The project experimentally investigated two molecular systems in which the linkage of a small number of rigid subunits by flexible connectors produces new or unusual macroscopic phase behavior and interesting physical properties. Thermotropic (“sensitive to temperature”) liquid crystalline oligomers consist of a few small, rigid aromatic molecular units connected by flexible hydrocarbon chains, which collectively assemble to form conventional liquid crystalline phases that depend on temperature (like the orientationally ordered “nematic” phase widely utilized in laptop computer displays, where the molecular units are typically just single molecular units). The linkages within the oligomers may be made in such a way as to enforce a conformational bend over the oligomer length, and in such cases an unusual liquid crystalline state, called the “twist-bend nematic” phase, may occur. The molecules in the twist-bend phase bend along, and simultaneously twist around, the preferred axis of orientation in a heliconical fashion that resembles a barber pole. Our team elucidated key properties of the twist-bend phase, including both fundamental aspects, such as a more ordered state wherein a mass density wave coexists with the heliconical orientational structure (“twist-bend smectic” phase), and certain properties with potential technological application, such as light-induced variations of the bulk alignment of the heliconical axis and/or pitch in liquid crystalline dimers containing light sensitive moieties.

The team also investigated a quite different system: nucleic acid-based liquid crystals whose constituents possess a similar architecture to the small molecule oligomers, though on an approximately ten times greater length scale. In this system, a “rigid-flexible-rigid” construct, dubbed “gapped DNA”, is created by the connection of relatively stiff double-stranded DNA molecules with a flexible single-strand of unpaired bases. Although elementary liquid crystal formation in concentrated solutions of fully paired DNA has been known for decades, our team and collaborators recently uncovered evidence of a more highly ordered, layered liquid crystalline phase (called a “smectic” phase) in solutions of gapped DNA. In the present project, the team discovered variants of the layer structure, whose occurrence depends on factors such as the length of the single strand linker and the overall concentration of DNA in the solution, and observed reversible, temperature driven transitions between the different layer structures (thereby establishing a remarkable combination of thermotropic and “lyotropic”, or concentration-dependent, phase behavior in this system). The team demonstrated that the thermotropic behavior can be used as a sensitive gauge of the relative strength of both end-to-end and side-to-side interactions between DNA duplexes, at near physiological concentration, as a function of the specific terminal base-pairings on the duplexes, the concentration of divalent salt in the solution, or the degree of non-randomness in certain base sequences along the duplex – factors that are biologically significant. These results may draw broader interest toward exploiting liquid crystalline behavior to develop “assays” of various compositional factors that affect duplex-duplex interactions and DNA condensation in dense DNA solutions.

During the early stages of the project, the announcement and subsequent confirmation of a fluid ferroelectric phase in a class of highly polar liquid crystalline molecules – a long-sought goal in liquid crystals’ science – inspired an expansion in the scope of the team’s research on small-molecule liquid crystals. One key outcome of the expanded research direction was the team’s discovery in certain polar compounds, synthesized by a colleague at Kent State, of multiple ferroelectric phases that are distinguished by the nature of the short-range positional order between the molecules. The team also showed that the fluid ferroelectric state can be extended to lower (near room) temperatures, with a remarkable improvement of the relaxation time for electro-optical switching, when the host liquid crystal is doped with a chiral molecular additive. At higher dopant concentration, the ferroelectric phase exhibits a chiral structure that selectively reflects light within the visible range. The reflection band is reversibly tunable by a much smaller electric field than required for reflection band tuning in conventional chiral liquid crystal systems. These results have promising implications for various types of electro-optic device development.

Six graduate students in physics or chemical physics programs at Kent State participated in project-related research activities and were mentored toward their Ph.D. degrees under support from the grant. Four of these students graduated, and three secured jobs in US industry, during the grant period. The participating students all received multidisciplinary training in the laboratories of the PIs and most had opportunities to conduct research at national laboratories, including the National Synchrotron Light Source II (Brookhaven National Lab) and the Advanced Light Source (Lawrence Berkeley National Lab). Project or project-related research resulted (so far) in nineteen peer-reviewed journal publications, as well as a large number of academic conference presentations (the majority given by graduate students on the team).

Last Modified: 12/28/2023
Modified by: Samuel N Sprunt