Two new compounds, 4-alkanoyloxybenzylidene- -fluoroaniline and 4-fluorobenzylidene- -n-alkanoyloxyaniline comprising a terminal fluorosubstituent were studied. The fluoro substituent contributes to the molecular polarizability, thus affecting intermolecular interactions and hence resulting in smectic mesomorphism. The mesomorphic properties were studied using differential scanning calorimetry and polarizing optical microscopy techniques. The mesomorphic properties of compounds studied are strongly dependent on the orientation of the imine (CH=N) linkage. The former Schiff base exhibited smectic A phase whereas the latter compound did not display any mesophase. Reversed imine linkage has caused depression of mesomorphic property in the compound studied (4-fluorobenzylidene- -n-alkanoyloxyaniline). The mesomorphic properties of the present compounds were compared with other structurally related series to establish the chemical structure-mesomorphic property relationship. 1. Introduction Liquid crystals (LCs) have many practical applications in scientific and technological areas, in particular as display devices, OLEDs, anisotropic networks, photoconductors, and semiconductor materials [1–3]. Strong demand of new LCs for applications has led to the synthesis of wide range of mesogens, in particular, thermotropic liquid crystals [4, 5]. Most thermotropic liquid crystals belonged to the calamitic molecules which have a rigid core that consisted of two or more phenyl rings and one or more flexible terminal alkyl chains. Schiff base is one of the most well-known linking groups used in connecting the rigid core groups. Although it provides a stepped core structure, it maintains molecular linearity, therefore providing better stability and inducing formation of mesophase [6, 7]. Numerous works have been reported in this area ever since the discovery of MBBA which exhibited room temperature nematic phase [8]. Recently, ester type of Schiff bases has received considerable attentions owing to their interesting properties and substantial temperature range [9–18]. Structure-property relationship is essential knowledge for molecular modifications of the synthesis of new mesogens with targeted properties and future practical applications [6]. Typical terminal substituents exhibiting LC properties are those with electronegative atoms, such as halogens. Halogens are polar substituents possessing strong dipole moments, thus having the ability to promote mesomorphic properties [19–21]. The increased dipole moment enhances the stability of the lattice and melting
References
[1]
L. Petti, M. Rippa, A. Fiore, L. Manna, and P. Mormile, “Optically induced light modulation in an hybrid nanocomposite system of inorganic CdSe/CdS nanorods and nematic liquid crystals,” Optical Materials, vol. 32, pp. 1011–1016, 2010.
[2]
N. A. Shurpo, M. S. Vakshtein, and N. V. Kamanina, “Effect of CdSe/ZnS semiconductor quantum dots on the dynamic properties of nematic liquid-crystalline medium,” Technical Physics Letters, vol. 36, no. 4, pp. 319–321, 2010.
[3]
M. H. Hoang, M. J. Cho, K. H. Kim, T. W. Lee, J.-I. Jin, and D. H. Choi, “Semiconducting 2,3,6,7,10,11-Hexakis {[4-(5-dodecylthiophen-2-yl)phenyl] ethynyl} triphenylene and its discotic liquid crystalline properties,” Chemistry Letters, vol. 39, no. 4, pp. 396–397, 2010.
[4]
F. Yuksel, D. Atilla, and V. Ahsen, “Synthesis and characterization of liquid crystalline unsymmetrically substituted phthalocyanines,” Polyhedron, vol. 26, no. 15, pp. 4551–4556, 2007.
[5]
B. Y. Zhang, F. B. Meng, M. Tian, and W. Q. Xiao, “Side-chain liquid-crystalline polysiloxanes containing ionic mesogens and cholesterol ester groups,” Reactive and Functional Polymers, vol. 66, pp. 551–558, 2005.
[6]
P. J. Collings and M. Hird, Introduction to Liquid Crystals Chemistry and Physics, Taylor & Francis, London, UK, 1998.
[7]
S. Singh and D. A. Dunmur, Liquid Crystals: Fundamentals, World Scientific, London, UK, 2002.
[8]
H. Kelker and B. Scheurle, “A liquid crystalline (nematic) phase with a particularly low solidification point,” Angewandte Chemie International Edition, vol. 8, pp. 884–885, 1969.
[9]
B. B. Eran, A. Nesrullajev, and N. Y. Canli, “Characterization and investigation of the mesogenic, thermo-morphologic and thermotropic properties of new chiral (S)-5-octyloxy-2-[{4-(2-methylbuthoxy)-phenylimino(methyl]phenol liquid crystalline compound,” Materials Chemistry and Physics, vol. 111, no. 2-3, pp. 555–558, 2008.
[10]
S. T. Ha, L. K. Ong, S. T. Ong et al., “Synthesis and mesomorphic properties of new Schiff base esters with different alkyl chains,” Chinese Chemical Letters, vol. 20, no. 7, pp. 767–770, 2009.
[11]
S. T. Ha, G. Y. Yeap, and P. L. Boey, “Synthesis and liquid crystalline properties of new schiff bases N-[4-(4-n-alkanoyloxybenzoyloxy)benzylidene]-4-cyano-, 4-hydroxy-, 4-thio-and 4-nitroanilines,” Australian Journal of Basic & Applied Sciences, vol. 3, no. 4, pp. 3417–3422, 2009.
[12]
M. Parra, J. Vergara, C. Zuniga, E. Soto, T. Sierra, and J. L. Serrano, “New chiral Schiff's bases with a 1,3,4-thiadiazole ring in the mesogenic core: synthesis, mesomorphic and ferroelectric properties,” Liquid Crystals, vol. 32, no. 4, 462 pages, 2004.
[13]
A. K. Prajapati and M. C. Varia, “Azomesogens with polar chloro, nitro and phenolic -OH substituents,” Liquid Crystals, vol. 35, no. 11, pp. 1271–1277, 2008.
[14]
R. Vora, A. K. Prajapati, and J. Kevat, “Effect of terminal branching on mesomorphism,” Molecular Crystals and Liquid Crystals, vol. 357, no. 1, pp. 229–237, 2001.
[15]
G.-Y. Yeap, S.-T. Ha, P.-L. Lim et al., “Synthesis and mesomorphic properties of schiff base esters ortwo-hydroxy-para-alkyloxybenzylidene-para-substituted anilines,” Molecular Crystals and Liquid Crystals, vol. 423, pp. 73–84, 2004.
[16]
G. Y. Yeap, S. T. Ha, P. L. Boey, W. A. K. Mahmood, M. M. Ito, and Y. Youhei, “Synthesis and characterization of some new mesogenic schief base esters N-[4-(4-n-hexadecanoyloxybenzoyloxy)-benzylidene]-4-substituted anilines,” Molecular Crystals and Liquid Crystals, vol. 452, no. 1, pp. 73–90, 2006.
[17]
G.-Y. Yeap, S.-T. Ha, P.-L. Lim et al., “Nematic and smectic a phases in ortho-hydroxy-para- hexadecanoyloxbenzylidene-para-substituted anilines,” Molecular Crystals and Liquid Crystals, vol. 452, no. 1, pp. 63–72, 2006.
[18]
G.-Y. Yeap, S.-T. Ha, P.-L. Lim et al., “Synthesis, physical and mesomorphic properties of Schiff's base esters containing ortho-, meta- and para-substituents in benzylidene- - alkanoyloxyanilines,” Liquid Crystals, vol. 33, no. 2, pp. 205–211, 2006.
[19]
Z. Galewski, “Molecular crystals and liquid crystals science and technology. Section A. molecular crystals and liquid crystals,” Molecular Crystals and Liquid Crystals, vol. 249, pp. 43–49, 1994.
[20]
Z. Galewski and H. J. Coles, “Liquid crystalline properties and phase situations in 4-chlorobenzylidene- -alkylanilines,” Journal of Molecular Liquids, vol. 79, no. 1, pp. 77–87, 1999.
[21]
S. Sakagami and M. Nakamizo, “Liquid crystalline properties of N-(4-alkoxybenzylidene)-4-halogenoanilines,” Bulletin of the Chemical Society of Japan, vol. 53, no. 1, 266 pages, 1980.
[22]
J. S. Dave and M. Menon, “Azomesogens with a heterocyclic moiety,” Bulletin of Material Science, vol. 23, pp. 237–238, 2000.
[23]
V. F. Petrov, M. Duan, H. Okamoto, J. Mu, Y. Shimizu, and S. Takenaka, “Halogenation in achiral liquid crystals: terminal and linking substitutions,” Liquid Crystals, vol. 28, no. 3, pp. 387–410, 2001.
[24]
H. Sackmann and D. Demus, “The polymorphism of liquid crystals,” Molecular Crystals and Liquid Crystals, vol. 2, pp. 81–102, 1966.
[25]
G. W. Gray and J. W. Goodby, Smectic Liquid Crystals: Textures and Structures, Leonard Hill, London, UK, 1984.
[26]
C. Gandolfo, D. Grasso, G. Buemi, and G. Torquati, “Phase transitions and structural studies of p-n-alkoxybenzyliden- -fluoro-anilines,” Il Nuovo Cimento D, vol. 10, no. 11, pp. 1363–1371, 1988.
[27]
S. Sakagami, T. Koga, and A. Takase, “Liquid crystalline properties and photochromism of 4-halogeno-N-(4-alkoxysalicylidene) anilines,” Molecular Crystals and Liquid Crystals, vol. 373, no. 1, pp. 71–81, 2002.
[28]
S.-T. Ha, L.-K. Ong, Y. Sivasothy et al., “Mesogenic schiff base esters with terminal chloro group: synthesis, thermotropic properties and X-ray diffraction studies,” International Journal of Physical Sciences, vol. 5, no. 5, pp. 564–575, 2010.
[29]
S. T. Ha, T. L. Lee, H. C. Lin et al., “Mesogenic Schiff base with bromo end group: synthesis and thermotropic properties,” Asian Journal of Chemistry, vol. 24, no. 8, pp. 3679–3684, 2012.
[30]
S.-T. Ha, T.-L. Lee, S.-L. Lee, S. Sreehari Sastry, and Y.-F. Win, “Schiff base liquid crystals with terminal iodo group: synthesis and thermotropic properties,” Scientific Research and Essays, vol. 6, no. 23, pp. 5025–5035, 2011.
[31]
T. W. G. Solomons, Fundamentals of Organic Chemistry, John Wiley & Sons, New York, NY, USA, 1994.
[32]
S. Kumar, Liquid Crystals: Experimental Study of Physical Properties and Phase Transitions, Cambridge University Press, Cambridge, UK, 2001.
[33]
S. A. R. Krishnan, W. Weissflog, G. Pelzl et al., “DFT and MD studies on the influence of the orientation of ester linkage groups in banana-shaped mesogens,” Physical Chemistry Chemical Physics, vol. 8, no. 10, pp. 1170–1177, 2006.
[34]
Y. Sakurai, S. Takenaka, H. Miyake, H. Morita, and T. Ikemoto, “Molecular structure and smectic properties. Part 1. The effect of linkages on smectic A thermal stability in three aromatic ring compounds linked by ester groups,” Journal of the Chemical Society, Perkin Transactions 2, no. 9, pp. 1199–1204, 1989.
