||Discotic liquid crystals can assemble into columnar structure by means of π-π interaction between molecules. In the highly ordered columnar structure, the charge-carrier mobility along the discotic columns is high (10−3 − 1 cm2V−1s−1), whereas the mobility in the transverse direction of the column is rather low. Consequently, discotic liquid crystals with a highly ordered columnar structure can be regarded as a one-dimension conductor, and are particularly suitable for the devices that demand the unidirectional and efficient charge-carrier transport property, such as photovoltaic cells, organic light emitting diodes, organic semiconductors. In the applications mentioned above, a long-range molecular order of discotic liquid crystals and the suitable molecular orientation of discotic columns are demanded in order to enhance the performance for these devices. However, there is no practical and reliable techniques can be used to align the discotic liquid crystals. In this study, we investigate the decisive factors that dominate the molecular arrangement of discotic molecules, and develop the effective techniques for aligning the discotic liquid crystals, in which the discotic mesogen hexaalkoxydibenzo[a,c]phenazine is used as the experimental object.|
Firstly, we observed the effect of the cooling procedure on the stacking of discotic molecules. Our experimental results show that a highly ordered molecular stacking of discotic liquid crystals may form when the cooling rate of the discotic layer is controlled to below 0.1 oC/min. Secondly, we studied the effect of the physicochemical properties of substrate on the molecular orientation of discotic molecules. Our investigation results indicate that the surface free energetic states of substrates play a decisive role in determining the molecular anchoring type of the discotic liquid crystals. We find that the surface with higher energetic state larger than 60 mJ/m2 supports the disk-face-on anchoring of discotic liquid crystals. With the surface free energy decreasing, the anchoring type varies. The disk-edge-on anchoring of discotic molecules forms when the surface free energy of substrate decreases to below a value of 20 mJ/m2.
The homeotropic alignment of discotic liquid crystals can be achieved by increasing the surface energetic state of substrates and on the help of slowly cooling process; whereas the homogeneous alignment of discotic layers can be obtained by aligning the discotic molecules using the microstructure surface or the patterned self-assembled monolayers. Our experimental results indicate that, under the conditions that the size of microstructure and the layout of the self-assembled monolayers are well designed, these alignment surfaces can align the discotic molecules into the homogeneous arrangement. Furthermore, the orientation of discotic columns in the discotic layer can be locally controlled, realizing the multi-domain molecular alignment of discotic liquid crystals.
We measured the characteristics of the charge-carrier transport of the aligned discotic liquid crystals. The experimental results show that the mobility of hole and electron are both on the order of 10−3 cm2V−1s−1, showing a good ambipolar charge-carrier transport property; whereas the mobility in the transverse direction of discotic columns is rather low. Therefore, the discotic liquid crystalline layer shows a clearly anisotropic conductivity. These results reveal that the alignment methods we used have the potential of realizing the one-dimensional charge-carrier transport properties of discotic molecules.