Temperature-responsive photoluminescence of quinoline-labeled poly(N-isopropylacrylamide) in aqueous solution Ioannis Thivaios a , Ioannis Diamantis a , Georgios Bokias a,⇑ , Joannis K. Kallitsis a,b a Department of Chemistry, University of Patras, GR-26504 Patras, Greece b Foundation of Research and Technology Hellas, Institute of Chemical Engineering and High-Temperature Chemical Processes (ICE/HT FORTH), P.O. Box 1414, GR-26504 Patras, Greece article info Article history: Received 7 October 2011 Received in revised form 11 April 2012 Accepted 21 April 2012 Available online 30 April 2012 Keywords: Quinoline Poly(N-isopropylacrylamide) Luminescence pH-control Temperature-control Sensors abstract The pH- and temperature-responsive optical properties of a quinoline-labeled poly(N-iso- propylacrylamide) copolymer are explored in aqueous solution and compared to the respective behavior of a similar quinoline-labeled poly(N,N-dimethylacrylamide) copoly- mer. 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-dimeth- ylacrylamide (DMAM), respectively. Both copolymers exhibit the well-known pH-con- trolled 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. Inter- estingly, this LCST behavior also imposes the partial deprotonation of the otherwise pro- tonated 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-isopropylacryl- amide) chain. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction As a consequence of their important photonic, elec- tronic and conducting properties [1–6], quinoline-based homopolymers and copolymers [7–10], are used in a variety of applications, including polymeric light-emitting diodes [11–13], organic photovoltaic devices [14,15] and selective chemosensors [16]. While solid state quinoline- based materials are needed for most of these applications, the interesting pH-responsive photoluminescence features, arising from the weak basic character of the quinoline group [9,10,17], render soluble quinoline-based polymers promising materials for applications in solution. In fact, it has been well established that, when the respective org- ano-soluble polymer is dissolved in an organic acid like formic acid, the protonation of the quinoline group leads to a strong red-shift (70–100 nm) of the emission spec- trum, as compared to the corresponding spectrum when the polymer is dissolved in a typical organic solvent like chloroform. Moreover, the emission peak of the polymer in the organic acid is much broader and significantly weak- er, a behavior usually attributed to excimer formation when quinoline is protonated in organic solvents [7]. Recently, we have demonstrated [18] that the free radi- cal copolymerization of vinyl functionalized quinoline derivatives with adequate water-soluble co-monomers, like N,N-dimethylacrylamide (DMAM), is a simple and ver- satile procedure to obtain quinoline-labeled water-soluble 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.04.014 ⇑ Corresponding author. E-mail addresses: bokias@upatras.gr, bokias@chemistry.upatras.gr (G. Bokias). European Polymer Journal 48 (2012) 1256–1265 Contents lists available at SciVerse ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj