chapter iv synthesis, characterization and optoelectronic...
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Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
230 | P a g e
4.1 INTRODUCTION
Organic Light-Emitting Diodes:
Organic light-emitting diodes (OLEDs) have generated a large interest in the
research community for the last twenty years. OLEDs consist of a thin film of an organic
compound placed between two electrodes. By applying voltage to the electrodes, charges
get injected into the organic material where they form excited states that recombine and
generate light. In the past decade, OLED displays have become commercially available in
portable small electronics applications, such as mobile phones, MP3 players, car radios,
digital cameras, and TVs. OLED displays are especially suited for such applications
because of their reduced power consumption compared to LCDs. In OLED displays, only
active pixels are turned on while inactive pixels do not use any power, whereas LCD
displays require the same power for their backlight independent of whether a black or a
white picture is shown. Furthermore, since the color of an OLED can be tuned, no filters
are necessary in the fabrication of an OLED display and very thin displays can be
fabricated, which is another important factor for portable devices that have to be packed
as efficiently as possible.
Whereas small OLED displays can be produced cost-efficiently nowadays and
Samsung has already shown prototypes of OLED displays with diagonals up to 50 inches,
the fabrication of larger displays is still very cost-intensive and will have to be improved
significantly if OLEDs should ever become competitive with other technologies in the
computer or TV display market. Nevertheless, the first commercial OLED TV has been
introduced to the market by Sony Corp. (Figure 4.1).1 However, the price tag of $2,500
for this 11-inch TV at the time of its introduction into the market in the year 2008 is
nowhere near competitive to other display technologies.
Advantages of OLEDs:
OLEDs have many advantages over other display technologies. For example,
OLEDs are very thin devices with the thickness of the organic layers in the range of about
100 nm. As mentioned, no backlight or color filters are needed for OLED displays either,
which leads to unusually thin displays like the Sony TV with a display thickness of 3 mm
and Sony’s newest prototypes with a display thickness of only 0.3 mm.1 Because of the
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
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small thickness of these devices, displays can also be made much lighter, and the main
weight comes from the device substrate.
Fig. 4.1: Sony XEL-1 OLED TV
Furthermore, the close relationship of organic materials to plastics and the
thinness of the devices allow for flexibility and thus make OLEDs compatible with plastic
substrates. Much work has therefore been done on flexible substrates and first prototypes
of flexible color displays have already been shown.1 Because of the vertical device
structure of an OLED, with electrodes on top and on the bottom of the device, OLEDs
also have the advantage that they are theoretically not limited in the lateral dimensions.
However, with current fabrication processes OLED devices with an area of only up to
100 cm2 seem feasible.
2
OLED devices show high brightness that is suitable for display applications as
well as for lighting. The direct emission of every single pixel also leads to wide viewing
angle with every angle receiving the same amount of light (Lambertian emitter), which
makes OLEDs stand out compared to LCD displays with an increasing but still limited
viewing angle.
The biggest disadvantage of OLEDs is their degradation in air. Hence, proper
encapsulation with very low leakage of oxygen and moisture is needed. For a long time,
even the lifetime of OLEDs in inert atmosphere was considered a serious issue. However,
by optimizing the materials and the device structures, OLED lifetimes have now reached
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
232 | P a g e
a point where their lifetime is comparable or exceeds the expected lifetime of commercial
products.2,3
History of OLED Technology
The early history of OLEDs goes back to the 1950s and 1960s.4,5
In experiments
on μm to mm-thick organic crystals, electroluminescence was observed when voltages of
a few hundreds of volts were applied.5 Since such voltages are impractical for most
applications, these early results went almost forgotten until the technical progress in
semiconductor processing allowed the fabrication of thin organic films where
electroluminescence could be observed at applied voltages of only 30V.6 Nevertheless, it
took another 5 years until the first OLEDs with a reasonable power efficiency were
demonstrated.7 Whereas these first devices were all based on organic small molecules,
electroluminescence was shown in polymers only a few years later.8 The reports by Tang
et al.7 and Burroughes et al.
8 sparked research in OLEDs, and increasing efficiencies
were reported at a steady pace by using more efficient device architectures and,
especially, by synthesizing materials with higher photoluminescence quantum yields.
However, the increase in efficiency resulted from the introduction of phosphorescent dyes
into OLED devices, which multiplied the efficiencies.9 Further optimization of these
devices recently led to the improved power efficiencies. Such high-efficiency OLEDs are
typically based on small molecules that are evaporated in vacuum and that emit in the
green color spectrum since the eye is most sensitive at these wavelengths. The power
efficiencies of OLEDs in other colors are still inferior to green devices, but they have also
been increased and even white OLEDs now reach efficiencies that are close to those of
fluorescent lamps and therefore make OLEDs also a potential candidate for lighting
applications. On the other hand, OLEDs with a solution-processed emissive layer
generally show lower efficiencies and require some evaporated organic layers to
maximize the efficiency (hybrid OLEDs).
Mechanism and Structure of Organic Light Emitting Diodes (OLEDs)
Electroluminescence is obtained from light-emitting diodes (LEDs) when the
light-emitting layer is incorporated between the anode and cathode. Single layer OLED
device includes anode, light-emitting layer and cathode, which is the basic and simplest
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
233 | P a g e
OLED structure. However, due to different mobility between holes and electrons, the
combining areas tend to close to one electrode, causing charge consumed on the electrode
surface and thus affecting the device efficiency. Improved device performance was
achieved when a more complicated multilayer device configuration was adopted (Figure
5.2).10
Hole injection/transport layer (HTL) and electron injection/transport layer (ETL)
were inserted to balance the charge injection and transport and control the recombination.
In order to confine charges in active layer, hole-blocking layer (HBL) and electron-
blocking layer (EBL) were added to prevent holes and electrons leakage. Multilayer
structures permit improvement in charge injection, transport and recombination. When a
voltage is applied onto the device, holes are injected from the anode and electrons from
the cathode, then they migrate through the hole transport layer and electron transport
layer, respectively. Finally they recombine in the organic light-emitting layer to form
excitons. The relaxation of the excitons from excited state to ground state will produce
light emission and the color of light depends on the energy difference between the excited
states and the ground states. In short, the fundamental physical process of the OLEDs can
be divided into four steps: charge injection, transport, recombination and radiative exciton
decay.
Fig. 4.2: Sandwich structure of OLEDs.
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
234 | P a g e
For OLEDs, indium-tin-oxide (ITO) coated glass substrate is a universal choice
for their anode. Up to now, other non-ITO anodes are seldom used. ITO is composed of
indium oxide (In2O3) and a small amount of tin oxide (SnO2). Its high work function, high
transparency (90%) to visible light, wide band gap (Eg=3.5 - 4.3 eV), conductive and
good adhesion ability with organic layer are the main considerations. Before using, ITO
must be cleaned ultrasonically in detergent solution and rinsed in deionized water in
sequence. After cleaning, the surface treatment, such as, using plasma or UV-ozone to
enhance its work function further to 5 eV and facilitate its hole injection. The subsequent
ITO treatment is very important, which will improve the efficiency and stability of
OLED. 11-13
However, the work function of treated ITO is still lower than the highest occupied
molecular orbital (HOMO) of most hole transport materials. For further improved device
performance, a hole-injection layer is inserted between ITO and hole transporting layer.
This layer will enhance hole injection at interface. Copper phthalocyanine (CuPc)14,15
and
poly(3,4-ethylene dioxythiophene)–poly(styrene sulfonic acid) (PEDOT/PSS)16,17
are
popular choices, especially the latter, PEDOT/PSS can smooth the surface of ITO,
decrease device turn-on voltage, reduce the probability of electrical short circuits. The
structures of PEDOT/PSS and CuPc are shown in Figure 4.3.
S
O On
+
SO3H
**n
PEDOT/PSS
N
N
N
N N
N
N
N
Cu
CuPc
Fig.4.3 Structures of PEDOT/PSS and CuPc
For the cathode, usually electropositive and low work function metals are used,
because they minimize the energy barrier for electron injection from cathode to the
organic materials and offer high current density.18,19
The attempt to use Ca, K and Li for
effective cathode materials revealed that they exhibit poor corrosion resistance and high
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
235 | P a g e
chemical reactivity with the organic layer. One solution is to use low-work function metal
alloys such as Mg-Ag and Al-Li, which have better stability. Currently, bilayer cathode,
such as LiF/Al was adopted and exhibited pronounced boost in device performances, thus
it has been widely used in OLEDs.
According to the mechanism and structure of OLEDs, the performance of an OLED
depends on two key factors:
1. Device configuration and
2. Light-emitting material.
Overview of Small molecule’s application for OLED:
Pioneering work using the low molecular weight organic materials on
electroluminescent (EL) devices has triggered extensive research and development in this
field. The organic molecular solids may form uniform, transparent and amorphous thin
film by vapour deposition or spin-coating methods. In contrast to polymers, small
molecules are pure materials with well defined molecular structures and definite
molecular weights without any distribution. The OLED materials should meet the
following requirements:
(1) Suitable ionization potential and electron affinity so that the charge can be
easily injected from the electrodes.
(2) Appropriate carrier mobility, not too high or too low.
(3) Thermally stable.
To attain high quantum efficiency for EL, it is necessary to achieve efficient charge
injection from the anode and the cathode into the adjacent organic layers at low drive
voltage, good charge balance, and confinement of the injected charge carriers within the
emitting layer to increase the probability of the desired emissive recombination. The
insertion of hole-transport and electron-transport layers between the electrodes and the
emitting layer reduces the energy barriers for the injection of charge carriers from the
electrodes into the emitting layer by a stepwise process, resulting in efficient charge
injection and charge balance. That is, charge carriers injected from the electrodes into the
adjacent charge-transport layers are transported through the charge transport layers and
then injected into the emitting layer. The hole and electron transport layers can also act as
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
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electron and hole blocking layers, respectively, thus confining the electrons and holes
within the emitting layer and preventing them from escaping to the adjacent carrier
transport layers.
The performance of OLEDs, therefore, depends upon various materials
functioning in specialized roles such as charge-injection and charge-transporting, charge-
blocking, and emission. Generally materials for use in OLEDs should meet the following
requirements:
(a) Materials should possess suitable ionization potentials and electron affinities, that
is well-matched energy levels for the injection of charge carriers from the
electrodes or the organic layer into the adjacent organic layers.
(b) They should be capable of forming smooth, uniform thin films without pinholes.
(c) They should be morphologically and thermally stable.
(d) In addition to these general requirements, materials should meet further
specialized needs depending upon the roles that they play in devices, for
example, hole transport, electron transport, charge blocking, and light
emission.20-27
Electron transport materials:
The electron-transport layer is used for attaining efficient electron injection from the
metal cathode, which is a usually low work-function metal such as calcium, magnesium
and aluminum.
Electron-transporting materials are those with high electron affinities together
with high ionization potentials usually function as electron-transporting materials. These
materials accept electron carriers with a negative charge and transport them. In other
words, materials with electron-accepting properties usually serve as electron transporting
materials.38
Materials for use in the electron-transport layer in OLEDs should fulfill several
requirements.
1. They should have high electron drift mobility to transport electrons.
2. They should meet the energy level matching the electron injection from the cathode.
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
237 | P a g e
3. The cathodic reduction processes of electron-transporting materials should be
reversible to form stable anion radicals.
4. They should form homogeneous thin films with morphological and thermal stability.
Electron-transporting materials containing electron-withdrawing Oxadiazole and
Triazole units:
1,3,4-Oxadiazoles (1-4) are amongst the most widely studied classes of electron
transport materials due to their electron deficiency, high photoluminescence quantum
yield, good thermal, and chemical stabilities. Triazoles (5) are another interesting class of
electron deficient thermostable material. Triazole derivatives have been demonstrated to
have more efficient electron-transport characteristics and have a higher stability to high
current density than the oxadiazole derivatives (e.g., PBD) in OLEDs.27-32
Some of the well known electron-transporting materials containing oxadiazole and
triazole units are given below.
NN
O
1
NN
O
2
O
NNNN
O
N
N
O
RR
R3
3aN N
OO
N N3b 3c
R O
NNNN
O
4
R =
H
H
N
4a 4b 4c
N
NN
Si
N
NN
5
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Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
238 | P a g e
Electron transport and hole blocking material:
The electron-transport layer in OLEDs plays a role in hole blocking as well as in
acceptance and transport of electrons. The presence of an electron-transport layer with an
effective hole-blocking ability is required to facilitate electron injection from the cathode
into the emitting layer and to block hole carriers from escaping from the emitting layer.
In case electron-transporting materials lack effective hole blocking ability, an
independent hole-blocking material is used together with a suitable electron transporter
that facilitates electron injection from the cathode. Hole-blocking materials should fulfill
several requirements. They should have weak electron-accepting and electron-
transporting properties. Their anion radicals should be stable. They should possess proper
energy levels of HOMO and LUMO to be able to block holes from escaping from the
emitting layer into the electron-transport layer but to pass on electrons from the electron-
transport layer to the emitting layer. In other words, the difference in the HOMO energy
levels between the emitting material and the hole-blocking material should be much
larger than that in their LUMO energy levels. In addition, they should not form any
excitation with emitting materials having electron-donating properties. 26
Use of oxadiazole in OLED is typically determined by their electron transport and
hole blocking qualities. Due to extremely low hole affinity, PBD (6) has been widely used
as an electron transporting and hole blocking material in OLED. The employment of hole
blocking triazoles as electron transport layer in OLEDs enables exciton confinement at
the organic interface. This material is therefore having advantages with respect to a high
efficient performance of the corresponding devices. Many oxadiazole and triazole unit
6-11 containing electron transporting along with hole blocking materials have been
developed.38,39
NN
O
NN
OO
NN
6
7
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
239 | P a g e
O
NN
NN
O
8
NN
NN
9
N
NN
R
10
10a R = H
10b R = C2H5
N
N
N
N N
O R
11
11a Ar = 1-napthyl, R = 9,9-di(n-butyl)-2-fluorenyl
11b Ar = 1-pyrenyl, R = 9,9-di(n-butyl)-2-fluorenyl
Ar
Ambipolar Charge-Transporting Materials:
There are a number of materials that exhibit ambipolar character, that is, materials
that can transport both holes and electrons. Molecules containing both the electron
donating and accepting moieties exhibit ambipolar character, readily accepting both holes
and electrons. These materials usually function as materials for the emitting layer in
OLEDs. Since the emitting layer in OLEDs acts as the recombination center for holes and
electrons injected from the anode and cathode, respectively, materials for use in the
emitting layer should accept both hole and electron carriers, and transport them. That is
the emitting materials should have bipolar character, permitting the formation of both
stable cation and anion radicals. The emitting materials should have high luminescence
quantum efficiencies. In addition to these requirements, they should be capable of
forming smooth, uniform thin films with thermal and morphological stability. The use of
emitting materials that fulfill these requirements is expected to lead to enhanced
performance and improved durability of devices.33,34
Several attempts have been carried out to combine electron transport and hole
transport material in a single molecule. The most successful approach, based on a
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
240 | P a g e
molecular structure with both electron transporting oxadiazole unit and hole transporting
triphenylamine groups is 12-14 reported.
N
O
NN
O
NN
N
12
NN
NN
N
13
R1R2
R1=
N N N
OR2=
14
Amorphous small organic molecules are good candidates for use in OLEDs35-39
and also have other advantages over polymers and inorganic compounds like easy
synthesis, purification and analysis. But the efficiency and life-time are two of the main
limitations restricting the large scale, low cost manufacturing of multi-layered OLED
device. Incarpoarating unsymmetric connection in small organic compounds prevents
from crystallizing and yield higher thermal stability over that of symmetric
derivatives.35,40
The development of the palladium-catalyzed cross-coupling Suzuki
reaction of arylhalides with boronic acids provides an efficient and versatile means of
extending π- π conjugation in organic compounds.
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
241 | P a g e
4.2 PRESENT WORK
During the present investigation, we have designed and synthesized a series of
novel unsymmetrical small organic molecules XXIXa–f and XXXa-f with indene and
carbazole substituted oxadiazole core moiety as an electron transporter, which are
connected through a phenyl spacer with para linkages. The introduction of the indene and
carbazole moieties extends the π- π conjugation and oxadiazole moiety enhances the
electron transporting capability because of the two withdrawing C=N groups, and also
improves the thermal stability for better morphology. We have thoroughly investigated
the optoelectronic properties like UV–Vis spectra, fluorescence emitting spectra, quantum
yields, HOMO–LUMO calculations, life-time measurements and quenching studies. The
molecular structures of target unsymmetrical indene and carbazole substituted oxadiazole
derivatives (XXIXa-f, XXXa-f) are illustrated in Figure 4.4.
R O
NN
XXIXa-f
R O
NNN
XXXa-f
R =S
F F
FF
F
a b c d e f
Fig. 4.4: Molecular structures of the designed indene and carbazole substituted
oxadiazole derivatives (XXIXa-f and XXXa-f).
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
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Br
O
OH
R O
NNBr
R O
NN
R O
NN
N
O
NH
RNH2
R = S
F F
F
F
F
Suzuki Buchwald
i
ii iii
XXVIIIa-f
a b c d e f
XXIXa-f XXXa-f
Reagents and conditions:
i) POCl3, reflux, 12 h.
ii) 2-Indenylboronic acid, Pd(dppf)Cl2, K2CO3, dioxane/water, 80 oC, 2h.
iii) Carbazole, Pd2(dba)3, BiNAP, Cs2CO3,dioxane, 100 oC, 12 h.
SCHEME 13.
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
243 | P a g e
4.3 RESULTS AND DISCUSSION
Relatively simple and efficient synthetic protocols were used to synthesize the
target compounds and the synthetic details are outlined in scheme 13. The 2-indenyl-
phenyl and carbazole-phenyl group is kept constant in the second postion of oxadiazole
ring and in the fifth position we have incorporated different substituents such as p-tert-
butylphenyl (a), thiophen-2-yl (b), biphenyl (c), 2-napthyl (d), pentafluorophenyl (e) and
anthracene (f) to modify their spectral properties. The required starting material 4-
bromobenzohydrazide was obtained from commercially available 4-bromobenzoic acid
on esterification followed by treating with hydrazine hydrate as per the reported
literature.41,42
The key intermediates OXD-bromides (XXVIIIa-f) were synthesized upon
treatment of 4-bromobenzohydrazide with various aromatic carboxylic acids by refluxing
in POCl3.43,44
Subsequent Pd-catalyzed Suzuki cross-coupling reaction between the OXD-
bromides (XXVIIIa-f) and 2-Indenylboronic acid afforded the unsymmetrical target
compounds (XXIXa-f) in 75-85% yields. The Pd-catalysed Buchwald-Hartwig amination
reaction between OXD-bromides (XXVIIIa-f) and carbazole in the presence of BiNAP as
ligand and Cs2CO3 as base afforded the unsymmetrical carbazole substituted oxadiazoles
(XXXa-f) in 60-65% yield. All compounds were purified by column chromatography on
silica gel followed by recrystallization in ethanol before spectral characterization. All
these compounds are amorphous in nature and are stable to routine purification and can
be stored under ambient conditions for long term without any detectable decomposition.
These compounds are readily soluble in common organic solvents like EtOH, CHCl3,
DCM and THF etc. Their structural identities and purities were confirmed by 1H NMR,
13C NMR, IR and LC/MS and elemental analysis (Figures 4.11 to 4.21).
4.3.1 UV-Vis Absorption and Fluoroscence Spectra
The UV-Vis absorption/emission spectra (see Figure 4.5) of compounds XXIXa-f
and XXXa-f were recorded in ethanol (HPLC grade) at room temperature and the
corresponding photophysical data are presented in Table 4.1. The electronic absorption
spectra of all compounds have similar absorption peaks in the range from 341 to 355 nm
and these characteristic vibronic bands are assigned to the π-π* transitions of the extended
conjugative indene-OXD-aryl and carbazole-OXD-aryl chain. Additionally, strong
absorption bands at the high energy 200-220 nm region, corresponding to the spin-
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
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allowed, π-π*, the so-called K band absorption of the indene group. In the series, the
biphenyl derivative XXIXc showed least λmaxabs
of 341 nm and the 2-naphtyl derivative
XXIXd exhibited highest λmaxabs
of 355 nm.
The normalized photoluminescence (PL) spectra (see Figure 4.6), which could
provide a good deal of information on the electronic structure of the conjugated
compounds, as oxadiazole itself is an electron deficient system having three electron rich
atoms delocalized over the ring that can act as a π-electron acceptor; incorporation of
electron rich indene and aryl groups at the 2nd
and 5th
postion of oxadiazole ring shifts the
wavelength to the longer wavelength (red shift). This implies interaction of the electron
rich bulky groups at 2nd
and 5th
positions of oxadiazole backbone and internal charge
transfer along the oxadiazole backbone in the excited state to enhance luminescence
intensity. The least overlapping of the emission and absorption spectra of these
compounds indicates that reabsorption of the emitted light by the compounds is
negligible. The PL spectra of all these compounds (XXIXa-f, XXXa-f) in ethanol at room
temperature were found to exhibit excellent blue emission with the peak maxima λmaxemi
in the range 410 to 490 nm. An anthracene derivative XXIXf displayed a significant red-
shifted PL emission with a broad λmaxemi
at 490 nm (emits blue-green light) compared to
other compounds in the series. This is due to the bulky anthracene group, which makes
the compound XXIXf to have extended conjugation length and the degree of
intermolecular interaction could lead to the formation of excimers or aggregates.
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Fig. 4.5: Normalized UV-Vis absorption spectra of compounds XXIXa-f in ethanol at 5
µM concentration at room temperature, compared with Reference (Coumarin 440).
Fig. 4.6: Normalized PL spectra of compounds XXIXa-f in ethanol at 5 µM
concentration at room temperature, compared with Reference (Coumarin 440).
400 500 600
0.0
0.5
1.0
No
rm
ali
sed
In
ten
sity
(a
.u.)
Wavelength (nm)
3a
3b
3c
3d
3e
3f...... Ref
200 250 300 350 400 4500.0
0.2
0.4
0.6
0.8
1.0
No
rm
ali
zed
Ab
sorp
tio
n (
a.
u.)
Wavelength (nm)
3a
3b
3c
3d
3e
3f ...... Ref
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The Stoke’s shift, indicating the extent of the red shift of the fluorescence
maximum (λmaxemi
) compared to the absorption maxima (λmaxabs
), is in the range of 61 –
145 nm. The lowest Stroke’s shift for compounds XXIXa and XXIXd is 61 nm and it is
larger for XXIXc (75 nm) and is largest for XXIXf (145 nm) indicating more significant
structural changes between the ground and an excited state of XXIXf compared to other
compounds in the series and is therefore connected with the difference in the internal
charge transfer (ICT) character of the ground state of these molecules.
Table 4.1: Photo physical & Time resolved measurements of compounds XXIXa-f.
Compounds Absorption
λmaxabs
(nm)a
Emission
λ maxemi
(nm)a
Stroke
shift
Δλ
(nm)
Quantum
Yield
Life Time
τ
(ns)
Optimized
Fluoroscence
Concentration
(µM)
XXIXa 349 410 61 0.45 1.286 8
XXIXb 351 423 72 0.40 1.332 80
XXIXc 341 416 75 0.40 1.305 10
XXIXd 355 416 61 0.51 1.378 20
XXIXe 349 421 72 0.27 1.653 4
XXIXf 345 490 145 0.26 4.514 4
Refb 351 427 76 0.98 nd nd
Refc nd nd nd nd 4.63 nd
Refd nd nd nd nd 4.10 nd
aThe absorption and emission spectra were measured in ethanol at 5 µM concentration at
room temperature.
nd - not determined. bCoumarin 440.
cCoumarin 540A.
dFluorescein 548.
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4.3.2 Computational methods
To have a deeper understanding on the structure–property relationship, we
performed theoretical calculations on the frontier molecular orbitals via DFT/B3LYP/6-
31G method using the Guassian 09 program45
for the geometry optimization. The ground
state optimized molecular structures and frontier molecular orbitals for all compounds
shown in Figure 4.7. The Highest Occupied Molecular Orbitals (HOMO) and Lowest
Unoccupied Molecular Orbitals (LUMO) and their energy band gap (Eg) values are
tabulated in Table 4.2. It is interesting to note that the HOMOs of all compounds are
mostly localized on the electron-donating indene-phenyl center, whereas the LUMOs are
shifted to the peripheral electron- accepting oxadiazole moieties, leading to an obvious
spatial separation of frontier orbitals. In contrast, the HOMO and LUMO of anthracene
derivative XXIXf are effectively delocalized over the electron donating and accepting
moieties, leading to a weak trend of charge transfer upon photo-excitation. The
computationally calculated HOMO and LUMO energy values are in the range of −5.39 to
−5.78 eV and 1.99–2.64 eV, respectively. Small variations in HOMO and LUMO energy
values for all compounds indicate a similar electronic structure. Optical band gaps
obtained from absorption spectrum are in the range of 3.49–3.64 eV which is in good
agreement with the band gaps (3.14 – 3.54 eV) obtained using DFT method.
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Compound Ground state
optimized structure H O M O L U M O
XXIXa
(3a)
XXIXb
(3b)
XXIXc
(3c)
XXIXd
(3d)
XXIXe
(3e)
XXIXf
(3f)
Fig. 4.7: Ground state optimized structures and Frontier molecular orbitals (HOMO and
LUMO) of XXIXa-f calculated by the DFT/B3LYP/6-31G method.
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
249 | P a g e
Table 4.2: Optical band gap obtained from DFT and UV-Vis absorption spectrum for
comparison
Compounds HOMO (eV)a LUMO (eV)
a ΔE (eV)
a ΔEopt (eV)
b
XXIXa -5.53838 -1.99869 3.54 3.55
XXIXb -5.56532 -2.15924 3.41 3.53
XXIXc -5.55334 -2.07597 3.48 3.64
XXIXd -5.54681 -2.07244 3.47 3.49
XXIXe -5.78247 -2.64089 3.14 3.55
XXIXf -5.39769 -2.22346 3.17 3.59
a Obtained using DFT/B3LYP/6-31G method, ΔE = HOMO−LUMO (eV).
b Optical band gap energies were calculated from the equation Eopt = hc/λ = 1240/λ (eV),
where λ is the wavelength (in nm) of the UV-Vis absorption spectrum.
4.3.3 Quantum Yield (Φ) and Life Time Measurements (τ)
Fluorescence quantum yields (Φ) of the all the compounds were measured in
ethanol at room temperature by comparison with a standard dye Coumarin 440 (C120) of
known quantum yield (Φ = 0.98)31,46
using the equation 1.
Where I is the integrated intensity, OD is the optical density and n is the refractive
index, the subscript R refers to the reference fluorophore of known quantum yield. The
quantum yields of all the compounds are in the range of 0.26 to 0.51 (Table 4.1). The 2-
napthyl derivative XXIXd exhibited higher quantum yield of 0.5 and there is a substantial
decrease in the quantum yield for the anthracene derivative (XXIXf) to be 0.26, which is
attributed to the photoinduced ICT process resulting from the fluorescence quenching by
electron exchange according to the Dexter mechanism.47,48
The decrease in quantum yield
resulting from the photoinduced ICT process is a common phenomenon for organic
compounds.49,50
Chapter IV
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oxadiazole based Indene and Carbazole derivatives for OLED applications
250 | P a g e
We have experimentally determined the fluorescence lifetimes (τ) for XXIXa-f
compounds (Figure 4.8) and it is in the range of 1.28 – 4.51 ns. The p-tert-butylphenyl
derivative XXIXa has shown the lowest life time of 1.28 ns, an anthracene derivative
XXIXf showed highest life time of 4.51 ns and the remaining compounds have shown life
time between 1.30 to 1.65 ns. The life time of anthracene derivative XXIXf (4.51 ns) is
very close to the standard dye Coumarin-540A (4.63 ns) and which is better than
Fluorescein-548 (4.10 ns) indicating that it has good lasing property.
10 20 30 40
102
103
104
3a
3b
3c
3d
3e
3f
Co
un
ts
Time (ns)
Fig. 4.8: Log scale plot of time-resolved PL traces of XXIXa-f (3a-f).
Chapter IV
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oxadiazole based Indene and Carbazole derivatives for OLED applications
251 | P a g e
4.3.4 Quenching studies
To evaluate the effect of concentration on absorption-emission properties of all
synthesized compounds XXIXa-f, we have recorded the absorption and emission spectra
of compounds in different concentrations. In the absorption spectra, the intensity of
absorption gradually increased with increasing concentrations from 0.05µM to 10 µM
which is an ideal condition for good OLED compounds. The representative absorption
spectrum of XXIXf which shows the effect of concentatration is shown in Figure 4.9. In
the emission spectrum of all the compounds, initially the intensity of emission gradually
increased with increasing concentration and reached a maximum (optimized
concentration) and then gradually decreased due to self quenching. We have identified the
optimized fluoroscence concentration for all the synthesized compounds and are
summarized in Table 4.1 and representative PL spectrum of XXIXf which shows the
effect of concentration on the intensity of emission is shown in Figure 4.10. The
thiophene derivative (XXIXb) has the highest optimized fluoroscence concentration of 80
µM and the pentafluorophenyl derivative (XXIXe) has the lowest optimized fluoroscence
concentration of 4 µM.
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Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
252 | P a g e
300 350 400 450 5000.0
0.2
0.4
Ab
sorp
tio
n (
a.u
.)
Wavelength (nm)
1
2
3
4
5
6
7
8
9
Fig. 4.9: Effect of concentration on the absorption spectra of XXIXf in ethanol; (1) 0.5
µM, (2) 1 µM, (3) 2 µM, (4) 4 µM, (5) 6 µM, (6) 8 µM, (7) 10 µM, (8) 20 µM, (9) 40
µM. λmaxabs
= 349 nm.
400 450 500 550 600 650 700
0.0
3.0x106
6.0x106
9.0x106
Inte
nsi
ty (
a.u
.)
Wavelength (nm)
1
2
3
4
5
6
7
8
9
Fig. 4.10 Effect of concentration on the emission spectra of XXIXf in ethanol; (1) 0.5
µM, (2) 1 µM, (3) 2 µM, (4) 4 µM, (5) 6 µM, (6) 8 µM, (7) 10 µM, (8) 20 µM, (9) 40
µM. λmaxabs
= 349 nm.
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
253 | P a g e
4.4 EXPERIMENTAL
4.4.1 General procedure for the synthesis of intermediate OXD-bromides
(XXVIIIa-f)
A mixture of 4-bromobenzohydrazide (5.0 g, 2.50 mmol) and different aryl / heteroaryl
carboxylic acids (5.0g, 2.5 mmol) in POCl3 (50 mL) was refluxed for 10 h at 100 oC. The
progress of the reaction was monitored by TLC. After completion, the reaction mixture
was allowed to reach room temperature (RT) and the quenching of reaction mixture in
crushed ice was carefully carried out under efficient fume hood. The solid that separated
was collected by filtration, washed with excess of H2O and then washed with aqueous
saturated NaHCO3 solution, dried and recrystallized from ethanol to obtain the desired
intermediate OXD-bromides (XXVIIIa-f) in 80-90% yield.
2-(4-bromophenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (XXVIIIa)31,51,52
White solid, Yield: 88 %. MP: 146-147 °C. 1H NMR (400 MHz, CDCl3) δ: 8.09 (d, J =
8.4 Hz, 2H), 8.05 (d, J=8.4 Hz, 2H), 7.72 (d,
J= 8.4 Hz, 2H), 7.59 (d, J=8.0 Hz, 2H), 1.40
(s, 9H); 13
C NMR (100 MHz, CDCl3) δ:
164.12, 150.17, 132.35, 130.01, 126.95,
125.70, 125.18, 123.22, 39.89, 31.25; LC/MS (ESI): m/z calculated for C18H17BrN2O
[M+H] 358.24. Found 358.47; Anal. Calcd (%) for C18H17BrN2O: C 60.52, H 4.84, N
7.84. Found: C 60.58, H 4.83, N 7.75. [Fig. 4.11]
2-(4-bromophenyl)-5-(thiophen-2-yl)-1,3,4-oxadiazole (XXVIIIb)53
Off white solid, Yield: 80 %. MP: 142-144 °C. 1H NMR (400 MHz, CDCl3) δ: 8.03 (d,
J=8.8 Hz, 2H), 7.88 (d, J=4 Hz, 1H), 7.72 (d,
J=8.4 Hz, 2H), 7.62 (d, J=4.8, 1H), 7.24 (t,
J=4.4 Hz, 1H); 13
C NMR (100 MHz, CDCl3)
δ: 164.31, 161.10, 132.22, 129.71, 127.95,
127.42, 125.44, 125.10, 123.15; LC/MS (ESI): m/z calculated for C12H7BrN2OS [M+H]
308.17. Found 308.31; Anal. Calcd (%) for C12H7BrN2OS: C 46.92, H 2.30, N 9.12.
Found: C 46.84, H 2.35, N 9.27. [Fig. 4.12]
O
NN
Br
O
NN
BrS
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oxadiazole based Indene and Carbazole derivatives for OLED applications
254 | P a g e
2-(4-bromophenyl)-5-(biphenyl)-1,3,4-oxadiazole (XXVIIIc)54
White solid, Yield: 82 %. MP: 204-206 °C. 1H NMR (400 MHz, CDCl3) δ: 8.24(d,
J=8.4 Hz, 2H), 8.07 (d, J=8.4 Hz, 2H), 7.791
(d, J=11.6 Hz, 2H), 7.716 (d, J=8.8, 2H), 7.683
(d, J=7.6, 2H), 7.450 (d, J=4, 2H), 7.508 (t,
J=14.8, 1H); 13
C NMR (100 MHz, CDCl3) δ:
164.32, 136.44, 132.35, 130.01, 129.82, 128.61, 128.04, 127.91, 127.75, 125.28, 125.14,
123.21; LC/MS (ESI): m/z calculated for C20H13BrN2O [M+H] 378.23. Found 378.37;
Anal. Calcd (%) for C20H13BrN2O: C 63.68, H 3.47, N 7.43. Found: C 63.72, H 3.51, N
7.46. [Fig. 4.13]
2-(4-bromophenyl)-5-(naphthalen-2-yl)-1,3,4-oxadiazole (XXVIIId)55
White solid, Yield: 85 %. MP: 145-147 °C. 1H NMR (400 MHz, CDCl3) δ: 8.640(s,
1H), 8.212 (d, J=8.4 Hz, 1H), 8.073 (d,
J=8.4 Hz, 2H), 8.002 (d, J=8.4, 1H), 7.923
(d, J=6.4, 2H), 7.719 (d, J=8.4, 2H), 7.615
(t, J=7.6, 1H); 13
C NMR (100 MHz,
CDCl3) δ: 164.41, 134.33, 134.10, 133.91, 132.34, 129.72, 128.63, 128.14, 126.30,
125.82, 125.33, 124.60, 123.11; LC/MS (ESI): m/z calculated for C18H11BrN2O [M+H]
352.2 Found 352.25; Anal. Calcd (%) for C18H11BrN2O: C 61.56, H 3.16, N 7.98. Found:
C 61.61, H 3.14, N 7.87.
2-(4-bromophenyl)-5-(perfluorophenyl)-1,3,4-oxadiazole (XXVIIIe)
White solid, Yield: 87 %. MP: 203-204 °C. 1H NMR (400 MHz, CDCl3) δ: 7.926 (d,
J=8.8, 2H), 7.627 (d, J=8.4, 2H); 13
C NMR (100
MHz, CDCl3) δ: 164.58, 144.72, 143.44, 138.27,
132.56, 129.75, 125.22, 123.21, 120.51; 19
F
NMR (400 MHz, CDCl3) δ: -135.33, -147.23, -
159.49; LC/MS (ESI): m/z calculated for
C14H4BrF5N2O [M+H] 392.09, found 392.18; Anal. Calcd (%) for C14H4BrF5N2O: C
43.00, H 1.03, N 7.17. Found: C 43.13, H 1.15, N 7.22. [Fig. 4.14]
O
NN
Br
O
NN
Br
FO
NN
Br
FF
FF
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oxadiazole based Indene and Carbazole derivatives for OLED applications
255 | P a g e
2-(anthracen-10-yl)-5-(4-bromophenyl)-1,3,4-oxadiazole (XXVIIIf)54
White solid, Yield: 86 %. MP: 188-190 °C. 1H NMR (400 MHz, CDCl3) δ: 8.225(d,
J=8.4, 2H), 8.053 (d, J=8.4 Hz, 2H), 7.791 (d,
J=11.6 Hz, 2H), 7.716 (d, J=8.8, 2H), 7.683 (d,
J=7.6, 2H), 7.450 (d, J=4, 2H), 7.508 (t, J=14.8,
1H); 13
C NMR (100 MHz, CDCl3) δ: 164.39,
139.23, 136.10, 132.31, 130.24, 129.71, 128.77,
128.35, 127.78, 127.35, 126.35, 125.52, 123.31;
LC/MS (ESI): m/z calculated for C22H13BrN2O [M+H] 402.26 Found 402.34; Anal.
Calcd (%) for C22H13BrN2O: C 65.85, H 3.27, N 6.98. Found: C 66.03, H 3.37, N 6.85.
4.4.2 General procedure for the synthesis of indene-substituted oxadiazole
derivatives (XXIXa-f)
Under nitrogen atmosphere, OXD-bromides XXVIIIa-f (0.5 g, 1.39 mmol), 2-
indenylboronic acid (0.25 g, 1.54 mmol) and Pd(dppf)Cl2 (0.051 g, 0.07 mmol) as a
catalyst were added to a mixure of 1,4 dioxane (10 mL) and aqueous 2M K2CO3 (5 mL).
The reaction was heated to 80 oC for 8 h. The progress of the reaction was monitored by
TLC. The solvent was evaporated under reduced pressure. The residue was dissolved in
DCM (25 mL), washed with H2O (25 mL) and brine (25 mL). The organic phase was
dried over anhydrous Na2SO4 and the solvent was evaporated, the residue was purified by
column chromatography by eluting with Hexane/DCM (8:2, v/v). The title compounds
were obtained as amorphous solids in 80-85% yield.
2-(4-(1H-inden-2-yl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (XXIXa):
Yield: 82%, Light ash colour. MP: 240-242
°C. 1H NMR (400 MHz, CDCl3) δ: 8.07 (d,
J = 8.4, 2H), 8.01 (d, J = 8.4 Hz, 2H), 7.71
(d, J = 8.4 Hz, 2H), 7.49 (d, J = 8.4, 2H),
7.44 (d, J = 7.2, 1H), 7.38 (d, J = 7.6, 1H), 7.30 (s, 1H), 7.23 (t, J = 7.4 Hz, 1H), 7.18 (d,
J = 7.4 Hz, 1H), 3.76 (s, 2H), 1.30 (s, 9H); 13
C NMR (100 MHz, CDCl3) δ: 164.61,
164.27, 155.36, 145.03, 144.92, 143.32, 139.10, 128.83, 127.25, 126.84, 126.81, 126.06,
125.48, 123.81, 122.59, 121.49, 121.16, 38.91, 35.12, 31.15.; LC/MS (ESI): m/z
O
NN
Br
O
NN
Chapter IV
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oxadiazole based Indene and Carbazole derivatives for OLED applications
256 | P a g e
calculated for C27H24N2O [M+H] 393.19, found 393.3; Anal. Calcd (%) for C27H24N2O: C
82.62, H 6.16, N 7.14. Found: C 82.41, H 6.13, N 7.11. [Fig. 4.15 (a-c)]
2-(4-(1H-inden-2-yl)phenyl)-5-(thiophen-2-yl)-1,3,4-oxadiazole (XXIXb):
Yield: 85%, Brown colour. MP: 210-212 °C. 1H NMR (400 MHz, CDCl3) δ: 8.05 (d, J
= 8.0 Hz, 2H), 7.78 (d, J = 3.2 Hz, 1H),
7.70 (d, J = 8.4 Hz, 2H), 7.51 (d, J = 4.8,
1H), 7.44 (d, J = 7.2 Hz, 1H), 7..38 (d, J =
7.2 Mz, 1H), 7.31 (s, 1H), 7.23 (t, J = 7.4
Hz, 1H), 7.13 (t, J = 7.4 Hz, 1H), 3.76 (s, 2H); 13
C NMR (100 MHz, CDCl3) δ: 162.80,
159.65, 143.83, 143.76, 142.20, 138.08, 129.11, 128.63, 127.80, 127.07, 126.15, 125.72,
124.93, 124.39, 124.19, 122.69, 121.07, 120.39, 37.77.; LC/MS (ESI): m/z calculated for
C21H14N2OS [M+H] 343.08, found 343.2; Anal. Calcd (%) for C21H14N2OS: C 73.66, H
4.12, N 8.18. Found: C 73.41, H 4.09, N 8.11. [Fig. 4.16 (a-c)]
2-(4-(1H-inden-2-yl)phenyl)-5-biphenyl-1,3,4-oxadiazole (XXIXc):
Yield: 78%, Light brown colour. MP: 243-245 °C. 1H NMR (400 MHz, CDCl3) δ: 8.17
(d, J = 7.6 Hz, 2H), 8.11 (d, J = 8.0 Hz, 2H), 7.72 (t, J = 7.0 Hz, 4H), 7.61 (d, J = 8.0,
2H), 7.45-7.33 (m, 6H), 7.25 (t, J = 7.4
Hz, 1H), 7.17 (t, J = 7.4 Hz, 1H), 3.78 (s,
2H); 13
C NMR (100 MHz, CDCl3) δ:
164.52, 164.35, 145.05 143.21, 139.12,
136.09, 136.51 129.29, 129.01, 128.38, 128.01, 129.0, 127.85, 127.65, 127.25, 126.84,
126.81, 126.06, 125.39, 125.24, 123.56, 122.43, 121.51, 121.21, 38.41.; LC/MS (ESI):
m/z calculated for C29H20N2O 412.16, found 412.9; Anal. Calcd (%) for C29H20N2O: C
82.62, H 6.16, N 7.14. Found: C 82.41, H 6.13, N 7.11. [Fig. 4.17 (a-b)]
O
NN
S
O
NN
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oxadiazole based Indene and Carbazole derivatives for OLED applications
257 | P a g e
2-(4-(1H-inden-2-yl)phenyl)-5-(naphthalen-2-yl)-1,3,4-oxadiazole (XXIXd):
Yield: 85%, Color: Light brown. MP: 248-250 °C. 1H NMR (400 MHz, CDCl3) δ:
8.57 (s, 1H), 8.15 (m, 3H), 7.94 (d, J =
8.4 Hz, 2H), 7.84 (s, 1H), 7.74 (d, J =
8.0, 2H), 7.53 (m, 2H), 7.45 (d, J = 7.2
Hz, 1H), 7.39 (d, J = 7.6 Hz, 1H), 7.33 (s,
H), 7.24 (t, J = 7.4 Hz, 1H), 7.17 (t, J = 7.4 Hz, 1H), 3.78 (s, 2H); 13
C NMR (100 MHz,
CDCl3) δ: 164.52, 145.21, 143.15, 139.41, 136.79, 134.35, 133.78, 129.32, 129.12, 128.6,
128.11, 127.88, 127.37, 126.91, 126.29, 126.22, 125.78, 123.89 122.71, 121.85, 121.32,
38.95; LC/MS (ESI): m/z calculated for C27H18N2O [M+H] 387.14, Found 387.3; Anal.
Calcd (%) for C27H18N2O: C 83.92, H 4.69, N 7.25. Found: C 83.78, H 4.57, N 7.28.
[Fig. 4.18 (a-b)]
2-(4-(1H-inden-2-yl)phenyl)-5-(perfluorophenyl)-1,3,4-oxadiazole (XXIXe):
Yield: 75%. Color: Off white. MP: 233-235 °C. 1H NMR (400 MHz, CDCl3) δ: 8.07 (d,
J = 8.4 Hz, 2H), 7.73 (d, J = 8.4 Hz, 2H),
7.45 (d, J = 7.2 Hz, 1H), 7.39 (d, J = 7.6
Hz, 1H), 7.34 (s, 1H), 7.24 (t, J = 7.4 Hz,
1H), 7.17 (d, J = 7.4 Hz, 1H) 3.77 (s, 2H);
13C NMR (100 MHz, CDCl3) δ: 165.68,
144.77, 143.34, 139.97, 129.39, 127.66, 126.91, 126.21, 125.68, 123.84, 121.61, 121.45,
38.91; 19
F NMR (400 MHz, CDCl3) δ: -135.33, -147.23, -159.49; LC/MS (ESI): m/z
calculated for C23H11F5N2O [M+H] 427.08, found 427.0; Anal. Calcd (%) for
C23H11F5N2O: C 64.80, H 2.60, N 6.57. Found: C 64.69, H 2.56, N 6.51. [Fig. 4.19 (a-d)]
2-(4-(1H-inden-2-yl)phenyl)-5-(anthracen-10-yl)-1,3,4-oxadiazole (XXIXf):
Yield: 84%. Color: Yellow. MP: 254-256 °C. 1H NMR (400 MHz, CDCl3) δ: 8.63 (s,
1H), 8.14 (d, J = 8.4 Hz, 2H), 8.03 (m, 4H),
7.74 (d, J = 8.4 Hz, 2H), 7.50 (m, 4H), 7.41
(m, 2H), 7.33 (s, 1H), 7.24 (t, J = 7.4, 1H),
7.16 (t, J = 7.4, 1H), 3.78 (s, 2H); 13
C NMR
O
NN
F
F
F
F
F
O
NN
O
NN
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oxadiazole based Indene and Carbazole derivatives for OLED applications
258 | P a g e
(100 MHz, CDCl3) δ: 164.48, 145.11, 143.13, 139.39, 139.21, 136.81, 136.01, 130.22,
129.32, 129.10, 128.62, 128.25, 127.88, 127.37, 126.91, 126.29, 126.22, 125.78, 123.89
122.64, 121.715, 121.22, 39.13; LC/MS (ESI): m/z calculated for C31H20N2O [M+H]
437.16, found 437.2; Anal. Calcd (%) for C31H20N2O: C 85.30, H 4.62, N 6.42. Found: C
85.42, H 4.33, N 6.51. [Fig. 4.20 (a-b)]
4.4.3 General procedure for the synthesis of Carbazole-substituted oxadiazole
derivatives (XXXa-f)
Under nitrogen atmosphere, OXD-bromides XXVIIIa-f (0.5 g, 1.39 mmol), carbazole
(1.2 eq), Cs2CO3 (3.0 eq), Pd2(dba)3 (5 mol %) and BiNAP (1 mol %) in 1,4-dioxane was
heated to 100 oC for 8 h. The progress of the reaction was monitored by TLC. The solvent
was evaporated under reduced pressure. The residue was dissolved in DCM (25 mL),
washed with H2O (25 mL) and brine (25 mL). The organic phase was dried over
anhydrous Na2SO4 and the solvent was evaporated, the residue was purified by column
chromatography by eluting with Hexane/DCM (8:2, v/v). The title compounds were
obtained as amorphous solids in 60-65% yield.
9-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)phenyl)-9H-carbazole (XXXa):
Yield: 62%, Light brownish. MP: 265-267 °C. 1H NMR (400 MHz, CDCl3) δ: 7.65 (d,
J = 8.4, 2H), 7.58 (d, J = 8.4 Hz, 2H), 7.47
(d, J = 8.4 Hz, 2H), 7.40 (d, J = 8.4, 2H),
7.31 (d, J = 8.4, 2H), 7.14 (t, J = 7.4, 2H),
7.05 (t, J = 7.4, 2H), 1.32 (s, 9H); 13
C NMR
(100 MHz, CDCl3) δ: 164.56, 150.1,
141.13, 138.78, 128.12, 127.21, 125.60, 123.11, 123.02, 122.19, 122.05, 121.12, 120.05,
119.04, 111.51, 39.14, 31.61 ; LC/MS (ESI): m/z calculated for C30H25N3O [M+H]
444.54, found 444.24; Anal. Calcd (%) for C30H25N3O: C 81.24, H 5.68, N 9.47. Found:
C 81.13, H 5.62, N 9.50
O
NN
N
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oxadiazole based Indene and Carbazole derivatives for OLED applications
259 | P a g e
9-(4-(5-(thiophen-2-yl)-1,3,4-oxadiazol-2-yl)phenyl)-9H-carbazole (XXXb):
Yield: 65%, Brown colour. MP: 230-232 °C.
1H NMR (400 MHz, CDCl3) δ: 7.68 (d, J = 8.0
Hz, 2H), 7.55 (d, J = 8.0 Hz, 2H), 7.47 (d, J =
8.4 Hz, 2H), 7.36 (d, J = 8.4, 2H), 7.11 (t, 7.4
Hz, 2H), 7.01(m, 4H; 13
C NMR (100 MHz,
CDCl3) δ: 164.56, 161.21, 141.15, 139.74, 132.20, 128.19, 127.91, 127.63, 125.57,
123.04, 122.26, 122.11, 121.20, 119.05, 111.15; LC/MS (ESI): m/z calculated for
C24H15N3OS [M+H] 394.46, found 394.21; Anal. Calcd (%) for C24H15N3OS: C 73.26, H
3.84, N 10.68. Found: C 73.31, H 3.75, N 10.56
9-(4-(5-(biphenyl)-1,3,4-oxadiazol-2-yl)phenyl)-9H-carbazole (XXXc):
Yield: 70%, Light brown colour. MP: 273-275 °C. 1H NMR (400 MHz, CDCl3) δ:
7.65-7.40 (m, 12H), 7.35 (t, J = 7.4 Hz,
2H), 7.31 (d, J = 7.4 Hz, 2H), 7.22 (t, J =
7.0 Hz, 1H), 7.12 (d, J = 8.0, 2H), 7.04 (t,
J = 7.4 Hz, 2H).; 13
C NMR (100 MHz,
CDCl3) δ: 164.52, 164.35, 145.05 143.21,
139.12, 136.09, 136.51 129.29, 129.01, 128.38, 128.01, 129.0, 127.85, 127.65, 127.25,
126.84, 126.81, 126.06, 125.39, 125.24, 123.56, 122.43, 121.51, 121.21, 111..; LC/MS
(ESI): m/z calculated for C32H21N3O 464.53, found 464.46; Anal. Calcd (%) for
C32H21N3O: C 82.92, H 4.57, N 9.07. Found: C 82.87, H 4.52, N 9.12
9-(4-(5-(naphthalen-3-yl)-1,3,4-oxadiazol-2-yl)phenyl)-9H-carbazole (XXXd):
Yield: 66%, Color: Light brown. MP: 280-282 °C. 1H NMR (400 MHz, CDCl3) δ:
8.14 (s, 1H), 8.15 (m, 4H), 7.94 (d, J = 8.4
Hz, 2H), 7.76 (d, J = 8.0, 2H), 7.55 (m, 2H),
7.48 (d, J = 7.2 Hz, 2H), 7.41 (d, J = 7.6 Hz,
2H), 7.26 (t, J = 7.4 Hz, 2H), 7.17 (t, J = 7.4
Hz, 2H).; 13
C NMR (100 MHz, CDCl3) δ:
164.52, 145.21, 143.15, 139.41, 136.79, 134.35, 133.78, 129.32, 129.12, 128.6, 128.11,
127.88, 127.37, 126.91, 126.29, 126.22, 125.78, 123.89 122.71, 121.85, 121.32, 111.14;
O
NN
NS
O
NN
N
O
NN
N
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oxadiazole based Indene and Carbazole derivatives for OLED applications
260 | P a g e
LC/MS (ESI): m/z calculated for C30H19N3O [M+H] 438.49, Found 438.53; Anal. Calcd
(%) for C30H19N3O: C 82.36, H 4.38, N 9.60. Found: C 82.28, H 4.32, N 9.56.
9-(4-(5-(perfluorophenyl)-1,3,4-oxadiazol-2-yl)phenyl)-9H-carbazole (XXXe):
Yield: 75%. Color: Off white. MP: 233-235 °C. 1H NMR (400 MHz, CDCl3) δ: 8.41 (d,
J = 10.8 Hz, 2H), 8.17 (d, J = 9.6 Hz, 2H),
7.83 (d, J = 11.2 Hz, 2H), 7.42-7.56 (m,
4H), 7.40 (t, J = 9.4 Hz, 2H).; 13
C NMR
(100 MHz, CDCl3) δ: 165.68, 144.77,
143.34, 139.97, 129.39, 127.66, 126.91,
126.21, 125.68, 123.84, 122.34, 121.61, 121.45, 120.51, 119.21, 111.15; 19
F NMR (400
MHz, CDCl3) δ: -135.24, -146.79, -159.26; LC/MS (ESI): m/z calculated for
C26H12F5N3O [M+H] 478.38, found 478.30; Anal. Calcd (%) for C26H12F5N3O: C 65.41,
H 2.53, N 8.80. Found: C 65.45, H 2.48, N 8.76. [Fig. 4.21 (a-c)]
9-(4-(5-(anthracen-9-yl)-1,3,4-oxadiazol-2-yl)phenyl)-9H-carbazole (XXXf):
Yield: 60%. Color: Yeloow. MP: 295-297 °C. 1H NMR (400 MHz, CDCl3) δ: 8.22 (s,
1H), 7.88 (d, J = 8.4 Hz, 4H), 7.50 (d, J = 8.4
Hz, 2H), 7.42 (m, 4H), 7.32 (d, J = 8.4 Hz,
2H), 7.41 (m, 2H), 7.24 (t, J = 7.4, 4H), 7.16 (t,
J = 7.4, 2H).; 13
C NMR (100 MHz, CDCl3) δ:
164.48, 145.11, 143.13, 139.39, 139.21, 136.81,
136.01, 130.22, 129.32, 129.10, 128.62, 128.25,
127.88, 127.37, 126.91, 126.29, 126.22, 125.78, 123.89 122.64, 121.715, 121.22, 120.33,
119.75, 111.16.; LC/MS (ESI): m/z calculated for C34H21N3O [M+H] 437.16, found
437.2; Anal. Calcd (%) for C34H21N3O: C 83.76, H 4.34, N 8.62. Found: C 83.80, H 4.38,
N 8.60.
O
NN
N
FO
NN
N
FF
FF
Chapter IV
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oxadiazole based Indene and Carbazole derivatives for OLED applications
261 | P a g e
Fig. 4.11:
1H NMR Spectrum of XXVIIIa in CDCl3
Fig. 4.12:
1H NMR Spectrum of XXVIIIb in CDCl3
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
262 | P a g e
Fig. 4.13:
1H NMR Spectrum of XXVIIIc in CDCl3
Fig. 4.14:
1H NMR Spectrum of XXVIIIe in CDCl3
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
263 | P a g e
Fig. 4.15(a):
1H NMR Spectrum of XXIXa in CDCl3
Fig. 4.15(b):
13C NMR Spectrum of XXIXa in CDCl3
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
264 | P a g e
Fig. 4.15(c): LC/MS Spectrum of XXIXa
Chapter IV
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oxadiazole based Indene and Carbazole derivatives for OLED applications
265 | P a g e
Fig. 4.16(a):
1H NMR Spectrum of XXIXb in CDCl3
Fig. 4.16(b):
13C NMR Spectrum of XXIXb in CDCl3
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
266 | P a g e
Fig. 4.16(c): LC/MS Spectrum of XXIXb
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
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Fig. 4.17(a): 1H NMR Spectrum of XXIXc in CDCl3
Fig. 4.17(b): Mass Spectrum of XXIXc
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oxadiazole based Indene and Carbazole derivatives for OLED applications
268 | P a g e
Fig. 4.18(a): 1H NMR Spectrum of XXIXd in CDCl3
Fig. 4.18(b): Mass Spectrum of XXIXd
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
269 | P a g e
Fig. 4.19(a):
1H NMR Spectrum of XXIXe in CDCl3
Fig. 4.19(b):
13C NMR Spectrum of XXIXe in CDCl3
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
270 | P a g e
Fig. 4.19(c): 19
F NMR Spectrum of XXIXe in CDCl3
Fig. 4.19(d): Mass Spectrum of XXIXe
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
271 | P a g e
Fig. 4.20(a): 1H NMR Spectrum of XXIXf in CDCl3
Fig. 4.20(b): Mass Spectrum of XXIXf
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
272 | P a g e
Fig. 4.21(a): 1H NMR Spectrum of XXXe in CDCl3
Fig. 4.21(b): 19
F NMR Spectrum of XXXe in CDCl3
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
273 | P a g e
Fig. 4.21(c): LC/MS Spectrum of XXXe
Chapter IV
Synthesis, Characterization and Optoelectronic studies of novel unsymmetrical
oxadiazole based Indene and Carbazole derivatives for OLED applications
274 | P a g e
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