A study of the conformational distribution of substituted 2,2'-spirobiindan-1,1'-diones in ferroelectric liquid crystals

Abstract

Ferroelectric liquid crystals (FLCs) exhibit a bulk electric polarization (PS) that can be coupled to an electric field to produce an ON-OFF light shutter, and are being investigated as an alternative to nematic liquid crystals in display applications. Commercial FLC mixtures normally consist of a small amount of chiral dopant in an achiral smectic C (SmC) liquid crystal host. Because the switching time of FLC display is inversely proportional to the induced polarization, the design of chiral dopants with high polarization powers (p) is a key aspect of FLC research. Such work requires an understanding of the relationship between molecular structure and polar order in the chiral SmC* phase. Previous work in the Lemieux group focused on 2,2’-spirobiindan-1,1’-diones dopants, and a conformational model was proposed to explain the observed host dependence of the polarization power (p) of these dopants. In order to test this model, the 2,2’-spirobiindan-1,1’-dione core has been modified by introducing polar substituents and by modifying the functional groups linking the core to the alkyl side-chains. Specifically, this thesis focuses on implementing this approach via the synthesis and characterization of chloro- and methyl-substituted 2,2’-spirobiindan-1,1’-dione dopants.

Four chiral dopants (2.1a, 2.1b, 2.2a and 2.2b) were synthesized, resolved and their absolute configurations assigned by CD spectroscopy. Their polarization powers were measured in four SmC hosts with different core structures. For both the ether-linked and ester-linked dopants, the addition of a substituent at the 6-position of one indanone ring results in lower polarization powers regardless of the size and polarity of the substituent, which is contrary to the original conformational distribution model. A comparative study of the data suggests that the ester-linked dopants exert much stronger perturbations on the host environment than the ether-linked dopants, especially when the 6-position is substituted. We postulate that this perturbation is chiral in nature, and that the feedback effect of chirality transfer causes a shift in the conformational distribution of the dopant favoring conformers with negative polarity. Probe experiments were performed to detect the effect of chirality transfer feedback (CTF) in the case of the chloro-substituted diester dopant (2.1b), showing consistent results with the postulate.