<p>A new fluorescent chemosensor based on 1,8-naphthalimide containing an iminodiacetate receptor fragment (N(CH₂COOH)₂) and capable of selectively detecting copper(<span>ii</span>) cations in acetonitrile solution was synthesized. The sensor demonstrates a low level of fluorescence in the absence of metal cations, which is related to photoinduced electron transfer (PET) from the receptor to the naphthalimide chromophore. Binding of Cu²⁺ ions leads to the formation of two complexes with a metal: ligand ratio of 2: 1 and 1: 1 ([Cu₂L]²⁺ and CuL, respectively) and is accompanied by fluorescence enhancement due to the suppression of the PET process in [Cu₂L]²⁺. Spectrofluorimetric titration was used to determine the stability constants of CuL and [Cu₂L]²⁺ particles, which were found to be 7.9•10⁶±2.6•10⁶ L mol⁻¹ and 1.0•10¹²±0.3•10¹² L² mol⁻², respectively. The observed spectral effects were interpreted using the results of quantum chemical calculations.</p>