Chapter IV Synthesis, Characterization and Optoelectronic...

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

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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

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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

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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.

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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

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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

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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.

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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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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

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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

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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.

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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).

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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.

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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-

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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.

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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

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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



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.

Chapter IV



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



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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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


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

Chapter IV



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



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


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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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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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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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


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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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


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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Fig. 4.11:

1H NMR Spectrum of XXVIIIa in CDCl3

Fig. 4.12:

1H NMR Spectrum of XXVIIIb in CDCl3

Chapter IV



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Fig. 4.13:

1H NMR Spectrum of XXVIIIc in CDCl3

Fig. 4.14:

1H NMR Spectrum of XXVIIIe in CDCl3

Chapter IV



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

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Fig. 4.15(c): LC/MS Spectrum of XXIXa

Chapter IV



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Fig. 4.16(a):

1H NMR Spectrum of XXIXb in CDCl3

Fig. 4.16(b):

13C NMR Spectrum of XXIXb in CDCl3

Chapter IV



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Fig. 4.16(c): LC/MS Spectrum of XXIXb

Chapter IV



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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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Fig. 4.18(a): 1H NMR Spectrum of XXIXd in CDCl3

Fig. 4.18(b): Mass Spectrum of XXIXd

Chapter IV



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Fig. 4.19(a):

1H NMR Spectrum of XXIXe in CDCl3

Fig. 4.19(b):

13C NMR Spectrum of XXIXe in CDCl3

Chapter IV



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Fig. 4.19(c): 19

F NMR Spectrum of XXIXe in CDCl3

Fig. 4.19(d): Mass Spectrum of XXIXe

Chapter IV



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Fig. 4.20(a): 1H NMR Spectrum of XXIXf in CDCl3

Fig. 4.20(b): Mass Spectrum of XXIXf

Chapter IV



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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



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Fig. 4.21(c): LC/MS Spectrum of XXXe

Chapter IV



274 | P a g e

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