10 h at room
temperature with continuous vigorous stirring leads to the
formation of required bipolar fuorophore CBZ-OXA-IV via
Wittig reaction. The product was readily purifed on silica-
gel column chromatography by using ethyl acetate/hexane
as eluent to get light green coloured solid in good yield.
The synthesized compound CBZ-OXA-IV is stable in an
ambient atmosphere containing oxygen and moisture. The
fuorophore CBZ-OXA-IV is fully characterized by using
various spectroscopic techniques like 1H, 13C NMR, FT-IR
and LC–MS/ESI–MS. The data obtained were well consist-
ent with the proposed chemical structure of the bipolar com-
pound of CBZ-OXA-IV.
(Fig. 1A), displayed two distinct absorption bands centred at
one in higher wavelength region (strong band) 353 nm and
in the lower wavelength region (weak band) 292 nm. The
relatively weak band observed at 292 nm can be ascribed to
the localized π-π* transition of the electron rich carbazole
donor species of the dye. While, the intense band observed
at 353 nm is attributed to delocalized π-π* electronic transi-
tion or ICT (intermolecular charge transfer) from the donor
to acceptor unit (carbazole to oxadiazole group through
extended π-conjugation) [31, 35].
Photoluminescence spectrum of fuorophore CBZ-OXA-
IV displayed a single emission peak at 470 nm in ethanol
solution at room temperature have been shown in Fig. 1(B)
emi
max
and the resultant emission maxima (ꢀ ) is summarized in
Table 1. The bipolar fuorophore CBZ-OXA-IV have deep
blue emission. As compared to the absorption spectrum the
emission maxima was red shifted. Furthermore, the Stokes
shift was calculated from the diference between ꢀabs and
emi
max
ꢀ
values and it was found to be 7052 cm−1. Usmuaaxlly a
small Stokes shift can cause self-quenching of fuorescence
and measurement error by excitation light and scattered
light [36]. Fascinatingly, the CBZ-OXA-IV dye shown high
Stokes shift about 7052 cm−1. The main advantage of the
high Stokes shift organic fuorophores is the minimization
of the reabsorption of fuorescence photons by the dye. High
fuorescence Stokes shift is critical for expands the feld of
their practical application, for example, solid state fuoro-
phores including those applied in OLEDs, fuorescence
imaging measurements, fuorescent sensing compounds,
several biophysical and analytical applications, etc. [37].
The new CBZ-OXA-IV dye has some important features,
such as a large Stokes shift (7052 cm−1) and signifcantly
stronger deep blue fuorescence. Here, the higher stokes shift
value of CBZ-OXA-IV can be accounted to an excited-state
intramolecular charge transfer (ICT) between the electron
donor carbazole group to electron acceptor 1,3,4-oxadiazole
group through π-extended conjugation in the dye.
The extensive applicability of organic materials that
show strong fluorescence in the solid-state have broad
applications, such as OLEDs, OFETs, organic lasers and
solid-state fuorescence sensors [38, 39]. On the other hand,
organic materials that exhibit fuorescence in dilute solu-
tions generally sufer from quenched or reduced fuorescence
intensity in their solid-state because of aggregation-caused
quenching (ACQ) [38]. In order to achieve strong fuores-
cence in solid-state form of dye requires the prevention of
intermolecular interactions between neighboring fuoro-
phores, which causes fuorescence quenching [38]. In the
light of this factor, introduction of bulky substituent groups
into a backbone of fuorophore is an efective strategy to
enhance the solid-state fuorescence by preventing intermo-
lecular interactions, such as π-π interactions [38]. Utilizing
this approach, we have reported solid-state fuorescent dye
Photophysical Properties
UV–Visible Absorption and Photoluminescence Properties
Optical spectroscopy is a powerful tool to study the interac-
tion of light and matter [34]. For the better understanding
of nature of electronic transitions and to get deeper inside
the optical properties of organic bipolar fuorophore CBZ-
OXA-IV for the feasible applications in optoelectronic
devices/OLEDs, the UV–vis absorption and photolumines-
cence spectra were measured in ethanol (EtOH) solution are
shown in Fig. 1(A and B), respectively. The corresponding
emi
optical data (ꢀabs , ꢀ
and Stokes shift) are summarized in
max
Table 1. In thmeaxabsorption spectrum of CBZ-OXA-IV dye
1 3

Journal of Fluorescence
Fig. 1 (A) UV–vis absorption spectrum and (B) Photoluminescence spectrum of the fuorophore CBZ-OXA-IV in ethanol solution at r.t (C)
Normalized fuorescence spectrum of CBZ-OXA-IV in solid state (thin flm). Ref—Coumarin 480 dye is used for comparison
CBZ-OXA-IV containing bulky carbazole as donor group
which is twisted by about 56.96° from the plane of the rest
of the dye (see in Table 3). Emission spectrum of bipolar
fuorophore CBZ-OXA-IV recorded in solid state form (thin
flms) is shown in Fig. 1(C) and resultant emission maxima
It is quite interesting to notice, that there is no shift in the
emission maxima form the solution to solid state emission
maxima. This is presumably due to lesser intermolecular
interactions that take place between neighboring molecules
in the solid state.
emi
(ꢀ ) is observed at 470 nm (violet to red color) (Table 1).
max
Table 1 Summary of photophysical properties, optical bandgap energies (Eogpt), fuorescence quantum yields (Φf) and thermal properties of bipo-
lar fuorophore CBZ-OXA-IV
Tg [oC][e]
Tm [oC][e]
Tc [oC][e]
T5d [oC][e]
Egopt
emi
max
emi
max
abs
max
[d]
Compound
Stokes shift
Quantum
ꢀ
ꢀ
ꢀ
(cm−1
)
Yield ( Φf)
(nm)[b]
(nm)[c]
(nm)[a]
(eV)
CBZ-OXA-IV
Ref. C480
353
354
470
428
470
-
7052
4884
3.01
-
0.40
0.98
98
-
207
-
-
-
387
-
[a]
abs
Absorption maxima (ꢀ ) in ethanol at 10 µM concentration.
max
emi
Emission maxima (ꢀ ) in ethanol at 10 µM concentration.
[b]
max
[c]Emission maxima measured in solid state (thin flm).
[d]Optical band gap energies (Egopt) were calculated from the long edge of the absorption spectrum using Egopt = hc/λ=1240/λ (eV).
[e]Results from DSC and TGA measurements; Tg-glass transition temperature, Tm-melting temperature, Tc-crystallization temperature and T5d-5%
weight loss temperature. Ref-Coumarin 480.
1 3

Journal of Fluorescence
Optical Band Gap (ꢀꢄꢁꢂꢃ
)
CBZ-OXA-IV have been carried out in various solvents
such as, polar/less polar protic solvents methanol, ethanol,
propanol, butanol, pentanol, hexanol, octanol, nonanol,
decanol and in polar aprotic solvents like DMSO, acetoni-
trile and DMF. Additionally, weakly/non-polar solvents
namely chloroform, toluene, 1,4-dioxane, octane, decane,
tetradecane and hexadecane were also studied. The normal-
ized absorption spectra and fuorescence emission spectra
of the compound CBZ-OXA-IV in various solvents with
varying polarities are shown in Fig. 2(A, B and C). Corre-
sponding photophysical data (absorption maxima, emission
maxima and Stokes shift) have been enlisted in Table 2. The
compound CBZ-OXA-IV exhibited a hypsochromic shift
(blue-shift) of about 17 nm as the polarity of the solvents
(in case of polar/less polar protic) increased gradually from
decanol to methanol. This clearly suggests that higher the
polarity of the solvents, higher is the energy of excitation.
This is very similar to the nature we have observed in our
work on pyrene substituted 1,3,4-Oxadiazole derivatives,
whose results reveal, irrespective of functionalization, the
solvent efects remain same for 1,3,4-oxadiazole based dyes
[31]. The ground state is highly stable in alcohols, which
can be verifed looking into the small shifts in peaks (maxi-
mum 9 nm shift between methanol and ethanol; in all other
solvents, the shift is 1–3 nm). We can observe very sim-
ilar trends in case of weakly polar solvents. For solvents
with more polar nature the excitation energy is high. And
as we move towards more weakly polar solvents, we can
observe the shifts towards red region. Contrarily, in case of
highly polar solvents, we observe a hypsochromic shift, to
which the intermolecular dipole interaction may be made
responsible.
The determination of the optical band gap (Egopt) is a nec-
essary parameter to be considered for the application of
organic probes in the feld of photovoltaics and organic light-
emitting devices/optoelectronic devices, as the Egopt provide
basics in the understanding of the optical as well as the elec-
trical process occurring within the organic materials [40].
The value of Egopt describes the fundamental light absorp-
tion edge, which gives the information about the amount
of light energy required to promote electrons from the
highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO) [41]. As the optical
bandgap (Egopt) of organic molecules mainly depends on the
bond length alternation pattern, planarity, aromaticity and
electron-withdrawing/releasing substitutions [42]. Therefore
tuning the optical band gap into the desired value for appli-
cations is of utmost importance. Optical band gap (Egopt) of
the bipolar fuorophore CBZ-OXA-IV is estimated from the
absorption edge (long wavelength edge) of the correspond-
ing absorption spectra (λonset, solution) to the baseline make
use of the Eq. (1) [43] and the corresponding value is tabu-
lated in Table 1.
1240
Egopt
=
(1)
ꢀonset
The optical band gap (Egopt) value of the bipolar fuoro-
phore CBZ-OXA-IV is found to be 3.01 eV. The low opti-
cal band gap (Egopt) value of the fuorophore CBZ-OXA-IV
reveals the increase in the extended π-conjugation from the
electron donor carbazole to electron acceptor 1,3,4-oxadiazole
group. Due to that reason, it is encouraging to use of
fuorophore CBZ-OXA-IV for the possible application in
the optoelectronic devices.
In case of fuorescence, we have observed positive solva-
tochromism (red shifted emission) in polar protic solvents
(polarity of the solvents increased gradually from decanol
to methanol) and polar aprotic solvents (polarity of the sol-
vents increased gradually from DMF to DMSO) are mainly
due to the fact that the excited state of the compound CBZ-
OXA-IV is infuenced by the dipole–dipole interaction and
solute–solvent interaction. This indicates a larger change in
dipole moment and stronger intramolecular charge transfer
character of bipolar fuorophore CBZ-OXA-IV between
the carbazole donor unit to the electron acceptor oxadia-
zole moiety through π-conjugation. Also, it is evident by
the large Stokes shifts of the fuorophore CBZ-OXA-IV
found to be in the range 3887- 8365 cm−1 (Table 2). Fur-
thermore, intramolecular charge transfer (ICT) character of
bipolar fuorophore CBZ-OXA-IV is confrmed by using
theoretical density functional theory (DFT) computations.
However, we observed bathochromic/hypsochromic shifts in
the emission properties of the molecule solvated in weakly/
non polar solvents. A major distinction is dual fuorescence
Solvatochromism Studies
Solvatochromism is a simple yet robust phenomenon through
which we can study few important properties of the solute.
The interaction between solute and solvents largely refect
the real nature of the solute molecule. Solvatochromism
can shed light on the efects of molecular environment on
the photophysical behavior of the solute. Solvents with dif-
ferent polarities show diferent peak intensities, which can
be accounted to H-bonding, intramolecular charge trans-
fer (ICT) and many other complex phenomena that occurs
between molecules [44]. In addition, the change in a solvent
is complemented by a change in polarity, dielectric constant
and polarizability of the medium. Therefore, the ground
and excited state of the organic materials are afected by
changing solvent [45]. In order to investigate the solvent
polarity efects (solvatochromism) on bipolar fuorophore
1 3

Journal of Fluorescence
Fig. 2 (A) Normalized absorption and emission spectra of organic
fuorophore CBZ-OXA-IV in polar/less polar protic solvents. (B)
Normalized absorption and emission spectra of organic fuorophore
CBZ-OXA-IV in polar aprotic solvents. (C) Normalized absorp-
tion and emission spectra of organic fuorophore CBZ-OXA-IV in
weakly/non polar solvents
1 3

Journal of Fluorescence
Table 2 Solvatochromism: Absorption and emission parameters of
organic fuorophore CBZ-OXA-IV in diferent solvents
as hole/electron injection, absorption and emission. Further,
the energy levels of the FMOs of the organic compounds are
dependent on the distribution of the frontier molecular orbit-
als. In addition, the optical behavior and electronic proper-
ties of the organic materials rely mainly on the changes in the
ground state electronic structure (S0), making it imperative
to inspect the highest occupied molecular orbital (HOMO),
and the lowest unoccupied molecular orbital (LUMO) and
band gaps (HOMO–LUMO gap) of the organic fuorophore
in the ground state (S0) [47, 48]. In order to understand the
electronic states, molecular geometries and intramolecular
charge transfer (ICT) properties of the synthesized bipolar
fluorophore CBZ-OXA-IV, Density Functional Theory
(DFT) calculations were performed by using the Gaussian
09 software [49]. The ground state (S0) geometries of CBZ-
OXA-IV dye were optimized using CAM-B3LYP/6-311G
(d,p) level of theory.
Solvent
CBZ-OXA-VI
λa
λf
Stokes
shift
Polar protic solvents
Methanol
Ethanol
Propanol
Butanol
Pentanol
Hexanol
Octanol
Nonanol
346
353
356
358
358
359
361
363
363
485
470
466
463
461
457
451
447
446
8283
7052
6630
6334
6240
5973
5527
5176
5126
Decanol
Polar aprotic solvents
DMSO
359
351
354
487
476
473
7321
7481
7106
Acetonitrile
DMF
Frontier Molecular Orbitals (FMO)
Weakly/Nonpolar solvents
Chloroform
Toluene
1,4-dioxane
Octane
Decane
Tetradecane
Hexadecane
Frontier molecular orbitals (FMO) amplitude plots of high-
est occupied molecular orbitals and lowest unoccupied
molecular orbitals of fuorophore CBZ-OXA-IV show in
Fig. 3 and corresponding data are summarized in Table 3.
As shown in Fig. 3 the electron density of highest occu-
pied molecular orbitals (HOMO) are mainly located on the
electron-donating carbazole moietie. On the other hand,
the lowest occupied molecular orbitals (LUMO) on the
electron-defcient 1,3,4-oxadiazole ring. This clearly indi-
cates that the presences of intramolecular charge transfer
(ICT) properties of the fuorophore CBZ-OXA-IV from
electron-donating carbazole group (D) to electron accepting
1,3,4-oxadiazole group (A) through phenyl π-linker, thereby
reducing to the D-π-A form. Almost complete separation
of HOMO/LUMO energy levels (Fig. 3) suggests that the
separation of electron cloud of HOMO/LUMO could aford
the transporting channels between hole and electrons, which
may endow the fuorophore CBZ-OXA-IV with the ability
of bipolar charge transport. The separation of the HOMO
and LUMO is efcient for charge carrier transfer and balance
in the emitter layers of OLEDs [50]. The theoretical HOMO
and LUMO energy levels of CBZ-OXA-IV were found to
be -5.82 and -1.76 eV, respectively. The theoretical band gap
(Eg) determined from the diference between the HOMO and
LUMO energy value from DFT calculations is found to be
4.06 eV (Table 3).
356
361
344
360
362
364
364
415/495
418/435
413/483
399/422
389/465
402/424
393/465
7887
4712
8365
4081
6118
3887
5967
λa Absorption maxima in nm, λf Emission maxima in nm and Stokes
shift in cm−1
peaks (Fig. 2(C)), which signify various intramolecular
processes such as protonated species, excimers, dimers,
excited state photoisomers and TICT [46]. Additionally, we
can also observe a fuctuation in shifts, which are irrespec-
tive of polarity values. This may be attributed to various
solvent–solute interactions. In case of highly polar aprotic
solvents, a bathochromic shift has been returned by the
system. Overall, the performance of this novel molecule is
useful in case of highly polar solvents (shown positive solva-
tochromism). This suggests that the molecule is a good opti-
cal material with a highly stabilized structure and molecular
orbitals. But the variations observed in emission properties
of the molecule in weakly/non polar solvents needs a higher
level of micro environmental study, which may be the scope
of our future work.
From Table4, by using the ground state optimized
molecular structure of compound CBZ-OXA-IV, the
dihedral angles (DA) [o] are calculated around the difer-
ent groups are found to be i/ii 56.96, ii/iii 0.04 and iii/iv
0.17. The large dihedral angle observed between the carba-
zole group and adjacent phenyl ring about 56.96 o indicates
the twisted nature of the carbazole plane from the rest of
Theoretical Computations
The investigation of frontier molecular orbitals (FMOs) is
recognized to be signifcant since they are directly connected
to the photophysical properties of the organic materials, such
1 3

Journal of Fluorescence
Fig. 3 Density functional theory
(DFT) computations: HOMOs
and LUMOs levels of bipolar
fuorophore CBZ-OXA-IV at
the CAM-B3LYP/6-311G (d,p)
level of theory in the gas phase
1,3,4-oxadiazole core. Large DA angles are benefcial for
suppressing quenching of their fuorescent caused by the
aggregation of molecules in the solid states [50].
bonding interactions. The calculated electrostatic poten-
tial surface (ESP) plot of CBZ-OXA-IV dye shown in
Fig. 4, computed using their ground state optimized
geometry on the corresponding molecular surface. In
addition, the obtained ESP maps demonstrate the three
dimensional charge distributions in the fluorophore.
In general, the areas of electrostatic potential in terms
of color increase from blue ˃ green ˃ yellow ˃ orange ˃
red [28]. The most electronegative potential (red color)
are mainly localized above the electron accepting 1, 3,
4-oxadiazole unit, thus indicating the abundance of elec-
trons in the red region of fluorophore. While, the most
Molecular Electrostatic Potential Surface (MESP)
Analysis
The molecular electrostatic potential (MESP) analysis is
an important tool to predict the molecular properties and
reactivity of the organic dyes. MEP surface plot helps
to exemplify chemical phenomenon such as reactiv-
ity, bonding, resonance, inductive effect and hydrogen
Table 3 Calculated frontier molecular energy levels and electrochemical properties of bipolar fuorophore CBZ-OXA-IV
[f]
HOMO[a]
LUMO[a]
Eoxonset [b] (eV)
HOMO[C]
LUMO[d]
E g
Eopt
[e]
Compound
(egV)
3.01
(eV)
(eV)
(eV)
(eV)
(eV)
CBZ-OXA-IV
-5.82
-1.76
1.28
-5.72
-2.71
4.06
[a]The HOMO and LUMO values from DFT calculations.
[b]The onset oxidation potential.
[
]
[c]Calculated HOMO from the onset oxidation potential of the compound EHOMO = − Eonset + 4.44 (eV).
ox
[
]
[d]Estimated LUMO using empirical equation ELUMO = − HOMO + Egopt (eV).
[e]Optical band gap energies were calculated from the equation Egopt = hc/λ=1240/λ (eV), where λ is the edge wavelength (in nm) of the UV–Vis
absorption spectrum.
[f]The theoretical band gap (Eg) determined from the diference between the HOMO and LUMO energy values from DFT calculations.
1 3

Journal of Fluorescence
Table 4 Dihedral angles (DA) [o] are calculated around carbazole group, phenyl and 1,3,4-oxadiazole group from ground state optimized molec-
ular structure of CBZ-OXA-IV
GS
DA i/ii
56.96
0.04
0.17
DA iii/iv
DA iv/v
positive electrostatic potential (blue color) localized
above the electron donating carbazole group and phenyl
rings. This gives the additional confirmation for the pres-
ence of strong intramolecular charge transfer properties
from donor (D) to acceptor (A) group of the π-conjugated
CBZ-OXA-IV dye.
with reference to the experimentally obtained data. How-
ever, the position and relative intensities of the peak justify
that both of these results can be compared and put together
to confrm the structural and vibrational properties of the
molecule. The simulated data leads the experimental values,
whose diference goes on decreasing as we move towards to
lower wavenumber region. The absorption due to C–O–C
asymmetric oscillation in oxadiazole ring has the minimum
shift. Since the oxadiazole ring is the dominant piece of
structure in the molecule, environmental efects and other
perturbations during experiment may have reduced, leading
to the minimum shift in the wavenumbers.
Comparison of IR Spectra
We have obtained optimized structure of CBZ-OXA-IV,
combined with frequency calculations to attain more stabi-
lized global minima. Here we consider studying the FT-IR
spectra which is indeed absorption spectrum due to various
signifcant vibrations in the molecule. In order to get more
precise results, comparison of simulated and experimental
outcomes can be considered. We have presented a combi-
nation of graphs put together in Fig. 5, so that the similari-
ties can be easily identifed. Both the plots have transmit-
tance peaks at relatively similar distances with an overall
shift due to diferences in the surroundings. The important
peaks which actually characterize the molecule are given
in Table 5 with comparison between experimental data and
simulated data obtained using DFT/CAM-B3LYP theory
with 6-311G (d,p) basis set. The compared data reveal that
there is a shift in DFT simulated data of around 50 cm−1
Natural Bond Orbital (NBO) Analysis
Natural bond orbital analysis stands one of the best methods
to understand the molecular orbitals very similar to the clas-
sical Lewis orbital structure [51]. It is a multi-step calcula-
tion process, in which the initial calculations consider each
atom as unbound and their atomic orbitals are taken. Such
simple atomic orbitals are then combined with the molecular
bonding and antibonding orbitals [51]. This efectively gives
the terminal step of determining the natural bond orbitals of
the molecule thus formed [51]. We did NBO calculations
1 3

Journal of Fluorescence
Fig. 4 Ground state (S0)
optimized geometry and MEP
surface plot for compound
CBZ-OXA-IV at CAM‐B3LYP
and 6‐311G (d,p) level of theory
Fig. 5 The experimental and
DFT calculated Infrared spectra
of fuorophore CBZ-OXA-IV
by the use of DFT/CAM-
B3LYP theory with 6‐311G
(d,p) basis set
1 3

Journal of Fluorescence
Table 5 Comparison of IR peaks of simulated and experimental data for CBZ-OXA-IV with assignment of their groups
FT-IR Peaks (cm−1
)
Assignment
Experimental
Simulated
3052
2955
1600
1547
1514
1227
1067
3090
3022
1654
1590
1542
1230
1078
C-H Aromatic
C-H Aliphatic
C–C Olefnic
C=C Aromatic
C=N Aromatic
C–O–C Asymmetric Oxadiazole ring
C–O–C Symmetric Oxadiazole ring
using DFT/CAM-B3LYP/6-311G (d,p) level in Gaussian 09
package to get this information. It also gives useful informa-
tion on intramolecular bonding and the interactions between
them, which can also be further extended to understand the
charge transfer process in the molecule [52].
Electrochemical Properties
In order to choose new organic materials for optoelectronic
devices/OLEDs applications, it is very important to know
the energy levels of highest occupied molecular orbital
(HOMO), lowest unoccupied molecular orbital (LUMO)
and the band gap between them. In this regard, the most
useful method to characterize the organic materials and esti-
mate the energy band gap is cyclic voltammetry. For organic
materials, HOMO represents the energy required to remove
an electron from a molecule, which is an oxidation process
and LUMO is the energy required to inject an electron to a
molecule, thus implying a reduction process [55]. Thus, the
electrochemical property of bipolar fuorophore CBZ-OXA-
IV was probed by cyclic voltammetry (CV). The cyclic vol-
tammogram of CBZ-OXA-IV is presented in Fig. 6 and
the corresponding electrochemical data are summarized in
Table 3. CV was carried out in a three-electrode cell with
a Pt counter electrode, an Ag/AgCl reference electrode and
a glassy carbon working electrode at a scan rate of 100
mVs−1 with 0.1 M nBu4NPF6 as the supporting electrolyte,
in anhydrous acetonitrile solution at room temperature under
N2 atmosphere. Fluorophore CBZ-OXA-IV showed oxida-
tion onset potential at 1.28 eV. The compound exhibits both
reversible oxidation and reduction behaviour, which sug-
gests its potential for efcient hole/electron transport. On the
basis of the obtained oxidation onset potential, the HOMO
and LUMO energy levels of fuorophore CBZ-OXA-IV is
calculated by using the Eqs. (2) and (3) [56].
The results of NBO can also be used to get the informa-
tion about interactions of flled and virtual orbitals, which
help in analysis of intra and intermolecular interactions. In-
order to get the donor–acceptor interactions through NBO,
we have employed second order Fock matrix as the base as
we have seen the same in few literature [53, 54]. By second
order perturbation theory, for every acceptor (j) and donor
(i), the stabilization energy (E (2)) associated with i → j
delocalization can be estimated to be,
(Fij)2
E(2) = ΔEij = q
(2)
i (Ej − Ei)
Here, (Fij)2 is the fock matrix element which is of-diagonal
between i and j, qi is the donor orbital occupancy and
Ei, Ej are the energies of diagonal elements.
In this particular case, we have considered the donor
acceptor pairs with distinctively high stabilization energy
(E (2)) only. The lists of all such important pairs are
enlisted in Table 6. Some notable donor–acceptor pairs are
π*(C54-C58)→π*(C64-C68) with an energy (E (2)) value
of 344.00 kcal/mol and π*(C53-N56) → π*(C61-C65)
with an energy of 344.76 kcal/mol. Some of the other
important donor → acceptor interactions are σ (Lone
pair N73) → π*(C54-C58) and π*(C53-C56), π (Lone
pair O18) → π*(C15-N16) and π*(N17-C19) with
energies with 42.68 and 42.70, 43.41 and 43.33 kcal/
mol respectively. The electron occupancy ratios and the
percentage of p and s-character in the above NBOs have
been collected in Table 7. We have observed almost 100%
p-character in the natural atomic orbitals of donors with
E(2) higher than 35 kcal/mol. The labels and symbols can
be associated with the optimized structure shown before
in Fig. 4.
[
]
ꢀHOMO = − Eonset + 4.44 eV
(
)
(3)
ox
[
]
ꢀLUMO = − HOMO + Eogpt (eV)
(4)
The HOMO energy level of CBZ-OXA-IV dye determined
from the onsets of the oxidation potential is found to be -5.72 eV
and the LUMO energy level of CBZ-OXA-IV dye is estimated
[
]
using empirical equation ELUMO = − HOMO + Egopt (eV) is
1 3

Journal of Fluorescence
1 3

Journal of Fluorescence
Table 7 NBO results exhibiting
the formation of Lewis and non-
Lewis orbital for CBZ-OXA-IV
with S and P orbital formation
percentage
Bond (A–B)
π* C54-C58
ED/energy (a.u.)
0.45585
EDA%
52.19
EDB%
47.81
NBO
s %
p %
0.7224(sp1.00)C54
-0.6915(sp1.00)C58
0.7224(sp1.00)C53
-0.6915(sp1.00)C56
0.6840(sp1.00)C20
-0.7295(sp1.00)C25
0.6460(sp1.00) N17
-0.7634(sp1.00)C19
0.7616(sp1.00)C15
-0.6481(sp1.00)N16
0.7087(sp1.00)C9
-0.7055(sp1.00) C10
0.7019(sp1.00)C4
-0.7122(sp1.00) C5
sp1.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
26.36
26.36
24.50
24.44
99.96
99.96
99.96
99.96
99.98
99.95
99.79
99.78
99.80
99.96
99.95
99.96
99.96
99.95
100.0
99.93
99.94
99.95
73.55
73.55
75.34
75.39
π* C53-C56
π* C20-C25
π* N17-C19
π* C15-N16
π* C9-C10
π* C4-C5
0.45586
0.37666
0.27824
0.28025
0.38182
0.36970
52.19
46.78
41.73
58.00
50.22
49.27
47.81
53.22
58.27
42.00
49.78
50.73
σ LP N73
π LP 018
π C23-C24
1.66034
1.71952
1.64854
-
-
-
-
sp1.00
48.37
51.86
0.6952(sp1.00) C23
-0.7189(sp1.00) C24
0.7727(sp2.79) C7
+0.6347(sp2.79) H48
0.7074(sp3.08) N16
+0.7068(sp3.08) N17
σ C7-H48
1.97585
1.97143
59.71
50.04
40.29
49.96
σ N16-N17
found to be -2.71 eV, indicating that deep HOMO/LUMO
energy levels possess good electron-transporting and the good
hole transporting properties. In addition, HOMO/LUMO
energy levels of CBZ-OXA-IV dye calculated from the DFT
computations were found to be -5.82 and -1.76 eV, respec-
tively (Table 3), which are in good agreement with the HOMO/
LUMO energy level values measured using CV. Also, it is inter-
esting to notice that the theoretical band gap (Eg) determined
from the diference between the HOMO and LUMO energy
values from DFT calculations is found to be 4.06 eV and is in
good agreement with the optical energy bandgap (Eogpt) esti-
mated from the absorption edge of UV–Vis spectra 3.01 eV.
These results revealed the facile reversible redox behaviour of
the synthesized bipolar fuorophore CBZ-OXA-IV and the
energy band gap was determined by both experimentally and
theoretically which is found to be an average of 3.53 eV, sug-
gesting that they can be applied as bipolar transport materials
for electroluminescence applications in optoelectronic devices/
OLEDs.
Thermal Properties
In order to use the organic materials for OLED/optoelec-
tronic device applications, the thermal stability of organic
materials is crucial factor in order to achieve high device
stability and lifetime. For example, thermal instability or low
glass transition temperatures Tg of an amorphous organic
layer may result in the faster degradation of organic devices
due to morphological changes and eventually device fail-
ure, fostered by the high temperatures that can occur during
device operation [57, 58]. In order to address such chal-
lenges, it is required to synthesise novel organic materials
having high thermal stability, glass transition temperatures
(Tg) and 5% weight loss temperature (T5d) that ensure stable
OLED/optoelectronic devices. The thermal stability of the
new CBZ-OXA-IV dye was investigated by thermogravi-
metric analysis (TGA) and diferential scanning calorimetry
Fig. 6 Cyclic voltammogram of bipolar fuorophore CBZ-OXA-IV
1 3

Journal of Fluorescence
Fig. 7 (A) DSC and (B) TGA curves of bipolar fuorophore CBZ-OXA-IV was carried out from 25–450 °C under N2 atmosphere with a heating
rate of 10 °C/min
(DSC) under N2 atmosphere at scan rates of 10 °C/min in
the temperature range from 25–450 °C, respectively. The
obtained results were shown in Fig. 7(A and B) and the
corresponding data were tabulated in Table 1. As shown
in Fig. 7(A), the DSC curve reveal that endothermic peak
is observed which correspond to melting point tempera-
ture (Tm) of the target fuorophore CBZ-OXA-IV found at
207 °C. The Tm of CBZ-OXA-IV was further confrmed by
melting point instrument to be 210–211 °C. In addition, no
exothermic peak corresponding to crystallization tempera-
ture (Tc) of the compound CBZ-OXA-IV was observed. In
DSC curve, the synthesized compound reveal endothermic
baseline shift due to the glass transition temperatures (Tg)
for the compound CBZ-OXA-IV at 98 °C. It is quite inter-
esting to notice that the glass transition temperatures (Tg) of
CBZ-OXA-IV higher than popular and most often employed
host materials in the literature, i.e., mCP (Tg =60 °C), CBP
(Tg = 62 °C), BCz1 (Tg = 67 °C), BCz3 (Tg = 77 °C) and
TCz1 (Tg = 88 °C). Also, it is important to mention that
the glass transition temperature (Tg) of CBZ-OXA-IV is
higher than those of the most popular oxadiazole-based
electron-transporting materials, such as, 2-(4-biphenylyl)-
5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, Tg =60 °C),
1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene
(OXD-7, Tg =77 °C) and is higher than the recently reported
non-bipolar host-emitters that contain oxadiazoles (BOBP,
Tg =45 °C) [59, 60]. The TGA curve reveal that the decom-
position temperature (T5d temperature, defned as the tem-
perature at which organic compound loses its 5% weight)
of bipolar fuorophore CBZ-OXA-IV under N2 atmosphere
was found at 387 °C. The high Tg and T5d temperature of the
CBZ-OXA-IV dyad may be due to a rigid and twisted struc-
ture of the carbazole branching unit attached to thermally
stable 1,3,4-oxadiazole group. These results suggest that
synthesized bipolar molecule CBZ-OXA-IV provide desir-
able thermal properties, which are favourable for use in the
OLED/organic electronic devices for fabrication of devices
via vacuum sublimation without any decomposition and also
as possible host materials in PHOLED.
Experimental Section
Fluorescence Quantum Yield (Фf)
The quantum yield (Φ) is an important measure of lumines-
cent/organic materials, which provides signifcant informa-
tion about the excited state, coupling of electronic to vibra-
tional states and radiationless transitions. Referring to the
number of emitted photons per absorbed photons, it is an
essential parameter that allows for primary classifcation of
materials and detailed analyses of materials. However, it is
also utilised to fnd out the appropriateness of laser material
for media, sample purity and chemical structure [16, 61].
The fuorescence quantum yield (Φf) of the bipolar CBZ-
OXA-IV dye is determined in ethanol solution by using a
comparison method relative to a standard dye Coumarin
480 (2,3,5,6-1H, 4H-tetrahydro-8-methylquinolazino-[9,
9a,l-gh] coumarin) (Фref = 0.95) according to the Eq. (4)
shown below [16]:
ꢀ2
Fu (ODs)
u
ꢀf = ꢀ
(5)
s F (OD )
2
ꢀs
s
u
where Φs is the quantum yield of the standard reference,
Fu and Fs are the integrated areas under the curve of
1 3

Journal of Fluorescence
emission spectra of unknown and standard sample,
ODu and ODs is the optical density of the unknown and
standard sample, ηu and ηs are the refractive index of
both unknown and standard sample. The fluorescence
quantum yield of CBZ-OXA-IV dyad is 0.40 in
ethanol solution as depicted in Table 1. In the CBZ-
OXA-IV dyad the presence of tert-butyl alkyl group
results in better solubility as well as carbazole and
1,3,4-oxadiazole groups connected through phenyl and
olefnic π spacer results in the extended π-π conjugation.
Due to the above reasons, it results in high fuorescence
quantum yield of the bipolar compound and making it an
excellent candidate for use in further applications like
optoelectronic devices and OLEDs, so on.
ethanol was charged into a single neck round-bottomed fask.
The reaction mixture temperature was raised to refux and con-
tinued for 10–12 h with stirring. After completion of the reac-
tion as monitored by TLC, the reaction mass was allowed to
cool to r.t. The reaction mixture was quenched with ice water
and the precipitate obtained was fltered to give required inter-
mediate 4-methylbenzohydrazide with 92% yield.
General Procedure for the Synthesis of Intermediate
4‑(9H‑cacrbazol‑9‑yl)benzaldehyde:
The intermediate 4-(9H-carbazol-9-yl)benzaldehyde was
synthesized according to the reported procedure [33]. A
solution of 4-fuorobenzaldehyde (0.222 g, 1.79 mmol),
carbazole (0. 30 g, 1.79 mmol), K2CO3 (500 mg) and DMSO
(10 mL) was heated at 150 ºC overnight. After completion
of the reaction as confrmed by thin layer chromatographic
analysis (TLC), the solution was cooled to room temperature
and water was added to the reaction mixture. The mixture
was extracted thrice with DCM (3 X 10 mL) and the organic
layer was separated and dried over anhydrous Na2SO4. After
the solvent was evaporated a yellow solid was obtained,
which is followed by recrystallization from EtOH yielding
intermediate compound 4-(9H-carbazol-9-yl)benzaldehyde
with 74% yield.
Materials and Synthesis
All required starting materials and reagents were pur-
chased from commercially available sources unless
otherwise stated and used without further purification.
Solvents used for synthesis and analytical/spectroscopic
measurements were pre-dried and freshly distilled over
appropriate drying reagents. The primary key interme-
diates ethyl 4-methylbenzoate, 4-methylbenzohydrazide
and 2-(4-(tert-butyl)phenyl)-5-(p-tolyl)-1,3,4-oxadiazole
were prepared according to the our previously published
method [31, 32]. Carbazole, 4-methylbenzoic acid,
4-(tert-butyl)benzoic acid and 4-fluorobenzaldehyde
were purchased from Sigma-Aldrich.
General Procedure
for the Synthesis of 2‑(4‑(tert‑butyl)
phenyl)‑5‑(p‑tolyl)‑1,3,4‑oxadiazole (I):
Compound 4-methylbenzohydrazide (4.00 g, 26.63 mmol)
and 4-(tert-butyl)benzoic acid (4.74 g, 26.63 mmol) were
added to a 100 mL one-neck round bottom fask containing
POCl3 (20 mL). The mixture was refuxed for about 12 h at
100 °C with constant stirring. And then reaction mixture was
cooled down to room temperature, the mixture was quenched
with crushed ice under efcient fume hood. The obtained
solid was fltered and washed with saturated sodium bicar-
bonate solution to remove unreacted carboxylic acid. The
crude solid was recrystallized from EtOH to obtain the pre-
ferred intermediate compound 2-(4-(tert-butyl)phenyl)-5-(p-
tolyl)-1,3,4-oxadiazole (I) in 86% as a white solid.
General Procedure for the Synthesis
of Intermediates Ethyl 4‑methylbenzoate
and 4‑methylbenzohydrazide:
To a suspension of 4-methylbenzoic acid (10.00 g,
73.50 mmol), dissolved in 80 mL of EtOH taken in a
250 mL RBF fitted with a dropping funnel and reflux
condenser. Thionyl chloride (5.83 mL, 80.85 mmol) was
added to the above reaction mixture slowly from the
dropping funnel for 30 min at room temperature under
constant stirring. The reaction mixture was refluxed for
8 h at 80 oC. The reaction mass was then cooled to room
temperature and quenched with ice water under efficient
fume hood. The resultant mixture was then extracted
with ethyl acetate (3 X 30 mL), organic layer was washed
with NaHCO3 solution and dried over anhydrous Na2SO4.
Then organic layer was evaporated by rotary evaporation
to afford intermediate compound ethyl 4-methylbenzoate
in 95% yield.
2‑(4‑(tert‑butyl)phenyl)‑5‑(p‑tolyl)‑1,3,4‑oxadiazole
(I):
Colour: White solid. Obtained yield: 86%. MP=150–152 °C.
1H NMR (500 MHz, CDCl3) δ: 8.06 (d, J=8.5 Hz, 2H), 8.03
(d, J=8.2 Hz, 2H), 7.55 (d, J=8.5 Hz, 2H), 7.34 (d, J=8.4 Hz,
2H), 2.44 (s, 3H), 1.37 (s, 9H); 13C NMR (126 MHz, CDCl3)
δ: 164.5, 155.0, 146.2, 138.4, 129.6, 127.4, 127.1, 125.6, 123.2,
123.1, 40.2, 31.5, 24.3; LC–MS: m/z calculated for C19H20N2O
292.38200, found 292.36491 (M+).
A mixture of ethyl 4-methylbenzoate (8.00 g, 48.72 mmol)
and hydrazine hydrate (2.60 mL, 53.59 mmol) in 60 mL
1 3

Journal of Fluorescence
General Procedure for the Synthesis
of 2‑(4‑(bromomethyl)phenyl)‑5‑(4‑(tert‑butyl)
phenyl)‑1,3,4‑oxadiazole (II):
General Procedure for the Synthesis
of (E)‑2‑(4‑(4‑(9H‑carbazol‑9‑yl)styryl)
phenyl)‑5‑(4‑(tert‑butyl)phenyl)‑1,3,4‑oxadiazole
(CBZ‑OXA‑IV):
A mixture of 2-(4-(tert-butyl)phenyl)-5-(p-tolyl)-1,3,4-
oxadiazole (I) (3.0 g, 10.26 mmol) and catalytic amount of
DBP (0.496 g, 0.2 equ) were added to CCl4 (30 mL). To the
above reaction mixture NBS (2.00 g, 11.28 mmol) was added
and the reaction mixture was heated for 6–8 h at 80 °C. And
then cooled and completion of the reaction was checked by
TLC. The solvent was removed via rotary evaporation to get
solid compound. Crude solid was washed repetitively with
H2O to remove excess succinamide. The obtained crude
solid was dried and recrystallized from ethanol to give pure
2-(4-(bromomethyl)phenyl)-5-(4-(tert-butyl)phenyl)-1,3,4-
oxadiazole (II) in 87% yield as a white solid.
A mixture of phosphonium salt (III) (0.30 g, 0.48 mmol)
and 4-(9H-carbazol-9-yl)benzaldehyde (0.131 g, 0.48 mmol)
were charged in a round bottom fask containing DCM:water
(60:40) mixture. The reaction mixture was vigorously stirred
and 50% NaOH (1 mL) was added dropwise to the mixture.
The reaction was lasted for another 10 h at room temperature
with continuous vigorous stirring. After the reaction
completed, 20 mL DCM and 20 mL water respectively, were
added to the reaction mixture. The organic layer was collected
and washed with brine three times. The organic solvent was
then dried over anhydrous Na2SO4 and removed by rotary
evaporator yielded a solid product. The crude product was
purifed by silica-gel column chromatography by using ethyl
acetate/hexane as eluent to get target compound (E)-2-(4-(4-
(9H-carbazol-9-yl)styryl)phenyl)-5-(4-(tert-butyl)phenyl)-
1,3,4-oxadiazole (CBZ-OXA-IV) in 83% yield as light
green colour solid. The synthetic method is simple and highly
efcient. The obtained target compound CBZ-OXA-IV is
the trans confguration across the double bond is confrmed
by 1H NMR spectroscopy. 1H NMR spectrum of the target
compound CBZ-OXA-IV exhibit a doublet signals about
6.78 and 7.39 δ ppm with a typical coupling of cal. 16.1 and
16.3 Hz, respectively. This value is in good agreement with
the expected value for a 3Jtrans coupling of the vinylic protons.
Further structural characteristics were confrmed by FT-IR,
13C NMR and ESI–MS.
2‑(4‑(bromomethyl)phenyl)‑5‑(4‑(tert‑butyl)
phenyl)‑1,3,4‑oxadiazole (II):
Colour: White solid. Obtained yield: 87%. MP=157–158 °C.
1H NMR (500 MHz, CDCl3) δ: 8.12 (d, J=8.2 Hz, 2H), 8.07
(d, J=8.5 Hz, 2H), 7.57–7.54 (d, 4H), 4.54 (s, 2H), 1.38 (s,
9H); 13C NMR (126 MHz, CDCl3) δ: 164.79, 163.88, 155.48,
141.34, 129.75, 127.31, 126.82, 126.10, 123.95, 120.98,
39.70, 35.13, 31.15; LC–MS: m/z calculated for C19H19BrN2O
371.27800, found 371.27610 (M+).
General Procedure for the Synthesis of Wittig Salt
(III):
2-(4-(bromomethyl)phenyl)-5-(4-(tert-butyl)phenyl)-1,3,4-
oxadiazole (II) (2.0 g, 5.40 mmol) and PPh3 (1.69 g,
6.48 mmol) were taken in ethyl acetate (20 mL) and stirred at
r.t. The resulting mixture was refuxed for about 8 h at 80 °C.
After the reaction, the obtained solid was fltered and washed
with excess of EtOAc to remove un-reacted triphenyl phos-
phine to get the intermediate compound phosphonium salt
(III) in 90% yield as white solid.
(E)‑2‑(4‑(4‑(9H‑carbazol‑9‑yl)styryl)
phenyl)‑5‑(4‑(tert‑butyl)phenyl)‑1,3,4‑oxadiazole
(CBZ‑OXA‑IV):
Colour: Light green colour. Obtained yield: 83%. MP:
210–211 °C. FT-IR (KBr, cm−1): 3052 (C-H aromatic),
2955 (C-H aliphatic), 1600 (C = C olefnic), 1547, 1514
(C=C and C=N aromatic), 1227, 1067 (C–O–C asym and
C–O–C sym, oxadiazole ring); 1H NMR (400 MHz, CDCl3)
δ: 8.17 (d, J = 4.2 Hz, 2H), 8.15 (d, J = 3.3 Hz, 2H), 8.14
– 8.03 (m, 4H), 7.79 (d, J=8.6 Hz, 2H), 7.73 (d, J=8.5 Hz,
2H), 7.61 (d, J = 8.6 Hz, 2H), 7.57 (d, J = 8.7 Hz, 2H),
7.39 (d, J=16.3 Hz, 1H), 7.32 (d, J=8.0 Hz, 2H), 7.28 (d,
Wittig Salt (III):
Colour: White solid. Obtained yield: 90%. 1H NMR
(500 MHz, CDCl3) δ: 7.94 (d, J=8.5 Hz, 2H), 7.87 – 7.74
(m, 12H), 7.63 (m, 5H), 7.50 (d, J=8.5 Hz, 2H), 7.38 (dd,
J=8.4, 2.6 Hz, 2H), 5.78 (d, J=15.3 Hz, 2H), 1.36 (s, 9H);
13C NMR (126 MHz, CDCl3) δ: 164.66, 163.69, 155.34,
134.98, 134.95, 134.61, 134.53, 132.57, 132.52, 131.65,
131.58, 130.17, 130.07, 126.75, 126.72, 126.68, 125.98,
123.55, 123.52, 120.72, 117.95, 117.26, 35.07, 31.11, 30.50,
30.13; LC–MS: m/z calculated for C37H34N2OP+ 553.24033,
found 553.25242 (M+).
J=8.5 Hz, 2H), 6.78 (d, J=16.1 Hz, 1H), 1.39 (s, 9H); 13
C
NMR (100 MHz, CDCl3) δ: 164.68, 164.25, 155.41, 140.69,
140.37, 135.86, 131.16, 130.37, 129.88, 129.56, 128.12,
127.26, 127.13, 126.81, 126.10, 126.01, 123.49, 123.01,
121.10, 120.37, 120.10, 109.81, 35.13, 31.12; ESI–MS: m/z
[M+H]+ calculated for C38H31N3O 546 found 546.
1 3

Journal of Fluorescence
under UGC-SRF (Sr. No. 2121510180 Ref. No. 20/12/2015) and
DST-Purse Phase-II program for providing necessary facilities. The
authors, Tilakraj T S is thankful to UGC-JRF for funding (Sr. No.
2061651259 Dated. 26/10/2017) and Mallikarjun K. Patil would like to
thank KSTePS Govt. of Karnataka for Providing DST-Ph.D fellowship.
Conclusion
In conclusion, we have designed and synthesized a novel
π-conjugated carbazole-1,3,4-oxadiazole-based D-π-A type
bipolar fuorophore CBZ-OXA-IV. The dye CBZ-OXA-IV
emits intense deep blue fuorescence with quantum yields
(Φ) 0.40 and large Stokes shifts about 7052 cm−1, which
is comparable to the reported Cz–OXD dyads having blue
emission bands with fuorescence quantum yields in the
range of 0.80 to 0.99 [23]. Optical band gap (Egopt) value
of dye CBZ-OXA-IV is 3.01 eV. It is noteworthy that the
Egopt for CBZ-OXA-IV is lower than the popular and most
often employed host materials, i.e., mCP (Egopt = 3.56eV)
and CBP (Egopt = 3.49eV) [60]. The electrochemical proper-
ties of CBZ-OXA-IV dye have been investigated in detail
by using cyclic voltammetry (CV) measurements. Inter-
estingly, HOMO/LUMO energy levels of CBZ-OXA-IV
dye calculated from the DFT computations were found to
be -5.82 and -1.76 eV, respectively, which are good agree-
ment with the HOMO/LUMO energy level values measured
using CV to be -5.72 and -2.71 eV, respectively. Almost
complete separation of HOMO/LUMO energy levels is ef-
cient for charge carrier transfer and balance in the emitter
layers of OLEDs [50], which may endow the fuorophore
CBZ-OXA-IV with the ability of bipolar charge transport.
The glass transition temperatures (Tg) for the compound
CBZ-OXA-IV at 98 °C. It is quite interesting to notice
that the glass transition temperatures (Tg) of CBZ-OXA-
IV higher than popular and most often employed host
materials in the literature, i.e., mCP (Tg = 60 °C), CBP
(Tg = 62 °C), BCz1 (Tg = 67 °C), BCz3 (Tg = 77 °C) and
TCz1 (Tg = 88 °C). Also, it is important to mention that
the glass transition temperature (Tg) of CBZ-OXA-IV is
higher than those of the most popular oxadiazole-based
electron-transporting materials, such as, 2-(4-biphenylyl)-
5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, Tg =60 °C),
1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene
(OXD-7, Tg = 77 °C) and is higher than the recently
reported non-bipolar host-emitters that contain oxadiazoles
(BOBP, Tg =45 °C) [59, 60]. The decomposition tempera-
tures (Td5%) for the compound CBZ-OXA-IV at 387 °C.
The obtained overall results demonstrated that the novel
bipolar fuorophore CBZ-OXA-IV is a potential candidate
for optoelectronics applications and possibly can be applied
as bipolar transport materials in the feld of optoelectronic
devices/OLEDs.
Authors' Contributions Mahesh S. Najare: Conceptualization, design
of study and Writing-Original draft preparation. Mallikarjun K. Patil:
Acquisition of data and Writing-Original draft preparation. Tilakraj T
S: Analysis, interpretation of data and Formal analysis. Mohammed
Yaseen: Resources and Software. AfraQuasar A. Nadaf: Data Cura-
tion and Validation. Shivaraj Mantur: Analysis and interpretation of
data. Sanjeev R. Inamdar: Visualization, Investigation, Reviewing and
Editing. Imtiyaz Ahmed M. Khazi: Supervision.
Funding No specifc funding was received for this project. Mahesh
Sadashivappa Najare is supported by a University Grants Commission
(UGC), New Delhi, India, under UGC-SRF (Sr. No. 2121510180 Ref.
No. 20/12/2015). All authors confrm their work is independent from
the funders.
Availability of Data and Materials All data generated or analysed during
this study are included in supplementary information fle.
Declarations
Conflicts of Interest Authors declare that there are no conficts of in-
terest.
References
1. Belavagi NS, Deshapande N, Pujar GH, Wari MN, Inamdar SR,
Khazi IM (2015) Design, Synthesis and Optoelectronic Prop-
erties of Unsymmetrical Oxadiazole Based Indene Substituted
Derivatives as Deep Blue Fluoroscent Materials. J Fluoresc
25:1323–1330
2. Zhao Y, Guo Q, Li X, Wang Q, Ma D (2017) Infuence of the link-
age mode and D/A ratio of carbazole/oxadiazole based host mate-
rials on phosphorescent organic light-emitting diodes. J Lumin
188:612–619
3. Tan Y, Rui B, Li J, Zhao Z, Liu Z, Bian Z, Huang C (2019)
Blue thermally activated delayed fuorescence emitters based
on a constructing strategy with diversed donors and oxadiazole
acceptor and their efcient electroluminescent devices. Opt Mater
94:103–112
4. Kim JH, Park JW (2017) Designing an electron-transport layer
for highly efcient, reliable, and solution-processed organic light-
emitting diodes. J Mater Chem C 5:3097–3106
5. Arunchai R, Sudyoadsuk T, Prachumrak N, Namuangruk S,
Promarak V, Sukwattanasinitta M, Rashatasakhon P (2015)
Synthesis and characterization of new triphenylamino-1,8-
naphthalimides for organic light-emitting diode applications. New
J Chem 39:2807–2814
6. Ko KJ, Lee HB, Kim HM, Lee GJ, Shin SR, Kumar N, Song YM,
Kang JW (2018) High-performance, color-tunable fber shaped
organic light-emitting diodes. Nanoscale 10:16184–16192
7. Jong FD, Daniels M, Castillo LV, Kennes K, Martín C, Miguel
GD, Cano M, Morales MP, Hofkens J, Dehaen W, Auweraer MV
(2019) 5,10-Dihydrobenzo[a]indolo[2,3-c]carbazoles as Novel
OLED Emitters. J Phys Chem B 123:1400–1411
Supplementary Information The online version contains supplemen-
tary material available at https://doi.org/10.1007/s10895-021-02778-1.
Acknowledgements 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 Com-
mission (UGC), New Delhi, India, for providing fnancial support
8. Li Z, Li W, Keum C, Archer E, Zhao B, Slawin AM,
Huang W, Gather MC, Samuel IW, Colman EZ (2019)
1 3

Journal of Fluorescence
Volyniuk D, Grazulevicius JV (2018) 1,3,5-Triazine and carbazole
derivatives for OLED applications. Dyes Pigm 149:804–811
25. Chen P, Zhu H, Kong L, Tian Y, Yang J (2019) Efect of solid-
state packing on the photophysical properties of two novel carba-
zole derivatives containing tetraphenylethylene and cyano groups.
J Lumin 212:212–218
1,3,4-Oxadiazole-based Deep Blue Thermally Activated Delayed
Fluorescence Emitters for Organic Light Emitting Diodes. J Phys
Chem C 123:24772–24785
9. Marghad I, Bencheikh F, Wang C, Manolikakes S, b Rerat A,
Gosmini C, Kim DH, Ribierre JC and Adachi C, (2019) Control of
the dual emission from a thermally activated delayed fuorescence
emitter containing phenothiazine units in organic light-emitting
diodes. RSC Adv 9:4336–4343
26. Kadam MM, Patil D, Sekar N (2018) Carbazole based NLOphoric
styryl dyes- synthesis and study of photophysical properties by
solvatochromism and viscosity sensitivity. J Lumin 202:212–224
27. Kim CY, Lee C, Kim HJ, Hwang JH, Godumala M, Jeong J, Woo
HY, Cho MJ, Park S, Choi DH (2020) Achievement of High
Efciency with Extremely Low Efciency Roll-of in Solution-
Processed Thermally Activated Delayed Fluorescence OLEDs
manufactured using Xanthone-based Bipolar Host Materials. J
Mater Chem C 8:6780–6787
10. Cias P, Slugovc C, Gescheidt G (2011) Hole Transport in Triph-
enylamine Based OLED Devices: From Theoretical Modeling to
Properties Prediction. J Phys Chem A 115:14519–14525
11. Gefroy B, Roy PL, Prat C (2006) Organic light-emitting diode
(OLED) technology: materials, devices and display technologies.
Polym Int 55:572–582
12. Jou JH, Kumar S, Agrawal A, Lia TH, Sahooa S (2015)
Approaches for fabricating high efciency organic light emitting
diodes. J Mater Chem C 3:2974–3002
28. Babu DD, Naik P, Keremane KS (2020) A simple D-A-π-A con-
fgured carbazole based dye as an active photo-sensitizer: A com-
parative investigation on diferent parameters of cell. J Mol Liq
310:113189
13. Najare MS, Patil MK, Nadaf AA, Mantur S, Garbhagudi M,
Gaonkar S, Inamdar SR, Khazi IM (2020) Photophysical, thermal
properties, solvatochromism and DFT/TDDFT studies on novel
conjugated D-A-p-A-D form of small molecules comprising thio-
phene substituted 1,3,4-oxadiazole. J Mol Struct 1199:127032
14. Bodedla GB, Thomas KJ, Kumar S, Jou JH, Li CJ (2015)
Phenothiazine-based bipolar green-emitters containing benzimidazole
units: synthesis, photophysical and electroluminescence properties.
RSC Adv 5:87416–87428
29. Tao Y, Gong S, Wang Q, Zhong C, Yang C, Qina J, Ma D (2010)
Morphologically and electrochemically stable bipolar host for
efcient green electrophosphorescence. Phys Chem Chem Phys
12:2438–2442
30. Huda MK, Dolui SK (2010) Luminescence property of poly(1,3-
bis(phenyl-1,3,4-oxadiazole))s containing polar groups in the
main chain. J Lumin 130:2242–2246
31. Najare MS, Patil MK, Nadaf AA, Mantur S, Inamdar SR, Khazi
IM (2019) Synthesis, characterization and photophysical proper-
ties of a new class of pyrene substituted 1,3,4-oxadiazole deriva-
tives. Opt Mater 88:256–265
15. Li Y, Xu Z, Zhu X, Chen B, Wang Z, Xiao B, Lam JW, Zhao Z,
Ma D, Tang BZ (2019) Creation of Efcient Blue Aggregation-
Induced Emission Luminogens for High-Performance Nondoped
Blue OLEDs and Hybrid White OLEDs. ACS Appl Mater Inter-
faces 11:17592–17601
32. Deshapande N, Pujar GH, Sunagar MG, Gaonkar S, Belavagi NS,
Inamdar SR, Bathula C, Khazi IM (2017) Synthesis and Opto-
electronic Exploration of Highly Conjugated 1,3,4-Oxadiazole
Containing Donor-p-Acceptor Chromophores. ChemistrySelect
2:1793–1801
16. Pramod AG, Renuka CG, Nadaf YF, Rajaramakrishna R (2019)
Impact of solvents on energy gap, photophysical, photometric
properties for a new class of 4-HCM coumarin derivative: Non-
linear optical studies and optoelectronic applications. J Mol Liq
292:111383
33. Zheng Y, Batsanov AS, Jankus V, Dias FB, Bryce MR, Monkman
AP (2011) Bipolar Molecules with High Triplet Energies:
Synthesis, Photophysical, and Structural Properties. J Org Chem
76:8300–8310
17. Goel A, Kumar V, Singh SP, Sharma A, Prakash S, Singh C,
Anand RS (2012) Non-aggregating solvatochromic bipolar
benzo[f]quinolines and benzo[a]acridines for organic electron-
ics. J Mater Chem 22:14880–14888
34. Leong YS, Ker PJ, Jamaludin MZ, Nomanbhay SM, Ismail A,
Abdullah F, Looe HM, Lo CK (2018) UV-Vis Spectroscopy: A
New Approach for Assessing the Color Index of Transformer
Insulating Oil. Sensors 18:2175
18. Priyanka B, Anusha V, Bhanuprakash K (2015) Toward Designing
Efcient Multifunctional Bipolar Molecules: DFT Study of Hole
and Electron Mobilities of 1,3,4-Oxadiazole Derivatives. J Phys
Chem C 119:12251–12261
35. Tagare J, Boddula R, Kumar Yadav RA, Dubey DK, Jou J-H, Patel
S, Vaidyanathan S (2021) Novel Imidazole-Alkyl Spacer-Carbazole
based Fluorophores for Deep-Blue Organic Light Emitting
Diodes: Experimental and Theoretical Investigation. Dyes Pigm
185:108853. https://doi.org/10.1016/j.dyepig.2020.108853
36. Peng X, Song F, Lu E, Wang Y, Zhou W, Fan J, Gao Y (2005)
Heptamethine Cyanine Dyes with a Large Stokes Shift and Strong
Fluorescence: A Paradigm for Excited-State Intramolecular
Charge Transfer. J AM CHEM SOC 127:4170–4171
19. Zhao F, Ma D (2017) Approaches to high performance white
organic light-emitting diodes for general lighting. Mater Chem
Front 1:1933–1950
20. Deksnys T, Simokaitiene J, Keruckas J, Volyniuk D, Bezvikonnyi
O, Cherpak V, Stakhira P, Ivaniuk K, Helzhynskyy I, Baryshnikov
G, Minaevc B, Grazulevicius JV (2017) Synthesis and characteri-
sation of a carbazole-based bipolar exciplex-forming compound
for efcient and color-tunable OLEDs. New J Chem 41:559–568
21. Hwang J, Yoon J, Kim CY, Choi S, Kang H, Kim JY, Yoon
DW, Han CW, Park S, Cho MJ, Choi DH (2020) Structural
isomers of 9-(pyridin-2-yl)-9H-carbazole in combination with
9’H-9,3’:6’,9”-tercarbazole and their application to high efciency
solution processed green TADF OLEDs. Dyes Pigm 179:108403
22. Hung WY, Tu GM, Chen SW, Chi Y (2012) Phenylcarbazole-
dipyridyl triazole hybrid as bipolar host material for
phosphorescent OLEDs. J Mater Chem 22:5410–5418
23. Linton KE, Fisher AL, Pearson C, Fox AM, Palsson LO, Bryce MR,
Petty MC (2012) Colour tuning of blue electroluminescence using
bipolar carbazole–oxadiazole molecules in single-active-layer organic
light emitting devices (OLEDs). J Mater Chem 22:11816–11825
24. Zassowski P, Ledwon P, Kurowska A, Herman AP, Lapkowski
M, Cherpak V, Hotra Z, Turyk P, Ivaniuk K, Stakhira P, Sych G,
37. Iliashenko RY, Borodin OO, Wera M, Doroshenko AO (2015)
2,5-bis[2-(2-phenyl-1,3-oxazol-5-yl)phenyl]-1,3,4-oxadiazole –
new sterically hindered high Stokes shift fuorophore sensitive to
media viscosity. J Photochem Photobiol A 298:68–77
38. Kubota Y, Matsui M, Funabiki K, Kasatani K, Takai H (2015)
Strategy to enhance solid-state fuorescence and aggregation-
induced emission enhancement efect in pyrimidine boron com-
plexes. Dalton Trans 44:3326–3341
39. Duan C, Zhou Y, Shan G, Chen Y, Zhao W, Yuan D, Zeng L,
Huang X, Niu G (2019) Bright solid-state red-emissive BODIPYs:
facile synthesis and their high-contrast mechanochromic property.
J Mater Chem C 7:3471–3478
40. Najare MS, Patil MK, Garbhagudi M, Yaseen M, Inamdar SR,
Khazi IM (2021) Design, synthesis and characterization of
1 3

Journal of Fluorescence
50. Zhang L, Bai Y, Liu Z, Jiang W, Lei T, Yang R, Islam A, Zhang
Y, Ouyang X, Ge Z (2017) Substituted Diindenopyrazinediones
with Polyalkyl-carbazole symmetrically for high-efciency bluish
green solution-processable OLED. Dyes Pigm 142:544–551
51. Glendening ED, Landis CR, Weinhold F (2012) Natural bond
orbital methods. WIREs Comput Mol Sci 2:1–42
π-conjugated 2,5-diphenylsubstituted-1,3,4-oxadiazole-based
D-π-A-π’-D′ form of efcient deep blue functional materials:
Photophysical properties and fuorescence “Turn-of” chemsen-
sors approach. J Mol Liq 328:115443
41. Najare MS, Patil MK, Mantur S, Nadaf AA, Inamdar SR, Khazi
IM (2018) Highly conjugated D-π-A-π-D form of novel benzo[b]
thiophene substituted 1,3,4-oxadiazole derivatives; Thermal,
optical properties, solvatochromism and DFT studies. J Mol Liq
272:507–519
52. Weinhold F, Landis CR (2001) Natural bond orbitals and exten-
sions of localized bonding concepts. Chem Educ Res Pract
2:91–104
53. Szafran M, Komasa A, Adamska EB (2007) Crystal and molecular
structure of 4-carboxypiperidinium chloride (4-piperidinecarboxylic
acid hydrochloride). J Mol Struct 827:101–107
42. Botelho AL, Shin Y, Liu J, Lin X (2014) Structure and Opti-
cal Bandgap Relationship of p-Conjugated Systems. PLoS One
9:e86370. https://doi.org/10.1371/journal.pone.0086370
43. Chouk R, Bergaoui M, Jaballah N, Majdoub M, Khalfaoui M
(2019) Shedding light on structural, optoelectronic and charge
transport properties of PPV stereoisomers for multilayer OLED
application: A frst principle computational studies. J Mol Liq
284:193–202
54. Xavier RJ, Gobinath E (2012) Experimental and theoretical spec-
troscopic studies, HOMO–LUMO, NBO and NLMO analysis of
3,5-dibromo-2,6-dimethoxy pyridine. Spectrochim Acta A Mol
Biomol Spectrosc 97:215–222
55. Leonat L, Sbarcea G, Branzoi IV (2013) Cyclic voltammetry for
energy levels estimation of organic materials. U P B Sci Bull
Series B 75:111–118
44. Marini A, Losa AM, Biancardi A, Mennucci B (2010) What is
Solvatochromism? J Phys Chem B 114:17128–17135
56. Deshapande N, Belavagi NS, Sunagar MG, Gaonkar S, Pujar GH,
Wari MN, Inamdar SR, Khazi IM (2015) Synthesis, characteriza-
tion and optoelectronic investigations of bithiophene substituted
1,3,4-oxadiazole derivatives as green fuorescent materials. RSC
Adv 5:86685–86696
45. Kadam MM, Patil DS, Sekar N (2019) Red emitting coumarin
based 4, 6-disubstituted-3-cyano-2-pyridones dyes – Synthesis,
solvatochromism, linear and non-linear optical properties. J Mol
Liq 276:385–398
46. Math NN, Naik LR, Suresh HM, Inamdar SR (2006) Dual fuo-
rescence and laser emissions from fuorescein-Na and eosin-B.
Journal of Luminescence J Lumin 121:475–487
57. Prachumrak N, Pojanasopa N, Tarsang R, Namuangruk S,
Jungsuttiwong S, Keawin T, Sudyoadsuk T, Promarak V
(2014) Synthesis and characterization of carbazole dendronized
coumarin derivatives as solutionprocessed non-doped emitters
and holetransporters for electroluminescent devices. New J Chem
38:3282–3294
47. Liu Y, Sun X, Wang Y, Wu Z (2014) The infuence of num-
bers and ligation positions of the triphenylamine unit on the
photophysical and electroluminescent properties of homoleptic
iridium (III) complexes: a theoretical perspective. Dalton Trans
43:11915–11924
58. Torres E, Lukasik RB, Santos MN, Hofe S, Colsmannd A, Brites
MJ (2016) N, N’-Diaryl-perylene-3,9-diamine derivatives: syn-
thesis, characterization and electroluminescence properties. RSC
Adv 6:107180–107188
48. Velmurugan G, Venuvanalingam P (2015) Luminescent Re(I)
terpyridine complexes for OLEDs: what does the DFT/TD-DFT
probe reveal? Dalton Trans 44:8529–8542
59. Venkatakrishnan P, Natarajan P, Moorthy JN, Lin Z, Chow TJ
(2012) Twisted bimesitylene-based oxadiazoles as novel host
materials for phosphorescent OLEDs. Tetrahedron 68:7502–7508
60. Tsai MH, Hong YH, Chang CH, Su HC, Wu CC, Matoliukstyte A,
Simokaitiene J, Grigalevicius S, Grazulevicius JV, Hsu CP (2007)
3-(9-Carbazolyl)carbazoles and 3,6-Di(9-carbazolyl)carbazoles as
Efective Host Materials for Efcient Blue Organic Electrophos-
phorescence. Adv Mater 19:862–866
49. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman AR, Scalmani G, Barone V, Mennucci B, Petersson
GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF,
Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K,
Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao
O, Nakai H, Vreven T, Montgomery JA, Peralta JE, Ogliaro F,
Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN,
Keith T, Kobayashi R, Normand J, Raghavachari K, Rendell A,
Burant JC, Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM,
Klene M, Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J,
Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R,
Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski
VG, Voth GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels
AD, Farkas O, Foresman JB, Ortiz JV, Cioslowski J and Fox DJ,
Gaussian, Inc., Wallingford CT (2013)
61. Fries F, Reineke S (2019) Statistical treatment of Photolumines-
cence Quantum Yield Measurements. Sci Rep 9:15638. https://
doi.org/10.1038/s41598-019-51718-4
Publisher's Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
1 3