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, ASP10 (DS=0.12), ASP40 and ASP40PA60 (DS=0.40), and ASP100 (DS=0.97), are synthesized through azide-alkyne cycloaddition of an alkynyl-functional SP derivative onto C6-azidated-amylose (AN3), with the unconverted azide in ASP40PA60, ASP100, and a sample of the AN3 precursor quenched by clicking with propargyl alcohol (PA). All ASPs are poorly soluble in trifluoroethanol, ethanol and water but soluble in DMSO, DMF and hexafluoroisopropanol irrespective of UV irradiation switching reversibly the grafted SP into zwitterionic merocyanine (MC). In the low polarity tetrahydrofuran only ASP10 and ASP40 are soluble, with ASP40 showing a marked photochromism and reduction of solubility on SP→MC switching. The photochromism observed 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 in such unplasticized rigid amylose-based matrices. The dynamics of both processes, evaluated both for a thin spin coated ASP100 film and thicker ASP40 and ASP100 photoswitchable coatings on glass or paper is more clearly affected than in solution by the grafting density. Finally, switching the nearly apolar SP into the zwitterionic MC does not significantly affect the slightly hydrophobic ASP40 coating wettability