Development of visible-light driven photo-switching copolymers with pH indicating performance

Abstract

Visible-light-driven azobenzene had been reported by theoretical calculation, but could not be detected by any characterization method due to unstable cis structure. Herein, copolymerization was applied to solve the problem based on a time-dependent characteristic of macromolecular motion. Therefore, two copolymers, namely, MRAA-PEG₂₀ copolymer and MRAA-NIPAM copolymer, were synthesized with a definite segment ratio. Both copolymers had photosensitive groups, which were confirmed by UV-vis spectra. Furthermore, copolymer concentration had a linear relationship with absorbance value at 425 nm. Firstly, blue light as an excitation source to induce the isomerization of copolymers. At the same time, repeated interval irradiation was used to evaluate the fatigue performance of the copolymer. Consequently, both copolymers could quickly transform to their cis isomers upon irradiation, which could be reversible recovered to their trans isomers in operable recovery time after removing irradiation. However, the MRAA-PEG₂₀ copolymer had an obvious photobleaching phenomenon along with the circle index, which did not exist in the MRAA-NIPAM copolymer. Secondly, the effects of excitation light source on isomerization were investigated. It was found that blue light was the most efficient excitation source for both copolymers though other light could also induce trans-to-cis transition. Thirdly, the influence of light intensity and temperature was respectively studied. With increasing of light intensity, the absorbance ratio before and after irradiation monotonously decreased, the irradiation response time shortened and the recovery response time prolonged. Moreover, higher temperature resulted in a higher absorbance ratio, shorter irradiation response time, and shorter recovery response time. Finally, synthesized copolymers had characteristic of pH indicator with a critical point of pH 5.0.