New Self-Organizing Optical Materials and Induced Polymorphic Phases of Their Mixtures Targeted for Energy Investigations

Herein, a new homologues series of fluorinated liquid crystal compounds, In, 4-(((4-fluorophenyl)imino)methyl)-2-methoxyphenyl 4-alkoxybenzoate were synthesized and its mesomorphic properties were investigated both experimentally and theoretically. The synthesized compounds were characterized by elemental analyzer, NMR, and FT-IR spectroscopy to deduce the molecular structures. The differential scanning calorimetry was employed to examine mesophase transitions whereas the polarized optical microscopy was used to identify the mesophases. The obtained results revealed that the purely nematic phase observed in all terminal side chains. All homologues showed to possess monotropic nematic mesophases except the derivative I8 exhibits enantiotropic property. The optimized geometrical structures of the present designed groups have been derived theoretically. The experimental data was explained using density functional theory computations. The estimated values of dipole moment, polarizability, thermal energy, and molecule electrostatic potential demonstrated that the mesophase stability and type could be illustrated. Binary phase diagram was constructed and addressed in terms of the mesomorphic temperature range and obtained polymorphic phases. It was found that incorporation of the terminal F-atom and lateral CH3O group influence both conformation and steric effect in pure and mixed states. The absorption and fluorescence emission spectra of fabricated films were recorded to elucidate the impact of terminal side chain on photophysical properties of synthesized liquid crystal. It was noted that the increase of terminal side chain length lead to reduction of optical band gap, whereas charge carrier lifetime increases.

Grey [25] discovered that increasing the molecule's width lowers the stability of both the smectic and nematic phases. The fluorine atom's small size allows the observation of liquid crystalline mesophases without causing sterical disruption. Additionally, the high polarity of fluorine atom influences the optical morphology, melting temperature, transition events, dielectric anisotropy, and other physical features. Furthermore, depending on the position and orientation, the polar fluorine atom also impacts the polarisability and dipole moment of the entire molecule structure. The mesomorphic features of the resulting molecule clearly reflect this. A small variation in molecular structure lead to changes in the polarity and direction of dipole moments, consequently, improvement of optical characteristics and observance of new mesophase behavior. The molecules, on the other hand, tend to orientate in a parallel arrangement as the length of the terminal substituent grows [26]. Furthermore, the twist-bend nematic and heliconical phases are influenced by the length of the terminal chains [27][28][29][30].
Remarkably, the computational assumption for new designing materials [31][32][33][34] shows a degree of attractiveness. To provide a wide range of optical properties, stimulated information about the molecule orbital energies and molecular geometries of liquid crystalline materials is required. Density functional theory (DFT) has recently emerged as a viable technique owing to its superior performance and reproducible computational results. Further, the knowledge of optical and photophysical properties of liquid crystal material is important to find its application in optoelectronic devices. The absorption spectroscopy is the primary tool to check whether a material is suitable for solar cells active layer or not by evaluating its optical band gap and absorption coefficient [35]. The steady state and time resolved fluorescence spectroscopy is used to illustrate the charge carrier dynamics in a semiconducting material under illumination [36].
The goal of the present work is to synthesize new fluorinated liquid crystals, 4-(((4-fluorophenyl)imino)methyl)-2-methoxyphenyl 4-alkoxybenzoate, (In), possessing two un-symmetrical terminal substituents ( Figure 1). The first terminal is an alkoxy chains with 6, 8, 10, and 12 16 carbons connected to the phenyl benzoate moiety. The second wing is the F-atom attached to the phenylimine mesogen. Experimentally, we investigate the mesomorphic and photophysical properties of synthesized materials and theoretically, predicted parameters by employing DFT calculation and investigated the impact of the linking group, alkoxy chain length as well as the position of lateral and terminal substituents. The optical properties of synthesized materials was investigated by recording absorption spectra, whereas steady state and time resolved fluorescence spectroscopy were employed to examine the impact of alkoxy side chain length on photophysical properties of liquid crystal.

Synthesis
The liquid crystals In were synthesized according to Scheme 1.        The isomerization of the present laterally substituted Schiff bases (In) has been investigated. The presence of only one isomer (E-isomer) has been confirmed by NMR spectra. No duplicate peaks neither for azomethine protons nor for aromatic protons have been observed. This is owing to the steric hindrance between the bulky two phenyl groups which destabilizes the Z-isomer relative to the E-isomer (Scheme 2). The isomerization of the present laterally substituted Schiff bases (In) has been investigated. The presence of only one isomer (E-isomer) has been confirmed by NMR spectra. No duplicate peaks neither for azomethine protons nor for aromatic protons have been observed. This is owing to the steric hindrance between the bulky two phenyl groups which destabilizes the Z-isomer relative to the E-isomer (Scheme 2).

Characterization
Details are given in Supplementary Data.

Computational Method
DFT calculations for the investigated substances were performed using Gaussian 09 software [39]. The computations were performed using the DFT/B3LYP methods with the 6-31G (d,p) basis set. Without imposing any molecular symmetry limitations, the geometries were optimized by minimizing the energies with respect to all geometrical parameters. Gauss View [40] was used to design the structures of the optimized geometries. In addition, the same level of theory was used to calculate frequencies. In the geometry optimization method, the frequency calculations revealed that all structures were stationary points with no imaginary frequencies.

Liquid Crystalline Properties
The mesomorphic properties of the present synthesized series (In) were investigated. The results of the transition temperatures and enthalpies, as determined from the DSC investigations are collected in Table 1. DSC data were estimated from the second heating/cooling cycles to study the stability of the synthesized compounds. The second heating scan was used to record all of the thermal properties of these substances. DSC thermogram of the synthesized homologue I8 is presented in Figure 2. On heating, the homologue showed two endothermic peaks assigned to the crystal-to-mesophase and mesophase-to-isotropic transitions while on cooling, a reversed exothermic peak was observed corresponds to isotropic to mesophases transition as presented in Figure 2 and the mssophase still present until room temperature. The POM textures was also confirmed by the DSC data. POM showed schlieren textures of the nematic (N) phase (

Characterization
Details are given in Supplementary Data.

Computational Method
DFT calculations for the investigated substances were performed using Gaussian 09 software [39]. The computations were performed using the DFT/B3LYP methods with the 6-31G (d,p) basis set. Without imposing any molecular symmetry limitations, the geometries were optimized by minimizing the energies with respect to all geometrical parameters. Gauss View [40] was used to design the structures of the optimized geometries. In addition, the same level of theory was used to calculate frequencies. In the geometry optimization method, the frequency calculations revealed that all structures were stationary points with no imaginary frequencies.

Liquid Crystalline Properties
The mesomorphic properties of the present synthesized series (In) were investigated. The results of the transition temperatures and enthalpies, as determined from the DSC investigations are collected in Table 1. DSC data were estimated from the second heating/cooling cycles to study the stability of the synthesized compounds. The second heating scan was used to record all of the thermal properties of these substances. DSC thermogram of the synthesized homologue I8 is presented in Figure 2. On heating, the homologue showed two endothermic peaks assigned to the crystal-to-mesophase and mesophase-to-isotropic transitions while on cooling, a reversed exothermic peak was observed corresponds to isotropic to mesophases transition as presented in Figure 2 and the mssophase still present until room temperature. The POM textures was also confirmed by the DSC data. POM showed schlieren textures of the nematic (N) phase ( Figure 3). All homologues exhibits the monotropic monomorphic characteristics except the derivative I8 shows enantiotropic property. Figure 4 shows a graphical illustration of DSC temperature of transitions to evaluate the effect of the terminal alkoxy chains (n) on the mesomorphic behavior of the formed derivatives [41,42]. All examined compounds of the present series (In) are nematogenic, as shown in Table 1 and behavior for any designed liquid crystalline molecular architecture [26,38]. As shown from Table 1 and Figure 4, the melting transitions temperature follow a random pattern. The homologue I12 possesses the lowest melting point (78.1 • C) whereas the homologue I6 has the highest melting transition temperature (116.3 • C). All compounds of the prepared group are nematogenic and display purely N mesophases and their N phase stability decreases with the terminal alkoxy chain, n; these results are in consistent with previous documents [25][26][27][28][29][30][31][32][33][34][35][36][43][44][45]. ure 3). All homologues exhibits the monotropic monomorphic characteristics except th derivative I8 shows enantiotropic property. Figure 4 shows a graphical illustration o DSC temperature of transitions to evaluate the effect of the terminal alkoxy chains (n) o the mesomorphic behavior of the formed derivatives [41,42]. All examined compound of the present series (In) are nematogenic, as shown in Table 1 and Figure 4. In genera the type of linking spacers and the length of terminal chains as well as the size of at tached substituents determine the mesomorphic behavior for any designed liquid crys talline molecular architecture [26,38]. As shown from Table 1 and Figure 4, the meltin transitions temperature follow a random pattern. The homologue I12 possesses the low est melting point (78.1 °C) whereas the homologue I6 has the highest melting transitio temperature (116.3 °C). All compounds of the prepared group are nematogenic and dis play purely N mesophases and their N phase stability decreases with the termina alkoxy chain, n; these results are in consistent with previous documents [25][26][27][28][29][30][31][32][33][34][35][36][43][44][45].       In general, the mesogenic core of the molecule's polarity and/or polarisability plays the most important roles in determining mesophase behavior. In comparison of present derivatives, homologue I6 has the highest nematic stability (112.9 • C), whereas homologue I12 has the lowest N phase stability (53.0 • C). Wide N temperature ranges are observed for all homologues upon cooling. From the present results, it could be concluded that, as the molecular anisotropy increases, as a result of the change in the mesogenic core of the molecule, the temperature range of the produced mesophases is increased.
On the other hand, the geometrical parameters such as dipole moment, polarizability, and molecular shape of the prepared homologues In are highly affected on the association of molecules that leads to enhance the formation of wide N mesophase temperature range on cooling.
The normalized entropy changes, ∆S/R, upon cooling of the prepared compounds (In) are collected in Table 1. Independent of the alkoxy chains length (n), the entropy of N transition is of small magnitude. The lower value of entropy changes for N transitions of homologous series In can be explained by the lower degree of linear alignments of the molecules at high temperatures. Furthermore, the terminal chains play a significant part in the molecule's multi-conformational modifications [46].

Geometrical Structures
Because fluorine atom is one of smallest atoms, lateral fluorine inside the mesogenic part of the molecule will have a slight steric effect. Furthermore, the fluoro substituent has the maximum electronegativity and is very polar (3.98). It also has a low polarizability (5.57 × 10 −25 cm −1 ), which means that intermolecular dispersion interactions are minimal. The effect of fluoro substitution in a laterally substituted mesogen, on the other hand, is largely reliant on its location.
The optimized geometrical shape of designed fluorinated derivatives I6, I8, I10 and I12 are shown in Figure 5. The computational calculations were performed by DFT/B3LYP method using 6-31G (d,p) basis set and GAUSSIAN 09W. The calculated data predict that the all compounds are linear. The synthesized homologues (In) should be in planar conformation because they are mesomorphic. The frequency calculation confirmed that the optimized structure of each member of the current series is stable, as no imaginary frequency was predicted for any of the members ( Figure 5).
It is widely known that the polarizability, dipole moment and total thermal energy of liquid crystalline materials have a significant impact on the type and mesophase stability [13]. Furthermore, these parameters have an impact on some properties that are significant for nonlinear optical (NLO) liquid crystal applications such as optical interconnections, telecommunications, and signal processing [47,48]. The computed energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), as well as the energy difference between them, using the same approach and the 6-31G (d,p) basis set for all compounds have been calculated. The predicted geometrical and thermal parameters values are collected in Tables 2 and 3. Results revealed that the competition between the intermolecular terminal and lateral molecular interactions impacts the mesomorphic characteristics. This competition leads to the predomination of one of them according to the geometrical structure optimization. The linear-shape of the investigated homologues (In) enhances the terminal interactions over the lateral ones. This consequently resulted in the production of the N mesophase, covering all of the prepared fluorinated derivatives (In). It is widely known that the polarizability, dipole moment and total thermal energy of liquid crystalline materials have a significant impact on the type and mesophase sta bility [13]. Furthermore, these parameters have an impact on some properties that are significant for nonlinear optical (NLO) liquid crystal applications such as optical inter connections, telecommunications, and signal processing [47,48]. The computed energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecula orbital (LUMO), as well as the energy difference between them, using the same ap proach and the 6-31G (d,p) basis set for all compounds have been calculated. The pre dicted geometrical and thermal parameters values are collected in Tables 2 and 3. Re sults revealed that the competition between the intermolecular terminal and lateral mo lecular interactions impacts the mesomorphic characteristics. This competition leads to the predomination of one of them according to the geometrical structure optimization The linear-shape of the investigated homologues (In) enhances the terminal interactions over the lateral ones. This consequently resulted in the production of the N mesophase   It was found that, linear dependency of the polarizability and dipole moment of the whole molecule as the terminal alkoxy chain length (n) increases (Table 3). Another linear dependency of the estimated total thermal energy as the stability of the N phase increases ( Figure 6). It was found that, linear dependency of the polarizability and dipole moment whole molecule as the terminal alkoxy chain length (n) increases (Table 3). Anoth ear dependency of the estimated total thermal energy as the stability of the N ph creases ( Figure 6).  Figure 7 depicts the frontier molecular orbitals HOMO (highest occupied) and LUMO (lowest unoccupied) diagrams for the current fluorinated derivatives designs In. Table 3 lists the calculated energies and energy gap (∆E) values. The energy gap predicts the ability of electron transfer from HOMO to LUMO during the electronic excitation mechanism and is inversely connected with reactivity [49]. The electron densities of the sites involved in the production of HOMOs and LUMOs are localized on the fluoro substituent and azomethine moiety as well as lateral methoxy group, according to the expected values. Furthermore, the two terminals of molecules have a minor effect on the placement of the FMOs' electron distributions. In addition, the energy gap of FMOs is not affected by the length of the alkoxy chain (n). the ability of electron transfer from HOMO to LUMO during the electronic excitation mechanism and is inversely connected with reactivity [49]. The electron densities of the sites involved in the production of HOMOs and LUMOs are localized on the fluoro substituent and azomethine moiety as well as lateral methoxy group, according to the expected values. Furthermore, the two terminals of molecules have a minor effect on the placement of the FMOs' electron distributions. In addition, the energy gap of FMOs is not affected by the length of the alkoxy chain (n).

Molecular Electrostatic Potential (MEP)
The distribution of electron density at atomic locations of LC materials affects molecule polarizability, electronic structure, dipole moment, and many other properties [36]. In addition, the molecular electrostatic potential (MEP) is a significant element for studying the electron density distribution in molecules [47][48][49] Furthermore, MEP is one of the most effective methods for determining whether or not a molecule has intermolecular and/or intramolecular interactions. The B3LYP/6311G(d,p) method was used to compute the MEP of the examined compounds (In, Figure 8). The position of the polar attached terminal F-group as well as the connecting azomethine and lateral methoxy groups of the examined compounds had slight effect on the localization of the iso-electronic density of the electron rich and electron deficient areas. The ester group of the benzoate component, on the other hand has the most electrons. While, the terminal alkoxy chain shows the least negatively charged atomic sites.
compute the MEP of the examined compounds (In, Figure 8). The position of the polar attached terminal F-group as well as the connecting azomethine and lateral methoxy groups of the examined compounds had slight effect on the localization of the iso-electronic density of the electron rich and electron deficient areas. The ester group of the benzoate component, on the other hand has the most electrons. While, the terminal alkoxy chain shows the least negatively charged atomic sites.  Figure 9 shows an example of a binary phase diagram derived by DSC investigations of two components bearing same alkoxy chain lengths, present derivative I8 and laterally neat, II8. The binary mixtures was produced from the enantiotropic terminal chain homologues (I8/II8). Laterally neat derivative II8 exhibit the SmA and N mesophases enantiotropically. As seen in this figure, the binary phase diagram exhibits a negative depression of the N phase from the normal behavior. The little disruption of the N phase can be attributable to the great difference between the geometry the two components of the mixture that disturbs to some extent the linear arrangement of the molecules. Moreover, induced polymorphic phases have been produced at compositions 40 and 60 mol % of II8. Figure 9 also demonstrates that the solid mixture with eutectic composition of 60.0 mol % of I16 has a eutectic melting point of 85.7 °C and mesomorphic temperature range of 41.3 °C. It may be deduced that incorporation of the terminal F-atom and lateral methoxy group influence both conformation and steric effect in pure and mixed states that lead to formation of induced polymorphic phases.  Figure 9 shows an example of a binary phase diagram derived by DSC investigations of two components bearing same alkoxy chain lengths, present derivative I8 and laterally neat, II8. The binary mixtures was produced from the enantiotropic terminal chain homologues (I8/II8). Laterally neat derivative II8 exhibit the SmA and N mesophases enantiotropically. As seen in this figure, the binary phase diagram exhibits a negative depression of the N phase from the normal behavior. The little disruption of the N phase can be attributable to the great difference between the geometry the two components of the mixture that disturbs to some extent the linear arrangement of the molecules. Moreover, induced polymorphic phases have been produced at compositions 40 and 60 mol % of II8. Figure 9 also demonstrates that the solid mixture with eutectic composition of 60.0 mol % of I16 has a eutectic melting point of 85.7 • C and mesomorphic temperature range of 41.3 • C. It may be deduced that incorporation of the terminal F-atom and lateral methoxy group influence both conformation and steric effect in pure and mixed states that lead to formation of induced polymorphic phases.

UV-Vis Absorption Spectra
The UV-Visible absorption spectra of prepared samples on glass slides are illustrated in Figure 10a. The spectra exhibit two regions: high energy absorption from 280 nm to 430 nm and low energy absorption from 430 nm to 950 nm. The peak absorption in higher energy region for all samples was observed at~293 nm which correspond to π − π * transition in aromatic ring. The absorption in this region also exhibits a shoulder~361 nm corresponds to absorption by side alkyl chain. As the length of side chain increases shoulder becomes more prominent and its position red shifted to 365 nm, 383 nm and 392 nm, for samples I8, I10 and I12, respectively. The direct energy band gaps of fabricated samples were evaluated from Tauc plot shown in Figure 10b

UV-Vis Absorption Spectra
The UV-Visible absorption spectra of prepared samples on glass slides are illustrated in Figure 10a. The spectra exhibit two regions: high energy absorption from 280 nm to 430 nm and low energy absorption from 430 nm to 950 nm. The peak absorption in higher energy region for all samples was observed at ~293 nm which correspond to * π π − transition in aromatic ring. The absorption in this region also exhibits a shoulder ~361 nm corresponds to absorption by side alkyl chain. As the length of side chain increases shoulder becomes more prominent and its position red shifted to 365 nm, 383 nm and 392 nm, for samples I8, I10 and I12, respectively. The direct energy band gaps of fabricated samples were evaluated from Tauc plot shown in Figure 10b

Fluorescence Spectra
To illuminate the influence of alkoxy side chain length on photophysical properties of prepared sample, we recorded steady-state photoluminescence (PL) spectra by exciting the sample via 320 nm laser diode. The normalized PL spectra of fabricated samples are displayed in Figure 11a which show a broad emission in the range 400-575 nm. The peak emission of I6 sample was observed at 462 nm and noted to be red shifted with increase of flexible alkoxy side chain length. The peak emission of I8, I10 and I12 samples were noted at 467 nm, 473 nm and 480 nm, respectively. The fluorescence decay spectra of these samples are shown in Figure 11b which were recorded at 465 nm. It can be noted from the Figure that as the alkyl side change length increases the fluorescence decay

Fluorescence Spectra
To illuminate the influence of alkoxy side chain length on photophysical properties of prepared sample, we recorded steady-state photoluminescence (PL) spectra by exciting the sample via 320 nm laser diode. The normalized PL spectra of fabricated samples are displayed in Figure 11a which show a broad emission in the range 400-575 nm. The peak emission of I6 sample was observed at 462 nm and noted to be red shifted with increase of flexible alkoxy side chain length. The peak emission of I8, I10 and I12 samples were noted at 467 nm, 473 nm and 480 nm, respectively. The fluorescence decay spectra of these samples are shown in Figure 11b which were recorded at 465 nm. It can be noted from the Figure that as the alkyl side change length increases the fluorescence decay becomes slower. The excited state charge carrier lifetime in these samples were evaluated by fitting the decay spectra with single exponential decay: where τ is lifetime. The charge carrier lifetimes in I6 sample was evaluated to be 107 ns. It can be noted from Table 4 that with increase of alkyl side chain length the lifetime increases attributed to the stearic hindrance of alkyl side chain. The lifetime I8, I10 and I12 samples were calculated to 133 ns, 150 ns and 178 ns, respectively.

Conclusions
New nematiogenic fluorinated liquid crystal homologues series were prepared and examined for their mesophase and photophysical behavior for energy investigations. All materials are exhibit purely nematogenic mesophase monotropically except the derivative I8 possess enantiotropic nematic phase. Theoretical calculations revealed that, the linear-geometry of the investigated compounds enhances the terminal interactions and resulted in the production of the N mesophase covering all the terminal chains. The optical band gap of liquid crystal material was found to be decreases from 2.97 eV for I6 to 2.70 eV I12 attributed to improved crystallinity with increase of alkoxy side chain. The red shift in steady state fluorescence spectra and increase of charge carrier lifetime further confirm improvement of crystallinity of liquid crystal material with increase of terminal side chain length.

Conclusions
New nematiogenic fluorinated liquid crystal homologues series were prepared and examined for their mesophase and photophysical behavior for energy investigations. All materials are exhibit purely nematogenic mesophase monotropically except the derivative I8 possess enantiotropic nematic phase. Theoretical calculations revealed that, the lineargeometry of the investigated compounds enhances the terminal interactions and resulted in the production of the N mesophase covering all the terminal chains. The optical band gap of liquid crystal material was found to be decreases from 2.97 eV for I6 to 2.70 eV I12 attributed to improved crystallinity with increase of alkoxy side chain. The red shift in steady state fluorescence spectra and increase of charge carrier lifetime further confirm improvement of crystallinity of liquid crystal material with increase of terminal side chain length.