Stereochemistry, Conformational Dynamics, and the Stability of Liquid Crystal Phases.

Although discotic liquid crystal dimers have been widely targeted as organic semiconductors and as LC-glass formers, the role of conformational dynamics on the self-assembly of these flexible mesogens remains poorly understood. In an effort to probe this effect, we investigated the impact of linker stereochemistry on the phase behavior of discotic liquid crystalline dimers. Diastereomeric dibenzo[a,c]phenazine diesters were prepared from (2R,3R)- and meso-2,3-butanediol. While both dimers form columnar phases, the meso-isomer had a clearing temperature (Tc) that was 31 °C higher than that of its chiral diastereomer. Conformational analysis via DFT calculations, 1H-NMR, and DOSY experiments indicated that both compounds adopt predominantly extended conformations but that the meso-dimer shows a stronger preference to unfold in solution. To probe how conformation alters phase stability, we prepared derivatives in which catechol and hydroquinone act as rigid linkers that lock the dimers in a folded or an extended conformation, respectively. The diester of hydroquinone possessed a Tc that was nearly 100 °C higher than the catechol derivative, consistent with a model where extended conformations stabilize the LC phase. Extended dimers also exhibited higher transition enthalpies at the Tc, an indication that their columnar phases are more ordered than folded structures.