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

A weakly fluorescent thiosemicabazone (L1H) was found to be a selective optical and turn-on fluorescent chemodosimeter for Cu2+ ion in aqueous medium. A significant fluorescence enhancement along with change in color was only observed for Cu2+ ion; among the other tested metal ions (viz. Na+, K+, Mg2+, Ca2+, Cr3+, Zn2+, Cd2+, Hg2+, Pb2+, Ag+, Ni2+, Co2+, Fe3+ and Mn2+). The Cu2+ selectivity resulted from an oxidative cyclization of the weak fluorescent L1H 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.