PREPARATION OF UQUID CRYSTALS CONTAINING
AZOBENZENE AND ALKYL CHAINS
AS TERMINAL GROUPS
LEE SWEE TING
PERPUSTAKAAN . U"'YERSITI MALAYSIA SABA ..
THIS DISSERTATION IS SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENT FOR THE DEGREE OF
BACHELOR OF SCIENCE WITH HONOURS
INDUSTRIAL CHEMISTRY PROGRAMME
SCHOOL OF SCIENCE AND TECHNOLOGY
UNlVERSITI MALAYSIA SABAH
May 2008
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UNIVERSITI MAbAVSIA SABAH
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SESI PENGAJIAN: __ CS/ 0"
mengaku membenarkan tesis (LPSMl~ahamObOi Fal3ltfM) ini disimpan di Perpustakaan Universiti Malaysia Sabah dengan syarat-syarat kegunaan seperti berikut:-
1. Tesis adalah hakmilik Universiti Malaysia Sabah. 2. Perpustakaan Universiti Malaysia Sabah dibenarkan membuat salinan untuk tujuan pengajian
sahaja. 3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara institutsi
pengajian tinggi. 4. Sila tandakan (/)
(TANDATJ\!GAN PENULIS)
Alamat Tetap: ~·4011 ~AN . ~,..:l1@ ~IN Nf.(~6fJ .$QV\!MIE\ 1
Tarik11:¥--1M CAT A T AN:- ·Potong yang tidak berkenaan .
(Mengandungi maklumat yang berdarjah keselamatan atau Kepentingan Malaysia seperti yang termaktub di dalam AKT A RAHSIA RASMI 1972)
(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)
Disahkan Oleh NURUlAIN BI ~TIISMAIL 1
~ LlBRAFAN t
~ lINIVERSITI MAL (\YSIA SABA,! (T ANDA ANGAN PUST AKA WAN) ! ,
_ Or. ~ .. L.LtHbr r<anmon. Nama Penyelia
Tarikh:.7/-4{ 08'
•• Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa /organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai SULIT dan TERHAD.
@Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana seeara penyelidikan atau disertai bagi pengajian seeara kerja kursus dan Laporan Projek Sarjana Muda (LPSM).
UMS UNIVERSITI MALAYSIA SABAH
11
DECLARA TION
I hereby declare that all the materials in this dissertation are original except for
quotations, excerpts and summaries which have been duly acknowledged.
May 2008
LEE SWEE TING
HS2005-1855
Name
Title
MAY, 2008
111
VERIFICATION
Lee Swee Ting
Preparation of Liquid Crystals Containing Azobenzene and Alkyl
Chains as T errninal Groups
Dr. Md. Lutfor Rahman
Marcus Jopony • -
S ·IM~1t.vatJ.~lJ'~-:~ ..
Dean,
School of Science and
Technology
UMS UNIVERSITI MALAYSIA SABAH
IV
ACKNOWLEDGEMENTS
I would like to thank my supervisor, Dr. Md. Lutfor Rahman for his advice
and suggestions in writing my dissertation as well as his technical guidance in this
work.
Special thanks to the laboratory assistants, Mr. Sani Gorudun and Mr. Samudi
from the School of Science and Technology, for providing me the equipments to do
the experimental work. For her guidance and patience, I must thank Monira Moly
Lizu. Lastly, I wish to thank my family and friends for their continuous
encouragement and support during the course of this work.
v
ABSTRACT
Liquid crystals have long dominated in the display devices, due to their large optical
anisotropies and ability to undergo electric-field induced switching. In particular,
liquid crystals containing azobenzene offers faster switching times than electric-field
induced switching. The synthesis of liquid crystals containing azobenzene and alkyl
chains as terminal groups is studied. The target compounds of ethyl-4-{ 4-
hexyloxyphenylazo )benzoate (compound 2), ethyl-4-{ 4-decyloxyphenylazo )benzoate
(compound 3), and ethyl-4-(4-hexadecyloxyphenylazo)benzoate (compound 4) were
prepared. These liquid crystals were prepared through several organic syntheses steps
which are diazonium coupling and Williamson ether synthesis reactions. The yield
obtained for all the compounds synthesized was in the range 70-80010. Infrared
spectroscopy was used to identify the functional groups of all the intermediates and
target compounds. IH NMR spectroscopy was carried out only for compound 2 in
order to confirm its molecular structure. The mesomorphic behaviours of the target
compounds were investigated using differential scanning calorimetry (DSC). The
DSC thermograms reveal that only compound 2 is a liquid crystalline molecule as it
exhibited mesophase. As for compounds 3 and 4, they are crystalline solids because
no mesophase was discovered. The transition temperatures of these compounds were
influenced by different lengths ofthe alkyl chains.
VI
PENYEDIAAN HABLUR CECAIR YANG MENGAIVDUNGI RAN6lUlAN
ALKIL SEBAGAI KUMPULAN TERMINAL
ABSTRAK
Hablur cecair telah lama mendominasi teknologi paparan disebabkan oleh anisotropi
optikalnya yang tinggi serta keupayaannya untuk melakukan penukaran apabila
dikenakan medan elektrik. Secara khususnya. hablur cecair yang mengandungi
azobenzena menunjukkan masa penukaran yang lebih pantas berbanding dengan
penukaran yang berlaku apabila dikenakan medan elektrik. Sintesis hablur cecair yang
mengandungi azobenzena dan rangkaian alkil sebagai kumpulan terminal telah
dilaporkan. Etil-4-( 4-heksiloksifenilazo )be02oat (compound 2), etil-4-( 4-
deksiloksifenilazo )benzoat (compound 3), dan etil-4-( 4-
heksadeksiloksifenilazo)benzoat (compound 4) disediakan sebagai molekul sasaran
melalui beberapa langkah sintesis organik iaitu tindakbalas pemasangan diazonium
serta sintesis eter Williamson. HasH yang diperoleh bagi semua sebatian berada dalam
lingkungan 70-80010. Spektroskopi infra merah digunakan untuk menentukan
kumpulan berfungsi bagi sebatian intermediari dan molekul-molekul sasaran tersebut.
IH NMR hanya digunakan untuk mengesahkan struktur molekul bagi sebatian 2. Sifat
mesomorfik molekul-molekul sasaran tersebut dikaji dengan menggunakan
kalorimetri pengimbasan bezaan (DSC). Termogram DSC menunjukkan bahawa
sebatian 2 ialah molekul hablur cecair kerana ia mempamerkan kewujudan mesofasa.
Sebatian 3 dan 4 merupakan pepejal kristalin kerana tiada penemuan mesofasa. Suhu
transisi molekul-molekul tersebut adalah dipengaruhi oleh panjang rangkaian alkil
yang berbeza.
UMS UNIVEASITI MALAYSIA SABAH
DECLARATION
VERIFICATION
ACKNOWLEDGEMENTS
ABSTRACT
ABSTRAK
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
TABLE OF CONTENTS
LIST OF ABBREVIATIONS
CHAPTER 1 INTRODUCTION
1.1 An Introduction to Liquid Crystals
1.2 Research Objectives
1.3 Scope of Research
CHAPTER 2 LITERATURE REVIEW
2.1 History of Liquid Crystals
2.2 Liquid Crystals
2.3 Properties of Liquid Crystals
2.3.1 Electronic Properties
2.3.2 Chemical Properties
2.3.3 Physical Properties
2.4 Classification ofThennotropic Liquid Crystals
2.5 Liquid Crystalline Phases
2.5.1 Nematic Phases
2.5.2 Smectic Phases
2.5.3 Columnar Phases
2.6 Liquid Crystals Containing Azobenzene
2.7 Alkyl Chains as Terminal Groups in Liquid Crystals
2.8 Synthesis of Liquid Crystals
VII
Page Number
II
III
IV
v
VI
Vll
x
XI
Xlll
1
1
4
4
5
5
6
7
7
9
10
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12
12
13
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UMS UNIVEASITI MALAYSIA SABAH
2.8.1 Diazonium Coupling Reaction
2.8.2 Williamson Ether Synthesis
2.9 Characterization of Liquid Crystals
VllI
21
23
24
2.9.1 Identification of Structures of Liquid Crystals 24
2.9.2 Phase Transition Temperatures and Enthalpies 24
CHAPTER 3 METHODOLOGY 26
3.1 Chemicals 26
3.2 Synthesis 27
3.2.1 Preparation of Ethyl-4-( 4-hydroxyphenylazo )benzoate
Through Diazonium Coupling Reaction (Compound 1) 27
3.2.2 Preparation ofEthyl-4-( 4-hexyloxyphenylazo )benzoate
Through Williamson Ether Synthesis (Compound 2) 28
3.2.3 Preparation ofEthyl-4-(4-decyloxyphenylazo )benzoate
Through Williamson Ether Synthesis (Compound 3) 29
3.2.4 Preparation ofEthyl-4-( 4-hexadecyloxyphenylazo )benzoate
Through Williamson Ether Synthesis (Compound 4) 29
3.3 Spectroscopic Analysis 30
3.4 Determination of Meso phases 30
CHAPTER 4 RESULTS AND DISCUSSION 31
4.1 Synthesis 31
4.1.1 Yield ofEthyl-4-(4-hydroxyphenylazo)benzoate (Compound 1) 32
4.1.2 Yield ofEthyl-4-(4-hexyloxyphenylazo)benzoate (Compound 2) 33
4 .1.3 Yield ofEthyl-4-(4-decyloxyphenylazo)benzoate (Compound 3) 34
4.1.4 Yield ofEthyI-4-( 4-hexadecyloxyphenylazo )benzoate
(Compound 4) 35
4.2 Infrared Spectra Analysis 36
4.2.1 Infrared Spectrum ofEthyl-4-(4-hydroxyphenylazo )benzoate
(Compound 1) 36
4 .2.2 Infrared Spectrum ofEthyl-4-(4-hexyIoxyphenylazo)benzoate
(Compound 2) 38
4.2.3 Infrared Spectrum ofEthyl-4-(4-decyloxyphenylazo)benzoate
(Compound 3) 40
1» 1LMA~
lX
4.2.4 Infrared Spectrum ofEthyl-4-{4-hexadecyloxyphenylazo)benzoate
(Compound 4) 42
4.3 IH NMR Analysis 44
4.3.1 IH NMR Analysis ofEthyl-4-(4-hexyloxyphenylazo)benzoate
(Compound 2) 44
4.4 Mesomorphic Properties 51
CHAPTER 5 CONCLUSIONS
REFERENCES
APPENDIX
56
58
62
x
LIST OF TABLES
Page
Table 2.1 Classification of liquid crystals 12
Table 2.2 Phase types of achiral smectic liquid crystals 15
Table 2.3 Transition temperatures for alky1cyanobiphenyl homologues 20
Table 3.1 Chemicals and reagents used in this work 26
Table 4.1 Summary of infrared spectrum of ethyl-4-( 4-
hydroxyphenylazo )benzoate (compound 1) 38
Table 4.2 Summary of infrared spectrum ofethyl-4-(4-
hexyloxyphenylazo)benzoate (compound 2) 40
Table 4.3 Summary of infrared spectrum of ethyl-4-( 4-
dexyloxyphenylazo)benzoate (compound 3) 42
Table 4.4 Summary of infrared spectrum of ethyl-4-( 4-
hexadecyloxyphenylazo )benzoate (compound 4) 44
Table 4.5 Summary of I H NMR data analysis of ethyl-4-( 4-
hexyloxyphenylazo)benzoate (compound 2) 51
Table 4.6 Phase transition temperatures (T 1°C) and associated enthalpy
changes (MIIJg- l) in parentheses for the second DSC
scans of compounds 2, 3 and 4 55
Xl
LIST OF FIGURES
Page
Figure 2.1 1t -+ 1t. electronic transitions in a benzene molecule 8
Figure 2.2 Molecular structure of a typical liquid crystal 9
Figure 2.3 Nematic phase for rod-shaped (on the left) and disc-shaped
molecules (on the right) 13
Figure 2.4 (a) Smectic A and (b) Smectic C phases 14
Figure 2.5 Conversion of a primary amine to diazonium salt 22
Figure 2.6 Mechanism for diazonium coupling reaction of phenol with a
diazonium ion electrophile 23
Figure 2.7 Mechanism for Williamson ether synthesis 24
Figure 2.8 DSC heating and cooling traces of a liquid crystal
25
Figure 3.1 Coupling reaction of phenol with the diazotized
ethyl-4-aminobenzoate 28
Figure 3.2 Synthesis of compound 2 through Williamson ether synthesis 28
Figure 3.3 Synthesis of compound 3 through Williamson ether synthesis 29
Figure 3.4 Synthesis of compound 4 through Williamson ether synthesis 29
Figure 4.1 The scheme for the synthesis of liquid crystals containing
azobenzene and alkyl chains as terminal groups 32
Figure 4.2 Ethyl-4-(4-hydroxyphenylazo)benzoate (compound 1) 33
Figure 4.3 Ethyl-4-( 4-hexyloxyphenylazo )benzoate (compound 2) 34
Figure 4.4 Ethyl-4-(4-decyloxyphenylazo)benzoate (compound 3) 35
Figure 4.5 Ethyl-4-( 4-hexadecyloxyphenylazo )benzoate (compound 4) U S ....
UNIVERSITI MALAYSIA SABAH
Figure 4.6 The infrared spectrum of ethyl-4-(4-hydroxyphenylazo )benzoate
(compound I)
Figure 4.7 The infrared spectrum of ethyl-4-( 4-hexyloxyphenylazo )benzoate
(compound 2)
Figure 4.8 The infrared spectrum of ethy 1-4-( 4-dexyloxyphenylazo )benzoate
(compound 3)
Figure 4.9 The infrared spectrum of ethyl-4-( 4-
hexadecyloxyphenylazo)benzoate (compound 4)
Figure 4.10 IH NMR spectrum of ethyl-4-(4-hexyloxyphenylazo)benzoate
(compound 2)
Figure 4.11 Expansion 1 H NMR spectrum for aromatic ring proton resonances
of ethyl-4-( 4-hexyloxyphenylazo )benzoate (compound 2)
Figure 4.12 Expansion IH NMR spectrum for aromatic ring proton resonances
of ethyl-4-( 4-hexyloxyphenylazo )benzoate (compound 2)
Figure 4.13 Expansion IH NMR spectrum for methylene group resonances of
ethyl-4-( 4-hexyloxyphenylazo )benzoate (compound 2)
Figure 4.14 Expansion IH NMR spectrum for alkyl group resonances of
ethyl-4-( 4-hexyloxyphenylazo )benzoate (compound 2)
Figure 4.15 DSC thermogram of ethyl-4-( 4-hexyloxyphenylazo )benzoate
(compound 2)
Figure 4.16 DSC thermogram of ethyl-4-(4-decyloxyphenylazo)benzoate
(compound 3)
xu
36
39
41
43
46
47
48
49
50
52
53
Figure 4.17 DSC thermogram of ethyl-4-( 4-hexadecyloxyphenylazo )benzoate
(compound 4) 54
LC
LCD
DSC
FT-IR
IHNMR
UV
N
S
SA
SB
Sc
S(
SF
SL
SJ
So
SE
S~
SH
I
TNJ
HN02
HCl
t
m
LIST OF ABBREVIATIONS
liquid crystals
liquid crystal display
differential scanning calorimetry
Fourier transform infrared
proton nuclear magnetic resonance
ultraviolet
nematic
smectic
smectic A
smectic B
smectic C
smectic I
smectic F
smectic L
smectic J
smectic G
smectic E
smectic K
smectic H
isotropic
nematic to isotropic transition temperature
nitrous acid
hydrochloric acid
hydrobromic acid
bimolecular nucleophilic substitution
potassium bromide
crystal
doublet
triplet
multiplet
XIlI
UMS UNIVEASITI MALAYSIA SA BAH
CHAPTER 1
INTRODUCTION
1.1 An Introduction to Liquid Crystals
It wasn't until the early 1970's that the field of liquid crystals (LCs) had really taken
off following the wholesale commercialization of liquid crystal displays (LCDs) due
to the realization by Gray and co-workers at the University of Hull, in which they
synthesized alkylcyanobiphenyls, alkoxycyanobiphenyls and alkylcyanoterphenyls.
These materials possess the right combination of physical properties and chemical
inertness for exploitation in displays (Bruce, 2000; Woollins, 2003). Since then,
research has boomed and this subject has now become one of the most sought after in
view of its expanding technological potentials in organic light emitting diodes,
electro-optical and information-storage devices, and in nonlinear optical systems
(Galli et 01., 1994; Ringsdorf et 01., 1992; Wang et 01., 2000). This is due to their high
carrier mobilities, the anisotropic transport, and polarized emission resulting from the
self-assembling properties and supermolecular structures (phases) of LCs (Gong &
Wan, 2005).
2
LCs are fascinating materials because they exhibit intermediate phases in
symmetry and structure between the solid crystalline state and the amorphous liquid
state (Khoo, 1995). They are typical self-organizing materials which possess exotic
and excellent properties such as a self-organizing nature within a certain temperature
range with fluidity and long-range order, a cooperative effect, a large optical
anisotropy, and an alignment change by an external field at surface and interface
(yamamoto et al., 1999).
In particular, LCs dominate in display devices because of their large optical
anisotropies and ability to undergo electric-field induced switching. LCOs have now
become an indispensable man-machine interface in our life. Numerous efforts are
sought to improve LCD performance. For instance, the relatively slow switching times
of such devices led to the development of liquid crystalline materials wherein the
optical properties can be reversibly controlled by light-activated processes, where
their switching times can be several orders of magnitude faster than electric-field
induced switching. Thus, synthesis of photoresponsive LC systems are currently of
interest because of the advantage of not only in displays, but also in high-speed
information processing, high density optical data storage, optical image processing,
dynamic holography, optical computing. parallel optical logic and pattern recognition
(Davis et al., 2003~ Lutfor et al., 2005; Shishido et al., 1997~ Sung et al., 2002~
Y oshizawa el al., 2005).
Photosensitive moieties such as azobenzene molecules are therefore used to
control LCs by light because they undergo change in structural property upon light
irradiation. Azobenzene compounds are well known for s UNIVERSITI MALAYSIA SABAH
3
between trans isomers with a rod-like shape and cis isomers with a bent shape
(Kanazawa et aI., 1997; Kurihara et aI., 2001). For example, the trans form can
stabilize the LC phase when dissolved in a host LC whereas the cis fonn destabilizes
the LC phase by disorganizing the molecular orientation of the host LC. Therefore, the
trans-cis photoisomerization of the azobenzene in the LC phase can induce
disorganization of the phase structure based on the change in molecular shape of the
photochromic azobenzene molecules (Kurihara et al., 2001; Sung et al., 2002;
Tsutsumi et al., 1997). Although the cis-azobenzene destroys the LC phase, the LC
phase is further enhanced by the trans-azobenzene since it is aligned with the LC
phase. This means that the mesophase of an azobenzene-doped LC can be controlled
by photoirradiation (park et al., 2002). Such photonic control has been mainly applied
in the nematic phase by means of transmission, reflection, and light-scattering modes
(Lee et aI., 2000; Liu & Yang, 2005).
In this study, liquid crystals containing azobenzene and alkyl chains as
tenninal groups will be synthesized. It has been established that the transition
temperatures and phase behaviour depend on the nature oftenninal groups. The length
of tenninal alkyl chain can have a pronounced effect on the mesomorphic properties
of LCs. A systematic study by Date and co-workers on a series of symmetric dimers
composed of two rod-like mesogenic units has revealed interesting behaviour
regarding the dependence of mesomorphic behaviour on the length of the terminal
groups (Achten et al., 2004; Cha et al., 2002; Umadevi & Sadashiva, 2007).
Introducing alkyl chains as terminal groups may improve isomerisation behaviour of
the system and mesophase stabilization may be achieved (Ohta et al., 2007; Pugh el
UMS UNIVERSITI MALAYSIA SA BAH
4
al., 1997). In addition, the length of the alkyl chains has a considerable influence on
the photoresponsiveness ofLC (Tamada et al., 2002).
1.1 Research Objectives
The objectives of this study are:
(a) To prepare the azo containing liquid crystals with alkyl chains.
(b) To identify the molecular structure of the liquid crystals by usmg
spectroscopic methods.
(c) To determine the mesophase of the liquid crystals by usmg differential
scanning calorimetry (DSC).
1.3 Scope of Research
In this study, liquid crystals containing azobenzene and alkyl chains as terminal
groups is carried out by a series of organic syntheses methods such as diazonium
coupling and Williamson ether synthesis, leading to the desired compound. Then, a
characterization study will be introduced on this compound, in which its structural
confirmation is determined by the use of spectroscopic methods, which are the Fourier
transform infrared (FT-IR) and proton nuclear magnetic resonance eH NMR)
spectroscopy. Finally, its mesophase is determined by using DSC in which its phase
transition temperatures and enthalpy changes are evaluated.
UMS UNIVERSITI MALAYSIA SABAH
CHAPTER 1
LITERA TURE REVIEW
1.1 History of Liquid Crystals
Friedrich Reinitzer who was an Austrian botanist came up with the first scientific
description of liquid crystals in 1888. Reinitzer observed an unexpected sequence of
phase transition where two melting points were observed while investigating some c:
! esters of cholesterol. Cholesteryl benzoate melted from a solid to a cloudy liquid at e ~
~rrI -:g 14S.S0C and it turned to a clear liquid at 178.S0 C. Besides, some odd colour behaviour ~ §
r-:»a
~e was detected upon cooling where a pale blue colour first appeared as the clear liquid ~:3 en )10
:a turned cloudy and then a bright blue-violet colour as the cloudy liquid crystallized. ~
Then, a German physicist, Otto Lehmann who was studying the crystallization
properties of various substances received the samples of this substance from Reinitzer.
By using his polarizing microscope, Lehmann noticed that they flow like liquids and
also show optical properties like that of a crystal. The following studies confirmed that
these observed intermediate phases embody a new thermodynamic state of matter.
Lehmann cited them as flowing crystals in the earliest and then utilized the expression
'liquid crystals' (Collyer, 1992; Singh, 2002).
UMS UNIVERSITI MALAYSIA SABAH
6
Throughout 1901 to 1934, the Gennan chemist, Daniel Vorlander, and his co-
workers produced over 80 doctoral dissertations which resulted from work in hi s
group on the syntheses of many new liquid crystalline materials. Lehmann introduced
Georges Friedel to LCs in 1909. In 1922, he published a seminal paper, (Arm. Phys. 18,
273 (1922»), which projected the current classification of liquid crystalline phases
using the words nematic, smectic and cholesteric. He also described LCs in terms of
positional and orientational ordering as well as the effects of magnetic and electric
fields on the LC phases (Fisch, 2006).
1.1 Liquid Crystals
LCs constitute an intennediate state of order (mesophase) between crystals and liquids.
Mesophase is an abbreviation of mesomorphic phases introduced by Friedel in 1922,
in which he defined them as phases with microscopic structures between solids and
ordinary isotropic liquids (Collyer, 1992~ Pestov & Vill, 2005).
In solids, the molecules are very close together and vibrate about their
equilibrium positions. There is long-range positional and orientational order in
crystals. In contrast, the molecules of a liquid can easily move about with random
arrangements. This means that there is no long-range positional and orientational
order in an isotropic liquid. Based on this characterization of the order present in a
crystalline solid and an isotropic liquid, we can define LCs as phases of matter that
have some liquid-like order, some degree of long-range orientational order, and at
times, some positional order (Fisch, 2006). The molecular axes remain parallel to each
other on average, giving rise to a preferred direction in space. The moltyu s s UNIVERSITI MALAYSIA SABAH
7
small tendency to point in more than one direction than others or to spend more time
in various positions than others (Singh, 2002).
2.3 Properties of Liquid Crystals
2.3.1 Electronic Properties
The electronic properties of constituent molecules will directly affect the electronic
properties and processes taking place in LCs (Khoo, 1995).
a. Electronic Transition and Ultraviolet Absorption
The energy levels are referred to as orbitals in common. There are 1[, n. and (1 orbitals
with their excited states named as 7r: ~ n -. and u -. The resonant frequencies of the
molecule are caused by the energy differences between these electronic states which
are connected by dipole transitions. The bands are formed if the levels are large
enough to give rise to absorption bands. The energy levels of aromatic rings act as a
major character since most of the LCs are aromatic compounds that contain one or
more aromatic rings. The 1t -+ 1t- transitions in a benzene molecule have been
substantially studied. Figure 2.1 shows three possible 1t -+ x- transitions in a benzene
molecule (Khoo, 1995).
UMS UNIVERSITI MALAYSIA SABAH
• 1..\ A.;: 1..3
lEI ..
IBI ..
IB:u ).\ = 180 om
A.;: = 230 run
I All! >"3 = 256 om
Figure 2.11t -+ 1t. electronic transitions in a benzene molecule (Khoo, 1995).
8
Commonly, these transitions correlate to the absorption of light in the near
ultraviolet (UV) spectral region (~ 200 nm). Hence, a benzene molecule can also be
used for interpreting absorption of LCs containing phenyl rings. On the other hand,
normally only (j electrons are comprised in a saturated cyclohexane ring. Hence, the
(J -+ (J. transitions correlate to absorption of light of shorter wavelength (~ 180 nm).
Besides, these electronic properties can be observed by the existence of conjugation
such as the 7t electron's wave function in conjugated molecule is delocalized along the
conjugation length, causing a longer wavelength region in absorption of light
compared to those without conjugation (Khoo, 1995).
b. Visible and Infrared Absorption
To some extent, LCs are absorptive in the UV region especially for organic molecules.
LCs are somewhat transparent in the visible and near-infrared regime which is from
0.4 J.1m to 5 J.1m since there are fewer absorption bands. The rovibrational transitions
start to dominate as the wavelength is increased toward the infrared (~ 9 J.1m) resulting
quite absorptive behaviours ofLCs in the infrared regime (Khoo, 1995).
9
2.3.2 Chemical Properties
Fundamentally, most of the LCs are aromatic and comprise of benzene rings. Figure
2.2 shows the basic structure of the most commonly occurring LC molecules.
Generally, aromatic LC molecules consist of a side chain R, two or more aromatic
rings A and A' which are connected by linkage groups X and Y, and at the other end
connected to a terminal group R' (Khoo, 1995).
R A x
Side chain
A' R'
Terminal group
Figure 2.2 Molecular structure of a typical liquid crystal (Khoo, 1995).
The examples of side-chains and terminal groups are the alkyl (CnH2n+l),
alkoxy (CnH2o+10), and others such as acyloxyl, alkylcarbonate, alkoxycarbonyl, and
the nitro and cyano groups. Whereas the X' s of the linkage groups are simple bonds
such as azoxy (-N=N-), Schiff base (-CH=N-), stilbene (-CH=CH-), esters, tolane,
acetylene, and diacetylene. The central linkage group is the key to define the names of
LCs. The aromatic rings include saturated cyclohexane or unsaturated phenyl,
bipheny~ and terphenyl in various combinations (Khoo, 1995).
Typically, all the physical and optical properties of LCs are affected by the
properties of these constituent groups and how they are chemically synthesized
together that give rise to dielectric constants, elastic constants, viscosities, absorption
spectra, and transition temperatures, existence of mesophases, aniso s UNIVERSITI MALAYSIA SA BAH
10
optical nonlinearities. The chemical stability depends more on the central linkage
group. Schiff-base LCs are often quite unstable whereas ester, azo, and azoxy
compounds are more stable but are also quite susceptible to moisture, temperature
change, and UV radiation. The most stable Les are compounds without a central
linkage group such as 5CB (pentylcyanobiphenyl), pyrimide. and phenylcyclohexane
(Khoo. 1995).
2.3.3 Physical Properties
The physical properties of LCs can be characterized into scalar and nonscalar
properties. The common scalar properties are the thermodynamic transition
parameters such as transition temperature, transition enthalpy, and entropy changes,
transition density and fractional density changes. The major nonscalar properties are
dielectric. diamagnetic, optical, elastic, and viscous coefficients (Singh, 2002). The
long range order of molecules in a mesophase is distinguished by the order parameter
S = 0.5[3(cos26) - 1] where 6) is the momentary angle between the long axis of the
molecule and the director. The order parameter S equals one in an ideal crysta~ and it
equals zero in an isotropic liquid. The order parameter lies in the range 0.5-0.7 in a
nematic phase (pestov & VilL 2005).
The liquid crystalline states can be distinguished through their orientation,
positional and conformation orders. The orientational order plays the significant role
since it exists in all the LC phases. The positional order is essential to distinguish the
various phases (nematic, smectic A, smectic C, etc.). How the rings, fractional groups
UMS UNIVERSITI MALAYSIA SA BAH
58
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