Helical Network Polymers Embodying High Dissymmetry Factors in Circularly Polarized Luminescence: Photocrosslinking Polymerization of Acrylate Derivatives in Chiral Smectic Liquid Crystals

Abstract

Radical photocrosslinking polymerizations of functional acrylate derivatives are achieved in a chiral smectic C liquid crystal (SC*-LC) used as an asymmetric medium to synthesize helical network polymers (HNPs) with thermally and chemically stable chiroptical properties that are represented by circularly polarized luminescence (CPL). Two types of acrylate derivatives that bear an LC moiety and a fluorene moiety as luminophores are synthesized and used as functional monomers for photocrosslinking polymerizations. A trisubstituted acrylate derivative is used as a crosslinker to enhance the amount of photocrosslinking to produce a spatially linked network structure of polymers. The synthesized HNPs show a striated fan-shaped texture that is characteristic of the SC*-LC, and they exhibit notable Cotton effects owing to their circular dichroism in the absorption and luminescence processes. The CPL of the (R)- and (S)-HNP films shows considerably high dissymmetry factors (|glum|) of 2.4 × 10–2 and 2.5 × 10–2, respectively. The CPL further increases in intensity by more than 4 times after heating the HNP film at 150 °C to provide |glum| values of 1.0 × 10–1 and 1.1 × 10–1 for (R)- and (S)-HNP films, respectively. This increase in CPL intensity is ascribed to the thermally stimulated progress of the crosslinking between the monomers that still remain unreacted during photopolymerization at room temperature, thus leading to the completion of the helical network in the HNP film. The higher structural order of the HNP films prepared with the SC*-LC than those prepared with a chiral nematic LC (N*-LC) is confirmed through X-ray diffraction measurements. The present work is the first successful asymmetric polymerization using the SC*-LC as a chiral medium.