Glassy Liquid Crystals for Reflective Coloration and Optical Storage

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Glasses are traditionally classified as amorphous materials having isotropic properties. As a class of noncrystalline solids, glassy liquid crystals are characterized by a uniaxial, lamellar, helical, or columnar molecular arrangement underlying liquid crystalline mesomorphism. Thanks to optical anisotropy and device robustness, glassy liquid crystals are well suited for optics, photonics, and optoelectronics. However, most liquid crystals tend to crystallize on cooling from melts, thereby losing the desired order and yielding grain boundaries that scatter light and limit charge transport. Under prior NSF support, a definitive set of materials has been generated following an empirical approach in which liquid crystalline groups are chemically bonded to a volume-excluding central core. Unique features of glassy liquid crystals include: a well-defined molecular structure with a low to medium molecular weight, superior chemical purity and film-forming ability, and favorable rheological properties that facilitate processing into large area monodomain thin films. In addition, feasibility has been demonstrated for latching electro-optic devices, such as optical switches, attenuators and tunable filters; efficient circular polarizers and polarized light emitters; and high-performance optical notch filters and reflectors. To take on new intellectual and technological challenges, the PI's propose to develop glassy nematics and cholesterics with a high glass transition temperature (80 to 150 C) and a wide mesomorphic fluid temperature range; to furnish molecular-level understanding of vitrification, as opposed to crystallization, of liquid crystals; to functionalize glassy liquid crystals with photoresponsive moieties that are thermallystable and fatigue resistant; and to demonstrate concepts for reversible tunability of reflective coloration and patterning as well as rewritable optical data storage. Glassy liquid crystals are particularly promising because of their ability to undergo repeated heating-cooling cycles across a wide temperature range (through the glass transition temperature) without encountering crystallization. The project involves implementation of deterministic synthesis strategies for multifunctional materials; preparation and characterization of ordered solid films; analysis of material stability against thermally activated crystallization; determination of crystalline lattice structures with x-ray crystallography and powder diffractometry; and reversible modulation of optical properties through photoinduced ring closure ring opening of diarylethene moieties. Aimed at cultivating new knowledge crucial to the development of novel optical materials, the proposed research is highly relevant to optical information technology that is undergoing a rapid growth worldwide. Transition of academic research to civilian and defense applications will be facilitated through collaboration and partnership with government and industry laboratories. From an educational perspective, students participating in the project will develop unique skills from conducting their thesis research in a multidisciplinary environment, gaining a broad perspective in a strategically important area of research and development.