Tuning the functionalization degree of amylose and amylopectin with photochromic spiropyran by CuAAc reaction

Amylose and amylopectin were functionalized with an alkynyl derivative of spiropyran (SPCC) and/or propargyl alcohol by a two step procedure consisting in the derivatization of glucopyranose units with azide at the C6 position followed by Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAc). Substitution degrees with spiropyran (DSSP) between 0.10 and 0.95 were achieved by simply modulating the molar ratio between alkyne species and polysaccharide in the CuAAc reaction, as it was confirmed by FT-IR and NMR spectroscopy characterizations. The UVeVis spectra of SPCC-modified amylose in DMF solution kept in the dark exhibited an absorbance band with maximum at ~346 nm, ascribed to spiropyran units. The intensity of the absorbance was found to increase linearly with DSSP. Upon irradiation with UV light, a band at ~560 nm appeared, due to the conversion of spiropyran to zwitterionic merocyanine, its intensity linearly increasing with DSSP too. ICP-MS analysis showed the presence of a small amount (<270 ppm) of residual Cu from the catalyst in the functionalized polysaccharides. However, EPR spectroscopy indicated that Cu2+ species are bonded to different moieties and they do not significantly interfere with the optical behaviour. These findings indicate that the proposed synthetic strategy can be exploited to prepare photochromic materials with the desired signal intensity by using natural amylose as raw material.