X-Ray Diffraction Study of Hexacyclene and Triphenylene Based Discotic Liquid Crystals

Discotic liquid crystal phases are formed when the disk-like molecules stack on top of each other to form columns. These molecules within each column are disordered, yet the columns can be arranged on an hexagonal lattice (D_{rm hd} phase) or a rectangular lattice (D_{rm rd}) phase. High resolution synchrotron powder x-ray diffraction experiments have been performed on various hexacyclene cored liquid crystals. We present the first conclusive evidence that demonstrates the existence of an hexagonal columnar phase in this class of compounds. We also show that diffraction data from certain hexacyclene derivatives can be modeled by a smectic phase or by a columnar phase. Computer simulations of isolated molecules were used to show that a linear conformation for these molecules was preferred over a radial conformation. These simulations also show that the core of the molecule is not open, but in fact collapses, so a tubular phase is not formed. Hexa-n-alkylthiotriphenylenes have been studied using high resolution powder x-ray diffraction, x-ray diffraction from singly oriented strands, and differential scanning calorimetry. We found that C7 forms a D_ {rm hd} phase with short range helical order between molecules in the columns. A phase diagram for the range n = 4,cdots,7 was constructed and several crystal phases identified. The helicity seen in the helical phase (previously called the D_ {rm ho} phase) was studied using unoriented powder samples. We found that the helical pitch was commensurate with the underlying lattice, in contrast with measurements of the helicity in free standing strands, which is incommensurate.