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.