Experimental and theoretical approaches of new nematogenic chair architectures of supramolecular H-bonded liquid crystals

O.A. Alhaddad 1, H.A. Ahmed 2,3*, M. Hagar 2,4*,  1. College of Sciences, Chemistry Department, Madina Monawara, Taibah University, Saudi Arabia  2. College of Sciences, Chemistry Department, Yanbu, Taibah University, Saudi Arabia., (HA)  ahoda@sci.cu.edu.eg, (MH) mohamedhaggar@gmail.com  3. Faculty of Science, Department of Chemistry, Cairo University, Cairo, Egypt.  4. Faculty of Science, Chemistry Department, Alexandria University, Alexandria, Egypt  * Correspondence: H.A. Ahmed, ahoda@sci.cu.edu.eg; M. Hagar , mohamedhaggar@gmail.com.  Received: date; Accepted: date; Published: date

H-bonding intermolecular interactions are a well-established strategy to design self-assembly LCs through several non-covalent bonds [10][11][12][13][14]. Among the hydrogen bond acceptors and donors, the pair of a carboxylic acid and a pyridine derivative is the best choice in several studies. Moreover, using of multifunctional components in the formation of non-covalent interaction can produce better characteristics supramolecular LC network architectures [6,7]. Azopyridine molecules are incorporated into liquid-crystal materials to make them photoresponsive [15,16]due to their ability for trans-cis-isomerization upon thermal and photo irradiation. Modifying the core structure or adding lateral substituents to azopyridine-based derivatives can lead to marked changes in photophysical and photochemical properties. [15,16] An incorporation of lateral groups with different size and polarity widely improves many characteristics of liquid crystalline materials. It could be attributed to the disturbance in the molecular packing that decreases the melting temperature and thermal stability of liquid crystal mesophases. [17][18][19][20][21][22][23][24]. Lately, azopyridines have been used in the formation of nano fiber supramolecular self-assembling and hydrogen/halogen-bonding LCs with photo induced transition phenomena. [25][26][27][28][29]Designing of photosensitive SMLCs through intermolecular interactions using the suitable H-bond donors and acceptors are concerns of our area of interest. [30][31][32][33][34][35][36]Anisotropic structures are produced from the overall molecular shape of architectures and the combination of rigid (aromatic) and flexible segments (alkyl chains). Those changes in the characteristics of the LCs may be impact the mesomorphism as well as the properties essential for technical uses. Recently, construction of materials according to computational prediction has a high attention of many researchers [18,[37][38][39][40][41][42][43][44][45][46]. Mutual influence of the many optical parameters requires stimulated information about the energies of molecular orbitals as well as the molecular geometries of the LCs. Moreover, density functional theory (DFT) becomes effective popular method for its excellent performance and consistent with the experimental results. [18,40,47,48] In order to understanding and controlling the mesomorphic properties of the soft material complexes, the goal of present work focus on designing new H-bonded supramolecular architectures of new conformation and discuss the geometrical as well as the thermal parameters of the investigated complexes. Also to study the stability of different spatial oriented lateral polar groups on the thermal and optical behavior of prepared intermolecular H-bonded complexes, which oriented with different angles on the central ring of the Azopyridine-based moiety. Moreover, DFT theoretical calculations will be discussed to predict the molecular conformation for the formed complexes as well as their thermal parameters. In addition, these calculations will be used to explain the effect the position and orientation of the lateral group as well as the length of the alkoxy chain on the type and the stability of the observed mesophase. Finally, to investigate the impact of the estimated thermal parameters of H-bonded complexes and how these parameters could affect their thermal and optical properties. 4-n-Alkoxy benzoic acids (An), and lateral chloro pyridine-based derivatives (Im and IIm) were checked to exhibit identical transition temperatures as previously reported. [8,49]  Measurements were made twice and the results were found to have accuracy in transition temperature and enthalpy within ± 0.2 o C.

Computational Methods and calculations
The theoretical calculations for the investigated compounds were carried out by Gaussian 09 software [51]. DFT/B3LYP methods using 6-31G (d,p) basis set was selected for the calculations. The geometries were optimized by minimizing the energies with respect to all geometrical parameters without imposing any molecular symmetry constraints. The structures of the optimized geometries had been drawn with Gauss View [52]. Moreover, the calculated frequencies were carried out using the same level of theory. The frequency calculations showed that all structures were stationary points in the geometry optimization method with none imaginary frequency.

FT-IR spectroscopic confirmation of SMHB complexes formation
The formation of the supramolecular complexes has been confirmed by FTIR spectral data. The     Table 2 and

3.2.Effect of polarity and orientation of lateral substituent on the supramolecular hydrogen-bonded complexes stability
In order to study the effect of polarity and position (spatial orientation) of the lateral group on the mesophase thermal stability (TC) of 1:1 supramolecular hydrogen-bonded complexes, a comparison was constructed between mesophase stabilities (TC) of present lateral Cl complexes (An/Im and An/IIm) and their corresponding lateral CH3 supramolecular H-bonded complexes (An/IIIm and An/IVm) [8,63], as well as the laterally neat (An/Vm) [33]. All data were represented graphically in Figure 5a-d. It had been found that, the location and the inductive effect of the lateral substituent incorporated in base complement impacts the polarizability between H-donors and H-acceptor and thus affects the strength of the hydrogen bond [55]. However, the polarity of both components was not affected by the length of the terminal alkoxy chain (Figure 5a-d). Also, the laterally neat supramolecular H-bonded complexes (An/Vm) have the highest thermal stability with respect to the derivatives of electron donating CH3 and electron withdrawing Cl lateral substituents.
In addition, the nematic mesophase in the present investigation (lateral Cl complexes, An/Im and An/IIm) is observed instead of the smectic C of the lateral CH3 and neat supramolecular complexes.
Thus the nature of intermolecular interactions between molecules affects the stability as well as the type of the mesophase. The lateral electron withdrawing Cl-atom of the complexes An/Im and An/IIm predominates the end to end interaction to enhance a less ordered phase (nematic), while the strong backing side by side interactions in case of lateral CH3 (An/IIIm and An/IVm)and laterally neat (An/Vm) complexes to observe more ordered mesophase ( SmC).

Relationship between experimental and theoretical parameters
The theoretical DFT calculations were performed in gas phase by DFT/B3LYP method at 6-31G  Table 3.   Table 3 and Figure 6,the length the alkoxy chain of the homologues series enhancement the calculated thermal energy. As the chain length increases more packing of the molecules is permitted and consequently, the stability of the molecules increases [36,47,49,53,[64][65][66]. Obviously, there is no significant effect of the alkoxy chain length on the dipole moment.
However, the position and the spatial orientation of the Cl-atom has high impact on the magnitude of the dipole moment, 6.8408 and 8.8598 Debye for ortho (A12/ I16) and meta (A12/ II16) chloro with respect to the carboxylate linkage, respectively. On the other hand, Figure 7 illustrates the relationship between the alkoxy chain length of acid moiety (n) and the polarizability. As the chain length increases the polarizability increases, and so, the candidate of the highest chain length showed the maximum polarizability and could be predicted to have the best characteristics in NLO applications. Moreover, the position and the orientation of the chloro atom affects the predicted stability as well as the polarizability, the ortho chloro derivative with respect to the ester group (An/ Im) showed higher polarizability and lower stability rather than that of the other isomer (An/ IIm), the difference was 38.4 Bohr 3 and 424.36 Kcal/mole, respectively, for n=12 ,m =16. The higher stability of the ortho chloro derivatives could be illustrated in the term of its high degree of interaction of the molecules which permits more packing of the compounds rather than that of the meta derivatives.    Figure 9 shows that, independent on the terminal flexible chains, an irregular entropy change was observed. That irregular change may be explained to the intermolecular interactions due to the location and rotation as well as the polarity of lateral substituent affect on the ordering of whole complex. [67,68] The high dipole moment of An/II16 than An/I16 is accompanied by more conformational entropy changes due to good packing of lateral meta Cl supramolecular complexes molecules than the ortho Cl isomers. In contrast for the lateral electron donating CH3 group, lower entropy transitions observed for meta CH3 SMHB complexes than the ortho CH3 isomeric complexes. These results could be explained in terms of the high degree of alignment of the molecules in case of electron donating lateral substituent (CH3) in the smectic mesophase that highly decreases the entropy with respect to the less ordered nematic mesophase in case of lateral electron withdrawing group (Cl).
The large value of entropy in many cases may be explained by the intermolecular interactions due to the location and rotation as well as the polarity of the lateral Cl-atom which enhancement the ordering of whole supramolecular complex. Moreover, non-correlation between the entropies and the terminal alkoxy-chain length may be due to the irregular change of lateral adhesion upon the increase of the total molecular length.    The charge distribution map for the complexes A12/ I16, A16/ I16and An/IIm was calculated under the same basis sets according to molecular electrostatic potential (MEP) (Figure 11). The red region (negatively charged atomic sites) was distributed on the aromatic moiety and the maximum was carbonyl oxygen of the H-bonded carboxylic group, while alkoxy chains showed the least negatively charged atomic sites (blue regions). As shown from Figure 11, there is no significant effect of either the location, the orientation of Cl atom or the alkoxy-chain length on the charge distribution. This could explain the reason of alteration of the type of the mesophase of the compounds under investigation in the term of the competitive interaction between end-to-end and side-side interaction by increasing of the chain length rather than the change of the charge distribution.

Conclusion
Four new isomeric series of 1:1 SMHB complexes in chair-shaped liquid crystalline were constructed based on laterally Cl azopyridine derivatives and 4-alkoxybenzoic acids. All

Conflicts of Interest
"The authors declare no conflict of interest"