Liquid crystalline properties of all symmetric p-phenylene and 2,5-thiophene pentamers

In order to understand the mesophase behaviour of the p-phenylene pentamer p-quinquephenylene (PPPPP), all symmetric pentamers composed of p-phenylene (P) and 2,5-thiophene (T) have been synthesised and investigated. The effects of substituting 1,4-phenylene by 2,5-thiophene units are the introduction of kinks along the main axis of the molecule, often resulting in mixtures of stable conformers, and planarisation of the molecule by avoiding twisted PP sequences. The mesophase behaviour of the compounds was determined by optical polarised microscopy and differential scanning calorimetry. Nematic phases were found for six out of eight compounds, but some of these compounds are monotropic. Quinquephenylene has the highest phase transition temperatures at 393 degrees C and 426 degrees C, for the crystal to nematic and nematic to isotropic phase transition, respectively. Introduction of one or two kinks in the pentamers lowers the phase transition temperatures, typically by 60 and 130 degrees C, respectively. It was also observed that 2,5-thiophene units at the positions 1 and 5 widens the nematic window for these compounds. The two compounds with three successive thiophene units in the middle of the pentamer, which possess three stable conformations, did not show liquid crystal behaviour.