Tuning the Properties of Hydrazone/Isosorbide-Based Switchable Chiral Dopants.

ABSTRACT

The long-range supramolecular interactions in liquid crystals (LCs) can be used to amplify and subsequently propagate microscopic structural changes into macroscopic events. Here, we report on a systematic structure-property analysis using 16 chiral photoswitchable dopants composed of bistable hydrazones and chiral isosorbide moieties. Our findings showcase the relationship between the dopant's structure and its helical twisting power (ß), and hence, the photophysical properties of the host LC. We show that an increase in the hydrazone CNNH dihedral angle results in an increase in the ß value, while alkoxy chains do not lead to such an increase. These results contradict established rules of thumb, stating that structural rigidity and long alky chains are needed for high ß values. We also found that the position of the substitution, whether at the 2' or 5' positions of the isosorbide unit, or the attachment of the chiral unit to the rotor or stator phenyl units can have negative or positive additive effects that can either increase or decrease the ß values. These results made us hypothesize that unsymmetrically functionalized dopants should result in large Δß values, which we corroborated experimentally. Moreover, a fluorine-functionalized dopant resulted in higher overall ß values, most likely because of π-π interactions. Finally, the dopants were used in modulating and locking in the reflective properties of LC films, yielding multicolor LC canvases that can reflect light from the ultraviolet to the infrared range (i.e., a manipulation of up to ca. 1500 nm of reflected light).