Oxidative cyclization of thiosemicarbazone: an optical and turn-on fluorescent chemodosimeter for Cu(II)

A weakly fluorescent thiosemicabazone (L(1)H) was found to be a selective optical and "turn-on" fluorescent chemodosimeter for Cu(2+) ion in aqueous medium. A significant fluorescence enhancement along with change in color was only observed for Cu(2+) ion; among the other tested metal ions (viz. Na(+), K(+), Mg(2+), Ca(2+), Cr(3+), Zn(2+), Cd(2+), Hg(2+), Pb(2+), Ag(+), Ni(2+), Co(2+), Fe(3+) and Mn(2+)). The Cu(2+) selectivity resulted from an oxidative cyclization of the weak fluorescent L(1)H into highly fluorescent rigid 4,5-dihydro-5,5-dimethyl-4-(naphthalen-5-yl)-1,2,4-triazole-3-thione (L(2)). The signaling mechanism has been confirmed by independent synthesis with detail characterization of L(2).