A coloring tool for spiropyrans: solid state metal–organic complexation versus salification
In this work, we examine the nature of the spiropyran/merocyanine metal interaction in the solid state and highlight its various linking possibilities. A ring opening isomerisation of spiropyran results in the colored merocyanine, which is only rarely observed in the solid state. Introducing metal salts (ZnCl2, ZnBr2, and CoCl2) to four different spiropyran compounds (i.e. 1,3,3-trimethylindolinobenzopyrylospiran, 1,3,3-trimethylindolino-β-naphthopyrylospiran, 1,3,3-trimethylindolino-8′-methoxybenzopyrylospiran, and 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran) leads successfully to the stabilisation of merocyanine. The ability to form different interactions as a complex, salt or a mixed form was discovered as well as the requirement of different synthetic pathways. Changing the solid form has a strong impact on the absorption properties, resulting in a tool which allows coloring of spiropyrans in the solid state covering a large scale spectrum from red to green.