Synthesis, Optical and DFT Characterizations of Laterally Fluorinated Phenyl Cinnamate Liquid Crystal Non-Symmetric System

: A new laterally ﬂuorinated unsymmetric liquid crystalline homologous series, based on cinnamate linkage, named 2-ﬂuoro-4-(4-(alkoxy)phenyl)diazenyl)phenyl cinnamate ( I n ), was synthesized and evaluated via different experimental and computational tools. The series had different terminal alkoxy-chain lengths with a lateral F atom in the meta position with respect to the azo moiety. The experimental mesomorphic and optical investigations were carried out using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Theoretical calculations and geometrical parameter predictions were conducted using the DFT program method at B3LYP/6-311G** level of theory. The results revealed that all the designed compounds exhibited the nematic (N) mesophase enantiotropically. The nematic stability and temperature range were impacted by the terminal alkoxy chain length. Compounds with the shortest chains ( I 6 and I 8 ) showed a monotropic smectic A (SmA) phase, while the longest chain derivative, I 16 , possessed enantiotropic Sm A phase. Theoretical density functional theory (DFT) predictions were correlated with the practically observed data from the mesomorphic investigations. Data revealed that the terminal alkoxy and lateral F groups had an essential impact on the total energy of possible geometrical structures and their physical and thermal parameters.

Generally, the mesomorphic ranges of nematic (N) phases are strongly enhanced when a lateral polar substituent is inserted to the mesogenic cores [24][25][26]. The extent of such impact is dependent on the volume, location, and polarity of the lateral group. The lateral substituents decrement the intermolecular packing and effectively broaden the mesogenic core. This leads to lateral interaction depressions [25][26][27][28] and, consequently, the N thermal stability is reduced. It was documented by Gray et.al. [29] that the increase in the breadth of molecules will reduce the stability of both the N and smectic mesophases. Moreover, different locations of the lateral substituent result in the protrusion of the side group with oriented angles.
On the other hand, the kind of the symmetrical and unsymmetrical terminal substituents will affect the mesomorphic phenomena [30][31][32][33]. Thus, when a change of the length of the terminal alkyl/alkoxy chains is affected and the rigidity of the structural molecular core is altered, the linearity of the molecules will be accordingly changed, to some extent, due to the higher number of the chain configurations that lead to different interactions between linear molecules [34,35]. Additionally, lengthening of the terminal chain impacts the twist-bend N and heliconical phases [22,23,36,37].
The surface hydrophobicity of materials plays an important role in governing the stability of LC alignment of molecules [38]. In addition, it is mainly impacted by the surface chemical nature and surface topography. Moreover, such studies have mostly investigated polymeric LC systems [38,39].
The DFT calculations program is one of the more interesting applied theoretical methods for LC and optical systems [40][41][42]. Prediction of the geometrical parameters for the molecular structure offers more interesting correlations between the obtained experimental results and the estimated theoretical simulations [43][44][45]. This will offer the predictions of the preferred molecular shape in the gas phase, but the presence of these materials in the condensed phase leads to a slight difference in resulted energy [46].
Cinnamate-composed LC materials were mostly approved to be important in applications and exhibited nematogenic phases [47][48][49][50][51][52][53][54][55][56][57]. In addition, the optical behavior of nematogenic materials was shown to be strongly enhanced by the inclusion of a lateral fluorine (F) atom. This influence was mainly dependent on the location and orientation of the F atom. The small volume and high polarity of the lateral F atom played an essential role in affecting the physical and thermal characteristics of the resulting LC system, such as melting and phase transitions [58].
Our research lab has focused its interests on the computational studies of newly synthesized LC materials in order to investigate their mesomorphic behavior with the theoretical prediction relationship. Additionally, the design of new laterally substituted LC molecules resulted in a new geometrical property, and the N phases were observed [24][25][26][27]. In previous works [24][25][26][27], more investigations were directed towards analyzing the effect of introducing the lateral group in the central or terminal rings on their mesophase behaviors. These investigated compounds were found to exhibit low melting transitions and mesomorphic behavior depending on the structural shape of the prepared molecules.
Continuing our work concerning the design and investigation of azo LC materials, a non-symmetrical homologues series based on a latterally attached F atom, namely 2-fluoro-4-(4-(octyloxy)phenyl)diazenyl)phenyl cinnamate (I n ) (Scheme 1), was synthesized and evaluated experimentally and theoritically. The present study aims to investigate the mesomorphic and optical properties of the newly prepared derivatives and correlate their experimental outcome data with the computional predictions. Moreover, the study aims to compare the present investigated system with the previously reported latterally neat derivatives (II n ).

Mesomorphic and Optical Investigations
The mesophase properties of the designed laterally fluorinated homologous series (I n ) were investigated. Data on the phase transition temperatures, enthalpies, and normalized entropies of transitions, as measured by DSC, are collected in Table 1. An example of DSC thermograms for the synthesized compound I 8 , during heating and cooling cycles, is displayed in Figure 1. DSC analyses were performed from the second heating/cooling cycles to ensure the thermal stability of the designed materials. Figure 1 shows two endothermic transitions upon heating cycle and three exothermic transitions during cooling scans. Nematic and smectic A mesophases were observed in all the investigated derivatives under the POM ( Figure 2). The DSC transitions were confirmed by the POM investigations. DSC and POM examinations indicated that these materials exhibit dimorphic mesophases. Derivatives I 6 and I 8 exhibited enantiotropic N and monotropic SmA phases, while I 16 possessed enantiotropic SmA and N mesophases (Figures S1 and S2). Graphical representation of DSC transition temperatures for I 6 , I 8 , and I 16 homologues are displayed in Figure 3 to illustrate the effect of the flexible terminal length chain on the mesomorphic properties of the synthesized laterally F-substituted system (I n ). Table 1. Mesophase transition temperatures ( • C), (enthalpy, ∆H (kJ/mole)) and normalized entropy ∆S/R of transitions for system I n .

Upon Heating Upon Cooling
Comp Cr-SmA = solid to the smectic A phase transition; SmA-N = smectic A to nematic phase transition; N-I = nematic to the isotropic phase transition; N-SmA = nematic to smectic A phase transition; SmA-Cr = smectic A to solid phase transition.  Table 1 and Figure 3 show that the melting transitions of the designed system I n have an irregular behavior that is independent from the length of the terminal flexible alkoxy-chain (n). Melting points increase as the polarizability of the whole molecule within the same series increases and are dependent again on its molecular shape. Furthermore, all members of the series are mesomorphic in nature, with mesophase stability and temperature range depending on their terminal alkoxy chain length (n). All derivatives are dimorphic, possessing both the SmA and N mesophases. The thermal stability of the SmA mesophase increases as the terminal alkoxy chain length increases [59][60][61][62], while the N thermal mesophase stability decreases with n. The shortest terminal chain I 6 derivative exhibits a monotropic SmA phase, with the lowest thermal stability nearly 54.7 • C, and its enantiotropic N phase stability is high at about 195.5 • C, with temperature ranging up to 103.0 • C upon heating. In the case of the I 8 homologue, it also possesses the SmA phase monotropically, with stability close to 74.5 • C, and an enantiotropic N phase stability and temperature range of nearly 173.5 and 66.7 • C, respectively. However, the longest alkoxy chain bearing homologue I 16 exhibits enantiotropic behavior for both observed SmA and N mesophases. In addition, it possesses the highest thermal SmA stability of nearly 97.6 • C and smectogenic temperature range of 13.7 • C. Additionally, the homologue I 16 possesses N stability and a range of nearly 132.8 and 35.2 • C, respectively.  Generally, the molecular shape, polarizability, and dipole moment of the investigated system are highly affected by the electronic nature of the attached terminal groups. Additionally, the mesomorphic properties are influenced by an increment in the polarity and/or polarizability of the mesogenic portions. In the present study, the mesophase range and stability of the investigated series increased in this order: I 6 > I 8 > I 16 . The mesophase behavior of rod-like molecules was a direct result of molecular-molecular interactions that are mainly dependent on the molecular geometry of the terminal polar groups. The increment in the Van der Waals attraction forces between the terminal alkoxy chains facilitated their lamellar packing and the observation of the smectic A phase in addition to the N phase. Furthermore, the mesomorphic thermal stability reflected the nonsymmetric electronic properties of the different terminals of the molecule.
The data on normalized entropy changes of N-I transitions, ∆S/R, and of the investigated system (I n ) are collected in Table 1. Results indicate that the entropy changes of N-I transitions possess small values. This may be due to the possible biaxial configuration of the compounds [63][64][65]. In addition, the stereo configuration of the lateral F atom plays an essential role in the thermal parameters. This will be discussed in the computational part. Moreover, the thermal cis-trans isomerization of the azo moiety has an important role in the observed lower entropy changes, as documented before [64][65][66].

DFT Studies and Molecular Geometries Investigations
The chemical quantum parameters, estimated by the DFT method, and experimental findings were correlated for the present investigated homologues I n . The computational calculations were carried out in the gas phase at B3LYP level (using 6-311G** basis set) of the estimated geometrical structures. The present series (I n ) exhibited LC properties, so they should exist in a planar conformation. The optimized geometry of each member in the system proved to be stable due to the absence of the imaginary frequency ( Figure 4). All resulted thermal parameters of the computational predictions are collected in Table 2. The zero-point energy and other calculated quantum chemical parameters were predicted to increase when increasing the length of the molecule. Similarly, the polarity of the prepared compounds was predicted to increase with the length of the terminal chain, as indicated by the higher magnitude of the dipole moment with higher value of n. It was previously [24][25][26][27] reported that the planarity of the mesogenic portion of the LC compounds is enhanced by the mesomeric nature of the attached polar group. Hence, the conjugated π-cloud interactions, resulting from the terminal alkoxy group, offer a high thermal stability with suitable geometrical parameters, which is in agreement with the experimental findings in the present investigated series.  On the other hand, the polar attached groups, aspect ratio, polarizability, and geometrical structure of the LC molecule are considered as important parameters for the influence of the formed mesophase thermal stability.
Total thermal energy, dipole moments, and polarizability of the optimized conformers were related with the experimental finding values of the mesophase thermal stability and the length of terminal alkoxy chains (n) in order to evaluate the effect of n on the predicted thermal parameters. Figure 5 illustrates the correlation between the N stability and the total thermal energy (E tot ) of the molecule bearing changeable terminal chain length. Table 2 and Figure 5 show a pronounced decrement in E tot with increasing n, and an increment as the N stability increases. Figure 6 shows the relation between the N stability and the whole polarizability and dipole moment of molecular structures (I n ). The relation indicated that the mean polarizability has a pronounced decrease with increment in N stability and increases as the chain length increases. This could be ascribed to the increase in the aspect ratios, which influence the space filling of the LC materials and lead to the enhancement of the mean polarizability as the terminal chain length increases. Additionally, the dipole moment has nearly constant relation with the nematic range independent from the alkoxy chain length, as shown in Figure 6. The van der Waals week intermolecular attractions have a role in destabilizing the observed N mesophase as the length of alkoxy chain is increased. The lower dipole moments values allow the terminal aggregations to predominate, which enhances the N mesophase formation.

Frontier Molecular Orbitals (FMOs)
The investigations of frontier molecular orbital HOMO (highest occupied) and LUMO (lowest unoccupied) distributions for the present synthesized compounds, I n , are represented in Figure 7. Table 2 collects their resulting energies and energy gaps. The energy gap (∆E) between HOMO and LUMO levels is a function of the chemical reactivity of compounds. The lower its value of energy gap (∆E), the more reactive the molecule is. The predicted energy gaps ( Table 2) confirm that all members of the investigated series I n possess the same reactivity. In addition, they have the same softness because the energy gap is inversely correlated to the softness (softness = 1/∆E). Furthermore, all the designed homologues (I n ) showed similar electron cloud distributions over the carbon atoms and π-system of the azo linkage and lateral F atom.

Molecular Electrostatic Potential (MEP)
The molecular shape of the designed LC compounds is affected by the mesomeric configurations, which are normally impacted by the molecular-molecular interactions. The electronic structure, molecular polarizability, dipole moment, and many other parameters are influenced by the electron density distribution at atomic sites of LC materials [27]. Prediction of the presence of inter-or intra-molecular interactions of the designed molecules via the molecular electrostatic potential (MEP) is one of the best tools. MEP is an indicator which shows the distribution of electron density within the molecular structure. The charge distribution map for the investigated materials I n , was computationally predicted under the same B3LYP/6-311G** level of calculation according to MEP (Figure 8). The MEP examinations suggests high electrostatic potential but low electron density for the terminal polar alkoxy groups of all the prepared members (I n ); this is represented by the blue cloud. On the other hand, the red regions (which indicates high electron density) are expected to be localized on the mesogenic cores, which appear at the oxygen atoms of the ester linkage and the alkoxy chains.

Effect of the Introduction of Lateral F Atom on the Mesomorphic Behavior of Laterally-Neat Series
In order to evaluate the impact of the addition of the F atom within the center of the molecular structure and on the mesomorphic behavior of nonfluorinated derivatives (II n , Scheme 2), the thermal phase transition stabilities of the present laterally fluorinated system (I n ) were compared with the laterally neat derivatives, II n [47]. Thermal mesophase stabilities (T C ) data for both series I n and II n are collected in Table S1. The comparison was made between their mesophase stabilities (T C ) as a function of their terminal alkoxy chain length. All compounds of homologous series II n [47] are nematogenic, exhibiting wide N mesophases with high thermal stability. The N phase was detected upon cooling from the isotropic liquid by the observation of N droplets via POM. Two endothermic peaks were detected through the DSC technique for Cr-N and N-I transitions upon heating, and two other reversed exothermic peaks from I-N and N-Cr during cooling were observed [47]. The comparison revealed that the kind and stability of the formed mesophase are dependent on the enhanced dipole moment of the molecular mesogenic part, which is essentially dependent on the location and relative orientation of the lateral F moiety. Moreover, the introduction of the lateral F group into the mesogenic core enhances the SmA phase for all lengths of the terminal chains. In order to investigate the isomerization effect on the mesomorphic properties of laterally F (I n ) and laterally neat (II n ) derivatives, the total energies of homologues I 6 and II 6 were predicted by the same set B3LYP level using 6-311G** basis set. All estimated data were collected in Table S2. The optimized z-form structural geometries of both compounds (I 6 and II 6 ) showed non-planar conformations ( Figure 9). Moreover, their thermal energies were higher values than the E-isomers. Thus, the E-isomers proved to be stable and planar geometries, which satisfied the mesomorphic properties requirements. Figure 9. Optimized structures of z-forms for I 6 and II 6 , calculated at B3LYP/6-311G** level.

Synthesis
The liquid crystalline compounds I n were synthesized as the following Scheme 3: The isomerization of the present investigated derivatives (I n ) has been evaluated. The NMR spectra indicated that only one isomer (E-isomer) was observed that was confirmed by the singlet proton resonances of most aromatic protons of the synthesized materials. Thus, the incorporation of a high electron-withdrawing lateral F atom in the meta-position with respect to the azo-linkage may stabilize the E-isomer rather than the Z-isomer (Scheme 4). It will be discussed in the theoretical part.

Conclusions
In the present investigation, a new non-symmetric series of laterally fluorinated liquid crystalline materials based on cinnamate moiety, namely 2-fluoro-4-(4-(alkoxy)phenyl) diazenyl)phenyl cinnamate (I n ), was synthesized and mesomorphically investigated using DSC and POM. Mesomorphic and optical characterizations revealed that all of the prepared azo derivatives of the cinnamate set are dimorphic, exhibiting both the SmA and N mesophases. All homologues have been shown to possess monotropic SmA except the longest chain derivative (I 16 ), which possesses the enantiotropic SmA phase. The introduction of a lateral F atom on the mesogenic molecular structure has induced the SmA phase. The unsymmetric geometry of the designed molecules plays an important role in the resulted physical and thermal parameters. DFT simulations showed that the length of the terminal chain enhanced the thermodynamic energy as well as the geometrical parameters.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.