Design of photonic liquid crystal materials: synthesis and evaluation of new chiral thioindigo dopants designed to photomodulate the spontaneous polarization of ferroelectric liquid crystals

Irradiation of a ferroelectric S-C* liquid crystal phase induced by the photochromic dopants (R,R)-6,6'-bis(2-octyloxy)-5,5'-dinitrothioindigo and (R,R)-5,5'-dichloro-6,6'-bis(2-octyloxy)thioindigo at lambda = 514 and 532 nm, respectively, causes an increase in spontaneous polarization (P-S) by a factor ranging from 1.4 to 4.0. This increase in P-S is achieved without concomitant destabilization of the S-C* phase, and is consistent with an increase in transverse dipole moment of the thioindigo core as a result of trans-cis photoisomerization. The contribution of the thioindigo core towards P-S is achieved through stereo-polar coupling with the chiral side-chains, which is enhanced by the presence of the nitro and chloro substituents.