Highly sensitive and selective ratiometric fluorescent copper sensors: Different binding affinities modulated by three separate side chains of naphthalimide

A series of compounds 1-11 with different side chains of naphthalimide as fluorescent copper sensors were designed and synthesized. Compounds 1, 9, 10 and 11 presented a high selectivity to Cu²⁺ in a neutral aqueous environment. Here 1, 9 and 10 showed selectivity and affinity to Cu²⁺ with an association constant of about ̃10⁶. It gave somewhat response to Ag⁺, Co²⁺, Ni²⁺ and Fe²⁺ while 1 detected copper. 9 and 10 displayed better selectivity by changing their hydrophobic side chains to the hydrophilic ones on imide moieties. 11, with one flexible side chain, showed high selectivity and an association constant (K _a = 2.2 × 10⁸), which were much higher than those of 1, 9 and 10. These results indicated that the selectivity and affinity could be improved by changing side chains of naphthalimide. That might provide a novel strategy or method for the development of fluorescent sensors.