Simple indole-based colorimetric sensors with electron-withdrawing chromophores: Tuning selectivity in anion sensing

Three simple colorimetric anion sensors (1, 2, and 3) containing anthrone, 1,3-indanedione, and malononitrile as signaling chromophores and an indole binding site have been designed and synthesized. The introduction of electron-withdrawing groups can not only provide chromogenic signal output, but also tune the sensitivity and selectivity of indole-based anion sensors by electron push–pull features. Their anion binding and sensing properties were investigated in detail by dramatic color changes, UV–vis absorption, and 1H NMR. As results revealed, sensor 1 showed high selectivity for F− over AcO− and H2PO4− with a distinct change in color due to the deprotonation of indole NH group. The excellent selectivity of 1 for F− can be attributed to the fitness in the acidity of its NH-group, which is tuned to be able to distinguish the subtle difference in the affinity of F−, AcO−, and H2PO4− to NH proton.