Rational Design of Smectic Liquid Crystals for Electro-Optical Applications

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Chirality is a property that man-made and almost all living things possess. Chiral objects exist in right- and left-handed forms (e.g., gloves, golf clubs) and are non-superposable on their mirror images. Chirality is also manifest at the molecular scale. (e.g., amino acids, carbohydrates); these chiral building blocks are assembled by nature to form larger chiral structures (e.g., proteins, nucleic acids), which themselves form crystalline structures with chiral topographies. This manifestation of molecular chirality at the macroscopic level, which was first recognized by Pasteur with crystals of sodium ammonium tartrate, is revealed in lamellar (smectic) liquid crystal phases by form chirality and spontaneous polarization. For example, the chiral smectic C (SmC*) phase is characterized by a helical structure in which the molecular tilt orientation precesses about the layer normal; it also exhibits a spontaneous polarization (PS) that can be coupled to an electric field E to produce an electro-optical light shutter that is used in high-resolution ferroelectric liquid crystal (FLC) microdisplays. The non-tilted chiral smectic A (SmA*) phase exhibits an electroclinic effect (ECE) described by molecules tilting in response to an applied electric field E; the induced tilt angle scales with E, which enables the generation of a gray scale in displays on a time scale that is orders of magnitude faster than nematic LCDs. Both PS and ECE are chiral bulk properties that depend on the structure of the chiral constituent(s) in the SmC* and SmA* phases, respectively. My research program focuses on understanding the relationship between molecular structure and the bulk properties of liquid crystal phases as a foundation for the rational design of chiral and non-chiral (achiral) components of ferroelectric and electroclinic liquid crystal mixtures. One aspect of this research focuses on the induction of chirality in smectic phases, and how axially chiral molecules can propagate their handedness most effectively via intermolecular interactions in a SmC* liquid crystal phase in order to maximize the spontaneous polarization. Another aspect of this program focuses on the rational design of liquid crystals that undergo a transition from the non-tilted SmA* phase to the tilted SmC* phase with minimal layer contraction. These so-called 'de Vries-like' materials provide a solution to one of the main roadblocks to the use of ferroelectric liquid crystal (FLC) technology in projectors and large flat panel displays: the formation of zigzag defects caused by the buckling of smectic layers upon cooling a FLC mixture from the SmA* to the SmC* phase. These materials are also known to exhibit pronounced electroclinic effects with minimal optical stripe defects and we are leveraging our design expertise to develop new SmA* liquid crystal materials that exceed current standards in electroclinic switching.