Synthesis, Optical Characterizations and Solar Energy Applications of New Schiff Base Materials

A new set of laterally OCH3-substituted photoactive liquid crystalline analogues, 4-hexyloxy phenyl- imino-4ʹ-(3-methoxyphenyl)-4ʹ’-alkoxybenzoates, were synthesized and investigated for their mesomorphic behavior. The prepared set constitutes five analogues that differ from each other by the terminally attached compact polar group. Characterization of the synthesized derivatives is conducted using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and UV-spectroscopy. Molecular structures were elucidated by elemental analyses, FT-IR and NMR spectroscopy. DSC and POM investigations indicated that all the prepared derivatives are monomorphic possessing the nematic (N) phase, except for the unsubstituted derivative that is nonmesomorphic. On the other side, the photophysical study and the optical spectra measurements confirm the photoactivity of the present compounds under UV/visible irradiation. The measured optical spectra showed impressive enhancement in the optical absorption and reduction in the optical bandgap from 3.63 to 3.0 eV depending on the terminal group. From the study of the DC electric properties, the lowest resistance, 106.5 GΩ at scan rate 0.1 V/s, was observed for the I6d film with Cl terminal, which decreased to 49.5 GΩ by increasing the scan rate to 0.5 V/s. Moreover, the electrical conductance is decreased from 9.39 pS to 1.35 pS at scan rate 0.1 V/s by changing the terminal group from Cl to F. The enhanced optical absorption and the reduced energy gap make the optimized samples suitable material for solar energy applications.


Introduction
Nowadays, liquid crystalline (LC) compounds prove to have several applications as optical materials-areas of technological applications such as displays, light-emitting diodes based on organic derivatives, anisotropic networks, and semi-and photoconductors materials [1][2][3]. The most helpful characteristics of these LC compounds that are considered to be essential in applications in devices are the mesophase type, switching times and optical rotation-power near the working temperatures [4]. Geometrical-property relationships could be important tools to synthesize suitable architectures to achieve the desired properties for device-displays [5][6][7][8]. Consequently, the selection of the flexible alkoxy/alkyl groups, terminal wings, as well as mesogenic linkers, is an important criterion in the formation of thermotropic LCs for proper characteristic industries. Moreover,

Synthesis
The synthesis pathway leading to the title compounds I6a-e are presented in Scheme 1

Synthesis
The synthesis pathway leading to the title compounds I6 a-e are presented in Scheme 1 and details of preparations with spectroscopic analyses were inserted in Supplementary Data. I6 a-e I6 a , X=OCH 3; I6 b , X=CH 3 ; I6 c , X=H; I6 d , X=Cl; I6 e , X=F

Synthesis
The synthesis pathway leading to the title compounds I6a-e are presented in Scheme 1 and details of preparations with spectroscopic analyses were inserted in Supplementary Data. NH

Films Preparation
A very thin layer of the sample was prepared by sandwiching them between a glass slide and a coverslip. The dimensions of the cell were 22 mm × 22 mm × 0.03 mm, i.e., the film thickness was ~30 μm. The temperature of the cell was controlled using a temperature controller with an accuracy ±0.1 °C. Scheme 1. Synthesis route of title compounds I6 a-e .

Films Preparation
A very thin layer of the sample was prepared by sandwiching them between a glass slide and a coverslip. The dimensions of the cell were 22 mm × 22 mm × 0.03 mm, i.e., the film thickness was~30 µm. The temperature of the cell was controlled using a temperature controller with an accuracy ±0.1 • C.
Molecular structures of products I6 a-e were elucidated via their spectroscopic data αFT IR, 1 H-NMR and 13 C-NMR, see Supplementary Data) and elemental analyses.
The FT-IR spectra of compounds I6 a-e revealed in each case two characteristic absorption bands in the regions υ 1720-1731, 1606-1612 cm −1 attributed to the C=O and C=N groups. The FT-IR results revealed that compounds I6 d and I6 e (X=Cl or F, electron withdrawing groups) had no pronounced effect on the absorption band of C=O group compared to the unsubstituted derivative I6 c (X=H; v = 1723, X=Cl; v = 1725 and X=F; v = 1722 cm −1 ), while compounds I6 a and I6 b (X=OCH 3 or CH 3 , electron donating groups) showed marked increase in carbonyl absorption bands to 1731 and 1728 cm −1 , respectively). In addition, the polarity of X has a slight effects on the absorption band of C=N group. The wave number of the C=N group for the unsubstituted derivative I6 c appeared at 1612 cm −1 while the electron donating derivatives (X=CH 3 O or CH 3 ) appeared near 1608 cm −1 instead of 1606 cm −1 for the electron withdrawing groups (X=F or Cl). Thus, the substituent X, regards of electron donating or electron withdrawing did not mesomerically affect the imino group.

Mesomorphic and Optical Properties
The mesophase properties of all the synthesized derivatives have been evaluated by DSC and POM. Typical heating/cooling DSC thermograms of prepared compound I6 d are displayed in Figure 2. Upon heating scans, significant endothermic peaks were observed, dependent on the polarity of terminal group (X), which are ascribed to mesomorphic transition, and the cooling cycle confirmed those observed upon decreasing the temperature. DSC curves confirm the mesophase transitions from Cr→ N, and N→ I according to the structural shape of synthesized compounds, I6 a-e . POM optical images of some derivatives are illustrated in Figure 3 as examples. The nematic mesophase was identified by schlieren/threads textures observations upon heating and cooling cycle.
The mesophase transition temperatures, as measured from DSC thermal analysis and their associated enthalpies for all the synthesized compounds, I6 a-e , are tabulated in Table 1, and the phases transitions are depicted in Figure 4. As can be seen from Table 1 and Figure 4, all prepared compounds are mesomorphic in nature with mesomorphic range of stability dependent on their terminal substituent, except the unsubstituted derivative (I6 c , X=H) which is nonmesomorphic. Moreover, all the compounds are monomorphic, possessing only the nematic phase. The mesomorphic range and stability increased in this order:     The mesophase transition temperatures, as measured from DSC thermal analysis and their associated enthalpies for all the synthesized compounds, I6a-e, are tabulated in Table  1, and the phases transitions are depicted in Figure 4. As can be seen from Table 1 and Figure 4, all prepared compounds are mesomorphic in nature with mesomorphic range of stability dependent on their terminal substituent, except the unsubstituted derivative (I6c, X=H) which is nonmesomorphic. Moreover, all the compounds are monomorphic, possessing only the nematic phase. The mesomorphic range and stability increased in this order: It can also be seen from Table 1 and Figure 4 that the melting temperature varies irregularly. Compound I6a (X=OCH3) has enantiotropic nematic phase with the highest nematic stability and temperature range 155.9 and 51.4 °C, respectively. The second derivative bearing the electron-donating group (I6b, X=CH3) also possesses enantiotropic nematic mesophase with stability and temperature range nearly 141.3 and 51.1 °C, respectively, while the derivatives bearing the electron-withdrawing groups (X=Cl and F) possess less thermal stability. Compound I6d (X=Cl) exhibits enantiotropic N phase with thermal stability 137.4 °C and range 38.7 °C. For terminal fluorine atom (I6e), it exhibits monotropic N phase with lower range of thermal stability. In general, the molecular architecture and polarizability as well as the dipole moment of the synthesized materials are highly affected by the electronic nature of the terminals and their volumes. In case of unsubstituted derivative (I6c), transitions are directly from solid to isotropic liquid upon heating and the reverse on cooling, meaning that it is nonmesomorphic. The mesomorphic stability is enhanced by an increment in the polarity and/or polarizability of the mesogenic part of the molecule. Thus, the nature of the terminal moieties (X=OCH3, CH3, Cl and F) influences the mesophase stability and induces liquid crystalline mesophases when compared to the unsubstituted derivative (X=H). In addition, the mesophase behavior of calamitic molecules is a direct impact of molecular-molecular interactions that depend mainly on geometrical structure and space filling of the molecules, polarizability anisotropy of the polar terminal and lateral groups, the stereo electronic characters of the whole molecular shape. These factors contribute in varies extents to the mesomorphic properties observations. Cr-N = transition from solid to the nematic phase. N-I = transition from nematic to the isotropic liquid phase. * monotropic phase. It can also be seen from Table 1 and Figure 4 that the melting temperature varies irregularly. Compound I6 a (X=OCH 3 ) has enantiotropic nematic phase with the highest nematic stability and temperature range 155.9 and 51.4 • C, respectively. The second derivative bearing the electron-donating group (I6 b , X=CH 3 ) also possesses enantiotropic nematic mesophase with stability and temperature range nearly 141.3 and 51.1 • C, respectively, while the derivatives bearing the electron-withdrawing groups (X=Cl and F) possess less thermal stability. Compound I6 d (X=Cl) exhibits enantiotropic N phase with thermal stability 137.4 • C and range 38.7 • C. For terminal fluorine atom (I6 e ), it exhibits monotropic N phase with lower range of thermal stability. In general, the molecular architecture and polarizability as well as the dipole moment of the synthesized materials are highly affected by the electronic nature of the terminals and their volumes. In case of unsubstituted derivative (I6 c ), transitions are directly from solid to isotropic liquid upon heating and the reverse on cooling, meaning that it is nonmesomorphic. The mesomorphic stability is enhanced by an increment in the polarity and/or polarizability of the mesogenic part of the molecule. Thus, the nature of the terminal moieties (X=OCH 3 , CH 3, Cl and F) influences the mesophase stability and induces liquid crystalline mesophases when compared to the unsubstituted derivative (X=H). In addition, the mesophase behavior of calamitic molecules is a direct impact of molecular-molecular interactions that depend mainly on geometrical structure and space filling of the molecules, polarizability anisotropy of the polar terminal and lateral groups, the stereo electronic characters of the whole molecular shape. These factors contribute in varies extents to the mesomorphic properties observations.
The normalized entropy changes of transition, ∆S N-I /R, of the present lateral methoxy compounds (I6 a-e ) are collected in Table 1. Results showed that small values of the entropy changes are observed that depends on the kind of terminal substituent X. The small values observed for the entropy change can be attributed to the decrease of the length-to-breadth ratio resulting from their lower anisotropy in terms of their molecular geometry and the increment in their molecular biaxiality [47,48]. The induction, conjugation forces, the specific dipolar interactions as well as the π-π stacking interactions [49,50] play important roles in the molecular orientation and thus in the arrangement of molecules and formation of the mesophase.

Clearing Temperature and Polarizability Anisotropy of the C ar -X Bonds
The relationship between the polarizability anisotropy (∆α X ) of bonds to the terminal compact polar group (C ar -X) and the thermal stability of the mesophase (clearing temperature, T C ) was investigated by van der Veen [51].
The form of relation (Equation (1)): where T C is measured in Kelvin. The term ∆α M is the anisotropy of the polarizability for whole molecular structure except the terminal group, X. Equation (1) can be put in the form (Equation (2)) [52]: where "a" is the proportionality constant. Thus, if √ T C is correlated against ∆α X for any set of liquid crystalline materials, a straight line is expected, the slope of which equals "a" and intercept equals "a·∆α M ". Consequently, ∆α M will be given by ∆α M = intercept/slope. √ T C values are displayed as a function of ∆α X , for present analogous I6 a-e in Figure 5. It could be seen from Figure 5, a linear correlation was obtained for all members of the synthesized series I6 a-e , whereby the slope and intercept as well as ∆α M were calculated by the least squares method. The calculated slope and intercept have values 2.35 × 10 24 and 19.80 K 1/2 respectively, while the resulted ∆α M equal 1.18 × 10 23 cm 3 . Additionally, a linear dependency was observed in Figure 5 for present synthesized set. Thus, the selected terminal substituents were enhancing the polarizability of the ester moiety by positive inductive effect and, consequently, the polarizability anisotropy of the whole molecular architecture.
C T values are displayed as a function of ΔαX, for present analogous I6a-e in Figure 5. It could be seen from Figure 5, a linear correlation was obtained for all members of the synthesized series I6a-e , whereby the slope and intercept as well as ΔαM were calculated by the least squares method. The calculated slope and intercept have values 2.35x 10 24 and 19.80 K 1/2 respectively, while the resulted ΔαM equal 1.18 x10 23 cm 3 . Additionally, a linear dependency was observed in Figure 5 for present synthesized set. Thus, the selected terminal substituents were enhancing the polarizability of the ester moiety by positive inductive effect and, consequently, the polarizability anisotropy of the whole molecular architecture.

Effect of the Incorporation of the Lateral Methoxy Group on the Mesomorphic Behaviors
The attachment of lateral methoxy group into the mesogenic core of the molecule will have a slight steric impact. In addition, it is an electron-donating moiety, thus it will have an affect on the intermolecular dispersion interactions. To investigate the effect of addition a lateral OCH 3 group, in the middle ring of the molecule with special orientation, upon the mesophase and thermal behaviors of the compounds, a comparison is established between the presently prepared unsubstituted derivative (I6 c ) and the previously documented laterally neat analogous, II, Figure 6 [53] for their mesophase stabilities. Generally, the thermal stability of the formed mesophase varies according to the enhanced molecular dipole moment and polarizability of the mesogenic part, which are dependent upon the position of the lateral OCH 3 group. Moreover, the incorporation of the lateral OCH 3 group into the ortho position with respect to the azomethine linkage of I6 c will disrupt the smectic A molecular arrangement for II and shows a nonmesomorphic behavior. The comparison revealed that the mesophase type and stability as well as range depends on the protrusion of the lateral OCH 3 group which is attached within the mesogenic skeleton of the molecule.

Effect of the Incorporation of the Lateral Methoxy Group on the Mesomorphic Behaviors
The attachment of lateral methoxy group into the mesogenic core of the molecule will have a slight steric impact. In addition, it is an electron-donating moiety, thus it will have an affect on the intermolecular dispersion interactions. To investigate the effect of addition a lateral OCH3 group, in the middle ring of the molecule with special orientation, upon the mesophase and thermal behaviors of the compounds, a comparison is established between the presently prepared unsubstituted derivative (I6c) and the previously documented laterally neat analogous, II, Figure 6 [53] for their mesophase stabilities. Generally, the thermal stability of the formed mesophase varies according to the enhanced molecular dipole moment and polarizability of the mesogenic part, which are dependent upon the position of the lateral OCH3 group. Moreover, the incorporation of the lateral OCH3 group into the ortho position with respect to the azomethine linkage of I6c will disrupt the smectic A molecular arrangement for II and shows a nonmesomorphic behavior. The comparison revealed that the mesophase type and stability as well as range depends on the protrusion of the lateral OCH3 group which is attached within the mesogenic skeleton of the molecule.

Photophysical Studies
The reasonable thermal stabilities of liquid crystalline materials have been extensively reported for electronic and photonic applications due to their photophysical characteristics [54]. UV-vis absorption spectra were investigated for the present series I6a-e) in C = 1.0 × 10 −3 mol L −1 dichloromethane solution and displayed graphically in Figure 7. As can be seen from Figure 7, the radiation of light in the wavelength range 260-600 nm and the derivatives possess two strong absorption bands at ~280 nm and 360 nm, according to the dependent on the terminally attached group (X). The more intense bands observed in the analogous set, observed at a wavelength around 359-362 nm, may be assigned to the π-π* transition in the π-electronic system throughout the whole mesogenic moiety, with a suitable charge transfer (CT) property, whereas the weak absorption band that appeared around 277-280 nm may be related to the n-π* transition of the Schiff base moiety [55][56][57]. Moreover, the intensity of the peak and absorption bands is dependent on the geometrical structure of the molecule that absorbs the light at a given wavelength. Further, the present synthesized nematogenic series has promise to be photoactive organic materials for many industries fields.

Photophysical Studies
The reasonable thermal stabilities of liquid crystalline materials have been extensively reported for electronic and photonic applications due to their photophysical characteristics [54]. UV-vis absorption spectra were investigated for the present series I6 a-e ) in C = 1.0 × 10 −3 mol L −1 dichloromethane solution and displayed graphically in Figure 7. As can be seen from Figure 7, the radiation of light in the wavelength range 260-600 nm and the derivatives possess two strong absorption bands at~280 nm and 360 nm, according to the dependent on the terminally attached group (X). The more intense bands observed in the analogous set, observed at a wavelength around 359-362 nm, may be assigned to the π-π* transition in the π-electronic system throughout the whole mesogenic moiety, with a suitable charge transfer (CT) property, whereas the weak absorption band that appeared around 277-280 nm may be related to the n-π* transition of the Schiff base moiety [55][56][57].
Moreover, the intensity of the peak and absorption bands is dependent on the geometrical structure of the molecule that absorbs the light at a given wavelength. Further, the present synthesized nematogenic series has promise to be photoactive organic materials for many industries fields.

DC Electrical Properties
The electrical properties of the studied samples are measured using the Keithley measurement source unit αModel 2400 SMU (Keithley Instruments, Cleveland, OH, USA). Our samples were measured in the nematic phase upon cooling. The samples were melted to 160 °C (isotropic liquid) and then allowed to cool to 115 °C for 30 min except for the nonmesomorphic sample (I6c) that was measured at room temperature (solid phase). The samples were provided with Ohmic contacts using silver paste (Resistivity < 0.04 Ω.cm). The current-voltage (I-V) response of the present synthesized analogs series, I6a-e films, are recorded by varying the applied voltage (V) from −10 V to 10 V with scan rates of 0.1 V and 0.5 V/s, as shown in Figure 8A,D. As the scan rate increased, the detected current increased, and the highest values are detected for the I6d film. The smallest values are detected for the I6d film. That indicates the existence of the terminal Cl group which greatly improves the flow of the charges in the film, and the opposite observation occurs with the existence of the F terminal group. It is evident that the observed behaviors are non-Ohmic (nonlinear). Thus, the change on the resistance of the materials occurred based on the current moving through it. Moreover, it has been recently noticed that the polymeric and organic systems are of Schottky diode behavior at low voltage. In the present investigation, the relation between log (I) and V 1/2 shows two linear stages as illustrated in Figure 9, which implies that our prepared films follow the Schottky diode behavior. DC resistance and electrical conductance values of I6a-e films at a scan rate of 0.1 V/s and 0.5 V/s were measured and presented in Figure 8B-F. The samples have electrical resistances in the same order of magnitude as it has been shown in Figure 8B,E, even though the I6e film presents the highest resistance (743.3 GΩ) and I6d film shows the lowest resistance (106.5 GΩ). The resistance of I6e and I6d films are changed to 807.0 and 49.5 GΩ by increasing the scan rate to 0.5 V/s. Additionally, it is clearly shown in Figure  8C that the electrical conductance is decreased from 9.39 pS to 1.35 pS at scan rate 0.1 V/s by changing the terminal group from Cl to F. By increasing the scan rate to 0.5 V/s, the

DC Electrical Properties
The electrical properties of the studied samples are measured using the Keithley measurement source unit αModel 2400 SMU (Keithley Instruments, Cleveland, OH, USA). Our samples were measured in the nematic phase upon cooling. The samples were melted to 160 • C (isotropic liquid) and then allowed to cool to 115 • C for 30 min except for the nonmesomorphic sample (I6 c ) that was measured at room temperature (solid phase). The samples were provided with Ohmic contacts using silver paste (Resistivity < 0.04 Ω.cm). The current-voltage (I-V) response of the present synthesized analogs series, I6 a-e films, are recorded by varying the applied voltage (V) from −10 V to 10 V with scan rates of 0.1 V and 0.5 V/s, as shown in Figure 8A,D. As the scan rate increased, the detected current increased, and the highest values are detected for the I6 d film. The smallest values are detected for the I6 d film. That indicates the existence of the terminal Cl group which greatly improves the flow of the charges in the film, and the opposite observation occurs with the existence of the F terminal group. It is evident that the observed behaviors are non-Ohmic (nonlinear). Thus, the change on the resistance of the materials occurred based on the current moving through it. Moreover, it has been recently noticed that the polymeric and organic systems are of Schottky diode behavior at low voltage. In the present investigation, the relation between log (I) and V 1/2 shows two linear stages as illustrated in Figure 9, which implies that our prepared films follow the Schottky diode behavior. DC resistance and electrical conductance values of I6 a-e films at a scan rate of 0.1 V/s and 0.5 V/s were measured and presented in Figure 8B-F. The samples have electrical resistances in the same order of magnitude as it has been shown in Figure 8B,E, even though the I6 e film presents the highest resistance (743.3 GΩ) and I6 d film shows the lowest resistance (106.5 GΩ). The resistance of I6 e and I6 d films are changed to 807.0 and 49.5 GΩ by increasing the scan rate to 0.5 V/s. Additionally, it is clearly shown in Figure 8C that the electrical conductance is decreased from 9.39 pS to 1.35 pS at scan rate 0.1 V/s by changing the terminal group from Cl to F. By increasing the scan rate to 0.5 V/s, the electric conductance changes to 20.22 pS for I6 d and to 1.24 pS for I6 e . These behaviors refer to the role of the terminal substituent in controlling the electric conductance, which depends mainly on the number and mobility of charge carriers [58,59]. The change in the density and mobility of charge carriers of the I6 a-e samples is related to their anisotropy, molecular geometry, molecular biaxiality, molecular orientation, dipolar and π-π stacking interactions [43,50]. Hao et al. [60] reported robust dependency of the vertical charge carriers' mobility on the π-π stacking gaps of the organic molecule/graphene heterojunction. In the same context, A.L. Briseno et al. [61] reported that the trapping of the charge carriers in the dipolar fields reduces the mobility of the films; Z. Ma et al. [62] conclude the increase of the charge mobility with the chain length which ascribed to the enhancement of the π-π interactions alongside the stacking directions and the expatriate stacking alongside the molecular plane short axis.

Optical Spectra and Energy Gap Calculation of I6 a-e Films
The optical absorbance and transmittance spectra of the prepared I6 a-e films were measured using a Perkin Elmer spectrophotometer (Lambda 900 UV-VIS-NIR) (PerkinElmer, Boston, MA, USA) within a wavelength range from 250 to 1500 nm. Figure 10A,B shows the dependence of the absorbance and transmittance of the films on the wavelength. From the optical spectra shown in Figure 10A, there is a very strong absorption band observed at 294 nm for I6 a , b , at 298 nm for I6 c , at 320 nm for I6 d , and at 348 nm for I6 e . The intensive UV absorption band is ascribed to the π-π* transition in the π-electronic system. The absorption in the UV and visible regions is higher than the absorption in the IR region. The samples showed almost plateau absorption in the visible light region, which decreased following the order: OCH 3 > CH 3 > Cl > H > F. Then, the absorption of the samples is decreased exponentially in the near IR range to reach almost a plateau absorption for λ > 1000 nm. From the transmission spectra shown in Figure 10B, the transmission of these samples is very low (< 10%) in the UV and visible regions. After that, the values of the transmission increase exponentially. The transmission of the samples is in order of H > F > Cl > CH 3 > OCH 3. The values of the maximum transmission in the IR region are in order of 55%, 47%, 28%, 15%, and 13%. This behavior confirms the role of the selected terminal substituents in enhancing the optical properties, which depend mainly on the morphology and chemical composition. Moreover, the observed red-shift in the absorption peak is mainly attributed to the size effect, in which small size reduces spin-orbit coupling and moderates the exciton positions [63]. This red-shift and high absorption in UV and visible regions is a desirable feature in energy-efficient solar cells [64].   The relationship between the absorption coefficient (α) and photon energy (hv) for the direct allowed transitions is defined by the optical absorption theorem (Equation (3)) [65]: where h is the Planck's constant (6.625 × 10 −34 J/s), A is a constant, and Eg is the optical bandgap. The values of direct Eg for the prepared I6a-e films are obtained by extrapolating the linear portions of the plot of (α·hν) 2 vs. hν to α = 0 as shown in Figure 11A. The linear parts observed in this figure indicate that the transitions are performed directly. The obtained values of Eg are displayed in Table 2. The smallest Eg value was determined to be 3.0 eV for I6e film and the highest value was determined to be 3.63 eV for I6c film. Due to the presence of the terminal fluorine atom, the observed decreasing in the bandgap of the nematogenic I6e film is attributable to the effect of the density of localized states. This  The relationship between the absorption coefficient (α) and photon energy (hv) for the direct allowed transitions is defined by the optical absorption theorem (Equation (3)) [65]: where h is the Planck's constant (6.625 × 10 −34 J/s), A is a constant, and Eg is the optical bandgap. The values of direct Eg for the prepared I6a-e films are obtained by extrapolating the linear portions of the plot of (α·hν) 2 vs. hν to α = 0 as shown in Figure 11A. The linear parts observed in this figure indicate that the transitions are performed directly. The obtained values of Eg are displayed in Table 2. The smallest Eg value was determined to be 3.0 eV for I6e film and the highest value was determined to be 3.63 eV for I6c film. Due to the presence of the terminal fluorine atom, the observed decreasing in the bandgap of the nematogenic I6e film is attributable to the effect of the density of localized states. This The relationship between the absorption coefficient (α) and photon energy (hv) for the direct allowed transitions is defined by the optical absorption theorem (Equation (3)) [65]: where h is the Planck's constant (6.625 × 10 −34 J/s), A is a constant, and E g is the optical bandgap. The values of direct E g for the prepared I6 a-e films are obtained by extrapolating the linear portions of the plot of (α·hν) 2 vs. hν to α = 0 as shown in Figure 11A. The linear parts observed in this figure indicate that the transitions are performed directly. The obtained values of Eg are displayed in Table 2. The smallest Eg value was determined to be 3.0 eV for I6 e film and the highest value was determined to be 3.63 eV for I6 c film.
Due to the presence of the terminal fluorine atom, the observed decreasing in the bandgap of the nematogenic I6e film is attributable to the effect of the density of localized states. This result is compliant with the previously mentioned studies [66]. It is worthy to note the importance of the reduction of the bandgap values for the solar energy applications such as photoelectrochemical hydrogen generation solar cells. result is compliant with the previously mentioned studies [66]. It is worthy to note the importance of the reduction of the bandgap values for the solar energy applications such as photoelectrochemical hydrogen generation solar cells. The width of the exponential absorption edge is referred to by Urbach energy (EU) or the Urbach tail. The tails of the valence and conduction bands are attributed to the disorder in the material [64]. The EU's exponential dependence can be measured using the following Equation (4) [67]: where α0 is the band tailing parameter that can be obtained by (Equation (5) where c is the speed of light, σo is electrical conductivity at absolute zero, ΔE refers to the width of the tail of the localized state in the forbidden gap. Figure 11B shows the plot of ln(α) vs. hν for the prepared I6a-e films. The values of EU were obtained from the slopes of the linear fitting of these curves and presented in Table 2. The minimum value of EU was determined to be 160.9 mV for I6a, whereas the maximum value was determined to be 1858.2 mV for I6d.

Conclusions
New laterally methoxy-substituent photoactive liquid crystalline analogues series, named 4-hexyloxy phenyl-imino-4ʹ-(3-methoxyphenyl)-4ʹ'-alkoxybenzoates, were synthesized and mesomorphically characterized as well as optically evaluated. The prepared series included five analogues derivatives that differ from each other by the terminally compact polar substituent (OCH3, CH3, Cl, F) including the unsubstituted derivative (H). A laterally protruded OCH3 group is introduced into the central benzene ring and makes an angle of 120° with respect to the ester moiety. Molecular structures were elucidated by elemental analyses, FT-IR and NMR spectroscopy. Mesomorphic,  The width of the exponential absorption edge is referred to by Urbach energy (EU) or the Urbach tail. The tails of the valence and conduction bands are attributed to the disorder in the material [64]. The EU's exponential dependence can be measured using the following Equation (4) [67]: where α 0 is the band tailing parameter that can be obtained by (Equation (5)) [68]; where c is the speed of light, σ o is electrical conductivity at absolute zero, ∆E refers to the width of the tail of the localized state in the forbidden gap. Figure 11B shows the plot of ln(α) vs. hν for the prepared I6 a-e films. The values of E U were obtained from the slopes of the linear fitting of these curves and presented in Table 2. The minimum value of E U was determined to be 160.9 mV for I6 a , whereas the maximum value was determined to be 1858.2 mV for I6 d .

Conclusions
New laterally methoxy-substituent photoactive liquid crystalline analogues series, named 4-hexyloxy phenyl-imino-4 -(3-methoxyphenyl)-4 '-alkoxybenzoates, were synthesized and mesomorphically characterized as well as optically evaluated. The prepared series included five analogues derivatives that differ from each other by the terminally compact polar substituent (OCH 3 , CH 3 , Cl, F) including the unsubstituted derivative (H). A laterally protruded OCH 3 group is introduced into the central benzene ring and makes an angle of 120 • with respect to the ester moiety. Molecular structures were elucidated by elemental analyses, FT-IR and NMR spectroscopy. Mesomorphic, optical, and electrical meaurements were conducted using DSC, POM, UV spectrophotometer, Keithley measurement-source unit, and UV/Vis/IR Perkin Elmer spectrophotometer. DSC and POM investigations indicated that all synthesized series are monomorphic, possessing the nematic mesophase, except the unsubstituted member (I6 c ) that is nonmesomorphic. Moreover, the polarizability anisotropy of the polar compact terminal and lateral groups influences the mesomeric characters of the whole molecular architecture. Furthermore, this nature of the terminal moieties influences the mesophase properties and induces phase transition phenomena when comparing to the unsubstituted derivative.
From the study of the electrical properties, the I6 e film showed the highest resistance value of 743.3 GΩ and the I6 d film showed the lowest resistance value of 106.5 GΩ at a scan rate of 0.1 V/s. The resistance of I6 d film is decreased to be 49.5 GΩ by increasing the scan rate to 0.5 V/s. Moreover, the electrical conductance is decreased from 9.39 pS to 1.35 pS at scan rate 0.1 V/s by changing the terminal group from Cl to F. Moreover, there is a very strong absorption band centered at 294 nm for I6 a , b , and 348 nm for I6 e . The I6 d showed very strong absorption relative to the other samples. Besides, the direct bandgap is tuned from 3.63 eV for I6 c to 3.00 eV for I6 e with band tails of 570.3 mV, and 1134.8 mV. Finally, the enhanced optical absorption and the reduced energy gap make the optimized samples suitable material for solar energy applications.

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