Investigations of optical activity of natural products and chiral pharmaceuticals using liquid crystal technologies

A liquid crystal-based passive badge for personal monitoring of exposure to hydrogen sulfide

A new liquid crystal (LC)-based passive dosimeter badge for personal monitoring of exposure to hydrogen sulfide (H2S) gas is reported. When a thin film of LC supported on a surface functionalized with lead perchlorate Pb(ClO4)2 (the LC sensor) is exposed to H2S, the orientation of LC molecules in the film changes from perpendicular to parallel. This reorientation induces a change in the appearance of the LC film when viewed between crossed polarizers. A H2S dosimeter was fabricated by pairing a LC sensor with a glass substrate forming a headspace between the two surfaces, to control diffusion of H2S across the LC film. When the dosimeter is exposed to H2S, a bright front appears as a function of exposure time. An algorithm has been developed to correlate this response length and exposure dose. The dosimeters are functionally stable when subjected to extreme temperature and humidity fluctuations, and are immune to a number of potentially interfering chemicals, except mercaptans. These dosimeters detect H2S at 0.2 ppm TWA (8 hr) with ±20% overall accuracy. The dosimeters were used to monitor the personal exposure of personnel working in an oil refinery. The TWA concentrations measured by the LC-based dosimeters correlate strongly with the NIOSH 1063 method that uses a sorbent tube and a pump followed by laboratory analysis. Thus, the LC-based dosimeters can provide a sensitive tool for on-site assessment of personal exposure to H2S in different environments.

Twisted and frustrated states of matter

The World, and much of Nature that we see within it, experiences an environment of reduced symmetries. For example, living organisms are dependent on asymmetric or dissymmetric structures for their life processes. In the solid state, a large number of space groups are chiral. Conversely, in liquids, the effects of reduced symmetries are smeared out owing to the dynamical fluctuations of the constituent molecules, atoms or ions. Thus, on progressing from the strongly ordered solid to the amorphous liquid state, the effects of reduced symmetries weaken as the molecular or atomic correlations and penetration lengths fall. Between these two states of matter, the fourth state of organized fluids can be markedly affected by chirality, and over substantial length scales, owing to both the fluidity and partial ordering of the molecules. In effect, complex fluids can amplify the effects of chirality at the molecular level. Broken symmetries in self-organizing systems can lead to the formation of novel phases of matter and to the creation of structured liquids, and to the generation of nonlinear properties such as heli-, ferro-, ferri- and antiferro-electricity, and electroclinism, which can be harnessed in a wide range of applications including thermal sensors, imaging devices and information displays, to name but a few.

The interplay of configuration and conformation in helical perylenequinones: Insights from chirality induction in liquid crystals and calculations

The chirality transfer in liquid crystals induced by two helical perylenequinones (namely, the natural compounds cercosporin and phleichrome) was investigated by integrating measurements of helical twisting power with a conformational analysis by DFT calculations and with the prediction of their twisting ability by the surface-chirality method. The two quasi-enantiomeric derivatives induce oppositely handed cholesteric phases when introduced as dopants in nematic solvents. We evaluated the role of the different conformations of the chiral hydroxyalkyl side chains in determining the helical twisting power: They were found to affect the strength of the chirality transfer, although the handedness of the induced cholesteric phase is essentially determined by the axial chirality (helicity) of the core of the perylenequinones.

Light-driven reversible handedness inversion in self-organized helical superstructures

We report here a fast-photon-mode reversible handedness inversion of a self-organized helical superstructure (i.e., a cholesteric liquid crystal phase) using photoisomerizable chiral cyclic dopants. The two light-driven cyclic azobenzenophanes with axial chirality show photochemically reversible trans to cis isomerization in solution without undergoing thermal or photoinduced racemization. As chiral inducing agents, they exhibit good solubility, high helical twisting power, and a large change in helical twisting power due to photoisomerization in three commercially available, structurally different achiral liquid crystal hosts. Therefore, we were able to reversibly tune the reflection colors from blue to near-IR by light irradiation from the induced helical superstructure. More interestingly, the different switching states of the two chiral cyclic dopants were found to be able to induce a helical superstructure of opposite handedness. In order to unambiguously determine the helical switching, we employed a new method that allowed us to directly determine the handedness of the long-pitched self-organized cholesteric phase.