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(13). 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 oft 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. (C) 2010 Elsevier Ltd. All rights reserved.