Synthesis and liquid crystalline properties of new triazine-based π-conjugated macromolecules with chiral side groups

In this study, we reported the synthesis of a new tribranched macromolecule liquid crystal with triazine in the centre. The central triazine core is bonded via sequences of Sonigashira coupling to 3 triazine unites through acetylenic bridges. The triazines at the periphery are substituted with 2 chiral citronellyloxy side groups. The salt of the resulting star-shaped macromolecule, which was oily at room temperature, with 4-dodecyloxybenzoic acid at 1:1 ratio exhibited a Smectic C (SmC) mesophase. The liquid crystalline properties of organic salt were investigated using DSC (differential scanning calorimetry) and POM (polarizing optical microscopy).


Introduction
Liquid crystals possessing properties of both liquids and solids have been focus of intensive research activities from industrial and academic points of views due to their technological importance and wide commercial application in last few decades. Their widespread use includes in consumer electronic, office equipment, calculators, watches, stereos, etc. Perhaps the most important applications are found on liquid crystal displays (LCDs), which have a dominant position in consumer electronics [1][2][3][4][5][6].
There is a growing interest in the synthesis and investigation of nonconventional liquid crystals to discover new LC phases and low-temperature applications, which have been regarded as new models in the progression of LC science and technology as they are capable of exhibiting unique physical properties and uncommon phase transitions [1,3,4,6].
In the studies on liquid crystal materials, intensive efforts have been devoted to the synthesis of liquid crystals with star-shaped topologies to investigate their potential applications, as they may possess unique properties. Many examples of such liquid crystalline compounds, which include benzene and triazine cores and alkyl chains in the periphery, exhibit interesting LC phases [7][8][9][10][11][12][13][14][15].
Ionic liquid crystals can be considered as the materials that combine the properties of liquid crystals and ionic liquids. A growing interest has been devoted to the research activities in the field of ionic liquids worldwide [16][17][18]. One of the important properties of ionic liquids is that these compounds have very low vapor pressures and can substitute volatile organics. Other applications include the use of solvents and batteries for * Correspondence: ltorun@yildiz.edu.tr This work is licensed under a Creative Commons Attribution 4.0 International License. extraction processes, as an electrolyte for fuel cells and dye-sensitive solar cells [18]. A large number of literature reports are available on the liquid crystalline properties of the charged materials containing anions and cations [15,19,20].
One approach to obtain ionic liquids is by converting an organic material into its salt. This can be made by mixing the organic substance, which has hydrogen bond accepting heteroatoms, with a benzoic acid derivative with long alkyl chain in appropriate ratios. This is a useful and practical way of converting nonliquid crystal materials into materials with mesomorphic properties [21,22].
In this study, we reported the synthesis and the LC properties of a triarmed organic material utilizing 1,3,5-triazine units in the core and on the periphery, which were substituted with chiral citronellyloxy groups. The 1:1 mixture of this compound with 4-dodecyloxybenzoic acid presented a liquid crystal material, which was characterized by FTIR, 1 H NMR, and 13 C NMR analyses. The mesomorphic properties were investigated using POM and DSC.
Compound 4 was synthesized according to literature [23]. Converting compound 3 into its alkoxy benzoic acid salt 5 was necessary in order to obtain a solid material at room temperature, which made it possible to obtain an observable phase transitions under POM (Scheme 2). Structural characterizations for compounds 3 and 5 were confirmed by 1 H NMR, 13

Result and discussion
The formation of the salt 5 between the macromolecule 3 and the mesogenic carboxyl group was mainly studied by FTIR, H NMR, and C NMR. The comparison of FTIR spectra of macromolecule 3, free acid 4, and salt 5 are provided in Figure 1. A sharp peak belonging to the asymmetric stretching of carbonyl peak of the carboxylic acid appeared at 1680 cm −1 , which shifted in the salt form to 1650 cm −1 [26]. The NMR spectroscopic analysis indicated the interaction between macromolecule 3 and 4-DBA. Evidences were found on the chemical shift of the aromatic hydrogens of 4-DBA shifted from 8.05 ppm and 6.95 ppm to 7.98 ppm and 6.86 ppm, indicating a change in the electron density after the formation of salt 5. Likewise, the signals of -OCH 2 -protons of 4-DBA in the complex shifted to a higher ?eld at 3.95 ppm compared to the signals of pure 4-DBA at 4.05 ppm ( Figure 2). As expected, the multiplet signals in H NMR spectra at 4.15-3.97 ppm of compound 3 showed no shifting after the complexation since their electronic environment was not influenced.  The mesomorphic properties of the compound 4-DBA and organic salt 5 were investigated by using POM and DSC. The phase transitions of the corresponding molecules are given in Table. Table . Mesophases and phase transition temperatures as observed on heating (H →) and cooling ( ← C) and corresponding transition enthalpies of the 4-DBA and organic salt 5. (The peak temperatures were expressed in degree Celsius and the numbers in parentheses referred to the transition enthalpy ( ∆ H) in kJ mol −1 ) .

Comp.
T As shown in Figure 4, compound 4-DBA with n -dodecyloxy alkyl chain showed enantiotropic liquid crystalline properties that were in agreement with the behaviour observed for the analogous benzoic acids carrying an alkoxy chain with different numbers of carbon atoms at the 4-position of the aromatic ring [21][22][23][24][25][26][27]29]. The organic salt 5 exhibited a phase transition sequence of Cr 1 -Cr 2 -SmC-Iso which was in agreement with 3 endotherms in DSC cooling curves (Figure 4 centre). Upon heating from the crystal, the SmC texture was observed at 93-115°C by POM ( Figure 5). The texture composed of a disc-like shaped π -conjugated system based on 1,3,5-triazine central core and the mesogenic carboxyl group referred to here as the Smectic C phase [30][31][32][33].  The creation of SmC mesophase is explained by the ionic interaction between the triarmed π -conjugated system and the mesogenic carboxyl group. In this noncovalent intermolecular interaction, the star-shaped compound 3 acts as proton acceptor and the benzoic acid with n -dodecyloxy group acts as the proton donor. The ionic interactions between macromolecule 3 and 4-DBA have striking influence on the mesogenic properties such as melting and clearing temperatures as well as prompting SmC phase at lower temperatures.

Conclusion
A new triazine-based macromolecular π -conjugated system carrying chiral citronellyloxy chains positioned at the peripheries was synthesized via Sonogashira and Negishi cross coupling reactions. Triazines at the periphery bearing citronellyloxy groups were connected to the central triazine unit by 3 acetylenic bridges. Its organic salt was also prepared, which was resulted from hydrogen bonding interaction between macromolecule 3 and 4-DBA mixed with 1:1 ratio in THF. The resulting organic salt was characterized by 1 H NMR, 13 C NMR, and FTIR. The organic salt exhibited enantiotopic SmC phase texture at lower temperatures.
The presence of chiral citronellyloxy groups on the periphery lowered the mesophase transitions from SmC 132.43°C for linear alkyl chain substituted analogue structure [13] to SmC 115.45°C in the organic salt 5. Low temperature mesophase transitions are desirable for low temperature applications of liquid crystals.