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) complexes was synthesized and the Λ- or Δ-enantiomers were isolated. The complex shows reversible E/Z isomerization using UV (λmax = 365 nm) or visible light (λmax = 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 system can be reversibly modulated, leading to very high changes in terms of HTP values. Corresponding to this change the HTP value of the doped liquid crystals varied between 615 µm-1 and 346 µm-1 or between - 563 µm-1 and - 352 µm-1 at 30 °C, where the positive or negative sign corresponds to the P and M helix, respectively. Thus, the Co(III) complex possesses both high HTP values and large changes upon photoisomerization which enables induction of the chiral nematic phase with low dopant concentrations, which can be useful for the development of novel optical applications.