Fluoride indicator that functions in mixed aqueous media: hydrogen bonding effects

A urea-based fluoride anion indicator, 3, that functions in mixed aqueous media is reported. Under these conditions, indicator 3 and the two control systems 1 and 2 are colorless. Addition of fluoride anions produces an easy-to-visualize colorimetric response (colorless to yellow) in the case of 3, but not 1 or 2 as the result of anion-induced Si–O bond cleavage. Most of the colorimetric response is believed to stem from an alteration in the hydrogen bonding network, rather than changes to the π-conjugation pathway per se.