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1979 (1)
1Author    MagedA. Osman, M. A. OsmanRequires cookie*
 Title    Nematic Liquid Crystals with High Positive Dielectric Anisotropy * Nematic Liquid Crystals 1093 *>-®-CH-N-©-CN  
 Abstract    Alkylamino-and formyl alkylamino-cyano Schiff's base, biphenyl and phenylbenzoate liquid crystals were synthesized. The mesomorphic behaviour of these compounds is described. The introduction of the formyl group in the higher homologues lead to the disappearance of the mesophase. The alkylamino and formylalkylamino substituents are strong electron donating groups and lead to nematic phases with remarkably high positive dielectric anisotropy. Interest in electrooptical displays based on the twisted nematic effect [1] has lead to a growing demand for nematic liquid crystals with positive dielectric anisotropy. Non-polar or almost non-polar nematic compounds of the type 1 were found to possess a very small positive dielectric anisotropy (As = 0.05-0.5) [5-7], X =-CH= N-|21 , -C00-131,—N=N— |4| 1 Nematic phases of the Schiff's base and phenyl-benzoate type with larger A a were synthesized by A. Boiler [8, 9] through replacement of one of the alkyl groups by the cyano group which possess a larger dipole moment (—4 D). The remaining alkyl group on the other end of the molecule, which has a dipole moment of -f-0.3 D, was not changed. Recently, G. Gray [10] and R. Eidenschink [11] prepared analogous nematic phases of the biphenyl and phenyl cyclohexane types. All these compounds have A e values from 8 to 20 [12-14] which allow an operating voltage of 3 volts for a twisted nematic display. Since the threshold voltage and, correspond-ingly, the operating voltage is proportional to \J11A e, it is clear that new nematic liquid crystals with larger dielectric anisotropics are required if the operating voltage is to be substantially reduced. This can be achieved by replacing the remaining alkyl group with an electron donating substituent having a stronger dipole moment along the molecu-lar axis. The first attempt along this line was made by introducing the dimethylamino group (-(-1.6 D) * Reprint requests to Dr. M. A. Osman. 0340-5087/79/0800-1092/$ 01.00/0 at the other end of the benzyliden-aminobenzo-nitrile molecule, but this gave an isotropic com-pound melting at 183 °C [15]. The disappearance of the liquid crystalline properties is probably due to the high melting point of this compound and to the bulkiness of the dimethylamino group which lowers its clearing point. The aim of this work is to introduce the less bulky monoalkylamino group in molecules of type 1 and to study the effect of this substitution on their mesomorphic behaviour and on the magni-tude of the dielectric anisotropy. Results and Discussion N-(4-Methylaminobenzylidene)-4'-aminobenzo-nitrile (2) was prepared by condensing 4-methyl-aminobenzaldehyde with 4-aminobenzonitrile in bulk [16]. CH3HN-^)-cho+H2N-<Q>-CN-^CH3HN-Q>-CH-N^Q>-CN z The aldehyde was prepared by the Vilsmeier re-action [17]. 2"c> ~HjC :N CN H0 ~ H 3 CHN <> CHO Compound 2 melted at 163.1 °C and showed a monotropic nematic phase clearing at 138.0 °C. This confirmed that the disappearance of the meso-phase in the dimethylamino derivative was caused by the bulkiness of this group. However, the melting point of compound 2 is too high for practical applications and for conveniently measuring the dielectric anisotropy. The higher homologues of alkylaminobenzaldehyde and their Schiff's bases were therefore prepared in an attempt to lower the melting point. It seemed also of interest to 
  Reference    Z. Naturforsch. 34b, 1092—1095 (1979); received March 23 1979 
  Published    1979 
  Keywords    Liquid Crystals, Nematic Phases, High Positive Dielectric Anisotropy, Alkylamino-cyano-biphenyls, Alkylamino-cyano-phenylbenzoates 
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 TEI-XML for    default:Reihe_B/34/ZNB-1979-34b-1092.pdf 
 Identifier    ZNB-1979-34b-1092 
 Volume    34