Synthesis and photochromic response of a new precisely functionalized chitosan with “clicked” spiropyran

A new light responsive polysaccharide based on a precisely defined N-phthaloyl chitosan with a covalent bound photochromic spiropyran moiety in C-6 was synthesized through Cu(I) catalyzed azide–alkyne [3 + 2] dipolar cycloaddition between 6-azido-6-deoxy, N-phthaloyl chitosan and a new spiropyran derivative containing an alkynyl group (SPCC). The reaction product was characterized by NMR and FT-IR spectroscopies for confirming the molecular structure. The UV–vis spectroscopy analyses of films prepared by solution casting on glass from DMF solutions of the functionalized polysaccharide, before and after exposure to an UV lamp (in the 280–350 nm spectral range) or sunlight, showed very slow thermal decay of the photo-induced colored form which was substantially maintained after few months at room temperature. On the contrary, fast color fading was observed for SPCC in ethanol or diethyl ether solution where the color disappeared completely in few minutes at the same temperature. Intermediate color durability was found in diethyl ether suspensions, which indicated the solid polysaccharide phase as responsible of the observed color durability. Considering that the absorption spectra of the polymers are similar under the different conditions examined, the observed effect in the solid state is discussed with reference to the chromophore environment providing molecular and supramolecular hindrance by steric effect and weak binding.