**2.1 Materials and apparatus**

Compound **A3** and **AS3** were obtained from a commercial source. All the chemicals and reagents used in this study were all of analytical reagent grade. The anions of Cl− , CN− , OH− , AcO− , Br− , I− , H2PO4 − , HSO4 − , N3 − , NO3 − , OCN− and F− were purchased as tetrabutylammonium salts from Sigma–Aldrich. Absorption measurements were performed on Perkin Elmer Lambda 35 spectrophotometer in a standard 3.0 ml quartz cuvette with 1 cm path length at room temperature. Fluorescence measurements were carried out on a steady state excitation and emission spectra on the Molecular Device SpectraMax M2, Plate Reader.

#### **2.2 Procedures for UV: Vis experiments**

All UV–Vis spectra were recorded in acetonitrile (CH3CN) solvent on a Perkin Elmer Lambda 35 spectrometer by adding Tetrabutylammonium salts while keeping the concentration of **AS** or **AS3** constant (1 × 10−5 M). Tetrabutylammonium salt (TBA) anions of Cl<sup>−</sup> , CN− , OH− , AcO− , Br− , I− , H2PO4 − , HSO4 − , N3 − , NO3 − , OCN− and F− were used for UV–Vis experiments. In addition, a range of heavy metal cation salts (AgNO3, Al(NO3)3, Co(NO3)2, Cr(NO3)3, Cu(NO3)2, Fe(NO3)2, Hg(NO3)2, MnCl2, Ni(NO3)2, Pb(NO3)2, SnCl2, Zn(NO3)2, CdCl2) were used for UV–Vis titrations.
