**Abstract**

Chemosensing properties of Alizarin (**A3**) and Alizarin S (**AS3**) towards anions and cations in acetonitrile are reported. The absorption and fluorescence properties of the two molecular entities were investigated in CH3CN. Based on the excited state intermolecular proton transfer system (ESIPT), the probes were able to collectively discriminate specific cations and anions via colorimetric observations and spectrometric activities. The investigation revealed that **A3** was selective to Cu2+, Fe3+, and Fe2+, compared to Cu2+, Zn2+, Fe3+, and Ni2+ for **AS3.** The disagreement in spectral responses were ascribed to the strong electron withdrawing group present in **AS3**, hence the difference in behaviors. Moreover, the emission properties displayed by **A3** and **AS3** upon molar titrations with cations, were closely similar for all cations, which all nearly experienced fluorescence quenching, except for Zn2+ with **A3**, which exhibited fluorescence enhancement. Similarly, a two-step fluorescence effect was observed in **A3** towards anions, which experienced both fluorescence quenching and enhancement, with incremental additions. The simultaneous fluorescence effects were ascribed to the deprotonation activities experienced by **A3**, as excess anion quantities were added. Thus, the sulfonyl electron withdrawing group had an effect on the Alizarin structure, towards the discrimination of anions and cations, both colorimetrically and fluorometrically.

**Keywords:** ESIPT process, alizarin probe, Fluorogenic sensor, molecular chameleon
