Synthesis of Biindole-Diazo Conjugates as a Colorimetric Anion Receptor

A colorimetric anion receptor based on an indole scaffold has been synthesized by incorporating an azobenzene chromophore to the 5-position of indole. The receptor binds anions by hydrogen bonds which are effectively transmitted through resonance to the chromophore to give rise to color changes. The cyanide ion induced a pronounced color change from light yellow to reddish orange, but less intense or negligible changes were observed with the other anions examined here.