Two Cholesterol-Containing Pyrene Derivatives: Subtle Spacer Difference, Diverse Stimuli-Responsive Luminescence, Chirality, and Self-Assembly Behaviors

Two chiral molecules 1 and 2 were designed
and synthesized with a pyrene moiety directly linked to a chiral cholesterol
moiety and connected through a methylene spacer, respectively. Influence
of the spacer on their stimuli-responsive luminescence, chirality,
and self-assembly behaviors was systematically investigated. Molecules 1 and 2 had similar aggregation-induced emission
enhancement (AIEE) in solution, because of carrying the same fluorescence
moiety. Both molecules displayed mechanochromism (MC) property but
with different color contrast, whereas only 2 showed
mechanoluminescence (ML) activity. When doping in liquid crystal molecule
5CB, both molecules induced the formation of chiral nematic liquid
crystals (N*-LCs) with strong circularly polarized luminescence (CPL).
Molecule 2 induced single handedness signal, irrespective
of doping ratios, while 1-doped N*-LCs showed an inversion
of CPL signal from negative to positive upon the increase of doping
ratios. Molecules 1 and 2 also self-assembled
into different coassemblies with 5CB. Their distinct behaviors were
attributed to the influence of the methylene spacer, which caused
different molecular conformation and steric bulkiness; accordingly,
it changed intermolecular interactions and molecular packing of the
two molecules and led to diverse chirality and luminescence. This
work provided important model molecules to better understand the molecular
structure–property relationship and guide the design of novel
functional molecules.