A colorimetric and absorption ratiometric anion sensor based on indole & hydrazide binding units

• An effective anion sensor L bearing indole & hydrazide units was synthesized.
• Sensor L displayed the colorimetric and absorption ratiometric responses towards F−, AcO− and H2PO4− in DMSO.
• Sensor L underwent NH deprotonation event upon addition of excessive F− in DMSO-d6.

A colorimetric and absorption ratiometric anion sensor (L) based on indole and hydrazide binding units was designed and synthesized, and its recognition & sensing properties towards different anions were studied by naked-eye observations, UV–vis and 1H NMR titration spectra. Sensor L could selectively recognize biologically important F−, AcO− and H2PO4− in DMSO over other anions, along with a significant change in its color and absorption spectrum, resulting from the formation of corresponding 1:2 (L/F−) and 1:1 (L/AcO− and L/H2PO4−) complexes. The 1H NMR titration experiments proved that sensor L experienced deprotonation of NH fragment and produced [HF2]− species, whereas a stable H-bonding complex was formed in the presence of AcO− and H2PO4−.

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