Dual stimuli-responsive viologen-containing poly(2-isopropyl-2-oxazoline) and its multi-modal electrochromic phase transition

Abstract

A viologen-containing thermo-responsive poly(2-isopropyl-2-oxazoline) (PiPOx-V) was synthesized by a simple one-pot reaction. During the cationic ring opening polymerization of 2-isopropyl-2-oxazoline, living cationic polymer chains were reacted with 4,4-bipyridyl to produce dual thermo- and redox-responsive PiPOx-V. An aqueous solution of PiPOx-V showed decreasing transmittance on increasing the temperature to 50 °C through the hydrophilic–hydrophobic phase transition. When sodium dithionite was added to the solution of PiPOx-V, the transparent solution was immediately changed to a blue colored solution, indicating the formation of a radical cation by single-electron reduction. The blue color gradually disappeared by aerobic oxidation. In addition to the color change, the thermal transition temperature of PiPOx-V also changed by redox state change of the viologen unit. Because PiPOx-V exhibited reversible redox behavior with color change, electrochromic behavior was investigated using a sandwich-type ITO glass cell containing an aqueous solution of PiPOx-V (180 mg mL⁻¹) with potassium ferricyanide (20 mM) as an electrolyte. When we applied a 1.5 V electric potential to the ITO glass cell, the solution changed to the violet phase. The violet phase was reversibly changed to the transparent state by the removal of the applied voltage. The PiPOx-V-containing ITO glass cell exhibited four different visual states due to variations in the temperature and electric potential. The thermal and electrochromic changes of PiPOx-V were fully reversible for 100 cycles of repeated voltage shifts.