Colorimetric response of spiropyran derivative for anions in aqueous or organic media

We previously found that a simple spiropyran derivative (1:1′,3′,3′-trimethyl-6-nitro-spiro-[2H-1-benzopyran-2,2′-indoline]) behaves as a selective and sensitive cyanide anion (CN−) receptor in aqueous media under UV irradiation. The receptor, when irradiated by UV light in a water/MeCN mixture, creates a CN−-selective absorption band via a nucleophilic addition of CN− to 1 (formation of the 1–CN− species) and allows quantitative determination of very low levels of CN−. In the present work, effects of pH and water content on the response of 1 to anions were studied to clarify the detailed properties of 1. In aqueous media, 1 reacts selectively with CN−regardless of pH and water content, but the reaction is suppressed by a decrease in pH and an increase in water content due to the protonation of CN−. In contrast, in pure MeCN, addition of F− also creates a new absorption band, as does CN−. This is promoted via a nucleophilic interaction between 1 and F− in a 1:2 stoichiometry (formation of the 1–2F− species). The 1–CN− and 1–2F− species have different photochemical properties; the 1–CN− species is stable upon UV irradiation, while the UV irradiation of the 1–2F− species leads to a decomposition of the spiropyran platform.