Design and characterization of novel bis-benzamide liquid crystalline materials

Article abstract of DOI:10.1016/j.molstruc.2017.08.106

A new homologous series of symmetric, bent-shaped bis-benzamide dimers have been prepared. Several 1,n-bis(p-aminophenoxy)alkanes (n = 3, 5, 9,10,11) were employed as spacers and p-hexyloxy tails have been synthesized and appended to the spacers by amide linking groups. Different important parameters were explored using computational analysis by semi empirical method. The experimental results were correlated with theoretical studies and relationship between molecular structure and mesogenic behavior has been established. The mesomorphic properties of the resultant dimers were characterized by differential scanning calorimetry (DSC) and polarized optical microscopy (POM) equipped with a hot stage. Change in mesomorphic properties with change of methylene spacers was observed. Enantiotropic mesogenic behavior was exhibited by D₃A₆, D₁₀A₆ and D₁₁A₆ and the needle like and blurred schleiren textures were observed. It was observed that increased methylene spacers chain length decreased the melting temperatures. Thermogravimetric analysis revealed the thermal stability of dimers upto 360 °C.

Full text of DOI:10.1016/j.molstruc.2017.08.106

Accepted Manuscript
Design and characterization of novel bis-benzamide liquid crystalline materials
Asma Iqbal, Humaira Masood Siddiqi, Zareen Akhter, Muhammad Qaiser Fatmi
PII:
S0022-2860(17)31161-4
10.1016/j.molstruc.2017.08.106
MOLSTR 24244
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 19 April 2017
Revised Date: 21 August 2017
Accepted Date: 24 August 2017
Please cite this article as: A. Iqbal, H.M. Siddiqi, Z. Akhter, M. Qaiser Fatmi, Design and characterization
of novel bis-benzamide liquid crystalline materials, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.08.106.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Design and characterization of novel bis-benzamide liquid crystalline
materials
Asma Iqbala, Humaira Masood Siddiqia*, Zareen Akhtera and Muhammad Qaiser
Fatmib
aDepartment of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan;
bDepartment of Biosciences, COMSATS Institute of information technology Chak
Shahzad, Islamabad, Pakistan.
Corresponding author* Email: humaira_siddiqi@yahoo.com, humairas@qau.edu.pk
Graphical Abstract:
Abstract
A new homologous series of symmetric, bent-shaped bis-benzamide dimers
have been prepared. Several 1,n-bis(p-aminophenoxy)alkanes (n= 3, 5, 9,10,11) were
employed as spacers and p-hexyloxy tails have been synthesized and appended to the
1

ACCEPTED MANUSCRIPT
spacers by amide linking groups. Different important parameters were explored using
computational analysis by semi empirical method. The experimental results were
correlated with theoretical studies and relationship between molecular structure and
mesogenic behavior has been established. The mesomorphic properties of the
resultant dimers were characterized by differential scanning calorimetry (DSC) and
polarized optical microscopy (POM) equipped with a hot stage. Change in
mesomorphic properties with change of methylene spacers was observed.
Enantiotropic mesogenic behavior was exhibited by D3A6, D10A6 and D11A6 and the
needle like and blurred schleiren textures were observed. It was observed that
increased methylene spacers chain length decreased the melting temperatures.
Thermogravimetric analysis revealed the thermal stability of dimers upto 360 oC.
Keywords: symmetric bis-benzamides; enantiotropic mesogens; blurred
schleiren; thermally stable.
1. Introduction:
Liquid crystal materials are one of the extensively investigated dynamic research
area playing an important technological role in new and emerging electro-optics1.
Thermotropic liquid crystals are the type of liquid crystals which consist of either rod
like or disc like organic molecules. Rod like thermotropic liquid crystals contain
linearly linked ring systems and flexible chains having polarizable moieties2,3. As the
functional groups influences the mesophase behaviour so functional groups having
permanent dipole moment are commonly incorporated to maintain the linearity as
well as the stability of mesogens. Azo4-6, azomethine7,8, esters9 are among the
commonly used linking groups and are known for mesomorphic properties.
Flexible spacers having similar groups on either side are termed as dimeric
mesogens which possess the decisive fundamental components of many thermotropic
main group polymers10,11. Additionally, liquid crystalline behaviour of dimeric
compounds is mostly controlled by its molecular design and a minor change in the
molecular architecture can significantly amend its properties12. Modification in spacer
length and terminal chain length brings about variation in liquid crystalline phases. A
variety of mesogenic compounds based on p-alkoxybenzoic acid tails and their
derivatives with different flexible spacers have been widely explored for the design of
diversity in liquid crystalline behaviour13-18
.
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The compounds having amide linking groups with aromatic rings are thought
to promote the formation of hydrogen bonding between different molecules. Such
arrangement of molecules may support the formation of layered structures thus
imparting mesomorphic behaviour. An extensive literature survey revealed that liquid
crystals having an amide linkage along with a flexible methylene spacer may exhibit
unique properties like supramolecular arrangement and stability19. Distinctive
macroscopic polarization within layers of bent shape molecules are capable of special
steric interactions causing liquid crystalline behaviour20-22
.
H-bonding between molecules both in calamitic and bent-shaped materials
supports smectic mesomorphism23. Very recently, the amino group has been also
utilized for the formation of an amide linking unit in bent-shaped materials which
exhibited a smectic A phase whereas no liquid crystalline properties appeared in
corresponding N-methyl derivatives owing to an altered molecular conformation and
missing hydrogen bonding24. It is generally well acknowledged that the amide group
exhibits higher rigidity due to the partial double bond character of the C–N bond,
which results in higher clearing temperatures of mesogens in comparison to the
analogous materials possessing different linking groups like esters, schiff bases etc.25.
However, there are only a few examples of low molecular weight mesogens with an
amide linking group to evaluate the role of the amide group on mesomorphic
properties. In another work three series of novel bent-shaped liquid crystals based on
a 3-aminophenol central core possessing five aromatic units have been reported26. In
literature so far, no such symmetrical dimeric benzamide mesogens have been
investigated, so keeping in mind the importance of this class of compounds27 we have
synthesized various bis-benzamides. Our newly prepared bis-benzamides have
different methylene spacers with ether linkage (to induce flexibility in molecular
chains), two amide groups as linking groups and four benzene rings to induce rigidity
in structures.
2 Experimental
2.1 Materials
Alkyl di bromides (1,3- 1,5- 1,9- 1,10- 1,11-), hexylbromide , 4-
hydroxybenzoic acid, hydrazine monohydrate, 4-nitrophenol, palladium on activated
carbon (Pd/C, 10%), anhydrous ethanol (EtOH) were purchased from Aldrich
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Chemical Co. and used as received. All other solvents and reagents were purchased
commercially and used without further purification.
2.1.1. General Procedure for the p-Aminophenoxy Alkanes synthesis (Dn)
The synthesis of p-aminophenoxy alkanes was accomplished in two steps, as
shown in Figure. 1. In the first step various p-nitrophenoxyalkanes were synthesized
according to the given procedure28. Finely powdered anhydrous potassium carbonate
(7.86 g, 57 mmol) was added to a solution of p-nitrophenol (8.0 g, 57 mmol) in DMF
(50 mL) and toluene (30 mL). The reaction mixture was stirred at room temperature
for one hour under an inert atmosphere of nitrogen. Afterwards, dropwise addition of
the corresponding dibromoalkane (28.5 mmol) was made over a period of 30 minutes
and the mixture was maintained at 120 ºC for 12 hours. The conversion of reactants
into the product was monitored by TLC (n-hexane: ethyl acetate 1:4). After the
complete consumption of reactants, the reaction mixture was cooled to room
temperature and then precipitated in distilled water, filtered and recrystallized from
ethanol. In the second step, 10% Pd/C (0.03 g) was added in solution of p-
nitrophenoxy alkanes (4 mmol) in 90 mL of ethanol followed by the dropwise
addition of hydrazinium monohydrate (3.8 mL) to the reaction mixture over a period
of 1 hour. The reaction mixture was refluxed for three hours after which the catalyst
was filtered off. The p-aminophenoxy alkanes (D3,D5,D9,D10 and D11) were obtained
by precipitation in distilled water, followed by drying and recrystallization in the
smallest possible volume of ethanol29.
COCH3
C6H13 Br
COOH
i.K2CO3
,DMF
ii. 0.5M KOH,EtOH
HO
C6H13O
A6
Hydrazinemonohydrate
Pd/C, Ethanol
NO2
O
O
O
O
K2CO3
DMF
C
C
n
n
Br
Br
n
H2
H2
C
3 hr Reflux
12hr Reflux
H2
O2N
NO2
H2N
NH
2
Dn
OH
n= 3,5,9,10,11
O
O
n
COOH
C
O
O
H2
i.SOCl2
C N
H
N C
H
C6H13O
ii.Dn, THF,Et3N
C6H13O
OC6H13
DnA6
Figure 1. Synthesis of bis-benzamides
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2.1.2 Synthesis of 4-hexyloxybenzoic acid (A6)
4-hexyloxy benzoic acid (A6) was prepared by hexylation and hydrolysis of ethyl 4-
hydroxybenzoate using standard procedure. To the solution of ethyl 4-hydroxybenzoate
(120.4 mmol, 20 g) in acetone (80 mL) and 10% dry DMSO, 20 g of finely powdered
anhydrous K2CO3 was added. Afterwards, dropwise addition of 1-bromohexane (120.4
mmol, 19.8 g) was made while subsequently purging with nitrogen gas and refluxing the
reaction mixture for 48 hours and monitored by TLC using (n-hexane:ethylacetate:
methanol 9:4:1). After successful completion reaction mixture was cooled to room
temperature poured into 200 mL of water, acidified with few drops of dil. HCl. Later
product was separated by extracting with DCM and dried over MgSO4 anhydrous,
filtered and then solvent was evaporated. (Yield 78 %). In next step, a mixture of
hexyloxy ester (3g, 0.012 mol), 0.5M KOH in ethanol and 5 % H2O (150 mL) was
heated under reflux for 12 hours. The mixture was acidified with 10 % HCl and
precipitated in water. The solid was filtered and recrystallized in absolute ethanol.
Product was dried under vacuum at room temperature. Yield 82 %, m.p. 100-1050C
(lit.103- 1050C)
2.1.3 Synthesis of bis-benzamides (DnA6)
The synthetic route used in the preparation of bis-benzamides is illustrated in
Figure. 1. p-hexyloxy benzoic acid was converted into p-hexyloxy benzoyl chloride
and then coupled with 1,n-bis(p-aminophenoxy)alkanes to yield respective bis-
benzamides. The synthetic details for bis-benzamide compounds are as follows, p-
hexyloxy benzoic acid (A6) was reacted with five diamines including D3, D5, D9, D10
and D11 to synthesize five novel dimeric bis-benzamides (D3A6, D5A6, D9A6, D10A6
and D11A6). For this purpose, A6 was converted into respected acid chloride, under
anhydrous condition. Afterwards an equimolar amount of diamine completely
dissolved in dry THF and triethylamine, was added in one portion into the acid
chloride, while stirring for 24 hours under inert conditions. The precipitates formed
when reaction mixture was poured into acidified water and filtered. Products thus
obtained were found to be pure enough by TLC (EtOAc: n-hexane 2:1). The purity of
products further confirmed by NMR, FTIR, and CHN analysis.
2.1.3.1 Synthesis of 4,4’-bis[4-(dihexyl)oxy]benzamidediphenoxypropane (D3A6)
Yield 75%, m.p.166-203 °C, Rf = 0.6. Anal. for (C41H50N2O6) in wt% Calc. C =
73.85, H = 7.56, N = 4.20 and found to be C = 74.01, H = 7.48, N = 3.99. FTIR:
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(ῡ/cm-1) N-H (3343), Csp3-H (2917, 2850), C=O (1639), C=C (1607, 1506), C-O
1
(1251), para disubstitution (840). H NMR (300MHz, DMSO) δ (ppm): 9.95 (s, 1H,
H-N), 7.93(d, 4H, ortho to C=O) 7.63(d, 4H, ortho to NH-C=O), 6.93(d, 8H, ortho to
O-Ph), 4.05(t, -OCH2), 1.729 (quint. -CH2 next to -OCH2), 2.14 (pent, 1H, H12),
1.24(m, 6H, methylene H’s), 0.84(t, 3H, -CH3).
2.1.3.2. Synthesis of 4,4’-bis[4-(dihexyl)oxy]benzamidediphenoxypentane (D5A6)
Yield 78%, m.p.208°C, Rf = 0.62. Anal. for (C43H54N2O6) in wt% Calc. C = 74.32, H =
7.83, N = 4.03 and found to be C = 74.01, H = 7.98, N = 3.99. FTIR: (ῡ/cm-1) N-H
(3276), Csp3-H (2927, 2857), C=O, (1643), C=C (Ar) (1607, 1513), C-O (1240), p-
1
disubstitution (838). H NMR (300 MHz, DMSO) δ (ppm): 9.94(s, 2H, H-N), 7.91(d
,4H,ortho to C=O), 7.63(d, 4H, ortho to NH-C=O), 6.90 (d, 8H, ortho to O-Ph), 4.08 (t, -
OCH2), 3.96 (t, (b), 2H, H11), 1.74 (quint.-CH2 next to -OCH2), 1.42(m, 7H, Methylenic
H’s), 0.82(t, 3H, CH3).
2.1.3.3. Synthesis of 4,4’-bis[4-(Dihexyl)oxy]benzamidediphenoxynonane(D9A6).
Yield 78%, m.p.148-199 °C, Rf = 0.56. Anal. for (C47H62N2O6) in wt% Calc. C = 75.17,
H = 8.32, N = 3.66 and found to be C = 74.97, H = 7.98, N = 3.59 , FTIR: (ῡ/cm-1): N-H
(3303), Csp3-H (2920, 2850), C=O, (1637), C=C (Ar) (1604, 1508), C-O (1245), p-
disubstitution (836).1H NMR (300MHz, CDCl3) δ (ppm): 7.83 (d, 4H, ortho to C=O),
7.52 (d, 4H, ortho to NH-C=O), 6.82 (d, 8H, ortho to O-Ph) 4.006 (t, -OCH2), 1.77
(quint.-CH2 next to -OCH2), 1.37(m, 11H, Methylene H’s), 0.90 (t, 3H, -CH3).
2.1.3.4. Synthesis of 4,4’-bis[4-(dihexyl)oxy]benzamidediphenoxydecane (D10A6).
Yield 75%, m.p.112-198 °C, Rf = 0.68. Anal. for D10A6 (C48H64N2O6) in wt% Calc. C =
75.36, H = 8.43, N = 3.66 and found to be C = 75.60, H = 8.24 , N = 3.49 FTIR: (ῡ/cm-
1): N-H (3309), Csp3-H (2918, 2849), C=O, (1637), C=C (Ar) (1609, 1512), C-O (1247),
1
p-disubstitution (836). H NMR (300MHz, DMSO) δ (ppm): 9.92 (s, 2H, H-N), 7.91(d,
4H, ortho to C=O),7.68 (d, 4H, ortho to NH-C=O), 6.95 (d, 8H, ortho to O-Ph),4.02 (t, -
OCH2), 1.80 (quint.-CH2 next to -OCH2), 1.33 (m, 13H, Methylene H’s), 0.81 (t,3H,-
CH3).
2.1.3.5. Synthesis of 4,4’-bis[4-(dihexyl)oxy]benzamidediphenoxyundecane
(D11A6) Yield 75%, m.p.112-198 °C, Rf = 0.71. Anal. for D11A6, (C49H66N2O6) in wt%
Calc. C = 75.54, H = 8.54, N = 3.60 and found to be C = 75.77, H = 8.68, N = 3.55,
FTIR: (ῡ/cm-1) N-H (3307), Csp3-H (2917, 2849), C=O, (1638), C=C (Ar) (1608, 1513),
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C-O (1247), para-di substitution (836).1H NMR (300MHz, DMSO) δ (ppm): 9.94 (s, 2H,
H-N), 7.90(d,4H,ortho to C=O), 7.67 (d, 4H, ortho to NH-C=O), 6.97 (d, 8H, ortho to O-
Ph), 4.03 (t, -OCH2), 1.70 (quint., 4H,) 1.31(m, 13H, Methylene H’s), 0.84(t, 3H, -CH3).
2.2. Characterization
The structures of all intermediates and final compounds were verified using
1H-NMR and 13C-NMR on a Bruker AM (300 MHz) NMR spectrometer in DMSO-d6
solution, using TMS as an internal standard reference. IR spectra were recorded on
Thermo scientific Nicolet 6700 ATR spectrophotometer. Gallenkamp (Sanyo) model
MPDBM 3.5 with digital thermometer was used to determine the melting points. The
elemental analysis were performed on CHNS analyzer, Thermoscientific (Flash
2000series). Thermogravimetric analyses were carried out on Mettler Toledo thermal
analysis system TGA-851. Phase transition temperatures of the bis-benzamides were
determined by Mettler Toledo DSC-823 system in an inert nitrogen atmosphere.
o
Heating and cooling rates were maintained at 5 C/min. The thermal transition
temperatures were taken from the position of peak maxima. The textures of the
mesophases were examined on a Leica DM-2500p polarizing microscope fitted with a
temperature controller and a hot stage. Computational studies were carried out as
mentioned by Khushi et. al.30 by using the same semi-empirical method available in
molecular orbital package (MOPAC2009).
3
Results and Discussions
3.1
Synthesis and structural confirmation of bis-benzamides.
The synthesis of bis-benzamides was designed and prepared to investigate their LC
properties. As the presence of hydrogen bonding in amides is a well-established fact 25
the synthesized compounds possess bent conformations due to the presence of flexible
methylene spacers and formation of the intermolecular hydrogen bonding as shown in
Figure.2. 31,32 The two hexyl groups were introduced at the two ends of these flexible
spacers to test the effect of these segments on the LC behavior of the resulting dimers.
Aliphatic chains, including n-propyl, n-pentyl, n-nonyl, n-decyl and n-undecyl, were
appended to the backbones and linked to the aromatic rings via amide and ether
linkage from the two sides of the spacers. These bis-benzamides were designed and
studied for their concomitant influence on the LC property. The synthetic routes for
the new dimesogens are provided in Figure 1. In the reported series, the terminal
7

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hexyloxy chain length was kept constant while varying the methylene unit spacers and
explored their thermal and LC properties.
Figure2. Bent structure of bis-Benzamides
The synthesized p-hexyloxybenzoic acid (A6) was reacted with various diamines
like D3, D5, D9, D10 and D11 (where 3,5,9,10,11 represents CH2 spacer units) in a
stoichiometric amount which led to the synthesis of several novel dimeric bis-
1
benzamides. The structures of final compounds were established by the H-NMR,
13C-NMR, IR spectroscopy and elemental analysis. IR spectral data revealed strong
IR absorption bands at about 3343 cm−1 for the –NH of amide pointing towards the
formation of required bis-Benzamides. The other IR stretching vibrations of the
prepared dimeric compounds were 2917, 2850cm−1 for the Csp3-H stretching vibration
1
and at 1639 cm-1 C=O, Aromatic C=C 1607, 1506, C-O 1251 cm- 1. The H-NMR
spectral data exhibited analogous splitting patterns in all the bis-benzamides showing
a diagnostic NH-C=O- signal at around 9 ppm. The aromatic region was confirmed by
the presence of signals in between 6.8-7.9 ppm. Alkoxy protons resonated at 4.0 ppm
whereas the protons resonated at 0.9 and 4.1 ppm were attributed to aliphatic protons.
bis-benzamides were only partially soluble in DMSO which resulted the signal
1
broadening in H-NMR. The NMR spectrum of a representative bis-benzamide is
given in S1.
3.2
Mesomorphic properties
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Differential scanning calorimetry was employed to investigate phase
transitions in the synthetic dimeric compounds. DSC thermograms of D3A6 and
D11A6 compounds are displayed in Figure 3, whereas, the phase transition
temperatures and enthalpy changes are presented in table 1. From the second heating
cycles of the DSC thermograms transition temperatures for melting (Table 1) were
obtained. Better results were obtained for the compounds having spacer lengths of 3,
10 and 11 as compared to other members of series.
Cooling scan
Cooling scan
Figure 3. DSC scans of D3A6 and D3A11 showing heating and cooling scans.
A large endothermic peak showing melting transition and another smaller peak
which was thought to be the liquid crystalline transition were observed in the heating
scan of D3A6. In the cooling scan of D3A6, a large exothermic peak and smaller
exothermic peaks appeared displaying an enantiotropic mesogenic behavior. As
shown in the Figure.3 in the cooling curve of D3A6, it displays an isotropic liquid
o
phase to nematic phase transition at 206 C, a nematic to smectic phase transition at
185 oC and smectic to crystal at 124 oC. D3A6 when observed under polarizing optical
microscope with (1ºC/min) heating and cooling rate, the conical fan like and dendritic
9

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thread like texture characteristic of the smectic and nematic mesophases were
observed which are presented in Figure.5 (a).
Table 1. Phase Transitions of bis-benzamides
Transition
ꢀT=Ti-Tm
Transitions
Temperature oC/ ꢀH
(Jg-1)
Compound
oC
Cr-Sm
Sm-Nem
Nem-Isotropic
Cr-I
143(10.13)
185(3.50)
D3A6
76
219 (85.46)
208 (27.85)
192 (73.36)
128 (1.82)
204 (44.01)
146(1.46)
D5A6
D9A6
-
-
Cr-I
Cr-Sm
D10A6
76
Sm-I
Cr-nem
D11A6
173 (9.70)
194 (24.07)
48
Nem-isotropic
Ti= Isotropic liquid temperature, Tm =mesophase temperature, ꢀT= mesophase range , Cr= Crsytal,
Sm= Smectic, Nem= Nematic, I= Isotropic
Figure 4. Dependence of melting points of bis-benzamides -CH2 spacer length.
10

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Since the molecules have different methylene spacers it was anticipated that
increasing methylene spacer chain length will increase the length-to-breadth ratio
significantly, which may result in liquid crystal phase changes. The results
demonstrated that the increased methylene spacers decreased the melting range, as
o
D3A6 has melting point at 219 oC whereas D10A6 and D11A6 has 204 oC and 194 C
respectively as shown in the Figure. 4.
o
As shown in the Figure 5a conical fan like texture was observed at 160 C
which showed another transition into dendritic thread like and mosaic texture at
o
around 200 C and converted into isotropic liquid at 219 oC. The compounds having
spacer lengths of 5 and 9 exhibited only a single large peak which corresponds to the
melting transition in their DSC thermographs. Whereas the heating and cooling scans
of D10A6 and D11A6 also showed enantiotropic behavior and two peaks indicating the
presence of mesophase were observed for these compounds. POM studies revealed
clear finger print like (smectic) texture which on further cooling converted to the
needle like texture for D10A6 as shown in (Figure. 5 b,c).The DSC thermograph of the
o
highest homologue D11A6 first phase transition appeared from the 190 to 175 C and
the other phase transition originated from 175 to 140 oC, which was ascertained to be
a liquid crystal to crystal transition. Blurred line texture at 145 oC was observed for
o
D11A6 which converted into blurred schleiren nematic at around 180 C when D11A6
was observed under POM. (Figure. 5d)14.
11

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12

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Figure 5. POM images 5a. showing nematic and smectic images D3A6, b,c. needle like
structure of D10A6 are shown , and d showing nematic thread like structures of D11A6.
13

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These results validate that the tendency towards the mesomorphic thermal
stability of mesogenic phase decreased with increasing spacer chain length which may
account for the increasing flexibility of spacers. The presence of terminal hexyloxy
chains at an angle to the long molecular axis may bring about the molecular tilt
between neighbouring molecules consequently decreasing the intermolecular
hydrogen bonding33.
3.3 Thermogravimetric analysis
Thermogravimetric analysis (TGA) was carried out to examine the thermal
stability of the obtained bis-benzamides. The TGA thermograms of the examined
compounds are presented in the (Figure 6). The mass loss associated with the
processes of degradation up to 600 oC in nitrogen atmosphere were estimated in terms
of in terms of 5%, 10% and 50% weight loss (Td, T10, T50) as summarized in Table 2.
The TGA curves of bis-benzamides, indicated main one step degradation process with
o
no noteworthy weight losses up to 360 C pointed towards the thermal stability of
these novel bis-benzamides up to this temperature. The Td and T10 varied from 362-
o
o
395 C and 390-408 C, respectively. The temperature at which the maximum
decomposition occurred (Tmax) ranged between 445-455 oC as shown in Table 2.
Out of all the bis-benzamides, D3A6 exhibited a higher thermal stability than other
members of the series, which is owed to the inclusion of the only three methylene
spacers units. However, the introduction of the longer methylene spacers units
augmented the molecular flexibility consequently reducing the thermal stability of
higher homologues of synthesized bis-benzamides.
Figure 6. TGA graphs of bis-benzamides.
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Table 2. TGA data of compounds (DnA6 )
Char yield at
o
Sr.
Td C T10 oC TmaxoC
490oC
D3A6
D5A6
D9A6
D10A6
D11A6
395
374
3682
370
362
408
400
379
399
404
448
445
451
455
449
22
18
17
15
16
(Td) (based on 5% weight loss), 10% weight loss temperatures (T10)
3.4 Semiempirical Calculations
Theoretical studies were carried out to optimize the geometry of prepared bis-
benzamides by employing semi-empirical method (MOPAC2009). Different
characteristics of synthesized bis-benzamides including the soft core length (F) and
molecular core arm (R), soft to rigid parts ratio (F:R ratio), mean polarizability and
enthalpies of formation were estimated and the results were very much alike as
reported by Muhammad K. et. al.30 as presented in Table 3.
The relative sizes of soft parts to hard core play an important role in imparting
mesogenic properties. The total length of the flexible unit was achieved by adding up
the lengths of terminal hexyloxy groups to the lengths of methylene spacers. The hard
core length was determined from terminal hexyloxy’s oxygen atom to the oxygen
group of methylene spacers, which also includes the amide linkage, as displayed in
the Figure 7.
The size of the flexible units were varied between 19.46 to 29.4Ao whereas hard
core was calculated to be 23.98 Å. The F/R ratio ranged between 0.81 for three
methylene spacers (D3A6) and 1.23 for eleven methylene units substituted ligand
(D3A11). It has also been reported that the compounds having F:R ratio ranging from
0.76 to 1.62 showed liquid crystalline properties, similarly our prepared bis-
benzamides displayed the F/R ratio from 0.81 to 1.23. Among all the compounds
D3A6, D10A6, and D11A6 displayed mesomorphic properties whereas rest of the
members besides having reasonable soft to hard ratio revealed no mesomorphism
indicating the involvement of some other factors. Polarizability of the molecule also
15

ACCEPTED MANUSCRIPT
influences the optical anisotropy of a macroscopic material, particularly, the phase
transition temperature34. Maier–Saupe theory suggests that the nematic-isotropic
phase transition is mainly stabilized and dependent on the anisotropy of the molecular
polarizability of mesogens. It could be inferred that the increasing polarizability
causes the lowering of melting temperature as mentioned in table 1. The increase in
length of methylene spacer groups caused an average raise of 1.75 A3 per methylene
(CH2) group thus increasing the polarizability. The polarizability values were ranged
from 86.09 to 100.46.
N' and 'n' stand for number of carbon atoms
Figure 7. The hard and soft core is mentioned in the image. Grey, blue, red and white
colours represent carbon, nitrogen, oxygen, and hydrogen atoms, respectively. N and
'n' stand for number of carbon atoms in spacer and terminal chain respectively.
Table 3. Calculated parameters of DnA6 using semi empirical method.
Ho
F
Structure
Soft Core
(F)
Molecular
Core Arm
(R)
Ratio
(F:R)
Polarizability
Ang3
Sr.
(2n+N)
Ang
Flexible Flexible
Kcal/mol
spacer
chain
(N)
3
(n)
6
D3A6
D5A6
D9A6
D11A6
19.461
22.001
27.078
29.616
23.984
23.984
23.984
23.984
0.811
0.917
1.129
1.235
86.07912
89.62968
96.83429
100.46970
-231.35057
-243.14108
-264.05429
-274.33424
5
6
9
6
11
6
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Enthalpy of formation is also an important factor as it controls the viscosity of the
material. The switching times of liquid crystal displays and their power consumption
is also influenced by the viscosity of the material35. Consistent increase in the
estimated enthalpy values were observed with increasing methylene spacer’s chain
length. The D3A6 has smaller Hf value than rest of the members of the series and
hence expected to be less viscous than higher members of the series35.
From these computational results and experimental observations, it is inferred that
the polarizability, enthalpy of formation, and F:R ratio values are in good agreement
with the liquid crystals reported in literature30,36
.
Conclusions
A series of new bent-core dimers having four benzene rings with ether and
amide linking groups have been synthesized and their thermal and mesomorphic
o
properties were investigated. The dimers were also thermally stable up to 360 C
which make their use as multifunctional materials. Significant effect on the thermal
stabilities of the reported bis-benzamides is also attributable to the length of
methylene spacers. D3A6 was found to be the most stable of all exhibiting the onset
o
degradation at 395 C. Among all the presented compounds only D3A6, D10A6 and
D11A6 showed enantiotropic mesogenic behaviour was and longest liquid crystalline
o
temperature range of 76 C was exhibited by D3A6.It was also observed that clearing
temperature was decreased as the spacer length was increased from three to eleven.
The POM studies revealed the existence of smectic and nematic phase transitions
which were confirmed by the observation of lined and thread like textures. It can be
established that the amide group serve as an appropriate linking group owing to the
development of hydrogen bonds in bent-core molecules there by imparting good
thermal stability as well as mesogenic properties. These newly prepared bis-
benzamides may act as models for the preparation of polymeric benzamide liquid
crystals with potential mesomorphic properties. This facile and inexpensive synthetic
route could be followed and tailored by employing heterocycles to prepare a number
of non-linear optical materials with unique mesomorphic properties. Furthermore, the
thermal stability as well as the unique liquid crystalline textures make these materials
multifunctional for microelectronics, non-linear optical properties and ferroelectric
applications.
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This research did not receive any specific grant from funding agencies in the
public, commercial or not-for-profit sectors.
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Design and characterization of novel bis-benzamide liquid crystalline
materials
• Symmetrical dimeric benzamides with four benzene rings, different
methylene spacers and hexyloxy tail have been proposed to be
potential mesogens.
• The synthesized compounds possess bent conformations due to the
presence of flexible methylene spacers and formation of the
intermolecular hydrogen bonding between the amide groups of
different molecules thus forming layered structures.
o
• The prepared bis-benzamides are thermally stable up to 360 C
which make their use as multifunctional materials.
• The prepared compounds displayed enantiotropic mesogenic
behavior. It is observed that clearing temperature decreased as the
spacer length increased from three to eleven methylene units.