Mesomorphic properties and X-ray diffraction studies of 4-alkanoyloxybenzylidene-4′-fluoroaniline

Article abstract of DOI:10.14233/ajchem.2014.17676

A new homologous series of Schiff base esters comprising a terminal fluoro substituent was studied. Molecular polarizability is enhanced by the presence of fluoro substituent, subsequently, affecting intermolecular interactions and resulting in smectic molecular arrangement. All members can be differed from the number of carbons at the alkanoyloxy chain (C_n-1H_2n-1COO^-, n = 10, 12, 14, 16, 18). The structure of the title compound was confirmed by spectroscopic (IR and NMR) techniques along with mass spectrometric analysis. Their mesomorphic properties were studied by using differential scanning calorimetry and optical polarizing microscopy techniques. Early members (n = 10, 12 and 14) exhibited monotropic smectic A phase while the longer chain members (n = 16 and 18) are non mesogenic compounds. X-ray diffraction analysis was used and confirmed monolayer of smectic A phase in n-dodecanoyloxy derivative.

Full text of DOI:10.14233/ajchem.2014.17676

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Asian Journal of Chemistry; Vol. 26, No. 22 (2014), 7737-7740
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.17676
Mesomorphic Properties and X-Ray Diffraction
Studies of 4-Alkanoyloxybenzylidene-4'-fluoroaniline
1,*
1
2
3
4
5
SIE-TIONG HA , TECK-LEONG LEE , SIEW-LING LEE , GUAN-YEOW YEAP , HONG-CHEU LIN and MASATO M. ITO
1Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia
2Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia
3Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
4Department of Material Science & Engineering, National Chiao-Tung University, 1001 Ta-Hsueh Road, Hsinchu 300, Taiwan
5Faculty of Engineering, Soka University, 1-236 Tangi-cho, Hachioji, Tokyo 192-8577, Japan
*Corresponding author: Fax: +60 5 4661676; E-mail: hast_utar@yahoo.com
Received: 27 March 2014;
Accepted: 13 May 2014;
Published online: 6 November 2014;
AJC-16224
A new homologous series of Schiff base esters comprising a terminal fluoro substituent was studied. Molecular polarizability is enhanced
by the presence of fluoro substituent, subsequently, affecting intermolecular interactions and resulting in smectic molecular arrangement.
All members can be differed from the number of carbons at the alkanoyloxy chain (Cn-1H2n-1COO–, n = 10, 12, 14, 16, 18). The structure
of the title compound was confirmed by spectroscopic (IR and NMR) techniques along with mass spectrometric analysis. Their mesomorphic
properties were studied by using differential scanning calorimetry and optical polarizing microscopy techniques. Early members (n = 10,
12 and 14) exhibited monotropic smectic A phase while the longer chain members (n = 16 and 18) are non mesogenic compounds. X-ray
diffraction analysis was used and confirmed monolayer of smectic A phase in n-dodecanoyloxy derivative.
Keywords: X-ray diffraction analysis, Monotropic, Smectic A, 4-Alkanoyloxybenzylidene-4'-fluoroaniline.
In the previous studies13-18, it was concluded that linking
INTRODUCTION
groups such as Schiff base and ester are good for generating
Liquid crystals (LCs) are known as fourth state of matter,
mesomorphism in two and three unsaturate rings thermotropic
which own the anisotropic properties of crystals as well as
liquid crystals. This continuation project involves the study of
fluid properties of liquids. It was first reported by Otto Lehman
4-alkanoyloxybenzylidene-4'-fluoroaniline possessing even
in 1889 after he investigated the double-melting points
number of carbon atoms at alkanoyloxy chain (Cn-1H2n-1COO-,
phenomenon of molten cholesteryl benzoate1. The self-assembly
n = 10, 12, 14, 16, 18). The mesomorphic properties of octa-
properties of liquid crystals play a significant role in producing
noyloxy (C8) and dodecanoyloxy (C12) derivatives are reported
specific properties to a particular phase. This gives different
in the previous paper19. As for current work, new homologous
order parameters in different mesophases2. Liquid crystals
members are prepared and characterized along with X-ray
found to be applied in display technology, advanced technolo-
diffraction technique for confirming the smectic phase in C12
gical devices and prototypical self-organizing molecular
derivative.
materials3.
A minor structure modification results a great effect on
EXPERIMENTAL
the mesomorphic property. Therefore, in order to prepare a
FT-IR spectra were collected using a Perkin-Elmer System
2000 FT-IR Spectrometer. 1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were obtaing using Bruker Avance 400
MHz Spectrometer. The solvent used was deuterated chloro-
form. EI mass spectrum (70 eV) were obtained with a Mass
Spectrometer Finnigan MAT95XL-T.
Liquid crystal study: Phase transition temperatures and
associated enthalpies were obtained using either differential
scanning calorimeter Mettler Toledo DSC823e or SII Nano-
new mesogen with desirable property and application, it is
essential to understand chemical structure-mesomorphic
property relationships4. Low-mass calamitic liquid crystals
consisting of two aromatic rings with at least one terminal
substituent, for instance, 4-methoxybenzylidene-4'-butyl-
aniline (MBBA), are capable of showing liquid crystal pro-
perties5,6. The discovery of 4-methoxybenzylidene-4'-butyl-
aniline has sparked a renewed interest of researcher on Schiff
bases due to its room temperature nematic phase7-12
.

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7738 Ha et al.
Asian J. Chem.
technologies Co. DSC6100. Carl Zeiss polarizing optical
microscope attached with Linkam hotstage was used for liquid
crystal texture studies.Video camera (Video Master coomo20P)
attached to the polarizing microscope was coupled with video
capture card, allowing real-time video capturing and image
saving. Textures exhibited by the samples were observed using
polarized light with crossed polarizers. Samples were sand-
wiched between glass slide and cover slip as thin films. Liquid
crystal texture confirmation was made by comparing the
Synthesis of 4-alkanoyloxybenzylidene-4'-fluoroaniline
(nABFA): 4-Alkanoyloxybenzylidene-4'-fluoroaniline were
synthesized via one pot esterification method23. 4-Hydroxy-
benzylidene-4'-fluoroaniline (3 mmol), fatty acid (3 mmol,
CnH2n-1COOH where n = 10, 12, 14, 16, 18), DMAP (1 mmol)
and DCC (3 mmol) were mixed in suitable amount of THF
and stirred for 6 h. Then, the solvent was evaporated in the
fume hood. Product obtained was recrystallized several times
with hexane and ethanol in order to yield pure compound. IR,
13
experimental with the literature textures20,21
.
NMR (1H and C) and mass spectral data of representative
X-ray diffraction analysis: Synchrotron powder X-ray
diffraction measurements were performed at beamline BL17A
of the National Synchrotron Radiation Research Center in
Taiwan. The analysis was done according the methods described
in previous work16.
Materials and synthesis: Fatty acids (CnH2n-1COOH
where n = 10, 12, 14, 16, 18) and 4-dimethylaminopyridine
(DMAP) were obtained from Merck Chemicals (Germany).
Dicyclohexylcarboiimide (DCC), 4-hydroxybenzaldehyde and
4-fluoroaniline were supplied by Acros Organics (USA).
The synthetic route of 4-alkanoyloxybenzylidene-4'-
fluoroaniline, nABFA is illustrated in Fig. 1.
compound, 12ABFA, are given below: IR (KBr, νmax, cm-1):
2956, 2921, 2851 (C-H aliphatic), 1751 (C=O ester), 1624
(C=N), 1205, 1096 (C-O ester). 1H NMR (400 MHz, CDCl3):
δ/ppm 0.9 (t, 3H, CH3-), 1.3-1.5 (m, 16H, CH3-(CH2)8-), 1.8
(pentet, 2H, -CH2-CH2-COO-), 2.6 (t, 2H, -CH2-COO-), 7.1
(d, 2H, Ar-H), 7.2 (m, 4H, Ar-H), 7.9 (d, 2H, Ar-H), 8.4 (s,
13
1H, -CH=N-). C NMR (100 MHz, CDCl3): δ/ppm 171.91
(C=O ester,) 158.85 (-C=N-), 162.90, 153.21, 147.94, 133.67,
129.91, 122.22, 116.01, 115.71 for aromatic carbons, 34.43
(-COO-CH2-), 31.90 (-COO-CH2-CH2-), 29.59, 29.44, 29.32,
29.24, 29.10 for methylene carbons [-(CH2)6 CH2CH2CH3],
24.89 (-CH2CH2CH3), 22.67 (-CH2CH3), 14.09 (-CH3). EI-MS
m/z (rel. int. %): 397(6) (M)+, 215 (100).
O
RESULTS AND DISCUSSION
H2N
F
HO
+
Mesomorphic properties: Under polarized optical
microscope, n-decanoyloxy to n-tetradecanoyloxy derivatives
exhibited smectic A phase while n-hexadecanoyloxy and
n-octadecanoyloxy derivatives did not display mesophase.
Optical photomicrograph of 14ABFA is shown in Fig. 2.
14ABFA showed co-existence of homogenous fan-shaped and
homeotropic (dark region) textures of smectic A phase. The
results from microscope observation were confirmed with the
DSC data. DSC thermogram of 14ABFA is given in Fig. 3 as
representative illustration. Phase transition temperatures and
enthalpy changes upon heating and cooling scans obtained
from DSC measurements are summarized in Table-1.
Plot of transition temperatures against number of carbon
atoms in alkanoyloxy chain is shown in Fig. 4. Based on the
plot, it can be observed that mesophase properties greatly
depended on the length of alkanoyloxy chain. Crystal-isotropic
transition temperatures exhibited ascending trend as length of
CH3OH
N
F
HO
HBFA
1. Cn-1H2n-1COOH
2. DCC, DMAP
3. THF
N
F
TABLE-1
PERCENTAGE YIELD OF SYNTHESIS, PHASE TRANSITION
TEMPERATURES AND ENTHALPY CHANGES OF nABFA
UPON HEATING AND COOLING SCANS
Cn-1H2n-1COO
nABFA
Yield
(%)
Phase transition temperature, °C
(enthalpy change, kJ mol–1)
Heating: Cr 68.3 (23.0) I
Cooling: I 84.7 (0.2) SmA 36.7 (25.1) Cr
Heating: Cr 80.5 (53.2) I
Cooling: I 74.1 (7.4) SmA 49.6 (43.3) Cr
Heating: Cr 84.3 (52.2) I
Cooling: I 67.4 (5.4) SmA 58.7 (44.9) Cr
Heating: Cr 81.9 (55.2) I
Cooling: I 66.6 (53.8) Cr
Heating Cr 86.8 (59.7) I
Cooling: I 77.5 (61.4) Cr
Compound
10ABFA
12ABFA
14ABFA
16ABFA
18ABFA
where n = 10, 12, 14, 16, 18
Fig. 1. Synthetic route for the 4-alkanoyloxybenzylidene-4'-fluoroaniline
38
41
41
44
45
Synthesis of 4-hydroxybenzylidene-4'-fluoroaniline,
(HBFA): 4-Hydroxybenzylidene-4'-fluoroaniline was prepared
according to a previously described method22. Equal molar
(5 mmol) of 4-hydroxybenzaldehyde and 4-fluoroaniline,
along with 30 mL of methanol, were refluxed with stirring for
3 h. Then, the mixture was allowed to cool to room temperature
and filtered. The yellow product was washed several times
with methanol.
Note: Cr = crystal; SmA = smectic A; I = isotropic.

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Vol. 26, No. 22 (2014)
Mesomorphic Properties and X-Ray Diffraction Studies of 4-Alkanoyloxybenzylidene-4'-fluoroaniline 7739
derivatives during cooling cycle indicated the presence of
monotropic phase. The melting point of monotropic liquid
crystal was always equal to or higher than the clearing points,
hence displaying supercooling effect24. SmecticA phase range
decreased from n-decanoyloxy (∆SmA = 48 °C) to n-tetrade-
canoyloxy (∆SmA = 8.7 °C) derivatives before the mesophase
is diminished in longer alkyl chain derivatives (n-hexade-
canoyloxy and n-octadecanoyloxy derivatives). This indicated
that 10ABFA having the optimum balance of rigidity and
flexibility which contributed to its widest mesophase range
than 12ABFA and 14ABFA.
16ABFA and 18ABFA are not mesogens due to excessive
length of alkanoyloxy chain, resulting in high melting point,
thus suppressing formation of mesophase25.
X-ray diffraction studies: In order to study the molecular
arrangement of the mesophase, temperature-dependent XRD
analysis was carried out for 12ABFA. The XRD pattern of the
representative compound 12ABFA is shown in Fig. 5 while
the powder X-ray diffraction data is given in Table-2. Two
pseudo-Bragg low angle diffraction peaks, 2° and 4° are notice-
able from the XRD diffractogram. Ratio of angle of diffraction
(2θ, °) of these peaks was found to be 1 and 2 which indicate
a smectic phase (1-D stack of layers)26. Conversely, diffuse
maxima, broad diffraction peak in wide angle region indicates
the characteristic of liquid-like order of alkyl chains within
smectic planes. Distance for wide angle is 4.5 Å which is the
typical values (4.4 to 4.6 Å) for calamitic mesophase. It also
means that there is no regular intermolecular distance inside
the layers, which confirmed is smectic A phase27. In addition,
d-layer spacing (31.1 Å) was found to be larger than molecular
length (31.4 Å) predicted by MM2 molecular calculation. The
calculated d/l ratio is 1.08 (approximately to 1), therefore,
confirmed monolayer arrangement of smectic A phase in
12ABFA28. Hence, it can be assumed that on an average, the
long molecular axis of 12ABFA pointed in one favorable
direction with a small interaction coefficient.
Fig. 2. Optical photomicrograph of 14ABFA exhibiting homogenous fan-
shaped and homeotropic textures of smectic A phase
Heating
Cr
I
SmA
I
Cr
Cooling
30 40
20
50 60
70
80 90 100
Temperature (°C)
110 120 130 140
Fig. 3. DSC thermogram of 14ABFA
90
85
80
75
70
65
60
55
50
45
40
35
30
TABLE-2
POWDER X-RAY DIFFRACTION DATA OF 12ABFA
Phase observed
Temperature (oC)
2θ (o)
Smectic A
90
2.03
d-spacing (Å)
Length, l (Å)
Aspect ratio (d/l)
Phase
31.1
31.4
0.99
Smectic A
Monolayer
C10
C12
C14
C16
C18
Arrangement
Number of carbon atoms (n) alkanoyloxy chain
Cr-to-I
I/SmA-to-Cr
Structural studies: Structural identification of nABFA
was carried out by employing IR and NMR spectroscopic and
EI-MS spectrometric techniques. EI-MS spectrum of 12ABFA
is shown in Fig. 6 as representative illustration. Molecular ion
peak at m/z 397 suggested molecular formula of C25H32NO2F,
which supported the proposed structure of 12ABFA.
Based on FT-IR spectrum of 12ABFA, absorption peaks
at 2956, 2921, 2851 cm-1 can be assigned to aliphatic groups.
Three absorption peaks owing to the presence of ester group
were observed at 1751, 1205, 1096 cm-1. Absorption peak
belonged to imine (C=N) group was observed at 1624 cm-1.
Fig. 4. Plot of phase transition temperature against number of carbon atoms
(n) in alkanoyloxy chain of nABFA
alkanoyloxy chain increased from n-decanoyloxy to n-octade-
canoyloxy except n-hexadecanoyloxy derivative. Same pheno-
menon was observed for isotropic or mesophase-to-crystal
transition temperatures (Fig. 4).
Out of five members, only three members (n = 10, 12,
14) exhibited mesophase. Additional peak of low enthalpy
change were observed on n-decanoyloxy to n-tetradecanoyloxy

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7740 Ha et al.
Asian J. Chem.
n-decanoyloxy to n-tetradecanoyloxy derivatives. Excessive
length of alkyl chain in n-hexadecanoyloxy and n-octadeca-
noyloxy derivatives has caused the mesophase diminished;
therefore, these compounds are non-mesogens.
2500
2000
SmA, cooling, at 65 °C, 12ABFA
ACKNOWLEDGEMENTS
1500
1000
500
0
The authors thank Universiti Tunku Abdul Rahman and
Ministry of Higher Education Malaysia for the financial
supports and research facilities. The powder XRD measurements
were supported by Beamline BL17A (charged by Dr. Jey-Jau
Lee) of the National Synchrotron Radiation Research Center,
Taiwan.
0
2
4
6
8
10 12 14 16 18 20 22 24
(°)
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1H NMR spectrum of 12ABFA shows two triplet signals
at δ = 0.9 and 2.6 ppm, which were, respectively ascribed
to the methyl (CH3–) and methylene protons of -CH2COO-Ar.
Methylene protons due to the long alkyl chain {-(CH2)8-}
exhibited the multiplet peaks at δ = 1.3-1.5 ppm. Eight aromatic
protons exhibited multiplet signals at the chemical shifts range
of δ = 7.1-7.9 ppm. Imine proton was observed as singlet signal
at the most downfield region, δ = 8.4 ppm18. The 13C signal at
δ = 14.09 ppm was attributed to the methyl carbon while the
signals observed at chemical shifts range of δ = 22.67-34.43
ppm are belonged to the methylene carbons of the long alkyl
chain. Signals resonated between δ = 115.71-162.90 ppm is
belonged two twelve aromatic protons. Whilst signal attributed
to azomethine carbon showed signal at δ = 158.85 ppm, carbonyl
carbon exhibited signal at δ = 171.91 ppm.
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The synthesis and mesomorphic behavior of homologous
series of 4-alkanoyloxybenzylidene-4-fluoroaniline is reported
in the current paper. Smectic A phase was observed from the