Unique morphology and structure of new organic porphyrin based discotic liquid crystals.

摘要

Organic materials are part of the 3rd generation photovoltaics which focus on providing cost-effective energy production and ease of use. Moreover, organic materials are easy to process, are chemically flexible, and are easy to handle. One method to overcome an intrinsic disadvantage in organics, small exciton diffusion lengths, is to utilize pi-pi stacking in aromatic materials. One example is discotic liquid crystals which can self-assemble to form continuous charge conduction pathways.;A new series of porphyrin based discotic liquid crystals had been previously synthesized and had shown photovoltaic efficiencies of 0.7%. This was very high for a potentially unaligned discotic phase. To understand the reason for this, the structure and morphology of these materials was characterized using DSC, WAXD, SAED and PLM amongst other techniques. The alignment of discotic liquid crystals has been problematic in the past due to their high viscosities. Only thermal gradient alignment and magnetic field alignment proved successful in large scale orientation of these materials.;This series showed a stable liquid crystalline phase at room temperature and also a metastable solvent induced crystal phase. The peripheral n-alkyl chain length was varied from C8 to C12. The liquid crystalline phase for all samples showed a ribbon-like morphology. WAXD showed a hexagonally packed columnar structure with significant helical ordering within the columns. A coiled coil structure has been proposed as the only possible structure. This stable Col*hh phase was seen in all the samples. This work is the first observed evidence of a coiled coil structure for an achiral discotic liquid crystal. This would indicate that discotic columnar systems mimic biological systems where coiled coils are found commonly.;Sample QE12C also showed a unique undulating morphology. To understand the morphology the n-C14 alkyl chain length molecule was investigated. Characterization showed that the undulating phase was formed by two simultaneous phase transitions occurring from the isotropic melt to a simple coil to a coiled coil structure. These transitions forced a compressive strain on the columns which then collapsed into the observed morphology. The results gave a unique molecular insight to the morphological evolution and further reinforced the existence of the proposed coiled coil structure.