Temperature-induced phase transition and intramolecular charge-transfer process based on poly(N-isopropylacrylamide) hydrogel with covalently attached D–π–A type dye

A thermoresponsive poly(NIPAM-co-dye) copolymer with covalently attached D–π–A type dye was prepared by typical radical copolymerization. Software was used to calculate the electron density distribution of the push–pull, intramolecular charge transfer (ICT) operating in donor–π-conjugation–acceptor (D–π–A) configurations of dye monomer 3. It can be constructed an acid/base-induced molecular switch by modulation of intramolecular charge transfer with protonation/deprotonation. The lower critical solution temperature (LCST) behavior was investigated by means of UV–vis spectroscopy that allows the measurement of the phase transition from 25 to 40 °C in aqueous solution. The poly(NIPAM-co-dye) copolymer also exhibited color change when used an acid/base-induced molecular switch via control of intramolecular charge transfer (ICT). The morphology of the internal microstructure of the poly(NIPAM-co-dye) hydrogel was observed by scanning electron microscopy (SEM). The reversible switch could be obtained by thermal and acid/base stimuli.