New wide-stability four-ring azo/ester/Schiff base liquid crystals: synthesis, mesomorphic, photophysical, and DFT approaches

Abstract

New four-groups-based azo/ester/Schiff base liquid crystals, ((4-substitutedphenylimino)methyl)phenyl 4-[2-(4-alkoxyhenyl)diazenyl]benzoate, In _a-d, were synthesized and analyzed for their mesomorphic stability and optical activity. In these compounds, a terminal alkoxy group of variable chain length from n = 6 to n = 16 carbons is attached to the end of a phenylazo benzoate moiety and the other end of the molecules is connected to a different polar compact substituent X (CH₃O, CH₃, H, and Cl). FT-IR, ¹H NMR, mass spectroscopy and elemental analysis were carried out for molecular structure confirmation of the prepared compounds. The mesomorphic properties were confirmed using a combination of differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The photophysical property was studied by UV-vis spectroscopy. All the prepared homologous series exhibited high thermal stability with a wide-temperature mesomorphic range. The thermal and geometrical parameters of the investigated compounds were estimated by density functional theory (DFT). The results revealed that all the compounds were not completely planar with a relatively high twisting moiety at the CH[double bond, length as m-dash]N part and their twist angles were affected by the electronic nature of the attached X group. Moreover, the calculated quantum chemical parameters as determined by the DFT approach of the investigated compounds were related to the experimentally determined values of the mesophase thermal stability (T _c) and mesophase temperature ranges (ΔT _SmA and ΔT _N) as well as the type of the mesophase.

Scheme 1. Synthesis of (4-substituted phenylimino)methyl)phenyl 4-[2-(4-alkoxyphenyl)diazenyl]benzoate, In_a–d.

Fig. 1. TG and DTG curves of the I10_a–d group.

Fig. 2. DSC thermograms of some representative compounds: (a) recorded from second heating and (b) from cooling at a rate of ±10 °C min⁻¹.

Fig. 3. PLM textures of: (a) the nematic phase of I12_b at 240.0 °C upon cooling; (b) smectic A phase of I16_d at 299.0 °C upon heating; (c) smectic A phase of I16_d at 280.0 °C upon cooling.

Fig. 4. Effect of the alkoxy-chain length on the mesophase behavior of the individual homologous series of In_a–d.

Fig. 5. Comparison of ΔT_c of the 4-ring (In_a–d) and 3-ring (IIn_a–d) analogues.

Fig. 6. Calculated geometrical structures of compounds I12_a–d.

Fig. 7. Relation of the mesophase thermal stability and the mesophase range of the homologues series I12_a–d with their calculated polarizability.

Fig. 8. Relation of the mesophase stability and the mesophase range of the homologues series I12_a–d with their calculated twist angle between rings A–B.

Fig. 9. Dependence of the mesophase stability of the homologues series I12_a–d with the calculated dipole moment.

Fig. 10. Dependence of the mesophase stabilities and their ranges of In_d on the calculated aspect ratio.

Fig. 11. Dependence of the mesophase stabilities and their ranges of In_d on the polarizability.

Fig. 12. Calculated ground-state isodensity surface plots for the Frontier molecular orbitals of I12_a–d.

Fig. 13. Calculated ground-state isodensity surface plots for the Frontier molecular orbitals of In_d.

Fig. 14. UV-vis absorption spectra of the homologues series of the investigated compounds I12_a–d.

Fig. 15. Resonating structures of I12_a.

Fig. 16. Molecular electrostatic potentials (MEP) of I12_a–d and In_d.