Unexpected chromogenic properties of 1,3,3-trimethylspiro(indoline-2,3′-[3H]naphtho [2,1-b][1,4]oxazine) in the solid phase: photochromism, piezochromism and acidichromism

Abstract

New chromogenic properties of a spirooxazine {1,3,3-trimethylspiro(indoline-2,3′-[3H]naphtho[2,1-b][1,4]oxazine), SO} are presented in this work. This molecule was found to be photochromic and piezochromic in the pure microcrystalline solid phase and also exhibited acidichromism when kept in contact with a hydrogen-donating solid phase. Photochromism was observed under continuous UV irradiation. Biexponential thermal bleaching followed the interruption of irradiation. Piezochromism was detected by applying a mechanical force to the solid powder, using either a hydraulic press or manual crushing. After discontinuing pressure, biexponential bleaching kinetics were observed. The biexponential nature of the fading processes is interpreted in terms of merocyanine (M) micro-spot formation inside the crystalline phase. The third chromogenic property of the solid spirooxazine, acidichromism, is due to the extraction of protons from an acidic solid matrix, alpha zirconium phosphate [α-Zr(HPO₄)₂·H₂O]. Proton transfer to SO in the solid phase causes the appearance of a purple colour. The spectral evolution is qualitatively similar to that observed in CH₃CN solution when HClO₄ is added at room temperature and in the dark. Temporal evolution of the colour bands, in both solid and liquid phases, is interpreted in terms of an initial protonation, inducing ring-opening, and a second protonation, leading to a colourless product.