Photo-isomerization and light-modulated aggregation behavior of azobenzene-based ionic liquids in aqueous solutions

Abstract

Light modulation of the isomerization and aggregation behavior of ionic liquids in aqueous solution is of great importance from a fundamental and technical point of view. In this work, 4 kinds of azobenzene-based photoresponsive ionic liquids 2-hydroxyethyl-dimethyl-[10-(4-phenylazo-phenoxy)-decyl]-ammonium bromide (ChoC₁₀Azo), 2-hydroxyethyl-dimethyl-[6-(4-phenylazo-phenoxy)-hexyl]-ammonium bromide (ChoC₆Azo), 2-hydroxyethyl-dimethyl-[4-(4-phenylazo-phenoxy)-butyl]-ammonium bromide (ChoC₄Azo) and 2-hydroxyethyl-dimethyl-[2-(4-phenylazo-phenoxy)-ethyl]-ammonium bromide (ChoC₂Azo) were designed, synthesized and characterized. The photo-isomerization and light modulation of the aggregation behavior of these ionic liquids were investigated in water using UV-vis spectroscopy, conductivity, dynamic light scattering and small-angle X-ray scattering measurements. The results showed that these ionic liquids had rapid photo-responsive performance, and their photoisomerization efficiencies were greater than 83% after 5 s of UV irradiation. The equilibrium time of the isomerization reaction was about 30 s for ChoC₆Azo, ChoC₄Azo and ChoC₂Azo, and about 10 min for ChoC₁₀Azo. However, there was no significant difference between the equilibrium isomerization efficiencies of these materials. In addition, only ChoC₁₀Azo could form micelles in water among the studied ionic liquids, UV/visible light irradiation only changed the size of its aggregates, but could not change its structure. The results have been used to understand the effect of azobenzene group position and alkyl chain length in the ionic liquids on the photoisomerization and self-assembly of the azobenzene-based ionic liquids in aqueous solutions.