Hydrogen-bonding between a dichroic dye and a liquid crystal-forming molecule, for application to LCDs

Abstract

The performance of guest–host type liquid crystal displays (LCDs) is governed by the solubility and dissolution stability of dichroic dyes in the liquid crystal medium in guest–host kind of LCDs. The solubility and stability is influenced by hydrogen-bonding between the dye and the liquid crystal host. An earlier work was carried out to study the hydrogen-bonding behavior of 4-pentyl-4′-cyanobiphenyl (5CB) (liquid crystal), benzonitrile, and acetonitrile, with 1-hexanol in CCl4 using FTIR spectroscopy, ab initio and equation of state calculations [Ind. Eng. Chem. Res. 42 (2003) 1129]. To gain further thermodynamic understanding, hydrogen-bonding behavior of 5CB (liquid crystal), benzonitrile, and acetonitrile, with a dichroic dye Disperse Red 1 (DR1) {2-[ethyl[4-[(4-nitrophenyl)azo]phenyl]amino]ethanol} in CCl4 were studied in this work. In all three mixtures, -OH acts as a hydrogen-bond donor and -CN acts as hydrogen-bond acceptor. Based on FTIR analysis for DR1/acetonitrile, DR1/benzonitrile, and DR1/5CB, the enthalpies of hydrogen-bond formation are −24.2, −23.7, and −3.7kJ/mol, respectively; and the entropies of hydrogen-bond formation are −58.9, −58.1, and −0.4J/(molK), respectively. Though, for DR1/5CB, enthalpy of H-bonding is not too negative, the entropy of H-bonding is more favorable; hence the H-bonding appears to be entropically driven. Lattice fluid hydrogen-bonding (LFHB) parameters obtained from the spectroscopic data were observed to match well with the experimental trends. LFHB equation of state was then used to predict hydrogen-bonding for a wider range of conditions.