Optical investigations and photoactive solar energy applications of new synthesized Schiff base liquid crystal derivatives

New homologues series of liquid crystalline materials namely, (E)-3-methoxy-4-[(p-tolylimino)methyl]phenyl 4-alkloxybenzoates (I-n), were designed and evaluated for their mesomorphic and optical behavior. The prepared series constitutes three members that differ from each other by the terminally attached alkoxy chain group, these vary between 6 and 12 carbons. A laterally OCH3 group is incorporated into the central benzene ring in meta position with respect to the ester moiety. Mesomorphic characterizations of the prepared derivatives are conducted using differential scanning-calorimetry (DSC), polarized optical-microscopy (POM). Molecular structures were elucidated by elemental analyses and NMR spectroscopy. DSC and POM investigations revealed that all the synthesized derivatives are purely nematogenic exhibiting only nematic (N) mesophase, except for the longest chain derivative (I-12) that is dimorphic possesses smectic A and N phases. Moreover, all members of the group have a wide mesomorphic range with high thermal nematic stability. A comparative study was established between the present derivative (I-6) and their previously prepared isomer. The results indicated that the location exchange of the polar compact group (CH3) influences the N mesophase stability and range. The electrical measurements revealed that all synthesized series I-n show Ohmic behaviors with effective electric resistances in the GΩ range. Under white light illumination, the effective electric conductivity for the compound I-8 is five times that obtained in dark conditions. This derivative also showed two direct optical band gaps in the UV and visible light range. In addition, I-6 has band energy gaps of values 1.07 and 2.79 eV, which are suitable for solar energy applications.

The physical analyses data of products I-n are given in Supplementary Materials.

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 was 22 mm × 22 mm × 0.03 mm. " i.e., the film thickness was ~ 30 μm. The temperature of cell was controlled using temperature controller with an accuracy ± 0.1 °C.

Results and discussion
Mesomorphic and optical investigations of present derivatives, I-n. The mesomorphic and optical characteristics of the investigated synthesized derivatives have been analyzed by DSC and POM. Figure 2 displayed typical heating/cooling DSC thermograms of prepared compound I-6 as a representative example. Figure 2 was observed that, the mesophase transitions from Cr → N, and N → I for short-chain length I-6 derivative. Transition peaks observations vary according to the structural shape of synthesized materials, I-n. Significant endothermic and exothermic peaks were observed depending on the attached terminal alkoxy chain length group that is ascribed to mesomorphic transition and the cooling cycle confirmed those observed upon decrement the temperature. Optical images of the I-6 derivative under POM are illustrated in Fig. 3. Schlieren/threads textures of the N mesophase were identified upon heating and cooling scans. The phase transition temperatures, as measured from DSC analysis, and their associated enthalpies for all the investigated compounds, I-n, are col-     www.nature.com/scientificreports/ lected in Table 1. The effect of terminal alkoxy chain length on their mesomorphic behavior has been depicted in Fig. 4. Table 1 and Fig. 4 show that all prepared members of the series I-n are mesomorphic in nature with high mesomorphic thermal stability and a wide mesophase range dependent on their terminal chain length. Moreover, Compounds I-6 and I-8 are monomorphic possessing purely N phase while the longer chain compound I-12 possesses two mesomorphic transitions (dimorphic) enantiotropically defined as SmA and N mesophases. It can also be seen from Table 1 and Fig. 4 that the melting point of compounds varies regularly with the chain length (n). Compound I-6 exhibits an enantiotropic nematic phase with the highest nematic thermal stability and temperature range 203.1 and 96.8 °C, respectively. For I-8 derivative, it has possesses also an enantiotropic N mesophase with nematogenic stability and range nearly 162.1 and 64.3 °C, respectively. While the derivative bearing the longest chain length (I-12) possesses less thermal nematic stability (151.9 °C) and the lowest melting temperature 61.3 °C. So that, compound I-12 has induced smectic A mesophase and its mesomorphic range has been broader (90.6 °C). In general, the molecular architecture, polarizability, and dipole moment of the synthesized materials are highly impacted by the electronic nature of the terminals. In addition, the mesomorphic character is influenced by an increment in the polarity and/or polarizability of the molecular mesogenic moieties. The mesomorphic range of present investigated homologue increased in the order: I-6 > I-12 > I-8. The mesophase behavior of rod-like molecules is directly impacted by molecular-molecular interactions that depend essentially on their geometrical structure of the polar terminal and lateral groups and their special orientation. Mesomorphic properties observations results of the contribution of these factors to different extents. On the other hand, the DSC examination indicated to, the investigated imine derivatives exhibit high thermal stabilities more than 300 °C, which covers the transition window of mesophase temperature that detected thermally and extends over this transition too.
On the other hand, the normalized transition entropy changes, ΔS N-I /R, of the present series (I-n) are collected in Table 1. Data showed that small entropy changes values are observed that mainly depend on the kind of terminal substituents. 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 [61][62][63][64] . The induction, conjugation forces, the specific dipolar interactions as well as the π-π stacking interactions 61-64 play important roles in the molecular orientation and thus in the arrangement of molecules and formation of the mesophase. In addition, the thermal cis/trans isomerization of the azomethine linkage was an essential factor in the lower entropy changes observed, as documented in previous studies 65,66 . Moreover, due to their nematic nature, this of the mesophase, this was exhibits of the lowest order mesophase. While the higher entropy changes of SmA-N transition for compound I-12 are attributed to the increment in its molecular biaxiality 67,68 . Table 1. Mesophase transition temperatures, o C (enthalpy of transition), mesomorphic range (ΔT, o C), and the normalized entropy of transition, ΔS/R, for present series In. Cr-N = solid to the nematic mesophase transition. Cr-SmA = solid to the smectic A mesophase transition. SmA-N = smectic A to the nematic mesophase transition. N-I = nematic to the isotropic liquid mesophase transition. ΔH= enthalpy of transition, kJ/mole; ΔS/R= normalized entropy of transition, unitless (due to the entropy change ΔS is divided by R= gas constant).

Comp
T Cr-SmA T Cr-N T SmA-N T N-I ΔT ΔS N-I /R  Figure 4. Impact of terminal chain-length on the mesomorphic transitions of the present homologue, I-n. www.nature.com/scientificreports/

Comparison between the present investigated In series and their isomeric derivatives.
In order to investigate the effect of exchange the mesogenic part between the aromatic rings on the mesophase and thermal behaviors of the compounds, thus a comparison is conducted between the presently prepared member I-6 and their previously corresponding isomer, II-6 69 for their mesophase behaviors. Compound II-6 possesses enantiotropic N mesophase with stability and temperature range nearly 141.3 and 51.1 °C, respectively. While the present investigated derivative I-6 has a wide nematic range with high thermal stability. In addition, the conjugated Schiff 's bases I-6 and II-6 ( Fig. 5) suggests that the insertion of one more double bond stabilities of the mesophases and increment the phase transition temperatures. It seems that the increase in length of molecule contributes to these effects. The comparison revealed that the thermal stability of the formed mesophase varies according to the enhanced molecular dipole moment and polarizability of the mesogenic part, which is dependent upon the location of polar groups. Moreover, the mesophase range and stability depend on the location of the terminal and linking groups in the mesogenic skeleton of the molecule.

II-6
Optical spectra and energy gap calculation. Because liquid crystals are anisotropic materials and mesogenic materials cannot be aligned using existing techniques, the measured physical properties are referred to as effective optical absorbance and effective electrical conductivity. A Perkin Elmer spectrophotometer (Lambda 950 UV-VIS-NIR) was used to measure the effective optical absorbance and transmission spectra of the present investigated series, I-n, over a wavelength range of 250 to 2500 nm utilizing a blank glass substrate in the reference beam. The samples were sandwiched between two glass substrates. Figure 6A,B shows how the effective absorbance and transmittance spectra of the films are affected by wavelength. In comparison to I8 and I12, the effective absorbance spectra in Fig. 6A show that I-6 has a high absorption behavior. For present homologue I-n, all films display high absorbance up to 402, 416, and 450 nm for I-6, I-8, and I-12, respectively. The absorbance then drops to a plateau at about 850 nm, before dropping again to a minimum absorbance around 1268 nm.  www.nature.com/scientificreports/ exciton positions 70 . This red-shift and high absorption in UV and visible regions is a desirable feature for design energy-efficient solar cells 71 . The optical spectra refer to the homogeneity of the prepared films by decreasing the terminal length of the prepared series because the optical properties depend mainly on morphology and chemical composition. All films showed transmission less than 5% in the wavelength range from 300 to 800 nm, Fig. 6B. Then, the transmission increased exponentially in the near IR region to reach maxima of ~ 14%, 9%, and 8%@1266 nm for I-12, I-8, and I-6, respectively. After that, the transmission decreased as the wavelength increased. The concept of the crystalline solids band gap (E g ) can be expanded to include disordered and highly deficient phases, such as amorphous bodies (glass) and liquids. In this scenario, Eg can be called quasigap (Eg*), which refers to the existance of both localized and widespread electronic states near the edges, as well as in its depth 72  where hν is the photon energy and α a is the absorption coefficient. The values of E g for I-6, I-8, and I-12 are obtained by extending the linear segments of the plot of (α a hυ) 2 vs. hυ to zero as shown in Fig. 7A-C. Interestingly as reported in Table 2, there are two values of the band gaps for the I-6, I-8, and I-12. For the shortest terminal chain compound (I-6), its values of the band gaps are 1.07 and 2.79 eV, which are suitable for solar energy applications [10][11][12][13][14] . By increasing the terminal length of the flexible chain, the values of the band gaps are shifted to 1.13 and 3.14 eV for the I-12 derivative.
The observed increase in the main bandgap from 2.79 eV for I6 to 3.14 eV for I12 is ascribed to the influence of the density of localized states. This behavior is consistent with the previously reported studies 75 . The reduction of the bandgap is very important for solar energy applications, specially photoelectrochemical hydrogen generation, and solar cells [76][77][78] .
Urbach energy (E U ) refers to the disorder in the material and represents the width of the exponential absorption edge (Urbach tail of the valence and conduction bands 79 . I.e., Urbach energy is the energy that refers to the creation of localized energy states at the boundaries of the energy gap due to structural disorder of the material and gives the spectral dependence of the absorption coefficients at photon energies less than the bandgap of the material. The exponential dependency of the E U can be determined according to the following equation 79 :  www.nature.com/scientificreports/ where α ao is the band tail parameter that can be given by 80 : where c is the speed of light, σ o is electrical conductivity at absolute zero, ΔE represents the width of the tail of the localized state in the forbidden gap. Figure 8A and B shows the plot of ln(α) vs. hν for the two band gaps of I-6, I-8, and I-12. The values of E U1 and E U2 were obtained from the slopes of the linear fitting of these curves and reported in Table 2. The statistical parameters, standard deviation (SD) and correlation coefficient (R 2 ), are also reported in this table. The values are 0.857 ± 0.009 and 0.990 ± 0.009 eV for I-6 and 0.956 ± 0.010 and 0.629 ± 0.040 eV for I-12, which refers to the extension of the bandgap edges to cover a wide range of the spectral range.
Electrical properties. The effective electrical properties of the investigated films are tested using a Keithley measurement source unit (Model 4200 SMU). The samples were provided with Ohmic contacts using silver paste (Resistivity < 0.04 Ω.cm). Variation of the applied voltage (V) from − 10 to 10 V with different scan steps, 1V to 0.005V, is used to record the current-voltage (I-V) characteristics of the I-6, I-8, and I-12 films, as shown in Fig. 9A-C. The behaviors are almost linear (Ohmic behaviors). As a result, the materials' resistances are nearly constant and independent of the current flowing through them. Recent research has discovered that at low voltage, polymeric and organic systems behave like Schottky diodes. The Schottky diode is a semiconductor diode made up of a semiconductor and a metal junction that has a low forward voltage drop and a fast switching operation. But in the present investigation, the relation between log (I) and V 1/2 is non-linear as illustrated in Fig. 10A, which implies that our films do not follow the Schottky diode behavior. Under white light illumination, Fig. 9A, the values of the current increased, and the I-V behavior is shifted from the ohmic relation. As the scan step increased, the current intensity is also increased, insets of Figs. 9B and 10A. Figure 10B shows  Table 1)of prepared sample where the liquid crystalline phase range increasing in order I-6 > I-12 > I-8. The mesomorphic range of liquid crystalline materials is affected by many parameters as the polarity and polarizability of whole molecular shape this would be reflected in the resulting resistance data. The observation of more ordered smectic phase for the longer chain length derivative (I-12) could be explained in the term of the enhancement of the (2) α a = α ao exp E ph /E u → E u = δE ph /δ(ln(α a )) www.nature.com/scientificreports/ polarity and the polarizability with lengthening of the alkoxy terminal chain 32,33 . The effective electric resistance of I-8 film is increased from 20.8 GΩ to 22.8 GΩ by decreasing the scan step from 1 V to 0.01 V as shown in Fig.10C which has the lowest entropy change value (see Table 1). The values of the effective electric conductance (σ) are obtained and shown in Fig. S1 (Supplementary Data). The value of the effective electrical conductance is increased from 0.91 nS in dark to 4.60 nS under white light illumination since the electrical conductance depends mainly on the number and mobility of charge carriers 81,82 . This indicates the coherent photocurrent generation, which is the basis of the photovoltaic cell 83 .

Conclusion
New mesomorphic laterally methoxy-substituent homologues series named, (E)-3-methoxy-4-[(p-tolylimino) methyl]phenyl 4-alkoxybenzoate (I-n), were synthesized and characterized by different thermal, optical, and electrical tools. The prepared series included three materials that differ from each other by the terminal length of the flexible chain. A lateral OCH 3 group is inserted into the central benzene ring. Elucidations of structures were carried out by elemental analyses, FT-IR, and NMR spectroscopy. Characterizations of present compounds are investigated using DSC, POM, UV spectrophotometer, Keithley measurement-source unit, and UV/Vis/ IR Perkin Elmer spectrophotometer. DSC and POM investigations indicate that all synthesized compounds are enantiotropic monomorphic exhibiting only N mesophase, except for the longest chain derivative (I-12) that is dimorphic possesses smectic A and N phases. Additionally, all compounds have a broad mesomorphic range with high thermal nematic stability. A comparative study was made between the present derivative (I-6) and their corresponding isomer and results indicated that the exchange of the location of the polar CH 3 group influences the N mesophase range and stability. The effective electrical measurements revealed that, with electric resistances in the GIGA range, all investigated derivatives (I-n) exhibited Ohmic behaviors. The effective electric conductivity of the compound I-8 is five times higher under white light illumination than in dark conditions. Moreover, In the UV and visible light ranges, this member (I-8) has revealed two direct optical band energy gaps. Further, it was found that the band energy gaps for I-6 are 1.07 eV and 2.79 eV, which confirming that it is appropriate for solar energy applications.   www.nature.com/scientificreports/