Cholesterol-based photo-switchable mesogenic dimers. Strongly bent molecules versus an intercalated structure

Abstract

Herein, a homologous series of cholesterol-based liquid crystalline bent-shaped dimers with different lengths of a terminal alkoxy chain were synthesized and characterized by polarizing optical microscopy, differential scanning calorimetry, and powder and single-crystal X-ray diffraction. The photosensitivity of the mesogenic molecules provides an opportunity to manipulate the structure of the liquid crystalline phase using light. The compound with the shortest alkoxy tail exhibited the heliconical twist-bend nematic phase. Elongation of the terminal alkoxy chain prevented the formation of the N_tb phase; for compounds with longer tails, instead of the N_tb phase, SmA was observed, which was rather unique for bent-shaped dimeric mesogens. The crystal structures presented herein shed new light on the possible molecular conformations and arrangements of molecules in the liquid crystalline phase. We have shown that in addition to the model of intercalated molecules, special attention should be paid to the model of strongly bent molecules. A new model of the molecular packing of bent-shaped dimers with flexible linkers in the SmA phase is proposed.