Alkylthio-based asymmetric liquid crystals: unravelling the substituent effects and intercalated cybotactic nematic and smectic phases

Abstract

To reveal not only the effects of alkylthio groups but also an unknown layered smectic (Sm) phase formed by short-chain-based liquid crystals (LCs), a novel series of asymmetric 1,4-phenylene bis(benzoate)s possessing 4,4′-alkylthio and alkoxy groups with various chain lengths (SOn, n = 1-8) was established. All members formed well-defined enantiotropic LC phases. The SOn homologs with n = 2-7 exhibited only a nematic (N) phase, and the longest SO8 showed an inclined smectic C (SmC) phase in addition to the N phase. Notably, the shortest SO1 formed an enantiotropic orthogonal smectic A (SmA) phase as well as the N phase. The primary effects of the alkylthio and alkoxy groups were identified for each phase transition over varied chain lengths and through comparison with those of their symmetric alkoxy- and alkylthio-based counterparts (OOn and SSn, respectively). In addition, X-ray diffractometry unraveled surprising results, wherein the N and SmA phases of SO1 were found to possess a layer period that was half of the molecular length, thereby suggesting half-intercalation of the intermolecular cores. These phases are distinct from the conventional N and Sm phases without such core intercalation. The remarkable formation of half-intercalated LC phase was discussed based on plausible intermolecular interactions (e.g., multiple hydrogen bonds due to the presence of esters) implied by single-crystal structural analyses. Furthermore, the birefringence increased when replacing the alkoxy group with the more polarizable alkylthio group.