Approaches to spontaneously polar materials: Investigations of bowlic liquid crystals based on calix(4)arenes

Bing Xu, University of Pennsylvania

Abstract

An extensive investigation has been conducted on the mesophase behavior of bowlic liquid crystals based on calix(4) arenes. We have developed high yield synthetic routes for the calix(4) arene inetallomesogens based-on tungsten(VI)-oxo azocalix(4) arenes and these compounds exhibit bowlic columnar phases which are stable over approximately a 200$\sp\circ$C range. The electron deficient nature of the tungsten center imparts Lewis acidic character. As a result, a number of Lewis bases form host-guest complexes with these materials. The most dramatic consequence of host-guest complexation is its effect Lipon the mesomorphic behavior. Host-guest complexation with DMF prevents mesophasc formation and these materials melt directly to isotropic liquids at temperatures about 200$\sp\circ$C less than the clearing temperature of their uncomplexed form. This fact combined with the columnar structure, is a unique proof of head-to-tail order in bowlic liquid crystals. Host-guest interactions have also been used to enforce a bowl shape in conformationally flexible azocalix(4) arenes and thereby stabilize liquid crystallinity. We have developed a novel method for stabilization of mesomorphic behavior which make use of the calix(4) arene's receptor properties. In particular we have found that host-guest interactions between azocalix(4) arene and 1,4-(diformamide)butane, stabilize a bowlic hexagonal columnar phase (B$\sb{\rm hd}).$ This mesophase has very low clearing enthalpies and high fluidity. The system also displays the unusual properties that the range of mesomorphism is independent of the host/guest ratio and that only a fractional amount of guest is required to stabilize the mesophase. The latter feature is indicative of a cooperative transition to the mesomorphic state. To create intercolumn interactions which favor polar structures we have introduced chirality since the dipolar frustration in a hexagonal structure is not sufficient for the generation of spontaneous polar order. We have designed general synthetic methods to chiral bowlic liquid crystals. We developed a novel synthetic route to 3-methylcalix(4) arene which is a new versatile precursor to inherently chiral calix(4) arenes. We also have successful developed new resolution methodology to obtain the optical active metallacalix(4) arenes. We have applied this method to produce optically pure bowlic liquid crystals which are predicted to spontaneously organize into polar liquid crystal phases. Polar order should be established in the liquid crystals phases due to the lack of a center of symmetry in the chiral substances. In order to elucidate the polar structure, future investigations will focus on the electro-optical properties of the B$\sb{\rm h}*$ phases.

Subject Area

Chemistry|Organic chemistry

Recommended Citation

Xu, Bing, "Approaches to spontaneously polar materials: Investigations of bowlic liquid crystals based on calix(4)arenes" (1996). Dissertations available from ProQuest. AAI9628031.
https://repository.upenn.edu/dissertations/AAI9628031

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