Azobenzene-Based Ionic Liquid Switches Phase Separation of Poly(N-isopropylacrylamide) Aqueous Solutions as a Molecular Trigger, Leading to UV Shutdown of Ionic Transport

Abstract

This paper presents a light-switchable phase separation of poly­(N-isopropylacrylamide) (PNIPAm) in aqueous solutions using an azobenzene-based ionic liquid as a molecular trigger. A different UV response was obtained with use of an azobenzene molecular trigger. A UV-induced aggregation of PNIPAm was observed in the current system, which is opposite to that reported for azobenzene-functionalized PNIPAm systems. According to in situ FT-IR, cis-isomers destabilized the hydrogen bonding between water and PNIPAm, contributing to a light-induced aggregation of PNIPAm. This polarity effect was quite similar to salting effects of hofmeister anions on clouding points. Furthermore, a UV shutdown of ion transport of the azobenzene-based ionic liquid was demonstrated with a significant reduction factor of 0.03 at ambient temperature, which can be potentially applied in biotechnology, optoelectronics, and ionic circuits.

Specific Ion Effects of Azobenzene Salts on Photoresponse of PNIPAm in Aqueous Solutions

“…Temperature‐dependent 1 HNMR also suggested that azobenzene‐based ILs stayed in PNIPAm aggregated phase rather than in water phase. [ 18 ]…”

“…[ 16,17 ] Very recently, we reported another method that an azobenzene‐based ionic liquid(IL) located in a “solvent,” rather than in a “solute” (azobenzene‐functionalized polymers), could tune LCST behaviors of PNIPAm in water. [ 18 ] It showed light controlled LCST phase transitions of PNIPAm in water at ambient temperatures. This method could avoid polymerization of azobenzene components since azobenzene monomers were not always conducive to the polymerization process and azobenzene in a molecular position was in favor of rapid phase separation behaviors.…”

“…[ 19 ] Previously, we reported that one ionic liquid functionalized azobenzene compound could switch the LCST behaviors of PNIPAm in aqueous solutions as a molecular trigger. [ 18 ] And a UV‐induced aggregation process was observed, it was due to high polarity of the cis ‐azobenzene‐based ionic liquid attracting the water molecules which were originally interplayed with water to accelerate the dehydration of PNIPAm in water. Such photoresponsive LCST behaviors were intrinsically related to isomerization of azobenzene and ionization of ions.…”

Azobenzene-containing liquid crystalline composites for robust ultraviolet detectors based on conversion of illuminance-mechanical stress-electric signals

“…Till now, the reported works mainly concentrated on the polarity modulation of photoisomerizable moieties via illumination to control the ionic conductivity. Generally, it will take some time for photoisomerizable moiety-containing polymers to reach the photostationary state upon illumination 19 , 20 , 23 . The response time is substantially decided by the polymer selection and composite structures.…”

Azobenzene-Containing Liquid Crystalline Composites for Robust Ultraviolet Detectors Based on Conversion of Illuminance-Mechanical Stress-Electric Signals

“…Generally, it will take some time for photoisomerizable moiety-containing polymers to reach the photostationary state upon illumination. 19,20,23 The response time is substantially decided by the polymer selection and composite structures. Therefore, an elaborate polymer composite of UV responsive polymers effectively integrated with ILs is highly desired for sensitive UV detectors.…”