Tris(indolyl)methene molecule as an anion receptor and colorimetric chemosensor: tunable selectivity and sensitivity for anions

Simple tris(indolyl)methene receptors 1-3 containing conjugated bisindole skeletons have been designed and synthesized. The anion binding and sensing properties have been studied using UV-vis spectroscopy and (1)H NMR titration technique. Compared with 3,3'-bis-indolyl phenylmethene (4), tris(indolyl)methene receptors could highly selectively detect F(-) based on two stages of proton transfer, along with stepwise drastic color changes. The introduction of the electron withdrawing or donating groups into indole unit, which tunes the acidities of the hydrogen bond sites, partially enhanced or inhibited the occurrence of the deprotonation of receptor and has a positive effect on the selectivity and sensitivity of such "proton-transfer" chemosensors for anions.