Photoregulated optical switching of poly(N-isopropylacrylamide) hydrogel in aqueous solution with covalently attached spironaphthoxazine and D-π-A type pyran-based fluorescent dye

A thermoresponsive poly(NIPAM-co-SPO-co-fluorophore) hydrogel comprising poly(N-isopropylacrylamide) that was partially modified using an electron donor-π-conjugated-electron acceptor (D-π-A) pyran-based fluorescent dye using radical copolymerization as well as a photochromic spironaphthoxazine unit displayed a lower critical solution temperature transition from 28 to 50 °C in aqueous solution. The poly(NIPAM-co-SPO-co-fluorophore) copolymer exhibited excellent photochromic behavior in both aqueous solution and hydrogel state. The fluorescence intensity of the poly(NIPAM-co-SPO-co-fluorophore) was temperature-dependent. The relationship between fluorescence intensity change and size of the polymer aggregates was determined using dynamic light scattering. Reversible modulation of fluorescence intensity was achieved using alternating irradiation with UV and visible light. The internal microstructure of the poly(NIPAM-co-SPO-co-fluorophore) hydrogel was investigated using SEM.