Novel naphthalimide–aminobenzamide dyads as <i>OFF/ON</i> fluorescent supramolecular receptors in metal ion binding

<p>A series of novel naphthalimide–aminobenzamide (NAPIM-2ABZ) dyads <b>3</b> connected by different length polymethylene chains were synthesized and studied as fluorescent supramolecular receptors in metal ion binding. The photophysical properties were evaluated and compared with separated chromophores. The electronic absorption spectra of dyads <b>3</b> showed no interaction between chromophores in the ground state. The fluorescence quantum yields were lower in dyads <b>3</b> in comparison with <i>N</i>-propyl-2-aminobenzamide (<b>8</b>). The fluorescence quenching is attributed to a PET mechanism between fluorophores (from 2ABZ to NAPIM), which is dependent on the polymethylene chain length. In metal binding study was found a response towards transition metal ions such as Hg(II), Cu(II), Zn(II) and Ni(II). Dyad <b>3b</b> presented selectivity towards Cu(II). The UV-vis, IR and <sup>1</sup>H-NMR studies demonstrated the interaction with 2ABZ moiety in the ground state, and interestingly dyads with shorter polymethylene chains <b>3a</b> (<i>n</i> = 0), <b>3b</b> (<i>n</i> = 1) and <b>3c</b> (<i>n</i> = 2) exhibited an <i>OFF/ON</i> fluorescence behaviour due to the PET inhibition and the quenching of 2ABZ fluorescence. Dyads <b>3d</b> (<i>n</i> = 4) and <b>3e</b> (<i>n</i> = 6) presented opposite response <i>ON/OFF</i> in the complex with metal ions evidencing the absence of PET in these dyads.</p>