Effect of Grafted Spiropyran on the Solubility and Film Properties of Photochromic Amylose

Four photochromic spiropyran-amylose (ASP) biobased polymers with different spiropyran (SP) grafting density, DS, ASP(10) (DS = 0.12), ASP(40) and ASP(40)PA(60) (DS = 0.40), and ASP(100) (DS = 0.97), are synthesized through azide-alkyne cycloaddition of an alkynyl-functional SP derivative onto C6-azidated-amylose (AN(3)), where the unconverted azide in ASP(40)PA(60), ASP(100), and the AN(3) precursor are quenched by clicking with propargyl alcohol (PA). All ASPs are poorly soluble in trifluoroethanol, ethanol, and water but soluble in dimethyl sulfoxide, N,N-dimethylformamide, and hexafluoroisopropanol irrespective of UV irradiation, switching reversibly the grafted SP into zwitterionic merocyanine (MC). Only ASP(10) and ASP(40) are slightly soluble in the low polarity tetrahydrofuran, with ASP(40) showing a marked photochromism and reduction of solubility on SP -> MC switching. The photochromism in solution is preserved in the solid state, although with significant differences between the still relatively fast SP -> MC photoisomerization and the much slower thermal retro-isomerization. The dynamics of both processes, evaluated in a thin spin-coated ASP(100) film and a thicker ASP(40) photoswitchable coating on glass and paper, is more clearly affected than in solution by the grafting density. Switching the nearly apolar SP into the zwitterionic MC does not significantly affect wettability of the slightly hydrophobic ASP(40) coating.