New two rings Schiff base liquid crystals; ball mill synthesis, mesomorphic, Hammett and DFT studies

Article abstract of DOI:10.1016/j.molliq.2019.112161

New analogues series of two rings Schiff base comprising different terminal polar substituents changed between N(CH₃)₂, CH₃O, H, Cl, F, CN and NO₂ were prepared using conventional heating and a ball-milling meth

Full text of DOI:10.1016/j.molliq.2019.112161

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New two rings Schiff base liquid crystals; ball mill synthesis,
mesomorphic, Hammett and DFT studies
Sherif S. Nafee, Mohamed Hagar, Hoda A. Ahmed, O.A.
Alhaddad, Reda M. El-Shishtawy, Bahaaudin M. Raffah
PII:
S0167-7322(19)33303-3
DOI:
https://doi.org/10.1016/j.molliq.2019.112161
MOLLIQ 112161
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
11 June 2019
2 November 2019
16 November 2019
Please cite this article as: S.S. Nafee, M. Hagar, H.A. Ahmed, et al., New two rings Schiff
base liquid crystals; ball mill synthesis, mesomorphic, Hammett and DFT studies, Journal
of Molecular Liquids(2019), https://doi.org/10.1016/j.molliq.2019.112161
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New two rings Schiff base liquid crystals; Ball mill synthesis, Mesomorphic, Hammett and DFT studies
Sherif S. Nafee a, Mohamed Hagar b,c *, Hoda A. Ahmed b,d,*, and O.A. Alhaddad e, Reda
M. El-Shishtawy f and Bahaaudin M. Raffah a
a. Physics Department, Faculty of Science, King Abdulaziz University, Jeddah 21589,
Saudi Arabia
b. Chemistry Department, College of Sciences, Taibah University, Yanbu 30799, Saudi
Arabia
c. Chemistry Department, Faculty of Science, Alexandria University, Alexandria 21321,
Egypt
d. Department of Chemistry, Faculty of Science, Cairo University, Cairo 12613, Egypt
e. College of Sciences, Chemistry Department, Al-Madina Al-Munawarah, Taibah
University, Al-Madina 30002, Saudi Arabia.
f. Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah
21589, Saudi Arabia
*Authors for correspondence; M. Hagar, mohamedhaggar@gmail.com; H.A. Ahmed,
ahoda@sci.cu.edu.eg.
Abstract
New analogues series of two rings Schiff base comprising different terminal polar
substituents changed between N(CH3)2, CH3O, H, Cl, F, CN and NO2 were prepared
using conventional heating and a ball-milling method. An alkoxy chain substituent
contributes one end of their molecular structure. Mesomorphic and optical properties as
well as their thermal stability were investigated by differential scanning calorimetry
1
(DSC) and polarized optical microscopy (POM). FT-IR, H-NMR, 13C-NMR, and
elemental analysis were carried out to elucidate and confirm the molecular structures of
the synthesized compounds. The mesophase type and stability of the prepared compounds
are rationalized based on the varying Van der Waal volume and polarizability of the
terminal groups as well as the intermolecular interactions. After compiling all resulting
experimental data and discussing it in the performance of Hammett and Density
functional theoretical estimated parameters, it found that the molecular structure and the
dipole moment as well as the polarizability of the prepared compounds are highly
affected by the electronic nature of the terminal substituent X rather than its volume.
Moreover, the mesophase type and its stability were explained in the term of the DFT
data and Hammett as well as Van der Waal's volume of the terminal substituents.
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Keywords: Calamitic liquid crystals, Schiff base, Hammett, DFT, Geometrical
structure, Ball mill synthesis.
1. Introduction
During the last few decades, liquid crystals containing Schiff base (or
azomethine) linkage have received massive amounts of attention from the discovery of
the room temperature-liquid crystal, 4-methoxybenzylidene-4’-butylaniline [1]. Recently,
it has been reported that the Schiffs base mesogenic core liquid crystals are known to
drive smectic phase formation and has resulted in the discovery of a new family of
smectic phases [2, 3]. In the designing of new thermotropic liquid crystals the choosing
of a mesogenic core, terminal groups and a flexible chain are the essential specifications
[4]. To secure the fact that the molecules have structural linearity and large molecular
polarizability a para-substituted phenyl ring is used. [5] Ball milling is a modern
environmentally-friendly method and is a mechanical method widely-used to granulate
minerals into extremely fine particles and for the preparation or changing of inorganic
solids, [6-9] several organic transformations have been achieved by using the ball-milling
technique [10-13] including reductions [14] condensations [15, 16] and oxidations [17,
18]. Ball milling is useful for the quick and efficient synthesis of various imines [19, 20].
Generally, the common synthesis by condensation of primary amines with carbonyl
compounds in solution needs acid catalysis, elimination of solvent and produced water
after the isolation of the product. As a result of the increasing interest that many of the
above reactions can be done in a single step with quantitative yields by different states of
matter such as gas-solid, liquid-solid or solid-solid reactions [21-24] This approach
showed very good performance in comparison to many of the traditional methods in
terms of yield, reaction conditions, and time. It has been reported that calamitic low
molar mass liquid crystal compounds with two aromatic rings and one or more terminal
substituent, i.e. 4-methoxybenzylidene-4'-butylaniline (MBBA), are able to show nematic
mesophase at room temperature [25-28]. Furthermore, the polarity effects of the small
compact size terminal substituents on the mesomorphic behaviours have been more
attention recently [29-35]. Polar substituents, possessing strong dipole moments, have the
ability to promote mesomorphic properties [36]. The increased dipole moment enhances
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the stability of the lattice and melting temperatures [37]. Lowering the transition
temperatures of the mesophase and extension of its temperature range has always been an
important aim of studies for many practical applications. One approach to achieve that is
mixing compounds with various molecular structures and properties in binary systems
[38-46]. Recently, several low molar mass thermotropic Schiff base liquid crystals have
been reported and investigated [47-56] Generally, the central linkages and the terminal
groups of the liquid crystalline compound play important role for the formation, thermal
stability, type, and stability range of the mesophases. [57-65].Thus, the mesogenic
portion, terminal groups and flexible chains are the important points in building new
mesomorphic compounds. [4] Moreover, studying theoretical calculations for predicting
the molecular geometry affects on the thermal properties and connecting them with
experiment data is one of our interest [34, 56, 66-76].
Continuing our interest, the goal of the present studies firstly, was to prepare new
linear two rings Schiff base liquid crystals through two different ways, conventional and
ball-milled methods. Secondly, was to investigate their mesomorphic, thermal and
optical behavior with respect to different polarity/size of terminal substituents. Moreover,
some parameters estimated from density functional theory (DFT) calculations and
Hammett parameters of the attached substituents as well as Van der Waal volume of the
terminal X were used to explain the obtained experimental results.
X
O
N
A-G
X = Me2N, MeO, H, Cl, F, CN, NO2
2. Experimental
N-arylidene-4-(hexyloxy)benzenamines (A-G) were prepared according to the
following scheme:
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O
N
CHO
O
Ehanol
Reflux
+
X
NH2
X
X = Me2N, MeO, H, Cl, F, CN, NO2
A-G
A- Conventional method
A mixture of the 4hexyloxyaniline (790 mg, 4.1 mmol) and 4-substituted
benzaldehyde (4.1 mmol) in ethanol (10 ml) were refluxed for two hours. The reaction
mixture was cooled and filtered and washed with cold ethanol. The obtained solid was
purified by recrystallization twice from hot ethanol to give pure compounds.
B- Ball mill method
N-arylidene-4-(hexyloxy)benzenamines (A-G) have been synthesized according to
the optimum conditions of our pervious method [12]. A mixture of the 4hexyloxyaniline
(790 mg, 4.1 mmol) and 4-substituted benzaldehyde (4.1 mmol) was introduced into
stainless steel vials with 56.6 g weight of stainless steel balls [4 (12.7 mm), 4 (6.3 mm),
4 (3.2 mm)] at 20 Hz frequency for 2 min milling time. The obtained paste was purified
by recrystallization twice from hot ethanol to give pure compounds.
3. Results and discussion
3.1 Methods of preparation
The synthesis of N-arylidene-4-(hexyloxy)benzenamines (A-G) have been
accomplished by two methods, the first method is the conventional method under reflux
conditions in ethanol for two hours. The other method is the ball milling method under
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solvent free conditions for very short reaction time (2 mins). The reaction proceeded
effectively to afford the final product in excellent product yields (92-98%). The
equimolar amounts of hexyloxyaniline and substituted benzaldehydes have been
subjected to milling under our previous optimum conditions [12]. The reactants have
been subjected into 250 cm3 stainless steel vials with 56.6 g weight of stainless steel
balls [4 (12.7 mm), 4 (6.3 mm), 4 (3.2 mm)] then the reaction proceeded at 20 Hz for 2
mins to afford quantitative amounts of the final products. To the best of our knowledge
there is no report for the synthesis of liquid crystals under ball milling conditions, so this
reaction could be considered as the first real synthesis of liquid crystalline materials
under solvent free ball mill method.
Obviously from the NMR spectroscopy the reaction proceeded under both methods to
give a mixture of two geometrical isomers (E and Z) their ratio depends on the electronic
nature of the terminal substituent (see supplementary data). The investigation of such
effects on the stability of the obtained isomers could be an important point for future
perspectives either theoretically or experimentally.
3.2. Mesophase behavior
Temperatures of transition and associated enthalpies, as measured by DSC, and phases
identified by polarized optical microscopy (POM), for synthesized compounds A-G are
tabulated in Table 1. DSC thermograms upon cooling for all prepared derivatives A-G
are depicted in Figure 1 as a representative example. The dependence of mesophase
transition temperatures and their ranges of stability on the terminal substituents are
depicted in Figure 2. Data in Table 1 and Figure 2 revealed that the analogous
derivatives A-G are mesomorphic except the C (X=H) and G (X=NO2) compounds. For
electron donating groups, compound B analogue is dimorphic showing smectic A (SmA)
and nematic phases (N) montropically while compound B (X=OCH3) is purely
nematogenic which exhibiting only monotropic nematic phase with small range of
stability. Conversely, all members of electron withdrawing groups are mesomorphic
exhibiting suitable range of stability except the terminal nitro-substituent (G) is non-
mesomorphic. Cl substituted compound (D) observed dimorphic phases upon heating
and cooling exhibiting semectic A and nematic mesophases. CN derivative (F) is purely
smectogenic possessing enantiotropic SmA phase while E analogues (F-substituent) is
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monomorphic showing monotropic SmA phase. The stability of the mesophase is
influenced by an increase in the polarity and/or polarizability of the mesogenic core of
the molecule. Thus the electron-donating (N(CH3)2 and CH3O) and electron-withdrawing
(Cl, F, and CN) groups enhance the mesophase stability over their unsubstituted
analogues (H). Moreover, the low clearing enthalpy associated with compounds A, B
(X= N(CH3)2 and CH3O) is presumably associated with its enhanced molecular biaxiality
[67, 77]. In case of NO2 substituent may be the high molecular packing prevent the
formation of the mesophase. It was reported that [78], the stability and types of the
mesophase formation are mainly dependent upon the dipole moment of the mesogenic
part of the molecule which dependent on the attached terminal polar group and the steric
one that varies according to the position and volume of the substituent. Moreover, the
introduction of terminal polar substituent with different size and polarity into this type of
two ring systems of Schiff base linkage, with suitable terminal alkoxy group, may be
effective approaches to offer and improve the mesophase formation and or/ stability of
phase transition phenomena depending on its position as well as its donating, as in
N(CH3)2, or withdrawing, as CN, power of the substituent. Thus, the introduction of a
substituent into a terminal position of a liquid crystalline compound would have two
opposing effects: (a) a decrease in the mesophase stability due to steric effect of terminal
group [79-83] and (b) an increase or decrease in its polar anisotropy due to its polarizing
effect molecule, the dipole moment will depend on the position and polarity of the group
[84, 85]. Furthermore, it is clear that terminal groups play an important role in the
influence the melting points of the prepared materials.
Table 1: Phase transition temperatures (oC), (enthalpy of transition ΔH, kJ/mole),
normalized entropy of transition ΔS/R and √TC (K1/2), for compounds A-G
Comp.
X
TCr-I
TCr-SmA
TCr-N
TSmA-N
91.1*
(3.04)
TSmA-I
TN-I
94.3*
(1.45)
101.0
(1.60)
ΔS/R
√TC
N(CH3)2
-
110.6
(30.72)
-
-
-
1.85 19.17
A
CH3O
H
-
104.7
(37.76)
-
-
-
1.91 19.34
B
C
60.5
-
-
-
-
-
-
(36.48)
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Cl
-
89.9
(32.90)
- 84.4
(34.63)
- 77.3
(30.27)
-
97.0
(4.01)
-
-
105.6
(2.96)
3.37 19.46
3.88 18.44
2.61 19.54
D
E
F
F
- 66.9*
(2.16)
- 108.6
(2.36)
-
-
-
CN
NO2
-
-
90.9
-
-
-
-
-
G
(35.34)
Cr-I denotes transition from solid to the isotropic phase.
Cr-SmA denotes transition from solid to the SmA phase.
Cr-N denotes transition from solid to the N phase.
SmA-N denotes transition from SmA to the N phase.
SmA-I denotes transition from SmA to the isotropic phase.
N-I denotes transition from Nematic to the isotropic phase.
* denotes to monotropic phase transition.
40
30
20
10
0
A
B
C
D
E
F
-10
-20
G
0
50
100
150
200
250
300
o
Temperature ( C)
Figure 1: DSC thermograms of compounds prepared (A-G) upon cooling scan with
rate 10 oC /min.
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N
Cr
SmA
A
B
N
Cr
Cr
C
D
E
F
SmA
Cr
N
Cr
SmA
Cr
SmA
Cr
G
0
20
40
60
80
100
120
Temperature (oC)
Figure 2: DSC transitions for all synthesized series A-G.
o
Figure 3: POM texture of (a) nematic phase of B at 99.0 C upon cooling; (b)
Smectic A phase of F at 106.0 oC upon heating.
Mesophase behaviour of calamitic mesogens is impact the molecular–molecular
interactions that essentially depends on the molecular geometry of compounds,
polarizability of the terminal polar substituent X, as well as the stereo electronic
properties of the whole molecule. Moreover, the shape of the liquid crystalline
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compounds resulting from the introducing of polar substituent is also an important factor
affecting many characteristics [86, 87]. From this point, in the present studied analogues,
molecular association of the calamitic molecules, and consequently their mesophases
stability (TC) and types of mesophase formation depends mainly on the lateral adhesion
of linear molecules that could be increase with the increase of the alkoxy chain length (n).
Moreover, the end-to-end interactions differ according to the polarity and size of the
polar substituent X. The higher molecular polarizability contributed by CN stabilizes the
mesophase and consequently enhanced the TC value. The terminal substituent (X) in the
molecules can be placed in the order of enhancing the mesophase stability as: CN ˃ Cl ˃
CH3O ˃ N(CH3)2 ˃ F ˃ H ≈ NO2. However, the order of enhancing the mesophase range
as: CN ˃ N(CH3)2 ˃ F ˃ Cl ˃ CH3O ˃ H ≈ NO2.
3.3. Tc and polarizability anisotropy of the Car-X bonds
The relationship between the polarizability anisotropy (X) of bonds to small
compact terminal substituent (Car-X) and the mesophase stability (TC) was studied by
van der Veen [88]. The relation has the form:
TC  (M+X)2
(1)
where TC is measured in Kelvin. The term M is the anisotropy of the
polarizability for all the molecular structure except the terminal substituent, X.
Equation (1) can be put in the form [89]:
TC  (M +X) = a .M + a.X
(2)
where ―a‖ is the proportionality constant. Thus, if TC is plotted against X for
any series of liquid crystalline compounds, a straight line is expected, the slope of
which equals ―a‖ and intercept equals ―a M‖. Consequently, M will be given by:
M = intercept/slope.
The values of TC (see Table 1) are plotted as a function of X, for the series A-G
in Figure 4. As shown in Figure 4, linear correlation was estimated for all mesomorphic
compounds, whereby the slope and the intercept, and consequently M, were calculated
by the least squares method and resulted positive values. It was found that TC increases
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linearly with the polarizability anisotropy, X, of the Car–X bond for the investigated
series (A-G), suggesting that the proper choice of the terminal polar groups with different
polarity and size play an important role in the stability of the mesophase. Furthermore,
the presence of terminal substituents with different polarity and volume are more
essential to enhancement the polarizability of the imino linkage by positive inductive
effect (+I), and, consequently the polarizability anisotropy of the whole molecular
structure [84].
Figure 4: Dependency of the mesophase stability (TC ) on the polarizability
anisotropy (X) of the terminal substituent (X) for series A-G.
3.3. DFT calculations
3.3.1. The geometrical structure
Gaussian 09 software was used for DFT calculations for the studied compounds
[90]. 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 constraints. The
structures of the optimized geometries had been drawn with Gauss View [91]. Also,
calculations frequencies were carried out by the same level of theory. The frequency
calculations showed that all structures were stationary points in the geometry
optimization method with none imaginary frequencies.
In order to investigate the effect of substituents on many properties, the relationship
between Hammett substituent constants (σ) and variable parameters such as λmax of UV-
Vis absorption spectra, the energy gap between the FMO, liquid crystalline mesophase
stability .........etc has been studied. Hammett substituent coefficient (σ) is one of the
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major tools that are used to evaluate the electronic effect of the substituents upon the
reaction rate [92]. The results revealed that all compounds are having non-planar
geometry. Generally, the deviation from linearity being variable depends upon the type of
the compound. The twist angle between the two phenyl rings, which predicted from DFT
calculations, is highly affected by the electronic effect of the terminal substituent X,
Figure 5, Table 2. However, all calculations of the optimum and stable molecular
structure of compounds under investigation have been performed for individual
molecules in gas phase, consequently, the calculated twist angles could be different in
condensed phases such as liquid crystals and crystals [93].
Figure 5: Estimated geometrical structure and twist angles of compounds A-G.
Recently, we have reported the relationship between the molecular geometry twist
angle and thermal and mesomorphic properties of synthesized Schiff base ester liquid
crystals, where the molecular geometry twist angle is highly affected with the
electronic nature of the substituent [34]. The relationship between the calculated
angles and Hammett substituent coefficient (σ) [94] of compounds A-G, Figure 6a,
showed that as the Hammett substituent coefficient goes to more positive value the
twist angle decreases while the highly electron donating substituent NMe2 showed the
most non-planar compound with twist angle 33.00o. On the other hand, while the
substituent X in terminal position there is no effect of Van der Waal volume on the
planarity of the prepared compounds and this could be good evidence that the twisting
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of the two phenyl rings is due to the electronic effect not the steric hindrance one,
Figure 6b.
Table 2. Hammett substituent coefficient (σ), Van der Waal volume of terminal X and
estimated twist angle of compounds A-G
Van der Waal volume
Twist angle
Hammett substituent
Compound
coefficient (σ)
37.07
14.37
3.50
-0.13
-0.11
0.00
0.24
0.06
0.67
0.79
33.00
30.92
30.32
29.68
29.16
26.95
22.43
A
B
C
D
E
F
12.00
5.80
14.70
16.80
G
Figure 6: Correlation between the twist angles against (a) Hammett substituent
coefficients σ and (b)Van der Waal volume of terminal X of compounds A-G.
The calculated thermal parameters, dipole moment and the polarizability of the
prepared liquid crystalline compounds A-G are summarized in Table 3. As shown
from Figure 7, the dipole moment is highly impacted by the electronic nature of the
substituent X. excluding the unsubstituted derivative C, which is more positive
Hammett constant and more polar compound. The most electron withdrawing group
(NO2) showed the highest dipole moment (8.2 Debye). The results of the calculated
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dipole moment could illustrate the non mesomorphic properties of the unsubstituted
and the nitro derivatives. The terminal neat derivative ,C, showed the least polarity
with µ = 1.1 Debye, where this small value of the dipole moment cannot permit a
significant interaction neither end to end nor side by side to arrange the molecules in
nematic or semectic phases. On the other hand, the large dipole moment resulted by
the highest electron withdrawing NO2-group could makes a very high lateral
interaction that opposes the liquid crystalline mesophase property, thus prevent the
mesophase formation. Moreover, the high value of dipole moment of the F- and CN-
groups allows a high parallel interaction to enhance a smectic A mesophase, while the
lower dipole moment in case of Cl-, MeO, and Me2N predominates the terminal
interaction to give a nematic mesophase. Such variation in the polarity is expected to
be accompanied by a parallel behaviour in the stability of the mesophase. However,
the presence of the smectic mesophase in addition to the nematic one could be due to
the large Van der Waal volume of the Me2N group that permits a favorable space
filling and thus enhance the molecular packing of molecules, Figure 7.
Table 3. Thermal parameters (Hartree/Particle), Dipole Moment (Debye) and polarizability of compounds A-G.
Parameter
A
B
C
D
E
F
G
Ecorr
0.451539
0.410489
-981.92166
-981.898482
-981.897538
-981.978376
1.7389
0.378469
0.368535
0.369890
0.376839
0.380132
-1071.91841
-1071.895335
-1071.894390
-1071.975600
8.2444
ZPVE
-1001.32539
-1001.300529
-1001.299585
-1001.383911
3.2188
-867.43777
-867.417306
-867.416362
-867.490811
1.1403
-1327.05298
-966.69402
-966.672656
-966.671712
-966.748249
3.4335
-959.67370
Etot
-1327.031211
-1327.030267
-1327.108036
4.0362
-959.651417
-959.650473
-959.729290
6.8760
H
G
Dipole moment
Polarizability α
307.04
278.35
252.00
270.77
253.22
284.89
296.04
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.
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Figure 7: Dependence of calculated dipole moments and Hammett substituent
coefficient (σ) of compounds A-G.
Figure 8: Correlation between the calculated polarizability α against (a) Hammett
substituent coefficients σ and (b) Van der Waal volume of terminal X of compounds A-
G.
Other factors are the polarizability [78, 95, 96] and Van der Waal volume of the
terminal substituent [94] also affect the type of the mesophase as well as the phase
stability temperature, Tc. To illustrate the impact of these factors, firstly, we correlate
the calculated polarizability to Hammett substituent coefficient σ (Figure 8a) and the
calculated polarizability to Van der Waal volume of the substituent (Figure 8b) then
the Tc values are displayed graphically against Van der Waal volume of the
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substituent (Figure 9a) and the polarizability of substituent X (Figure 9b). The results
suggested that the Van der Waal volume and polarizability resulted due to the
attachment of the substituent, X, impact the mesophase stability of the compounds
under investigation. The higher mesophase stability and mesophase range of NMe2
and CN derivatives could be attributed to their high polar nature as well as the extra
conjugation due to the nitrogen atom donation of the NMe2 or the presence of the
triple bond of CN group in conjugation with the π-cloud, which increases the overall
polarizability compared with unsubstituted and halo derivatives. Moreover, the large
size of NMe2 with respect to CH3O enhances their molecular packing due to higher
space filling. The lower mesophase thermal stability of halo derivative could be due to
their lower polarizability as well as smaller size. It could be concluded that
comparable effects on the type as well as the thermal stability of the mesophases due
to the presence of polarizable atom that affect on the dipole moment and polarizability
to enhance the intermolecular attraction between molecules and a bulk terminal groups
those facilitate the molecular space-filling at the terminal moiety. [29]
Figure 9: Correlation between the Tc against (a) Hammett substituent coefficients σ
and (b) Van der Waal volume of terminal X
3.3.2. Frontier molecular orbitals and UV–visible spectroscopy
Optical applications of non-linear optical (NLO) liquid crystals are highly affect by
the energy difference between the frontier molecular orbitals (FMOs), HOMO (highest
occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) [97, 98].
Moreover, this energy gap between FMOs is a good tool for prediction of important
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parameters such as chemical hardness (η), global softness (S) and polarizability (α).
Figure 10 illustrates the calculated ground state isodensity surface plots for the FMOs of
A-G. Table 3 summarizes the values of the FMO energy gap and the global softness (S)
and the experimental λmax. As shown from Table 4 and Figures 10, 11 the global softness
as well as the experimental maximum absorption (λmax) were affecting by the FMO
energy gap and the electronic nature of the terminal substituent. The experimental results
of the UV-Vis absorption are constituent with that of the theoretical calculations of the
energy gab between the FMO. As the energy gab between the FMO decreases the λmax
decreases. The attachment of electron donating substituents (Me2N, MeO) of the phenyl
ring (A, B) produces bathochromic shifts of the band wavelength absorption maximum as
compared to the unsubstituted derivative C. The lowest energy FMO electronic
transitions of n-π* electronic transition due to the resonance that present a double bond
character of the methoxy and dimethylamino groups enhancing the push-pull character of
molecules and consequently, makes red shift (see Figure 12). [99, 100] On the other
hand, if σ value is negative, which have electron withdrawing substituent F, Cl, CN and
NO2, they cause red shift of the λmax of these molecules. The electron donation of the Cl
group is higher than that of the F-group of the more electronegative character and so the
λmax of the chloro derivative (D) is longer than that of the flouro (E). While, the extra
conjugation of the CN and NO2 derivatives (F, G) decreases the energy gaps of the FMO
that causes a bathochromic shift. As shown from Figure 13 a, the relation between the
λmax and the energy gap between the FMO is almost linear except for the NMe2 (A) this
could be explained in the term of the high donation of this group. Moreover, the deviation
of the highest electron donation NMe2 and the highest electron withdrawing NO2 group
from the linear relationship of the λmax with Hammett substituent coefficient (σ) could be
illustrated in term of their high electron nature as well as their large Van de Waal volume
Figure 13 b.
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Figure 10: The calculated ground state isodensity surface plots for frontier molecular
orbitals of compounds A-G
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Figure 11: UV–vis spectra of compounds A-G in dichloromethane at 25°C.
X
X
N
N
O
C6H13
O
C6H13
X = NMe2, MeO
Figure 12: Resonating structures of compounds A, B
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Figure 13: Correlation between the λmax against (a) ΔE(ELUMO-EHOMO) and (b) Hammett
substituent coefficient (σ) of compounds A-G.
Table 4. Molecular orbital energies, hardness (η) and global softness (S) of compounds A-G
Compound
EHOMO (a.u) ELUMO (a.u) ΔE(ELUMO-EHOMO) (a.u) S= 1/ ΔE
λmax exp
-0.18580
-0.20267
-0.20946
-0.21592
-0.21389
-0.22201
-0.22721
-0.04575
-0.05620
-0.06311
-0.07341
-0.06877
-0.08971
-0.11804
0.14005
0.14647
0.14635
0.14251
0.14512
0.13230
0.10917
7.140307
6.827337
6.832935
7.017051
6.890849
7.558579
9.160026
358.2
333.6
333.2
340.4
334.0
356.4
379.8
A
B
C
D
E
F
G
3.3.3. Molecular electrostatic potential (MEP)
The charge distribution map for the compounds A-G was calculated under the same basis
sets according to molecular electrostatic potential (MEP, Figure 14). The red region
(negatively charged atomic sites) was localized on the oxygen atoms and the nitrogen atom
of the imino group, while alkyl chains showed the least negatively charged atomic sites (blue
regions). As shown from Figure 14, the conformation of the electronic nature of the terminal
substituent (X) that affects the orientation of the charge distribution map; this could impact
on the type and stability 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 [101-103]. The
alteration of the charge distribution on the molecules due to more electron donation or
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electron acceptance could predominate the terminal aggregation to enhance the nematic
mesophase or the parallel interaction to give a semectic mesophase. Moreover, the high
separation of the charges as in case of the nitro derivative G is a good reason for its non
mesomorphic.
Figure 14: Molecular electrostatic potentials (MEP) of compounds A-G.
4. Conclusions
A new Schiff base series, N-4-substitutedbenzylidene-4-(hexyloxy)benzenamines was
successfully synthesized and prepared by ball milling method. Molecular structures were
confirmed via elemental analyses, FT-IR, and 1H and C13-NMR spectroscopy.
Mesomorphic and optical behaviors of prepared compounds were investigated by DSC,
POM and UV- spectroscopy.
The study revealed that:
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1- All prepared Schiff base compounds are mesomorphic except the NO2 and
unsubstituted derivatives not exhibited any liquid crystal phases.
2- The molecular structures of the prepared compounds are highly affected by the
electronic nature of the terminal substituent X rather than its volume.
3- The mesophase formation and/or stability of the phase depends mainly upon the
size and polarity of terminal group as well as the polarizability of whole structure and
other parameters that impacted by the terminal substituent, X.
4- The high dipole moment of the NO2 derivative and the least of the terminally neat
compound explain their non mesomorphic behaviors.
5- Finally, the estimated charge distribution varied by the polarity of the terminal
substituent, X has high effects on the type and stability of the observed mesophase.
Acknowledgments: This project was funded by the National Plan for Science,
Technology, and Innovation (MAARIFAH)—King Abdulaziz City for Science and
Technology—the Kingdom of Saudi Arabia—award number (14-MAT1702-03). The
authors also acknowledge, with thanks, the Science and Technology Unit, King
Abdulaziz University for technical support.
Conflicts of Interest: ―The authors declare no conflict of interest."
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Figure 1

Figure 2

Figure 3