New All-Aromatic Liquid Crystal Architectures

We have explored oxadiazole and thiophene modifications of the classic calamitic mesogens, p -quinquephenyl, p -sexiphenyl, and p -septiphenyl (P n, n = 5, 6, and 7). Replacing a phenyl ring with 2,5-substituted 1,3,4-oxadiazole and 2,5-substituted thiophene introduces a bend, or kink, into the parent mesogens of 134 and 148°, respectively. When oxadiazole was placed at penultimate positions on both ends of the mesogen (P O P n O P, n = 1, 2, and 3), a significant reduction in melt transitions for all model compounds could be observed. p -Septiphenyl (P 7 ) is an intractable and infusible molecule but its oxadiazole analogue P O P 3 O P melts at 312 °C, exhibiting a broad nematic range 170° wide. Replacing the central para-substituted phenylene ring in P O P 3 O P with an ortho- or a meta-substituted phenylene, however, destroys the LC behavior completely; the ortho-derivative forms an amorphous glass ( T g = 83 °C). Comparisons of the P 6 derivatives with the oxadiazoles in P O PP O P replaced with 2,4- and 2,5-substituted thiophenes show that the LC phase is completely lost for the 2,4-thiophene derivative (2,4-P T PP T P). The more linear 2,5-thiophene isomer (2,5-P T PP T P), with an exocyclic bond angle ϵ = 148°, shows a broad LC range with both nematic and SmA phases. This suggests that in addition to mesogen shape, electronic conjugation and electrostatic considerations are important mesophase stabilizing factors in this class of all-aromatic liquid crystals. A 1,3,5-tris(oxadiazole) discoid, i.e., P( O P) 3, was synthesized but no mesophase behavior could be detected; this absence of mesogenicity was consonant with the X-ray determined packing motif for P( O P) 3 .