Unraveling the Origin of Differentiable 'Turn-On' Fluorescence Sensing of Zn2+ and Cd2+ Ions with Squaramides.

A squaramide ring conjugated with Schiff-bases decorated with hydroxy and methoxy functional groups differentially senses zinc and cadmium ions, which turn on the fluorescence. The feebly emitting free ligands light up in the presence of zinc and cadmium acetates, with the acetate ion playing a pivotal role as a conjugate anion. The selective and differentiable emission responses for zinc and cadmium ions make these ligands efficient multi-analyte sensing agents. Furthermore, these ligands could be used to differentially sense zinc and cadmium ions even in aqueous environments. The NMR investigations reveal marginal differences in the binding of zinc and cadmium ions to the ligands, whereas density functional theory calculations suggest the different extent of ligand-to-metal charge transfer (LMCT) contributes to the differential behavior. Finally, comparison of the excited-state dynamics of free ligand and the metal complexes reveal the appearance of longer lifetime (about 500-700 ps) component with complexation, due to rigidified molecular skeleton, thereby impeding the non-radiative processes.