Highly quadrupolar derivatives of the [closo-B10H10]2- anion: Investigation of liquid crystalline polymorphism in an homologous series of 1,10-bis(4-alkoxypyridinium) zwitterions

Evidence of Counterion Size Effect on the Stability of Columnar Phase of Ionic Liquid Crystals Based on Pyridinium Salts Derived from N-3,4,5-Tri(alkyloxy)-benzyl-4-pyridones

The synthesis and characterization of novel ionic liquid crystals based on pyridinium salts with Br− and PF6− counterions are described in this work. These pyridinium salts were derived from 4-hydroxypyridine, both by N- and O-alkylation. The 3,4,5-tri(alkyloxy)-benzyl mesogenic unit was attached to the nitrogen atom of the pyridinium ring. Alkyl chains with a different number of carbon atoms (6, 8, 10, 12 and 14) were employed in order to show the effect on the stability of mesophase. The POM (polarizing optical microscopy) and XRD (powder X-ray diffraction) studies indicated that bromide salts with shorter chains C6, C8 and C10 do not show mesomorphic properties, while longer chain analogues with C12 and C14 exhibit two enantiotropic columnar phases. Surprisingly, the pyridinium salts with the larger size PF6− counterion do not exhibit liquid crystal properties.

Polar derivatives of [closo-1-CB9H10]- and [closo-1-CB11H12]- anions as high Δε additives to a nematic host: a comparison of the CH2CH2 and COO linking groups

A series of liquid crystalline pyridinium and sulfonium derivatives of the [closo-1-CB9H10]- and [closo-1-CB11H12]- anions containing the CH2CH2 linking group was prepared and their molecular and crystal structures were determined by single crystal XRD methods. Thermal and dielectric properties of the series were evaluated in a weakly polar nematic host. The highest extrapolated dielectric anisotropy, Δε, was observed for pyridinium zwitterions (up to 56.0). The dielectric data were analyzed with the Maier-Meier formalism augmented with density functional theory calculations, and the results were compared to those obtained for the analogous ester derivatives (COO linking group). The effect of the linking group on thermal and electrooptical properties is discussed.

C(1)-Phenethyl Derivatives of [closo-1-CB11 H12 ]- and [closo-1-CB9 H10 ]- Anions: Difunctional Building Blocks for Molecular Materials

C(1)-vinylation of [closo-1-CB₉ H₁₀ ]^- (A) and [closo-1-CB₁₁ H₁₂ ]^- (B) with 4-benzyloxystyryl iodide followed by hydrogenation of the double bond and reductive deprotection of the phenol functionality led to C(1)-(4-hydroxyphenethyl) derivatives. The phenol functionality was protected as the acetate. The esters were then treated with PhI(OAc)₂ and the resulting isomers were separated kinetically (for derivatives of anion A) or by chromatography (for derivatives of anion B) giving the difunctionalized building blocks in overall yields of 9 % and 50 %, respectively. A similar series of reactions was performed starting with anions A and B and 4-methoxystyryl bromide and iodide. Significant differences in the reactivity of derivatives of the two carborane anions were rationalized with DFT computational results. Application of the difunctionalized carboranes as building blocks was demonstrated through preparation of two ionic liquid crystals. The extensive synthetic work is accompanied by single crystal XRD analysis of six derivatives.

Thermal and Photophysical Properties of Highly Quadrupolar Liquid-Crystalline Derivatives of the [closo-B12 H12 ]2- Anion

Two series of 1,12-bis-zwitterionic derivatives of the [closo-B₁₂ H₁₂ ]^2- anion (B), containing either two 4-alkoxypyridinium groups (1B[n]-p) or one 4-alkoxypyridinium and one 4-pentylthianium groups (2B[n]-p), were prepared and their structural (XRD, DFT), thermal, and photophysical properties were compared with those of the analogous derivatives of the [closo-B₁₀ H₁₀ ]^2- anion (1A[n]-p and 2A[n]-p). Some 1,7-derivatives of B were isolated and investigated. Both series 1[n] and 2[n] exhibit nematic and crystalline polymorphism; the 12-vertex derivatives (B) have higher transition temperatures than those of the 10-vertex analogues (A). All compounds fluoresce with quantum yields higher for 1B (Φ_F =0.37 for 1B[7]-p and Φ_F =0.27 for 2B[7]-p) than those for the 10-vertex analogues (Φ_F =0.04 for 2A[5]-p). DFT calculations demonstrate an order of magnitude lower first hyperpolarizability, β_(-ω,ω,0) , for 2B[7]-p than that for the 10-vertex analogue 2A[7]-p (1.7×10^-30 vs. 18.9×10^-30  esu at ω=0).