A reversible conductivity modulation of azobenzene-based ionic liquids in aqueous solutions using UV/vis light

Abstract

Photo-induced conductivity modulation of stimuli-responsive materials is of great importance from the viewpoint of fundamental research and technology. In this work, 5 new kinds of azobenzene-based photo-responsive ionic liquids were synthesized and characterized, and UV/vis light modulation of their conductivity was investigated in an aqueous solution. The factors affecting the conductivity modulation of the photo-responsive fluids, such as photo-isomerization efficiency, photo-regulation aggregation, concentration and chemical structure of the ionic liquids, were examined systematically. It was found that the conductivity of the ionic liquids in water exhibited a significant increase upon UV light irradiation and the ionic liquids with a shorter alkyl spacer in the cation showed a more remarkable photo-induced conductivity enhancement with a maximum increase of 150%. In addition, the solution conductivity was restored (or very close) to the initial value upon an alternative irradiation with visible light. Thus, the solution conductivity can be modulated using alternative irradiation with UV and visible light. Although the reversible photo-isomerization of the azobenzene group under UV/vis irradiation is the origin of the conductivity modulation, the photo-regulated aggregation of the ionic liquid in water is indispensable for the maximum degree of conductivity modulation because UV irradiation can weaken, even break the aggregated cis-isomers of the ionic liquids in an aqueous solution.

Photo-Triggered Reversible Phase Transfer of Azobenzene-Based Ionic Liquid Surfactants between Oil and Water

“…On the other hand, by alternatively irradiating with visible light, the azobenzene-based IL surfactants could move back to organic phase because of the reverse isomerization of azobenzene group from cis- to trans -isomer. Therefore, the transfer of the photo-responsive IL surfactants between the organic phase and water phase originates from the increase in hydrophilicity of them after isomerism of its azobenzene groups [30]. The degree of isomerization of the IL surfactants from trans- to cis -isomers would have an important effect on the transfer efficiency of them.…”

“…In addition, the aggregation results could be used to explain the effect of IL surfactant concentration on transfer efficiency of [C 4 AzoC 2 DMEA]Br presented in Figure 3. Our previous work [30] shows that the critical aggregation concentration (CAC) of [C 4 AzoC 2 DMEA]Br in water is 2.30 × 10 −3 mol/kg before UV irradiation, and the CAC value increases to 1.30 × 10 −2 mol/kg after UV irradiation (see Figure S16 in Supplementary Materials). This result suggests that trans -[C 4 AzoC 2 DMEA]Br could form aggregates in water before UV irradiation within the concentration range investigated here.…”

“…The azobenzene-based IL surfactants were prepared and purified by using the procedures described in our previous work [30]. The chemical structures of them were confirmed by 1 H NMR, and their purity was found to be greater than 97% in mass fraction.…”

Stimuli‐Responsive Ionic Liquids and the Regulation of Aggregation Structure and Phase Behavior†

“…Light‐responsive ILs refer to the light responsive‐ability of photosensitive groups, also known as photosensitive ILs, mainly including azobenzene, [ 52 ] cinnamic acid, [ 53 ] spirogyra, [ 54 ] diarylethene, [ 55‐56 ] etc . The structure of light‐responsive ILs can be changed by light, further affecting the melting point, [ 55,57‐59 ] rheological properties, [ 60 ] hydrophilicity‐hydrophobicity, [ 61 ] and conductivity [ 61‐63 ] of ILs. Furthermore, it can also regulate the structure of the solution and the phase behavior of samples based on ILs.…”

“…However, compared with [C 4 AzoC 2 DMEA][Br] with shorter cationic alkyl spaces, for [C 4 AzoC 4 DMEA][Br] and [C 4 AzoC 6 DMEA][Br] with more extended cationic alkyl spaces, the aggregation of cis ‐isomers under UV light irradiation was only slightly weakened, and the conductivity increased less in aqueous solution (Figure 8). [ 61 ]…”