Photomodulation of a Chiral Nematic Liquid Crystal by the Use of a Photoresponsive Ruthenium(III) Complex

We have attempted to control the helical pitch of a chiral nematic liquid crystal by the use of a photoresponsive chiral metal complex. For this purpose, a novel Ru(III) complex [Ru(acac) 2 (L)] was synthesized, where acac and L denote acetylacetonato and 1,3-bis-{4-[6-(4-phenylazo-phenoxy)-hexyloxy]-phenyl}-propane-1,3 -dionato, respectively. The complex underwent cis−trans photoisomerization in methanol when under UV (λ max = 360 nm) or visible (λ max = 450 nm) light illumination, respectively. When the complex was doped into a room-temperature nematic liquid crystal (ZLI-1132), it induced a chiral nematic phase. Under illumination with UV or visible light, the helical pitch of the chiral nematic phase could be changed reversibly by up to 50%. Corresponding to this change, the helical twisting power (HTP) of the complex varied between 34.0 and 22.0 μm - 1, or between −43.9 and −26.6 μm - 1 at 35.0 °C, where the positive and negative signs corresponded to the Λ- and Δ-enantiomers, respectively. This has been the first attempt at combining the ΔΛ chirality of an octahedral metal complex with the cis−trans isomerization of an azobenzene group in the development of a photoresponsive dopant.