Photomodulation of the spontaneous polarization of ferroelectric liquid crystals using chiral thioindigo dopants

Ferroelectric liquid crystals (FLC) are chiral smectic C (S{dollar}sb{lcub}rm C{rcub}sp*){dollar} phases that exhibit a spontaneous polarization (Ps). This permanent dipole moment is a macroscopic manifestation of molecular chirality. Between crossed polarizers, a FLC becomes a light valve by switching the orientation of Ps using an applied electric field. Due to the bistability exhibited by FLC light valves, any digital information written in such devices is kept for an indefinite period of time in the absence of the electrical field. The switching of the orientation of Ps can also be achieved optically if the FLC media is photoresponsive. Thus, an erasable optical memory device can be developed.; Optically addressed FLC light valves have been developed based on the photomodulation of Ps via the so-called photomechanical effect, which works on the principle of destabilizing the FLC phase using azobenzene dopants that absorb in the UV region of the spectrum. In the work described here in, we propose the following approach: to photomodulate the spontaneous polarization of a ferroelectric liquid crystal by modulating the transverse dipole moment of a chiral thioindigo dopant. This dopant undergoes a reversible cis-trans isomerization when irradiated in the visible range of the spectrum, and maintains a rod-like shape in both isomeric forms.; In the first part of the project, we synthesized a series of 6,6-dialkoxythioindigo dopants and tested their solubility properties in a variety of S{dollar}sb{lcub}rm C{rcub}{dollar} and nematic liquid crystals. Based on these results, we synthesized the first chiral thioindigo dopant (R,R)-1a with improved solubility in S{dollar}sb{lcub}rm C{rcub}{dollar} hosts. This compound was doped in a phenyl benzoate S{dollar}sb{lcub}rm C{rcub}sp*{dollar} host to induce a ferroelectric S{dollar}sb{lcub}rm C{rcub}sp*{dollar} phase and irradiated to cause trans-cis photoisomerization, which resulted in negligible photomodulation of Ps due to the absence of steric coupling between the thioindigo core and the chiral side-chains.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI); In order to harness the transversal dipole modulation of the core towards Ps modulation, we synthesized a second generation of chiral thioindigo dopants with additional polar substituents at the 5,5{dollar}spprime{dollar} positions, e.g. (R,R)-1b, -1c, which provide the necessary steric coupling with the chiral side-chains. As a dopant in a S{dollar}sb{lcub}rm C{rcub}{dollar} host, R,R)-1b caused Ps to increase by ca. 85% upon trans-cis photoisomerization at 514 nm, while (R,R)-1c caused Ps to increase by ca. 300% upon trans-cis photoisomerization at 532 nm. Control experiments showed that Ps photomodulation is achieved without destabilization of the S{dollar}sb{lcub}rm C{rcub}{dollar} layers. The prototype of a FLC optical switch based on photoinduced Ps sign reversal using dopant (R,R)-1c has been built and tested.