Photoresponsive Polycations Bearing an Arylazopyrazolium Dye

A set of cationic photoresponsive monomers is synthesized from a phenylazopyrazol dye by N-methylation followed by ion exchange and is converted by free radical polymerization to the corresponding arylazopyrazolium-bearing polycations. By an appropriate choice of the anion, ionic liquid behavior can be implemented in the monomers. Both the monomers and the polymers retain the outstanding spectroscopic properties of their noncharged arylazopyrazole analogs regarding their quantitative reversible E-Z (trans-cis) photoisomerization by alternating irradiation with ultraviolet (UV) and green light. At this, they maintain a rather long half-life of the metastable Z-isomer despite their much more polar character compared to the noncharged chromophore. Furthermore, the polymers show a characteristic solubility behavior in water. While they dissolve only at elevated temperatures, thus showing an upper critical solution temperature, the polymers remain in solution at temperatures as low as 4 °C. Only rapid cooling or freezing and subsequent thawing induce macroscopic phase separation. Characterization by ¹H NMR spectroscopy and by measurements of the ζ-potential, the ion mobility, the surface tension, and cryogenic scanning electron microscopy (cryo-SEM) suggests that the monomer is a hydrotrope and that the particular solution behavior is related to the self-assembly of the dye moieties in the aqueous environment.