Cross-linkable liquid crystal monomers containing hydrocarbon 1,3-diene tail systems

The unprecedented use of polymerizable hydrocarbon tail systems containing 1,3-diene groups for the design of thermotropic and lyotropic liquid crystal (LC) monomers is described. Crosslinkable LC dienes are synthesized by attaching LC core units to modular omega -bromoalka-1,3-diene tails of variable length. These modular diene tails are synthesized by the oxidation of long chain omega -bromoalkanl-ols to the corresponding omega -bromoalkanals. Reaction of the omega -bromoalkanals with Matteson's reagent, followed by treatment with triethanolamine and deoxysilylation under Peterson elimination conditions, affords the desired omega -bromoalka-1,3-diene tails. The effect of the 1,3-diene group on the mesogenic behavior of certain thermotropic and lyotropic LC systems was determined by examining 1,3-diene analogues of a thermotropic calamitic LC diacrylate and a taper-shaped lyotropic LC triacrylate. Compared to their diacrylate analogues, the thermotropic LC bis(1,3-diene)s exhibit the same progression of nematic and smectic phases but with higher smectic C to nematic transition temperatures and higher clearing temperatures. Replacement of the three acrylate groups in the tapered-shaped lyotropic LC monomer with 1,3-diene moieties had little effect on its tendency to form the inverted hexagonal phase at room temperature in the presence of water. The lyotropic LC diene phases also exhibit higher clearing temperatures than the corresponding LC triacrylate. The 1,3-diene group was found to be an efficient cross-linking unit for the photopolymerization of lyotropic LC phases at ambient temperature because of its hydrophobicity, minimal phase perturbation, and the high degree of photopolymerization. With thermotropic LC systems, however, Diels-Alder dimerization of adjacent diene units was found to occur upon heating the thermotropic LC bis(diene) monomers to ca. 90 degreesC or higher. Thus, as a photopolymerizable group in LC monomer design, the practical utility of the 1,3-diene group appears to be limited to temperature regimes below 90 degreesC.