Morphology, Mesophase, and Molecular Order of 3‑Hexyl Thiophene-Based π‑Conjugated Mesogens

Molecular materials that are built with π-conjugated moieties which self-assemble to form organic semiconductors have gained greater recognition for their applications in organic electronics. By judicious molecular design, liquid crystallinity can be incorporated into them to realize π-conjugated liquid crystals. In this work, we report 3-hexyl thiophene-based π-conjugated liquid crystals in which alkoxy biphenyls are introduced into the core by palladium acetate-catalyzed direct arylation. The π-conjugated molecules exhibit self-assembly in solution and enantiotropic nematic phase upon melting. Therefore, the morphological studies of molecules in solution by atomic force microscopy and scanning electron microscopy, mesophase properties by hot-stage optical polarizing microsocpe and differential scanning calorimetry, as well as molecular order by solid-state 13C NMR is performed. The morphological studies indicate the formation of supramolecular fibers due to the self-assembly of molecules through assisted π–π stacking and van der Waals interactions. The static one-dimensional and 13C NMR studies reveal the molecular organization and orientational order in the nematic phase. Because of the asymmetry of the thiophene ring due to the 3-hexyl chain, the orientational order in the nematic phase is characterized by three values, i.e., S zz , S xx –S yy , and S xz . The important feature of the 13C NMR investigations is the contrasting orientational constraints for terminal octyloxy and lateral hexyl chains. The large difference in 13C–1H dipolar couplings of the individual carbons of the terminal and lateral chains reveal that the mean orientation of the former is in fully trans conformation, while the lateral hexyl chain carbons prefer cis conformation to align with the core unit. A plot of alignment-induced chemical shifts versus temperature follows a decreasing trend with rise in temperature. The optical absorption and emission in solution indicate that the absorption band is structureless while the fluorescence spectrum is structured with two maxima, located at 422 and 445 nm, which are independent of terminal chain length. The investigations point out that the 3-hexylthiophene in combination with alkoxy biphenyls can result in π-conjugated low melting mesogens which undergo self-assembly in solution in addition to displaying fluorescence emission.