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

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

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 (E_g^opt) value is found to be 3.01 eV and the

fuorescence 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 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 (T_d5%) 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 decom position 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 t he 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

o f 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 pr operty, good thermal

and photochemical stability [24–27]. D-A configured

carbazole based dyes used as activ e 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, thereb y increasing the quantum yield of

fluorescence (Φ_f) [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 (E_g^opt) and fluorescence quantum yields (Ф_f)

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 POCl₃and

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 CCl₄heating 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 K₂CO₃were 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 spectr um 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 ꢀ^absand

emi

max

ꢀ

values and it was found to be 7052 cm⁻¹. Us^mu^aa^xlly 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, OFET s, organic lasers and

solid-state fuorescence sensors [38, 39]. On the other hand,

organic materials that exhibit fuor escence 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 f orm 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 th^me^axabsorption spectrum of CBZ-OXA-IV dye

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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 (E^o_g^pt), fuorescence quantum yields (Φ_f) and thermal properties of bipo-

lar fuorophore CBZ-OXA-IV

T_g[^oC]^[e]

T_m[^oC]^[e]

T_c[^oC]^[e]

T_5d[^oC]^[e]

E_g^opt

emi

max

emi

max

abs

max

[d]

Compound

Stokes shift

Quantum

ꢀ

(cm⁻¹

)

Yield ( Φ_f)

(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 (E_g^opt) were calculated from the long edge of the absorption spectrum using E_g^opt= hc/λ=1240/λ (eV).

^[e]Results from DSC and TGA measurements; T_g-glass transition temperature, T_m-melting temperature, T_c-crystallization temperature and T_5d-5%

weight loss temperature. Ref-Coumarin 480.

1 3

Journal of Fluorescence

Optical Band Gap (ꢀ_ꢄ^ꢁꢂꢃ

)

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 (E_g^opt) 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 E_g^optprovide

basics in the understanding of the optical as well as the elec-

trical process occurring within the organic materials [40].

The value of E_g^optdescribes 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 (E_g^opt) 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 (E_g^opt) of

the bipolar fuorophore CBZ-OXA-IV is estimated from the

absorption edge (long wavelength edge) of the correspond-

ing absorption spectr a (λ_onset, solution) to the baseline make

use of the Eq. (1) [43] and the corresponding value is tabu-

lated in Table 1 .

1240

E_g^opt

=

(1)

ꢀonset

The optical band gap (E_g^opt) value of the bipolar fuoro-

phore CBZ-OXA-IV is found to be 3.01 eV. The low opti-

cal band gap (E_g^opt) 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

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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

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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 g ap) 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

λ_a

λ_f

Stokes

shift

Polar protic solvents

Methanol

Ethanol

Propanol

Butanol

Pentanol

Hexanol

Octanol

Nonanol

346

353

356

358

359

361

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

(E_g) 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

415/495

418/435

413/483

399/422

389/465

402/424

393/465

7887

4712

8365

4081

6118

3887

5967

λ_aAbsorption maxima in nm, λ_fEmission 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 ^oindicates

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]

E_ox^{onset [b]}(eV)

HOMO^[C]

LUMO^[d]

E _g

E^opt

[e]

Compound

(e^gV)

3.01

(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 E_HOMO= − E^onset+ 4.44 (eV).

ox

[

]

^[d]Estimated LUMO using empirical equation E_LUMO= − HOMO + E_g^opt(eV).

^[e]Optical band gap energies were calculated from the equation E_g^opt= hc/λ=1240/λ (eV), where λ is the edge wavelength (in nm) of the UV–Vis

absorption spectrum.

^[f]The theoretical band gap (E_g) 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⁻¹with 0.1 M nBu₄NPF₆as the supporting electrolyte,

in anhydrous acetonitrile solution at room temperature under

N₂atmosphere. 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 fuor ophor e 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 matr ix as the base as

we have seen the same in few literature [53, 54]. By second

order perturbation theory, for every accept or (j) and donor

(i), the stabilization energy (E (2)) associated with i → j

delocalization can be estimated to be,

(F_ij)²

E(2) = ΔE_ij= q

(2)

ⁱ(E_j− E_i)

Here, (F_ij)²is the fock matrix element which is of-diagonal

between i and j, q_iis the donor orbital occupancy and

E_i, E_jare 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= − E^onset+ 4.44 eV

(

)

(3)

ox

[

]

ꢀ_LUMO= − HOMO + E^o_g^pt(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 E_LUMO= − HOMO + E_g^opt(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(sp^1.00)C54

-0.6915(sp^1.00)C58

0.7224(sp^1.00)C53

-0.6915(sp^1.00)C56

0.6840(sp^1.00)C20

-0.7295(sp^1.00)C25

0.6460(sp^1.00) N17

-0.7634(sp^1.00)C19

0.7616(sp^1.00)C15

-0.6481(sp^1.00)N16

0.7087(sp^1.00)C9

-0.7055(sp^1.00) C10

0.7019(sp^1.00)C4

-0.7122(sp^1.00) C5

sp^1.00

0.00

26.36

24.50

24.44

99.96

99.98

99.95

99.79

99.78

99.80

99.96

99.95

99.96

99.95

100.0

99.93

99.94

99.95

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

-

sp^1.00

48.37

51.86

0.6952(sp^1.00) C23

-0.7189(sp^1.00) C24

0.7727(sp^2.79) C7

+0.6347(sp^2.79) H48

0.7074(sp^3.08) N16

+0.7068(sp^3.08) 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 (E_g) 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 (E^o_g^pt) 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 t o synthesise novel organic materials

having high thermal stability, glass transition temperatures

(Tg) and 5% weight loss temperature (T_5d) 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 N₂atmosphere with a heating

rate of 10 °C/min

(DSC) under N₂atmosphere 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 (T_m) of the target fuorophore CBZ-OXA-IV found at

207 °C. The T_mof 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 (T_c) 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 hos t-emitters that contain oxadiazoles (BOBP,

Tg =45 °C) [59, 60]. The TGA curve reveal that the decom-

position temper ature (T_5dtemperature, defned as the tem-

perature at which organic compound loses its 5% weight)

of bipolar fuorophore CBZ-OXA-IV under N₂atmosphere

was found at 387 °C. The high Tg and T_5dtemperature 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 (Ф_f)

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 mater ial

for media, sample purity and chemical structure [16, 61].

The fuorescence quantum yield (Φ_f) 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

F_u(OD_s)

u

ꢀ_f= ꢀ

(5)

^sF (OD )

2

ꢀ_s

s

u

where Φ_sis the quantum yield of the standard reference,

F_uand F_sare the integrated areas under the curve of

1 3

Journal of Fluorescence

emission spectra of unknown and standard sample,

OD_uand OD_sis the optical density of the unknown and

standard sample, η_uand η_sare 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-buty l)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 NaHCO₃solution and dried over anhydrous Na₂SO_4.

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 Na₂SO₄and 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 ³J_transcoupling 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 (E_g^opt) value

of dye CBZ-OXA-IV is 3.01 eV. It is noteworthy that the

E_g^optfor CBZ-OXA-IV is lower than the popular and most

often employed host materials, i.e., mCP (E_g^opt= 3.56eV)

and CBP (E_g^opt= 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 hos t-emitters that contain oxadiazoles

(BOBP, Tg =45 °C) [59, 60]. The decomposition tempera-

tures (T_d5%) for the com pound 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.

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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

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