Five novel soluble chemosensors in EtOH/H₂O based on Schiff-base diaminomaleonitrile derivatives which were modified by different aromatic functional moieties have been synthesized. These chemosensors were fully characterized by ¹H NMR, ¹³C NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction. The recognition abilities of these chemosensors with a range of metal ions at different pH values were evaluated and their photophysical properties have been systematically investigated. DFT theoretical calculations were employed to understand the behaviors of the chemosensors toward Hg²⁺. The sensing mechanism was derived through experimental and theoretical calculations. All the results consistently indicated that these diaminomaleonitrile derivatives were ideal chemosensors for the detection of Hg²⁺ in aqueous solutions.