Fluorescence ratiometric selective recognition of Cu(2+) ions by dansyl-naphthalimide dyads

Chimeric dyads 1a and 1b based on dansyl and naphthalimide units linked through the polymethylene group were synthesized, and their photophysical and interactions with various metal ions were investigated under different conditions. These dyads showed dual emission centered at around 375 and 525 nm, respectively, due to the locally excited state of the naphthalimide chromophore and energy-transfer-mediated emission (FRET) from the dansyl moiety. When titrated with various metal ions, these systems exhibited unusual selectivity for Cu(2+) ions as compared to Na(+), Li(+), K(+), Zn(2+), Pb(2+), Hg(2+), Co(2+), Fe(2+), Cd(2+), Mg(2+), and Ba(2+) ions and signaled the binding event through inhibition of FRET mediated emission at 525 nm with concurrent enhancement in the emission intensity of the naphthalimide chromophore at 375 nm. The uniqueness of these dyads is that they form stable 2:1 stoichiometric complexes involving sulfonamide functionality and act as visual fluorescence ratiometric probes for the selective recognition of Cu(2+) ions.