Conformational dynamics and self-assembly of discotic dimers

Disc-shaped (discotic) liquid crystals (DLCs) predominantly form columnar phases, which have been utilized as organic semiconductors in opto-electronic devices; yet these materials have been hampered by their thermal stability, often exhibiting liquid crystallinity at high and narrow temperature ranges. Discotic dimers, where two discotic moieties are tethered, overcome this hinderance through their unique ability to supercool and retain liquid crystallinity at device operating temperatures; however, the flexible linker that helps promote supercooling also makes dimers difficult to study – the dimer can adopt multiple conformations in equilibrium, including folded and extended structures. The effect these conformational dynamics have on the liquid crystal (LC) properties is still poorly understood. A series of dibenzo[a,c]phenazine diesters with subtle changes to the linking group were prepared and their conformational dynamics and self-assembly were probed. We observed a strong sensitivity between linker stereochemistry and supramolecular self-assembly. For a pair of 2,3-butyl dimers, the meso isomer, which extends more than its diastereomer, formed a substantially more thermally stable and ordered columnar phase. The different properties arose from differences in their conformational equilibria, as self-assembly from extended shapes was shown to stabilize and increase the order of the columnar hexagonal phase. In addition to the conformational equilibrium, the geometries had a significant impact on the supramolecular self-assembly. For a pair of 1,2-cyclohexyl diastereomers, one isomer was liquid crystalline, and the other was amorphous. The disparate phase behaviour stemmed from their unfolded shapes, as the mesogenic isomer formed an extended and fairly planar structure while the amorphous isomer could only adopt non-planar unfolded geometries. We also investigated the effect adjacent groups have on the conformational dynamics and self-assembly, as the majority of discotic dimers contain an ether group ortho to the linker. In discotic monomers, adjacent ether groups broadened and lowered the columnar thermal range through depression of the melting temperature and elevation of the clearing temperature. In dimers, the additional ether group deterred folding. While we anticipated this would enhance the columnar thermal stability, the opposite was observed. We discovered the adjacent ether group destabilizes the columnar phase from extended geometries. The ether group likely perturbs planarity in the columnar hexagonal array to some extent, highlighting again the importance of planarity for columnar self-assembly from extended shapes. Finally, the lessons learned from discotic dimer self-assembly was utilized to design discotic nematic materials. To promote the rarely observed phase, discotic dimers with a short direct ester bridge and an adjacent ether group were synthesized. The key was the non-planar, extended geometry, which allowed promotion of nematic ordering over columnar hexagonal assembly.