Temperature-responsive photoluminescence of quinoline-labeled poly(N-isopropylacrylamide) in aqueous solution

The pH- and temperature-responsive optical properties of a quinoline-labeled poly(N-isopropylacrylamide) copolymer are explored in aqueous solution and compared to the respective behavior of a similar quinoline-labeled poly(N,N-dimethylacrylamide) copolymer. These copolymers, P(NIPAM-co-SDPQ) and P(DMAM-co-SDPQ), were prepared through free radical copolymerization of 2,4-diphenyl-6-(4-vinylphenyl)quinoline (SDPQ) with the thermosensitive N-isopropylacrylamide (NIPAM) and the hydrophilic N,N-dimethylacrylamide (DMAM), respectively. Both copolymers exhibit the well-known pH-controlled optical response of quinoline unit in aqueous solution and the emitted color changes from blue to green upon decreasing pH. Nevertheless, a ～20 nm emission shift is observed upon heating the aqueous P(NIPAM-co-SDPQ) solution, regardless of pH, due to the formation of hydrophobic microdomains (Nile Red probing), as a consequence of the Lower Critical Solution Temperature (LCST) behavior of this copolymer in water. Interestingly, this LCST behavior also imposes the partial deprotonation of the otherwise protonated SDPQ unit at pH = 2 and the emission of the basic form appears upon increasing temperature, suggesting that the acid/base equilibrium of the quinoline unit is significantly temperature-controlled, when introduced in the thermosensitive poly(N-isopropylacrylamide) chain.