Square and Hexagonal Columnar Liquid Crystals Confined in Square and Triangular Pores

Columnar liquid crystals confined in cylindrical pores are known to orient perpendicular to the pore axis, although for potential nanoelectronic applications they would need to be aligned axially. The X-ray diffraction study reveals that while in all three compounds examined, the columns arrange in concentric circles in circular pores, axial alignment is indeed achieved for the hexagonal phase of a discotic hexa-peri-hexabenzocoronene in "triangular pores," and for the square phase of a T-shaped amphiphile in "square pores." For the third compound, a carbazole dendron with very soft hexagonal columns, a new three-circle configuration is observed by atomic force microscopy (AFM) in triangular pores. The complex structure of domain walls in columnar liquid crystals is thus seen for the first time on the level of individual columns. A simple estimation of free energies confirms that it is the distortion of polygonal columnar lattice that prevents axial orientation in circular pores. The model also explains the appearance of the three-circle arrangement of soft columns.