Impact of Smectic Fluidity on the Development of Continuous Packing during Polymorphous Behavior of Alkyl-Substituted Terthiophene Derivative

For a newly designed liquid-crystalline (LC) organic semiconductor of alkyl-substituted terthiophene derivative, abbreviated as NTETB, polymorphous phase behavior has been identified, including the growth of orthorhombic crystals at low temperature and a highly ordered smectic phase above 110 degrees C. The preferentially developed structural scheme of layer stacking within both ordered phases results in uniform homeotropic orientation of molecular packing within thin film. As a result of fluid smectic packing, the slippage of packing planes along the crystallographic b-axis was recognized. This slippage behavior caused stacking faults and two coexistent orthorhombic lattices within smectic layers. For later transformed crystals, inherited statistic stacking faults are uniformly along the a-axis as indicated by diffraction streaks on the electron diffraction pattern. The orientational slippage of packing planes accordingly divides the smectic phase into several parallel subdomains, creating straight boundaries along the b-axis. The bond-orientation order among subdomains was found to decline, while slight thermal agitation was activated upon reaching 160 degrees C. In addition to inducing the slippage of the packing plane, the gained smectic fluidity upon heating is able to trigger the coalescence of randomly oriented platelets of smaller size and leads to the formation of larger board-like domains. The long axis of the coalescent board-like domain is always along the b-axis of unified molecular packing, viewed as a morphological feature linked to the anisotropic packing within smectic layers. These disclosed impacts of smectic fluidity on creating stacking faults and the mobility of subdomain helps to better understand delayed charge carrier transportation within herringbone packing of oligothiophene derivatives as a result of polymorphous behavior.