A naphthylidene-diimine L 2 was newly designed, and its structure was identified by elemental analysis and spectroscopic methods. The effect of temperature, acid–base and light on enol–keto tautomerism in this Schiff base was evaluated by colorimetry, UV–Vis and fluorescence spectroscopy. Under irradiation 365 nm, L 2 emitted yellow, orange and strong green emission in pure, basic and aqueous DMSO media (v/v, 1/1), respectively. Its ionochromic behavior against various cations (Fe3+, Al3+, Cr3+, Cu2+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+, Ba2+ and Ag+) and anions (F−, Cl−, CH3COO−, SO32–, S2O32−, HSO4−, H2PO4−, NO3−, CN−, and OH−) was investigated in aqueous DMSO media (v/v, 1/1) by UV–Vis and fluorescence experiments. Dark yellow color of L 2 changed to colorless for Fe3+, Cr3+ and HSO4− ions, and turned to light yellow for Al3+ and Cu2+ ions, and to orange for CN− and OH− ions. According to UV–Vis data, the chemosensor displayed selective recognition towards Fe3+, Al3+, Cu2+, HSO4−, CN− and OH− with a 1:1 stoichiometric ratio. At the excitation wavelength of 365 nm, L 2 gave strong yellowish white emission (λem = 445 and 539 nm) in the presence of Al3+, and the intensity increased about 12.5 times. On the other hand, the chemosensor displayed one emission band at 452 nm and 450 nm in the presence of CN− and OH− with 1.9 fold and 2.3 fold fluorescence enhancement, respectively.