Mesophase behavior and DFT conformational analysis of new symmetrical diester chalcone liquid crystals

Article abstract of DOI:10.1016/j.molliq.2019.04.083

Synthesis, geometrical structures, conformational analysis, mesomorphic and optical characterizations of new angular chalcone liquid crystals, 4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-alkoxybenzoates, were investigated. These compounds contain chalcone core and diester phenyl rings attached to symmetrical terminal alkoxy chains with different number (n) of carbons from 6 to 16. Mesomorphic and optical properties were studied for the prepared homologues by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Elemental analyses, FT-IR, ¹H, and ¹³C NMR spectroscopy were used for molecular structure confirmation. Density functional theory (DFT) theoretical calculations were estimated to confirm the experimental data and to deduce the most stable confirmal of prepared compounds. The results indicate that the stability of the conformers and the type of the mesophase are dependent on the length of the terminal alkoxy chains. Thermal parameters, dipole moments and polarizability of all estimated conformers were discussed. The relationships between the values of these parameters and the conformer type as well as the mesophase stability were illustrated.

Full text of DOI:10.1016/j.molliq.2019.04.083

Accepted Manuscript
Mesophase behavior and DFT conformational analysis of new
symmetrical diester chalcone liquid crystals
M. Hagar, H.A. Ahmed, T.H. El-Sayed, R. Alnoman
PII:
S0167-7322(19)31329-7
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2019.04.083
MOLLIQ 10806
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
6 March 2019
13 April 2019
15 April 2019
Please cite this article as: M. Hagar, H.A. Ahmed, T.H. El-Sayed, et al., Mesophase
behavior and DFT conformational analysis of new symmetrical diester chalcone liquid
crystals, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.04.083
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ACCEPTED MANUSCRIPT
Mesophase behavior and DFT conformational analysis of new
symmetrical diester chalcone liquid crystals
M. Hagar*a,b, H. A. Ahmed *a,c, T. H. El-Sayed a,d, and R. Alnoman a
a. Chemistry Department, College of Sciences, Taibah University, Yanbu,
Saudi Arabia.
b. Chemistry Department, Faculty of Science, Alexandria University,
Alexandria, Egypt.
c. Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt.
d. Chemistry Department, Faculty of Women for Arts, Science and Education,
Ain Shams University, Heliopolis, Cairo 11757, Egypt
*Authors for correspondence; M. Hagar, mohamedhaggar@gmail.com; H.A. Ahmed,
ahoda@sci.cu.edu.eg
Abstract
Synthesis, geometrical structures, conformational analysis, mesomorphic and
optical characterizations of new angular chalcone liquid crystals, 4-(1,5-(3-oxopenta-
1,4-dienyl))diphenyl bis-4-alkoxybenzoates, were investigated. These compounds
contain chalcone core and diester phenyl rings attached to symmetrical terminal alkoxy
chains with different number (n) of carbons from 6 to 16. Mesomorphic and optical
properties were studied for the prepared homologues by differential scanning
calorimetry (DSC) and polarized optical microscopy (POM). Elemental analyses, FT-
1
IR, H, and 13C NMR spectroscopy were used for molecular structure confirmation.
Density functional theory (DFT) theoretical calculations were estimated to confirm the
experimental data and to deduce the most stable confirmal of prepared compounds. The
results indicate that the stability of the conformers and the type of the mesophase are
dependent on the length of the terminal alkoxy chains. Thermal parameters, dipole
moments and polarizability of all estimated conformers were discussed. The
relationships between the values of these parameters and the conformer type as well as
the mesophase stability were illustrated.
Keywords: Chalcones; Angular liquid crystals; DFT; Conformational analysis.
1

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1.
Introduction
Liquid crystalline molecules have been intensively investigated for scientific and
technological potential applications such as optoelectronic display materials, sensing
materials, biological active molecules [1-3] A spacious sort of devices, covering
photovoltaic solar cells,[4] and light-emitting diodes [5, 6]. Liquid crystalline materials
can be formed of disc-like, [7] linear [8] or bent-shaped [9] core structures. Factors that
influence the cores electronic structure also affect the molecules ability to form liquid
crystalline mesophases. [10] It has been observed that alkyl side-chain length [11, 12],
electronics [13] and core size [14] have a significant effect on mesophase behaviour. In
latest years, angular-shaped liquid crystals have been given much attention in various
aspects such as fast polarization reorientation, ferroelectricity and antiferroelectricity,
and small threshold voltage when applied to liquid crystal display mode. [15] Chalcones
and their derivatives have been used as biogenetic precursors to cure human diseases
[16-22]. Many derivatives of chalcone were described for excellent nonlinear optical
property [23] and liquid crystal displays [24, 25]. Chalcones has been used in
developing light transmittance [26] and good crystal ability [27]. The relationship
between structure-property and mesophase properties of chalcone-based liquid crystals
has been recently reported [24, 28-34]. Many thermotropic symmetric and
nonsymmetrical chalcone liquid crystals have been synthesized and investigated with
verity of mesogenic linkages.[35-40] Thus, the mesogenic portion, terminal groups and
flexible chains are the important points in building new thermotropic mesomorphic
compounds. [41] Four homologous series of symmetrical 1,5-bis(4-alkoxyphenyl)-
penta-1,4-dien-3-ones were reported [35] to investigate the role of the proportionating
terminal alkoxy groups on the mesophase behaviour. Since the exchange of terminal
alkoxy groups [42] would not affect the polarizability of the central linkages and
consequently that of the whole molecular structure. This would be reflected in the
resulting data, it was found that the mono-alkoxy dibenzylidene acetone derivatives [35]
were non-mesomorphic. Such a study has encouraged us to prepare and investigate new
symmetrically terminally substituted diester chalcone core, to obtain further and good
opportunity of the structure-property relationship to build up chalcone/ester central
linkages liquid crystalline compounds. Moreover, studying theoretical calculations for
predicting the molecular geometry confirmations effects on the characteristic properties
and connecting them with determined experimental values is one of our interest [43-
53].
2

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The aim of the present work is to prepare new derivatives of angular-shaped
chalcones and investigate their liquid crystalline properties by using differential
scanning calorimetry (DSC) and polarizing optical microscopy (POM). Moreover, DFT
conformational analysis has been performed to deduce, firstly, the most probable
conformal for each molecule. Secondly, is to relate the mesophase stability and the
alkoxy chain length of the prepared compounds with the estimated thermal parameters,
dipole moments and polarizability for each conformer.
2.
Experimental
2.1. Materials and instrumentation
 4-n-Alkoxybenzoic acids were obtained from Merck (Germany). 4-
hydroxybenzaldehyde, N,N'- dicyclohexylcarbodiimide (DCC) and
4-
dimethylaminopyridine (DMAP) were purchased from Aldrich (Wisconsin, USA).
All solvents are purchased from Aldrich (Wisconsin, USA) and used pure grade.
 Differential Scanning Calorimeter, TA instrument Co. Q20 (DSC; USA), was used
for calorimetric measurements. The melting point and enthalpy of indium and lead
was used for DSC calibration. Aluminum pans and (2–3 mg) sample amounts were
used for DSC measurements investigation. (30 ml/min) nitrogen gas inert
atmosphere and 10°C/min heating rate were selected for all measurements and all
transition were recorded from the second heating scan. The types of the mesophase
texture were identified by a standard polarized light microscope (PLM, Wild,
Germany) with Mettler FP82HT hot stage and the temperature controller was
attached thermocouple for temperature measurements. All recorded values were
made twice and the results have accuracy ± 0.2°C for transition temperature.
 The purity of the prepared compounds was checked by Thin-layer chromatography
(TLC, E Merck). FT-IR (Nicolet iS 10 Thermoscientific), NMR spectra (Varian EM
350L 300 MHz spectrometer (Oxford, UK)) and elemental analyses (Thermo
Scientific Flash 2000 CHS/O Elemental Analyzer, Milan, Italy) were used to
confirm the structures. NMR as well as FT-IR spectroscopy showed the presence of
the expected aliphatic proton for all investigated compounds.
3

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H2n+1CnO
O
HO
O
O
H
DCC, DMAP, DCM
H2n+1CnO
O
NaOH
O
+
COOH
O
OH
HO
O
O
I
1: CnH2n+1 = C6H13
2: CnH2n+1 = C8H17
3: CnH2n+1 = C10H21
4: CnH2n+1 = C12H25
5: CnH2n+1 = C16H33
H2n+1CnO
1-5
Scheme 1: Synthesis of 4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-alkoxybenzoates 1-5.
2.2. Synthesis
Synthesis of 1,5-bis(4-hydroxyphenyl)penta-1,4-dien-3-one I:
A mixture of 4-hexyloxy-benzaldehyde (0.02 mol) and acetone (0.01mol) was
stirred in ethanol (30 mL) and to it aqueous solution of sodium hydroxide (30%, 5 mL)
was added. The mixture was allowed to stir at room temperature for 72 h and it was
acidified with hydrochloric acid (3M) until brown precipitated was formed , then it was
filtered under vacuum and the product was washed with water and recrystallized by
ethanol to give pure compound, as indicated by TLC analysis. Brown crystal, Yield:
75%, mp : 244–246°C [54].
Synthesis of 4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-alkoxybenzoates 1-5:
A mixture of 1,5-bis(4-hydroxyphe-nyl)penta-1,4-dien-3-one (I) (4.1 mmol) and
4-alkoxybenzoic acid (4.1 mmol) was dissolved in dry methylene chloride (DCM) (25
ml). N, N′dicyclohexylcarbodiimide (DCC, 0.02 mol) and few crystals of
4dimethylaminopyridine (DMAP), as catalyst, were added. The mixture kept under
stirring for 72 hours at room temperature. The solid residue obtained filtered and the
filtrate was evaporated till dryness and the solid was recrystallized twice from ethanol.
4

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4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-hexyloxybenzoates 1:
Yield: 95.0 %; mp 104.9 °C, FTIR (ύ, cm−1): 2931- 2865 (CH2 stretching), 1722 (C=O),
1596 (C=C), 1508 (CO Asym), 1256 (C-O Sym). 1H NMR (400 MHz, CDCl3): δ/ppm: 1H
NMR (400 MHz, CDCl3): δ/ppm: 0.94 (t, 6H, CH3(CH2)3CH2CH2, J = 6.5 Hz), 1.21-
1.54 (m, 12H, CH3(CH2)3CH2CH2), 1.78-1.84 (m, 4H, CH3(CH2)3CH2CH2), 4.07 (t, 4H,
CH3(CH2)3CH2CH2, J = 6.5 Hz), 7.00 (d, 4H, ArH, J = 8.6 Hz), ), 7.09 (d, 2H,
CH=CH-C=O, J = 15.9 Hz), 7.30 (d, 4H, ArH, J = 8.3 Hz), 7.71 (d, 4H, ArH, J =
8.3 Hz), 7.78 (d, 2H, CH=CH-C=O, J = 15.9 Hz), 8.16 (d, 4H, J = 8.6 Hz, ArH). 13C
NMR (101 MHz, CDCl3) δ 164.67, 163.74, 152.80, 142.44, 132.39, 129.61, 125.43,
122.50, 121.12, 114.38, 68.38, 31.56, 29.06, 25.67, 22.61, 14.06. Elemental analyses:
Found (Calc.): C, 76.48 (76.53); H, 6.84 (6.87).
4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-octyloxybenzoates 2:
°
Yield: 92.9 %; mp 78.5 C, FTIR (ύ, cm−1): 2922-2854 (CH2 stretching), 1719 (C=O),
1
1597 (C=C), 1471 (CO Asym), 1251(C-O Sym). H NMR (400 MHz, CDCl3): δ/ppm:
0.84 (t, 6H, CH3(CH2)5CH2CH2, J = 6.8 Hz), 1.21-1.54 (m, 20H, CH3(CH2)5CH2CH2),
1.78-1.84 (m, 4H, CH3(CH2)5CH2CH2), 4.03 (t, 4H, CH3(CH2)5CH2CH2, J = 7.0 Hz),
6.95 (d, 4H, ArH, J = 8.5 Hz), ), 7.03 (d, 2H, CH=CH-C=O, J = 15.9 Hz), 7.25 (d,
4H, ArH, J = 8.5 Hz), 7.66 (d, 4H, ArH, J = 8.6 Hz), 7.72 (d, 2H, CH=CH-C=O, J =
15.6 Hz), 8.12 (d, 4H, J = 8.5 Hz, ArH). 13C NMR (101 MHz, CDCl3) δ 164.68,
163.70, 152.79, 142.48, 132.36, 129.57, 125.43, 122.47, 121.15, 114.37, 68.37, 31.78,
29.31, 29.21, 29.08, 25.97, 22.64, 14.09. Elemental analyses: Found (Calc.): C, 77.20
(77.23); H, 7.42 (7.45).
4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-decyloxybenzoates 3:
Yield: 93.7 %; mp 100.8 °C, FTIR (ύ, cm−1): 2945- 2854 (CH2 stretching), 1724 (C=O),
1
1595 (C=C), 1471 (CO Asym), 1225 (C-O Sym). H NMR (400 MHz, CDCl3): δ/ppm:
0.91 (t, 6H, CH3(CH2)7CH2CH2, J = 6.9 Hz), 1.21-1.51 (m, 28H, CH3(CH2)7CH2CH2),
1.70-1.83 (m, 4H, CH3(CH2)7CH2CH2), 4.05 (t, 4H, CH3(CH2)7CH2CH2, J = 6.8 Hz),
7.02 (d, 4H, ArH, J = 8.5 Hz), ), 7.02 (d, 2H, CH=CH-C=O, J = 15.8 Hz), 7.33 (d,
4H, ArH, J = 8.3 Hz), 7.64 (d, 4H, ArH, J = 8.5 Hz), 7.78 (d, 2H, CH=CH-C=O, J =
5

ACCEPTED MANUSCRIPT
15.9 Hz), 7.98 (d, 4H, J = 8.3 Hz, ArH). Elemental analyses: Found (Calc.): C, 77.79
(77.83); H, 7.92 (7.94).
4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-dodecyloxybenzoates 4:
°
Yield: 95.1 %; mp 81.5 C, FTIR (ύ, cm−1): 2922-2849 (CH2 stretching), 1722 (C=O),
1
1594 (C=C), 1470 (CO Asym), 1272 (C-O Sym). H NMR (400 MHz, CDCl3): δ/ppm:
0.89 (t, 6H, CH3(CH2)9CH2CH2, J = 6.7 Hz), 1.19-1.56 (m, 36H, CH3(CH2)9CH2CH2),
1.71-1.84 (m, 4H, CH3(CH2)9CH2CH2), 4.09 (t, 4H, CH3(CH2)9CH2CH2, J = 7.0 Hz),
6.99 (d, 4H, ArH, J = 8.3 Hz), ), 7.03 (d, 2H, CH=CH-C=O, J = 15.9 Hz), 7.35 (d,
4H, ArH, J = 8.5 Hz), 7.68 (d, 4H, ArH, J = 8.4 Hz), 7.67 (d, 2H, CH=CH-C=O, J =
15.7 Hz), 8.10 (d, 4H, J = 8.5 Hz, ArH). 13C NMR (101 MHz, CDCl3) δ 164.52,
164.20, 150.01, 149.74, 133.03, 129.09, 125.10, 123.01, 120.99, 114.46, 68.43, 32.98,
31.93, 30.82, 29.99, 29.04, 26.32, 26.01, 25.43, 24.91, 22.53, 14.13. Elemental
analyses: Found (Calc.): C, 78.33 (78.35); H, 8.34 (8.37).
4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-hexadecyloxybenzoates 5:
Yield: 96.3 %; mp 90.4 °C, FTIR (ύ, cm−1): 2914- 2852 (CH2 stretching), 1723 (C=O),
1595 (C=C), 1470 (CO Asym), 1262 (C-O Sym). 1H NMR (400 MHz, CDCl3): δ/ppm: 1H
NMR (400 MHz, CDCl3): δ/ppm: 0.93 (t, 6H, CH3(CH2)13CH2CH2, J = 6.9 Hz), 1.19-
1.56 (m, 52H, CH3(CH2)13CH2CH2), 1.74-1.89 (m, 4H, CH3(CH2)13CH2CH2), 4.05 (t,
4H, CH3(CH2)13CH2CH2, J = 6.9 Hz), 6.98 (d, 4H, ArH, J = 8.4 Hz), ), 7.05 (d, 2H,
CH=CH-C=O, J = 15.8 Hz), 7.34 (d, 4H, ArH, J = 8.4 Hz), 7.72 (d, 4H, ArH, J =
8.5 Hz), 7.68 (d, 2H, CH=CH-C=O, J = 15.8 Hz), 8.09 (d, 4H, J = 8.6 Hz, ArH). 13C
NMR (101 MHz, CDCl3) δ 164.47, 163.86, 150.23, 149.85, 132.45, 129.07, 124.98,
122.81, 120.90, 114.36, 68.32, 32.41, 31.95, 30.85, 29.82, 29.54, 29.21, 29.09, 26.28,
25.99, 25.66, 25.35, 24.97, 24.53, 22.72, 14.16. Elemental analyses: Found (Calc.): C,
79.18 (79.20); H, 9.04 (9.07).
2.3.
DFT Method and calculations
The theoretical calculations for the investigated compounds were carried out by
Gaussian 09 software [55]. DFT/B3LYP methods using 6-31G (d,p) basis set was
selected for the calculations. The geometries were optimized by minimizing the energies
with respect to all geometrical parameters without imposing any molecular symmetry
6

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constraints. The structures of the optimized geometries had been drawn with Gauss
View [56]. Moreover, the calculated frequencies were carried out using the same level
of theory. The frequency calculations showed that all structures were stationary points
in the geometry optimization method with none imaginary frequency.
3. Results and Discussion
3.1. Characterization
Molecular formulae of the prepared compounds were confirmed via their
elemental analyses, FT-IR data, and NMR spectroscopy. The results were consentient
with the expected structures (see Figure 1).
100
80
60
40
20
4000
3000
2000
1000
Wavelength (cm-1)
Figure 1: FT-IR spectrum of 4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl 4-
octyloxybenzoates 2.
3.2. Mesophase and optical behavior
The newly symmetrical set 4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-
alkoxybenzoates were investigated by DSC and POM to study their thermal and optical
properties. Transition temperatures and their associated enthalpies were tabulated in
Table 1. A representative example showed the DSC curve of heating/cooling cycles for
compound 2 was depicted in Figure 2. The corresponding detected mesophases of
prepared compounds were identified by observing their textures through POM (Figure
3). Effect of increasing the alkoxy-chain length on the thermal transitions of prepared
derivatives (1-5) is represented in Figures 4.
7

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It is showed that from Table 1 and Figure 4, melting temperatures irregularly
varies, as usual, with the alkoxy-chain length (n). Moreover, all the symmetrical
chalcone derivatives are enantiotropic mesomorphic exhibit wide ranges of thermal
stabilities. Compounds of short alkoxy chain length (1 and 2) are purely nematogenic
with mesophase ranges 79.6 and 94.1oC on heating, respectively. The derivative bearing
alkoxy chain with 10 carbons (3) are dimorphic exhibiting smectic C and nematic
phases, while the derivatives of longer alkoxy chains (4 and 5) are monomorphic
o
exhibiting only smectic C phase with high thermal stability ranges 121.5 and 114.5 C
on heating, respectively. It is obvious that the SmC transition rises gradually with
increasing the alkoxy chain length, while the N thermal transition decreases, as the same
trend reported by Gary et al. [57] and Imire et al. [58]. The end-to-end aggregation due
to the alkoxy group oxygen and the ester carbonyl strong mesomeric intermolecular
interaction and side by side cohesive forces between molecules that decreases by
increasing the alkoxy chain length, is one of the important factors determine the type of
the phase [58]. As will be discussed in the theoretical calculation part, the end-to-end
intermolecular association results from the change of the confirmation by incorporation
of the terminal hexyloxy chain is a good reason for the pure nematic phase observed
for the short lengthen homologues 1 and 2. The competitive effect end-to-end
aggregation due to the alkoxy group and the carbonyl ester and side by side cohesive
interaction is highly affected by the alkoxy chain length. The end-to-end aggregation for
short lengthen derivative 1 is more pronounced than the side by side interaction. It was
reported that [35], increasing the core size of some chalcones and the number of side
chains helped to induce mesophase formation. In addition, the increment of the length
of the terminal alkoxy chain expands the temperature range of the mesophase. In the
present series, 1-5, these molecules present in non-linear geometry which have been
deduced from estimated angles. Theoretically, the angle values are predicted from DFT
calculations. In general, the deviation from linearity being variable depends upon the
type of compound. Obviously, the angle between the alkoxy chain arms was dependent
on the chain length. A small entropy changes are found in all members of investigated
compounds (Table 1), which may be result from the reduction in the interaction
strength parameter of the core and/or the presence of some sort of molecular biaxiality
[58, 59]. The increment of the alkoxy chain length results in increasing of the lateral
interaction in semectic phase and so the entropy decreases, while the end to end
interaction has less order and so the entropy increases in nematic phase.
8

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4
2
Cooling cycle
Nematic
0
Isotropic liquid
Nematic
Crystal
-2
Heating cycle
-4
-6
-8
-10
-12
-14
0
50
100
150
200
250
300
Temperature (oC)
Figure 2: DSC of compound 2 during second heating/cooling cycles at rate 10°C min−1.
Figure 3: Mesophase textures for compounds 2 and 3 obtained by POM, (a) Nematic
texture of compound 2 at 120..0°C ; (b) Nematic texture of compound 3 at 140..0°C;(c)
Smectic C texture of compound 3 at 181.0°C.
Figure 4: DSC phase transitions of prepared derivatives (1-5).
9

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In comparison to simple C2-symmetric chalcone compounds with different core
sizes and varying alkoxy chains [35], present symmetrical four phenyl ring chalcone
esters are thermally high stable. These materials [35], two phenyl ring chalcones, follow
the typical liquid crystal behavior appeared only for dialkoxy chain sides and lengthen
core compound homologous exhibiting very narrow nematic phases, while the
monoalkoxy chains are non-mesomorphic. As the molecular architecture of the present
series is different and exhibit angular shape structure, their mesophase behaviors are
different and have wide range of stability for nematic and smectic phases. Overall, the
present four phenyl ring chalcones evidenced that the substitution of symmetrical
terminal alkoxy chains at both ends with elongated chalcone esters promised the
mesophase in the homologous series of compounds.
Table 1:.Phase transition temperatures (°C), enthalpy of transitions (kJ/mol), and transition
entropy for compounds 1-5.
Compound TCr-SmC
TCr-N
TSmC-N TSmC-I
TN-I
∆SSmC-I/R ∆SN-I/R
1
104.9
(52.56)
184.5
(2.40)
-
-
-
1.56
1.30
0.96
-
-
-
-
78.5
(52.31)
172.6
(1.87)
2
-
-
-
3
4
100.8
(42.12)
143.4
(3.57)
193.5
(1.54)
81.5
(46.00)
203.0
(1.94)
1.15
1.64
-
-
-
-
-
-
5
90.4
(62.66)
204.9
(2.8)
-
-
Abbreviations: Cr-N = crystal to Nematic transition; Cr-SmC crystal to smectic c transition; SmC -N= smectic c to Nematic
=
transition; SmC-I = smectic c to isotropic liquid transition; N-I= Nematic to isotropic liquid transition.
3.3. DFT calculations and Conformational Analysis
3.3.1. Thermal parameters and geometrical structure
The theoretical calculations were carried out in the gas phase using method
DFT/B3LYP at basis set 6-31G (d,p). Since the chalcone homologous series (1-5) are
mesomorphic, hence they should all exist in a planar conformation [53, 60-62]. That is
the four rings of each molecule should be co-planar; otherwise, any possible rotation
around the core by different angles would result a non-linear conformation. The
calculations were preformed for all compounds 1-5. The two most proper conformers
were estimated for each compound, V-shape (a) and U-shape (b), Figures 5 &6. All
optimized structures were approved to be stable according to the absence of the
10

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imaginary frequency. The results of the theoretical calculations for both conformers are
collected in Table 2.
Table 2. Thermal parameters (Hartree/Particle) and Dipole Moment (Debye) of 1-5a &1-5b.
Parameter
1a
2a
3a
4a
5a
1b
2b
3b
4b
5b
1.369260
0.794361
0.912624
1.026619
1.140889
1.369326
0.798307
0.912432
1.026659
1.140869
Ecorr
ZPVE
Etot
-2191.949429
-2191.906135
-2191.905191
-2192.031346
0.0045567
2.9477154
-0.0373377
7.4930
-2349.125687
-2349.070671
-2349.069727
-2349.232681
0.0030967
2.9584975
-0.0351306
7.5203
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-2506.198426
-2506.376087
0.0039549
2.9557476
-0.0238003
7.5130
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-2663.328052
-2663.327107
-2663.517784
0.004052
-2977.662258
-2977.585472
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0.0036514
-2.9615847
-0.0371671
7.5284
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-2191.940672
-2192.090228
0.0283764
4.7652362
-0.0641395
12.1134
-2349.125483
-2349.071342
-2349.070398
-2349.229131
0.0242879
4.7823741
-0.1877154
12.1134
-2506.259631
-2506.199134
-2506.198189
-2506.376213
0.0265227
4.7962083
-0.1962565
12.2011
-2663.393793
-2663.327872
-2663.326928
-2663.517521
0.0276753
4.8053315
-0.1933241
12.2240
-2977.662137
-2977.585354
-2977.584410
-2977.801828
4.7734768
-0.2798747
-0.6018418
12.2500
H
G
X
Dipole
Mome
nt
2.9578462
-0.0241093
7.5184
Y
Z
Tot
634.20
682.77
730.26
777.81
872.52
629.15
676.93
724.52
771.89
866.28
Polarizability
ZPVE: Sum of electronic and zero-point energies; Etot: Sum of electronic and thermal energies; H: Sum of electronic and thermal enthalpies; G: Sum of electronic and thermal free energies.
Figure 5: Calculated geometrical structure of 1a, 1b, 5a, and 5b.
Figure 6: Predicted lateral interactions of V-shape and U-shape conformers of 5.
As shown from Figure 5 the length between the terminal carbon atoms of the
alkoxy chains were predicted to be 34.9, 41.1, 63.6 and 49.0 A0 for 1a, 1b, 5a, and 5b,
11

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respectively. While the angle between the alkoxy chain arms was 135.30, 129.50, 106.80
and 91.50, respectively. It could be concluded that V-shape has larger angle than that of
the U-shape conformer. Moreover, as the alkoxy chain increases the distance between
the terminals increases while the angle decreases. This geometrical shape could permits
the maximum side-side interaction for longer chain lengths while for shorter one this
interaction could be less pronounced with respect to end to end aggregation of alkoxy
chains and the ester carbonyl moieties. Consequently, semictic mesophase was
predicted for the longer alkoxy chains while the nematic for shorter. The increment of
the distance with decreasing of the angle between the alkoxy chain arms permits more
aggregation with stronger lateral interaction due to the stacking of the aromatic moieties
of the core part resulted in more ordered phase (SmC phase), Figure 6.
3.3.2. Relationship of experimental and theoretical parameters
Thermal parameters, dipole moments and polarizability of both estimated
conformers were related with the experimentally determined values of the mesophase
stability and the length of alkoxy chains to study the effect of the terminal length on the
stability of possible conformers. As shown from Figure 7, the stability of the deduced
conformers increases as the length of the alkoxy chain increases. These results could be
explained in terms of the strength stacking of the aromatic rings and the aggregation of
the alkoxy chains. At shorter alkoxy chains (n= 6, 8) the stacking of the aromatic rings
is more pronounced than the aggregation of the chains. On the other hand, the increment
of the alkoxy chains, the strength of the aggregation increases with respect to the
stacking of the benzene rings. The V-shape permits more strength of the aggregation of
the alkoxy chains; this phenomenon could explain the higher stability of the V-shape at
longer chain lengths. While, for the conformers of shorter chain length (1 & 2), the
staking of the aromatic rings could be more effective to stabiles the structure than the
aggregation of the chains and this could be achieved properly in case of the conformer
of the U-shape (b) rather than the V-shape (a). On the other hand, as the energy
decreases the mesophase stability increases, Figure 8. Moreover, the dipole moments as
well as the polarizability were calculated with the same method at the same basis set.
The results revealed that the V-stable (a) is less polar than the U-shape (b) for all
compounds. However, the increment of the dipole moment with increasing of the chain
length in case of V-shape (a) is lesser than that of U-shape conformer (b), Figures
9&10. On the other hands, the polarizability increases as the chain length increases,
12

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however, the V-stable (a) is more polarizable than the U-shape (b) for both compounds,
Figure 11. As shown from Figure 10 & 11, as the polarizability increases the
mesophase stability increases except for 2a. The presence of nematic mesophase (less
ordered) with the shorter alkoxy chains and semectic phase (more ordered) for the
longer chain lengths could be explained in the term of the increment of the polarity and
the polarizability with increasing of the alkoxy chain length, Table 1 & 2. More lateral
interaction aggregation of the alkoxy chains in the ordered semectic phase as the alkoxy
chain length increases, Figure 6.
1
-2192.6
2
-2392.6
5
3
3
4
2
-2592.6
-2792.6
-2992.6
-3192.6
4
-5
-10
-15
-20
-25
-30
5
1
6
8
10
12
14
16
6
8
10
12
14
16
Number of carbons in alkoxy chain
Number of carbons in alkoxy chain
Figure 7: Dependence of the akoxy chain-length of homologues series (1-5) on the
thermal energies for both conformers a and b.
1
-2192.6
-2392.6
2
3
-2592.6
-2792.6
-2992.6
-3192.6
4
5
170
180
190
200
210
Mesophase stability temperature, oC
Figure 8: Dependence of the mesophase stability of homologues series (1-5) on the
thermal energies.
13

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7.55
7.50
7.45
12.3
5b
5a
4b
2a
4a
3b
3a
12.2
12.1
12.0
1a
2b
1b
6
8
10
12
14
16
6
8
10
12
14
16
Number of carbons in alkoxy chain
Number of carbons in alkoxy chain
Figure 9: Effect of the akoxy chain-length of homologues series (1-5) on the calculated
dipole moment for each conformer a and b.
7.55
7.50
7.45
12.3
12.2
12.1
12.0
5b
5a
2a
4b
3b
4a
3a
1a
2b
1b
170
180
190
200
210 170
180
190
200
210
Mesophase stability temperature, OC
Mesophase stability temperature, OC
Figure 10: Relation of the mesophase stability of homologues series (1-5) with the
calculated dipole moment for each conformer a and b.
5a
5b
5a
5b
862
857.4
812
807.4
4a
4a
762
757.4
707.4
657.4
607.4
4b
3a
3a
4b
712
662
612
3b
2a
2a
2b
3b
2b
1a
1b
1a
1b
6
8
10
12
14
16
170
180
190
200
210
Mesophase stability temperature, oC
Number of carbons in alkoxy chain
Figure 11: Effect of the akoxy chain-length / mesophase stability of homologues series
(1-5) on the calculated polarizability for each conformer a and b.
14

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3.3.2. Frontier molecular orbitals and molecular electrostatic potential
Optical interconnections, telecommunications and signal processing are important
applications of nonlinear optical (NLO) liquid crystals [63, 64]. These applications are
highly affect by the energy difference between the frontier molecular orbitals (FMOs),
HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular
orbital). Furthermore, the prediction of the energy deference between FMOs is a
wonderful sign for a lot of important variables such as chemical hardness (η), global
softness (S) and polarizability (α). It is well known that low energy gap of the FMO as
well as high polarizability are better characteristics for good NLO compounds [65].
Figure 12 illustrates the calculated energies difference and the ground state isodensity
surface plots for the FMOs of 1a, 1b, 5a, and 5b. As shown from Table 3, FMO energy
gap and the global softness (S) were affecting by the length and the conformation the
alkoxy chain. As the length of terminal wings increase the global softness and the
polarizability increase. Consequently, it is predicted that as the length of the alkoxy
chains increases, their characteristics improve to be more proper candidates in nonlinear
optical applications, Table 3.
Figure 12 : The calculated ground state isodensity surface plots for frontier molecular
orbitals of 1a, 1b, 5a, and 5b
15

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Table 3. Molecular orbital energies, hardness (η) and global softness (S) of 1a,
1b, 5a, and 5b
Compound EHOMO (a.u) ELUMO (a.u)
ΔE(ELUMO-EHOMO) (a.u) η=ΔE(ELUMO HOMO
-
)/2 S= 1/ ΔE = (1/2η)
7.417847
-0.09683
-0.09434
-0.09374
-0.09431
-0.23164
-0.22873
-0.22816
-0.22876
0.13481
0.13439
0.13442
0.13445
0.067405
1a
1b
5a
5b
0.067195
0.067210
0.067225
7.441030
7.439369
7.437709
The charge distribution map for the 1a, 1b, 5a, and 5b was calculated under the
same basis sets according to molecular electrostatic potential (MEP) (Figure 13). The red
region (negatively charged atomic sites) was localized on the oxygen atoms and the
maximum was one the ketonic oxygen atom, while alkyl chains showed the least negatively
charged atomic sites (blue regions). As shown from Figure 13, the conformation of the
attached alkoxy chain affects the orientation of the charge distribution map, this could effect
on the type of the mesophase by alteration of the competitive interaction between end-to-
end and side-side interaction. Recently, we reported the relationship between the theoretical
charge distribution and the experimental mesophase type [49-51].
Figure 13: Molecular Electrostatic potentials (MEP) of 1a, 1b, 5a, and 5b.
4. Conclusion
New symmetrical homologous set of diester chalcone liquid crystals were
synthesized and mesomorphically investigated. The prepared compounds possessing
alkoxy groups at each end of the molecule with variable chain lengths. All compounds
exhibited broad range of mesophase stability in addition to the short terminal alkoxy
chains exhibit pure nematic phase while the longer chains possessing smectic C phase.
16

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DFT calculations and the conformational analyses were discussed. V-shaped and U-
shaped, the most proper conformers, were estimated from theoretical analysis for each
molecule. The conformations deduced were found to vary according to the angle of the
central chalcone molecule and definitely dependent on the length of terminal alkoxy
chains. Finally, the conformer type is reflected on the resultant of the thermal
parameters, dipole moments and polarizability of the prepared chalcone set and
consequently on their mesomorphic properties. The results of the DFT calculations
revealed that the estimated polarizability, dipole moment, stability and the experimental
mesophase stability increase as the chain length increases.
Conflicts of interest
Authors declare “There are no conflicts of interest”.
Acknowledgements
Authors gratefully acknowledge the Deanship of Scientific Research, Taibah
University for support of our research group (60334).
17

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Highlights
1. New angular chalcone liquid crystals were prepared
2. Shorter chains showed nematic phase while the longer semectic C.
3. V-shaped and U-shaped conformers were estimated from theoretical
calculations.
4. The theoretical studies revealed that the polarizability, dipole moment and
stability increase with the chain length increment.
5. The mesophase stability also increases as the chain length increases.
24