INFLUENCE OF CORE AND SIDECHAIN SUBSTITUENTS OF BIPHENYL BENZOATES ON PHASE APPEARANCE AND SPONTANEOUS POLARISATION OF FERROELECTRIC LIQUID CRYSTALS

Sergiy Pakhomov, Miroslav Kaspar, Vera Hamplova

Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 180 40 Praha 8 - Liben, Czech Republic.

Keywords: ferroelectric liquid crystals, substituents, structure-properties relationship.

Liquid crystals can form a wide variety of different phases. From the practical point of view, we are interested in the smectic phases that form defined layers positionally ordered along one direction. Molecules of several smectic phases (SmC*, SmI*, SmF*, SmJ*, etc.) built up of chiral compounds were found to be tilted within the layers that allows ferroelectricity. The molecular dipoles are not completely cancelled and the spontaneous polarisation PS appears in the each layer in the direction perpendicular to the molecular director. Antiferroelectric (AF) phase, molecules of which are aligned antiparallel in the neighbouring layers, belongs to the novell smectic modifications we are interested in.

Ferroelectric (FE) phases are usually compoused of the chiral rod-like molecules with a rigid core. One of the most widely used motive, biphenyl benzoate

 

has been applied in design of ferroelectric liquid crystals for many years. However, we are not aware of comprehensive studies devoted to the influence of structural modifications of this backbone on appearance of liquid crystalline phases and their characteristics. This influence is not necessarily transferable to the compounds possessing other core, but the effects seem to be rather general.

Now we present the first approach of such a work, where the attempt to compare the results of the other authors already published and ours is done.

Results of single crystal structure determinations [1] of some materials in several tilted smectic phases were also taken into account in the discussion.

To simplify the situation we divided the structural modifications into four types:

  1. Modifications of endchains: the mutual changes of the lengths of alkyl chains and moving the chiral centre off the rigid core and its attachment to the core by the various linkages or without them (the linkages most often utilised are: -COO- conjugated with the aromatic system of the core, -COO- without conjugation (of the aliphatic acid origin), -O- ether linkage and highly polarised -CO- keto group); double branching in the chiral chain and the influence of the distance between two chiral centres; influence of the various groups in the endchains (ether bridges, fluoro and chloro atoms, hydroxy groups as well as bulkier groups of acrylate or metacrylate origin); opposite attachment of both chiral and achiral chains with respect to the mesogenic core.
  2. Lateral substitution of the mesogenic core: in the off-centre position next to the chiral moiety or placed at the opposite end of the molecule as well as centrally positioned. Last time the substitution with the fluoro atoms to improve the FE and AF properties and the nitro group in compounds proposed to be used as materials for non-linear optics or SmA materials [2] is used most frequently. Inserting other substituting groups is rear enough and we are aware several works only devoted to methyl [3], methoxy [4], cyano [5], amino [6,7], dimethylamino [7] and chloro [5,8] substitutions.
  3. Central ester linkage modifications: substitution of the central ester linkage for -CH2O- group; introduction of the double bond between two rings of biphenyl unit as well as between the phenyl ring and the carboxy group [9,10] and its modification; insertion of the triple bond in the same position [11].
  4. Introduction of the heteroatoms into the aromatic core [5].

Compounds containing various chiral side chains were studied. They were derivatives of sec-alkohols, 2-methylbutanol, 2-methylalkanoic and lactic acids, chloro, fluoro and heterocyclic (oxirane and thiirane) compounds and so on.

Some effects found will be presented on the conference.

The references presented here are not comprehensive, of course, and represent the subjective selection made by the authors.

  1. Hori, K., Ohashi, Y.: Bull. Chem. Soc. Jpn., 62, 3216 (1989).
  2. Ratna, B.R., Crawford, G.P., Naciri, J., Shashidhar, R.: SPIE Proceedings, 2175, 79 (1994).
  3. Kašpar, M., Hamplová, V., Pakhomov, S.A., Stibor, I., Sverenyák, H., Bubnov, A.M., Glogarová, M., Vanik, P.: Liq. Cryst., 22, 557 (1997).
  4. Kašpar, M., Sverenyák, H., Hamplová, V., Glogarová, M., Pakhomov, S.A., Vanik, P., Trunda, B.: Liq. Cryst., 19, 775 (1995).
  5. Furukawa, K., Terashima, K., Ichihashi, M., et al.: Int. Symp. on Ferroel. Liq. Cryst., Bordeaux - Arcachon, Abstracts, P-7.
  6. Walba, D.M., Blanca Ros, M., Sierra, T., Rego, J.A., Clark, N.A., Shao, R., Wand, M.D., Vohra, R.T., Arnett, K.E., Velsco, S.P.: Ferroelectrics, 121, 247 (1991).
  7. Schmitt, K., Herr, R.-P., Schadt, M., Fünfschilling, J., Buchecker, R., Chen, X.H., Benecke, C.: Liq. Cryst., 14, 1735 (1993).
  8. Kašpar, M., Górecka, E., Sverenyák, H., Hamplová, V., Glogarová, M., Pakhomov, S.A.: Liq. Cryst., 19, 589 (1995).
  9. Tuffin, R.P., Goodby, J.W., Bennemann, D., Heppke, G., Loetzsch, D., Scherowsky, G.: Mol. Cryst. Liq. Cryst., 260, 51 (1995).
  10. Hamplová, V., Sverenyák, H., Kašpar, M., Novotná, V., Pakhomov, S.A., Górecka, E., Bubnov, A.M., Glogarová, M.: SPIE Proceedings, 3319, 92 (1997).
  11. Nishiyama, I., Yoshizawa, A., Fukumasa, M., Hirai, T.: Jpn. J. Appl. Phys., 28, L2248 (1989).