Photoregulation of the Chiral Nematic Phase by Using a Hexa-Arylazopyrazole-Substituted Co(III) Complex

A photoresponsive hexa-arylazopyrazole-substituted tris-(β-diketonato) Co-(III) complex was synthesized, and the Λ- and Δ-enantiomers were isolated. The complex shows reversible E/Z isomerization using UV (λ_max = 365 nm) and visible light (λ_max = 520 nm), respectively. Using the Co-(III) complex as a dopant in the commercially available liquid crystal N-(4-methoxybenzylidene)-4-butylaniline (MBBA), the chiral nematic phase can be induced. Upon photoisomerization, the helical pitch of the doped liquid crystal can be reversibly modulated, leading to very high changes in terms of helical twisting power (HTP or β _M ). Corresponding to this change, the HTP value of the doped liquid crystals varied between 615 and 346 μm^-1 or between -563 and -352 μm^-1 at 30 °C, where the positive and negative signs correspond to the P and M helix, respectively. Thus, the Co-(III) complex effects both high HTP values of the liquid crystal and large changes upon photoisomerization, enabling induction of the chiral nematic phase at very low dopant concentrations, which can be useful for the development of optical applications.