New ester-type banana (bent-shaped) monomers, 1,3-phenylene bis[4'-(alken-1-yloxy)-biphenyl-4-carboxylate] s Ia - Vb, with different substituent(s) on the central phenyl ring (H, CH3, Cl or NO2) and alkenyl tails in the side arms ( decenyl or undecenyl) were prepared. The analogues IIIa - IVb with 4-chloro- or 4,6-dichloro-substituents exhibited a nematic phase, while Va, Vb with 2-nitro-substituent showed a B-7 phase at relatively low temperature. All the compounds were stable, no degradation or polymerization was observed under applied electric fields or heat treatments. Electro-hydrodynamic instabilities were observed in the nematic phase of each sample. In the B-7 phase of Vb there was an electro-optical switching in the range 112 - 114.5 degreesC with a switching time of about 150 mus at 20 V mum(-1) field. At lower temperatures no electro-optical switching occurred, but a ferroelectric-type spontaneous polarization of 22 nC cm(-2) could be detected. The bend angle was calculated by the semiempirical CAChe/MOPAC/PM5 method.
Fluorine Containing Nonsymmetrical Five-Ring Achiral Banana-Shaped Compounds with Columnar and Synclinic Antiferroelectric Layered Phases07/27/2006
The phase transitional behavior of two homologous series of five-ring banana-shaped compounds comprising fluorine substituents synthesized through covalent linking of two chemically dissimilar rod-like anisometric cores (arms) to central 1,3-phenylene is reported. The novelty of these molecules originates from the fact that the molecules are highly nonsymmetrical. One of the arms, which is either salicylaldimine or Schiff base core, possesses two vicinal fluorine atoms at the terminal ring having a n-decyloxy tail; while the n-alkyl tail attached to the other arm was varied to realize a homologous series of compounds. The mesophases have been characterized by several complementary studies. The banana-shaped systems having short or medium alkyl chain lengths form apolar columnar (two-dimensional) structures, while on ascending the series, a polar smectic phase is stabilized. Detailed electro-optical investigations on one of the polar smectic phases revealed a synclinic antiferroelectric (racemic) ground state structure, which switches, as expected, to an anticlinic ferroelectric state by the application of an electrical field. At higher field strengths applied for an extended time interval, the anticlinic ferroelectric state switches to a synclinic ferroelectric (chiral) state. Upon field removal, these domains switch to an anticlinic antiferroelectric (chiral) state, which eventually nucleates to the original antiferroelectric synclinic (racemic) state. Remarkably, the associated spontaneous polarization value exceeds 800 nC cm(-2), which is among the highest reported hitherto.