Nematic Phase Induced from Symmetrical Supramolecular H-Bonded Systems Based on Flexible Acid Core

New symmetrical 1:2 supramolecular H-bonded liquid crystals (SMHBLCs) interactions, A/2Bn, were formed between adipic acid and 4-(4′–pyridylazophenyl) 4”-alkoxybenzoates. Optical and mesomorphic behaviors of the prepared SMHBLC complexes were investigated using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). FT-IR spectroscopy was carried out to confirm the H-bond interactions of the prepared complexes via Fermi bands formation. Their photo-physical properties were investigated by UV-spectroscopy and the observed absorbance values were found to be mainly dependent on the length of the terminal alkoxy chain. Mesomorphic behaviour for all A/2Bn complexes revealed that all complexes are dimorphic-exhibiting enantiotropic mesophases with induced nematic phases, except for the long chain terminal complex which exhibits only a monomorphic smectic A phase. In order to investigate the effect of mesogenic core geometry on the mesophase properties, a comparison was established between the mesomeric behaviors of the present complexes and previously reported rigid core acid complexes. Finally, the XRD pattern confirmed the POM and DSC results.

The molecular architecture and the length of the terminal flexible chain of the liquid crystalline molecule play a significant role in the formation, type, thermal stability and temperature range of the mesophase. In such cases, as the terminal chain length increases, the molecules will be oriented in a parallel arrangement [26]. In addition, the terminal chain length has an important role in influencing Scheme 1. 1:2 supramolecular H-bonded liquid crystal (SMHBLC) complexes A/2Bn.

Experimental
The azo-pyridine base derivatives Bn were synthesized according to the previous procedure [29,40], which is attached in the Supplementary Materials.

Preparation of 1:2 SMHBLC Complexes
A/2Bn supramolecular complexes were formed from one mole of adipic acid (A) and two moles of azo-pyridine base (Bn) bearing different terminal alkoxy chain lengths that varied between n = 8 to n = 14. The solid mixture was melted with stirring to form an intimate blend and then allowed to cool to room temperature (Scheme 2). The formation of the supramolecular complexes (A/2Bn), were confirmed via DSC and FT-IR spectroscopy. Scheme 2. Preparation of 1:2 SMHBLC complexes (A/2Bn).

FT-IR Characterizations of 1:2 SMHBLC Complexes
SMHBLC complex formation via H-bonding interactions of azo-pyridine (Bn) and adipic acid (A) with molar ratio 2:1 was confirmed by FT-IR spectral data. FT-IR measurements were performed for the two individual components as well as their 1:2 SMHBLC mixtures. FT-IR spectra of A, B10 and their complex A/2B10 are given in Figure 1 as representative examples. The signal at 1649 cm −1 is assigned to the C=O group of the adipic acid (A). The important evidence of the H-bonded interactions formation is the C=O, OH stretching vibration. It has been reported [41][42][43][44][45][46][47][48] that the presence of three Fermi-resonance vibration bands (A-, B-and C-types) of the H-bonded OH groups is a confirmation of the SMHB formation. The A-type Fermi-band of the complex A/2B10 is overlapped with that of the C-H vibrational peaks at 2922 to 2853 cm −1 . Furthermore, the observed peak at 2354 cm −1 for (A/2B10) could be assigned to the B-type of the in-plane bending vibration of the OH group. On the other hand, the band at 1920 cm −1 that corresponds to C-type Fermi-band is attributed to the molecular interactions between the overtone of the fundamental stretching vibration of the OH group and the torsional effect.

FT-IR Characterizations of 1:2 SMHBLC Complexes
SMHBLC complex formation via H-bonding interactions of azo-pyridine (Bn) and adipic acid (A) with molar ratio 2:1 was confirmed by FT-IR spectral data. FT-IR measurements were performed for the two individual components as well as their 1:2 SMHBLC mixtures. FT-IR spectra of A, B10 and their complex A/2B10 are given in Figure 1 as representative examples. The signal at 1649 cm −1 is assigned to the C=O group of the adipic acid (A). The important evidence of the H-bonded interactions formation is the C=O, OH stretching vibration. It has been reported [41][42][43][44][45][46][47][48] that the presence of three Fermi-resonance vibration bands (A-, B-and C-types) of the H-bonded OH groups is a confirmation of the SMHB formation. The A-type Fermi-band of the complex A/2B10 is overlapped with that of the C-H vibrational peaks at 2922 to 2853 cm −1 . Furthermore, the observed peak at 2354 cm −1 for (A/2B10) could be assigned to the B-type of the in-plane bending vibration of the OH group. On the other hand, the band at 1920 cm −1 that corresponds to C-type Fermi-band is attributed to the molecular interactions between the overtone of the fundamental stretching vibration of the OH group and the torsional effect.

Mesophase Behavior of 1:2 SMHBLC Complexes, A/2Bn
Mesomorphic and optical behaviors for the present symmetrical 1:2 SMHBCs (A/2Bn) were investigated by DSC and POM. The results determined by DSC, given as phase transition temperatures (T), associated enthalpy (∆H) and their normalized entropy (∆S/R), for all prepared mixtures (A/2Bn), are summarized in Table 1. Representative examples of DSC curves taken from the second heating/cooling scans are represented in Figure 2. Textures of mesophases and optical behaviour of the present 1:2 SMHBCs were also investigated by POM. Moreover, POM observation textures are depicted in Figure 3. In order to ensure the stability of the prepared complexes, the DSC measurements were performed for two heating-cooling cycles and found to be the same. All thermal analyses were recorded from the second heating scan for all complexes. DSC measurements were confirmed by the POM texture observations. Transition temperature dependences on the terminal chain-length were graphically represented for the characterized complexes, A/2Bn ( Figure 4). Figure 4 was constructed in order to investigate the effect of the terminal flexible chain lengths of the base components on the mesomorphic behaviour.
It should be mentioned here that the adipic acid spacer (A) is non-mesomorphic (converted from solid to liquid state directly upon heating), whereas the azo-pyridine derivatives (Bn) are smectogenic, exhibiting the smectic A (SmA) phase, except for the homologue B8 which have dimorphic mesophases (SmA and N phases, see Table S2 in supplementary materials) [29,40]. Therefore, it was interesting to investigate the mesophase properties of the complexes resulting from mixing derivatives A and Bn.
The characteristic textures under POM ( Figure 3) have solid phases that converted to smetic A and were followed by N mesophase observations for the prepared SMHBCs A/2B8, A/2B10 and A/2B12. Meanwhile, the A/2B14 homologue only transitions from the solid phase to SmA then to the isotropic liquid phase upon heating, and returns on cooling.
As can be seen from Table 1 and Figure 4, all the formed SMHBCs are enantiotropic and mesomorphic, exhibiting high thermal stabilities with an induced nematic phase. Moreover, the

Mesophase Behavior of 1:2 SMHBLC Complexes, A/2Bn
Mesomorphic and optical behaviors for the present symmetrical 1:2 SMHBCs (A/2Bn) were investigated by DSC and POM. The results determined by DSC, given as phase transition temperatures (T), associated enthalpy (∆H) and their normalized entropy (∆S/R), for all prepared mixtures (A/2Bn), are summarized in Table 1. Representative examples of DSC curves taken from the second heating/cooling scans are represented in Figure 2. Textures of mesophases and optical behaviour of the present 1:2 SMHBCs were also investigated by POM. Moreover, POM observation textures are depicted in Figure 3. In order to ensure the stability of the prepared complexes, the DSC measurements were performed for two heating-cooling cycles and found to be the same. All thermal analyses were recorded from the second heating scan for all complexes. DSC measurements were confirmed by the POM texture observations. Transition temperature dependences on the terminal chain-length were graphically represented for the characterized complexes, A/2Bn ( Figure 4). Figure 4 was constructed in order to investigate the effect of the terminal flexible chain lengths of the base components on the mesomorphic behaviour.            It should be mentioned here that the adipic acid spacer (A) is non-mesomorphic (converted from solid to liquid state directly upon heating), whereas the azo-pyridine derivatives (Bn) are smectogenic, exhibiting the smectic A (SmA) phase, except for the homologue B8 which have dimorphic mesophases (SmA and N phases, see Table S2 in supplementary materials) [29,40]. Therefore, it was interesting to investigate the mesophase properties of the complexes resulting from mixing derivatives A and Bn.
The characteristic textures under POM ( Figure 3) have solid phases that converted to smetic A and were followed by N mesophase observations for the prepared SMHBCs A/2B8, A/2B10 and A/2B12. Meanwhile, the A/2B14 homologue only transitions from the solid phase to SmA then to the isotropic liquid phase upon heating, and returns on cooling.
As can be seen from Table 1 and Figure 4, all the formed SMHBCs are enantiotropic and mesomorphic, exhibiting high thermal stabilities with an induced nematic phase. Moreover, the results revealed that the 1:2 complexes possess irregular melting temperatures with respect to the terminal chains (n). It can also be seen from Figure 4, that the nematic phase stability decreases, as usual, with the chain length of terminal base and vanishes at n = 14. In general, the terminal chain length and the mesogenic core of the H-bonded complexes play important roles in impacting the stability of the formed mesophase. In addition, it was found that the difference in polarity between H-donors and H-acceptors affects the H-bonding strength and influences the molecular anisotropy then promotes broadening of the mesomorphic range [49]. However, the length of the terminal alkoxy chain of the mixture does not affect the polarity of either component.
In the case of the A/2B8 complex, the investigated data revealed that it is dimorphic, exhibiting the smectic A phase followed by an induced nematic phase, with a relatively broad temperature range (~12.1 • C). The smectic and nematic phase stabilities are 159.1 and 171.2 • C, respectively. Thus, the H-bonding interactions that enhance the stability of the SmA and lead to the formation of the induced N phase are found to be effective. Also for the homologues A/2B10 and A/2B12 complexes, the 1:2 mixtures are dimorphic, possessing SmA and N mesophases with thermal stabilities 156.8, 166.1, 149.7 and 161.7 • C, respectively. Thus, the increment of the molecular anisotropy will impact the nematic phases range, agreeing with previously reported work [49] which states that the increment of the mesogenic core length enhances the nematic phase stability.
In the case of the A/2B14 mixture, only the SmA phase has been observed with mesophase stability 138.4 • C and smectogenic range~11.9 • C. The vanishing of the N phase at terminal chain length n = 14 can be attributed to the rigid mesogenic core dilution [28]. Additionally, the higher attractions of Van der Waals between longer alkoxy chains increases the terminal interaction and alkyl group aggregation, which affects the formed mesophase.
It is well known that the mesophase behaviour of a certain liquid crystalline molecule depends mainly on its mesomeric properties, namely, inter molecular interactions and the molecular shape. In the present SMHBCs, A/2Bn, the mesophase stability depends mainly on several factors:

1.
Lateral adhesion of molecules which increases with the increase of the base alkoxy-chain length or aspect ratio. In addition, the alkoxy chain length seems to play a significant role in the stabilization of mesophases of the SMHBCs.

2.
Molecular geometry which is actually a function of the structure of the complexes, which in many cases is affected by the steric hindrance of the attached groups.

3.
End-to-end interaction which depends mainly on the polarity and the length of the terminal substituents that result in the variation of the polarizability.
On the other side, the linking groups greatly affect the conjugation within the mesogenic core of the molecule.

Entropy Changes
As driven from DSC measurements, the normalized transition entropies (∆S/R) were estimated for the present symmetric 1:2 SMHBCs (A/Bn) and collected in Table 1. The graphical dependence of the entropy change on the alkoxy chain length of the azo-pyridine derivatives is depicted in Figure 5. As can be seen from Table 1 and Figure 5, the decrement of the entropy change (∆S/R) with the terminal alkoxy chain length (n) have been observed for both smectic and nematic transitions. In addition, the entropy changes of N-isotropic transitions, ∆S N /R, possess lower values compared to their corresponding smectic A transition entropy changes, ∆S SmA /R; this is in agreement with the previous reports [28]. It can be concluded that the decrement of the entropy changes with the increase in the terminal alkoxy-chain lengths is due to the increase in the biaxiality of the mesogenic part, which results in decreasing the conformational entropy. Moreover, this may be further attributed to the change in geometrical interactions between individual components of the complex, which is accordingly affected by the polarizability and the molecular shape of the molecule.

Entropy Changes
As driven from DSC measurements, the normalized transition entropies (∆S/R) were estimated for the present symmetric 1:2 SMHBCs (A/Bn) and collected in Table 1. The graphical dependence of the entropy change on the alkoxy chain length of the azo-pyridine derivatives is depicted in Figure 5. As can be seen from Table 1 and Figure 5, the decrement of the entropy change (∆S/R) with the terminal alkoxy chain length (n) have been observed for both smectic and nematic transitions. In addition, the entropy changes of N-isotropic transitions, ∆SN/R, possess lower values compared to their corresponding smectic A transition entropy changes, ∆SSmA/R; this is in agreement with the previous reports [28]. It can be concluded that the decrement of the entropy changes with the increase in the terminal alkoxy-chain lengths is due to the increase in the biaxiality of the mesogenic part, which results in decreasing the conformational entropy. Moreover, this may be further attributed to the change in geometrical interactions between individual components of the complex, which is accordingly affected by the polarizability and the molecular shape of the molecule.

Effect of Changes in the Mesogenic Cores
In order to investigate the effect of an exchange in the mesogenic core of the flexible acid component on the mesomorphic properties, a comparison was made between the mesophase behaviour of present investigated five rings SMHBCs A/2Bn, with a flexible acid spacer core, and the previously reported 1:1 complexes Cm/Bn, possessing the rigid core acid moieties (4-alkoxy benzoic acid) [40] (Figure 6). The comparison indicated that the addition of a flexible chain core in the molecular geometry destabilizes the mesomorphic stability and induces new nematic transitions in most of the complexes. Meanwhile, all reported SMHBLC complexes Cm/Bn [40] exhibit high transition stabilities for the SmA and SmC mesophases. The stability increments of alkoxy benzoic acid complexes ( Figure 6) are attributed to the increase in polarizability of their molecules, as well as their rigidity, which leads to the increase in the intermolecular interactions between individual molecules.

Effect of Changes in the Mesogenic Cores
In order to investigate the effect of an exchange in the mesogenic core of the flexible acid component on the mesomorphic properties, a comparison was made between the mesophase behaviour of present investigated five rings SMHBCs A/2Bn, with a flexible acid spacer core, and the previously reported 1:1 complexes Cm/Bn, possessing the rigid core acid moieties (4-alkoxy benzoic acid) [40] ( Figure 6). The comparison indicated that the addition of a flexible chain core in the molecular geometry destabilizes the mesomorphic stability and induces new nematic transitions in most of the complexes. Meanwhile, all reported SMHBLC complexes Cm/Bn [40] exhibit high transition stabilities for the SmA and SmC mesophases. The stability increments of alkoxy benzoic acid complexes ( Figure 6) are attributed to the increase in polarizability of their molecules, as well as their rigidity, which leads to the increase in the intermolecular interactions between individual molecules.

Photo-Physical Study
The photo-physical investigation of the present 1:2 SMHBCs A/2Bn was carried out by measuring UV-vis spectra. A solution of C = 1.8 × 10 −6 mol/L in chloroform was used to estimate the spectro-photometric absorption spectra. Azopyridines undergoes cis/trans isomerization when irradiated with light of an appropriate wavelength. Those phenomena make it a suitable component of various molecular devices as well as functional materials [50,51]. Figure 7 shows the effects of UV irradiation on the UV-vis spectra of the base derivatives (Bn) in chloroform solution. It can be seen from Figure 7 that the light radiation in the wavelength range 290-500 nm is found to be strongly absorbed with the maximum at 349-350 nm, with another small peak at 456 nm for all derivatives with trans-conformation, which can be attributed to the π-π* transition of the chromophore in the molecule. [50,51] The resulting absorption bands of the present complexes (A/2Bn) are graphically represented in Figure 8. As shown from Figure 8, the formed 1:2 SMHBCs A/2Bn possess maximum absorption bands within~450-454 nm according to the terminal alkoxy chain length of the base component (n). The highly delocalized electronic systems and π-π* transitions were attributed to the maximum absorption in the present H-bonding interactions for A/2B12 complex. Moreover, the absorption and intensity of the peak in the absorption spectrum depend on the structure of the molecule that absorbs the light at a given wavelength. The change occurred in the UV-vis spectra of the base homologues (Bn), confirming the complexation. The complex A/2B12 absorption spectra ( Figure 8) showed the maxima bands at 454 nm, which is attributed to the electronic transition from the highest occupied molecular orbitals (HOMO) to lowest unoccupied molecular orbital (LUMO), π-π transition [47][48][49][52][53][54].

Photo-Physical Study
The photo-physical investigation of the present 1:2 SMHBCs A/2Bn was carried out by measuring UV-vis spectra. A solution of C = 1.8 × 10 −6 mol/L in chloroform was used to estimate the spectro-photometric absorption spectra. Azopyridines undergoes cis/trans isomerization when irradiated with light of an appropriate wavelength. Those phenomena make it a suitable component of various molecular devices as well as functional materials [50,51]. Figure 7 shows the effects of UV irradiation on the UV-vis spectra of the base derivatives (Bn) in chloroform solution. It can be seen from Figure 7 that the light radiation in the wavelength range 290-500 nm is found to be strongly absorbed with the maximum at 349-350 nm, with another small peak at 456 nm for all derivatives with trans-conformation, which can be attributed to the π-π* transition of the chromophore in the molecule. [50,51] The resulting absorption bands of the present complexes (A/2Bn) are graphically represented in Figure 8. As shown from Figure 8, the formed 1:2 SMHBCs A/2Bn possess maximum absorption bands within ~450-454 nm according to the terminal alkoxy chain length of the base component (n). The highly delocalized electronic systems and π-π* transitions were attributed to the maximum absorption in the present H-bonding interactions for A/2B12 complex. Moreover, the absorption and intensity of the peak in the absorption spectrum depend on the structure of the molecule that absorbs the light at a given wavelength. The change occurred in the UV-vis spectra of the base homologues (Bn), confirming the complexation. The complex A/2B12 absorption spectra ( Figure 8) showed the maxima bands at 454 nm, which is attributed to the electronic transition from the highest occupied molecular orbitals (HOMO) to lowest unoccupied molecular orbital (LUMO), π-π transition [47][48][49][52][53][54].

X-Ray Diffraction Investigation
Another tool to confirm the mesophase assignments (SmA and N phases), X-ray diffraction investigation at different temperatures upon cooling from the isotropic liquid phase, was performed for the homologue A/2B10 as an example for the present 1:2 SMHBCs complexes series ( Figure 9). As can be seen from Figure 8, XRD analysis patterns showed a broad peak at angle 2θ = 23.5° indicating the presence of a nematic transition phase upon cooling for both recorded temperatures which related to the lateral interactions between mesogenic groups. Another small peak in high angle value was observed (2θ = 50.0°) corresponding to the SmA transitions. The formation of the smectic phase is proved by the existence of the X-ray small-angle reflection corresponding to the layer distance. Generally, the wide-peak angles reflect to lateral interactions of the N phases [55]. Temperature-dependent XRD measurements of the prepared complexes confirmed the presence of the SmA and N phases. The XRD pattern was consistent with the intrinsic character of each mesophase. By combining the XRD results and POM investigations, the existence of both the smectic A and N mesophases in the formed SMHBCs A/2Bn series was confirmed.

X-ray Diffraction Investigation
Another tool to confirm the mesophase assignments (SmA and N phases), X-ray diffraction investigation at different temperatures upon cooling from the isotropic liquid phase, was performed for the homologue A/2B10 as an example for the present 1:2 SMHBCs complexes series ( Figure 9). As can be seen from Figure 8, XRD analysis patterns showed a broad peak at angle 2θ = 23.5 • indicating the presence of a nematic transition phase upon cooling for both recorded temperatures which related to the lateral interactions between mesogenic groups. Another small peak in high angle value was observed (2θ = 50.0 • ) corresponding to the SmA transitions. The formation of the smectic phase is proved by the existence of the X-ray small-angle reflection corresponding to the layer distance. Generally, the wide-peak angles reflect to lateral interactions of the N phases [55]. Temperature-dependent XRD measurements of the prepared complexes confirmed the presence of the SmA and N phases. The XRD pattern was consistent with the intrinsic character of each mesophase. By combining the XRD results and POM investigations, the existence of both the smectic A and N mesophases in the formed SMHBCs A/2Bn series was confirmed.
Crystals 2020, 10, x FOR PEER REVIEW 10 of 13 Figure 9. X-ray diffraction patterns of A/2B10 at different temperatures upon cooling from the isotropic liquid phase.

Conclusions
A new series of 1:2 SMHBLC complexes, bearing symmetric alkoxy group terminals with different lengths was prepared, and investigated by different techniques. The H-bonding interactions were confirmed via Fermi-bands observations in FT-IR spectroscopic analysis. The mesomorphic behaviors of present SMHBLC complexes were investigated by DSC, POM and XRD measurements. The results showed that N phase is induced in most of complexes, with temperature

Conclusions
A new series of 1:2 SMHBLC complexes, bearing symmetric alkoxy group terminals with different lengths was prepared, and investigated by different techniques. The H-bonding interactions were confirmed via Fermi-bands observations in FT-IR spectroscopic analysis. The mesomorphic behaviors of present SMHBLC complexes were investigated by DSC, POM and XRD measurements. The results showed that N phase is induced in most of complexes, with temperature ranges that decrease with the length of alkoxy chain, until they vanish at n = 14. All complexes examined are found to be dimorphic, exhibiting enantiotropic SmA and N phases, except the homologue A/2B14 which is smectogenic, possessing only the SmA phase. Moreover, the temperature-dependent XRD measurements were inconsistent with the intrinsic transition of each mesophase, which is in agreement with both the DSC and POM results. Photo-physical properties of the complexes were carried by UV-spectroscopy and the investigations revealed that shift absorption bands occurred in the UV-vis spectra for the base components, confirming the complexation. Finally, the decrement of entropy changes for present complexes was attributed to the geometrical interactions difference between individuals.